Genesis of the ultradian rhythm of GH secretion: a new model
unifying experimental observations in rats
Clemens
Wagner1,
S. Roy
Caplan1,2, and
Gloria S.
Tannenbaum1
1 Departments of Pediatrics, and Neurology and
Neurosurgery, McGill University and the Neuropeptide Physiology
Laboratory, McGill University-Montreal Children's Hospital Research
Institute, Montreal, Quebec, Canada H3H 1P3; and
2 Department of Biological Chemistry, Weizmann
Institute of Science, Rehovot, Israel 76100
 |
ABSTRACT |
Growth hormone (GH) induces growth in animals and humans and
also has important metabolic functions. The GH neuroendocrine axis
consists of a signaling cascade from the hypothalamus to the pituitary,
the liver, and peripheral tissues, including two major feedback
mechanisms. GH is secreted from the pituitary into the circulating
blood according to the effect on the somatotrophs of two hypothalamic
peptides, GH-releasing hormone (GHRH) and its antagonist, somatostatin
(SRIF). The typical GH profile in the male rat shows secretory episodes
every 3.3 h, which are subdivided into two peaks. Focusing on the
mechanisms for generation of this ultradian GH rhythm, we simulated the
time course of GH secretion under a variety of conditions. The model
that we propose is based on feedback of GH on its own release mediated
both by GH receptors on SRIF neurons in the brain and by a delayed SRIF
release into both the brain and portal blood. SRIF, with a resultant
periodicity of 3.3 h, affects both the somatotroph cells in the
pituitary and the GHRH neurons in the hypothalamus. The secretion of
GHRH is postulated to occur in an ~1-h rhythm modulated by the level of SRIF in the hypothalamus. The model predicts a possible mechanism for the feminization of the male GH rhythm by sex steroids and vice
versa, and suggests experiments that might reveal the proposed intrinsic 1-h GHRH rhythm.
somatostatin; growth hormone-releasing hormone; hypothalamus; growth hormone receptor; mathematical model
 |
INTRODUCTION |
GROWTH HORMONE (GH) is secreted in
a pulsatile manner from the pituitary gland, resulting in an
oscillating time course of GH concentration in the circulating blood.
In the adult male rat, the regular time course of GH is characterized
by biphasic secretion episodes every 3.3 h separated by intervening
trough periods with undetectable basal levels (39). The secretion is
governed by the two hypothalamic neuropeptides, GH-releasing hormone
(GHRH) and somatostatin (SRIF), which have stimulatory and inhibitory effects, respectively, on GH release. SRIF neurons are located in the
periventricular nucleus (PVN), as well as in the arcuate nucleus (ARC),
of the hypothalamus, whereas GHRH cells predominantly reside in the
ARC. The axons of these neurons course to the median eminence where
they release the neurohormones into the portal blood. GH exerts a
negative feedback on its own secretion at the level of the hypothalamus
by regulating the secretion of SRIF (short loop feedback). In
the liver, GH regulates the secretion of insulin-like growth factors
(IGF-I and IGF-II), which exert growth-promoting effects in peripheral
tissues; IGF-I and IGF-II constitute the long loop feedback of the GH
neuroendocrine axis by suppressing GH release at the level of the
pituitary and/or hypothalamus (for reviews, see Refs. 14 and
35).
Although many experiments were performed to elucidate the
interrelationship between SRIF and GHRH in GH regulation, the
mechanisms underlying normal pulsatile GH secretion remain obscure.
Chen et al. (10) derived an elaborate model involving eight
differential equations and five arbitrary feedback functions, which was
too complex to explain the actual function of the system. We address the problem using a model that focuses on the generation of the 3.3-h
rhythm of GH secretion. Due to the time scale examined, short-term
alterations were assumed to be in pseudoequilibrium with the relevant
variables, whereas long-term effects were treated as constants. This
allows us to propose a model of the ultradian rhythm of GH secretion
using only two differential equations: one for GH and one for SRIF.
GHRH is assumed to be generated independently of GH and possesses an
intrinsic rhythm with a mean period of ~1 h. We found that the major
feedback mechanism for generation of the 3.3-h rhythm is the short
feedback loop via SRIF. The release of SRIF due to GH is delayed, which
reflects the kinetics of the signaling pathway in these cells. SRIF,
with a resultant periodicity of 3.3 h, affects both the somatotroph
cells in the pituitary and the GHRH neurons in the hypothalamus. The
inclusion of two sites of SRIF action in this model resolves
experimental results that have so far eluded explanation. The model
also predicts a possible mechanism for the change of the male GH rhythm
into the female pattern by sex steroids and vice versa, and it suggests experiments that might reveal the proposed intrinsic 1-h GHRH rhythm.
 |
METHODS |
Basis of the Model
Pituitary GH secretion.
The interrelationship of SRIF and GHRH in GH regulation at the level of
the pituitary was investigated using an adult rat model. From these
experiments, Tannenbaum and Ling (38) postulated that SRIF and GHRH are
released in reciprocal 3- to 4-h cycles from the hypothalamus into the
hypophyseal portal blood to generate the ultradian GH rhythm. These
conclusions were corroborated by measuring the concentrations of
immunoreactive GHRH and SRIF in hypophyseal portal plasma (30). Our
model is based on these well-established findings.
GH feedback via SRIF.
GH given centrally (34) or peripherally (44) suppresses the spontaneous
bursts of GH in the rat. Experimental evidence indicates that the
feedback effect of GH on its own secretion is exerted by increasing
hypothalamic SRIF release into the portal blood (11). In vitro studies
show that the pituitary gland is not the site of GH feedback inhibition
(20). The GH feedback effect also results in a high-frequency pulse
train of GH (~1 h), unmasked by injecting SRIF antibody (Ab) after GH
treatment (21), suggesting that the GH rhythm is generated by an
intrinsic high-frequency (~1 h) GHRH release.
Mediation of GH feedback by GH receptors on SRIF cells.
GH receptor (GHR) transcripts have been found to colocalize with SRIF
transcripts in the PVN (5). Because there is experimental evidence that
GH does not affect GHRH cells in the ARC (25), we assume that the
feedback of GH is mediated via GHR on SRIF neurons. The binding of GH
to its receptor results in a dimerization of GHR and activates a
variety of signaling molecules (7). Central injection of GH increases
SRIF release into portal blood with a delay of 40-80 min (11),
which is determined by the above-mentioned signaling cascade. In the
simulations, we used a generally accepted model for the GHR turnover
(1, 16).
Further mediation of GH feedback by SRIF receptors in brain.
Evidence has been provided for the association of SRIF receptors with a
subpopulation of GHRH-containing ARC neurons (3, 23, 41), implying
direct regulation of the GHRH hypothalamo-hypophyseal system by SRIF.
In addition, ultradian oscillation in SRIF binding within the ARC in
synchrony with the pattern of GH secretion has been shown (36). These
results suggest that the feedback of GH on GHRH neurons in the ARC is
mediated via SRIF, at least on the timescale of a secretion episode.
Long loop feedback involving IGFs.
Although there is convincing in vitro evidence that the IGFs affect GH
secretion (2), the effect of the IGFs in vivo is still poorly
understood. At the level of the brain, it appears that a synergistic
interaction of IGF-I and IGF-II is required to reduce the amplitude of
GH pulses 2-3 h after injection (15). Furthermore, no modification of
plasma IGF-I levels was observed after acute or chronic GH
administration (21), indicating that GH-induced negative feedback can
operate independently of changes in IGF-I. We assume that the IGFs are
involved in a feedback loop but that it is not essential for generation
of the 3.3-h rhythm of GH.
Male-female dimorphism.
In adult rats, there is a striking sex difference in the pulsatile
pattern of GH secretion. In contrast to the regular 3.3-h periodicity
in the male, females exhibit more frequent, irregular, low-amplitude GH
pulses with an elevated baseline (see Ref. 19 for review). The level of
GHR in the liver is lower in males than in females (31). It has been
postulated that, in the female rat, SRIF release into portal blood is
continuous, whereas GHRH release is erratic with a high frequency (12,
27). Administration of the female sex steroid estradiol to male rats
converts the male GH rhythm to a female-like pattern (28), whereas,
conversely, the male sex steroid testosterone masculinizes the female
GH secretory profile (35). Because the high-frequency pattern of GH
secretion is observed in the female rat, we used the transition between the male and female GH profiles to justify the assumed intrinsic 1-h
rhythm of GHRH in the male rat.
GH secretagogues.
A novel class of synthetic compounds with potent GH-releasing activity,
termed GH secretagogues (GHSs), has been described (4, 33). The
cellular mechanisms involved in the actions of GHSs are different from
those of GHRH (33). A receptor for GHS (GHS-R) has been cloned recently
(18), and there is anatomic evidence that the GHS-R is expressed by
GHRH neurons (37), suggesting that an additional neuroendocrine pathway
may exist to regulate pulsatile GH secretion. However, the endogenous
ligand for GHS-R has not, as yet, been identified. Thus GHS is not
included in the model.
Description of the Model
Mathematical implementation.
The scheme of the model used for the simulation is presented in Fig.
1. The numbering convention refers to SRIF
as species 1, GHRH as species 2, GH as species
3, and GH receptors as species 4. The symbol A refers to
the hypothalamus, B to the portal blood or pituitary, and C to the
circulation. Concentrations are denoted by c, with an appropriate
subscript to indicate the species and superscript to indicate the zone.
All concentrations are time dependent, unless they are doubly
subscripted, in which case they are parameters of the
model with assigned values (see Table 1).

View larger version (31K):
[in this window]
[in a new window]
|
Fig. 1.
Schematic drawing of the signaling pathway of the growth hormone (GH)
neuroendocrine axis for generation of the ultradian GH rhythm. From the
hypothalamus, somatostatin (SRIF) and GH-releasing hormone (GHRH) are
released into the portal blood according to the concentration time
courses indicated (~3.3-h intervals). At the pituitary, the
reciprocal release patterns of GHRH and SRIF result in a GH profile in
the circulation as shown. Feedback of GH on the release of SRIF is
mediated via GH receptors on SRIF cells in the periventricular nucleus
(PVN) of the hypothalamus, delayed by the time . SRIF is secreted
both into the portal blood and within the hypothalamus at the
level of the arcuate nucleus (ARC), according to a partition
coefficient denoted by . In the ARC, the GHRH neurons show an ~1-h
rhythm, the release being amplitude modulated (M) by SRIF. SRIF neurons
are located in the ARC and the PVN, as suggested by the vertical dashed
line dividing the hypothalamus. The numbers and letters in parentheses
and brackets correspond to the notation used for the equations (see
text).
|
|
In our model, the 1-h rhythm of GHRH is taken to be generated
intrinsically. This means that the mechanism by which the GHRH secretory profile is realized is independent of GH on the time scale
investigated and remains to be elucidated. The amplitude of the GHRH
concentration pulses, cB2, is governed by the
SRIF level in the hypothalamus, cA1, in such a way that a high concentration of SRIF results in a low GHRH amplitude
and vice versa. The duration of the secretion is taken from a time
series analysis (see Parameters in
RESULTS). Hence the time course of the GHRH
concentration in portal blood can be described by the
function
|
(1)
|
where cB2,max and
cB2,min denote the maximal and minimal
amplitude of GHRH, respectively, t denotes time, and the modulo
function (mod) denotes the fractional remainder on dividing t
by T. The periodicity of the GHRH profile is given by
T, whereas T1 represents the secretion
duration. To account for the modification of the GHRH amplitude by SRIF
in the brain, we invoke a downregulatory function f incorporating a
Hill function with a threshold cA1,th and an
exponent n1
|
(2)
|
Note that Eq. 1 is simply an algebraic construct
("ansatz") describing the train of modulated GHRH pulses
generated in the arcuate nucleus, as suggested by the study of Lanzi
and Tannenbaum (21). Equation 2 accounts for the modulation, a
putative downregulation of GHRH release by SRIF binding, presumably to
GHRH neurons. The binding here (and elsewhere) is described by a Hill
function, since this is the simplest possible model of cooperative
ligand binding. The fractional degree of saturation of the receptors, given by the ratio in Eqs. 2, 4 (which also represents
downregulation), and 5, is 50% at the threshold concentration,
which is also close to the point of inflection of the sigmoid curve,
especially at high values of the Hill coefficient where cooperativity
is high. Hence this concentration marks the threshold between low
degrees of saturation and high degrees of saturation. The antagonizing effect of SRIF on GHRH-induced release of GH is represented by an
appropriate combination of two Hill functions. The clearance of GH in
the circulation can be described rather well by a single exponential
(9). The time course of GH is therefore given by
|
(3)
|
where the down- and upregulatory functions f and g are,
respectively,
|
(4)
|
|
(5)
|
Here n2 and n3
represent Hill coefficients. The rate constants for clearance and
release are k3,cl and k3,r,
respectively. The thresholds used to distinguish whether GHRH or SRIF
are high or low are denoted by cB2,th and
cB1,th.
The mechanism for the turnover of the GHR is shown in Fig.
2. We use the following two simplifying
assumptions: 1) the total amount of GHR is not changed during
the observed time, which supposes that synthesis and degradation cancel
each other, and 2) the factor that determines the kinetics in
the feedback loop is the delay between the binding of GH and the
release of SRIF (see Parameters) and not the
dynamics of the receptors. Thus we approximate the time course of the
receptors by the steady-state solution. Furthermore, we assume that the
signal transmitted in the SRIF cells that governs the release of SRIF
is proportional to the concentration of internalized receptors,
cA4, given by the following equation (see APPENDIX)
|
(6)
|
with
|
(7)
|
|
(8)
|
where
and
are receptor functions, and
k4,i, k4,ri, and
k4,d are the rate constants for internalization,
reinsertion of GHR, and dissociation of GH, respectively. The binding
constant is denoted by Ka, and c4,tot
is the total amount of GHR in the hypothalamus. For the male and female
parameter sets, the dependence of internalized receptors on GH
normalized by c4,tot is shown in Fig. 2, inset.

View larger version (17K):
[in this window]
[in a new window]
|
Fig. 2.
Model for the turnover of GH receptors on SRIF cells in the
hypothalamus. GH binds to the receptors with a 1:2 stoichiometry.
Symbols in parentheses correspond to the notation used for the
equations in text. Inset shows the concentration of
internalized receptors, cA4, as a function of
GH in the circulation both for the male ( = 0.05;
c4,tot = 0.00094 ml/ng; solid line) and the female
( = 8; c4,tot = 0.94 ml/ng; dashed line),
normalized by c4,tot.
|
|
Due to the signaling pathway in the cells, the secretion of SRIF into
the portal blood and the hypothalamus is delayed by
with respect to
the concentration of internalized receptors (11, 34). Furthermore,
is taken to represent the sum of all delays in the GH feedback loop
(26); including the fast clearance observed for SRIF, its time course
is given by
|
(9)
|
where k1,cl and k1,r
denote the rate constants for clearance and release of SRIF,
respectively, and the incorporation of the delays in the time
dependence of cA4 is shown explicitly. The
secretion of SRIF is considered to be divided into two parts. One part
is released into the portal blood, cB1,
whereas the other part acts on the GHRH neurons in the hypothalamus,
cA1. This yields the following partitioning
|
(10)
|
|
(11)
|
where
denotes the partition coefficient.
 |
RESULTS |
Simulations and Comparisons with Experiments
Calculations.
Simulations of time-dependent concentrations of SRIF, GHRH, and GH were
performed using the Euler algorithm with a time step of 0.01 min.
Figures 3-6 do not show the initial transient, and the zero point
of the time window presented was shifted to match the experimental
conditions. Note that the two differential equations describe a
"limit cycle," which is independent of initial conditions after
the initial transient.
Parameters.
The differential equation system was calibrated in such a way that the
simulated concentrations are comparable to measured values. Hence the
release rate constants are given in nanograms per milliliter per minute
for GH and picograms per milliliter per minute for SRIF. They were
chosen to reproduce physiological concentrations. In contrast, the
clearance rate constants were calculated according to the measured
half-life. For GH, we used a half-life of 8-10 min (9), and, for
SRIF, we used a value of 0.5 min (10). The amplitude variation of GHRH
and the maximal amount of SRIF in the portal blood correspond to
experimental findings (30).
The generation of the GHRH pulse train requires the parameters
T, the distance between two pulses, and T1,
the duration of secretion. To estimate these parameters, we performed a
time series analysis of typical normal GH profiles of six different
rats with two secretion episodes each. It revealed a periodicity of the GH secretory episodes of 201 ± 11 min, and if the secretion period was composed of two consecutive GH pulses the peak-to-peak distance was
found to be 59 ± 15 min. The periodicity T was adjusted to 50 min to simulate the 3.3-h (200 min) rhythm of the GH profile. The
time of increasing GH concentration was considered to be due to
secretion events. Thus we obtained a secretion time period of 20 ± 9 min for the first peak and 21 ± 7 min for the second peak. In
accordance with the time series analysis, a secretion duration of
T1 = 20 min was used. This value was corroborated using the deconvolution algorithm developed by Veldhuis et al. (42) on
five GH profiles (data not shown).
The delay between the increase of GH in the brain and the rise of SRIF
in the portal circulation,
, was estimated by several authors (11,
34, 44) to lie within 40-80 min. The value
= 62 min imposed on
the feedback in the simulation was chosen such that, first, at low
levels of SRIF, the secretion of GH is induced by more than one GHRH
pulse (
> 50 min) and, second, on increasing levels of SRIF, the
second pulse of GHRH is cut off, resulting in a lower GH peak amplitude
(
< 70 min).
The Hill coefficient n3 (Eq. 5) was
determined by fitting a Hill function to an experimental data set (24)
that shows the GH response curve induced by different amounts of GHRH.
This yielded a Hill coefficient of approximately two. SRIF acts via a
similar second messenger mechanism on the somatotrophs, leading to the assumption of equal Hill coefficients of the two neuropeptides at the
pituitary (n2 = 2). The dense network of SRIF
neurons in the ARC suggests a high degree of cooperativity and is
represented in the simulation by a Hill coefficient
n1 = 4. If the value of n1 is
less than four, adequate suppression of GHRH release by SRIF cannot be attained.
The thresholds and the two parameters of the receptor function were
chosen arbitrarily to fit experimental data. The partition coefficient
lies between zero and one. A summary of the parameter values used
is given in Table 1. An examination of the
sensitivity of the system to the parameter values showed that it is
insensitive to small changes in all of the parameters shown in Table 1
except the Hill functions (provided that the constraints on the time constants are fulfilled). However, increasing the Hill coefficients decreases the sensitivity to the thresholds.
Unperturbed GH rhythm.
The simulated time courses of the unperturbed SRIF, GHRH, and GH
rhythms in the male rat compared with experimental GH data are
presented in Fig. 3. The simulated GH
bursts exhibit a periodicity of 3.3 h, and each burst is subdivided
into two GH pulses separated by 50 min. The duration of the nadir
period is ~90 min with almost undetectable GH levels.

View larger version (23K):
[in this window]
[in a new window]
|
Fig. 3.
Simulation of the unperturbed SRIF (A), GHRH (B),
and GH (C) profiles in the male rat compared with
experimental GH data (D) taken from Tannenbaum et al. (40).
In the simulations, data points are represented every 5 min for SRIF
and GHRH in the portal blood and every 15 min for GH in the circulation
to compare with the experiment. Dashed horizontal line in A
marks the threshold of SRIF at the level of the pituitary. Vertical
dotted lines on left in A-C denote the
commencement of a GH secretory episode, and dotted lines on
right denote the increased level of SRIF and the cut off of the
hourly GHRH pulses. Distance between the two dotted lines corresponds
to the time delay .
|
|
The differential equation system generates the 3.3-h rhythm of GH
secretion in the following manner. In Fig. 3A, the dashed horizontal line marks the threshold of SRIF in the pituitary, cB1,th; in the hypothalamus this threshold,
cA1,th, is fivefold less (see Table 1). If
the actual concentration of SRIF is less than the respective
thresholds, the inhibiting action of SRIF is suspended both in the
hypothalamus, where the pulses of GHRH are no longer suppressed, and in
the pituitary, where GHRH pulses may induce the release of GH. The
commencement of a GHRH pulse and the concomitant release of GH during
low levels of SRIF are indicated in Fig. 3 by the first vertical dotted
line. The increase of GH in the circulation results in a rise of
internalized receptors and, after the time delay, in an increase of
SRIF release. Because the delay of the feedback signal is larger than
the time between two GHRH pulses, a second pulse of GHRH starts to
stimulate the secretion of GH; however, the increasing level of SRIF
cuts off the second GHRH pulse, as indicated by the second vertical dotted line in Fig. 3, and the somatotrophs become sealed so that the
secretion of GH is stopped. This results in a lower peak amplitude for
the second GH peak in the circulation. The double peak of a GH
secretory episode is reflected in the shape of the SRIF surge. After
the decay of SRIF below the threshold, the cycle starts again. The
decay of SRIF is determined mostly by the decay of GH due to the high
clearance rate constant of SRIF and to the almost constant
amplification of elevated GH levels by the receptor function.
Perturbation by exogenous GHRH.
The male rat shows a synchronized GH rhythm so that administration of
exogenous GHRH during a GH secretory episode results in an amplified GH
peak, whereas during a trough period the response is negligible (38).
Injections of GHRH were modeled, and the simulations as well as the
experimental result are presented in Fig.
4. The model shows two high GH peaks at
1100 and 1500 (Fig. 4C). The suppression of the GH response
to GHRH at 1300, due to the high level of SRIF, fits well with the
experimental data. The extended tail of GH secretion after the peak at
1100 and the increased level of GH preceding the peak at 1500 are due
to endogenous pulses of GHRH.

View larger version (21K):
[in this window]
[in a new window]
|
Fig. 4.
Simulation of the effect of GHRH injections at 1100, 1300, and 1500 on
SRIF (A), GHRH (B), and GH (C) profiles
in the male rat compared with experimental data (D) taken
from Lanzi and Tannenbaum (22). Data points are represented as
described in legend to Fig. 3. Dotted lines in B denote the
simulation of exogenous GHRH injections, whereas in the experiment in
D the administration of GHRH is indicated by arrows.
|
|
Perturbation by human GH.
By administering exogenous human (h) GH, the periodicity of endogenous
rat GH can be changed reproducibly. The response due to hGH
perturbation is a suppression of the GH pulses and a prolongation of
the GH trough period by ~1.5 h (21). The effects of the GH negative
feedback loop are presented in Fig. 5. The
injection of hGH at 0800 results in a shift of the spontaneous GH
burst, which appears at ~1230 (Fig. 5, C and D). In
addition, the GH peak amplitudes are often reduced while the system
returns to the normal rhythm (21). The change in periodicity is not due to the high peak amplitude of hGH (~400 ng/ml) but to the slower decay of hGH in the circulating blood. This causes a long tail in the
SRIF surge (Fig. 5A) that suppresses GH pulses for 1.5 h. The
elevated SRIF level in the recovery phase of GH reduces the GHRH pulses
and diminishes the amplitude of GH secretion.

View larger version (21K):
[in this window]
[in a new window]
|
Fig. 5.
Simulation of the effect of a human (h) GH injection at 0800 on SRIF
(A), GHRH (B), and GH (C) profiles in the
male rat compared with experimental data (D) taken from Lanzi
and Tannenbaum (21). Data points are represented as described in legend
to Fig. 3. Dashed lines in C and D are scaled according
to the ordinate for exogenous (exo) hGH on the right-hand side.
|
|
Feminizing the male rhythm.
Despite the different GH periodicities in male (3.3 h) and female (~1
h) rats, the regulatory mechanisms (Fig. 1) are probably the same since
the ~1-h rhythm of the female can be detected within the GH secretory
episode of the male. The transformation of a male GH profile into a
female profile is presented in Fig. 6. By
introducing a 1,000-fold increase in the product
c4,tot,
to make the receptor function almost independent of GH (see
APPENDIX), and by increasing the ratio
to eight (see
Table 1), which adjusts the SRIF level (see APPENDIX), the
simulations exhibit a GH profile similar to that observed in the
feminized male rat (28). According to the simulations, the feminized GH
rhythm is generated by an almost constant release of SRIF into the
hypothalamus (Fig. 6A), where it reduces the GHRH peak
amplitudes, and into the portal circulation, resulting in partly sealed
somatotrophs. Consequently, the GH peak amplitude is much smaller
compared with the normal male profile, and the system exhibits the
intrinsic 1-h rhythm (Fig. 6C). The characteristic properties
of the GH pattern in the feminized male rat, i.e., lower peak
amplitude, 1-h periodicity, and an elevated baseline, are well
represented by the simulations.

View larger version (28K):
[in this window]
[in a new window]
|
Fig. 6.
Simulation of the male-female transition (A-C)
compared with experimental data for the male (D) and for the
feminized (estradiol-treated) male (E) taken from Painson et
al. (28). Data points are represented as described in legend to Fig. 3.
Left-hand vertical dotted line corresponds to the time point of
switching the receptor parameter set from male to female (see Fig. 2).
Right-hand vertical dotted line indicates the termination of the period
of adaptation to new parameter values in the model.
|
|
 |
DISCUSSION |
GH is essential for normal body growth and metabolism in both animals
and humans. Hence, an understanding of the mode of generation of the GH
secretion profile is of physiological and pathophysiological importance.
Based on experimental results, we modeled the GH neuroendocrine axis as
a combination of a linear pathway that describes the sequential
signaling between GHRH and GH and a cyclic pathway involving the
feedback of GH on its own release via GH receptors on SRIF neurons in
the hypothalamus. The GHRH-GH axis is based on an intrinsic 1-h rhythm
while the SRIF-GH cycle yields the 3.3-h periodicity. The ultradian GH
rhythm observed in the male rat is obtained by modulating the 1-h GHRH
rhythm with an overall 3.3-h periodicity of SRIF release into both the
brain and portal blood.
The 3.3-h GH rhythm is composed of the following components. Starting
at the beginning of a secretion period, the concentration of SRIF
increases after a delay of 62 min and remains at a high level for 70 min, i.e., the duration of two pulses. Thereafter, SRIF follows the
decay of GH in the circulation until the latter declines below the 1%
threshold after 63 min
(5 × 1/k3,cl). This yields a total
of 195 min, so the next pulse of GHRH (after 200 min) induces the
release of GH and the cycle starts again. To obtain a 3.3-h rhythm, an
integral number of GHRH pulses must occur within 200 min. Because the
delay time
and the clearance rate constant
k3,cl are fixed parameters of the system, the 3.3-h rhythm can be disrupted either by changing the duration of the high
level of GH in the circulation (e.g., administration of hGH) or by
altering the parameters of the feedback function (resulting in a
decreased sensitivity of SRIF to GH as in the case of the male-female transition).
The intriguing fact that the 3.3-h GH rhythm is conserved after an
intravenous injection of SRIF-Ab with an elevated baseline (27) can be
readily explained in terms of the model. SRIF-Ab is assumed to
neutralize SRIF only at the level of the pituitary but not at the level
of the hypothalamus (due to the blood-brain barrier). In the absence of
SRIF in the pituitary, the GH profile in the circulation represents the
GHRH pulses in the portal blood. Because the feedback of GH on
hypothalamic SRIF release is not distorted by the perturbation, a rise
in circulating GH still results in an increase of SRIF after the delay,
which in turn suppresses GHRH pulses at the level of the hypothalamus.
Therefore, the 3.3-h periodicity of SRIF in the brain remains conserved.
The model is designed so that release of GH can only be induced by
GHRH. It follows that administration of GHRH-Ab results in an
undetectable level of GH in the circulation, as observed in many
experiments (27, 43).
The hypothesis of an intrinsic 1-h rhythm generated by GHRH neurons is
based on the following arguments. In the male rat, a high-frequency
pattern of GH is often observed in perturbation studies (13, 21),
which is similar to that seen within a GH secretory episode.
Because the GH profile in blood shows a periodicity of 3.3 h, it is
unlikely that the high frequency of the GHRH pulses is generated by a
GH feedback loop. It remains possible that the high-frequency GHRH
profile is the result of mutual activation or inhibition between GHRH
and SRIF neurons in the brain. Unfortunately, such a
mechanism appears to require Hill coefficients greater than seven (10),
which are usually not observed in biology. Thus we arrived at a 1-h
rhythm intrinsic to a black box system involving GHRH neurons, which
could be the result of a feedback circuit within the GHRH neuronal
network (17) or via other neuropeptides such as the putative GHS (4,
33) and neuropeptide Y (8).
The model predicts a possible mechanism for the change of the male GH
rhythm into the female pattern by sex steroids and vice versa. The
transition was obtained by an appropriate change of the parameters
(
,
c4,tot) of the receptor function (see Fig. 2).
The latter becomes independent of GH on sufficiently increasing
c4,tot, which can be achieved by a higher binding
constant or by increasing the number of GHR; indeed, there is
experimental evidence that the total amount of GHR is elevated in the
female rat (31). Therefore, intravenous administration of hGH to a female rat has no effect on the rat GH profile (6) (the feedback of GH
is disrupted), and the system gives a constant response to hourly
injections of GHRH (12, 27) (constant SRIF release).
There is no doubt that pulsatility in GH secretion is important for
growth (32). The typical GH profile shows two frequencies, namely the
3.3-h periodicity of the GH secretory episodes and the 1-h rhythm
within the episodes. In the model, the origin of the multiple peaks per
GH episode is the time delay
. If
were <50 min, only one GH
peak per period would be observed. Therefore, one might speculate that
the double peak within a secretion episode is due to an intrinsic
limiting parameter (
), whereas the 3.3-h periodicity is the result
of an evolutionary process to optimize growth.
Finally, the model suggests two experiments that might reveal the 1-h
rhythm of GHRH neurons. Disrupting the feedback of GH on its own
release (which entails the obliteration of internalized receptors)
should cause the profile of GH in the circulation to show a 1-h
periodicity according to the pulses of GHRH in the portal blood, since
release of SRIF ceases. Indeed, Pellegrini et al. (29) recently
demonstrated that central administration of a GHR mRNA antisense
increases the pulsatility of GH and decreases hypothalamic SRIF
expression. Second, antagonizing the activity of SRIF in the
hypothalamus and in the pituitary, using a pure SRIF antagonist, should
result in a 1-h rhythm of GH secretion in the circulating blood. This
experiment is now in progress.
 |
APPENDIX |
Derivation of the Receptor Function
(cA4)
In general, it is assumed that two receptors bind to one GH molecule to
activate the signaling pathway within the cell. Assuming the total
amount of receptors, c4,tot, is constant, the kinetic equations corresponding to the model shown in Fig. 2 read
|
(A1)
|
|
(A2)
|
|
(A3)
|
where x and y are the concentrations of free and bound
receptors, respectively. The rate constants k4,b
and k4,d denote binding and dissociation of GH,
whereas k4,i and k4,ri
represent the internalization and the reinsertion steps, respectively.
Applying the condition given in the text, c4,tot = x + 2y + 2cA4, the steady-state solution (Eq. 6) can be calculated.
The following two interesting limits of Eq. 6 should be noted:
1) if cC3 tends to zero, it can
readily be shown that cA4 tends to zero as
well, and 2) if
c4,tot tends to infinity, the
value of cA4/c4,tot becomes
constant and independent of the GH concentration
|
(A4)
|
 |
ACKNOWLEDGEMENTS |
This work was supported by Grant MT-6837 (to G. S. Tannenbaum) from
the Medical Research Council of Canada. G. S. Tannenbaum is a Chercheur
de Carrière of the Fonds de la recherche en santé de
Québec. C. Wagner is the recipient of postdoctoral fellowship awards from The Swiss National Foundation and the
Ciba-Geigy-Jubilaeumsstiftung.
 |
FOOTNOTES |
Present address of C. Wagner: Biozentrum, University of Basel, Dept. of
Biophysical Chemistry, Klingelbergstrasse 70, CH-4056 Basel, Switzerland.
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: G. S. Tannenbaum, Neuropeptide Physiology
Laboratory, McGill University-Montreal Children's Hospital Research
Institute, 2300 Tupper St., Montreal, Quebec, Canada H3H 1P3.
Received 22 May 1998; accepted in final form 21 August 1998.
 |
REFERENCES |
1.
Alberts, B.,
D. Bray,
A. Johnson,
J. Lewis,
M. Raff,
K. Roberts,
and
P. Walter.
Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. New York: Garland, 1998.
2.
Berelowitz, M.,
M. Szabo,
L. Frohman,
S. Firestone,
and
L. Chu.
Somatomedin C mediates growth hormone negative feedback by effects on both the hypothalamus and the pituitary.
Science
212:
1279-1281,
1981[Medline].
3.
Bertherat, J.,
P. Dournaud,
A. Bérod,
E. Normand,
B. Block,
W. Rostène,
C. Kordon,
and
J. Epelbaum.
Growth hormone-releasing hormone-synthesizing neurons are a subpopulation of somatostatin receptor-labeled cells in the rat arcuate nucleus: a combined in situ hybridization and receptor light-microscopic radioautographic study.
Neuroendocrinology
56:
25-31,
1992[Medline].
4.
Bowers, C. Y.,
F. A. Momany,
G. A. Reynolds,
and
A. Hong.
On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone.
Endocrinology
114:
1537-1545,
1984[Abstract].
5.
Burton, K. A.,
E. B. Kabigting,
D. K. Clifton,
and
R. A. Steiner.
Growth hormone receptor messenger ribonucleic acid distribution in the adult male rat brain and its colocalization in hypothalamic somatostatin neurons.
Endocrinology
131:
958-963,
1992[Abstract].
6.
Carlsson, L. M. S.,
R. A. Clark,
and
I. C. A. F. Robinson.
Sex difference in growth hormone feedback in the rat.
J. Endocrinol.
126:
27-35,
1990[Abstract].
7.
Carter-Su, C.,
J. Schwartz,
and
L. S. Smit.
Molecular mechanism of growth hormone action.
Ann. Rev. Physiol.
58:
187-207,
1996[Medline].
8.
Chan, Y. Y.,
R. A. Steiner,
and
D. K. Clifton.
Regulation of hypothalamic neuropeptide-Y neurons by growth hormone in the rat.
Endocrinology
137:
1319-1325,
1996[Abstract].
9.
Chapman, I. M.,
A. Helfgott,
and
J. O. Willoughby.
Disappearance half-life times of exogenous and growth hormone-releasing factor-stimulated endogenous growth hormone in normal rats.
J. Endocrinol.
128:
369-374,
1991[Abstract].
10.
Chen, L.,
J. D. Veldhuis,
M. L. Johnson,
and
M. Straume.
Systems-level analysis of physiological regulatory interactions controlling complex secretory dynamics of the growth hormone axis: a dynamical network model.
In: Methods in Neurosciences. New York: Academic, 1995, p. 270-310.
11.
Chihara, K.,
N. Minamitami,
N. Kaji,
A. Arimura,
and
T. Fugita.
Intraventricularly injected growth hormone stimulates somatostatin release into rat hypophyseal portal blood.
Endocrinology
109:
2278-2281,
1981.
12.
Clark, R. G.,
G. Chambers,
J. Lewin,
and
I. C. A. F. Robinson.
Automated repetitive microsampling of blood: growth hormone profiles in conscious male rats.
J. Endocrinol.
111:
27-35,
1986[Abstract].
13.
Clark, R. G.,
and
I. C. A. F. Robinson.
Growth hormone responses to multiple injections of a fragment of human growth hormone-releasing factor in conscious male and female rats.
J. Endocrinol.
106:
281-289,
1985[Abstract].
14.
Frohman, L. A.,
T. R. Downs,
and
P. Chomczynski.
Regulation of growth hormone secretion.
Front. Neuroendocrinol.
13:
344-405,
1992[Medline].
15.
Harel, Z.,
and
G. S. Tannenbaum.
Synergistic interaction between insulin-like growth factors-I and -II in central regulation of pulsatile growth hormone secretion.
Endocrinology
131:
758-764,
1992[Abstract].
16.
Hochberg, Z.,
T. Bick,
T. Amit,
R. J. Barkey,
and
M. B. H. Youdim.
Regulation of growth hormone receptor turnover by growth hormone.
Acta Paediatr. Scand.
367:
148-152,
1990.
17.
Horvàth, S.,
and
M. Palkovits.
Synaptic interconnections among growth hormone-releasing hormone (GHRH)-containing neurons in the arcuate nucleus of the rat.
Neuroendocrinology
48:
471-476,
1988[Medline].
18.
Howard, A. D.,
S. D. Feighner,
D. F. Cully,
J. P. Arena,
P. A. Liberator,
C. I. Rosenblum,
M. Hamelin,
D. L. Hreniuk,
O. C. Palyha,
J. Anderson,
P. S. Paress,
C. Diaz,
M. Chou,
K. K. Liu,
K. K. McKee,
S.-S. Pong,
L.-Y. P. Chaung,
A. Elbrecht,
M. Dashkevicz,
R. Heavens,
M. Rigby,
D. J. S. Sirinathsinghi,
D. C. Dean,
D. G. Melillo,
A. A. Patchett,
R. P. Nargund,
P. R. Griffin,
J. A. DeMartino,
S. K. Gupta,
J. M. Schaeffer,
R. G. Smith,
and
L. H. T. Van der Ploeg.
A receptor in pituitary and hypothalamus that functions in growth hormone release.
Science
273:
974-977,
1996[Abstract].
19.
Jansson, J.-O.,
S. Edén,
and
O. Isaksson.
Sexual dimorphism in the control of growth hormone secretion.
Endocr. Rev.
6:
128-150,
1985[Abstract].
20.
Kraicer, J.,
B. Lussier,
B. C. Moor,
and
J. S. Cowan.
Failure of growth hormone (GH) to feed back at the level of the pituitary to alter the response of the somatotrophs to GH-releasing factor.
Endocrinology
122:
1511-1514,
1988[Abstract].
21.
Lanzi, R.,
and
G. S. Tannenbaum.
Time course and mechanism of growth hormone's negative feedback effect on its own spontaneous release.
Endocrinology
130:
780-788,
1992[Abstract].
22.
Lanzi, R.,
and
G. S. Tannenbaum.
Time-dependent reduction and potentiation of growth hormone (GH) responsiveness to GH-releasing factor induced by exogenous GH: role for somatostatin.
Endocrinology
130:
1822-1828,
1992[Abstract].
23.
McCarthy, G. F.,
A. Beaudet,
and
G. S. Tannenbaum.
Colocalization of somatostatin receptors and growth hormone-releasing factor immunoreactivity in neurons of the rat arcuate nucleus.
Neuroendocrinology
56:
18-24,
1992[Medline].
24.
McCormick, G. F.,
W. J. Millard,
T. M. Badger,
C. Y. Bowers,
and
J. B. Martin.
Dose-response characteristics of various peptides with growth hormone-releasing activity in the unanesthetized male rat.
Endocrinology
117:
97-105,
1985[Abstract].
25.
Minami, S.,
J. Kamegai,
H. Sugihara,
and
O. Hasegawa.
Systemic administration of recombinant human growth hormone induces expression of the c-fos gene in the hypothalamic arcuate and periventricular nuclei in hypophysectomized rats.
Endocrinology
131:
247-253,
1992[Abstract].
26.
Murray, J. D.
Biomathematics.
In: Mathematical Biology, edited by S. A. Levin. New York: Springer-Verlag, 1993, p. 166-175.
27.
Painson, J.-C.,
and
G. S. Tannenbaum.
Sexual dimorphism of somatostatin and growth hormone-releasing factor signaling in the control of pulsatile growth hormone secretion in the rat.
Endocrinology
128:
2858-2866,
1991[Abstract].
28.
Painson, J.-C.,
M. O. Thorner,
R. J. Krieg,
and
G. S. Tannenbaum.
Short-term adult exposure to estradiol feminizes the male pattern of spontaneous and growth hormone-releasing factor-stimulated growth hormone secretion in the rat.
Endocrinology
130:
511-519,
1992[Abstract].
29.
Pellegrini, E.,
M. T. Bluet-Pajot,
F. Mounier,
P. Bennett,
C. Kordon,
and
J. Epelbaum.
Central administration of a growth hormone (GH) receptor mRNA antisense increases GH pulsatility and decreases hypothalamic somatostatin expression in rats.
J. Neurosci.
16:
1-9,
1996[Abstract].
30.
Plotsky, P. M.,
and
W. Vale.
Patterns of growth hormone-releasing factor and somatostatin secretion into the hypophysial-portal circulation of the rat.
Science
230:
461-463,
1985[Medline].
31.
Postel-Vinay, M. C.
Binding of human growth hormone to rat liver membranes: lactogenic and somatotropic sites.
FEBS Lett.
69:
137-140,
1976[Medline].
32.
Robinson, I. C. A. F.,
and
R. G. Clark.
The secretory pattern of GH and its significance for growth in the rat.
In: Growth Hormone-Basic and Clinical Aspects, edited by O. Isaksson,
C. Binder,
K. Hall,
and B. Hökfelt. Amsterdam: Elsevier, 1987, p. 109-127.
33.
Smith, R. G.,
L. H. T. Van der Ploeg,
A. D. Howard,
S. D. Feighner,
K. Cheng,
G. J. Hickey,
M. J. Wyvratt Jr,
M. H. Fisher,
R. P. Nargund,
and
A. A. Patchett.
Peptidomimetic regulation of growth hormone secretion.
Endocr. Rev.
18:
621-645,
1997[Abstract/Free Full Text].
34.
Tannenbaum, G. S.
Evidence for autoregulation of growth hormone secretion via the central nervous system.
Endocrinology
107:
2117-2120,
1980[Abstract].
35.
Tannenbaum, G. S.
Multiple levels of cross-talk between somatostatin (SRIF) and growth hormone (GH)-releasing factor in genesis of pulsatile GH secretion.
Clin. Pediatr. Endocrinol.
3:
97-110,
1994.
36.
Tannenbaum, G. S.,
F. Farhadi-Jou,
and
A. Beaudet.
Ultradian oscillation in somatostatin binding in the arcuate nucleus of adult male rats.
Endocrinology
133:
1029-1034,
1993[Abstract].
37.
Tannenbaum, G. S.,
M. Lapointe,
A. Beaudet,
and
A. D. Howard.
Expression of growth hormone secretagogue-receptors by growth hormone-releasing hormone neurons in the mediobasal hypothalamus.
Endocrinology
139:
4420-4423,
1998[Abstract/Free Full Text].
38.
Tannenbaum, G. S.,
and
N. Ling.
The interrelationship of growth hormone (GH)-releasing factor and somatostatin in generation of the ultradian rhythm of GH secretion.
Endocrinology
115:
1952-1957,
1984[Abstract].
39.
Tannenbaum, G. S.,
and
J. B. Martin.
Evidence for an endogenous ultradian rhythm governing growth hormone secretion in the rat.
Endocrinology
98:
562-570,
1976[Abstract].
40.
Tannenbaum, G. S.,
G. F. McCarthy,
P. Zeitler,
and
A. Beaudet.
Cysteamine-induced enhancement of growth hormone-releasing factor (GRF) immunoreactivity in arcuate neurons: morphological evidence for putative somatostatin/GRF interactions within hypothalamus.
Endocrinology
127:
2551-2560,
1990[Abstract].
41.
Tannenbaum, G. S.,
W.-H. Zhang,
M. Lapointe,
P. Zeitler,
and
A. Beaudet.
Growth hormone-releasing hormone neurons in the arcuate nucleus express both sst1 and sst2 somatostatin receptor genes.
Endocrinology
139:
1450-1453,
1998[Abstract/Free Full Text].
42.
Veldhuis, J. D.,
M. L. Carlson,
and
M. L. Johnson.
The pituitary gland secretes in bursts: appraising the nature of glandular secretory impulses by simultaneous multiple-parameter deconvolution of plasma hormone concentrations.
Proc. Natl. Acad. Sci. USA
84:
7686-7690,
1987[Abstract].
43.
Wehrenberg, W. B.,
P. Brazeau,
R. Luben,
P. Böhlen,
and
R. Guillemin.
Inhibition of the pulsatile secretion of growth hormone by monoclonal antibodies to the hypothalamic growth hormone releasing factor (GRF).
Endocrinology
111:
2147-2148,
1982[Medline].
44.
Willoughby, J. O.,
M. Menadue,
P. Zeegers,
P. H. Wise,
and
J. R. Oliver.
Effects of human growth hormone on the secretion of rat growth hormone.
J. Endocrinol.
86:
165-169,
1980[Abstract].
Am J Physiol Endocrinol Metab 275(6):E1046-E1054
0002-9513/98 $5.00
Copyright © 1998 the American Physiological Society