Department of Anatomy and Cell Biology, Hebrew University, Hadassah Medical School, Jerusalem 91120, Israel
Submitted 17 March 2003 ; accepted in final form 27 May 2003
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ABSTRACT |
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L-type channels; mechanosensitivity; somatotrophs; lactotrophs
Hyposmotic swelling may enhance calcium influx either by direct modulation
of calcium channels, caused by increase in membrane tension, or by indirect
modulation of calcium channels, caused by alterations in membrane potential.
There are several examples for the latter possibility. Hyposmotically induced
calcium influx in GH3 pituitary cells was attributed to osmotic activation of
cationic stretch-activated channels that, in turn, caused membrane
depolarization (10).
Similarly, hyposmotically induced calcium influx in chromaffin cells
(38) and in pancreatic
-cells (4,
14) was attributed to the
activation of swell-dependent chloride channels that, in turn, caused membrane
depolarization. However, there are also several examples that demonstrate
direct hyposmotic enhancement of voltage-gated calcium currents, in smooth
muscle cells (26,
31,
54), cardiac myocytes
(36), pancreatic
-cells
(14), and hippocampal neurons
(49).
Because voltage-gated calcium channels play a key role in regulating the secretion of pituitary hormones (8, 27, 50), it was of interest to investigate whether or not hyposmotically induced cell swelling directly modulates voltage-gated calcium currents in pituitary cells. Both L-type and T-type calcium currents (IL and IT, respectively) were observed in anterior pituitary cells (2, 9, 13, 32, 40, 51). In a previous study we demonstrated that hypertonicity decreases calcium influx through L-type and T-type channels in anterior pituitary cells (37), presumably because of hyperosmotic effects on the gating of L-type channels (3). In this study we show that hypotonicity directly enhances calcium influx through IL and IT, as previously suggested from secretion studies. A preliminary account of this work in abstract form has appeared elsewhere (1).
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EXPERIMENTAL PROCEDURES |
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Electrophysiological recording and analysis. Barium currents
(IBa) through calcium channels were recorded with an
Axopatch 1C amplifier (Axon Instruments, Union City, CA) by using the whole
cell mode of the patch-clamp technique at room temperature (20-24°C).
Patch electrodes were pulled from 1.6-mm (outer diameter) borosilicate glass
(Hilgenberg, Malsfeld, Germany) on a two-stage puller (L/M-3P-A;
List-Electronic, Darmstadt, Germany), and their resistance ranged from 3 to 6
M when filled with the "intracellular solutions" (see
Solutions). Membrane currents were sampled with an analog-to-digital
converter (Digidata 1320A, Axon Instruments) at 5 kHz, filtered with a
four-pole low-pass Bessel filter with a cutoff frequency (-3 dB) of 1 kHz, and
stored in the hard disk of an IBM-based computer. Capacitive currents and
access resistances (Ra) were electronically compensated
with the potentiometers provided with the amplifier. Final access resistance
was usually <15 M
. Linear leak currents (and residual capacitive
currents) were digitally subtracted after extrapolation of averaged leak
currents that were obtained in response to test pulses (P) divided by 2 or 4
(P/2 or P/4 pulse protocols). The pCLAMP8 suite of programs (Axon Instruments)
was used for on-line acquisition and for off-line analysis of the membrane
currents.
IBa through L-type and T-type channels were usually activated with 200-ms voltage steps (interval 10-15 s) from a holding potential (Vh) of -80 mV to various test potentials (Vt). In some of the experiments double-pulse protocols were used to activate IT (Vt = -30 mV) and IL (Vt = 0 mV) simultaneously. To obtain instantaneous current-voltage (I-V) relationships of calcium channel currents, we used 500-ms voltage ramps ranging from -100 to +80 mV.
For simultaneous monitoring of IBa, cell membrane
capacitance (Cm), and Ra we used
double-pulse protocols. The first subthreshold pulse, a 20-ms depolarization
from -80 to -70 mV, activated capacitive currents. The second pulse, a 300-ms
voltage ramp from -100 to +60 mV, activated IBa. In these
experiments currents were sampled at a rate of 100 kHz (filter 10 kHz) and the
capacitive currents were not compensated with the dials of the amplifier. The
uncompensated capacitive currents were used to calculate the values of
Cm and Ra by using the relation =
Ra*Cm, where
is the
time constant of the decay of the capacitive currents
(33). The value of
was
calculated by fitting a monoexponential function to the decay of the
capacitive currents. Cm was calculated from the relation
C = Q/V, where Q is the amount of charge
needed to discharge the cell membrane (obtained by integrating the capacitive
current) and V is the amplitude of the subthreshold pulse.
Ra was calculated from the relation Ra
=
/Cm.
The among-group differences of tested parameters were examined either by
paired or unpaired t-tests. Multiple-comparison tests were performed
by one-way analysis of variance (ANOVA). When significant differences were
indicated in the F-test (P < 0.001), the significance of
differences between the means of any of these groups was determined by the
Tukey method for multiple comparisons with = 0.05 (see
Fig. 4). Results are reported
as means ± SE.
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Solutions. Two types of hyposmotic stimuli were used to examine the effects of hyposmolarity on calcium channel currents: moderate hyposmotic stimuli (18-22% decrease in [Os]e) and strong hyposmotic stimuli (31-32% decrease in [Os]e). To study hyposmotic effects while keeping ionic composition and strength constant we used as controls isosmotic media containing mannitol. Hyposmotic media were obtained by omitting mannitol from these isosmotic media. The compositions of the extracellular media used are given in Table 1. Barium was used as the charge carrier through calcium channels in all these experiments.
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The pipette "intracellular" solution for whole cell recordings of IBa contained (in mM) 138 CsCl, 11 EGTA, 10 HEPES, 2 Mg-ATP, 0.16 GTP, and 1.5 lidocaine N-ethyl chloride (QX-314) [adjusted to pH =7.2-7.3 with Cs(OH), osmolarity 305 mosM]. The pipette solution for perforated patch recordings contained (in mM) 75 CsCl, 75 Cs(OH), 75 L-aspartic acid, 0.1 EGTA, 10 HEPES, 1 MgCl2, and 200-300 µg/ml nystatin [adjusted to pH = 7.2-7.3 with Cs(OH), osmolarity 300 mosM].
In some of the experiments we examined the hyposmotic effects on IBa in chloride-free media. In these experiments chloride was substituted with glutamate and acetate. The extracellular isosmotic media in these experiments contained (in mM) 95 L-glutamic acid, 10 Ba(acetate)2, 10 HEPES, 10 glucose, 115 mannitol [adjusted to pH 7.2-7.3 with tetraethylammonium (TEA)(OH), osmolarity 309 mosM]. The hyposmotic media (214 mosM) were obtained by omitting mannitol from this chloride-free isosmotic medium. The pipette intracellular solution in these experiments contained (in mM) 130 L-glutamic acid, 5 TEA(acetate), 10 HEPES, 11 EGTA, 2 Mg-ATP, 0.16 GTP, 1.5 QX-314 [adjusted to pH = 72-7.3 with Cs(OH), osmolarity 310 mosM]. All chemicals for the extracellular and intracellular solution were purchased from Sigma except QX-314 (Alomone Labs, Jerusalem, Israel), which was used to block sodium currents.
Before each experiment coverslips containing pituitary cells were placed in
a perfusion chamber (RC-16, Warner Instruments). Cells were exposed to control
(isosmotic) or hyposmotic solutions by perfusing the chamber at a rate of
1 ml/min. The volume of solution in the perfusion chamber was
0.4
ml, and the cells were exposed to the different experimental solutions for 2-4
min.
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RESULTS |
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In some of these experiments cells were repeatedly exposed to the same moderate hyposmotic stimulus (while IL were recorded). Figure 3A illustrates a pituitary cell that was exposed three consecutive times to the same moderate hyposmotic stimulus. The first exposure resulted in a biphasic response, a decrease in IL (by 30%) that was followed by a recovery to control levels. The second exposure (5 min later) resulted also in a biphasic response. However, at this time, the decrease (by 23%) was followed by a substantial increase (by 30%) in IL. In the third exposure only an increase (by 40%) in IL was observed. Similar results were obtained in 10 cells that were exposed three consecutive times to the same moderate hyposmotic stimulus, as illustrated in Fig. 3B. The first 10 exposures resulted mostly in biphasic responses (9 of 10); both the first and second phases exhibited decrease in IL. The second 10 exposures also resulted mostly in biphasic responses (8 of 10 cells). However, at this time, the second phase exhibited a clear increase in IL. The third 10 exposures resulted in biphasic responses (with a substantial increase in IL in the second phase; 6 of 10 cells) and in type III responses, increase in IL (3 of 10 cells). A similar trend of enhancement was observed in 20 cells that were exposed two consecutive times to the same moderate hyposmotic stimulus (not shown). Thus it appears that repetitive exposures to the same moderate hyposmotic stimulus result in enhanced manifestation of type III responses, increase in IL.
One way to explain this shift toward type III responses is by assuming that the biphasic responses (type II) are actually composed of two superimposed responses, decreases (type I) and increases (type III) in IL. Repeated exposures to moderate hyposmotic stimuli cause a use-dependent inactivation of type I responses, thus leading to better manifestation of type III responses, increase in IL. Support for this hypothesis comes from the nature of the biphasic response. The decrease in IBa (first phase) always preceded the increase in IBa (second phase). In addition, the distribution of delay times to the onset of these two phases in type II response is similar to the distribution of delay times to the onset of type I and III responses, respectively, as illustrated in Fig. 4.
Effects of strong hyposmotic stimuli on calcium channel currents. Because repeated exposures to the same moderate hyposmotic stimulus resulted in better manifestation of type III response (increase in IL), we thought that it was of interest to examine the effects of stronger hyposmotic stimuli on calcium channel currents. One of these experiments is illustrated in Fig. 5. IT and IL were activated by a double-pulse protocol, as shown in Fig. 1. Decrease in [Os]e by 32% resulted in a substantial increase in the amplitude of both IT and IL (Fig. 5A). The full time course of this experiment shows that the hyposmotic enhancement in the amplitude of IT (by 43%) and IL (by 63%) was reversible and that it was not associated with significant changes in holding currents (Fig. 5B). Similar results were observed in additional experiments. IL were increased by 44 ± 5% (n = 20), whereas IT were increased by 28 ± 4% (n = 7) (P < 0.04, unpaired t-test). Hence, only increases in calcium channel currents were observed in response to strong hyposmotic stimuli. This difference between strong and moderate hyposmotic stimuli in their effects on IL may stem from differences in the extent of pituitary cell swelling. Indeed, Table 2 shows that swelling in response to strong hyposmotic stimuli was larger than swelling in response to moderate hyposmotic stimuli.
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Hyposmotic effects on IL: voltage independence. The hyposmotic effects on IL may have resulted from hyposmotically induced changes in the voltage dependence of IL. We therefore examined the effects of hyposmolarity on I-V relationships of IBa that were activated with voltage ramps from -100 to 80 mV (ramp duration 0.5 s, interval 15 s).
Figure 6A shows
I-V curves that were activated by voltage ramps before and during
exposure to a strong hyposmotic stimulus. The small hump on the rising phase
of the I-V curve (at about -30 mV) may represent the activation of
T-type barium currents, whereas the maximal peak of the I-V curve
(defined as Imax, at 5 mV) represents mainly the
activation of L-type barium currents. Exposure to the strong hyposmotic
stimulus resulted in a substantial increase in Imax (by
30%) that was not associated with a significant voltage shift in the
I-V relationship. This voltage independence of the hyposmotic effects
is further illustrated in the normalized I-V curves
(Fig. 6B) that were
obtained by normalizing the I-V curves in
Fig. 6A to
Imax. It is clear from these normalized curves that the
hyposmotic effect was not associated with any significant changes in the
voltage at which IBa/Imax reached 50%
(V0.5) or in the voltage at which
IBa/Imax reached 100%
(Vpeak). The full time course of the experiment
(Fig. 6C) shows that
the hyposmotic effects on the I-V relationships (on
Imax) are fully reversible. Similar effects were observed
in eight cells that were exposed to strong hyposmotic stimuli and in six cells
that were exposed to moderate hyposmotic stimuli.
Table 3 shows that the voltage
shifts in V0.5, Vpeak, and apparent
reversal potential (Er) in these experiments were
insignificant (<2 mV). Therefore, it is reasonable to suppose that the
effects of osmotic swelling on IBa result from hyposmotic
effects on the activity of calcium channels rather than voltage-dependent
shifts in the activation of calcium channels.
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Hyposmotic effects on IL: chloride-free media. To rule out the possibility that activation of a swell-dependent chloride conductance contributes to the hyposmotic effects on calcium channels, we examined hyposmotic effects on IBa in chloride-free media. Figure 7A shows that exposure of a pituitary cell to a strong hyposmotic stimulus in a chloride-free medium resulted in a reversible increase in the amplitude of IL. The full time course of this experiment is illustrated in Fig. 7B. The hyposmotic stimulus (214 mosM) caused a reversible increase in IL (by 35%) without changing the holding currents. Similar results were observed in additional experiments that were performed in chloride-free media with strong hyposmotic stimuli. IL were increased by 26 ± 5% (n = 5), and IT were increased by 19 ± 3% (n = 4).
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Hyposmotic effects on calcium channel currents in intact pituitary cells: perforated patch recordings. To examine hyposmotic effects on IBa under conditions that are more similar to the normal physiological conditions of the cell, we used the perforated patch method (21). With this method the cell remains intact and retains most of its intracellular components. Figure 8 summarizes experiments that were performed with the perforated patch method. Figure 8A shows the effects of a strong hyposmotic stimulus on IT and IL that were activated by a double-pulse protocol. Exposure to the strong hyposmotic stimulus (207 mosM) resulted with a reversible increase in both IT (by 17%) and IL (by 29%). Similar effects were observed in additional cells. IL were increased by 25 ± 3% (n = 11), and IT were increased by 17 ± 4% (n = 5) (P < 0.05, unpaired t-test). Figure 8B shows the effects of a strong hyposmotic stimulus on I-V curves that were elicited by voltage ramps as described above (see Fig. 6). Exposure to the strong hyposmotic stimulus (207 mosM) resulted in an increase in the peak current of the I-V curve (by 12%) without any significant shift in V0.5 or Vpeak (as judged from the normalized I-V curves seen below).
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Similar effects were observed in four cells that were exposed to strong hyposmotic stimuli and in four cells that were exposed to moderate hyposmotic stimuli. Table 3 shows that the voltage shifts in V0.5, Vpeak, and Er in these experiments were insignificant (<2 mV). Hence, the hyposmotic effects on calcium channel currents in intact pituitary cells cannot be attributed to hyposmotically induced changes in the voltage dependence of the calcium channels. Figure 8C summarizes the hyposmotic effects on IL and IT that were obtained with perforated patch recording. Interestingly, the histogram in Fig. 8C shows only increases in IL in response to moderate hyposmotic stimuli, in contrast to the three response types that were observed with whole cell recordings (Fig. 2). Thus it appears that in intact pituitary cells exposure to either moderate or strong hyposmotic stimuli results only in increases in calcium channel currents.
Hyposmotic swelling is not accompanied by increase in cell surface
area. The hyposmotic effects on calcium channel currents were
consistently accompanied by an increase in pituitary cell diameter (measured
at the end of exposure to hyposmotic stimuli by using a scale that was
inserted into the ocular of the microscope). Pituitary cell diameter increased
from 11.9 ± 0.3 µmin control to 14.5 ± 0.4 µm (n
= 37) during exposure to hyposmotic media (P < 0.001, paired
t-test) and returned back to control levels (11.8 ± 0.4 µm,
n = 26) after washout of hyposmotic media (see also
Table 2). Because pituitary
cells are spherical cells, this increase in cell diameter corresponds to
80% increase in pituitary cell volume and
48% increase in pituitary
cell surface area. These changes in cell volume and cell membrane surface area
can affect our experimental results. We therefore monitored in several
experiments simultaneous changes in IBa,
Cm, and Ra. In these experiments we
used a double-pulse protocol. The first subthreshold pulse activated
capacitive currents (uncompensated) to measure changes in
Cm and Ra, and the second pulse, a
voltage ramp, activated IBa (see EXPERIMENTAL
PROCEDURES). Figure 9
shows that a strong hyposmotic stimulus elicited a 26% increase in peak
IBa (Fig. 9, A
and C) without significant changes in
Cm and Ra
(Fig. 9B).
Cm in control conditions (8.0 pF) was not much different
from Cm measured during exposure to the hyposmotic
stimulus (7.4 pF). Similarly, Ra in control conditions
(9.1 M
) was not much different from Ra measured
during exposure to the hyposmotic stimulus (9.6 M
). Similar results
were obtained in seven experiments. Exposure to strong hyposmotic stimuli
resulted in significant increases in peak IBa (by 48
± 6%) but not in significant changes in Cm and
Ra. Cm values in control conditions
(8.5 ± 0.4 pF) were not significantly different from
Cm values during the hyposmotic stimuli (8.4 ± 0.4
pF; P < 0.709, paired t-test). Similarly,
Ra values in control conditions (18.1 ± 2.9
M
) were not significantly different from Ra values
during the hyposmotic stimuli (16.3 ± 2.7 M
; P <
0.089, paired t-test). Thus the hyposmotically induced changes in
IBa observed in this study cannot be attributed to changes
in either Cm or Ra. This result goes
along with previous studies that demonstrated that hyposmotic cell swelling
(11,
15, 83) is not accompanied by
significant increases in Cm (or cell membrane surface
area).
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DISCUSSION |
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The three response types in IL that were observed in response to moderate hyposmotic stimuli (Fig. 2) may be explained by two opposing hyposmotic effects on IL: decrease (type I response) and increase (type III response) in IL. Simultaneous activation (or superposition) of these two responses may result in the appearance of the biphasic response (type II response). Indeed, type I response was observed only while pituitary cells were exposed to moderate hyposmotic stimuli. It was attenuated or disappeared upon repetitive exposures to the same moderate hyposmotic stimulus (Fig. 3). Its onset was always faster than the onset of type III response (Fig. 4), and, in addition, it was associated with smaller increases in pituitary cell volume (Table 2). Thus it appears that the mechanism underlying type I response is different from that underlying type III response and that type I response is possibly overridden (or inactivated) by strong hyposmotic stimuli. Interestingly, the use-dependent inactivation of type I response resembles the use-dependent adaptation of mechanosensitive ion channels in Xenopus laevis oocytes (17) and in snail neurons (48).
Are calcium channels in pituitary cells mechanosensitive? The sensitivity of calcium channels in the membrane of pituitary cells to hyposmotic swelling implies that they are mechanosensitive. This sensitivity to hyposmotic swelling corroborates previous studies that demonstrated that calcium channels in pituitary cells are sensitive to hyperosmotic shrinkage (37), to a stream of physiological solution (2), and to membrane tension produced during seal formation (41). Interestingly, the use of hyposmotic swelling as a tool to study mechanosensitivity of ion channels gained support from recent studies that demonstrated, by using laser tweezers, that hyposmotic swelling is associated with increase in membrane tension (11, 12). Indeed, the hyposmotic enhancements of IL in smooth muscle cells (26, 31, 54) and in cardiac myocytes (36) were mimicked by increase in mechanical membrane tension, suggesting that L-type channels in these cells are mechanosensitive. Furthermore, it was demonstrated recently that recombinant L-type (35) and N-type (6) channels are mechanosensitive, suggesting that the mechanosensitivity of voltage-gated calcium channels might be a general phenomenon. Therefore, it is reasonable to suppose that the hyposmotic enhancements of IL and IT observed in this study reflect their sensitivity to mechanical membrane tension.
Possible mechanisms for effects of hyposmolarity on calcium channels. Our study shows that the hyposmotic enhancements of IBa cannot be attributed to a voltage shift in the activation of calcium channel currents (Figs. 6 and 8), to activation of an anionic conductance (Fig. 7), or to an increase in membrane surface area (Fig. 9). In addition, because [Ca]i was strongly buffered (see EXPERIMENTAL PROCEDURES), the hyposmotic enhancements of IBa cannot be attributed to release of calcium from internal stores.
The biophysical basis for the hyposmotic effects is unknown at this stage. However, it is reasonable to suppose that osmotic swelling alters the activity of calcium channels (NPo, where N is the number of channels contributing to the calcium current and Po is the open probability of single calcium channels). The voltage independence of the hyposmotic effects on IL implies that N might be the parameter affected. However, a detailed kinetic study will be needed to distinguish between the role of parameters N and Po in the hyposmotic effects. Interestingly, it was recently reported that membrane stretch increased the activity of recombinant L-type (35) and N-type (6) calcium channels. In the case of L-type channels this increase in activity was thought by the authors to reflect a change in Po (35). In the case of N-type channels, neither I-V shifts nor activation rates of whole cell currents accompanied this increase in activity, implying that N might be the parameter affected (6).
The hyposmotic enhancements of IL and
IT may stem from direct mechanical effects of membrane
expansion on the calcium channel protein or, alternatively, from various
indirect effects. It has been shown that the effect of membrane stretch on
recombinant L- and N-type channels is not dependent on interactions between
the 1-subunit and its accessory subunits
(6,
35). Thus it appears that
membrane tension may be transferred to the
1-subunit either
directly or indirectly via other proteins that interact with the
1-subunit. The cytoskeleton of the cell is a natural
candidate for this transfer of membrane tension
(16,
42). Indeed, it has been shown
that hyposmotic enhancement of IL in smooth muscle cells
is reduced by the actin-cytoskeleton disrupter cytochalasin D (Cyt-D) and
potentiated by the actin cytoskeleton stabilizer phalloidin
(55). Interestingly, several
studies in recent years pointed to a functional link between calcium channels
and the cytoskeleton (22,
23). In two recent studies it
was demonstrated that actin cytoskeleton disruptors and stabilizers can
modulate the amplitude of IBa in cardiac cells
(29) and in vascular smooth
muscle cells (39). Cyt-D
produced a decrease in IBa, whereas phalloidin increased
IBa.
Increased tension in the cytoskeleton may affect the calcium channel directly, leading to changes in the conformational state of the channel protein and thus altering its activity. Alternatively, increased tension in the cytoskeleton may affect the activity of phosphorylating enzymes known as regulators of L-type calcium channels. In a recent study it was shown that inhibitors of protein tyrosine kinase (PTK) modulate hyposmotic enhancements of IBa in smooth muscle cells (26). On the basis of these results these authors suggest that c-SRC, a cytoplasmic PTK that may be associated with the cytoskeleton, is involved in the hyposmotic modulation of L-type calcium currents in these smooth muscle cells.
Membrane tension may also be conferred to the calcium channel protein via the phospholipid bilayer (16, 42). In fact, it has been suggested that altering membrane stiffness by using lipophilic compounds can modulate influx through N-type channels in neuroblastoma cells (34). However, cholesterol, which was used to mimic membrane stretch in that study, had no effect on the extent of activation of N-type currents, in contrast to the effect of osmotic swelling on calcium currents described here (and to the effects of membrane stretch on recombinant channels). Interestingly, it has been suggested that membrane phospholipids, rather than the cytoskeleton, mediate osmomechanical effects on glutamate receptors (7).
Possible physiological significance. The idea that hyposmotically induced membrane expansion directly modulates calcium influx through L-type channels in pituitary cells and thereby [Ca]i and hormone secretion was first proposed by Greer and colleagues (43). Previous studies suggested that hyposmotic swelling may affect calcium influx indirectly through L-type channels by altering membrane potential (4, 10, 38). The present study demonstrates direct effects of hyposmotic swelling on calcium influx through L-type channels. Calcium influx through L-type channels plays a key role in regulating both basal and stimulated hormone secretion from pituitary somatotrophs and lactotrophs (8, 27, 50). Therefore, it is expected that hypotonic swelling would enhance hormone secretion from these pituitary cells. However, the physiological significance of this sensitivity to membrane tension is not clear. Although the hyposmotic stimuli used in this study were not excessively high (18-32% decrease in [Os]e), it is not feasible that pituitary cells would be challenged with similar hyposmotic stimuli during normal physiological life. Pituitary cells, like most mammalian cells (except kidney cells and intestinal cells), experience very small alterations in [Os]e during normal physiological conditions (30), and decrease in [Os]e by >10% is considered to be pathological. Nevertheless, alteration in membrane tension may occur also under normal physiological conditions. One interesting possibility is that changes in the metabolic state of pituitary cells may be associated with changes in intracellular osmolarity, and as a result with changes in pituitary cell volume, under normal physiological conditions (19, 30). According to this hypothesis, volume regulatory mechanisms are continuously activated during normal physiological life and calcium channels may participate in these volume regulatory mechanisms (43). Another interesting possibility is that calcium channels sense alteration in membrane tension during the process of exocytosis (12, 25) and that these alterations in membrane tension participate in regulating calcium influx during the exocytotic event, thus providing a novel autoregulatory mechanism. This latter hypothesis is based on the recently reported physical and functional links between calcium channels and proteins of the exocytotic machinery. Most of this evidence was obtained for neuronal calcium channels (47), and similar evidence exists also for L-type calcium channels (53).
Interestingly, it was proposed recently that mechanosensitive L-type calcium channels in human intestinal smooth muscle cells participate in a feedback mechanism that controls intestinal muscle contraction during normal or pathological digestive activity (20). In addition, it has been proposed that mechanosensitivity of L-type channels in cardiac myocytes may affect the contraction of cardiac muscle under normal and pathophysiological conditions such as ischemic heart disease (36).
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DISCLOSURES |
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FOOTNOTES |
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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. Section 1734 solely to indicate this fact.
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