Institut für Veterinär-Physiologie, Universität
Giessen, D-35392 Giessen, Germany
enteric nervous system; chloride transport; potassium transport; prostaglandins
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INTRODUCTION |
THE EPITHELIUM OF THE COLON is able to both absorb and
secrete K+. To absorb
K+,
K+ enters the colonocyte from the
colonic lumen via an
H+-K+-ATPase;
K+ probably leaves the cell by
passing through basolateral K+
channels. K+ secretion is driven
by the intracellular accumulation of
K+ via the basolateral
Na+-K+-2Cl
cotransporter and the
Na+-K+-ATPase;
the exit into the colonic lumen is mediated by apical K+ channels (for review, see Ref.
1).
These transport pathways are under the extracellular control of
neurotransmitters, hormones, and paracrine substances. One of these
extracellular messengers affecting intestinal
K+ transport is epinephrine. This
adrenergic agonist has been shown to induce a
K+ secretion in rabbit (12, 22)
and guinea pig colon (18), which leads to a decrease in short-circuit
current (Isc).
Also, in the rat colon, epinephrine has been shown to induce a decrease in Isc (17). In
addition to the induction of K+
secretion, in guinea pig colon, epinephrine has been observed to
transiently stimulate Cl
secretion (18). In other intestinal segments such as rabbit ileum (10),
rat colon (17), or rabbit proximal colon (21) epinephrine has been
reported to stimulate Na+ and
Cl
absorption
and/or to inhibit
Cl
secretion, whereas in
rabbit distal colon no changes in
Na+ and
Cl
transport were evoked
(22). No data are available concerning the effect of epinephrine on
K+ transport across the rat colon.
Therefore, the aim of the present experiments was to study the action
of epinephrine on the transport of
K+,
Na+,
Cl
, and
HCO
3 across the rat colon using
electrophysiological and pharmacological approaches. Because of the
known segmental differences across the longitudinal axis of the large
intestine (see, e.g., Ref. 15), the experiments were performed with the distal and the proximal colon.
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METHODS |
Solutions.
For the Ussing chamber experiments, a Parsons solution was used
containing (in mmol/l) 107 NaCl, 4.5 KCl, 25 NaHCO3, 1.8 Na2HPO4, 0.2 NaH2PO4,
1.25 CaCl2, 1 MgSO4, and 12 glucose. The
solution was gassed with carbogen (5%
CO2 in 95%
O2); pH was 7.4. When 86Rb+
fluxes were measured, KCl was replaced by RbCl. For the
Cl
-poor buffer, NaCl was
replaced with sodium gluconate (elevating the
Ca2+ concentration to 5.75 mmol/l
to compensate for the
Ca2+-buffering properties of
gluconate). For the indirect measurement of
HCO
3 transport, an unbuffered Parsons
solution was used consisting of (in mmol/l) 135.8 NaCl, 4.5 KCl, 1.25 CaCl2, 1 MgSO4, and 12 glucose. This
solution was gassed with O2 and was prepared from degassed distilled water.
Tissue preparation.
Rats of both sexes were used (weight of 180-220 g). The animals
had free access to water and food until the day of the experiment. Animals were stunned by a blow on the head and killed by exsanguination (approved by Regierungspräsidium Giessen, Giessen, Germany). The
appearance of palmlike foldings distinguished between the border and
the distal and proximal colon (14). The serosa and muscularis propria
were stripped away by hand to obtain a mucosa-submucosa preparation.
Isc measurement.
The mucosa-submucosa preparation was fixed in a modified Ussing
chamber, bathed with a volume of 3.5 ml on each side of the mucosa (9).
The tissue was incubated at 37°C and short-circuited by a voltage
clamp (Ing. Büro für Mess- und Datentechnik, Dipl. Ing. K. Mussler, Aachen, Germany) with correction for solution resistance.
Isc was
continuously recorded every 6 s; tissue conductance (Gt) was
measured every minute. To compare different experimental periods during
measurement of unidirectional ion fluxes, the means of the electrical
parameters (measured every 5 min) over the whole period were calculated.
Experimental design.
In some experiments, i.e., the concentration response and the
desensitization experiments, epinephrine was administered repeatedly to
the same tissue. In this case, the serosal compartment was washed three
times in 5-min intervals with five times the chamber volume before the
drug was administered a second or third time. In all other experiments,
epinephrine was administered only once to the same preparation. The
maximal increase (first phase) and the maximal decrease in
Isc (second
phase) evoked by epinephrine were measured in the presence and absence
(control) of putative inhibitors. Inhibitors were administered to the
compartment indicated in the text;
Isc was allowed
to stabilize, which usually took 15-20 min, before epinephrine was
added. In some experiments, the half time of the epinephrine response
was calculated by linear interpolation of the change in
Isc.
Unidirectional flux measurements.
After an equilibration period of 60 min,
22Na (59 kBq) and
36Cl (29 kBq) were added to one
side of the epithelium (9). After an additional 20 min to allow isotope
fluxes to reach a steady state, unidirectional ion fluxes were
determined in two sequential 20-min periods. The protocol was as
follows: first phase, control; second phase, fluxes 5 min after
administration of epinephrine (5 × 10
6 mol/l at the serosal
side). From the measured unidirectional fluxes, net ion fluxes
(Jnet) were
calculated. Residual ion flux (J Rnet), i.e., the sum of
the movement of all ions other than Na+ and
Cl
, was calculated according to the following:
J Rnet = Isc
J Nanet + J Clnet. A positive
J Rnet
indicates either the absorption of a cation or the secretion of an
anion. With the same protocol, the effect of epinephrine on
unidirectional Rb+ fluxes was
determined. The amount of
86Rb+
added into the chambers was 37 kBq.
Serosal-to-mucosal HCO
3 flux was
estimated from the base transport into the mucosal compartment (24). The serosal half chamber was filled with standard Parsons solution gassed with
CO2-O2,
whereas the fluid in the mucosal compartment was an unbuffered Parsons
solution (see Solutions) gassed with O2, from which samples were taken
and replaced by fresh solution in regular intervals. The base transport
was measured in four sequential 30-min periods, i.e., two control
periods and two periods in the absence (control) or presence of
epinephrine (5 × 10
6
mol/l at the serosal side).
The volume of HCl (0.5 mmol/l) needed to titrate the mucosal sample to
a pH of 7.0 was measured; from this value, the amount of
HCO
3 in the sample was calculated
using a calibration curve with samples of known
HCO
3 concentration. The calibration
curve was linear (r > 0.996) over the complete concentration range tested (0.1, 0.2, 0.4, 0.6, 0.8, and
1.0 mmol/l).
Chemicals.
Methazolamide and phentolamine methanesulfate (Aldrich, Steinheim,
Germany) were dissolved in DMSO (final concentration 0.3%, vol/vol).
Bumetanide, indomethacin, quinine hydrochloride, and yohimbine
hydrochloride were dissolved in ethanol (final maximal concentration
0.1%, vol/vol). Atenolol (gift from Zeneca, Plankstadt, Germany),
atropine sulfate, epinephrine bitartrate, hexamethonium chloride,
ICI-118551
{1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride; Tocris, Bristol, UK}, prazosin hydrochloride
(gift from Pfizer, Karlsruhe, Germany), and propranolol (Aldrich) were dissolved in aqueous stock solutions diluted in salt buffer just before
use. TTX was dissolved as a stock solution in citrate buffer (20 mmol/l). Tetraethylammonium (TEA) was added as
Cl
salt. If not indicated
differently, drugs were from Sigma (Deisenhofen, Germany).
Radiochemicals were obtained from NEN Life Science (Köln, Germany). Specific activity was 3.9 TBq/g
Na+ and 550 MBq/g
Cl
; the initial activity of
86Rb+
amounted to 411 GBq/g.
Statistics.
Values are given as means ± SE. Significances were tested by paired
or unpaired two-tailed Student's
t-test or a
U test, respectively. An
F test decided which test method was
to be used; P < 0.05 was considered
to be statistically significant. Statistical comparison of qualitative
data, i.e., the absence or the presence of a first or a second phase of
the epinephrine response, was tested by a
2 test. The quality of linear
regressions was checked by the linear regression coefficient
(r).
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RESULTS |
Effect of epinephrine: basic properties.
After an equilibration time of 1 h, the distal colon exhibited a
spontaneous baseline
Isc of 3.4 ± 0.1 µeq · h
1 · cm
2
(n = 86) and a
Gt of 15.7 ± 0.8 mS/cm2 (n = 86). In
the proximal colon, baseline
Isc amounted to
3.4 ± 0.1 µeq · h
1 · cm
2
(n = 85) at a
Gt of 32.4 ± 2.3 mS/cm2 (n = 85, P < 0.05 vs. distal colon).
Epinephrine (5 × 10
6
mol/l at the serosal side) induced a biphasic change in
Isc in both
colonic segments (Fig. 1). A first
transient increase in Isc, which
amounted to 0.2 ± 0.0 µeq · h
1 · cm
2
in the distal (n = 86, P < 0.05) and 0.3 ± 0.0 µeq · h
1 · cm
2
in the proximal colon (n = 85, P < 0.05), was followed by a
long-lasting decrease in
Isc of 0.9 ± 0.1 µeq · h
1 · cm
2
in the distal (n = 86, P < 0.05) and of 0.7 ± 0.1 µeq · h
1 · cm
2
in the proximal colon (n = 85, P < 0.05; Table
1). The first phase of the effect of
epinephrine developed rapidly with a half time of about 30 s, whereas
the half time of the second phase amounted to
140-160 s (Table 1). The second phase of the epinephrine effect
was accompanied by an increase in
Gt, which was
about nine times greater in the proximal compared with the distal colon
(P < 0.05; Table 1).

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Fig. 1.
Effect of epinephrine (5 × 10 6 mol/l at the serosal
side) on short-circuit current
(Isc) in the
distal (A) and proximal
(B) rat colon. Tracings are
representative for 85-86 experiments with each tissue; for
statistics, see Table 1.
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The first phase of the epinephrine response exhibited a more pronounced
variability compared with the second phase. When the presence of each
individual phase was defined as change of at least 0.1 µeq · h
1 · cm
2,
in the distal colon only 60 of 86 tissues (70%) showed a first phase,
whereas 80 of 86 tissues (93%) exhibited a second phase of the
epinephrine response (P < 0.05 vs.
first phase,
2 test). In the
proximal colon, the numbers were 79% (67 of 85) for the first and 92%
(78 of 85) for the second phase (P < 0.05 vs. first phase,
2 test).
The effect of epinephrine, especially the second phase, i.e., the
decrease in Isc
induced by the drug, showed a relative flat concentration dependence
(Fig. 2). However, a clear first phase, i.e., an
increase in Isc,
was only observed when the highest concentration (5 × 10
6 mol/l) was
administered. Increasing the concentration to 5 × 10
5 mol/l in an additional
series of experiments did not further increase the electrical response
to the adrenergic agonist (n = 6, data
not shown). Therefore, a concentration of 5 × 10
6 mol/l was chosen for
all subsequent experiments.

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Fig. 2.
Concentration-response curve for the first (maximal increase in
Isc) and second
(maximal decrease in
Isc) phase of
the effect of epinephrine (administered at the serosal side) in the
distal ( ) and proximal colon ( ). Epinephrine was administered in
increasing concentrations to the same tissue with an intermediate
washing step between the individual administrations; therefore, a
possible desensitization cannot be ruled out. Values are means
(symbols) ± SE (error bars); n = 6-8. The threshold at which epinephrine induced a first
significant change in
Isc amounted to 5 × 10 8 mol/l in the
distal colon (for both phases) and 5 × 10 7 mol/l in the proximal
colon (for both phases).
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When epinephrine was administered three times to the same tissue with
an intermediate washing step between the individual administrations,
the Isc response
decreased from administration to administration, although this
desensitization reached statistical significance only in the case of
the distal colon (n = 6, data not
shown). Therefore, for all further experiments, epinephrine was
administered only once to the same preparation.
Ionic nature of the Isc
response.
Ion substitution experiments were performed to investigate the ionic
basis of the biphasic
Isc response
induced by epinephrine. The increase in
Isc was totally
dependent on the presence of
Cl
in both colonic segments
(Fig. 3, Table 2). When NaCl
was substituted by sodium gluconate on both sides of the tissue,
epinephrine (5 × 10
6
mol/l at the serosal side) did not increase
Isc, whereas the decrease in Isc
induced by the adrenergic agonist was still present. In the absence of
Cl
, the decrease in
Isc developed
even faster than in the presence of this anion; the half time for the
decrease in Isc
amounted to 52 ± 5 s (P < 0.05 vs. the half time in
Cl
-containing buffer;
n = 8) in the distal and 92 ± 23 s
in the proximal colon (P < 0.05 vs.
the half time in
Cl
-containing buffer;
n = 8). In contrast, the inhibitor of
the Na+-K+-2Cl
cotransporter, bumetanide
(10
4 mol/l at the serosal
side), inhibited both phases of the epinephrine response (Table 2).
These results are compatible with the assumption that the first phase
of the epinephrine-induced increase in
Isc is due to
Cl
secretion. Bumetanide
itself led to a decrease in
Isc and
Gt in both
colonic segments (Table 3).

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Fig. 3.
Effects of epinephrine (5 × 10 6 mol/l at the serosal
side) on Isc in
the distal (A) and proximal
(B) rat colon in the absence of
Cl . Tracings are
representative of 8 experiments with each tissue; for statistics, see
Table 2.
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The involvement of K+ channels in
the epinephrine-induced
Isc response was
investigated by the use of different
K+ channel blockers. Quinine
(10
3 mol/l at the mucosal
side) inhibited the second phase of the Isc response in
the proximal colon and even reversed the usual decrease into an
increase of Isc
in the distal colon (Fig. 4, Table 2),
suggesting the induction of K+
secretion via apical quinine-sensitive
K+ channels during the late phase
of the epinephrine effect. The reason for the paradox, the long-lasting
increase in Isc
evoked by epinephrine in the presence of quinine, is unknown. In
contrast, TEA (5 × 10
3 mol/l at the mucosal
side) was ineffective (Table 2).

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Fig. 4.
Effects of epinephrine (5 × 10 6 mol/l at the serosal
side) on Isc in
the distal (A) and proximal
(B) rat colon in the presence of
quinine (10 3 mol/l at the
mucosal side). Tracings are representative of 5-7 experiments with
each tissue; for statistics, see Table 2.
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Unidirectional fluxes of Na+ and
Cl
were measured to study
the effect of epinephrine on the transport of both ions. The flux measurements were performed 5 min after administration of epinephrine, i.e., during the second phase of the
Isc response
induced by the adrenergic agonist (Fig. 5).
Under control conditions, both in the distal and in the proximal colon,
the mucosal-to-serosal fluxes (Jm
s) of
Na+ and
Cl
exceeded the
corresponding serosal-to-mucosal fluxes
(Js
m), leading to a net absorption of both ions (Table
4). Epinephrine (5 × 10
6 mol/l) induced a
significant decrease of
J Clm
s and
J Cls
m in the distal
but not in the proximal colon (Table 4). In parallel, there was a
significant increase in
J Nas
m in this colonic segment.

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Fig. 5.
Averaged Isc
(A and
B) and tissue conductance
(Gt;
C and
D) in distal
(A and
C) and proximal
(B and
D) colon during measurement of
unidirectional Na+ and
Cl fluxes. Values are means
(lines) ± SE (shaded area); n = 16.
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Unidirectional fluxes of
86Rb+
were measured as a marker for K+
transport (11). In both colonic segments,
J Rbnet was not
significantly different from zero (Table
5). Epinephrine increased
J Rbs
m in both
colonic segments (P < 0.05), i.e., it stimulated
K+ secretion.
The alkalinization of an unbuffered solution at the mucosal side of the
tissue was used to indirectly measure the effect of epinephrine on
HCO
3 secretion (24). The experiments were performed in the presence of bumetanide
(10
4 mol/l at the serosal
side) to eliminate the effects of epinephrine on
Cl
secretion. Under control
conditions, there was a spontaneous alkalinization of the mucosal
compartment, which, under the assumption that this alkalinization
completely represents HCO
3 secretion,
gave a transport rate of HCO
3 in the
range of 2.5-5
µmol · h
1 · cm
2
at the start of the experiment (Fig. 6).
Under control conditions, HCO
3
secretion gradually declined during the time of measurement. This time
course was not altered by administration of epinephrine (5 × 10
6 mol/l at the serosal
side). The validity of the method was verified by comparing the effect
of epinephrine with that of forskolin, which induced a strong
HCO
3 secretion measured with the
identical protocol (20).

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Fig. 6.
Serosal-to-mucosal flux of HCO 3
(J HCO3s m,
determined titrimetrically) in the absence ( ) and presence ( )
of epinephrine (5 × 10 6 mol/l) in the distal
(A) and proximal
(B) rat colon. Data are means
(symbols) ± SE (bars); n = 11-15.
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Involvement of subepithelial structures in the epinephrine-induced
effect on Isc.
Indomethacin (10
6 mol/l at
the mucosal and the serosal side), a cyclooxygenase inhibitor,
abolished the first phase, i.e., the
Isc increase, of
the epinephrine response in the distal colon, whereas the second phase,
i.e., the subsequent decrease in
Isc, was
unaffected (Fig. 7, Table
6). In contrast, indomethacin was ineffective in the proximal colon. Conversely, pretreatment with the
neuronal toxin, TTX (10
6
mol/l at the serosal side) had no effect on the first phase but abolished the second phase of the epinephrine response in the distal
colon; the blocker, however, was ineffective in the proximal colon
(Fig. 7, Table 6). Consequently, the first phase of the epinephrine
effect is mediated by prostaglandins and the second phase by enteric
neurons in the distal but not in the proximal colon.

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Fig. 7.
Effects of epinephrine (5 × 10 6 mol/l at the serosal
side) on Isc in
the distal (A and
C) and proximal
(B and
D) rat colon in the presence of
indomethacin (10 6 mol/l at
the mucosal and the serosal side; A
and B) or in the presence of TTX
(10 6 mol/l at the serosal
side; C and
D). Tracings are representative of 6 experiments with each tissue; for statistics, see Table 5.
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To further characterize the neuronal component of the epinephrine
response in the distal colon, the effects of cholinergic antagonists
were tested. Pretreatment of the tissue with the nicotinic antagonist
hexamethonium (10
5 mol/l at
the serosal side) significantly inhibited the epinephrine-induced decrease in Isc
in the distal colon (Table 6), whereas the first phase was unaltered.
Atropine (10
6 mol/l at the
serosal side), the inhibitor of muscarinic receptors, was ineffective
(Table 6). As should be expected from the results with TTX, none of the
inhibitors affected the response to epinephrine in the proximal colon
(Table 6).
Characterization of the adrenergic receptors mediating the
epinephrine effect.
The nonselective
-receptor blocker phentolamine
(10
4 mol/l at the serosal
side; see Ref. 4 for references concerning the adrenoceptor blockers)
did not inhibit the effect of epinephrine in the distal colon. Instead,
a paradoxical increase in the first phase of the epinephrine-induced
Isc response was
observed (Table 7); a similar phenomenon
has already been reported for the rabbit ileum (10). In contrast, in
the proximal colon, phentolamine inhibited the second phase of the
epinephrine effect while leaving the first phase unaltered. The effect
of phentolamine was mimicked by the
2-receptor blocker yohimbine
(10
5 mol/l at the serosal
side; Fig. 8, Table 7) in the proximal colon but not by the
1-antagonist, prazosin
(10
6 mol/l at the serosal
side; Table 7), suggesting a mediation of the epinephrine response by
2-receptors in the proximal
colon. None of these inhibitors was effective in the distal colon
(Table 7).

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Fig. 8.
Effects of epinephrine (5 × 10 6 mol/l at the serosal
side) on Isc in
the distal (A and
C) and proximal
(B and
D) rat colon in the presence of the
2-receptor blocker yohimbine
(10 5 mol/l at the serosal
side; A and
B) or in the presence of the
2-receptor blocker ICI-118551
(10 5 mol/l at the serosal
side; C and
D). Tracings are representative of 6 experiments with each tissue; for statistics, see Table 6.
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The second phase of the epinephrine effect was inhibited in the distal
colon by pretreatment of the tissue with the nonselective
-receptor blocker, propranolol (5 × 10
6 at the serosal side;
Table 7). Inhibition was mimicked by the
2-receptor blocker, ICI-118551
(10
5 mol/l at the
serosal side; Fig. 8); the specific
1-receptor blocker atenolol
(10
4 mol/l at the serosal
side) was ineffective (Table 7), indicating the involvement of
2-receptors in the mediation of
the epinephrine effect in the distal colon. None of the
-receptor
blockers was effective in the proximal colon (Table 7).
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DISCUSSION |
Epinephrine induces a biphasic change of
Isc in the rat
colon, which consists of an initial, transient increase followed by a
long-lasting decay (Fig. 1). The first phase of the action of epinephrine is caused by a
Cl
secretion, as indicated
by anion substitution experiments and by the sensitivity to bumetanide,
an inhibitor of the basolateral Na+-K+-2Cl
cotransporter (Fig. 3, Table 2). Depending on the individual colonic
segment, only 70-79% of the tissues investigated exhibited a
rapid increase in
Isc. Such a
variability was already observed by Racusen and Binder (17) for the rat
colon. The ability of epinephrine to induce
Cl
secretion seems to be
species dependent: it is absent in rabbit ileum (10) and in rabbit
colon (22) but present in guinea pig colon (18).
In contrast, the second phase of the epinephrine response could be
regularly evoked in more than 90% of the tissues. The decrease in
Isc was
suppressed by apical administration of the
K+ channel blocker quinine (Fig.
4) and was paralleled by an increase in
J Rbs
m
(Table 5), indicating that epinephrine induces a
K+ secretion, a response that has
already been observed in several other intestinal epithelia such as
rabbit (12, 22) or guinea pig colon (18). The decrease in
Isc was
suppressed by bumetanide but not in the absence of
Cl
(Table 2). At first
glance, this paradoxical result may be explained by the fact that the
Cl
-poor buffer still
contained Cl
due to the
presence of K+ and
Ca2+ salts of this anion;
obviously, this Cl
is
sufficient to allow K+ influx via
the
Na+-K+-2Cl
cotransporter.
The K+ secretion begins earlier
than the observed decrease in
Isc. Under
Cl
-poor conditions, i.e.,
under conditions in which epinephrine is not able to induce
Cl
secretion, the half time
for the development of the decrease in
Isc is
significantly shortened, suggesting that under control conditions the
effect of the induction of K+
secretion, which leads to a decrease in
Isc, is partially
counteracted by the induction of
Cl
secretion leading to an
increase in Isc.
Except for a decrease in
J Cls
m in the
distal colon, representing an inhibition of spontaneous
Cl
secretion, which was
accompanied by a (smaller) decrease in
J Clm
s (Table 4),
no relevant changes in the transport of
Na+ and
Cl
(Table 4) or the
transport of HCO
3 (Fig. 6) were
observed during the long-lasting decrease in
Isc induced by
epinephrine. This raises the question concerning the nature of the part
of the Isc
response, which is several times greater than the change in net
K+ transport (Table 5),
independent of K+ secretion. This
"unexplained
Isc" might
represent either changes in the electrogenic transport of other ions
present in the buffer solution or, more probably, the combinations of
subtle changes in Na+,
Cl
, and
HCO
3 transport not resolved in the
corresponding flux studies.
Despite the phenomenologically similar action of epinephrine on
Isc in the distal
and the proximal colon (cf. Fig. 1, A
and B, respectively), there are
characteristic differences in the mediation of the response to the
adrenergic agonist. In the distal colon, both phases of the effect of
epinephrine are mediated by extraepithelial action sites. In this
segment, the first phase of the epinephrine-induced change in
Isc is suppressed
by the cyclooxygenase inhibitor, indomethacin (Fig. 7),
indicating the involvement of prostaglandins in the induction of
Cl
secretion. The
predominant part of prostaglandin synthesis in the rat colon takes
place in the connective tissue of the submucosa (6), suggesting an
effect of the adrenergic agonist on subepithelial structures. In
contrast, the second phase of the epinephrine response was insensitive
to indomethacin but was abolished by the neurotoxin TTX (Fig. 7) in the
distal colon, indicating that the drug stimulates secretomotoneurons of
the enteric nervous system, which induce K+ secretion at the epithelium.
The nature of the transmitter finally mediating the secretory signal to
the colonocyte is not known; an involvement of ACh can be excluded by
the missing effect of atropine on the epinephrine-induced decrease in
Isc (Table 6).
Obviously, a nicotinic synapse is involved in the neuronal secretory
pathway, as shown by the partial sensitivity to the nicotinic
antagonist, hexamethonium (Table 6). In contrast, none of these
inhibitors was effective in the proximal colon, suggesting a direct
epithelial site of action of epinephrine. At first glance, these
results seem to be in conflict with previous observations in rat colon,
in which the decrease in
Isc evoked by
epinephrine was resistant to a somewhat lower concentration (2 × 10
7 mol/l) of TTX (17).
However, in this study, no distinction was made between the distal and
the proximal colon, which may easily explain the apparent discrepancy.
In other species such as rabbit (22) or guinea pig colon (18), the
decrease in Isc evoked by epinephrine is TTX insensitive.
Different receptors are involved in the epithelial and nonepithelial
actions of epinephrine in both colonic segments. In the proximal colon,
the decrease in
Isc evoked by
epinephrine did not happen in the presence of phentolamine, a
nonselective
-blocker (Table 7), and in the presence of yohimbine, a
selective
2-blocker (Fig. 8),
suggesting the involvement of
2-receptors in the induction of
K+ secretion. A similar
sensitivity to adrenoceptor blockers has been observed in rabbit ileum
(5) in which yohimbine abolishes the decrease in
Isc evoked by
epinephrine. In contrast, in the rabbit colon, the TTX-resistant
K+ secretion is mediated by a
1-receptor (22).
In the distal colon, the TTX-sensitive decrease in
Isc evoked by
epinephrine was suppressed by the nonselective
-blocker, propranolol
(Table 7), and by the selective
2-blocking drug, ICI-118551
(Fig. 8), indicating mediation by a
2-receptor. Neuronal effects of
epinephrine on enteric neurons have been described, although they are
in general attributed to the stimulation of
2-receptors (16). However,
-receptors on other nerve structures are well known (see e.g., Ref.
23); therefore, the most simple explanation of these data is the
assumption of a
-receptor on an enteric neuron, although a
localization on another cell type, which then triggers
secretomotoneurons, is not excluded.
Surprisingly, the first phase of the epinephrine response was not
inhibited in either the distal or the proximal colon by any of the
tested
- or
-blockers (Table 7). The reason for this failure is
unknown. There is, however, the speculative possibility that this phase
of the action of epinephrine may be mediated by another type of
adrenoceptor, e.g.,
3-receptors
as observed in fatty tissue or intestinal smooth muscle (for
references, see Ref. 3), which are in general quite resistant against
even nonselective
-blockers such as propranolol. Because of the
transient (and variable) nature of this part of the epinephrine
response, no further studies were performed to clarify this point.
We can only speculate about the nature of the second messenger(s)
involved in the induction of K+
secretion in both colonic segments. An increase in both the
intracellular Ca2+ as well as in
the intracellular cAMP concentration induces
K+ secretion by different
mechanisms (8, 19).
2-Receptors, which mediate the
action of epinephrine on K+
transport in the proximal colon and which are most likely located at
the epithelium, are known to be coupled to an increase in the intracellular Ca2+ concentration
as well as an inhibition of adenylate cyclase (for references, see Ref.
13); therefore, it may be possible that Ca2+ is the second messenger
inducing the K+ secretion in the
proximal colon. The neurotransmitter, which is released by the
secretomotoneurons stimulated by epinephrine in the distal colon, is
not known; therefore, all hypotheses about the intracellular mediation
of the epinephrine response in this colonic segment remain speculative.
Taken together, these data indicate a segment-specific regulation of
transepithelial ion transport in rat colon by adrenergic stimuli: a
direct epithelial action of epinephrine in the proximal colon and an
indirect action in the distal colon, mediated by the enteric nervous
system in the case of epinephrine-induced K+ secretion and mediated by
prostaglandins in the case of epinephrine-induced Cl
secretion. Thus colonic
K+ transport is, in addition to
Cl
transport (7), a further
example of several elements of the intestinal wall, i.e., the
eicosanoid-producing submucosal tissue, the enteric nervous system, and
the ion-transporting cell (i.e., the epithelium), showing a strong
interaction in the regulation of intestinal functions.
We are grateful for the diligence of B. Brück, E. Haas, A. Metternich, and B. Schmitt.
This work was supported by Deutsche Forschungsgemeinschaft Grant Di
388/3-1.
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article were defrayed in part by the
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Address for reprint requests: M. Diener, Institut für
Veterinär-Physiologie, Universität Giessen, Frankfurter
Str. 100, D-35392 Giessen, Germany.