Inhibition of muscle cell relaxation by somatostatin: tissue-specific, CAMP-dependent, pertussis toxin-sensitive L. McHENRY, K. S. MURTHY, J. R. GRIDER, AND G. M. MAKHLOUF Departments of Medicine and Physiology, Medical College of Virginia, Richmond, Virginia 23298-9601

L., K. S. MURTHY, J. R. GRIDER, AND G. M. Inhibition of muscle cell relaxation by somatostatin: tissue-specific, CAMP-dependent, pertussis toxin-sensitive. Am. J. Physiol. 261 (Gastrointest. Liver Physiol. 24): G45-G49, 1991.-The direct action of somatostatin on smooth muscle was examined in muscle cells isolated from the stomach and intestine of human and guinea pig. Somatostatin inhibited relaxation in gastric but not intestinal muscle cells of the two species, and its mechanism of action was explored in more detail in gastric muscle cells of the guinea pig. Somatostatin inhibited relaxation induced by vasoactive intestinal peptide (VIP, 83 t 7%, P < 0.001) and isoproterenol (85 t 5%, P c O.OOl), as well as the concomitant increase in adenosine 3’,5’cyclic monophosphate (CAMP) production [81 t 25% inhibition with VIP (P < 0.02) and 68 t 12% inhibition with isoproterenol (P c O.Ol)]. Inhibition of relaxation and CAMP production was abolished by pretreatment of the cells with pertussis toxin. Relaxation induced by the permeant derivative of CAMP, W,2’O-dibutyryladenosine 3’,5’-cyclic monophosphate, by sodium nitroprusside, which acts by increasing levels of guanosine 3’,5’-cyclic monophosphate, or by ATP, which acts by opening of K+ channels, was not affected by somatostatin. The fact that inhibition by somatostatin and its reversal by pertussis toxin was confined to agonists that stimulate an increase in the levels of CAMP implied that somatostatin acts by inhibiting the generation and not the action of CAMP. It is concluded that somatostatin receptors on gastric muscle cells mediate inhibition via a GTP-binding, pertussis-sensitive regulatory protein, Gi, coupled to adenylate cyclase. MCHENRY, MAKHLOUF.

somatostatin receptors; vasoactive intestinal terenol; adenosine 3’,5’-cyclic monophosphate; nal transduction

peptide; isoproG protein; sig-

has a wide range of endocrine, paracrine, autocrine, and neural actions that reflects its mode of delivery to target cells. All these actions are manifest in various regions of the gut (4, 5, 17, 18, 20, 21, 26). Paracrine action is evident in the regulation of gastrin cells of the antrum and parietal cells of the fundus of the stomach (17, 20, 21); autocrine action is evident in the ability of somatostatin to regulate its own release from paracrine cells (16). Neural action can be expected from the presence of somatostatin in neurons of the myenteric and submucosal plexuses of the gut (4, 5, 7, 22-24, 26). In the intestine, somatostatin neurons of the myenteric plexus project their fibers caudad to other neurons within the plexus but not into the circular or longitudinal muscle layers (3, 4, 10, 25). In the stomach, however, somatostatin neurons project fibers mainly to other neurons within the plexus, but they also project fibers,

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$1.50 Copyright

though sparsely, into both muscle layers. The topography of somatostatin neurons implies that in the stomach, somatostatin might exert a direct effect on smooth muscle, as well as an indirect effect via other neurons. Neurally mediated effects include inhibition of release of the excitatory (contractile) transmitter acetylcholine (7, 24, 26) and stimulation of release of the inhibitory (relaxant) transmitter vasoactive intestinal peptide (VIP) (5). At the cellular level, the action of somatostatin, independently of its mode of delivery, is inhibitory. Where the stimulus involves activation of adenylate cyclase, the inhibitory action of somatostatin is mediated by a guanosine nucleotide (GTP) -binding, pertussis toxin-sensitive protein, Gi, coupled to adenylate cyclase (11, 12, 15, 18, 19). Inhibitory action can also be mediated by other pertussis toxin-sensitive and insensitive mechanisms: one pertussis toxin-sensitive mechanism involves an increase in K+ channel activity and hyperpolarization of the plasma membrane that leads to a decrease in Ca”’ influx and cytosolic Ca”+ levels (11, 12, 19). In the present study, we have used dispersed smooth muscle cells from the stomach and intestine of the guinea pig to determine the presence and functional characteristics of somatostatin receptors and the nature of the signal transduction pathway to which they are coupled. Somatostatin was examined for its ability to modulate the effect of relaxant agents: these included VIP and the ,&adrenergic agonist isoproterenol, which act by increasing the intracellular levels of adenosine 3’,5’-cyclic monophosphate (CAMP; 1, 9, 28), N’,2’-O-dibutyryladenosine 3’,5’-cyclic monophosphate (dibutyryl CAMP), a permeant derivative of CAMP; sodium nitroprusside, which acts by increasing the intracellular levels of guanosine 3’,5’-cyclic monophosphate (cGMP; 22); and ATP, which acts via cyclic nucleotide-independent mechanisms (27). The results show that somatostatin inhibits relaxation induced by agents coupled only to adenylate cyclase and that it acts by inhibiting the generation not the action of CAMP. METHODS

Dispersion of gastric smooth muscle cells. Muscle tissue was obtained from the stomach and intestine of male guinea pigs weighing 200-400 g. Human gastric and intestinal muscle tissue were obtained from patients undergoing intestinal bypass surgery for morbid obesity; informed consent was obtained in each instance. Muscle cells were dispersed as described previously (1, 2,6). The

63 1991 the American

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mucosa was removed by blunt dissection, and the circular muscle layer was cut into strips 2 mm wide and 15 mm long. The strips were incubated for two successive 45 min periods at 31°C in 15 ml of medium containing 0.1% collagenase (CLS type II) and 0.01% soybean trypsin inhibitor. The composition of the medium was as follows (in mM): 120 NaCl, 4 KCl, 2.6 KHZP04, 2 CaC12, 0.6 MgCIZ, 25 N-2-hydroxyethylpiperazine-N’-2-ethanesulfonic acid (HEPES), 14 glucose, and 2.1% Eagle’s essential amino acid mixture. After the second incubation, the partially digested muscle strips were washed with 50 ml of enzyme-free medium and reincubated for 30 min in 15 ml of enzyme-free medium to allow the muscle cells to disperse spontaneously. The cells were harvested by filtration through 500~pm Nitex mesh. Measurement of contraction and relaxation in isolated muscle cells. Contraction was measured as described previously (2, 6). Aliquots of cells ( lo4 in 0.5 ml of suspension) were added to 0.2 ml of medium containing the contractile agent and the reaction stopped after 30 s with 0.1 ml of acrolein (final concn 1%). The length of the first 50 cells encountered randomly in successive microscopic fields was measured by scanning micrometry and compared with the lengths of control cells treated in the same manner but without addition of contractile agent. Contraction was expressed as percent decrease in cell length from control. Relaxation was measured in cells maximally precontracted for 30 s with 1 nM cholecystokinin octapeptide (CCK-8; 1). The cells were preincubated for 60 s with the relaxant agent before addition of CCK-8. The following relaxant agents were used: 1 PM VIP, 100 PM isoproterenol, 0.1 and 1 mM ATP, 1 and 10 nM sodium nitroprusside (SNP), and 0.01, 0.1, and 1 mM dibutyryl CAMP. The response of the cells to relaxant agents was examined with and without simultaneous addition of somatostatin (1 ,uM). The same experiments were repeated in cells preincubated for 1 h with pertussis toxin (200 rig/ml). Finally, the potency of somatostatin was examined in the range of 100 pM to 1 PM. Relaxation was expressed as percent inhibition of CCK-induced maximal contraction. Measurement of CAMP. Cellular CAMP was measured by radioimmunoassay as described previously (1,8). The same time sequence for incubation of cells and addition of various agents was used as for measurement of contraction and relaxation, except that the reaction was terminated with 6% cold trichloracetic acid instead of acrolein. 3-Isobutyl-1-methylxanthine (IBMX, 1 PM) was included in each aliquot at the time of addition of relaxant agents. Measurements were made in duplicate using 0.25-ml aliquots of cell suspension containing l1.2 X lo6 cells/ml. After the reaction was terminated, i.e., 90 s after addition of the relaxant agent, the suspension was vortexed, placed on ice for 15 min, and then centrifuged for another 15 min at 800 g. The supernatant was extracted three times with 1 ml ethyl ether, and the samples were lyophilized for subsequent radioimmunoassay. CAMP levels were expressed as pmol/106 cells or as percent increase above basal levels. Chemicals. ATP, dibutyryl CAMP, IBMX, SPN, and isoproterenol were obtained from Sigma Chemical (St.

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Louis, MO); synthetic VIP, CCK, and somatostatin from Bachem (Torrance, CA); soybean trypsin inhibitor and collagenase type II from Worthington Biochemical (Freehold, NJ); acetic anhydride and HEPES from Mallinckrodt (Paris, KY); Eagle’s essential amino acid mixture from Whitaker Bioproducts (Walkersville, MD); and pansorbin (human immunoglobulin G to Staphylococcus aureus) from Calbiochem Boehring Diagnos&cs (La Jolla, CA). ‘“‘I-CAMP was a gift from Dr. Robert W. Downs, Medical College of Virginia (Richmond, VA). Data analysis. Contractile and relaxant responses and changes in CAMP were evaluated for statistical difference using Student’s t test for paired data. In thi s study, n refers to the n.umber of experiments ; cells for each- experiment were derived from separate animals. RESULTS

Inhibition of VIP- and isoproterenol-induced relaxation by somatostatin. VIP (10B6 M) caused relaxation, that is, decreased CCK-induced maximal contraction by 74 t 4% (P C 0.001, n = 6) in circular gastric muscle cells and by 63 t 4% (P < 0.001, n = 6) in circular intestinal muscle cells of the guinea pig. Somatostatin added at the same time as VIP inhibited the relaxant response to VIP in a concentration-dependent manner; the concentration of somatostatin causing 50% inhibition of relaxation (i.e., ICsO) was 30 nM (Fig. 1). The highest concentration of somatostatin used in this study (1 PM) inhibited the relaxant response to VIP by 83 t 7% (P < 0.001, n = 6) in gastric muscle cells but had no effect on the relaxant response in intestinal muscle cells (Fig. 2). Somatostatin had no effect on CCK-induced contraction in either gastric or intestinal muscle cells. Control cell length in different experiments ranged from 102 t 4 to 112 t 4 pm. CCK-8 (1 nM) decreased cell length in different experiments by 30-33%. Similar results were obtained in circular muscle cells isolated from human stomach and intestine (Fig. 3). VIP caused a relaxation of 74 t 2% (P < 0.001, n = 4) in gastric muscle cells and 73 t 5% (P < 0.001, n = 4) in intestinal muscle cells. Somatostatin inhibited the relaxant response to VIP by 86 t 2% (P < 0.001, n = 4) in

SOMATOSTATIN

( M)

FIG. 1. Effect of somatostatin on relaxation induced by vasoactive intestinal peptide (VIP, 1 PM) in muscle cells isolated from circular muscle layer of guinea pig stomach. Relaxant response to VIP in absence of somatostatin was denoted as 100. Data are means k SE of 3 experiments.

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SOMATOSTATIN 100

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a x 4

T

z Y z 0 Ia x 4

6o

40-

W

a

209

O-

VIP

* A VIP + ss

g

VIP + ss

FIG. 2. Effect of somatostatin (SS, 1 PM) on relaxation induced by VIP (1 PM) in muscle cells isolated from circular muscle layers of stomach and intestine of guinea pig. In this and subsequent figures, relaxation is expressed as percent inhibition of maximal contraction. Data are means t SE of 6 experiments. * Significant inhibition. GASTRIC

100 -

n 8 u 5 i= a x

T

GO409

20-

VIP

VIP

VIP

s’s

s’s + PTX

IS0

FIG. 4. Inhibition by somatostatin (1 PM) of relaxation induced by VIP (1 PM) and isoproterenol (ISO, 100 ,uM) in muscle cells isolated from guinea pig stomach and reversal of inhibition in cells preincubated for 1 h with pertussis toxin (PTX, 200 rig/ml). Data are means k SE of 6 experiments with VIP and 4 experiments with isoproterenol. * Significant inhibition.

1. Lack of effect of somatostatin on relaxation induced by various concentrations of sodium nitroprusside, ATP, and dibutyryl CAMP

TABLE

T

Effect

6040-

4 k

CIRCULAR INTESTINE

I

80-

-

O-

VIP

G47

CELLS

T

L

F

MUSCLE

100 -

u

E

GASTRIC

80

80-

#

ON

CIRCUIAR INTESTINE

-

cc

RECEPTORS

L 20

*

-

OVIP

VIP + ss

VIP + ss

3. Effect of somatostatin (1 PM) on relaxation induced by VIP in muscle cells isolated from circular muscle layers of human stomach and intestine. Data are means k SE of 4 experiments. * Significant inhibition. FIG.

(1 PM)

gastric muscle cells but had no effect on the relaxant response in intestinal muscle cells. The mode of action of somatostatin was examined further in gastric muscle cells of the guinea pig using pertussis toxin. Preincubation of the cells for 1 h with pertussis toxin (ZOO rig/ml) abolished the inhibitory effect of somatostatin (74 t 4% relaxation with VIP vs. 82 + 6% with VIP plus somatostatin) (Fig. 4). By itself, iertussis toxin had no effect on control length of cells, CCK-induced contraction, or VIP-induced relaxation. A similar pattern of inhibition of relaxation by somatostatin and reversal of inhibition by pertussis toxin was observed when relaxation was induced by the padrenergic agonist isoproterenol (100 PM). Isoproterenol caused a relaxation of 79 t 9% (P < 0.01, n = 4) that was inhibited 85 -I- 5% (P < 0.001, n = 4) by somatostatin (Fig. 3). Preincubation of the cells for 1 h with pertussis toxin abolished the inhibitory effect of somatostatin (79 + 9% relaxation with isoproterenol vs. 88 t 9% with Goproterenol plus somatostatin) (Fig. 4). As shown in Table 1, relaxation induced by 1 and 10

Sodium nitroprusside 1 nM 10 nM ATP 0.1 mM 1.0 mM Dibutyryl CAMP 0.01 mM 0.1 mM 1.0 mM Values are means 0dibutyryladenosine

of Relaxant,

5%

Alone

+Somatostatin (1 PM)

53*4 78t8

64_t2 78k4

651k7 78*9

65k6 83kll

41tlO 67&l 90&9

41k-11 74t2 100t14

k SE of 3-4 experiments. Dibutyryl 3’ ,Y -cyclic monophosphate.

CAMP,

w,a’-

nM SNP, by 0.1 and 1 mM ATP, and by 0.01, 0.1, and 1 mM dibutyryl CAMP was not affected by somatostatin (1 PM). Mechanism of action of somatostatin. Relaxation induced by both VIP and isoproterenol was accompanied by a significant increase in CAMP within 90 s, at the time of optimal relaxation. As previously shown (1), VIP (1 PM) increased basal CAMP production (4.6 t 0.4 pmol/106 cells) by 176 t 36% (P < 0.01, n = 5) in the presence of a low concentration of IBMX (1 PM). Isoproterenol (100 PM) increased basal CAMP production by 196 t 9% (P < 0.001, n = 5). Somatostatin inhibited the CAMP response to VIP by 81 t 25% (P < 0.02, n = 5) and to isoproterenol by 68 t 12% (P < 0.01, n = 5) but had no effect on basal CAMP levels (Fig. 5). Preincubation of the cells for 1 h with pertussis toxin (200 ng/ ml) abolished the inhibitory effect of somatostatin (Fig. 5). By itself, pertussis toxin had no effect on basal or agonist-induced production of CAMP. DISCUSSION

This study confirms that VIP causes relaxation of muscle cells isolated from human and guinea pig stomach

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G48

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200

RECEPTORS

-

-L -

150-

ai 5: 3 y s 5 -

loo

*

60’

O-

i

VIP

VIP

VIP

s’s

s’s + PTX

IS0

IS0 ii3

IS0 z!s P:x

w:. 5. Inhibition by somatostatin (1 pM) of CAMP production sitmulated by VIP (1 PM) and isoproterenol (100 PM) in muscle cells isolated from guinea pig stomach and reversal of inhibition in cells preincubated for 1 h with pertussis toxin (200 rig/ml). Results expressed as percent increase in CAMP production above basal level. Data are means t SE of 5 experiments. * Significant inhibition.

and intestine and demonstrates that, in both species, somatostatin inhibits relaxation in muscle cells of the stomach but not of the intestine. The potency of somatostatin was evident in a low-threshold concentration of -10 pM and an I& of 30 nM. The inhibition of relaxation by somatostatin was abolished by preincubation of gastric muscle muscle cells with pertussis toxin. A similar pattern of relaxation, inhibition of relaxation by somatostatin and reversal of inhibition by pertussis toxin, was also observed with the P-adrenergic agonist isoproterenol. The changes in relaxation were accompanied by parallel changes in the intracellular levels of CAMP. Both VIP and isoproterenol caused a prompt increase in CAMP production that coincided with the time of optimal relaxation, consistent with the notion that relaxation induced by these agents is mediated by an increase in intracellular CAMP (1, 9, 28). Somatostatin inhibited the increase in CAMP production, and the inhibition was reversed by preincubation of the cells with pertussis toxin. Inhibition of CAMP production by somatostatin and its reversal by pertussis toxin is consistent with the mode of action of somatostatin in a variety of cell types (11, 15, 18, 19, 21) and implies that in gastric muscle cells, somatostatin interacts with receptors coupled to adenylate cyclase by an inhibitory GTP-binding protein Gi. The fact that somatostatin had no effect on relaxation induced by dibutyryl CAMP, a permeant derivative of CAMP, confirms that somatostatin acts to inhibit the generation rather than the intracellular action of CAMP. In other cell types, somatostatin exerts its inhibitory effect via other mechanisms that are not dependent on changes in CAMP and that may or may not be sensitive to pertussis toxin (11, 12, 19, 21). Relaxation induced by nitroprusside that acts by increasing the levels of cGMP (22), or by ATP that acts by opening membrane K+ channels (27), was not affected by somatostatin. Preliminary studies suggest that relaxation induced by ATP is not accompanied by an increase

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in CAMP and is inhibited by the K+ channel blocker apamin (Grider, McHenry, and Makhlouf, unpublished studies). The fact that the inhibitory effect of somatostatin in gastric muscle cells was confined to agents, such as VIP and isoproterenol, that induce relaxation by increasing the levels of CAMP provides further evidence for a singular mechanism of action in these cells mediated by inhibition of adenylate cyclase activity. It is not clear why the inhibitory effect of somatostatin could not be detected in intestinal muscle cells. It is possible that somatostatin receptors are not present on intestinal muscle cells or that somatostatin is more rapidly degraded by these cells (14). An inhibitory signal transduction pathway involving Gi, however, is present in intestinal muscle cells as shown by recent studies on inhibitory adenosine A, receptors (13). The physiological relevance of a direct action of somatostatin on muscle cells should be viewed in light of the topography of somatostatin-containing neurons. In both stomach and intestine, somatostatin neurons project their fibers mainly to other neurons within the myenteric plexus; only a few fibers, particularly in the stomach, project into the adjacent muscle layers (3, 10, 25). Projections within the plexus constitute a pathway whereby somatostatin could regulate smooth muscle function by modulating the release of contractile and relaxant neurotransmitters. Projections into the muscle layer could also modulate neurotransmitter release via axo-axonal interactions and would allow somatostatin to exert a direct action on smooth muscle cells. Finally, somatostatin might reach muscle cells via the circulation on release from endocrine cells. However, it is not known whether at the concentrations that prevail in the circulation somatostatin has any effect on smooth muscle cells or neurons. This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-28300. Address for reprint requests: G. M. Makhlouf, Box 711, MCV Station, Medical College of Virginia, Richmond, VA 23298-0711. Received

16 October

1990; accepted

in final

form

14 February

1991.

REFERENCES 1. BITAR, K. N., AND G. M. MAKHLOIJF. Relaxation of isolated gastric smooth muscle cells by vasoactive intestinal peptide. Science Wash. IX’ 216: 531-533, 1982. 2. BITAR, K. N., ANI) G. M. MAKHLOUK Receptors on smooth muscle cells: characterization by contraction and specific antagonists. Am. J. Physiol. 242 (&strointest. Liuer Physiol. 5): G400-G407, 1982. 3. COSTA, M., J. B. FIJRNESS, I. tJ. L. SMITH, B. DAVIES, AND ,J. OLIVER. An immunohistochemical study of the projections of somatostatin-containing neuron in the guinea-pig intestine. Neuroscience 5: 841-852, 1980. 4. FURNESS, J. B., AND M. COSTA. Actions of somatostatin on excitatory and inhibitory nerves in the intestine. Eur. J. Pharmacol. 56: 69-74, 1979. 5. GRIDER, J. R., A. ARIMURA, AND G. M. MAKHLOUF. Role of somatostatin neurons in intestinal peristalsis: facilitatory interneurons in descending pathways. Am. J. Physiol. 253 (Gastrointest. Liver Physiol. 16): G434-G438, 1987. 6. GRIDER, J. R., AND G. M. MAKHLOUF. Contraction mediated by Ca”’ release in circular and Ca”’ influx in longitudinal intestinal muscle cells. J. Pharmacol. Exp. Ther. 244: 432-437, 1987. 7. GUILLEMIN, R. Somatostatin inhibits the release of acetylcholine induced electrically in the myenteric plexus. J. Physiol. Lord. 303: 315323, 1980.

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8. HARPER, J. F., AND G. BROOKER. Femtomole sensitive radioimmunoassay for cyclic AMP and cyclic GMP after 2’0 acetylation by acetic anhydride in aqueous solution. J. Cyclic Nucleotide Res. 1: 207-218, 1975. 9. HONEYMAN, T., P. MERRIAM, AND F. S. FAY. The effects of isoproterenol on adenosine cyclic 3’,5’-monophosphate and contractility in isolated smooth muscle cells. Mol. Pharmacol. 14: 8698, 1978. 10. KEAST, J. R., J. B. FURNESS, AND M. COSTA. Somatostatin in human enteric nerves. Cell Tissue Res. 237: 299-308, 1984. 11. KOCH, B. D., L. J. DORFLINGER, AND A. SCHONBRUNN. Pertussis toxin blocks both cyclic AMP-mediated and cyclic AMP-independent actions of somatostatin. J. Biol. Chem. 260: 13138-13145,1985. 12. LEWIS, D. L., F. F. WEIGHT, AND A. LUINI. A gaunine nucleotidebinding protein mediates the inhibition of voltage-dependent calcium current by somatostatin in a pituitary cell line. Proc. Natl. Acad. Sci. USA 83: 9035-9039, 1986. 13. MCHENRY, L., J. R. GRIDER, AND G. M. MAKHLOUF. Coexistence of adenosine stimulatory (AZ) and inhibitory (A,) receptors on intestinal muscle cells (Abstract). Castroenterology 95: A334, 1989. 14. MENOZZI, D., AND N. W. BUNNETT. Degradation of enkephalins by isolated gastric muscle cells (Abstract). Gastroenterology 98: A510, 1990. 15. MICHIO, V. I. Islet-activating protein, pertussis toxin: a probe for functions of the inhibitory g-uanine nucleotide regulatory component of adenylate cyclase. Trends Pharmacol. Sci. 5: 227-229, 1984. 16. PARK, J., T. CHIBA, K. YOKOTANI, J. DELVALLE, AND T. YAMADA. Somatostatin receptors on canine fundic D-cells: evidence for autocrine regulation of gastric somatostatin. Am. J. Physiol. 257 (Gastrointest. Liver Physiol. 20): G235-G241, 1989. 17. SAFFOURI, S., G. WEIR, K. BITAR, AND G. MAKHLOUF. Stimulation of gastrin secretion from the perfused rat stomach by somatostatin antiserum. Life Sci. 25: 1749-1754, 1979. 18. SAKAMOTO, C., T. MATOZAKI, M. NAGAO, AND S. BABA. Coupling of guanine nucleotide inhibitory protein to somatostatin receptors on pancreatic acinar membranes. Am. J. Physiol. 253 (Gastrointest. Liver Ph.ysiol. 16): G308-G314, 1987. 19. SCHONBRUNN, A. Somatostatin action in pituitary cells involves

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two independent transduction mechanisms. Metabolism 39, Suppl. 2: 96400, 1990. SCHUBERT, M. L., N. F. EDWARDS, A. ARIMURA, AND G. M. MAKHLOUF. Paracrine regulation of gastric acid secretion by fundic somatostatin. Am. J. Physiol. 252 (Gastrointest. Liver Physiol. 15): G485-G490,1987. SCHUBERT, M. L., J. HIGHTOWER, AND G. M. MAKHLOUF. Linkage between somatostatin and acid secretion: evidence from the use of pertussis toxin. Am. J. Physiol. 256 (Gastrointest. Liver Phrsiol. 19): G418-G422, 1989. SCHULTZ, K.-D., K. SCHULTZ, AND G. SCHUI,TZ. Sodium nitroprusside and other smooth muscle relaxants increase cyclic GMP levels in rat ductus deferens. Nature Lond. 265: 750-751, 1977. TAKEDA, T., K. TANIYAMA, S. BABA, AND C. TANAKA. Put,ative mechanisms involved in excitatory and inhibitory effects of somatostatin on intestinal motility. Am. J. Physiol. 257 (Gastrointest. Liver Physiol. 20): G532G538, 1989. TEITELBAUM, D. H., T. M. O’DORISIO, W. E. PERKINS, AND T. S. GAGINELLA. Somatostatin modulation of peptide-induced acetylcholine release in guinea pig ileum. Am. J. Physiol. 246 (Gastrointest. Liver Physiol. 9): G509-G514, 1984. WATTCHOW, D. A., J. B. FURNESS, AND M. COSTA. Distribution and coexistence of peptides in nerve fibers of the external muscle of the human gastrointestinal tract. Gastroenterology 95: 32-41, 1988. WILEY, J., AND C. OWYANG. Somatostatin inhibits CAMP-mediated cholinergic transmission in the myenteric plexus. Am. J. Phcysiol. 253 (Gastrointest. Liver Physiol. 16): G607-G612, 1987. YAMANAKA, K., K. FURUKAWA, AND K. KITAMURA. The different mechanisms of action of nicorandil and adenosine triphosphate on potassium channels of circular smooth muscle of the guinea pig small intestine. Naunyn-Schmiedebergs Arch. Pharmakol. 331: 96103,1985. ZHANG, L., R. T. JENSEN, AND P. N. MATON. Characterization of &adrenoceptors on smooth muscle cells from guinea pig stomach. Am. J. Physiol. 259 (Gastrointest. Liver Physiol. 22): G436-G442, 1990.

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Inhibition of muscle cell relaxation by somatostatin: tissue-specific, cAMP-dependent, pertussis toxin-sensitive.

The direct action of somatostatin on smooth muscle was examined in muscle cells isolated from the stomach and intestine of human and guinea pig. Somat...
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