Somatostatin inhibits pancreatic exocrine secretion via a neural mechanism

Use and perceived effectiveness of non-analgesic medical therapies for chronic pancreatitis in the United States

BACKGROUND

Effectiveness of medical therapies in chronic pancreatitis has been described in small studies of selected patients.

AIM

To describe frequency and perceived effectiveness of non-analgesic medical therapies in chronic pancreatitis patients evaluated at US referral centres.

METHODS

Using data on 516 chronic pancreatitis patients enrolled prospectively in the NAPS2 Study, we evaluated how often medical therapies [pancreatic enzyme replacement therapy (PERT), vitamins/antioxidants (AO), octreotide, coeliac plexus block (CPB)] were utilized and considered useful by physicians.

RESULTS

Oral PERT was commonly used (70%), more frequently in the presence of exocrine insufficiency (EI) (88% vs. 61%, P < 0.001) and pain (74% vs. 59%, P < 0.002). On multivariable analyses, predictors of PERT usage were EI (OR 5.14, 95% CI 2.87-9.18), constant (OR 3.42, 95% CI 1.93-6.04) or intermittent pain (OR 1.98, 95% CI 1.14-3.45). Efficacy of PERT was predicted only by EI (OR 2.16, 95% CI 1.36-3.42). AO were tried less often (14%) and were more effective in idiopathic and obstructive vs. alcoholic chronic pancreatitis (25% vs. 4%, P = 0.03). Other therapies were infrequently used (CPB - 5%, octreotide - 7%) with efficacy generally <50%.

CONCLUSIONS

Pancreatic enzyme replacement therapy is commonly utilized, but is considered useful in only subsets of chronic pancreatitis patients. Other medical therapies are used infrequently and have limited efficacy.

The islet-acinar axis of the pancreas: more than just insulin

Although the role of the islets in the regulation of acinar cell function seemed a mystery to investigators who observed their dispersion among pancreatic acini, over time an appreciation for this intricate and unique structural arrangement has developed. The last three decades have witnessed a steadily growing understanding of the interrelationship of the endocrine and the exocrine pancreas. The islet innervation and vascular anatomy have been more fully characterized and provide an appropriate background for our current understanding. The interrelationship between the endocrine and exocrine pancreas is mediated by islet-derived hormones such as insulin and somatostatin, other humoral factors including pancreastatin and ghrelin, and also neurotransmitters (nitric oxide, peptide YY, substance P, and galanin) released by the nerves innervating the pancreas. Although considerable progress has been achieved, further work is required to fully delineate the complex interplay of the numerous mechanisms involved. This review aims to provide a comprehensive update of the current literature available, bringing together data gleaned from studies addressing the actions of individual hormones, humoral factors, and neurotransmitters on the regulation of amylase secretion from the acinar cell. This comprehensive view of the islet-acinar axis of the pancreas while acknowledging the dominant role played by insulin and somatostatin on exocrine secretion sheds light on the influence of the various neuropeptides on amylase secretion.

Microinjection of exogenous somatostatin in the dorsal vagal complex inhibits pancreatic secretion via somatostatin receptor-2 in rats

Previous studies have suggested that somatostatin inhibits pancreatic secretion at a central vagal site, and the dorsal vagal complex (DVC) is involved in central feedback inhibition of the exocrine pancreas. The aim of this study was to investigate the effect of exogenous somatostatin in the DVC on pancreatic secretion and the somatostatin receptor subtype(s) responsible for the effect. The effects of somatostatin microinjected into the DVC on pancreatic secretion stimulated by cholecystokinin octapeptide (CCK-8) or 2-deoxy-d-glucose (2-DG) were examined in anesthetized rats. To investigate the somatostatin inhibitory action site, a somatostatin receptor antagonist [SRA; cyclo(7-aminoheptanoyl-Phe-d-Trp-Lys-Thr)] was microinjected into the DVC before intravenous infusion of somatostatin and CCK-8/2-DG. The effects of injection of a somatostatin receptor-2 agonist (seglitide) and combined injection of somatostatin and a somatostatin receptor-2 antagonist (CYN 154806) in the DVC on the pancreatic secretion were also investigated. Somatostatin injected into the DVC significantly inhibited pancreatic secretion evoked by CCK-8 or 2-DG in a dose-dependent manner. SRA injected into the DVC completely reversed the inhibitory effect of intravenous administration of somatostatin. Seglitide injected into the DVC also inhibited CCK-8/2-DG-induced pancreatic protein secretion. However, combined injection of somatostatin and CYN 154806 did not affect the CCK-8/2-DG-induced pancreatic secretion. Somatostatin in the DVC inhibits pancreatic secretion via somatostatin receptor-2, and the DVC is the action site of somatostatin for its inhibitory effect.

Autonomic pathways regulating pancreatic exocrine secretion

The parasympathetic (PNS) and sympathetic (SNS) and nervous systems densely innervate the exocrine pancreas. Efferent PNS pathways, consisting of central dorsal motor nucleus of the vagus (DMV) and peripheral pancreatic neurons, stimulate exocrine secretion. The DMV integrates cortical (olfactory, gustatory) and gastric, and intestinal vagal afferent input to determine central PNS outflow during cephalic, gastric and intestinal phases of exocrine secretion. Pancreatic neurons integrate DMV input with peripheral enteric, sympathetic, and, possibly, afferent axon reflexes to determine final PNS input to all exocrine effectors. Gut and islet hormones appear to modulate both central and peripheral PNS pathways. Preganglionic sympathetic neurons in the intermediolateral (IML) column of the spinal cord receive inputs from brain centers, some shared with the PNS, and innervate postganglionic neurons, mainly in prevertebral ganglia. Sympathetic innervation of the exocrine pancreas is primarily indirect, and inhibits secretion by decreasing blood flow and inhibiting transmission in pancreatic ganglia. Interactions between SNS and PNS pathways appear to occur in brain, spinal cord, pancreatic and prevertebral ganglia, and at neuroeffector synapses. Thus, the PNS and SNS pathways regulating the exocrine pancreas are directly or indirectly antagonistic at multiple sites: the state of exocrine secretion reflects the balance of these influences. Despite over a century of study, much remains to be understood about the connections of specific neurons forming pancreatic pathways, their processes of neurotransmission, and how disruption of these pathways contributes to pancreatic disease.

Endogenous somatostatin inhibits interaction of insulin and cholecystokinin on exocrine secretion of isolated, perfused rat pancreas

INTRODUCTION

Although somatostatin inhibits pancreatic exocrine secretion, the inhibitory mechanism of endogenous somatostatin is not clearly understood.

AIM

To investigate the effect of endogenous somatostatin on the interaction between endogenous insulin and exogenous cholecystokinin (CCK) in exocrine secretion of the totally isolated, perfused rat pancreas.

METHODOLOGY

Endogenous releases of somatostatin and insulin were induced by 18 mM glucose. Streptozotocin (75 mg/kg) or cysteamine (300 mg/kg) was injected into rats 24 hours before the experiment to deplete insulin or somatostatin in the pancreas.

RESULTS

Glucose (18 mM) enhanced CCK (10 pM)-stimulated secretions of fluid and amylase in the normal pancreas, which was further elevated by a somatostatin antagonist. Exogenous insulin (100 nM) also enhanced CCK-stimulated secretions in the streptozotocin-treated pancreas, which was also markedly increased by the somatostatin antagonist. The glucose (18 mM)-enhanced CCK-stimulated secretions were much higher in the cysteamine-treated pancreas than in the normal pancreas, which was dose-dependently reduced by exogenous somatostatin (30, 100 pM). However, endogenous or exogenous somatostatin did not modify the pancreatic responses to CCK alone.

CONCLUSION

Endogenous somatostatin inhibits the interaction of endogenous insulin and CCK on pancreatic exocrine secretion in the rat rather than reducing the action of CCK alone or endogenous release of insulin.

Substance P inhibits pancreatic exocrine secretion via a neural mechanism

We investigated the effects of the sensory neuropeptide substance P (SP) on amylase and fluid secretion in the isolated vascularly perfused rat pancreas. SP inhibited CCK-induced amylase release and secretin-induced juice flow via the pancreatic duct in a dose-related fashion. Threshold inhibition occurred following addition of 10(-10) M SP to the perfusate, and maximal inhibition was seen with 10(-8) M SP. The effects of SP were partially blocked by both the neurokinin-1 (NK1) and neurokinin-2 (NK2) receptor antagonists. Atropine and TTX blocked SP-induced effects on both amylase secretion (26 and 63% blockade, respectively) and pancreatic juice flow (21 and 79% blockade, respectively). Excitation of pancreatic sensory nerves using capsaicin (in the absence of SP) inhibited both amylase and pancreatic juice flow via activation of the NK1 receptor. We conclude that SP inhibits exocrine secretion via an indirect neural mechanism.

Inhibition by somatostatin of amylase secretion induced by calcium and cyclic AMP in rat pancreatic acini

It has recently been shown that somatostatin inhibits amylase secretion from isolated pancreatic acini by reducing cyclic AMP (cAMP) production [Matsushita, Okabayashi, Hasegawa, Koide, Kido, Okutani, Sugimoto and Kasuga (1993) Gastroenterology 104, 1146-1152]. To date, however, little is known as to the other mechanism(s) by which somatostatin inhibits amylase secretion in exocrine pancreas. To investigate the action of somatostatin independent of cAMP generation, we examined the effect of somatostatin in isolated rat pancreatic acini stimulated by 1 microM calcium ionophore A23187 and 1 mM 8-bromo-cyclic AMP (8Br-cAMP). Somatostatin inhibited amylase secretion evoked by a combination of A23187 and 8Br-cAMP in a dose-dependent manner. The maximum inhibition was obtained by 10(-7) M somatostatin, and at this concentration somatostatin inhibited the effect of A23187 and 8Br-cAMP by approximately 30%. In electrically permeabilized acini, an elevation of free calcium concentration resulted in an increase in amylase secretion and cAMP enhanced the secretion evoked by calcium. cAMP shifted the dose-response curve for calcium-induced secretion leftwards and elevated the peak value of secretion. Somatostatin inhibited the effect of cAMP on calcium-induced amylase secretion by shifting the dose-response curve to the right. To determine the involvement of a G-protein(s), we examined the effect of somatostatin in acini pretreated with pertussis toxin. Pretreatment of acini with pertussis toxin completely blocked somatostatin-inhibition of amylase-secretion evoked by A23187 and 8Br-cAMP. These results indicate that somatostatin decreases amylase secretion induced by cAMP and calcium by reducing the calcium sensitivity of exocytosis. A pertussis toxin-sensitive G-protein is also involved in this step.

Somatostatin inhibits secretin-induced canine pancreatic response via a cholinergic mechanism

BACKGROUND

Somatostatin inhibits pancreatic exocrine secretion in intact animals but not in vitro, suggesting an indirect effect. The present study examined the influence of extrapancreatic nerves and intrapancreatic cholinergic activity on somatostatin-induced inhibition of pancreatic exocrine secretion in conscious dogs.

METHODS

Seven dogs underwent extrapancreatic denervation and creation of pancreatic fistulae, while a second group of 6 dogs had pancreatic fistulae created without denervation. The pancreatic responses to graded doses of secretin (16-500 ng.kg-1.h-1), both alone and during background infusions of somatostatin-14 (400 and 800 pmol/L.kg-1.hr-1), were determined in all dogs. The secretin dose response was then repeated with a continuous infusion of bethanechol (90 micrograms.kg-1.h-1) both with and without somatostatin-14 (800 pmol/L.kg-1.h-1).

RESULTS

Secretin-induced bicarbonate and protein outputs were significantly inhibited by somatostatin-14 in both the innervated and denervated animals. The inhibitory effects of somatostatin-14 were partially reversed by bethanechol in the innervated animals and completely reversed in the denervated animals. Bethanechol alone potentiated secretin-induced bicarbonate output from both the innervated and denervated pancreas.

CONCLUSIONS

The data suggest that extrapancreatic nerves do not mediate the inhibitory effects of somatostatin-14. Rather, somatostatin-14 appears to inhibit secretin-induced pancreatic response by an intrapancreatic cholinergic mechanism.

Effect of cysteamine on insulin release and exocrine pancreatic secretion in vitro

Cysteamine is known to deplete somatostatin from pancreatic D cells. In the isolated perfused rat pancreas we investigated its effects on somatostatin and insulin release as well as exocrine pancreatic secretion in the presence of 16.7 mM glucose and 180 pM CCK-8. At a concentration of 0.1 mM, cysteamine had no significant effect on pancreatic endocrine and exocrine functions. At 10 mM, however, cysteamine released somatostatin (380 +/- 70 vs 100 +/- 20 fmol/20 min), inhibited insulin output (890 +/- 120 vs 13210 +/- 3260 mu units/20 min) and reduced exocrine pancreatic secretion (volume: 12 +/- 2 vs 20 +/- 2 microliters/20 min; lipase: 31 +/- 3 vs 60 +/- 7 units/20 min). We conclude that the complex changes induced by cysteamine are consistent with a physiological role of endogenous somatostatin in the regulation of insulin release. The reduction of exocrine pancreatic secretion, however, was at least in part, if not completely, mediated via the insuloacinar axis rather than a direct effect of cysteamine-released somatostatin on pancreatic acinar cells.

Pharmacodynamic effects of Sandostatin in the gastrointestinal tract

Somatostatin (SST) is widely distributed throughout the human gastrointestinal system. There, it is found in neurons and fibers of both the submucosal and myenteric plexus and the pancreas, and also in the D cells of the stomach, gut, and pancreatic islets. Whereas in the intestinal nervous system, duodenum, and pancreas, somatostatin-14 (SST-14) appears to be the predominant molecular form, the endocrine-type D cells of the intestine primarily contain somatostatin-28 (SST-28). SST peptides may act very differently at different sites, as hormones, paracrine substances, or neurotransmitters. Because of this complexity of action, very little is known about the physiological effects of SST in the gastrointestinal tract. In contrast, the pharmacological actions of natural synthetic SST have been thoroughly studied and have given rise to many therapeutic applications. Octreotide, an analogue with a longer half-life and higher potency, has greatly facilitated the clinical application of SST. This review deals with the pharmacological effects of octreotide on different gastrointestinal functions. The SST analogue exerts a long-lasting inhibitory action on gastric acid, pancreatic enzyme, bicarbonate secretion, and on bile flow. It also inhibits stimulated intestinal secretion, ie, the release of neuropeptides from the gut and pancreas. It can also prolong orocecum transit time and prevent gallbladder contraction. It inhibits absorption of nutrients and exerts inhibitory effects on splanchnic hemodynamics. It is because of these actions that SST has attracted so much attention in the treatment of different gastrointestinal disorders.

Selective release of somatostatin by calcitonin gene-related peptide and influence on pancreatic secretion

Calcitonin gene-related peptide is a potent inhibitor of stimulated pancreatic exocrine secretion in vivo. The mechanism of this inhibitory action was studied in dogs and rats. The questions examined were: (1) is the inhibitory action of CGRP on pancreatic secretion mediated by somatostatin? (2) is the inhibition direct, via action on acinar cells, or indirect? and (3) is a neuronal mechanism involved, and, if so, by what pathway? In dogs with chronic pancreatic fistulae, CGRP caused significant inhibition of the outputs of pancreatic protein (63-68%) and of pancreatic bicarbonate (74-89%) and a simultaneous dose-related rise (40-102 fmol/ml) in plasma somatostatin-like immunoreactivity. A similar degree of inhibition was found when exogenous somatostatin was infused to achieve similar levels of plasma somatostatin-like immunoreactivity. More direct evidence of somatostatin mediation of CGRP action was sought in conscious rats with pancreatic fistulae using a potent and specific monoclonal antibody to somatostatin. The latter studies suggest that CGRP has both a somatostatin-dependent and a somatostatin-independent mechanism of action. In isolated rat acini, CGRP did not inhibit CCK-stimulated amylase release, suggesting that its in vivo action is indirect. In the isolated vascularly perfused rat pancreas, CGRP (10(-10)-10(-7) M) inhibited in a dose-dependent manner volume and protein output stimulated by a mixture of CCK-8 and secretin. The inhibitory action of CGRP was blocked by tetrodotoxin (10(-7) M) and by atropine (10(-7) M), but not by hexamethonium (10(-7) M). We conclude that CGRP action: (1) is partly explained by release of somatostatin; (2) is indirect; (3) is neurally mediated; and (4) involves cholinergic muscarinic neurons within the pancreas.

Galanin inhibits rat pancreatic amylase release via cholinergic suppression

The effects of galanin on pancreatic exocrine function were examined using rat pancreatic tissues. In anesthetized rats, galanin (40 micrograms/kg/h) decreased amylase secretion stimulated by 2-deoxy glucose (5.8 +/- 0.1 vs. 3.1 +/- 0.1 times basal) and cholecystokinin octapeptide (21.5 +/- 0.6 vs. 16.8 +/- 0.5), while not inhibiting bethanechol-stimulated secretion. In dispersed acini, there was no effect of galanin alone (10(-8) to 10(-13) M) on amylase release, nor did galanin (10(-6) or 10(-8) M) coincubation affect amylase release stimulated by bethanechol (10(-3) to 10(-7) M) or CCK-8 (10(-8) to 10(-13) M). Using pancreatic lobules, coincubation with galanin (10(-6) M) suppressed 75 mM KCl-stimulated amylase secretion and ACh release (10.1 +/- 0.6% vs. 7.3 +/- 0.4%). Veratridine-stimulated (10(-4) M) amylase secretion and ACh release (12.4 +/- 1.7% vs. 8.5 +/- 0.7%) were similarly diminished.