Cholinergic secretory and inhibitory mechanisms in canine pancreatic secretion

Dose-response relationships for pancreatic stimulants and the interactions by atropine were studied in conscious gastric and duodenal fistula dogs. Secretin, caerulein, and bethanechol, the two latter against background secretin, induced similar maximal secretions of water and bicarbonate, and maximal protein outputs with the two latter were not different. Actions of atropine differed according to type and dose of stimulant, dose of atropine, and secretory variable studied. In the dose interval of 10-40 microg x kg(-1) x h(-1), atropine suppressed the secretin-stimulated water and bicarbonate, but these or lower doses enhanced the response to submaximal caerulein. The secretion after 200 microg x kg(-1) x h(-1) or less bethanechol was unaffected by atropine, but the suppressed response to higher bethanechol doses was reversed and enhanced. These findings are compatible with the presence of one ductal secretory process sensitive to cholinergic influence in three ways: one secretory, partly atropine-sensitive, required for submaximal secretin's optimal action; one secretory, stimulated by bethanechol, and atropine-resistant; and one inhibitory, atropine-sensitive, triggered by caerulein (and high doses of bethanechol). Atropine at low doses inhibited the protein output by bethanechol but enhanced the submaximal caerulein response, which again indicates the presence of an inhibitory atropine-sensitive cholinergic principle. It is proposed that pancreatic polypeptide may be the mediator of this inhibition and that vasoactive intestinal polypeptide could be the mediator of the atropine-resistant cholinergic stimulation of water and bicarbonate secretion.     

Corticotropin-release inhibitory factor Evidence for dual stimulatory and inhibitory hypothalamic regulation over adrenocorticotropin secretion and biosynthesis.

The hypothalamus is currently thought to stimulate the synthesis and secretion of ACTH uniquely by secreting neuropeptides into the hypophysial-portal circulation, of which the most important are CRH and arginine vasopressin. However, analysis of the effects of pituitary isolation on the pituitary-adrenal axis in a variety of species suggests that the hypothalamus exerts both stimulatory and inhibitory regulation over ACTH secretion and POMC biosynthesis, and that the inhibitory control is dominant. Because none of the currently known inhibitory factors in the hypophysial-portal circulation consistently decreases basal ACTH secretion and POMC mRNA levels in normal anterior pituitary cells, it is suggested that this inhibition is mediated by a currently unidentified hypothalamic substance, presumably a neuropeptide, which we have termed corticotropin-release inhibitory factor (CRIF). The possible roles in clinical medicine of agonists and antagonists of this putative CRIF are discussed.     

Effect of atropine on feedback regulation of pancreatic enzyme secretion in rats

The effect of atropine on the feedback regulatory mechanism of pancreatic enzyme secretion exerted by intraluminal trypsin was investigated in conscious rats. Intravenous atropine infusion (50 micrograms/kg/h) suppressed pancreatic enzyme secretion to the same extent in both the presence and the absence of pancreatic juice in the intestine. However, with or without atropine infusion, pancreatic secretory rate was higher throughout diversion of pancreatic juice than during intraduodenal return of the juice. Atropine also inhibited the stimulatory response to intraduodenal trypsin inhibitor and intravenous caerulein. The atropine-induced inhibitory effect was not significantly different between the two experimental conditions. Regardless of atropine administration, both trypsin inhibitor and caerulein caused a significant increase in pancreatic secretion. The results suggest that cholinergic mechanisms have little influence on the feedback regulation. Cholinergic mechanisms may play an important role in maintaining the physiologically basal secretion because the basal secretion is atropine-sensitive.     

Effects of secretin and caerulein on luminal feedback regulation of pancreatic enzyme secretion in rats

The stimulatory pancreatic response to exclusion of pancreatic proteases from the intestine was compared with the response to stepwise increasing doses of secretin and caerulein in conscious rats. Secretin stimulated pancreatic fluid secretion in a dose-related manner with or without intraduodenal return of pancreatic juice, while it could not significantly affect enzyme secretion. The dose response curve for enzyme secretion to caerulein was smooth during return of the juice. However, the already increased enzyme secretion by pancreatic juice diversion was only stimulated with the smallest dose of caerulein. The maximal dose of caerulein for enzyme secretion during return had been supramaximal dose during diversion. Intraduodenal trypsin inhibitor failed to stimulate enzyme secretion during diversion but induced the same stimulatory effect as the submaximal dose of caerulein during return. Different doses of intraduodenal trypsin caused an almost dose-related inhibition. It is concluded that a submaximal level of endogenous CCK might participate in the feedback regulation of pancreatic enzyme secretion in rats.     

Mechanism of galanin's inhibitory action on pancreatic enzyme secretion: modulation of cholinergic transmission--studies in vivo and in vitro.

This study examined the inhibitory mechanism of galanin, a 29 amino acid polypeptide on pancreatic enzyme secretion in anaesthetised rats, isolated pancreatic acini, and lobules. Urethane anaesthetised rats with pancreatic fistulas pretreated with 3-0-methyl-glucopyranose (500 mg/kg/h) were stimulated with an intravenous bolus of 2-deoxyglucose (2-DG) (75 mg/kg). Maximal amylase secretion was mean (SEM) 274 (19)% of basal secretion. Atropine (150 micrograms/kg/h) and galanin (10 nmol/kg/h) almost completely inhibited 2-DG stimulated amylase secretion suggesting an inhibition of cholinergic transmission. To further test this possibility this study investigated the effect of galanin on carbachol and cholecystokinin stimulated amylase release from isolated pancreatic acini. Galanin did not suppress carbachol or cholecystokinin stimulated amylase release, indicating that galanin inhibits exocrine secretion by indirect mechanisms. The cholinergic pathway was assessed by using pancreatic lobules containing intrapancreatic neurons. Veratridine, a sodium channel activator, dose dependently stimulated amylase release. Veratridine (100 microM) stimulated amylase release by 411 (10)% of basal secretion. Atropine (1 microM) or tetrodotoxin (1 microM) almost completely blocked veratridine stimulated amylase release. Galanin (1 microM) significantly inhibited veratridine stimulated amylase release with a maximal inhibition of 50% (p < 0.05). In addition, when lobules were incubated with [3H]-choline, galanin significantly (p < 0.05) inhibited veratridine stimulated release of newly synthesised [3H]-acetylcholine. Thus galanin inhibits pancreatic secretion by inhibiting cholinergic transmission. These studies show that galanin inhibits rat pancreatic enzyme secretion by an indirect mechanism by reducing cholinergic transmission.     

Influence of adrenalectomy on pancreatic enzyme secretion

Both adrenalectomy and chemically induced diabetes mellitus cause a marked decrease of pancreatic amylase synthesis in rats. Diabetes is further associated with alterations of cholecystokinin (CCK)-stimulated enzyme secretion. Using isolated pancreatic acini prepared from adrenalectomized male rats, we investigated the effects of adrenalectomy on pancreatic enzyme secretion. CCK8-stimulated amylase secretion showed a typical biphasic dose-response curve in acini from both adrenalectomized and sham-operated animals with similar basal secretions, similar sensitivities to various CCK8 concentrations, but a statistically significant elevation of maximal amylase secretion after adrenalectomy. Receptor-binding studies with 125I-BH-CCK8 revealed an increase of binding to the high-affinity part of the CCK receptor in adrenalectomized rats. While carbachol-stimulated secretion showed no changes in maximal secretion rates, it did show a decrease in sensitivity in adrenalectomized animals with a statistically significant shift to the right of the dose-response curves. Competitive inhibition curves with 3H-N-methylscopolamine and carbachol as the competitive receptor agonist showed no differences in receptor binding between controls and adrenalectomized rats. We propose that complex alterations in hormone/neurotransmitter-stimulated pancreatic enzyme secretion are found in glucocorticoid depletion, explainable via a postreceptor defect in carbachol-stimulated secretion. The functional and binding data with regard to CCK are more difficult to explain.     

Effect of somatostatin on pancreatic enzyme secretion

The effect of somatostatin (SS) on the pancreatic enzyme secretion was studied in a perfusion system using dispersed pancreatic rat acini in vitro. In addition the effect of SS on pancreatic secretion in vivo was also studied in conscious rats for comparison. In an in vitro study, 6 x 10(-7) M SS-14 caused no significant change in amylase release when added 20 min before stimulation by 10(-5) M carbamylcholine (Cch), 10(-6) M A23187, 5 x 10(-7) M secretin and 2 mM dibutyryl cyclic AMP. The addition of 6 x 10(-7) M SS-28 also caused no significant change in amylase release stimulated by 10(-5) M Cch. High performance liquid chromatographic examination indicated that no degradation of either SS-14 or SS-28 occurred after reaction with dispersed acini. In an in vivo study SS-14 caused marked inhibition of basal pancreatic secretion and stimulated pancreatic secretion by bile-pancreatic juice diversion. These results indicate that SS has no direct inhibitory action on rat pancreatic secretion, and that SS may inhibit the pancreatic secretion by indirect mechanisms.     

Effect of somatostatin on pancreatic enzyme secretion

The effect of somatostatin (SS) on the pancreatic enzyme secretion was studied in a perfusion system using dispersed pancreatic rat acini in vitro. In addition the effect of SS on pancreatic secretion in vivo was also studied in conscious rats for comparison. In an in vitro study, 6×10^-7M SS-14 caused no significant change in amylase release when added 20 min before stimulation by 10^-5M carbamylcholine (Cch), 10^-6M A23187, 5×10^-7M secretin and 2mM dibutyryl cyclic AMP. The addition of 6×10^-7M SS-28 also caused no significant change in amylase release stimulated by 10^-5M Cch. High performance liquid chromatographic examination indicated that no degradation of either SS-14 or SS-28 occurred after reaction with dispersed acini. In an in vivo study SS-14 caused marked inhibition of basal pancreatic secretion and stimulated pancreatic secretion by bile-pancreatic juice diversion. These results indicate that SS has no direct inhibitory action on rat pancreatic secretion, and that SS may inhibit the pancreatic secretion by indirect mechanisms.     

Relative importance of cholecystokinin and cholinergic nerve mechanisms in the regulation of pancreatic secretion in rats

Several substances modulate the function of pancreatic acinar cells in vitro through specific receptors. All of them may also act in vivo, but the relative importance of their effect under physiological conditions is highly variable. The aim of this study was to assess the respective participation of cholinergic nerves and cholecystokinin (CCK) in the interdigestive pancreatic exocrine secretion and in the pancreatic response to intragastric or intraduodenal nutrients. Unanesthetized rats, equipped with a semichronic (5 days) pancreatic fistula, were used. They received a test-meal in the stomach (42 kJ) or the duodenum (21 kJ), and were infused intravenously with an antagonist of muscarinic (atropine, pirenzepin), nicotinic (hexamethonium), or CCK (L364718, lorglumide, Boc-Tyr(SO3)-Nle-Gly-D-Trp-Nle-Asp-NH-CH2-CH2-C6H5) receptors. Basal interdigestive pancreatic exocrine secretion decreased significantly after atropine, pirenzepine or hexamethonium. Protein output decreased more (peak inhibition 60 to 80 percent according to the drugs, p less than 0.01 to p less than 0.001) than volume or bicarbonate output (peak inhibition 25 to 65 percent according to the drugs, p less than 0.05 to p less than 0.01). CCK antagonists did not change the interdigestive pancreatic secretion. The cumulated response to the intraduodenal meal did not change after hexamethonium or pirenzepin, but increased after atropine (by approximately 50 percent, p less than 0.001 for volume and bicarbonate output, and nearly 100 percent, p less than 0.05 for protein output). The CCK antagonists entirely suppressed the protein response to the intraduodenal meal, decreased the volume response by 70 percent (p less than 0.01), and the bicarbonate response by 50 percent (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of inhibitory and stimulatory control of prolactin secretion in the rhesus monkey.

In the unanesthetized, ovariectomized rhesus monkey, gonadotropin secretion is episodic while prolactin (Prl) release is relatively constant. Under pentobarbital anesthesia, however, Prl secretion also becomes pulsatile in synchrony with the discharges of LH. This finding suggests that the anesthesia has unmasked normally inhibited Prl releasing stimuli which are driven by the circhoral 'clock' that also times the discharges of GnRH.     

Negative control of human pancreatic secretion: physiological mechanisms and factors

The negative control of pancreatic exocrine secretion in man occurs during the interdigestive and postprandial periods of the digestive cycle. The physiological mechanisms involved include negative feedback mechanisms, well described and accepted in animals, and controlled by the cholecystokinin- and secretin-releasing factors of pancreatic and duodenal origin, along with the active pancreatic proteases present in the upper gut. The presence of these factors and their efficacy in humans, however, have their supporters and detractors, with a possibility for reconciliation among opponents. Besides these releasing factors, hormones, mostly from the intestine, are also involved in this inhibitory process of pancreatic secretion. Somatostatin, peptide YY, pancreatic polypeptide, glucagon, ghrelin, and leptin were described as potentially involved from studies mostly performed on animals. Finally, bile and bile salts have mixed responses on this inhibition, and their effects seem to be at the intestine level with gastrointestinal hormones involved. Future studies will have to be performed in humans to determine the presence of cholecystokinin- and secretin-releasing factors and their role. Finally, the demonstrated modulatory action of hormones and bile acids in other species needs to be confirmed in humans.     

The stimulatory and inhibitory effects of dopamine on prolactin secretion involve different G-proteins.

The reverse hemolytic plaque assay (RHPA) was used in this study to further characterize the mechanism whereby low concentrations of dopamine (DA) stimulate PRL secretion in vitro. Female Sprague-Dawley rats were used as a source of anterior pituitary cells for the RHPA. Pituitary cells were infused into Cunningham chambers along with a suspension of protein-A-coated ovine red blood cells. Excess cells were rinsed from the chambers leaving a monolayer of cells attached to the glass. The cells were then incubated with solutions containing PRL antiserum (1:40) and various concentrations of DA. After 4 h, a solution containing guinea pig complement (1:60) was infused into the chambers. Thirty minutes later, the cells were fixed and plaques (zones of hemolysis) surrounding PRL-producing cells (lactotrophs) were measured and used as an index of the amount of PRL secreted. Control cells that received no DA had a mean plaque area of 8,000 microns 2 and two distinct subpopulations of plaque sizes. This biphasic population of cells consisted of a small and a large plaque producing population. The mean plaque area surrounding lactotrophs was significantly (P less than 0.05) decreased if 1 microM or 10 microM DA was present (4,500 microns 2 and 3,500 microns 2, respectively). These cells which received inhibitory concentrations of DA demonstrated a monophasic distribution of plaque-forming cells. On the other hand, mean plaque area was significantly (P less than 0.05) increased if 0.1 nM or 1 nM DA was presented to the cells (15,000 microns 2 and 14,500 microns 2, respectively). These cells receiving stimulatory doses of DA exhibited a multiphasic distribution of plaque-forming cells. The possibility that the two physiological opposing actions of DA on PRL secretion might be mediated by different GTP binding proteins was also examined using cholera toxin (CTX) and pertussis toxin (PTX). Anterior pituitary cells were pretreated with either CTX (50 micrograms/ml) or PTX (5 micrograms/ml) for 1 h before initiation of the RHPA. In the RHPA, cells received no DA, a stimulatory dose of DA (0.1 nM), or a inhibitory dose of DA (10 microM). The effects of toxin pretreatment on mean plaque area of DA-treated cells was determined. PTX pretreatment significantly attenuated the inhibitory effects of DA while having no effect on the stimulatory effects of DA on PRL secretion. CTX significantly (P less than 0.05) potentiated the stimulatory effects of DA on PRL secretion and had no effect on inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)     

Prostaglandin E analogue inhibition of pancreatic enzyme secretion

The effect of native prostaglandins and their methylated analogues on pancreatic enzyme secretion remains unclear, with previous studies reporting inconsistent results. To determine whether the E series prostaglandins directly influence pancreatic secretion, we studied the effect of rioprostil, a prostaglandin E1 analogue, and 16,16-dimethyl prostaglandin E2 (DMPGE2), a prostaglandin E2 analogue, on enzyme release from dispersed guinea pig pancreatic acini. Basal amylase release (4.3 +/- 0.6% of total acinar content) was not altered by either analogue (10(-10)-10(-5) M). A 50% inhibition of maximal cholecystokinin stimulation (10(-9) M; 28.8 +/- 1.2%) was seen with rioprostil (10(-7) M; 14.6 +/- 1.3%) and DMPGE2 (10(-6) M; 15.9 +/- 0.7%) (both p less than 0.005). Prostaglandin inhibition of carbachol-stimulated amylase was less pronounced. The most effective inhibitory dosage with maximal carbachol (10(-5) M; 30.2 +/- 1.9%) was 10(-6) M for both rioprostil (19.2 +/- 1.6%) and DMPGE2 (22.4 +/- 1.7%) (both p less than 0.005). Incubation of acini with A23187, phorbol ester, and 1-oleoyl-2-acetyl-glycerol resulted in a dose-dependent increase in amylase release that was not altered by maximal concentrations of either prostaglandin analogue. Our results indicate that rioprostil and DMPGE2 can directly inhibit pancreatic acinar secretion. This effect appears to occur before activation of the inositol phospholipid system.