Galanin inhibits pancreatic amylase secretion in the pentobarbital-anesthetized rat.

The potent inhibitory effect of galanin on basal and pentagastrin-stimulated gastric acid secretion in vivo, and the presence of galanin-containing nerves in gastrointestinal tract and pancreas, suggested that this peptide may regulate the exocrine secretion of the GI system. Male rats were anesthetized with pentobarbital and the dose-dependent inhibitory effects of galanin on basal and stimulated pancreatic protein and amylase secretions were investigated in separate experiments. Galanin was administered intravenously in the following doses: 3, 6, 10, 15 and 20 micrograms/kg/h (0.93, 1.86, 3.1, 4.65 and 6.2 nmol/kg/h), and pancreatic secretions measured. The maximal effective dose of galanin (3.1 nmol/kg/h) on basal pancreatic secretions was found, and was used for evaluating the inhibitory effect of galanin on pancreatic protein and amylase secretions stimulated by bombesin, secretin and cholecystokinin. Galanin potently inhibited basal, bombesin-, secretin- and cholecystokinin-stimulated pancreatic protein and amylase secretion. Inhibitory effect of galanin was dose-dependent and biphasic.     

Factors regulating amylase secretion from chicken pancreatic acini in vitro

In mammals, cholecystokinin regulates pancreatic exocrine secretion under physiological conditions. We have shown, however, that cholecystokinin at physiological concentrations does not induce pancreatic amylase secretion in birds. Therefore, we investigated the effects of various neurotransmitters and gut hormones on the pancreatic amylase secretory response in isolated chicken pancreatic acini. Acetylcholine (half-maximal stimulation at 800 nM) and vasoactive intestinal polypeptide (half-maximal stimulation at 40 pM) produced a concentration-dependent increase in amylase secretion at physiological concentrations. The combination of acetylcholine and vasoactive intestinal polypeptide produced an additive response in amylase secretion. Sodium nitroprusside, a spontaneous nitric oxide releaser, and bombesin, induced amylase secretion at concentrations greater than 10 nM and 100 nM, respectively. Gastrin and secretin increased amylase secretion at pharmacological concentrations (10 to 100 nM). Our findings suggest that neural regulation is important for pancreatic enzyme secretion in birds and the contribution of gut hormones seems to be physiologically unimportant.     

Effect of dopamine on amylase secretion from guinea pig pancreatic acinar cells in vitro

Dopamine has been shown to effect pancreatic flow, protein output and amylase secretion in a variety of species. However, there is conflicting evidence regarding the role of dopamine on amylase release in vitro. Specific studies were conducted to evaluate the effect of dopamine and to compare its effects with other substances on basal- and secretagogue-stimulated amylase secretion in a guinea pig dispersed pancreatic acinar cells preparation. Dopamine (10(-6) M) induced a small, but significant (P less than 0.05) increase of amylase secretion. Established secretagogues (10(-6) M) including bombesin, cholecystokinin-octapeptide (CCK-8) and carbachol as anticipated induced significantly larger responses. Other substances tested (10(-6) M) including thyrotropin-releasing hormone (TRH) and muscimol were without effect. Complete dose-response studies (10(-11)-10(-3) M) in the presence of bombesin, CCK-8 and carbachol revealed that dopamine does not affect amylase release in response to these secretagogues. These findings suggest that dopamine is a weak stimulant of amylase secretion in vitro, and that it may therefore play a minor role in regulation of pancreatic enzyme secretion. Several factors including vascular, hormonal and neural have been implicated in regulation of pancreatic exocrine secretion. In particular, autonomic nervous system activity, notably cholinergic, has been shown to affect the secretory status of the pancreatic acinar cell. In addition, several biologically active peptides including bombesin, cholecystokinin (CCK), secretin, vasoactive intestinal peptide (VIP), substance P, gastrin and stimulation of cholinergic (muscarinic) receptors with carbachol have been shown to stimulate pancreatic enzyme secretion both in vivo and in vitro. Certain controversy regarding the role of the sympathetic nervous system in regulation of pancreatic exocrine secretion does exist. For example, several studies with agonists and antagonists of noradrenergic and dopaminergic receptor subtypes suggest a stimulatory effect on pancreatic fluid, electrolyte and enzyme secretion. However, these responses are species-specific and variations inherent to the model have been described. Dopamine administration has been shown to stimulate pancreatic bicarbonate and enzyme secretion in a variety of species including mice, dogs, and man. Radioligand binding studies with 3H-dopamine have revealed the presence of high- and low-affinity dopamine binding sites in dog pancreatic acinar cells. Stimulation of these receptors has been correlated with dose-dependent increases in intracellular cAMP levels.(ABSTRACT TRUNCATED AT 400 WORDS)     

Regulation of pancreatic exocrine secretion in vitro: the action of secretagogues

Pancreatic acinar cells possess two functionally distinct mechanisms by which secretagogues can increase enzyme secretion. One mechanism is mediated by mobilization of cellular calcium and can be activated by any one of four different classes of receptors. The other mechanism is mediated by cyclic AMP and can be activated by either of two different classes of receptors. In addition to stimulating enzyme secretion, a secretagogue can cause potentiation of secretion, desensitization to the subsequent stimulation caused by the same or other secretagogues as well as residual stimulation of enzyme secretion. Although each class of secretagogue receptors can cause the same final effect, stimulation of enzyme secretion, the existence of multiple classes of receptors and the different mechanisms of action endow the acinar cell with a wide range of patterns of response depending on which of the several classes of receptors are activated.     

Photodynamic action of rose bengal on isolated rat pancreatic acini: stimulation of amylase release

The halogenated fluorescein derivative, rose bengal, upon photon activation, elicits amylase secretion from isolated, perifused pancreatic acini. This effect is due to production of highly reactive singlet delta oxygen which can permeabilize the cell membrane and may also react chemically with secretagogue receptors, or other functional components of the membrane such as the G-proteins. The profile of photodynamically induced amylase secretion is anion-dependent: it becomes biphasic when the chloride ion is substituted by the glutamate ion, an effect attributed to the action of glutamate on the ionic transport systems of the zymogen granule membrane.     

Inhibition by mepacrine and amylase secretion from intact and permeabilized rat pancreatic acini.

In intact rat pancreatic acini, the phospholipase A2 inhibitor mepacrine did not affect basal amylase release but dose-dependently inhibited the carbachol (IC50 65 microM) and CCK-8 (IC50 210 microM)-stimulated amylase release. In permeabilized acini, mepacrine shifted the dose-response curve for calcium to the right by a factor 2 and inhibited the release of amylase stimulated by GTPrS. From these results we conclude that carbachol, CCK-8 and GTPrS probably activate a phospholipase A2 closely coupled to exocytosis.     

Mode of stimulatory action of deoxycholate in signal transduction system of isolated rat pancreatic acini.

Mode of stimulatory action of deoxycholate (DCA) on the secretagogue-induced amylase release and the phospholipase C reaction in isolated rat pancreatic acini was investigated using sodium fluoride (NaF), which is a direct activator of GTP-binding proteins (G proteins). DCA enhanced the amylase release induced by submaximal concentrations of NaF without affecting the maximal level of this reaction. Under the similar conditions, DCA enhanced the NaF-induced phospholipase C reaction. These stimulatory effects of DCA on the NaF-induced amylase release and phospholipase C reaction are comparable to those on the secretagogue-induced reactions reported previously. These results suggest that DCA acts on the coupling of a G protein(s) to the phospholipase C in the membrane transduction mechanism in isolated rat pancreatic acini.     

Interactions of intracellular mediators of amylase secretion in permeabilized pancreatic acini.

Mouse pancreatic acini were permeabilized with streptolysin O to investigate amylase secretion stimulated by various intracellular mediators and the kinetics of secretion as a function of temperature. Amylase secretion was temperature dependent in that the initial rate of Ca2(+)-stimulated secretion increased with increasing temperature. In addition, there was no enhancement of Ca2(+)-stimulated secretion by GTP[gamma S] at 14 degrees C, while enhancement was maximal at 30 degrees C. GTP[gamma S]-mediated enhancement of secretion at a given temperature was mostly due to sustained secretion with a small increase in secretory rate. At 30 degrees C Ca2(+)-stimulated secretion was also enhanced by cAMP and phorbol ester (TPA) to similar extents as by GTP[gamma S]. The maximally effective concentration of cAMP was 1-10 microM in the presence of 0.1 mM isobutylmethylxanthine. The enhancements of Ca2(+)-stimulated amylase secretion by all combinations of cAMP (100 microM plus 0.1 mM isobutylmethylxanthine), TPA (1 microM), and GTP[gamma S] (30 microM) were fully additive. In Ca2(+)-free buffer, cAMP, TPA or GTP[gamma S] individually had no effect on amylase secretion. Together, TPA and GTP[gamma S] stimulated Ca2(+)-independent secretion, which was 187 +/- 38% of basal. Cyclic AMP together with TPA and GTP[gamma S] in the absence of Ca2+ stimulated 329 +/- 30% of basal secretion. Ca2(+)-stimulated amylase secretion was decreased about 50% by metabolic inhibition, while the enhancement by cAMP, TPA or GTP[gamma S] was totally blocked by metabolic inhibitors. These data demonstrate that amylase secretion in the acinar cell is mediated by multiple intracellular pathways which act in parallel and probably converge at a distal step in the exocytotic process.     

Influence of thyroid status on Ca2+ mobilization and amylase secretion in rat pancreatic acini

Thyroid hormones influence Ca2+ homeostasis in both skeletal and cardiac muscle. Since secretory cells, like muscle cells, store and use Ca2+ in stimulus-response coupling, we have studied the effects of thyroid status on Ca2+ mobilization and secretion in a model secretory tissue, the pancreatic acinar cell. Hyperthyroidism was induced by rats by daily, subcutaneous injections of triiodothyronine for 8 days and hypothyroidism by adding 6-n-propyl-2-thiouracil to the drinking water for 14 days. Pancreatic acini were prepared by collagenase digestion of pancreatic tissue from hyper- and hypo-thyroid animals and from euthyroid controls. Ca2(+)-mobilization was assessed using Quin-2 fluorescence and secretion by assaying amylase release. The data indicate that the amount of Ca2+ mobilized by the muscarinic agonist carbachol or by cholecystokinin octapeptide increases with increasing thyroid hormone concentrations. Only in hypothyroidism was this change in Ca2+ homeostasis reflected by a parallel change in amylase secretion. This implies the existence of some compensatory mechanism which stabilizes secretory rate in the face of stimulus-evoked increases in intracellular Ca2+ concentration.     

Evaluation of pancreatic amylase mRNA upon cholinergic stimulation of secretion.

The primary function of the exocrine pancreas consists of the synthesis and secretion of several digestive enzymes. It is well established that amylase secretion by rat pancreatic tissue or by isolated acinar cells in culture can be stimulated by the cholinergic agonist carbachol. However, the effect of this secretagogue on enzyme synthesis remains unclear. Some studies demonstrated increases in rates of synthesis, whereas others reported increases in secretion with or without decreases in synthesis. We have evaluated changes in pancreatic amylase mRNA and total RNA after a single injection of carbachol and under fasting conditions. Two approaches in molecular morphology were applied on rat pancreatic tissue: in situ hybridization and RNase A-gold. Both revealed decreases in RNA labeling at the level of the rough endoplasmic reticulum (RER) 5 min after stimulation of secretion and after fasting. Gradual recovery was registered 15 and 30 min after stimulation of secretion. Northern blotting confirmed drastic decreases in amylase mRNA 5 min after stimulation and after fasting. The combination of such different approaches has demonstrated drastic decreases in RNA at the RER level, reflecting declines in rates of synthesis at the translational level under all conditions tested.     

Effects of an inhibitor of myosin light chain kinase on amylase secretion from rat pancreatic acini

Ca(2+)/calmodulin-dependent protein (CaM) kinases play an important role in Ca(2+)-mediated secretory mechanisms. Previously, we demonstrated that a CaM kinase II inhibitor KN-62 had a small inhibitory effect on amylase secretion stimulated by CCK. In the present study, we investigated the effects of a myosin light chain kinase (MLCK) inhibitor on amylase secretion and Ca(2+) signaling in rat pancreatic acini. A specific inhibitor of MLCK, wortmannin, inhibited amylase secretion stimulated by CCK-8 (30 pM) in a concentration-dependent manner. Wortmannin (10 microM) had no effects on basal secretion but reduced amylase secretion stimulated by CCK-8 (30 pM) by 67 +/- 3%. Wortmannin inhibited amylase secretion stimulated by calcium ionophore (A23187) and phorbol ester (TPA). Wortmannin also inhibited amylase response to thapsigargin by 76 +/- 8% and to both thapsigargin and TPA by 52 +/- 10%. Ca(2+) oscillations evoked by CCK-8 (10 pM) were inhibited by wortmannin (10 microM). Wortmannin had a little inhibitory effect on an initial rise in [Ca(2+)](i), and abolished a subsequent sustained elevation of [Ca(2+)](i) evoked by 1 nM CCK-8. In conclusion, MLCK plays a crucial role in amylase secretion from pancreatic acini and regulates Ca(2+) entry from the extracellular space.     

Combined effect of adenosine,alpha adrenergic and adenosine antagonists on serum insulin and insulin secretion from rat pancreatic islets

• 1.1. The effect of adenosine separately or in combination with alpha-1 adrenergic antagonist prazosin and alpha-2 adrenergic antagonist yohimbine as well as adenosine antagonists 8-phenyltheophylline and xanthine amine conjugate on glucose-induced insulin secretion from isolated rat pancreatic islets was studied.
• 2.2. Their in vivo effects on serum glucose and insulin levels were also investigated. Adenosine at 10 and 100 μM inhibited significantly, insulin secretion from the isolated islets whereas at 10 mM slightly increased the secretion of insulin.
• 3.3. Prazosin used at 100 μM inhibited insulin secretion. When it combined with adenosine (10 μM) it augmented the inhibitory effect of adenosine.
• 4.4. In vivo prazosin (21 mg/kg bodywt) caused a hyperglycaemia which was accompanied by hypoinsulinaemia.
• 5.5. Concurrent administration of this drug with adenosine neither affect the hyperglycaemic nor the hypoinsulinaemic effects of adenosine.
• 6.6. On the other hand, yohimbine (100 μM) has no effect neither separately nor in combination with adenosine (10 μM) in modulating the inhibitory effect of adenosine on insulin secretion.
• 7.7. When Yohimbine administered at 19.5 mg/kg body wt it did not alter serum glucose but it markedly increased the serum insulin level. Its combined administration with adenosine reduced the hyperglycaemic effect of adenosine with a remarkable increase in serum insulin.
• 8.8. Both adenosine-antagonists were ineffective in alteration of insulin secretion.
• 9.9. However, combination of 8-phenyltheophylline with adenosine (10 μM) totally blocked the inhibitory effect of adenosine on insulin secretion while xanthine amine conjugate failed to prevent this effect of adenosine.
• 10.10. These results indicate that the inhibitory effect of adenosine on insulin secretion is neither mediated via alpha-1 nor alpha-2 adrenoceptors. It might be via activation of specific adenosine receptors on rat islets which are sensitive to blockade by 8-phenyltheophylline.