Effect of nicotine on insulin secretion in the isolated perfused rat pancreas]

Insulin secretion induced by glucose (1.5 g/l) is changed by nicotine infusion; the recorded changes depend on the nicotine concentration uses. 1) At a low concentration (0.05 mM) nicotine provokes an immediate, progressively increasing and lasting stimulation of insulin secretion. This stimulation is inhibited by hexamethonium (0.1 mM) and atropine (0.3 micrometer). 2) At a high concentration (1 mM) nicotine has a triphasic effect on insulin secretion : brief decrease, peak of stimulation and prolonged decrease. Hexamethonium decreases the stimulation and suppresses the prolonged inhibition.     

Effects of thiamine pyrophosphate and sodium dichloroacetate on the increase of lactate and insulin outputs induced by phenformin on the isolated and perfused rat pancreas]

Thiamine pyrophosphate and sodium dichloroacetate, substances which promote the aerobic metabolism of pyruvate, reduce the increase in lactate output induced by phenformin (100 mg/l) on the isolated and perfused rat pancreas. Both substances provoke a decrease in insulin secretion produced by phenformin.     

Adrenergic modulation of insulin and glucagon secretion from the isolated perfused rat pancreas

In order to observe the effect of the adrenergic system on pancreatic glucagon secretion in the isolated perfused rat pancreas, phenylephrine, an alpha-adrenergic agonist, and isoproterenol, a beta-adrenergic agonist, were added to the perfused solution. 1.2 microM phenylephrine suppressed glucagon secretion at 2.8 mM glucose, and it also decreased insulin secretion at 11.1 mM glucose. 240 nM isoproterenol enhanced glucagon secretion not only at 2.8 mM glucose, but also at 11.1 mM glucose, as well as insulin secretion at 11.1 mM. In order to study the role of intra-islet noradrenalin, phentolamine, an alpha-adrenergic antagonist, and propranolol, a beta-adrenergic antagonist, were infused with the perfused solution. 10 and 100 microM phentolamine caused an increase in insulin secretion, and 25 microM propranolol decreased insulin secretion, while they did not cause any change in glucagon secretion. From these results, it can be concluded that alpha-stimulation suppresses not only insulin but also glucagon secretion, while beta-stimulation stimulates glucagon secretion, as well as insulin secretion. Intra-islet catecholamine may have some effect on the B cell, whereas it seems to have no influence on the A cell.     

Biotin enhances glucose-stimulated insulin secretion in the isolated perfused pancreas of the rat

The effects of biotin on insulin secretion in pair-fed control rats and biotin-deficient rats were investigated using the method of isolated pancreas perfusion. Isolated pancreas perfusion was performed using 20 mM glucose, 10 mM arginine, and 20 mM glucose plus various concentrations of biotin (20 mM glucose + biotin solution) as stimulants of insulin secretion. The insulin response to 20 mM glucose in biotin-deficient rats was approximately 22% of that seen in control rats. The level of the insulin response to 10 mM arginine was also significantly lower in biotin-deficient rats than in control rats. These results indicate that insulin release from the pancreas was disturbed in biotin-deficient rats. The insulin responses to 20 mM glucose + 1 mM biotin in biotin-deficient and control rats increased to 165% and 185%, respectively, of that to 20 mM glucose. These biotin-induced increases in glucose-stimulated insulin release were evident within the first few minutes of the infusion. An enhancement of the arginine-induced insulin response in control rats was not found when arginine and biotin was administered. These results suggest that biotin may play an important role in the mechanism by which glucose stimulates insulin secretion from the beta cells of the pancreatic islets.     

Effects of salicylate on insulin and glucagon secretion by the isolated and perfused rat pancreas

The effects of sodium salicylate, a prostaglandin synthesis inhibitor, on glucose-induced secretion of insulin and glucagon by the isolated perfused rat pancreas have been studied. Sodium salicylate inhibited both basal (2.8 mM glucose) and stimulated (16.7 mM glucose) insulin release in a dose dependent manner (1, 5 and 10 mM). This inhibition is not interpretable in terms of a simple inhibition of cyclooxygenase by sodium salicylate. Basal glucagon release was not changed by 1 mM sodium salicylate but the latter partially blocked its inhibition by 16.7 mM glucose. Higher doses of sodium salicylate (5 and 10 mM) inhibited basal glucagon secretion without affecting its response to 16.7 mM glucose. These findings suggest a predominant stimulatory action of endogenous prostaglandins on glucagon release.     

Effects of lipoxygenase and cyclooxygenase inhibitors on glucose-stimulated insulin secretion from the isolated perfused rat pancreas

Some of the metabolites of arachidonic acid formed in the lipoxygenase and cyclooxygenase pathways stimulate insulin release. We studied the relative importance of each of these pathways in the modulation of glucose-induced insulin release by using inhibitors of arachidonate metabolism. Perfusion of the isolated rat pancreas with two chemically different inhibitors of cyclooxygenase, flurbiprofen and sodium salicylate, markedly inhibited prostaglandin E2 release, but had little effect on glucose-induced insulin release or on potentiation of insulin release caused by prior exposure to glucose. On the other hand, nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, not only inhibited both phases of glucose-induced insulin release but also abolished the potentiation effect. These effects of NDGA prevailed, when it was administered together with flurbiprofen, which caused profound inhibition of prostaglandin E2 release. We conclude that 1) lipoxygenase pathways play a dominant role in glucose-stimulated insulin release, and 2) endogenous lipoxygenase metabolites influence the potentiating effect of glucose on the release of insulin in response to a subsequent stimulation.     

Characterization of acetylcholine-induced insulin secretion in the isolated perfused dog pancreas.

In the presence of a nonstimulatory concentration of glucose, a 60-min perfusion with 50 muM acetylcholine was shown to elicit a monophasic release of insulin in the isolated dog pancreas preparation. A decline in secretory response, which may be due to desensitization of the beta-cell to acetylcholine, was noted during the latter part of the perfusion interval. The potent insulin secretory response elicited by acetylcholine during the 60-min period was abolished 0y 25 muM atropine. Inhibition of the insulinotropic action of acetylcholine was also noted with administration of the mitotic spindle inhibitor, colchicine. When compared to 20-min control perfusions, addition of 1 mM colchicine resulted in a 50% reduction in acetylcholine-induced insulin release. These results suggest that insulin secretion stimulated by acetylcholine can be considered to be due to a muscarinic action of this agent which is dependent, at least in part, upon the microtubular system of the beta-cell.     

Action of glibenclamide on glucagon and insulin secretions studied on isolated and perfused pancreas of the rat]

Glibenclamide stimulates the insulin secretion by the isolated and perfused rat pancreas, but does not inhibit glucagon secretion when the perfusion liquid contains 1.5 g/I glucose. In the absence of glucose in the perfusion medium, glibenclamide stimulates both insulin and glucagon secretions.     

Somatostatin, insulin and glucagon secretion by the perfused pancreas from the cysteamine-treated rat

In rats, administration of a single dose of cysteamine (300 mg/kg, intragastrically) induces a depletion of pancreatic somatostatin content (approximately 60%) without modifying pancreatic insulin or glucagon content. In perfused pancreases from cysteamine-treated rats, there was a lack of somatostatin response to glucose, arginine or tolbutamide. In the absence of stimulated somatostatin release, the secretory responses of insulin and glucagon to glucose, to arginine, and to tolbutamide were not significantly different from those observed in pancreases from control rats. Our data do not support the concept that pancreatic somatostatin plays a major role in the control of insulin and glucagon release.     

Islet amyloid polypeptide (IAPP) does not inhibit glucose-stimulated insulin secretion from isolated perfused rat pancreas

Islet amyloid polypeptide (IAPP) is a recently discovered pancreatic islet hormone which is stored with insulin in the secretory vesicles of beta cells. Several lines of evidence suggested that IAPP might affect glucose-stimulated insulin secretion and, therefore, might play a role in the development of impaired insulin secretion which is typical of type 2 diabetes. In this study, the effects of human IAPP (amide) on glucose-stimulated insulin secretion was evaluated in the isolated perfused rat pancreas. IAPP in concentrations from 5 x 10(-12) to 10(-7) M had no significant effects on insulin secretion. IAPP, therefore, does not appear to be a significant modulator of glucose-stimulated insulin secretion at concentrations that are physiologically relevant.     

Exendin-4 dose-dependently stimulates somatostatin and insulin secretion in perfused rat pancreas.

We have investigated the effect of exendin-4, a GLP-1 analogue, on somatostatin and insulin secretion in perfused rat pancreas. At constant glucose concentration within the type 2 diabetic range (9 mM), exendin-4 stimulated somatostatin and insulin secretion in a dose-dependent manner. Dose-response curves were sigmoidal (R (2) = 0.9954 and R (2) = 0.9973, respectively; p < 0.01) and the EC (50) was 4.3 nM for somatostatin secretion and 1.4 nM for insulin secretion. Exendin-4 stimulated somatostatin output at low (3.2 mM), normal (5.5 mM) and high (9 mM) glucose concentrations, while the insulinotropic effect of exendin-4 was not found at low glucose levels. On the other hand, exendin-4 potentiated somatostatin and insulin responses to an increase in perfusate glucose levels, and to arginine and carbachol. Finally, the insulinotropic effect of exendin-4 was maintained in the absence of a somatostatin response as induced by cysteamine pretreatment, indicating a direct effect of exendin-4 on the B-cell. In summary, exendin-4 behaves as a general stimulatory agent of both insulin and somatostatin release in the perfused rat pancreas. Given that exendin-4 has also been shown to increase gastric somatostatin secretion, it is tempting to speculate that exendin-4 might behave as a general stimulator of D-cell function in other tissues, a point worthy of further investigation.     

Effect of beta-hydroxybutyrate and acetoacetate on insulin and glucagon secretion from perfused rat pancreas

To elucidate the physiological significance of ketone bodies on insulin and glucagon secretion, the direct effects of beta-hydroxybutyrate (BOHB) and acetoacetate (AcAc) infusion on insulin and glucagon release from perfused rat pancreas were investigated. The BOHB or AcAc was administered at concentrations of 10, 1, or 0.1 mM for 30 min at 4.0 ml/min. High-concentration infusions of BOHB and AcAc (10 mM) produced significant increases in insulin release in the presence of 4.4 mM glucose, but low-concentration infusions of BOHB and AcAc (1 and 0.1 mM) caused no significant changes in insulin secretion from perfused rat pancreas. BOHB (10, 1, and 0.1 mM) and AcAc (10 and 1 mM) infusion significantly inhibited glucagon secretion from perfused rat pancreas. These results suggest that physiological concentrations of ketone bodies have no direct effect on insulin release but have a direct inhibitory effect on glucagon secretion from perfused rat pancreas.