Modulation by alloxan of glucagon and insulin secretion in the isolated perfused rat pancreas.

The acute in vitro effect of alloxan on glucagon and insulin secretion from the isolated perfused rat pancreas was examined. Alloxan alone produced transient insulin secretion. Pretreatment with alloxan attenuated both the stimulatory effect of glucose on insulin secretion and the inhibitory effect of glucose on glucagon secretion. Exposure to alloxan in varying doses either partially or completely inhibited insulin secretion induced by arginine in the presence or absence of glucose. On the contrary, pretreatment with alloxan produced complex effects on arginine-induced glucagon secretion. In the absence of glucose, the response of glucagon to arginine infusion was lower in the pancreas exposed to alloxan than in the control experiment. In the presence of glucose, however, an apparently augmented response of glucagon to arginine was observed after exposure to higher doses of alloxan, suggesting an impaired inhibitory effect of glucose on arginine-induced glucagon secretion. These effects of pretreatment with alloxan on glucagon secretion can not be explained by earlier or simultaneous insulin secretion. Therefore, we conclude that alloxan acts not only on beta-cells, but also directly on alpha-cells, although the latter are less sensitive to this agent.     

Different effects of hypothermia on insulin and glucagon secretion from the isolated perfused rat pancreas

Summary Two series of experiments with the isolated perfused rat pancreas were performed in parallel. The conditions differed only with respect to temperature, which was 37.5 °C in one series and 28 °C in the other. The lowering of the temperature decreased insulin secretion induced by glucose as well as the insulin response to tolbutamide and acetylcholine. Unlike insulin, glucagon secretion was not significantly modified by hypothermia. Our results suggest that the mechanisms involved in glucagon and insulin secretion are different.Equipe de Recherche Associée au CNRS n 786     

Acidotic pH augments glucagon secretion and gluconeogenesis in the isolated perfused rat pancreas and liver.

To study effects of acidosis on glucagon secretion and gluconeogenetic action of glucagon, rat pancreas and liver were perfused with media of pH 6.4, 6.9, 7.4 and 7.9. Glucagon secretion from the pancreas during glucopenic perfusion (1 mmol/l) was blunted at alkalotic pH, and was augmented at acidotic pH; 0.28+/-0.18 at pH 7.9 (P<0.01), 3.57+/-0.94 at pH 6.9 (P<0.01) and 1.72+/-0.36 at pH 6.4 (P<0.01), vs. 0.66+/-0.25 pmol for 15 min at pH 7.4. Incorporation rate of 14C of lactate-U-14C into glucose carbon one was decreased at pH 7.9 (1.2+/-0.2% for 15 min, P<0.05) and was increased at pH 6.9 (2.8+/-0.5%, P<0.05) compared to that at pH 7.4 (1.9+/-0.3%). Percent increasing rate of lactate gluconeogenesis by 1 nmol/l glucagon was not different within a range of pH 6.4-7.9. Thus, glucagon-stimulated gluconeogenesis from lactate was smaller at pH 7.9 (2.2+/-0.6%) and was significantly greater at pH 6.9 (4.9+/-0.9%, P<0.05) than that at pH 7.4 (3.2+/-0.6%). These results suggest that the pancreatic glucagon secretion and the glucagon-stimulated hepatic gluconeogenesis play more important roles in the maintainance of blood glucose level in the stress states associated with acidosis than without acidosis.     

Pancreatic polypeptide,glucagon and insulin secretion from the isolated perfused canine pancreas

Summary The release of pancreatic polypeptide (PP) by gut hormones, acetyl choline and adrenaline was investigated in an isolated perfused pancreas preparation. PP was potently released by 1 nmol/l caerulein (186±12%, p<0.001) and gastric inhibitory peptide (GIP) (211±31%, p<0.005) as well as by 1 [mol/l acetyl choline (1097±59%, p<0.001). A significant two-fold release of PP was also evoked by 1 nmol/l vasoactive intestinal peptide (VIP) (129±38%, p<0.02 and gastrin (108±25% p<0.01). Insulin release, induced by high glucose concentration was enhanced by both GIP (210 ±38%, p<(0.01) and VIP (48±5%, p<0.001). In addition GIP enhanced the release of glucagon by 179±18% (p<0.001) at 1.4 mmol/l glucose and by 127±24% (p<0.005) at 8.3 mmol/l glucose. Thus no simple inter-relationship appears to exist between the control of the three circulating islet hormones.     

Parathyroid hormone enhances glucagon secretion from the isolated perfused rat pancreas preparation

We examined whether PTH could increase glucagon secretion in an in vitro system, the isolated perfused rat pancreas. Since the response of the A cell has been shown to be modulated by antecedent exposure to elevated concentrations of glucose, bovine PTH (Beckman 1-34) was superimposed upon 15-min infusions of glucose followed by arginine or upon infusions of arginine alone. In the presence of PTH (44 ng/ml) and when the ambient calcium concentration was 9.0 mg/dl, arginine (168 mg/dl)-induced glucagon secretion was augmented. This occurred regardless of whether arginine was preceded by glucose (150 mg/dl). The glucagonotropic effect of PTH was absent in the presence of a low ambient calcium concentration (3.0 mg/dl). PTH failed to affect glucose-induced glucagon suppression.     

Modulation by glucose and D-glyceraldehyde of glucagon and insulin secretion in the isolated perfused streptozotocin-treated rat pancreas

Summary In order to compare the effects of D-glyceraldehyde or glucose on glucagon secretion in insulin deficiency, the isolated streptozotocin-treated rat pancreas was perfused with arginine alone and arginine plus either glucose or D-glyceraldehyde. The glucagon secretion induced by arginine alone was not modified by pretreatment with streptozotocin, but the glucagon secretion induced by arginine plus either glucose or D-glyceraldehyde was less inhibited in the streptozotocin-treated pancreas. We conclude, therefore, that insulin deficiency may interfere with the metabolism of D-glyceraldehyde as well as glucose in the pancreatic A-cells, thus interfering with the inhibitory effect of glucose and D-glyceraldehyde of glucagon secretion.     

Effects of endotoxin and interleukin-1 on glucagon and insulin secretion from the perfused rat pancreas.

This study evaluated the in vivo effects of endotoxin and interleukin-1 (IL-1) on the simultaneous secretion of glucagon and insulin. The hypothesis that endotoxin, or IL-1 as a mediator, induces hyperglucagonemia secondary to pancreatic hypersecretion of glucagon was examined. Hormone secretion was measured using the in vitro perfused rat pancreas preparation. In response to an arginine stimulus, glucagon secretion was neither stimulated nor inhibited significantly by endotoxin or IL-1. Insulin secretion was significantly potentiated with both endotoxin and IL-1. In response to a low-glucose stimulus, glucagon secretion was significantly inhibited by endotoxin treatment, while insulin secretion was increased by endotoxin or IL-1. These results indicate that neither endotoxin nor IL-1 treatment resulted in glucagon hypersecretion, although either of these agents could induce insulin hypersecretion. Thus, the mechanism of endotoxin-induced hyperglucagonemia cannot be explained by a hypersecretory state of glucagon secretion. The parallel respective effects of endotoxin and IL-1 on glucagon and insulin secretion are consistent with the concept that IL-1 mediates some of the effects of endotoxin on the endocrine pancreas.     

Effect of insulin in vitro on the isolated,perfused alloxan-diabetic rat liver

Summary Withdrawal of exogenous insulin and a subsequent fast (24 h) of alloxan diabetic rats stimulated rates of gluconeogenesis, ureogenesis, ketogenesis, and amino acid release by in situ perfused livers when compared to those from normal, fasted rats. The contribution of liver glycogen to the high rates of gluconeogenesis observed with the diabetic liver could be excluded. Perfusate lactate concentrations remained constant during the period when the elevated rate of gluconeogenesis was observed with diabetic liver. Addition of insulin as a bolus (750 mU) and continuous infusion (12.5 mU/min) to the perfusion medium of diabetic livers resulted in constant perfusate levels of glucose, urea and -amino nitrogen indicating a suppression of the catabolic processes present in the fasted, diabetic liver. The rate of ketogenesis was also slowed by insulin to about half the rate prior to addition of the hormone. These data indicate that insulin has an immediate anti-catabolic effect in the perfused, diabetic liver.     

Antithyrotropin-releasing hormone serum inhibits secretion of glucagon from isolated perfused rat pancreas: an experimental model for positive feedback regulation of glucagon secretion.

TRH is synthesized in the islets of Langerhans and was found in the perfusate of isolated rat pancreas. In the present study, designed to determine the role of endogenous TRH, we first characterized chromatographically the identity of immunoreactive TRH with synthetic pGlu-His-Pro-NH2. Since endogenous TRH secretion may mask the effects of exogenous TRH, we performed, in parallel to dose-response studies, immunoneutralization experiments using anti-TRH serum to neutralize the endogenous TRH secretion from isolated perfused rat pancreas. The data indicate that exogenous TRH enhances basal glucagon secretion; inversely, anti-TRH serum inhibits glucose plus arginine-induced glucagon secretion and produces a concomitant slight inhibition of somatostatin secretion. The present study shows a physiological contribution for endogenous TRH as a local modulator of intraislet hormone regulation; from these observations, we postulate a direct effect of pancreatic TRH on glucagon-containing (alpha) cell secretion, which, in turn, may produce the fluctuation in somatostatin secretion. Local TRH secretion provides a model for positive feedback regulation of glucagon secretion, frequently associated with diabetes.     

Inhibition of glucagon and insulin secretion from the perfused rat pancreas by a B-cell-selective somatostatin analog

The isolated perfused rat pancreas was used to study the effects of somatostatin and the analog des-Asn⁵-[D-Trp⁸, D-Ser¹³]-somatostatin on arginine-stimulated insulin and glucagon secretion. Even though the analog was found to inhibit glucagon secretion at concentrations of 10 and 100 ng/ml, it had a relatively more inhibitory effect on the B cell than on the A cell than did somatostatin itself. These data suggest that the A- and B-cell receptors for these two peptides differ.     

Independence of GIP-induced insulin secretion from sympathetic and parasympathetic innervation in the isolated perfused rat pancreas.

The incretin candidate GIP (gastric inhibitory polypeptide) is released from the gut by nutrients and can stimulate insulin secretion. Metabolic and hormonal factors have been shown to modulate insulin response to GIP. It is unknown, however, whether the autonomic nervous system, which itself controls insulin secretion, can modulate the insulinotropic effect of GIP. In the isolated perfused rat pancreas, we therefore investigated the influence of sympathetic and parasympathetic agonists and antagonists on the insulin response to GIP. As compared to control (6990 +/- 890 microU/10 min), the effect of either acetylcholine (29030 +/- 4600 microU/10 min), atropine (5880 +/- 1740 microU/10 min), norepinephrine (2520 +/- 750 microU/10 min), phentolamine (11380 +/- 1910 microU/10 min), isoproterenol (12740 +/- 2090 microU/10 min), propranolol (5600 +/- 880 microU/10 min), or GIP (29660 +/- 4490 microU/10 min) on insulin secretion was consistent with previous reports. The effects of the combined administration of GIP and either acetylcholine (48140 +/- 7540 microU/10 min), phentolamine (43930 +/- 4490 microU/10 min), norepinephrine (9000 +/- 1740 microU/10 min), or isoproterenol (36280 +/- 5210 microU/10 min) on insulin release were additive. Insulin response to GIP was resistant to atropine (24210 +/- 9470 microU/10 min) and propranolol (26450 +/- 4930 mu/10 min). We conclude that both GIP and the autonomic nervous system influence insulin secretion, but that they do so independently from each other.     

Differential sensitivity to somatostatin of pancreatic polypeptide, glucagon and insulin secretion from the isolated perfused canine pancreas

This dose-response study deals with the relative inhibitory effect of somatostatin on the acetylcholine-stimulated release of pancreatic polypeptide (PP), glucagon, and insulin from the isolated canine pancrease. Somatostatin in picomolar doses potently inhibited insulin and glucagon secretion, whereas PP secretion was relatively insensitive. Also, in the absence of acetylcholine, somatostatin exerted a preferential inhibition of the release insulin and glucagon compared with PP. These findings point to a physiologically important role of somatostatin for the secretion of insulin and glucagon, but probably not for PP.     

Multiple effects of leucine on glucagon, insulin, and somatostatin secretion from the perfused rat pancreas

The effects of increasing concentrations of leucine (0.2, 2.0, and 15.0 mmol/liter) on glucagon secretion from the perfused rat pancreas were examined at various glucose levels (0, 3.3, or 8.3 mmol/liter) and in the absence or presence of either arginine (5.0 mmol/liter) or glutamine (10.0 mmol/liter). At a low glucose concentration (3.3 mmol/liter), leucine caused a dose-related biphasic increase in glucagon output in the absence of arginine, but only a transient increase in the presence of the latter amino acid. These positive responses were markedly reduced and, on occasion, abolished at a high glucose concentration (8.3 mmol/liter). Moreover, at a low glucose concentration (3.3 mmol/liter) and in the presence of arginine, the highest concentration of leucine (15.0 mmol/liter) provoked a sustained and reversible inhibition of glucagon release. Likewise, leucine (15.0 mmol/liter) reversibly inhibited glucagon secretion evoked by glutamine in the absence of glucose. Thus, leucine exerted a dual effect on the secretion of glucagon, the inhibitory effect of leucine prevailing at a high concentration of the branched chain amino acid and when glucagon secretion was already stimulated by arginine or glutamine. At a physiological concentration (0.2 mmol/liter), however, leucine was a positive stimulus for glucagon release, especially in the absence of another amino acid. Concomitantly, leucine was always a positive stimulus for both insulin and somatostatin secretion. The intimate mechanisms involved in the dual effect of leucine on glucagon secretion remain to be elucidated.     

Somatostatin,glucagon, and insulin secretion from isolated perfused pancreas of new genetically obese hyperglycemic rats

Insulin, somatostatin, and glucagon release from the perfused pancreas was studied in the newly developed genetically obese hyperglycemic hyperinsulinemic (Wistar fatty) rat. Insulin and somatostatin levels rose significantly compared to those in lean littermate controls during arginine infusion. The glucagon increase, however, was significantly less when total amounts during arginine infusion were calculated. These results show that hypersecretion of insulin and somatostatin in vitro may suppress glucagon release in Wistar fatty rats.