Inhibitory action of rat insulin and synthetic rat C-peptide on insulin secretion in the perfused rat pancreas

Summary In order to determine whether both insulin and C-peptide have an inhibitory action on insulin secretion, the isolated rat pancreas was perfused with exogenous rat insulin or synthetic rat C-peptide in the presence of high or low concentration of glucose. In the presence of 2.8 mM glucose, exogenous rat insulin (4 or 10 ng/ml) and C-peptide 1 and 2 (100 ng/ml) show no effect on the insulin levels in the outflow from the perfused pancreas. However, the insulin response to 16.7 mM glucose decreased in the presence of exogenous rat insulin or synthetic rat C-peptide, showing a biphasic pattern of glucose-induced insulin release. When rat insulin or C-peptide were added at 20 min and removed at 40 min while the isolated pancreas was exposed to a 60-min glucose infusion (16.7 mM), glucose-induced insulin secretion decreased during the infusion of rat insulin or C-peptide. The present study clearly showed that exogenous rat insulin and synthetic rat C-peptide 1 and 2 inhibited glucose-induced insulin secretion. Although the suppressive mechanisms of the exogenous insulin and the C-peptide on insulin release are not yet proved, the inhibitory process is considered to be related to cyclic AMP in the pancreatic B-cell.     

Effect of fructose feeding on insulin secretion and insulin action in the rat

Insulin secretion and insulin action were studied in rats fed either a diet containing (as percent of calories) 66% fructose, 22% protein, and 12% fat, or standard rat chow (60% vegetable starch, 29% protein, 11% fat) for 7 days. Plasma glucose concentration following either an oral glucose or fructose load ($\text{180 mg}\text{100 g body weight}$) were slightly higher in the fructose-fed rats, and this was associated with a much greater elevation of plasma insulin concentrations. The ability of insulin to stimulate disposal of glucose load was determined during the continuous infusion of epinephrine, propranolol, glucose, and insulin. Under these conditions the steady state plasma insulin levels were the same in the two groups of rats, whereas the steady state plasma glucose levels were almost twice as high in the fructose fed rats. Thus, fructose feeding for 7 days resulted in an increase in the insulin response to an oral carbohydrate challenge, as well as to a loss of normal insulin sensitivity.     

The inhibition of insulin secretion from the perfused rat pancreas after thyroxine treatment

Summary Thyroxine treatment did not significantly affect the immediate insulin secretory response of the perfused rat pancreas, but it inhibited the late phase of D-glucose-induced insulin secretion. Thyroxine treatment did not inhibit D-glyceraldehyde-, D-mannose-, and tolbutamide-induced insulin release from the perfused pancreas. An increase in the D-glucose concentration of the perfusion medium as well as feeding of the rats did not restore insulin secretion after thyroxine treatment. The inhibition of D-glucose-induced insulin release in response to thyroxine treatment was reversed after addition of either D-glyceraldehyde, dihydroxyacetone, DL-glyceric acid, pyruvate, or -ketobutyrate to the perfusion medium. Tolbutamide, L-glucose, D-fructose, D-mannose, L-lactate, and propionic acid were not able to overcome the inhibition of D-glucose-induced insulin secretion. Except for -ketobutyrate all substances which were effective in reversing the inhibition of D-glucose-induced insulin release were glycolytic intermediates. Comparing the glycolytic -ketoacid pyruvate and the non-glycolytic ketoacid -ketobutyrate, the only part common to both substances was the ketoacid moiety. It is concluded from these findings that the ketoacid moiety of the -ketoacids plays an important role in reversing the effect of thyroxine on D-glucose-induced insulin release.This work was presented in part at the 11^th Annual Meeting of the European Association for the Study of Diabetes, Munich, September, 1975     

Thyroxine treatment facilitates prolactin secretion and induces a state of photorefractoriness in thyroidectomized starlings

Sexually immature male starlings were radiothyroidectomized while held under short day-lengths. They were then subjected to long (18-h) photoperiods and the testes developed rapidly and apparently normally to full maturity. As expected, thyroidectomy prevented an onset of photorefractoriness and after 140 days the testes were still fully developed (testicular width 6.5 +/- 0.8 mm). Half of the birds were then given thyroxine (0.011 mmol/l) in the drinking water for 14 days while the others were maintained as controls. At the end of this time the birds still had fully developed gonads (testicular width 7.8 +/- 0.3 mm) but after a further 14 days testicular regression began in the thyroxine-treated birds. The testes of all individuals in this group were fully regressed (width 1.8 +/- 0.1 mm) by 56 days after the end of thyroxine administration and moulting of the flight feathers had begun. No recrudescence of the gonads was subsequently noted in the next 2 months and the birds were apparently photorefractory. The short period of thyroxine treatment also caused a rapid and prolonged increase in plasma prolactin levels from 2.0 +/- 0.3 to 16.8 +/- 2.6 micrograms/l. No testicular regression or moulting was observed in the control birds and their plasma prolactin levels remained very low (below 4.0 micrograms/l) throughout the experiment. In thyroidectomized and castrated starlings held on 18-h daylengths, 14 daily injections of thyroxine (100 micrograms/bird per day) caused a rapid and permanent decrease in circulating FSH to basal levels (reached about 36 days after thyroxine treatment began, at which time the birds moulted). The treatment also caused a marked increase in plasma prolactin lasting for about 50 days.     

Effects of galanin on insulin and glucagon secretion in the rat

Galanin-like immunoreactivity has been visualized in nerve fibers in the islets of Langerhans, suggesting an involvement of galanin in the neural regulation of islet function. In this study, we investigated the effects of galanin on basal and stimulated insulin and glucagon secretion by infusing the peptide at three different dose rates in rats. We also studied the direct effect of galanin on insulin secretion from freshly isolated rat islets. At 320 pmol/kg/min, but not at 20 or 80 pmol/kg/min, galanin lowered basal plasma insulin levels. In contrast, basal plasma glucagon levels were lowered by galanin already at 20 and 80 pmol/kg/min. Furthermore, galanin inhibited both glucose- and arginine-induced insulin release at all three dose levels, whereas arginine-induced glucagon release was not affected by galanin. Glucose-stimulated insulin secretion from isolated rat islets was dose-dependently suppressed by galanin (10(-6)-10(-8) M). Therefore, it is concluded that galanin in rats inhibits insulin secretion, both in vivo and in vitro, and that at lower dose levels, the peptide also inhibits basal glucagon release.     

Effects of galanin on insulin and glucagon secretion in the rat

Galanin-like immunoreactivity has been visualized in nerve fibers in the islets of Langerhans, suggesting an involvement of galanin in the neural regulation of islet function. In this study, we investigated the effects of galanin on basal and stimulated insulin and glucagon secretion by infusing the peptide at three different dose rates in rats. We also studied the direct effect of galanin on insulin secretion from freshly isolated rat islets. At 320 pmol/kg/min, but not at 20 or 80 pmol/kg/min, galanin lowered basal plasma insulin levels. In contrast, basal plasma glucagon levels were lowered by galanin already at 20 and 80 pmol/kg/min. Furthermore, galanin inhibited both glucose- and arginine-induced insulin release at all three dose levels, whereas arginine-induced glucagon release was not affected by galanin. Glucose-stimulated insulin secretion from isolated rat islets was dose-dependently suppressed by galanin (10^-6-10^-8M). Therefore, it is concluded that galanin in rats inhibits insulin secretion, both in vivo and in vitro, and that at lower dose levels, the peptide also inhibits basal glucagon release.     

Mebendazole and insulin secretion from isolated rat islets

In a preliminary communication we reported that mebendazole, a vermicide, decreased plasma glucose and free fatty acid concentrations and increased plasma C peptide concentrations in both type II diabetic patients. Therefore, we suggested that mebendazole was an insulin secretagogue. However, these were uncontrolled studies, and improved metabolic control in these patients due to spontaneous remission rather than drug-induced insulin secretion was a possibility. To investigate the direct effect of mebendazole on insulin secretion we used intact islets isolated from normal rat pancreata. Mebendazole in concentrations as low as 10 to 20 mumol/L caused a twofold to threefold increase in acute-phase insulin release from isolated perifused rat islets. This heightened insulin release occurred in the presence of glucose-stimulated insulin secretion.     

Effect of progesterone on insulin secretion in the rat.

Implants of progesterone resulted in an increased amount of insulin in plasma in response to intravenous administration of glucose in the rat. Isolated islets of Langerhans from progesterone-treated animals showed a greater maximum secretory response to glucose than islets from control animals but their sensitivity to low concentrations of glucose was unchanged. Theophylline increased glucose-induced secretion of insulin to a greater extent in progesterone-treated than in control rats and also produced a greater increase in the concentration of cyclic AMP in isolated islets from hormone-treated animals. These results suggest that the effect of progesterone on insulin secretion may be mediated by a change in cyclic AMP levels in the beta cell. The possible role of progesterone in increasing the secretion of insulin in pregnancy is discussed.     

Characteristic features of insulin secretion in the streptozotocin-induced NIDDM rat model

Male Wistar neonatal rats at age 1.5 days (Streptozotocin [STZ] group 1) and 5 days (STZ group 2) received a subcutaneous injection of 90 mg/kg STZ. After 10 weeks, the rats were subjected to an oral glucose tolerance test (OGTT) (2 g/kg) in a conscious state. The pancreas perfusion experiments were conducted 2 weeks after the OGTT. There was no statistical difference in insulin response between the STZ group 1 and the control group. On the contrary, in the STZ group 2, the plasma glucose response to OGTT showed a typical diabetic pattern, and the plasma insulin response was markedly blunted. In the isolated perfused rat pancreas, the infusion of glucose evoked a biphasic insulin secretion, but the peak insulin levels induced by 16.7 mmol/L glucose in the STZ group 1 were significantly lower than in the controls. We further investigated characteristics of insulin secretion in response to different secretagogues in these animal models using isolated islets. The insulin content of the islets of the STZ group 1 were about one half that of the control group. Insulin secretion in the STZ group 1 was impaired in response to glucose stimulation, but remained normal in response to arginine and forskolin. These results suggest that insulin secretion of non-insulin-dependent diabetes mellitus (NIDDM) rat model is selectively impaired in response to glucose stimulation, possibly due to a disorder of signaling mechanism other than adenylate cyclase.     

Potassium ion concentration alters glucagon secretion independently of insulin secretion in the isolated rat pancreas

Summary In the arginine-stimulated perfused rat pancreas, elevated concentrations of potassium ion inhibited glucagon secretion while stimulating the secretion of insulin. Decreased potassium ion produced the reverse effect. The observed inverse correlation between changes in insulin and glucagon secretion (r = -0.64; p<0.001) was suggestive of local interactions between islet hormones, and prompted us to determine whether potassium-induced changes in glucagon secretion were dependent upon concurrent changes in insulin release. We found that when insulin secretion was greatly suppressed, either through acute induction of diabetes with streptozotocin or by utilization of a glucose-free perfusate, no qualitative differences in glucagon responsiveness to altered potassium ion were evident, although the amplitude of these glucagon changes was enhanced. Similarly, when exogenous insulin (20,000 mU/l) was added to the perfusate in order to render potassium-induced changes in endogenous insulin secretion insignificant, glucagon responsiveness to altered potassium ion was maintained. Exogenous insulin alone had no effect on arginine-stimulated glucagon secretion. We conclude that any indirect effects of potassium ion on arginine-stimulated glucagon secretion are not mediated by insulin, but could be related to changes in somatostatin secretion.     

Insulin secretion in insulin-requiring diabetics before and during insulin treatment

Endogenous insulin secretion after different stimuli was determined in insulin requiring diabetics without circulating insulin antibodies. Four groups of non-obese diabetics were investigated and compared with 111 controls. Group I: 14 patients with mild diabetes, not yet requiring insulin; diagnosis before the age of 30 years. Group II: 19 ketonuric patients just before being started on insulin treatment. Group III: 18 patients during remission after an average of 16.5 months' insulin treatment. Group IV: 13 patients with no remission period or relapse after an average of 19.5 months on insulin treatment. Blood glucose and immunoreactive insulin were measured during fasting and after iv secretin, iv tolbutamide, iv GTT, and oral GTT, followed by combined iv tolbutamide and glucagon stimulation. A considerable insulin secretion could be demonstrated in group I, whereas in group II only a very low insulin peak was obtained after secretin and the combined injection of glucagon and tolbutamide. In group III considerable insulin secretion was demonstrated, whereas in group IV only a very low insulin peak was obtained. A significant correlation between the degree of metabolic control and endogenous insulin secretion was found.     

Prolactin regulates adenylyl cyclase and insulin secretion in rat pancreatic islets

A role for prolactin (PRL) in the regulation of adenylyl cyclase (AC), cyclic AMP (cAMP) formation and insulin secretion was studied in isolated rat pancreatic islets cultured for 4 days at 5.5 mM glucose in the absence (control) or presence of PRL (500 ng/ml). In PRL-treated islets, stimulation by glucose (8 mM), carbamylcholine chloride (CCh) and phorbol dibutyrate increased cAMP levels 40, 89, and 151%, respectively, above similarly stimulated control islets without PRL. Moreover, insulin secretion in PRL-treated islets was more than doubled in response to 8 mM glucose plus glucagon-like peptide 1 compared with control islets. PRL also increased protein kinase C (PKC) activity in cultured islets. When islets were cultured at an insulin secretion desensitizing concentration of glucose (11 mM) for 4 days, there was a decrease in forskolin-stimulated cAMP production. However, the presence of PRL with 11 mM glucose prevented the glucose-induced decrease in cAMP production. Insulin secretion in response to 17 mM glucose was also higher (P<0.02) in islets cultured with 11 mM glucose plus PRL compared with islets cultured with 11 mM glucose alone. Islet AC types -III, -V, and -VI mRNA levels increased relative to 18s rRNA following PRL treatment. In contrast, culture at 11 mM glucose decreased relative AC-III, -V and -VI mRNA levels by as much as 50%. Culture with PRL prevented the decrease in AC expression during islet culture with 11 mM glucose, and the mRNA levels remained similar to control islets cultured at 5.5 mM glucose. Thus, PRL not only increased islet AC expression and activity and insulin secretory responsiveness, but also protected islets from chronic glucose-induced inhibition of these beta-cell activation parameters.     

Indirect evidence for short-loop negative feedback of insulin secretion in the rat

Feedback inhibition of glucose-mediated insulin release has repeatedly been demonstrated in isolated pancreatic islets and in the perfused pancreas. It was the aim of the present study to determine whether inhibition occurs through a long-loop (plasma concentration of insulin) or a short-loop (local concentration) action of insulin. The perfused rat pancreas was used, with different perfusion rates and different insulin concentrations in the medium. Increasing the flow rate from 1 to either 3 or 6 ml/min gradually decreased the insulin concentration in the effluent, at stimulatory concentrations of glucose (11.1 and 16.7 mmol/l). Under the same conditions, however, the integrated amount of insulin released over a period of 30 min was significantly enhanced. When exogenous insulin (2.7 and 5.4 mumol/l) was added to the perfusion medium, insulin secretion in the presence of 11.1 or 16.7 mmol glucose/l at flow rates of 3 and 6 ml/min was diminished. This effect was most prominent with 11.1 mmol glucose/l and 2.7 mumol exogenous insulin/l at all flow rates (except 1 ml/min), as well as at the high perfusion flow rates with other glucose concentrations. Insulin secretion was not affected by 5.4 mumol exogenous insulin/l at 1 ml/min or by 2.7 mumol exogenous insulin/l at 3 ml/min. The data support a negative feedback inhibition of insulin secretion by secreted insulin, since insulin secretion was decreased by either adding exogenous insulin or by lowering endogenous insulin as the consequence of increased flow rates.(ABSTRACT TRUNCATED AT 250 WORDS)