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The effect of serotonin on insulin secretion and biosynthesis was studied using isolated islets of mice. Serotonin produced a small stimulatory effect on insulin secretion when glucose was present in the incubation medium at a low concentration. On the other hand, an inhibition of insulin secretion was obtained with serotonin when glucose in the medium reached 3.0 mg/ml concentration. No significant effect of serotonin was obtained on insulin biosynthesis, neither in the presence of low nor with a high glucose concentration. These results suggest that the effect of this monoamine on insulin secretion is not mediated via its effect on insulin biosynthesis.

The effect of glucose and glucose plus glucagon on the incorporation of H3-L-leucine into proinsulin and insulin was examined in isolated islets of twenty-one-day old fetal and five- and ten-day old newborn rats. Maximal stimulation of (pro-) insulin biosynthesis was achieved with 100 mg. per cent of glucose in isolated islets of twenty-one-day old fetal rats. No additional effect was observed with 300 mg. per cent of glucose. On the other hand, in islets of five- and ten-day old newborn rats the incorporation of H3-L-leucine into proinsulin and insulin was gradually augmented by glucose up to concentrations of 300 mg. per cent. Addition of glucagon to the various glucose concentrations only enhanced the synthesis of insulin in ten-day old newborn islets, whereas it had no effect on the islets of the younger age groups. The results show a different pattern of insulin biosyntheses in fetal and newborn islets, which may be related to the varied plasma glucose concentrations of the perinatal period.

Long term feeding of a sucrose rich diet to rats is accompanied by a decreased glucose assimilation rate, despite high plasma insulin levels. Hyperinsulinism is at least partially based on a relative obesity, with increased amounts of abdominal- and retroperitoneal fat tissue, but unchanged total body weight compared to starch fed controls. The secretory pattern of insulin release was studied following glucose, arginine, fructose and sulfonylurea administration in the isolated perfused pancreas of sucrose and isocaloric starch fed rats. In addition, isolated islets of Langerhans were used to demonstrate the effects of glucose on insulin secretion and the incorporation of H-3 leucine into the proinsulin and insulin fraction of islet proteins. Following 11 mM glucose, the dynamics of insulin release in the isolated perfused pancreas of sucrose fed rats is characterized by a markedly elevated, late plateau-like response, usually seen only at higher glucose concentrations. Hyperinsulinism, as compared to starch fed controls, can also be demonstrated following arginine and the sulfonylurea HB-419, whereas fructose has no effect in the presence of low glucose concentrations. During incubation of the pancreatic islets, the hyperinsulinism in sucrose-, compared to starch fed rats, is more pronounced at 11 mM glucose than at 5.5 mM glucose. The incorporation of H-3 leucine into the proinsulin-insulin fraction of islet proteins in sucrose compared to starch fed rats, however, is significantly greater with glucose 5.5 mM than at high glucose level. In sucrose fed rats, secretion and biosynthesis of insulin thus appear to be elevated but closely linked only at physiological glucose concentration.

The influence of arginine, lysine, tolbutamide and glibenclamide on (pro-) insulin biosynthesis and release of newly synthesized (pro-) insulin was studied in isolated islets of rat pancreas. Islets were incubated with 3H-leucine and glucose in the presence and absence of the test agents. Proinsulin and insulin of islets and incubation media were separated by gel filtration on Sephadex G 50. Estimations were carried out for radioactivity and immunoreactivity for insulin. All four substances were able to enhance insulin release whereas no stimulation of leucine incorporation into (pro-) insulin was found. Arginine and tolbutamide even markedly reduced (pro-) insulin aynthesis. Conversion of proinsulin to insulin was not affected by any of the test agents. For studying the influence of the 4 substances on secretion of newly synthesized (pro-) insulin two experimental models were used: 1) Labelling of the islets in the presence of the test agents, followed by uniform stimulation with glucose alone in the presence or absence of Ca++. 2) Addition of the 4 test substances after uniform prelabelling of the islets. 1) Presence of arginine and sulfonylureas during labelling resulted in a significantly enhanced relative fractional release of newly synthesized (pro-) insulin, although the bulk of secreted hormone appeared to stem from the storage pool also under these conditions. The enhanced fractional release was persistent also during the postlabelling period when the islets had been labelled in the presence of arginine or glibenclamide. On the other hand, a decreased release of newly synthesized (pro-) insulin was obsevered during the postlabelling period in islets labelled in the presence of tolbutamide. Lysine was without significant effects in both periods. Omission of calcium ions during the postlabelling period inhibited the release of both immunoreactive and radioactive hormone. 2) When amino acids or sulphonylureas were added after prelabelling no significant changes were found in the specific radioactivity of released (pro-) insulin or in the fractional release of newly synthesized hormone. Enhanced release of fresh granules from the beta cell might explain the increased fractional release of newly synthesized (pro-)insulin when lebelling is carried out in the presence of arginine and sulphonylureas, especially glibenclamide.

Based on the clinical observation that biguanide treatment of obese patients may alter insulin levels, the influence of metformin and phenformin on basal and glucose stimulated insulin secretion, as well as on insulin biosynthesis, was studied in isolated islets of rats. — Biguanide concentrations of 100 μg/ml, or higher, significantly reduced glucose stimulated insulin secretion. Both dose dependence and a difference in the intrinsic activities of metformin and phenformin were demonstrated. Incubating the same islets for a second period without biguanides, glucose stimulated insulin secretion was still decreased. Addition of glibenclamide during this second period increased insulin secretion, but did not overcome complete inhibition achieved after incubation at very high biguanide concentrations. Glucose stimulated biosynthesis of proinsulin and insulin was decreased in the presence of biguanides and completely suppressed at very high concentrations. Inhibition of cell respiration in the islet cells effected by high biguanide doses may be the reason for the inhibition of secretion and biosynthesis of insulin. — On the other hand, an insulin release was found at the highest phenformin concentration of 10 mg/ ml and during perfusion of the isolated rat pancreas with higher biguanide doses. — Biguanide concentrations found to be effective in this study are very high compared with therapeutic levels. Moreover, biguanide actions are known to be highly dependent on species, concentration and metabolic situation. — Definite conclusions from these findings regarding clinical significance, therefore, seem unwarranted.

Secretion and biosynthesis of (pro-) insulin were studied in isolated pancreatic mouse islets in the presence of secretin, pancreozymincholecystokinin, gastrin and glucagon, at 0, 100 and 300 mg⁄100 ml glucose. Incorporation of aHleucine into the proinsulin and insulin fraction obtained after gel nitration of the islet proteins on a Sephadex G 50 fine column was taken as parameter of insulin synthesis. Specific radioactivity of free intracellular leucine pool in the isolated islets was determined under different conditions. In the absence of glucose, pancreozymin was the only substance to stimulate insulin secretion.When glucose was present in the medium, glucagon was able to potentiate insulin output. Secretin and gastrin did not change insulin release. Insulin synthesis, found to be stimulated by glucose like insulin secretion, was unaffected by all 3 gastrointestinal hormones whether or not glucose was present. (Pancreatic) glucagon, on the other hand, potentiated glucose-induced insulin synthesis. Spe...

Incorporation of H-3-leucine into proinsulin and insulin and insulin secretion were determined in isolated pancreatic islets of mice. Batches of forty islets were incubated for three hours at 0, 100 or 300 mg. per cent glucose alone or in the presence of glucagon, theophylline or cyclic AMP. Proinsulin and insulin were separated by gel filtration on columns of Sephadex C50fine in 1 M. acetic acid. In the absence of glucose, no stimulation of insulin biosynthesis as judged from the incorporation of H-3-leucine was observed with glucagon, cyclic AMP or dibutyryl cyclic AMP; insulin, secretion was not significantly stimulated. At 100 and 300 mg. per cent glucose, both leucine incorporation and insulin secretion were potentiated by the substances tested; the effect of glucagon was pronounced mainly at high concentrations of glucose. However, leucine incorporation and insulin secretion were not stimulated to the same extent in all cases. Biosynthesis of insulin thus appears to be not always directly linked to insulin secretion. It is suggested that glucagon and the adenylcyclase system, besides regulating insulin secretion, are involved in insulin biosynthesis.

Hypophysectomized rats were substituted with varying doses of human or porcine growth hormone (GH) as well as with ACTH for 6 or 12 days. Hypophysectomy was performed in animals of 80 or 170 g body weight either 12 or 4 weeks prior to the onset of the therapy. Increase in weight and the width of the epiphyseal cartilage were determined, insulin secretion and biosynthesis of proinsulin and insulin, were investigated in isolated pancreatic islets of the animals. — No differences were found between the effects of human and poreine GH preparations. Weight gain was similar in rats which had been hypophysectomized at a weight of 80 g either 12 or 4 weeks prior to the substitution. Secretion and biosynthesis of insulin which were both found to be reduced in isolated islets of untreated, hypophysectomized rats, were improved or normalized after substitution with GH (1 mg/kg/day) for 12 days. On the other hand, a therapy with GH for 6 days, even in tenfold daily dose (10 mg/kg), was ineffective in all rats which had been hypophysectomized at a weight of 80 g. Normalisation of lowered levels of blood sugar was the only positive effect observed after an administration of ACTH for 6 or 12 days. — It appears from our findings that, in contrast to the administration of ACTH, GH given to hypophysectomized rats for a longer period in relatively low doses may normalize both reduced secretion and biosynthesis of insulin.

In adult rat islets of Langerhans glucose is an important stimulus for insulin biosynthesis and release. In contrast fetal islets secrete only small amounts of insulin when challenged acutely with glucose while the biosynthesis has not been evaluated. Therefore the incorporation of H3-leucine into proinsulin and insulin was studied in 21-day old fetal, 5 day old and 10 day old newborn rat islets. In fetal islets the incorporation of H3-leucine into insulin was enhanced by 50 mg% and 100 mg%, while 300 mg% or the addition of glucagon was without effect. The biosynthesis of insulin in 5 day old newborn islets was augmented by all glucose concentrations tested, again glucagon had no effect, 10 day old newborn islets reacted similar to adult islets: glucose stimulated the incorporation of H3-leucine into the proinsulin and insulin peak from 50 mg to 300 mg% and glucagon further enhanced this effect at the high glucose concentration. The results suggest separate mechanisms for insulin biosynthesis and release.