H Schatz

(Pro‐)Insulin Biosynthesis and Release of Newly Synthesized (Pro‐)Insulin from Isolated Islets of Rat Pancreas in the Presence of Amino Acids and Sulphonylureas+,++

Abstract. 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 test 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 synthesis. 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 observed 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 signifcant 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 labelling is carried out in the presence of arginine and sulphonylureas, especially glibenclamide.

Insulin secretion and biosynthesis in sucrose fed rats

SummaryLong 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.

Insulin biosynthesis in isolated pancreatic islets of fetal and newborn rats.

The effect of glucose and glucose plus glucagon on the incorporation of H-3-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 H-3-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.