Hatsumi Niki

Insulin release independent of a rise in cytosolic free Ca2+ by forskolin and phorbol ester

The role of cytosolic free Ca2+ in insulin release was evaluated using isolated rat pancreatic islets permeabilized with digitonin and incubated in Ca‐EGTA buffers to fix free Ca2+ concentration at arbitrary levels. Ca2+ induced insulin release in a concentration‐dependent manner with the threshold being between 0.1 and 1 μM. The hormone release was increased by forskolin and 12‐O‐tetradecanoyl phorbol‐13‐acetate (TPA), a potent activator of adenylate cyclase and that of protein kinase C, respectively. The findings suggest that activation of both protein kinase A and protein kinase C modulate insulin release without a concomitant increase in cytosolic free Ca2+.

Insulin Secretion by Anomers of D-Glucose

Isolated rat islets were incubated for 5 minutes in the media containing either the α or β anomer of D-glucose (2 milligrams per milliliter). The amounts of secreted insulin and changes of anomers ratio were concomitantly determined. In spite of rapid mutarotation, significantly greater stimulation of insulin secretion was observed by α-D-glucose as compared with β-D-glucose.

Interaction of Alloxan and Anomers of D-Glucose on Glucose-induced Insulin Secretion and Biosynthesis in Vitro

The direct effects of alloxan on glucose-induced insulin secretion and biosynthesis and the interaction of alloxan and D-glucose anomers were studied in vitro by use of isolated islets from rat pancreas. Islets were pretreated by incubation for five minutes in media containing alloxan (0.2 mg./ml.) alone or alloxan with either the α or β anomer of D-glucose (3 mg./ml.). After washing, batches of five islets were incubated in the medium supplemented with glucose (1.8 mg./ml.) for 60 minutes to observe insulin secretion and for 90 minutes to observe insulin biosynthesis. Prior exposure to alloxan alone produced marked inhibition of subsequent glucose-induced insulin secretion and biosynthesis. A significantly greater protection against these inhibitory effects of alloxan was observed by using the α anomer of D-glucose than the β anomer. The anomeric preference of D-glucose for protecting islet cells from the inhibitory effect of alloxan on glucose-induced insulin secretion and biosynthesis was similar to that for triggering insulin secretion. Possible mechanisms of the inhibitory effect of alloxan and the protective effect of D-glucose anomers in connection with those of other sugars are discussed. It is suggested that a glucoreceptor, stereospecific to the α anomer of D-glucose, may exist for both insulin secretion and biosynthesis.

Insulin release by glucose anomers in a rat model of non-insulin-dependent diabetes

SummaryThe effects of the α and β anomers of D-glucose on insulin release were studied in a rat model of non-insulin-dependent diabetes, which was induced by streptozotocin injection at 2 days of age. Glucose tolerance of the streptozotocin-treated rats at 8–10 weeks of age was mildly diabetic. Insulin release from the isolated perfused pancreas of the diabetic rats in response to 10 mmol/l α-D-glucose was markedly impaired, while insulin response to 10 mmol/l β-D-glucose in the diabetic pancreas was only slightly reduced as compared to that in the control pancreas.

Induction of insulin biosynthesis by anomers of D-glucose

D-glucose is considered to be the most important stimulus for both the releases and the biosynthesis of insulin [l] . We have previously reported that the (Yanomer of D-glucose was more effective than the /I-anomer in triggering insulin release from isolated rat pancreatic islets [2]. This phenomenon was also observed in other experimental systems in vitro, as well as in vivo including man (for a review, see ref. [ 11). Similar anomeric preference was reported in vivo in preventing the diabetogenic effects of alloxan by D-glucose [3,4]. In in-vitro experiments using isolated islets, we have further observed the preference for the cr-anomer of D-glucose in protecting pancreatic B cells from the inhibitory effects of alloxan on both glucoseinduced insulin release and biosynthesis, and suggested that the cr-anomer of D-glucose may also be more effective than the fi-anomer in inducing insulin biosynthesis [5]. However, it is difficult to directly compare between the effects of the (Yand /I-anomers on insulin biosynthesis with a conventional method using batch incubated isolated islets, since the incorporation of the labeled amino acid into (pro)insulin does not occur in a measurable amount within a few minutes, whereas the anomers mutarotate rapidly. In the present study, free cells were prepared from pancreatic islets to expose the B cells directly to glucose, thus avoiding a lag time of its unphysiological diffusion into the islet tissue, and the effects of the two

Insulin release by D-glucose anomers in a non-insulin-dependent diabetes rat model.

The alpha anomer of D-glucose is more potent than the beta anomer in stimulating insulin release. We have studied the effects of D-glucose anomers on insulin release from the perfused pancreas isolated from a rat model of non-insulin-dependent diabetes (NIDD) induced by streptozotocin injection at 2 days of age. Insulin release from the pancreas of the diabetic rat in response to 10 mM alpha-D-glucose was markedly impaired, while insulin response to the same concentration of beta-D-glucose was only slightly reduced as compared to that in the control pancreas. Thus, the pancreatic B cell of the diabetic rat did not discriminate between the alpha and beta anomers of D-glucose. Insulin release induced by 5 mM D-glyceraldehyde was decreased in the diabetic pancreas, while insulin release induced by 0.15 mg/ml tolbutamide did not differ from that in the control pancreas. Glucose oxidation in the islets isolated from the diabetic pancreas was not lower than that in comparable control islets. Treatment of the diabetic pancreas with 5 microM forskolin or 0.15 mg/ml tolbutamide did not restore its defective discrimination between the two anomers, although forskolin potentiated insulin release more markedly in the diabetic pancreas. The findings may provide some insight into the pathophysiology of the pancreatic B cell in NIDD.