Inhibition of phosphodiesterase 3B in insulin-secreting cells of normal and streptozocin-nicotinamide-induced diabetic rats: implications for insulin secretion.

Cyclic adenosine monophosphate (cAMP) plays important role in the potentiation of insulin secretion in pancreatic B-cells. However, the relevance of cAMP-degrading enzymes in the regulation of insulin secretion is not fully elucidated. The present work was undertaken to determine effects of inhibition of phosphodiesterase 3B (PDE3B) by amrinone on insulin secretion from pancreatic islets and perfused pancreas of normal and mildly diabetic rats. Inhibition of this enzyme was demonstrated to substantially increase insulin-secretory response to 6.7 mM glucose in the isolated islets and perfused pancreas of non-diabetic rats. Increment in glucose-induced insulin secretion resulting from inhibition of PDE3B was accompanied by an increase in islet cAMP levels and was suppressed by inhibition of protein kinase A. It was also demonstrated that insulin secretion stimulated by glucose and 1 µM forskolin was only slightly elevated in the presence of amrinone. Moreover, insulin release induced by succinate instead of glucose was also augmented by inhibition of PDE3B in rat islets. However, exposure of the pancreatic islets of streptozotocin-nicotinamide-induced diabetic rats to amrinone appeared to be without any effect on glucose-induced insulin secretion. Similar lack of response was shown in the perfused pancreas of diabetic rats. These results indicate that inhibition of PDE3B by amrinone significantly augments insulinotropic action of physiological glucose in B-cells of normal rats. This effect is mediated via protein kinase A and may be also induced in the presence of metabolizable stimuli other than glucose. Effects generated by amrinone were demonstrated to be, however, insufficient to enhance glucose-induced insulin secretion in B-cells of streptozotocin-nicotinamide-induced diabetic rats.

Metabolic disturbances and defects in insulin secretion in rats with streptozotocin-nicotinamide-induced diabetes.

Rats with diabetes induced by streptozotocin (STZ) and nicotinamide (NA) are often used in animal studies concerning various aspects of diabetes. In this experimental model, the severity of diabetes is different depending on doses of STZ and NA. Moreover, diabetic changes in rats with STZ-NA-induced diabetes are not fully characterized. In our present study, metabolic changes and insulin secretion were investigated in rats with diabetes induced by administration of 60 mg of STZ and 90 mg of NA per kg body weight. Four to six weeks after diabetes induction, insulin, glucagon and some metabolic parameters were determined to evaluate the severity of diabetes. Moreover, insulin secretory capacity of pancreatic islets isolated from control and diabetic rats was compared. It was demonstrated that administration of 60 mg of STZ and 90 mg of NA per kg body weight induced relatively mild diabetes, since insulin, glucagon and other analyzed parameters were only slightly affected in diabetic rats compared with control animals. In vitro studies revealed that insulin secretory response was preserved in pancreatic islets of diabetic rats, however, was lower than in islets of control animals. This effect was observed in the presence of different stimuli. Insulin secretion induced by 6.7 and 16.7 mmol/l glucose was moderately reduced in islets of diabetic rats compared with control islets. In the presence of leucine with glutamine, insulin secretion appeared to be also decreased in islets of rats with STZ-NA-induced diabetes. Insulinotropic action of 6.7 mmol/l glucose with forskolin was also deteriorated in diabetic islets. Moreover, it was demonstrated that at a non-stimulatory glucose, pharmacological depolarization of plasma membrane with a concomitant activation of protein kinase C evoked significant rise in insulin release in islets of control and diabetic rats. However, in diabetic islets, this effect was attenuated. These results indicate that impairment in insulin secretion in pancreatic islets of rats with mild diabetes induced by STZ and NA results from both metabolic and nonmetabolic disturbances in these islets.

Effects of adenosine A(1) receptor antagonism on insulin secretion from rat pancreatic islets.

Adenosine is known to influence different kinds of cells, including beta-cells of the pancreas. However, the role of this nucleoside in the regulation of insulin secretion is not fully elucidated. In the present study, the effects of adenosine A(1) receptor antagonism on insulin secretion from isolated rat pancreatic islets were tested using DPCPX, a selective adenosine A(1) receptor antagonist. It was demonstrated that pancreatic islets stimulated with 6.7 and 16.7 mM glucose and exposed to DPCPX released significantly more insulin compared with islets incubated with glucose alone. The insulin-secretory response to glucose and low forskolin appeared to be substantially potentiated by DPCPX, but DPCPX was ineffective in the presence of glucose and high forskolin. Moreover, DPCPX failed to change insulin secretion stimulated by the combination of glucose and dibutyryl-cAMP, a non-hydrolysable cAMP analogue. Studies on pancreatic islets also revealed that the potentiating effect of DPCPX on glucose-induced insulin secretion was attenuated by H-89, a selective inhibitor of protein kinase A. It was also demonstrated that formazan formation, reflecting metabolic activity of cells, was enhanced in islets exposed to DPCPX. Moreover, DPCPX was found to increase islet cAMP content, whereas ATP was not significantly changed. These results indicate that adenosine A(1) receptor blockade in rat pancreatic islets potentiates insulin secretion induced by both physiological and supraphysiological glucose concentrations. This effect is proposed to be due to increased metabolic activity of cells and increased cAMP content.