Acute and chronic effects of glucose and carbachol on insulin secretion and phospholipase C activation: studies with diazoxide and atropine

Abstract

The acute and chronic effects of 20 mM glucose and 10 microM carbachol on beta-cell responses were investigated. Acute exposure of rat islets to 20 mM glucose increased glucose usage rates and resulted in a large insulin-secretory response during a dynamic perifusion. The secretory, but not the metabolic, effect of 20 mM glucose was abolished by simultaneous exposure to 100 microM diazoxide. Glucose (20 mM) significantly increased inositol phosphate (IP) accumulation, an index of phospholipase C (PLC) activation, from [(3)H]inositol-prelabeled islets. Diazoxide, but not atropine, abolished this effect as well. Unlike 20 mM glucose, 10 microM carbachol (in the presence of 5 mM glucose) increased IP accumulation but had no effect on insulin secretion or glucose (5 mM) metabolism. The IP effect was abolished by 50 microM atropine but not by diazoxide. Chronic 3-h exposure of islets to 20 mM glucose or 10 microM carbachol profoundly reduced both the insulin-secretory and PLC responses to a subsequent 20 mM glucose stimulus. The adverse effects of chronic glucose exposure were abolished by diazoxide but not by atropine. In contrast, the adverse effects of carbachol were abolished by atropine but not by diazoxide. Prior 3 h of exposure to 20 mM glucose or carbachol had no inhibitory effect on glucose metabolism. Significant secretory responses could be evoked from 20 mM glucose- or carbachol-pretreated islets by the inclusion of forskolin. These findings support the concept that an early event in the evolution of beta-cell desensitization is the impaired activation of islet PLC.

Contribution of Calcium Influx in Mediating Glucose-Stimulated Oxygen Consumption in Pancreatic Islets

“…However, we found that nimodipine did not inhibit protein synthesis, which is consistent with studies where it has been observed that calcium has little effect on insulin biosynthesis rates (48). An intriguing possibility that phospholipase C activation could be involved will be considered in future studies, based on data showing that blocking calcium influx also blocks phospholipase C activation by glucose (49). Implications for glucose-stimulated ISR.…”

Divergent effects of epinephrine and prostaglandin E2 on glucose-induced insulin secretion from perifused rat islets

“…We have reported recently [1], in agreement with other studies [15][16][17], that sustained increases in the glucose level bathing the beta cell induces a defect in the subsequent insulin secretory response. Although our studies with diazoxide and carbachol suggest the proximal involvement of PLC in the secretory lesion, we decided to assess whether epinephrine altered the induction of desensitization.…”

“…Although most experimental attention has been placed on those compounds such as glucose, glucagon-like peptide-1, and cholinergic agonists that positively impact on the secretory process, dramatic effects can also be induced by a number of inhibitors as well. We reported recently [1], in agreement with prior studies [2], that diazoxide, by maintaining the patency of the adenosine triphosphatesensitive potassium channel, not only abolished the shortterm impact of glucose on insulin secretion but also protected the beta cell from the adverse impact of chronic sustained hyperglycemia. We attributed this sparing effect of diazoxide to its ability to impair the activation of beta cell phospholipase C (PLC).…”

Physiologic Implications of Phosphoinositides and Phospholipase C in the Regulation of Insulin Secretion

“…A series of experiments was conducted with the hypothesis that the impaired activation of PLC occurred simultaneously with the evolution of impaired secretion. This has been verified (36,49,64). Interestingly, with sustained 3-h exposure to a physiological glucose level (7 mM), both insulin secretion and PLC activation were impaired (65).…”