Distinct mechanisms for two amplification systems of insulin release

Abstract

The mechanisms whereby activation of the cyclic AMP-dependent protein kinase A or the Ca2+-phospholipid-dependent protein kinase C amplifies insulin release were studied with mouse islets. Forskolin and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) were used to stimulate adenylate cyclase and protein kinase C respectively. The sulphonylurea tolbutamide was used to initiate insulin release in the presence of 3 mM-glucose. Tolbutamide alone inhibited 86Rb+ efflux, depolarized ,-cell membrane, triggered electrical activity, accelerated 45Ca2+ influx and efflux and stimulated insulin release. Forskolin alone only slightly inhibited 86Rb+ efflux, but markedly increased the effects of tolbutamide on electrical activity, 45Ca2+ influx and effilux, and insulin release. In the absence of Ca2+, only the inhibition of 86Rb+ efflux persisted. TPA (100 nM) alone slightly accelerated 45Ca2+ efflux and insulin release without affecting 45Ca2+ influx or fl-cell membrane potential. It increased the effects of tolbutamide on 45Ca2+ efflux and insulin release without changing 86Rb+ efflux, 45Ca2+ influx or electrical activity. Omission of extracellular Ca2+ suppressed all effects due to the combination of TPA and tolbutamide, but not those of TPA alone. Though ineffective alone, 10 nM-TPA amplified the releasing action of tolbutamide without affecting its ionic and electrical effects. In conclusion, the two amplification systems of insulin release involve at least partially distinct mechanisms. The cyclic AMP but not the protein kinase C system increases the initiating signal (Ca2+ influx) triggered by the primary secretagogue.

Comparative effects of amino acids and glucose on insulin secretion from isolated rat or mouse islets

“…Carbachol activates PLC, increases IP accumulation but has little, if any, sustained effect on release in the presence of substimulatory glucose (Best & Malaisse 1983, 1984, Henquin et al 1988, Zawalich et al 1989b, Gilon & Henquin 2001. Forskolin activates adenylate cyclase (Seamon & Daly 1981) and potentiates insulin secretion in response to a large number of compounds (Wiedenkeller & Sharp 1983, Henquin et al 1987. Like carbachol, however, it is impotent at low glucose concentrations (Wiedenkeller & Sharp 1983).…”

“…Information flow in both systems is activated by glucose and they appear integral for the maintenance of islet responsiveness (Henquin et al 1987). The studies with carbachol were based on our hypothesis that the activation of PLC provides those signals that play an important role in the regulation of several -cell response patterns, including a rising second phase of insulin release and the induction of TDP, in rat islets.…”

“…Because both the PLC/PKC and the PKA/cAMP second messenger system serve to amplify nutrientinduced secretion (Henquin et al 1987, we probed the effect of elevating cAMP levels using the diterpene forskolin. This pharmacological tool is a direct activator of adenylate cyclase that in the absence of added nutrient has little, if any, independent stimulatory effect (Wiedenkeller & Sharp 1983, Yamada et al 2002.…”

Effects of Glucose, Exogenous Insulin, and Carbachol on C-peptide and Insulin Secretion from Isolated Perifused Rat Islets

“…Insulin secretion from the pancreatic ␤-cell is tightly regulated by stimulatory signals generated during the intracellular metabolism of glucose and by neurohumoral agonists operative at the cell membrane (1)(2)(3)(4)(5). Most recently an additional layer of complexity has been added by reports suggesting that insulin exerts an autocrine stimulatory effect on insulin secretion from the ␤-cell (6,7).…”

“…This concept was based primarily on amperometric measurements using ␤-cells preincubated in 5-hydroxytryptamine (5HT). 1 Because 5HT exposure exerts inhibitory effects on insulin release (8 -10), the precise physiologic significance of findings made with 5HT-preloaded ␤-cells is unclear. Moreover, previous studies exploring the potential role of insulin on stimulated secretion showed no effect (11) or supported the concept that insulin exerts a negative, not positive, feedback on its own secretion (12)(13)(14)(15).…”