Effects of adrenaline and tolbutamide on insulin secretion in INS-1 cells under voltage control

The aim of the present study was to investigate whether mechanisms distal t o the regulation of Ca2+-influx are involved in tolbutamide-induced stimula tion and adrenaline- and somatostatin -induced inhibition of insulin secret ion in INS-1 cells. Using the patch clamp method. the membrane voltage was either kept constant at -70 mV, or Ca2+-influx was activated by short depol arising pulses to 0 mV. These pulses induced an increase in cellular capaci tance (C-m) caused by fusion of secretory granules with the plasma membrane . Tolbutamide did not alter, neither C-m under voltage clamp at -70 mV nor increases of C-m due to voltage pulses. The inhibitors of secretion, adrena line and somatostatin, counteracted the augmentation of [Ca2+](i) which was induced by glucose, tolbutamide and forskolin. In the voltage clamp mode, however, where no changes of [Ca2+](i) were observed, adrenaline but not so matostatin inhibited the increase of C-m caused by depolarizing voltage pul ses. The adrenaline effect on C-m was dependent on the addition of GTP to t he pipette solution. When GTP was replaced by GDP beta S or GTP gamma S, th e effect of adrenaline on C-m was abolished. The blockade of calcineurin, b y the addition of calcineurin inhibitory peptide (CIP) to the pipette solut ion, did not affect the adrenaline-induced inhibition of C-m. Moreover, aft er incubation of the cells with deltamethrin, a calcineurin inhibitor, the stimulation of secretion was attenuated, but the adrenaline-induced inhibit ion was not affected. Our results suggest that adrenaline-induced inhibitio n of insulin secretion involves a site of action directly related to the ex ocytotic membrane fusion. In contrast, the stimulator tolbutamide and the i nhibitor somatostatin had no direct effect on exocytosis in INS-1 cells. Co pyright (C) 2000 S. Karger AG, Basel.