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 to the regulation of Ca2-influx are involved in tolbutamide-induced stimulation and adrenaline- and somatostatin- induced inhibition of insulin secretion 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 depolarising pulses to 0 my. These pulses induced an increase in cellular capacitance (Cm) caused by fusion of secretory granules with the plasma membrane. Tolbutamide did not alter, neither Cm under voltage clamp at -70 mV nor increases of Cm due to voltage pulses. The inhibitors of secretion, adrenaline 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 somatostatin inhibited the increase of Cm caused by depolarizing voltage pulses. The adrenaline effect on Cm was dependent on the addition of GTP to the pipette solution. When GTP was replaced by GDPbetaS or GTPgammaS, the effect of adrenaline on Cm was abolished. The blockade of calcineurin, by the addition of calcineurin inhibitory peptide (CIP) to the pipette solution, did not affect the adrenaline-induced inhibition of Cm. Moreover. After incubation of the cells with deltamethrin, a calcineurin inhibitor, the stimulation of secretion was attenuated, but the adrenaline-induced inhibition was not affected. Our results suggest that adrenaline-induced inhibition of insulin secretion involves a site of action directly related to the exocytotic membrane fusion. In contrast, the stimulator tolbutamide and the inhibitor somatostatin had no direct effect on exocytosis in INS-1 cells.