The ionic, electrical, and secretory effects of endogenous cyclic adenosine monophosphate in mouse pancreatic B cells: studies with forskolin.

Forskolin, an activator of adenylate cyclase, was used to study the effects of endogenous cAMP on the stimulus-secretion coupling in mouse pancreatic B cells. Forskolin produced a rapid, dose-dependent (0.05-50 microM) increase in islet cAMP, which was not influenced by the prevailing concentration of glucose and did not require extracellular Ca2+. At a nonstimulatory concentration of glucose (3 mM), a high concentration of forskolin (20 microM) barely doubled basal insulin release, marginally decreased 86Rb+ efflux from the islets, was without effect on the B cell membrane potential, and did not affect 45Ca2+ uptake (5 min). High concentrations of endogenously formed cAMP thus failed to mimic these early steps of the B cell response to glucose and other stimuli. At a threshold concentration of the sugar (7 mM), 5 microM forskolin slightly depolarized the B cell membrane, induced electrical activity (slow waves and spikes), stimulated 45Ca2+ uptake, and triggered insulin release. At a stimulatory concentration of glucose (10 mM), forskolin potentiated insulin release; half maximal and maximal effects were observed at 1 and 20 microM, respectively. Forskolin also increased the rate of 45Ca2+ and 86Rb+ efflux from islet cells, augmented 45Ca2+ uptake, and potentiated the electrical activity triggered by glucose in B cells. Its effects on insulin release, 45Ca2+ fluxes, and electrical activity were inhibited by omission of extracellular Ca2+ and/or Ca channels blockers. At a high concentration of glucose (25 mM), forskolin augmented the amplitude and the duration of the spikes occurring in the depolarized B cell membrane, slightly increased 45Ca2+ uptake, and potentiated insulin release. At threshold or stimulatory concentrations of glucose, endogenously formed cAMP can thus induce or enhance the Ca2+-dependent events normally triggered by the sugar. It is suggested that, besides its action on intracellular Ca stores, cAMP facilitates Ca2+ influx in B cells, by modulating the gating properties of the Ca channels.