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.
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D003035:Cobalt / Q000494:pharmacology; D005576:Colforsin; D000242:Cyclic AMP / Q000502:physiology*; D004224:Diterpenes / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D005260:Female; D005711:Gallopamil / Q000494:pharmacology; D005947:Glucose / Q000378:metabolism; D007328:Insulin / Q000378:metabolism; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D012413:Rubidium / Q000378:metabolism*
D002121:Calcium Channel Blockers; D004224:Diterpenes; D007328:Insulin; D005576:Colforsin; D005711:Gallopamil; D003035:Cobalt; D000242:Cyclic AMP; D005947:Glucose; D012413:Rubidium; D002118:Calcium