PATCH-CLAMP STUDY ON MECHANISM OF ADENOSINE-INDUCED INHIBITORY EFFECTS IN FROG PITUITARY MELANOTROPHS

Our laboratory demonstrated that adenosine inhibits the activation of adenylyl cyclase and the secretion of the alpha-melanocyte-stimulating hormone (alpha-MSH) from the intermediate robe of the frog pituitary. This paper showed the bioelectric effects induced by adenosine, the i onic conductances modulated by adenosine, and the possible involvement of intracellular messengers, indicated the mechanism by which adenosi ne controls the secretion of alpha-MSH. The results show that adenosin e acting on A(1) adenosine receptor subtype reduced the Ca2+ influx ne cessary for the secretion, through 4 distinct mechanisms: 1) a hyperpo larization resulting from the activation of a voltage-insensitive K+ c onductance, 2) a reduction of the duration of spontaneous action poten tials due to an increase of the outward delayed rectifyer K+ current ( I-K), 3) a diminution of the cellular excitability by an activation of the transient outward K+ current (I-A), and 4) an inhibition of the L - and N-type Ca2+ currents, with a predominant action on the N-type co mponent. Cell dialysis with GTP gamma S rendered irreversible the effe cts of adenosine on the K+ conductances and Ca2+ channels, whereas PTX pretreatment totally abolished the response to adenosine, suggesting all bioelectric effects of adenosine were mediated by pertussis toxin- sensitive G proteins. Whether the implicated G proteins regulate the K + and Ca2+ channels by tight-coupling or via a second-messenger system remains to be solved. With our results, the involvement of adenylyl c yclase can be excluded because addition of cAMP and IBMX, an inhibitor of phosphodiesterases, in the intracellular solution, or application of dibutyryl cAMP in the extracellular solution did not modify the ade nosine-induced responses.