ADENOSINE INHIBITS L-TYPE AND N-TYPE CALCIUM CHANNELS IN PITUITARY MELANOTROPHS - EVIDENCE FOR THE INVOLVEMENT OF A G-PROTEIN IN CALCIUM-CHANNEL GATING

It has been previously demonstrated that activation of A(1) adenosine receptors in frog melanotrophs causes inhibition of spontaneous action potential discharges and alpha-melanocyte-stimulating hormone secreti on. In the present study, we have investigated the effect of adenosine on high-voltage-activated (HVA) calcium currents In cultured melanotr ophs, using the whole-cell variant of the patch-clamp technique with b arium as a charge carrier. Adenosine and the specific A(1) adenosine r eceptor agonist R-PIA (50 mu M each) produced a decrease of the amplit ude of the barium current, while the selective A(2) adenosine receptor agonist CGS 21680 did not affect the current. The inhibitory effect o f R-PIA was observed throughout the activation range of the current, w ith stronger responses at more positive potentials. R-PIA inhibited bo th the L- and N-type components of the current, the effect on the N-co mponent being two-fold higher than on the L-component, The inhibitory effect of R-PIA was rendered irreversible by addition of GTP gamma S ( 100 mu M) to the intracellular solution. Pre-treatment of the cells wi th pertussis toxin (1 mu g/ml; 12 h), totally abolished the effect of R-PIA on the HVA calcium channels. Conversely, addition of a high conc entration of cAMP (100 mu M) together with the phosphodiesterase inhib itor IBMX (100 mu M) to the intracellular solution did not modify the effect of R-PIA on the current. It is concluded that, in frog melanotr ophs, adenosine induces inhibition of L- and N-calcium currents and th at this effect is mediated by a pertussis toxin-sensitive G protein. O ur data also indicate that the inhibitory effect of adenosine on the c alcium currents is not mediated by inhibition of adenylyl cyclase.