ALPHA(1D)L-TYPE CA2-CHANNEL CURRENTS - INHIBITION BY A BETA-ADRENERGIC AGONIST AND PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE (PACAP) IN RAT PINEALOCYTES()

In this study the subunits of the dihydropyridine-sensitive L-type Ca2 + channels (L-channels) expressed in rat pinealocytes were characteriz ed using reverse transcription (RT)-PCR analysis, and the modulation o f these channels by adrenergic agonists and by pituitary adenylate cyc lase-activating polypeptide (PACAP) was studied using the patch-clamp technique. RT-PCR analysis showed that rat pinealocytes expressed alph a(1D), alpha(2b), beta(2), and beta(4) Ca2+-channel subunit mRNAs. Oth er alpha(1) subunit transcripts were either not expressed or present a t very low levels, indicating that the pinealocytes express predominan tly alpha(1D) L-channels. Electrophysiological studies confirmed that the pineal expressed a single population of L-channels. The L-channel currents were inhibited by two agonists that elevate cyclic AMP: the b eta-adrenergic agonist isoproterenol and PACAP. Similar inhibition was observed with a cyclic AMP analogue, 8-bromo-cyclic AMP. The presence of a cyclic AMP antagonist, Rp-adenosine 3',5'-cyclic monophosphoroth ioate, blocked the inhibition by isoproterenol and PACAP. Norepinephri ne, a mixed alpha- and beta-adrenergic agonist, also inhibited the L-c hannel currents, but the inhibition was smaller. The smaller inhibitio n by norepinephrine was secondary to the simultaneous activation of al pha- and beta-adrenergic receptors. These results indicate that (a) pi nealocytes express predominantly alpha(1D) L-channels, and (b) the bet a-adrenergic agonist isoproterenol and PACAP inhibit the L-channel cur rents through elevation of cyclic AMP. However, an alpha-adrenergic-me diated mechanism also appears to be involved in the effect of norepine phrine on the L-channel currents.