Inhibition of voltage-gated Ca²⁺ current by PACAP in rat adrenal chromaffin cells

It is well established that pituitary adenylate cyclase-activating polypeptide (PACAP) can stimulate catecholamine biosynthesis and secretion in adrenal chromaffin cells. Recent studies from this laboratory demonstrated that PACAP pretreatment inhibits nicotine (NIC)-induced intracellular Ca²⁺ transients and catecholamine secretion in porcine adrenal chromaffin cells. Mechanistically, this effect is mediated by protein kinase C (PKC), and based on indirect evidence, is thought to primarily target voltage-gated Ca²⁺ channels. The present study used whole-cell patch-clamp analysis to test this possibility more directly in rat chromaffin cells. Consistent with the porcine data, pretreatment with PACAP or with phorbol ester [phorbol myristate acetate (PMA)] significantly suppressed NIC-induced intracellular Ca²⁺ transients and catecholamine secretion in rat chromaffin cells. Exposure to PACAP and PMA significantly reduced peak Ca²⁺ current in rat cells. The effects of both PACAP and PMA on Ca²⁺ current could be blocked by treating cells with the PKC inhibitor staurosporine. Exposure to selective channel blockers demonstrated that rat chromaffin cells contain L-, N- and P/Q-type Ca²⁺ channels. PACAP pretreatment significantly reduced Ca²⁺ current gated through all three channel subtypes. These data suggest that PACAP can negatively modulate NIC-induced catecholamine secretion in both porcine and rat adrenal chromaffin cells.