Modulation of stimulus-secretion coupling in porcine adrenal chromaffin cells by receptor-mediated increases in protein kinase C activity

Abstract

Catecholamine (CAT) secretion by adrenal chromaffin cells is primarily triggered by nicotinic receptor-dependent increases in cytosolic Ca(2+). The principal aim of the present study was to determine whether pituitary adenylate cyclase activating peptide (PACAP), which is coreleased with acetylcholine from the splanchnic nerve, can modulate nicotinic receptor-dependent Ca(2+) signaling and catecholamine secretion in porcine adrenal medullary chromaffin (PAMC) cells. Activation of protein kinase C (PKC) with phorbol myristate acetate (PMA) dose- and time-dependently inhibited nicotine (NIC)-induced Ca(2+) transients. At 100 nM PMA, peak Ca(2+) levels were reduced by 27% +/- 2% (P < 0.05) and 41% +/- 3% (P < 0. 05) after 10 and 20 min exposure, respectively. The inhibitory effects of PMA were significantly reduced by preincubation with the PKC inhibitor staurosporine. KCl-induced Ca(2+) transients were also reduced by 20 min PMA treatment (Delta -27% +/- 4%; P < 0.05), suggesting that PKC affects voltage-gated Ca(2+) channel activity. Pretreatment with PACAP also resulted in both time- and concentration-dependent suppression of Ca(2+) transients. After 20 min exposure to 1 microM PACAP, NIC- and KCl-induced transients were reduced by 36% +/- 5% (P < 0.05) and 51% +/- 6% (P < 0.05), respectively. These effects could also be prevented by staurosporine pretreatment. NIC-induced CAT secretion was significantly reduced by pretreatment with both PMA (Delta -56% +/- 2%; P < 0.05) and PACAP (Delta-53% +/- 7%; P < 0.05). This suppressive effect on secretion could be prevented by pretreatment with staurosporine. These data suggest that, in addition to having direct stimulatory effects on catecholamine synthesis and secretion, PACAP can also negatively modulate nicotinic receptor-dependent Ca(2+) signaling and secretion in PAMC cells.