Phorbol ester facilitates ⁴⁵Ca accumulation and catecholamine secretion by nicotine and excess K⁺ but not by muscarine in rat adrenal medulla

Abstract

Several investigators have shown that tumour promoter phorbol esters mimic the effects of endogenous diacylglycerol to activate a second messenger, protein kinase C (refs 1–3). These phorbol esters have proved to be valuable tools for exploring the role of protein kinase C in many cellular functions^3–8. We demonstrate here that secretion of catecholamines evoked from the rat adrenal gland^9,10 by stimulation of splanchnic nerves, excess potassium (K⁺) and nicotine is facilitated by phorbol 12,13-dibutyrate. An inhibitor of protein kinase C, polymixin B, produced concentration-dependent inhibition of the evoked secretion, and the effect was reversed by the phorbol ester. Furthermore, we show that an increase in the accumulation of radioactively labelled calcium (⁴⁵Ca) obtained in the adrenal medulla after stimulation with nicotinic agonists and excess K⁺ is further enhanced by phorbol ester. Muscarine-evoked secretion of catecholamines, which depends on mobilization of intracellularly bound Ca²⁺, was not associated with an increase in ⁴⁵Ca²⁺ uptake, and phorbol ester did not facilitate either catecholamine secretion or ⁴⁵Ca²⁺ accumulation. We suggest that protein kinase C is involved in the exocytotic secretion of catecholamines by regulating the influx of Ca²⁺ through voltage-sensitive and nicotine receptor-linked Ca²⁺ channels of rat chromaffin cells.