Regulation of cyclic AMP levels by phorbol esters in bovine adrenal medullary cells

Cyclic AMP responses to phorbol esters were studied in cultured bovine adrenal medullary cells. Phorbol esters that activate protein kinase C (PKC: phorbol 12,13-dibutyrate, phorbol 12,13-didecanoate) increased cellular cyclic AMP levels by up to 100% over 5 min, and this was maintained for up to 3 h. The effect was mimicked by 1,2-dioctanoyl-sn-glycerol but not by inactive phorbol esters. The effect of active phorbol esters was concentration dependent over the range 50–500 nM, and was abolished by the PKC inhibitor, Ro 31–8220 (10μM). The response was enhanced by 3-isobutyl-1-methylxanthine (1 mM) and by forskolin (0.3 μM), was enhanced following pertussis toxin pretreatment (100 ng/ml, 7.5 h) and was unaffected by removing extracellular Ca²⁺. The phorbol ester cyclic AMP response was additive with that to K⁺ depolarisation, and synergised with those to prostaglandin E₁ and dimaprit. The results indicate PKC activation increases cyclic AMP formation in bovine adrenal medullary cells, probably by a direct action on adenylate cyclase or G_s.