Acetylcholine and phorbol esters inhibit potassium currents evoked by adenosine and cAMP in Xenopus oocytes

In Xenopus laevis oocytes, adenosine and other purinergic agonists induce a K⁺-conductance increase that is fully mimicked by intracellular application of cAMP. Acetylcholine suppressed the K⁺-conductance increase caused by adenosine, by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, or by intracellular injection of cAMP. This effect of acetylcholine is not mimicked by intracellular injection of Ca²⁺ or the Ca-mobilizing agent inositol 1,4,5-triphosphate. However, adenosine and cAMP responses are inhibited by 4β-phorbol 12,13-dibutyrate and 4β-phorbol 12-myristate 13-acetate. These results suggest that, in Xenopus oocytes, the muscarinic inhibition of purinergic and cAMP responses is mediated through the activation of the phospholipid-dependent, Ca-activated protein kinase (protein kinase C).