Phorbol esters increase GTP-dependent adenylate cyclase activity in rat brain striatal membranes

Abstract

4 beta-Phorbol 12-myristate 13-acetate (PMA), added to a lysed mitochondrial fraction of rat striatum, stimulates adenylate cyclase activity with an apparent time lag of approximately 30 s. Half-maximal and maximal enzyme stimulations are obtained with 8 and 200 nM PMA, respectively. The PMA stimulation is GTP dependent, reaching a maximum of approximately 60% at 50 microM GTP, and is associated with disappearance of the enzyme inhibition induced by micromolar concentrations of GTP. Enhancement of enzyme activity by cholera toxin and 3,4-dihydroxyphenylethylamine is amplified by PMA only at micromolar concentrations of GTP. PMA does not affect the enzyme stimulation by forskolin but reverses the inhibition of forskolin-stimulated enzyme by GTP. When guanyl-5'-yl-imidodiphosphate is substituted for GTP, PMA does not modify adenylate cyclase activity. Enzyme inhibition by acetylcholine, Leu-enkephalin, and R(-)N6-(2-phenylisopropyl)adenosine is magnified by PMA. Stimulation of adenylate cyclase by PMA is markedly reduced following EGTA treatment, is not observed when adenyl-5'-yl-imidodiphosphate is substituted for ATP as substrate for adenylate cyclase, and is enhanced by L-alpha-phosphatidyl-L-serine. Like PMA, 4 beta-phorbol 12,13-dibutyrate and 1-oleoyl-2-acetylglycerol stimulate striatal adenylate cyclase, whereas 4 beta-phorbol and 4 beta-phorbol 13-acetate are ineffective. The results indicate that phorbol esters increase striatal adenylate cyclase activity by reducing the GTP-induced inhibition of the enzyme, presumably as a result of protein kinase C activation.