Influence of phorbol esters on stimulation-induced noradrenaline release and contractile force of isolated guinea-pig atria.

Phorbol-12,13-dibutyrate (PDB) and phorbol-12-myristate-13-acetate (PMA) increased the field-stimulation-induced efflux of radioactivity from guinea-pig atria preloaded with 7-[3H]-noradrenaline. The efflux was more than doubled by 10(-7) mol/l of either compound. Phorbol-12-myristate-13-acetate-4-O-methylether (PME), which has no effect on the protein kinase C (PKC), did not modify the stimulation-induced efflux of radioactivity, while a slight reduction was seen with 70 microM of the PKC inhibitor polymyxin B. In the presence of polymyxin B, the effects of PMA and PDB were greatly attenuated. In addition, PDB had a concentration-dependent negative inotropic effect (EC50 7.0 x 10(-10) mol/l). Pretreatment with polymyxin B shifted the concentration-response curve for PDB to the right (EC50 4.6 x 10(-9) mol/l). No negative inotropic effect was seen with PMA or PME. The results suggest that all effects of the phorbol esters were mediated by a stimulation of PKC. The different lipophilicity of PMA and PDB or a different influence on the various isozymes of PKC may account for the diverging postjunctional effects of the two compounds.