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Mechanisms for cardiac depression induced by phorbol myristate acetate in working rat hearts

1 The effects of the phorbol ester, phorbol myristate acetate (PMA) were examined on function and energy metabolism in the isolated working heart of the rat. 2 At a concentration of 10−9 m. PMA produced a rapid loss in cardiac function in terms of aortic flow rate (AFR) and coronary flow rates (CFR) whereas a similar concentration of 4α‐phorbol 12,13‐didecanoate was ineffective. At a concentration of 10−10 m., the PMA‐induced depression was more gradual but nevertheless very pronounced with an almost total loss in AFR after 30 min perfusion. The reduction in CFR was more moderate than that observed with respect to AFR. 3 The protein kinase C (PKC) inhibitor (±)‐l‐O‐hexadecyl‐2‐O‐acylglycerol significantly attenuated the loss in AFR and CFR following addition of PMA. 4 Two inhibitors of Na+/H+ exchange, amiloride and quinacrine, totally prevented the reduction in AFR. Although the PMA‐induced depression in CFR was also attenuated by both amiloride and quinacrine, these effects were not significant, probably reflecting the less pronounced effect of PMA on this parameter. 5 Nifedipine, a dihydropyridine calcium channel blocker reduced PMA toxicity to a similar degree as Na+/H+ exchange inhibition whereas the calcium channel agonist Bay K 8644 was without effect. 6 Tissue content of energy metabolites including high energy phosphates, total adenine nucleotides or lactate were not significantly affected by PMA perfusion. 7 We conclude that PKC activation is necessary for phorbol ester‐induced cardiac dysfunction. The consequence of PKC stimulation includes (1) Na+/H+ exchange activation and a subsequent elevation in intracellular calcium [Ca2+]i via Na+/Ca2+ exchange and (2) PKC‐dependent phosphorylation of the calcium channel, both of which would produce toxicity by elevation of [Ca2+]i. Pharmacological manipulation of any of these steps prevents PMA toxicity by virtue of a reduction in the accumulation of [Ca2+]i. PMA effects or their prevention are unrelated to any changes in energy metabolism.