Role of intracellular Ca2+ and pH in positive inotropic response of cardiomyocytes to diacylglycerol

Diacylglycerol has been hypothesized to mediate the positive inotropic response of myocardium to the alpha-adrenergic agonists angiotensin II and endothelin. The mechanism of action of diacylglycerol was examined here in adult rat ventricular myocytes by releasing dioctanoylglycerol (diC(8)) intracellularly from a caged compound while monitoring Ca2+ transients and pH with fluorescent indicators. DiC(8) caused a three- to fourfold increase in twitch amplitude and a twofold increase in the systolic Ca2+ transient. No other parameter was consistently influenced by diC(8), including the kinetics of Ca2+ cycling, the Ca2+ content of the sarcoplasmic reticulum, or the myofilament Ca2+ sensitivity. DiC(8) also had no detectable effect on intracellular pH or Na+/H+ antiport activity. Consistent with this finding, the Na+H+ exchange inhibitor N-ethylisopropyl amiloride was without effect on the positive inotropic response to diC(8). The marked enhancement of systolic Ca2+ by diC(8)suggests that the process of excitation-contraction coupling is an important and possibly preferred target of diacylglycerol-protein kinase C signaling in myocardium.