Metabolic inhibition reduces cardiac L-type Ca(2+) channel current due to acidification caused by ATP hydrolysis

Metabolic stress evoked by myocardial ischemia leads to impairment of cardiac excitation and contractility. We studied the mechanisms by which metabolic inhibition affects the activity of L-type Ca(2+) channels (LTCCs) in frog ventricular myocytes. Metabolic inhibition induced by the protonophore FCCP (as well as by 2,4- dinitrophenol, sodium azide or antimycin A) resulted in a dose-dependent reduction of LTCC current (I(Ca,L)) which was more pronounced during β-adrenergic stimulation with isoprenaline. I(Ca,L) was still reduced by metabolic inhibition even in the presence of 3 mM intracellular ATP, or when the cell was dialysed with cAMP or ATP-γ-S to induce irreversible thiophosphorylation of LTCCs, indicating that reduction in I(Ca,L) is not due to ATP depletion and/or reduced phosphorylation of the channels. However, the effect of metabolic inhibition on I(Ca,L) was strongly attenuated when the mitochondrial F(1)F(0)-ATP-synthase was blocked by oligomycin or when the cells were dialysed with the non-hydrolysable ATP analogue AMP-PCP. Moreover, increasing the intracellular pH buffering capacity or intracellular dialysis of the myocytes with an alkaline solution strongly attenuated the inhibitory effect of FCCP on I(Ca,L). Thus, our data demonstrate that metabolic inhibition leads to excessive ATP hydrolysis by the mitochondrial F(1)F(0)-ATP-synthase operating in the reverse mode and this results in intracellular acidosis causing the suppression of I(Ca,L). Limiting ATP break-down by F(1)F(0)-ATP-synthase and the consecutive development of intracellular acidosis might thus represent a potential therapeutic approach for maintaining a normal cardiac function during ischemia.