Role of Calcium and ADP Infusion Rates in Cardiac Mitochondrial Fuel Selection

Abstract

Calcium ion concentration modulates the function of several mitochondrial enzymes. Specifically, the kinetic operations of the decarboxylating dehydrogenases pyruvate dehydrogenase, isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase are all affected by [Ca2+]. Previous studies have shown that, despite its ability to affect the function of specific dehydrogenases, [Ca2+] does not substantially alter mitochondrial ATP synthesis in vitro or in vivo in the heart. We hypothesize that, rather than contributing to respiratory control, [Ca2+] plays a role in contributing to fuel selection. Specifically, cardiac mitochondria are able to use different primary carbon substrates (carbohydrates, fatty acids, and ketones) to synthesize ATP aerobically in the living cells. To determine if and how [Ca2+] affects the relative use of carbohydrates versus fatty acids in vitro we measured oxygen consumption and TCA cycle intermediate concentrations in suspensions of cardiac mitochondria with different combinations of pyruvate and palmitoyl-L-carnitine in the media and at various calcium concentrations and ADP infusion rates. Stoichiometric analysis of the data reveals that when both fatty acid and carbohydrate substrates are available, fuel selection is sensitive to both the rate of ADP infusion and to the calcium concentration. Under low-flux (leak state) conditions and with zero added Ca2+, β-oxidation provides roughly 70% of acetyl-CoA for the citrate synthase reaction, with the rest coming from the pyruvate dehydrogenase reaction. With both increasing rates of oxidative ATP synthesis and with increasing [Ca2+], the fuel utilization ratio shifts to increased fractional consumption of pyruvate. The effects of ATP synthesis load and [Ca2+] are shown to be interdependent.