Control of Cardiac Mitochondrial Fuel Selection by Calcium

Abstract

Calcium ion concentration modulates the function of pyruvate dehydrogenase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. Previous studies have shown that despite its ability to affect the function of these dehydrogenases, [Ca²⁺] does not substantially alter mitochondrial ATP synthesis in vitro under physiological sub-strate conditions. We hypothesize that, rather than contributing to respiratory control, [Ca²⁺] governs fuel selection. Specifically, cardiac mitochondria are able to use different primary carbon substrates to synthesize ATP aerobically. To determine if and how [Ca²⁺] affects the relative use of carbohydrates versus fatty acids we measured oxygen consumption and tricarboxylic acid cycle intermediate concentrations in suspensions of cardiac mitochondria with different combinations of pyruvate and palmitoyl-L-carnitine in the media at various [Ca²⁺] and ADP infusion rates. Results reveal that when both fatty acid and carbohydrate substrates are available, fuel selection is sensitive to both calcium and ATP synthesis rate. When no Ca²⁺ is added under low ATP-demand conditions, β-oxidation provides roughly half of acetyl-CoA for the citrate synthase reaction with the rest coming from the pyruvate dehydrogenase reaction. Under low demand conditions with increasing [Ca²⁺], the fuel utilization ratio shifts to increased fractional consumption of pyruvate, with 83±10% of acetyl-CoA derived from pyruvate at the highest [Ca²⁺] evaluated. With high ATP demand, the majority of acetyl-CoA is derived from pyruvate, regardless of the Ca²⁺ level. Our results suggest that changes in work rate alone are enough to effect a switch to carbohydrate use while in vivo the rate at which this switch happens may depend on mitochondrial calcium.Key Points: Despite its effects on activity of mitochondrial dehydrogenases, Ca²⁺ does not substantially alter mitochondrial ATP synthesis in vitro under physiological substrate conditions. Nor does is appear to play an important role in respiratory control in vivo in the myocardium. We hypothesize that Ca²⁺ plays a role mediating the switch in fuel selection to increasing carbohydrate oxidation and decreasing fatty acid oxidation with increasing work rate. 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 purified rat ventricular mitochondria with carbohydrate, fatty acid, and mixed substrates at various [Ca²⁺] and ATP demand rates. Our results suggest that changes in work rate alone are enough to effect a switch to carbohydrate use in vitro while in vivo the rate at which this switch happens may depend on mitochondrial calcium.