Simulation of cardiac work transitions, in vitro:: effects of simultaneous Ca2+ and ATPase additions on isolated porcine heart mitochondria

During increases in cardiac work there are net increases in cytosolic [Ca2+] and ATP hydrolysis by myofiliments and ion transport ATPases. However, it is still unclear what role Ca2+ or the ATP hydrolysis products, ADP and Pi, have on the regulation of mitochondrial ATP production. In this study, work jumps were simulated by simultaneous additions of Ca2+ and ATPase to porcine heart mitochondria. The net effects on the mitochondrial ATP production were monitored by simultaneously monitoring respiration (mVo(2)), [NADH], [ADP] and membrane potential (Delta psi) at 37 degreesC. Addition of exogenous ATPase (300 mlU.ml(-1)) + ATP (3.4 mM) was used to generate a 'resting' background production of ADP. This resting metabolic rate was 200% higher than the quiescent rate while [NADH] and Delta psi were reduced. Subsequent ATPase additions (1.3 IU.ml(-1)) were made with varying amounts of Ca2+ (0 to 535 nM) to simulate step increases in cardiac work. Ca2+ additions increased mVo(2) and depolarized Delta psi, and were consistent with an activation of F-0/F(1)ATPase. In contrast, Ca2+ reduced the [NADH] response to the ATPase addition, consistent with Ca2+-sensitive dehydrogenase activity (CaDH). The calculated free ADP response to ATPase decreased <2-fold in the presence of Ca2+. The addition of 172 nM free Ca2+ + ATPase increased mVo(2) by 300% (P0.05, n = 8) while Delta psi decreased by 14.9 +/- 0.1 mV without changes in [NADH] (P>0.05, n=8), consistent with working heart preparations. The addition of Ca2+ and ATPase combined increased the mitochondrial ATP production rate with changes in Delta psi, NADH and [ADP], consistent with an activation of CaDH and F-0/F(1)ATPase activity. These balancing effects of ATPase activity and [Ca2+] may explain several aspects of metabolic regulation in the heart during work transitions in vivo. (C) 2001 Harcourt Publishers Ltd.