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, wo rk jumps were simulated by simultaneous additions of Ca2+ and ATPase to por cine heart mitochondria. The net effects on the mitochondrial ATP productio n were monitored by simultaneously monitoring respiration (mVo(2)), [NADH], [ADP] and membrane potential (Delta psi) at 37 degreesC. Addition of exoge nous 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. Subsequen t 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 inc reased mVo(2) and depolarized Delta psi, and were consistent with an activa tion of F-0/F(1)ATPase. In contrast, Ca2+ reduced the [NADH] response to th e ATPase addition, consistent with Ca2+-sensitive dehydrogenase activity (C aDH). The calculated free ADP response to ATPase decreased <2-fold in the p resence of Ca2+. The addition of 172 nM free Ca2+ + ATPase increased mVo(2) by 300% (P<less than or equal to>0.05, n = 8) while Delta psi decreased by 14.9 +/- 0.1 mV without changes in [NADH] (P>0.05, n=8), consistent with w orking heart preparations. The addition of Ca2+ and ATPase combined increas ed the mitochondrial ATP production rate with changes in Delta psi, NADH an d [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 viv o. (C) 2001 Harcourt Publishers Ltd.