Role of calcium in metabolic signaling between cardiac sarcoplasmic reticulum and mitochondria in vitro

The role of Ca2+ as a cytosolic signaling molecule between porcine cardiac sarcoplasmic reticulum (SR) ATPase and mitochondrial ATP production was evaluated in vitro. The Ca2+ sensitivity of these processes was determined individually and in a reconstituted system with SR and mitochondria in a 0.5: 1 protein-to-cytochrome aa(3) ratio. The half-maximal concentration (K-1/2) of SR ATPase was 335 nM Ca2+. The ATP synthesis dependence was similar with a K-1/2 of 243 nM for dehydrogenases and 114 nM for overall ATP production. In the reconstituted system, Ca2+ increased thapsigargin-sensitive ATP production (maximum similar to5-fold) with minimal changes in mitochondrial reduced nicotinamide adenine dinucleotide (NADH). NADH concentration remained stable despite graded increases in NADH turnover induced over a wide range of Ca2+ concentrations (0 to similar to500 nM). These data are consistent with a balanced activation of SR ATPase and mitochondrial ATP synthesis by Ca2+ that contributes to a homeostasis of energy metabolism metabolites. It is suggested that this balanced activation by cytosolic Ca2+ is partially responsible for the minimal alteration in energy metabolism intermediates that occurs with changes in cardiac workload in vivo.