Regulation of mitochondrial free Ca2+ by metabolite and pH‐dependent Ca2+ buffering in the matrix: analysis by a computational model of mitochondrial Ca2+ handling

Mitochondrial free Ca 2+ (mfCa 2+ ) is regulated by ion fluxes through Ca 2+ uniporter (CU), Na + /Ca 2+ exchanger (NCE), Na + /H + exchanger (NHE), and Ca 2+ /H + exchanger (CHE) as well as by mitochondrial protein buffering. Regulation of mfCa 2+ by metabolite and pH‐dependent dynamic Ca 2+ buffering in the matrix during transient respiration, however, is not well known. To gain a quantitative understanding of these phenomena, we developed a biophysical model of mitochondrial Ca 2+ handling and bioenergetics by developing and integrating kinetic models of the CU, NCE, NHE, and CHE into our well‐validated model of mitochondrial bioenergetics. Experiments were performed to spectrofluorometrically measure mfCa 2+ , pH m , membrane potential, and NADH redox state in guinea pig heart mitochondria suspended in Na + and Ca 2+ free buffer with 2.5 mM EGTA and 0.5 mM pyruvic acid (PA). Dynamics were inferred with addition of CaCl 2 , ADP, and CCCP with and without ruthenium red (CU blocker). Model analysis of the data on a) decreased mfCa 2+ with addition of Na + ‐independent substrate PA, and b) transient increases of mfCa 2+ with added ADP and CCCP, implicates metabolite (ATP, ADP, P i and TCA cycle intermediates) and pH‐dependent dynamic Ca 2+ buffering in the cardiac mitochondrial matrix. The model will be helpful to understand mechanisms by which mfCa 2+ both regulates, and is modulated by, mitochondrial energy metabolism.