Analysis of Cardiac Mitochondrial Na+/Ca2+ Exchanger Kinetics with a Biophysical Model of Mitochondrial Ca2+ Handing
Suggests a 3:1 Stoichiometry

 

Abstract

Calcium is a key ion and is known to mediate signaling pathways between cytosol and mito-chondria and modulate mitochondrial energy metabolism. To gain a quantitative, biophysical understanding of mitochondrial Ca2+ regulation, we developed a thermodynamically-balanced model of mitochondrial Ca2+ handling and bioenergetics by integrating kinetic models of mito-chondrial Ca2+ uniporter (CU), Na+/Ca2+ exchanger (NCE), and Na+/H+ exchanger (NHE) into an existing computational model of mitochondrial oxidative phosphorylation (Beard, PLoS Comp Biol 1:e36, 2005). Kinetic flux expressions for the CU, NCE and NHE were developed and indi-vidually parameterized based on independent data sets on flux rates measured in purified mito-chondria. While available data supports a wide range of possible values for the overall activity of the CU in cardiac and liver mitochondria, even at the highest estimated values, the Ca2+ current through the CU does not have a significant effect on mitochondrial membrane potential. This integrated model was then used to analyze additional data on the dynamics and steady-states of mitochondrial Ca2+ governed by mitochondrial CU and NCE. Our analysis of the data on the time course of matrix free [Ca2+] in respiring mitochondria purified from rabbit heart with addi-tion of different levels of Na+ to the external buffer medium (with the CU blocked) with two separate models—one with a 2:1 stoichiometry and the other with a 3:1 stoichiometry for the NCE—supports the hypothesis that the NCE is electrogenic with a stoichiometry of 3:1. This hypothesis was further tested by simulating an additional independent data set on the steady state variations of matrix free [Ca2+] with respect to the variations in external free [Ca2+] in purified respiring mitochondria from rat heart to show that only the 3:1 stoichiometry model predictions are consistent with the data. Based on these analyses, it is concluded that the mitochondrial NCE is electrogenic with a stoichiometry of 3:1.