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This Article Full Text Full Text (PDF) Supplemental Data All Versions of this Article: 586/13/3267    most recent jphysiol.2008.151977v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Dash, R. K. Articles by Beard, D. A. PubMed PubMed Citation Articles by Dash, R. K. Articles by Beard, D. A. Related Collections Integrative

¹ Biotechnology and Bioengineering Center and Department of Physiology, Medical College of Wisconsin, Milwaukee, WI 53226, USA

Calcium is a key ion and is known to mediate signalling pathways between cytosol and mitochondria 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 mitochondrial Ca²⁺ uniporter (CU), Na⁺–Ca²⁺ exchanger (NCE), and Na⁺–H⁺ exchanger (NHE) into an existing computational model of mitochondrial oxidative phosphorylation. Kinetic flux expressions for the CU, NCE and NHE were developed and individually parameterized based on independent data sets on flux rates measured in purified mitochondria. While available data support 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 analyse 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 addition 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.

(Received 30 January 2008; accepted after revision 7 May 2008; first published online 8 May 2008)
Corresponding author D. A. Beard: Biotechnology and Bioengineering Center, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226-6509, USA. Email: dbeard{at}mcw.edu