Modeling Dynamic Regulation of Mitochondrial free Ca 2+ : Effects of Ca 2+ Sequestration and Precipitation

One of the most intriguing features of cardiac mitochondria is their ability to accumulate, retain and release enormous amount of Ca 2+ via sophisticated Ca 2+ transport and buffering systems. Mitochondrial free Ca 2+ ([Ca 2+ ] m ) is mainly regulated by integrated function of Ca 2+ uptake and extrusion pathways and Ca 2+ sequestration and precipitation mechanisms. In spite of significant studies on the kinetics and regulation of mitochondrial Ca 2+ transport systems, the mechanisms involved in mitochondrial Ca 2+ sequestration and precipitation remain obscure. Here we developed a detailed computational model by integrating our recent biophysical models of cation transporters and a novel Ca 2+ sequestration mechanism into our existing model of mitochondrial bioenergetics to quantitatively understand the dynamic regulation of [Ca 2+ ] m . Experiments were conducted to measure time course of [Ca 2+ ] m , [Ca 2+ ] e , [NADH] m and pH in guinea pig heart mitochondria suspended in buffer media subjected to incremental concentrations of CaCl 2 (0–40 μM) and NaCl (0–20 mM). Model analyses of the data suggest (1) with a constant Ca 2+ buffering capacity, it is not possible to explain the dynamics of [Ca 2+ ] m , (2) [Ca 2+ ] m is largely influenced by a Ca 2+ ‐dependent Ca 2+ sequestration mechanism and least affected by Ca 2+ ‐phosphate precipitation and, (3) there may be additional pathways for Ca 2+ uptake (RaM), apart from the Ca 2+ uniporter.