Role of Mitochondria in Calcium Regulation of Spontaneously Contracting Cardiac Muscle Cells

Mitochondrial involvement in the regulation of cytosolic calcium concentration ([Ca2+]i) in cardiac myocytes has been largely discounted by many authors. However, recent evidence, including the results of this study, has forced a reappraisal of this role. [Ca2+]i and Ca2+ in the mitochondria ([Ca2+]m) were measured in this study with specific fluorescent probes, fluo-3 and di-hydro-rhod-2, respectively; mitochondrial membrane potential (ΔΨm) was monitored with JC-1. Addition of uncouplers or inhibitors of the mitochondrial respiratory chain was found to cause a twofold decrease in the rate of removal of Ca2+ from the cytosol after a spontaneously generated Ca2+ wave. These agents also caused a progressive elevation of [Ca2+]i, an increase in the number of hotspots of Ca2+ release (Ca2+ sparks), and depression of mitochondrial potential. The Ca2+-indicative fluorophore dihydro-rhod-2 has a net positive charge that contributes to selective accumulation by mitochondria, as supported by its co-localization with other mitochondrial-specific probes (MitoTracker Green). Treatment of dihydro-rhod-2-loaded cells with NaCN resulted in rapid formation of “black holes” in the otherwise uniformly banded pattern. These are likely to represent individual or small groups of mitochondria that have depressed mitochondrial potential, or have lost accumulated rhod-2 and/or Ca2+; all of these eventualities are possible upon onset of the mitochondrial permeability transition. Release of Ca2+ from the sarcoplasmic reticulum and the resultant spontaneous contractility of cardiac muscle are proposed to be triggered by the induction of the mitochondrial permeability transition and the subsequent loss of [Ca2+]m.