ROLE OF MITOCHONDRIA IN CALCIUM REGULATION OF SPONTANEOUSLY CONTRACTING CARDIAC-MUSCLE-CELLS

Mitochondrial involvement in the regulation of cytosolic calcium conce ntration ([Ca2+](i)) in cardiac myocytes has been largely discounted b y many authors. However, recent evidence, including the results of thi s study, has forced a reappraisal of this role. [Ca2+], and Ca2+ in th e mitochondria ([Ca2+](m)) were measured in this study with-specific f luorescent probes, fluo-3 and di-hydro-rhod-2, respectively; mitochond rial membrane potential (Delta Psi(m)) was monitored with JC-1. Additi on of uncouplers or inhibitors of the mitochondrial respiratory chain was found to cause a twofold decrease in the rate of removal of Ca2+ f rom the cytosol after a spontaneously generated Ca2+ wave. These agent s also caused a progressive elevation of [Ca2+](i), an increase in the number of hotspots of Ca2+ release (Ca2+ sparks), and depression of m itochondrial 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 mitoch ondrial-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 repr esent individual or small groups of mitochondria that have depressed m itochondrial potential, or have lost accumulated rhod-2 and/or Ca2+; a ll of these eventualities are possible upon onset of the mitochondrial permeability transition. Release of Ca2+ from the sarcoplasmic reticu lum and the resultant spontaneous contractility of cardiac muscle are proposed to be triggered by the induction of the mitochondrial permeab ility transition and the subsequent loss of [Ca2+](m).