Integration of rapid cytosolic Ca signals by mitochondria in cat ventricular myocytes

Sedova, Marina, Elena N. Dedkova, and Lothar A. Blatter. Integration of rapid cytosolic Ca signals by mitochondria in cat ventricular myocytes. Am J Physiol Cell Physiol 291: C840–C850, 2006. First published May 24, 2006; doi:10.1152/ajpcell.00619.2005.—Decoding of fast cytosolic Ca concentration ([Ca ]i) transients by mitochondria was studied in permeabilized cat ventricular myocytes. Mitochondrial [Ca ] ([Ca ]m) was measured with fluo-3 trapped inside mitochondria after removal of cytosolic indicator by plasma membrane permeabilization with digitonin. Elevation of extramitochondrial [Ca ] ([Ca ]em) to 0.5 M resulted in a [Ca ]em-dependent increase in the rate of mitochondrial Ca accumulation ([Ca ]em resulting in half-maximal rate of Ca accumulation 4.4 M) via Ca uniporter. Ca uptake was sensitive to the Ca uniporter blocker ruthenium red and the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone and depended on inorganic phosphate concentration. The rates of [Ca ]m increase and recovery were dependent on the extramitochondrial [Na ] ([Na ]em) due to Ca extrusion via mitochondrial Na /Ca exchanger. The maximal rate of Ca extrusion was observed with [Na ]em in the range of 20–40 mM. Rapid switching (0.25–1 Hz) of [Ca ]em between 0 and 100 M simulated rapid beat-to-beat changes in [Ca ]i (with [Ca ]i transient duration of 100–500 ms). No [Ca ]m oscillations were observed, either under conditions of maximal rate of Ca uptake (100 M [Ca ]em, 0 [Na ]em) or with maximal rate of Ca removal (0 [Ca ]em, 40 mM [Na ]em). The slow frequency-dependent increase of [Ca ]m argues against a rapid transmission of Ca signals between cytosol and mitochondria on a beat-to-beat basis in the heart. [Ca ]m changes elicited by continuous or pulsatile exposure to elevated [Ca ]em showed no difference in mitochondrial Ca uptake. Thus in cardiac myocytes fast [Ca ]i transients are integrated by mitochondrial Ca transport systems, resulting in a frequencydependent net mitochondrial Ca accumulation.