Beat-to-beat oscillations of mitochondrial [Ca2+] in cardiac cells

The Ca²⁺-sensitive photoprotein aequorin and the new green fluorescent protein-based fluorescent Ca²⁺ indicators 'ratiometric-pericam' were selectively expressed in the mitochondria, cytosol and/or nucleus of spontaneously beating ventricular myocytes from neonatal rats. This combined strategy reveals that mitochondrial [Ca²⁺] oscillates rapidly and in synchrony with cytosolic and nuclear [Ca²⁺]. The Ca²⁺ oscillations were reduced in frequency and/or amplitude by verapamil and carbachol and were enhanced by isoproterenol and elevation of extracellular [Ca²⁺]. An increased frequency and/or amplitude of cytosolic Ca²⁺ spikes was rapidly mirrored by similar changes in mitochondrial Ca²⁺ spikes and more slowly by elevations of the interspike Ca²⁺ levels. The present data unequivocally demonstrate that in cardiac cells mitochondrial [Ca²⁺] oscillates synchronously with cytosolic [Ca²⁺] and that mitochondrial Ca²⁺ handling rapidly adapts to inotropic or chronotropic inputs.