Intramitochondrial free Ca 2+ concentration ([Ca 2+]m) is an important parameter in the regulation of the energy economy of the muscle cell. There is convincing evidence that [Ca 2+]m activates three dehydrogenases which catalyze irreversible reactions in the complete oxidation of carbohydrate and fatty acid, viz. the pyruvate dehydrogenase complex, and NAD-isocitrate and alpha-ketoglutarate dehydrogenases. In a recent development we have been able to measure [Ca 2+]m continuously in single living cardiac myocytes. The method involves the loading of cells with the membrane-permanent ester form of indo-1, followed by the selective quenching of the cytosolic indo-1 signal with Mn 2+. This technique allowed us to monitor the response of [Ca 2+]m not only to low Na+ superfusion (to induce cell calcium loading by Na+ -Ca 2+ exchange), but also to monitor the response over a range of electrical stimulation frequencies. With increasing frequencies of stimulation, values of [Ca 2+]m are recorded which span the range over which dehydrogenase-activation occurs. Further, [Ca 2+]m rises in response to beta-adrenergic stimulation. These findings are consistent with an increase in [Ca 2+]m acting as a signal to increase dehydrogenase activity, and hence flux through oxidative phosphorylation in response to increased workload.
