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The aim of the present investigation was to contrast the Ca2+ dependence of cardiac energy metabolism in two species with differential reliance on extracellular Ca2+ for excitation–contraction coupling.We measured energy expenditure as the rate of oxygen consumption (V̇O2) of isolated, Langendorff-perfused hearts of rats and guinea-pigs during KCl arrest. In parallel experiments, we indexed intracellular Ca2+ concentration ([Ca2+]i) of isolated right-ventricular trabeculae, using the Ca2+ fluorophore fura-2 and ratiometric spectrofluorometry. By varying extracellular Na+ concentration ([Na+]o), V̇O2–[Na+]o and [Ca2+]i–[Na+]o relationships were constructed for each species.Reduction of [Na+]o during K+ arrest caused pronounced species-dependent elevations of both V̇O2 and [Ca2+]i. Despite the species dependence of both V̇O2 and [Ca2+]i on [Na+]o, a single species-independent V̇O2–[Ca2+]i relationship obtained.We infer that elevation of the metabolic rate of the arrested heart above its basal value is determined primarily by [Ca2+]i and is not species dependent.