Pyruvate has been shown to be a metabolic inotrope in the myocardium. In millimolar concentrations, it has been shown to increase both myocardial phosphorylation potential and the cytosolic [NAD+]-to-[NADH] ratio. To determine if changes in these parameters can alter intracellular Ca2+ concentration ([Ca2+]i) and hence contractile function, Ca2+ transients and cell shortening (CS) were measured in isolated rat ventricular myocytes superfused with a physiological N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer (11 mmol/l glucose) with and without additional pyruvate, L-lactate, acetate, or isoproterenol. The addition of 5 mmol/l pyruvate resulted in a 33% increase in CS and a 39% increase in systolic [Ca2+]i. These pyruvate effects were 70% of those observed with 100 nmol/l isoproterenol. The mitochondrial monocarboxylate transport inhibitor alpha-cyano-4-hydroxycinnamate (250 mumol/l) strongly inhibited pyruvate inotropy, suggesting a substantial obligatory coupling between pyruvate inotropism and its oxidation by the mitochondria. A possible role of the cytosolic [NAD+]-to-[NADH] ratio was assessed by comparing the effects of 20 mmol/l L-lactate to those of equimolar pyruvate. In contrast to 20 mmol/l pyruvate, excess L-lactate failed to appreciably increase CS or systolic [Ca2+]i. The findings imply that, at levels substantially above 5 mmol/l, a portion of pyruvate inotropism might be due to extreme cytosolic [NAD+]-to-[NADH] ratios. This study is the first evidence that augmented [Ca2+]i transients are most likely the mechanism of cardiac pyruvate inotropism.