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Systolic properties and coronary flow were studied in Langendorff preparations of normal and septic rat hearts paced at 100, 200, 300, and 400 beats per minute. In addition, the effects of amino acid formulations differing in their branched chain amino acid (BCAA) concentration in normal and septic rat hearts were investigated. Our experiments demonstrated the following: in the normal isolated rat heart, Krebs plus glucose and Krebs plus glucose plus 42% BCAA are most effective in maintaining systolic properties, while Krebs plus glucose plus 15% or 100% BCAA were considerably less effective. Sepsis results in a significant decrease in the systolic properties of the isolated rat heart, and in a loss of the negative linear correlation between contractility and heart rate, probably the result of a diminishing intracellular load of contractile calcium. In the isolated septic rat heart, mechanical washout during perfusion has a beneficial effect, suggesting the presence of a myocardial depressant substance in sepsis. The use of a balanced amino acid mixture containing 42% BCAA exerts the greatest benefit in maintaining systolic properties and in improving coronary flow in the isolated septic rat heart.