The work of the perfused rat heart was acutely increased by raising the aortic pressure in the Langendorff preparation from 50 to 120mmHg; within 1 min in perfusions with media containing glucose or glucose+acetate, rates of oxygen consumption and tricarboxylate-cycle turnover increased 2.5-fold, glycolysis rate doubled and oxidation of triglyceride fatty acid was strikingly enhanced. 2. Increased cardiac work had no significant effects on the heart concentrations of creatine phosphate, ATP, ADP or 5'-AMP. The only significant changes in tricarboxylate-cycle intermediates were a decrease in malate in perfusions with glucose and decreases in acetyl-CoA and citrate and an increase in aspartate in perfusions with glucose+acetate. 3. Measurements of intracellular concentrations of hexose phosphates, glucose and glycogen indicated that work accelerated glycolysis by activation of phosphofructokinase and subsequently hexokinase; the activation could not be accounted for by changes in the known effectors of phosphofructokinase. 4. Acetate at either perfusion pressure increased heart concentrations of acetyl-CoA, citrate, glutamate and malate and decreased that of aspartate; acetate increased tricarboxylate-cycle turnover by 50-60% and inhibited glycolysis and pyruvate oxidation. 5. In view of the markedly different effects of acetate and of cardiac work on the concentrations of cycle intermediates the changes that accompany acetate utilization may be specifically concerned with the regulatory functions of the cycle in control of glycolysis and pyruvate oxidation and not with the associated increase in cycle turnover. It is suggested that the concentrations of key metabolites controlling the rate of cycle turnover may fluctuate with each heart beat and that this may explain why no significant changes (for example, in adenine nucleotide concentrations) have been detected with increased work in the present study.
