The Flux of Pyruvate in Perfused Rat Heart

It was possible to find a steady‐state lactate concentration in a closed‐circuit heart perfusion containing added glucose. Insulin but not growth hormone (4 μg/ml) increased the equilibrium concentration in perfusate by 200%. Intracellular lactate also remained constant and was about twice that in perfusate: this distribution is difficult to explain whether ionic or undissociated lactate is the permeating species. Pyruvate accumulated linearly in perfusate, reaching a peak after 30 min perfusion and then declined non‐linearly. Insulin increased the peak concentration and growth hormone slowed the decline after the peak. The intracellular pyruvate concentration stayed constant and, except in the first few minutes, was lower than the perfusate concentration. Pyruvate addition at zero time could inhibit pyruvate accumulation but altered neither the non‐linear decline, the intracellular pyruvate level nor the lactate equilibrium. The [NAD]/[NADH] ratio, after 10 min of pre‐perfusion on open circuit, was 23. During 2 h of perfusion the ratio rose slowly and then declined. If either insulin or growth hormone were present in the medium, the ratio fell to about 11 during the first few minutes of perfusion and remained low for 90 min. These values could not easily be correlated with either the varying extracellular, or the constant intracellular [lactate]/[pyruvate] ratios. The glucose and oxygen consumption of the hearts during perfusion was measured, and the change in glycogen concentration in the tissue. It was then possible to draw up a balance sheet which indicated that the control hearts oxidised little other than carbohydrate for the 60 min of observation, but both insulin and growth hormone‐treated hearts oxidised significant quantities of fatty acid during the first 15 min. It appears that growth hormone, like insulin, has an immediate effect on heart metabolism. The addition of pyruvate at zero time appeared to stimulate carbohydrate oxidation in hearts perfused with insulin, even though the hormone had been present in the pre‐perfusion fluid. Consideration of the variation in the extracellular [lactate]/[pyruvate] ratio with length of perfusion, and its lack of relationship to the intracellular [lactate]/[pyruvate] ratio, together with similar reports of other workers, leads us to conclude that neither lactate nor pyruvate establish a concentration gradient across the heart cell membrane which is based on passive diffusion. The discrepancy is particularly marked with respect to pyruvate. Our results strongly suggest that the extracellular [lactate]/[pyruvate] ratio cannot be used as an index of the cytoplasmic redox potential in perfused heart. A tracer study of pyruvate metabolism in the reproducible non‐steady‐state conditions described can offer an explanation for the behaviour of the observed metabolic parameters: this study is reported in the following papers.