Futile cycles in isolated perfused rat liver and in isolated rat liver parenchymal cells.

Isolated livers from fed rats were perfused with a medium containing glucose labeled uniformly with ¹⁴C and specifically with ³H. There was considerable formation of glucose from endogenous sources but simultaneously uptake of about half of the ¹⁴C in glucose. After 2 hours the ${}^3\text{H}{}^{14}\text{C}$ ratios in perfusate glucose decreased by 55–60% with (2-³H, U-¹⁴C), 40–50% with (5-³H, U-¹⁴C), 25–30% with (3-³H or 4-³H, U-¹⁴C) and by 10–15% with (6-³H, U-¹⁴C) glucose. Qualitatively comparable patterns were obtained with rat hepatocytes. These results demonstrate recycling of carbon between glucose and pyruvate. Superimposed upon this there is an extensive futile cycle between glucose and glucose 6-P. There is also futile cycling between fructose 6-P and fructose 1,6 P₂ and to a small extent between phosphoenol pyruvate and pyruvate.