FUTILE CYCLING IN CARBOHYDRATE-METABOLISM .1. BACKGROUND AND CURRENT CONTROVERSIES ON PYRUVATE CYCLING

Isotopic evidence for futile cycling in gluconeogenic cells was obtain ed nearly 30 years ago. Rigorous tests were devised to prove that futi le cycling was not simply the result of isotopic exchange reactions, b ut rather the result of opposing reactions. The in vitro evidence sugg ested however, that futile cycling was low in tissues obtained from fa sted animals and incubated with physiological levels of physiological substrates. Current whole animal studies from other laboratories, on t he other hand, have been interpreted as demonstrating high rates of py ruvate cycling in the liver in the fasted state. We challenge these in terpretations here. However, we concur with the recently demonstrated (Petersen et al., Am. J. Physiol. 267: E273-E277, 1994) large increase in pyruvate cycling in the fasted hyperthyroid state. Also, pyruvate cycling is markedly increased in the fasted to fed transition.