Delayed recycling of plasma FFA in mice: revised model of turnover and oxidation

We have reexamined the concept of the irreversible disposal rate (IDR) or fractional catabolic rate (FCR) in relation to the oxidation of a major metabolic fuel, plasma free fatty acids (FFA), in mice. We measured the disappearance of intravenously injected [1-14C]palmitate complexed to mouse serum albumin from the circulation of normal mice fed ad libitum and fasted approximately 4 h before tracer injection. We also measured the appearance of expired 14CO2 in the breath. Using multicompartmental analyses (SAAM) we found that, in contrast to earlier studies in this and other mammals where the estimated IDR has always been two to three times greater than the oxidative rate, the IDR (FCR) corresponded closely to the fractional rate of plasma FFA oxidation to CO2. This rate could be estimated accurately in a study of 10- to 15-min duration. In an extended study (5-h duration) we obtained kinetic evidence of a major transport pathway that involves delayed recycling of FFA through at least several unidentified relatively slowly turning compartments.