Fetal fuels II: contributions of selected carbon fuels to oxidative metabolism in rat conceptus.

The effects of fasting on the oxidative disposition of selective fuels in tissues of the rat conceptus were examined on day 20 of gestation. Placentas and portions of fetal liver and brain from fed and 48-h fasted mothers were incubated in vitro with artificial mixtures containing glucose, lactate, and beta-hydroxybutyrate in concentrations simulating those that obtain in fed or 48-h fasted animals in vivo. Oxidative contributions from individual components were evaluated by separate incubations in which only one of the three fuels was 14C-labeled. As judged by the evolution of 14CO2, rates of oxidation of individual fuels by tissues of the conceptus appeared to be conditioned by ambient fuel concentrations rather than the dietary status of the mother. Additional studies indicated that evolution of 14CO2 from glucose or lactate may be depressed directly by adding beta-hydroxybutyrate to the incubation medium. This substitutive property of beta-hydroxybutyrate may "spare" glucose and lactate within the placenta for transfer to the fetus and preserve glucose and lactate availability for biosynthetic rather than oxidative disposition within the fetus.