Inhibitory effect of glucose on the maturation of rat liver mitochondria at birth. Phospholipid and oxidative metabolism

Mammalian adaptation to extrauterine environment: mitochondrial functional impairment caused by prematurity

In this paper we report that, compared with term rat neonates, both mitochondrial content and function are diminished in liver of preterm neonates (delivered 24 h before full term) compromising cellular energy provision in the postnatal period. In addition, there is a parallel reduction in the content of mRNAs encoding mitochondrial proteins in preterm rats. Also, efficient oxidative phosphorylation is not attained in these pups until 3 h after birth. Although isolated liver mitochondria from preterm neonates show a two-fold increase in F1-ATPase beta-subunit and cytochrome c oxidase activity 1 h after birth, the abnormal coupling efficiency between respiration and oxidative phosphorylation (ADP/O ratio) is due to maintenance of high H(+)-leakage values in the inner mitochondrial membrane. Postnatal reduction of the H+ leak occurs concomitantly with an increase in intra-mitochondrial adenine nucleotide concentration. Accumulation of adenine nucleotides in preterm and term liver mitochondria parallels the postnatal increase in total liver adenine nucleotides. Delayed postnatal induction of adenine biosynthesis most likely accounts for the lower adenine nucleotide pool in the liver of preterm neonates. The delayed postnatal accumulation of adenine nucleotides in mitochondria is thus responsible for the impairment in oxidative phosphorylation displayed by organelles of the preterm liver.

Cyclic GMP, cyclic AMP, glucose at birth, and maturation of rat liver mitochondria

The improvement in mitochondrial functions which normally occurs in newborn rat liver in vivo during the few hours following delivery is inhibited by a glucose injection at birth (Meister, R., Comte, J., Baggetto, L., G., Godinot, C. and Gautheron, D.C. (1983) Biochim. Biophys. Acta 722, 36-42). To test whether this improvement could be correlated to changes in cyclic nucleotides, the levels of cAMP and cGMP have been measured during the 2 h following birth. At birth, a short rise followed by a decrease of cAMP occurs, then a significant increase of cAMP level is observed between 45 min and 2 h. The cAMP level for animals injected at birth with glucose is lower than for control animals at each time studied. The cGMP level is not significantly affected in control animals, while in glucose-treated animals a significant decrease of cGMP is observed in the postnatal 2 h. The present work shows also that the glucose-induced inhibition of mitochondrial maturation is mimicked by injection at birth of either 8-Br-cGMP or nitroprusside. The latter transiently increases intracellular cGMP. In contrast, the glucose-induced inhibition is prevented by the injection at birth of either dbcAMP or alkylxanthines together with glucose (Comte, J., Meister, R., Baggetto, L.G., Godinot, C. and Gautheron, D.C. (1986) Biochem. Pharmacol. 35, 2411-2416). It is concluded that the postnatal improvement of mitochondrial functions is stimulated by cAMP and inhibited by cGMP, and that glucose-induced inhibition of the maturation is at least partly supported by a decrease in cAMP but not correlated to an increase in cGMP.

Reversal of glucose-induced inhibition of newborn rat liver mitochondrial maturation by administration of alkylxanthines at birth

A glucose injection given immediately after birth delays the maturation which normally occurs in rat liver mitochondria and which increases the rate of ATP synthesis coupled to succinate oxidation from a low value at birth to the adult value a few hours after birth [R. Meister, J. Comte, L. Baggetto, C. Godinot and D. C. Gautheron, Biochim. biophys. Acta 722, 36 (1983)]. Alkylxanthine (pentoxifylline, HWA 285) administration at birth has no effect on the maturation of mitochondria prepared from 2-hr-old rat livers while DBcAMP administration increases their RCR and their rate of ATP synthesis. On the contrary, both alkylxanthines and DBcAMP reverse the glucose-induced inhibition of mitochondrial maturation. This DBcAMP effect cannot be mimicked by butyrate and is therefore related to cAMP. The cAMP content of rat liver increases during this postnatal period in both control and glucose-treated rats, although glucose administration tends to decrease the level of cAMP. Alkylxanthine administration restores after 2 hr the cAMP level in glucose-treated animals. The variations of RCR could not be completely correlated with the level of cAMP. The possible involvement of other factors in the mitochondrial maturation and the glucose effect is discussed.

High phosphate requirement for oxidative phosphorylation and low affinity for phosphate transport in newborn rat liver mitochondria

Rat liver mitochondria are not fully functional at birth. The relationship between this deficiency and the affinity for phosphate, in oxidative phosphorylation or in phosphate transport, have been studied. The phosphate concentration necessary to observe maximal rate of succinate oxidation in the presence of ADP was higher for newborn than for adult rat liver mitochondria. After preincubation of newborn rat liver mitochondria with ATP, the rate of succinate oxidation in the presence of ADP increased with phosphate concentration similarly for newborn and adult rat liver mitochondria. The maximal rate of phosphate-acetate exchange, which is an indirect measure of the rate of phosphate transport across the mitochondrial membrane, was not significantly different for adult and newborn rat liver mitochondria. On the contrary the apparent affinity for phosphate was about ten-fold lower for newborn than for adult mitochondria.