Regulation of organic nutrient metabolism during transition from late pregnancy to early lactation.

Conceptus energy and nitrogen demands in late pregnancy are mostly met by placental uptake of maternal glucose and amino acids. The resulting 30 to 50% increase in maternal requirements for these nutrients is met partly by increased voluntary intake and partly by an array of maternal metabolic adaptations. The latter include increased hepatic gluconeogenesis from endogenous substrates, decreased peripheral tissue glucose utilization, increased fatty acid mobilization from adipose tissue, and, possibly, increased amino acid mobilization from muscle. Within 4 d of parturition, mammary demands for glucose, amino acids, and fatty acids are several-fold those of the pregnant uterus before term. Even unusual postparturient increases in voluntary intake cannot satisfy this increased nutrient demand. Therefore, rates of hepatic gluconeogenesis and adipose fat mobilization are greatly accelerated. Concomitant changes in amino acid metabolism include increased hepatic protein synthesis and, possibly, decreased amino acid catabolism, and increased peripheral mobilization of amino acids. Insulin resistance in adipose tissue and muscle, developed during late pregnancy, continues postpartum; adipose lipolytic responsiveness and sensitivity to adrenergic agents are increased postpartum beyond their levels during late pregnancy. Before parturition, these homeorhetic adjustments may be coordinated with lactogenesis by increased secretion of estradiol and prolactin. Their amplification and reinforcement at and soon after parturition may be regulated mostly by somatotropin.