Endocrine control of metabolic adaptation during lactation

In this brief essay I have therefore focused mainly on recent developments, especially in changes in endocrine responsiveness of tissues with lactation.In addition, I have tried to highlight some of the important differences and similarities between domestic ruminants and laboratory rodents in their metabolic adaptations and their endocrine control during lactation. AdaptationsLactation in both rodents and ruminants increases nutrient requirements-nutrient output in milk may equal or exceed nutrient utilization by the rest of the body!This considerable challenge is met primarily by increasing food intake and also the capacity to digest, absorb and process the additional nutrients in both types of species (Table 1).However, adaptations of liver metabolism are markedly different in rodents and ruminants (Table 2), reflecting differences in nutrients absorbed; there is a massive increase in glucose absorption in rats and mice whereas, as a result of rumen fermentation, little or no glucose is absorbed in ruminants, while in both glucose requirements are greatly increased for lactose synthesis.The need for additional nutrients is decreased by certain adaptations.A major energy-saving device in rodents is a decrease in non-shivering thermogenesis (Trayhurn, 1985); this arises from both a diminished capacity of brown adipose tissue to produce heat (Trayhurn & Jennings, 1987) and a decrease in sympathetic nervous activity in the tissue (Trayhurn & Wusteman, 1987) during lactation.There does not appear to be any brown adipose tissue in ruminants, hence this option is not available.However, in both ruminants and rodents there is a decrease in the rate of lipid synthesis and in the capacity (i.e., activity of limiting enzymes) for lipid synthesis in white adipose tissue (see Bauman & Elliot, 1983; Vernon & Flint, 1984;Williamson, 1986;Vernon, 1988), which both decreases the requirement for lipid precursors and also ensures their preferential Table 1.