Endocrine adaptations to demanding physiological states in ruminants.

Highly productive ruminants rely on hormonally driven adaptations to prioritize the use of limiting nutrients during the demanding phases of the pregnancy-lactation cycle. Glucose, the predominant oxidative fuel of fetal life and the absolute precursor of mammary lactose synthesis, illustrates the need and benefit of such adaptations. Endocrine mechanisms such as insulin resistance and/or hypoinsulinemia favor the diversion of maternal glucose to the placenta or mammary gland where uptake is independent of insulin. Research in dairy cows in the 1980s and 1990s identified growth hormone as a peripherally acting signal opposing the effects of insulin. The following decades have seen the discovery of a new generation of signals secreted almost exclusively by adipose tissue, skeletal muscle or liver, dynamically regulated by metabolic challenges, and engaged in cross-organ communication. The understanding of these signals in the coordination of metabolism in ruminants has been limited by the availability of assays to measure their circulating concentrations and materials to perform functional studies. Nevertheless, emerging data point to their importance during demanding physiological states in ruminants, including early lactation in dairy cows and late pregnancy in sheep. Examples include modulation of insulin action by liver-derived fibroblast growth factor 21 (FGF21) and regulation of energy allocation among tissues by the action of the adipose-derived hormone leptin via its ability to control the hypothalamic-pituitary-thyroid axis. Recent studies investigating the regulation and action of FGF21 and leptin in dairy cows and sheep will be used to illustrate the potential of recently discovered signals to coordinate metabolism during physiologically demanding states such as early lactation.