A review of energy metabolism in producing ruminants. 2. Control of nutrient partitioning.

Insulin is the primary short-term hormonal regulator of metabolism in the resting ruminant. The concentration of plasma insulin is positively correlated with energy intake. Diets producing hyperinsulinaemia, direct the balance towards body gain (anabolic). However, in lactating animals, the postprandial rise in insulin is reduced, thereby favouring movement of nutrients to the mammary gland and promoting gluconeogenesis. Similar mechanisms balance the demands of foetal and maternal growth. Glucagon, on the other hand, stimulates both glycogenolysis and gluconeogenesis in the liver from glucogenic amino acids, thereby indirectly diminishing protein synthesis in muscle. Homeorhetic hormones from both the pituitary and reproductive glands, play a major role in the long-term control of nutrient partitioning. Oestrogens appear to affect feed intake, promote RNA and protein synthesis and inhibit gluconeogenesis in the liver, thereby promoting the metabolic adaptations necessary for pregnancy. Progesterone, on the other hand, appears to block the action of the oestrogens at cellular level, and may actually increase feed intake. The pituitary hormones, prolactin and somatotropin, bring about significant improvements in production, especially in milk yield. The action of the somatomedins appears to be responsible for the paradoxical spectrum of effects attributed to somatotropin.