Tissue protein synthesis in lactating and dry goats.
Intravenous infusion of L-[3H]phenylalanine (Phe) was carried out for 8 h in dry, non-pregnant and lactating dairy goats. Nitrogen balance was positive in the dry group and negative in the lactating group. Whole-body Phe flux was 50% greater in lactating goats (P less than 0.01). Fractional synthesis rates (Ks) of tissue proteins were estimated from plasma- (Ksp) and tissue- (Ksh) specific radioactivities of Phe. In lactating goats, Ksp for mammary gland, duodenum and diaphragm was increased (P less than 0.05). Ksp also tended to increase in liver, kidney and rumen (P less than 0.08) of lactating goats, but was not different in uterus, spleen, caecum or heart. Values of Ksh were higher than Ksp; however, these measures agreed qualitatively. When absolute rates of protein synthesis were calculated, an increased contribution of mammary and visceral organs was seen in lactating goats. Ks and absolute rates of protein synthesis of hind-limb skin were less in lactating goats (P less than 0.05). A decreased proportion of skeletal muscle (P less than 0.05). Decreased rates of muscle and skin protein synthesis would appear to participate in alterations of protein metabolism, permitting lactation to occur at the expense of body reserves.