Methionine flux and tissue protein synthesis in lactating and dry goats

Whole-body methionine flux (rate of irreversible loss from plasma) and tissue protein synthesis were estimated in dry and early lactating goats (10-14 d postpartum) by intravenous infusion of L-[35S]methionine. Tissue protein mass was significantly (p less than 0.05) higher for mammary gland and liver but lower for carcass in lactating animals. The plasma methionine flux was higher during lactation (8.5 vs. 5.1 g/d). The fractional synthesis rates of tissue proteins (Ksp: %/d) were lower during lactation for some muscles, especially the masseter muscle (1.46 vs. 2.15), and for skin (0.59 vs. 1.22) and the pooled head plus feet fraction (1.64 vs. 2.31), but the rates were greatly increased in mammary gland (42 vs. 3). The non-mammary methionine flux (plasma flux minus the flux corresponding to milk methionine output and methionine utilization for mammary protein synthesis) was significantly (p less than 0.05) lower for the lactating goats than for the dry group (93 vs. 131 mg.d-1.kg empty body weight-1). This is in agreement with the lower rates of protein synthesis in carcass (542 vs. 948 mg.d-1.kg empty body weight-1) and skin (93 vs. 189) for lactating compared to dry goats. It can be inferred from these data that in early lactation, when nutrient requirements of animals are not adequately met, an adaptative mechanism occurs that allows amino acids to be available for the mammary gland by a decrease of their utilization in some extramammary tissues.