METABOLISM OF SKIN AND MUSCLE PROTEIN IS REGULATED DIFFERENTLY IN RESPONSE TO NUTRITION

We have measured skin and muscle protein kinetics and amino acid (AA) transport in anesthetized rabbits during 1) 64-h fast, 2) AA infusion, 3) AA plus fat emulsion infusion, and 4) AA plus hyperinsulinemia. L- [ring-C-13(6)]phenylalanine was infused as the tracer, and the ear and hindlimb were used as arteriovenous units to reflect skin and muscle protein kinetics, respectively. Skin protein net balance was not diffe rent from zero in all groups, indicating a maintenance of protein mass . In contrast, the muscle net balance differed over a range from -1.6 +/- 0.6 after fasting to 0.2 +/- 0.2 mu mol.100 g(-1).h(-1) during hyp erinsulinemia. In the skin, 59-66% of intracellular free phenylalanine came from proteolysis, and phenylalanine availability from proteolysi s was positively correlated to the protein synthesis rate. In conclusi on, normal skin maintains its constant protein mass by efficient reuti lization of AAs from proteolysis. In contrast to muscle, skin protein is relatively insensitive to control by nutritional and hormonal facto rs. Because of the metabolic differences, when limb models are used fo r muscle protein metabolism the potential contribution by limb skin sh ould be considered.