Protein-Synthesis and Breakdown in Skin and Muscle - a Leg Model of Amino-Acid Kinetics

In the postabsorptive state, amino acids are released from the periphery to provide precursors for protein synthesis in the splanchnic organs. To evaluate the differential role of the most important peripheral tissues, i.e., skin and muscle, in the interorgan amino acid exchange, we have developed a model to simultaneously measure the rates of protein synthesis and degradation in these tissues. Anesthetized dogs were studied using the arteriovenous catheterization of the leg in combination with muscle and skin biopsies. L-[alpha-15N]lysine and L-[ring-2H5]phenylalanine were infused as independent markers of both skin and muscle protein kinetics. Model structure described leg skin and muscle as tissues arranged in parallel and accounted for blood flow distribution. Lysine data show that, in the postabsorptive state, the fractional rate (%/h) of skin protein synthesis (0.543 +/- 0.218) was comparable to the fractional rate of degradation (0.507 +/- 0.157), whereas, in muscle, degradation (0.454 +/- 0.116) was greater (P < 0.05) than synthesis (0.318 +/- 0.109). Similar conclusions were apparent from the phenylalanine data. Skin protein synthesis and degradation accounted for approximately 10-15% of the total leg protein kinetics.