DO NONESTERIFIED FATTY-ACIDS REGULATE SKELETAL-MUSCLE PROTEIN-TURNOVER IN HUMANS

We examined whether elevated plasma nonesterified fatty acid (NEFA) le vels exert a direct effect on protein metabolism by measuring [H-2(5)] -phenylalanine skeletal muscle exchange and whole body turnover. [H-2( 5)] phenylalanine was infused (0.5 mg.kg-1.h) for 300 min in seven hea lthy subjects on two occasions. Intralipid (10%; 30 ml/h) or 0.154 mol /l NaCl was infused in random order from 120 min. Measurements were ta ken during basal (90-120 min) and infusion (270-300 min) periods. Intr alipid infusion increased plasma NEFA levels [1.31 +/- 0.13 vs. 0.49 /- 0.05 (SE) mmol/l; P < 0.05] and forearm NEFA uptake [45 +/- 76 vs. -51 +/- 44 nmol . 100 ml forearm-1.min-1; P < 0.05]. Serum insulin and blood ketone body levels were similar with the two treatments. Elevat ed plasma NEFA levels were associated with a comparable decrease in fo rearm phenylalanine uptake (11 +/- 2 vs.17 +/- 2 nmol.100 ml forearm-1 .min-1: lipid vs. control, P < 0.05) and release (20 +/- 2 vs. 26 +/- 3 nmol.100 ml forearm-1.min-1; lipid vs. control, P < 0.05). However, there were no significant changes in net forearm phenylalanine exchang e and whole body phenylalanine turnover. Therefore, elevated plasma NE FA levels were associated with a comparable decrease in the rates of s keletal muscle protein synthesis and breakdown but did not appear to i nfluence overall protein balance, as assessed using [H-2(5)]phenylalan ine.