Platelet-activating factor (PAF) has been shown to reduce rat skeletal muscle amino acid uptake, which may restrict intracellular amino acid availability for protein synthesis and amino acid oxidation during endotoxemia. We investigated in rats the effect of PAF infusion on amino acid and protein metabolism by measuring (a) whole-body and tissue leucine kinetics; (b) plasma amino acid profile; and (c) muscle RNA activity (protein synthesis efficiency) and relative abundance of myofibrillar proteins. Fasted male Sprague-Dawley rats (250+/-20 g) were given a 4-h i.v. continuous infusion of L-(1-14C)-leucine to determine leucine kinetics during the infusion of PAF (2 microg/kg PAF as a priming i.v. bolus 1 h before a 4-h i.v. infusion of 2 microg/kg/h PAF) or vehicle. PAF infusion caused sustained hypotension, hyperglycemia, hematological alterations, and hyperlacticacidemia. Whole-body protein synthesis was decreased by 24% (P < 0.05) and leucine flux oxidized was increased by 23% (P < 0.05). Leucine flux was reduced, although not significantly (P = 0.07), in PAF-treated rats (n = 8) compared with controls (n = 8). PAF significantly decreased fractional protein synthesis in the rectus abdominus (33%), soleus (30%), and extensor digitorum longus (26%) muscles, but not in the liver. Plasma branched-chain amino acid levels decreased (approximately 30%, P < 0.05) in PAF-treated rats. Muscle RNA activity was 32% lower and myosin relative abundance declined whereas actin was unchanged in PAF-treated rats. PAF induced net protein catabolism as a result of elevated leucine oxidation at the expense of protein synthesis. PAF had the cumulative effects in the skeletal muscle of (a) attenuating amino acid uptake, (b) reducing protein synthesis efficiency, (c) decreasing fractional protein synthesis rate, and (d) decreasing myosin relative abundance. Thus, PAF may be an important mediator of decreased protein synthesis in skeletal muscle during endotoxic and septic shock.