Abstract
Leukocyte endogenous mediator (LEM), a low-molecular-weight peptide synthesized by monocytic cells during phagocytosis, has been implicated as the host's initiator of the protein metabolic response to infection and inflammation. To determine whether administration of LEM would alter protein kinetics, appearance and oxidation of plasma tyrosine as well as the rates of protein synthesis in liver and skeletal muscle were determined in fasted rats that received a 30-hour continuous infusion of either physiologic saline, LEM, or heat-inactivated LEM. The LEM was obtained from rabbit peritoneal exudate and the treatment solutions supplied 2.8 × 10 8, cell equivalents 100 g of body weight (BW) per day. Endogenous tyrosine oxidation increased from 4.0 ± 0.4 μ mol 100 g , BW/h to 5.4 ± 0.7 μ mol 100 g , BW/h in animals infused with heat-inactivated LEM and to 7.5 ± 1.5 μ mol 100 g , BW/h in rats receiving LEM ( P < 0.01). Nonsecretory protein synthesis in the liver was greatest in rats administered LEM (2239 ± 325 mg/d) when compared with control groups receiving physiologic saline (1122 ± 195 mg/d) or heat-inactivated LEM (1374 ± 62 mg/d; P < 0.01), whereas skeletal protein synthetic rates were unchanged. Rates of muscle and collagen protein breakdown were estimated from the urinary excretion rate of N t-methylhistidine and hydroxyproline, respectively, and their excretion rose by 30% ( P < 0.05) and 42% ( P < 0.05) with LEM administration. These results suggest that administration of LEM stimulates a mobilization of amino acids from peripheral tissues to support increased visceral protein anabolism while whole body amino acid oxidation is also enhanced. Since similar effects follow fever and infection, these results suggest that LEM may play an underlying role in the protein metabolic response to infection and inflammation.
Published Version
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