FEATHERSTON, WR, Q. R. ROGERS, AND RA FREEDLAND. Relative importance of kidney and liver in synthesis of arginine by the rat. Am. J. Physiol. 224 (l): Yl27-129. 1973.-The relative contributions of the kidneys and liver to the total synthesis of arginine in the rat were studied by following the conversion of intravenously injected L-citrulline-ureido-14C to L-arginine guanido-14C in intact controls or in rats with either the kidneys, or liver, or both completely ligated. The incorporation of the labeled arginine into the trichloroacetic acid-precipitable protein from liver, kidney, skeletal muscle, brain, and lung was determined. A greater decrease in the specific activity of skeletal muscle, lung, and liver protein was observed when the kidneys were ligated than occurred in the skeletal muscle, lung, and kidney protein when the liver was ligated. The specific activity of skeletal muscle protein was approximately 3.3 times higher when the liver was ligated than when the kidneys were ligated. Ligation of either the kidney, liver, or both had less of an effect on lowering the specific activity of brain protein than the ligation had on the specific activity of skeletal muscle protein indicating the brain may be synthesizing a portion of its own arginine from citrulline. The results provide evidence that the kidneys synthesize a considerably larger portion of the endogenous arginine used for body protein synthesis than does the liver. skeletal muscle; brain; lung; citrulline

THE ABILITY OF THE KIDNEY of the rat to synthesize arginine from citrulline has been well established (1, 3, 9). Rogers et al.(10) have recently presented evidence that in the liver the conversion of citrulline to arginine is limited and that much of the arginine synthesized is broken down to urea and ornithine. Their work suggested that the kidney may be supplying a significant portion of the arginine needed for tissue synthesis as a result of arginine synthesis in the kidney from citrulline produced in the liver. The present study was designed to measure the relative contributions of the kidneys and liver to the total-body synthesis of arginine. Results indicating arginine synthesis by the brain for use within this organ are also presented.