Renal Reabsorption and Metabolic Conversion of Galactose in the Cat

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Abstract
Extrahepatic elimination of galactose occurs mainly in the kidney. Urinary excretion as well as metabolic conversion may be of significance. At plasma concentrations of galactose between 60 and 1200 mg per liter in cats urine to plasma concentration ratios below one were never observed. At low filtered loads the reabsorption fraction was close to 1.0. At higher loads the reabsorption fraction decreased and seemed to converge towards 0.6. The rate of metabolic conversion of galactose in the kidney — calculated as the RBF times the arterio‐venous concentration difference minus the excretion rate — exceeded considerably the reabsorption rate at low filtered loads. At increasing loads the metabolic rates approached maximum values between 50 and 100 μg per minute and gram kidney weight. The capacity of the kidney for metabolic conversion of galactose appeared (per gram organ weight) to be in the same order of magnitude as that of the liver. Push‐flow experiments indicated that reabsorption of galactose takes place exclusively in the proximal tubular system. When, at low plasma concentrations, the metabolic rate exceeds the reabsorption rate the intracellular concentration of galactose in the tubular wall cells may stay low and this may promote diffusion into the cells and account for the high reabsorption fractions found (conversion reabsorption). At higher concentrations, where the reabsorption rate exceeds the metabolic rate, reabsorption is probably due increasingly to carrier‐facilitated transcellular backdiffusion.