The sulfur contained in plasma is derived from sulfalipids, sulfatides, amino acids, and glycoproteins as well as from ionizable inorganic sulfate. The plasma concentration of inorganic sulfate is partially determined by the kidneys, since the ion is freely filterable at the glomerulus (1, 2) and reabsorbed by the renal tubules. In dogs (3) the excretory processes have been fairly well defined and the tubular reabsorptive mechanism has been shown to be characterized by a maximal transfer rate (Tmso4). The present study was undertaken to ascertain if similar mechanisms are operating in man. To elucidate this problem, the filtered load, tubular reabsorption, and urinary excretion of inorganic sulfate were measured prior to and during the infusion of sodium sulfate solution. Further, the readily exchangeable sulfate mass was determined using isotope dilution techniques. The results of this study show that the tubular reabsorption of inorganic sulfate in man is limited by a maximal transfer rate (Tm). MATERIALS AND METHODSSubjects. The subj ects were convalescent patients without apparent cardiovascular, renal, or metabolic disorders, and healthy young adult volunteers. All subjects had been on an ordinary diet and were fasted 12 hours prior to the study. concentration and the inulinoid blank in urine and plasma. To measure the glomerular filtration rate (GFR) a priming dose of inulin, administered to raise the blood level to approximately 0.25 mg per ml, was followed by a sustaining infusion containing inulin in saline, delivered by a Bowman constant infusion pump at a rate of 1.0 ml per minute.After an appropriate equilibration time, at least two urine collection periods were obtained for measurement of control glomerular filtration and sulfate excretion. Five per cent sodium sulfate 1 was subsequently administered by a second constant infusion pump to insure an independent rate of infusion of 2 to 10 ml per minute. Throughout each study, which lasted for about 4 hours, blood samples were obtained and urine collections were made at intervals of 30 to 40 minutes. Complete urine collections were obtained by urethral catheterization and bladder washout with 20 ml of sterile distilled water followed by an equal volume of air.Analytical methods. Inulin was determined by Schreiner's (4) modification of the method of Roe, and inorganic sulfate was analyzed by the method of Power and Wakefield (5).Determtiniation of the exchangeable sulfate miass. The readily available inorganic sulfate mass was calculated as the product of the fasting plasma inorganic concentration (Pso,) and the simultaneously measured volume of distribution of radiosulfate (Vsmo4). The volume of distribution of radiosulfate was derived by dividing the total administered radioactivity by the plasma radioactivity at the time of injection (6). This latter value was obtained by extrapolating the exponential disappearance curve of the injected radiosulfate to zero time. No correction was made for urinary loss of radiosulfate during the equilibration time. A...

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