EXCRETION OF SOLUTES AND OSMOTIC WORK DURING OSMOTIC DIURESIS OF HYDROPENIC MAN. THE IDEAL AND THE PROXIMAL AND DISTAL TUBULAR WORK; THE BIOLOGICAL MAXIMUM OF WORK

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Abstract
The excretion of solutes and the renal work under conditions of osmotic diuresis in man were studied. The loading solutes were admd. intraven. in amts. of 500-2000 m.Osm./1.73 m2 body surface. They were glucose, sucrose, mannitol, sorbitol, sorbose, xylose, urea, creatinine, Na para-aminohippurate, Na2SO4 and NaCl. The urine flow increased widely up to 22.8 ml./min. The plasma osmolarity averaged 330 [plus or minus] 30 m.Osm./l. while the urine osmolarity decreased. The concn. of the loading solute in plasma rose as high as 60 m.Osm./l. in the case of mannitol. In the urine it averaged 330 m.Osm./l. for most loading solutes. As the urine flow rose the excretion of Na and Cl rose proportionately, while that of K and phosphate tended to remain constant. Na salts produced an increased K loss, while sulfate decreased the chloride loss. The calculated ideal osmotic work rose during loading to a max. of 4 cal./min., a value which was not increased by further augmentation of urine flow or load or plasma level of the loading solute. The same max. value was reached with all loading solutes excepting NaCl, NaPAH and urea. NaPAH was not admd. in adequate amts., but in the case of urea and NaCl, theoretical reasons exist for the failure to reach maximum work. A subdivision of the renal work in proximal and distal portions, based on the observed flow-load relations of urine has been attempted. It is suggested, that while the proximal tubular work is essentially "chemical" in nature, depending on the concn. in plasma and urine of individual solutes, that in the distal tubule is "osmotic," depending on the total osmolarity of plasma and urine. The proximal work is closely similar in magnitude to the ideal total work. The distal work is usually a small component of the total work. The relations among the renal work, the plasma concn. and the urinary load under the simplifying assumption of a single urinary solute are graphically presented.