The role of the distal nephron mechanisms in the concentration of contrast medium in the urine

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Abstract
The concentrating mechanisms affecting contrast medium in the distal nephron were studied using 3 contrast media of differing osmolality (monomer, dimer and nonionic compounds) in antidiuretic dogs with 3 different basal solute excretion rates. Evidence has been obtained that, under conditions of contrast medium osmotic diuresis, the loop of Henle generates more free water. If the free water generated in response to the contrast medium solute load is reabsorbed in the collecting ducts, this can in part mask the different proximal tubular effects of contrast media of different osmolality. As basal solute excretion increases, the compensatory mechanism is overwhelmed and urine osmolality falls exponentially. At the higher basal solute excretion rates, different osmolar loads of contrast medium no longer produce different urine osmolalities and different urine contrast medium concentrations. Apparently, contrast media of reduced osmolality will not produce significant increases in urine contrast medium concentration under the conditions of solute diuresis which occur in renal failure.