Influence of prehydration on the changes in renal tissue composition induced by water diuresis in the rat.

1. The composition of renal tissue was determined in rats before and immediately after intravenous infusion of dextrose (2.5 g/100 ml.) in amounts sufficient to administer a positive fluid load of 4% body weight over 2 hr. The rats were classified into three groups, according to the preinfusion urine osmolality: hydropaenia, normal and moderately diuretic (over 2400, 800-1500 and below 800 mu-osmoles/g H(2)O, respectively).2. In non-infused rats, the steepness of the corticomedullary osmolal gradient varied, due to differences in both water and solute (sodium and urea) contents, and was related to urinary osmolality. Whereas differences in medullary and papillary solute contents occurred between all three groups, papillary water content was significantly higher only in the moderately diuretic animals.3. Dextrose infusion caused the induction of water diuresis, the lowest urinary osmolalities being produced in the previously moderately diuretic animals.4. Dextrose infusion caused a considerable reduction in the steepness of the corticomedullary osmolal gradient in all rats, particularly in the previously hydropaenic animals, due to changes in both solute (sodium and urea) and water contents. Whereas reductions in medullary and papillary solute contents occurred in all three groups, there was no further increase in papillary water content from the already high values seen in the noninfused diuretic animals.5. Thus, dextrose infusion largely abolished any previous differences in tissue water content, whereas significant, though small, differences in osmolal (particularly urea) content persisted.6. These data are discussed in terms of changes and differences in endogenous antidiuretic hormone (A.D.H.) release.7. Changes in the magnitude and direction of the urinary-papillary urea concentration difference are discussed in terms of passive transport, with probable A.D.H.-induced changes in nephron urea permeability.