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1. The time course and extent of changes in the composition of renal tissue slices in water diuresis were determined by sacrificing groups of rats before and during the intravenous infusion of dextrose (2·5 g/100 ml.) in amounts sufficient to administer over 2 hr, and subsequently to maintain for up to 7
hr, a positive fluid load of 4% body weight.
2. The corticomedullary osmolal gradient characteristic of the nondiuretic rats was progressively dissipated until, at 7 hr, only papillary tip concentrations were higher than those of other segments.
3. The changes in individual constituents followed different time courses: (i) an increase in water content in all segments, particularly the papilla, was almost complete by 1 hr, preceding the maximal increases in urine flow; (ii) a marked decrease in papillary and medullary urea content in the first hour was followed by a slower, progressive decrease leading to an almost complete dissipation of the urea gradient by 7 hr; (iii) small, non-significant decreases in sodium content occurred in all segments in the first hr, followed by a further small, progressive decrease in papillary sodium content; (iv) changes in ammonium and potassium concentrations were mainly related to those in water content, since the contents of these solutes showed only small changes.
4. By 2 hr, differences in the rates of decline of osmolal and urea concentrations in urine and papilla led to urinary concentrations significantly lower than papillary values. The steep papilla-urine urea concentration difference became smaller, but remained significant even at 7 hr.
5. The findings are discussed in terms of changes in countercurrent mechanisms, particularly as influenced by anti-diuretic hormone.
6. The development of papilla/urine urea concentration ratio greater than unity is also considered in terms of passive transport with changes in membrane permeability.
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