Tubular reabsorption and sodium excretion during urine reinfusion

Background. The mechanisms responsible for the natriuresis that follows uri ne reinfusion was investigated in rats by clearance and micropuncture techn iques, Methods. In each animal two urine reinfusion periods (R-1 and R-2) were per formed and compared to a non-urine-reinfusion, saline infusion period (S) s andwiched between them. Results. Switching from urine reinfusion to an equivalent rate of saline lo ading was followed by a fall in Na excretion from 1.9+/-0.5 to 0.5+/-0.2% o f filtered load, p<0.002, Urine osmolality rose, and urine to plasma inulin concentration ratio rose significantly from 73+/-14 to 147+/-21 (p<0.002). The changes in GFR, SNGFR, absolute and percent proximal reabsorption coul d not account for these findings. A reduced Na excretion coupled to increas ed urine osmolality indicates enhanced transport along a segment responsibl e for the urinary concentrating mechanism. Thus the data can be interpreted then as due to enhanced reabsorption along the ascending limb of Henle's l oop. These changes were reversed by reinstituting urine reinfusion after th e S period. The consensual changes in Na+ and K+ excretion excluded an effe ct of urine reinfusion on the distal exchange site. There was a continuous fall in proximal reabsorption from R-1 (76+/-3%) to S (69+/-3%) to R-2 (62/-5%) which was inversely correlated with the changes in hematocrit (R=0.49 , p<0.026). This indicates that part of the late diuresis and natriuresis w as due to volume expansion. An osmotic effect of reinfused urine solutes wa s suggested by a late rise in plasma osmolality, from 312+/-13 to 323+/-8 m Osm/kg. Osmotic diuresis could have exerted additive effects upon those of volume expansion, accounting for the late fall in proximal reabsorption. Conclusions. We conclude that the acute effects of urine reinfusion are due to changes in transport of solutes and permeability to water along distal tubular segments. The changes in plasma osmolality during the last period o f the present acute experiments, suggest the possibility that solute retent ion may be linked to the chronic effects of urine reinfusion.