Tubular reabsorption and sodium excretion during urine reinfusion

The mechanisms responsible for the natriuresis that follows urine reinfusion was investigated in rats by clearance and micropuncture techniques.

In each animal two urine reinfusion periods (R1 and R2) were performed and compared to a non-urine-reinfusion, saline infusion period (S) sandwiched between them.

Switching from urine reinfusion to an equivalent rate of saline loading was followed by a fall in Na excretion from 1.9 +/- 0.5 to 0.5 +/- 0.2% of 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 could not account for these findings. A reduced Na excretion coupled to increased urine osmolality indicates enhanced transport along a segment responsible for the urinary concentrating mechanism. Thus the data can be interpreted then as due to enhanced reabsorption along the ascending limb of Henle's loop. These changes were reversed by reinstituting urine reinfusion after the S period. The consensual changes in Na+ and K+ excretion excluded an effect of urine reinfusion on the distal exchange site. There was a continuous fall in proximal reabsorption from R1 (76 +/- 3%) to S (69 +/- 3%) to R2 (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 was due to volume expansion. An osmotic effect of reinfused urine solutes was suggested by a late rise in plasma osmolality, from 312 +/- 13 to 323 +/- 8 mOsm/kg. Osmotic diuresis could have exerted additive effects upon those of volume expansion, accounting for the late fall in proximal reabsorption.

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 of the present acute experiments, suggest the possibility that solute retention may be linked to the chronic effects of urine reinfusion.