Effect of hydration state on renal responses to head-out water immersion in conscious dogs.

Renal responses to head-out water immersion (WI) (37 degrees C, WI 100 min) were studied in conscious, instrumented dogs during volume repletion (R), when all blood and urine losses were replaced with 0.9% NaCl, or without volume repletion (NR), to determine the influence of hydration state. The lithium clearance method was used to estimate the locus of the renal tubular fractional sodium excretion (FENa) responses. WI in the R condition increased urine flow (V) from 0.9 (+/- 0.1 SE) to 4.2 (+/- 0.6) ml/min and FENa from 0.7 (+/- 0.1) to 3.2 (+/- 0.8)%. Fractional proximal sodium reabsorption (FPRNa) decreased from 0.82 (+/- 0.03) to 0.69 (+/- 0.1)% and fractional distal sodium reabsorption decreased from 0.96 (+/- 0.01) to 0.88 (+/- 0.04)%. By comparison, WI in the NR condition elicited smaller increments in V and FENa, no change in FPRNa and a significant decrease of FDRNa from 0.97 (+/- 0.01) to 0.93 (+/- 0.01). Although there were quantitative differences in the renal responses in the R and NR conditions, there were similar increments in both arterial and atrial pressures as well as plasma atrial natriuretic peptide concentration; plasma arginine vasopressin was unaltered in either situation, and plasma renin activity was depressed in both conditions. Since plasma protein concentration was significantly lower during the R condition, the differing renal responses are probably related to differing levels of volume expansion in the R vs. NR condition.