Dissociation of saline-induced natriuresis from urea washout in the rat

Medullary urea washout has been postulated to play a major role in the natriuretic response to volume expansion (VE). To examine this hypothesis, renal tissue solute composition was measured in a natriuretic and nonnatriuretic model of VE. Renal tissue composition was analyzed during hydropenia, acute VE, acute VE with renal artery pressure reduced to 70 mmHg at the onset of saline loading (immediate clamping), and acute VE with renal artery pressure reduced to 70 mmHg after 45 min of saline loading (delayed clamping). Immediate clamping, a nonnatriuretic model of VE, prevented VE-induced urea washout and the increase in sodium and water content in the cortex and outer medulla. Delayed clamping, a natriuretic model of VE, produced a pattern of tissue composition very similar to immediate clamping. Tissue urea content was not significantly different in the two protocols and only minor differences in sodium and water content were noted. Thus, under these experimental conditions, VE-induced natriuresis can be dissociated from medullary urea washout, and other mechanisms must be invoked to explain the increased sodium excretion.