Amplification of the natriuretic response to acute unilateral nephrectomy by volume expansion

We examined the consequences of acute unilateral nephrectomy (AUN) on sodium (UNaV) and potassium excretion (UKV) in anesthetized rats which received either no volume replacement or an infusion of 1, 1.5, or 2% body weight of an albumin-saline solution during surgical preparation. Both UNaV and UKV during the initial control period increased progressively with graded volume repletion. AUN caused a significant increase in UNaV in each group, and the magnitude of this increment was related to the degree of volume repletion, and hence baseline UNaV: in rats replaced with 2% body weight of albumin-saline solution, UNaV rose from 1,801 +/- (SD) 260 to 3,435 +/- 1,748 nEq/min (p less than 0.005), while in rats receiving no replacement, it rose only from 213 +/- 106 to 469 +/- 186 nEq/min (p less than 0.005). In contrast, the increment in UKV was the same in each group despite higher baseline UKV related to graded volume repletion. In no group did GFR increase significantly. We conclude that volume expansion amplifies the natriuretic response to AUN by decreasing reabsorption in the proximal or distal nephron, or both. However, the postnephrectomy kaliuresis is not influenced by the degree of volume repletion, suggesting that it is mediated by a separate mechanism independent of volume expansion-induced changes in tubular function.