Micropuncture analysis of single-nephron function in NHE3-deficient mice

The Na/H exchanger isoform 3 (NHE3) is expressed in the proximal tubule and thick ascending limb and contributes to the reabsorption of fluid and electrolytes in these segments. The contribution of NHE3 to fluid reabsorption was assessed by micropuncture in homozygous (Nhe3(-/-)) and heterozygous (Nhe3(+/-)) knockout mice, and in their wild-type (WT, Nhe3(+/+)) littermates. Arterial pressure was lower in the Nhe3(-/-) mice (89 +/- 6 mmHg) compared with Nhe3(+/+) (118 +/- 4) and Nhe3(+/-) (108 +/- 5). Collections from proximal and distal tubules demonstrated that proximal fluid reabsorption was blunted in both Nhe3(+/-) and Nhe3(-/-) mice (WT, 4.2 +/- 0.3; Nhe3(+/-), 3.4 +/- 0.2; and Nhe3(-/-), 2.6 +/- 0.3 nl/min; P < 0.05). However, distal delivery of fluid was not different among the three groups of mice (WT, 3.3 +/- 0.4 nl/min; Nhe3(+/-) 3.3 +/- 0.2 nl/min; and Nhe3(-/-), 3.0 +/- 0.4 nl/min; P < 0.05). In Nhe3(-/-) mice, this compensation was largely attributable to decreased single-nephron glomerular filtration rate (SNGFR): 10.7 +/- 0.9 nl/min in the Nhe3(+/+) vs. 6.6 +/- 0.8 nl/min in the Nhe3(-/-), measured distally. Proximal-distal SNGFR differences in Nhe3(-/-) mice indicated that much of the decrease in SNGFR was due to activation of tubuloglomerular feedback (TGF), and measurements of stop-flow pressure confirmed that TGF is intact in Nhe3(-/-) animals. In contrast to Nhe3(-/-) mice, normalization of early distal flow rate in Nhe3(+/-) mice was not related to decreased SNGFR (9.9 +/- 0.7 nl/min), but rather, to increased fluid reabsorption in the loop segment (Nhe3(+/+), 2.6 +/-. 0.2; Nhe3(+/-) 3.6 +/- 0.5 nl/min). We conclude that NHE3 is a major Na/H exchanger isoform mediating Na(+) and fluid reabsorption in the proximal tubule. In animals with NHE3 deficiency, normalization of fluid delivery to the distal tubule is achieved through alterations in filtration rate and/or downstream transport processes.