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

Abstract

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.

Regulation of Na+/H+ Exchanger Isoform 3 by Protein Kinase A in the Renal Proximal Tubule

“…Second, the pattern of sensitivity to different inhibitors of Na + /H + exchange in renal brush border vesicles and of bicarbonate absorption in microperfused proximal tubules is most consistent with the properties of NHE3 among the known NHE isoforms (Vallon et al, 2000). Third, micropuncture analysis on NHE3 knockout mice revealed a remarkable reduction of fluid and bicarbonate reabsorption in renal proximal tubule further supporting the concept that NHE3 is the isoform that accounts for most Na + /H + exchange in this nephron segment (Lorenz et al, 1999;Wang et al, 1999). Indeed, mice lacking NHE3 display hypotension and had a mild hyperchloremic metabolic acidosis (Schultheis et al, 1998).…”

The Effects ofSyzygium aromaticum-Derived Oleanolic Acid on Kidney Function of Male Sprague–Dawley Rats and on Kidney and Liver Cell Lines

“…Na þ is mainly reabsorbed across the proximal tubular cell in a process mediated by proximal tubule apical NHE3 followed by uphill active extrusion via basolateral Na þ /K þ ATPase. 34,35 The literature on natriuretic hormones' actions in the proximal tubule, however, shows that transport inhibition is effected via parallel independent inhibition of apical NHE3 and basolateral Na þ /K þ ATPase. 36 We, therefore, speculate that OA inhibited apical NHE3 and basolateral Na þ /K þ ATPase accounting for Na þ excretion since NHE3 is a major Na þ transport pathway in the renal proximal tubule.…”

Tethering, recycling and activation of the epithelial sodium–proton exchanger, NHE3

“…Both microperfusion and micropuncture studies on NHE3-null mice revealed a dramatic decrease (Ϸ50%) in proximal tubular Na + and bicarbonate absorption (Lorenz et al, 1999;Wang et al, 1999). If not for a significant reduction in their glomerular filtration rate and increased distal Na + absorption, these mice would not be viable (Lorenz et al, 1999;Schnermann, 1999).The movement of Na + across the apical membrane creates a slight but significant osmotic gradient that is responsible for the osmotically driven flux of water (approximately 120 l day -1 ) (Lorenz et al, 1999). NHE3 should therefore also contribute to the passive paracellular flux of other ions -such as calcium and magnesium -from the lumen of the proximal tubule.…”