NHE3 null mice are only mildly acidotic, despite a profound defect in sodium bicarbonate reabsorption in the proximal tubule (Schultheis et al. 1998). Recent evidence suggests that a significant fraction of bicarbonate reabsorption in the distal convoluted tubule is mediated by NHE2 (Wang et al. 2001). To assess the possible role of distal tubule NHE2 in compensating for the proximal defect in NHE3 null mice, we have assessed the impact of HOE694, an inhibitor with affinity for NHE2, on excretion of sodium and bicarbonate in both NHE3 null and wild-type mice.
Mice (n = 12 in each group) were anaesthetised (Inactin, 100 mg kg-1 I.P.) and infused intravenously with a saline solution containing [3H]inulin for the measurement of glomerular filtration rate (GFR). After 1 h of measurements, mice in each group received either HOE694 (3 mg kg-1; 3 mg kg-1 h-1) or vehicle (1 % DMSO) alone, and measurements were performed during the subsequent hour. At the end of the experiment, animals were killed by an overdose of anaesthetic. Data are means ± S.E.M. Comparisons were made by ANOVA.
GFR (0.62 ± 0.07 vs. 1.05 ± 0.05 ml min-1 100 g-1; P < 0.05) and sodium excretion (0.14 ± 0.02 vs. 0.21 ± 0.03 mmol min-1; P < 0.05) were lower in null mice than in wild-type. Net acid excretion was also reduced in NHE3 null mice, reflecting a reduction in phosphate excretion (0.6 ± 0.3 vs. 73.7 ± 8.4 nmol min-1; P < 0.05) and an increase in bicarbonate excretion (15.0 ± 2.2 vs. 2.5 ± 0.7 nmol min-1; P < 0.05). Ammonium excretion was similar in both groups. In wild-type mice, HOE694 had no effect on electrolyte excretion rate. In contrast, both sodium excretion (0.24 ± 0.05 mmol min-1; P < 0.05) and bicarbonate excretion (33.5 ± 4.9 nmol min-1) were higher in NHE3 null mice receiving HOE694 than in time controls. The drug did not affect GFR.
These data indicate that the acidosis observed in NHE3 null mice reflects impaired renal acid-base handling. Our results suggest that increased NHE2 activity contributes to a compensatory increase in renal sodium bicarbonate reabsorption although the mechanism of upregulation remains unclear.
M.A.B. was funded by The Wellcome Trust.
All procedures accord with current National and local guidelines.