Renal tubule‐specific knockdown of Na+/H+ exchanger 3 attenuates diabetes‐induced increase in kidney weight and blood pressure

Abstract

BackgroundThe Na+/H+ exchanger 3 (NHE3) plays a major role in sodium and bicarbonate reabsorption in renal tubules. However, the role of tubular NHE3 in the diabetic kidney remains incompletely understood.MethodsMice with floxed NHE3 heterozygously expressing Cre recombinase under the control of Pax8 promoter, in which NHE3 is knocked down along the entire tubular system (NHE3‐KO), were crossed with heterozygous Ins2Akita mice, a genetic model of type 1 diabetes, to generate 4 experimental groups; i) WT (n=11), ii) NHE3‐KO (n=7), iii) diabetic WT (n=5), and iv) diabetic NHE3‐KO (n=7). Blood and spot urine samples were collected for acid base analysis. Systolic blood pressure (SBP) was measured by automated tail cuff, and glomerular filtration rate (GFR) by plasma elimination kinetics of FITC‐sinistrin, both in conscious mice at 18–20 weeks of age. Then, kidneys were harvested to determine kidney weight.ResultsIn non‐diabetic mice, tubular NHE3‐KO markedly increased urine pH and HCO3− concentration vs WT (7.94±0.18 vs 6.69±0.15; 109±41 vs 7±2 mmol/L; each P<0.01), while blood pH and HCO3− concentration were maintained (7.40±0.01 vs 7.37±0.01; 26±1 vs 26±1 mmol/L). This was associated with lower GFR and left kidney weight (KW) in NHE3‐KO vs. WT (10.5±3.7 vs 18.5±1.5 ul/min/g, P<0.05; 5.0±0.2 vs 5.9±0.2 g/kg, P<0.01), while SBP was preserved (121±1 vs 122±2 mmHg). In NHE3‐KO and WT mice, diabetes increased blood glucose (~500 and 600 mg/dl at 9 and 17 weeks of age, respectively) and food and fluid intake (not shown), and decreased body weight (not shown), to the same extent and similar levels. In WT, diabetes lowered blood pH (7.26±0.03, P<0.01), tended to decrease blood HCO3− concentration (22.6±2.3, P=0.06), and reduced urine pH (5.98±0.15) and HCO3− concentration (0.9±0.3 mmol/L)(both P<0.05) vs non‐diabetic WT. This was associated with glomerular hyperfiltration (28.4±3.5 ul/min/g, P<0.05) and increased KW (11.7±1.6 g/kg, P<0.01) and SBP (128.3±1.1 mmHg, P<0.05) in diabetic vs nondiabetic WT. In NHE3‐KO, diabetes induced similar changes in acid base status and GFR as in WT: it lowered blood pH (7.33±0.01, P<0.01), while blood HCO3− centration was not significantly changed (25.0±1.2, P=0.41), reduced urine pH (6.93±0.11) and HCO3− concentration (5.1±1.1 mmol/L), and increased GFR (23.1±3.8 ul/min/g, P<0.05). In NHE3‐KO, diabetes did not change SBP (123.3±2.9 mmHg), and the increase in KW (8.3±0.5 g/kg, P<0.01) was likewise blunted in NHE3‐KO vs WT (increase of 65% vs 98%).Conclusion1) In the absence of diabetes, tubular KO of NHE3 induced urine HCO3− loss, associated with reduced GFR and KW, compensated blood acid base status, and preserved SBP. 2) Type 1 diabetes induced a metabolic acidosis and increased GFR, KW and SBP. 3) The effect of diabetes on acid base status and GFR occurred independent of tubular NHE3. 4) Tubular NHE3 was a determinant of the diabetes‐induced increase in KW and SBP.Support or Funding InformationNational Institutes of Health (R01DK56248, R01HL094728, P30DK079337)