Preserved survival of renal tubulus‐specific sodium hydrogen exchanger isoform 3 knockout mice in response to sodium restriction
Authors: Chavez Samantha , Poulsen Søren Brandt , Soleimani Manoocher , Fenton Robert A , Rieg Timo 
Published: November 2023

The Na+/H+ exchanger isoform 3 (NHE3) facilitates Na+ absorption and H+ secretion. NHE3 is expressed in the intestine, proximal tubule and thick ascending limb of the kidney. Conventional NHE3 knockout mice show severe signs of dehydration or die when deprived of NaCl for 3 days (Am J Physiol Renal Physiol. 2001;281(4):F718‐27). However, the importance of kidney‐specific NHE3 inactivation in this context is unknown. To test the role of kidney‐specific NHE3 knockout in Na+ homeostasis we studied tubulus‐specific NHE3 knockout mice which have been previously generated by us (Am J Physiol Renal Physiol. 2015;308(12):F1409‐20). NHE3loxlox (Con) and NHE3loxloxPax8Cre (NHE3loxloxCre) mice were randomized and studied at baseline and for 10 days on zero (<0.01%) or high (4%) NaCl intake (N=5–7/group). At baseline and the end of each diet regimen blood was collected and analyzed for Na+, K+, aldosterone as well as glomerular filtration rate (GFR) determined via FITC‐Sinistrin clearance. Body weight was recorded daily. Under baseline conditions there was no difference between genotypes in body weight, fluid or food intake, plasma Na+ or K+, or blood pH. However, plasma aldosterone was significantly lower in NHE3loxloxCre compared to Con mice (204±25 vs 288±25 pg/ml, P<0.05) while urinary pH was significantly elevated (8.1±0.1 vs 7.2±0.1, P<0.05). Neither zero or high NaCl intake affected body weight during the experimental period in both genotypes. NaCl intake had no impact on plasma Na+ in Con mice; however, plasma Na+ in NHE3loxloxCre mice was susceptible to the effects of zero (−3.9±1.0 mM, P<0.05) and high NaCl (+2.2±0.6 mM, P<0.05) compared to baseline. Zero NaCl decreased plasma K+ in Con (−0.5±0.2 mM, P<0.05) but to a significant greater amount in NHE3loxloxCre mice (−1.2±0.1 mM, P<0.05). Aldosterone was not significantly different between Con and NHE3loxloxCre under zero (345±96 vs 498±78 pg/ml) and high NaCl intake (60±19 vs 86±20 pg/ml). Possibly as a consequence of impaired proximal tubular Na+ reabsorption and consequently activation of tubuloglomerular feedback, GFR was lower in NHE3loxloxCre mice under baseline conditions (457±20 vs 358±17 μl/min, P<0.05). Zero NaCl decreased GFR in Con (−110±13 μl/min, P<0.05) and NHE3loxloxCre (−99±8 μl/min, P<0.05) mice, whereas high NaCl was without effect on GFR in either genotype. The results indicate that maintenance of plasma Na+ requires functional NHE3 in the proximal tubule; otherwise dietary NaCl directly impacts total body NaCl possibly by disturbing glomerulo‐tubular balance. In addition, the role of renal NHE3 for overall Na+ homeostasis is less detrimental than previously reported for NHE3 knockout mice.Support or Funding InformationThis work was supported by an APS STRIDE Undergraduate Summer Research Fellowship 1R25HL115473‐01, the O'Brien Center for Acute Kidney Injury Research Grant P30DK079337, Diabetes Endocrinology Research Center P30DK063491, American Heart Association 15BGIA22410018, Satellite Healthcare (a not‐for‐profit renal care provider), Department of Veterans Affairs Merit Review Award, and Center on Genetics of Transport and Epithelial Biology at University of Cincinnati. Further funding is provided by the Lundbeck Foundation, the Danish Medical Research Council, and the Novo Nordisk Foundation.