Renomedullary Interstitial Cell Endothelin A Receptors Regulated Blood Pressure and Renal Function

Abstract

Renomedullary interstitial cells (RMIC) may modulate blood pressure (BP) and renal function through release of natriuretic factors and through direct effects on medullary blood flow (RMIC encircle vasa recta). Endothelin‐1 (ET‐1) modulates RMIC vasoactive factor release and contraction via ETA receptors (ETAR) in vitro. Consequently, we tested the hypothesis that knockout of ETAR in RMIC regulates renal function and BP in vivo. We bred ETARf/f mice containing loxP‐flanked exons 6–8 of the EDNRA gene with tamoxifen‐inducible tenascin‐C‐CreER2 knockin mice. Tenascin‐C‐CreER2:ETARf/f mice were injected with tamoxifen (160 mg/kg IP) for 5 consecutive days at 3 months of age to obtain RMIC ETAR KO. Littermates of the same genotype without injection served as controls. EDNRA gene recombination occurred in inner medulla, but not cortex. Inner medulla ETAR mRNA content fell by 80%, while ETBR mRNA levels were unaffected. ETRA KO did not affect BP in mice on normal (0.3% Na+) or low (0.03% Na+) salt diets. In contrast, ETAR KO reduced salt‐sensitive systolic BP elevation in mice on a high (3.2% Na+) salt diet (ETAR KO 127±3 vs. control 134±2 mmHg, n=13, 15, p<0.05). Urine volume and urinary Na+ and K+ excretion were higher in high‐salt diet‐treated ETAR KO mice compared with controls during the first 3 days of high salt intake. Urinary ET‐1, nitrite/nitrate, and prostaglandin E2 excretion were increased in high salt diet‐treated ETAR KO mice vs. controls. No significant differences were found in GFR, vasopressin, aldosterone, or renin levels between high salt fed ETAR KO and control mice. Inner medullary protein and mRNA levels of total NKCC2 and ENaC‐α and ‐β were decreased, and inner medullary cyclooxygenase‐2 (COX‐2), but not COX‐1, protein levels were increased, in high salt fed ETAR KO vs. control mice (n=8). Exogenous ET‐1 did not detectably alter RMIC intracellular Ca2+ concentration or cause RMIC contraction as assessed by ex vivo inner medulla confocal imaging. We conclude that disruption of RMIC ETAR reduces salt‐sensitive hypertension and this is associated with reductions in NKCC2 and ENaC together with elevations in several autocrine and paracrine natriuretic factors. These effects of RMIC ETAR, at least based on ex vivo studies, may not involve RMIC contraction. While further study is required, our data suggest a novel regulatory pathway for maintaining water/electrolyte homeostasis.Support or Funding InformationNHLBI P01 HL136267