Proteolytic Activation of ENaC Mediates Fructose‐Induced Salt‐Sensitive Hypertension

Abstract

Proteolytic cleavage of the amiloride‐sensitive epithelial sodium channel (ENaC) by serine proteases such as trypsin leads to channel activation. In nephrotic syndrome, glomerular injury allows filtration of such enzymes causing ENaC‐dependent Na retention and hypertension(PMID: 23216619). Dietary fructose also causes Na retention and salt‐sensitive hypertension at least, in part, via altered hormonal signaling and metabolism in the proximal nephron. Proximal tubules normally secrete proteases into the forming urine but whether they are a source of proteases that cleave and activate ENaC in fructose‐induced salt‐sensitive hypertension is unknown. We hypothesized that dietary fructose increases expression and release of proteases from the proximal nephron, and that these enzymes activate ENaC thereby causing Na retention and salt‐sensitive hypertension. To test this hypothesis we first measured proximal nephron protease expression and urinary protease excretion in rats on either a high‐salt diet containing 4% NaCl without fructose (high salt) or one containing high salt plus 20% fructose in the drinking water (high salt/fructose) for 7 days. High salt/fructose treatment increased proximal nephron expression of trypsin I, an enzyme known to cleave and activate ENaC, by 70% (p < 0.01) compared to high salt alone. Urinary excretion of trypsin I was 8.4±1.3 arbitrary units/μg protein in rats fed high salt while it was 20.3±4.6 arbitrary units/μg protein in those on the high salt/fructose diet, 142% greater (p < 0.02). There was no difference in total urinary protein excretion between groups. Finally, we examined the effect of high salt/fructose and high salt plus 20% glucose (high salt/glucose) on blood pressure before and after oral amiloride. After 7 days, the systolic blood pressure of rats on high salt/fructose was 142±6 mm Hg while it was only 121±6 mm Hg in those on high salt/glucose (p < 0.05). After 10 days, the systolic blood pressure of rats on high salt/fructose was 148±6 mm Hg while it was only 124±5 in those on high salt/glucose (p < 0.02). Amiloride treatment for 24 hrs reduced systolic blood pressure in rats on the high salt/fructose diet from 148±6 to 134±5 mm Hg but did not decrease it in the high salt/glucose group. We conclude that proteolytic cleavage of ENaC contributes to fructose‐induced salt‐sensitive hypertension and that the source of the protease(s) is likely the proximal tubule rather than glomerular filtration.Support or Funding InformationNLHBI HL128053