TISSUE NA+ EXCESS UPON EXPERIMENTAL SALT-LOADING IS SYSTEMIC AND REFLECTS ISOTONIC OEDEMA

Abstract

Objective: Skin was identified as a large depot for Na+ accumulation. Its proposed water-independent (hypertonic) nature has the potential to induce local pathogenic changes, including activation of the osmotic stress sensor TonEBP-mediated lymphangiogenic signalling. Blockade of this response led to salt-sensitive hypertension. Our study tested the hypertonic nature and organ-specific distribution of tissue Na+ accumulation upon salt-loading. Design and method: Twelve weeks-old male and female stroke-prone spontaneously hypertensive (SHRSP) and control Wistar-Kyoto (WKY) rats were treated with 1% NaCl in drinking water (high salt, HS) or normal water (NS) for 3 weeks (n = 8–10/sex/group). Tissue water and Na+/K+ content, as well as TonEBP gene expression in abdominal skin, liver, lung, myocardium and skeletal muscle were measured by gravimetric approach (wet–dry weight), flame photometry and qRT-PCR, respectively. The functional effects of in-vivo and ex-vivo exposure of resistance arteries to salt excess (HS treatment and incubation in +15 mM NaCl, respectively) were assessed and compared by wire myography. Results: Tissue Na+ increased in skin (+20.7%), but also liver (+10.3%), lungs (+14.8%) and skeletal muscle (+23.6%) in HS-SHRSP compared to NS; apart from skeletal muscle, it was consistently paralleled by water (skin: +14.1%; liver: +4.8%; lungs: +9.0%; p < 0.05 for all). Patterns were similar between males and females and absent in WKY. In both strains and all organs, K+ + Na+ concentration was not affected by HS and sat within physiological limits, thus pointing against hypertonicity. Nevertheless, TonEBP gene expression changes matched Na+ and water isotonic accumulation in SHRSP-HS skin, liver and lungs. Hypertonic incubation did not induce hypercontractile responses but exacerbated the sensitivity to NO-mediated relaxation in arteries from both strains (δIC50, p < 0.001). The effect of in-vivo HS, in keeping with classic hypertensive vascular changes, was opposite (p < 0.001, in SHRSP only). Conclusions: If water independent, interstitial Na accumulation is unlikely to underpin the development of experimental hypertension via a direct impact on vascular function. Tissue Na+ excess upon HS rather reflects isotonic oedema accumulation and systemic changes in tissue architecture, not solely confined to skin, and sensed by TonEBP.