Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB014

Poster Communications

A high salt diet results in augmented osmolal gradients in rat skin

E. Nikpey1,2, O. Tenstad2, H. Wiig2

1. Medicine,, Haukeland University Hospital, Bergen, Norway. 2. Medicine, University of Bergen, Bergen, Norway.

Excessive salt (NaCl) ingestion or retention in the body is linked with hypertension. Although the kidney is the major regulator for body Na+, it has also been shown that the skin is involved in Na+ homeostasis and can be an important reservoir in case of Na+ retention. Little is known about the local distribution of sodium during high salt conditions. In previous experiments in rats we found that a high salt diet gave a significant increase in blood pressure, and resulted in increased Na+ accumulation in the skin along with an increased skin osmolality when compared with rats given low salt diet (Nikpey et al, Hypertension, 69, 660-668, 2017). The plasma osmolality did not differ significantly between the groups. These studies suggested that excessive Na+ accumulation in the skin creates a hypertonic microenvironment, but also that there might be gradients within the skin that may have consequences for the local handling and storage of osmolytes, including Na+. We wanted to explore this gradient in further detail because of its potential physiological importance for skin electrolyte homeostasis. Male Sprague Dawley rats received either high-salt diet; HSD (chow having 8% salt and 1% saline) or low salt-diet (control); LSD (chow having <0.1% salt and tap water) for 14 days. Upon termination of the feeding period, back skin was harvested and frozen. Contiguous sections of skin parallel to the surface from epidermis (40mm) through upper dermis to lower hypodermis (all 100mm) were prepared with a cryomicrotome, and were eluted in distilled (milli-Q) water for 24 h and analyzed for Na+/K+ in the eluate by HPLC. Assuming that there is an interstitial fluid volume of 40% in all layers, we found that there is a Na+ gradient from epidermis to hypodermis in both HSD (epidermis; 147mM - dermis; 173mM, n=1) and LSD (epidermis; 143mM - dermis; 99mM, n=1) rats. Simultaneously, the Na+ content in each layer of HSD skin was higher compared with LSD skin. The Na+ gradient was significantly higher in the skin layers of the HSD rat (149mM, n=1) compared to the LSD rat (90mM, n=1) (p<0.05, ANOVA). Because urea might also be a contributor to skin osmolality and a potential gradient, we assessed urea using an assay kit (Urea colorimetric assay kit, Biovision) in eluted skin sections from HSD and LSD rats. Urea concentration was higher in the superficial than in the deeper layers in both HSD and LSD skin. Thus urea concentration averaged 10 and 8.3 mmol/ml/mg dry weight in epidermis-dermis and deeper dermis, respectively, in HSD (n=2) with corresponding values of 10 and 1.7 mmol/ml/mg dry weight in LSD (n=2). Our preliminary experiments suggest that urea as well as Na+ may contribute to the observed osmolal gradient and supports the notion of countercurrent exchange in the skin.

Where applicable, experiments conform with Society ethical requirements