Do Counter‐Current Exchange and Glycosaminoglycan Buffering Contribute in Salt Storage in Skin of Rats?

We have previously shown that there is an osmolality and urea gradient from epidermis to dermis and subcutis. This gradient may be a result of the proposed counter‐current mechanism for electrolyte homeostasis in skin. Our aims were to test whether there are gradients in salt concentration in skin generated by counter‐current exchange and whether glycosaminoglycans (GAGs) can store sodium making it ‘inactive’ in relation to fluid exchange and thereby act as buffers during high salt intake. Sprague‐Dawley rats were given low salt diet (LSD, <0.1% NaCl in chow and tap water), high salt diet (HSD, 8% NaCl in chow and 1% saline to drink), and DOCA‐salt diet (50mg/week deoxycorticosterone acetate, 150mg/21 days and 1% saline to drink). Rats were randomly assigned to either LSD (n=15), HSD (n=7) or DOCA salt (n=8) diet for measurements of extracellular volume (ECV) in tangential sequential skin sections. Two weeks into the diet regimen the rats were anesthetized, neprectomized and given the extracellular tracer 51 Cr‐EDTA i.v. that was allowed 120 min circulation time. Directly thereafter, shaved back skin was further processed by cutting sequential sections of 40 µm (epidermis) and 100 µm (dermis and subcutis) thickness in a cryostat taking care to avoid evaporation. The skin sections were weighed and counted in a gamma counter and the extracellular volume found as the plasma equivalent space of 51 Cr‐EDTA. After counting, the samples were dried until constant weight, then eluted in ultrapure water for seven days. The eluate was analyzed in an ion chromatograph to determine sodium and potassium content in each layer. Sulfated GAGs were determined in whole skin, dermis and subcutis using Blyscan sGAG colorimetric assay to determine sGAG content. When relating sodium content to ECV we found that there was a gradient from epidermis to dermis, and that this gradient was more pronounced in the DOCA rats. The LSD group had a mean sodium of 332.7±194.3 mM in epidermis and 118.3±20.15 mM in the following layer at 140±mM, with corresponding concentrations of 219.2±52 mM and 121.1±41.4 mM in HSD, and 757.8±426.1 mM and 123.7±11.45 mM in DOCA. There was no difference in sulfated GAGs in skin between LSD, HSD and the DOCA salt group in any of the skin regions. Our results suggest that there may be a gradient of salt in the skin with a higher concentration in ECV in epidermis supporting the notion of a counter‐current mechanism. However, our data do not support that an increased concentration of sulfated GAGs contribute to salt buffering during high salt conditions.