P6 Skin Na+ and water channels as potential determinants of salt and blood pressure homeostasis

Abstract

Introduction Recent studies have suggested that the skin may play an important role in aging and hypertensive disease by offering an extensive depot for water-independent sodium storage. However, we do not know if local (skin) regulation of salt/water balance affects this phenomenon. Thus, we hypothesised that protein channels implicated in systemic salt and water balance are differentially expressed in the skin in aging and hypertension, and that this expression can be regulated by local milieu. Methods Skin samples from healthy young volunteers, 55–80 years old hypertensive patients and age-matched healthy controls were analysed by immunohistochemistry (IHC) for expression of epithelial sodium channel (ENaC), aquaporin 3 (AQP3) and aquaporin 1 (AQP1). The same analysis was performed on additional skin samples from healthy young volunteers, after a 6 days whole-tissue culture protocol in one of 4 randomly assigned conditions - i) control (DMEM +10% charcoal-stripped FBS) ii) salt-enriched medium (+30 mmol/L NaCl) iii) aldosterone (1 × 10–7) iv) salt-enriched medium +aldosterone. Rat and human kidney sections were used as positive controls. ImageJ was used for image analysis. Results No obvious differences for ENaC and AQP3 staining could be observed across young healthy and aged healthy or hypertensive subjects. Cultured samples from 17 young healthy volunteers (control, n=6; salt-enriched media, n=5; aldosterone media, n=4; salt-enriched media+aldosterone, n=2) showed AQP3 channels to be primarily expressed at the basal epidermal layers and minimally expressed at the more superficial layers, with an opposite pattern for ENaC, frequent changes in tissue architecture but no difference across culture groups. In contrast, along with a trend for reduction in the interdigitation in epidermal-dermal junction, we observed a higher AQP1 expression in aged hypertensive vs aged healthy subjects (p=0.069). Discussion For the cultured samples, the overall whole-tissue culture stress, suggested by marked changes in the tissue architecture and the AQP3/ENaC staining pattern, without differences across groups, could have masked the specific effect of different culture conditions. However, the lack of differences for AQP3 and ENaC across clinical groups also suggests that their expression could be kept constant by homeostatic factors in a wide range of conditions. At variance, although final interpretation is hampered by limited statistical power, the higher expression of AQP1 in aged hypertensive patients points to a potential role for AQP1 in extracellular fluid balance in the skin as well as in blood pressure regulation. Alternatively, an increase in endothelial AQP1 may be an adaptive mechanism in response to hypertension.