Frog skin function revisited: physiology of cutaneous surface fluid of Rana esculenta

Amphibians are adapted to living on dry land where body fluid turnover is governed by evaporative water loss. We aimed at testing the hypothesis [EH Larsen (2011) Acta Physiologica 202: 435–464] that water is evaporating from the cutaneous surface fluid (CSF) secreted by subepidermal glands. Samples of CSF contained, [Na] = 64.5±5.1 and [K] = 14.9±1.6 mM (mean±s.e.m., n = 16) with osmotic pressures of CSF and hemolymph, 168±4 (n = 22) and 238±1.7 (n = 25) mOsm/Kg. The relatively high [K] of CSF and an inwardly directed driving force for transepithelial water flux confirm that gland secretions characterized by high [K] constitute the source of water evaporating from the body surface. Thus, on land CSF is maintained by a balance between fluid secretion by subepidermal glands, water evaporation into the atmosphere, and reabsorption of water and Na by principal cells and Cl by mitochondria-rich cells of the epidermis. These mechanisms have evolved pari passu with life alternating between aquatic and terrestrial habitats associated with permeabilities of the skin controlled by external ion- and osmotic concentrations (loc. cit.). This allows for fast switching of the cutaneous uptake of chloride between active and passive transport associated with dynamic electrical coupling of active sodium uptake by principal cells and passive chloride uptake by mitochondria-rich cells. Supported by the Carlsberg Foundation.