Osmotic pressure of the cutaneous surface fluid of Rana esculenta

The osmotic pressure of the cutaneous surface fluid (CSF) in vivo was measured for investigating whether evaporative water loss (EWL) derives from water diffusing through the skin or fluid secreted by exocrine subepidermal mucous glands. EWL was stimulated by subjecting R. esculenta to 30–34 °C. A solute-free paper disc was placed on the skin for sampling of the clear CSF uniformly covering the body surface. The osmolality measured in a Wescor Vapro Vapor Osmometer was, 173 ± 9 mosmol/Kg, mean ± se, n = 21 samples. The osmolality of CSF of isoproterenol injected frogs at 20 °C was, 149 ± 5 mosmol/Kg, n = 16. Osmolality of lymph was, 239 ± 4 mosmol/Kg, n = 8. Thus the flow of water across the epidermis would be in the direction from CSF to the interstitial fluid driven by the above osmotic gradients and/or coupled to the inward active Na+ flux via the slightly hyperosmotic paracellular compartment [EH Larsen et al. (2009) Acta Physiologica 195: 171–186]. It is concluded that the source of EWL of the frog on land is the fluid secreted by the mucous glands and not water diffusing through the skin. The study supports the hypothesis [EH Larsen (2011) Acta Physiologica 202: 435–464] that volume and composition of CSF are regulated by subepidermal exocrine gland secretion balanced by EWL into the atmosphere and solute coupled fluid uptake by the epidermal epithelium, respectively