Quantitative relation between hydrostatic pressure gradient, extracellular volume and active sodium transport in the epithelium of the frog skin ( R. temporaria)

Abstract

With a method previously described we have studied the quantitative relation between hydrostatic pressure gradients, area of cellular interspaces and active sodium transport in the epithelium of the isolated frog skin ( R. temporaria). It was shown that in frog skin epithelium, the extracellular space expands when the hydrostatic pressure on the inside of the skin is raised relative to that on the outside. If the intercellular area of the sections is taken to be a measure of the intercellular volume, a linear relationship holds between interspace volume and pressure (up to 30 cm H 2O). The time course of the expansion of the interspaces was followed by observations in vivo, using the swelling of the epithelium as a measure of the expansion of the interspaces. With NaCl Ringer as the medium, the interspaces are partly open, even at zero pressure. If sodium is replaced in the outside solution by the non-transported choline ion, the interspaces are totally collapsed at zero pressure, but expand in the usual way as the pressure on the inside is raised. With zero pressure gradient we further find, in short-circuited skins, a proportionality between short-circuit current and interspace volume, indicating that it is the active transport of sodium from the outside into the interspaces which determines their volume. We may assume that the transfer of sodium into the interspaces leads to a local increase in hydrostatic pressure by osmotic transfer of water from cells to interspaces. Due to the permeability to anions, notably chloride, enough sodium salts are transported into the interspaces, even at open circuit, to keep them slightly expanded. However, with choline as the cation, the interspaces collapse. The excess fluid collecting in the interspaces during sodium transport probably drains off through pores in the basement membrane. Since the net transfer of water between Ringer solutions is known to be exceedingly small, most of the water in this excess fluid must be recycled through the basal cells of the epithelium.