Amphibian ventral epithelia can perform significant Na+ transport, as characterized in the now-classic two-membrane model first demonstrated by Ussing and colleagues. This transport is normally demonstrated by using short-circuit current (Isc) to negate transport-generated potentials across epithelia suspended in Ussing chambers. While this model and method have been supported through decades of experimentation, the exact relationship between the transport current and the epithelial potential generated is often ignored, as potentials typically are treated as a factor to be systematically eliminated. Here, leopard frog (Rana spp.) ventral epithelium were utilized in Ussing chambers for tests of specifically how the relationship between epithelial potentials and transport currents depends upon external medium ionic content. Stable skin potentials were recorded using 300 mOsm NaCl; potentials were then reduced to 0 mV via Isc in order to estimate the magnitude of the ionic transport current. A subset of the epithelia prepared was then tested in one or more alternative ionic solutions (300 mOsm KCl, NaHCO3, and CaCl2). While identifiable skin potentials were detected in all solutions tested, only in NaCl were the magnitudes of the epithelial potential and short-circuit current statistically significantly related. Detectable skin potentials in non-Na+-containing solutions indicates the existence of non-Na+ electrogenic activity in this tissue, whose presence explains the relatively poor ability of transport current magnitudes to explain the magnitudes of observed skin potential. Estimates of molecular transport rates for Na+ exceed 4 x 1014 molec sec-1 cm-1, demonstrating the enormous osmoregulatory challenge faced by fresh water amphibians in maintaining ionic homeostasis.

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