Printed in Great Britain THE ROLES OF SODIUM TRANSPORT AND ANION PERMEABILITY IN GENERATING TRANSEPITHELIAL POTENTIAL DIFFERENCES IN LARVAL SALAMANDERS*

Recent investigation has shown that the fully aquatic, larval form of the salamander Ambystoma tigrinum handles osmotic and ionic regulatory problems as do other fresh water and semiterrestrial animals. Faced with osmotic water entry from a hypotonic environment, it excretes a dilute urine to remain in water balance. Uptake of NaCl from artificial-pond water occurs at the rate of about i/i-equiv. (10 g.J^hr.-1. This maintains an ionic steady state by compensating for losses in the urine and by diffusion across the skin and gills (Alvarado, 1962; Alvarado & Kirschner, 1963). Animals in a steady state in pond water ([NaCl] = 1-2 mM/1.) have a potential difference across the body surface of 10-20 mV., body fluids positive to the external solution. Nothing is known about the mechanism generating electrical asymmetry in these animals. Transepithelial potential differences (TEPs) have been studied for many years, and our understanding of the ionic mechanisms underlying them is derived largely from studies on isolated frog skins and toad urinary bladders bathed on both sides by Ringer's solution. Little attention has been paid to the origin of TEPs in intact animals bathed by dilute solutions. Barker-Jorgensen, Levi & Zerahm (1954) showe