The Kinetics of Na²⁴ Flux Across Amphibian Skin and Bladder¹

Abstract

Abstract The sudden addition of Na 24 to the solution bathing one side of isolated frog skin or toad urinary bladder results in a steady increase in the labelled flux with time to a final steady state value. The build‐up of the labelled flux follows a single exponential course for both influx and outflux. This is consistent with a model involving a single sodium pool in the membrane. The build‐up half‐times for toad urinary bladder and for frog skin averages about 2–5 minutes and is not accountable as diffusion delay through the connective tissue. The steady state flux, divided by the time constant of flux build‐up, gives an estimate of the minimal amount of sodium in the pool, if only one pool is responsible for the delay. The calculated sodium pool size for urinary bladder and skin is in the same range, from 0.01 to 0.42 uMeq Na/cm 2 . This amount exceeds any amount imaginable as residing in the 250 Å thick basement membrane which has been proposed as the site of active transport. The pool is probably located in the epithelial cells, and the actively transported sodium passes through and not between the epithelial cells.