Na⁺ transport and flux ratio through apical Na⁺ channels in toad bladder

Abstract

In many epithelia, the rate of NaCl reabsorption is determined by the activity of a Na⁺ transport system in the outer (apical) membrane. In frog skin and toad bladder, this system is thought to involve transmembrane pores through which Na⁺ ions move down an electrochemical activity gradient^1–5. It has been shown, however, that increased intracellular Na⁺ concentration leads to reduction in unidirectional Na⁺ influx^6,7, raising the possibility that ions do not move through the channel independently. The Ussing flux ratio equation is one test for independence of passive ion movements⁸. In particular, single-filing of ions through long pores can account for effects such as reduction of unidirectional fluxes by ions on the opposite (trans) side of the membrane; this type of transport mechanism is characterized by flux ratio exponents >1 (refs 9–11). I have evaluated the flux ratio exponent (n′) for the apical Na⁺ channel in the toad bladder as the ratio of tracer permeability to electrical conductance at electrochemical equilibrium^9–11, that is, n′=RT/F²(G_Na/J_Na), where J_Na and G_Na are, respectively, unidirectional flux and the conductance for Na⁺ions through the channel in the absence of net Na⁺ movement. I report here values of n′ of 1.15±0.10 and 1.08±0.07 for activities of 40mM and 10 mM Na⁺, respectively, in the outer solution. This channel is therefore occupied by at most one Na⁺ ion at these activities.