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 gradient1–5. It has been shown, however, that increased intracellular Na+ concentration leads to reduction in unidirectional Na+ influx6,7, 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 equilibrium9–11, that is, n′=RT/F2(GNa/JNa), where JNa and GNa 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.
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Palmer, L. Na+ transport and flux ratio through apical Na+ channels in toad bladder. Nature 297, 688–689 (1982). https://doi.org/10.1038/297688a0
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DOI: https://doi.org/10.1038/297688a0
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