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Apical membrane permeability and kinetic properties of the sodium pump in rabbit urinary bladder.
Author(s) -
Lewis S A,
Wills N K
Publication year - 1983
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1983.sp014799
Subject(s) - sodium , rabbit (cipher) , permeability (electromagnetism) , chemistry , biophysics , urinary bladder , membrane , urinary system , urology , endocrinology , biochemistry , medicine , biology , computer science , organic chemistry , computer security
Previous studies have shown that aldosterone stimulates the rate of Na+ transport across the rabbit urinary bladder epithelium by increasing the apical membrane permeability to Na+. Paradoxically, ion‐sensitive and conventional micro‐electrode measurements demonstrated that intracellular Na+ activity aiNa+ was essentially unchanged by aldosterone, i.e. aiNa+ was constant regardless of the rate of Na+ transport. The present study was designed to resolve this apparent contradiction. The effects of elevated, endogenous aldosterone levels produced by low‐Na+ diet (Lewis & Diamond, 1976) on urinary bladder Na+ transport were investigated in vitro using Ussing‐type chambers and intracellular conventional and ion‐sensitive microelectrodes. Apical membrane selectivity and kinetics of the Na+ pump were assessed as a function of hormone stimulation. The aldosterone‐stimulated increase in Na+ transport was accounted for by increases in both the relative selective permeability of the apical membrane to Na+ and an increase in its absolute Na+ permeability. The kinetics of the Na+ pump were evaluated electrically by loading the cells with Na+ (monitored with Na+‐sensitive micro‐electrodes) or alternatively by manipulating serosal solution K+ concentration and measuring changes in the basolateral membrane electromotive forces and resistance. From these measurements the current generated by the pump was calculated as a function of intracellular Na+ or extracellular K+. The kinetics of the pump were not altered by aldosterone. A model of highly co‐operative binding estimated Km for Na+ as 14.2 mM and 2.3 mM for K+. Hill coefficients for these ions were 2.8 and 1.8, respectively, consistent with a pump stoichiometry of 3 Na+ to 2 K+. The kinetic properties of the Na‐K pump indicate that physiological levels of aiNa+ are poised at the foot of a step kinetic curve which energetically favours Na+ extrusion.