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Competitive blocking of epithelial sodium channels by organic cations: The relationship between macroscopic and microscopic inhibition constants

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Summary

Fluctuation analysis of Na current passing the apical membrane in the skin ofRana ridibunda was used to study the kinetics of Na-channel blocking by several organic cations present in the outer solution together with 60mm Na. The ratios of the apparent off-rate and on-rate constants (the microscopic inhibition constants) thus obtained for triamterene, triaminopyrimidine (TAP), 5,6-diCl-amiloride, 5H-amiloride and amiloride itself are found to be in the mean about sevenfold smaller than the corresponding inhibition constants obtained from macroscopic dose-response curves. The apparent discrepancy is explicable by competition of the organic blocker with the channel block by Na ions (the self-inhibition effect). The type of interaction between extrinsic blockage and self-inhibition may be purely competitive or mixed. However, in case of mixed inhibition the competitive component must dominate the noncompetitive component by at least seven to one.

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Abbreviations

Nao :

Na ion activity in apical (mucosal) solution [mm]

Ao :

amiloride concentration in apical solution [μm]

AAo :

concentration of an amiloride analog [μm ormm]

TAP:

triaminopyrimidine

T:

triamterene

BIG:

benzimidazolyl-guanidine

PCMPS:

parachloromercuri-phenylsulfonate

I Na :

amiloride-blockable component of the macroscopic current passing the apical membrane [μamp cm−2]

a :

membrane area [cm2]

P Na :

permeability of the apical ensemble of amiloride-blockable Na channels [cm sec−1]

F :

Faraday's constant [coul mol−1]

K N :

inhibition constant [mm] of Na self-inhibition obtained macroscopically but in the absence of added organic blockers

K maAA :

macroscopic inhibition constant (or Michaelis Menten constant) [μm ormm] of the extrinsic blocker AA (amiloride analog) as obtained from the inflection point of a dose-response curve

K miAA :

microscopic inhibition constant [μm ormm] of blocker AA as obtained from noise analysis as the negative intercept of alinear rate-concentration plot with the abscissa (=k off/k on)

K AA :

ratio of the true rate constants (e.g.K A=k 20/k 02,K N=k 10/k 01)

K′AA :

ratio of blocking rate constants of AA at a

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  1. 2nd Department of Physiology, Universität des Saarlandes, 6650, Homburg/Saar, Germany

    Jack H. -Y. Li & Bernd Lindemann

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  1. Jack H. -Y. Li
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  2. Bernd Lindemann
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Li, J.H.Y., Lindemann, B. Competitive blocking of epithelial sodium channels by organic cations: The relationship between macroscopic and microscopic inhibition constants. J. Membrain Biol. 76, 235–251 (1983). https://doi.org/10.1007/BF01870366

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  • DOI: https://doi.org/10.1007/BF01870366

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