Molecular Pharmacology, Vol 20, 543-550, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Kinetic Analysis of the Amiloride-Sodium Entry Site Interaction in Rabbit Colon

¹ Pharmakologisches Institut der Universität Wien, A-1090 Vienna, Austria

The diuretic amiloride inhibits apical sodium entry in isolated epithelia of rabbit decending colon primarily by decreasing the maximal sodium transport capacity of the tissue. Since in addition the apparent affinity of sodium to the transport system is also reduced, the type of inhibition exerted by amiloride has to be termed mixed. Increasing sodium concentrations cause a rightward shift of the amiloride concentration-response relationship. Elevation of cellular sodium by ouabain inhibits the action of amiloride noncompetitively. The concentration-response relationships of amiloride in the presence of various sodium concentrations yield Hill coefficients between 0.93 and 1.03, indicating a stoichiometry of the amiloride-sodium entry site interaction of 1:1. It is concluded that the sodium translocation site and the amiloride binding site are separate and that extracellular sodium decreases the apparent affinity of amiloride to its binding site, whereas increased intracellular sodium depresses the maximal amiloride response.

Note:
ACKNOWLEDGMENTS Amiloride was a generous gift of Merck Sharp & Dohme, Rahway, N. J. The technical assistance of Mr. P. Krivanek is gratefully acknowledged.