Inhibition by amiloride and quinacrine of specific [3H]nitrendipine binding to rat cardiac membranes
Stokke, M.; Hagelin, E.M.; Poulsson, C.; Patel, R.; Haile, Y.; Brørs, O.
Journal of Pharmacology and Experimental Therapeutics 260(3): 1366-1372
1992
ISSN/ISBN: 0022-3565 PMID: 1545399 Document Number: 400028
Binding studies were designed to test if and how amiloride and quinacrine affected the specific binding of [3H]nitrendipine (NIT) to rat cardiac membranes. Specific binding of NIT was inhibited in a dose-dependent manner by amiloride [Hill coefficient (nH), 1.46; concentration of inhibitor producing 50% inhibition (K0.5) = 9.2 x 10(-4) M] and quinacrine (nH = 0.54, K0.5 = 7.7 x 10(-6) M). The inhibitions were incomplete in the presence of 10(-3) M Ca ions. The Hofstee plot was convex upwards for amiloride and concave upwards for quinacrine. Amiloride increased the Kd, decreased the maximum specific binding and increased the k-1. Quinacrine increased the Kd without changing the maximum specific binding and increased the k-1. The effects of amiloride and quinacrine on k-1 were nonadditive. We conclude that amiloride and quinacrine bind to or close to the L-type Ca channel, and inhibit the specific binding of NIT by allosteric, complex interactions influenced by the free concentration of Ca++. The nonadditive allosteric effects suggest a shared mechanism of interaction for amiloride and quinacrine with the site(s) of NIT. Several mechanisms are discussed to explain how amiloride and quinacrine can produce such inhibition of NIT binding.