[en] Bepridil, an antiarrhythmic agent, inhibits Na-Ca exchange in cardiac sarcolemmal membrane vesicles by a novel mechanism, different from that determined for amiloride analogues. Bepridil causes partial inhibition of Na/sub o/-dependent ⁴⁵Ca²⁺ efflux. Inhibition of Na-Ca exchange is noncompetitive vs Ca²⁺ but competitive vs Na⁺ in both K⁺ and sucrose. Bepridil also blocks Ca-Ca exchange, with or without K⁺ present. However, K⁺ has two effects on inhibition: it reduces the potency of bepridil and causes inhibition to become partial. Inhibition of Ca-Ca exchange displays noncompetitive kinetics vs Ca²⁺ in either sucrose of K⁺. Dixon analyses of Na-Ca exchange inhibition caused by mixtures of bepridil and amiloride analogues demonstrate that these compounds produce a competitive interaction at a common carrier site that is evident only a low concentrations of amiloride inhibitors. Hill plots of bepridil inhibition of Na-Ca and Ca-Ca exchange display unitary Hill coefficients. These results indicate that bepridil interacts at only one substrate-binding site, the site selective for Na⁺, where amiloride analogues also preferentially interact. However, unlike amiloride, bepridil does not interact at the common Na⁺, Ca²⁺-binding site of the carrier. During stimulation of Ca-Ca exchange, K⁺ may bind at other sites besides the site selective for Na⁺, because in addition to being competitive with bepridil, it prevents complete abolition of Ca-Ca exchange, suggesting that this Na⁺ site is not involved in carrier turnover. These findings indicate the bepridil is a mechanisms-based Na-Ca exchange inhibitor that interacts at a transporter site which binds Na⁺ but not Ca²⁺

[en] The mechanism by which terminal guanidino nitrogen substituted analogues of amiloride inhibit Na-Ca exchange in purified cardiac sarcolemmal membrane vesicles has been investigated. These inhibitors block both Na/sub i/-dependent Ca²⁺ uptake and Na₀-dependent Ca²⁺ efflux. Inhibition of Na-Ca exchange monitored in K⁺ is noncompetitive vs ⁴⁵Ca²⁺ but competitive vs ²²Na⁺. Substitution of sucrose for K⁺ results in mixed kinetics of inhibition vs ⁴⁵Ca²⁺, suggesting a complex interaction between inhibitor and carrier under this condition. Amiloride derivatives also block two other modes of carrier action: Na-Na exchange is inhibited in a competitive fashion with Na⁺ and kinetics of Ca-Ca exchange inhibition are mixed vs Ca²⁺ in either sucrose or K⁺. However, Ca-Ca exchange inhibition can be alleviated by increasing K⁺ concentration. Dixon analyses of Na-Ca exchange block with mixtures of inhibitors suggest that these agents are interacting at more than one site. In addition, Hill plots of inhibition are biphasic with Hill coefficients of 1 and 2 at low and high inhibitor concentration, respectively. These results indicate that amiloride derivatives are mechanisms-based inhibitors that interact at two classes of substrate-binding sites on the carrier; at low concentration they bind preferentially to a site that is exclusive for Na⁺, while at higher concentration they also interact at a site that is common for Na⁺, Ca²⁺, and K⁺