Inhibition of sodium-calcium exchange in cardiac sarcolemmal membrane vesicles. 1. Mechanism of inhibition by amiloride analogues.
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 Nai-dependent Ca2+ uptake and Nao-dependent Ca2+ efflux. Inhibition of Na-Ca exchange monitored in K+ is noncompetitive vs Ca2+ but competitive vs Na+. Substitution of sucrose for K+ results in mixed kinetics of inhibition vs Ca2+, 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 concentrations, respectively. These results indicate that amiloride derivatives are mechanism-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+.
Slaughter
R S
RS
Department of Biochemistry, Merck Institute for Therapeutic Research, Rahway, New Jersey 07065.
Garcia
M L
ML
Cragoe
E J
EJ
Reeves
J P
JP
Kaczorowski
G J
GJ
D000584:Amiloride / Q000031:analogs & derivatives* / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002417:Cattle; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism*; D012508:Sarcolemma / Q000187:drug effects / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger; D013329:Structure-Activity Relationship; D013552:Swine
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D000584:Amiloride; D012964:Sodium; D002118:Calcium