title: Mechanistic distinction between activation and inhibition of (Na(+)+K(+))-ATPase-mediated Ca2+ influx in cardiomyocytes.
abstract: (Na(+)+K(+))-ATPase (NKA) mediates positive inotropy in the heart. Extensive studies have demonstrated that the reverse-mode Na(+)/Ca(2+)-exchanger (NCX) plays a critical role in increasing intracellular Ca(2+) concentration through the inhibition of NKA-induced positive inotropy by cardiac glycosides. Little is known about the nature of the NCX functional mode in the activation of NKA-induced positive inotropy. Here, we examined the effect of an NKA activator SSA412 antibody on (45)Ca influx in isolated rat myocytes and found that KB-R7943, a NCX reverse-mode inhibitor, fails to inhibit the activation of NKA-induced (45)Ca influx, suggesting that the Ca(2+) influx via the reverse-mode NCX does not mediate this process. Nifedipine, an L-type Ca(2+) channel (LTCC) inhibitor, completely blocks the activation of NKA-induced (45)Ca influx, suggesting that the LTCC is responsible for the moderate increase in intracellular Ca(2+). In contrast, the inhibition of NKA by ouabain induces 4.7-fold (45)Ca influx compared with the condition of activation of NKA. Moreover, approximately 70% of ouabain-induced (45)Ca influx was obstructed by KB-R7943 and only 30% was impeded by nifedipine, indicating that both the LTCC and the NCX contribute to the rise in intracellular Ca(2+) and that the NCX reverse-mode is the major source for the (45)Ca influx induced by the inhibition of NKA. This study provides direct evidence to demonstrate that the activation of NKA-induced Ca(2+) increase is independent of the reverse-mode NCX and pinpoints a mechanistic distinction between the activation and inhibition of the NKA-mediated Ca(2+) influx path ways in cardiomyocytes.
authors.affiliation: Department of Surgery, University of Maryland School of Medicine, Baltimore, MD 21201, USA. kxu002@umaryland.edu
authors.forename: Kai Y
authors.initials: KY
authors.lastname: Xu
authors.forename: Weizhong
authors.initials: W
authors.lastname: Zhu
authors.forename: Ling
authors.initials: L
authors.lastname: Chen
authors.forename: Christopher
authors.initials: C
authors.lastname: DeFilippi
authors.forename: Jin
authors.initials: J
authors.lastname: Zhang
authors.forename: Rui-Ping
authors.initials: RP
authors.lastname: Xiao
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D017136:Ion Transport; D009200:Myocardial Contraction; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000201:enzymology*; D051381:Rats; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013890:Thiourea / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: C101670:2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)isothiourea methanesulfonate; D000254:Sodium-Potassium-Exchanging ATPase; D013890:Thiourea; D002118:Calcium