title: Pharmacological inhibition of na/ca exchange results in increased cellular Ca2+ load attributable to the predominance of forward mode block.
abstract: Block of Na/Ca exchange (NCX) has potential therapeutic applications, in particular, if a mode-selective block could be achieved, but also carries serious risks for disturbing the normal Ca2+ balance maintained by NCX. We have examined the effects of partial inhibition of NCX by SEA-0400 (1 or 0.3 micromol/L) in left ventricular myocytes from healthy pigs or mice and from mice with heart failure (MLP-/-). During voltage clamp ramps with [Ca2+](i) buffering, block of reverse mode block was slightly larger than of forward mode (by 25+/-5%, P<0.05). In the absence of [Ca2+](i) buffering and with sarcoplasmic reticulum (SR) fluxes blocked, rate constants for Ca2+ influx and Ca2+ efflux were reduced to the same extent (to 36+/-6% and 32+/-4%, respectively). With normal SR function the reduction of inward NCX current (I(NCX)) was 57+/-10% (n=10); during large caffeine-induced Ca2+ transients, it was larger (82+/-3%). [Ca2+](i) transients evoked during depolarizing steps increased (from 424+/-27 to 994+/-127 nmol/L at +10 mV, P<0.05), despite a reduction of I(CaL) by 27%. Resting [Ca2+](i) increased; there was a small decrease in the rate of decline of [Ca2+](i). SR Ca2+) content increased more than 2-fold. Contraction amplitude of field-stimulated myocytes increased in healthy myocytes but not in myocytes from MLP-/- mice, in which SR Ca2+ content remained unchanged. These data provide proof-of-principle that even partial inhibition of NCX results in a net gain of Ca2+. Further development of NCX blockers, in particular, for heart failure, must balance potential benefits of I(NCX) reduction against effects on Ca2+ handling by refining mode dependence and/or including additional targets.
authors.affiliation: Division of Experimental Cardiology, University of Leuven, Belgium.
authors.forename: Semir
authors.initials: S
authors.lastname: Ozdemir
authors.forename: Virginie
authors.initials: V
authors.lastname: Bito
authors.forename: Patricia
authors.initials: P
authors.lastname: Holemans
authors.forename: Laurent
authors.initials: L
authors.lastname: Vinet
authors.forename: Jean-Jacques
authors.initials: JJ
authors.lastname: Mercadier
authors.forename: Andras
authors.initials: A
authors.lastname: Varro
authors.forename: Karin R
authors.initials: KR
authors.lastname: Sipido
mesh_terms: D000814:Aniline Compounds / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002316:Cardiotonic Agents / Q000494:pharmacology; D002478:Cells, Cultured; D004195:Disease Models, Animal; D004558:Electric Stimulation / Q000379:methods; D005260:Female; D006333:Heart Failure / Q000378:metabolism*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009200:Myocardial Contraction / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010647:Phenyl Ethers / Q000494:pharmacology*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013552:Swine
chemical_list: D000814:Aniline Compounds; D002121:Calcium Channel Blockers; D002316:Cardiotonic Agents; D010647:Phenyl Ethers; C430918:SEA 0400; D019831:Sodium-Calcium Exchanger; D002118:Calcium