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 Ca balance maintained by NCX. We have examined the effects of partial inhibition of NCX by SEA-0400 (1 or 0.3 μmol/L) in left ventricular myocytes from healthy pigs or mice and from mice with heart failure (MLP). During voltage clamp ramps with [Ca]i buffering, block of reverse mode block was slightly larger than of forward mode (by 25±5%, P<0.05). In the absence of [Ca]i buffering and with sarcoplasmic reticulum (SR) fluxes blocked, rate constants for Ca influx and Ca efflux were reduced to the same extent (to 36±6% and 32±4%, respectively). With normal SR function the reduction of inward NCX current (INCX) was 57±10% (n=10); during large caffeine-induced Ca transients, it was larger (82±3%). [Ca]i transients evoked during depolarizing steps increased (from 424±27 to 994±127 nmol/L at +10mV, P<0.05), despite a reduction of ICaL by 27%. Resting [Ca]i increased; there was a small decrease in the rate of decline of [Ca]i. SR Ca 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 Ca content remained unchanged. These data provide proof-of-principle that even partial inhibition of NCX results in a net gain of Ca. Further development of NCX blockers, in particular, for heart failure, must balance potential benefits of INCX reduction against effects on Ca handling by refining mode dependence and/or including additional targets. © 2008 American Heart Association, Inc.