Mechanism of shortened action potential duration in Na⁺-Ca²⁺ exchanger knockout mice

Abstract

.-In cardiac-specific Na ϩ -Ca 2ϩ exchanger (NCX) knockout (KO) mice, the ventricular action potential (AP) is shortened. The shortening of the AP, as well as a decrease of the L-type Ca 2ϩ current (ICa), provides a critical mechanism for the maintenance of Ca 2ϩ homeostasis and contractility in the absence of NCX (Pott C, Philipson KD, Goldhaber JI. Excitation-contraction coupling in Na 1378 -1403, 2004) to determine the relative contributions of increased I to, reduced ICa, and reduced NCX current (I NCX ) on the shape and kinetics of the AP. Reduction of I Ca and elimination of INCX had relatively small effects on the duration of the AP in the computer model. In contrast, AP repolarization was substantially accelerated when I to was increased in the computer model. Thus, the increase in I to, and not the reduction of ICa or INCX, is likely to be the major mechanism of AP shortening in KO myocytes. We reported previously (27) that there are two fundamental mechanisms of adaptation to the absence of NCX: 1) a decrease in L-type Ca 2ϩ current (I Ca ) with an increased ECC gain and 2) a shortened and more rapidly repolarizing action potential (AP) that further reduces Ca 2ϩ influx. The more rapid repolarization of the AP renders it as effective as the wild-type (WT) AP in triggering sarcoplasmic reticular (SR) Ca 2ϩ release. These two mechanisms independently but synergistically limit Ca 2ϩ influx so that Ca 2ϩ homeostasis can be maintained by a low-capacity efflux mechanism (i.e., the plasma membrane Ca 2ϩ ATPase) while at the same time maintaining contractility.It is important to understand the mechanism responsible for the shortened AP in KO mice, since this mechanism plays a key role in assuring both Ca 2ϩ homeostasis and effective ECC in the absence of NCX. Both the elimination of NCX current (I NCX ) and the reduction of I Ca could potentially explain the abbreviated AP in KO myocytes. However, the dominant repolarizing current in cardiac ventricular myocytes is the transient outward current (I to ) (20). Recent studies (8,23,(35)(36)(37) have suggested that some of the cardiac K ϩ channel subunits that generate I to are tightly regulated by cytosolic Ca 2ϩ in their expression and activity, thus making regulation of the AP by I to in response to changes in Ca 2ϩ produced by knockout of NCX an attractive possibility.We report here that I to is increased in NCX KO mice and that the expression of the I to generating voltage-dependent K ϩ channel subunit Kv4.2 and the K ϩ channel interacting protein (KChIP) are upregulated. With the use of computer modeling, we further demonstrate that I to upregulation is the main determinant of AP shortening in KO mice. We hypothesize that altered Ca 2ϩ handling in NCX KO mice leads to upregulation of I to via an unknown mechanism. This in turn reduces AP duration and limits Ca 2ϩ influx. I to upregulation may comprise an important negative feedback mechanism against Ca 2ϩ overload in situations of reduced myocyte Ca 2ϩ extrusion capacity. METHODSGeneration of transgenic mi...

Activation of L-type Ca2+channels by protein kinase C is reduced in smooth muscle-specific Na+/Ca2+exchanger knockout mice

“…Similar results were previously described in cardiac-specific NCX1 knockout mice, where the reduced LVGC current is due to faster and more complete Ca 2ϩ -dependent inactivation (9,20). In contrast, in ASMCs, the decreased LVGC conductance is not due to an increase in channel inactivation or to a decreased expression of the channels.…”

“…The LVGCs are also downregulated in cardiac-specific NCX1 knockout mice (9,20), but the mechanism of the current decline is apparently different in ASMCs. In the heart, Ca 2ϩ -dependent inactivation of LVGCs is faster and more complete in the NCX1 knockout mouse myocytes than in control myocytes, resulting in reduced LVGC current.…”

Knockout of Na+/Ca2+ exchanger in smooth muscle attenuates vasoconstriction and L-type Ca2+ channel current and lowers blood pressure

“…In cardiac myocytes, NCX1 makes a major contribution to Ca 2ϩ extrusion during diastole (3, 11). Nevertheless, cardiac-specific NCX1 knockout mice thrive: the loss of NCX1 is compensated by a reduced L-type voltage-gated Ca 2ϩ channel (LVGC) current and a shortened action potential because of accelerated Ca 2ϩ -dependent LVGC inactivation and augmented transient outward K ϩ current (18,31,32). Importantly, cardiac NCX1-mediated Ca 2ϩ flux must be inward during depolarization and systole (5).…”

Induced overexpression of Na+/Ca2+ exchanger transgene: altered myocyte contractility, [Ca2+]i transients, SR Ca2+ contents, and action potential duration

“…Indeed, although NCX1 overexpression in isolated adult rabbit myocytes (by adenovirus-mediated gene transfer) results primarily in impaired myocyte contractility compared with control myocytes (25,30), in adult rat myocytes NCX1 overexpression can result in either decreased, no change, or increased contraction and intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) transient amplitudes, depending on prevailing [Ca 2ϩ ] o (45). Over the last decade, Philipson and his colleagues have made major contributions toward our understanding of NCX1 function in vivo and in vitro by the generation of constitutively expressed NCX1 transgenic (1,27,28,36,40) and the cardiacspecific NCX1 knock-out (17,(22)(23)(24) mice. During the same period, it became increasingly clear that alterations in NCX1 expression and/or activity are associated with many models of cardiac hypertrophy and heart failure (for review, see Ref.…”