Sodium-calcium exchanger (NCX-1) and calcium modulation: NCX protein expression patterns and regulation of early heart development

Ouabain-induced inhibition of early heart development indicated that Na/K-A T-Pase plays an important role in maintaining normal ionic balances during differentiation of cardiomyocytes (Linask and Gui [1995] Dev Dyn 203: 93-10 5). Inhibition of the sodium pump is generally accepted to affect the activ ity of the Na+-Ca++ exchanger (NCX) to increase intracellular [Ca++]. These previous findings suggested that Ca++ signaling may be an important modula tor during differentiation of cardiomyocytes. In order to identify a connec tion between heart development and NCX-mediated Ca++ regulation, we determi ned the embryonic spatiotemporal protein expression pattern of NCX-1 during early developmental stages. In both chick and mouse embryos, NCX-1 (the ca rdiac NCX isoform) is asymmetrically expressed during gastrulation; in the right side of the Hensen's node in the chick, in the right lateral mesoderm in the mouse. At slightly later stages, NCX-1 is expressed in the heart fi elds at comparable stages of heart development, in the chick at stage 7 and in the mouse at embryonic day (ED) 7.5. By ED 8 in the mouse, the exchange r protein displays a rostrocaudal difference in cardiac expression and an o uter curvature-inner curvature ventricular difference. By ED 9.5, cardiac e xpression has increased from that seen at ED8 and NCX-1 is distributed thro ughout the myocardium consistent with the possibility that it is important in regulating initial cardiac contractile function. Only a low level of exp ression is detected in inflow and outflow regions. To substantiate a role f or the involvement of calcium-mediated signaling, using pharmacologic appro aches, ionomycin (a Ca++ ionophore) was shown to perturb cardiac cell diffe rentiation in a manner similar to ouabain as assayed by cNkx2.5 and sarcome ric myosin heavy chain expression. In addition, we show that an inhibitor o f NCX, KB-R7943, can similarly and adversely affect early cardiac developme nt at stage 4/5 and arrests cardiac cell contractility in la-somite embryos . Thus, based upon NCX-1 protein expression patterns in the embryo, experim ental Ca++ modulation, and inhibition of NCX activity by KB-R7943, these re sults suggest an early and central role for calcium-mediated signaling in c ardiac cell differentiation and NCX's regulation of the initial heartbeats in the embryo, (C) 2001 Wiley-Liss, Inc.