Expression and regulation of the sodium-calcium exchanger in cardiac microvascular endothelial cells.

1. The sodium-calcium exchanger (NCX) plays an important role in Ca2+ homeostasis. In the heart, NCX participates in the control of contraction and relaxation and in large vessel endothelial cells some data suggest that NCX could influence nitric oxide (NO) generation. In this context, the cardiac microvasculature has received considerable attention as a mediator of myocardial performance, via the release of paracrine acting factors such as NO. Therefore, the aim of the current study was to characterize NCX expression and regulation in cardiac microvascular endothelial cells (CMEC). The NCX expression was also examined in neonatal ventricular cardiomyocytes where aspects of its function and regulation have been well characterized. 2. The presence of functional NCX in CMEC was confirmed by the presence of a consistent rise in intracellular Ca2+ concentration ([Ca2+]i) in response to removal of extracellular Na+. Furthermore, NCX mRNA expression was readily detectable in CMEC. 3. In order to examine the role of possible physiological regulators of NCX expression, the effect of intracellular Ca2+ loading, caused by 24 h exposure to 10 mumol/L ouabain, was investigated. In Ca(2+)-loaded CMEC, there was a substantially greater rise in [Ca2+]i during exposure to Na(+)-free buffer: 33 +/- 6 versus 124 +/- 25 nmol/L% (P < 0.05), consistent with increased protein expression. Consistent with these findings, northern blot analysis confirmed the presence of a two-fold increase in NCX mRNA in these cells. 4. These data indicate the presence of functional NCX in CMEC and identify [Ca2+]i as a potential physiological regulator of expression.