Voltage-gated sodium channels in cardiac microvascular endothelial cells

The goal of this study was to determine whether inward Na+ or Ca+ currents could be measured in cardiac microvascular endothelial cells (CMEC). CMEC were isolated ii om rat ventricular muscle and studied during days 1-4 in culture. Differential uptake of fluorescently labeled acetylated low-density lipoproteins (LDL) indicated that the primary culture contained >90% CMEC. Membrane currents were measured with the use of the whole cell arrangement; of the patch-clamp technique with a Cs+ internal solution to prevent contamination by outward K+ currents. Voltage steps positive to -30 mV resulted in the activation of a fast, inward Na+ current (I-Na) Tn 20 cells examined, the peak inward current measured at 0 mV was 2.1 pA/pF. The half-maximal voltage required for inactivation of I-Na was -45 mV, and the current recovered from inactivation with a time constant of 10 ms. Inward currents were eliminated by replacement of external sodium with N-methylglucamine and were blocked by both tetrodotoxin (TTX) (dissociation constant = 5 nM) and saxitoxin (50 nM), Stimulation of protein kinase C, through application of phorbol 12,13-dibutyrate, resulted in an increase in the amplitude of I-Na without any change in the voltage dependence of current activation. Thus the endothelium of cardiac microvessels may be unique in expressing voltage gated, TTX-sensitive Na+ channels.