Activation of reverse Na+-Ca2+ exchanger by skeletal Na+ channel isoform increases excitation-contraction coupling efficiency in rabbit cardiomyocytes.

ABSTRACT

Our prior work has shown that Na+ current (INa) affects sarcoplasmic reticular (SR) Ca2+ release by activating early reverse of the Na+-Ca2+ exchanger (NCX). The resulting Ca2+ entry primes the dyadic cleft, which appears to increase Ca2+ channel coupling fidelity. It has been shown that the skeletal isoform of the voltage-gated Na+ channel (Nav1.4) is the main tetrodotoxin (TTX)-sensitive Nav isoform expressed in adult rabbit ventricular cardiomyocytes. Here, I tested the hypothesis that it is also the principal isoform involved in the priming mechanism. Action potentials (APs) were evoked in isolated rabbit ventricular cells loaded with fluo-4, and simultaneously recorded Ca2+ transients before and after the application of either relatively low doses of TTX (100 nM), the specific Nav1.4 inhibitor µ-Conotoxin GIIIB or the specific Nav1.1 inhibitor ICA 121430. Although APs changes after the application of each drug reflected the relative abundance of each isoform, the effects of TTX and GIIIB on SR Ca2+ release (measured as the transient maximum upstroke velocity) were no different. Furthermore, this reduction in SR Ca2+ release was comparable with the value that we obtained previously when total INa was inactivated with a ramp applied under voltage clamp. Finally, SR Ca2+ release was unaltered by the same ramp in the presence of TTX or GIIB. In contrast, application of ICA had no effect of SR Ca2+ release. These results suggest that Nav1.4 is the main Nav isoform involved in regulating the efficiency of excitation-contraction coupling in rabbit cardiomyocytes by priming the junction via activation of reverse-mode NCX.NEW & NOTEWORTHY A number of studies suggest that the Na+-Ca2+ exchanger (NCX) activated by Na+ currents is involved in the process of excitation-contraction (EC) coupling in cardiac ventricular myocytes. Although insufficient to trigger sarcoplasmic Ca2+ release alone, the Ca2+ entering through reverse NCX during an action potential can prime the dyadic cleft and increase the Ca2+ current coupling fidelity. Using specific Na+ inhibitors in this study, we show that in rabbit ventricular cells the skeletal Na+ channel isoform (Nav1.4) is the main isoform responsible for this priming. Our study provides insights into a mechanism that may have an increased relevance where EC coupling is remodeled.