Gap Junction-associated Na+ Influx Is Involved in the Mediation of Ca2+ Transients in Neonatal Rat Ventricular Myocytes

Gap junction (GJ) coupling is involved in the regulation of spontaneous Ca(2+) transients in neonatal rat ventricular myocytes (NRVMs); however, the underlying mechanism(s) remains incompletely clear. In this study, we investigated Na influx mediated by Cx43-associated GJ modulating the spontaneous NRVM excitation. Intracellular Na(+) and Ca(2+) concentrations ([Na(+)](i) and [Ca(2+)](i)) were assessed using CoroNa Green and Fluo-4 fluorescent probes, respectively, by confocal microscopy. GJ function was evaluated by measuring fluorescence recovery after photobleaching. The results showed that [Na(+)](i) and spontaneous Ca(2+) oscillating were significantly decreased by the treatment of NRVMs with GJ uncouplers or adenovirus-mediated Cx43 gene knockdown using short hairpin RNA, while both of the intracellular cation levels were increased by adenovirus-mediated Cx43 overexpression. Neither Na(+) channel blocker (tetrodotoxin) nor the GJ uncoupler (Gap27) or Cx43-short hairpin RNA adenovirus could completely suppress the Ca(2+) transients, but their combination usage could. In addition, Ca(2+) transients disappeared in NRVMs incubated in Na(+)-free, but not in Ca(2+) -free medium, in which the remained transients could be further abolished by Gap27. Collectively, our findings suggest that Cx43-associated GJ function is involved in the regulation of Na(+) influx into cardiomyocytes, which contributes, at least in part, to triggering spontaneous excitation and regulation of cardiomyocyte automaticity.