Molecular identification of a TTX-sensitive [Ca.sup.2+] current

Molecular identification of a TTX-sensitive [Ca.sup.2+] current. Am J Physiol Cell Physiol 280: C1327-C1339, 2001.--The TTX-sensitive [Ca.sup.2+] current [[I.sub.Ca(TTX)]] observed in cardiac myocytes under [Na.sup.+]-free conditions was investigated using patch-clamp and [Ca.sup.2+]-imaging methods. [Cs.sup.+] and [Ca.sup.2+] were found to contribute to [I.sub.Ca(TTX)], but [TEA.sup.+] and N-methyl-D-glucamine ([NMDG.sup.+]) did not. HEK-293 cells transfected with cardiac [Na.sup.+] channels exhibited a current that resembled [I.sub.Ca(TTX)] in cardiac myocytes with regard to voltage dependence, inactivation kinetics, and ion selectivity, suggesting that the cardiac [Na.sup.+] channel itself gives rise to [I.sub.Ca(TTX)]. Furthermore, repeated activation of [I.sub.Ca(TTX)] led to a 60% increase in intracellular [Ca.sup.2+] concentration, confirming [Ca.sup.2+] entry through this current. [Ba.sup.2+] permeation of [I.sub.Ca(TTX)], reported by others, did not occur in rat myocytes or in HEK-293 cells expressing cardiac [Na.sup.+] channels under our experimental conditions. The report of block of [I.sub.Ca(TTX)] in guinea pig heart by mibefradil (10 [micro]M) was supported in transfected HEK-293 cells, but [Na.sup.+] current was also blocked (half-block at 0.45 [micro]M). We conclude that [I.sub.Ca(TTX)] reflects current through cardiac [Na.sup.+] channels in [Na.sup.+]-free (or 'null') conditions. We suggest that the current be renamed [I.sub.Na(null)] to more accurately reflect the molecular identity of the channel and the conditions needed for its activation. The relationship between [I.sub.Na(null)] and [Ca.sup.2+] flux through slip-mode conductance of cardiac [Na.sup.+] channels is discussed in the context of ion channel biophysics and 'permeation plasticity.' tetrodotoxin; excitation-contraction coupling; sodium channel; calcium channel; ventricular myocyte