Molecular identification of a TTX-sensitive Ca²⁺ current

The TTX-sensitive Ca²⁺ current [I_Ca(TTX)] observed in cardiac myocytes under Na⁺-free conditions was investigated using patch-clamp and Ca²⁺-imaging methods. Cs⁺ and Ca²⁺ were found to contribute to I_Ca(TTX), but TEA⁺ and N-methyl-D-glucamine (NMDG⁺) did not. HEK-293 cells transfected with cardiac Na⁺ channels exhibited a current that resembled I_Ca(TTX) in cardiac myocytes with regard to voltage dependence, inactivation kinetics, and ion selectivity, suggesting that the cardiac Na⁺ channel itself gives rise to I_Ca(TTX). Furthermore, repeated activation of I_Ca(TTX) led to a 60% increase in intracellular Ca²⁺ concentration, confirming Ca²⁺ entry through this current. Ba²⁺ permeation of I_Ca(TTX), reported by others, did not occur in rat myocytes or in HEK-293 cells expressing cardiac Na⁺ channels under our experimental conditions. The report of block of I_Ca(TTX) in guinea pig heart by mibefradil (10 μM) was supported in transfected HEK-293 cells, but Na⁺ current was also blocked (half-block at 0.45 μM). We conclude that I_Ca(TTX) reflects current through cardiac Na⁺ channels in Na⁺-free (or "null") conditions. We suggest that the current be renamed I_Na(null) to more accurately reflect the molecular identity of the channel and the conditions needed for its activation. The relationship between I_Na(null) and Ca²⁺ flux through slip-mode conductance of cardiac Na⁺ channels is discussed in the context of ion channel biophysics and "permeation plasticity.".