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.”