Regulation of Ca2+ homeostasis by atypical Na+ currents in cultured human coronary myocytes

Primary cultured human coronary myocytes (HCMs) derived from ischemic human hearts express an atypical voltage-gated tetrodotoxin (TTX)-sensitive sodium current (I-Na). The whole-cell patch-clamp technique was used to study the properties of I-Na in HCMs. The variations of intracellular calcium ([Ca2+](i)) and sodium ([Na+](i)) were monitored in non-voltage-clamped cells loaded with Fura-2 or benzofuran isophthalate, respectively, using microspectrofluorimetry. The activation and steady-state inactivation properties of I-Na determined a "window" current between -50 and -10 mV suggestive of a steady-state Na+ influx at the cell resting membrane potential. Consistent with this hypothesis, the resting [Na+](i) was decreased by TTX (1 mu mol/L). In contrast, it was increased by Na+ channel agonists that also promoted a large rise in [Ca2+](i). Veratridine (10 mu mol/L), toxin V from Anemonia sulcata (0.1 mu mol/L), and N-bromoacetamide (300 mu mol/L) increased [Ca2+](i) by 7- to 15-fold. This increase was prevented by prior application of TTX or lidocaine (10 mu mol/L) and by the use of Na+-free or Ca2+-free external solutions. The Ca2+-channel antagonist nicardipine (5 mu mol/L) blocked the effect of veratridine on [Ca2+](i) only partially. The residual component disappeared when external Na+ was replaced by Li+ known to block the Na+/Ca2+ exchanger. The resting [Ca2+](i) was decreased by TTX in some cells. In conclusion, I-Na regulates [Ca2+](i) in primary cultured HCMs, This regulation, effective at baseline, involves a tonic control of Ca2+ influx via depolarization-gated Ca2+ channels and, to a lesser extent, via a Na+/Ca2+ exchanger working in the reverse mode.