Regulation of Ca 2+ Homeostasis by Atypical Na + Currents in Cultured Human Coronary Myocytes

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Abstract
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 ([Ca 2+ ] 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 μmol/L). In contrast, it was increased by Na + channel agonists that also promoted a large rise in [Ca 2+ ] i . Veratridine (10 μmol/L), toxin V from Anemonia sulcata (0.1 μmol/L), and N -bromoacetamide (300 μmol/L) increased [Ca 2+ ] i by 7- to 15-fold. This increase was prevented by prior application of TTX or lidocaine (10 μmol/L) and by the use of Na + -free or Ca 2+ -free external solutions. The Ca 2+ -channel antagonist nicardipine (5 μmol/L) blocked the effect of veratridine on [Ca 2+ ] i only partially. The residual component disappeared when external Na + was replaced by Li + known to block the Na + /Ca 2+ exchanger. The resting [Ca 2+ ] i was decreased by TTX in some cells. In conclusion, I Na regulates [Ca 2+ ] i in primary cultured HCMs. This regulation, effective at baseline, involves a tonic control of Ca 2+ influx via depolarization-gated Ca 2+ channels and, to a lesser extent, via a Na + /Ca 2+ exchanger working in the reverse mode.