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 micromol/L). In contrast, it was increased by Na+ channel agonists that also promoted a large rise in [Ca2+]i. Veratridine (10 micromol/L), toxin V from Anemonia sulcata (0.1 micromol/L), and N-bromoacetamide (300 micromol/L) increased [Ca2+]i by 7- to 15-fold. This increase was prevented by prior application of TTX or lidocaine (10 micromol/L) and by the use of Na(+)-free or Ca(2+)-free external solutions. The Ca(2+)-channel antagonist nicardipine (5 micromol/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.
Boccara
G
G
Centre National de la Recherche Scientifique, Institut de Genetique Humaine, Montpellier, France.
Choby
C
C
Frapier
J M
JM
Quignard
J F
JF
Nargeot
J
J
Dayanithi
G
G
Richard
S
S
D000328:Adult; D001158:Arteries; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D003331:Coronary Vessels / Q000166:cytology / Q000378:metabolism*; D004553:Electric Conductivity; D006706:Homeostasis; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism; D008297:Male; D008875:Middle Aged; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000378:metabolism*; D009994:Osmolar Concentration; D012964:Sodium / Q000378:metabolism / Q000502:physiology*; D062687:Sodium Channel Agonists; D015222:Sodium Channels / Q000502:physiology
D062687:Sodium Channel Agonists; D015222:Sodium Channels; D012964:Sodium; D002118:Calcium