Calcium channel currents in isolated guinea-pig ventricular cells superfused with Ca-free EGTA solution.
Changes in membrane currents seen in Ca-free, EGTA (1 mM)-containing Tyrode solution (EGTA Tyrode), were studied in isolated guinea-pig ventricular cells, under the voltage clamp performed with a "G omega seal" patch electrode. Application of the EGTA Tyrode (calculated [Ca]0 = 1.3 X 10(-9) M) first eliminated the usual calcium current, but induced an extra inward current within 2 min. The reversal potential of this current, as judged by the direction of the current change, was about +25 mV (without correction of a liquid junction potential of -12 mV), but above this voltage a decaying outward current was observed. The decay of these inward and outward currents during depolarization was slow, but a large, nearly time-independent component was evident. These currents, regardless of their polarity and time course, were reduced by application of verapamil (10(-5) M) and Mg (5 mM), and were inactivated by pre-depolarizations. In Na-free EGTA Tyrode, the inward current disappeared but the outward current persisted at high voltages. These results suggest that in ventricular cells, reduction of external Ca concentrations to a nanomolar range induces a Ca channel current composed of an inward current carried by Na, and an outward current, presumably carried by K ions. Because of the persistence of the apparently non-inactivating Ca channel current, the net membrane current evoked at voltages around 0 mV remained close to zero, or even inward, after the decay of the time-dependent component, which was completed within a few hundreds ms. This characteristic I-V relation was considered to be linked to the development of the long-lasting action potentials, with a plateau maintained at around 0 mV, in EGTA Tyrode.
Y
Y
Imoto
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T
Ehara
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M
Goto
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004533:Egtazic Acid / Q000494:pharmacology*; D005026:Ethylene Glycols / Q000494:pharmacology*; D006168:Guinea Pigs; D006321:Heart / Q000502:physiology*; D006352:Heart Ventricles / Q000187:drug effects; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D010477:Perfusion; D016276:Ventricular Function
D005026:Ethylene Glycols; D007473:Ion Channels; D004533:Egtazic Acid; D002118:Calcium