Abstract
Changes in membrane currents seen in Ca-free, EGTA (1mM)- containing Tyrode solution (EGTA Tyrode), were studied in isolated guinea-pig ventricular cells, under the voltage clamp performed with a "GΩ seal" patch electrode. Application of the EGTA Tyrode (calculated [Ca]0=1.3×10-9M) first eliminated the usual calcium current, but induced an extra inward current within 2min. The reversal potential of this current, as judged by the direction of the current change, was about +25mV (without correction of a liquid junction potential of -12mV), 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 (10-5M) and Mg (5mM), 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 0mV 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 0mV, in EGTA Tyrode.
