The initial inward current in spherical clusters of chick embryonic heart cells.

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Abstract
The rapid inward Na current in spherical clusters of 11 day embryonic chick heart cells, ranging in size from 65-90 .mu.g diameter, was studied using the 2-microelectrode voltage-clamp technique. Using these preparations, it was possible to resolve the activation phase of the rapid inward current for potentials negative to -25 mV at 37.degree. C. The rapid inward current exhibited a voltage and time dependence similar to that observed in other excitable tissues. It was initiated at potential steps more positive than -45 mV. The magnitude of the current reached its maximum value at a potential of .apprx. -20 mV. The measured reversal potential was that predicted by the Nernst equation for Na ions. The falling phase of the current followed a single exponential time-course with a time constant of inactivation, .tau.h, ranging from 2.14 ms at -40 mV to 0.18 ms at -5 mV. .tau.h, determined by a single voltage-step protocol was compared to the time constant, .tau.c, determined by a double voltage-step protocol and no significant difference between the 2 constants of inactivation was found. The time constants of inactivation and reactivation at the same potential in the same preparation were similar. The Na current of heart cells recorded at 37.degree. C evidently can be described by Hodgkin-Huxley kinetics with speeds approximately 4 times faster than the squid giant axon at 15.degree. C.