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AJP - Heart and Circulatory Physiology, Vol 256, Issue 5 1478-H1492, Copyright © 1989 by American Physiological Society
ARTICLES |
Y. Hirano, H. A. Fozzard and C. T. January
Department of Medicine (Cardiology), University of Chicago, Illinois 60637.
Two types of Ca2+ currents were recorded in single dialyzed canine cardiac Purkinje cells using a whole cell voltage clamp technique. T-type current was easily separated from L-type current, because its voltage dependence of inactivation and activation was more negative and it decayed rapidly. L-type current was available at more depolarized holding potentials, activated at more positive voltages, and decayed slowly. In 2 mM extracellular Ca2+ concentration [( Ca]o), the average peak T- and L-type current density was 1.70 and 2.87 pA/pF, respectively. T-type current was relatively insensitive to modification by Ca2+, nifedipine, Cd2+, BAY K 8644, or isoproterenol. T-type current was more sensitive to block by Ni2+ and amiloride. Replacement of Ca2+ by Ba2+ or Sr2+ did not increase T-type current. Changes in the Ca2+ or Ba2+ concentration caused parallel shifts in the voltage dependence of several kinetic parameters for L- and T-type current. In 2 mM [Ca]o, the V1/2 (Boltzmann fit) for inactivation of T-type current was -68 mV with a slope of 3.9, and for L-type current the V1/2 was -31 mV with a slope of 5.5. Recovery from inactivation of L- and T-type current was voltage dependent, and for similar conditions L-type current recovered from inactivation more rapidly than T-type current. These findings show that T- and L-type currents are large in cardiac Purkinje cells, and they can easily be separated by their voltage, kinetic, and pharmacological differences. Both may have important physiological roles.
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