Characteristics of L- and T-type Ca2+ currents in canine cardiac Purkinje cells.
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.
Hirano
Y
Y
Department of Medicine (Cardiology), University of Chicago, Illinois 60637.
Fozzard
H A
HA
January
C T
CT
D000818:Animals; D001464:Barium / Q000378:metabolism; D002118:Calcium / Q000494:pharmacology; D015220:Calcium Channels / Q000145:classification / Q000187:drug effects / Q000502:physiology*; D004285:Dogs; D004594:Electrophysiology; D006329:Heart Conduction System / Q000502:physiology*; D006706:Homeostasis; D007473:Ion Channels / Q000378:metabolism / Q000502:physiology; D007477:Ions; D009994:Osmolar Concentration; D011690:Purkinje Fibers / Q000502:physiology*; D012996:Solutions
D015220:Calcium Channels; D007473:Ion Channels; D007477:Ions; D012996:Solutions; D001464:Barium; D002118:Calcium