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Todd Franklin Cardiac Research Laboratory, The Children's Heart Center, Department of Pediatrics, Emory University, Atlanta, Georgia 30322
Action potential
conduction through the atrium and the ventricle of the heart depends on
the membrane properties of the atrial and ventricular cells,
particularly with respect to the determinants of the initiation of
action potentials in each cell type. We have utilized both current- and
voltage-clamp techniques on isolated cells to examine biophysical
properties of the two cell types at physiological temperature. The
resting membrane potential, action potential amplitude, current
threshold, voltage threshold, and maximum rate of rise measured from
atrial cells (
80 ± 1 mV, 109 ± 3 mV, 0.69 ± 0.05 nA,
59 ± 1 mV, and 206 ± 17 V/s, respectively; means ± SE) differed significantly (P < 0.05) from those values measured from ventricular cells (82.7 ± 0.4 mV, 127 ± 1 mV, 2.45 ± 0.13 nA, 46 ± 2 mV,
and 395 ± 21 V/s, respectively). Input impedance, capacitance, time
constant, and critical depolarization for activation also were
significantly different between atrial (341 ± 41 M
, 70 ± 4 pF, 23.8 ± 2.3 ms, and 19 ± 1 mV, respectively) and ventricular
(16.5 ± 5.4 M, 99 ± 4.3 pF, 1.56 ± 0.32 ms, and 36 ± 1 mV, respectively) cells. The major mechanism of these differences is
the much greater magnitude of the inward rectifying potassium current
in ventricular cells compared with that in atrial cells, with an
additional difference of an apparently lower availability of inward Na
current in atrial cells. These differences in the two cell types may be
important in allowing the atrial cells to be driven successfully by
normal regions of automaticity (e.g., the sinoatrial node), whereas
ventricular cells would suppress action potential initiation from a
region of automaticity (e.g., an ectopic focus).
electrophysiology; arrhythmia; inward rectifier current; cell coupling
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