AJP - Heart and Circulatory Physiology, Vol 247, Issue 6 936-H945, Copyright © 1984 by American Physiological Society
Cycle length-dependent action potential duration in canine cardiac Purkinje fibers
V. Elharrar, H. Atarashi and B. Surawicz
We studied the effects of pharmacologic probes that affect predominantly
the Na inward current [tetrodotoxin (TTX), lidocaine], the slow inward
current [cobalt, isoproterenol, verapamil], and the potassium currents
[tetraethylammonium chloride (TEA), SG-75] on the duration of the action
potential (APD) of canine cardiac Purkinje fibers during steady state and
restitution. A schema is proposed in which the APD during steady state or
restitution is determined by three factors: maximum action potential
duration (APDmax), kinetics of restitution, and "memory." The predicted
APDmax was 469 +/- 34 (SE) ms (n = 27) in control. It was prolonged (P less
than 0.05) by cobalt, verapamil, and TEA and shortened (P less than 0.05)
by TTX, lidocaine, isoproterenol, and SG-75. In control, the kinetics of
restitution were described by a sum of two exponentials with time constant
T1 = 137 +/- 9 ms and T2 = 1,665 +/- 135 ms (n = 27), respectively. T1 was
prolonged (P less than 0.05) by TTX, lidocaine, and verapamil but was not
changed by other probes. None of the probes studied altered the T2 of
restitution or the memory factor, computed at a cycle length of 500 ms from
the predicted APDmax and the plateau of restitution. Low temperature (31
degrees C) prolonged APDmax and T1 and reduced the memory. We conclude that
each of the proposed three factors is controlled by different mechanisms
and that a TTX-sensitive current appears to contribute to the process of
restitution of APD.
