AJP - Heart and Circulatory Physiology, Vol 259, Issue 2 290-H299, Copyright © 1990 by American Physiological Society
Rate dependence and supernormality in excitability of guinea pig papillary muscle
J. M. Davidenko, R. J. Levi, G. Maid, M. V. Elizari and M. B. Rosenbaum
Hospital Ramos Mejia, Buenos Aires, Argentina.
It is well known that in most cardiac tissues an increase in rate results
in a decrease of excitability and, eventually, conduction block. We used
microelectrode techniques to evaluate the rate and time dependence of
excitation latency in 27 isolated guinea pig papillary muscles (GPPM).
Latency was measured as the interval between the stimulus onset and action
potential upstroke. When the intensity of current was just suprathreshold,
prolongation of the basic cycle length (BCL) from 300 to 1,000 ms produced
an increase in latency or failure of excitation. Such behavior was observed
with extracellular as well as intracellular stimulation. Rate-dependent
changes in latency were maximal during the first 10-20 s following the rate
change and reached a steady state in approximately 200 s. Application of
premature beats revealed the presence of a "supernormal phase" in which
latency abbreviated. Strength-interval and strength-duration curves
demonstrated that changes in excitability accurately paralleled those
observed in latency. Hence, supernormal excitability at the end of the
phase 3 repolarization was consistently observed in all ventricular muscle
experiments. Deceleration-induced decrease of excitability was attended by
hyperpolarization, increase of action potential upstroke velocity (Vmax)
and action potential amplitude, and decrease in membrane resistance. Our
data suggest that paradoxical rate-related changes of excitability in GPPM
are the result of changes in the passive membrane properties. Under
conditions of depressed conductivity, this particular behavior may account
for the occurrence of bradycardia-dependent block.
