AJP - Heart and Circulatory Physiology, Vol 237, Issue 2 204-H212, Copyright © 1979 by American Physiological Society
Participation of slow inward current in the Purkinje fiber action potential overshoot
L. Mary-Rabine, B. F. Hoffman and M. R. Rosen
We used microelectrode techniques to study the relationship of canine
Purkinje fiber membrane potential and the action potential (AP) overshoot.
At the maximum diastolic potential, -93.0 +/- 0.5 (SE) mV, AP overshoot was
+37.7 +/- 0.4 mV. There was a range of membrane potentials (MP) less
negative than the maximum diastolic potential from which action potentials
were elicited with an overshoot greater than the control. Starting at an MP
of less than -78.7 +/- 0.4 mV, AP overshoot was less than control. A
maximum overshoot of +40.2 +/- 0.4 mV occurred at an MP of -85.4 +/- 0.4
mV. The relationship of the maximum upstroke velocity (Vmax) of phase 0
depolarization to MP was sigmoidal. Peak Vmax, 497 +/- 13 V/s, occurred at
MP greater than or equal to -89.3 +/- 0.5 mV. The increase in overshoot was
enhanced as perfusate [Ca2+] increased and decreased as [Ca2+] decreased.
Slow-channel blocking agents and tetrodotoxin (TTX) depressed the peak of
the curve relating overshoot to MP. TTX also decreased Vmax. The effect of
TTX on overshoot but not on Vmax was reversed with Ca2+, 8.1 mM. The
increase in overshoot for action potentials initiated during the terminal
part of phase 3 was due to a slow, delayed component of the upstroke and
appears to result from the slow inward current.
