Participation of slow inward current in the Purkinje fiber action potential overshoot

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.