Two levels of resting potential in cardiac Purkinje fibers.

Open Access
Publisher Website
Google Scholar
Abstract
In an appropriate inoic environment, the resting potential of canine cardiac Purkinje fibers may have either of 2 values. By changing the external K concentration, [K]o, in small steps, it was shown that in the low (1 mM) Cl, Na-containing solutions used in this study, the 2 levels of resting potential could be obtained only within a narrow range of [K]o values; that range was usually found between 1 and 4 mM. Within the critical [K]o range the resting potential could be shifted from either level to the other by the application of small current pulses. Under these conditions the steady-state current-voltage relationship was N-shaped, and a region of both negative slope and negative chord, conductance lay between the 2 stable zero-current potentials. The negative chord conductance was largely due to inward Na current, only part of which was sensitive to tetrodotoxin (TTX). Under appropriate conditions, the negative chord conductance could be abolished by several experimental interventions and the membrane potential thereby shifted from the lower to the higher resting level. Those interventions which were effective by presumably diminishing the steadystate inward current included reducing the external Na concentration, adding TTX, or adding lidocaine. Those which presumably increased the steady-state outward current included small increases in [K]o, brief depolarizations to around -20 mV or the addition of acetylcholine chloride.