Membrane permeability during low potassium depolarization in sheep cardiac Purkinje fibers

Exposure of sheep Purkinje fibers to low [K]o leads to marked depolarization to a stable potential of about -40 mV. This level is equivalent to the plateau of the Purkinje fiber action potential. The low [K]o depolarization could be prevented by removal of [Na]o and was modified by tetrodotoxin. The membrane potential in the depolarized state was unresponsive to changes in [Cl]o or [Ca]o and it was poorly responsive to changes in [K]o between 0 and 2 mM. Repolarization was induced by decrease in [Na]o with a slope response of 30 mV/10-fold change in [Na]o. Average internal K activity (aK) in the resting state with a [K]o of 5 mM was 121.4 mM for a membrane potential of -80 mV. During low K depolarization aK was 119.7 mM with a membrane potential of -34 mV. The depolarization was therefore due to a change in membrane permeability, with little change in aK. Upon restoration of [K]o the fiber repolarized to values transiently more negative than the prior resting potential. These transient potentials were more negative than the K equilibrium potential (VK), if it is calculated assuming a uniform [K]o. The hyperpolarization was reduced by ouabain [10(-6)] or by low [Ca]o.