Actions of barium and rubidium on membrane currents in canine Purkinje fibres.

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Abstract
The actions of Ba2+ and Rb+, 2 blockers of background K+ conductance, were investigated. Recent studies performed on ungulate Purkinje fibers have suggested that the pacemaker current is an inward current activated by hyperpolarization. This hypothesis is based on the assumption that Ba2+ reduces the inwardly rectifying background K+ conductance without affecting the pacemaker current. Addition of 5 mM-BaCl2 to the bathing Tyrode solution decreases background K+ permeability and eliminates the reversal of the pace-maker current. The reversal reappears on return to Ba2+-free Tyrode solution. 5 mM-BaCl2 also reduces the time-dependent current at pacemaker potentials positive to about -95 mV in 4 mM-K+ Tyrode solution. The pacemaker current in Ba2+ Tyrode solution usually does not have an exponential time course, and often decays non-monotonically. It can take > 2 min to reach a steady state. The fast initial component of membrane current, which is observed on hyperpolarizing in the pacemaker potential range in Purkinje fibers and which has been called the depletion current, is still present in Ba2+ Tyrode solution, but is reduced or eliminated if 10 mM-CsCl is added to the Ba2+ Tyrode solution. The addition of Cs+ is accompanied by an outward shift in membrane current in Ba2+ Tyrode solution. Ba2+ reduces the background K+ permeability in a dose-dependent manner. Addition of between 0.5 and 1 mM-BaCl2 achieves a maximum effect. Raising the amount of BaCl2 above this level reduces the time-dependent current even when no further effect on background permeability is observed. Rb+ substitution for K+ reduces the magnitude of the pacemaker current at potentials positive to -100 mV, eliminates the reversal of the pacemaker current, shifts the activation range to more negative potentials, and decreases the voltage dependence of pacemaker current kinetics. Rb+ addition to Tyrode solution has little effect on pacemaker current magnitude or time course positive to -90 mV, but does shift the reversal to more negative potentials. The pacemaker current in Ba2+ Tyrode solution apparently is an inward current activated by hyperpolarization. However, Ba2+ blocks an unknown fraction of the pacemaker current in a dose-dependent, and possibly voltage-dependent manner. Also, the presence of a slow component of pacemaker decay suggests that the standard Hodgkin-Huxley formalism for the analysis of pacemaker currents is inappropriate.