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Actions of barium and rubidium on membrane currents in canine Purkinje fibres.
Author(s) -
Cohen I S,
Falk R T,
Mulrine N K
Publication year - 1983
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1983.sp014691
Subject(s) - purkinje fibers , conductance , chemistry , current (fluid) , biophysics , membrane potential , electrophysiology , physics , neuroscience , biology , biochemistry , thermodynamics , condensed matter physics
The actions of Ba2+ and Rb+, two blockers of background K+ conductance, were investigated. Recent studies performed on ungulate Purkinje fibres have suggested that the pace‐maker 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 pace‐maker 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 pace‐maker potentials positive to about ‐95 mV in 4 mM‐K+ Tyrode solution. The pace‐maker current in Ba2+ Tyrode solution usually does not have an exponential time course, and often decays non‐monotonically. It can take more than two minutes to reach a steady state. The fast initial component of membrane current, which is observed on hyperpolarizing in the pace‐maker potential range in Purkinje fibres 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 pace‐maker current at potentials positive to ‐100 mV, eliminates the reversal of the pace‐maker current, shifts the activation range to more negative potentials, and decreases the voltage dependence of pace‐maker current kinetics. Rb+ addition to Tyrode solution has little effect on pace‐maker current magnitude or time course positive to ‐90 mV, but does shift the reversal to more negative potentials. The available evidence suggests that the pace‐maker current in Ba2+ Tyrode solution is an inward current activated by hyperpolarization. However, Ba2+ blocks an unknown fraction of the pace‐maker current in a dose‐dependent, and possibly voltage‐dependent manner. Also, the presence of a slow component of pace‐maker decay suggests that the standard Hodgkin‐Huxley formalism for the analysis of pace‐maker currents is inappropriate.

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