title: Two levels of resting potential in canine cardiac Purkinje fibers exposed to sodium-free solutions.
abstract: Canine cardiac Purkinje fibers exposed to sodium-free solutions containing 16 mM CaCl2, 20 mM tetraethylammonium chloride, 108 mM tetramethylammonium chloride, and 2.7 mM KCl may be quiescent at a resting potential of either -50 mV or -90 mV. The membrane potential of these fibers can be switched from -50 mV to -90 mV by a hyperpolarizing current pulse and from -90 mV to -50 mV by a depolarizing current pulse. The transition from -50 mV to -90 mV depends on a voltage-dependent increase in potassium conductance, that conductance being low at -50 mV and high at -90 mV. A reduction in potassium conductance causes the fiber to depolarize from -90 mV to -50 mV because of the presence of an inward current which apparently is carried mainly by Ca. Fibers that show a high resting potential cannot be excited except by depolarizing stimuli strong enough to move the membrane from -90 mV to a threshold potential of about -40 mV. Fibers that show a low resting potential are more easily excited and may show rhythmic activity sustained by afterpotentials that appear only if the low membrane potential is accompanied by a low potassium conductance. Slow changes in membrane potential also are seen; these changes may result from movements of chloride.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002463:Cell Membrane Permeability / Q000187:drug effects; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005071:Evoked Potentials / Q000187:drug effects; D005260:Female; D006329:Heart Conduction System / Q000187:drug effects*; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects*; D009200:Myocardial Contraction / Q000187:drug effects; D011188:Potassium / Q000494:pharmacology; D011690:Purkinje Fibers / Q000187:drug effects*; D012032:Refractory Period, Electrophysiological / Q000187:drug effects; D012964:Sodium / Q000494:pharmacology*; D009435:Synaptic Transmission / Q000187:drug effects
chemical_list: D012964:Sodium; D008274:Magnesium; D011188:Potassium; D002118:Calcium