Electrical properties associated with wide intercellular clefts in rabbit Purkinje fibres.

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Abstract
Rabbit Purkinje fibers were studied using micro-electrode recordings of electrical activity or a 2-micro-electrode voltage clamp. Previous studies suggested that these preparations offer structural advantages for analysis of ionic permeability mechanisms. Viable preparations were obtained consistently by exposure to a K glutamate Tyrode solution during excision and recovery. In NaCl Tyrode solution, the action potential showed a large overshoot and fully developed plateau, but no pacemaker depolarization at negative potentials. The passive electrical properties were consistent with morphological evidence for the accessibility of cleft membranes within the cell bundle. Electrotonic responses to intracellular current steps showed the behavior expected for a simple leaky capacitative cable. Capacitative current transients under voltage clamp were changed very little by an 8-fold reduction in the external solution conductivity. Slow current changes attributable to K depletion were small compared to those in other cardiac preparations. The amount of depletion was close to that predicted by a cleft model which assumed free K diffusion in 1 .mu.m clefts. Step depolarizations over the plateau range of potentials evoked a slow inward current which was resistant to tetrodotoxin but blocked by D600 [.alpha.-isopropyl-.alpha.-[(N-methyl-N-homoveratryl)-.gamma.-aminopropyl]-3,4,5-trimethoxyphenyl-acetonitr ile]. Strong depolarizations to potentials near 0 mV elicited a transient outward current and a slowly activating late outward current. Both components resembled currents found in sheep or calf Purkinje fibers. Previous interpretations of slow plateau currents as genuine permeability changes are supported. The rabbit Purkinje fiber may allow study of various ionic channels with relatively little interference from radial non-uniformities in membrane potential or ion concentration.