Two levels of resting potential in canine cardiac Purkinje fibers exposed to sodium-free solutions.

Abstract

SUMMARY Canine cardiac Purkinje fibers exposed Co sodiumfree solutions containing 16 m,\i CaCL, 20 mM tetraethvlammonium chloride, 108 m\i tetramethylammonium chloride, and 2.7 m\i KCI may be quiescent at a resting potential of either -5 0 mV or -9 0 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 raV 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 -9 0 mV to -5 0 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 -9 0 mV to a threshold potential of about -4 0 raV. 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.CARDIAC PURKINJE FIBERS bathed in Na-free solutions can show two stable resting potentials, one near -90 mV and one near -50 mV.1 2 We now report that within a critical range of [KJ 0 the membrane potential can be shifted from either level to the other by the application of hyperpolarizing or depolarizing current pulses. This transition appears to be governed primarily by a voltage-dependent change in potassium conductance, that conductance being high at the -9 0 mV level and low at the -5 0 mV level. This phenomenon assumes special interest because cardiac fibers can produce two distinct types of propagated action potentials. One type, dependent on a rapid increase in sodium conductance, is abolished by voltage-dependent inactivation when the resting potential is low. The other type, called the slow response, depends on an increase in permeability which occurs at membrane potentials between -5 0 and +10 mV. Various cardiac arrhythmias arise in fibers in which a loss of resting potential causes the rapid upstroke to be replaced by the slow response.' The possibility that cardiac fibers are characterized not only by an ability to produce two types of action potential, but also by an ability to display the two levels of resting potential from which those action potentials can arise, is therefore intriguing. MethodsMongrel dogs of either sex weighing 15-20 kg were anesthetized with an intravenous injection of pentobarbital sodium (30 mg/kg). The heart was rapidly excised and immersed in Tyrode's solution (Table 1). Bundles of Purkinje fibers (false tendons) 4-12 mm long were removed from the right and left ventricles, placed in a tissue bath, and perfused with Tyrode's solution at 36-37 c C. Fibers which, had a membrane potential of less than -8

Action of Tetrodotoxin and Verapamil on Cardiac Purkinje Fiber Action Potentials in a Sodium-Free, Calcium-Rich Medium

“…On the other hand, when the pH of the superfusate was 7.4, our action potentials were produced from normal rest ing potential levels. Our results were similar to those of Wiggins and Cranefield [1976]. The potentials produced from high rest levels on an average required 68% higher voltage stimuli to reach threshold.…”

Induction and removal of inward‐going rectification in sheep cardiac Purkinje fibres

“…The existence of two stable resting potentials in K-free, Na-free solution prompted the present investigation. Two stable potential levels in cardiac Purkinje fibres have been described in a number of experimental conditions: low temperature (Chang & Schmidt, 1960); low K, low Cl (Gadsby & Cranefield, 1977), Na-free, Ca-rich, TEA-containing solutions (Wiggins & Cranefield, 1976). In terms of changes in I-V relation the existence of two stable potentials in K-free, Na-free solution is different 306 E. CARMELIET INWARD RECTIFICATION from the situation described by Gadsby & Cranefield (1977).…”

Regulation of chloride in quiescent sheep‐heart Purkinje fibres studied using intracellular chloride and pH‐sensitive micro‐electrodes.

“…ratio (e.g. Wiggins & Cranefield, 1976) and is said to be facilitated by the reduction of external C1 (Gadsby & Cranefield, 1977 Effect of removing bicarbonate/CO2 on the recovery of aic Fig. 5 shows an experiment where the recovery of at , was compared in both bicarbonate-and HEPES-buffered solutions.…”