Effect on Membrane Potential and Electrical Activity of Adding Sodium to Sodium-Depleted Cardiac Purkinje Fibers

Abstract

Canine cardiac Purkinje fibers exposed to Na-free solutions containing 128 mM TEA and 16 mM Ca show resting potentials in the range -50 to -90 mV; if the concentration of Na in the perfusate is raised from 0 to 4 to 24 mM, hyperpolarization follows. If the initial resting potential is low, the hyperpolarization tends to be greater; the average increase in the presence of 8 mM Na is 14 mV. Such hyperpolarization is not induced by adding Na to K-free solutions, is not seen in cooled fibers, or in fibers exposed to 10 -3 M ouabain, nor is it induced by adding Li and thus may result from electrogenic sodium extrusion. Fibers exposed to Na-free solutions are often spontaneously active; if they are quiescent they often show repetitive activity during depolarizing pulses. Such spontaneous or repetitive activity is suppressed by the addition of Na. This suppression may or may not be related to the hyperpolarization.Propagated action potentials dependent on Ca as a carrier of inward current can occur in canine cardiac Purkinje fibers exposed to sodium-free solutions (Aronson and Cranefield, 1973). Such action potentials presumably result from the flow of inward current through the so-called "slow channel." Voltage-clamp studies suggest that the slow channel is permeable both to Ca and to Na (Rougier et al., 1969;Vitek and Trautwein, 1971; for review see Reuter, 1973, andTrautwein, 1973). Since a very small amount of Na may remain in bundles of Purkinje fibers exposed to Na-free solutions (Aronson and Cranefield, 1973;Reuter, 1973), the action potentials seen in fibers exposed to Nafree solutions might depend at least in part on Na as a carrier of inward current. To examine this possibility we added small amounts of NaCl to initially Na-free perfusates. Such an addition of Na may transiently increase the overshoot of the action potential but the almost invariable result is the abolition of spontaneous activity followed by a marked increase in resting potential. The hyperpolarization suggests that the addition of Na to the previously Na-free perfusate induces electrogenic extrusion of sodium. We report below results consistent with this hypothesis, supporting the belief that electrogenic

Activation of the electrogenic sodium pump in guinea‐pig atria by external potassium ions

“…Somewhat similar observations, i.e. a depolarization of cardiac cells upon addition of K-poor solution during enhanced Na pumping have been reported from Purkinje fibres (Hiraoka & Hecht, 1973;Wiggins & Cranefield, 1974) and guinea-pig atria (Glitsch & Klare, 1977).…”

Origin of the background sodium current and effects of sodium removal in cultured embryonic cardiac cells.

“…Existence of a background sodium current implies that a cell should hyperpolarize if extracellular so- (Carmeliet and Vereecke, 1979), and that depolarization is frequently observed instead (Aronson and Cranefield, 1973;Wiggins and Cranefield, 1974). The present study addresses this paradox by showing that abrupt removal of sodium from tissue cultured ventricular cells elicits both hyperpolarizing and depolarizing current in a reproducible sequence.…”

“…Electrogenicity of the sodium-potassium pump is well established in the heart (see Glitsch, 1982), so there is little doubt that a portion of the sodium-removal current must be due to this mechanism. However, as pointed out by Wiggins and Cranefield (1974), only a fraction of the sodium ions entering a cell are extruded electrogenically. Interruption of the pump by sodium removal could not, therefore, produce net inward current unless most of the resting sodium influx were by way of an electroneutral carrier.…”

5-(N,N-dimethyl)amiloride-sensitive Na-Li exchange in isolated specimens of human atrium.

“…For this study, the ionic selectivity of intracellular Na, K-pump sites is crucial for interpretation of the experimental findings. Previous studies (3,17) have provided evidence against substantial outward transport of LiW by the sarcolemmal Na, K-pump. However, glycoside-induced pump inhibition was not utilized in those studies.…”

“…Hyperpolarization of Em to levels more negative then EK is usually attributed to enhanced electrogenic Na, K-pump activity (16,17). However, to explain the hyperpolarization depicted in Figs.…”