Characterization of the electrogenic sodium pump in cardiac Purkinje fibres

Abstract

The Na pump is examined in sheep cardiac Purkinje fibers using a 2 microelectrode voltage clamp technique. After reducing the external K concentration, [K]o, to zero for 2 min or more, subsequent addition of an activator cation (known to activate the Na pump in other preparations) produces a transient increase of outward current. This outward current transient is abolished by 10-5 M-strophanthidin. This transient increase of outward current is a result of a transient stimulation of the Na pump by the raised [Na]1 following exposure to 0-Ko. Increasing [Rb]o or [Cs]o over the range of 0-40 mM increases the rate of decay of the electrogenic Na pump current transient. Using a simple model the decay rate constant of the electrogenic Na pump current transient is apparently a good measure of the degree of activation of the external site of the Na pump. At a given concentration of activator cation, Rbo produces a greater activation of the Na pump than does Cso. The coupling ratio of the Na pump is independent of Rbo or Cso over the range 2-40 mM. The coupling ratio is independent of Na1 over the range considered. The electrogenic Na pump current transient provides a good measure of the activity of the Na pump when Rb or Cs are used as activator cations. This measure can be used in the intact preparation to investigate the relationship between Na pump rate and other cellular events, such as the regulation of tension.