The dependence of sodium pumping and tension on intracellular sodium activity in voltage‐clamped sheep Purkinje fibres.

Abstract

Intracellular Na activity .**GRAPHIC**. was measured in sheep cardiac Purkinje fibers using a recessed-tip Na+-sensitive micro-electrode. The membrane potential was controlled with a 2 microelectrode voltage clamp. Tension was measured simultaneously. Removing external K produced a rise of .**GRAPHIC**. and both twitch and tonic tension. On adding 4-10 mM-[Rb]o to reactivate the Na-K pump .**GRAPHIC**. and tension declined. An electrogenic Na pump current transient accompanied the fall of .**GRAPHIC**. The half-time of decay of the electrogenic Na pump current transient was similar to that of .**GRAPHIC**. (mean .**GRAPHIC**. = 0.97 .+-. 0.03 (SEM; n =28)). Following re-activation of the Na-K pump, the electrogenic Na pump current transient was linearly related to .**GRAPHIC**. The duration of exposure to K-free, Rb-free solutions was varied to change the level of .**GRAPHIC**. On subsequently re-activating the Na-K pump with 10 mM-[Rb]o, the ratio of charge extruded to the total change of .**GRAPHIC**. was constant. The fraction of Na extruded electrogenically is unaffected by changes of .**GRAPHIC**. Approximately 26% of the total Na extrusion appeared as charge transfer. The relationship between tonic tension and .**GRAPHIC**. was usually different during Na-K pump inhibition in a K-free, Rb-free solution compared with the relationship during Na-K pump re-activation. In general, a given .**GRAPHIC**. was associated with a greater level of tonic tension during Na-K pump inhibition compared with that during pump re-activation. A similar hysteresis was often seen between twitch tension and .**GRAPHIC**.