The relationship between sodium pump activity and twitch tension in cardiac Purkinje fibres.

1. Sheep cardiac Purkinje fibres were studied under voltage clamp conditions to examine the relationship between the Na pump rate and twitch tension.2. The Na pump activity was measured following a period of decreased Na pump rate and increased internal Na activity, Na(i), by examining the electrogenic Na pump current transient produced by reactivating the Na pump (see Eisner & Lederer, 1980).3. Various concentrations of Cs or Rb were used to reactivate the Na pump in the absence of extracellular K, K(o). Results from such experiments showed that these ;activator cations' produced a monotonic increase in Na pump rate with increasing concentration while producing monotonic decreases in steady-state twitch tension. A given concentration of Rb was more potent than the same concentration of Cs in its effects on both Na pump rate and tension. Nevertheless, varying [Rb](o) or [Cs](o) produced the same relationship between Na pump activity and tension.4. After the preparation was exposed to low K(o) (below 4 mM), the Na pump was reactivated with 10 mM-Rb(o) (0-K(o)). The area under the resulting electrogenic Na pump current transient gives a measure of the increase of [Na](i) that occurred when the preparation was exposed to the test solution compared to the steady-state Na(i) in 10 mM-Rb(o). Increasing the duration of exposure to low K(o), further augments the twitch tension achieved at the end of the test period and the area under the electrogenic Na pump current transient. Similarly, for equal periods of exposure, the lower the [K](o) in the test solution, the greater the increase of twitch tension at the end of the test period and the greater the area under the electrogenic Na pump current transient.5. The relationship between tension and the area under the electrogenic Na pump current transient is the same for a variable duration exposure to 0-K(o) or for a constant exposure to various low K(o). It is concluded that a rise in [Na](i) is the rate limiting step linking Na pump activity and twitch tension.6. In an experiment similar to the one described in (4), the fibre was exposed to a test solution containing a variable concentration of one of the activator cations of the Na pump for a fixed period. The effects of different cations were compared with the effects of a test solution containing no activator cation. Increasing the concentration of a particular activator cation from zero reduced the twitch tension augmentation in the test solution and also reduced the area under the electrogenic Na pump current transient on subsequently reactivating the Na pump with 10 mM-Rb(o). It is concluded that the ability of a test solution to reduce the area of the electrogenic Na pump current transient reflects the ability of the test solution to activate the Na pump. Equivalent concentrations of the activator cations that are required to activate the Na pump are: 2 mM-K(o) = 2 mM-Rb(o) = 6 mM-Cs(o) = 6 mM-NH(4o) = 22 mM-Li(o). Tl(o) was more effective than K(o). The order of these cations to reduce twitch tension is found to be the same as that to reduce the area under the electrogenic Na pump current transient.7. We conclude that the effects of the activator cations on twitch tension are determined by their effects on the Na pump and thereby on [Na](i).