The quantitative relationship between twitch tension and intracellular sodium activity in sheep cardiac Purkinje fibres.

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Abstract
Tension was measured in voltage clamped sheep cardiac Purkinje fibers while simultaneously measuring the intracellular Na activity .**GRAPHIC**. with a recessed-tip, Na-selective micro-electrode. Inhibiting the Na-K pump either by exposing the preparation to a K-free solution or by adding the cardioactive steroid strophanthidin increased both .**GRAPHIC**. and twitch tension and resulted in the development of tonic tension, after-contractions and a transient inward current (ITI). The increase of twitch tension was present at lower .**GRAPHIC**. than that required to produce the other phenomena. The steep dependence of twitch tension on .**GRAPHIC**. observed means that maneuvers which produce even small changes of .**GRAPHIC**. will have significant effects on contraction.