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Tension was measured in voltage clamped sheep cardiac Purkinje fibres while simultaneously measuring the intracellular Na activity (aiNa) 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 aiNa 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 aiNa than that required to produce the other phenomena. The relationship between the magnitude of the twitch tension and aiNa was always non-linear. Twitch tension increased steeply with aiNa at first but the relationship flattened off at higher aiNa and tension eventually decreased. Over the steep range, the relationship between tension and aiNa could be represented as: twitch tension = b (aiNa)y where y had a mean value of 3.2. Changing membrane potential or [Ca2+]o changed b but had little effect on y. Mn (2 mmol/l) greatly decreased twitch tension but, at least initially, had little effect on tonic tension. The steep relationship between twitch tension and aiNa was seen, irrespective of whether the Na-K pump was inhibited either by exposure to K-free solution or to strophanthidin and whether the relationship was measured either when aiNa was increasing or after it had reached a steady state. The steep dependence of twitch tension on aiNa observed in the present work means that manoeuvres which produce even small changes of aiNa will have significant effects on contraction.
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