The effects of low concentrations of cardiotonic steroids on membrane currents and tension in sheep Purkinje fibres

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Abstract
1. Simultaneous measurements of voltage‐clamp currents and tension were made in shortened sheep Purkinje fibres exposed to various concentrations of strophanthidin, ouabain and digoxin. 2. In 5·4 mM‐K moderate doses (mean 2·4 × 10−7M) of the drugs produced an inward shift of the current—voltage relationship at very negative potentials, consistent with an increase in cleft K concentration (Cohen, Daut & Noble, 1976b), which was always accompanied by an increase in tension. This change, which has been attributed to Na—K pump inhibition, was often better correlated with an increase in voltage‐dependent tonic tension than in twitch tension. 3. Exposure to dihydro‐ouabain gave a monotonic increase in tension but a delayed increase in inward current. This suggests (cf. Lee, Kang, Sokol & Lee, 1980) that minor changes in pump activity may not always change the current—voltage relationship. 4. Low concentrations of strophanthidin (5 × 10−9 to 5 × 10−7 M) produced an outward current shift at very negative potentials, this change becoming smaller with a more rapid onset and reversing on increasing the dose. This change is attributed to pump stimulation. 5. The outward current shift was often associated with a negative inotropic effect, which always reversed either spontaneously or on removal of the drug. 6. The alternative response at a narrower dose range (1 × 10−8 to 2 × 10−7 M) was an increase in twitch (not tonic) tension, termed the low‐dose positive inotropic effect. 7. After a low concentration of cardiotonic steroid had given an early negative inotropic effect the bulk Ca concentration was reduced and the drug re‐applied. The low‐dose positive inotropic mechanism was then observed. 8. Outward current shifts and negative inotropy were also obtained with low concentrations of the clinically used glycosides digoxin and ouabain. 9. Low concentrations of strophanthidin applied to externally stimulated sheep ventricular trabeculae produced negative inotropy with lengthening of the action potential duration. Positive inotropy and action potential shortening occurred with higher doses. 10. A computer model of ionic currents and distributions in Purkinje fibres satisfactorily reproduced the changes in membrane currents and ionic gradients observed with cardiotonic steroids. The only perturbations capable of explaining our results were Na pump stimulation and inhibition. 11. It is concluded that cardiotonic steroids possess two inotropic mechanisms. The first is a low‐dose positive inotropic mechanism causally unrelated to changes in sodium pump activity and possibly a direct release of a membrane‐associated calcium fraction. Should this mechanism be unavailable then net pump stimulation at low doses will produce negative inotropy. The second mechanism is the well known Na‐lag process.

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