Activity-dependent changes of slow inward current in ventricular heart muscle

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Abstract
The relationships between membrane voltage, contractile force and slow inward current were studied in cat and dog papillary muscles or trabeculae employing the double sucrose gap voltage clamp technique. The experiments were performed at 30°C and the preparations were stimulated at a frequency of 0.5 Hz. The known relationships between steady state contractile force, slow inward current and membrane voltage were confirmed. Under non-steady state conditions the slow inward current decreases during ascending and increases during descending contraction staircases when the clamp steps of the test train exceed about 60 mV from resting level. Depolarization clamp steps below 60 mV produce parallel changes of the slow inward current and contractile force. Those clamp conditions which increase the contractile force shift the threshold of Isi and of contraction towards more negative values. During ascending staircases an increasing background outward current was regularly observed together with diminshing slow inward current. The reported current transients agree with the changes of action potential configuration during mechanical transients: the prolongation of plateau during descending staircases corresponds to an increase, and the shortening of action potential during late repolarization corresponds to a decrease of slow inward current in the respective voltage ranges. The slow inward current was tentatively separated into two components. The main component is inversely proportional to contractile force and it exhibits the well known current-voltage relationship for this current. The other one is directly proportional to contractile force and may be related to a regenerative response of reticular membranes.