A down then up staircase in frog ventricle due to altered excitation-contraction coupling.

1. Repetitive stimulation of frog ventricle strips bathed in low calcium (< 1mM) fluids produced a decrease followed by an increase in successive tension responses which was not due to changes in the amplitude or the duration of the cardiac action potential.2. The depression of contraction reached its peak 5-10 sec after a conditioning contraction and decayed to one half in about 0.5 min; it was greater during a series of 5-10 stimuli when the frequency was increased from 3/min to 12/min.3. Lowering the external calcium concentration increased the amount of depression.4. The initial depression could be converted to facilitation by: (a) increasing the external calcium; (b) lowering the external sodium; (c) lowering the external potassium; and (d) the addition of strophanthidin.