The calcium and frequency dependence of the slow inward current 'staircase' in frog atrium.

1. Changes in the magnitude of the slow inward current in the frog atrium were monitored at different stimulation frequencies, using a double sucrose-gap technique. 2. After short rest periods (1.5-3.0 min), repetitive clamp depolarizations applied at frequencies ranging from 0.33 to 1 Hz (20-60/min) resulted in a progressive increase in the slow inward current towards a new level. Action-potential amplitudes and plateau levels usually showed similar increases under these conditions. 3. Changes in the magnitude of the slow inward current were also found when the frequency was changed during constant stimulation. 4. Replacement of calcium ions by strontium or barium ions led to an augmentation or reduction, respectively, of the 'staircase' effect, relative to the effect in calcium-containing solutions. Barium ions were found to greatly increase the slow inward channel 'recovery' time. 5. The results suggest that calcium influx into amphibian atrial fibres contributes to the regulation of the slow inward conductance mechanism. Progressively increasing currents may underlie positive tension staircases.