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

Abstract

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. 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. Changes in the magnitude of the slow inward current were found when the frequency was changed during constant stimulation. Replacement of Ca2+ by Sr2+ or Ba2+ led to an augmentation or reduction, respectively, of the staircase effect, relative to the effect in Ca-containing solutions. Ba2+ greatly increased the slow inward channel recovery time. Ca2+ influx into amphibian atrial fibers contributes to the regulation of the slow inward conductance mechanism. Progressively increasing currents may underlie positive tension staircases.