Observations on the staircase phenomenon in guinea pig atrium

The staircase phenomenon, occurring after a change in frequency was studied in isolated trabeculae from guinea pig atrium. The effects on tension, action potential form and function as well as ionic currents were investigated. 1. In the ascending part of the frequency-force relationship a sudden change to a new driving frequency resulted in a staircase response which consisted of two exponential phases (tau 1 = 1-2 s; tau 2 = 20-30 s). 2. The build-up or decline of twitch tension in response to either an increase or reduction of [Ca2+]0 followed a similar composed time course. 3. After a reduction in stimulation frequency the action potentials changed time dependently: In the first response the peak of the overshoot was reached faster and the plateau phase was shortened; afterwards these parameters remained constant while the repolarization phase continually shortened during the following 5-10 action potentials. 4. In voltage clamp experiments an analogous reduction in the frequency of depolarizing voltage clamp pulses induced an immediate increase of the slow inward current (Isi). In the following 10 pulses the increased Isi remained constant, while the late outward current continually increased. 5. The time course of recovery of the Isi-system was found to be slow in atrial trabeculae (tau = 300-500 ms at -70 mV). Thus the increase in Isi, observed after a reduction in frequency, could be explained by a more complete recovery of the Isi-system during the interval between two stimuli. 6. the increase in Isi during these recovery experiments was accompanied by an accelerated inactivation. 7. It is concluded that after a reduction in stimulation rate the faster development of the overshoot and the shortening of the plateau phase are due to an augmentation and a faster inactivation of the Isi, respectively. The shortening of the late repolarization phase which developed during successive action potentials is most probably related to the observed increase in late outward current.