Short-term effects of stimulus interval changes in guinea-pig and rat atrial muscle.

Isometric force and action potentials were recorded in thin atrial strips from guinea-pigs and rats (32 degrees C). The restitution of peak force and action potential duration, after a regular contraction, was determined (test interval 0.1-120 s), together with the post-extrasystolic potentiation. The mechanical restitution could be described with an exponential function in two phases as: force = A(I-e-k1t) + B(I-e-k2t). By increasing the basic stimulation rate in guinea-pig atria from 0.2 to 2 Hz, the size of A was approximately doubled while B was only slightly affected. When [Ca2+] was increased from 0.9 to 3.6 mmol l-1, the size of A increased approximately 3.4 times while B decreased only slightly. There was a close correlation between steady-state contractility of the muscle and parameter A but not parameter B. In a similar fashion post-extrasystolic potentiation can be described as: force = Ce-kt + D. This potentiation was greater in guinea-pig than in rat hearts. In both species the rate of potentiation decay (k) was usually similar to the rate of the first phase of restitution (k1). It seems reasonable to interpret the parameters A and B as reflections of two separate intracellular compartments for activator calcium.