Post-rest adaptation of electrical and mechanical activity in the isolated guinea-pig papillary muscle

In guinea-pig papillary muscle the time course of the changes in contractile force and action potential duration (APD) were studied after periods of rest of variable duration. After a long period of rest, the force of contraction adapted to pre-rest control values in a monophasic manner whereas the time-course of the APD was clearly biphasic. The post-rest adaptation of the APD could be described mathematically by a simple model, which considers the action potential duration during steady state as the sum of a resting value (APDR) plus a lengthening effect of activation (LEA) minus a shortening effect of activation (SEA). LEA and SEA are assumed to occur immediately, with each excitation and to decay continuously. During repetitive stimulation, both effects will accumulate. Using the constants found for the post-rest adaptation of the APD, the steady-state frequency-dependence of the APD could also be described with this model.