The inotropic memory of amphibian myocardium

(1) Experimentally observed changes of contractile force induced by changes in the pattern of stimulation of frog ventricular myocardium were compared with predictions computed on the basis of a model for the contractile conditioning, proposed in a previous paper. (2) For this purpose, two functions (Potentiation and Inhibition) which describe in the model the effect of previous contractions, were determined experimentally. (3) It is shown that the model adequately predicts : force-frequency curve Inotropic Effect Curves at different basal frequencies, effect of the suppression of a contraction in a sequence of definite frequency, frequency-staircases, and strength-interval curves. (4) The formal characteristics of the model are discussed, and it is suggested that the potentiation mechanism results from the recirculation of activator calcium in two types of compartments, from which calcium passes to the myofilaments during the contractions or is lost to the exterior with first order kinetics during the rest period. Possible locations for these compartments are proposed. Various hypothesis on the nature of the inhibitory mechanism are considered.