[Experimental-theoretical study of the force-interval relationship in the developing chicken myocardium]

The force-interval relationship was studied on myocardium preparations from chick embryos and hatched chickens. It is shown that the force-interval relationships of myocardium change during ontogenesis. A negative staircase (a decrease in the isometric force with increasing stimulation rate) in the chick embryo myocardium and a positive steady-state relationship in hatched stage myocardium were revealed. Changes in the force after switching from one stimulation frequency to another, the effects of poststimulation potentiation, as well as responses to the introduction of pauses and extrasystols at a constant stimulation rate were recorded. All the effects observed in the transient processes in preparations from hatched stage myocardium were more pronounced than in embryo myocardium. Our previous mathematical model of calcium recirculation in cardiomyocytes was adapted for simulating the main features of force-interval relationships in embryonal and relatively developed myocardium. The main source of regulatory calcium in the model of hatched stage myocardium is sarcoplasmic reticulum. In the model of embryo myocardium, it was postulated, based on data available in literature, that the main regulator of contractile response of the muscle is calcium that enters cardiomyocytes from extracellular medium. To describe force-interval relationships, by this model, the decreasing dependence of the entry of extracellular calcium on the intervals between stimuli was introduced.