Action potentials (APs) and the force of isometric contractions of the right ventricular papillary muscles were measured in adult and newborn guinea-pigs. The measurements were carried out in the steady state with the rate of stimulation of 0.1, 1, and 2 Hz, and further some measurements were done in which Sr2+ was substituted for Ca2- The duration of APs of the newborn animals without pharmacological treatment was significantly shorter in comparison with that of the adults at all the used stimulation frequencies. An analogous sensitivity was found in the contractile force to increased stimulation frequency and when the steady state stimulation was discontinued by the insertion of interpolated extrasystoles in papillary muscles of adult or newborn animals. The biphasic contractions of papillary muscles were evoked in both groups of animals by the incomplete substitution of Sr2+ by Ca2+ in the presence of isoprenaline. The early component of the biphasic contractions had a faster course as compared to the late component and disappeared in the presence of caffeine in both groups of animals. Our results suggest that the heart cells of newborn guinea-pigs probably possess the sarcoplasmic reticulum (SR), whose function does not differ in quality from that of the adult guinea-pigs. The postnatal prolongation of APs is therefore not probably the result of postnatal development changes of the functions of SR, but could be related to changes in the relations between the surface and volume of the heart cell during its growth.
The biochemical model of excitation-contraction coupling in cardiomyocyte is presented and the validity of simulations of both physiological and pathological processes is discussed. The model of regulatory and actomyosin subsystems, even if it is rather simple in its regulatory subunit, gives results well consistent with experimental data. Specifically, intracellular free calcium levels ([Ca2+]i) were computed under various states of sarcoendoplasmic reticular Ca2+-ATPase (SERCA2) and compared to experimental findings. Computed results reproduced well both the increase in resting [Ca2+]i level and the attenuation of [Ca2+]i decline commonly observed in heart failure. Thus the computational simulations could help to identify core relations in studied systems by comparing results obtained using similar models of various complexities., M. Mlček, J. Neumann, O. Kittnar, V. Novák., and Obsahuje bibliografii