The kinetics of cytosolic calcium in the right ventricular myocardium of guinea pigs and rats

The characteristics of tension increase and decline, as well as those of the calcium transients, have been measured in the trabeculae of the right cardiac ventricle of the guinea pig and rat in the isometric contraction mode with different preloads. Measurements were performed at different temperatures of physiological saline and the effects of inhibition of calcium removal from the cytosol mediated by Na⁺–Ca²⁺ exchange and the ATP-dependent Ca²⁺ pump of the sarcoplasmic reticulum (SERCA2a) were analyzed. Emergence of the “bump” phase (a phase of brief deceleration of the decay of the calcium transient) was observed in the guinea pig myocardium as the temperature was increased from 25 to 30°C; earlier observations of this phenomenon were reported only for rats. As the temperature was elevated to 35°C, the “bump” phase in the guinea pig myocardium transformed into a “plateau” phase of the calcium transient. The effect of temperature on the course of the decay of the calcium transient in the rat myocardium was negligible. In contrast, a gradual stretching of the right ventricular trabecula of the rat was accompanied by the emergence of the “bump” phase and a gradual increase of its parameters (amplitude, integral intensity, and duration), whereas preload did not exert a similar effect on the guinea pig myocardium. Selective inhibition of the reverse mode of Na⁺–Ca²⁺ exchange did not affect the characteristics of the decay of the calcium transient in guinea pig myocardium. Selective inhibition of SERCA2a in the guinea pig and rat myocardium had a significant modifying effect on the decay phase of the calcium transient and resulted in emergence of the “bump” phase or an increase in the intensity of this phase in the myocardium of these animal species. The characteristics of this phase can be used to quantify the length-dependent activation of myocardial contraction.