Species- and Temperature-Dependency of the Decrease in Myofilament Ca2+ Sensitivity Induced by β -Adrenergic Stimulation

Although beta-adrenergic stimulation has been shown in many studies to decrease myofilament Ca2+ sensitivity in various types of cardiac muscle such as rat and rabbit ventricles, other studies disagree with this conclusion. In the present study, we aimed to explain these contradictory findings. We examined the effect of beta-adrenoceptor stimulation on Ca2+ sensitivity using guinea pig and rat ventricles. We performed the experiment at two different temperatures and compared the results. In guinea pig ventricles, isoproterenol and forskolin did not alter the relationship between [Ca2+]i and muscle force during the relaxation phase of tetanic contraction at either 24 degrees C or 30 degrees C. In rat ventricles, in contrast, isoproterenol shifted the [Ca2+]i-force curve to the right at 24 degrees C, but not at 30 degrees C. In guinea pig ventricles permeabilized by alpha-toxin, in which the cAMP/PK-A system is intact, the addition of cAMP did not decrease Ca2+ sensitivity. These results suggest that there are species- and temperature-dependent differences in the regulation of myofilament Ca2+ sensitivity by beta-adrenergic stimulation.