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

Abstract

Although β-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 β-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°C or 30°C. In rat ventricles, in contrast, isoproterenol shifted the [Ca²⁺]_i-force curve to the right at 24°C, but not at 30°C. In guinea pig ventricles permeabilized by α-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 β-adrenergic stimulation.