Mechanism of Tachyphylaxis on the Inhibitory Effect of Manganese on Agonist-Induced Contractions in the Isolated Vas Deferens of the Guinea Pig

抄録

In the isolated vas deferens of the guinea pig, the inhibitory effect of manganese (Mn²⁺) on contractions induced by norepinephrine decreased when the contractions were induced repeatedly in the presence of Mn²⁺. This phenomenon, "tachyphylaxis" to the inhibitory effect of Mn²⁺, was minimally observed when contractions were induced by acetylcholine. Contractions induced by acetylcholine as well as by norepinephrine in a Mn²⁺-free medium were augmented in preparations in which manganese had accumulated intracellularly by repetitive applications of high potassium in the presence of Mn²⁺. The magnitudes of their augmentation were dependent on the manganese contents of the preparations. The augmented contractions were remarkably resistant to the inhibitory effect of Mn²⁺ applied to the medium. These results suggest that intracellular manganese augments the contractions and this augmentation results in an apparent decrease in the inhibitory effect of Mn²⁺ in the medium.