Mechanism of tachyphylaxis on the inhibitory effect of manganese on agonist-induced contractions in the isolated vas deferens of the guinea pig

Effects of phorbol-12,13-dibutyrate and protein kinase C inhibitors on Mn(2+)-dependent norepinephrine-induced contractions involving increase in Mn2+ sensitivity in Ca(2+)-depleted vas deferens of the guinea pig

1. In Ca(2+)-depleted Mn(2+)-loaded vas deferens from the guinea pig (Mn-loaded preparations), norepinephrine (NE) induced a tonic contraction dose dependently without extracellular Ca2+ and Mn2+. 2. In the beta-escin skinned vas deferens, Mn2+ as well as Ca2+ induced contractions. Guanosine triphosphate and NE increased the sensitivity of contractile mechanisms to these divalent cations. 3. Phorbol-12,13-dibutyrate (PDBu) did not induce contractions in normal (Mn(2+)-unloaded Ca(2+)-contained) preparations, whereas it induced slow sustained contractions dose dependently in Mn-loaded preparations. Although cumulative applications of PDBu desensitized the preparations to this phorbol ester, the desensitization did not affect Mn(2+)-dependent NE-induced contractions. PDBu did not affect the dose-response relation of NE in Mn-loaded preparations. 4. Staurosporine (10-100 nM) preferentially inhibited NE-induced contractions in normal and in Mn-loaded preparations to that induced by K+ in normal preparations. However, bisindolylmaleimide I (1 microM) did not inhibit NE-induced contractions in normal and Mn-loaded preparations but abolished PDBu-induced contractions. 5. These results suggest that the NE-induced increase in the Mn2+ sensitivity of contractile mechanisms contributes to Mn(2+)-dependent NE-induced contractions, which may not involve the activation of protein kinase C.

The specific effect of Mn2+ on the tonic components of receptor-mediated contractions in isolated vas deferens of the guinea pig

Intracellular accumulation of Mn2+ augmented the contractions induced by norepinephrine and acetylcholine in the guinea pig isolated vas deferens. Contractions repeatedly induced by norepinephrine, acetylcholine, or a high concentration of K+ decreased depending on the incubation time in Ca(2+)-free medium. The rate of decrease of all contractions was delayed by intracellularly accumulated manganese. In the Mn(2+)-loaded preparations, the tonic components of the contractions induced by norepinephrine and acetylcholine, but not K+, were highly resistant to extracellular Ca2+ elimination. Ryanodine abolished the initial phasic component but did not affect the tonic component of norepinephrine- and acetylcholine-contractions in Mn(2+)-loaded preparations in Ca(2+)-free medium. In Ca(2+)-depleted preparations, the tonic contraction induced by norepinephrine was restored after the Mn(2+)-loading procedure, and the magnitude of this tonic contraction was comparable to the tonic component of the norepinephrine contraction in the normal medium before Mn2+ loading. The tonic contraction was reproducible in medium without either Mn2+ or Ca2+. These results suggested that intracellular Mn2+ can support norepinephrine-induced tonic contractions. In the Ca(2+)-depleted Mn(2+)-loaded preparations, K+ also induced a tonic contraction in the presence of extracellular Mn2+. However, this contraction was much smaller than that induced by norepinephrine. These results suggested that intracellular Mn2+ augmented contractions not only via an increase in intracellular Ca2+ availability but also via the direct action of Mn2+ on contractile mechanisms, and that this action is highly specific for developing and/or maintaining tonic contractions mediated by receptor activation in the guinea pig isolated vas deferens.