Effects of ryanodine and nifedipine on contractions induced by norepinephrine, acetylcholine, and potassium in the isolated vas deferens of the guinea pig

Effects of heptanol and carbenoxolone on noradrenaline induced contractions in guinea pig vas deferens

We examined the effects of two putative gap junction blockers, heptanol and carbenoxolone, on noradrenaline-induced contractions in guinea pig vas deferens. The force generated due to the exogenously added noradrenaline (20 microM) consisted of two components: the tonic and the oscillatory. 2 mM heptanol abolished the oscillatory contractions and drastically suppressed both the maximum force (by 85.4 +/- 18.2%) as well as the tonic component (by 28.8 +/- 5.1%) (P<0.01, n=7). However, the effects of carbenoxolone (50 microM) were strikingly different, with the spikes of the oscillatory component being merged into a steady, "fused" contraction, without affecting the maximum force developed. The L-type Ca(2+) channel blocker nifedipine (2 microM) abolished the oscillatory component of the contractions and significantly reduced the maximum force and tonic component (by 82.4 +/- 6.8% and 19.7 +/- 6.4% respectively; P<0.01, n=4), in a manner similar to that elicited by heptanol. Our results indicate that (i) while carbenoxolone specifically blocks gap junctions, heptanol appears to exert its actions through non-gap junctional mechanisms, possibly by blocking VGCCs in smooth muscle; (ii) gap junctions play a significant modulatory role in the generation of noradrenaline-induced contractions in guinea pig vas deferens, particularly in the emergence of oscillatory contractions, while the maximum force developed may be independent of gap junctional contribution.

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.