The relaxing effect of SKF 38393 on the catch contraction of Mytilus smooth muscle.

The relaxing effect of SKF 38393 on the catch contraction in the anterior byssus retractor muscle (ABRM) of Mytilus and the effect of SKF 38393 on the cyclic AMP (cAMP) levels in the ABRM were investigated. The catch contraction was relaxed by SKF 38393 in a dose-dependent manner. The dose-response curves of SKF 38393 were shifted in parallel to the right by butaclamol (3 X 10(-6) and 6 X 10(-6) M) and haloperidol (3 X 10(-6) and 3 X 10(-5) M). Their pA2 values were 5.69 +/- 0.05 and 5.80 +/- 0.07. The cAMP levels in the ABRM were not altered by SKF 38393 at a concentration (10(-5) M) sufficient to relax. These findings indicate that the dopamine receptors of the ABRM are D-1 like receptors but somewhat different from those of the vertebrates.