Pharmacological evidence for the existence of presynaptic alpha-like receptors in Mytilus muscle.

The effects of alpha-adrenoceptor agonists and antagonists on contractions of the ABRM of M. edulis were examined. Naphazoline (10(-11)-10(-7)M) as well as octopamine potentiated both contractions in response to ACh and to repetitive electrical stimulation with brief pulses, while clonidine (10(-12)-10(-6)M), imidazole (10(-4)-10(-3)M) and tolazoline (10(-6)-10(-4)M) potentiated only the contraction in response to repetitive electrical stimulation. Potentiating actions of octopamine and naphazoline on ACh contraction were blocked by tolazoline but were little affected by clonidine and imidazole. Potentiating after-effect of repetitive electrical stimulation on subsequent ACh-contraction was not affected by clonidine and imidazole but was blocked by tolazoline. These results suggest that in the ABRM there are probably at least two classes of alpha-like receptors for octopamine; one may be on the muscle fibres and the other may be on the excitatory nerve terminals. Activation of the latter class of receptors may result in enhancement of ACh release from the terminals.