Structure and function of the molluscan myoactive tetradecapeptides.

Effects of myoactive tetradecapeptides, Achatina excitatory peptide 2 and 3 (AEP2 and AEP3) and Fusinus excitatory peptide 4 (FEP4), on several molluscan muscles and neurons were investigated. In the penis retractor and radula retractor muscles of Achatina fulica (pulmonate), the three peptides enhanced the tetanic contraction elicited by nerve stimulations. The order of potency was AEP2 > AEP3 > FEP4, although the effects of AEP3 and FEP4 on the radula retractor were somewhat irregular. AEP2 also induced rhythmic bursts of activity in the buccal ganglionic neuron B4 known as a cholinergic motoneuron of the radula retractor. In the radula protractor and retractor muscles of Fusinus ferrugineus (prosobranch), FEP4 was most potent in enhancing the contraction. The enhancement was greater in the protractor than in the retractor. It was suggested that myoactive tetradecapeptides modulate mainly the cholinergic transmission in molluscan muscles.