Inhibitory Action of Crustacean Red Pigment Concentrating Hormone on Mytilus Muscles

It has been suggested that a peptide which belongs to the family of the arthropod neuropeptides, red pigment concentrating hormone (RPCH) and adipokinetic hormone (AKH), is also present in mollusc. In the present study, the effects of RPCH and AKH on the anterior byssus retractor muscle (ABRM), the pedal retractor muscle (PRM) and the heart of a bivalve, Mytilus edulis, were examined.
In most of the ABRMs examined, RPCH inhibited contractile response or catch-relaxing response or both, which were evoked by repetitive electrical pulses of stimulation. The threshold concentrations for the inhibition of contraction and relaxation were about 10^-7 M and 10^-8 M, respectively. In some of the muscles, however, both of the inhibitory effects or one of them was not found at 10^-6 M or less.
In most of the PRMs examined, RPCH also inhibited contractile response to repetitive electrical stimulation at 3 x 10^-8 M or higher, but in some of the muscles, it did not show the inhibitory effect at 3 x 10^-7 M.
Rhythmic activity in some hearts was inhibited by 10^-7 M or higher RPCH, but in the other hearts, the activity was not affected at 3 x 10^-6 M or was enhanced at 10^-6 M or higher.
AKH was found to be ineffective at 10^-6 M in all of the muscles of three kinds.