Pharmacological properties of ACh receptors on the heart of the marine bivalve Meretrix lusoria

The cholinergic receptors on the heart of the marine bivalve Meretrix lusoria were used as a model to investigate their pharmacological properties in bivalves. By recording method, we measured the contractile force (CF) and the heart rate (HR) of the heart to examine the characteristics of cholinergic receptors of a marine bivalve. The known ACh receptor agonists or antagonists in vertebrate nervous system were then introduced into the heart by perfusion, to monitor their effects upon heart activities. Of the six tested agonists, two mAChR agonists (OXA-22, bethanechol) and two nAChR agonists (methylcarbachol, nicotine) mimicked the response induced by ACh. Among the seven tested antagonists, three mAChR antagonists (tribexyphenidyl, atropine and scopolamine) and three nAChR antagonists (TEA, curare and hexamethonium bromide) showed the antagonistic effects on the inhibition induced by ACh. However, there was one mAChR agonist, metoclopramide, that behaved like an ACh antagonist. Both APE (mAChR agonist) and PTMA (nAChR antagonist) showed no significant effects. This differences suggest that the pharmacological characteristics of ACh receptors on the heart of bivalve are different from those in vertebrates. It may be a novel type of ACh receptors.