Cardiac responses of Pacific oyster Crassostrea gigas to agents modulating cholinergic function

To examine the functional effects of cholinergic modulation compounds in oyster hearts and to explore their possible use in monitoring intoxication with acetylcholine-esterase (AChE) inhibitors such as organophosphates, tests were performed with in situ oyster heart preparations. The endogenous cholinergic agonist acetylcholine (ACh), AChE-resistant synthetic agonist carbachol, and the reversible carbamate type of AChE inhibitor physostigmine, all potently depressed spontaneous cardiac contractility. The depression was reversed by extensive washout, or prevented by muscarinic cholinergic antagonist atropine. The irreversible organophosphate type AChE inhibitor parathion or its active metabolite paraoxon at concentrations up to 100 μM failed to depress cardiac contractility. While other reversible AChE inhibitors such neostigmine and pyridostigmine also depressed the contractility, organophosphate AChE inhibitors malathion, diazinon, or phenthoate did not. Despite the differential effect in depressing cardiac function between the reversible and irreversible inhibitors, both of these inhibitors effectively inhibited cardiac AChE activity. The results suggest that the activation of muscarinic cholinergic receptors is coupled to inhibitory cardiac modulation, and organophosphate AChE inhibitors may inhibit only an AChE isozyme located at sites that are not important for control of cardiac activity in oysters.