CHOLINERGIC INHIBITORY INNERVATION OF THE CARDIOARTERIAL VALVES IN THE ISOPOD BATHYNOMUS-DOEDERLEINI

In the isopod Bathynomus doederleini, the cardioarterial valves of all five pairs of lateral arteries and the pair of anterior lateral arter ies are innervated by inhibitory (dilator) nerves which consist of one or two axons arising from the central nervous system. Stimulation of the valve dilator nerves produced inhibitory junctional potentials (IJ Ps) in valve muscle cells which arose one-to-one in response to stimul us pulses. Acetylcholine (ACh) hyperpolarized muscle cells of the valv es. Both the IJPs and ACh-induced hyperpolarization brought about an i ncrease of haemolymph pressure in the arteries, through relaxation of valve muscles. The muscarinic agonists, muscarine, carbamylcholine and arecoline, mimicked ACh-induced hyperpolarizing responses of the valv e muscle cells. Atropine and methylxylocholine antagonized both the IJ Ps and ACh-induced hyperpolarizing potentials, while d-tubocurarine di d not antagonize IJPs. These results indicate that ACh may be the tran smitter for the valve dilator nerves. IJPs did not invert in Cl--free saline. Amplitude of IJPs increased in low K+ salines, and decreased i n high K+ salines. It is likely that IJPs are mediated predominantly b y K+ ions. This could be the first case of cholinergic inhibitory tran smission at neuromuscular junctions in crustaceans.