Cholinergic Inhibitory Innervation of the Cardioarterial Valves in the Isopod Bathynomus doederleini

Abstract

In the isopod Bathynomus doederleini, the cardioarterial valves of all five pairs of lateral arteries and the pair of anterior lateral arteries 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 (IJPs) in valve muscle cells which arose one-to-one in response to stimulus pulses. Acetylcholine (ACh) hyperpolarized muscle cells of the valves. Both the IJPs and ACh-induced hyperpolarization brought about an increase 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 valve muscle cells. Atropine and methylxylocholine antagonized both the IJPs and ACh-induced hyperpolarizing potentials, while d-tubocurarine did not antagonize IJPs. These results indicate that ACh may be the transmitter for the valve dilator nerves. IJPs did not invert in Cl−-free saline. Amplitude of IJPs increased in low K salines, and decreased in high K salines. It is likely that IJPs are mediated predominantly by K ions. This could be the first case of cholinergic inhibitory transmission at neuromuscular junctions in crustaceans.