Muscarinic M3-like receptors, cyclic AMP and L-type calcium channels are involved in the contractile response to cholinergic agents in gut smooth muscle of the rainbow trout, Oncorhynchus mykiss

Cholinergic signalling in mammalian gut smooth muscle usually involves M3 m uscarinic receptors for direct contraction via phospholipase C activation a nd M2 muscarinic receptors to reduce cyclic AMP levels. However, the propor tion of receptor subtypes and second messengers involved varies among tissu es and animals and studies in non-mammalian species will provide informatio n on the conservation of pathways and consequently on their importance for signal transduction. In the present study we investigated receptor subtypes , involvement of calcium, phospholipase C and cyclic AMP in the cholinergic contraction of the rainbow trout gut. Intestinal and gastric smooth muscle strip preparations, with the mucosa removed, were used in functional studi es, and homogenised strips were used for measurements of cyclic AMP. Calciu m-free medium, the L-type calcium-channel inhibitor verapamil, the cyclic A MP-phosphodiesterase inhibitor IBMX + isoprenaline, and the M3-antagonist 4 -DAMP methiodide all caused a partial or marked reduction of the response t o cholinergic agonists. Neomycin, an inhibitor of phospholipase C, and SKF9 6365, an inhibitor of receptor-operated calcium channels, had no effect. Ca rbachol (0.1 mM) reduced the levels of cyclic AMP transiently. It is conclu ded that the cholinergic signal transduction in rainbow trout gut smooth mu scle involves (1) binding to M3-like receptors, (2) a transient reduction i n cyclic AMP levels, (3) influx of extracellular calcium, in part through L -type calcium-channels and (4) no involvement of phospholipase C.