Effects of modulatory agents on neurally-mediated ...

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Title Effects of modulatory agents on neurally-mediated responses of trout intestinal smooth muscle in vitro
Author(s) John F. Burka, H. A. Briand, C. A. Wartman, J. G. Hogan, W. P. Ireland
Journal Fish Physiology and Biochemistry
Date 1996
Volume 15
Issue 2
Start page 95
End page 104
Abstract Mediators and mechanisms responsible for the inhibitory modulation of trout intestinal smooth muscle were examined using a series of putative mediators and substances known to modulate neurotransmission in mammalian systems. Frequency response relationships to transmural stimulation and concentration response relationships to 5-hydroxytryptamine, carbachol, and substance P were established on paired segments of rainbow trout intestine in vitro in the presence and absence of putative modulatory agents. Modulation of neurally-mediated contractions of trout intestine was achieved with dibutyryl cyclic AMP and forskolin, agents that increase intracellular levels of cyclic AMP. The effect appears to be at the level of the smooth muscle, since the adenylate cyclase activator, forskolin, inhibited muscarinic and serotoninergic contractions as well as transmurally stimulated contractions. Substance P-induced contractions were unaffected by forskolin. The endogenous agonists/neurotransmitters which would increase cyclic AMP levels in rainbow trout intestinal smooth muscle are as yet unknown. The effects do not appear to be modulated by vasoactive intestinal peptide (VIP), calcitonin, calcitonin gene-related peptide (CGRP), or agents that activate beta-adrenoceptors. Prostaglandin E(2) (PGE(2)) and alpha(2)-adrenergic agonists are possible agents which will decrease contractility of the smooth muscle. They were only active in the proximal intestine and on transmurally stimulated contractions. The effects of both PGE(2) and alpha(2)-agonists appear to be prejunctional, decreasing release of contractile neurotransmitters in the enteric nervous system.

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