Effects of beta-endorphin, met-enkephalin and somatostatin on the neurotensin-induced neurogenic contraction in the guinea-pig ileum

At maximally effective concentrations, the opiate peptides β-endorphin (240 nm) and Met-enkephalin (1400 nM) virtually abolished the contractions induced by a maximally effective concentration of 60 nM neurotensin (NT), either in the longitudinal smooth muscle strip or in the intact segment of guinea-pig ileum. This inhibitory effect was concentration-dependent and was totally blocked by naloxone at 100 nM. In contrast a maximally effective concentration of somatostatin (60 nM) partially inhibited (50–60%) the contraction induced by 60 nM NT in either smooth muscle preparation. Somatostatin inhibition was concentration-dependent and was not blocked by naloxone at 100 nM. Atropine at 100 nM inhibited by 50% the contractions induced by 60 nM NT in the intact segment of guinea-pig ileum. The remaining contraction was abolished by β-endorphin and Met-enkephalin and partially reduced by somatostatin. Our results confirm that NT-induced contractions in the guinea-pig ileum are neurogenic and involve a cholinergic as well as a non-cholinergic component. Furthermore, we show that the release of mediators from both components