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Mechanical responses to noradrenaline (NA) were recorded in longitudinal muscle strips from the terminal and intermediate regions (3-10 cm and 30-40 cm from the ileo-caecal junction, respectively) of the guinea-pig ileum. NA(0.16-1,600 microM) produced a concentration-dependent contraction in the terminal ileum with the EC50 value of 11.9 +/- 4.3 microM (n = 5). In the intermediate ileum, NA produced relaxation at concentrations ranging from 0.016 microM to 1.6 microM. Responses to NA at 16 microM varied among preparations: no noticeable change in tension, a moderate relaxation and a small contraction. At higher concentrations, NA produced contractile responses and their peak tension increased in a concentration-dependent manner. The contractile effect of NA in the terminal ileum remained unaltered after treatment with propranolol (1.4 microM) or yohimbine (1.1 microM), or a combination of these drugs. In the intermediate ileum, the relaxant effect of NA was markedly reduced or abolished by propranolol (1.4 microM). Yohimbine (1.1 microM) had little effect on NA-induced relaxation. The contractile effect of NA in both the terminal ileum and intermediate ileum was inhibited by prazosin (1.1 microM), so that the contractions were converted to relaxation or markedly reduced. Propranolol abolished the relaxant response induced by NA at 0.16 microM, but did not inhibit those induced by NA at 1.6 microM. Methoxamine (0.16-1,600 microM) produced a concentration-dependent contraction in both the terminal ileum and the intermediate ileum. The EC50 was 93.5 +/- 28.5 microM (n = 8) in the terminal ileum and 83.3 +/- 27.7 microM (n = 10) in the intermediate ileum. There was no significant difference between the values. The results showed that NA is capable of producing contraction or relaxation in the longitudinal muscle layer of the terminal and intermediate regions of the guinea-pig ileum. The contraction, which is mediated by alpha 1-adrenoceptors, predominates in the terminal ileum, but relaxation, which is mediated by beta-adrenoceptors and uncharacterized adrenoceptors, predominates in the intermediate ileum.

journal of veterinary medical science

Pharmacological Characterization of Adrenoceptors Mediating Contractile and Relaxant Responses to Noradrenaline in the Longitudinal Muscle of Guinea-Pig Ileum.