title: Comparison of VIP and beta 2-adrenoceptor-induced relaxations in the circular and longitudinal smooth muscle layers of the rat portal vein.
abstract: The relative importance of VIP in reduction of vascular tone was studied in circular and longitudinal preparations of the VIP-innervated rat portal vein. Exogenous VIP inhibited the methoxamine-evoked contractures in the atropine-blocked preparations with a lower potency in the inner, circular (pD2 = 6.4 +/- 0.5, n = 6) than in the outer, longitudinal layer (pD2 = 7.7 +/- 0.1, n = 6). VIP was also a less efficient relaxant (intrinsic activity (alpha) = 0.60 +/- 0.16, n = 6) of the inner than of the outer layer (alpha = 1.00). The selective (salbutamol) and the non-selective (isoproterenol) beta 2-agonists completely relaxed the methoxamine contractures in both layers and the potency (isoproterenol) was higher in the inner (pD2 = 6.39 +/- 0.32, n = 6) than in the outer layer (pD2 = 5.67 +/- 0.34, n = 6). Plasma from the portal-mesenteric vein of anaesthetized, fasting rats contained 0.036 nM VIP (median, n = 17), that is, several orders of magnitude lower than the range of VIP concentrations relaxing the methoxamine contracted vein preparations via VIP receptors of the apamin-blockable category. The results support the hypothesis that alpha 1-adrenoceptor-induced contractions in the circular layer are predominately relaxed via beta 2-adrenoceptors while relaxation of the outer layer may occur via VIP receptors, probably activated by local release of the neuropeptide.
mesh_terms: D000420:Albuterol / Q000494:pharmacology*; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D005260:Female; D007545:Isoproterenol / Q000494:pharmacology*; D008729:Methoxamine / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects*; D009126:Muscle Relaxation / Q000187:drug effects*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D011169:Portal Vein / Q000187:drug effects*; D051381:Rats; D011919:Rats, Inbred Strains; D014660:Vasoactive Intestinal Peptide / Q000494:pharmacology*
chemical_list: D001030:Apamin; D014660:Vasoactive Intestinal Peptide; D008729:Methoxamine; D007545:Isoproterenol; D000420:Albuterol