ENDOTHELIUM-DEPENDENT AND ENDOTHELIUM-INDEPENDENT VASODILATATION OF THE HEPATIC-ARTERY OF THE RABBIT

1 The isolated hepatic artery of the rabbit contracted to exogenously applied noradrenaline (NA). There was no significant difference in the maximal contraction or the EC50 value in vessels where the endothelium was present and in endothelium-denuded preparations. 2 Acetylcholine (ACh) induced a vasodilatation of vessels preconstricted with NA which was entirely dependent on the endothelium. 3 Adenosine 5'-triphosphate (ATP), 2-methylthio ATP, adenosine and sodium nitroprusside induced concentration-dependent, sustained relaxations of vessels in which tone had been induced with NA. The relaxation responses were not reduced after removal of the endothelium. 8-Phenyltheophylline antagonized the relaxation response produced by adenosine, but not that due to ATP at lower concentrations. The maximum response to ATP was reduced in the presence of 8-phenyltheophylline. 4 alpha,beta-Methylene ATP produced further contraction of vessels preconstricted with NA in both endothelium-denuded preparations and in vessels where the endothelium remained intact. 5 Immunohistochemical analysis was used to show the presence of nerve fibres containing substance P (SP), calcitonin gene-related peptide (CGRP) and vasoactive intestinal polypeptide (VIP) in the hepatic artery. Application of SP induced a concentration-dependent relaxation which was entirely dependent on the presence of an intact endothelium. CGRP and VIP, however, elicited concentration-dependent relaxations which were independent of the endothelium. 6 It is concluded that in the rabbit hepatic artery, responses to ACh are dependent on the presence of intact endothelium. P1-, P2x- and P2y-purinoceptors, mediating relaxation to adenosine, vasoconstriction to ATP and vasodilatation to ATP respectively, are located on vascular smooth muscle. Furthermore, CGRP and VIP mediate a direct vasodilatation of smooth muscle both in the absence and the presence of the endothelium, whereas SP produces a relaxation via receptors located on the endothelium.