A study of NPY-mediated contractions of the porcine isolated ear artery

Evidence for a cyclic AMP-dependent pathway in angiotensin AT1-receptor activation of human omental arteries

Enhanced responses to vasoconstriction induced by neuropeptide Y and α2-adrenoceptor agonists have been seen following pharmacological activation of the adenylyl cyclase (AC) system. Since preliminary studies revealed only minor responses to angiotensin II (Ang II) in human omental arteries, we have investigated whether enhanced activity of AC may unravel further functional Ang II receptors. Human omental arteries were obtained in conjunction with elective gut surgery. After dissection of the vessel, the endothelium was removed by 10 sec of Triton X-100 treatment. Ring segments (1—2 mm long) were mounted on a myograph and studied. Ang II produced small contractions, 27±5% relative to the response elicited by 60 mM K+. However, enhanced Ang II (105±10%, p<0.001) responses were seen during AC activation by forskolin (0.1—1 µM). This enhanced contractile response to Ang II was not inhibited by the angiotensin II type 2 (AT2-receptor antagonist PD 123319 (0.1 µM), but was blocked in an insurmountable way by the angiotensin II type 1 (AT1)-receptor antagonist candesartan (1 nM) and in a surmountable manner by losartan (0.1 µM) and irbesartan (0.1 µM). Pertussis toxin (a Gi-protein blocker) and the protein kinase C inhibitor, RO31—8220 (0.01, 0.1 and 1 µM), markedly reduced this response, while the protein kinase A inhibitor, H89 (1, 10 µM), had no effect. RT-PCR provided evidence for the presence of mRNA for both AT1- and AT2-receptors. The results suggest that both a cAMP-dependent and a cAMP-independent mechanism are involved in the contractile responses to Ang II in human omental arteries and that both responses are mediated via the AT1-receptor.

Evidence for a non-adrenoceptor, imidazoline-mediated contractile response to oxymetazoline in the porcine isolated rectal artery

Imidazoline derivatives are known to elicit responses through both alpha(2)-adrenoceptor and non-adrenoceptor, imidazoline sites, though as yet there are no examples of the latter on vascular smooth muscle. In the presence of 0.3 microM prazosin, neither UK-14304 (0.01 - 3 microM) nor oxymetazoline (0.01 - 30 microM) caused a significant contraction of the porcine isolated rectal artery, a preparation with a low density of alpha(2)-adrenoceptors. In the presence of a combination of U46619 and forskolin, however, both agonists produced concentration-dependent contractions. Pretreatment with phenoxybenzamine (3 microM) abolished responses to UK-14304, but left those elicited by oxymetazoline largely unaffected. The putative I(3) imidazoline antagonist 2-(2,3 dihydro-2-benzofuranyl)-2-imidazole (KU-14R, 10 microM) caused a 6 fold rightward displacement of the phenoxybenzamine-insensitive concentration - response curve to oxymetazoline. Our data indicates that non-adrenoceptor, imidazoline sites, pharmacologically similar to the I(3) imidazoline site on islet cells, mediate vasoconstriction in the porcine isolated rectal artery.

alpha(2)-adrenoceptor and NPY receptor-mediated contractions of porcine isolated blood vessels: evidence for involvement of the vascular endothelium

1. Enhanced contractions to the alpha(2)-adrenoceptor agonist UK14304 and neuropeptide Y (NPY) in the porcine ear artery can be uncovered by pharmacological manipulation. The aim of this study was to determine whether similar pharmacological manipulation can uncover enhanced contractions in the porcine splenic artery, and to determine whether the endothelium modulates these responses. 2. UK14304 (0.3 microM) and NPY (0.1 microM) produced small contractions of the porcine splenic artery. After pre-contraction of the tissue with U46619, followed by relaxation with forskolin, the responses to both UK14304 and NPY were enhanced. Enhanced contractions to both UK14304 and NPY were also obtained after relaxation with SNP. These results demonstrate that, as in the porcine ear artery, alpha(2)-adrenoceptors and NPY receptors are able to produce enhanced contractile responses through both adenylyl cyclase-dependent and -independent signal transduction pathways. 3. Removal of the endothelium had no significant effect on responses to UK14304 either alone or in the presence of U46619 and forskolin in the porcine splenic artery. On the other hand, responses to UK14304 after relaxation with SNP were reduced after endothelium-denudation in both the porcine splenic artery and ear artery. Similar results were obtained with NPY in the porcine ear artery. 4. In conclusion, enhanced contractile responses to UK14304 and NPY in the porcine splenic artery can be uncovered using methods similar to those employed in the porcine ear artery. Under certain conditions the responses to both agents are modulated by the endothelium. These data highlight further the similarities in the signal transduction pathways used by both alpha(2)-adrenoceptors and NPY receptors to induce vasoconstriction.