ALPHA(2)-ADRENOCEPTOR-MEDIATED CONTRACTIONS OF THE PORCINE ISOLATED EAR ARTERY - EVIDENCE FOR A CYCLIC-AMP-DEPENDENT AND A CYCLIC AMP-INDEPENDENT MECHANISM

1 The aim of this study was to determine the conditions under which th e alpha(2)-adrenoceptor agonist UK14304 produces vasoconstriction in t he porcine isolated ear artery. 2 UK14304 (0.3 mu M) produced a small contraction of porcine isolated ear arteries which was 7.8 +/- 3.3% of the response to 60 mM KCl. Similar sized contractions were obtained a fter precontraction with either 30 nM angiotensin II, or 0.1 mu M U466 19 (8.2 +/- 1.8% and 10.2 +/- 2.6% of 60 mM KCl response, respectively ). However, an enhanced alpha(2)-adrenoceptor response was uncovered i f the tissue was precontracted with U46619, and relaxed back to baseli ne with 1-2 mu M forskolin before the addition of UK14304 (46.9 +/- 9. 6% of 60 mM KCl response). 3 The enhanced responses to UK14304 in the presence of U46619 and forskolin were not inhibited by the alpha(1)-ad renoceptor antagonist prazosin (0.1 mu M), but were inhibited by the a lpha(2)-adrenoceptor antagonist rauwolscine (1 mu M), indicating that the enhanced responses were mediated via postjunctional alpha(2)-adren oceptors. 4 In the presence of 0.1 mu M U46619 and 1 mM isobutylmethyl xanthine (IBMX), 1 mu M forskolin produced an increase in [H-3]-cyclic AMP levels in porcine isolated ear arteries. Addition of 0.3 mu M UK1 4304 prevented this increase. 5 The enhanced UK14304 response was depe ndent upon the agent used to relax the tissue. After reIaxation of ear arteries precontracted with 10 nM U46619 and relaxed with forskolin t he UK14304 response was 46.9 +/- 9.6% of the 60 mM KCl response, and a fter relaxation with sodium nitroprusside (SNP) the response was 24.8 +/- 3.3%. However, after relaxation of the tissue with levcromakalim t he UK14304 response was only 8.2 +/- 1.7%, which was not different fro m the control response in the same tissues (12.2 +/- 5.6%). An enhance d contraction was also obtained after relaxation of the tissue with th e cyclic AMP analogue dibutyryl cyclic AMP (23.2 +/- 1.3%) indicating that at least part of the enhanced response to UK14304 is independent of the ability of the agonist to inhibit cyclic AMP production. 6 Rela xation of U46619 contracted ear arteries with SNP could be inhibited b y the NO-sensitive guanylyl-cyclase inhibitor 1H-[1,2,4] oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ) indicating that production of cyclic GMP is necessary for the relaxant effect of SNP. However, ODQ had no effect on the relaxation of tissue by forskolin, suggesting that this compoun d does not act via production of cyclic GMP. Biochemical studies showe d that while forskolin increases the levels of cyclic AMP in the tissu es, SNP had no effect on the levels of this cyclic nucleotide. 7 In co nclusion, enhanced contractions to the alpha(2)-adrenoceptor agonist U K14304 can be uncovered in porcine isolated ear arteries by precontrac ting the tissue with U46619, followed by relaxation back to baseline w ith forskolin, SNP or dibutyryl cyclic AMP before addition of UK14304. There was a greater contractile response to UK14303 after relaxation with forskolin than with SNP or dibutyryl cyclic AMP, suggesting that cyclic AMP-dependent and- independent mechanisms are involved in the e nhancement of the UK14304 response.