α 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 the α 2 ‐adrenoceptor agonist UK14304 produces vasoconstriction in the porcine isolated ear artery. 2 UK14304 (0.3 μ m ) produced a small contraction of porcine isolated ear arteries which was 7.8±3.3% of the response to 60 m m KC1. Similar sized contractions were obtained after precontraction with either 30 n m angiotensin II, or 0.1 μ m U46619 (8.2±1.8% and 10.2±2.6% of 60 m m KC1 response, respectively). However, an enhanced α 2 ‐adrenoceptor response was uncovered if the tissue was precontracted with U46619, and relaxed back to baseline with 1–2 μ m forskolin before the addition of UK14304 (46.9±9.6% of 60 m m KC1 response). 3 The enhanced responses to UK14304 in the presence of U46619 and forskolin were not inhibited by the α 1 ‐adrenoceptor antagonist prazosin (0.1 μ m ), but were inhibited by the α 2 ‐adrenoceptor antagonist rauwolscine (1 μ m ), indicating that the enhanced responses were mediated via postjunctional α 2 ‐adrenoceptors. 4 In the presence of 0.1 μ m U46619 and 1 m m isobutylmethylxanthine (IBMX), 1 μ m forskolin produced an increase in [ 3 H]‐cyclic AMP levels in porcine isolated ear arteries. Addition of 0.3 μ m UK14304 prevented this increase. 5 The enhanced UK14304 response was dependent upon the agent used to relax the tissue. After relaxation of ear arteries precontracted with 10 n m U46619 and relaxed with forskolin the UK14304 response was 46.9±9.6% of the 60 m m KC1 response, and after relaxation with sodium nitroprusside (SNP) the response was 24.8±3.3%. However, after relaxation of the tissue with levcromakalim the UK14304 response was only 8.2±1.7%, which was not different from the control response in the same tissues (12.2±5.6%). An enhanced contraction was also obtained after relaxation of the tissue with the 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 Relaxation of U46619 contracted ear arteries with SNP could be inhibited by 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 compound does not act via production of cyclic GMP. Biochemical studies showed that while forskolin increases the levels of cyclic AMP in the tissues, SNP had no effect on the levels of this cyclic nucleotide. 7 In conclusion, enhanced contractions to the α 2 ‐adrenoceptor agonist UK14304 can be uncovered in porcine isolated ear arteries by precontracting the tissue with U46619, followed by relaxation back to baseline with forskolin, SNP or dibutyryl cyclic AMP before addition of UK14304. There was a greater contractile response to UK14304 after relaxation with forskolin than with SNP or dibutyryl cyclic AMP, suggesting that cyclic AMP‐dependent and‐ independent mechanisms are involved in the enhancement of the UK14304 response.British Journal of Pharmacology (1998) 124 , 1107–1114; doi: 10.1038/sj.bjp.0701935