1 The aim of this study was to determine the conditions under which the a 2 -adrenoceptor agonist UK14304 produces vasoconstriction in the porcine isolated ear artery. 2 UK14304 (0.3 mM) produced a small contraction of porcine isolated ear arteries which was 7.8+3.3% of the response to 60 mM KC1. Similar sized contractions were obtained after precontraction with either 30 nM angiotensin II, or 0.1 mM U46619 (8.2+1.8% and 10.2+2.6% of 60 mM KC1 response, respectively). However, an enhanced a 2 -adrenoceptor response was uncovered if the tissue was precontracted with U46619, and relaxed back to baseline with 1 ± 2 mM forskolin before the addition of UK14304 (46.9+9.6% of 60 mM KC1 response). 3 The enhanced responses to UK14304 in the presence of U46619 and forskolin were not inhibited by the a 1 -adrenoceptor antagonist prazosin (0.1 mM), but were inhibited by the a 2 -adrenoceptor antagonist rauwolscine (1 mM), indicating that the enhanced responses were mediated via postjunctional a 2 -adrenoceptors. 4 In the presence of 0.1 mM U46619 and 1 mM isobutylmethylxanthine (IBMX), 1 mM forskolin produced an increase in [ 3 H]-cyclic AMP levels in porcine isolated ear arteries. Addition of 0.3 mM 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 nM U46619 and relaxed with forskolin the UK14304 response was 46.9+9.6% of the 60 mM 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 di erent 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 e ect of SNP. However, ODQ had no e ect 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 e ect on the levels of this cyclic nucleotide. 7 In conclusion, enhanced contractions to the a 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 ...

1 The responses of the electrically stimulated guinea-pig ileum and vas deferens to human and rat calcitonin gene-related peptide (CGRP) and amylin were investigated. 2 The inhibition of contraction of the ileum produced by human aCGRP was antagonized by human cxCGRP8-37 (apparent pA2 estimated at 7.15+0.23)>human aCGRP19 37 (apparent pA2 estimated as 6.67 + 0.33)> [Tyr°-human aCGRP28-37. The amylin antagonist, AC187, was three fold less potent than CGRP8-37 in antagonizing human aCGRP. 3 Both human J-and rat aCGRP inhibited contractions of the ileum, but this was less sensitive to inhibition by CGRP8 37 than the effect of human aXCGRP. However, CGRP19 37 was twenty times more effective in inhibiting the response to rat aCGRP (apparent pA2 estimated as 8.0+0.1) compared to human aCGRP. 4 Rat amylin inhibited contractions in about 10% of ileal preparations; this effect was not antagonized by any CGRP fragment. Human amylin had no action on this preparation. 5Both human and rat aCGRP inhibited electrically stimulated contractions of the vas deferens, which were not antagonized by 3 giM CGRP8-37 or 10 /M AC187. 6 Rat amylin inhibited the stimulated contractions of the vas deferens (EC50= 77 + 9 nM); human amylin was less potent (EC50 = 213 + 22 nM). The response to rat amylin was antagonized by 10 gM CGRP8-37 (EC5o=242+25 nM) and 10 gM AC187 (EC50= 610 + 22 nM).7 It is concluded that human aCGRP relaxes the guinea-pig ileum via CGRPI-like receptors, but that human /CGRP and rat aCGRP may use additional receptors. These are distinct CGRP2-like and amylin receptors on guinea-pig vas deferens.

1 The receptors which mediate the e ects of calcitonin gene-related peptide (CGRP), amylin and adrenomedullin on the guinea-pig vas deferens have been investigated. 2 All three peptides cause concentration dependant inhibitions of the electrically stimulated twitch response (pD 2 s for CGRP, amylin and adrenomedullin of 7.90+0.11, 7.70+0.19 and 7.25+0.10 respectively). 3 CGRP 8 ± 37 (1 mM) and AC187 (10 mM) showed little antagonist activity against adrenomedullin. 4 Adrenomedullin 22 ± 52 by itself inhibited the electrically stimulated contractions of the vas deferens and also antagonized the responses to CGRP, amylin and adrenomedullin. 6 Although CGRP has been shown to act at a CGRP 2 receptor on the vas deferens with low sensitivity to CGRP 8 ± 37 , this antagonist displaced [ 125 I]-CGRP with high a nity from vas deferens membranes. This a nity was unaltered by increasing the temperature from 48C to 258C, suggesting the anomalous behaviour of CGRP 8 ± 37 is not due to temperature di erences between binding and functional assays.