1 A comparative study was carried out between the adenosine receptor mediating a stimulation of cyclic AMP formation in guinea-pig cerebral cortical slices with the adenosine receptor mediating relaxation of phenylephrine precontracted guinea-pig aortic rings. (104 ± 13). Maximal relaxations elicited by these agents were 71 ± 3, 98 ± 1, and 100± 1%, respectively. CGS 21680 and DPMA at 100 pM elicited smaller relaxations of the precontracted tissues (12 ± 2 and 43 ± 15%, respectively). 6 Antagonism by xanthine derivatives of the 5'-N-ethylcarboxamidoadenosine-induced relaxation of aortic rings showed the following rank order of potency (Ki, nM): xanthine amino congener (17 ± 4) > 8-cyclopentyl-1,3-dipropylxanthine (171 ± 36) > PD 115,199 (341 ± 64) > 3,7-dimethyl-1-propargylxanthine (5520 ± 820). 7 We conclude that the A2 adenosine receptor mediating relaxation of phenylephrine-contracted aortic rings is an A2b adenosine receptor which exhibits certain minor differences from the A2b receptor which stimulates cyclic AMP accumulation in cerebral cortical slices.
1 We have investigated the pharmacological profile of the adenosine receptor mediating relaxation of the carbachol pre-contracted guinea-pig trachea. 2 5'-N-Ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine elicited concentration-dependent relaxations with pD2 (-log,0 half-maximal values) of 6.37 + 0.04 and 5.25 + 0.09, with maximal relaxations of 73 + 7 and 208 + 38%, respectively. In the presence of 10 gM NECA, 2-chloroadenosine was able to relax the tissue further with a pD2 value of 4.74 + 0.11 and a maximal response of 252 + 68%. 3 CGS 21680, APEC and adenosine failed to elicit significant relaxations of precontracted tracheal rings at concentrations below 10 gM. At 10 gM, adenosine analogues elicited relaxations with the following order of magnitude (% relaxation): 2-chloroadenosine (75+ 16%) = NECA (69 + 16%)> APEC (25+8%)>CGS 21680 (11+2%)>adenosine (6+4%). 4 NECA-induced relaxation of precontracted trachea was antagonized by adenosine receptor antagonists with the rank order of apparent affinity (Ki, nM): PD 115,199 (27+8)=XAC (43+11)> CP 66,713(285+89)=DPCPX (316+114). 5 We conclude that the adenosine analogue-induced relaxation of guinea-pig tracheal rings fails to fit into the current classification of A2 adenosine receptors.
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