Characterization of P1‐purinoceptors on rat duodenum and urinary bladder

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Abstract
The P1‐purinoceptors mediating relaxation of the rat duodenum and inhibition of contraction of the rat urinary bladder were characterized by use of adenosine and its analogues 5′‐N‐ethylcarboxamidoadenosine (NECA), N6‐cyclopentyladenosine (CPA) and 2‐p‐((carboxyethyl)phenethylamino)‐5′‐carboxamidoadenosine (CGS 21680), as well as the A1‐selective antagonist 1,3‐dipropyl‐8‐cyclopentylxanthine (DPCPX). The stable analogue of adenosine 5′‐triphosphate (ATP), adenylyl 5′‐(β,γ‐methylene)diphosphonate (AMPPCP), was also used as previous work had indicated that it has a direct action on some P1 receptors in addition to its P2‐purinoceptor activity. In the rat duodenum, the order of potency of the adenosine agonists was NECA ≥ CPA > AMPPCP = adenosine > CGS 21680, and DPCPX antagonized CPA and AMPPCP at a concentration of 1 nm whereas equivalent antagonism of NECA and adenosine required a concentration of 1 μm. This suggests the presence of a mixture of A1 and A2 receptors in this tissue, with CPA and AMPPCP acting on the A1 and NECA and adenosine acting on the A2 receptors. In the rat bladder, the order of potency of the adenosine agonists for inhibition of carbachol‐induced contractions was NECA ≫ adenosine > CPA = CGS 21680, and a concentration of DPCPX of 1 μm was required to antagonize responses to NECA and adenosine. This suggests the presence of A2 receptors in this tissue. ATP and AMPPCP each caused contractions which were not enhanced by DPCPX (1 μm) which suggests that in this tissue AMPPCP was acting only via P2 receptors and had no P1 agonist activity. That AMPPCP was active on the A1 receptors in the duodenum but inactive on the A2 receptors in the bladder implies that it has selectivity for the A1 subtype. That CGS 21680, which has been reported to bind selectively to the high affinity A2a subclass of A2 receptors, had a very low potency on the A2 receptors in the duodenum and in the bladder suggests that these receptors are of the low affinity A2b subclass.