Molecular identification and pharmacological characterization of adenosine receptors in the guinea‐pig colon

The aim of this study is to elucidate the role of adenosine in the motor function of the guinea‐pig distal colon. To determine whether adenosine A 1 receptors and A 2B receptors are expressed in the guinea‐pig colon, we employed the reverse transcription‐polymerase chain reaction (RT–PCR). The gene expression of A 1 receptor and A 2B receptor was found for the first time in the guinea‐pig proximal and distal colon. Adenosine A 1 agonist N 6 ‐cyclopentyladenosine (CPA), and A 1 /A 2 agonist 5′‐N‐ethylcarboxamidoadenosine (NECA) concentration‐dependently inhibited neurogenic responses to electrical field stimulation (EC 50 =1.07×10 −8 and 2.12×10 −8   M ) in the longitudinal muscle, but A 2A agonist 2‐p‐(2‐carboxyethyl)phenylethylamino‐5′‐N‐ethycarboxamido‐adenosine (CGS21680) had only a slight inhibitory effect (25.9%, 1 μ M ). A 1 antagonist 8‐cyclopentyl‐1,3‐dipropylxanthine (DPCPX, 10 n M : A 1 selective concentration) antagonized responses to CPA and NECA. Furthermore, the affinity order of antagonists at inhibiting the effect NECA was: DPCPX>8‐phenyltheophylline (8‐PT: A 1 /A 2 antagonist). In the presence of tetrodotoxin (TTX, 0.3 μ M ), CPA and NECA relaxed myogenic precontraction induced by KCl (50 m M ) (EC 50 =1.26×10 −5 and 1.04×10 −5   M , respectively), but CGS21680 (1 μ M ) did not cause any relaxation. DPCPX did not affect responses to CPA and NECA at a concentration of 10 n M , but a higher concentration (1 μ M ) of DPCPX and 10 μ M of 8‐PT antagonized those responses. These data lead us to the hypothesis that adenosine may mediate relaxation through two different inhibitory receptor subtypes; A 1 receptors on the enteric neuron and A 2B receptor on the smooth muscle in the guinea‐pig distal colon.British Journal of Pharmacology (2000) 129 , 871–876; doi: 10.1038/sj.bjp.0703123