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

1 The aim of this study is to elucidate the role of adenosine in the motor function of the guinea-pig distal colon. 2 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. 3 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 held stimulation (EC50 = 1.07 x 10(-8) and 2.12 x 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 mu M). A(1) antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 10 nM: 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). 4 In the presence of tetrodotoxin (TTX, 0.3 mu M), CPA and NECA relaxed myogenic precontraction induced by KCl (50 mM) (EC50 = 1.26 x 10(-5) and 1.04 x 10(-5) M, respectively), but CGS21680 (1 mu M) did not cause any relaxation. DPCPX did not affect responses to CPA and NECA at a concentration of 10 nM, but a higher concentration (1 mu M) of DPCPX and 10 mu M of 8-PT antagonized those responses. 5 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.