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 A₁ 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 A₁ receptor and A(2B) receptor was found for the first time in the guinea-pig proximal and distal colon. 3. Adenosine A₁ agonist N⁶-cyclopentyladenosine (CPA), and A₁/A₂ agonist 5'-N-ethylcarboxamidoadenosine (NECA) concentration-dependently inhibited neurogenic responses to electrical field 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 μM). A₁ antagonist 8-cyclopentyl-1,3-dipropyxanthine (DPCPX, 10 nM: A₁ 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: A₁/ A₂ antagonist). 4. In the presence of tetrodotoxin (TTX, 0.3 μM), CPA and NECA relaxed myogenic precontraction induced by KCI (50 mM) (EC₅₀ = 1.26 x 10^-5 and 1.04 x 10^-5 M, respectively), but CGS21680 (1 μM) 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 μM) of DPCPX and 10 μM 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; A₁ receptors on the enteric neuron and A(2B) receptor on the smooth muscle in the guinea-pig distal colon.