Identification of purinoceptors in the isolated stomach and intestine of the three-spined stickleback Gasterosteus aculeatus L.

1. Adenine nucleosides and nucleotides were examined for pharmacological activity in isolated stomach and intestine from the stickleback Gasterosteus aculeatus L. 2. Adenosine and its stable analogues all concentration-dependently relaxed carbachol-contracted stomach and intestine, with no significant difference in the potency of the analogues. Only 8-(p-sulphophenyl) theophylline inhibited the relaxant response to adenosine in both tissues; other adenosine antagonists such as 1,3-dipropyl-8-cyclopentylxanthine were not active. 3. ATP, alpha, beta-methylene ATP (alpha, beta-MeATP) and 2-methylthio ATP (2-MeSATP) all caused concentration-dependent contractions of the stomach and intestine. 4. In the stomach, the order of potency was 2-MeSATP > alpha,beta-MeATP = ATP; the P2Y-purinoceptor antagonist reactive blue 2 inhibited responses to ATP. 5. In the intestine, the order of potency was alpha,beta-MeATP > 2-MeSATP = ATP; reactive blue 2 did not affect responses to ATP, nor did prolonged incubation with alpha,beta-MeATP. 6. It is concluded that in both the stomach and intestine, adenosine is acting through a non-specific or undifferentiated P1-purinoceptor. In the stomach, however, the P2-purinoceptor appears to be analogous to the mammalian P2Y-purinoceptor, and in the intestine, the receptor is more similar to the mammalian P2X-subtype, although it was not susceptible to desensitization.