Further investigations into adenosine A1 receptor‐mediated contraction in rat colonic muscularis mucosae and its augmentation by certain alkylxanthine antagonists

1 The alkylxanthine antagonists, 8‐phenyltheophylline (8‐PT), 8‐p‐sulphophenyltheophylline (8‐SPT) and 1,3,7‐trimethylxanthine (caffeine) produced rightward displacements of contractile concentration‐effect curves to 5′‐N‐ethylcarboxamidoadenosine (NECA) in rat isolated colonic muscularis mucosae (RCMM) with concentration‐ratios consistent with adenosine receptor blockade. The non‐xanthine antagonist, 9 fluro‐2‐(2‐furyl)‐5,6‐dihydro [1,2,4] triazo to [1,5‐c]‐quinazin‐imine (CGS15943A) also antagonized contractions to NECA with an affinity (pKB 8.1–8.5) consistent with adenosine A1 receptor blockade. 2 In addition to producing rightward shifts of the concentration‐response curves, the maximum contractions to 5′‐N‐ethylcarboxamidoadenosine (NECA) were also markedly increased in the presence of 8‐PT (by 83 ± 16% at 1 μm), 8–SPT (by 37 ± 7% at 10 μm) and caffeine (by 45 ± 5% at 100 μm) but were unaffected by CGS15943A (at 0.01 and 0.03 μm). 3 As with NECA, the maximum contractions to the adenosine A1 receptor agonists R‐phenylisopropyladenosine (R‐PIA) and N‐[(1S, trans)‐2‐hydroxyclopentyl] adenosine (GR79236) were both antagonized and augmented by 8‐PT. In addition, the contractions to NECA in the presence of 8–PT (1 μm) were inhibited by nanomolar concentrations of 8‐cyclopentyl‐1,3‐dipropylxanthine (DPCPX). 4 The non‐selective phosphodiesterase (PDE) inhibitor, 3‐isobutyl‐1‐methylxanthine (1 μm) produced a marked increase in the NECA maximum without producing a rightward shift in the NECA curve, whereas a higher concentration (10 μm) virtually abolished responses. The PDE type III inhibitor, milrinone (1 μm), the type IV inhibitor, rolipram (10 μm), and the type V PDE inhibitor, zaprinast (3 μm), were all without effect on NECA responses in RCMM. 5 Partial inhibitions of contractions to NECA were produced by indomethacin (at 3 or 10 μm) or piroxicam (at 3 μm). Responses to GR79236 were also partially inhibited by indomethacin. In the presence of indomethacin, 8‐PT was still able to enhance markedly the maximum contractions obtained to NECA in RCMM. 6 The present study has shown that certain alkylxanthine antagonists (but not the non‐xanthine CGS15943A) produced a marked augmentation of adenosine A1 receptor‐mediated contractions in RCMM. The mechanism of this augmentation is, as yet, not known but is unlikely to result from inhibition of PDE. This study has also shown that adenosine A1 receptor‐induced contractions in RCMM are mediated, in part, via product