Dual effect of (−)‐N6‐phenylisopropyl adenosine on guinea‐pig trachea

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Abstract
The effect of (−)‐N6‐phenylisopropyl adenosine (PIA), a metabolically stable P1‐receptor agonist, was investigated on guinea‐pig isolated trachea. PIA showed two opposite effects: contraction, evident at low concentrations (10−7 to 2–5 × 10−6m), and relaxation at higher doses. Relaxation by PIA was antagonized in an apparently competitive manner by two antagonists of extracellular (P1) adenosine receptors: theophylline (Theo) and 8‐phenyltheophylline (PT). Contraction by PIA was not inhibited by methylxanthines and was not mediated by stimulation of cholinergic or histaminergic systems. Inhibitors of arachidonic acid cascade acting at different levels, i.e. indomethacin, nordihyd‐roguaiaretic acid (NDGA) and BW755C, all inhibited the contraction by PIA, while they potentiated the relaxation in a concentration‐dependent manner. Mepacrine, an inhibitor of phospholipase A2, inhibited the contraction by PIA, but did not affect the relaxation. These results indicate that the contractile effect induced by PIA is supported by an indirect mechanism involving the release of arachidonic acid derivatives (via P2‐purinoceptor?). Thus the balance between the two opposite effects of adenosine on tracheal tone is possibly modulated by the prostaglandin turnover.