Purine receptors in the guinea‐pig internal anal sphincter

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Abstract
In the isolated internal anal sphincter of the guinea‐pig, adenosine 5′‐triphosphate (ATP) and adenosine induced a concentration‐dependent and tetrodotoxin‐insensitive relaxation. Pretreatment with theophylline (25–50 μm) had no significant effect on the concentration‐response curves obtained with either purine compound. Reactive blue 2 (25–100 μm) shifted the curve to ATP to the right in a dose‐dependent fashion leaving that to adenosine unaltered. The antagonism appeared to be non‐competitive. Neither reactive blue 2 nor purine receptor occupation by ATP or adenosine altered the electrically‐induced non‐adrenergic, non‐cholinergic inhibitory response. The actions of ATP and adenosine in the guinea‐pig internal anal sphincter appear to be mediated by separate receptors. These receptors are not involved in the nerve‐mediated relaxation.