Purinergic modulation in the flounder gut

1. Spontaneous mechanical responses of ileum and rectum of the flounder were recorded isometrically. 2. Both ileal and rectal segments showed positive dose-dependent inotropic but not chronotropic responses to ATP. 3. The P2 receptor antagonist alpha-beta-M-ATP induced contractures in flounder ileum and rectum. Pretreatment with this agent blocked the responses of both ileum and rectum to ATP. 4. Adenosine abolished rhythmic activity and lowered basal tonus in the ileum but in the rectum adenosine induced dose-dependent positive inotropic responses. 5. Theophylline pretreatment abolished the positive inotropic effects of adenosine in the rectum and it prevented the inhibitory actions of adenosine in the ileum. 6. It is concluded that the positive actions of ATP in ileum and rectum are mediated via P2 purinoreceptors. The positive response of the rectum and inhibition of the ileum seen with adenosine appear to be mediated via mixed populations of excitatory and inhibitory P1 purinoreceptors.