Haemodynamic effects of purinergic receptor stimulation in isolated fish gills

The haemodynamic responses of the isolated, perfused gill of the tropical cichlid, Oreochromas (Sarotherodon) niloticus, to exogenous application of adenosine and adenosine triphosphate (ATP) showed that both drugs are potent vasoactive agents. However, while adenosine had vasoconstrictory effect, ATP induced vasodilation. ATP-induced vasodilation was reversibily inhibited in the presence of the enzyme, adenosine deaminase, and the P1 receptor antagonist, theophylline, but was unaffected by the P2 receptor antagonist, quinidine. This indicates that ATP acts specifically through P1 receptor stimulation. The significance of the inhibition of ATP by adenosine deaminase remains obscure since in purine metabolism the enzyme catalyzes the conversion of the haemodynamically active adenosine to inactive inosine.