Purine receptors in the trachea: is there a receptor for ATP?

In guinea-pig trachea adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), adenosine 5'-phosphate (AMP), adenosine and adenine were similarly potent in causing relaxation of the smooth muscle. This is in contrast to gut where ATP and ADP are 30 times more potent than adenosine. Studies with dipyridamole suggest that in trachea, as in gut, nucleotides are rapidly metabolized to adenosine. A polyphosphate modified analogue of ATP, the alpha,beta-methylene isostere, which resists degradation to adenosine was inactive in trachea although it is a potent relaxant in gut. This result may suggest that the intact ATP molecule is also inactive in the tracheal preparation: i.e. ATP acts only via its adenosine metabolite implying that receptors for adenosine but not ATP are present in the tissue.