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Nucleotides released to the extracellular space stimulate purinergic receptors, and their effects are modulated by ectonucleotidases. The role of ATP in the allergic bronchospasm has been scantly studied.We used several techniques (plethysmography, organ baths, confocal microscopy, RT-PCR, ATP measurement) to explore the role of nucleotides and ectonucleotidases in the allergic bronchospasm in guinea pigs.While allergenic challenge with a low-dose ovalbumin (OVA) only produced a small bronchospasm (˜2-fold the basal lung resistance), previous inhibition of ectonucleotidases by ARL-67156 greatly intensified this response (˜11-fold the basal lung resistance, with 44% mortality). Bronchoalveolar lavage fluid obtained during this bronchospasm contained increased ATP concentration. This potentiation was abolished by antagonism of purinergic receptors (suramin+RB2) or TXA2 receptor (SQ29548), or by intratracheal apyrase. In tracheal rings and lung parenchyma strips, OVA caused a concentration-dependent contraction. Suramin+RB2 or levamisole produced a significant rightward displacement of this response, and ARL-67156 did not modify it. Platelets stimulated with OVA released ATP. Confocal images of nonsensitized tracheas showed slight fluorescence for P2Y6 receptors in epithelium and none for P2Y4. Sensitized animals showed strong fluorescence to both receptors and to alkaline phosphatase in the airway epithelium. This correlated with a large increment in mRNA for P2Y4 and P2Y6 receptors in sensitized animals.Nucleotides greatly potentiate the allergic bronchospasm when ectonucleotidases activity is diminished, and this effect is probably favored by the upregulation of P2Y4 and P2Y6 receptors in airway epithelium during sensitization. These results prompt for further research on these mechanisms in human asthma.