Functional evidence for involvement of P 2 purinoceptors in the ATP stimulation of phosphatidylcholine secretion in type II alveolar epithelial cells

Abstract

There is evidence that phosphatidylcholine secretion in type II pneumocytes is stimulated by adenosine and adenine nucleotides and that the effect of adenosine is mediated by the A 2 subtype of the p 1 purinoceptor. To determine if the effect of ATP is also mediated by the same receptor following its catabolism to adenosine or by the P 2 purinoceptor we compared the effects of adenosine and ATP. Adenosine and terbutaline stimulated phosphatidylcholine secretion approx. 2-fold, while ATP stimulated it by more than 3-fold, essentially to the same extent as the protein kinase C activator, 12- O-tetradecanoylphorbol 13-acetate. The stimulatory effect of adenosine but not of ATP was abolished by adenosine deaminase. The effect of ATP was markedly diminished by the P 2 desensitizing agent α,β-methylene ATP, but only slightly by the P 1 antagonist 8-phenyltheophylline. Adenosine increased the cAMP content of type II cells while ATP had little effect. The effects of ATP and terbutaline were additive while those of adenosine and terbutaline were not. These data show that ATP and adenosine stimulate phosphatidylcholine secretion via different mechansims. Therefore, the effect of ATP is not mediated via catabolism to adenosine. Metabolically resistant analogs of ATP also stimulated secretion in a concentration-dependent manner although none were as potent as ATP. The order of potency was ATP > β, γ-methylene ATP = 2-methylthio ATP = 2-deoxy ATP ≥ 8-bromo ATP > α, β-methylene ATP. The facts that ATP analogs also stimulate secretion and that the effect of ATP was antagonized by α,β-methylene ATP suggest that the stimulatory effect of ATP is mediated by the P 2 purinoceptor.