Adenosine A2-receptor-mediated phosphatidylcholine secretion in type II pneumocytes

Adenosine and its analogues stimulate phosphatidylcholine secretion in cultured rat type II pneumocytes. There is evidence that this effect is mediated by A2 receptors. We have now employed the radioligand 5'(N-ethylcarboxyamido)adenosine (NECA) in an attempt to study the adenosine receptor in a membrane fraction of type II cells. Specific binding of [3H]-NECA was saturable with a total binding capacity (Bmax) of 4.91 pmol/mg protein and a dissociation constant (Kd) of 240 nM. The Kd was similar to the concentration of NECA which half-maximally stimulated phosphatidylcholine secretion (EC50, 150 nM). Although the relative potency of adenosine analogues in displacing bound [3H]NECA was consistent with that expected at an A2 receptor, there were discrepancies between binding and function with respect to some agonists as well as the antagonist xanthine amine congener (XAC). This together with the unusually high Bmax suggests that the NECA binding site is not the adenosine receptor mediating phosphatidylcholine secretion. The receptor was further characterized functionally by measuring its antagonist dissociation constant (Kb). KbS for XAC inhibition of the effects of NECA, N6-(L-2-phenylisopropyl)adenosine, and adenosine on phosphatidylcholine secretion were 19 +/- 8, 24 +/- 11, and 6 +/- 12 nM, respectively, suggesting that all three agonists act at the same receptor. The Kb value allowed comparison of the receptor in type II cells with that in other tissues and revealed that it was similar to the A2 receptor previously described in human platelets. These functional data further characterize and are consistent with a physiological role for the adenosine A2 receptor in the regulation of lung surfactant secretion.