P-2U PURINOCEPTOR STIMULATION OF SURFACTANT SECRETION COUPLED TO PHOSPHATIDYLCHOLINE HYDROLYSIS IN TYPE-II CELLS

ATP is known to stimulate surfactant phospholipid secretion in type II cells, and there is evidence that this effect is mediated by a P-2 purinoceptor. At least five subtypes of the P-2 receptor have been reported, but it is not clear which one exists on the type II cell. To determine whether it is the P-2u subtype, at which UTP is equipotent with ATP, we have compared the effects of ATP and UTP on phosphatidylcholine secretion and second messenger formation in primary cultures of rat type II cells. ATP and UTP were equally potent in stimulating phosphatidylcholine secretion and phospholipase D activation. The potency order, UTP = ATP > ADP > 2-methylthio-ATP, was the same as that reported for the P-2u receptor. UTP stimulated diacylglycerol and phosphatidic acid formation to the same extent as ATP. ATP also increased choline formation. Formation of diacylglycerol was biphasic, and the first peak in response to ATP was previously shown to be associated with inositol trisphosphate formation. Northern analysis showed that the P-2u receptor gene was expressed to a greater extent in type II cells than in whole lung. These data suggest that ATP and UTP act via a P-2u receptor that is coupled to phosphoinositide-specific phospholipase C with subsequent activation of phospholipase D acting on phosphatidylcholine. ATP has also been reported to act at an additional type II cell receptor coupled to adenylate cyclase. In contrast, UTP did not promote adenosine 3',5'-cyclic monophosphate formation and therefore does not act at that receptor.