G protein-coupled prostaglandin receptor modulates conductive Na+ uptake in lung apical membrane vesicles

Because G protein-regulated cation channels in type II pneumocytes constitute the most likely pathway for alveolar Na+ entry, we explored the hypothesis that a G protein-coupled prostaglandin (PG) E2 receptor controls perinatal lung alveolar Na+ transport. [3H]PGE2 binding to the alveolar apical membrane was trypsin sensitive and showed a rank order of competitive inhibition: PGE2 = PGE1 > PGD2 > PGF2 alpha. Kinetic analysis demonstrated both high-affinity [dissociation constant (KD) = 2.1 +/- 0.7 nM; maximal binding (Bmax) = 27 +/- 7 fmol/mg protein] and low-affinity (KD = 28 +/- 2 nM; Bmax = 265 +/- 29 fmol/mg protein) binding sites. Modulation of high-affinity GTPase activity identified a similar potency order (IC50 = 11 mM for PGF2 alpha vs. 10-50 microM for other PGs), suggesting that the receptors are G protein coupled. Finally, 1 microM PGE2 (approximately IC25) increased conductive 22Na+ uptake into membrane vesicles only in the presence of 100 microM intravesicular GTP. The KD value for the high-affinity binding site together with the rank order of PG effect on ligand binding and G protein function places this PG receptor in the EP3 subtype, whereas Na+ uptake studies suggest that it helps maintain perinatal lung Na+ homeostasis.