Oxygen-dependent PAF receptor binding and intracellular signaling in ovine fetal pulmonary vascular smooth muscle.
Circulating levels of platelet-activating factor (PAF) are high in the fetus, and PAF is active in maintaining high PVR in fetal hypoxia (Ibe BO, Hibler S, Raj J. J Appl Physiol 85: 1079-1085, 1998). PAF synthesis by fetal pulmonary vascular smooth muscle cells (PVSMC) is high in hypoxia, but how oxygen tension affects PAF receptor (PAF-r) binding in PVSMC is not known. We studied the effect of oxygen tension on PAF-r binding and signaling in fetal PVSMC. PAF binding was saturable. PAF-r density (B(max): fmol/10(6) cells; means +/- SE, n = 6), 25.2 +/- 0.77 during hypoxia (Po(2) <40 Torr), was higher than 13.9 +/- 0.44 during normoxia (Po(2) approximately 100 Torr). K(d) was twofold lower in hypoxia than normoxia. PAF-r protein expression, 35-40% greater in hypoxia, was inhibited by cycloheximide, a protein synthesis inhibitor, suggesting translational regulation. IP(3) release, an index of PAF-r-mediated cell signaling, was greater in hypoxia (EC(50): hypoxia, 2.94 +/- 0.61; normoxia, 5.85 +/- 0.51 nM). Exogenous PAF induced 50-90% greater intracellular calcium flux in cells during hypoxia, indicating hypoxia augments PAF-r-mediated cell signaling. PAF-r phosphorylation, with or without 5 nM PAF, was 40% greater in hypoxia. These data show 1) hypoxia upregulates PAF-r binding, PAF-r phosphorylation, and PAF-r-mediated intracellular signaling, evidenced by augmented IP(3) production and intracellular Ca(2+) flux; and 2) hypoxia-induced PAF-r phosphorylation results in activation of PAF-r-mediated signal transduction. The data suggest the fetal hypoxic environment facilitates PAF-r binding and signaling, thereby promoting PAF-mediated pulmonary vasoconstriction and maintenance of high PVR in utero.
Ibe
Basil O
BO
Dept. of Pediatrics, University of California, Los Angeles, CA, USA. ibe@labiomed.org
Portugal
Ada M
AM
Chaturvedi
Shiva
S
Raj
J Usha
JU
D000818:Animals; D002118:Calcium / Q000378:metabolism; D005260:Female; D005311:Fetal Hypoxia / Q000378:metabolism* / Q000503:physiopathology; D000860:Hypoxia / Q000378:metabolism* / Q000503:physiopathology; D007295:Inositol Phosphates / Q000378:metabolism; D008168:Lung / Q000098:blood supply / Q000196:embryology; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism*; D010766:Phosphorylation; D010980:Platelet Membrane Glycoproteins / Q000378:metabolism*; D011247:Pregnancy; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism*; D012756:Sheep; D015398:Signal Transduction / Q000502:physiology
D007295:Inositol Phosphates; D010980:Platelet Membrane Glycoproteins; D043562:Receptors, G-Protein-Coupled; C039351:platelet activating factor receptor; D010100:Oxygen; D002118:Calcium