**Acknowledgements**

effect occurs after activation of its receptor. This relationship is relevant physiologically because it suggests that postnatally, cAMP will stimulate growth of PASMC and under this condition, the presence of PAF will be detrimental cell growth and pulmonary vascular development. Thus, we can speculate that cAMP/PKA-mediated inhibition of PAF effects, in vivo, may constitute one mechanism whereby the postnatal vasodilator properties of cAMP are maintained. Our studies with PKA siRNA demonstrate that cAMP acts at its receptor, PKA, to inhibit postnatal adverse PAFR-mediated responses in the pulmonary circulation of the

*Role of cGMP:* Endothelium-derived nitric oxide produced under basal conditions, or by a stimulus, readily diffuses into the contiguous smooth muscle to activate soluble guanylyl cyclase, resulting in an increase in cGMP synthesis and smooth muscle relaxation [37, 38]. Nitric oxide is a potent vasodilator in the pulmonary circulation and it is important in the transition of the pulmonary circulation from fetal to postnatal life. In this study, 8-Br-cGMP blunted PAFR binding to PASMC in physiologic oxygen level (normoxia) and in hypoxia. This suggests that cGMP produced in vivo will counteract the vasoconstricting properties of PAF. Interestingly, inhibition of endogenous cGMP activity with the inhibitor of cGMP-dependent PKG, the endogenous receptor of cGMP, resulted in abrogation of the ability of cGMP to inhibit PAF binding to its receptors, similar to the effect of PKA receptor antagonist. This shows that

cGMP has been shown to inhibit endothelin-stimulated inositol phosphate release in pulmo‐ nary artery of fetal lambs studied in organ bath [41]. Inositol phosphate is released downstream from PAFR effect. Both endothelin and PAF are potent endogenous vasoconstrictors in the pulmonary circulation. This shows that increased levels of endogenous PAF under normoxic conditions can inhibit cGMP effect and as a corollary, increased levels of cGMP after birth can inhibit PAF effect in normoxia, leading to increased vasodilation. Our data show that cAMP and cGMP produce different effects on cell proliferation. During normoxia, cGMP inhibited PAF stimulation of cell proliferation, but the effect in hypoxia seemed to be stimulatory. This physiological significance of this effect is not clear, but may indicate a protective role of cGMP

We can infer that in vivo, activation of PKA will result in inhibition of PAFR-mediated effects such as stimulation of inositol phosphate release, calcium mobilization, and vasoconstriction. As with cGMP effect, cAMP/PKA-mediated inhibition of PAF effects, in vivo, may constitute one mechanism whereby the postnatal vasodilator properties of cAMP are maintained.

**4.2. PAF and regulation of PKG and PKA activity and role in perinatal pulmonary adaptation** Previous reports have shown that the activities of PKG and PKA are upregulated by normoxic condition [36, 37, 42]. The physiologic implications of these findings are that upregulation of PKG and PKA activities by normoxia, after birth, is one mechanism by which fetal high pulmonary vasomotor tone is downregulated to facilitate postnatal pulmonary adaptation. It can be deduced that favorable perinatal pulmonary vascular adaptation can be achieved by both downregulation of mediators of pulmonary vascular constriction, such as PAF, and upregulation of mediators of pulmonary vasodilation such as cGMP/PKG and cAMP/PKA.

newborn lamb lung.

468 Muscle Cell and Tissue

cGMP acts via its receptor to inhibit PAFR binding.

against unwarranted cell growth in the presence of PAF.

This work was supported in part by a Bridge Grant (513292) from Los Angeles Biomedical Research Institute, Torrance, CA.
