Prostacyclin synthesis and stimulation of cyclic AMP production in ovine fetal vasculature: Heterogeneity in pulmonary and systemic arteries

Prostacyclin (PGI₂) is an important local mediator of vasomotor tone in the fetus and newborn, acting via stimulation of cAMP production by vascular smooth muscle (VSM) adenylate cyclase. In this investigation PGI₂ and prostaglandin (PG) E₂ synthesis and stimulation of cAMP production were compared in vitro in ovine fetal pulmonary versus systemic (mesenteric) arteries. PGI₂ synthesis was similar in the pulmonary and systemic arteries (2.4 ± 0.2 and 2.6 ± 0.3 ng/mg protein-h, respectively), as was PGE₂ synthesis (1.9 ± 0.2 and 1.5 ± 0.2 ng/mg protein h, respectively); synthesis was greater for PGI₂ versus PGE₂ in both artery types. 65-71% of PGI₂ synthesis and 51-59% of PGE₂ synthesis occurred in the endothelium. Basal (nonstimulated) cAMP production was fully attenuated by indomethacin, indicating that it is mediated exclusively by endogenous PG. Basal cAMP production was 3.8-fold less in pulmonary versus systemic arteries, and this was related to a 9.7-fold difference in responsiveness to PG. A 14.7-fold difference in the response to forskolin indicates that underlying mechanism may be a disparity in the quantity and/or function of the adenylate cyclase enzyme complex. Thus, prostacyclin and PGE₂ synthesis are comparable in ovine fetal pulmonary versus systemic arteries, but the cAMP response to the prostanoids is markedly greater in the latter artery type due to differences in the activation of adenylate cyclase. This heterogeneity in VSM intracellular signalling may contribute to differential vasomotor tone and responses in the fetal pulmonary and systemic circulation.