Prostacyclin biosynthesis in vascular endothelium is not inhibited by cyclic AMP. Studies with 3-isobutyl-1-methylxanthine and forskolin

We have previously reported ($\text{Proc. Natl. Acad. Sci.}$ 79, 495–499, 1982) that the cyclic nucleotide phosphodiesterase inhibitor, 3-isobutyl-l-methylxanthine (IBMX), stimulates cyclic AMP accumulation and inhibits prostacyclin (PGI₂) production in primary monolayer cultures of human umbilical vein endothelium. The present study was carried out to determine whether these effects are causally related. Incubation of endothelial monolayers with the diterpene, forskolin, increased the intracellular concentration of cyclic AMP by 10-fold. Despite this marked increase in cyclic AMP, neither baseline production of PGI₂ nor release in response to stimulation by thrombin or the divalent cation ionophore, A23187, was affected. Both forskolin and isoproterenol were found to potentiate the effect of IBMX on cyclic AMP accumulation without causing further inhibition of PGI₂ biosynthesis. Inhibition of cyclic nucleotide phosphodiesterase activity with 2,6-bis-(diethanolamino)-4-piperidinopyrimido-5,4-d]pyrimidine increased cyclic AMP levels to the same extent as IBMX; however, this agent had no effect on PGI₂ biosynthesis. These findings demonstrate that increases in the intracellular concentration of cyclic AMP have no short-term effects on PGI₂ biosynthesis in vascular endothelium and suggest that inhibition of PGI₂ production by IBMX is the result of some other, cyclic AMP-independent action of the drug.