Regulation by Dexamethasone

Recently, we have shown that dexamethasone treatment of rabbits specifically reduces vascular smooth muscle responsiveness to agonists that interact with the vascular thromboxane A2 / prostaglandin H2 (TXA2 / PGH2) receptor. One potential site at which dexamethasone can influence prostanoid-mediated vasoconstriction may be at the level of the vascular TXA2 IPGH2 receptor. Therefore, we characterized the vascular TXA2/PGH2 receptor in rabbit aortic membranes and examined the influence of dexamethasone treatment on vascular TXA2 IPGH2 receptor affinity and number. The binding of [1251] [1S-(1ac,2P(5Z),3a(IE,3R)4a)]-7-[3-(3hydroxy-4-(p-iodophenoxy)-1-butenyl)-7-oxabicyclo[2.2.1]heptan-2-yl]-5-heptanoic acid ([12511 BOP), a potent TXA2/PGH2 receptor agonist, to rabbit aortic membranes was saturable, displaceable, and dependent on protein concentration. Scatchard analysis of equilibrium binding data disclosed one class of high affinity binding sites with a Kd of 0.44±0.13 nM and a Bm. of 114.4±5.2 fmollmg protein (n=7). Removal of the endothelium before membrane preparation did not significantly alter the affinity or number of binding sites for ["SI]BOP. Kinetic analysis of the rates of [1251]BOP association/dissociation yielded a Kd of 0.62 nM. The ability ofvarious agonists at the TXA2/ PGH2 receptor to displace [1251]BOP from vascular membranes correlated well with their contractile potencies in rabbit aortic rings. Moreover, stereospecific displacement of [1251]BOP binding in aortic membranes and inhibition of U46619-mediated aortic contractions were obtained with the stereoisomers L657925(-) and L657926(+). Collectively, these data suggest that this binding site represents the functionally relevant vascular TXA2 IPGH2 receptor. In functional experiments, [1271]BOP induced concentration-dependent contractions ofthe rabbit aorta, which were reduced by 52% in vessels from dexamethasone-treated rabbits. Binding experiments performed in aortic membranes from dexamethasone-treated rabbits revealed a 25% reduction in vascular TXA2 /PGH2 receptor number with no change in affinity. Thus, the dexamethasone-induced decrease in TXA2 /PGH2 receptor number in aortic membranes from dexamethasone-treated rabbits may contribute to the accompanying decrease in vascular responsiveness to TXA2/PGH2 receptor agonists. (Circulation Research 1990; 67:1562-1569)