TESTOSTERONE INCREASES THROMBOXANE-A2 RECEPTORS IN CULTURED RAT AORTIC SMOOTH-MUSCLE CELLS

Previous studies have demonstrated increased contractile responses to thromboxane A2 (TXA2) mimetics in aortas obtained from male rats compared with those obtained from females. This study was designed to determine the effects of testosterone and 17-beta-estradiol treatment on TXA2 receptors in cultured rat aortic smooth muscle cells (RASMCs). TXA2 receptor affinity and density were determined through equilibrium binding experiments using the TXA2/prostaglandin H-2 mimetic [1S-(1-alpha,2-beta(5Z),3-alpha(1E,3R*),4-alpha)]-7-[3-(3-hydroxy-4-(4'-125iodophenoxy)-1-butenyl)-7-oxabicyclo[2.2.1]heptan-2-yl]-5-heptenoic acid (I-125-BOP). Incubation with testosterone (100 nM) for 24 or 48 hours resulted in a significant (p < 0.05) 31% and 48% increase in TXA2 receptor density without any change in affinity. 17-beta-Estradiol (100 nM) had no significant effect on either the density or affinity of TXA2 receptors. Coincubation with the testosterone receptor antagonist hydroxyflutamide (1-mu-M) blocked the testosterone-induced increase in TXA2 receptor density. The maximum increase in intracellular free calcium induced by I-BOP was significantly (p < 0.05) greater in testosterone-treated RASMCs than controls. Similarly, increases in inositol trisphosphate induced by the TXA2/prostaglandin H-2 mimetic U46619 were significantly (p < 0.05) greater in testosterone-treated RASMCs compared with controls. The results demonstrate that testosterone increases vascular TXA2 receptor density and support the notion that sex steroid hormones modulate the expression of this receptor.