Estrogen Receptor-Mediated Enhancement of Venous Relaxation in Female Rat. Implications in Sex-Related Differences in Varicose Veins

Background A greater incidence of varicose veins has been reported in premenopausal women than in men. We hypothesized that the sex differences in venous function reflect reduced constriction and enhanced venous dilation in women due to direct venous relaxation effects of estrogen on specific estrogen receptors (ER). Methods Circular segments of inferior vena cava (IVC) from male and female Sprague-Dawley rats were suspended between two wires, and isometric contraction (in mg/mg tissue) to phenylephrine, angiotensin II (AngII), and 96 mM KCl was measured. To investigate sex differences in venous smooth muscle, Ca 2+ release from the intracellular stores, and Ca 2+ entry from the extracellular space, the transient phenylephrine contraction in 0 Ca 2+ Krebs was measured. Extracellular CaCl 2 (0.1, 0.3, 0.6, 1, 2.5 mM) was added, and the [Ca 2+ ] e -dependent contraction was measured. To investigate sex differences in venous endothelial function, acetylcholine-induced relaxation was measured. To test the role of specific ERs, the amount of venous tissue ERs was measured using Western blots, and the venous relaxation in response to 17β-estradiol (E2, activator of most ERs), 4,4,′4′′-(4-propyl-[1H]-pyrazole-1,3,5-triyl)-tris-phenol (PPT; ERα agonist), 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN; ERβ agonist), and ICI 182,780 (ERα/ERβ antagonist, and G protein-coupled receptor 30 [GPR30] agonist) was measured in IVC segments nontreated or treated with the nitric oxide synthase (NOS) inhibitor N ω -nitro-L-arginine methyl ester (L-NAME). Results Phenylephrine caused concentration-dependent contraction that was less in female (max 104.2 ± 16.2) than male IVC (172.4 ± 20.4). AngII (10 −6 )-induced contraction was also less in female (81.0 ± 11.1) than male IVC (122.5 ± 15.0). Phenylephrine contraction in 0 Ca 2+ Krebs was insignificantly less in female (4.8 ± 1.8) than male IVC (7.2 ± 1.7), suggesting little difference in the intracellular Ca 2+ release mechanism. In contrast, the [Ca 2+ ] e -dependent contraction was significantly reduced in female than male IVC. Also, contraction to membrane depolarization by 96 mM KCl, which stimulates Ca 2+ influx, was less in female (129.7 ± 16.7) than male IVC (319.7 ± 30.4), supporting sex differences in Ca 2+ entry. Acetylcholine relaxation was greater in female (max 80.6% ± 4.1%) than male IVC (max 48.0% ± 6.1%), suggesting sex differences in the endothelium-dependent relaxation pathway. Western blots revealed greater amounts of ERα, ERβ, and GPR30 in female than male IVC. ER agonists caused concentration-dependent relaxation of phenylephrine contraction in female IVC. E2-induced relaxation (max 76.5% ± 3.4%) was more than DPN (74.8% ± 9.1%), PPT (71.4% ± 12.5%), and ICI 182,780 (67.4% ± 7.8%), and was similar in L-NAME-treated and nontreated IVC. Conclusion The reduced α-adrenergic, AngII, depolarization-induced, and [Ca 2+ ] e -dependent venous contraction in female rats is consistent with sex differences in the Ca 2+ entry mechanisms, possibly due to enhanced endothelium-dependent vasodilation and increased ER expression/activity in female rats. E2/ER-mediated venous relaxation in female rats is not prevented by NOS blockade, suggesting activation of an NO-independent relaxation pathway. The decreased venous contraction and enhanced E2/ER-mediated venous relaxation would lead to more distensible veins in female rats.