Relaxant effects of 17-beta-estradiol in cerebral arteries through Ca(2+) entry inhibition.

Estrogens account for gender differences in the incidence and outcome of stroke, but it remains unclear to what extent neuroprotective effects of estrogens are because of parenchymal or vascular actions. Because reproductive steroids have vasoactive properties, the authors assessed the effects and mechanisms of action of 17-beta-estradiol in rabbit isolated basilar artery. Cumulative doses of 17-beta-estradiol (0.3 micromol/L to 0.1 mmol/L) induced concentration-dependent relaxation that was larger in basilar than carotid artery, in male than female basilar artery, and in KCl-precontracted than UTP-precontracted male basilar artery. Endothelium removal did not modify relaxation induced by 17-beta-estradiol in basilar artery, whereas relaxation induced by acetylcholine (1 nmol/L to 0.1 mmol/L) was almost abolished. Neither the estrogen receptor antagonist ICI 182,780 (1 micromol/L), nor the protein synthesis inhibitor cycloheximide (1 micromol/L) affected 17-beta-estradiol-induced relaxations. Relaxations induced by the K(+) channel openers NS1619 and pinacidil in the same concentration range were greater and lower, respectively, when compared with relaxation to 17-beta-estradiol, which was not significantly modified by incubation with the K(+) channel blockers charybdotoxin (1 nmol/L and 0.1 micromol/L) or glibenclamide (10 nmol/L and 1 micromol/L). Preincubation with 17-beta-estradiol (3 to 100 micromol/L) produced concentration-dependent inhibition of CaCl(2)-induced contraction, with less potency than the Ca(2+) entry blocker nicardipine (0.01 to 10 nmol/L). The authors conclude that 17-beta-estradiol induces endothelium-independent relaxation of cerebral arteries with tissue and gender selectivity. The relaxant effect is because of inhibition of extracellular Ca(2+) influx to vascular smooth muscle, but activation of estrogen receptors, protein synthesis, or K(+) efflux are not involved. Relatively high pharmacologic concentrations of 17-beta-estradiol causing relaxation preclude acute vascular effects of physiologic circulating levels on the cerebral circulation.