1 Inhibition of the Na þ -K þ -2Cl À cotransporter (NKCC1) with bumetanide reduced contractile responses to phenylephrine (PE) in male rat aortas (12974% of 60 mM KCl-induced contraction control vs 10877% bumetanide; PE 10 À5 M; Po0.01) but did not change equivalent responses in female rat aortas. Removal of the endothelium blunted the effect of NKCC1 inhibition on the response to PE (10 À5 M) in males, whereas in denuded aorta from female rats, bumetanide reduced this response (16275% control vs 14673% bumetanide; Po0.05). 2 NKCC1 basal activity did not show gender differences in intact aortic rings, but in the presence of PE, bumetanide-sensitive 86 Rb þ /K þ uptake increased more in male than female aortas (17978 in males vs 15875 nmol 86 Rb þ /K þ min À1 (g aorta) À1 in females; Po0.05). PE did not stimulate NKCC1 activity in denuded aorta from male rats. However, in female rats, PE increased NKCC1 activity similarly in both denuded (169711 nmol 86 Rb þ /K þ min À1 (g aorta) À1 ) and intact aortas. 3 Ovariectomy increased the bumetanide-sensitive 86 Rb þ /K þ uptake increase elicited by PE (223717 nmol 86 Rb þ /K þ min À1 (g aorta) À1 ) and hormone replacement with 17b-estradiol prevented this effect (159729 nmol 86 Rb þ /K þ min À1 (g aorta) À1 ). 4 Na þ ,K þ -ATPase basal activity, measured as ouabain-sensitive 86 Rb þ /K þ uptake, was similar in male and female rats, but the effect of PE was significantly less in intact male aortas (232716 in males vs 296725 nmol 86 Rb þ /K þ min À1 (g aorta) À1 in females; Po0.05). 5 Our results suggest that PE induced activation of NKCC1 and Na þ ,K þ -ATPase in the rat aorta in a gender-dependent way.
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