Embryonic and hormonal control of ovine uterine arterial constriction Public Deposited

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  • The role of the conceptus and ovarian hormones in regulating ovine uterine vasoconstriction was investigated. The uterine artery ipsilateral to the ovary bearing the corpus luteum in ten nonpregnant and eight pregnant ewes was utilized in two experiments. Arteries of nonpregnant and pregnant ewes were removed at necropsy on day 13 of the estrous cycle or post-mating, respectively (day of detected estrus = day 0). The first experiment consisted of examining the responses of arteries to periarterial sympathetic nerve stimulation (NS) following in vitro perfusions of vehicle (saline), prostaglandin E₂ (PGE₂) and excess norepinephrine (NE). In the second experiment, the presynaptic release of neurotransmitter was bypassed with intraluminal injections of low concentrations of NE (1 to 8 μg). Uterine arteries from nonpregnant ewes responded to NS following perfusions of saline with greater constriction (P < 0.01) than arteries from pregnant ewes. Responses of arteries of pregnant and nonpregnant ewes to NS after perfusion of PGE₂ were not significantly different from the responses following saline perfusions. Although responses of arteries to NS after perfusion of excess NE were greater (P<0.01) than after saline, arterial responses to NE were not significantly different between pregnant and nonpregnant ewes. Magnitude of constriction of uterine arteries increased linearly with exposure to increasing concentrations of NE, however, the response of arteries from pregnant ewes at each concentration of NE was less (P< 0.01) than that of arteries of nonpregnant ewes. A third experiment of 2x2 factorial design involving the use of 20 ovariectomized ewes was conducted to determine the effects of estradiol-17β and progesterone on the in vitro constriction of uterine arteries and the arterial levels of α-adrenergic receptors. Animals received the following treatments for 4.5 days: 1) corn oil (controls); 2) estradiol-17β (silastic capsule implants); 3) progesterone (15 mg injected twice-daily); and 4) estradiol-17β plus progesterone. Vasoconstriction was monitored during perfusion of uterine arterial segments with Krebs Henseleit solution into which was injected saline, norepinephrine (8 μg) and saline, respectively, each at 15-min intervals. Arterial concentrations of α-adrenergic receptors were determined by use of an exchange assay. Treatment of ewes with progesterone increased (P < 0.05) while exogenous estradio1-17β tended to decrease (P < 0.08) vasoconstriction to norepinephrine compared to responses of arteries of control ewes. Arteries from control and estradiol-17β plus progesterone-treated ewes did not differ in their response to norepinephrine. No constriction of any artery occurred following exposure to saline. Control and steroid-treated ewes did not differ significantly in their arterial concentration of α-adrenergic receptors. Knowledge of the factors affecting uterine blood flow is requisite in understanding the causes of early embryonic mortality in domestic animals. Results of these experiments suggest that the presence of the conceptus exerts a dilatory effect on the uterine artery by inhibiting the response of smooth muscle cells to the neurotransmitter. The action of progesterone and estradiol-17β in altering constriction of uterine arteries may be independent of an effect of these steroids on a-adrenergic receptors in the smooth muscle cell membrane.
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