Muscle, Smooth, Vascular/physiology; Receptors, Adrenergic, alpha-1/physiology; Umbilical Veins/metabolism; Uterus/blood supply; Adrenergic alpha-Agonists/pharmacology; Adrenergic alpha-Antagonists/pharmacology; Animals; Arteries/drug effects; Arteries/metabolism; Binding, Competitive; Calcium Channel Blockers/pharmacology; Clonidine/analogs & derivatives; Clonidine/metabolism; Clonidine/pharmacology; Dioxanes/metabolism; Dioxanes/pharmacology; Drug Interactions; Drug Synergism; Female; Muscle Contraction/drug effects; Muscle, Smooth, Vascular/drug effects; Nifedipine/pharmacology; Norepinephrine/pharmacology; Prazosin/metabolism; Prazosin/pharmacology; Pregnancy; Receptors, Adrenergic, alpha-1/drug effects; Sheep; Umbilical Veins/drug effects; Uterus/drug effects; Uterus/metabolism; Vasoconstriction/drug effects