Uterine blood flow, myometrial activity and their response to adenosine during the peri-implantation period in sheep

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Abstract
Uterine blood flow and myometrial activity were measured simultaneously in anesthetized sheep 15 days after a sterile (non-pregnant group) or fertile (pregnant) mating. During the peri-implantation period uterine blood flow was similar in both groups of animals, but spontaneous myometrial activity was greatly reduced in pregnant ewes. This block of myometrial activity was associated with circulating levels of progesterone which were significantly higher (2.8 .+-. 0.8 ng/ml, mean .+-. standard error of the mean) than those in non-pregnant animals (0.4 .+-. 0.3 ng/ml). Adenosine injected into the uterine artery produced uterine vasodilation in both groups, but the log dose-response was significantly less in pregnant than in non-pregnant animals (P < 0.001). Myometrial activity was stimulated by adenosine, particularly in the pregnant group (P < 0.001). Vascular and myometrial effect were potentiated by a previous infusion of dipyridamole. Occlusion of the uterine artery produced reactive hyperemia, and estradiol infused close-arterially induced vasodilatation after a lag phase of .apprx. 30 min. Vascular and myometrial cells in the uterus may contain 2 types of adenosine receptor, 1 mediating excitatory and the other inhibitory responses, and both responses are modified by the presence of a conceptus. Estrogens may produce uterine vasodilatation by increasing the local concentration of vasoactive substance.