Metabolism of the Uterus of the Rat During Early Pseudopregnancy and Its Regulation by Estrogen and Progestogen

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Abstract
Uterine oxygen consumption and [beta]-glucuronldase activity were measured in adult rats during the estrous cycle, pseudopregnancy and after ovariectomy and steroid replacement therapy. Peak oxygen utilization occurred during diestrus-2 and proestrus of the estrous cycle. Following the induction of pseudopregnancy by cervical stimulation, 2 distinct patterns of metabolism were observed. The first 5 days of pseudopregnancy were characterized by an increased utilization similar to that during late diestrus and proestrus; this was termed the "active phase." From the sixth day of pseudopregnancy until luteal regression a prolonged decline in oxidative metabolism was measured, which was termed the "inactive phase." No changes in specific activity of [beta]-glucuronidase activity occurred during the estrous cycle. However, after cervical stimulation, a significant rise in specific activity was recorded between Day 0 (estrus) and Day 1 of pseudopregnancy, followed by a gradual but steady decline throughout the remainder of the luteal phase. Ovariectomy on Day 0 resulted in a decrease toward basal levels of oxygen consumption, water content and uterine weight. Marked differences between total and specific enzyme activity in the ovariectomized rat suggested that [beta]-glucuronldase is less sensitive to hormone deprivation than other parameters measured. Treatment of ovariectomized rats with 2.0 mg progesterone daily reproduced, in part, the pattern of oxygen consumption recorded in intact animals but prolonged the duration of uterine sensitivity to decidualization. The addition of estrone to the progesterone regimen resulted in a 24- to 48-hr, advance in the pattern of oxidative metabolism in comparison with progesterone controls, and a decrease in both the magnitude and duration of uterine sensitivity. Modified steroid combinations had similar effects on metabolism, but potentiated the magnitude of sensitivity, similar to that in intact rats. The present experiments revealed that: (a) the level of oxidative metabolism on Day 3 of pseudopregnancy may have a bearing on the development of maximum sensitivity to decidualization on Day 4; (b) the timing of the onset of progestational secretions, in relation to the regression of estrogenic dominance during estrus, may regulate the general pattern of uterine metabolism, as well as that for decidual sensitivity during the preimplantation period; and (c) the complex series of metabolic changes which are observed during the preimplantation period can be supported by a relatively constant ratio of estrogen: progestogen.

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