Endometrial biopsies obtained throughout the menstrual cycle of the Macaca arctoides show the glycogen content paralleling the serum progesterone fluctuations which occur during the menstrual cycle. Secretory phase samples contained a three‐fold higher concentration of glycogen when compared to follicular phase tissue. Changes in the activity levels of the glycogen metabolizing enzymes, glycogen phosphorylase and glycogen synthetase, during various stages of the menstrual cycle are in accord with the concept that the post‐ovulatory increase in endometrial metabolism is a function of progesterone influence on this tissue. Endometrial glycogen synthetase activity remains low during the early proliferative phase of the cycle and becomes significantly elevated (two‐to three‐fold) during the early secretory phase of the cycle. Glycogen phosphorylase shows a similar cyclicity later in the luteal phase, reaching maximal activity between the seventeenth to nineteenth day of the cycle and remaining elevated through the twenty‐sixth day of the cycle. The coincident nature of the rise in peripheral progesterone to increases in uterine glycogen metabolism suggest that progesterone may be the prime modulator of uterine endometrial metabolism during the post‐ovulatory phase.
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