Circadian and Ovarian Influences on Tissue Glycogen in Female Rats*

Influences of circadian rhythm and ovarian cycles upon the glycogen content of heart, liver, fast twitch red muscle, and fast twitch white muscle were studied in female rats, weighing between 150–300 g. Red vastus muscle demonstrated a significant circadian rhythm during estrus and metestrus, with peak glycogen levels at 0800 h and nadirs at 2400 h. When the data were expressed only as a function of time of day, a significant ultradian rhythm existed, with highs at 0800 and 2000 h and lows at 1200 and 2400 h. White vastus muscle demonstrated a circadian rhythm during estrus, metestrus, and diestrus. Fluctuations ranged from 7.75 mg/g (0800 h) to 2.75 mg/g (2000 h). Myocardial circadian rhythms existed during estrus and metestrus, with peak glycogen concentrations occurring during metestrus (7.23 mg/g at 0800 h) and low concentrations occurring during estrus (3.0 mg/g at 2000 h). Hepatic glycogen content demonstrated circadian rhythms throughout all phases of the estrus cycle. The most dramatic fluctuation occurred during estrus, 61 mg/g (0800 h) to 21 mg/g (2400 h), a change of 61% and 45%, respectively, from the 24 h mean. Blood glucose had a significant circadian rhythm corresponding marginally with the variation measured in liver glycogen. When data were analyzed disregarding the time of day when the animal was sacrificed, there was no noticeable ovarian hormonal influence on glycogen content in any tissue analyzed. It is concluded that definite circadian rhythms exist in tissue glycogen. However, ovarian hormones may cause alterations in the patterns, either accentuating or depressing these circadian fluctuations.