Abstract
Summary
Resting levels of bioassayable (BA) and radioimmunoassayable (RIA) pituitary growth hormone (GH), radioimmunoassayable plasma GH, and hypothalamic GH-releasing (GRF) activity were determined at intervals during a controlled 24-hr light-dark cycle in intact prepubertal female rats. A gradual parallel increase in BA and RIA pituitary GH was found from late morning through afternoon, with peak levels at 1800–2100 hr. Reattainment of values close to the morning levels took place during the night.
Radioimmunoassayable plasma GH exhibited wide fluctuations with peak concentration at 1800 and 0300 hr. Measurements of hypothalamic GRF activity were unable to evidence daily excursions in phase with the clear pituitary rhythm.
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