Abstract
Changes in pituitary prolactin (PRL) content were examined in pseudopregnant rats. Blood and anterior pituitary glands were harvested every 3 hr on day 6 of pseudopregnancy. There were 2 plasma PRL surges with peaks at 18:00 hr (diurnal surge) and 3:00 hr (nocturnal surge). Pituitary PRL contents, assessed by RIA of pituitary homogenate made in phosphate buffered saline with teflon glass homogenizer, showed single peaked variation with a peak at midnight and a nadir at the time of the nocturnal surge. The same pituitary homogenate samples were assessed for PRL again after 4 months' frozen storage. The value by this 2nd RIA was dramatically increased in samples obtained around the time of the diurnal surge (12:0021:00 hr) and the peak shifted from midnight to 15:00 hr. This change between 2 assessments was reproduced by the experiment in which the PRL content was assessed after 4 cycles of freezing and thawing homogenate sample. Since almost all PRL of pituitary homogenate was seen in 105, 000 × g pellet fraction, shown by the result of basic ethanol extraction, the observed effect of freezing is thought to be the result of rupturing undestroyed membrane particle existed in homogenate sample.
These data show the change in pituitary PRL content in pseudopregnant rats and suggest the diurnal variation in the activity of PRL synthesis, which accompanies the change in the distribution of PRL among intracellular compartments and the peak of which is assumed to be just prior to the diurnal surge.
