A Detailed in Vitro Characterization of the Basal Follicle-Stimulating Hormone and Luteinizing Hormone Secretion Rates during the Rat Four-Day Estrous Cycle*

Studies were undertaken to determine the relative basal anterior pituitary gland FSH and LH release rates in vitro for rats killed by decapitation at 1 of 15 times during the 4- day estrous cycle and to ascertain if these basal release rates correlated with changes in serum gonadotropin concentrations. Trunk blood was collected, and serum was assayed for FSH and LH. Half of the anterior pituitary gland was homogenized and assayed for FSH and LH. The other half was placed in culture medium for a 30-min preincubation and then placed in fresh media for a 2-h incubation (basal FSH and LH release). Serum FSH concentration rose in association with a rise in serum LH levels during the afternoon of proestrus (first phase of FSH release). Whereas serum LH levels fell during the evening of proestrus, the serum FSH concentration remained elevated through 2400 h on estrus (second phase of FSH release). The basal FSH release into medium increased markedly during late proestrus and showed a highly significant positive correlation (p < 0.00001) with changes in serum FSH levels during the period of the second phase of FSH release. The basal FSH release and serum FSH concentration also correlated positively throughout the estrous cycle. A small increment in basal FSH release occurred during the first phase of FSH release, but only after a marked increase in the serum FSH concentration. The basal LH secretion did not change significantly during the estrous cycle. The anterior pituitary gland FSH concentration decreased in association with the first phase of FSH release, and concentrations were fully restored by noon on estrus. The results indicate that the anterior pituitary gland undergoes marked changes during the afternoon and early evening of proestrus, resulting in a large increase in the basal FSH release rate which is independent of total radioimmunoassayable anterior pituitary gland FSH stores. Based on these observations and those of previous studies in this laboratory, we suggest that the second phase of increased serum FSH concentration is primarily the result of an increase in the basal FSH secretion rate.