Changes inβ-Endorphin-Like Immunoreactivity in Pituitary Portal Blood during the Estrous Cycle and after Ovariectomy in Rats*

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Abstract
.beta.-Endorphin-like immunoreactivity (.beta.-EP-LI) was measured by RIA [radioimmunoassay] in plasma collected from pituitary portal vessels of rats at various times in the estrous cycle and after ovariectomy. There were no appreciable differences between the mean .beta.-EP-LI concentration either in the afternoon (1500-1700 h) of estrus (4.1 .+-. 1.5 ng/ml) or on diestrous days 1 (4.1 .+-. 1.3 ng/ml) or 2 (5.4 .+-. 1.5 ng/ml). The concentration increased slightly (6.7 .+-. 1.3 ng/ml) but not significantly in the early afternoon (1400-1500 h) of proestrus. The concentration of .beta.-EP-LI then fell to 0.5 .+-. 0.1 ng/ml at 1700-1800 h on proestrus, a level significantly lower than at any other time of the cycle. Portal plasma .beta.-EP-LI was also low (1.9 .+-. 0.5 ng/ml) in animals ovariectomized for 3 wk. After gel filtration of the portal plasma extracts, most of the .beta.-EP-LI eluted in the same position as synthetic .beta.-EP. Dilution of portal plasma produced a displacement curve parallel to that of .beta.-EP and hypothalamic extract. Apparently, the secretion of hypothalamic .beta.-EP into the blood of pituitary portal vessels changes during the estrous cycle, possibly due to cyclic changes in sex steroids.