High levels of beta-endorphin in hypophyseal portal blood.

beta-Endorphin was measured by RIA in the hypophyseal portal blood of six pig tailed monkeys after pituitary stalk section. The mean beta-endorphin concentration was 4,770 pg/ml (range, 2,900-10,500 pg/ml). This was more than 100 times greater than the mean simultaneous peripheral venous concentration, which was less than or equal to 45 pg/ml. After gel filtration of the portal plasma extracts, the majority of beta-endorphin immunoactivity eluted as a single peak coincident with synthetic beta-endorphin standard. The demonstration of high levels of beta-endorphin in the hypophyseal portal blood suggests that endogenous opioids of hypothalamic origin are secreted into the portal blood and may directly affect the pituitary.