Stimulation of Pituitary Growth Hormone Release by a Hypothalamic Extractin Vitro1

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Abstract
Anterior pituitaries from mature female virgin rats were cultured in synthetic medium 199 at 35 C in an atmosphere of 95 % O2-5 % CO2. At the end of 18 days of culture, 167 % more growth hormone (STH) activity was recovered from the medium than was originally present in fresh anterior pituitary, as determined by the standard tibia assay in young hypophysectomized rats. In 2 separate trials, addition of an acid extract of rat hypothalamus to the medium during a 6-day culture period increased pituitary STH release 4- to 6-fold. These differences were highly significant statistically. An equivalent amount of extract of rat cerebral cortex had no effect on pituitary STH release. When an extract of rat hypothalamus was boiled for 15 min, ⅔ of the STH-releasing activity still remained in the extract, whereas boiling has been reported to destroy pituitary STH activity. Injections of hypothalamic extract directly into assay rats had no significant effect on cartilage width, also indicating that the extract per se was free of STH activity. Since acid extracts of rat hypothalamus inhibit pituitary prolactin release in vitro, and prolactin injections into assay rats produced only a slight increase in cartilage width, prolactin is not believed to have contributed significantly to the STH activity recovered from the culture medium. These results are believed to demonstrate the existence of an STH-releasing factor in rat hypothalamus. (Endocrinology74: 408, 1964)