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Male rat anterior pituitaries were incubated in vitro for 5 hr in Krebs-Ringer bicarbonate medium and the growth hormone (GH) released from the glands was estimated by the tibial epiphyseal cartilage assay. Addition of crude sheep or rat hypothalamic extract to the pituitaries increased the GH concentration in the medium on comparison to either diluent or cortical extract-treated controls. The ovine extracts also depleted pituitary GH concentration when injected into male rats in vivo. Gel filtration of either sheep or rat hypothalamic extract on a column of Sephadex G-25 resulted in the elution from the column of 2 zones which influenced the release of GH in vitro on addition to the incubated pituitaries. The first zone of activity to be eluted increased the release of GH from the glands severalfold, whereas the second zone inhibited the release of GH to levels about one half of that released by control glands incubated in the presence of the eluting buffer. Increasing the dose in the inhibitory zone resulted in a further decrease in GH release. The addition of either the stimulatory or inhibitory fractions to the incubation medium at the termination of the incubation failed to affect the value for GH obtained on assay. This result indicates that neither fraction altered the assay of GH. Consequently, the results obtained are attributable to altered rates of release of GH from the incubated glands. The findings are consistent with the hypothesis that hypothalamic extracts of rat and sheep origin contain a GH-inhibiting factor (GIF) in addition to the GH-releasing factor (GRF). The GH-releasing activity of crude extracts is explained by assuming that the relative concentration of GRF exceeds that of GIF. (Endocrinology83: 783, 1968)
Krulich et al. (Tue,) studied this question.