Pituitary growth hormone "synthesizing factor" in hypophysial portal blood

Immunoreactive growth hormone (GH) released into an incubation medium and (14)C-leucine incorporation into GH by the anterior pituitary were determined after incubation for 15 min and one hr with hypophysial portal plasma and with peripheral plasma from the same animals as a control substance. Twenty-three hemipituitaries in which (14)C-leucine incorporation was measured at IS min showed a mean difference of 2,980 +/- 1,030 SEM cpm/mg of GH (t = 2.89, p < .01). After one hr of incubation no significant difference could be determined. After one hr of incubation the amount of GH released into the medium containing the portal plasma was significantly greater than the amount released into the medium containing peripheral plasma. The mean difference was 409 +/- 179 SEM ng/mg pituitary (* = 2.87, p < .05). After 15 min of incubation no differences in GH released into the medium could be detected. Differences in GH content of the pituitary could not be detected at either time period. The study supports the hypothesis that hypophysial portal plasma contains a factor which stimulates GH synthesis by the pituitary. In addition, the earlier finding of Wilber and Porter that the hypophysial portal plasma contains a factor which will release GH from the pituitary is confirmed.