Paradoxical Enhancement of Pituitary Growth Hormone (GH) Responsiveness to GH-Releasing Factor in the Face of High Somatostatin Tone*

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Abstract
Pituitary GH secretion is regulated by a delicate interplay between stimulatory (GRF) and inhibitory [somatostatin (SRIF)] hypothalamic hormones, although the nature of the GRF/SRIF interaction remains to be elucidated. In the present study, we documented a significant elevation of plasma SRIF-like immunoreactivity in 72-h fasted rats compared to that in red controls (129.0 .+-. 17.9 vs. 38.2 .+-. 5.8 pg/ml; P < 0.01) and used this model of high SRIF tone to further delineate the interrelation between GRIF and SRIF in physiological regulation of pulsatile GH secretion. We examined pituitary GH responsiveness to GRF, both in vivo and in vitro, after 72-h exposure to nutritional deprivation and high SRIF secretion. In vivo, GRF-induced GH release was markedly enhanced in the face of high circulating SRIF; freely moving, starved rats released 4- to 8-fold more GH than fed controls in response to rat GRF iv. In vitro, both basal and human GRF-induced GH release were augmented 2- to 4-fold in perifused dispersed anterior pituitary cells of starved rats compared to those in fed controls, and this enhanced responsiveness persisted in the presence of 10-9 M SRIF. These results demonstrate that SRIF not only inhibits GH secretion stimulated by GRF, but that under different temporal conditions SRIF may act in a paradoxically positive manner to sensitize pituitary GH responsiveness to GRF. Such a cooperative interaction of the two peptides may be necessary to optimize pulsatile GH release. Our findings provide support for the hypothesis that the temporal patterning of hypothalamic GRF/SRIF signals to pituitary somatotrophs may be the major determinant for pulsatile GH secretion and ultimtely, body growth.