Suppression of pituitary gland hormone content by pituitary tumor hormones

Growth hormone increases somatostatin release and messenger ribonucleic acid levels in the rat hypothalamus

Growth hormone (GH) suppresses its own secretion by stimulating somatostatin (SRIF) release. Thus, the possible regulation of GH-releasing factor (GRF) and SRIF release and SRIF messenger ribonucleic acid (mRNA) by GH was studied in the hypothalamus of male rats in vitro. The median eminences (ME's) were incubated in buffer containing 10(-7)-10(-11) M GH for 30 min. SRIF and GRF released into the medium were quantitated by RIA. The release of SRIF from ME fragments was significantly increased (P < 0.001) by 10(-9) M GH; however, 10(-9) M GH also inhibited (P < 0.01) GRF release from the ME. To determine the effect of GH on SRIF mRNA levels, periventricular nucleus (PeN) explants were cultured during 6 h in medium with 10(-7)-10(-11) M GH. Levels of SRIF mRNA (determined by an S1 nuclease protection assay) were significantly elevated in the presence of 10(-10)-10(-7) M GH. Likewise, 10(-9) M GH significantly stimulated SRIF release from PeN explants at 30 min and at 6 h. Surprisingly, 10(-9) M GH also significantly increased GRF release from the PeN explants at these times as well. This GRF was not responsible for the increased SRIF release or SRIF mRNA induced by GH since GRF antibody did not modify the GH-induced increases in SRIF release and mRNA levels. These results demonstrate a negative short-loop feedback of GH mediated at the ME by suppression of GRF and stimulation of SRIF release, whereas in the PeN GH increased both SRIF release and SRIF mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution of lactogen receptors in the mammalian hypothalamus: an in vitro autoradiographic analysis of the rabbit and rat

The hypothalamus contains a high concentration of lactogen receptors as detected with in vitro radioreceptor assay techniques. In an effort to define the location of the lactogen receptors relative to specific hypothalamic nuclei, an in vitro autoradiography technique was applied to frozen sections of rat and rabbit brains. Three lactogenic hormones, i.e. human growth hormone (hGH), ovine prolactin (oPRL), and rat prolactin (rPRL), were radiolabeled with iodine-125. Competition for observed binding sites was assessed with unlabeled hGH, oPRL, and bovine growth hormone (bGH). Analysis of the autoradiographs with a microcomputer-based densitometry system revealed that the rabbit hypothalamus contains specific lactogen binding sites within the supraoptic, paraventricular, suprachiasmatic, ventromedial, arcuate, and dorsomedial nuclei and the medial preoptic area. Unlabeled bGH was effective in competing for binding sites in all areas when hGH but not oPRL was used as the radiolabeled ligand, suggesting the presence of growth hormone receptors in the rabbit hypothalamus with a distribution similar to that of the lactogen binding sites. In contrast to the rabbit, no lactogen binding sites were detected in the rat hypothalamus regardless of the ligand used in the assay. All of the ligands were successful, however, in detecting lactogen receptors within the rat choroid plexus and liver. The results from the rabbits indicate that the influences of prolactin on hypothalamic activity are mediated via lactogen receptors that are widely distributed throughout the various pertinent hypothalamic nuclei. The broad distribution of lactogen receptors in the rabbit hypothalamus attests to the extensive influence of prolactin on hypothalamic regulatory systems. The results from the rat raise questions as to the nature of rat brain prolactin receptors in comparison to prolactin receptors in rat peripheral tissues.

Hypothalamic growth hormone-releasing factor (GRF) participates in the negative feedback regulation of growth hormone secretion

Effects of growth hormone (GH) excess on immunoreactive hypothalamic GH-releasing factor (GRF) and somatostatin (SRIF) were studied in rats. Hypothalamic GRF content significantly reduced after 7-day daily treatment with 160 micrograms of rat GH or after inoculation of GH-secreting rat pituitary tumors, MtT-F4 for 9 or 13 days and GH3 for 3 months. Basal and 59 mM K+-evoked release of GRF from incubated hypothalami diminished, more than the content, by 43-51% in MtT-F4 tumor- or by 67-83% in GH3 tumor-bearing rats. In contrast, there was a small but significant increase in content or release of SRIF in rats harboring the GH3 or MtT-F4 tumor, respectively. These results indicate the existence of a negative feedback loop via hypothalamic GRF as well as SRIF in control of GH secretion.

Exogenous growth hormone inhibits growth hormone-releasing factor-induced growth hormone secretion in normal men

Previous studies from this laboratory and by others in rats, monkeys, and humans support the concept that growth hormone (GH) can regulate its own secretion through an autofeedback mechanism. With the availability of human growth hormone-releasing factor (GRF), the possible existence of such a mechanism was reexplored by examining the effect of exogenous GH on the GH response induced by GRF-44-NH2 in six normal men (mean age, 32.4 yr). In all subjects the plasma GH response evoked by GRF-44-NH2 (1 microgram/kg i.v. bolus) was studied before and after 5 d of placebo (1 ml normal saline i.m. every 12 h), and then before and 12 h after 5 d of biosynthetic methionyl human GH (5 U i.m. every 12 h). The GH response to GRF (maximal increment over time 0 value) was significantly inhibited after GH treatment (0-1.3 vs. 2.3-11.2 ng/ml before treatment, P = 0.05), but was not significantly affected by placebo. This impaired pituitary response to GRF persisted for at least 24 h following exogenous GH treatment in two subjects who underwent further study. Serum somatomedin-C concentrations were significantly increased after 5 d of GH treatment (2.66-5.00 vs. 0.92-1.91 U/ml before treatment, P = less than 0.01). The impaired pituitary response to GRF may be mediated indirectly through somatomedin, somatostatin, by a direct effect of GH on the pituitary somatotropes, or by all of these mechanisms. These data suggest that after GH treatment, the blunted GH response to synthetic GRF is not solely a consequence of the inhibition of hypothalamic GRF secretion.

Fine structural studies on the fate of "castration" cells in the rat adenohypophysis after bilateral oophorectomy

The fate of castration cells has been studied by electron microscopy after bilateral oophorectomy. The castration cells were characterized by a large-like dilatation of rough endoplasmic reticulum which occupied almost the entire cytoplasm, and first appeared at 5 weeks after operation. Castration cells showed cellular suppression after 15 weeks of the experiment but no degenerative changes were observed at any experimental period. At the later stage of the experiment numerous secretory granules reaccumulated in the cytoplasm of the gonadotrophs and concomitantly lake-like dilatation of rough endoplasmic reticulum in some gonadotrophs gradually decreased and finally disappeared from the cytoplasm. Proportion of the castration cells and mean diameter of the largest cisterane of rough endoplasmic reticulum in each cell attained maximum value at 9 weeks after the operation; thereafter both parameters returned toward normal, although they still remained at a higher level than in controls at 18 weeks after the operation.

The effect of administration of human growth hormone on the plasma growth hormone, cortisol, glucose, and free fatty acid response to insulin: evidence for growth hormone autoregulation in man

The effect of administration of human growth hormone (HGH) (3 mg every 6 hr for 6 days) on the endogenous GH response to insulin-induced hypoglycemia at 8, 12, 24, and 48 hr posttreatment was studied in 11 healthy male adults. Free fatty acid, cortisol, and glucose responses pre- and posttreatment with HGH were evaluated concurrently. Control subjects received saline injections to evaluate relationship of GH responses to the periodicity of insulin tolerance tests. The data were compared for each subject pre- and posttreatment with HGH as well as by comparison of the results of the saline-treated group with those of the HGH-treated group. The mean maximal GH concentration in response to insulin-induced hypoglycemia for all the subjects (n = 16) was 31.1 +/-3.6 ng/ml (+/-SEM) on day 1 of the control period and 23.4 +/-3.1 (SEM) on day 2, not statistically significant.A significant decrease in the maximal peak GH response (n = 8) after insulin-induced hypoglycemia was observed at 8 and 12 hr after HGH administration was terminated with mean peak values for GH of 4.6 +/-1.3 ng/ml and 10.4 +/-1.9 ng/ml, respectively (P < 0.01). A progressive return to control values was noted between 12 and 24 hr. The GH responsiveness of the saline-treated group (n = 5) was unchanged from that observed during the control period. The fasting glucose values were unchanged in the GH-treated group from those of the control period or of the saline-treated controls. Insulin resistance was apparent at 8 hr posttreatment with HGH. No differences in FFA response after insulin-induced hypoglycemia were observed in GH-treated or saline-treated subjects. The rise in plasma cortisol after insulin-induced hypoglycemia was comparable in the GH-treated and saline-treated group. Diurnal variation in plasma cortisol was maintained during the period of GH suppression. These observations support the concept that GH can modulate its secretion by means of an auto-feedback mechanism.