Hormonal properties of transplanted pituitary tumors and their relation to the pituitary gland

Effect of human prolactin administration on gonadotropin and thyrotropin secretion in normal men

To test the hypothesis that PRL is able to feedback negatively on its own secretion (short-loop feedback) in humans via augmentation of the turnover of tuberoinfundibular dopamine (TIDA), the effects of the administration of purified hPRL on endogenous LH, FSH and TSH were assessed. Purified hPRL, given in an i.v. loading dose of 90 micrograms followed by a continuous infusion of 1.39 micrograms/min to 4 normal male volunteers resulted in a tripling of PRL levels (10.5 +/- 1.9 micrograms/L increasing to 30.9 +/- 3.6 micrograms/L) at the end of 90 min. There were no changes in LH, FSH or TSH levels, however, during or following the infusion. Purified hPRL was also given in 1 and 8 micrograms/kg doses IM to 5 normal male volunteers. Although PRL levels did not rise significantly with the 1 microgram/kg dose, levels almost doubled with the 8 micrograms/kg dose (9.5 +/- 2.2 micrograms/L increasing to 17.4 +/- 1.5 micrograms/L). Again, LH, FSH and TSH levels did not change significantly over the three hour period of sampling with either dose. In conclusion, in this study we found that a 2-3 fold increase of circulating PRL levels maintained for 1.5-3 h exerted no apparent effects on the secretion of endogenous LH, FSH and TSH. This study provides direct evidence against the existence of a short-loop feedback occurring via TIDA activation in humans over this time interval but does not rule out the possibility that such feedback may occur with more prolonged states of hyperprolactinemia or via other mechanisms or the possibility of an effect on the hypothalamic pulse generator.

Ultrastructure of spontaneous and transplanted pituitary tumors in laboratory animals

The present review examines ultrastructural studies of pituitary tumors in laboratory animals. Such studies have been facilitated by the development of newer techniques such as immunocytochemistry and in situ hybridization. Using these methods it has been possible to correlate morphological features with function by characterizing specific pituitary hormones in secretory granules and analyzing the messenger ribonucleic acids produced by specific cell types. Analyses of spontaneous and experimentally induced tumors have provided a great deal of insight into the function and ultrastructure of these neoplasms. The description of tumor cells producing more than one hormone has been facilitated by immunocytochemical analyses. Studies of transgenic models of pituitary hyperplasia and tumor development are expanding our knowledge about the ultrastructure and about other aspects of pituitary tumors in a more controlled experimental setting. The production of various hormones by the same pituitary tumors and even the same tumor cells indicates that complex regulatory mechanisms must be analyzed by a combination of ultrastructural and other techniques to learn more about growth and hormone secretion in pituitary neoplasms.

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.

Somatic growth in hypophysectomized pituitary-homografted rats is promoted by prolactin

Hypophysectomized male rats bearing a homograft of two adenopituitaries under the kidney capsule showed a significant increase in b.wt as compared to hypophysectomized non-homografted animals. Radioimmunoassay of growth hormone (GH), ACTH, alpha-MSH, beta-endorphin and prolactin (PRL) revealed that only the latter was highly increased in the plasma of hypophysectomized homografted rats. These animals showed also increased levels of plasma corticosterone. However, daily injection of corticosterone failed to promote somatic growth in hypophysectomized non-homografted rats. These results suggest that PRL, and not other hormonal factors, promotes somatic growth in hypophysectomized homografted rats, and stress the concept that only PRL is secreted in significant amounts by pituitary homografts.

Studies of anterior pituitary-grafted rats: II. Normal growth hormone secretion

Of the various animal models used to study chronic hyperprolactinemia, the otherwise intact rat implanted with extra anterior pituitary glands (AP) under the kidney capsule is assumed to be normal except for excess circulating prolactin (PRL). Since the ectopic glands contain numerous somatotropes in addition to abundant and active lactotropes, it was important to assess growth hormone (GH) secretion as well in this model of hyperprolactinemia. The structural and functional similarities of PRL and GH are such that it is necessary to demonstrate that metabolic abnormalities noted in AP-implanted rats are due to hyperprolactinemia and not to altered GH secretion. AP-implanted female rats have significantly higher resting serum PRL concentrations when compared to sham-operated control rats, but baseline serum GH levels are similar in normal and pituitary-grafted rats. Suppression of GH by insulin and clonidine is comparable in AP-implanted and control rats. The intrasellar pituitary GH concentration is also similar (ca. 20 micrograms/mg wet weight) in hyperprolactinemic and normal rats. We conclude that GH secretion is normal in the non-hypophysectomized AP-implanted rat, in contrast to the hypophysectomized AP-implanted rat model which has been reported to have diminished GH secretion. Despite the presence of recognizable somatotropes, the ectopic anterior pituitary does not appear to secrete significant amounts of GH, making the intact rat bearing multiple pituitary grafts an excellent model of chronic hyperprolactinemia.

Acute inhibition of PRL and TSH secretion after intraventricular injection of PRL in ovariectomized rats

Prolactin (PRL) is under short-loop inhibitory control via the hypothalamus. However, earlier studies evaluated the effects on PRL secretion of PRL levels elevated for periods of days. In this study we evaluated the acute effects of intraventricular and systemic injection of PRL on the release of a variety of pituitary hormones. Ovariectomized (OVX) rats, bearing implanted third ventricular and jugular cannulas were used. Blood was withdrawn in unanesthetized, freely moving animals before and after intraventricular injection of 0.9% NaCl or 1 or 3 micrograms of bovine (b) or ovine (o) PRL. Prolactin was also administered intravenously in doses of 3 or 6 micrograms. No effect on plasma levels of any of the pituitary hormones occurred after intraventricular or systemic injection of saline. Intraventricular injection of both doses of bPRL or oPRL significantly lowered plasma PRL within 15-30 min. In animals with elevated initial PRL values because of stress or estradiol (E) priming, greater lowering of PRL occurred. Inconsistent reductions in plasma PRL occurred after intravenous injection of oPRL but not bPRL, which elevated PRL values via cross-reaction in the immunoassay. In contrast, only small and inconsistent declines in luteinizing hormone (LH) were seen after intraventricular injection of PRL in either OVX or OVX E-primed rats. Plasma follicle-stimulating hormone (FSH) and growth hormone (GH) were not affected by PRL in any of the experiments; however, a significant lowering of thyrotropin (TSH) occurred in OVX or OVX E-primed rats within 30 min after intravenous injection of 3 micrograms of oPRL, but no change occurred after intravenous PRL. The data indicate that PRL can acutely inhibit PRL and TSH release via a hypothalamic action, whereas release of LH is only slightly inhibited and that of FSH and GH is unaltered.

Altered dopaminergic regulation of thyrotrophin release in patients with prolactinomas: comparison with other tests of hypothalamic-pituitary function

This study was carried out to test the hypothesis that sustained hyperprolactinaemia in patients with prolactinomas stimulates hypothalamic dopaminergic activity via a short loop positive feedback effect of prolactin (PRL). The intensity of dopamine (DA) effects on the pituitary around the adenoma was evaluated by measuring thyroid stimulating hormone (TSH) responses to intravenous injection of domperidone (10 mg) a new DA receptor blocking drug that does not penetrate the blood-brain barrier. TSH responses have been compared with those of PRL to the same agent. Eight females with prolactinomas showed greater TSH release after domperidone than nine normal females (sum of TSH increments over 20 min 17.5 +/- 1.7 v. 8.9 +/- 1.5 mu/l, P less than 0.001) whilst PRL release was reduced (sum of PRL increments over 120 min 5.9 +/- 2.4 v. 21.8 +/- 3.8 mu/l x 10(-3), P less than 0.01). Amongst nineteen hyperprolactinaemic females with apparently normal pituitary fossae (plain skull X-ray), ten showed an exaggerated TSH response (delta TSH, 4.2 +/- 0.6 mu/l, range 2.5-9.0 mu/1) and reduced PRL response to domperidone, comparable with established tumor cases. In the remaining nine normal fossa hyperprolactinaemic females, the TSH and PRL responses to dopaminergic were similar to normal females. These results support the initial hypothesis and indicate the coexistence of a defect in the dopaminergic inhibition of PRL release and increased dopaminergic inhibition of TSH release in patients with prolactinomas. The presence of an exaggerated TSH response to DA antagonism in a euthyroid, radiologically normal (plain skull X-ray), hyperprolactinaemic patient is compatible with the presence of an autonomously-functioning, PRL secreting, pituitary microadenoma and the TSH changes seen in these patients after DA antagonist administration can be readily detected by sensitive TSH radioimmunoassay.

Some biochemical characteristics of hormone-secreting pituitary tumors and of the host's anterior pituitary gland

The activity of some glycolytic, oxidative, and degradative enzymes was studied in transplanted rat hormone-secreting pituitary tumors MtTW15 and 7315a and in the host pituitary gland. The elevated serum-hormone concentrations produced by 7315a tumor decreased the size of the host's pituitary gland, its hormone content, and G6P-DH, LDH, PK, and ICDH, but produced no changes in MDH, acid phosphatase, cathepsin-D, and LYSAR enzyme activities (mU/mg tissue). LDH and PK activities were greater in unit weight of pituitary tumors than in pituitary glands. Although more G6P-DH was found in MtTW15 tumor than in normal pituitary tissue, less of the enzyme was detected in 7315a pituitary tumor. It is concluded that elevated serum pituitary hormones selectively decrease hormone production and the activity of some enzymes in the pituitary gland, presumably through a feedback mechanism.

Heterogeneity of the MtTW15 mammosomatotropic tumor. I. Light microscopic evaluation of cell types by means of immunocytochemistry, morphometric quantitation, fluorescence cytophotometry and radioimmunoassay

Large MtTW15 pituitary tumors produced 200- to 800-fold elevations in serum growth hormone (GH) and prolactin (PRL) levels. Female tumor hosts showed doubling in body weight, milk secretion, and a 2-fold hepatosplenomegaly. Pituitaries of host animals were reduced by about 50% in both weight and concentrations of GH and PRL. Large tumors were well-encapsulated, multinodular and showed variable amounts of necrosis and hemorrhage. Cytofluorometric analysis revealed a range of 100-fold in nuclear DNA content of tumor parenchymal cells which were chromophobic, pleomorphic and frequently mitotic. Concentrations of hormones in tumors were less than in normal pituitaries and highly variable with the ratio of GH/PRL ranging up to 30-fold within the same tumor. Immunostaining and linear scanning quantitation showed that about 50% of the tumor cells contained immunodetectable hormones. Comparison of immunostained adjacent sections showed that hormone-containing tumor cells were pleomorphic, unequally distributed within nodules, lacking in distinctive identifying morphological characteristics and that they contained GH or PRL but not both hormones simultaneously. Collectively our results show that large MtTW15 tumors are comprised of a markedly heterogeneous population of tumor cells and they suggest that the hormone-containing cells are monohormonal secreting tumor cells which can produce GH or PRL but not both hormones.

The influence of long term estrogen treatment on plasma prolactin levels induced by ether anesthesia in ovariectomized rats

2 methods of continuous estrogen delivery, polyestradiol phosphate injection and implantation of Silastic capsules of estradiol-17 beta, in ovariectomized rats induced increases in plasma prolactin in the afternoon (15.00-17.00) beginning at 1 week and continuing for 4-8 weeks. In addition these methods of estrogen treatment potentiated the ether-induced increase in plasma prolactin in the morning (9.00-11.00) beginning on week 2 and continuing for 3-8 weeks. These results indicate that estrogen activates the mechanisms that cause an afternoon surge in prolactin before potentiating a morning elevation induced by ether anesthesia.

Prolactin

Prolactin exists in man as a distinct and separate anterior pituitary hormone from growth hormone. It is important in lactation and the control of gonadal function, although it may have a much wider and basic metabolic role, similar to its role in lower forms. In clinical endocrinology it is important as an index of pituitary and hypothalamic diseases; thus prolactin levels are elevated in association with these conditions and this reflects the normal tonic inhibitory hypothalamic control of prolactin by PIF; DA is the most important PIF. Hyperprolactinaemia causes hypogonadism in both men and women; it may present in women with amenorrhoea, oligomenorrhoea, polymenorrhoea, regular cycles with anovulation or a defective luteal phase, and impotence in men. In either sex galactorrhoea is reported to occur in only 30 per cent of patients. Neurotransmitter therapy, with dopamine agonists which act as functional analogues of PIF, restores prolactin levels to normal and leads to a return of normal gonadal function. The mechanism of the hypogonadism is not clear and is discussed together with the problems associated with inducing pregnancy in these patients, who may harbour microadenomata of the pituitary.

Localization of prolactin in chromophobe pituitary adenomas: study of human necropsy material by immunoperoxidase technique

In order to identify prolactin-producing tumours in human pituitary glands, 45 chromophobe adenomas, obtained from unselected necropsies, have been studied by various staining procedures including the immunoperoxidase technique for the demonstration of prolactin. The presence of immunoreactive prolactin was revealed in the cytoplasm of the tumour cells in six cases (13%), indicating that the occurrence of prolactin-producing adenomas is not rare. No correlations were established between tumours and clinical history. Two adenomas were detected in female and four in male patients. The age of the patients at necropsy ranged from 28 to 75 years. Three adenomas were associated with disseminated carcinoma, two with fatal liver disease, and one with diabetes mellitus, atherosclerosis, and pyelonephritis. Manifest endocrine symptoms were not disclosed, and endocrine investigations, including measurements of blood prolactin levels, were not undertaken. Thus, direct evidence is lacking as to whether or not these tumours were actively secreting prolactin. In the non-tumorous parts of the anterior lobes the number of prolactin cells was decreased in two cases, suggesting that prolactin released from the adenoma cells suppressed prolactin production in the non-tumorous pituitary. However, the number of prolactin cells of the non-tumorous adenohypophysis seemed to be unchanged in two and increased in another two cases. The present findings conclusively proved the existence of the prolactin-producing adenomas as a distinct entity. These tumours do not stain with acid or basic dyes, they are PAS or thionin negative, and do not contain immunoreactive growth hormone. Thus, by conventional staining procedures they are indistinguishable from other chromophobe adenoma types. Herlant's erythrosin and Brookes' carmoisine methods, claimed spedifically to stain prolactin cells, failed to provide reliable results, hence their use cannot be recommended in tumour identification. Immunoperoxidase staining of prolactin is the only technique which conclusively reveals the presence of immunoreactive prolactin in the cytoplasm of the tumour cells and permits diagnosis. It is proposed that this technique be introduced in pituitary morphological studies. Its application may lead to a better understanding of problems related to prolactin-producing tumours and their secretory activity.

Immunocytochemical localization of somatostatin in cell bodies of the rat hypothalamus

Cell bodies of small to moderate-sized neurons in the female rat hypothalamus were stained specifically for somatostatin (SRIF) by means of the unlabeled antibody-peroxidase-antiperoxidase immunocytochemical method. SRIF-positive perikarya were scattered throughout the periventricular nucleus in a limited region extending from the middle of the optic chiasm to the rostral margin of the median eminence. The same neurons were revealed with either rabbit (R) or guinea pig (GP) anti-SRIF antisera. Positive cell bodies were more readily assessed with GP antibodies because nonspecific background staining was much less with these than with R anti-SRIF. Positive perikarya were not observed in other hypothalamic nuclei and ependymal elements were also immunocytochemically negative.

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.

Release of pituitary growth hormone by prostaglandins and dibutyryl adenosine cyclic 3':5'-monophosphate in the absence of protein synthesis

Effects of prostaglandins on the incorporation of [4,5-(3)H]leucine into growth hormone and its subsequent release into the incubation medium were studied. Incubation of rat anterior pituitary glands with 10(-6) M prostaglandin PGE(1) in tissue culture medium 199 for 7 hr caused a 40-300% increase in the release of labeled growth hormone into the incubation medium. PGE(1) at 10(-8) M increased growth hormone synthesis but not release. At 10(-6) M, PGE(2) had effects similar to PGE(1); PGA(1) increased growth hormone synthesis but not release. PGF(2alpha) was without effect on either synthesis or release of growth hormone.Prolactin synthesis and release were not affected by prostaglandins. All of the prostaglandins, at 10(-4) M, increased adenyl cyclase activity in the pituitary gland but phosphodiesterase activity was unaltered. Dibutyryl cyclic AMP, with or without caffeine, caused an up to 300% increase in labeled growth hormone release. No consistent effect of prolactin was observed. If potassium concentration was increased 10-fold, a 215% increase in growth hormone release was observed. A combination of hypertonic potassium and 10(-6) M PGE(1) increased growth hormone release 325%, suggesting that potassium and prostaglandins act by independent mechanisms. Addition of theophylline to pituitary gland, incubated in vitro, increased both the synthesis and release of growth hormone. Although fluoride greatly stimulated growth hormone release, it completely inhibited the incorporation of leucine into the hormone. Similarly, puromycin inhibited synthesis of growth hormone but did not block release induced by prostaglandin, dibutyryl cyclic AMP, theophylline, or fluoride. Prostaglandins increase pituitary adenyl cyclase activity and, presumably via cyclic AMP, increase growth hormone release, independently of protein synthesis.