Estrogen and androgen elicit growth hormone release via dissimilar patterns of hypothalamic neuropeptide secretion

The dimorphic pattern of growth hormone (GH) secretion and somatic growth in male and female mammals is attributable to the gonadal steroids. Whether these hormones mediate their effects solely on hypothalamic neurons, on somatotropes or on both to evoke the gender-specific GH secretory patterns has not been fully elucidated. The purpose of this study was to determine the effects of 17beta-estradiol, testosterone and its metabolites on release of GH, GH-releasing hormone (GHRH) and somatostatin (SRIF) from bovine anterior pituitary cells and hypothalamic slices in an in vitro perifusion system. Physiological concentrations of testosterone and estradiol perifused directly to anterior pituitary cells did not affect GH releases; whereas, dihydrotestosterone and 5alpha-androstane-3alpha, 17beta-diol increased GH. Perifusion of testosterone at a pulsatile rate, and its metabolites and estradiol at a constant rate to hypothalamic slices in series with anterior pituitary cells increased GH release. The androgenic hormones increased GHRH and SRIF release from hypothalamus; whereas, estradiol increased GHRH but decreased SRIF release. Our data show that estradiol and the androgens generated distinctly different patterns of GHRH and SRIF release, which in turn established gender-specific GH patterns.     

Dynamic of the GRF-induced GH response in genetically obese Zucker rats: influence of central and peripheral factors

To determine the time onset of the growth hormone (GH) alteration in the genetically obese rat, we studied the in vivo and in vitro rat growth hormone releasing factor (rGRF(1-29)NH2)-induced GH secretion in 6- and 8-week-old lean and obese male Zucker rats. Under sodium pentobarbital anesthesia, rGRF(1-29)NH2 (GRF) was injected intravenously at two doses: 0.8 and 4.0 micrograms/kg b.w. Basal serum GH concentrations were similar in lean and obese age-matched animals. The GH response to both GRF doses tested was unchanged in 6-week-old obese rats as compared to their lean litter mates. In contrast, a significant decrease of the GH secretion in response to 4.0 micrograms/kg b.w. GRF was observed in the 8-week-old obese rats. The effect of GRF (1.56, 6.25 and 12.5 pM) was further studied in vitro, in a perifusion system of freshly dispersed anterior pituitary cells of lean and obese Zucker rats. Basal GH release was similar in the 6-week-old animal group. In contrast, it was significantly decreased in 8-week-old obese rats as compared to their lean litter mates. Stimulated GH response to 1.56 and 6.25 pM GRF was significantly greater in the 6-week-old obese group than in the age-matched control group. In contrast, the GH response to all GRF concentrations tested was significantly decreased in the 8-week-old obese rats as compared to their respective lean siblings. In 8-week-old obese rats, a decrease of GH pituitary content and an increase of hypothalamic somatostatin (SRIF) concentration were observed. Insulin and free fatty acid serum were significantly increased in 8-week-old obese rats. In contrast, lower insulin-like growth factor I serum levels were observed in the obese animals as compared to their lean litter mates. Finally, to further clarify the role of the periphery in the inhibition of GH secretion observed in the 8-week-old fatty rats, we exposed cultured pituitary cells of 8-week-old lean animals to 17% serum of their obese litter mates. A significant decrease of GRF-stimulated GH secretion of lean rat pituitary cells exposed to the obese serum was noted (P less than 0.05). This study demonstrates that, in the obese Zucker rat, an alteration of the GH response to GRF is evident by the 8th week of life. This defective GH secretion could be related to peripheral and central abnormalities.     

Growth hormone secretion during pregnancy: altered effects of growth hormone releasing factor and insulin-like growth factor-I in vitro

Growth hormone (GH) secretion is controlled by growth hormone releasing factor (GRF) but changes in the circulating level of this hormone are difficult to measure. Insulin-like growth factor (IGF-I) is a GH-dependent growth factor which significantly but slightly inhibits stimulated GH release in vitro. We have tested the effects of GRF and IGF-I on GH release in pregnancy, a state in which serum concentrations of GH are elevated and levels of IGF-I are lowered. We have found, in a system of acutely dispersed adenohypophysial cells prepared from pregnant (day 21-23) or control cycling female rats, that adenohypophysial cells from pregnant rats have an increased GH release with GRF. In contrast, IGF-I inhibition is similar but slightly smaller. These altered responses may result in elevated serum GH levels during pregnancy.     

Comparative in vivo and in vitro studies on abnormal GH secretion in patients with acromegaly

Eight patients with active acromegaly due to GH-producing pituitary adenoma were studied. GH secretory dynamics in vitro was evaluated by adding GRF, CRF, or a somatostatin analog, SMS 201-995 to the perifusate of dispersed cells from tumors. A comparison was made between the data obtained in preoperative tests for GH secretion and those obtained in experiments in vitro. Before operation, the GRF test (100 micrograms, iv) resulted in no GH response in three of six patients examined. The CRF test (100 micrograms, iv) resulted in a paradoxical GH increase in two of the same six patients. In vitro studies performed on adenoma cells revealed that exposure to GRF (100 ng/ml) elicited an increase in GH in seven of eight patients examined. Exposure to CRF (100 ng/ml) caused an enhanced GH secretion in four of the same eight patients. There were cases in which GH response to these hypothalamic hormones was observed in vitro but not in vivo, whereas there was only one case in which CRF caused an increase in GH in vivo but not in vitro. Thus, GH secretory dynamics was not always the same in vivo and in vitro. The discrepancy could be ascribed to the different secretory status of hypothalamic hormone (e.g., GRF or somatostatin) in vivo in each acromegalic patient.     

The episodic secretory pattern of growth hormone regulates liver carbonic anhydrase III. Studies in normal and mutant growth-hormone-deficient dwarf rats.

Carbonic anhydrase III (CAIII) occurs in male rat liver at concentrations twenty times those in the female, and is sensitive to the pattern of growth hormone (GH) release. Males release GH episodically and have high concentrations of CAIII; females produce GH in a more continuous fashion and have lower CAIII levels. In normal female rats, the endogenous GH secretory pattern was masculinized, either by regular injections of GH-releasing factor (GRF) or by intermittent infusions of somatostatin (90 min on/90 min off). Both treatments induced regular GH pulses and stimulated growth, but only intermittent somatostatin infusions raised CAIII levels (controls, 1.5 +/- 0.5; somatostatin-treated, 9.0 +/- 2.9 micrograms/mg; means +/- S.D.). GRF pulses (4 micrograms every 4 h) did not however raise CAIII levels (controls 1.8 +/- 0.5; GRF-treated 1.4 +/- 0.4 micrograms/mg). Surprisingly, hepatic CAIII is also sexually dimorphic (males, 18.8 +/- 3; females, 2.22 +/- 0.4 micrograms/mg) in a GH-deficient dwarf rat strain which has low plasma GH levels without 3-hourly GH peaks. Intermittent somatostatin infusions in female dwarf rats partially masculinized hepatic CAIII, an effect reduced by co-infusion with GRF. This CAIII response was not secondary to growth induction, since neither somatostatin nor GRF stimulated growth in dwarf rats, and pulses of exogenous GH stimulated growth in female dwarfs without masculinizing CAIII levels. Furthermore, continuous GH infusion in male dwarf rats partially feminized hepatic CAIII levels (to 9.1 +/- 2.4 micrograms/mg), whereas infusions of insulin-like growth factor-1, which induced the same body weight gain, did not affect hepatic CAIII (20.8 +/- 6 micrograms/mg). These results show that hepatic CAIII expression is highly sensitive to the endogenous GH secretory pattern, independent of growth. They also implicate the low basal GH levels between pulses, rather than the peak GH levels, as the primary determinant of the sexually dimorphic hepatic CAIII expression in the rat.     

Ontogeny of the response to growth hormone-releasing factor

Primary cell cultures were prepared from fetal, neonatal and adult rat pituitaries and evaluated for their ability to secrete growth hormone (GH) in response to growth hormone-releasing factor (GRF). Pituitary cells prepared from fetuses at days 19 and 21 of gestation, neonatal animals at the day of birth (day 0) or the following day (day 1) and peripubertal male rats showed full dose response curves to GRF with maximal GH release when stimulated with 1 X 10(-10) M rat GRF. At this concentration of GRF, the amount of GH released was not different from that elicited by activation of adenylate cyclase with 1 X 10(-5) M forskolin. In contradistinction, a preparation of cells from fetuses at day 18 of gestation did not show the same release of GH when challenged with 1 X 10(-10) M GRF and forskolin (0.057 +/- 0.001, compared to 0.076 +/- 0.003 micrograms/10(5) cells per 4.5 h), although the cells clearly responded to both secretagogues (basal levels of GH, 0.029 +/- 0.002 micrograms/10(5) cells per 4.5 h). While cells prepared from fetuses at day 21 of gestation or from animals after birth released 5-10% of their total cellular GH content, those prepared from 18- and 19-day fetuses released as much as 40% of their total GH suggesting there is a maturation of intracellular GH processing that occurs late in gestation. The results show that, in late pregnancy, the rat fetal pituitary is highly responsive to growth hormone-releasing factor and suggest that this peptide participates in regulating GH levels during the perinatal period.     

Stimulation of growth hormone release in dwarf and normal chickens by thyrotrophin releasing hormone (TRH) or human pancreatic growth hormone releasing factor (hpGRF)

The effect of thyrotrophin releasing hormone (TRH) or human pancreatic growth hormone releasing factor (hpGRF) on growth hormone (GH) release was studied in both dwarf and normal Rhode Island Red chickens with a similar genotype except for a sex-linked dw gene. Both TRH (10 micrograms/kg) and hpGRF (20 micrograms/kg) injections stimulated plasma GH release within 15 min in young and adult chickens. The increase in GH release was higher in young cockerels than that in adult chickens. The age-related decline in the response to TRH stimulation was observed in both strains, while hpGRF was a still potent GH-releaser in adult chickens. The maximal and long acting response was observed in young dwarf chickens, suggesting differences in GH pools releasable by TRH and GRF in the anterior pituitary gland. The pituitary gland was stimulated directly by perifusion with hpGRF (1 microgram/ml and 10 micrograms/ml) or TRH (1 microgram/ml). Repeated perifusion of GRF at 40 min intervals blunted further increase in GH release, but successive perifusion with TRH stimulated GH release. The results suggest the possibility that desensitization to the effects of hpGRF occurs in vitro and that the extent of response depends on the number of receptors for hpGRF or TRH and/or the amount of GH stored in the pituitary gland.     

Sex differences in fatty acid composition of rat liver phosphatidylcholine are regulated by the plasma pattern of growth hormone

The effects of continuous and intermittent (at 12 h intervals) administration of growth hormone (GH), and the effects of gonadal steroids on the regulation of the fatty acid composition of liver phosphatidylcholine were studied in gonadectomized and hypophysectomized adult female Sprague-Dawley rats. Gonadal steroids have been shown to influence the fatty acid composition of liver phosphatidylcholine in the rat. It is shown in the present study that neither testosterone nor estradiol had any effects on liver phosphatidylcholine in hypophysectomized rats. There was a 'masculinizing' effect of hypophysectomy of female rats on the fatty acid composition of liver phosphatidylcholine (i.e., an increase in the proportion of palmitic, oleic and linoleic acids and a decrease in the proportion of stearic and arachidonic acids). Continuous infusion of human GH and bovine GH partly reversed the 'masculinizing' effect of hypophysectomy. In contrast, there were no effects of intermittent administration of human GH. Also, there was no effect of prolactin infusion. It is concluded that the sexually dimorphic secretory pattern of GH may be involved in the regulation of the sexual differentiation of the fatty acid composition of liver phosphatidylcholine in the rat.     

Attenuation of growth hormone gene expression in desensitized somatotropes

The effects of GRF-induced desensitization of somatotropes on GH gene expression were investigated on pituitary cells derived from male rats. Pretreatment of monodispersed cells for 18 hr with GRF abolished both the acute release of GH and the stimulation of GH gene expression in response to a subsequent 4 hr challenge with GRF. Concomitant preincubation with GRF and SS resulted in restoration of the ability of GRF to stimulate release of GH but not to augment GH gene expression. These results demonstrate that desensitization by GRF affects both the release of GH and GH gene expression, whereas the resensitizing effects of SS appear to be directed exclusively at the release mechanism.     

Effects of testosterone and estradiol on serum somatomedin A and growth rate of rats

The mode of involvement of sex steroids in the growth spurt during adolescence was studied in Wistar rats with a special reference to the level of serum somatomedin A (SMA) determined by radioreceptor assay. Intramuscular administration of testosterone propionate (T; 1 mg/day, alternately for 10 days) to female or gonadectomized male rats provoked a small but significant increase in their body weight or body length without affecting the serum SMA level. In contrast, in hypophysectomized (hypox) male rats T caused a considerable increase in body weight and the serum SMA level only when administered concurrently with bovine growth hormone (bGH; 0.2 U/day, ip). T did not affect the sulfation activity in vitro. These results suggest that androgen participates in the growth spurt during adolescence by enhancing the SMA effect and/or potentiating the SMA production by GH. Estradiol benzoate (E2; 100 micrograms/day, alternately for 10 days) caused a decrease in the serum SMA level and the growth rate in normal male rats. However, E2 produced an increase in the SMA level when administered to hypox male rats, although the growth was retarded and sulfation potency of the serum was sharply reduced. These results indicate that E2 may suppress the growth by lowering SMA generation in normal rats and cause a production of biologically inactive SMA in hypox male rats.     

Characterization of the stimulatory effect of galanin on growth hormone release from the rat anterior pituitary.

The present study was undertaken to investigate the direct actions of rat galanin (R-GAL) on growth hormone (GH) release from the rat anterior pituitary in vitro. R-GAL modestly but significantly stimulated GH release without an increase in intra- and extracellular cyclic AMP levels in monolayer cultures of rat anterior pituitary cells. This stimulatory effect of R-GAL was dose-dependent but not additive with that of GH-releasing factor (GRF). R-GAL-stimulated GH release was less sensitive to the inhibitory effect of somatostatin than was GRF-stimulated GH release. In perfusions of rat anterior pituitary fragments, R-GAL induced a gradual and sustained increase of GH release. Incremental GH release derived in part from preformed stored GH. These data confirm that R-GAL acts at the pituitary level to stimulate GH release by a mechanism distinct from that of GRF.     

Development of the renal sexual segment in immature snakes: effect of sex steroid hormones

The renal sexual segment (RSS) of immature Northern and Diamondback Water Snakes and Red-Sided Garter Snakes exhibited varying responses to testosterone or 17beta-estradiol. In both male and female water snakes, kidney mass was not a reliable indicator of hormone treatment, whereas tubule diameter, epithelial height and number of sexual granules responded to hormone treatment. In male water snakes, either hormone initiated granule development by day 16; by day 23, only testosterone increased granule density. Female water snakes receiving either hormone exhibited a small number of granules by day 16; by day 23, granules increased only in Diamondback Water Snakes receiving testosterone. Hormones did not initiate RSS hypertrophy in female Red-Sided Garter Snakes. Tubule diameter and epithelial height of testosterone-treated males exhibited significant hypertrophy, while 17beta-estradiol initiated significant increases in tubule diameter. Garter snakes initiated sexual granule development in response to hormone treatment with males exhibiting a greater response than females and testosterone stimulating a greater response than 17beta-estradiol. Sex steroids appear to mimic sexual maturity in immature snakes initiating RSS development. Whereas the RSS of adult males respond to testosterone, our data suggest specific changes in the RSS of females during maturation effectively negates the effect of 17beta-estradiol evident in immature female RSS.     

Evidence for direct action of testosterone on rat liver cells: in vivo and in vitro induction of unusual estrogen-binding protein.

We demonstrate that on the rat liver, testosterone (T) induced differentiated functions and enhanced unusual estrogen-binding protein (UEBP) content through mechanisms dependent on cell activation by androgens, the presence of growth hormone (GH) and the hormonal status of the animal. To determine whether liver cells are a target for androgens, we measured T effects on UEBP in gonadectomized adult male and female rats in vivo and in vitro. In ovariectomized rats, T increased 8- to 9-fold UEBP levels that remained constant during 10 days. Also in vitro, using hepatocytes from ovariectomized rats, T alone increased UEBP levels 3-fold in a dose-response pattern. Combining a fixed low dose of GH with different concentrations of T increased UEBP 2-fold above T alone. Whereas GH alone had no effects in ovariectomized rats, hepatocytes were responsive to GH, in a dose dependent pattern that was abolished when T was used together with GH. On the other hand, T alone had no effect in hypophysectomized-ovariectomized animals. The latter group was rendered T responsive after the simultaneous injection of GH with T that increased UEBP content 6.6-fold in vivo. Castrated males revealed a marked responsiveness to T and GH in vivo and in vitro, when added separately or in combination. The results obtained suggest a complex regulatory system and we conclude that T acts directly on rat liver as: (1) an inducer of sex differentiation; and (2) a regulator of UEBP production in males. In addition, liver regeneration studies in castrated-hypophysectomized males revealed the UEBP phenotype in daughter cells in the absence of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

The molecular weight forms of rat growth hormone secreted in response to growth hormone-releasing factor

The different molecular weight forms of immunoreactive growth hormone (irGH) secreted by the anterior pituitary of rats were evaluated during basal and stimulated secretion in vitro and in vivo. Anterior pituitary cells maintained in a monolayer culture system secreted only a 22,000-Da form of GH based on Sephacryl S-200 column chromatography. This was also true for the irGH secreted in response to growth hormone-releasing factor and prostaglandin E2 stimulation. In contrast, the molecular weight forms of irGH found in plasma under basal conditions included an approximately 90,000-Da form as well as the expected 22,000-Da form. The concentrations of both forms increased following growth hormone-releasing factor stimulation although there was a shift in the ratio of the forms secreted. These results suggest that the larger molecular weight forms of rat GH observed in plasma may be the result of some postsecretory process which occurs in blood and suggests a possible regulatory function for the larger molecular weight forms as it pertains to the bioavailability of GH in vivo.     

Growth hormone releasing effect of hpGRF-40 in rats at different time intervals following ablation of the mediobasal hypothalamus

We have investigated the effect of hypothalamo-pituitary disconnection in the rat on the growth hormone (GH) responsiveness to human pancreatic GH-releasing factor (hpGRF). Adult female rats, sham-operated (sham-op) or bearing a complete mechanical ablation of the mediobasal hypothalamus (MBH-A) were challenged, while under urethane anesthesia, with hpGRF-40 (20,100,500 ng/rat i.v.) at different time intervals after surgery. In sham-op rats only 500 ng/rat of hpGRF-40 stimulated GH release, while in 1-and 7-day MBH-A rats the stimulation also occurred with the lower hpGRF doses and the rise in plasma GH was greater than in sham-op controls. Twenty-one and 42 days after the placing of the lesions the GH response to hpGRF-40 was still present at the 500 ng/rat dose, though it was smaller than in sham-op controls. Evaluation of pituitary GH content demonstrated a progressive and rapid decline starting the first day after the placing of the lesions. These data indicate that GH responsiveness to hpGRF is: 1) enhanced in the anterior pituitary shortly after hypothalamo-pituitary disconnection and, 2) despite a striking reduction of the pituitary GH stores, it is maintained after these lesions.The physiologic growth hormone (GH) releaser in the rat is GH-releasing factor and, recently, a group of peptides has been characterized from human pancreatic tumors (hpGRFs) (1,2) which are potent and specific GH-releasers in both animals (3) and man (4). The availability of these peptides, which show a high degree of homology with the physiologic rat hypothalamic GRF (5), offers the unique opportunity to assess somatotrope responsiveness to GRF molecules in rats with hypothalamo-pituitary disconnection.In this study we have first evaluated the GH pituitary responsiveness to increasing doses of hpGRF-40 in rats following mechanical ablation of the mediobasal hypothalamus (6). These rats, by definition, lack the effect of both central nervous system (CNS) inhibitory (e.g. somatostatin) and stimulatory (e.g. GRF) influences to GH release. With the aim to ascertain how the lack of these two opposing inputs reflects on the secretory capacity of the somatotropes, we also investigated the GH response to hpGRF-40 at different time intervals after the lesioning. In a study in rats with electrolytic lesions of the ventromedial-arcuate region of the hypothalamus Tannenbaum et al (7) had shown persistence of the GH response to huge doses of a hpGRF analog.     

Robust growth hormone (GH) secretion in aged female rats co-administered GH-releasing hexapeptide (GHRP-6) and GH-releasing hormone (GHRH)

Aging is associated with a blunted growth hormone (GH) secretory response to GH-releasing hormone (GHRH), in vivo. The objective of the present study was to assess the effects of aging on the GH secretory response to GH-releasing hexapeptide (GHRP-6), a synthetic GH secretagogue. GHRP-6 (30 micrograms/kg) was administered alone or in combination with GHRH (2 micrograms/kg) to anesthetized female Fischer 344 rats, 3 or 19 months of age. The peptides were co-administered to determine the effect of aging upon the potentiating effect of GHRP-6 on GHRH activity. The increase in plasma GH as a function of time following administration of GHRP-6 was lower (p less than 0.001) in old rats than in young rats; whereas the increase in plasma GH secretion as a function of time following co-administration of GHRP-6 and GHRH was higher (p less than 0.001) in old rats than in young rats (mean Cmax = 8539 +/- 790.6 micrograms/l vs. 2970 +/- 866 micrograms/l, respectively; p less than 0.01). Since pituitary GH concentrations in old rats were lower than in young rats (257.0 +/- 59.8 micrograms/mg wet wt. vs. 639.7 +/- 149.2 micrograms/mg wet wt., respectively; p less than 0.03), the results suggested that GH functional reserve in old female rats was not linked to pituitary GH concentration. The differential responses of old rats to individually administered and co-administered GHRP-6 are important because they demonstrate that robust and immediate GH secretion can occur in old rats that are appropriately stimulated. The data further suggest that the cellular processes subserving GH secretion are intact in old rats, and that age-related decrements in GH secretion result from inadequate stimulation, rather than to maladaptive changes in the mechanism of GH release.     

Dynamics of secretory granules in somatotrophs of rats after stimulation with growth hormone-releasing factor: a stereological analysis

The anterior pituitary tissue of male rats injected with growth hormone-releasing factor (GRF) was either processed for stereology at the light-and electron-microscopic levels, or homogenized for growth hormone (GH) assay 2–60 min after GRF injection. Secretory granules of somatotrophs became smaller but increased in numerical density 2 min after GRF injection. Their volume density began to increase at 5 min. The frequency of exocytosis of the granules was most prominent as early as 2 min after GRF injection and reduced thereafter. GH levels in the tissue were lowest at 2–5 min, and returned to the control value by 60 min. Serum GH levels were highest at 15 min; even at 60 min, this value was higher than in the controls. These findings suggest that secretory granules in somatotrophs are stimulated to divide by GRF, resulting in a decrease in size and an increase in number. The discrepancy between the earlier formation of new secretory granules and the later restoration of intracellular GH levels implies that GRF first stimulates the synthesis of constituents of granules other than GH, and only later the synthesis of GH, and that newly formed small secretory granules contain less GH. From the clearance rate of serum GH and the frequency of granule exocytosis, it can be estimated that about a half million granules are released to maintain 1 ng/ml of serum GH in rats.     

Effects of testosterone on hormonal content and calcium-dependent basal secretion in female rat pituitary cells

In vivo and in vitro effects of elevated androgens on agonist-induced gonadotropin secretion have been addressed previously. Here we investigated the effects of testosterone on hormonal content and basal (in the absence of agonists) hormone release in pituitary lactotrophs, somatotrophs and gonadotrophs from female rats. Furthermore we tested the hypothesis that testosterone action is dependent on the pattern of spontaneous and Bay K 8644 (a L-type calcium channel agonist) -induced calcium signalling. Mixed anterior pituitary cells were cultured in steroid containing or depleted media, and testosterone (1pM to 10nM) was added for 48h. Cells were studied for their spontaneous and Bay K 8644-induced calcium signalling pattern and total hormone levels (release and hormonal content). In lactotrophs, somatotrophs and gonadotrophs testosterone did not affect the pattern of spontaneous calcium signalling. Bay K 8644-induced calcium signalling and hormone release were not affected by testosterone. In both steroid-depleted and -containing medium, testosterone inhibited prolactin (PRL), luteinizing hormone (LH) and growth hormone (GH) cellular content and release in a dose-dependent manner, with IC(50)s in a sub-nanomolar concentration range. These results indicate that testosterone inhibits basal hormone release from lactotrophs, somatotrophs and gonadotrophs without affecting intracellular calcium signalling. This action of testosterone is not dependent on the presence of other steroid hormones.     

Inhibitory effects of cysteamine on neuroendocrine function

The action of cysteamine on anterior pituitary hormone secretion was studied in vivo using conscious, freely moving male rats and in vitro using anterior pituitary cells in monolayer culture. Administration of 500 micrograms cysteamine into the lateral cerebral ventricles of normal rats caused the complete inhibition of pulsatile GH secretion for a minimum of 6 h. This treatment also significantly decreased plasma concentrations of LH for at least 6 h in orchiectomized rat, TSH in short-term (0.5 month) thyroidectomized rats, and PRL in long-term (6 months) thyroidectomized rats. The in vivo stimulation of GH, LH, TSH and PRL with their respective releasing hormones 60 min after administration of cysteamine was not different from the response observed in rats pretreated with saline except for PRL where cysteamine pretreatment significantly inhibited the expected PRL increase. In vitro, 1 mM cysteamine decreased basal and TRH stimulated PRL release while not affecting basal or stimulated GH, LH, TSH and ACTH secretion. These data demonstrate the dramatic and wide-ranging effects of cysteamine on anterior pituitary hormone secretion. This action appears to be mediated through hypothalamic pathways for GH, LH and TSH and through a pituitary pathway for PRL.