EFFECTS OF GRF(1–40) AND DOMPERIDONE ON GH SECRETION IN NORMAL MAN

In eight normal adult men pituitary secretion following GRF(1-40) was studied. GRF administration (50 micrograms i.v.) was followed by an increase in GH release with a peak value between the 15 and 60 min. No effects were noticed on LH, FSH, PRL, TSH and ACTH secretion. GH and PRL release was also studied after domperidone (DOM) (5 mg i.v./h), and GRF plus DOM. PRL increased significantly after DOM and GRF plus DOM. During GRF plus DOM a more marked GH release was observed in comparison with the hormone response to GRF alone at 15-45 and 120 min (P less than 0.05). This phenomenon was found in in six out of eight subjects studied. Mean peak and secretory area was greater (P less than 0.05) after GRF plus DOM than after GRF alone. These data suggest that GRF(1-40) at the dose used is a useful tool in the study of GH secretion. The GH pattern during GRF plus DOM seems to indicate that dopaminergic tone may play a direct inhibitory role on GH secretion in man.     

The effects of growth hormone-releasing factor (GRF) and dopamine on growth hormone (GH) secretion in acromegaly

GRF (1-40), dopamine (DA), DA plus GRF and placebo were administered to 6 acromegalic patients. The GRF administration induced a highly variable GH release (GH delta % 167.3 +/- 21.4; mean +/- SE). GRF did not provoke any change in PRL serum levels. During simultaneous GRF and DA administration GH release was found to be reduced (GH delta % 80.2 +/- 17.8) compared to that observed for a corresponding period of time after GRF alone (p less than 0.05). Our data underscore that in acromegaly the DA tonus inhibits GH secretion after GRF by acting directly at the pituitary level.     

Phorbol ester, not growth hormone releasing factor, consistently stimulates growth hormone release from somatotroph adenomas in culture

In order to study the mechanism of GH secretion from somatotroph adenoma cells, we have compared the effect of 12–O-tetradecanoyl phorboi-13-acetate (TPA) with that of growth hormone releasing factor (GRF) on GH secretion from human somatotroph adenoma cells cultured in monolayer. Pituitary adenoma cells were obtained from 13 patients with acromegaly undergoing surgery. On the 7th day of culture, the cells were exposed for 2 h to secretagogues. All 13 adenoma cell cultures (100%) responded to TPA (1·6–16·0 nmol/I) with a two- to six-fold increase in GH release (240·37% Increase of control: mean±SE). The response was detectable within 10 min, and was maximal at 2 h. Phosphollpase C (7·7 mmol/I) also stimulated a two-to ten-fold Increase In GH release in all four adenomas examined (100%). GH release was stimulated by GRF (2·0 nmol/I) in eight out of 12 adenoma cells (67%), but the magnitude of the responses to GRF (60·18% Increase of control: mean ± SE) were much smaller than that of TPA. Five out of 13 adenomas secreted detectable amount of PRL Into the medium and these five adenomas (100%) responded to TPA (16·0 nmol/I) with a two- to six-fold Increase. These observations indicate that the activation of protein kinase C is the consistent stimulator in GH and PRL secretion In human somatotroph adenoma cells. However, It is not determined whether the protein kinase C     

Inhibitory influence of thyrotropin releasing hormone administration on growth hormone response to low doses of growth hormone-releasing hormone in normal man

Literature data show that TRH may have either stimulatory or inhibitory actions on GH release according to pathophysiological conditions of the subject. In view of this dual effect of TRH, we studied the possible interaction of TRH and GRF on GH secretion. Six healthy male volunteers received iv in different occasions and in random order: 1) GRF 0.05 micrograms/Kg; 2) GRF 0.1 micrograms/Kg; 3) GRF 1 microgram/Kg; 4) GRF 0.05 micrograms/Kg + TRH 400 micrograms, simultaneously; 5) GRF 0.05 micrograms/Kg + TRH 20 micrograms, simultaneously; 6) GRF 1 microgram/Kg + TRH 400 micrograms, simultaneously, 7) the vehicle as control treatment. Blood samples were obtained at several time intervals and plasma GH, PRL and TSH were measured by RIA methods. Plasma GH significantly increased in all subjects after all the tested doses of GRF and after the combination of the highest and of the lowest doses of GRF + TRH (treatments 6 and 5). GH responses increased progressively with the dose of GRF administered, even if a clear dose-response relationship could not be demonstrated, owing to the considerable interindividual variability in the responsiveness. The administration of GRF 0.05 micrograms/Kg increased significantly plasma GH levels vs control treatment. The simultaneous administration of a low effective dose of GRF (0.05 micrograms/kg) plus a high dose of TRH (400 micrograms) was able to significantly inhibit the GH secretion elicited by GRF 0.05 micrograms/Kg alone. The other GRF + TRH combinations tested (treatments 5 and 6) did not modify the GH response to the same doses of GRF given alone. Plasma PRL and TSH did not change either after GRF at any dose or after the vehicle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sexual differentiation of growth hormone feedback effects on hypothalamic growth hormone-releasing hormone and somatostatin

To investigate possible sex differences in the feedback regulation of growth hormone (GH) secretion, concentrations of immunoreactive GH-releasing hormone (GRF) and somatostatin (SS) were measured in the median eminence (ME) and the hypothalamus of male and female rats bearing the MtTW15 tumor, which secretes high amounts of GH and prolactin (PRL). Four weeks after tumor implantation in male rats, the GRF concentration in the whole hypothalamus, including the ME, was decreased by 37% (0.29 +/- 0.02 vs. 0.46 +/- 0.02 ng/mg protein in intact male controls; p less than 0.001) and the concentration of SS was increased by 40% (11.5 +/- 0.7 vs. 8.1 +/- 0.3 ng/mg protein in male controls; p less than 0.01). In female rats, the presence of tumor for 4 weeks caused a smaller (18%) reduction in GRF concentrations (0.27 +/- 0.02 vs. 0.33 +/- 0.03 ng/mg protein in intact female controls; p less than 0.05) and no significant change in SS concentrations (10.2 +/- 0.08 vs. 9.7 +/- 0.8 ng/mg protein in female controls). Tumor-related changes in GRF and SS concentrations were also more pronounced in male rats than in females, when determined separately in the microdissected ME and in the remaining hypothalamus. These differences occurred despite similar increases in serum GH, PRL and insulin-like growth factor I concentrations in male and female tumor-bearing rats. To assess which hormone (GH or PRL) was responsible for these changes, intact male rats were treated for 10 days with 2 daily s.c. injections of rat GH (rGH; 100 and 250 micrograms/day), rat PRL (100 and 250 micrograms/day) or vehicle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Potentiation of cholinergic tone by pyridostigmine bromide re-instates and potentiates the growth hormone responsiveness to intermittent administration of growth hormone-releasing factor in man

It is known that in normal subjects repeated administration of the growth hormone-releasing factor (GRF) induces a state of partial refractoriness of the somatotropes to GRF. Studies were conducted to verify whether the cholinergic system plays a role in the mechanism(s) underlying the reduced GH responsiveness to the neuropeptide. In five healthy men, the GH response to three consecutive injections of GRF (50 micrograms iv), administered at 2 h intervals, was considerably blunted after the second and third GRF bolus. Administration of the inhibitor of cholinesterase, pyridostigmine bromide (120 mg orally) 30 min before the second GRF bolus, not only restored but greatly potentiated the GH responsiveness to the second GRF bolus. The GH response to the third GRF bolus was not apparently influenced by pre-treatment with pyridostigmine. These data reinforce the view that cholinergic neurotransmission plays an important role in the control of GH secretion in human.     

Potency and specificity of a growth hormone-releasing factor in a primate and in vitro

The potency and specificity of the 44-amino acid human pancreatic tumor GRF were tested in six adult female rhesus monkeys and in a perifusion system containing a suspension of rat pituicytes. In vivo, plasma GH levels were elevated in a dose-dependent fashion, with an ED50 of approximately 5 micrograms/kg, a value of the same order of magnitude as other hypothalamic releasing hormones. The magnitude of the GH response after GRF treatment was similar to that observed during insulin-induced hypoglycemia, with peak plasma GH concentrations occurring 5-15 min after GRF administration. High doses of GRF slightly stimulated PRL release, but had no effect on arterial blood pressure, heart rate, or plasma cortisol or glucose concentrations. In vitro, GRF released GH in a dose-dependent manner, but no PRL was released even at the highest GRF concentrations employed (100 nM). It thus appears that stimulation of PRL in vivo may be an indirect effect of GRF. Alternatively, there may be species differences in responsiveness to GRF.     

Growth hormone response to growth hormone releasing factor in Alzheimer's disease

Because of the well-established reduction in the concentration of somatostatin in several brain areas of patients with histologically verified Alzheimer's disease, we sought to determine if growth hormone (GH) secretion is altered in Alzheimer's disease. In order to study this, we assessed the GH response to growth hormone releasing factor (GRF) in 8 patients with Alzheimer's disease and 8 age-matched controls. Although there was no difference between the magnitude of the GRF-induced GH response (delta max GH response or area under the curve) between the Alzheimer's disease patients and the controls, the Alzheimer's patients exhibited a delayed GH response to GRF.     

Plasma growth hormone response to human growth hormone releasing factor in rats administered with chlorpromazine and antiserum against somatostatin. Effects of hypo- and hyperthyroidism

The effect of hypo- and hyperthyroidism on the plasma growth hormone (GH) response to synthetic human growth hormone releasing factor (GRF) was determined in conscious, freely moving rats pretreated with chlorpromazine and antiserum against somatostatin. Chlorpromazine plus somatostatin antiserum pretreated rats gave consistent response to GRF which was not observed in untreated rats. Chlorpromazine alone has no effect on GH secretion induced by GRF in rat pituitary monolayer culture. In rats made hypothyroid by thyroidectomy, both basal and peak plasma GH responses to a small (0.25 microgram/kg bw) and a moderate dose of GRF (1 microgram/kg bw) were significantly reduced as compared to controls. In rats made hyperthyroid by the administration of thyroxine, basal and peak plasma GH responses to a small but not to a moderate dose of GRF were significantly reduced as compared to controls. A reduced plasma GH response to a small dose of GRF was observed 8 days after the cessation of thyroxine administration. The pituitary GH reserve was markedly reduced in hypothyroid but not in hyperthyroid rats as compared to their respective controls. These results indicate that plasma GH response to GRF is reduced both in hypo- and hyperthyroidism. The mechanism involved in the phenomenon appears to be different between the two conditions.     

Adenohypophysial changes in mice transgenic for human growth hormone-releasing factor: a histological, immunocytochemical, and electron microscopic investigation

The effect of protracted GH-releasing factor (GRF) stimulation on adenohypophysial morphology was investigated in six mice transgenic for human GRF (hGRF). All animals had significantly higher plasma levels of GH and GRF and greater body weights than controls. Eight-month-old mice were killed, and the markedly enlarged pituitaries were studied by histology, immunocytochemistry, electron microscopy, and immunogold method, using double labeling at ultrastructural level. In all pituitaries, a massive hyperplasia, chiefly of mammosomatotrophs, was found. These bihormonal cells, containing GH and PRL, were demonstrated by light microscopy and ultrastructural immunocytochemistry. Electron microscopy revealed the presence of cells with characteristics of GH cells in three pituitaries and cells resembling human adenomatous mammosomatotrophs in the other three glands. All of these cells, regardless of their ultrastructural features, contained secretory granules heavily labeled for GH by immunogold technique; PRL labeling varied from cell to cell, with the predominance of a weak immunostaining and was colocalized with GH in secretory granules. These results indicate that chronic exposure to GRF excess leads to mammosomatotroph hyperplasia. It is suggested that GH cells proliferate and transform to mammosomatotrophs in response to GRF stimulation. Focal PRL cell hyperplasia noted in three pituitaries could also be due to a GRF effect. Longer exposure to GRF is needed to clarify whether GRF can cause adenoma.     

Intrahypothalamic actions of somatostatin and growth hormone releasing factor on growth hormone secretion

Possible effects of GRF on somatostatin neurons and of somatostatin on GRF neurons were examined by measuring the effects of localised intracerebral injections of these peptides on growth hormone (GH) secretion. Serial GH concentrations were measured in plasma in unanaesthetized male rats chronically prepared with venous and intracerebral cannulae, before and after treatment with bilateral intracerebral injections of somatostatin or GRF in the preoptic anterior hypothalamic area (PO/AHA) and medial basal hypothalamus. Injections of 0.1 and 1 nmol of GRF in medial basal hypothalamus or 10 nmol somatostatin in the PO/AHA, respectively, had stimulatory or inhibitory effects on GH, which were assumed to be due to diffusion of the peptide from the injection site to the median eminence and pituitary gland. Injection of lower doses of somatostatin around GRF neurons in the medial basal hypothalamus were without significant effect on secretion of GH, but 0.1 nmol somatostatin in the PO/AHA resulted in an increase in GH concentrations from 128 +/- 61 to 524 +/- 103 ng/ml, p less than 0.02. Injections of GRF in lower doses amongst somatostatin neurons in the PO/AH or amongst GRF neurons in the medial basal hypothalamus were both without effect on GH secretion. We conclude that somatostatin may stimulate GH secretion by an effect on or close to somatostatin neurons in the PO/AHA. Somatostatin, though present in terminals on GRF neurons, is without effect at this site in our model. Furthermore, we have been unable to demonstrate any significant intrahypothalamic effect of GRF on GH regulation.     

Post-somatostatin rebound secretion of growth hormone is dependent on growth hormone-releasing factor in unrestrained female rats

To examine the characteristics of GH secretion following the termination of the infusion of somatostatin, unrestrained adult female Wistar rats were subjected to repeated infusions of somatostatin separated by 30-min control periods. When somatostatin was infused for 150 min at a dose of 3, 30 or 300 micrograms/kg body wt per h, the magnitude of the rebound GH secretion increased in a dose-dependent manner. The infusion of somatostatin at a dose of 300 micrograms/kg body wt per h for 60, 150 or 240 min progressively augmented the size of the rebound GH secretion. When an antiserum to rat GH-releasing factor (GRF) was injected i.v. 10 min before the end of the infusion, the peak amplitude of the rebound GH secretion (300 micrograms/kg body wt, 150 min) was reduced to less than 20% of that of control rats. The rebound GH secretion (300 micrograms/kg body wt per h, 150 min) was augmented by a bolus injection of human GRF (1 microgram/kg body wt). The combined effect of the end of infusion of somatostatin and a bolus injection of GRF on the amount of GH secreted was additive. The plasma GH response to GRF was completely inhibited when human GRF (3 micrograms/kg body wt per h) and somatostatin (300 micrograms/kg body wt per h) were infused simultaneously for 150 min. The magnitude of the rebound GH secretion following the termination of the co-administration was larger than that following the somatostatin infusion alone, but this rebound was not enhanced by a bolus injection of human GRF.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth hormone and prolactin secretion in cultured somatomammotroph cells

A somatomammotropic cell line (P0) derived from adult rat pituitaries has been maintained in culture for 2 yr. Secretion of GH and PRL by this cell line has been studied in response to hypophysiotropic peptides known to affect the release of both hormones as well as agents that affect second messenger systems in an attempt to characterize the stimulus-secretion mechanisms used by these cells. GH and PRL release during short term (4 h) incubations of P0 cells and primary cultures of dispersed rat pituitary cells was initially measured in response to GRF, TRH, vasoactive intestinal peptide (VIP), and SRIF. In P0 cells, the minimal effective dose of each of the hypophysiotropic peptides was comparable with respect to GH and PRL secretion. The effects of TRH and VIP were similar to those in freshly dispersed cells with respect to PRL release, whereas those of GRF and SRIF were less potent with respect to GH release. The stimulation of GH and PRL release in P0 cells by adenylate cyclase-related agents ((Bu)2 cAMP and forskolin) was comparable to that for GH secretion in mature somatotrophs but much greater than that of PRL release in mature lactotrophs. Stimulation of GH and PRL release in P0 cells by protein kinase C-related agents (diacylglycerol and phorbol ester) was also similar to that observed for GH release from mature pituitary cells, whereas minimal or undetectable effects were observed on PRL release from mature cells. The results indicate that the P0 somatomammotropic cell line possesses receptors, second messenger systems, and secretory characteristics of both somatotrophs and lactotrophs, although where differences exist, there is more resemblance to somatotrophs. They also demonstrate that the responses to each of the agents studied are bihormonal and appear to be regulated by a common mechanism.     

Pre-exposure of rat anterior pituitary cells to somatostatin enhances subsequent growth hormone secretion.

The precise roles of GH-releasing factor (GRF) and somatostatin (SRIF) in the orchestration of pulsatile GH secretion have not yet been fully determined. We examined the interactions of rat GRF and SRIF in the concentration ranges present in rat hypophysial-portal blood, on the secretion of GH from dispersed male rat anterior pituitary cells in monolayer culture. The effects of exposing cells to GRF and/or SRIF (0.01-1.nmol/l) for 1 h were compared with the effects of preincubation of cells with SRIF before experimental incubations. As anticipated, the stimulatory effects of 0.1-1 nmol GRF/1 were abolished by concurrent incubation with SRIF at an equimolar concentration, although SRIF, at these concentrations, did not significantly inhibit basal GH secretion. Conversely, pre-exposure to 0.1 nmol SRIF/1 for 30 or 60 min, resulted in an increase in GH secretion during a subsequent 60-min incubation period, both in the absence or in the presence of GRF (0.01-1 nmol/l). Pretreatment with GRF caused increased responsivity to GRF rather than significant sensitization of the GH response to GRF. These observations demonstrate actions of SRIF, at low and probably physiological concentrations, which are more complex than those of a pure inhibitor of GH secretion. Pre-exposure of the pituitary to SRIF enhances subsequent GH secretion, suggesting that SRIF may play an additional physiological role in amplifying the GRF signal.     

GROWTH HORMONE-RELEASING FACTOR INCREASES SERUM PROLACTIN CONCENTRATIONS IN NORMAL SUBJECTS AND IN PATIENTS WITH PITUITARY ADENOMAS

We have examined the serum growth hormone (GH) and prolactin (PRL) response to growth hormone releasing factor (hGRF-(1-44)NH2 (GRF) 1 microgram/kg i.v. bolus) in 16 acromegalic patients (eight of whom were hyperprolactinaemic), 13 patients with microprolactinoma, and 14 healthy subjects. The GH responses to TRH and to the somatostatin analogue SMS 201-995 were also studied in acromegalic patients. In these, and in patients with microprolactinoma, GH responses after GRF (P less than 0.001 vs saline) were variable. The absolute GH increase (calculated as area under the curve) in acromegalic patients (2489 +/- 920 micrograms/l min), or in patients with microprolactinoma (1322 +/- 279 micrograms/l min) was not different from that in controls (2238 +/- 633 micrograms/l min). In addition, a significant increase in PRL release was observed after GRF in comparison to saline in acromegalic patients (P less than 0.01), in patients with microprolactinoma and in normal subjects (P less than 0.001). The PRL increase was significantly correlated with basal PRL levels in acromegalic patients (r = 0.99, P less than 0.001) and in patients with microprolactinomas (r = 0.61, P less than 0.05). Furthermore, a significant correlation was found between GH rise after GRF and basal GH, and between GH rise after GRF and GH decrement after SMS in patients with acromegaly. These results suggest that GRF can stimulate PRL release by actions on the normal pituitary and on pituitary adenomas, including microprolactinomas. Moreover, the data suggest that in acromegaly there is a relative functional deficiency of hypothalamic somatostatin.     

Effects of intraventricular growth hormone-releasing factor on growth hormone release: further evidence for ultrashort loop feedback

We examined the effects of cerebroventricular injection of synthetic human GH-releasing factor [hGRF-(1-44)] on regulation of GH release in conscious male rats. These results were compared with the direct effects of hGRF on hormone released from dispersed anterior pituitary cells. Administration of two higher doses of hGRF (200 and 2000 ng) into the third ventricle (3V) produced a dose-related increase in plasma GH levels (P less than 0.001). Injection of hGRF into the 3V at two lower doses actually reduced GH release. Infusion of 20 ng (5 pmol) hGRF reduced plasma GH from 5-60 min (P less than 0.005), with a maximum suppression of 66%. The 2-ng (0.5-pmol) dose decreased GH secretion by 45% (P less than 0.05). hGRF stimulated a significant and dose-dependent release of GH from dispersed pituitary cells at concentrations of 10(-10) and 10(-9) M (P less than 0.025). The specificity of GRF for GH control, whether stimulatory or inhibitory, was seen by the failure of GRF to modify PRL, TSH, or LH release. Our results indicate that injection of larger doses of GRF into the 3V produce GH release, but at lower doses, 3V GRF may exert an action centrally to inhibit GH release. We propose that hypothalamic GRF may decrease its own neurosecretion by negative ultrashort loop feedback.     

Effect of estrogen on the growth hormone (GH) secretory response to GH-releasing factor in the castrate adult female rat in vivo

Five groups (n = 11) of 250-g female rats were oophorectomized and immediately thereafter received daily sc injections of estradiol benzoate (EB; 0.05, 0.5, 5.0, and 50.0 micrograms) or vehicle for 28 days. A sixth group underwent sham operation and received injections of vehicle. Somatomedin-C (SmC) concentrations were determined before EB administration. After 4 weeks of EB treatment, the GH response to human GH-releasing factor (1-44) (GRF; 5 micrograms/kg, iv) was determined under pentobarbital anesthesia in seven animals from each group. Serum PRL, LH, and estradiol and plasma SmC concentrations were also measured. The GH secretory response to GRF (delta GH) was greatest in castrated animals receiving vehicle (P less than 0.05) and was significantly blunted in animals receiving 5.0 and 50.0 micrograms EB (P less than 0.05) compared to that in sham-operated animals. A significant negative correlation was observed between delta GH and serum PRL concentrations (r = -0.53; P less than 0.0001). SmC concentrations after treatment were significantly lower in animals receiving 5.0 and 50.0 micrograms EB (P less than 0.01), than in sham-operated animals and were elevated compared to those in sham-operated controls in the group receiving the lowest dose of EB (0.05 microgram; P less than 0.01). Posttreatment SmC levels correlated positively with delta GH (r = 0.58; P less than 0.001) and negatively with serum estradiol concentrations (r = -0.47; P less than 0.01). Pituitary glands from the remaining animals in each group (n = 4) were weighed and assayed for GH, PRL, and LH content. Pituitary PRL content increased with increasing doses of EB replacement and correlated strongly (r = 0.82; P less than 0.0001) with pituitary weight. In the castrated adult female rat, high doses of estrogen inhibited the GH secretory response to GRF in vivo and decreased SmC concentrations. Low dose estrogen increased SmC concentrations, although the GH secretary response to GRF in this group was similar to that in sham-operated rats. The latter observation suggests that the rise in SmC levels associated with low dose estrogen may not be mediated through a change in GH secretion.     

Cellular mechanism of caprylic acid-induced growth hormone suppression

To determine the dynamic secretory pattern of growth hormone (GH) in the presence of free fatty acids (FFA), we studied the effect of caprylic acid on basal and rGRF(1-29)NH2 (GRF)-induced GH secretion in acutely dispersed and perifused rat pituitary cells. At a concentration of 3.0 mmol/L, caprylic acid inhibited both basal (P less than .05) and GRF-stimulated GH secretions (P less than .01), except when the maximal (near the EC100) GRF concentration of 100 pmol/L was used. Lower concentrations of caprylic acid such as 0.3 and 1.0 mmol/L significantly inhibited, in a concentration-dependent manner, GH secretion induced by a 6.25-pmol/L GRF. However, at a GRF concentration of 25 pmol/L, this inhibitory effect was abolished. The time-course of GH response to GRF was similar in both control and caprylic acid-treated cells. To elucidate the mechanism(s) of action of the caprylic acid-induced blockade of GH secretion, in 3-day cultured rat pituitary cells, the effect of caprylic acid on basal and GRF-stimulated GH and 3',5'-cyclic adenosine monophosphate (cAMP) release. We also tested its effect on the Ca2+ ionophore, A23187-induced GH release. Caprylic acid (0.3 to 3.0 mmol/L) significantly inhibited basal GH release and GRF- or A23187-induced GH secretion. Furthermore, it decreased both basal and GRF-stimulated cAMP release (P less than .05). In addition, the effect of caprylic acid on rGRF(1-29)NH2 affinity to GRF pituitary binding sites was determined using [125I-Tyr10]hGRF(1-44)NH2 as radioligand.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endogenous testosterone enhances growth hormone (GH)-releasing factor-induced GH secretion in vitro

The influence of endogenous gonadal steroids in male and female rats on basal and growth hormone-releasing factor (GRF)-stimulated GH secretion from perifused anterior pituitaries was studied. After 75 min of perifusion with basal medium, freshly dissected pituitaries were exposed to human GRF(1-44) (10 nmol/l) for 15 min. Neonatal (day 1-2) or prepubertal (day 25) gonadectomy of male rats suppressed baseline GH release (ng/min per mg dry weight) as well as GRF-stimulated GH release by 40-70%. This effect was slightly more pronounced in neonatally gonadectomized animals. In prepubertally gonadectomized male rats, the suppression of GH release was completely reversed by testosterone replacement therapy. In female rats, prepubertal gonadectomy did not affect GH secretion from perfused pituitaries. However, treatment of ovariectomized female rats with oestradiol reduced baseline and GRF-induced GH release to levels lower than those observed in sham-operated or vehicle-treated ovariectomized animals. The data suggest that testicular androgen secretion in adult male rats increases the pituitary GH release in response to GRF in vitro, whereas ovarian oestrogen secretion is of less importance for the GRF responsiveness of female rat pituitaries.     

Restoration of juvenile baseline growth hormone secretion with preservation of the ultradian growth hormone rhythm by continuous delivery of growth hormone-releasing factor.

The ability of continuously delivered GH-releasing factor (GRF) to enhance GH secretion while maintaining the normal ultradian GH rhythm was investigated. Synthetic human GH-releasing factor (hGRF(1-44)NH2) was continuously infused for 4 days by means of i.v. catheters to 11-week-old broiler chickens. At this age, overall endogenous GH secretion is low, and baseline GH is barely detectable. Six birds per treatment received vehicle (control), 0.324 mg hGRF(1-44)NH2/kg body weight per day (low dose) or 3.24 mg hGRF(1-44)NH2/kg body weight per day (high dose). After 4 days of GRF conditioning, concurrent with continued GRF infusion, serial blood samples were removed via atrial catheters at 15-min intervals for 6 h and GH plasma profiles determined. High dose GRF significantly increased GH plasma concentrations over tenfold compared with controls; however, most of this increase reflected an increase in basal GH, which was reinstated to juvenile baseline levels. Augmentation of pulse amplitude above this increased baseline was not proportionately as high, and failed to reach juvenile levels. The ultradian rhythm of GH was not altered by continuous GRF administration. Both low and high dose GRF treatments resulted in significant enlargement of the anterior pituitary gland. Total pituitary GH mRNA levels, although elevated over twofold by GRF treatment, were not significantly different from controls. Measures of plasma GH magnitude (overall and baseline mean, and peak amplitude) were significantly correlated with pituitary GH mRNA for control birds, but were not correlated for GRF treatments. Feed intake was markedly depressed (33%) on the high dose GRF treatment, in conjunction with total inhibition of body weight gain over the 4-day period of administration. Longitudinal bone growth and width of the epiphyseal growth plate were also significantly reduced by high dose GRF treatment, probably reflecting the reduced level of nutrient intake, despite high circulating concentrations of GH.