Pituitary secretion of growth hormone in response to opioid peptides and opiates is mediated through growth hormone-releasing factor

Passive immunization of rats with an antiserum raised against rat growth hormone-releasing factor (GRF) completely inhibited the growth hormone (GH) response to morphine and beta-endorphin but did not alter the prolactin (PRL) response to those two stimuli. These results demonstrate that opiate and opioid peptide stimulation of pituitary GH secretion is mediated through hypothalamic GRF and presents an animal model in which the stimulated secretion of GH and PRL can be specifically dissociated.     

Evidence of a circulating growth hormone stimulating factor other than growth hormone releasing hormone in a patient with pituitary tumour and acromegaly

This study is a report on the growth hormone (GH) stimulatory effect of serum and plasma from a patient with notably active acromegaly due to a GH producing pituitary adenoma. Pituitary adenomatous tissue from 7 patients with GH producing adenomas, one with a prolactin (Prl) producing adenoma, one with a TSH producing adenoma, and one with a non-secreting adenoma, were cultured in vitro for 8-10 days. Media were changed every 48-72 h and contained Neumann Tytell buffer with the addition of 1) foetal calf serum, 2) patients' own serum or plasma, 3) serum or plasma from the patient with notably active acromegaly. GH release expressed as microgram GH/1/48-72 h between day 6 and 8 in culture did not differ when adenomatous tissue was cultured in buffer, foetal calf serum or the patients' own serum or plasma. In contrast, GH release was increased in 9/10 patients, when media contained serum or plasma from the patient with notably active acromegaly. This GH stimulatory effect was demonstrated in vitro in human pituitary adenomatous tissue from patients with pathological as well as normal GH secretion in vivo. Furthermore, this GH releasing plasma in a concentration of 10% increased GH release in cultures of dispersed rat anterior pituitary cells. In the same system, synthetic growth hormone-releasing hormone (GRF)-44 stimulated the release of GH in a dose-dependent manner. However, at all dose levels including maximally stimulating doses of GRF, an additive effect on GH release was seen with 10% of the GH releasing plasma.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth hormone (GH)-releasing factor does not regulate GH release or GH mRNA levels in GH3 cells

Previous studies have shown that GH-releasing factor (GRF) regulates both GH production and GH mRNA levels in primary cultures of rat pituitary cells. Investigations were carried out to ascertain the ability of GRF to regulate GH production or mRNA levels in a clonal strain of rat pituitary tumor (GH3) cells. Incubation of the cells with GRF at 1-1000 nM for 4 h to 10 days did not result in a stimulation of GH or PRL production, nor did it affect the cytoplasmic levels of the corresponding mRNAs. The lack of response to GRF was not affected by dexamethasone, T3, or serum. We conclude that GH3 cells do not provide a useful model system for studies of the mechanism(s) of action of GRF on either GH release or GH gene expression.     

Stimulation of growth hormone release by vasoactive intestinal peptide and peptide PHI in rat anterior pituitary reaggregates. Permissive action of a glucocorticoid and inhibition by thyrotropin-releasing hormone

In superfused rat anterior pituitary cell reaggregates, cultured for 5 days in serum-free defined medium, vasoactive intestinal peptide (VIP) concentration-dependently stimulated prolactin (Prl) release but had only a marginal effect on growth hormone (GH) release. When reaggregates were cultured in the presence of 80 nM dexamethasone (Dex) VIP strongly stimulated GH release from a concentration as low as 0.1 nM. VIP did not stimulate LH release. Peptide PHI also stimulated GH release but thyrotropin-releasing hormone (TRH) or angiotensin II did not. In fact, TRH slightly but transiently inhibited basal GH release and strongly inhibited VIP-stimulated GH release. GH-releasing factor (GRF) stimulated GH more potently and with higher intrinsic activity than VIP but GRF did not increase Prl release. The present data indicate that under defined hormonal conditions VIP and PHI are capable of stimulating GH release and that TRH can antagonize this effect by a direct action on the pituitary.     

Quantitative in-situ hybridization histochemistry studies on growth hormone (GH) gene expression in acromegalic somatotrophs: effects of somatostatin, GH-releasing factor and cortisol

We have examined the effects of human GH-releasing factor (1-44) (GRF), cortisol and somatostatin-(1-14) on GH gene expression in solid tissue and dispersed cells from human pituitary adenomas using quantitative in-situ hybridization histochemistry. Sections cut from tissue obtained at hypophysectomy from three acromegalic patients were hybridized to probes directed against mature alpha-subunit, GH, prolactin, pro-opiomelanocortin, TSH beta-subunit and LH beta-subunit mRNA. Only one biopsy contained GH mRNA in isolation. A second was found to coexhibit GH, prolactin and alpha-subunit mRNA, and a third was found to contain prolactin, TSH beta-subunit, alpha-subunit and LH beta-subunit mRNA, with GH mRNA below the limit of specific detection, indicating that the sample was composed of normal rather than adenomatous pituitary tissue. GH mRNA in individual dispersed cells derived from the latter declined to barely detectable levels over 287 h, both in cultures containing GRF (10 ng/ml) or GRF (10 ng/ml) plus somatostatin (10 ng/ml) and in controls, but increased fourfold in cultures containing GRF (10 ng/ml) plus cortisol (0.5 mumol/l). GH mRNA remained unchanged in both adenoma samples over 138 and 450 h, irrespective of the addition of GRF or GRF plus hydrocortisone. In these samples, somatostatin plus GRF had no consistent effect. These studies confirm that quantitative in-situ hybridization histochemistry can be used to investigate hormone gene regulation in small samples of human tissue and should enable us to define more clearly the level at which abnormal gene regulation occurs.     

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.     

Synergistic interaction in bovine pituitary cultures between growth hormone-releasing factor and other hypophysiotrophic factors

Synthetic human pancreatic GH-releasing factor (1-44)NH2 (GRF) and acetylcholine (ACh) were shown to evoke a dose-related release of GH from cultured bovine pituitary cells with half-maximal effective doses of 0.3 and 500 nmol/l respectively. Concentrations of ACh (10 mumol/l) and GRF (25 nmol/l) which were shown to give near maximal responses when presented alone, produced highly synergistic responses when tested in combination. This synergism was related to the ACh concentration employed, and both the ACh-induced release and ACh-induced synergism were abolished by the muscarinic antagonist, atropine. A synergistic interaction was also demonstrated between GRF and concentrations of thyrotrophin-releasing hormone (TRH) and bombesin which, in the absence of GRF, failed to elicit significant GH release. Acetylcholine stimulated a similar dose-dependent release of prolactin, but GRF was ineffectual in either directly stimulating prolactin release or affecting the response to ACh or TRH. No synergistic interaction could be detected between combinations of ACh and TRH or between ACh and bombesin. The data suggest that, in the somatotroph, GRF acts through a different second messenger pathway to ACh, TRH and bombesin and that these two pathways can be activated to produce a potentiated response. Growth hormone-releasing factor is, therefore, not only a specific GH secretagogue, but may act in concert with other hypophysiotrophic factors to regulate GH secretion from the bovine anterior pituitary.     

Isolation and characterization of hypothalamic growth-hormone releasing factor from common carp, Cyprinus carpio.

A growth hormone-releasing factor (GRF)-like peptide was isolated from the hypothalamus of common carp, Cyprinus carpio, by acid extraction, gel filtration chromatography, immunoaffinity chromatography using antiserum directed against rat GRF, and multiple steps of HPLC using octadecyl columns. Based on Edman degradation and peptide mapping, this teleost GRF was established to be a 45-residue peptide with the following primary structure: His-Ala-Asp-Gly-Met-Phe-Asn-Lys-Ala-Tyr-Arg-Lys-Ala-Leu-Gly-Gln-Leu-Ser- Ala-Arg - Lys-Tyr-Leu-His-Thr-Leu-Met-Ala-Lys-Arg-Val-Gly-Gly-Gly-Ser-Met-Ile-Glu- Asp-Asp-Asn-Glu-Pro-Leu-Ser. Carp GRF is closely related structurally to peptides of the glucagon-secretin superfamily, and more particularly to mammalian vasoactive intestinal peptide (VIP) precursors and the N-terminal portion of mammalian GRFs. A synthetic replicate of this peptide is highly potent [50% effective dose (ED50) approximately 0.08 nM] in stimulating GH release from cultured goldfish pituitary glands and in elevating serum GH levels 30 min after injection (0.1 micrograms/g) in goldfish.     

Pituitary vasoactive intestinal peptide regulates prolactin secretion in the hypothyroid rat

In this study, we demonstrated that the cell content and basal secretion of vasoactive intestinal peptide (VIP) in primary rat pituitary cell cultures were increased in hypothyroidism. VIP release from hypothyroid pituitary cells in vitro was stimulated by thyrotropin releasing hormone (TRH 10(-8) to 10(-6) M) and growth hormone (GH)-releasing hormone (GHRH 10(-9) to 10(-8) M) but not by corticotropin-releasing hormone or luteinizing hormone-releasing hormone in concentrations up to 10(-6) M. In the presence of anti-VIP antisera, there was a significant decrease in basal prolactin secretion from cultured hypothyroid pituitary cells (p less than 0.005) indicating that VIP exerts a tonic stimulatory effect on prolactin (PRL) secretion. The increment in PRL secretion following TRH was not affected by exposure to anti-VIP indicating that PRL release after TRH is not mediated by VIP at the pituitary level. In contrast to changes in PRL, exposure to anti-VIP had no effect on basal GH secretion, indicating that the PRL changes are hormone specific. Similarly, GHRH-induced GH release was unaffected by VIP immunoneutralization.     

Effects of growth hormone-releasing factor (1-44) on growth hormone release from human somatotrophinomas in vitro: interaction with somatostatin, dopamine, vasoactive intestinal peptide and cycloheximide

The effect of GH-releasing factor(1-44)(GRF) alone, or together with somatostatin (SRIF), dopamine (DA), vasoactive intestinal peptide (VIP) or cycloheximide was studied in a total of ten human somatotrophinomas using a static cell culture system. Growth hormone-releasing factor (2.0 X 10(-8) mol/l) significantly (P less than 0.05) stimulated GH release from nine out of ten tumours over 4-h incubations, and a dose-related effect (2.0 X 10(-10) -2.0 X 10(-8) mol/l) was observed in five tumours thus studied. Repeated GRF (2.0 X 10(-8) mol/l)-mediated GH release was seen during 96% (n = 25) of experiments performed on six tumours over 4 h and up to 27 days in culture. Growth hormone-releasing factor (2.0 X 10(-8) mol/l) also stimulated GH release from five out of seven somatotrophinomas during 60-min incubations. Somatostatin (6.1 X 10(-9) mol/l) completely inhibited GRF-induced GH secretion from four tumours studied over 4 h, but in each case there was significant (P less than 0.05) 'rebound' of GH release from cultures exposed to both GRF and SRIF during a subsequent recovery period. Dopamine suppressed basal GH release from two out of four tumours, but in each case had a greater inhibitory effect on GRF-mediated GH release. Vasoactive intestinal peptide directly stimulated GH release from two out of three tumours, and the effects were additive to maximal stimulatory doses of GRF. Cycloheximide significantly (P less than 0.01) enhanced GRF-stimulated release of GH during a 60-min incubation, but inhibited both basal and GRF-stimulated release over 4 and 8 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

RESOLUTION OF ACROMEGALY AFTER REMOVAL OF A BRONCHIAL CARCINOID SHOWN TO SECRETE GROWTH HORMONE RELEASING FACTOR

A 29 year old woman with an enlarged pituitary fossa and classical acromegaly, possibly present for ten years, had biochemical and partial somatic resolution of the disorder after removal of a bronchial carcinoid tumour. In addition, galactorrhea stopped, menstruation returned after two years, and amenorrhea and elevated prolactin levels fell towards normal.
Immunocytochemistry showed numerous growth hormone releasing factor (GRF) staining cells in the tumour. The tumour cells, when cultured, produced a supernatant selectivity stimulating human pituitary somatotrophic cell cultures to produce growth hormone (GH). The bronchial carcinoid did not secrete detectable GH, but extracts of it, and preoperative serum contained GRF immunoreactivity which coeluted with synthetic human pancreatic GRF.     

Changes in growth hormone secretion in spontaneously hypertensive rats

Secretion of the growth hormone (GH) in spontaneously hypertensive (SH) male rats has been determined and compared with that of normotensive Wistar-Kyoto (WKY) controls. In a first set of experiments, plasma GH concentration and pituitary GH content were measured in SH rats 30, 60 and 90 days old. 30-day rats showed reduced GH plasma and pituitary levels, whereas in 60- and 90-day-old rat no differences in GH plasma concentration and increased GH pituitary content were observed. In a second set of experiments, 90-day-old SH male rats anesthetized with sodium pentobarbital and intravenously injected with growth hormone-releasing factor (GRF) showed an amplitude and duration of response to injected hormone higher than WKY controls. In a third set of experiments, hemipituitaries of 90-day-old SH males were incubated for 2 h in Krebs-Ringer-bicarbonate either in the presence or in the absence of GRF. In the absence of GRF stimulation, the in vitro release of GH was higher than in WKY controls, whereas in the presence of GRF the sensitivity and the maximum response to GRF was reduced in comparison with normotensive male rats. These results indicate that SH rats have decreased pituitary content and plasma GH concentration before puberty. Besides, they showed increased pituitary GH content in adulthood and opposite changes in the in vivo and in vitro response to GRF.     

Growth hormone releasing factor, somatocrinin, releases pituitary growth hormone in vitro.

Purified (rat) hypothalamic growth hormone releasing factor (GRF), native human GRF isolated from an islet cell tumor of the pancreas that had caused acromegaly, and the synthetic replicates of the human material are potent secretagogues of immunoreactive growth hormone (GH) by primary cultures of rat pituitary cells. Native or synthetic peptides give identical dose-response curves, with identical slopes and identical maximal effects. The median effective dose of the tumor-derived GRF is 15 x 10(-12) M. The effect of hypothalamic GRF or of a synthetic replicate of tumor-derived GRF is immediate, being demonstrable in less than or equal to 30 sec after contact in a pituitary cell perifusion system. The effect of hypothalamic GRF or of tumor-derived GRF is highly specific for stimulating release of immunoreactive growth hormone; there is no demonstrable concomitant effect on the secretion of other pituitary hormones. Somatostatin-28 and somatostatin-14 inhibit the release of growth hormone produced by hypothalamic GRF or tumor-derived GRF in typical noncompetitive antagonism. On the basis of the results reported here, hypothalamic GRF and tumor-derived GRF are qualitatively indistinguishable in their ability to stimulate the secretion of immunoreactive growth hormone in vitro. The name "somatocrinin" is proposed to replace the acronym GRF.     

Growth hormone-releasing factor does not stimulate phosphoinositides breakdown in primary cultures of rat and human pituitary cells

It has been reported that rat growth hormone releasing factor (rat GRF-43), similarly to the two human GRFs (GRF-40 and 44) stimulates adenylate cyclase activity in pituitary cells. Controversial findings have been presented by two different groups on the action of GRF on phosphoinositides (PI) metabolism, a phenomenon linked to Ca-- mediated intracellular mechanisms. In the work to be reported, we evaluated the accumulation of inositol phosphates induced by GRF exposure in primary cultures of rat and human pituitary cells. Addition of rat GRF-43 to rat pituitary cells at doses up to 1 microM had no effect on inositol phosphates accumulation, while already at a dose as low as 0.05 nM it increased growth hormone secretion in the incubation medium significantly. In the same cell system, TRH, a known activator of PI breakdown, significantly increased [3H]inositol phosphates. In primary cultures of human somatotrophs from acromegalic subjects as in rats, addition of hpGRF-40 and also of TRH did not elicit any modification in the accumulation of [3H]inositol phosphates. Consistent with in vivo findings, both peptides induced a significant release of GH in the medium. Our results show that the GH releasing effect of GRF does not involve the hydrolysis of phosphatidylinositol in normal rat as well as in tumoral human somatotrophs. In addition it appears that the anomalous response of TRH on adenomatous cells from acromegalic patients is differently mediated in respect to the action of the tripeptide on normal lactotrophs and thyrotrophs.     

Effects of rat growth hormone (rGH)-releasing factor and somatostatin on the release and synthesis of rGH in dispersed pituitary cells

The effects of rat hypothalamic GH-releasing factor (GRF) and somatostatin (SRIF) on the release and biosynthesis of rat GH were studied by RIA and quantitative immunoprecipitation using monolayer cultures of rat anterior pituitary cells. In kinetic studies, GRF stimulation of GH release appeared at the first sampling time (20-min incubation) and the effect began to diminish after 2-h incubation with GRF. On the other hand, total (cell plus medium) content of GH significantly increased only after 24-h incubation. To examine the GH-synthesizing effect of GRF more directly, newly synthesized GH labeled by [35S]methionine during incubation with GRF was quantified by immunoprecipitation. The amount of immunoprecipitable GH increased significantly and specifically (compared with the total amount of labeled proteins) also only after 24-h incubation. When GH pools were labeled with [35S]methionine under different schedules, the basal release of newly synthesized GH, which was labeled for 1 h immediately before chase incubation was lower during the first 15 min than stored GH which had been labeled earlier. Basal newly synthesized GH secretion exceeded stored GH secretion after 30 min. GRF stimulated the release of GH from both pools but the stimulation of stored GH was greater. In this system, SRIF suppressed both the basal and stimulated release of GH but did not modify GH biosynthesis under either condition. Newly synthesized GH showed significant degradation during 24-h incubation; neither GRF nor SRIF affected the rate of GH degradation during the same incubation period. These results indicate that 1) GRF stimulates both release and synthesis of GH; 2) these two effects have different kinetics and different sensitivities to SRIF; and 3) GRF stimulates the release of GH from heterogeneous pools disproportionally.     

In vitro responses of rainbow trout (Oncorhynchus mykiss) somatotrophs to carp growth hormone-releasing factor (GRF) and somatostatin

To study the hypothalamic control of growth hormone (GH) release in lower vertebrates, we employed an in vitro technique using a monolayer cell culture system of rainbow trout pituitary glands. Two newly purified carp brain growth hormone-releasing factors, carp GRF(1-45) and carp GRF(1-29), and synthetic somatostatin-14 (SST-14) were applied to the cultured pituitary cells. The results indicate that: (1) The carp GRFs had a dose-related potency in stimulating growth hormone release. The dose of half maximum effect (ED50) for carp GRF(1-45) was 0.107 nM, and an equal potency for carp GRF(1-29) was 0.388 nM. (2) SST-14 inhibited GH release having a dose-dependent potency with an ED50 of 0.186 nM. (3) Osmotic pressure did not influence SST-14 inhibited GH secretion but did affect spontaneous GH release. (4) The response of cultured cells was not affected by length of incubation period with SST-14 or carp GRF but was affected by cell density.     

Effects of acute and prolonged glucose excess on growth hormone release by cultured rat anterior pituitary cells.

The aim of this study was to verify whether prolonged exposure of cultured rat anterior pituitary cells to high glucose can alter growth hormone (GH) release and responsiveness to secretagogues. Therefore, we cultured anterior pituitary cells obtained from normal male Sprague-Dawley rats in presence of normal (6 mM) or high (22 mM) glucose concentrations. After 3 days, the acute effects of glucose, growth hormone-releasing factor (GRF), dibutyryl cyclic AMP(db-cAMP) and somatostatin were studied during 2-hour incubations. High glucose did not alter basal GH release from cells cultured in 6 mM glucose. However, basal GH release from cells cultured in 22 mM glucose was moderately higher in the 2-hour incubation (by 46%) than in cells cultured in 6 mM glucose. In contrast, GH stimulation by GRF or db-cAMP was significantly reduced in cells cultured in 22 mM as compared to cells cultured in 6 mM glucose. This inhibitory effect of high glucose on GRF-stimulated GH release was completely reversible after 24 h of exposure of the cultured cells to 6 mM glucose and testing on the 4th day of culture. Finally, GH inhibition by somatostatin was also attenuated in cells cultured with high glucose. We conclude that prolonged exposure to high glucose could act directly at the pituitary level to modulate GH release and responsiveness.     

Interaction of carp growth hormone-releasing factor and somatostatin on in vitro release of growth hormone in rainbow trout (Oncorhynchus mykiss).

Possible antagonism between somatostatin (SS) and carp growth hormone-releasing factor (GRF) on growth hormone (GH) secretion was examined by radioimmunoassay in a dispersed rainbow trout pituitary cell culture system. SS (3 nM) significantly antagonized carp GRF(1-29; 1 nM, 10 nM)-induced GH secretion. The slope of the dose-response curve for carp GRF(1-29) with SS was statistically different from that of carp GRF(1-29) alone (p less than 0.05) suggesting a noncompetitive antagonism of SS to carp GRF. The carp GRF(1-29) was also indicated to be a noncompetitive antagonist to SS (p = 0.056). Carp GRF(1-29; 100 nM) was unable to restore the inhibitory effect of SS on GH release after pre-exposure of SS (30 nM) to the pituitary cells. We conclude that SS antagonizes carp GRF on GH release at the pituitary level in rainbow trout and this antagonism is noncompetitive. SS has a postantagonism to carp GRF which may implicate some important physiological adaptations in teleosts.     

Transgenic mice over-expressing galanin exhibit pituitary adenomas and increased secretion of galanin, prolactin and growth hormone

Galanin is a biologically active 29 amino acid peptide, widely distributed in the central and peripheral nervous system, and most abundantly in the hypothalamus where it may serve in the regulation of anterior pituitary hormones. We herein report that mice carrying the rat preprogalanin cDNA specifically targeted to the somatomammotroph cell lineage, under the control of the rat GH promoter, over-express and over-secrete galanin. Galanin peptide is localised within the GH and prolactin secretory granules. GH and prolactin release is increased as well, predominantly in males, while older transgenic animals develop pituitary hyperplasia and adenoma. In both male and female transgenic mice there is a significant increase in serum galanin (P<0.00003 and P<0.001 respectively) and prolactin (P<0.002 and P<0.05 respectively) levels, while only in male transgenic mice is there a significant increase in the serum levels of GH. Furthermore, in male transgenic mice serum prolactin levels are significantly correlated with the serum galanin levels (P<0.03). We conclude that galanin plays a key role in the process of pituitary hyperplasia, acting as a growth factor to promote pituitary cell proliferation, and participates in pituitary adenoma formation not necessarily dependent on oestrogens. Targeted over-expression and over-secretion of galanin in the somatomammotroph cell lineage stimulates predominantly hyperprolactinaemia in an oestrogen-independent manner.     

Roles of somatostatin and growth hormone-releasing factor in ether stress inhibition of growth hormone release.

In order to evaluate the release of somatostatin (SRIF) and growth hormone-releasing factor (GRF) into the pituitary gland in response to ether stress, a push-pull perifusion (PPP) technique has been used in freely moving rats. Push-pull cannulae (PPC) were implanted into the anterior pituitary (AP) glands of male rats. After a 7-day recovery period the rats were fitted with indwelling jugular catheters. The next day the animals were subjected to PPP of the AP during 1 h followed by ether stress (2 min) and another hour of perifusion. The perifusion flow was 20 microliters/min and 10-min fractions were collected and assayed for SRIF and GRF by RIA. Plasma growth hormone (GH) levels were assayed every 10 min. At the end of the experiments, the accuracy of PPC tip placements was ascertained with a dissecting microscope. Under basal conditions there were 2.9 pulses/h of SRIF with an amplitude of 12.76 +/- 0.46 pg. The output of SRIF and GRF in the 10-min period beginning with application of ether was increased 2-fold (p less than 0.005 and p less than 0.01, respectively). Interestingly, the increased release of SRIF continued for an additional 10 min, whereas GRF output decreased and was almost undetectable. The release of both GRF and SRIF had returned to basal values 20-30 min after stress. Mean plasma GH levels were significantly lowered 10 min after stress. Each of the 9 animals showed a restoration of pulsatile GH release to basal levels within 20-30 min after stress. Our findings provide compelling evidence that SRIF plays a prominent role in stress-induced inhibition of GH release in the rat by blocking the response to the transient elevation of GRF and continuing to suppress GH release for 20 min.