Human Pancreatic Growth Hormone Releasing Research Articles

Localization of growth hormone releasing factor-like immunoreactivity in the hypothalamo-hypophyseal system of the frog ( Rana temporaria) and the Sea Bass ( Dicentrarchus labrax)

Using two different antisera, one raised against total human pancreatic growth hormone releasing factor (hpGRF) coupled through a two-step glutaraldehyde method and the other one raised against rat hypothalamic growth hormone releasing factor 1–10 (rGRF1-10), GRF-like immunoreactivity was localized in the hypothalamo-hypophyseal system of the frog ( Rana temporaria) and the sea bass ( Dicentrarchus labrax). In the frog immunoreactive neurons were found in the nucleus preopticus, pars magnocellularis. The immunopositive fibers were localized in the lateral wall of the preoptic recess, the pars ventralis of the tuber cinereum, the internal and external zone of the median eminence, and the neural lobe. Positive-stained neurons in the sea bass were located in the preoptic nucleus, in the pars magnocellularis as well as in the pars parvocellularis, and in the nucleus lateralis tuberis, pars rostralis. GRF-ir nerve fibers, originating in the hypothalamus, projected to the rostral and proximal pars distalis, the posterior neurohypophysis, and the pars intermedia (PI). Double stainings with anti-GRF and anti-ACTH or anti-trout GH showed some close relationship between GRF immunoreactive nerve fibers and adenohypophyseal cell types. In the PI both the MSH and the PI “PAS” positive cells seemed to be directly innervated by the GRF-ir axons. These results show that a GRF-like system is present in the hypothalamohypophyseal system of amphibians and teleosts and that in these lower vertebrates GRF-like material may be secreted directly in the systemic circulation. The function of this GRF, however, is not yet clear.

Hypothalamic hormones that release growth hormone stimulate hepatic 5'-monodeiodination activity in the chick embryo.

Plasma GH, tri-iodothyronine (T3), thyroxine (T4) and liver 5'-monodeiodination (5'-D) activity were measured in 18-day-old chick embryos injected with thyrotrophin-releasing hormone (TRH) and human pancreatic growth hormone releasing factor (hpGRF). Injections of 0.1 and 1 microgram TRH and 1.5 micrograms hpGRF increased the concentration of plasma GH while injection of 15 micrograms hpGRF had no effect. Concentrations of plasma T3 were raised after injection of TRH or hpGRF. Injections of TRH but not of hpGRF raised the concentration of plasma T4. The increases in concentration of plasma T3 after injection of TRH or hpGRF were parallelled by increases in liver 5'-D activity. An injection of 0.25 micrograms T4 significantly raised the concentration of T4 in plasma but had no effect on plasma T3 or liver 5'-D activity. It is concluded that the release of chicken GH by TRH or hpGRF is responsible for the observed increase in plasma concentration of T3 and liver 5'-D activity.

Growth hormone releasing factor evokes rhythmic hyperpolarizing currents in rat anterior pituitary cells.

1. The effect of human pancreatic growth hormone releasing factor (hpGHRF) on the electrical activity of dissociated rat anterior pituitary cells in culture was studied, using both the cell-attached and whole-cell modes of the patch-clamp recording technique. 2. To avoid possible wash-out of the responses, extracellular records were made from cell-attached patches. Application of hpGHRF to the cells produced rhythmic inward currents through the patches, attributable to rhythmic hyperpolarizations of the cell membrane outside the patch. The amplitude of the current oscillations was 1-8 pA and the frequency 0.05-0.4 Hz. 3. Flooding the cells with K+ ions from a small pipette containing 50 mM or 100 mM-K+ resulted in a reversible attenuation or block of the rhythmic inward currents evoked by hpGHRF, indicating that changes in K+ conductance were involved in the responses. 4. Flooding the cells with a solution containing 10 mM-EGTA blocked these rhythmic inward currents reversibly, suggesting the involvement of Ca2+ in the responses. In addition, responses were blocked by adding Co2+ (5-10 mM) to the bathing medium. The presence of tetrodotoxin (3 microM) had no effect, ruling out the participation of voltage-gated Na+ channels. 5. With whole-cell recording, the resting potential (-41.46 +/- 7.78 mV) and input resistance (5.34 +/- 3.73 G omega) of anterior pituitary cells in culture were found to be similar to those previously reported for pituitary cells and chromaffin cells with the same recording method. 6. In whole-cell experiments, application of hpGHRF (shortly prior to intracellular penetration) evoked rhythmic outward currents, associated with conductance increases, when the cells were clamped at their resting potential. The persistence of these currents in the 'voltage-clamped' cell indicated that the rhythmicity was not related to voltage-dependent phenomena. The currents disappeared within 4 min after breaking into the cell, presumably because of 'washout' of cell constituents into the pipette. 7. The reversal potential (-60 mV) of the hpGHRF-induced currents was negative to the resting potential of the cells (-41 mV), further indicating that hpGHRF would evoke rhythmic hyperpolarizations in unclamped cells, possibly due to periodic increases in K+ conductance. 8. The possible relation of these rhythmic currents to hpGHRF-induced secretion of growth hormone is discussed.

Influence of sex and age on growth hormone (GH) secretion stimulated by human pancreatic growth hormone releasing hormone (hpGRF)

Influence of sex and age on growth hormone (GH) secretion stimulated by human pancreatic growth hormone releasing hormone (hpGRF)

Vasoactive intestinal polypeptide regulation of rabbit renal adenylate cyclase activity in vitro.

1. The effect of vasoactive intestinal polypeptide (VIP) upon adenylate cyclase activity was determined in purified cortical basolateral membranes and in glomeruli and tubular elements obtained from rabbit kidney. 2. In purified basolateral membranes prepared from cortex, 1 microM-VIP consistently stimulated adenylate cyclase activity above basal levels (1.55 +/- 0.09-fold (mean +/- S.E. of mean), n = 10 animals). Half-maximal stimulation was observed at 17 +/- 11 nM-VIP (S.D., n = 9). 3. Related peptides, e.g. secretin, glucagon, gastric inhibitory peptide, human pancreatic growth hormone releasing factor, and peptide having N-terminal histidine and C-terminal isoleucine amide (PHI), were without effect or gave lower stimulations of adenylate cyclase activity when tested at 1 microM. 4. Significant VIP degradation was observed under the assay conditions used but this did not substantially alter the response or selectivity to VIP. 5. In separate preparations of isolated glomeruli and proximal tubules addition of 1 microM-VIP resulted in a 3.3 +/- 1.1-fold (S.D., n = 3) and 2.2 +/- 1.0-fold (S.D., n = 3) stimulation (respectively) of adenylate cyclase activity. 6. In isolated medullary tubule suspensions, isolated by collagenase-hyaluronidase digestion of outer (red) medulla, and in thick ascending-limb-enriched preparations prepared by Percoll density gradient fractionation, 1 microM-VIP significantly increased adenylate cyclase activity by 2.4 +/- 0.6-fold (S.D., n = 3) and 2.1 +/- 0.7-fold (S.D., n = 3) respectively. 7. A possible role for VIP in the regulation of renal function in the rabbit is discussed in relation to the occurrence of VIP stimulation of adenylate cyclase activity in several renal cellular elements.

Growth hormone releasing factors and secretion of growth hormone in sheep, calves and pigs

Human pancreatic growth hormone releasing factors (hpGRF) (1–40) and (1–44) were administered iv in sheep, pigs and calves to determine their effectiveness in stimulating GH release in these species. Both peptides produced a rapid increase in plasma GH concentration in all three species at dose levels ranging from .0065 to .65 nmol/kg. Moreover, there was no difference in the GH-secretory response observed between hpGRF(1–44)NH 2 and (1–40)OH in sheep. Sheep also responded to hpGRF(1–40)NH 2 and (1–40)OH as well as [his 1]- and [tyr 1]-hpGRF(1–40)NH 2 in a similar manner. Rat hypothalamic GRF was less effective than [his 1]-hpGRF(1–40), while the response to bGRF was not significantly different from hpGRF(1–40) in stimulating GH secretion in sheep. Although all three species responded to hpGRF, the elevation in plasma GH levels above baseline were greater after hpGRF injection in sheep than in pigs or calves. Subcutaneous injection of hpGRF in sheep was an effective mode of administration of the peptide, although the effect was not as long-lasting as that after iv injections and higher doses were required to stimulate GH secretion.

Human pancreatic growth hormone releasing hormone fails to stimulate human growth hormone both in Cushing's disease and in Cushing's syndrome due to adrenocortical adenoma.

An absent or severely blunted hGH response to an i.v. bolus injection (100 micrograms) of human pancreatic growth hormone releasing hormone (hpGRF 1-44) was found in seven female patients with Cushing's syndrome (five with pituitary dependent Cushing's disease and two due to an adrenal adenoma) and four men with pituitary dependent Cushing's disease. Three of the female and three of the male patients had an adequate hypoglycaemia after insulin administration. All these patients showed an absent or blunted hGH response after insulin induced hypoglycaemia. The GHRH data in these patients are in agreement with those in older literature on hGH responsiveness to stimuli such as L-dopa, arginine and insulin induced hypoglycaemia. It is concluded that hypercortisolism inhibits hGH release to various stimuli at the pituitary level.

Growth hormone response to hpGRF-40 in different forms of growth retardation and endocrine-metabolic diseases

The effect of one of the new human pancreatic growth hormone releasing factors (hpGRFs) was assessed in children or young adults with different forms of growth retardation or endocrine-metabolic diseases. Intravenously administered synthetic hpGRF-40 (1 microgram/kg) induced a clear-cut and prompt rise in plasma growth hormone (GH) levels in 8 normal prepubertal children and a definite GH rise in 11 out of 14 children with isolated GH deficiency (IGHD) and one child with the Silver-Russel syndrome. In two out of three subjects with craniopharyngioma hpGRF-40 did not induce any plasma GH increase. In seven out of ten children with constitutional growth delay (CGD), hpGRF-40 induced a biphasic GH response, with a prompt small GH increment followed by a second, more consistent rise. Both in children with IGHD and with CGD the rise in plasma GH following hpGRF-40 was markedly lower than in controls. In children with CGD the GH response to hpGRF-40 was defective, despite the fact that in most of them the GH response to standard pharmacological stimuli was normal according to generally accepted criteria. hpGRF induced a small but sustained plasma GH rise in four hypothyroid subjects, while in three out of four children with idiopathic obesity the GH response to hpGRF was strikingly reduced. These data demonstrate that hpGRF is a potent stimulus of GH release in normal prepubertal children and a physiological means of investigating GH function in diseases associated with growth impairment.

Medianosomes as integrative units in the external layer of the median eminence. Studies on grf/catecholamine and somatostatin/catecholamine interactions in the hypothalamus of the male rat☆

Somatostatin/catecholamine as well as growth hormone releasing factor/catecholamine interactions have been characterized in the hypothalamus and the preoptic area using morphometrical and quantitative histofluorimetrical analyses. The combined results of the present and previous studies have led us to put forward the medianosome concept. The medianosome is defined as an integrative unit, which consists of well defined aggregates of transmitter identified nerve terminals interacting with one another in the external layer of the median eminence. Our present data indicate the existence of putative medianosomes consisting predominantly of growth hormone releasing factor nerve terminals costoring dopamine as well as of somatostatin and dopamine nerve terminals, which interact locally to control growth hormone secretion. A complementary control of growth hormone secretion may be exerted by noradrenaline mechanisms in the subependymal layer, in the ventral zone and/or in the suprachiasmatic preoptic nucleus. However, further analyses in view of the differential effects seen with the present doses of rat hypothalamic and human pancreatic growth hormone releasing factor have to be done. The results also indicate the possible existence of growth hormone releasing factor receptors in the median eminence which may participate in the feedback control of the growth hormone releasing factor immunoreactive neurons in the ventral zone of the hypothalamus.

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.

The Response of Obese Subjects to Continuous Infusion of Human Pancreatic Growth Hormone Releasing Factor 1–44 (hpGRF 1–44)

The Response of Obese Subjects to Continuous Infusion of Human Pancreatic Growth Hormone Releasing Factor 1–44 (hpGRF 1–44)

Vasoactive intestinal peptide effects on GH 3 pituitary tumor cells: high affinity binding, affinity labeling, and adenylate cyclase stimulation. Comparison with peptide histidine isoleucine and growth hormone-releasing factor

The vasoactive intestinal polypeptide (VIP) receptor was characterized on the GH 3 rat pituitary tumor cell line using competitive binding studies with peptides having sequence homology with VIP. Further studies investigated receptor coupling to the adenylate cyclase complex by measurement of cAMP levels. Finally, the molecular weight of the receptor was estimated by affinity labeling techniques. Studies using 125I-VIP and unlabeled competing peptides revealed a single class of high affinity binding sites with a dissociation constant ( K D) of 17 ± 2 nM (mean ± S.E.M.) for VIP, 275 ± 46 nM for peptide histidine isoleucine (PHI), and 1380 ± 800 nM for human pancreatic growth hormone releasing factor (GHRF). VIP and PHI each stimulated intracellular cAMP accumulation in a dose-dependent manner; both peptides demonstrated synergism with forskolin. In contrast, GHRF neither stimulated accumulation of cAMP nor demonstrated synergism with forskolin. VIP plus PHI (1 μM each) caused no significant increase in cAMP over either VIP or PHI alone, implying that the two peptides act through the same receptor. Covalent crosslinking of 125I-VIP to its binding site using either disuccinimidyl suberate (DSS) or ethylene glycol bis(succinimidyl succinate) (EGS) was followed by SDS-PAGE and autoradiography. The result is consistent with an M r 47 000 VIP-binding subunit comprising or being associated with the VIP receptor of GH 3 pituitary tumor cells.

Potent interaction between thyrotropin releasing hormone (TRH) and human pancreatic growth hormone releasing factor (hpGRF) in stimulating chicken growth hormone (cGH) in vivo: Hypothalamic noradrenergic mediation in TRH stimulation of cGH release

The interaction of human pancreatic growth hormone releasing factor (hpGRF) and thyrotropin releasing hormone (TRH) on chicken growth hormone (cGH) release in vivo and possible noradrenergic involvement on TRH-induced stimulation of cGH in vivo were examined. Four-week old cockerels (∼1 kg) were injected intravenously with hpGRF (1.0 μg/bird), TRH (0.1 μg/bird), or hpGRF (1.0 μg/bird) in combination with TRH (0.1 μg/bird). Five min after the injection, blood samples were collected and serum concentrations of cGH were determined by a homologous RIA. The results showed that hpGRF and TRH were potent stimulators of cGH release, 5- and 6-fold over the control birds, respectively, and that hpGRF and TRH administered in combination produced a synergistic stimulation of cGH release (>20 fold). In separate experiments, pretreatment with alpha-methyl-para-tyrosine (250 mg/bird) for 2 hours resulted in complete suppression of the TRH stimulatory effect on cGH release but not the stimulatory effect of hpGRF. Pretreated with phenoxybenzamine hydrochloride (20 mg/bird) or diethyl-dithiocarbamate (500 mg/bird) also resulted in complete suppression of TRH-induced cGH release. These results indicate that hpGRF acts directly at the pituitary and TRH acts at the hypothalamus in addition to the pituitary in stimulating cGH release, possibly mediated through the noradrenergic neurons. HpGRF and TRH were potent releasers of cGH and their stimulation was potentiated when administered together.

ChemInform Abstract: STRUCTURE‐ACTIVITY STUDIES ON THE N‐TERMINAL REGION OF GROWTH HORMONE RELEASING FACTOR

In previous reports illustrating the effects of conformational restriction of the N-terminal region of human pancreatic growth hormone releasing factor, we demonstrated that D-amino acid substitutions in either of positions 1, 2, or 3 resulted in greatly increased growth hormone releasing activity both in vivo and in vitro. The most active compound, [D-Ala-2]GRF(1-29)NH2, was 51 times more active than the parent 29 amino acid peptide in the sodium pentobarbital anesthetized rat. These observations have now been extended to analogues containing multiple D-amino acid replacements in these three positions. Once again, peptides with superagonist potencies ranging from 1200% to 3800% were obtained after solid-phase synthesis and purification by medium-pressure reverse-phase liquid chromatography. In addition, it was found that [D-Asn-8]- and [D-Ala-4]GRF(1-29)NH2 were, respectively, 2.43 and 1.1 times more active than GRF(1-29)NH2 itself. In contrast, [D-Phe-6] and [D-Thr-7] analogues were virtually inactive. Chou-Fasman structural predictions suggest that the first three residues of the peptide assume no fixed type of conformation but that a reverse turn could be present between residues 6 and 10. Attempts are made to rationalize the biological results with these calculations. The effects of other side chains on the D-amino acid in position 2 were also investigated. Both the Ac-[D-Phe-2]- and Ac-[D-Arg-2]peptides had very low activity. Several of the inactive peptides were tested as possible antagonists of GRF; however, none was able to block the stimulatory effects of GRF(1-29)NH2 after combined administration.

Cyclic AMP-dependent protein kinase in Molt 4b lymphoblasts: identification by photoaffinity labeling and activation in intact cells by vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI).

Molt 4b lymphoblasts have previously been shown to possess a single class of pharmacologically specific, high affinity receptors for vasoactive intestinal polypeptide (VIP). This study further explores the molecular basis for modulation of human lymphocyte function by VIP. Dose-dependent stimulation of adenylate cyclase was observed in Molt lymphoblasts over the range of 0.1 nM to 1 microM VIP. VIP-mediated by guanine nucleotide. Accumulation of intracellular cAMP was observed in the presence of either VIP or the diterpene, forskolin. The effects of these two agonists were synergistic. Two neuropeptides that share sequence homology with VIP were also studied; both peptide histidine isoleucine (PHI) and human pancreatic growth hormone releasing factor (1-44 GHRF) competed for 125I-VIP binding to Molt cells. PHI stimulated intracellular cAMP accumulation and demonstrated synergism with forskolin, whereas GHRF had no effect on cAMP. Photoaffinity labeling of 100,000 X G soluble proteins with 8-N3-[32P]cAMP followed by SDS gel electrophoresis demonstrated the presence of cAMP-dependent protein kinases I and II. Cyclic AMP-dependent protein kinase II predominated in the soluble fraction and was the only isozyme observed in particulate fractions. Protein phosphorylation was studied in Molt 4b cells preincubated with [32P]PO43- followed by addition of media alone, 1 microM peptide, or 10 microM forskolin. Cells were lysed and subjected to two-dimensional electrophoresis. Increased phosphorylation of a specific 41,000 Mr protein was observed after addition of forskolin, VIP, or PHI. A much lower concentration of VIP (1 nM) also caused a significant net increase in phosphorylation, which was of a lower magnitude. In contrast, no net effect on protein phosphorylation was seen with GHRF. These data demonstrate the presence of a functional VIP receptor that is linked to the G protein-adenylate cyclase complex. The demonstration of cAMP-dependent protein kinase and of VIP- and PHI-mediated protein phosphorylation in Molt 4b lymphoblasts provides evidence on a molecular level for neuropeptide modulation of human lymphocyte function.

The effects of centrally administered neuropeptides on the development of gastric lesions in the rat

Centrally administered neuropeptides were investigated for their effects on the development of gastric lesions in rats. Thyrotropin releasing hormone (TRH), vasoactive intestinal peptide (VIP) and gonadotropin releasing hormone (LHRH) produced gastric lesions acutely, with TRH demonstrating the most pronounced effect in terms of incidence and severity. Ten-fold higher doses of the same peptides administered intravenously produced none or very few gastric lesions. Moreover, pretreatment with atropine partially inhibited their production. Corticotropin releasing factor (CRF) exhibited only mild ulcerogenic effets, and the gastric lesions induced with this peptide developed more slowly than with TRH, VIP and LHRH. Although ulcerogenic in their own right, none of these four neuropeptides significantly potentiated the potent ulcerogenic effects of cold-restraint stress. Since other neuropeptides, including somatostatin, human pancreatic growth hormone releasing factor (hpGRF), substance P, bombesin, and neurotensin, had no demonstrable effects on gastric mucosa, we can conclude that the lesions were not a general effect of intracisternal administration of neuropeptides. The results suggest that within the central nervous system, there are several neuropeptides that play a significant role in the development of gastric lesions via, at least in part, vagal-dependent mechanisms.

Structure-activity studies on the N-terminal region of growth hormone releasing factor.

In previous reports illustrating the effects of conformational restriction of the N-terminal region of human pancreatic growth hormone releasing factor, we demonstrated that D-amino acid substitutions in either of positions 1, 2, or 3 resulted in greatly increased growth hormone releasing activity both in vivo and in vitro. The most active compound, [D-Ala-2]GRF(1-29)NH2, was 51 times more active than the parent 29 amino acid peptide in the sodium pentobarbital anesthetized rat. These observations have now been extended to analogues containing multiple D-amino acid replacements in these three positions. Once again, peptides with superagonist potencies ranging from 1200% to 3800% were obtained after solid-phase synthesis and purification by medium-pressure reverse-phase liquid chromatography. In addition, it was found that [D-Asn-8]- and [D-Ala-4]GRF(1-29)NH2 were, respectively, 2.43 and 1.1 times more active than GRF(1-29)NH2 itself. In contrast, [D-Phe-6] and [D-Thr-7] analogues were virtually inactive. Chou-Fasman structural predictions suggest that the first three residues of the peptide assume no fixed type of conformation but that a reverse turn could be present between residues 6 and 10. Attempts are made to rationalize the biological results with these calculations. The effects of other side chains on the D-amino acid in position 2 were also investigated. Both the Ac-[D-Phe-2]- and Ac-[D-Arg-2]peptides had very low activity. Several of the inactive peptides were tested as possible antagonists of GRF; however, none was able to block the stimulatory effects of GRF(1-29)NH2 after combined administration.

Specific binding of synthetic human pancreatic growth hormone releasing factor (1–40-OH) to bovine anterior pituitaries

We have studied the specific binding of a synthetic 40 amino acid, free carboxy terminus analog of human pancreatic growth hormone releasing factor (hpGRE-40-OH) to partially purified homogenates of bovine anterior pituitaries. The binding of hpGRF-40-OH to pituitary receptors at 4°C reached maximal level in 4 hours and remained steady for the next 18 hours. Specific binding increased linearly with the amount of protein present in the assay. 125I-hpGRF-40-OH binding to pituitary homogenates was competitively inhibited by hpGRE-40-OH but not by unrelated hormones. The competition curve and Scatchard analysis suggest the presence of single class of receptors with a K d⋍3nM and binding capacity of ≈200 fmoles/mg protein. This is the first demonstration of specific receptors for GRF on anterior pituitary cells.

Growth hormone responsiveness to human pancreatic growth hormone releasing factor in acromegaly: modulatory effects of basal hormone levels and of concomitant somatostatin administration.

Human pancreatic growth hormone releasing factor 1-44 (hpGRF), 100 micrograms was administered as an i.v. bolus injection to eleven patients with acromegaly. The mean serum growth hormone (GH) levels rose (P less than 0.001) from 54 +/- 20 ng/ml to 215 +/- 126 ng/ml (+/- SEM) 20 min after the injection. Although the maximum response of GH levels was highly variable it correlated positively with the individual GH levels (P less than 0.01, Rs = +0.80). Thus the higher the GH levels, the greater the responsiveness to hpGRF. Administration of somatostatin (SRIF), 300 micrograms/h, lowered basal GH levels from 76 +/- 38 ng/ml to 13 +/- 5 ng/ml (P less than 0.01) after 1 h. hpGRF administration during concomitant SRIF infusion also led to highly variable growth hormone responses. The maximum GH responses again correlated positively with the GH level before hpGRF after 1 h of SRIF administration (P less than 0.05, Rs = +0.79). GH responses to hpGRF were completely blocked by SRIF in three out of four patients whose GH levels decreased to normal levels during SRIF infusion. Our data illustrate that the pituitary in acromegaly is normally responsive to both SRIF and hpGRF but at a higher setting of basal GH levels.

Effect of pancreatic growth hormone releasing factors on GH secretion by human somatotrophic pituitary tumours in cell culture.

Human pancreatic growth hormone releasing factors (hpGRF(1-40) and hpGRF(1-44) significantly stimulated GH secretion when added to cell cultures of human somatotrophic pituitary tumours for 2 h. There was little difference in potency between the two peptides, and the response of different tumours varied, ranging from 30% to 500% increases in GH secretion over control levels. This stimulatory effect was blocked by somatostatin (SRIF) and bromocriptine. The suppressive effect of bromocriptine, but not of SRIF, was overcome by high doses of hpGRF(1-44). TRH stimulated GH secretion by one of three somatotrophic tumours in cell culture, and it was found to potentiate the stimulatory effects of hpGRF(1-44). These results demonstrate that hpGRFs increase serum GH levels in man by a direct action at the pituitary somatotroph level.