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.     

Growth hormone and prolactin response to thyrotropin releasing hormone and growth hormone releasing factor in the immature turkey

Synthetic thyrotropin releasing hormone (TRH) and human pancreatic growth hormone releasing factor (hpGRF) stimulated growth hormone (GH) secretion in 6- to 9-week-old turkeys in a dose-related manner. TRH and hpGRF (1 and 10 micrograms/kg, respectively) each produced a sixfold increase in circulating GH levels 10 min after iv injection. Neither TRH nor hpGRF caused a substantial change in prolactin (PRL) secretion in unrestrained turkeys sampled through intraatrial cannulas. However, some significant increases in PRL levels, possibly related to stress, were noted.     

Synthetic human pancreatic growth hormone releasing factor (GRF) stimulates growth hormone secretion in the domestic fowl (Gallus domesticus)

Synthetic human pancreatic Growth Hormone-Releasing Factor (hpGRF) elevated the plasma concentration of growth hormone (GH) in young and adult domestic fowl. This in vivo effect of hpGRF appeared to be largely similar for both the 32 amino-acid (hpGRF 1-32) or 40 amino-acid (hpGRF 1-40) polypeptide, although the effect of hpGRF 1-32 was more prolonged than that of hpGRF 1-40 in adult domestic fowl. The increase in plasma GH concentrations following hpGRF administration (10 micrograms/kg) was somewhat greater in young than adult chickens (the increase in plasma concentration of GH being 230 ng/ml at 1 week old, 282 ng/ml at 6 week old, 241 ng/ml at 10 weeks and 150 ng/ml in adults). In the adult domestic fowl hpGRF stimulated a greater increase in the plasma concentration of GH than did thyrotropin-releasing hormone (TRH). However in the young chicks TRH was more active. The in vitro release of GH from dispersed chicken pituitary cells was elevated by hpGRF (1-32) and hpGRF (1-40).     

Sensitivity of rat forebrain neurons to growth hormone-releasing hormone

The effects of iontophoretically applied human pancreatic growth hormone-releasing factor (hpGRF), peptide histidine isoleucine (PHI-27), and somatostatin (SS) on the extracellular activity of single cells in the hypothalamus, thalamus, and cortex of the rat brain were studied in urethane-anesthetized, male rats. Neurons with membrane sensitivity to hpGRF, PHI-27, and SS were present in each brain region. Although neurons excited by these peptides were encountered in thalamus and hypothalamus, depression of neuronal firing was the predominant response observed. Overall, the neurons responding to hpGRF also possessed membrane sensitivity to PHI-27, whereas, the hpGRF sensitive neurons appeared to be more divided as to their ability to respond to SS. The results clearly demonstrate that hpGRF and PHI-27 are capable of affecting the membrane excitability of neurons in several brain regions. The distribution of neurons sensitive to hpGRF suggests that hypothalamic GRF, in addition to its well documented role in the regulation of pituitary growth hormone secretion, may subserve other physiological events in the rat central nervous system as a neurotransmitter and/or neuromodulator.     

Interaction of human pancreatic growth hormone-releasing factor, thyrotropin-releasing hormone, and somatostatin on growth hormone release in chickens

The effects of synthetic somatostatin (SRIF) on serum growth hormone (GH) concentrations stimulated by exogenous administration of synthetic thyrotropin-releasing hormone (TRH) and/or human pancreatic GH-releasing factor (hpGRF) were investigated in 4-week-old cockerels. In addition, the additive effects of TRH and hpGRF on serum GH were examined. TRH and hpGRF, when given in combination intravenously, produced an additive effect on serum GH concentration that peaked 10 min after the injection. The somatostatin did not significantly affect basal GH concentrations when given alone, but did significantly decrease the magnitude of the GH response to hpGRF. In contrast, SRIF did not significantly decrease the stimulatory effects of TRH on GH release. These results suggest that TRH and hpGRF are potent GH releasers in vivo and that their stimulating effects on GH release are additive, suggesting different mechanisms for their stimulation. The results obtained from the combination studies suggest that the main site of the stimulatory action of hpGRF is at the pituitary, and that SRIF significantly inhibited the rise in serum GH induced by a synthetic hpGRF, but not that induced by TRH.     

Effects of secretin and gastric inhibitory polypeptide on human pancreatic growth hormone-releasing factor(1-40)-stimulated growth hormone levels in the rat

Synthetic human pancreatic growth hormone-releasing factor containing 40 amino acids ([hpGRF (1-40)]-OH) significantly stimulated plasma growth hormone (GH) levels in both sodium pentobarbital and urethane anesthetized rats. Synthetic secretin, gastric inhibitory polypeptide (GIP), and glucagon significantly decreased plasma GH levels while synthetic vasoactive intestinal peptide (VIP) had no effect. Secretin and GIP also altered the in vivo plasma GH response to [hpGRF(1-40)]-OH. Whether this effect is the result of an interaction at the pituitary level or is due to an extra-pituitary effect of secretin and GIP awaits further study.     

Human pancreatic growth hormone-releasing factor (hpGRF) stimulates growth hormone release in goldfish

Synthetic human pancreatic growth hormone-releasing factor (hpGRF) was injected intraperitoneally in different dosages, either as one injection or two injections 3 hours apart, into goldfish. Serum GH levels were increased by certain dosages of hpGRF under both treatments. This is the first demonstration of GH-releasing activity of hpGRF in a teleost fish.     

In vitro pituitary hormone releasing activity of 40 residue human pancreatic tumor growth hormone releasing factor

The hypophysiotropic activities of a synthetic human pancreatic growth hormone releasing factor (hpGRF) with 40 residues was examined in vitro using rat pituitary halves. At concentrations from 10(-10) M to 10(-7) M the peptide stimulated GH release in a dose-dependent manner with the ED50 being 1.2 x 10(-9) M. The concentration of 10(-10) M hpGRF is comparable to the basal hypophyseal portal blood levels of other known hypothalamic hypophysiotropic hormones. However, GH release was enhanced three-fold by concentration as low as 10(-12) M, though no dose-response relationship was observed up to 10(-10) M. Thus, this peptide not only stimulates the release of GH in a dose-dependent manner, but at lower concentrations also maintains elevated GH levels. The release of ACTH, beta-endorphin, LH, and FSH was not affected by hpGRF at any of the concentrations tested. At hpGRF concentrations less than 10(-7) M, the release of TSH and PRL were unaffected. However, at 10(-6) M, TSH release was enhanced about 2.5 fold and prolactin release was elevated slightly.     

Growth hormone response of bull calves to growth hormone-releasing factor

Three experiments were conducted to determine serum growth hormone (GH) response of bull calves (N = 4; 83 kg body wt) to iv injections and infusions of human pancreatic GH-releasing factor 1-40-OH (hpGRF). Peak GH responses to 0, 2.5, 10, and 40 micrograms hpGRF/100 kg body wt were 7 +/- 3, 8 +/- 3, 18 +/- 7, and 107 +/- 55 (mean peak height +/- SEM) ng/ml serum, respectively. Only the response to the 40-microgram dose was greater (P less than 0.05) than the 0-microgram dose. Concentrations of prolactin in serum were not affected by hpGRF treatment. In calves injected with hpGRF (20 micrograms/100 kg body wt) at 6-hr intervals for 48 hr, GH increased from a mean preinjection value of 3.1 ng/ml serum to a mean peak response value of 70 ng/ml serum. Differences in peak GH response between times of injection existed within individual calves (e.g., 10.5 ng/ml vs 184.5 ng/ml serum). Concentrations of GH in calves infused continuously with either 0 or 200 micrograms hpGRF/hr for 6 hr averaged 7.4 +/- 3 and 36.5 +/- 11 ng/ml serum, respectively (P less than 0.05). Concentrations of GH oscillated markedly in hpGRF-infused calves, but oscillations were asynchronous among calves. We conclude that GH response of bull calves to hpGRF is dose dependent and that repeated injections or continuous infusions of hpGRF elicit GH release, although magnitude of response varies considerably. We hypothesize that differences in GH response to hpGRF within and among calves, and pulsatile secretion in the face of hpGRF infusion may be related to the degree of synchrony among exogenous hpGRF and endogenous GRF and somatostatin.     

Growth hormone-releasing factor on growth hormone secretion in prepubertal calves

The effects of iv administration of growth hormone-releasing factor (GRF) on growth hormone (GH) release and on nitrogen metabolism were measured in prepubertal calves. Crossbred beef heifers (111 kg) were used in a Latin square design to test the effects of 0, 0.01, 0.033, 0.067, and 0.1 microgram human pancreatic (hp) GRF [hpGRF (1,40)OH]/kg body wt on plasma GH concentrations. When they were given doses of 0.067 and 0.1 microgram hpGRF/kg body wt, plasma GH increased (P less than 0.05) within 5-15 min, compared with injections of control buffer, and then returned to preinjection concentrations. The response to 0.067 microgram hpGRF/kg body wt every 3 hr for 42 hr was studied in five heifers (137 kg body wt). The animals responded to 50% of the GRF injections with an increase in plasma GH during every 6-hr period measured. Nitrogen retention, hormone concentrations, and weight gain were measured in five bull calves (90 kg body wt) administered 0 or 0.067 microgram Nle rat hypothalamic GRF (1,29)NH2/kg body wt every 4 hr for 10 days. Metabolic parameters were interpreted to indicate an anabolic response to GRF even though increases of 16% in nitrogen retention, 23% in plasma somatomedin C concentrations, and 36% in weight gain with pulsatile GRF treatment were variable and statistically similar to those of controls. These results indicate that GRF induces peak GH secretion within 15 min in prepubertal calves and that calves can respond to multiple injections of GRF with an increase in plasma GH.     

Plasma growth hormone responses to repetitive administrations of growth hormone releasing factor in patients with pituitary dwarfism

Plasma growth hormone (GH) responses to the repetitive administrations of synthetic human pancreatic growth hormone releasing factor (hpGRF-44) were studied in 15 patients with GH deficiency (11 diagnosed as idiopathic and 4 diagnosed as secondary to hypothalamo-pituitary tumor). hpGRF-44 was administered by single iv bolus (2 micrograms/kg), repetitive im (100 micrograms, twice a day), and/or repetitive iv infusion (2.5 micrograms/min for 90 min, once a day) for three to six consecutive days. Three of the eleven idiopathic GH deficient patients had plasma GH responses to both single iv bolus injection and repetitive administrations by im, or iv infusion of hpGRF. In four of the remaining eight, who had not had peak plasma GH levels above 5 ng/ml to a single iv bolus of the peptide, repetitive administrations of hpGRF-44 by im injection and/or iv infusion induced GH responses to the peptide. In the four patients with secondary GH deficiency, three had plasma GH response to hpGRF administration but one patient, who had indications of pituitary disorder, did not show any plasma GH response to either single iv injection or repetitive administrations of hpGRF-44. These data show that repetitive administrations of hpGRF-44 can induce plasma GH responses in some GH deficient patients who do not respond to a single iv bolus of the peptide.     

Growth hormone-releasing factor stimulates adenylate cyclase activity in the anterior pituitary gland

Synthetic human pancreatic growth hormone-releasing factor (hpGRF) (1–40)-NH₂ stimulates adenylate cyclase activity in rat anterior pituitary particulate fraction at an ED₅₀ value of approximately 150 nM. GTP more than doubles the stimulatory effect of hpGRF aand PGE₂ on [³²p] cyclic AMP formation. The present data show that hpGRF as well as PGE₂, another potent stimulus of GH secretion, act at least partly, through GTP-dependent mechanisms in their coupling with adenylate cyclase.     

Synthetic hpGRF 1–40 stimulates growth hormone and inhibits prolactin secretion in normal children and children with isolated growth hormone deficiency

Intravenously administered synthetic hpGRF 1–40 at doses of 0.1, 0.33 and 1.0 μg/kg increased plasma GH in a dose-dependent fashion in 4 normal prepubertal children. hpGRF 1–40 at the dose of 1.0 μg/kg stimulated GH release, though to a lesser extent than in normals, in 7 children with isolated GH-deficiency (IGHD) but failed to do so in a patient with craniopharyngioma. In all normal children and 6/7 patients with IGHD, hpGRF 1–40 at all doses used induced a clear and sustained lowering of plasma prolactin levels; this effect was lacking in the patient with craniopharyngioma. hpGRF 1–40 had no effect on plasma FSH, LH, TSH or glucose levels nor did it influence pulse rate, blood pressure, or body temperature. These results indicate that hpGRF 1–40 is a potent stimulus to GH release in normal prepubertal children and holds promise for treatment of GH-deficient children. In addition, in both normal children and children with IGHD, hpGRF 1–40 is a potent suppressor of prolactin levels.     

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.     

The influence of albumin on vitamin D metabolism in fetal chick osteoblast-like cells

Human pancreatic growth hormone releasing factor (1-29)-amide [hpGRF (1-29)-NH2] and the following analogs: [D-Tyr-1]-hpGRF(1-29)-NH2, [D-Ala-2]-hpGRF(1-29)-NH2, [D-Asp-3]-hpGRF(1-29)-NH2, and [N-Ac-Tyr-1]-hpGRF (1-29)-NH2 were synthesized using solid phase methodology and tested for their ability to stimulate growth hormone (GH) secretion in the rat and the pig in vivo. [D-Ala-2]-hpGRF (1-29)-NH2 was approximately 50 times more potent than the parent molecule in eliciting GH secretion in the rat. The other analogs were less active, but all were more potent than the 1-29 amide in the rat. [D-Tyr-1]-hpGRF(1-29)-NH2 was 10 times more potent, [D-Asp-3]-hpGRF(1-29)-NH2 7 times more potent, and the acetylated molecule approximately 12 times more potent than hpGRF(1-29)-NH2.     

Enhanced growth hormone responses to growth hormone releasing hormone in male type I diabetic patients.

In a previous paper we have demonstrated that growth hormone (GH) responses to growth hormone releasing hormone (GHRH) are higher in premenopausal normal women than in age matched healthy men. As in type I diabetes mellitus various disturbances of GH secretion have been reported, the aim of our study was to assess the effect of sex on basal and GHRH stimulated GH secretion in type I diabetes mellitus. In 21 female and 23 male type I diabetic patients and 28 female and 30 male control subjects GH levels were measured before and after stimulation with GHRH (1 microgram/kg body weight i.v.) by radioimmunoassay. GH responses to GHRH were significantly higher in female than in male control subjects (p less than 0.02), whereas the GH levels following GHRH stimulation were similar in female and male type I diabetic patients. GH responses to GHRH were significantly higher in the male type I diabetic patients than in the male control subjects (p less than 0.001); in the female type I diabetic patients and the female control subjects, however, GH responses to GHRH were not statistically different. The absence of an effect of sex on GHRH stimulated GH responses in type I diabetes mellitus provides further evidence of an abnormal GH secretion in this disorder.     

The effects of testosterone and estrogen on the pituitary growth hormone response to growth hormone-releasing factor

The effects of testosterone and estrogen on the pituitary growth hormone response to hypothalamic growth hormone-releasing factor (GRF) were evaluated in vivo using male and female rats and in vitro using a pituitary cell monolayer culture system. In vivo the increase in plasma growth hormone (GH) concentration in response to a 500 ng/kg dose of GRF was similar in gonadectomized male and female rats. Pretreatment of intact and gonadectomized male rats with testosterone caused significant enhancement of the pituitary GH response to GRF, whereas pretreatment of gonadectomized female rats with 17 beta-estradiol did not alter the response. The GH response to GRF was not different between prepubertal (i.e., 30-day-old) male and female rats. However, following puberty (i.e., by 60 days of age), the response in male rats was significantly greater than that observed in female rats. The in vitro preincubation of anterior pituitary cells with either testosterone or 17 beta-estradiol did not cause any shift in the dose-response curve between GRF and GH. These results demonstrated that androgens play an active role in modulating the pituitary response to GRF in vivo.     

Developmental expression of growth hormone and prolactin genes in the bovine pituitary

Cell-free translation was used to initially characterize the major mRNA species present in the bovine anterior pituitary as a function of development. The only detectable change in translation products, which occurred during the transition from fetus to adult, was a reversal in the relative ratio of pituitary growth hormone and prolactin. Subsequent hybridization analysis with cloned growth hormone and prolactin cDNA probes indicated that growth hormone mRNA comprised over 40% of the total fetal mRNA and was 50- to 100-fold higher than prolactin mRNA. The steady state levels of growth hormone mRNA remained relatively constant throughout gestation. In contrast, prolactin mRNA levels, which were low early in gestation, increased during development to become the principal mRNA in the adult pituitary. Since growth hormone and prolactin are synthesized and secreted by specialized cells (somatotrophs and mammotrophs, respectively) immunochemical staining was used to determine whether the changes in the mRNA levels for these two hormones were a reflection of specific cell proliferation. For growth hormone, there was a close correlation between the number of somatotrophs and the relative levels of growth hormone mRNA. In contrast, the increase in prolactin mRNA exceeded the increase in the number of mammotrophs. Thus, the cellular concentration of growth hormone mRNA remains relatively constant during development, while the cellular concentration of prolactin mRNA increases by more than an order of magnitude.     

Pretranslational regulation of sex-dependent testosterone hydroxylases by growth hormone in mouse liver

The effects of growth hormone on the expression of sex-dependent testosterone 16 alpha- and 15 alpha-hydroxylases were studied in growth hormone-deficient Little (lit/lit) mice at the activity as well as at the mRNA levels. The male isozyme of testosterone 16 alpha-hydroxylase ("C"-P-450(16)alpha) was repressed in the liver of male lit/lit mice, and the injection of bovine growth hormone resulted in an increase of the isozyme at both activity and mRNA levels to those seen in control lit/+ male mice. On the other hand, the female isozymes of testosterone 16 alpha- ("I"-P-450(16)alpha) and 15 alpha-hydroxylase (P-450(15)alpha) were increased in livers of both male and female lit/lit mice. The increased I-P-450(16)alpha and P-450(15)alpha in lit/lit mice were suppressed by growth hormone but only when it was injected once every 12 h. Thus, the results indicate that growth hormone acts as a masculinizing factor for testosterone hydroxylase activity by activating and inhibiting the expression of male and female isozymes of testosterone hydroxylases in mice, respectively. When growth hormone was infused to simulate a continuous secretion pattern, it showed no significant effect on the expression of hydroxylases in lit/lit mice, suggesting that growth hormone may not be a feminizing factor for testosterone hydroxylase activity in female mice. The changes of specific hydroxylase activities modulated by growth hormone in the mice correlated well with those amounts of hydroxylase mRNAs. The action of exogenous growth hormone to regulate the hydroxylases was so slow that it took 2 days to show a significant effect.     

Central effects of growth hormone-releasing factor (GRF) on intestinal motility in dogs: involvement of dopaminergic receptors

The effects of intracerebroventricular (ICV) and intravenous (IV) administration of human pancreatic growth hormone-releasing factor (hpGRF) on gastro-intestinal motility were examined in fasted and fed conscious dogs equipped with chronically implanted strain-gauges on the antrum and the jejunum. During the fasted state, hpGRF injected ICV at 0.1 micrograms . kg-1 or IV at 0.5 micrograms . kg-1 did not affect the cyclic occurrence of the migrating motor complex (MMC). This pattern was normally disrupted for 8-10 hours by a daily standard meal. Injected ventricularly (0.1 micrograms . kg-1) but not intravenously (0.5 micrograms . kg-1) 10-15 min after the daily meal, hpGRF significantly reduced (p less than 0.01) the duration of the jejunal fed pattern (2.0 +/- 1.4 vs. 8.4 +/- 1.1 hours for control) but not that of the stomach. This effect persisted when hpGRF (0.1 micrograms . kg-1 ICV) was administered after indomethacin (2 mg . kg-1 IM), naltrexone (0.1 mg . kg-1 IV) or domperidone (1 mg . kg-1 IV) but was abolished by a previous IV injection of metoclopramide (1 mg . kg-1). It was concluded that hpGRF is able to act centrally to control the pattern of jejunal motility in fed but not in fasted dog, its effect being probably mediated through dopaminergic pathways.