Serotoninergic drugs affect prolactin and growth hormone secretion in the domestic fowl

Adult fowl of both sexes injected with the monoamine oxidase inhibitor pargyline showed elevated circulating prolactin concentrations and reduced growth hormone concentrations. Young cockerels injected with the serotonin agonist quipazine and the antagonist methysergide showed responses consistent with a serotoninergic stimulatory control of prolactin. Injection of the serotonin precursor tryptophan and the serotonin re-uptake blocker imipramine resulted in elevated prolactin and reduced growth hormone levels. The similarities and differences in the control of prolactin and growth hormone in birds and mammals were discussed.     

Episodic growth hormone secretion in the domestic fowl (Gallus domesticus): alpha adrenergic regulation

In animals the secretion of GH is characterized by a series of spontaneous peaks. The situation in avian species, however, is unknown. The present study examines this in the domestic fowl. Blood samples were obtained via a remote catheter at 5-min intervals from young male chickens. An episodic pattern of GH secretion was observed having an interpulse interval (frequency) of approximately 1 h. Episodic GH secretion was altered by peripheral administration of drugs which influence adrenergic function. Spontaneous GH peaks were not observed following the administration of (1) bis-(4-methyl-1-homopiperazinyl-thiocarbonyl) disulfide (FLA63) or diethyldithiocarbamate (DDC) (which blocks norepinephrine (NE) and epinephrine (E) synthesis by inhibiting dopamine-beta-hydroxylase), (2) phenoxybenzamine (an alpha 1-adrenergic antagonist) and (3) clonidine (an alpha 2-adrenergic agonist). Neither alpha-methyl-p-tyrosine, which blocks dopamine (DA), NE and E synthesis, nor yohimbine, a predominantly alpha 2-antagonist could completely suppress episodic GH secretion. These data support a role for NE/E, acting via alpha adrenergic receptors, in the control of epidosic GH secretion in the domestic fowl.     

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).     

Somatostatin immunoneutralization affects plasma metabolite concentrations in the domestic fowl

Young leghorn cockerels were injected with antiserum to somatostatin (anti-SRIF) and plasma glucose, free fatty acids and alpha-amino nitrogen concentrations determined. Plasma glucose concentrations increased rapidly after anti-SRIF and remained high for up to 2 hr. Two different antisera tested had hyperglycaemic activity. Plasma free fatty acids also increased rapidly after administration of the two different anti-SRIFs, and remained high for about 1 hr. Plasma alpha-amino nitrogen increased during the first 30 min after anti-SRIF, then declined to levels significantly lower than control by 1-2 hr after injection. Anaesthesia reduced plasma concentrations of glucose and alpha-amino nitrogen, and also reduced the changes of these metabolites following anti-SRIF. The results show the importance of endogenous somatostatin in the regulation of plasma metabolite concentrations.     

Insulin-like growth factor inhibition of growth hormone secretion

Somatomedins-insulin-like growth factors (SM/IGF) are growth hormone (GH) dependent serum growth factors. There is some evidence that IGF inhibit GH release (negative feedback) in 3- to 24-h incubations of cultured rat adenohypophysial cells. We have used acutely dispersed noncultured rat adenohypophysial cells to study the dynamics of IGF on GH secretion. In this system both IGF-I and IGF-II (100 ng/mL) slightly, but significantly, decrease the cumulative GH released by human pancreas growth hormone releasing factor 1-40 (GRF) and the phosphodiesterase inhibitor 3-isobutyl-1-methyl xanthine. The inhibition is small (16%) and usually not statistically significant until 2 h of incubation. The inhibition with IGF is additive to that produced with low concentrations of somatostatin. The IGF also significantly decrease the rate of GH release in all time periods tested (0-1, 1-2, 2-3 h). In addition, the IGF decrease the quantity of [14C]leucine protein eluted at the position of labelled rat GH on Sephadex G75, which would include newly synthesized GH extracted from the cells. Thus we conclude that the decreased GH released may be due to an effect of IGF on both rate of release and on GH synthesis.     

Somatostatin as a physiological regulator of pulsatile growth hormone secretion

Somatostatin plays an important role in the regulation of the episodic and ultradian rhythm of growth hormone (GH) secretion. Passive immunization of rats with specific antibodies to the 14 and 28 amino acid sequences caused a significant GH elevation. The fact that somatostatin antiserum was unable to block episodic GH surges indicates that this hormone's release must be regulated by a dual mechanism. Indeed, GH-releasing factor (GRF) seems to be instrumental in the maintenance of pulsatile GH secretion. Moreover, exogenous GRF induced a further GH increase predominantly during the period of active secretion. Neutralization of endogenous somatostatin eliminated this time-dependent effect, indicating that this peptide blocks periodical spontaneous GH release. Food deprivation and changes in glucose homeostasis virtually obliterate the ultradian GH rhythm. In this context, peripheral somatostatin seems to play an important role. Also the central GRF/somatostatin interplay is responsible for a short-loop feedback control on pituitary somatotrops.     

Somatostatin receptor subtype 1 modulates basal inhibition of growth hormone release in somatotrophs

Somatostatin (SST) inhibits the secretion of many peptide hormones including growth hormone (GH). The various functions of SST are mediated through at least five different receptor subtypes (SSTR1-5), their precise physiological roles have not been solved yet. Here we report on studies concerning the functional role of SSTR1 in the modulation of GH release from somatotrophs. Primary cell cultures from pituitaries of wild-type SSTR1 mice exposed to the SSTR1 selective somatostatin analog CH-275 show reduction of basal levels of GH secretion whereas somatotrophs isolated from SSTR1 null mutant mice did not respond to the agonist-mediated effect. This suggests that SSTR1 is involved in modulating basal GH levels in primary pituitary cell cultures and, together with SSTR2, may control the secretion of GH in the body.     

Some biological activities of recombinant DNA-derived growth hormone on plasma metabolite concentrations in domestic fowl

The biological activity of recombinant-DNA-derived chicken growth hormone (rcGH) has been examined in young broiler cockerels, by determining its effects on plasma concentrations of glucose, free fatty acids and alpha-amino nitrogen. A single injection of rcGH increased plasma glucose, which remained high for several hours, whereas daily treatment with rcGH for 1 week had no effect on basal plasma glucose concentrations but blunted the glucose response to a further rcGH challenge. Plasma free fatty acids were also promptly increased following acute rcGH treatment, and chronic exposure to rcGH again attenuated this response. The effects of rcGH on plasma alpha-amino nitrogen were more variable. The stress of repeated blood sampling tended to reduce alpha-amino nitrogen, and after rcGH, an increase relative to vehicle-injected controls was seen in both acute and chronically-treated birds. These data suggest that rcGH has both hyperglycaemic and lipolytic activity in chickens, and may also increase amino acid availability.     

Somatostatin and growth hormone regulation in cancer

Somatostatin analogues are used clinically in a variety of pituitary and gastroenteropancreatic tumours. In addition, they may influence breast and prostate growth either directly through somatostatin receptors or indirectly through inhibition of growth hormone and prolactin release. Somatostatin analogues may interfere with EGF/TGF -stimulated growth of these tumours and can suppress circulating levels of IGF-I in addition.     

Increase in pulsatile secretion of growth hormone during failure of catch-up growth following glucocorticoid-induced growth inhibition

The pattern of growth hormone (GH) secretion was determined in rats injected with cortisone acetate, 5 mg/rat/day subcutaneously, or with an equivalent volume of saline for 4 days from age 40 days. Cortisone injections resulted in inhibition of growth of body weight and tail length. During recovery the rats resumed a normal rate of growth but failed to show catch-up growth acceleration. From 17 to 27 days of recovery, plasma was sampled at 15-min intervals through the lights-on period, 06:00 to 18:00, via a catheter chronically implanted in the superior vena cava. During sampling each rat was housed singly in an insulated chamber, unrestrained, and with food and water ad lib. Cortisone-treated animals had a normal periodicity of GH plasma concentration, but they showed a reduction in values in the range of 50 to 99 ng/ml (P less than 0.01) and an increase of values in the range of 200 to 499 ng/ml (P less than 0.025) and above 1000 ng/ml (P less than 0.05). The area under the GH concentration curve of the cortisone-treated rats was significantly greater than that of the controls, 100.9 +/- 18.7 (mean +/- SE) units vs 55.3 +/- 7.4 (P less than 0.025). Thus, increased growth hormone secretion during the light phase persisted in spite of failure of catch-up growth acceleration. The findings indicate that the mechanism involved in GH release is linked to the catch-up growth control.     

The development of beta-adrenergic mediated inhibition of growth hormone secretion in the ovine fetus

The ontogeny of the suppressive effect of the beta-adrenergic agonist, isoprenaline, on fetal growth hormone (GH) release was examined in 14 chronically-catheterized ovine fetuses. Isoprenaline was administered as an intravenous infusion over 1 h (200 micrograms/kg). In seven fetuses between 72 and 99 days of gestation, isoprenaline had no effect on fetal plasma GH concentrations. In seven older fetuses between 114 and 140 days of gestation, isoprenaline infusion suppressed (P less than 0.02) fetal GH release. No effect was observed in five saline-treated control fetuses (119-131 days). Propranolol (250 micrograms/kg i.v.) administered 5 min prior to the isoprenaline infusion to four fetuses (117-136 days) delayed (P less than 0.05) the onset of the suppressive effect of isoprenaline demonstrating that the action of isoprenaline was mediated by the beta-adrenergic receptor. Propranolol alone (n = 6) had no effect. These observations demonstrate that the potential for beta-adrenergic inhibition of fetal GH release differentiates after 100 days of gestation. Comparison with previous studies of the ontogenesis of the control of GH secretion suggests that the hypothalamic beta-adrenergic control of GH release differentiates with an intermediate time course compared to other potential neuroendocrine controls.     

Brain serotonin turnover and plasma prolactin and growth hormone concentrations during changes in osmotic balance in the domestic fowl

Summary Young cockerels injected 24 h earlier with 0.9% saline,para-chorophenylalanine (pCPA, brain serotonin depletor) or alpha-methylpara-tyrosine (AMPT, brain catecholamine depletor) were deprived of access to water for 24 h. Plasma prolactin concentrations were markedly elevated by water deprivation and returned to normal on rehydration. pCPA, but not AMPT, significantly reduced the increase in prolactin. Concentrations of growth hormone were not affected by water deprivation. Brain serotonin concentrations were reduced by treatment with pCPA. Groups of cockerels were maintained under normal conditions or without access to drinking water for 12 h or 24h. Some were injected with the monoamine oxidase inhibitor pargyline, which increased the prolactin and decreased the growth hormone concentration in the plasma of the hydrated birds. The inhibitory effect of pargyline on growth hormone was augmented following water deprivation. Serotonin levels were not significantly affected by water deprivation but turnover (defined as accumulation of serotonin after pargyline treatment) was increased in the hypothalamus but not in remaining tissue. Injecting 30% saline solution intravenously markedly increased plasma prolactin whilst growth hormone concentrations were decreased. Serotonin turnover was increased in the hypothalamus but not in other brain regions. The results show that secretion of prolactin and growth hormone by the pituitary gland during osmotic imbalance in the fowl may be mediated by changes in hypothalamic scrotonin turnover.     

Cycloheximide blocks insulin-like growth factor I but not somatostatin inhibition of growth hormone secretion

Growth hormone secretion is controlled by the two hypothalamic hormones, growth hormone releasing factor (GRF) and somatostatin. In addition, the insulin-like growth factors (IGF or somatomedins) which are themselves growth hormone dependent, inhibit growth hormone release in vitro, therefore acting to close the negative feedback loop. The studies reported here examine some of the differences between inhibition of growth hormone secretion by somatostatin and IGF-I in vitro. The major finding is that cycloheximide, a protein synthesis inhibitor, blocks inhibition of GRF-stimulated growth hormone release caused by IGF-I, without changing the inhibition caused by somatostatin. The experiments were done by exposing mixed rat adenohypophysial cells to secretagogues with or without cycloheximide for 24 h in a short term culture. Somatostatin (0.6 nM) totally blocked rat GRF (1 nM) stimulated growth hormone release to values 48% of control (nonstimulated values), while IGF-I (27 nM) only reduced the GRF-stimulated growth hormone release by 27 +/- 3% (N = 5). Cycloheximide (15 micrograms/mL) totally blocked the effect of IGF-I but not somatostatin. A low concentration (0.12 nM) of somatostatin, which only partly inhibited growth hormone release, was also unaffected by cycloheximide. In purified rat somatotrophs, somatostatin (0.1 nM) inhibited GRF-stimulated cAMP levels slightly and reduced growth hormone release while IGF-I (40 nM) had no effect. We suggest that IGF-I inhibits only the secretion of newly synthesized growth hormone, while somatostatin inhibits both stored and newly synthesized growth hormone pools.     

The effect of agouti-related protein on growth hormone secretion in adult male rats

Agouti-related protein (AGRP) and neuropeptide Y (NPY) are synthesized in the same neurons in the hypothalamic arcuate nucleus. We have previously shown that NPY/AGRP neurons contain growth hormone (GH) receptor mRNA, and are activated following systemic GH administration. We also reported that NPY inhibits GH secretion when administered centrally. In this study, we have examined the effect of AGRP on GH secretion. Central administration of AGRP (83-132) as a single injection of 1 or 10 microg/rat, or chronic treatment of 1 microg/rat, every 12 h for 7 days, did not alter the GH secretory pattern of adult male rats. AGRP (83-132) at doses of 1-100 nM (4 h) did not alter baseline- and GHRH-induced GH secretion from the rat pituitary cell cultures. These results suggest that AGRP does not play a significant role in the feedback regulation of the GH secretion.     

Pharmacokinetics of growth hormone secretion in humans induced by growth hormone releasing hormone

This investigation compares the age- and sex-related changes in growth hormone (GH) response to growth hormone releasing hormone (GHRH) in normal subjects using an appropriate pharmacokinetic model. Twenty-five subjects (14 males and 11 females) aged 23-89 yr received a single intravenous bolus dose (1 microgram/kg) of GHRH-40 solution. Plasma GH concentration-time profiles are best characterized by a biexponential equation (or one-compartment model) with first-order release and disappearance rates and an equilibration lag time. The harmonic mean release rate half-life is similar for both sexes (males: 12.6 min vs. females; 11.4 min) but significantly different across age groups (23-35 yr: 7.2 min vs. 50-89 yr: 16.8 min). The mean disappearance rate half-life and GHRH-equilibration time lag for females (33.6 and 20.4 min, respectively) and the higher age group subjects (32.4 and 21.6 min, respectively) are significantly longer than those of males (22.8 and 9 min, respectively) and the lower age-group subjects (21.6 and 8.4 min, respectively). The mean metabolic clearance rate of GH is significantly lower (p less than 0.02) for females than for males (3.1 vs. 4.83 ml/hr.m2). However, the production rate and the amount of GH released by the pituitary for our subjects appear to be very similar for both males (8.7 micrograms/hr.m2 and 4.65 micrograms/m2) and females (9.33 micrograms/hr.m2 and 5.11 micrograms/m2).