Colin G. Scanes

The Effect of Thyrotropin-Releasing Hormone (TRH) and Somatostatin (GHRIH) on Growth Hormone and Prolactin Secretion in vitro and in vivo in the Domestic Fowl (Gallus domesticus)

The effects of thyrotropin-releasing hormone (TRH) on growth hormone (GH) and prolactin (Prl) secretion have been investigated in vitro and in vivo in domestic fowl. In both conscious and anaesthetized immature chickens the administration (i.v.) of TRH (2.5 and 25 microgram/kg) significantly increased the concentration of plasma GH. The simultaneous administration of somatostatin (GHRIH), 2.5 microgram/kg, to conscious birds significantly reduced the magnitude of the GH response to TRH treatment, but had no effect on the basal levels of plasma GH. The repeated injection of TRH (10 microgram/kg) every 20 min over a 100-min period failed to maintain the concentration of plasma GH at a high level. Prl secretion was not stimulated in any of these experiments, and in anaesthetized birds TRH (2.5 and 25 microgram/kg) treatment was followed by a depression in the level of plasma Prl. The effects of TRH and GHRIH on GH secretion by an in vitro dispersed pituitary cell suspension system were very similar to the in vivo studies. TRH stimulated Prl release in vitro, in contrast to the in vivo studies, and the response was dose related. GHRIH had no effect on the basal release of Prl in vitro but significantly inhibited the response to TRH treatment.

Growth hormone secretion: molecular and cellular mechanisms and in vivo approaches.

Growth hormone (GH) release is under the direct control of hypothalamic releasing hormones, some being also produced peripherally. The role of these hypothalamic factors has been understood by in vitro studies together with such in vivo approaches as stalk sectioning. Secretion of GH is stimulated by GH-releasing hormone (GHRH) and ghrelin (acting via the GH secretagogue [GHS] receptor [GHSR]), and inhibited by somatostatin (SRIF). Other peptides/proteins influence GH secretion, at least in some species. The cellular mechanism by which the releasing hormones affect GH secretion from the somatotrope requires specific signal transduction systems (cAMP and/or calcium influx and/or mobilization of intracellular calcium) and/or tyrosine kinase(s) and/or nitric oxide (NO)/cGMP. At the subcellular level, GH release (at least in response to GHS) is accomplished by the following. The GH-containing secretory granules are moved close to the cell surface. There is then transient fusion of the secretory granules with the fusion pores in the multiple secretory pits in the somatotrope cell surface.

Stimulatory Effect of Ghrelin on Isolated Porcine Somatotropes

Research on the mechanism for growth hormone secretagogue (GHS) induction of growth hormone secretion led to the discovery of the GHS receptor (GHS-R) and later to ghrelin, an endogenous ligand for GHS-R. The ability of ghrelin to induce an increase in the intracellular Ca2+ concentration – [Ca2+]i – in somatotropes was examined in dispersed porcine pituitary cells using a calcium imaging system. Somatotropes were functionally identified by application of human growth hormone releasing hormone. Ghrelin increased the [Ca2+]i in a dose-dependent manner in 98% of the cells that responded to human growth hormone releasing hormone. In the presence of (D-Lys3)-GHRP-6, a specific receptor antagonist of GHS-R, the increase in [Ca2+]i evoked by ghrelin was decreased. Pretreatment of cultures with somatostatin or neuropeptide Y reduced the ghrelin-induced increase of [Ca2+]i. The stimulatory effect of ghrelin on somatotropes was greatly attentuated in low-calcium saline and blocked by nifedipine, an L-type calcium channel blocker, suggesting involvement of calcium channels. In a zero Na+ solution, the stimulatory effect of ghrelin on somatotropes was decreased, suggesting that besides calcium channels, sodium channels are also involved in ghrelin-induced calcium transients. Either SQ-22536, an adenylyl cyclase inhibitor, or U73122, a phospholipase C inhibitor, decreased the stimulatory effects of ghrelin on [Ca2+]i transiently, indicating the involvement of adenylyl cyclase-cyclic adenosine monophosphate and phospholipase C inositol 1,4,5-trisphosphate pathways. The nonpeptidyl GHS, L-692,585 (L-585), induced changes in [Ca2+]i similar to those observed with ghrelin. Application of L-585 after ghrelin did not have additive effects on [Ca2+]i. Preapplication of L-585 blocked the stimulatory effect of ghrelin on somatotropes. Simultaneous application of ghrelin and L-585 did not cause an additive increase in [Ca2+]i. Our results suggest that the actions of ghrelin and synthetic GHS closely parallel each other, in a manner that is consistent with an increase of hormone secretion.

Comparative Stimulation of Growth Hormone Secretion in Anaesthetized Chickens by Human Pancreatic Growth Hormone-Releasing Factor (hpGRF) and Thyrotrophin-Releasing Hormone (TRH)

Plasma concentrations of growth hormone (GH) were elevated in anaesthetized male domestic fowl following the intravenous administration of either synthetic human pancreatic GH-releasing factor 1-44 (NH2) (hpGRF) or synthetic thyrotrophin-releasing hormone (TRH). In 6-week-old chicks the plasma GH level was elevated between 5 and 10 min after the injection of hpGRF at doses between 1 and 80 micrograms/kg. The magnitude of the response increased with doses of hpGRF between 1 and 10 micrograms/kg but declined with higher doses. The GH concentration rapidly declined between 10 and 20 min and between 20 and 40 min after injection. The administration of TRH had similar effects on GH secretion, although the responses were greater than with comparable doses of hpGRF, and the most effective dose (1-1.4 micrograms/kg) was less than with hpGRF. In anaesthetized adult cockerels GH secretion was also increased by the administration of hpGRF (1-20 micrograms/kg) or TRH (0.1-80 micrograms/kg) and in both cases the dose-response relationship was biphasic. The maximal response to TRH in adult birds was again greater than that produced by hpGRF although the response was less than that elicited in immature birds and required a higher dose (20 micrograms/kg) of TRH. The optimal dose of hpGRF and the magnitude of the GH response induced in adult birds was comparable with that in immature chicks. These results demonstrate provocative effects of TRH and hpGRF on GH secretion in the domestic fowl. The sensitivity of the GH response to TRH suggests that it may have a physiological role in the hypothalamic control of GH secretion.

angiogenic activity of anterior pituitary tissue and growth hormone on the chick embryo chorio-allantoic membrane : A novel action of GH

A useful system to evaluate the angiogenic activity of hormones and growth factors is the chorioallantoic membrane (CAM) of chick embryos. The present studies examined the angiogenic activity of chicken anterior pituitary glands and both fibroblast growth factor (FGF) and growth hormone (GH). Grafts of anterior pituitary gland evoked an angiogenic response on the CAM which was lost if the adenohypophyseal tissue was first boiled. The magnitude of the angiogenic response to anterior pituitary glands increased with the age of the donor (from a minimum 15 days of embryonic development to a maximum between 2 and 6 weeks old). In view of the similarity of the profile of the angiogenic response and the reported changes in GH secretion, the angiogenic activity of GH was then examined. Considerable angiogenic responses were observed with GH; there being increases (P < 0.05) in number of new blood vessels on the CAM of chick embryos on which native chicken GH or native bovine GH or recombinant bovine GH were added. These data support GH having an angiogenic action.

Hormonal responses and tolerance to cold of female quail following parathion ingestion

Abstract Thirty-week-old female bobwhite quail (Colinus virgininus), maintained at 26 ± 1°C, were provided diets containing 0,25, or 100 ppm parathion ad libitum. After 10 days, birds were exposed to mild cold (6 ± 1°C) for 4, 8, 12, 24, or 48 hr. Brain acetylcholinesterase activity was inhibited in a dose-dependent manner in birds receiving 25 and 100 ppm parathion. Body weight, egg production, and plasma luteinizing hormone and progesterone concentrations were reduced in birds receiving 100 ppm parathion compared with other groups. Cold exposure did not alter plasma corticosterone levels in the 0- and 25-ppm parathion groups, but a two- to fivefold elevation of plasma corticosterone was observed in birds fed 100 ppm parathion. These findings indicate that (i) short-term ingestion of parathion can impair reproduction possibly by altering gonadotropin or steroid secretion, and (ii) tolerance to cold may be reduced following ingestion of this organophosphate.

Enhanced Growth and Immune Development in Dwarf Chickens Treated with Mammalian Growth Hormone and Thyroxine

Abstract The effects of growth hormone and/or thyroxine treatments on antibody production, primary lymphoid organ development, and general body growth were examined in two dwarf strains (sex-linked dwarf—SLD, and autosomal dwarf—ADW) and in a normal-growing strain (K) of White Leghorn chickens. One-day-old male chicks were assigned to experimental groups and were treated either with thyroxine (T4) feed supplements or daily mammalian growth hormone (GH) injections or a combination of these treatments (T4/GH). Within the SLD strain, GH treatments resulted in a significant enhancement (P < 0.005) of humoral immune responsiveness and bursal growth while T4 treatments significantly (P < 0.05) stimulated thymic growth. Overall growth in the SLD was also stimulated by T4 treatments. GH and/or T4 treatments had no specific effects on primary lymphoid organ growth in the ADW strain but either treatment separately resulted in a significant (P < 0.05) increase in overall body size. None of the treatments significantly (P > 0.05) affected any of these parameters in the K-strain controls. Supplementation with T4 significantly elevated serum T4 levels within all strains. There were, however, no significant differences in the serum levels of T4 between strains within any of the treatment groups. Serum triiodothyronine (T3) levels were significantly lower in all treatment groups of the SLD as compared to the K-strain control. ADW serum T3 levels were significantly lower than the K-strain only in the T4-treated group. No differences (P > 0.05) were found between the dwarf and control strains in endogeneous GH levels, although mammalian GH treatments did produce significant changes in serum T3 and T4 levels within the K-strain. These results provide evidence that hormonal manipulations can significantly affect body growth, primary lymphoid organ development, and immune function in the dwarf strains studied and suggest these strains may be useful models for future studies on hormonal interactions and immune function.

Growth hormone and proclatin in avian species

Abstract Considerable progress has been made in study of the physiology of avian growth hormone (GH) and prolactin (PRL) following their purification and the consequent development of radioimmunoassay systems. Plasma GH concentrations are consistently high in the early rapid phase of growth while PRL levels are related to both reproductive and salt/water status. In the chicken, GH secretion appears to be under dual stimulatory and inhibitory hypothalamic influences while PRL is under predominantly stimulatory control. Stress can affect both GH PRL release, normally decreasing GH and elevating PRL levels.

Lipolytic activity of purified pituitary and bacterially derived growth hormone on chicken adipose tissue in vitro.

Abstract The ability of growth hormone (GH) to stimulate lipolysis was examined using chicken abdominal adipose tissue explants incubated in vitro and purified pituitary and bacterially derived chicken and bovine GH. Consistently in the fourth hour of incubation, lipolysis (as determined by glycerol release) was increased by the presence of GH (1 μg/ml), irrespective of pituitary or bacterial derivation or of chicken or bovine origins. This effect of GH was observed with adipose tissue originating from young (6-8 weeks old) intact and hypophysectomized chicks and adult (6-9 months old) male chickens. Glycerol release was also enhanced by lower doses of GH (10 ng/ml with tissue from young and 100 ng/ml with tissue from adult chickens).

Effect of Mammalian Growth Hormone and Prolactin on the Growth of Hypophysectomized Chickens

Body weight gain and shank-toe growth during a 26-day treatment period following hypophysectomy were 55 and 46%, respectively, of control values, but the body weight gain was unaffected and bone growth only slightly reduced when the hypophysectomized chickens were fed a low dose of corticosterone (5 ppm). Bovine growth hormone (0.5 mg GH/kg body wt/day for 18 days) enhanced body weight gain and shank-toe length increase (an estimate of bone growth) by 46 and 33%, respectively, compared to the growth of hypophysectomized chickens receiving only corticosterone. These same endpoints were increased approximately 24% after ovine growth hormone treatment in hypophysectomized chickens not receiving corticosterone. Body weight gain during 18 days of treatment with bovine prolactin (0.5 mg PRL/kg/day) was 27% greater than the value for corticosterone-treated hypophysectomized chickens, but bone growth was unaffected. The mammalian GH preparations increased heart weight of the hypophysectomized chickens (25-29%), but pectoralis muscle weight was unaffected. GH treatment enhanced thymal weights by 71% in corticosterone-treated hypophysectomized chickens, and by 93% in hypophysectomized animals not receiving corticosterone. GH had no significant effect on bursal weights, and PRL had no effect on either of these lymphoid organ weights in corticosterone-treated hypophysectomized chickens. GH increased liver and adipose tissue weights considerably more than the large increases that followed treatment of hypophysectomized chickens with corticosterone alone (69 and 126% greater, respectively), but had no effect on these endpoints in hypophysectomized chickens not receiving corticosterone. PRL also greatly increased liver and adipose tissue weights in corticosterone-treated hypophysectomized chickens (79 and 75%, respectively). These results provide evidence that mammalian GH enhances body weight gain, bone growth, and the growth of several organs in the hypophysectomized chicken. Mammalian PRL increased body weight gain, liver weight, and adipose tissue weight in corticosterone-treated hypophysectomized chickens, but did not influence bone growth or the weights of the heart, pectoralis, thymi, or bursa.