GHRH-SST-GH-IGF axis regulates crosstalk between growth and immunity in rainbow trout (Oncorhynchus mykiss) infected with Vibrio anguillarum

An energy trade-off is existed between immunological competence and growth. The axis of growth hormone releasing hormone, somatostatin, growth hormone, insulin-like growth factor (GHRH-SST-GH-IGF axis) regulates growth performances and immune competences in rainbow trout (Oncorhynchus mykiss). The salmonid-specific whole genome duplication event is known to result in duplicated copies of several key genes in GHRH-SST-GH-IGF axis. In this study, we evaluated the physiological functions of GHRH-SST-GH-IGF axis in regulating crosstalk between growth and immunity. Based on principal components analysis (PCA), we observed the overall expression profiles of GHRH-SST-GH-IGF axis were significantly altered by Vibrio anguillarum infection. Trout challenged with Vibrio anguillarum showed down-regulated igf1s subtypes and up-regulated igfbp1a1. The brain sst genes (sst1a, sst1b, sst3b and sst5) and igfpbs genes (igfbp4s and igfbp5b2) were significantly affected by V. anguillarum infection, while the igfbp4s, igfbp5s, igfbp6s and igf2bps genes showed significant changes in peripheral immune tissues in response to V. anguillarum infection. Gene enrichment analyses showed functional and signaling pathways associated with apoptosis (such as p53, HIF-1 or FoxO signaling) were activated. We further proposed a possible model that describes the IGF and IGFBPs-regulated interaction between cell growth and programmed death. Our study provided new insights into the physiological functions and potentially regulatory mechanisms of the GHRH-SST-GH-IGF axis, indicating the pleiotropic effects of GHRH-SST-GH-IGF axis in regulating crosstalk between growth and immunity in trout.

Thyrotropin-Releasing Hormone (TRH) and Somatostatin (SST), but not Growth Hormone-Releasing Hormone (GHRH) nor Ghrelin (GHRL), Regulate Expression and Release of Immune Growth Hormone (GH) from Chicken Bursal B-Lymphocyte Cultures

It is known that growth hormone (GH) is expressed in immune cells, where it exerts immunomodulatory effects. However, the mechanisms of expression and release of GH in the immune system remain unclear. We analyzed the effect of growth hormone-releasing hormone (GHRH), thyrotropin-releasing hormone (TRH), ghrelin (GHRL), and somatostatin (SST) upon GH mRNA expression, intracellular and released GH, Ser133-phosphorylation of CREB (pCREB^S133), intracellular Ca²⁺ levels, as well as B-cell activating factor (BAFF) mRNA expression in bursal B-lymphocytes (BBLs) cell cultures since several GH secretagogues, as well as their corresponding receptors (-R), are expressed in B-lymphocytes of several species. The expression of TRH/TRH-R, ghrelin/GHS-R1a, and SST/SST-Rs (Subtypes 1 to 5) was observed in BBLs by RT-PCR and immunocytochemistry (ICC), whereas GHRH/GHRH-R were absent in these cells. We found that TRH treatment significantly increased local GH mRNA expression and CREB phosphorylation. Conversely, SST decreased GH mRNA expression. Additionally, when added together, SST prevented TRH-induced GH mRNA expression, but no changes were observed in pCREB^S133 levels. Furthermore, TRH stimulated GH release to the culture media, while SST increased the intracellular content of this hormone. Interestingly, SST inhibited TRH-induced GH release in a dose-dependent manner. The coaddition of TRH and SST decreased the intracellular content of GH. After 10 min. of incubation with either TRH or SST, the intracellular calcium levels significantly decreased, but they were increased at 60 min. However, the combined treatment with both peptides maintained the Ca²⁺ levels reduced up to 60-min. of incubation. On the other hand, BAFF cytokine mRNA expression was significantly increased by TRH administration. Altogether, our results suggest that TRH and SST are implicated in the regulation of GH expression and release in BBL cultures, which also involve changes in pCREB^S133 and intracellular Ca²⁺ concentration. It is likely that TRH, SST, and GH exert autocrine/paracrine immunomodulatory actions and participate in the maturation of chicken BBLs.

Catecholaminergic axonal varicosities appear to innervate growth hormone-releasing hormone-immunoreactive neurons in the human hypothalamus: the possible morphological substrate of the stress-suppressed growth

CONTEXT

Stress is considered to be a major factor in the regulation of growth. Psychosocial dwarfism, characterized with short stature, delayed puberty, and depression, is typically preceded by psychological harassment or stressful environment. It has been observed that stress suppresses GH secretion, possibly via the attenuation of GHRH secretion. However, the exact mechanism of the impact of stress on growth has not been elucidated yet.

OBJECTIVE

Our previous studies revealed intimate associations between neuropeptide Y (NPY)-immunoreactive (IR) axonal varicosities and GHRH-IR perikarya in the human hypothalamus. Because NPY is considered to be a stress molecule, NPY-GHRH juxtapositions may represent an important factor of stress-suppressed GHRH release. In addition to NPY, catecholamines are among the major markers of stress. Thus, in the present study, we examined the putative juxtapositions between the catecholaminergic tyrosine hydroxylase (TH)-/dopamine-β-hydroxylase-/phenylethanolamine N-methyltransferase-IR and GHRH-IR neural elements in the human hypothalamus. To reveal these juxtapositions, double-label immunohistochemistry was used.

RESULTS

Our findings revealed that the majority of the GHRH-IR perikarya formed intimate associations with TH-IR fiber varicosities. The majority of these juxtapositions were found in the infundibular nucleus/median eminence.

CONCLUSIONS

The lack of phenylethanolamine N-methyltransferase-GHRH associations and the small number of dopamine-β-hydroxylase-GHRH juxtapositions suggest that the vast majority of the observed TH-GHRH juxtapositions represent dopaminergic associations. The density of the abutting TH-IR fibers on the surface of the GHRH perikarya suggests that these juxtapositions may be functional synapses, and thus, in addition to NPY, catecholamines may regulate GHRH secretion via direct synaptic mechanisms.

A pleiomorphic GH pituitary adenoma from a Carney complex patient displays universal allelic loss at the protein kinase A regulatory subunit 1A (PRKARIA) locus

Carney complex (CNC) is a familial multiple endocrine neoplasia syndrome associated with GH-producing pituitary tumours and transmitted as an autosomal dominant trait. Mutations of the PRKAR1A gene are responsible for approximately half the known CNC cases but have never found in sporadic pituitary tumours. Pituitary tissue was obtained from an acromegalic CNC patient heterozygote for a common (PRKARIA)i-inactivating mutation. Both immunohistochemistry and electron microscopy showed a highly pleiomorphic pituitary adenoma. The cell culture population appeared morphologically heterogeneous and remained so after more than 30 passages. The mixture was comprised of cells strongly immunostained for GH, spindle-shaped myofibroblast-like cells, and cuboid cells with large axonal projections (negative for GH). The population appeared to have both epithelial and mesenchymal cells. Both at baseline and at passage 30, cytogenetic analysis indicated the presence of normal 46, XY diploid karyotype, whereas losses of the PRKARIA(i) locus were demonstrated in more than 98% of the cells by fluorescent in situ hybridisation, supporting this gene's involvement in pituitary tumorigenesis. Allelic loss may have occurred in a single precursor cell type that differentiated and clonally expanded into several phenotypes. Epithelial-to-mesenchymal transition may also occur in CNC-associated pleiomorphic pituitary adenomas.

Long-term effects on bone of postnatal immunization against GHRH in female and male rats

The effects of neonatal passive immunization against GHRH on bone was examined in male and female rats. Pups were treated subcutaneously with GHRH-antiserum (GHRH-Ab) from day 1 to day 10 of age. Bone mineral content (BMC) and bone mineral density (BMD) were evaluated at monthly intervals until 7 months. Markers of bone resorption (urinary lysylpyridinoline, LP), bone formation (serum osteocalcin, OC) and serum IGF-I were measured at 2, 3 and 7 months. In male rats, GHRH-Ab did not modify BMC and BMD when compared with controls. In contrast, female rats demonstrated lower whole body and femoral BMC and BMD from 2 to 7 months of age. Reduced bone growth in the females was associated with lower IGF-I levels than controls at 2 and 3 months of age, whereas in males IGF-I titers did not change during the period of the study. LP excretion was higher in GHRH-Ab-treated rats at 2 and 3 months in both sexes. In females, no difference in OC values was recorded, whereas in GHRH-Ab-treated males, there was an increase in OC levels at 2 and 3 months. These data indicate that transient GHRH deprivation induces an osteopenic effect in female rats which is not evident in male rats.

Role for central ghrelin in food intake and secretion profile of stomach ghrelin in rats

Ghrelin, a 28-amino-acid peptide, has recently been isolated from the rat stomach as an endogenous ligand for the GH secretagogue receptor. We have reported previously that central or peripheral administration of ghrelin stimulates food intake, and the secretion of GH and gastric acid in rats. In the present study, we investigated how much endogenous centrally released ghrelin is involved in the control of food intake and body weight gain. We also examined the profile of ghrelin secretion from the stomach by RIA using two kinds of anti-ghrelin antiserum, one raised against the N-terminal ([Cys(12)]-ghrelin[1-11]) region and one raised against the C-terminal ([Cys(0)]-ghrelin [13-28]) region of the peptide. The former antibody recognizes specifically ghrelin with n- octanoylated Ser 3 (acyl ghrelin), and does not recognize des-acyl ghrelin. The latter also recognizes des-acyl ghrelin (i.e. total ghrelin). Intracerebroventricular treatment with the anti-ghrelin antiserum against the N-terminal region twice a day for 5 days decreased significantly both daily food intake and body weight. Des-acyl ghrelin levels were significantly higher in the gastric vein than in the trunk. Either fasting for 12 h, administration of gastrin or cholecystokinin resulted in increase of both acyl and des-acyl ghrelin levels. The ghrelin levels exhibited a diurnal pattern, with the bimodal peaks occurring before dark and light periods. These two peaks were consistent with maximum and minimum volumes of gastric content respectively. These results suggest that (1) endogenous centrally released ghrelin participates in the regulation of food intake and body weight, (2) acyl ghrelin is secreted from the stomach, (3) intestinal hormones stimulate ghrelin release from the stomach, and (4) regulation of the diurnal rhythm of ghrelin is complex, since ghrelin secretion is augmented under conditions of both gastric emptying and filling.

Regulation of hepatic insulin-like growth factor I leader exonusage in lambs: effect of immunization against growth hormone-releasing factor and subsequent growth hormone treatment

The establishment of a GH-responsive endocrine IGF-I network is essential for the regulation of postnatal growth. Transcripts of exons 1 and 2 of the mammalian IGF-I gene are alternately spliced onto exon 3, generating class 1 and class 2 mRNA, respectively, each encoding individual signal peptides. The liver is largely responsible for the synthesis of circulating IGF-I and is the main site of expression for class 2 mRNA. The aim of this study was to examine the regulation of hepatic class 1 and 2 mRNA levels in response to changed GH status. Lambs were actively immunized against GRF to suppress GH secretion; hepatic IGF-I mRNA leader exon usage was examined in the presence and absence of GH replacement and in control-immunized lambs. Lambs immunized against GRF exhibited a 17% (P < 0.001) decrease in growth rate as assessed by whole body weight gain, accompanied by decreased circulating IGF-I concentrations (P < 0.001), which were increased by subsequent GH treatment (P < 0.001). Hepatic class 1 and 2 IGF-I mRNA levels decreased in GRF-immunized lambs, although only class 2 transcripts decreased significantly (P < 0.001). Subsequent GH treatment induced increases in class 1 and 2 mRNA levels (P < 0.001) but the increase in class 2 message exceeded that for class 1 (P < 0.001). Thus, the percentage of total IGF-I mRNA accounted for by class 2 mRNA was 45% in control lambs, decreased to less than 20% in GRF-immunized lambs, but increased to 72% in the GRF-immunized lambs treated with GH and correlated with circulating IGF-I concentrations. These data suggest physiological significance for class 1 and 2 IGF-I mRNA species in GH action. Possible functions for such alternative splicing mechanisms are discussed.

Growth hormone-releasing factor affects macronutrient intake during the anabolic phase of zinc repletion: total hypothalamic growth hormone-releasing factor content and growth hormone-releasing factor immunoneutralization during zinc repletion

Growth hormone-releasing factor (GRF) is thought to perform two distinct functions within the brain. GRF synthesized in the median eminence (ME) stimulates the release of growth hormone (GH) from the pituitary, while GRF in the suprachiasmatic nucleus and median preoptic area (SCN/MPOA) may stimulate selection of dietary protein. These two functions may be coupled to regulate and enhance growth. During zinc repletion, a period characterized by increased protein intake and accelerated growth, we examined this coupling by measuring GRF peptide content in hypothalamic sites and neutralizing GRF function by infusing anti-GRF antibody into the hypothalamus during zinc repletion. Total GRF content and GRF content in the ME and SCN/MPOA were decreased in zinc-deficient (Zn-) rats compared to zinc-adequate (Zn+) rats (P < 0.05). There were no differences in GRF content during zinc repletion in either nuclei. Subsequently, we investigated the macronutrient feeding patterns of rats chronically infused with anti-GRF IgG into the lateral ventricle of the brain during zinc repletion. All Zn- and Zn+ rats administered anti-GRF IgG exhibited a reduction in protein intake during zinc repletion. The Zn- rats receiving anti-GRF-IgG consumed equal amounts of total diet compared to those receiving vehicle during the repletion period however they consumed less carbohydrate (P < 0.05) and considerably more fat (P < 0.02). There were no significant differences in carbohydrate or fat intake in Zn+ rats receiving anti-GRF antibody. These results suggest that GRF likely directs protein intake during normal growth, but may interact with additional appetite-controlling neuropeptides during zinc repletion.

Unequal autonegative feedback by GH models the sexual dimorphism in GH secretory dynamics

Growth hormone (GH) secretion, controlled principally by a GH-releasing hormone (GHRH) and GH release-inhibiting hormone [somatostatin (SRIF)] displays vivid sexual dimorphism in many species. We hypothesized that relatively small differences within a dynamic core GH network driven by regulatory interactions among GH, GHRH, and SRIF explain the gender contrast. To investigate this notion, we implemented a minimal biomathematical model based on two coupled oscillators: time-delayed reciprocal interactions between GH and GHRH, which endow high-frequency (40-60 min) GH oscillations, and time-lagged bidirectional GH-SRIF interactions, which mediate low-frequency (occurring every 3.3 h) GH volleys. We show that this basic formulation, sufficient to explain GH dynamics in the male rat [Farhy LS, Straume M, Johnson ML, Kovatchev BP, and Veldhuis JD. Am J Physiol Regulatory Integrative Comp Physiol 281: R38-R51, 2001], emulates the female pattern of GH release, if autofeedback of GH on SRIF is relaxed. Relief of GH-stimulated SRIF release damps the slower volleylike oscillator, allowing emergence of the underlying high-frequency oscillations that are sustained by the GH-GHRH interactions. Concurrently, increasing variability of basal somatostatin outflow introduces quantifiable, sex-specific disorderliness of the release process typical of female GH dynamics. Accordingly, modulation of GH autofeedback on SRIF within the interactive GH-GHRH-SRIF ensemble and heightened basal SRIF variability are sufficient to transform the well-ordered, 3.3-h-interval, multiphasic, volleylike male GH pattern into a femalelike profile with irregular pulses of higher frequency.

A construct of interactive feedback control of the GH axis in the male

Growth hormone (GH) secretion is controlled by GH-releasing hormone (GHRH), the GH release-inhibiting hormone somatostatin (SRIF), and autofeedback connections. The ensemble network produces sexually dimorphic patterns of GH secretion. In an effort to formalize this system, we implemented a deterministically based autonomous feedback-driven construct of five principal dose-responsive regulatory interactions: GHRH drive of GH pituitary release, competitive inhibition of GH release by SRIF, GH autofeedback via SRIF with a time delay, delayed GH autonegative feedback on GHRH, and SRIF inhibition of GHRH secretion. This formulation engenders a malelike pattern of successive GH volleys due jointly to positive time-delayed feedback of GH on SRIF and negative feedback of SRIF on GH and GHRH. The multipeak volley is explicated as arising from a reciprocal interaction between GH and GHRH during periods of low SRIF secretion. The applicability of this formalism to neuroendocrine control is explored by initial parameter sensitivity analysis and is illustrated for selected feedback-dependent experimental paradigms. The present construct is not overparameterized and does not require an ad hoc pulse generator to achieve pulsatile GH output. Further evolution of interactive constructs could aid in exploring more complex feedback postulates that confer the vivid sexual dimorphism of female GH profiles.

Interactions of growth hormone secretagogues and growth hormone-releasing hormone/somatostatin

The class of novel synthetic compounds termed growth hormone secretagogues (GHSs) act in the hypothalamus through, as yet, unknown pathways. We performed physiologic and histochemical studies to further understand how the GHS system interacts with the well-established somatostatin (SRIF)/growth hormone-releasing hormone (GHRH) neuroendocrine system for regulating pulsatile GH secretion. Comparison of the GH-releasing activities of the hexapeptide growth hormone-releasing peptide-6 (GHRP-6) and GHRH administered intravenously to conscious adult male rats showed that the pattern of GH responsiveness to GHRP-6 was markedly time-dependent, similar to that observed with GHRH. Immunoneutralization of endogenous SRIF reversed the blunted GH response to GHRP-6 at trough times, suggesting that GHRP-6 neither disrupts nor inhibits the cyclical release of endogenous hypothalamic SRIF. By striking contrast, passive immunization with anti-GHRH serum virtually obliterated the GH responses to GHRP-6, irrespective of the time of administration. These findings suggest that the GHSs do not act by altering SRIF release but, rather, stimulate GH release via GHRH-dependent pathways. Our dual chromogenic and autoradiographic in situ hybridization experiments revealed that a subpopulation of GHRH mRNA-containing neurons in the arcuate (Arc) nucleus and ventromedial nucleus (VMN) of the hypothalamus expressed the GHS receptor (GHS-R) gene. These results provide strong anatomic evidence that GHSs may directly stimulate GHRH release into hypophyseal portal blood, and thereby influence GH secretion, through interaction with the GHS-R on GHRH- containing neurons. Altogether, these findings support the notion that an additional neuroendocrine pathway may exist to regulate pulsatile GH secretion, possibly through the influence of the newly discovered GHS natural peptide, ghrelin.

Differential expression of gonadotropin and prolactin antigens by GHRH target cells from male and female rats

There is a 2- to 3-fold increase in luteinizing hormone-beta (LHbeta) or follicle-stimulating hormone-beta (FSHbeta) antigen-bearing gonadotropes during diestrus in preparation for the peak LH or FSH secretory activity. This coincides with an increase in cells bearing LHbeta or FSHbeta mRNA. Similarly, there is a 3- to 4-fold increase in the percentage of cells that bind GnRH. In 1994, we reported that this augmentation in gonadotropes may come partially from subsets of somatotropes that transitionally express LHbeta or FSHbeta mRNA and GnRH-binding sites. The next phase of the study focused on questions relating to the somatotropes themselves. Do these putative somatogonadotropes retain a somatotrope phenotype? As a part of ongoing studies that address this question, a biotinylated analog of GHRH was produced, separated by HPLC and characterized for its ability to elicit the release of GH as well as bind to pituitary target cells. The biotinylated analog (Bio-GHRH) was detected cytochemically by the avidin-peroxidase complex technique. It could be displaced by competition with 100-1000 nM GHRH but not corticotropin-releasing hormone or GnRH. In cells from male rats exposed to 1 nM Bio-GHRH, 28+/-6% (mean+/-s.d) of pituitary cells exhibited label for Bio-GHRH (compared with 0.8+/-0.6% in the controls). There were no differences in percentages of GHRH target cells in populations from proestrous (28+/-5%) and estrous (25+/-5%) rats. Maximal percentages of labeled cells were seen following addition of 1 nM analog for 10 min. In dual-labeled fields, GHRH target cells contained all major pituitary hormones, but their expression of ACTH and TRH was very low (less than 3% of the pituitary cell population) and the expression of prolactin (PRL) and gonadotropins varied with the sex and stage of the animal. In all experimental groups, 78-80% of Bio-GHRH-reactive cells contained GH (80-91% of GH cells). In male rats, 33+/-6% of GHRH target cells contained PRL (37+/-9% of PRL cells) and less than 20% of these GHRH-receptive cells contained gonadotropins (23+/-1% of LH and 31+/-9% of FSH cells). In contrast, expression of PRL and gonadotropins was found in over half of the GHRH target cells from proestrous female rats (55+/-10% contained PRL; 56+/-8% contained FSHbeta; and 66+/-1% contained LHbeta). This reflected GHRH binding by 71+/-2% PRL cells, 85+/-5% of LH cells and 83+/-9% of FSH cells. In estrous female rats, the hormonal storage patterns in GHRH target cells were similar to those in the male rat. Because the overall percentages of cells with Bio-GHRH or GH label do not vary among the three groups, the differences seen in the proestrous group reflect internal changes within a single group of somatotropes that retain their GHRH receptor phenotype. Hence, these data correlate with earlier findings that showed that somatotropes may be converted to transitional gonadotropes just before proestrus secretory activity. The LH and FSH antigen content of the GHRH target cells from proestrous rats demonstrates that the LHbeta and FSHbeta mRNAs are indeed translated. Furthermore, the increased expression of PRL antigens by these cells signifies that these convertible somatotropes may also be somatomammotropes.

Effects of active immunization against growth-hormone releasing factor on puberty and reproductive development in gilts

Hormones within the somatotropin cascade influence several physiological traits, including growth and reproduction. Active immunization against growth hormone-releasing factor (GRFi) initiated at 3 or 6 mo of age decreased weight gain, increased deposition of fat, and delayed puberty in heifers. Two experiments were conducted to investigate the effects of GRFi on puberty and subsequent ovulation rate in gilts. Crossbred gilts were actively immunized against GRF-(1-29)-(Gly)2-Cys-NH2 conjugated to human serum albumin (GRFi) or against human serum albumin alone (HSAi). In Exp. 1, gilts were immunized against GRF (n = 12) or HSA (n = 12) at 92 +/- 1 d of age. At 191 d of age, antibody titers against GRF were greater (P < .05) in GRFi (55.5 +/- 1.3%) than in HSAi (.4 +/- 2%) gilts. The GRFi decreased (P < .05) BW (86 +/- 3 vs 104 +/- 3 kg) by 181 d of age and increased (P < .05) backfat depth (15.7 +/- .4 vs 14.8 +/- .4 mm) by 130 d of age. At 181 d of age, GRFi reduced the frequency of ST release (1.0 +/- .5 vs 5.0 +/- .5, peaks/24 h; P < .0001) and decreased (P < .01) ST (1.1 +/- .06 vs 1.7 +/- .06 ng/mL), IGF-I (29 +/- 2 vs 107 +/- 2 ng/mL), and insulin concentrations (3.5 +/- .2 vs 6.3 +/- .2 ng/mL). The GRFi decreased (P < .05) feed conversion efficiency but did not alter age at puberty (GRFi = 199 +/- 5 d vs HSAi = 202 +/- 5 d) or ovulation rate after second estrus (GRFi = 10.7 +/- .4 vs HSAi = 11.8 +/- .5). In Exp. 2, gilts were immunized against GRF (n = 35) or HSA (n = 35) at 35 +/- 1 d of age. The GRFi at 35 d of age did not alter the number of surface follicles or uterine weight between 93 and 102 d of age, but GRFi decreased (P < .05) ovarian weight (.41 +/- .08 vs 1.58 +/- .4 g) and uterine length (17.2 +/- 1.1 vs 25.3 +/- 2.3 cm). Immunization against GRF reduced (P < .05) serum IGF-I (GRFi = 50 +/- 4 vs HSAi = 137 +/- 4 ng/mL) and BW (GRFi = 71 +/- 3 vs HSAi = 105 +/- 3 kg) and increased (P < .05) backfat depth (GRFi = .38 +/- .03 vs HSAi = .25 +/- .02 mm/kg). Age at puberty was similar in GRFi and HSAi gilts, but ovulation rate was lower (P < .05) after third estrus in GRFi (11.3 +/- .8) than in HSAi (13.8 +/- .8) gilts. Thus, GRFi at 92 or 35 d of age decreased serum ST, IGF-I, and BW in prepubertal gilts without altering age of puberty. However, GRFi at 35 d of age, but not 92 d of age, decreased ovulation rate. These results indicate that alterations in the somatotropic axis at 1 mo of age can influence reproductive development in pubertal gilts.

Immune function in transgenic mice overexpressing growth hormone (GH) releasing hormone, GH or GH antagonist

Effects of life-long exposure to high levels of homologous or heterologous growth hormone (GH) and effects of GH resistance on selected parameters of immune function were studied in adult male transgenic mice overexpressing GH releasing hormone (GHRH), bovine (b) GH or an antagonistic bGH analog. In metallothionein I (MT)-bGH transgenic mice with high peripheral levels of bovine GH, there were significant increases in the absolute weight of the thymus and the spleen and in the mitogenic responses of splenocytes to concanavalin A (ConA), lipopolysaccharide (LPS) and phytohemagglutinin (PHA), as compared to age-matched normal animals. There were no significant differences between MT-bGH transgenic and normal mice in splenocyte viability or in delayed-type hypersensitivity measured by the allergic contact dermatitis response to oxazolone. Similar results, including significant stimulation of splenocyte responses to ConA, LPS, and PHA, were obtained in MT-hGHRH transgenic mice in which overexpression of GHRH leads to striking pituitary enlargement and massive elevation of peripheral levels of homologous (mouse) GH. In MT-bGH-antagonist transgenic mice in which overexpression of an antagonistic bGH analog interferes with the actions of endogenous GH, spleen weight was reduced but proliferative responses of splenocytes to ConA, LPS, and PHA were not affected. It is concluded that overexpression of heterologous or homologous GH in transgenic mice can lead to significant stimulation of some parameters of immune function, whereas antagonism of GH action by expression of an antagonistic GH analog does not affect splenocyte responses to mitogens.

Humoral regulation of physiological sleep: cytokines and GHRH

Interleukin-1, tumour necrosis factor, and growth hormone releasing hormone form part of the humoral mechanisms regulating physiological sleep. Their injection enhances non-rapid-eye-movement sleep whereas their inhibition reduces spontaneous sleep and sleep rebound after sleep deprivation. Changes in their mRNA levels and changes in their protein levels in the brain are consistent within their proposed role in sleep regulation. Furthermore, results from transgenic and mutant animals also are suggestive of their role in sleep regulation. The sites responsible for the growth hormone releasing hormone somnogenic activity seem to reside in the anterior hypothalamus/basal forebrain. Somnogenic sites for interleukin-1 and tumour necrosis factor likely include the anterior hypothalamus, but also may extend beyond that area. These substances elicit non-rapid-eye-movement sleep via a biochemical cascade that includes other known sleep regulatory substances.

Suppression of growth hormone does not affect ongoing spermatogenesis in rats

Recent evidence suggests that growth hormone (GH) may enhance physiologic processes, such as spermatogenesis, in addition to causing classical anabolic effects. We have previously shown that testosterone restores spermatogenesis in rats that were made azoospermic by immunization against gonadotropin-releasing hormone (GnRH). In this study, we investigated whether suppression of GH affects spermatogenesis and the ability of testosterone to restore spermatogenesis following immunization against GnRH and/or growth hormone-releasing hormone (GHRH). Twelve rats were actively immunized against GnRH (anti-GnRH), twelve rats were actively immunized against GHRH (anti-GHRH), six rats were immunized against both GnRH and GHRH (anti-GnRH/GHRH), and six rats served as controls. Two weeks after the second booster, six rats each from the anti-GnRH and anti-GHRH groups as well as the six anti-GnRH/GHRH rats received 24-cm testosterone-filled Silastic implants (T), and the remaining six rats from each of these groups received empty Silastic implants. All rats were euthanized 2 months later. Weights of testes and testicular sperm counts were determined. Serum testosterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), and insulin-like growth factor-1 (IGF-1) concentrations were determined by radioimmunoassays. Serum GH and IGF-1 were suppressed in anti-GHRH rats. IGF-1 was partially restored by testosterone in anti-GHRH and in anti-GnRH/GHRH rats, but GH was restored to control value in anti-GnRH/GHRH rats. Serum LH and FSH were suppressed in anti-GnRH and anti-GnRH/GHRH rats, but only FSH was partially restored by testosterone. Suppression of GH did not affect maintenance of spermatogenesis. However, because T partially restored GH and IGF-1 levels in anti-GnRH/GHRH rats and because spermatogenesis was found to be restored in these rats, we conclude that GH does not play a role in the maintenance of spermatogenesis in adult rats, but it may be required for the replenishment of germ cells in experimentally induced regressed rat testes.

Hexarelin exhibits protective activity against cardiac ischaemia in hearts from growth hormone-deficient rats

Male rats were treated with growth hormone (GH)-releasing hormone antiserum to induce selective GH deficiency. The chronic administration of hexarelin to these GH-deficient rats had a pronounced protective effect against ischaemic and post-ischaemic ventricular dysfunction. Hexarelin prevented hyper-responsiveness of the coronary vascular bed to angiotensin II and also prevented the reduction in generation of 6-keto-prostaglandin F1alpha in perfused hearts from GH-deficient rats. The most plausible interpretation of these findings is that hexarelin acts via stimulation of specific cardiac and vascular receptors, triggering currently unknown cytoprotective mechanisms that are responsible for resistance to ischaemic insults and for the preservation of the integrity of the endothelial vasodilation function.

Cardiac ischemia and impairment of vascular endothelium function in hearts from growth hormone-deficient rats: protection by hexarelin

The ability of hexarelin, an effective growth hormone (GH)-releasing hexapeptide, to reverse the worsening of cardiac dysfunction in GH-deficient animals was studied in young male rats passively immunized by administration of an anti-GH-releasing hormone (GHRH) serum. Heart preparations from anti-GHRH serum-treated rats, undergoing low-flow ischemia and reperfusion, showed: (1) a progressive increase of left ventricular end-diastolic pressure during the ischemic period and a poor recovery of contractility at reperfusion with a consistent decrease of the left ventricular-developed pressure; (2) a decreased rate of formation of 6-keto-prostaglandin F1alpha (6-keto-PGF1alpha), a stable metabolite of prostacyclin, in perfusates from preischemic and reperfusion periods; (3) an increased vasopressor activity of angiotensin II. Hexarelin (80 microg/kg, bid, s.c.), administered for 15 days to anti-GHRH serum-treated rats, restored to normal the impaired somatotropic function and counteracted the ischemic damage, improving postischemic left ventricular developed pressure to values higher than those of controls. Furthermore, both the generation of 6-keto-PGF1alpha and the vasopressor activity of angiotensin II reverted to those of control preparations. Administration of hexarelin to control rats induced a considerable improvement of postischemic ventricular function of the perfused hearts which was similar to that present in preparations from anti-GHRH serum-treated rats given hexarelin. This protective activity was divorced from any further stimulation of somatotropic function. Collectively, these data indicate that, in GH-deficient rats, hexarelin is capable of restoring somatotropic function and has a beneficial effect in myocardial ischemia and reperfusion damage. In addition, the increased responsiveness of the coronary vasculature to angiotensin II and the decreased generation of prostacyclin in hearts from GH-deficient rats would indicate that for prevention of injury and dysfunction of the vascular endothelium a normal somatotropic function is mandatory.

Effects of GH and IGF-I administration on GHRH and somatostatin mRNA levels: II. A study in the infant rat

It is generally accepted that growth hormone influences its own secretion by modulating the activity of GHRH and SRIF neurons. To investigate if GH feedback mechanisms are already operating in the early postnatal life of the rat, we have studied in 10-day-old pups the effects of rhGH and rhIGF-I administration on GHRH and somatostatin mRNA levels. The same experiment was also performed in pups passively immunized with an anti-GHRH antiserum from the day of birth. The latter animal model had been previously characterized for presenting reduced levels of circulating GH and IGF-I. In control pups, neither rhGH (250 micrograms/kg, b.i.d., sc) nor rhIGF-I (150 micrograms/kg, b.i.d., sc) administration induced significant changes of GHRH and SRIF gene expression. The passive immunization against GHRH induced per se a trend toward an increase and a reduction of GHRH and SRIF mRNA levels, respectively. Also in these rats the treatment for 3 days with rhGH and rhIGF-I did not further modify the GHRH and SRIF mRNA levels. Based on these results, we conclude that in the 10-day-old rat GH feedback mechanisms are poorly operative, though a direct ultra-short loop mechanism involving the GHRH and SRIF systems seems already operating.

Hexarelin stimulation of growth hormone release and mRNA levels in an infant and adult rat model of impaired GHRH function

Hexarelin, a GH-releasing peptide, is an effective GH secretagogue in man and a variety of experimental animals. In the present study, we sought to characterize the effects of short-term Hexarelin treatment on GH release and GH mRNA levels in infant and young-adult rats and in rats of either age passively immunized with an antiserum against GHRH (GHRH-Ab). Hexarelin (80 micrograms/kg, b.i.d. s.c.), administered for 3, 5 or 10 days to 8-, 6- and 1-day-old rats, respectively, induced a progressive enhancement of the plasma GH rise elicited by a subsequent acute Hexarelin (80 micrograms/kg s.c) challenge when pups were 10 days old. As expected, GHRH-Ab treatment decreased GH concentrations in 10-day-old pups. In GHRH-Ab-treated pups, Hexarelin administration for 3-10 days significantly enhanced the GH response to the acute Hexarelin injection, though the mean plasma GH values remained significantly lower than in the respective control group. Hexarelin treatment did not alter GH mRNA levels in control pups. In GHRH-Ab-treated pups Hexarelin treatment for 3 and 5 days, but not 10 days, restored GH mRNA levels to control values. In young-adult male rats, regardless of antiserum treatment, Hexarelin administration for 5 or 10 days significantly suppressed the GH response to a subsequent acute challenge with the peptide. Yet, 5-10 days of Hexarelin treatment did not alter GH mRNA in control young-adult rats. In adult rats GHRH-Ab also decreased GH mRNA levels, but 10 days of Hexarelin treatment were necessary to return GH mRNA back to normal levels. These results indicate that: (1) the effects of Hexarelin on GH release and GH mRNA levels may be unrelated events; (2) deprivation of GHRH function discloses the ability of Hexarelin to stimulate GH mRNA levels; (3) age plays a crucial role in setting the pituitary responsiveness to short-term Hexarelin treatment, and (4) the different ability of Hexarelin to stimulate GH release and GH synthesis in neonatal and young-adult rats may have clinical relevance in the chronic administration of the peptide.

Characterization of the ovine LH beta-subunit gene: the promoter is regulated by GnRH and gonadal steroids in transgenic mice

We have previously demonstrated that 1.9 kb of ovine LH beta promoter fused to bacterial chloramphenicol transferase (CAT) coding sequence is sufficient to target expression of the transgene specifically to the gonadotroph cells of the anterior pituitary in mice with no expression being observed in other tissues. However, it is not known if this region of the ovine LH beta promoter contains the necessary elements that confer transcriptional regulation by gonadal steroids and GnRH. Following gonadectomy, both endogenous pituitary LH and CAT activity significantly (P > 0.001) increased as did plasma LH. This post-gonadectomy increase in CAT, pituitary and plasma LH could be suppressed in females by treatment with oestradiol alone or oestradiol and progesterone, with an additional significant (P < 0.05) reduction in CAT activity being observed in one line following the combined steroid treatment. In castrated males, testosterone suppressed CAT activity in one line. Treatment of transgenic ovariectomized females with oestradiol alone significantly suppressed plasma LH (P < 0.01) with no change in pituitary LH content. There was no difference in pituitary LH between oestradiol-treated ovariectomized transgenic and non-transgenic females. Treatment of intact females from both lines with either GnRH antiserum or agonist demonstrated a decrease in pituitary CAT activity whereas similar treatment in intact males had no effect. While endogenous pituitary LH concentrations were variable, plasma LH was lower in all treated animals irrespective of line, sex or expression of the transgene. In conclusion, these results indicate that (1) the presence of CAT protein is not toxic and does not compromise either endogenous LH synthesis, storage and secretion and (2) the ovine LH beta-CAT gene is regulated in a similar but more variable manner to the endogenous LH beta gene. This may relate to the use of CAT as a reporter where its release is not necessarily related to that of the endogenous hormone whose synthesis, storage and release may differ.

Regulation of pituitary growth hormone-releasing factor (GRF) receptor gene expression by GRF

We examined how growth hormone-releasing factor (GRF) regulated pituitary GRF receptor gene expression in the conscious rat. GRF receptor mRNA levels were significantly increased by immunoneutralization of endogenous GRF with its specific antiserum. This effect was dose dependent and the maximum level was 3.8-fold higher than that in control rats. A similar rise in GRF receptor gene expression was obtained by the depletion of noradrenaline, a neurotransmitter thought to stimulate GRF release, and was reversed by 87% by the repeated administration of synthetic GRF. These results indicate that pituitary GRF receptor gene expression was up- or down-regulated in vivo in the absence or presence of GRF, respectively.

Immunization against growth hormone releasing factor or chronic feed restriction initiated at 3.5 months of age reduces ovarian response to pulsatile administration of gonadotropin-releasing hormone at 6 months of age and delays onset of puberty in heifers

A severe or moderate suppression of serum insulin-like growth factor I (IGF-I) was induced in heifers, beginning at 104 days of age, by active immunization against growth hormone-releasing factor (GRFi) or by chronic feed restriction (RES), respectively. We hypothesized that reduced serum IGF-I results in decreased serum estradiol-17 beta (E2), which in turn delays onset of puberty. The objectives of this experiment were to determine 1) whether GRFi and RES would alter follicular development and delay onset of puberty through similar mechanisms, and 2) whether GnRH would enhance follicular growth in control, GRFi, and RES heifers at 6 mo of age. Changes in IGF-I, somatotropin, LH, FSH, and E2 were evaluated. Serum IGF-I was greater in control than in RES heifers, and was greater in both these groups than in GRFi heifers by 169 days of age. Basal LH decreased in control and RES but not in GRFi heifers from 136 to 157 days of age. During the same period, a decrease in mean FSH was detected in control but not in GRFi and RES heifers. RES decreased mean serum E2 from 148 to 183 days of age. At 6 mo of age, pulsatile administration of GnRH (5 micrograms every 2 h for 42-46 h) increased serum LH and FSH similarly across treatments but had no effect on the number of follicles > or = 8 mm in GRFi and RES heifers relative to saline treatment. Serum E2 and IGF-I in follicular fluid from follicles > or = 8 mm were increased in all GnRH-treated heifers; however, concentrations of both hormones were lower in GRFi than in control or RES heifers. The main effect of treatments on serum IGF-I was reflected in follicles < or = 7 mm; follicular fluid IGF-I was greater in control than in RES heifers and was greater in both these groups than in GRFi heifers. Serum E2 was lower in RES than in control and GRFi heifers from 253 to 281 days of age. Because of an interaction, E2 was lower in GRFi-GnRH than in control-GnRH heifers but similar in GRFi-saline and control-saline heifers. By 393 days of age, 0% of RES and 32% of GRFi heifers had reached puberty compared to 71% of control heifers. These data support our hypothesis that decreased serum IGF-I results in decreased serum E2. GRFi appears to delay puberty in heifers because decreased serum IGF-I impairs the ovary's ability to synthesize preovulatory concentrations of E2, thereby delaying stimulation of an LH surge. In contrast, RES may delay puberty by delaying follicular development at two stages: a) decreased IGF-I in follicles < or = 7 mm may delay predominant follicular growth, and b) decreased LH may delay maturation of the preovulatory follicle.

Ovarian expression of insulin-like growth factor-I (IGF-I), IGF binding proteins, and growth hormone (GH) receptor in heifers actively immunized against GH-releasing factors

Active immunization against GRF at 6 months of age delays puberty in beef heifers. The objectives of the present study were to determine whether active immunization against GRF at an earlier age would affect normal onset of puberty and follicular growth and to determine whether these changes were related to alterations in ovarian insulin-like growth factor I (IGF-I) or IGF binding protein (IG-FBP) messenger RNA (mRNA) levels. Heifers were immunized against human serum albumin (HSAi; n = 15) or against GRF conjugated to HSA (GRFi; n = 18) at 3 months of age. A third group of heifers was not immunized (CON; n = 16). Immunization against GRF delayed puberty beyond 13 months of age in 75% of treated heifers. Unilateral ovariectomy at 191 days of age revealed that the delay in puberty was associated with a reduction in the number of large ( > or = 7 mm in diameter) follicles. Large follicles were present in only 22% of GRFi heifers compared to 77% of HSAi heifers. The number of small ( < or = 3 mm in diameter) and medium (4 to 6 mm in diameter) follicles was not affected by GRFi. The percentage of 1- to 3-mm follicles that were atretic was not different between HSAi (65%) and GRFi (62%) heifers. Unilateral ovariectomy had no effect on age at puberty. Immunization against GRF decreased (P < 0.01) concentrations of IGF-I in serum (23 +/- 2 ng/ml) compared to HSAi heifers (109 +/- 11 ng/ml). IGF-I levels in follicular fluid (FFL) of medium and small follicles were also decreased by GRFi from 82 +/- 3 ng/ml in HSAi heifers to 48 +/- 6 ng/ml (P < 0.01). Levels of IGFBP-3 (determined by ligand blot analysis) in serum and FFL of small follicles were decreased by GRFi (P < 0.01). In contrast, IGFBP-2 serum levels were increased from 422 +/- 32 ng/ml in HSAi heifers to 657 +/- 6 ng/ml in GRFi heifers (P < 0.05). Likewise, IGFBP-2 levels in FFL from small and medium follicles were increased from 785 +/- 44 ng/ml to 926 +/- 44 ng/ml (P < 0.05). Ligand blot analysis indicated that IGFBP levels were lower in FFL from large vs. small follicles. The band intensities of IGFBP-4 and -5 were drastically reduced ( > 80%) while the decreases in IGFBP-2 and -3 were less marked ( < 50%). The decreased levels of IGFBP-5 in FFL from large follicles was not associated with an increase in proteolytic fragments detectable by immunoblot analysis. While mRNA transcripts for IGF-I, GH receptor, and IGFBP-2, -3, -4, and -5 were readily detectable in ovarian tissue, GRFi had no effect on ovarian levels of mRNA for each of these proteins. This suggests that the decrease in follicular development associated with GRFi may be related to changes in circulating IGF-I and/or IGFBPs.

Perfusion immunoaffinity chromatography and its application in analysis and purification of biomolecules

Immunoaffinity cartridges have been prepared by immobilizing the monoclonal antibody to bovine growth hormone (bGH) and bovine growth hormone releasing factor (bGRF) on the streptavidin-coated perfusion media. The cartridge-immobilized antibody to bGH has been used for the analysis of standard sample of recombinant bGH (r-bGH). The cartridge immobilized bGRF has been applied for purification of the C-and N-terminal antibodies and their conjugates with horseradish peroxidase. The purity of the antibody fractions purified by bGRF cartridges has been tested by capillary zone electrophoresis. It has been shown that the immunoaffinity cartridges prepared by immobilizing the antigen offer more biological specificity for the purification of antibody than the Protein G cartridges. The stability and life time of the immunoaffinity cartridges are mainly dependent on the stability of the biomolecules immobilized on them. Immunoaffinity cartridges based on the perfusion media offer us the tools for rapid analysis and purification of antigens or antibodies as well as for determination of the biological activity between them at a very low back pressure in the columns.

N-methyl-D,L-aspartate-induced growth hormone secretion in barrows: possible mechanisms of action

Four experiments were conducted to determine mechanisms by which n-methyl-d,l-aspartate (NMA) increases serum concentrations of growth hormone (GH). Blood samples were collected from barrows every 15 min for 2 h (Exp. 1, 2, and 3) or 3 h (Exp. 4) immediately before and immediately after i.v. treatments. In Exp. 1, barrows (n = 4/treatment) received either .9% saline or 1.25, 2.5, or 5 mg of NMA/kg of BW. The change in circulating GH concentrations was greater (P < .05) for barrows receiving 2.5 mg (by 883%) or 5.0 mg of NMA/kg of BW (by 1,095%) than for those injected with saline. In Exp. 2, barrows (n = 4/treatment) received NMA (2.5 mg/kg of BW) or injections of 1.25 mg of the pure d or pure 1 isomers of NMA/kg of BW. Growth hormone concentrations increased by 177% (P < .025) after NMA treatment and by 245% (P < .01) after injection of the pure d isomer of NMA. The pure 1 isomer of NMA had no effect (P > .1) on GH concentrations. In Exp. 3, barrows received NMA (2.5 mg/kg of BW) 10 min after i.m. injection of saline (n = 7) or ketamine hydrochloride ( n = 8; 19.9 mg/kg of BW), an n-methyl-d-aspartate (NMDA) receptor antagonist. The NMA increased (P < .01) GH concentrations by 289% in saline-pretreated barrows but had no effect (P > .1) on barrows pretreated with ketamine hydrochloride. In Exp. 4, barrows (n = 4/treatment) received NMA 3 h after i.v. pretreatment with antisera to GH-releasing factor (GRF; 154 mL) or no pretreatment. Serum GH concentrations increased by 166% (P < .05) after injection of NMA in barrows receiving no pretreatment. The NMA had no effect (P > .1) on GH concentrations in barrows receiving antisera to GRF. Our results support the concept that NMDA stimulates GRF, and hence GH secretion, by activating an NMDA receptor.

Aspartate and glutamate modulation of growth hormone secretion in the pig: possible site of action

The influence of excitatory amino acids (EAA) on growth hormone (GH) secretion and the possible site of action was investigated in the pig. In Experiment (Exp) I three replicates were conducted with 30 prepuberal gilts, 130 d of age and averaging 70.6 +/- 1.3 kg body weight (BW). Six gilts each received intravenously (i.v.) 0, 50, 100, or 150 mg/kg BW of aspartate (ASP) or glutamate (GLU) in saline. Blood samples were collected every 15 min for 2 hr before and 3 hr after treatment. In Exp II, mature ovariectomized gilts (163 +/- 10 kg BW) that had been immunized against growth hormone releasing factor (GRF) conjugated to human serum albumin (GRFi; n = 4) or against human serum albumin alone (HSAi; n = 5) received 150 mg/kg BW ASP or GLU i.v. in a 2 x 2 factorial arrangement of treatments, which was then repeated in a crossover design. One week later, all animals received 10 mg/kg N-methyl-D,L-aspartate (NMA; EAA agonist) in saline i.v. Blood samples were collected as described above. In Exp III, cultures of anterior pituitary cells from market-weight (averaging 105 kg BW) gilts were studied. On Day 4 of culture, cells (10(5) seeded/well) were challenged with 10(-8), 10(-6), or 10(-4) M ASP or GLU, 10(-6) M [Ala15]-human GRF (1-29)-NH2, or the EAA antagonist, 2-amino-5-phosphonopentanoic acid (10(-4) M; AP5), alone or in combination with ASP or GLU. In Exp I, all doses of ASP and the 100- and 150-mg doses of GLU increased (P < 0.05) GH secretion when compared with Time 0. However, serum GH concentrations were higher (P < 0.01) after 150 mg/kg of ASP when compared with those after 150 mg/kg of GLU. In Exp II, serum GH concentrations increased (P < 0.05) in HSAi but not in GRFi pigs (averaging 1.2 +/- 0.2 ng/ml before and 8.2 +/- 0.7, 6.3 +/- 0.5, and 9.2 +/- 0.5 ng/ml by 15 min after ASP, GLU, and NMA, respectively). In Exp III, relative to controls (40 +/- 6 ng/ml), GH increased (P < 0.05)1.6-, 1.9-, and 1.9-fold and 1.7-, 1.8-, and 2.0-fold after 10(-8), 10(-6), and 10(-4) M ASP and GLU, respectively. The EAA receptor antagonist AP5 failed to prevent the GH response to ASP or GLU, except for 10(-8) M ASP. In summary, ASP is a more potent secretagogue of GH secretion than is GLU in vivo, whereas each is equipotent in vitro. Because no stimulation of GH by EAA was observed in GRFi pigs and no specific dose-response effect of EAA was found in vitro, it may be concluded that modulation by EAA is mediated primarily at the level of the hypothalamus or higher brain centers.

Immuno-enhancement and -inhibition of GH-releasing factor by site-directed anti peptide antibodies in vivo and in vitro

It is now well established that specific antibodies and binding proteins can potentiate rather than inhibit hormone activity. In order to investigate this phenomenon further, the current study was undertaken using a hormone with a characterised structure, in terms of receptor binding, and for which activity has already been manipulated in specific ways (prolongation of half-life, increased receptor affinity) using synthetic hormone analogues. GH-releasing factor (GRF) is a 40 or 44 residue peptide and is, together with somatostatin, responsible for the regulation of GH secretion. The effects of site-directed anti peptide antibodies were determined on the activity of GRF in vivo and in vitro as GH release. The peptide regions of GRF were: 1-14 (part of putative receptor-binding region) and 31-44 and 35-44 (sites thought to be distant from the receptor-binding region). Five sheep were administered GRF (1 microgram/kg), anti peptide immunoglobulin (Ig; a calculated tenfold excess binding to GRF dose), or GRF together with anti peptide Ig (preincubated for 1 h). GRF induced a significant increase in plasma GH concentration over the next 240 min, this was abolished when GRF was administered with anti 1-14 Ig (P < 0.05) and augmented (P < 0.05) when GRF was administered with anti 35-44 Ig; anti 31-44 had no effect on GRF activity. Anti 35-44 Ig alone induced an increase in GH secretion which was equivalent to that for GRF alone, implying that the antibody had interacted and potentiated with endogenous GRF. The Ig effects on exogenous GRF activity were confirmed for GH release in vitro using primary cultures of sheep pituitary cells, except that anti 31-44 Ig also augmented GH release (P < 0.05) when co-administered with GRF. (ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic octreotide treatment on GH secretory dynamics and tumor growth in rats bearing an ectopic somatotroph (GC) tumor

The effects of octreotide, a long-acting somatostatin agonist selective of the sstr2/sstr3/sstr5 receptor subtypes, on ectopic GH secretion and tumor growth were investigated in Wistar-Furth female rats implanted with GH secreting (GC) cells which express mostly somatostatin receptors of the sstr1 and sstr2 subtypes. Octreotide dose dependently inhibited thymidine incorporation (-57%) and GH secretion (-41%) from GC cells in culture. In vivo, 6 weeks after GC cell implantation, plasma GH, IGF-1 and insulin levels were highly elevated. Cluster analysis of GH secretory dynamics revealed that GH secretion was less pulsatile in GC-implanted than in control animals. Furthermore, in GC-implanted animals, passive immunization either with SRIH or GHRH antisera, did not affect GH plasma levels. Three weeks after GC cell implantation, when tumors became palpable, octreotide (1 micrograms/h/kg BW) or saline was infused constantly for three weeks by osmotic minipumps. In octreotide treated rats, GH, IGF-1 and insulin levels were not different from sham-implanted animals and tumors weight were reduced by 80%. High affinity somatostatin binding sites were found in equivalent amounts on tumors from octreotide-treated or saline-treated animals. These findings indicate that GH secretion in GC-rats is mainly derived from the tumors and independent of hypothalamic control and that octreotide reduces both GH secretion and tumor growth. We conclude that the GC-implanted rat represents a good animal model to test the antisecretory and antitrophic properties of somatostatin analogs in vivo.

Endocrine and ovarian responses to exogenous estradiol-17 beta in 6-month-old heifers previously immunized against growth hormone-releasing factor

We hypothesized that estradiol-17 beta (E2) was capable of stimulating an LH surge in 6-mo-old heifers immunized against growth hormone-releasing factor (GRFi). To test this hypothesis Angus x Simmental heifers were immunized against GRF (n = 11) or human serum albumin (HSAi) (n = 6) at 113 d of age. Heifers were injected i.m. with 500 micrograms of E2 at 172 d of age. To characterize the LH response, samples were collected at frequent intervals for 1 h before and 32 h after E2. To assess ovarian responses, real-time ultrasonography was performed from d 9 to 20, and serum progesterone was evaluated from d 6 to 25 after E2, then weekly for 13 wk. Exogenous E2 stimulated (P < .001) LH surges, preceded by negative E2 feedback that decreased serum LH, in all GRFi and HSAi heifers. During the negative feedback period, serum LH was greater (P < .001), and the change in LH from before to after E2 was lower (P < .05), in GRFi than in HSAi heifers. Puberty occurred in 7/17 heifers within 66 d after receiving E2. We conclude that GRFi does not impair the release of an E2-induced LH surge, but it does decrease hypothalamo-hypophyseal sensitivity to the inhibitory effect of E2.

Effect of actively immunizing sheep against growth hormone-releasing hormone or somatostatin on spontaneous pulsatile and neostigmine-induced growth hormone secretion

The physiological role of endogenous circulating GH-releasing hormone (GHRH) and somatostatin (SRIH) on spontaneous pulsatile and neostigmine-induced secretion of GH was investigated in adult rams actively immunized against each neuropeptide. All animals developed antibodies at concentrations sufficient for immunoneutralization of GHRH and SRIH levels in hypophysial portal blood. In the anti GHRH group, plasma GH levels were very low; the amplitude of GH pulses was strikingly reduced, although their number was unchanged. No stimulation of GH release was observed after neostigmine administration. The reduction of GH secretion was associated with a decreased body weight and a significant reduction in plasma IGF-I concentration. In the anti-SRIH group, no changes in basal and pulsatile GH secretion or the GH response to neostigmine were observed as compared to controls. Body weight was not significantly altered and plasma IGF-I levels were reduced in these animals. These results suggest that in sheep, circulating SRIH (in the systemic and hypophysial portal vasculature) does not play a significant role in pulsatile and neostigmine-induced secretion of GH. The mechanisms of its influence on body weight and production of IGF-I remain to be determined.

Mechanism of action of hexarelin and GHRP-6: analysis of the involvement of GHRH and somatostatin in the rat

We have recently reported oral and parenteral bioactivity for a new GH-releasing peptide, hexarelin. In the present study, we have examined the neuroendocrine mechanism by which hexarelin and GHRP-6, two GH-releasing peptides, mediate their actions. Although previous studies have looked at the role of growth hormone-releasing hormone (GHRH) and somatostatin in regulating the action of GHRP-6 in culture and in stressed animals, our study looked at the role of both somatostatin and GHRH in regulating the action of hexarelin as well as GHRP-6 in conscious and freely-moving, nonstressed rats. Adult male rats, prepared with indwelling jugular catheters, were pretreated i.v. with either control antiserum (CTLas), growth hormone-releasing hormone antiserum (GHRHas), somatostatin antiserum (SSas), or both GHRHas and SSas. Animals were then treated i.v. with 25 micrograms/kg of either hexarelin or GHRP-6 4 h after i.v. antisera pretreatment. Blood samples were collected every 20 min for the 3 h prior to peptide treatment and at 5, 10, 15, 20, 40 and 60 min following hexarelin or GHRP-6 injection. The peak plasma GH responses in rats pretreated with CTLas were 552 +/- 125 ng/ml following hexarelin administration and 386 +/- 132 ng/ml following GHRP-6 administration. Rats pretreated with SSas exhibited peak GH responses following hexarelin or GHRP-6 of 702 +/- 115 and 312 +/- 42 ng/ml, respectively. These plasma GH responses were similar to those observed in the CTLas-pretreated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Mouse placental cells secrete immunoreactive growth hormone-releasing factor

The initial objective of this study was to establish a placental cell culture system in which the secretion of mouse growth hormone-releasing factor (mGHRF) could be examined during a several-day period. To determine when during pregnancy placental cells begin to express mGHRF, Northern blot analysis was carried out on total RNA from placentas collected on Days 6, 9, 11, 13, 15, 17, and 18 of pregnancy. Mouse GHRF mRNA could be detected as early as Day 11 of pregnancy. Its steady-state levels increased to maximum values on Days 15-17 and then declined slightly on Day 18. Placentas from Day 12 of pregnancy were selected for cell culture. The basal zone and labyrinth were dispersed in collagenase, and the cells were fractionated on a Percoll gradient. Two bands of cells were selected for further study. Both released significant amounts of immunoreactive mGHRF during a 5-day culture period. Effects of prolonged exposure of the cells to 8-bromo-cAMP and to agents that elevate intracellular cAMP concentration were then examined. Treatment of the cells with 0.5 mM 8-bromo-cAMP resulted in a significant decrease in the mGHRF concentration of the medium by the second day of culture. Mouse GHRF secretion was also inhibited by treatment of the cells with 100 ng/ml cholera toxin or 0.1 mM forskolin. The effect of 8-bromo-cAMP was concentration-dependent, with 0.1 mM being the lowest concentration that was active. 8-Bromo-cAMP treatment also reduced the steady-state level of mGHRF mRNA in the cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone (GH) deprivation induced by passive immunization against rat GH-releasing factor does not disturb the course of sexual maturation and fertility in the female rat

The importance of normal GH secretion for the onset of sexual maturation is a subject of controversy. Also, the need to achieve a minimal body size or body fat content has been postulated to be of importance for determining the timing of the onset of puberty. To evaluate the importance of GH secretion on the onset of sexual maturation in the female rat, GH deprivation has been induced by treating prepubertal rats with antirat GRF serum to passively immunize these animals against GRF. Chronic administration of anti-GRF serum produced in all series an impressive reduction in growth rate (from 5 to 2 g/day), resulting in a body weight averaging 50-60% the normal value at 50 days of life. Despite this deficit in growth, sexual maturation, as established by vaginal opening and first estrous cycles, occurred at the normal age in three of four series of rats; in one series, however, sexual maturation was delayed by 4 days, but thereafter, all parameters indicated that the gonadotropic axis was normally activated. In one series, fertility was tested at 59 days of age in females with a body weight corresponding to 51% of the control weight; these females conceived and delivered a reduced number of pups (9.4 +/- 0.7 instead of 14.2 +/- 0.8 in control dams), but the pups were of normal size. In a second experimental approach, the effect of GH deprivation was evaluated in a model of late sexual maturation obtained by severe food restriction followed by a switch to ad libitum feeding. Severe food restriction initiated at approximately 28 days, when the body weight was 75 g, drastically reduced the growth rate and completely prevented sexual maturation. A switch to ad libitum feeding at 50 days provoked an important compensatory growth and the occurrence of sexual maturation 4 days later. Passive immunization against GRF during this recovery phase did reduce the growth rate, but did not delay sexual maturation. Plasma insulin-like growth factor-I (IGF-I) secretion was very low in food-restricted rats and in each situation with induced GH deprivation. During food restriction, plasma IGF-binding protein-3 (IGFBP-3) and to a lesser extent IGFBP-1 were decreased, and IGFBP-2 was increased; after switching to ad libitum feeding, plasma levels of IGFBP-2 normalized, but levels of IGFBP-1 and IGFBP-3 remained low in the face of normalized plasma IGF-I levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth hormone-releasing hexapeptide is a potent stimulator of growth hormone gene expression and release in the growth hormone-releasing hormone-deprived infant rat

The growth hormone-releasing hexapeptide (GHRP-6) specifically stimulates growth hormone (GH) secretion in several animal species and humans. The mechanism of action of GHRP-6 is largely unknown, although experimental evidence indicates that it may modulate growth hormone-releasing hormone (GHRH) and somatostatin actions at the pituitary or hypothalamic level. To gain more insight into the mechanism(s) of action of GHRP-6, we studied the infant rat, an animal model highly responsive to GH-releasing stimuli. In 14-d-old rats GHRP-6 (32-600 micrograms/kg, s.c.) induced a marked and dose-dependent rise in plasma GH concentrations, maximal stimulation occurring with the dose of 300 micrograms/kg. Neither GHRH nor somatostatin antiserum prevented or modified the GH release elicited by GHRP-6. In pups passively immunized with GHRH antibodies, a 5-d treatment with GHRP-6 (80 micrograms/kg, s.c., twice daily) completely counteracted the inhibitory effect of GHRH deprivation on GH mRNA expression. In vitro GHRP-6 (10(-7) and 10(-6) M) induced a small and transient stimulation of GH release from cultured pituitary cells. These results indicate the following: 1) GHRP-6 is a potent stimulator of GH release in rat pups; 2) it stimulates GH gene expression in the GHRH-deprived pup; 3) during the neonatal period its action is not mediated by GHRH or somatostatin; and 4) its actions are not directed at the somatotrophs.

Effect of daily replacement therapy with recombinant bovine somatotropin on somatotropin, insulin-like growth factor I, and onset of puberty in beef heifers immunized against growth hormone-releasing factor

Two experiments examined whether replacement therapy with recombinantly derived bovine somatotropin (rbST) would induce puberty in heifers that had been actively immunized at 6 mo of age against growth hormone-releasing factor (GRF). Heifers received daily i.m. injections of 25 mg of rbST (Exp. 1, n = 6; Exp. 2, n = 4) or vehicle (VEH; Exp. 1, n = 6; Exp. 2, n = 4) for 56 d. Serum concentrations of somatotropin (ST, nanograms/milliter) were low in all heifers before first injection in Exp. 1 (1.56 +/- .04) and 2 (.95 +/- .03). During treatment, serum ST was greater (P < .01) in rbST than in VEH heifers (75.4 +/- 4.8 vs 2.8 +/- .1 ng/mL, respectively) in both experiments and remained increased through d 57 (32.2 +/- 6.4 vs .90 +/- .01 ng/mL). IN Exp. 1 and 2, concentrations of serum IGF-I were similar in rbST and VEH heifers before treatment, increased (P < .01) 12 h after first rbST, and remained increased (P < .01) through d 57 in rbST heifers. Concentrations of serum insulin (INS) and plasma glucose (GLU) were similar (P > .10) in rbST and VEH heifers before first injection (Exp. 1 and 2). Serum INS (micro-units/milliliter) was greater (P < .01) in rbST (61.7 +/- 3.7 and 36.0 +/- 2.4) than in VEH (12.4 +/- 1.6 and 8.1 +/- 1.0) heifers on d 1 or 2 only, in Exp. 1 and 2, respectively. In Exp. 1, GLU was increased (P < .05) by rbST on d 2 through 57, but only on d 1 in Exp. 2. Proportion of heifers pubertal by d 21 tended to be greater (P < .07) in rbST (3 of 6) than in VEH (0 of 5) heifers in Exp. 1, but not in Exp. 2 (1 of 4 vs 1 of 4, respectively). All heifers in Exp. 1 and 50% of the heifers in Exp. 2 attained puberty by d 56. Daily rbST increased ST, IGF-I, INS, and GLU but did not hasten onset of puberty in heifers immunized against GRF.

Antigenicity and immunogenicity of experimental equine influenza ISCOM vaccines

A comparison of the antigenicity and immunogenicity of ISCOM vaccines prepared from equine influenza viruses H3N8 and H7N7 was made with inactivated whole-virus vaccines containing equivalent amounts of virus haemagglutinin. ISCOMs stimulated superior antibody responses in terms of both amount and duration. As with conventional whole-virus vaccines, the levels of antibody to virus haemagglutinin induced by ISCOMs correlated with protection.

Concentrations of hormones and metabolites, estimates of metabolism, performance, and reproductive performance of sows actively immunized against growth hormone-releasing factor

Cyclic females actively immunized against growth hormone-releasing factor (GRF; n = 5) or human serum albumin (HSAi; n = 4) were used to determine the effects of reduced serum somatotropin (ST) and IGF-I on metabolism and production in gestating and lactating sows. Sows farrowed, pigs were weaned at 28 d of lactation, and sows were observed for estrus after weaning. Blood samples were collected at 15-min intervals for 5 to 6 h on d 110 of gestation and d 21 of lactation. Mean ST (nanograms/milliliter) was less (P < .05) in GRFi than in HSAi sows at d 21 of lactation, but it was similar at d 110 of gestation. Serum concentrations of IGF-I were less (P < .05) in GRFi than in HSAi sows at d 21 of lactation but not at d 110 of gestation. Serum thyroxine was greater (P < .05) in GRFi than in HSAi sows during gestation and lactation. Sows actively immunized against GRF weighed less (P < .01) and had more backfat (P < .01) at d 110 of gestation than HSAi sows; in vitro glucose oxidation and lipogenic rate of adipose tissue were greater (P < .05 for treatment x day interaction) in GRFi than in HSAi sows at d 110 of gestation. Across treatment, mammary gland oxidation of glucose and glucose clearance rates increased (P < .05), whereas adipose tissue utilization of glucose decreased (P < .05) during lactation. During lactation, GRFi sows mobilized more (P < .05) backfat than did controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Negative regulation of hypothalamic growth hormone-releasing factor messenger ribonucleic acid by growth hormone and insulin-like growth factor I

Increased growth hormone-releasing factor messenger ribonucleic acid (GRF mRNA) and decreased somatostatin (SRIF) mRNA levels have been reported in the hypothalamus of hypophysectomized rats as well as of dwarf mice. In order to elucidate the effect of the growth hormone-insulin-like growth factor I (GH-IGF-I) axis on hypothalamic GRF and SRIF synthesis, we measured levels of mRNA coding for GRF and SRIF and for pituitary GH in pubertal male rats treated for 3 weeks with antirat GRF gamma-globulin (GRF-ab), anti-SRIF gamma-globulin (SRIF-ab) or both. Immunoneutralization of circulating endogenous GRF resulted in a marked decrease in serum IGF-I and pituitary GH mRNA levels in Northern blot analysis, whereas it caused a significant increase in GRF mRNA levels in the arcuate nucleus as assessed by both Northern blot and in situ hybridization analysis. SRIF mRNA levels in the periventricular nucleus were slightly decreased by GRF-ab treatment when analyzed by in situ hybridization, but not significantly after Northern blot analysis. Immunoneutralization of circulating endogenous SRIF failed to affect mRNA levels of hypothalamic GRF and SRIF but caused a slight reduction in pituitary GH mRNA levels. Levels of mRNA coding for hypothalamic GRF and pituitary GH were also measured by Northern blot analysis in young male rats treated with rat GRF-ab for 2 weeks and replaced with rat GH or IGF-I for the second 1 week. Replacement with either rat GH or IGF-I suppressed the increased hypothalamic GRF mRNA levels. These data indicate that endogenous GRF is essential for normal synthesis of pituitary GH and that both GH and IGF-I negatively regulate the synthesis of hypothalamic GRF.

Characterization and localization of mouse hypothalamic growth hormone-releasing factor and effect of gold thioglucose-induced hypothalamic lesions

Hypothalamic growth hormone-releasing factor (GRF) in higher mammals, including human GRF, is a 44 amino acid residue peptide and is highly homologous in structure. By contrast, mouse GRF (mGRF) recently deduced by cDNA cloning consists of only 42 residues and shows relatively low homology to the GRFs of higher mammals and the same rodent species, rat. To characterize and localize the predicted mature mGRF peptide in the hypothalamus, we have generated its antiserum and developed a homologous radioimmunoassay. Immunoreactive mGRF in the acid hypothalamic extract was eluted as a single peak at a position identical to that of synthetic peptide on both gel filtration chromatography and reverse-phase high-performance liquid chromatography (HPLC). Secretion of immunoreactive mGRF from incubated hypothalami increased several fold in response to 50 mM K+, and this rise was abolished in the absence of medium Ca2+. Only a single peak of immunoreactive mGRF that coeluted with synthetic replicate was observed after the K(+)-stimulated medium was extracted on Bond Elut C18 cartridges and applied on reverse-phase HPLC. Immunohistochemistry identified many mGRF-positive cell bodies in the arcuate nucleus and dense bundles of immunoreactive fibers in the median eminence. Treatment of mice with gold thioglucose (GTG), a chemical agent known to cause hypothalamic lesions, markedly depleted both content and in vitro secretion of immunoreactive mGRF. The decline in mGRF secretion was greater in GTG obese than in nonobese mice, whereas somatostatin secretion was not affected by GTG treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of somatotropin and/or equine chorionic gonadotropin on serum and follicular insulin-like growth factor I and insulin-like growth factor binding proteins in cattle

We investigated the effect of administration of somatotropin (ST) and/or eCG on insulin-like growth factor I (IGF-I) and IGF-binding proteins (IGFBP) in serum and follicular fluid (FFL) of cattle actively immunized against growth hormone-releasing factor (GRF). Cyclic beef cattle, previously immunized against GRF-(1-29)-Gly-Gly-Cys-NH2 conjugated to human serum albumin (synthesized and provided by Hoffmann-LaRoche, Inc., Nutley, NJ; GRFi, n = 31) or to human serum albumin alone (HSAi, n = 26), received (i.m.): 1) 25 mg recombinantly derived methionyl somatotropin (rbST, n = 14; sometribove provided by Monsanto Co., St. Louis, MO); 2) 1100 IU eCG (n = 10); 3) rbST and eCG (rbST-eCG, n = 15); or 4) vehicle (VEH, n = 17) at 0 and 24 h after receiving prostaglandin F2 alpha (PGF2 alpha). Serum samples were collected at 0 and 40 h after PGF2 alpha, and the ovary bearing the largest follicle (DOM) was removed 44.0 +/- 0.5 h after PGF2 alpha; FFL was harvested from DOM and the subordinate follicle (SUB). Before treatment (0 h), GRFi cows had lower serum ST (0.6 +/- 0.2 vs. 2.2 +/- 0.2 ng/ml; p < 0.01) and IGF-I (26 +/- 4 vs. 72 +/- 4 ng/ml; p < 0.01), but greater IGFBP-2 (594 +/- 48 vs. 384 +/- 52 ng/ml; p < 0.01) than HSAi cows. Serum and FFL concentrations of IGF-I or IGFBP-2 were not different between rbST- and rbST-eCG-treated cows or between VEH- and eCG-treated cows at Hour 40 after the initial treatment injection; therefore, data were combined and designated as rbST and VEH, respectively. Serum IGF-I was increased to a greater extent (percentage increase above 0 h) by rbST treatment in GRFi (362 +/- 24) than in HSAi (176 +/- 16) cows (immunization by treatment, p < 0.01). Across GRFi and HSAi, rbST lowered serum IGFBP-2 (342 +/- 31 vs. 541 +/- 27 ng/ml, rbST vs. VEH; p < 0.01). Diameters of DOM or SUB were not affected by immunization or treatment. Concentrations of IGF-I and IGFBP-3 (determined by ligand blot analysis) in FFL from both DOM and SUB were lower (p < 0.05) in GRFi than in HSAi cows. In contrast, IGFBP-2 in FFL was elevated in SUB, but not DOM, in GRFi compared to HSAi cows.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term changes of somatotrophic function induced by deprivation of growth hormone-releasing hormone during the fetal life of the rat

We have studied the effects of intra-amniotic administration of an anti-GH-releasing hormone serum (GHRH-Ab) on day 16 of fetal life in the rat, when the ontogenetic development of the GHRH neuronal system occurs. Control animals received normal rabbit serum. Following delivery, body weight was monitored for the next 30 days as an index of somatic growth, and the following indices of somatotrophic function were determined: plasma and pituitary GH, pituitary GH mRNA, hypothalamic GHRH and somatostatin mRNA, and the in vivo GH responsiveness to GHRH. At birth, GHRH-Ab-treated rats had a body weight that was equivalent to that of control rats but, starting from postnatal day 6 up to day 30, they had a significantly reduced body weight. Pituitary weight, the absolute pituitary GH content and GH mRNA levels were lower in experimental compared with control rats, while pituitary GH concentrations were similar in the two groups, thus implying that there was a defect, not only in GH synthesis, but also in GH release. In agreement with this theory, basal GH levels and GHRH-stimulated GH secretion were reduced in GHRH-Ab-treated rats but, in contrast, hypothalamic regulation of GH secretion appeared to be working in these rats as they were still able to respond to the low plasma GH by increasing GHRH and decreasing somatostatin mRNA levels. These findings indicate that deprivation of GHRH during fetal life induces long-lasting changes of growth rate and somatotrophic function.(ABSTRACT TRUNCATED AT 250 WORDS)

Feedlot performance, carcass characteristics, hormones, and metabolites in steers actively immunized against growth hormone-releasing factor

Large-framed Simmental and Charolais steers were actively immunized against growth hormone-releasing factor (GRF) to evaluate the effect on growth, carcass characteristics (especially intramuscular fat deposition), and concentrations of somatotropin (ST) and IGF-I. Primary immunizations of 1.5 mg of GRF-(1-29)-Gly-Gly-Cys-NH2 conjugated to 1.5 mg of human serum albumin (GRFi, n = 12) or 1.5 mg of human serum albumin (HSAi, n = 12) were given at approximately 10 mo of age. Booster immunizations of .5 mg of the appropriate antigen were given at d 49 and 125. Weights of steers administered GRFi were less (P < .05) than those given HSAi at 126 d (34.6 kg) or at 262 d (48.2 kg) after treatment. Carcass weights were 28.2 kg less (P < .01) for GRFi than for HSAi steers. Dry matter intake was not affected by immunization treatment, whereas feed efficiency was reduced in GRFi steers. Marbling scores were higher (P < .05) for HSAi than for GRFi steers but similar percentages (83.3) of both treatments graded Low Choice or higher. Rib sections of GRFi steers contained more fat (31.2 vs 25.0%) and less lean (63.3 vs 68.4%) than those of HSAi steers (P < .05). A breed x treatment interaction was observed for percentage of fat within the trimmed longissimus muscle (P < .05); percentage of fat was similar for Charolais and Simmental steers when immunized against HSAi but was higher for Simmental than for Charolais when immunized against GRFi.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in serum somatotropin, somatotropin mRNA, and serum and follicular insulin-like growth factor-I in response to feed restriction in cows actively immunized against growth hormone-releasing factor

Cyclic cows immunized against growth hormone-releasing factor (GRFi, n = 19), human serum albumin (HSAi, n = 10), or not immunized (CON, n = 18) were used to investigate the effects of feed restriction on serum and pituitary somatotropin (ST), pituitary ST mRNA, and serum and follicular IGF-I. Cows were either fed 2.7 kg/d cottonseed hulls (R) or given ad libitum access to feed (AL) for 15 d. Ovaries bearing the largest follicle and pituitaries were collected on d 14, at 44 to 45 h after injection of prostaglandin F2 alpha. Data from CON and HSAi cows were similar; thus, data were combined (represented as CON). Serum ST (nanograms/milliliter) on d 13 was greater (P < .09) in CON-R (5.3) than in CON-AL (3.9), whereas ST in GRFi-AL (1.1) and GRFi-R (1.1; pooled SE = .4) were similar. Hemipituitary weight (grams) and ST mRNA (arbitrary units) were greater (P < .05) in CON (1.5 +/- .1 and 135 +/- 25) than in GRFi (1.0 +/- .1 and 90 +/- 18) cows. Across immunization, ST mRNA and pituitary ST concentration (mg/100 mg of tissue), respectively, were greater (P < .06) in R (152 +/- 22 and 22.5 +/- 1.9) than in AL (73 +/- 16 and 17.3 +/- 1.8) cows. Immunization and diet decreased (P < .05) serum IGF-I (nanograms/milliliter) on d 13 (CON, 176 +/- 7 vs GRFi, 42 +/- 8; AL, 120 +/- 7 vs R, 98 +/- 8). Concentrations of IGF-I in follicular fluid (FFL) from the largest follicle were lower in GRFi (29 +/- 3) than in CON (102 +/- 6) cows; however, IGF-I in FFL was similar in AL (70 +/- 9) and R (71 +/- 10) cows. In conclusion, GRFi decreased serum ST and IGF-I, and decreased ST mRNA. Feed restriction increased serum ST and ST mRNA, and decreased serum IGF-I. Although feed restriction and GRFi decreased serum IGF-I, concentrations of IGF-I in FFL were decreased only by GRFi.

Effect of feed restriction on serum somatotropin, insulin-like growth factor-I-(IGF-I) and IGF binding proteins in cyclic heifers actively immunized against growth hormone releasing factor

Feed restriction often increases serum somatotropin (ST) and decreases insulin-like growth factor-I (IGF-I) in ruminants; however, the mechanisms responsible for this change in ST and IGF-I are not well defined. We investigated the effects of feed restriction on serum ST, IGF-I, IGF binding proteins (IGFBP), insulin and nonesterified fatty acids (NEFA) in cyclic Angus and Charolais heifers (n = 15) previously immunized against growth hormone releasing factor (GRFi) or human serum albumin (HSAi). Cows were fed a concentrate diet ad libitum (AL) or were restricted to 2 kg cotton seed hulls (R) for 4 d. Each heifer received each dietary treatment in a single reversal design. As anticipated, GRFi decreased ST, IGF-I and insulin (P < .05). In addition, GRFi decreased serum IGFBP-3 (P < .01), but increased IGFBP-2 (P < .01). Feed restriction resulted in an increase in serum ST in HSAi, but not in GRFi heifers. Regardless of immunization treatment, feed restriction decreased serum IGF-I and insulin, and increased NEFA (P < .01). In conclusion, the increase in serum ST levels observed during feed restriction was blocked by active immunization against GRF. However, feed restriction resulted in decreased serum IGF-I in GRFi heifers in spite of initial low levels of IGF-I (due to GRFi). Although GRFi decreased levels of IGFBP-3 and increased levels of IGFBP-2, feed restriction for 4 d did not alter serum IGFBP.

Somatostatin reduces transcription of the growth hormone gene in rats

To examine whether somatostatin (SS) exerts influences on the steady state levels of GH-releasing factor (GRF), the effect of SS on GH gene transcription was examined in rats. This approach was used because it has been shown that GRF stimulates GH gene transcription independent of GH release, and SS does not inhibit basal or GRF-stimulated GH gene transcription. Therefore, it is assumed that an effect of SS on GH gene transcription would be mediated by a change in GRF levels. Adult female Sprague-Dawley rats were provided with right atrial cannulae. Studies were performed using unanesthetized rats. Pituitary GH gene transcription was measured by transcription assay. An iv administration of antiserum to rat GRF 150 min previously significantly decreased GH gene transcription compared with that in control rats given normal goat serum. A continuous infusion of SS (300 micrograms/kg.h) via the cannula for 150 min significantly decreased GH gene transcription compared with that in control rats receiving 0.9% NaCl. When GRF (3 micrograms/kg.h) was given simultaneously with SS (300 micrograms/kg.h), GH gene transcription increased significantly compared with that in rats receiving SS infusion alone. After the withdrawal of SS infusion, GH gene transcription rapidly and significantly increased. The data suggest that SS reduces the steady state levels of GRF.

Evidence for a common neural mechanism mediating growth hormone-releasing factor-induced and somatostatin-induced feeding

We have previously reported that the somatotropin regulatory peptides, growth hormone-releasing hormone (GRF) and somatostatin (SS) increase feeding when applied centrally in low picomole-range doses. The similarity between the dose-response characteristics and photoperiod sensitivities of these two peptide's feeding effects, together with indirect evidence suggesting that these peptides interact functionally within the hypothalamus, suggests that GRF and SS may induce feeding via a common neural mechanism. The present studies sought to investigate this possibility. In the first experiment combined intracerebroventricular (icv) injections of optimal doses of GRF and SS were found to be less potent than icv injections of GRF or SS alone, suggesting that the mechanisms mediating GRF-induced and SS-induced feeding are not totally independent. In further studies, icv injections of antiserum raised against SS blocked GRF-induced feeding. In contrast, icv injections of antiserum raised against GRF were ineffective in blocking SS-induced feeding. These results suggest that SS may act downstream to GRF in a putative neural feeding mechanism which utilizes both peptides. The possible anatomical basis for such a feeding mechanism involving GRF and SS is discussed.

Growth hormone-releasing hormone antibodies suppress sleep and prevent enhancement of sleep after sleep deprivation

Previous reports suggest that the hypothalamic growth hormone-releasing hormone (GHRH) promotes sleep, especially non-rapid-eye-movement sleep (NREMS). To evaluate the role of endogenous GHRH in sleep regulation, the effects of antibodies to rat GHRH (GHRH-ab) were studied on normal sleep, brain temperature (Tbr), and GH secretion in experiment I and on enhanced sleep after sleep deprivation in experiment II. In experiment I, affinity-purified GHRH-ab (50 and 200 micrograms) raised in goats and a control goat immunoglobulin G (IgG) preparation were injected intracerebroventricularly (icv) in rats 1 h before the onset of the light cycle, and sleep-wake activity and Tbr were recorded for the next 12 or 23 h. Both doses of GHRH-ab suppressed NREMS and REMS throughout the light cycle. Sleep durations at night were normal. Electroencephalographic (EEG) slow-wave activity, characterized by EEG slow-wave amplitudes, was reduced after GHRH-ab during both the light and the dark cycles. Plasma GH concentrations measured 6-12 h after injection of GHRH-ab (200 micrograms) were diminished. Both the control IgG and GHRH-ab elicited fever. In experiment II, the sleep-wake activity and Tbr of rats were recorded for 24 h in three experimental conditions: base-line with icv injection of IgG, 3-h sleep deprivation with icv IgG injection, and 3-h sleep deprivation with icv GHRH-ab (200 micrograms). After sleep deprivation (+IgG), a prompt increase in EEG slow-wave activity (power density analysis) and late increases in NREMS and REMS durations were found.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term failure of compensatory growth in rats following acute neonatal passive immunization against growth hormone-releasing hormone

Interruption of hypothalamic growth hormone-releasing hormone (GHRH) secretion by administration of antiserum against GHRH (GHRH-ab) decreases growth hormone (GH) secretion and inhibits growth in rats. The present study was undertaken to investigate whether there is a period of accelerated or catch-up growth following a period of growth arrest induced by GHRH-ab treatment. Neonatal male and female rats were injected daily on days 1-14 of age. Animals received normal rabbit serum (NRS) or GHRH-ab subcutaneously at a dose of 5 microliters/10 g body weight. Body weight, serving as an index of somatic growth, was monitored over the next 3 months. The increase in absolute body weight and growth velocity of GHRH-ab-treated rats, regardless of gender, was lower than the increase of NRS-treated animals. Significant decreases were observed by day 13 of age in the female rats and day 17 in the male rats. The percent differences and absolute difference in weight between the two treatment groups clearly demonstrated that the GHRH-ab-treated rats did not demonstrate any period of catch-up growth. A second group of animals was treated in a similar fashion to evaluate serum GH concentrations at three months of age. Pulsatile GH secretion, as assessed by peak frequency and amplitude, was normal in all of the rats, suggesting that the failure of catch-up growth in the GHRH-ab-treated animals was not due to decreased GH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)