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1.
This study was designed to examine the effects of the proportion of concentrate in the diet on the secretion of growth hormone (GH), insulin and insulin‐like growth factor‐I (IGF‐I) secretion and the GH‐releasing hormone (GHRH)‐induced GH response in adult sheep fed once daily. Dietary treatments were roughage and concentrate at ratios of 100:0 (0% concentrate diet), 60:40 (40% concentrate diet), and 20:80 (80% concentrate diet) on a dry matter basis. Mean plasma concentrations of GH before daily feeding (10.00–14.00 hours) were 11.4 ± 0.4, 10.1 ± 0.5 and 7.5 ± 0.3 ng/mL on the 0, 40 and 80% concentrate diet treatments, respectively. A significant decrease in plasma GH concentration was observed after daily feeding of any of the dietary treatments and these decreased levels were maintained for 8 h (0%), 12 h (40%) and 12 h (80%), respectively (P < 0.05). Plasma IGF‐I concentrations were significantly decreased 8–12 h and 4–16 h after the end of feeding compared with the prefeeding level in the 40 and 80% concentrate diet treatments, respectively (P < 0.05). GHRH injection brought an abrupt increase in the plasma GH concentrations, reaching a peak 10 min after each injection, but, after the meal, the peak plasma GH values for animals fed 40% (P < 0.05) and 80% (P < 0.01) concentrate diet were lower than that for roughage fed animals. The concentrate content of a diet affects the anterior pituitary function of sheep resulting in reduced baseline concentrations of GH and prolonged GH reduction after feeding once daily.  相似文献   

2.
The effects of l ‐DOPA, a precursor of dopamine (DA), and sulpiride, a D2‐type DA receptor blocker, on growth hormone (GH) and prolactin (PRL) secretion were investigated in steers. Eight Holstein steers (212.8 ± 7.8 kg body weight) were used. Lighting conditions were 12:12 L:D (lights on: 06.00–18.00 hours). Blood samplings were performed during the daytime (11.00–15.00 hours) and nighttime (23.00–03.00 hours). Intravenous injections of drugs or saline were performed at 12.00 hour for the daytime and 00.00 hour for the nighttime, respectively. Plasma GH and PRL concentrations were determined by radioimmunoassay. l ‐DOPA did not alter the GH secretion when it was injected at 12.00 hour (spontaneous GH level at its peak). On the other hand, l ‐DOPA increased GH secretion at 00.00 hour (GH level at its trough). Injection of sulpiride suppressed GH secretion at 12.00 hour but did not affect GH levels at 00.00 hour. l ‐DOPA inhibited and sulpiride stimulated PRL release during both periods. These results suggest that dopaminergic neurons have stimulatory action on GH secretion and inhibitory action on PRL secretion in cattle. In addition, injection time should be considered to evaluate the exact effects on GH secretion due to its ultradian rhythm of GH secretion in cattle.  相似文献   

3.
OBJECTIVE: To investigate the effects of long-term administration of recombinant bovine tumor necrosis factor-alpha (rbTNF) on plasma glucose and growth hormone concentrations, and to determine whether treatment with rbTNF causes insulin resistance in steers. ANIMALS: 5 steers treated with rbTNF and 5 steers treated with saline (0.9% NaCl) solution (control). PROCEDURES: In experiment 1, rbTNF (5.0 microg/kg of body weight) or saline solution (5 ml) was administered SC daily for 12 days. Blood samples were obtained before treatment, and plasma was harvested for determination of glucose, insulin, and growth hormone (GH) concentrations. In experiment 2, insulin, glucose, or growth hormone-releasing hormone (GHRH) was administered IV on days 7, 9, and 11, respectively, after initiation of rbTNF or saline treatment in experiment 1. Plasma glucose and insulin concentrations were measured before and at various times for 4 hours after insulin or glucose administration. Plasma GH concentrations were measured at various times for 3 hours after GHRH administration. RESULTS: In experiment 1, administration of rbTNF resulted in hyperinsulinemia without hypoglycemia and decreased plasma GH concentrations. In experiment 2, plasma glucose concentrations were higher in steers treated with rbTNF and insulin than in controls. Plasma GH concentrations were lower in steers treated with rbTNF and GHRH than in controls. CONCLUSIONS AND CLINICAL RELEVANCE: Prolonged treatment with rbTNF induced insulin resistance and inhibited GHRH-stimulated release of GH in steers. Results indicate that rbTNF is a proximal mediator of insulin resistance and inhibits release of GH during periods of endotoxemia or infection.  相似文献   

4.
Secretion of growth hormone (GH) is reduced for several hours after feeding when access to feed is restricted to a 2-hr period each day. We hypothesized that increased secretion of insulin after feeding inhibits release of GH from the anterior pituitary gland. Our objectives were to determine whether: 1) alloxan prevents concentrations of insulin from increasing after feeding steers; 2) concentrations of GH remain high after feeding alloxan-treated steers; and 3) GH-releasing hormone (GHRH) stimulates greater release of GH in alloxan-treated, than in control, steers after feeding. Steers were injected iv with either saline (control) or with alloxan (110 mg/kg) (n = 4 per group). Concentrations of insulin were not different (P = 0.61) between control and alloxan-treated steers before feeding (87.5 +/- 33.6 pmol/l). However, alloxan prevented insulin from increasing (P < 0.001) after feeding (131.8 pmol/1) compared with control steers (442.0 pmol/l) (pooled SEM = 47.5). Overall, GH was higher (P < 0.05) in alloxan-treated (6.4 ng/ml) than in control steers (3.7 ng/ml) (pooled SEM = 0.7), but GH decreased (P < 0.001) after feeding in both groups. Iv injection of GHRH stimulated release of GH 1 hr before, but not when injected 1 hr after feeding (P < 0.001). In addition, net areas under the GH curve were not significantly different between control and alloxan-treated groups. We conclude that increased concentrations of insulin after feeding do not mediate feeding-induced suppression of GH secretion in steers.  相似文献   

5.
Secretion of growth hormone (GH) is reduced for several hours after feeding when access to feed is restricted to a 2-hr period each day. We hypothesized that increased secretion of insulin after feeding inhibits release of GH from the anterior pituitary gland. Our objectives were to determine whether: 1) alloxan prevents concentrations of insulin from increasing after feeding steers; 2) concentrations of GH remain high after feeding alloxan-treated steers; and 3) GH-releasing hormone (GHRH) stimulates greater release of GH in alloxan-treated, than in control, steers after feeding. Steers were injected iv with either saline (control) or with alloxan (110 mg/kg) (n = 4 per group). Concentrations of insulin were not different (P = 0.61) between control and alloxan-treated steers before feeding (87.5 +/- 33.6 pmol/l). However, alloxan prevented insulin from increasing (P < 0.001) after feeding (131.8 pmol/1) compared with control steers (442.0 pmol/l) (pooled SEM = 47.5). Overall, GH was higher (P < 0.05) in alloxan-treated (6.4 ng/ml) than in control steers (3.7 ng/ml) (pooled SEM = 0.7), but GH decreased (P < 0.001) after feeding in both groups. Iv injection of GHRH stimulated release of GH 1 hr before, but not when injected 1 hr after feeding (P < 0.001). In addition, net areas under the GH curve were not significantly different between control and alloxan-treated groups. We conclude that increased concentrations of insulin after feeding do not mediate feeding-induced suppression of GH secretion in steers.  相似文献   

6.
Release of growth hormone (GH) is known to be regulated mainly by GH-releasing hormone (GHRH) and somatostatin (SRIF) secreted from the hypothalamus. A novel peripheral release-regulating hormone, ghrelin, was recently identified. In this study, differences of the GH secretory response to ghrelin and GHRH in growing and lactating dairy cattle were investigated and an alteration of plasma ghrelin levels was observed. The same amounts of ghrelin and GHRH (0.3 nmol/kg) were intravenously injected to suckling and weanling calves, early and mid-lactating cows and non-lactating cows. Plasma ghrelin levels were also determined in dairy cattle in various physiological conditions. The peak values of ghrelin-induced GH secretion were increased in early lactating cows compared to those in non-lactating cows. The relative responsiveness of GH secretion to ghrelin was also increased compared with that to GHRH in early lactating cows. GH secretory responses to GHRH were blunted in mature cows with and without lactation. Conversely, GHRH-induced GH secretory response was greater than that to ghrelin in calves, and also greater in calves than in mature cows. Plasma ghrelin concentrations were elevated in early lactating cows compared to those in non-lactating cows. Plasma GH concentrations were higher in suckling calves and early lactating cows compared with those in non-lactating cows. These results suggest that GHRH is an effective inducer of GH release in growing calves, and that the relative importance of ghrelin in contributing to the rise in plasma GH increases in early lactating cows.  相似文献   

7.
To understand the regulatory mechanism of the secretory rhythm of GH and the involvement of melatonin (MEL) in GH regulation in cattle, daytime and nighttime profiles of GH secretion and the effect of a photic stimulation on nocturnal GH and MEL secretion were investigated in Holstein steers. Steers were kept under a constant lighting condition of 12 h of light (LIGHT; 500 lx, 0600 to 1800 h):12 h of dark (DARK; 10 lx, 1800 to 0600 h). In Exp. 1, blood was taken for 4 h at 15-min intervals during LIGHT (1100 to 1500 h) and DARK (2300 to 0300 h), respectively. The sampling was also performed from 0500 to 0900 h, with the usual light transition (light onset at 0600 h; morning sampling). In Exp. 2, steers were exposed to light (500 lx) for 1 h from 0000 to 0100 h. Plasma GH and MEL concentrations were determined by RIA and enzyme immunoassay, respectively. Both GH (P < 0.05) and MEL (P < 0.01) concentrations in plasma for 4 h during DARK were greater than those during LIGHT. On the other hand, although MEL concentrations were decreased after the light onset at 0600 during the morning, GH release was not altered. Increased GH secretion during DARK was suppressed (P < 0.01) by the 1 h of light exposure, as were MEL concentrations (P < 0.05). Pineal MEL, which was affected by the photic condition, may play an important role in the secretory rhythm of GH secretion in cattle.  相似文献   

8.
Growth hormone (GH) is essential for postnatal somatic growth, maintenance of lean tissue at maturity in domestic animals and milk production in cows. This review focuses on neuroregulation of GH secretion in domestic animals. Two hormones principally regulate the secretion of GH: growth hormone-releasing hormone (GHRH) stimulates, while somatostatin (SS) inhibits the secretion of GH. A long-standing hypothesis proposes that alternate secretion of GHRH and SS regulate episodic secretion of GH. However, measurement of GHRH and SS in hypophysial-portal blood of unanesthetized sheep and swine shows that episodic secretion of GHRH and SS do not account for all episodes of GH secreted. Furthermore, the activity of GHRH and SS neurons decreases after steers have eaten a meal offered for a 2-h period each day (meal-feeding) and this corresponds with reduced secretion of GH. Together, these data suggest that other factors also regulate the secretion of GH. Several neurotransmitters have been implicated in this regard. Thyrotropin-releasing hormone, serotonin and gamma-aminobutyric acid stimulate the secretion of GH at somatotropes. Growth hormone releasing peptide-6 overcomes feeding-induced refractoriness of somatotropes to GHRH and stimulates the secretion of GHRH. Norepinephrine reduces the activity of SS neurons and stimulates the secretion of GHRH via alpha(2)-adrenergic receptors. N-methyl-D,L-aspartate and leptin stimulate the secretion of GHRH, while neuropeptide Y stimulates the secretion of GHRH and SS. Activation of muscarinic receptors decreases the secretion of SS. Dopamine stimulates the secretion of SS via D1 receptors and inhibits the secretion of GH from somatotropes via D2 receptors. Thus, many neuroendocrine factors regulate the secretion of GH in livestock via altering secretion of GHRH and/or SS, communicating between GHRH and SS neurons, or acting independently at somatotropes to coordinate the secretion of GH.  相似文献   

9.
The effects of melatonin (MEL) injection into the third ventricle (3V) on growth hormone (GH) secretion were investigated in conscious Holstein steers. A stainless steel cannula was stereotaxically implanted in the 3V based on the ventriculogram. In Exp. 1, three doses of MEL (100, 300 or 600 microg) were injected into the 3V through the cannula and the GH concentration after the injection was determined. In Exp. 2, intracerebroventricular (icv) and intravenous (iv) injections of MEL (100 microg) and GH-releasing hormone (GHRH; 0.25 microg/kg body weight), respectively, were performed simultaneously to examine the effect of MEL on GHRH-induced GH release. The icv injection of MEL significantly stimulated GH release at 100 microg. The increase in GH concentrations by 100 microg of MEL was persistent. Intravenous injection of GHRH dramatically increased GH release. The injection of MEL did not alter GHRH-induced GH release. These results suggest that MEL stimulates GH secretion possibly through the hypothalamus in cattle.  相似文献   

10.
The aim of the present study was to clarify the effect of photoperiod on the secretion of growth hormone (GH) in goats. Adult female goats were kept at 20°C with an 8‐h or 16‐h photoperiod, and secretory patterns of GH for 4 h (12.00 to 16.00 hours) were compared. In addition, the goats were kept under a 16‐h photoperiod and orally administered saline (controls) or melatonin, and the effects of melatonin on the secretion of GH were examined. GH was secreted in a pulsatile manner. There were no significant differences in pulse frequency between the 8‐ and 16‐h photoperiods; however, GH pulse amplitude tended to be greater in the group with the 16‐h photoperiod (P = 0.1), and mean GH concentrations were significantly greater in the 16‐h photoperiod (P < 0.05). The GH‐releasing response to GH‐releasing hormone (GHRH) was also significantly greater for the 16‐h photoperiod (P < 0.05). There were no significant differences in GH pulse frequency between the saline‐ and melatonin‐treated groups. However, GH pulse amplitude and mean GH concentrations were significantly greater in the saline‐treated group (P < 0.05). The present results show that a long photoperiod enhances the secretion of GH, and melatonin modifies GH secretion in female goats.  相似文献   

11.
Serotonin stimulates secretion of growth hormone (GH) in cattle, but the mechanism is unknown. In rats, thyrotropin-releasing hormone (TRH) mediates serotonin-induced secretion of GH. We hypothesized that the same is true in cattle. Cattle were fed for 2h daily to synchronize secretion of GH, such that concentrations of GH were high before and low after feeding. Our first objective was to determine whether or not feeding suppresses serotonin receptor agonist (quipazine) induced secretion of GH. Holstein steers were injected with quipazine (0.2 mg/kg BW) either 1 h before or 1 h after feeding. Quipazine-induced secretion of GH which did not differ in magnitude before and after feeding. If TRH mediates serotonin-induced secretion of GH, then magnitude of TRH-induced secretion of GH should not be different before and after feeding (our second objective). Sixteen meal-fed Holstein steers were injected with 0.3 microg TRH/kg BW either 1 h before or 1 h after feeding. Indeed, magnitude of TRH-induced secretion of GH before and after feeding was not different. Our third objective was to inhibit endogenous TRH with 3,5,3'-triiodothyronine (T(3)) and examine basal, GH-releasing hormone (GHRH)-, TRH- and quipazine-induced secretion of GH. Sixteen Holstein steers were injected daily with either T(3) (3 or 6 microg/kg BW) or vehicle for 20 days and then challenged sequentially with vehicle or GHRH, TRH, or quipazine. T(3) did not affect basal, GHRH- or TRH-induced secretion of GH, but reduced basal secretion of thyroxine. T(3) reduced but did not completely block quipazine-induced secretion of GH. In conclusion, TRH mediates, in part, serotonin-induced secretion of GH in cattle.  相似文献   

12.
The aim of the present study was to clarify the effect of photoperiod on secretory patterns of growth hormone (GH) in male goats. Adult male goats were kept at 20°C with an 8‐h or 16‐h light photoperiod, and secretory patterns of GH secretion were compared. In addition, plasma profiles of prolactin (PRL), insulin‐like growth factor‐I (IGF‐I) and testosterone (T) were also examined to characterize GH secretion. GH was secreted in a pulsatile manner. There was no significant difference in pulse frequency between the 8‐h and 16‐h photoperiods. However, GH pulse amplitude tended to be greater in the group with the 16‐h photoperiod (P = 0.1), and mean GH concentrations were significantly greater in the 16‐h photoperiod (P < 0.05). The GH‐releasing response to GH releasing hormone was greater in the 16‐h than 8‐h photoperiod (P < 0.05). Plasma PRL and IGF‐I levels were higher in the 16‐h than 8‐h photoperiod (P < 0.05). In contrast, plasma T levels were lower in the 16‐h photoperiod (P < 0.05). These results show that a long light photoperiod enhances the secretion of GH as well as PRL and IGF‐I, but reduces plasma T concentrations in male goats.  相似文献   

13.
The objective of this study was to determine the effect of a subtherapeutic level of chlortetracycline (CTC) fed to growing beef steers under conditions of limited and adequate dietary protein on plasma concentrations of GH, thyroid-stimulating hormone (TSH), and thyroid hormones before and after an injection of thyrotropin-releasing hormone (TRH) + GHRH. Young beef steers (n = 32; average BW = 285 kg) were assigned to a 2x2 factorial arrangement of treatments of either a 10 or 13% crude protein diet (70% concentrate, 15% wheat straw, and 15% cottonseed hulls) and either a corn meal carrier or carrier + 350 mg of CTC daily top dressed on the diet. Steers were fed ad libitum amounts of diet for 56 d, and a jugular catheter was then placed in each steer in four groups (two steers from each treatment combination per group) during four consecutive days (one group per day). Each steer was injected via the jugular catheter with 1.0 microg/kg BW TRH + .1 microg/kg BW GHRH in 10 mL of saline at 0800. Blood samples were collected at -30, -15, 0, 5, 10, 15, 20, 30, 45, 60, 120, 240, and 360 min after releasing hormone injection. Plasma samples were analyzed for GH, TSH, thyroxine (T4), and triiodothyronine (T3). After 84 d on trial, the steers were slaughtered and the pituitary and samples of liver were collected and analyzed for 5'-deiodinase activity. Feeding CTC attenuated the GH response to releasing hormone challenge by 26% for both area under the response curve (P<.03) and peak response (P<.10). Likewise, CTC attenuated the TSH response to releasing hormone challenge for area under the response curve by 16% (P<.10) and peak response by 33% (P<.02), and attenuated the T4 response for area under the curve by 12% (P<.08) and peak response by 14% (P<.04). Type II deiodinase activity in the pituitary was 36% less (P<.02) in CTC-fed steers than in steers not fed CTC. The results of this study are interpreted to suggest that feeding subtherapeutic levels of CTC to young growing beef cattle attenuates the release of GH and TSH in response to pituitary releasing hormones, suggesting a mechanism by which CTC may influence tissue deposition in cattle.  相似文献   

14.
Growth hormone is a key component of the somatotropic axis and is critical for the interplay between nutrition, regulation of metabolic functions, and subsequent processes of growth. The objective of this study was to investigate potential relations between meal feeding concentrates differing in the glycemic responses they elicit and GH secretory patterns in young growing horses. Twelve Quarter Horse weanlings (5.4 ± 0.4 mo of age) were used in a crossover design, consisting of two 21-d periods and two treatments, a high-glycemic (HG) or low-glycemic (LG) concentrate meal, fed twice daily. Horses were individually housed and fed hay ad libitum. On the final day of each period, quarter-hourly blood samples were drawn for 24 h to measure plasma glucose, insulin, non-esterified fatty acids, and GH. Growth hormone secretory characteristics were estimated with deconvolution analysis. After a meal, HG-fed horses exhibited a longer inhibition until the first pulse of GH secretion (P = 0.012). During late night hours (1:00 AM to 6:45 AM), HG horses secreted a greater amount of pulsatile GH than LG horses (P = 0.002). These differences highlight the potential relations between glycemic and insulinemic responses to meals and GH secretion. Dietary energy source and metabolic perturbations associated with feeding HG meals to young, growing horses have the potential to alter GH secretory patterns compared with LG meals. This may potentially affect the developmental pattern of various tissues in the young growing horse.  相似文献   

15.
The purpose of the present study was to clarify the hypothalamic action of leptin on the secretion of luteinizing hormone (LH) and growth hormone (GH) in cattle. Intracerebroventricular (the third ventricle) injections of leptin were given to fully fed castrated Holstein calves. Blood samples were collected at 10‐min intervals for 60 min after injection and 20‐min intervals for 60 min before injection and for 60–180 min after injection through an indwelling catheter in the external jugular vein. Plasma LH and GH levels were examined by homologous radioimmunoassay. The administration of 10 µg of leptin stimulated a significant (P < 0.05) release of GH but not LH. Average GH levels began to rise after 30 min and were significantly increased at 40, 50 and 60 min after the injection, compared with the respective control values (P < 0.05). The present result suggests that leptin may act partly on the hypothalamus to stimulate the release of GH in castrated calves.  相似文献   

16.
Pituitary adenylate cyclase-activating polypeptide (PACAP) is a hypothalamic neuropeptide that stimulates release of growth hormone (GH) from cultured bovine anterior pituitary gland cells, but the role of PACAP on the regulation of in vivo secretion of GH in cattle is not known. To test the hypothesis that PACAP induces secretion of GH in cattle, meal-fed Holstein steers were injected with incremental doses of PACAP (0, 0.1, 0.3, 1, 3, and 10 microg/kg BW) before feeding and concentrations of GH in serum were quantified. Compared with saline, injection of 3 and 10 microg PACAP/kg BW increased peak concentrations of GH in serum from 11.2 ng/ml to 23.7 and 21.8 ng/ml, respectively (P < 0.01). Peak concentrations of GH in serum were similar in steers injected with 3 or 10 microg PACAP/kg BW. Meal-fed Holstein steers were then injected with 3 microg/PACAP/kg BW either 1 hr before feeding or 1 hr after feeding to determine if PACAP-induced secretion of GH was suppressed after feeding. Feeding suppressed basal concentrations of GH in serum. Injection of PACAP before feeding induced greater peak concentrations of GH in serum (19.2 +/- 2.6 vs. 11.7 +/- 2.6 ng/ml) and area under the response curve (391 +/- 47 vs. 255 +/- 52 ng. ml(-1) min) than injection of PACAP after feeding, suggesting somatotropes become refractory to PACAP after feeding similar to that observed by us and others with growth hormone-releasing hormone (GHRH). We concluded that PACAP induces secretion of GH and could play a role in regulating endogenous secretion of GH in cattle, perhaps in concert with GHRH.  相似文献   

17.
The aim of the present study was to clarify the effects of hypothalamic dopamine (DA) on the secretion of growth hormone (GH) in goats. The GH‐releasing response to an intravenous (i.v.) injection of GH‐releasing hormone (GHRH, 0.25 μg/kg body weight (BW)) was examined after treatments to augment central DA using carbidopa (carbi, 1 mg/kg BW) and L‐dopa (1 mg/kg BW) in male and female goats under a 16‐h photoperiod (16 h light, 8 h dark) condition. GHRH significantly and rapidly stimulated the release of GH after its i.v. administration to goats (P < 0.05). The carbi and L‐dopa treatments completely suppressed GH‐releasing responses to GHRH in both male and female goats (P < 0.05). The prolactin (PRL)‐releasing response to an i.v. injection of thyrotropin‐releasing hormone (TRH, 1 μg/kg BW) was additionally examined in male goats in this study to confirm modifications to central DA concentrations. The treatments with carbi and L‐dopa significantly reduced TRH‐induced PRL release in goats (P < 0.05). These results demonstrated that hypothalamic DA was involved in the regulatory mechanisms of GH, as well as PRL secretion in goats.  相似文献   

18.
We previously reported that growth hormone (GH) pulses were negatively associated with neuropeptide Y (NPY) profiles in cerebrospinal fluid (CSF) of the third ventricle of Shiba goats. In addition, while most GH pulses were coincident with GH-releasing hormone (GHRH) pulses, there was no correlation between GH and somatostatin (SRIF) levels. The present study was performed to elucidate the relationship between GH pulses and these neuropeptide levels in CSF when estradiol (1.0 mg/head) was subcutaneously administered to ovariectomized goats. CSF and plasma samples were collected every 15 min for 18 h (from 6 h before to 12 h after injection). GH levels in peripheral blood and GHRH, SRIF and NPY levels in CSF were measured by radioimmunoassay. Pulse/trough characteristics and correlations were assessed by the ULTRA algorithm and cross-correlation analysis. Before estradiol was injected, significant coincidence was found between GHRH pulses and GH pulses, and negative coincidence was found between NPY troughs and GH pulses. Six to 12 h after estradiol injection, the amplitude and area under the curve (AUC) of the GH pulses were markedly increased. The duration and AUC of the GHRH pulses in the CSF were also increased, and stronger synchrony of GHRH with GH was observed. In contrast, the baseline of NPY was significantly decreased, and the negative correlation between the GH pulses and NPY troughs disappeared. The parameters of SRIF troughs were not clearly changed. These observations suggest that estrogen enhances the pattern of secretion of GH in the goat via enhancement of GHRH pulses and decrease of NPY levels.  相似文献   

19.
Corticosterone (CORT) can stimulate growth hormone (GH) secretion on embryonic day (e) 12 in the chicken. However, CORT failed to induce GH secretion on e20 in a single report, suggesting that regulation of GH production changes during embryonic development. Secretion in response to CORT during embryonic development is modulated by the thyroid hormones triiodothyronine (T3) and thyroxine (T4). Growth hormone responses on e12 involve both glucocorticoid (GR) and mineralocorticoid receptors (MR); however, involvement of MR has not been evaluated past e12. To further define changes in somatotroph responsiveness to CORT, pituitary cells obtained on e12–e20 were cultured with CORT alone and in combination with T3 and GH-releasing hormone (GHRH). Growth hormone mRNA levels and protein secretion were quantified by quantitative real-time polymerase chain reaction (qRT-PCR) and radioimmunoassay (RIA), respectively. Corticosterone significantly increased GH mRNA and protein secretion on e12; however, mRNA concentration and protein secretion were unaffected on e20. Contributions of GR and MR in CORT responses were evaluated using GR and MR antagonists. Treatment with a GR-specific antagonist effectively blocked the CORT-induced increase in GH secretion on e12. The same treatment on e20 had no effect on GH secretion. These findings demonstrate that GR is directly involved in glucocorticoid stimulation of GH secretion at the time of somatotroph differentiation but is not regulatory at the end of embryonic development. We conclude that positive somatotroph responses to CORT are lost during chicken embryonic development and that GR is the primary regulator of CORT-induced GH secretion.  相似文献   

20.
To investigate the effects of high and low somatostatinergic tone on GH-releasing peptide-2 (GHRP-2) and GH-releasing hormone (GHRH)-induced growth hormone (GH) secretion in swine, we examined GHRP-2- and GHRH-induced GH secretion after pretreatment with atropine or pyridostigmine. Pretreatment of swine with atropine (80 µg/kg bodyweight (BW), intravenous (i.v.)) 15 min before i.v. administration of saline, GHRP-2 (30 µg/kg BW), GHRH (1 µg/kg BW) or a combination of GHRP-2 and GHRH, reduced plasma GH area under the curve ( P  < 0.05), completely blocked GH response to GHRH, and attenuated GH response to GHRP-2 and GHRH combined ( P  < 0.05), without affecting GH response to GHRP-2 only. A synergistic effect of GHRP-2 and GHRH was not observed. In contrast, pretreatment of swine with pyridostigmine (100 µg/kg BW, i.v.), under the same pretreatment conditions as above, increased plasma GH concentration ( P  < 0.01), augmented GH response to GHRP-2 ( P  < 0.05), and GHRP-2 and GHRH combined ( P  < 0.05), but did not affect GH response to GHRH. These results suggest that the cholinergic muscarinic agents atropine and pyridostigmine modulate the GH response to GHRP-2 and GHRH, and that GHRP-2 acts antagonistically on the inhibitory effect of somatostatin in swine.  相似文献   

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