Effect of daily injections of growth hormone-releasing factor and thyrotropin-releasing hormone on growth and endocrine parameters in chickens

The control of growth is a complex mechanism regulated by several metabolic hormones including growth hormone (GH) and thyroid hormones. In avian species, as well as in mammals, GH secretion is regulated by hypothalamic hypophysiotropic hormones. Since thyrotropin-releasing hormone (TRH) and growth hormone-releasing factor (GRF) are potent GH secretagogues in poultry, we were interested in determining the influence of daily intravenous administration of either peptide or both simultaneously on circulating GH and IGF-I concentrations and whether an improvement in growth rate or efficiency would be obtained.

Male broiler chicks were injected once daily for a period of 21 days with either GRF (10 μg/kg), TRH (1 μg/kg) or both GRF and TRH (10 and 1 μg/kg respectively) between four and seven weeks of age. On the last day of the experiment, following intravenous injection of TRH, GRF or a combination of GRF and TRH, plasma GH levels were significantly (P<.05) increased to a similar extent in control chicks and in those which had received daily peptide injections for the previous 21 days. Circulating GH levels between 10 and 90 min post-injection were significantly (P<.05) greater and more than additive than GH levels in chicks injected with both GRF and TRH when compared to those injected with either peptide alone. Mean plasma T₃ concentrations during that same time period were significantly elevated (P<.05) above saline-injected control chick levels in birds treated with TRH or GRF and TRH respectively, regardless of whether the chicks had received peptide injections for the previous 21 days. There was no evidence of pituitary refractoriness to chronic administration of either TRH or GRF injection in terms of growth or thyroid hormone secretion.

Despite the large elevation in GH concentration each day, growth rate, feed efficiency and circulating IGF-I concentrations were not enhanced. Thus the quantity or secretory pattern of GH secretion induced by TRH or GRF administration was not sufficient to increase plasma IGF-I concentration or growth.     

Effects of pituitary adenylate cyclase-activating polypeptide (PACAP), prostaglandin E2 (PGE2) and growth hormone releasing factor (GRF) on the release of growth hormone from cultured bovine anterior pituitary cells in vitro

The effect of pituitary adenylate cyclase-activating polypeptide (PACAP) on growth hormone (GH) release was compared with that of prostaglandin E₂ (PGE₂) and growth hormone releasing factor (GRF) from cultured bovine anterior pituitary cells in vitro. Both PACAP and PGE₂ stimulated GH release at concentrations as low as 10⁻⁹ and 10⁻⁸ M, respectively, (P<0.01). However, GRF released GH at a concentration as low as 10⁻¹³ M (P<0.01). Percent increases of GH compared with controls were not significantly different among GRF, PACAP, and PGE₂ at 10⁻⁷ M; however, the increases of GH by the 10⁻⁸ M GRF, PACAP and PGE₂ were 196, 118, and 27%, respectively, (P<0.01), and 124, 65, and 1% in the 10⁻⁹ M media, respectively, (P<0.01). When GRF and somatostatin (SS) were added together, the GH releasing effect of GRF was blunted (P<0.01). Similar bluntness were observed in PACAP and PGE₂, when SS was added. The stimulatory effects of GRF and PGE₂ together were similar to that by either GRF or PGE₂ alone. When GRF and PACAP were added together, the GH released by both secretagogues was greater than that by PACAP alone (P<0.01); however, a synergistic effect was not clear when compared with GRF alone.

These findings suggest that PACAP and PGE₂ may modulate the release of GH in cattle.     

Effects of VIP and GRF on the release of growth hormone in perifused bovine adenohypophysis

The effects of vasoactive intestinal polypeptide (VIP) and growth hormone releasing factor (GRF:hpGRF(1–29)-NH₂) on the release of growth hormone (GH) from anterior pituitaries from cows were examined by using an in vitro superfusion system. The pituitaries were excised randomly from cycling cows, dissected to obtain medial portions, and minced to obtain cubes with approximate dimensions of 1.5mm on a side. For each perifusion setup, 5 pieces of pituitary tissues were chambered and flushed with modified KRB solution saturated with 95% O₂-5% CO₂ at 38C. Perifusion with media containing 10⁻⁶ and 10⁻⁷M VIP for 30 min induced a significant release of GH during the treatments (P<0.05). VIP (10⁻⁸M) increased GH levels significantly (P<0.05), but to a minor degree. Perifusion with the media containing 10⁻⁶, 10⁻⁷ and 10⁻⁸M GRF for 30 min markedly increased the GH concentration and the effects continued up to 90 min after termination of the perifusion of the peptide (P<0.05, P<0.01). The GH releasing effects of GRF could be seen at doses as low as 10⁻¹¹M GRF (P<0.05, P<0.01).

These findings indicate that the GH releasing effect of VIP is less potent that that of GRF in cows.     

Influence of age and sex on basal secretion of growth hormone (GH) and on GH-induced release by porcine GH-releasing factor pGRF(1–29NH2) in growing pigs

The aim of this study was to determine the effect of age and sex on basal secretory patterns of growth hormone (GH) and growth hormone-releasing factor (GRF) induced GH release. Eighteen pigs (9 castrated males and 9 females) were stimulated with pGRF(1–29)NH₂ at 7,11,15,19 and 23 weeks of age. Blood samples were taken from each animal via jugular vein cannulate every 20 min, from 6 hr before to 5 hr after iv GRF administration at a dose of 4 μg/kg. GH baseline levels, amplitude of the GH peaks, area under the GH peaks and the overall mean of GH serum levels decreased (P<.001) with age in both sexes. Age also had a marked effect on GRF-induced GH release: the amplitude of GH peaks and area under the GH peaks decreased (P<.001) with age. The GH response to pGRF(1–29)NH₂ varied considerably, depending on the timing of the episodic endogenous secretion of GH. An immediate response (<30 min) was observed when GRF was injected at the end of a trough period or at the beginning of a peak, but there was no immediate response when GRF was injected at the end of a peak or at the beginning of a trough period. Our results show that both endogenous GH secretion and pGRF(1–29)NH₂-induced GH release declines with age, suggesting a decreased sensitivity of the somatotroph cells to GRF with age; and that the high variability of the GH response to pGRF(1–29)NH₂ stimulation depends greatly on the timing of the episodic endogenous GH release, thus implying a possible episodic endogenous somatostatin secretion by the hypothalamus.     

Perturbed metabolism and hormonal profiles in calves infected with Sarcocystis cruzi

Plasma concentrations of growth hormone (GH), thyroid stimulating hormone (TSH), insulin (IN), thyroxine (T₄), and triiodothyronine (T₃) in addition to metabolic parameters [N balance (NB), urinary 3-methylhistidine (TMH), urinary creatinine (CR), and urinary hydroxyproline (HP)] were measured in 4-mo-old Holstein steers divided equally among groups infected with Sarcocystis (I), noninfected ad libitum fed (C), and noninfected pair fed to I (PF) (7 steers per treatment). Effects of infection on these parameters beyond those attributable to altered dietary intake were determined using orthogonal contrasts (effect of intake, C vs I + PF; effect of infection, PF vs I). NB was higher in C than I and PF (P<.05) and lower in I than PF (P<.02). Hydroxyproline and CR were influenced by intake (P<.05) and HP excretion was reduced in association with infection (P<.05). Reduced intake was associated with lowered mean basal plasma concentrations of GH, IN, T₃ and T₄ (P<.05). Infection further reduced (P<.001) plasma T₃ concentration.

Triiodothyronine and T₄ responses following an intravenous bolus of thyrotropin releasing hormone (TRH) were measured. The magnitude of the responses in I and PF were lower than those observed in C (P<.05). Plasma T₃ responses were further reduced in association with infection (P<.05). Insulin responses to intravenous arginine infusion (ARG) were also low in association with reduced intake. Growth hormone responses to TRH or ARG were affected by the level of feed intake only. These data suggest that hormonal perturbations associated with the insult of infection further compromise metabolism and the direction of nutrient partitioning that would ordinarily be associated with developmental growth in young steers beyond those responses anticipated from solely the reduction of feed intake.     

Birthweight determines intestinal microvasculature development and alters endothelial nitric oxide synthase density in young piglets

Blood supply to enterocytes dictates intestinal health and nutrient absorption. These two aspects are impaired in low birthweight (LBW) piglets, but whether the perfusion to intestinal tissues is implicated as well is still unknown. Thus, structural changes in the microvasculature of LBW and normal birthweight (NBW) piglets were investigated during early postnatal development. Additionally, the presence of endothelial nitric oxide synthase (eNOS) in the intestinal mucosa was assessed given its important role to assure perfusion. A total of 22 pigs (11 LBW and 11 NBW) were sacrificed at days 0, 3, 8 and 19 of life. Body weight and intestinal length were recorded and a piece of the small intestine was sampled for immunohistochemical analysis of von Willebrand Factor (vWF, an endothelial cell marker) and eNOS. LBW piglets had a relatively (to body weight) longer intestine than their NBW counterparts. Age did not affect microvasculature, which was more abundant (85% larger vWF-positive area) in NBW than LBW pigs. However, an interaction age*BW was observed for eNOS-IR, showing that eNOS presence peaked in NBW piglets on the first day of life and subsequently decreased. This pattern was not observed in LBW piglets. The less abundant intestinal endothelial mass and the different pattern of eNOS expression observed in LBW piglets suggests microcirculation as a contributing factor in the impaired digestive functioning and gut health of LBW pigs. However, revealing whether the origin of this alteration is prenatal or postnatal, for example due to a lower milk intake, needs further study.     

Effects of cimaterol administration on plasma concentrations of various hormones and metabolites in friesian steers

The aim of the experiment was to determine the acute and chronic effects of the β-agonist, cimaterol, on plasma hormone and metabolite concentrations in steers. Twelve Friesian steers (liveweight = 488 ± 3 kg) were randomly assigned to receive either 0 (control; n=6) or .09 mg cimaterol/kg body weight/day (treated; n=6). Steers were fed grass silage ad libitum. Cimaterol, dissolved in 140 ml of acidified distilled water (pH 4.2), was administered orally at 1400 hr each d. After 13 d of treatment with cimaterol or vehicle (days 1 to 13), all animals were treated with vehicle for a further 7 d (days 14 to 20). On days 1, 13 and 20, blood samples were collected at 20 min-intervals for 4 hr before and 8 hr after cimaterol or vehicle dosing. All samples were assayed for growth hormone (GH) and insulin, while samples taken at −4, −2, 0, +2, +4, +6 and +8 hr relative to dosing were assayed for thyroxine (T₄), triiodothyronine (T₃), cortisol, urea, glucose and non-esterified fatty acids (NEFA). Samples taken at −3 and +3 hr relative to dosing were assayed for IGF-I only. On day 1, cimaterol acutely reduced (P<.05) GH and urea concentrations (7.6 vs 2.9 ± 1.4 ng/ml; and 6.0 vs 4.9 ± 0.45 mmol/l, respectively; mean control vs mean treated ± pooled standard error of difference), and increased (P<.05) NEFA, glucose and insulin concentrations (160 vs 276 ± 22 μmol/l, 4.1 vs 6.2 ± 0.15 mmol/l and 29.9 vs 179.7 ± 13.9 μU/ml, respectively). Plasma IGF-I, T₃, T₄ and cortisol concentrations were not altered by treatment. On day 13, cimaterol increased (P<.05) GH and NEFA concentrations (7.7 vs 14.5 ± 1.4 ng/ml and 202 vs 310 ± 22 mEq/l, respectively) and reduced (P<.05) plasma IGF-I concentrations (1296 vs 776 ± 227 ng/ml). Seven-d withdrawal of cimaterol (day 20) returned hormone and metabolite concentrations to control values. It is concluded that : 1) cimaterol acutely increased insulin, glucose and NEFA and decreased GH and urea concentrations, 2) cimaterol chronically increased GH and NEFA and decreased IGF-I concentrations, and 3) there was no residual effect of cimaterol following a 7-d withdrawal period.     

Relationships between the thyroid and somatotropic axes in steers I: Effects of propylthiouracil-induced hypothyroidism on growth hormone, thyroid stimulating hormone and insulin-like growth factor I

The effects of propylthiouracil (PTU)-induced thyroid hormone imbalance on GH, TSH and IGF-I status in cattle were examined. In the first study, four crossbred steers (avg wt 350 kg) were fed a diet dressed with PTU (0, 1, 2 or 4 mg/kg/d BW) in a Latin square design with four 35-d periods. On day 29 in each period, steers were challenged with an intrajugular bolus of thyrotropin releasing hormone (TRH, 1.0 μg/kg). Blood samples were obtained to assess the change in plasma GH and TSH as affected by PTU. Plasma IGF-I was measured from blood samples obtained before and after (every 6 hr for 24 hr) intramuscular injection of bovine GH (0.1 mg/kg, day 31). Doses of 1 and 2 mg/kg PTU increased plasma T₄ (P<.01). At 4 mg/kg, PTU depressed T₄ concentrations to 30% of control (P<.01). Plasma T₃ linearly decreased with increasing doses of PTU (P<.01). Plasma TSH increased when PTU was fed at 4 mg/kg (P<.05) while the TSH response to TRH declined with increasing PTU (P<.02). Neither basal nor TRH-stimulated plasma concentration of GH was affected by PTU; the IGF-I response to GH tended to increase at the 1 and 2 mg/kg PTU (P<.01). In a second study 24 crossbred steers were fed PTU (1.5 mg/kg) for 119 d in a 2 × 2 factorial design with implantation of the steroid growth effector, Synovex-S (200 mg progesterone + 20 mg estradiol), as the other main effect. Basal plasma GH and IGF-I were not affected by PTU treatment. Synovex increased plasma concentration (P<.01) of IGF-I without an effect on plasma GH. The data suggest that mild changes in thyroid status associated with PTU affects regulation of T₃, T₄ and TSH more than GH or IGF-I in steers.     

Growth hormone secretory responsiveness to multiple injections of growth hormone-releasing factor in sheep

Growth hormone releasing factor (GRF) has been shown to be a potent and specific stimulant of growth hormone (GH) secretion in a variety of species. The objective of this series of experiments was to determine whether repeated iv bolus injections of GRF would increase circulating levels of GH, and thus have the potential to stimulate growth in young lambs. In lambs given three injections of hpGRF either one (.065 nmol/Kg) or two hours apart (.016 or .065 nmol/Kg), the GH secretory responses decreased significantly with each subsequent injection. Bolus injections of hpGRF (.016 nmol/Kg) given for three days in 15 Kg lambs resulted in development of refractoriness to subsequent injections. In lambs treated three and six times daily, basal plasma GH and SmC levels decreased progressively over the three day period. Older lambs (30 Kg) also became less responsive to hpGRF given once daily for three days, but the refractoriness was less pronounced than in 15 Kg lambs. In 30 Kg lambs treated once daily with bovine GRF (bGRF, .065 nmol/Kg), GH secretory responsiveness decreased progressively over a five day treatment period. Because of the severe refractoriness to GRF that developed in very young lambs, it was predicted that multiple injections over several weeks might result in suppression of growth. However, growth rate, feed intake and feed efficiency were not altered in 10 Kg lambs treated twice daily for three weeks with .065 nmol/Kg bGRF and there was no evidence of refractoriness to bGRF as determined every seven days. These results suggest that the refractoriness that develops with multiple bolus injections of GRF may not be long-lasting, and therefore will not have a negative impact on growth. However, there was no evidence that at the frequencies tested bolus injections of GRF could stimulate growth in young lambs.     

Effects of Long-term Growth Hormone-releasing Factor Administration on Plasma Growth Hormone, Luteinizing Hormone and Progesterone Profiles in Growing Female Buffaloes (Bubalus bubalis)

To investigate the effects of long-term growth hormone-releasing factor (GRF) administration on plasma growth hormone (GH), LH and progesterone and body weight gain in growing buffalo calves, 12 female Murrah buffaloes within the age group of 6-8 months of age were divided into two groups (treatment and control groups) of six each in such a way so that average body weights between the groups did not differ (p > 0.05). Control buffaloes were not given any hormonal treatment and treatment group buffaloes were treated with synthetic bovine GRF [bGRF (1-44)-NH(2)] at the rate of 10 microg/100 kg body weight intravenously at an interval of 15 days from week 6 (5-week pre-treatment period) till 18 injections were completed (week 6-42 treatment period) and thereafter, effect of exogenous GRF were observed for 10-week post-treatment period. Jugular blood samples were drawn twice a week at 3-4-day intervals for plasma GH, LH and progesterone quantification. Body weight of all animals was recorded twice a week. During pre-treatment period, mean plasma GH, LH and progesterone did not differ (p > 0.05) between the groups. But during treatment as well as post-treatment period, mean plasma GH levels were found to be significantly (p < 0.01) higher in treatment than control group of buffaloes. Administration of GRF for longer term sustained a higher level of plasma GH even after cessation of treatment. GRF-treated buffaloes attained higher (p < 0.01) body weight than the controls. Repeated GRF administration for long-term significantly (p < 0.01) increased plasma LH and progesterone. In conclusion, repeated long-term exogenous GRF administration induces and even enhances GH release without any sign of refractoriness. GRF may, therefore, be used to induce daily GH release without loss of responsiveness over an extended period of time in young growing female buffaloes and it may assist these animals to grow faster.     

Stimulation of pig growth performance by porcine growth hormone and growth hormone-releasing factor

The current study was undertaken to determine the effects of human growth hormone-releasing factor [hpGRF-(1-44)-NH2] on growth performance in pigs and whether this response was comparable to exogenous porcine growth hormone (pGH) treatment. Preliminary studies were conducted to determine if GRF increased plasma GH concentration after iv and im injection and the nature of the dose response. Growth hormone-releasing factor stimulated the release of pGH in a dose-dependent fashion, although the individual responses varied widely among pigs. The results from the im study were used to determine the dose of GRF to use for a 30-d growth trial. Thirty-six Yorkshire-Duroc barrows (initial wt 50 kg) were randomly allotted to one of three experimental groups (C = control, GRF and pGH). Pigs were treated daily with 30 micrograms of GRF/kg body weight by im injection in the neck. Pigs treated with pGH were also given 30 micrograms/kg body weight by im injection. Growth rate was increased 10% by pGH vs C pigs (P less than .05). Growth rate was not affected by GRF; however, hot and chilled carcass weights were increased 5% vs C pigs (P less than .05). On an absolute basis, adipose tissue mass was unaffected by pGH or GRF. Carcass lipid (percent of soft-tissue mass) was decreased 13% by GRF (P less than .05) and 18% by pGH (P less than .05). Muscle mass was significantly increased by pGH but not by GRF. There was a trend for feed efficiency to be improved by GRF; however, this was not different from control pigs. In contrast, pGH increased feed efficiency 19% vs control pigs (P less than .05). Chronic administration of GRF increased anterior pituitary weight but did not affect pituitary GH content or concentration. When blood was taken 3 h post-injection, both GRF- and pGH-treated pigs had lower blood-urea nitrogen concentrations. Serum glucose was significantly elevated by both GRF and pGH treatment. This was associated with an elevation in serum insulin. These results indicate that increasing the GH concentration in blood by either exogenous GH or GRF enhances growth performance. The effects of pGH were more marked than for GRF. Further studies are needed to determine the optimal dose of GRF to administer in growth trials and the appropriate pattern of GRF administration in order to determine whether GRF will enhance pig growth performance to the extent that exogenous pGH does.     

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.     

Effect of long-term administration of porcine growth hormone-releasing factor and(or) thyrotropin-releasing factor on growth hormone, prolactin and thyroxine concentrations in growing pigs

Long-term administration of porcine growth hormone-releasing factor (pGRF(1-29)NH2) and(or) thyrotropin-releasing factor (TRF) was evaluated on serum concentrations of growth hormone (GH) thyroxine (T4) and prolactin (PRL). Twenty-four 12-wk-old female Yorkshire-Landrace pigs were injected at 1000 and 1600 for 12 wk with either saline, pGRF (15 micrograms/kg), TRF (6 micrograms/kg) or pGRF + TRF using a 2 x 2 factorial design. Blood samples were collected on d 1, 29, 57 and 85 of treatment from 0400 to 2200. Areas under the GH, T4 and PRL curves (AUC) for the 6 h (0400 to 1000) prior to injection were subtracted from the postinjection periods (1000 to 1600, 1600 to 2200) to calculate the net hormonal response. The AUC of GH for the first 6 h decreased similarly (P less than .05) with age for all treatments. The GH response to GRF remained unchanged (P greater than .10) across age. TRF alone did not stimulate (P less than .05) GH release but acted in synergy with GRF to increase (P less than .05) GH release. TRF stimulated (P less than .001) the net response of T4 on all sampling days. Animals treated with the combination of GRF + TRF showed a decreased T4 AUC during the first 6 h on the last three sampling days. Basal PRL decreased (P less than .05) with age. Over the four sampling days, animals injected with TRF alone showed (P less than .01) a reduction (linear effect; P less than .01) followed by an increase (quadratic effect; P less than .05) in total PRL concentration after injection; however, when GRF was combined with TRF, such effects were not observed (P greater than .10). Results showed that 1) chronic injections of GRF for 12 wk sustained GH concentration, 2) TRF and GRF acted synergistically to elevate GH AUC, 3) TRF increased T4 concentrations throughout the 12-wk treatment period, 4) chronic TRF treatment decreased the basal PRL concentration and 5) chronic GRF + TRF treatment decreased the basal concentration of T4.     

Effects of winter nutrition and summer pasture or a feedlot diet on plasma Insulin-like Growth Factor I (IGF-I) and the relationship between circulating concentrations of IGF-I and thyroid hormones in steers

Effects of two winter nutritional levels (LOW, MOD) and two summer pastures (bahiagrass, BG; perennial peanut, PP) on plasma IGF-I, and the relationship between IGF-I and average daily gain (ADG), thyroid hormones, plasma urea, packed cell volume (PCV) and steer type were determined in 101 steers (217 kg) varying in breed composition, frame size and initial condition. Relationships between body composition or composition of gain and IGF-I were determined in 11 contemporary steers assigned directly to the feedlot. Initial IGF-I (57.9 ± 3.5 ng/ml) was positively correlated (P<.05) to initial condition, estimated percentage of Brahman and plasma T₃, but not related to subsequent ADG. During the 126-day wintering period, ADG was .21 kg for the LOW winter treatment and .47 kg for the MOD winter treatment. Concentration of IGF-I in the wintering period was affected (P<.01) by nutritional level (LOW = 71.8 ng/ml, MOD = 150.6 ng/ml) and was positively related to winter ADG in MOD steers (P<.01) but not in LOW steers. Concentration of IGF-I in winter was also positively related to condition at the end of the winter period (P<.01), T₃ (P<.05) and T₄ (P<.05). There were no effects of winter treatment on IGF-I during the subsequent summer pasture period. During the 145-d summer period, ADG was .53 kg for BG and .68 kg for PP. Concentration of IGF-I during the summer period was affected (P<.05) by pasture treatment (BG = 138.6 ng/ml, PP = 181.9 ng/ml), was positively related (P<.01) to PCV and percentage of Brahman, and was negatively related (P<.05) to estimated percentage of English breeding. In steers assigned directly to the feedlot, IGF-I was correlated with empty body (EB) weight (r=−.59, P<.10), EB water (r=−.59, P<.10) and EB protein (r=−.60, P<.10) at slaughter, and with days on feed (r=−.65, P<.05), but was not correlated with ADG or rate of component gain. These data indicate that IGF-I is related to nutritional status in steers as in other species, that there may be significant breed or cattle type differences in circulating concentrations of IGF-I, and that circulating concentration of IGF-I may be functionally related to plasma concentration of thyroid hormones.     

Dose effect of human growth hormone-releasing factor and thyrotropin-releasing factor on hormone concentrations in lactating dairy cows

Two experiments were conducted to study the effects of growth hormone-releasing factor (GRF) and thyrotropin-releasing factor (TRF) administration on hormone concentrations in dairy cows. In the first trial, 12 cows were used on 5 consecutive days to determine the effect of four sc doses of GRF (0, 1.1, 3.3 and 10 μg•kg⁻¹ BW) and three sc doses of TRF (0, 1.1 and 3.3 μg•kg⁻¹ BW) combined in a factorial arrangement. GRF and TRF acted in synergy (P = .02) on serum growth hormone (GH) concentration even at the lowest dose tested and GH response to the two releasing factors was higher than the maximal response observed with each factor alone. TRF increased (P<.01) prolactin (Prl), thyrotropin (TSH), triiodothyronine (T₃) and thyroxine (T₄) concentrations similarly at the 1.1 and 3.3 μg•kg⁻¹ doses and GRF did not interact (P>.40) with TRF on the release of these hormones. In the second trial, the effect of GRF (3.3 μg•kg⁻¹ BW, sc) and TRF (1.1 μg•kg⁻¹ BW, sc) was tested at three stages (18, 72 and 210 days) of lactation on serum Prl and TSH concentrations. Eighteen cows (n = 6 per stage of lactation) were used in two replicates of a 3 × 3 latin square. The TRF and GRF-TRF treatments were equipotent (P>.05) in increasing Prl and TSH concentrations. Prl and TSH responses were similar (P>.40) throughout lactation. In summary, GRF at doses ranging from 1.1 to 10.0 μg•kg⁻¹ and TRF at doses ranging from 1.1 to 3.3 μg•kg⁻¹ act in synergy on GH release and do not interact on Prl, TSH, T₃ and T₄ concentrations in dairy cows. Furthermore, Prl and TSH response to TRF are not affected by stage of lactation.     

Castration and testosterone effects on endogenous and somatocrinin-induced growth hormone release in intact and castrated male pigs

Growth hormone (GH) release is influenced mainly by two hypothalamic factors, growth hormone-releasing factor (GRF) and somatostatin and is modulated by other hormones such as gonadal steroids. The objective of this study was to determine if castration (CA) and exogenous testosterone (TE) affect endogenous and GRF-induced GH release. Purebred Yorkshire male pigs (n = 32) were assigned to one of the following treatments: T1:CA; T2:CA +/- TE; T3: intact (IN); T4: IN +/- TE, in a 2 x 2 factorial design. Piglets were castrated at 3 days of age. Testosterone propionate (1.0 mg/kg) in sesame oil (2 ml) or sesame oil alone was injected sc SID during a 10-day period before each sampling day at 9, 15 and 21 weeks of age. Jugular blood samples were collected for a 6-hr period preceding and following iv injection of hGRF (1-29)NH2 (10 micrograms/kg). These procedures were repeated at 9, 15 and 21 weeks of age. The overall mean GH levels and the area under the GH peaks before and after GRF stimulation were lower (P less than .05) in castrated animals than in intact animals. Testosterone treatment increased (P less than .05) circulating TE levels and increased the amplitude of the endogenous GH peaks but did not affect (P greater than .05) the GRF-induced GH release. Increasing age produced a marked reduction of the amplitude of the GH peaks, the area under the GH peaks, the baseline mean and the overall mean GH levels during the 6-hr period preceding GRF injection. The present data support the hypothesis that castration in pigs reduces circulating and GRF-induced GH release. Exogenous testosterone for 10 days did not stimulate endogenous or GRF-induced GH release with the exception of the amplitude of the endogenous GH peaks.     

Secretory patterns of growth hormone and insulin-like growth factor-I during peripubertal period in intact and castrate male cattle

Fifteen Angus bulls and 15 Angus steers 9 months of age and 275 kg of body weight were bled at 20-min intervals over a 6-hr period and serum GH and IGF-I concentrations were measured by RIA. There were no differences between bulls and steers in the mean GH concentration, pulse frequency and amplitude when analyzed by the computer program PULSAR. Mean IGF-I concentration was not different between the two sex phenotypes, nor was there a significant correlation between the integrated IGF-I and GH concentrations. Subsequently, five bulls and five steers were selected from the 30 animals, full-fed a diet for growth in individual pens for 3 months and bled at 15-min intervals over a 24-hr period. Bulls tended to show a greater weight gain and feed conversion efficiency (P<.10) than steers during the 3-month period. Serum GH concentrations had a pulsatile pattern in all animals with no apparent diurnal rhythm during the 24-hr bleeding. Although mean GH concentration was not different between the two sex phenotypes, bulls tended to have lower baseline levels (P<.10) and greater peak amplitudes than steers. Serum IGF-I concentrations fluctuated within a two-fold concentration range, with no obvious pulsatility similar to that of GH. Mean IGF-I concentrations of each of the 10 animals were correlated with mean peak GH amplitudes (r = .79), but not with mean GH. These results suggest that gonadal hormone(s) modulates the GH secretory pattern and increases IGF-I secretion which may be related to the greater growth rate of bulls compared with steers.     

Opioid modulation of gonadotropin releasing hormone release from the hypothalamic preoptic area in the pig

Two experiments (Exp) were conducted to examine in vitro the release of gonadotropin releasing hormone (GnRH) from the hypothalamus after treatment with naloxone (NAL) or morphine (MOR). In Exp 1, hypothalamic-preoptic area (HYP-POA) collected from 3 market weight gilts at sacrifice and sagitally halved were perifused for 90 min prior to a 10 min pulse of morphine (MOR; 4.5 × 10⁻⁶ M) followed by NAL (3.1 × 10⁻⁵ M) during the last 5 min of MOR (MOR + NAL; N=3). The other half of the explants (n=3) were exposed to NAL for 5 min. Fragments were exposed to KCl (60 mM) at 175 min to assess residual GnRH releasability. In Exp 2, nine gilts were ovariectomized and received either oil vehicle im (V; n=3); 10 μg estradiol-17β/kg BW im 42 hr before sacrifice (E; n=3); .85 mg progesterone/kg BW im twice daily for 6 d prior to sacrifice (P₄; n=3). Blood was collected to assess pituitary sensitivity to GnRH (.2 μg/kg BW) on the day prior to sacrifice. On the day of sacrifice HYP-POA explants were collected and treated as described in Exp 1 except tissue received only NAL. In Exp 1, NAL increased (P<.05) GnRH release. This response to NAL was attenuated (P<.05) by coadministration of MOR. Cumulative GnRH release after NAL was greater (P<.05) than after MOR + NAL. All tissues responded similarly to KCl with an increase (P<.05) in GnRH release. In Exp 2, pretreatment luteinizing hormone (LH) concentrations were lower (P<.05) in E gilts compared to V and P₄ animals with P₄ being lower (P<.05) than V gilts. LH response to GnRH was lower (P<.05) in E pigs than in V and P₄ animals, while the responses was similar between V and P₄ gilts. NAL increased GnRH release in all explants, whereas, KCl increased GnRH release in 6 of 9 explants. These results indicate that endogenous opioid peptides may modulate in vitro GnRH release from the hypothalamus in the gilt.