Synergism and diurnal variations of human growth hormone-releasing factor (1-29)NH2 and thyrotropin-releasing factor on growth hormone release in dairy calves

Sixteen male Holstein calves averaging 168 kg body weight (BW) were used to determine the effects of human growth hormone-releasing factor (1–29)NH₂ (hGRF (1–29)NH₂; .22 μg/kg BW), thyrotropin-releasing factor (TRF; .165 μg/kg BW) or hGRF (1–29)NH₂ plus TRF (.22 and .165 μg/kg BW, respectively) on growth hormone (GH) release in animals exposed to 16 hr of light (L): 8 hr of dark (D) (lights on at 0100 hr) and hGRF plus TRF (.22 and .165 μg/kg BW, respectively) in animals exposed to 8L:16D (lights on at 0900 hr). For each treatment, times of iv injection were 0400, 1000, 1600 and 2200 hr. In animals exposed to 16L:8D, average GH peaks reached after hGRF (1–29)NH₂ or TRF injections were 49.7 and 32.0 ng/ml while the area under the GH response curve (AUC) were 1247 and 1019 ng/ml^*min, respectively. There was no significant effect of times of injection on GH release following the separate injection of hGRF (1–29)NH₂ or TRF. In animals exposed to 16L:8D, GH peaks and AUC after hGRF plus TRF injections were 226.4, 189.2 and 116.8 ng/ml, and 4340, 3660 and 2415 ng/ml^*min at 0400, 1000 and 1600 hr (lights on), respectively but only 42.3 ng/ml and 1692 ng/ml^*min at 2200 hr (lights off). In animals exposed to 8L:16D, GH levels and AUC after hGRF plus TRF injections reached 177.5 and 180.5 ng/ml, and 2759 and 3704 ng/ml^*min at 1000 and 1600 hr (lights on) but only 84.0 and 72.7 ng/ml, and 1544 and 1501 ng/ml^*min at 0400 and 2200 hr (lights off), respectively. These results demonstrated that hGRF (1–29)NH₂ and TRF can act in synergy to potentiate GH release in dairy calves. This synergistic action occurred only when both peptides were injected during the lighted phase of short and long day photoperiods.     

The influence of methimazole on the thyrotrophic and peripheral activity of thyrotrophin and thyrotrophin-releasing hormone in the chick embryo and growing chicken

Plasma concentrations of thyroxine (T₄) and triiodothyronine (T₃) were profoundly depressed both in chick embryos and growing chickens after methimazole (MMI) treatment. There was no response of T₄ and T₃ levels to TRH or TSH injections in the MMI group, either in embryos or growing chickens.

Peroxidase activity measured in the thyroid gland was significantly higher in embryos and growing chickens treated with MMI. However, neither TRH nor TSH affected this activity 2 hr after injection in either control or the MMI-treated group.

Hepatic 5′-monodeiodinase activity was significantly stimulated in the MMI-treated groups of embryos and growing chickens but only when additional sulphydryl groups (DTT) were provided. In embryos, monodeiodination activity 2 hr after TSH injection was not significantly different from control values for either DTT-stimulated or unstimulated conditions within the control and MMI-infused groups. However, in both control and MMI-treated embryos monodeiodination activity significantly increased 2 hr after TRH injection. In the growing chickens, monodeiodination activity 2 hr after TRH or TSH injection was not significantly different from control values in either stimulated or unstimulated conditions of each group.     

Effect of a nonsteroidal aromatase inhibitor on in vitro and in vivo secretion of estradiol and on the estrous cycle in ewes.

Three experiments were performed to study effects of decreased concentrations of estradiol-17β (E₂) on lifespan and function of ensuing ovine corpora lutea (CL). In experiment 1, 52 follicles were collected from 10 ewes and placed into individual culture with 0 or .01 μCi ³H-androstenedione (10 ng; ³H-A) and 0, 10⁻¹¹, 10⁻⁹, 10⁻⁷, or 10⁻⁵ M of a nonsteroidal aromatase inhibitor, CGS16949A (CGS). Concentrations of E₂ secreted into the medium, and synthesis of estrogens as estimated by formation of ³H-water from ³H-A were decreased by 10⁻⁵ and 10⁻⁷ (P<.01), but not 10⁻⁹ or 10⁻¹¹ M CGS. In experiment 2, luteolysis was induced in 24 ewes by injection of PGF₂ on days 5 to 10 of the estrous cycle (0 hr). Ewes received 0, 0.5, 1.0, 2.0 or 4.0 mg CGS per kg BW i.v. at −12, 0, 12 and 24 hr, and an ovulatory dose of hCG at 36 hr. Jugular (P<.001) and vena caval (P<.001) concentrations of E₂ were decreased by CGS at all doses tested for 8 to 10 hr, but had returned to levels similar to control ewes by the time of the next injection. Concentrations of E₂ around the time of the LH surge were similar in control and treated ewes. During the subsequent luteal phase, concentrations of progesterone (P₄) were similar in control and treated ewes. Thus, transient decreases in E₂ during the follicular phase were not deleterious to the subsequent luteal phase. In experiment 3, luteolysis was induced in 18 ewes by injection of PGF₂ on days 6 or 7 (0 hr) of the estrous cycle. Ewes received 0 or 1 mg CGS per kg BW i.v. every 8 hr from 0 to 40 hr. Ovulation was induced with hCG at 36 hr. CGS reduced jugular (P<.001) and vena caval (P<.001) concentrations of E₂, prevented an endogenous surge of LH (P<.05) and increased (P<.001) concentrations of FSH. All ewes had ovulated a marked follicle by 72 hr, but onset of the luteal phase, as assessed by concentrations of P₄, was delayed (P<.01) in ewes receiving CGS. Delayed luteal phases were not solely attributable to the presence of new CL or to luteinization of follicular cysts. When data were aligned according to the day ewes were observed in estrus, profiles of P₄ did not differ with treatment. Therefore, normal luteal function ensued following estrus whether or not ewes re-ovulated. In conclusion, decreased secretion of E₂ by the preovulatory follicle was not involved in the ontogeny of CL of short lifespan or subnormal function. Instead, adequate production of E₂ or precisely timed E₂ secretion may be required during follicular development for subsequent functional luteinization.     

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.     

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.     

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.     

The effect of insulin and relaxin upon mitosis (in vitro) in mammary tissue from pregnant and lactating pigs

The rate of cellular proliferation in the mammary glands of pigs during late gestation and lactation was assessed by measuring the incorporation of ³H-thymidine (T₁) into the DNA of mammary gland explants in vitro. The T₁ showed a linear response over the first 9 hr in vitro, and was not affected by the addition of 500 ng insulin/ml medium. From day 100 to parturition the T₁ rose, reached a peak at 2 d after parturition and declined during lactation to the lowest levels seen at day 21 of lactation.

The inclusion of 0–1000 ng relaxin/ml medium on T₁ at 24–72 hr in vitro had no effect in stimulating T₁ in mammary tissue explants taken from either pregnant or lactating pigs.     

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.     

Relationship of thyroid status to growth hormone and insulin-like growth factor-I (IGF-I) in plasma and IGF-I mRNA in liver and skeletal muscle of cattle

Steers were made hyperthyroid or hypothyroid to study the effects of physiological alterations in thyroid hormone status on plasma growth hormone (GH) profiles, plasma insulin-like growth factor-I (IGF-I) concentrations, and relative abundance of IGF-I mRNA in skeletal muscle and liver. Eighteen yearling crossbred steers (360 to 420 kg) were randomly allotted to hyperthyroid (subcutaneous injection 0.6 μg/kg BW L-thyroxine for 10 d), hypothyroid (oral thiouracil; 0.25% diet plus 12.5 g capsule/d for 17 d), or control (subcutaneous injection 0.9% NaCl) treatment groups. Blood samples were taken for measurement of GH, IGF-I, thyroxine (T₄) and triiodothyronine (T₃) by RIA. Samples of liver and skeletal muscle were taken by biopsy for measurement of IGF-I mRNA by solution hybridization. Steers receiving thiouracil had 57 and 53% (P<.05) lower T₄ and T₃, respectively, than control steers (84.1 and 1.7 ng/ml). The hyperthyroid steers had 228 and 65% greater (P<.05) T₄ and T₃ than control steers. Neither increased nor decreased thyroid status had any significant effects on plasma GH profiles, liver IGF-I mRNA, or plasma concentration of IGF-I. There was no effect of thyroid hormone alteration on skeletal muscle IGF-I mRNA concentrations. The results of this study suggest that short-term changes in thyroid status of cattle had no major impact on the GH-IGF-I axis or skeletal muscle IGF-I mRNA.     

Extrathyroidal thyroxine-5′-monodeiodinase activity in cattle

The monodeiodination of thyroxine (T₄) to triiodothyronine (T₃) was studied in vitro using liver, kidney, and muscle obtained from two-year old Angus and Hereford steers. Tissues were homogenized in .1 M phosphate buffer-.25 M sucrose - 5 mM EDTA, pH 7.5, and centrifuged at 2000 × g for 30 min. Supernatants were incubated with T4 (1.3 μM) at 37 C and T₃ generated was measured by radioimmunoassay of an ethanol extract of the incubation mixture. The T₄ to T₃ conversion in Angus liver homogenate was dependent upon pH, temperature, duration of incubation (5–120 min), homogenate (.025–.20 g-eq tissue/ml), and substrate concentration (.32–6.43 μM T₄). The apparent K_m and V_max of the conversion were .64 μM T₄ and 1.87 ng T₃ generated/hr/mg protein, respectively. Mean T₄ to T₃ conversion in Angus liver and kidney was 1.37 and .22 ng T₃/hr/mg protein. The presence of 2 mM dithiothreitol (DTT), a sulfhydryl protective agent, significantly increased T₃ generation in liver and kidney (5.12 and 4.58 ng/hr/mg protein) and also revealed activity in muscle (05 ng/hr/mg protein). In liver and kidney from Hereford steers conversion activity was 2.89 and .48 in absence and 10.91 and 5.38 ng T₃/hr/mg protein in presence of DTT, respectively. These results demonstrate the presence of a very active enzymatic system responsible for the peripheral 5′-monodeiodination of T₄ to T₃ in cattle.     

Response of young broiler chickens to chronic injection of recombinant-derived human insulin-like growth factor-I

The purpose of this study was to determine if exogenous insulin-like growth factor-I (IGF-I) would improve growth rate or body composition of young broiler chickens. Broiler cockerels were given a daily intramuscular (im) injection of sodium acetate buffer (buffer control), 100 or 200 μg recombinant-derived human IGF-I (rhIGF-I) per kg body weight from 11 to 24 days of age. Exogenous IGF-I did not affect the average daily gain, average daily feed consumption, or the gain-to-feed ratio of broiler chickens. Although daily injection of 200 μg/kg of rhIGF-I reduced (P<0.05) body ash content, there was no significant effect of IGF-I treatment on either body fat or protein content. Plasma GH levels were depressed (P<0.05) by chronic treatment with rhIGF-I. In contrast, plasma levels of T₃ and T₄ were not affected by rhIGF-I treatment. The half-life of rhIGF-I in plasma was determined at 25 days of age in naive control or chronically-injected chickens after a single intravenous dose of 50 μg rhIGF-I/kg. We found a single compartment, first-order disappearance pattern of rhIGF-I from chicken plasma. The half-life (t_1/2) of rhIGF-I in plasma was similar (t_1/2 = 32.5 min) for naive controls (injected once) or chronically-treated chickens which had received a daily injection of rhIGF-I (100 or 200 μg/kg) for 14 d. These data indicate that daily injection of IGF-I cannot be used to enhance growth performance or body composition of broiler chickens when given during the early growth period. The depression of plasma GH levels in rhIGF-I-injected chickens supports a negative-feedback role of IGF-I on pituitary GH secretion.     

Effect of progressive cachectic parasitism and growth hormone treatment on hepatic 5'-deiodinase activity in calves

Thyroid status is compromised in a variety of acute and chronic infections. Conversion of thyroxine (T₄) into the metabolically active hormone, triiodothyronine (T₃), is catalyzed by 5′-deiodinase (5′D) mainly in extrathyroidal tissues. The objective of this study was to examine the effect of protozoan parasitic infection (Sarcocystis cruzi) on hepatic 5′D (type I) activity and plasma concentrations of T₃ and T₄ in placebo- or bovine GH (bGH)-injected calves. Holstein bull calves (127.5±2.0 kg BW) were assigned to control (C, ad libitum fed), infected (I, 250,000 S. cruzi sporocysts per os, ad libitum fed), and pair-fed (PF, non-infected, fed to intake of I treatment) groups placebo-injected, and three similar groups injected daily with pituitary-derived bGH (USDA-B-1, 0.1 mg/kg, i.m.) designated as C_GH, I_GH and PF_GH. GH injections were initiated on day 20 post-infection (PI), 3–4 days prior to the onset of clinical signs of the acute phase response (APR), and were continued to day 56 PI at which time calves were euthanized for liver collection. Blood samples were collected on day 0, 28, and 55 PI. Alterations in nutritional intake did not affect type I 5′D in liver. Treatment with bGH increased (P<0.05) 5′D activity in C (24.6%) and PF (25.5%) but not in I calves. Compared to PF calves, infection with S. cruzi reduced 5′D activity 25% (P<0.05) and 47.8% (P<0.01) in placebo- and bGH-injected calves, respectively. Neither nutrition nor bGH treatment significantly affected plasma concentrations of T₄ and T₃ on day 28 and 55 PI. However, plasma thyroid hormones were reduced by infection. On day 28 PI, the average plasma concentrations of T₃ and T₄ were reduced in infected calves (I and I_GH) 36.4% (P<0.01) and 29.4% (P<0.05), respectively, compared to pair-fed calves (PF and PF_GH). On day 55 PI, plasma T₃ still remained lower (23.7%, P<0.01 versus PF) in infected calves while plasma T₄ returned to control values. The data suggest that parasitic infection in growing calves inhibits both thyroidal secretion and extrathyroidal T₄ to T₃ conversion during the APR. After recovery from the APR, thyroidal secretion returns to normal but basal and bGH-stimulated generation of T₃ in liver remains impaired.     

Administration of recombinant porcine somatotropin (rpST) changes hormone and metabolic status during early pregnancy

The objective of this study was to examine the effects of somatotropin (ST) on porcine reproductive and metabolic statuses during early pregnancy. Four pregnant crossbred gilts received 6 mg of recombinant porcine somatotropin (rpST) daily from days 10 to 27 after artificial insemination while six pregnant gilts served as controls. Blood samples were taken on days 8, 10, 12, 14, 18, 22, and 27 prior to rpST injections (8:00 h) and subsequently at 9:00, 10:00, 12:00, 14:00, 16:00, 18:00, and 20:00 h. On all remaining days of treatment, samples were taken once daily before injections (8:00 h). The samples were assayed for the metabolic hormones: ST, insulin-like growth factor I (IGF-I), insulin, thyroxine (T₄), triiodothyronine (T₃), and cortisol; for metabolites: free fatty acids (FFA) and glucose; and for the reproductive hormones: luteinizing hormone (LH), progesterone, estradiol-17β, estrone sulfate, and prostaglandin F₂. Delivery of rpST daily induced a 20- to 40-fold increase in plasma ST concentrations. Moreover, repeated administration of rpST resulted in a continuous increase in plasma IGF-I concentration (P<0.001), from 191.0±22.3–340.0±15.3 ng/mL 24 h after initial injection to 591.3±46.8 ng/mL after final injections. Mean serum insulin tended to be greater in rpST-treated gilts. Blood concentrations of T₄ were reduced (P<0.05) from day 14 of gestation in treated gilts while T₃ concentrations remained unchanged. Concentrations of both glucose and FFA were greater (P<0.01) and cortisol concentrations were unchanged in treated gilts. Changes in reproductive steroid hormones were minimally affected. Circulating progesterone (P=0.078), and estradiol-17β (P=0.087) concentrations tended to be lower in treated animals. These data show that treatment of pregnant gilts with rpST during early gestation mainly impacts metabolic rather than reproductive status.     

Potent interaction between thyrotropin releasing hormone (TRH) and human pancreatic growth hormone releasing factor (hpGRF) in stimulating chicken growth hormone (cGH) in vivo: Hypothalamic noradrenergic mediation in TRH stimulation of cGH release

The interaction of human pancreatic growth hormone releasing factor (hpGRF) and thyrotropin releasing hormone (TRH) on chicken growth hormone (cGH) release in vivo and possible noradrenergic involvement on TRH-induced stimulation of cGH in vivo were examined. Four-week old cockerels (1 kg) were injected intravenously with hpGRF (1.0 μg/bird), TRH (0.1 μg/bird), or hpGRF (1.0 μg/bird) in combination with TRH (0.1 μg/bird). Five min after the injection, blood samples were collected and serum concentrations of cGH were determined by a homologous RIA. The results showed that hpGRF and TRH were potent stimulators of cGH release, 5- and 6-fold over the control birds, respectively, and that hpGRF and TRH administered in combination produced a synergistic stimulation of cGH release (>20 fold). In separate experiments, pretreatment with alpha-methyl-para-tyrosine (250 mg/bird) for 2 hours resulted in complete suppression of the TRH stimulatory effect on cGH release but not the stimulatory effect of hpGRF. Pretreated with phenoxybenzamine hydrochloride (20 mg/bird) or diethyl-dithiocarbamate (500 mg/bird) also resulted in complete suppression of TRH-induced cGH release. These results indicate that hpGRF acts directly at the pituitary and TRH acts at the hypothalamus in addition to the pituitary in stimulating cGH release, possibly mediated through the noradrenergic neurons. HpGRF and TRH were potent releasers of cGH and their stimulation was potentiated when administered together.     

The influence of exogenous pituitary growth hormone on the ovulatory response to PMSG and hCG and the LH response to estradiol benzoate in prepubertal gilts

Two experiments were performed to examine the influence of exogenous growth hormone on the reproductive axis in gilts. Experiment one employed 26 Yorkshire × Landrace prepubertal gilts, which were selected at 150 d and 86.5 ± 1.5 kg bodyweight (BW) and assigned equally to two treatments. Gilts received injections of either porcine growth hormone at 90 μg/kg BW, or vehicle buffer, from 150 to 159 d. At 154 d gilts received 500 IU PMSG, followed 96 hr later by 250 IU hCG. Gilts were slaughtered at 163 days and their ovaries recovered to determine ovulatory status. In each treatment, gilts failed to show any ovarian response to PMSG/hCG. All remaining control gilts ovulated and their ovaries appeared morphologically normal. In gilts receiving exogenous growth hormone, fewer ovaries (4/11, P<.01) appeared morphologically normal. The ovaries of all other growth hormone injected gilts had very large (12–25 mm) non-luteinized follicles. In experiment two, 20 prepubertal Yorkshire × Landrace gilts were selected at 138 days and 85 kg BW. These gilts received injections of growth hormone at 90 μg/kg BW (n=9) or vehicle (n=11) from 138 to 147 days. At 143 days, all gilts were given an injection of estradiol benzoate (EB) at 15 μg/kg BW. Blood samples were taken at the time of EB injection, at 24 and 36 hr and then at 6 hr intervals until 78 hr. All samples were assayed for serum LH concentrations. The EB induced LH peak height was lower (P<.04) in gilts receiving exogenous growth hormone than in controls. The results presented indicate that the daily injection of growth hormone at 90 μg/kg BW reduced the estradiol-induced release of LH in addition to reducing the number of corpora lutea in gonadotrophin stimulated gilts.     

Total and free iodothyronine levels of growing Thoroughbred foals: Effects of weaning and gender

The aim of this study was investigate the effect of growing associated with different gender on circulating total and free iodothyronine concentrations during the first 13 mo of age in foals. In addition, we investigated the evolution of circulating concentrations of thyroid hormones during the first 3 d of weaning. Blood was collected from 13 clinically healthy Thoroughbred foals every month. All foals were weaned at the 4 mo and blood samples were taken also at 24, 48 and 72 h after weaning. The results obtained showed growing effects for tri-iodothyronine (T₃), thyroxine (T₄), free tri-iodothyronine (fT₃) and free thyroxine (fT₄) values (P < .001).

Serum T₃ concentrations averaged respectively 2.89 and 0.29 nmol/L at 7 and 9 mo. Serum T₄ concentrations averaged respectively 100.17 and 21.77 nmol/L at 1 and at 10 mo. Serum fT₃ concentrations averaged respectively 6.96 and 1.50 pmol/L at 1 and 4 mo. Serum fT₄ concentrations averaged respectively 31.40 and 4.93 pmol/L at 1 and 9 mo. Significant correlations between T₃, T₄, fT₃ and fT₄ with body weight (BW) and between T₃, T₄ and fT₄ with age were observed.

Weaning effects (P < .001) were shown for T₃ and fT₄ levels. No differences (P > .05) in T₄ and fT₃ levels were observed over the 3-day period. Gender effects (P < .001) were shown for T₃, T₄, fT₃, and fT₄ levels. Significant correlations between T₄ and fT₄ with BW and age were observed in colts and fillies. T₃ concentrations were correlated with age only in colts and fT₃ with BW only in colts. The results obtained seem to lend support to the recognized effects of growing and weaning in modulating the thyroid function of Thoroughbred foals. In fact, significant and differentiated effects of growing and weaning on total and free iodothyronine levels have been demonstrated.     

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.     

Acute and chronic hormone and metabolite changes in lambs fed the beta-agonist, cimaterol

The objective of this study was to determine if acute and chronic changes in circulating metabolic hormone and metabolite concentrations are associated with β-agonist-induced nutrient repartitioning in young growing lambs. Two groups of 12 Dorset and Dorset-Finn cross ram lambs weighing 36 or 33 kg live weight were assigned to 3- or 6-week treatment intervals, respectively, to achieve similar slaughter weights. Six lambs within each treatment interval were fed ad libitum a complete mixed high-concentrate diet containing either 0 or 10 ppm cimaterol. During the first 12 hr of cimaterol administration plasma somatotropin (ST), thyroxine (T₄), and triiodothyronine (T₃) concentrations were not altered by treatment, but plasma insulin, glucose, non-esterified fatty acids (NEFA) and glycerol concentrations were elevated 2 hr after ingestion. These acute responses suggest direct stimulation of glycogenolysis and lipolysis by cimaterol, which is characteristic of β-adrenergic alteration of carbohydrate and lipid metabolism. Chronic administration of cimaterol significantly decreased insulin concentrations by 36% and 52% at 3 and 6 weeks, respectively, while glucose concentrations remained unchanged. Serum IGF-I concentrations were not significantly altered by cimaterol. T₄ levels were reduced 22.1% after 3 weeks of cimaterol treatment. Although plasma NEFA concentrations were chronically elevated 56% to 65% in lambs fed cimaterol, plasma glycerol concentrations remained at baseline levels. The relative changes in plasma NEFA and glycerol concentrations are consistent with a decreased rate of lipogenesis, rather than an increase in lipolysis.     

Characterization of and radioimmunoassay for canine thyroglobulin

Canine thyroglobulin (cTg) has been isolated and purified. It has similar electrophoretic patterns as Tg from other mammalian species. The main fraction had a MW of 660,000, whereas also fractions of a MW of approximately 1,300,000 (dimer) and 330,000 (subunit) were present. The iodine content was 0.8 to 1.0 % (w/w). cTg did not cross-react with antibodies against human Tg to a degree that would allow the use of a radioimmunoassay for human Tg for the determination of cTg in serum or plasma. Therefore a polyclonal antiserum was raised against cTg and a homologous radioimmunoassay was developed, which was sensitive (0.4 μg/l) and specific (cross-reactivity in cTg assay of human Tg, goat Tg, T₄, T₃, and DIT < 0.01 %).

Plasma Tg levels in normal dogs of both sexes and aged 3–15 years amounted to 192 ± 73 μg/l (mean ± SD, n=30). There was no relation between plasma Tg and T₄ levels.     

Endotoxin induces delayed ovulation following endocrine aberration during the proestrous phase in Holstein heifers

The effect of endotoxin on follicular growth and on secretion of LH, estradiol-17β, progesterone and cortisol during the proestrous phase in cattle was investigated. Holstein heifers were treated with PGF₂ at 11–13 d after ovulation to induce luteolysis. At 42 hr after PGF₂ treatment, heifers were administered either lipopolysaccharide (LPS; Escherichia coli, O111:B4, 5 μg/kg, n = 6) or saline (control; n = 6) by i.v. bolus injection. Ovarian structures were monitored daily by transrectal ultrasonography, and blood samples were collected at various times for hormonal analysis. The duration from PGF₂ treatment to ovulation was significantly longer in the LPS group (8.0 ± 1.3 d) than in the control group (4.2 ± 0.2 d). LPS significantly reduced the pulse frequency of LH for 6 hr after the administration, and increased the mean concentration and pulse amplitude of LH from 3 to 6 hr after the administration. The plasma concentrations of progesterone and cortisol were transiently increased after LPS administration. The plasma concentration of estradiol-17β was significantly decreased at 24 hr after LPS administration compared to that in the controls. Five of six LPS-treated heifers exhibited no preovulatory LH surge until 120 hr after PGF₂ treatment and the remaining heifer exhibited the surge at 108 hr after PGF₂ treatment, while the LH surge was observed at 54–78 hr after PGF₂ treatment in control heifers. These results suggest that endotoxin disrupts progression of the proestrous phase of cattle, interrupting the preovulatory estradiol rise and thus delaying the LH surge and the subsequent ovulation.