Effects of long-term administration of recombinant bovine tumor necrosis factor-alpha on glucose metabolism and growth hormone secretion in steers

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.     

Effects of human pancreatic and rat hypothalamic growth hormone-releasing factors on growth hormone secretion in steers

Potencies of human pancreatic growth hormone-releasing factor [hpGRF(1–40)-OH] and of a peptide corresponding to the N-terminal 29 residues of rat hypothalamic GRF, [rGRF(1–29)-NH₂] were compared in two experiments. Eight Angus steers averaging 297 days of age and 290 kg in February 1984 were used in Exp. 1. Five months later six of the steers, weighing 391 kg, were used in Exp. 2.In Exp. 1, hpGRF(1–40)-OH and rGRF(1–29)-NH₂ were infused for 5 min at rates of 0, 1.3, 2.6, 5.2, 7.8 and 13.3 pmol/min/kg. Two steers were infused simultaneously, one received hpGRF(1–40)-OH and the other the equivalent dose of rGRF(1–29)-NH₂. Four pairs of steers received each dose. Both peptides elicited rapid GH release. Plasma GH concentrations peaked 15 to 20 min following onset of GRF administration, and returned to baseline levels 60 to 90 min later. Minimum effective doses, the lowest dose tested that resulted in a statistically significant GH reponse, were 5.2 pmol/min/kg hpGRF(1–40)-OH and 13.3 pmol/min/kg rGRF(1–29)-NH₂. Magnitudes of GH responses to 5.2, 7.8 and 13.3 pmol/min/kg hpGRF(1–40)-OH and 13.3 pmol/min/kg rGRF(1–29)-NH₂ were similar; corresponding to respective peak concentrations of 79, 66, 57 and 56 ng/ml. Growth hormone levels before GRF administration averaged 16 ng/ml.Experiment two was designed like the first except steers were infused for 6 hr with hpGRF(1–40)-OH and rGRF(1–29)-NH₂ at rates of 0, .5 and 1 pmol/min/kg. Both peptides at both rates raised (P<.05) GH concentrations during the 6 hr infusion period. Mean GH levels were 7 ng/ml during saline infusion, 30 and 23 ng/ml during infusion of .5 pmol/min/kg hpGRF(1–40)-OH and rGRF(1–29)-NH₂, and 41 and 27 ng/ml during infusion of 1 pmol/min/kg of the respective peptides. The initial GH response was biphasic, after which GH levels decreased temporarily and then one or two more GH surges occurred during the latter portion of the infusion period. Results demonstrate that hpGRF(1–40)-OH and rGRF(1–29)-NH₂ are potent GH secretagogues in steers. Potency of rGRF(1–29)-NH₂ is about 40% of hpGRF(1–40)-OH. Intrinsic activities, their ability to stimulate maximum GH secretion, appear to be similar. Both peptides are effective in raising GH levels over a 6 hr constant infusion period.     

Effects of parachlorophenylalanine,quipazine and cyproheptadine on growth hormone and adrenocorticotropin secretion in steers

Adrenergic and perhaps dopaminergic neurons provide inhibitory regulation of growth hormone (GH) secretion in ruminants. This suggests that either serotonergic or other neurons regulate the stimulatory release of GH. The nature of neurotransmitter control of adrenocorticotropin (ACTH) secretion in ruminants has not been determined. Parachlorophenylalanine (PCPA; serotonin synthesis inhibitor), quipazine (serotonin receptor agonist) and cyproheptadine (serotonin receptor antagonist) were utilized in Holstein steers to determine whether serotonin receptors mediate stimulatory actions on GH and ACTH secretion. PCPA (100 mg/kg BW) administered each day at 1900 hr for three successive days did not alter mean GH concentrations, amplitude of GH peaks, nor the number of GH peaks. Likewise, PCPA altered none of these parameters for ACTH. Quipazine injected iv at .1 or .5 mg/kg BW increased plasma GH (P<.05) and ACTH (P<.001) concentrations. There was a dose effect of quipazine on both GH (P<.05) and ACTH (P<.001) secretion. Pretreatment of steers with cyproheptadine (.06 and .6 mg/kg BW) reduced the stimulation of GH by quipazine (P<.0001) and decreased basal GH concentrations (P<.0004). Cyproheptadine at .06 mg/kg BW did not alter quipazine effects on ACTH, however, the higher dose decreased the peak ACTH response (P<.02) to quipazine. Studies with quipazine and cyproheptadine indicated that serotonergic mechanisms are likely involved in the regulation of GH and ACTH secretion in steers.     

Twenty-four-hour growth hormone profiles in Angus steers

A study was conducted in February 1984 to characterize plasma growth hormone (GH) patterns in steers. Eight Angus steers averaging 285 d of age and 276 kg were housed in a sheltered pen and group-fed once daily. Animals gained at a mean rate of .78 kg/d, with individuals ranging from .34 to 1.02 kg/d. A jugular vein cannula was inserted the day before blood sampling, which commenced at 0600 h and continued at 15-min intervals for 24 h. Growth hormone patterns consisted of frequent GH surges of varying amplitude. Growth hormone surges occurred at an average frequency of .7/h. This rate did not differ markedly among steers nor hour of day. The magnitude of GH secretory surges varied significantly among steers and during the 24-h period. Growth hormone peaks averaged 47.0 and 27.2 ng/ml in steers having the highest and lowest GH surges, respectively. During the 24-h period there were two to four 2- to 3-h periods in which GH surges were reduced in amplitude. These trough periods occurred at approximately 6-h intervals in two steers, imparting rhythmicity to GH profiles of these animals. Periods of reduced surge amplitudes and intervening secretory episodes were less periodic in the other steers. Steers were fed at 1400 h and in all steers GH levels fell from 1400 to 1600 h, and then rebounded with two to four high amplitude surges. Peak and mean GH levels were associated positively (r = .93, P less than .01) and both were associated negatively with rates of gain (r = -.82 and -.74, respectively; P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of short- and long-term recombinant equine growth hormone and short-term hydrocortisone administration on tissue sensitivity to insulin in horses

OBJECTIVE: To determine the effects of short-term IV administration of hydrocortisone or equine growth hormone (eGH) or long-term IM administration of eGH to horses on tissue sensitivity to exogenous insulin. ANIMALS: 5 Standardbreds and 4 Dutch Warmblood horses. PROCEDURE: The euglycemic-hyperinsulinemic clamp technique was used to examine sensitivity of peripheral tissues to exogenous insulin 24 hours after administration of a single dose of hydrocortisone (0.06 mg/kg), eGH (20 microg/kg), or saline (0.9% NaCl) solution and after long-term administration (11 to 15 days) of eGH to horses. The amounts of metabolized glucose (M) and plasma insulin concentration (I) were determined. RESULTS: Values for M and the M-to-I ratio were significantly higher 24 hours after administration of a single dose of hydrocortisone than after single-dose administration of eGH or saline solution. After long-term administration of eGH, basal I concentration was increased and the mean M-to-I ratio was 22% lower, compared with values for horses treated with saline solution. CONCLUSIONS AND CLINICAL RELEVANCE: Increases in M and the M-to-I ratio after a single dose of hydrocortisone imply that short-term hydrocortisone treatment increases glucose use by, and insulin sensitivity of, peripheral tissues. Assuming a single dose of hydrocortisone improves sensitivity of peripheral tissues to insulin, it may be an interesting candidate for use in reducing insulin resistance in peripheral tissues of horses with several disease states. In contrast, long-term administration of eGH decreased tissue sensitivity to exogenous insulin associated with hyperinsulinemia. Therefore, increased concentrations of growth hormone may contribute to insulin resistance in horses with various disease states.     

Effects of serotonin injected into the third ventricle on prolactin and growth hormone secretion in Holstein steers

In order to clarify the role of serotonin (5-HT) in the regulation of pituitary hormones, the effects of 5-HT injected into the third ventricle (3V) on prolactin (PRL) and growth hormone (GH) release were investigated in Holstein steers. A chronic cannula was implanted in 3V by stereotaxic surgery under general anesthesia. After sufficient recovery from surgery, 5-HT (0, 0.1, 1.0, 2.0 mg) was injected into via the cannula and blood samples were collected over 4 h. Plasma PRL and GH concentrations were determined by radioimmunoassay. PRL release was significantly stimulated by the injection of 5-HT. The increase in PRL was observed at 20 min after the injection at three doses and the highest dose (2.0 mg) was the most effective in stimulating PRL release. The injection of 5-HT into 3V, at all doses tested, did not alter GH release significantly. Our results suggest that 5-HT is involved in the regulation of PRL release partly through the hypothalamus in cattle.     

The effects of bovine recombinant growth hormone administration on insulin-like growth factor-I and the haemopoietic system in thoroughbred geldings

The effect of intramuscularly administered recombinant bovine growth hormone (rbGH) on insulin-like growth factor-I (IGF-I) and white and red blood cell indices was studied in Thoroughbred geldings. An insulin-like growth factor binding protein (IGFBP)-blocked radioimmunoassay was modified and validated for the measurement of IGF-I in equine blood plasma. Baseline values of IGF-I and blood indices were determined over a 48 h period and then a single dose of 5 microg/kg, 10 microg/kg or 50 microg/kg of rbGH was administered. Insulin-like growth factor-I levels increased in a dose-dependent manner, with the highest values between 12 h and 24 h. The highest dose (50 microg/kg) yielded the greatest IGF-I response with a 90.2+/-10.8% increase at 24 h. White blood cell count increased following the three doses of rbGH with the highest white blood cell count at 12 h after the 50 microg/kg dose. Haemoglobin was significantly increased at 24 h (P< 0.05), when values following doses of 10 microg/kg and 50 microg/kg were significantly greater than after the vehicle or the dose of 5 microg/kg. Red blood cell count was not affected by any of the rbGH doses. These results indicated that rbGH is biologically active in the horse and that rbGH at a dose rate of 10 microg/kg or more could be used therapeutically.     

Effect of repeated administration of combination trenbolone acetate and estradiol implants on growth, carcass traits, and beef quality of long-fed Holstein steers

Our objective was to determine the effect of repeated use of implants on feedlot performance and carcass characteristics of Holstein cattle. Holstein steers (n = 128) weighing an average of 211 kg were blocked by weight and randomly assigned to 16 pens. At the start of the trial (d 0), pens were assigned to one of four treatments: 1) nonimplanted control (C); 2) implant on d 0, 112, and 224 (T3); 3) implant on d 112 and 224 (T2); and 4) implant on d 224 (T1). Component TE-S implants (120 mg of trenbolone acetate and 24 mg of estradiol per implant) were used for all treatments during the 291-d feeding period. Over the course of the study, T2 and T3 cattle had greater ADG and final weights than C and T1 cattle (P < 0.05). Steers were harvested at a commercial abattoir on d 291. Hot carcass weights of T3 steers were greater than those of C and T1 steers (P < 0.05). Dressing percentage, adjusted 12th-rib fat, percentage of kidney, pelvic, and heart fat, yield grade, and longissimus color were not different among treatments (P > or = 0.26). Longissimus muscle areas (LMA) of T2 and T3 carcasses were larger than LMA of C (P < 0.01). No USDA Select carcasses were produced from C cattle, whereas the percentage of Select carcasses from implanted cattle ranged from 10 to 18%. Skeletal maturity advanced (P < 0.05) progressively with each additional implant. Steaks from T3 carcasses had a higher percentage of protein than controls (P < 0.05) and were less tender than all other treatments (P < 0.05). Repeated administration of combination trenbolone acetate and estradiol implants increased ADG and resulted in heavier carcasses with larger LMA. Administration of three successive implants decreased tenderness of Holstein beef, and resulted in more advanced skeletal maturity scores.     

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.     

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.     

Effects of vitamin A on growth performance and carcass quality in steers

Vitamin A plays a critical role in many essential life processes. In herbivores, it is either derived from plant β-carotene or directly as a dietary supplement. In cattle, vitamin A has the potential to influence various carcass traits that are sought by specific beef markets. A group of 20 Angus steers was removed from pasture and fed a low β-carotene and vitamin A cereal-based ration on a feedlot for 308 days. Ten of the steers were supplemented with vitamin A (retinyl palmitate, 60 IU of vitamin A/100 kg body weight/day) and the other ten received no supplement. The results demonstrated that restriction of vitamin A intake changed intramuscular fat deposition without changing subcutaneous fat depots. Angus steers that had been depleted of vitamin A showed increased intramuscular fat in the longissimus thoracis et lumborum (LTL) by 35% (P < 0.026) and seam fat area at the quartering site by 33% (P < 0.0273), when compared with cattle supplemented with vitamin A. There were no changes in intramuscular fat in the semitendinosus. Visually assessed marbling scores were also higher (19%; P < 0.094) in the non-supplemented, depleted group. There was no effect of vitamin A depletion on cattle growth and other meat traits (eye muscle area, meat colour, pH, meat cut weight), meat eating attributes (tenderness, cooking loss) or muscle fibre diameter. The only difference (P < 0.0177) among the meat traits was fat colour where depleted animals had whiter fat than the controls. Moreover, the fat from the vitamin A depleted group was softer with a lower melting point. We conclude that the reduced vitamin A consumption, leading to vitamin A depletion, increases intramuscular fat. On the other hand, the vitamin A depletion did not increase subcutaneous fat depth or change other meat quality traits, suggesting that marbling and these other traits are not invariably related.     

The effect of pulsatile gonadotropin-releasing hormone and estradiol administration on luteinizing hormone and follicle-stimulating hormone concentrations in pituitary stalk-sectioned ovariectomized pony mares

Hourly pulses of gonadotropin-releasing hormone (GnRH) or bi-daily injections of estradiol (E2) can increase luteinizing hormone (LH) secretion in ovariectomized, anestrous pony mares. However, the site (pituitary versus hypothalamus) of positive feedback of estradiol on gonadotropin secretion has not been described in mares. Thus, one of our objectives involved investigating the feedback of estradiol on the pituitary. The second objective consisted of determining if hourly pulses of GnRH could re-establish physiological LH and FSH concentrations after pituitary stalk-section (PSS), and the third objective was to describe the declining time trends of LH and FSH secretion after PSS. During summer months, ovariectomized pony mares were divided into three groups: Group 1 (control, n = 2), Group 2 (pulsatile GnRH (25 μg/hr), n = 3), and Group 3 (estradiol (5 mg/12 hr), n = 3). All mares were stalk-sectioned and treatment begun immediately after stalk-section. Blood samples were collected every 30 min for 8 h on the day before surgery (DO) and 5 d post surgery (D5) to facilitate the comparison of gonadotropin levels before and after pituitary stalk-section. Additionally, jugular blood samples were collected every 12 hr beginning the evening of surgery, allowing for evaluation of the gonadotropin secretory time trends over the 10 d of treatment. On Day 10, animals were euthanized to confirm pituitary stalk-section and to submit tissue for messenger RNA analysis (parallel study). Plasma samples were assayed for LH and FSH by RIA. Mean LH secretion decreased from Day 0 to Day 5 in Groups 1 and 3, whereas LH secretion tended (P < 0.08) to decrease in Group 2 mares. On Day 5, LH was higher (P < 0.01) in Group 2 (17.26 ± 3.68 ng/ml; LSMEANS ± SEM), than either Group 1 (2.65 ± 4.64 ng/ml) or group 3 (4.28 ± 3.68 ng/ml). Group 1 did not differ from Group 3 on Day 5 (P < 0.40). Similarly, mean FSH levels decreased in all groups after surgery, yet Group 2 mares had significantly (P < 0.001) higher FSH concentrations (17.66 ± 1.53 ng/ml) than Group 1 or Group 3 (8.34 ± 1.84 and 7.69 ± 1. 63 ng/ml, respectively). Regression analysis of bi-daily LH and FSH levels indicated that the time trends were not parallel. These findings indicate: 1) Pituitary stalk-section lowered LH and FSH to undetectable levels within 5 d after surgery, 2) pulsatile administration of GnRH (25 μg/hr) maintained LH and FSH secretion, although concentrations tended to be lower than on Day 0, and 3) E2 did not stimulate LH or FSH secretion.     

Plasma growth hormone, insulin, glucocorticoids and thyroid hormones in large, medium and small breeds of steers with and without an estradiol implant

Growth hormone secretory patterns and plasma concentrations of insulin, adrenal corticosteroids, thyroxine and triiodothyronine were measured in three types of steers with and without estradiol implants. The three types of cattle were large (Simmental), medium (Limousin) and small (Angus) frame size with medium, low and high propensity to deposit fat. Simmental steers had higher (P<.01) concentrations of growth hormone and lower (P<.01) concentrations of insulin as compared with Angus. Implanting the steers with estradiol increased (P<.01) growth hormone concentrations in all three groups, tended to decrease glucocorticoids and had no effect on insulin or thyroid hormone concentrations. There tended to be less frequent release of growth hormone, with secretory spikes of greater magnitude in the Simmental steers. The implants tended to increase the frequency of release of growth hormone in all three groups.     

Analogs of growth hormone-releasing hormone induce release of growth hormone in the bovine

Biological potencies of three 29 amino acid growth hormone-releasing hormone analogs (GHRH[1-29]) were determined in the bovine and compared to synthetic human GHRH (44 amino acids; hGHRH[1-44]NH2) for their ability to increase serum growth hormone (GH) concentrations. Four prepubertal Holstein heifers (179 +/- 10 kg) received hGHRH(1-44)NH2 or analogs (D-Ala2, Nle27, Agm29 GHRH[1-29], [JG-73]; D-N-MeAla2, Nle27, Agm29 GHRH[1-29], [JG-75]; and desamino-Tyr1, D-Ala2, Nle27, Agm29 GHRH[1-29], [JG-77]) at the following doses: 0, 6.25, 25, 100 and 400 micrograms/animal. All treatment-dose combinations were administered to each heifer with at least a 1-d interval between treatments. Sixteen blood samples were collected via jugular cannulas 20 min before and up to 6 h after treatment injection. There was a linear dose-dependent GH release in response to hGHRH(1-44)NH2 and the three analogs. Growth hormone peak amplitudes for the three analogs were similar to those observed after administration of the hGHRH(1-44)NH2 (P greater than .05). However, when total area under the GH response curves for each treatment was averaged over all the doses, JG-73 stimulated greater GH release than hGHRH(1-44)NH2 (P less than .05) Heifers injected with the 400-microgram dose of hGHRH(1-44)NH2 or the three analogs showed a primary release of GH followed by a secondary release 1 h later. At all other doses, only a primary GH release was observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of growth hormone administration on vitamin D metabolism and vitamin D receptors in the pig

Twelve 36-kg pigs were given either 100 micrograms/kg porcine pituitary growth hormone (pGH) or placebo injections daily for 33 days. Serum was obtained weekly for analysis of minerals and vitamin D metabolites. On day 34, the pigs were sacrificed and renal and duodenal tissue were obtained for analysis of vitamin D receptor content (VDR). Animals treated with pGH grew faster and had a higher rate of bone accretion than did control animals. Serum concentrations of 1,25-dihydroxyvitamin D (1,25-(OH)2D) were significantly higher in pGH-treated pigs than in control pigs at all time points following initiation of treatment, with the greatest difference observed at day 28 (42.4 +/- 4.9 pg/ml in controls vs. 65.4 +/- 4.7 pg/ml in pGH-treated pigs). Serum 24,25-dihydroxy-vitamin D tended to be lower in pGH-treated pigs than in control pigs, being significantly lower on day 21 of the experiment (3.22 +/- .52 vs. 6.73 +/- 1.22 ng/ml, respectively). Serum concentrations of 25-hydroxyvitamin D and calcium were unaffected by pGH treatment. Kidneys of control pigs contained significantly more unoccupied vitamin D receptors than did kidneys from pGH-treated pigs (73.3 +/- 4.3 vs. 58.3 +/- 4.1 fmoles/mg protein). Duodenal tissue unoccupied vitamin D receptor content was similar in both pGH-treated (245 +/- 17.9 fmoles/mg protein) and control (263 +/- 21.8 fmoles/mg protein) pigs. Duodenal occupied vitamin D receptor concentration was similar in both pGH-treated (6.8 +/- .75 fmoles/mg protein) and control pigs (5.32 +/- .77 fmoles/mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of growth rate during the early fattening period on growth, carcass characteristics and circulating hormones in the different growth hormone genotypes of Japanese black steers

One of the bovine growth hormone (GH) genetic variants is a substitution of leucine (Leu) to valine (Val) at amino acid position 127 of the protein. The GH genotypes of 14 Japanese black steers used in the present study were Leu/Leu (A, n = 7) and Val/Val (B, n = 7). The steers in each genotype group were divided into two groups based on intended growth rate (high, 1.0 kg/day; low, 0.6 kg/day) during 10–17 months of age. The overall mean concentration of plasma GH was higher (P < 0.05) in the A group than in the B group. The serum concentration of insulin‐like growth factor‐I was higher (P < 0.05) in the B group than in the A group. The carcass weight of the A group was greater (P < 0.01) than that of the B group. However, there was no significant difference in carcass weights between the 1.0 kg/day and 0.6 kg/day groups (P > 0.05). The rib thickness of the 1.0 kg/day group was greater (P < 0.05) than that of the 0.6 kg/day group. The crude fat content of longissimus muscle was greater (P < 0.05) for the 0.6 kg/day group compared with the 1.0 kg/day group.     

Effects of oral chlortetracycline and dietary protein level on plasma concentrations of growth hormone and thyroid hormones in beef steers before and after challenge with a combination of thyrotropin-releasing hormone and growth hormone-releasing hormone.

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.     

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.     

Effects of weaning age and diet on growth and carcass characteristics in steers

Two experiments were conducted to determine the effects of diet on growth of steers weaned at approximately 100 vs 205 d of age. In Exp. 1, a 2 x 2 x 2 factorial experiment was conducted using 78 Angus crossbred cow-calf pairs. The factors examined were age at weaning (early, at 103+/-3 d [EW] vs normal, at 203+/-3 d [NW]), feeding strategy (ad libitum vs postweaning programmed intake), and dietary CP concentration (100 vs 120% of NRC [1984] recommended levels). Early-weaned calves had a greater (P < .001) ADG than NW calves from 103 to 203 d and reached market weight at 385 d vs 418 d for NW calves (P < .001). Likewise, steers offered feed for ad libitum consumption reached market weight at 394 d, compared with 409 d for programmed-intake steers (P < .05). In Exp. 2, 64 Angus crossbred steers were either weaned at 93+/-3 d and fed one of four diets, weaned at 210+/-3 d without access to creep feed, or weaned at 210+/-3 d with access to creep feed for 60 d prior to weaning. Early-weaned calves were heavier (P < .01) than NW calves at 210 d if fed either 100 or 90% concentrate diets, and they had greater (P < .001) backfat thickness at 210 d but no difference (P > .10) in longissimus muscle area compared to EW calves fed a 60% concentrate diet. At slaughter, 80 to 100% of steers on all treatments graded low Choice or higher. Feeding high-concentrate diets to EW beef calves accelerated growth rate and fat deposition early in the feeding period and may be a way to provide young cattle for a high-quality beef market.     

Effects of leptin on the release of luteinizing hormone, growth hormone and prolactin from cultured bovine anterior pituitary cells

The effects of leptin on the release of luteinizing hormone (LH), growth hormone (GH) and prolactin (PRL) were studied in cultured bovine anterior pituitary (AP) cells in vitro. The AP cells were obtained from fully‐fed Japanese Black steers and were incubated for 3 h with 10^?13 to 10^?7 mol/L of leptin after incubating in Dulbecco's modified Eagle's Medium for 3 days. Leptin significantly increased the concentration of LH in the culture medium by 45 and 44% at doses of 10^?8 and 10^?7 mol/L, respectively, compared with the controls (P < 0.05). Leptin significantly increased the concentration of GH in the culture medium by 14 and 12% at doses of 10^?8 and 10^?7 mol/L, respectively (P < 0.05). Leptin also significantly increased the concentration of PRL in the culture medium by 26% compared with the controls at a dose of 10^?7 mol/L (P < 0.05). These results show that leptin stimulates the release of LH, GH and PRL by acting directly on bovine AP cells from fully‐fed steers.