Rates of muscle protein breakdown in chickens selected for increased growth rate, food consumption or efficiency of food utilisation as assessed by N tau-methylhistidine excretion

1. N tau-methylhistidine excretion, growth rate, food consumption and body composition was determined in 12 4 to 5 week old chickens sampled from each of 4 lines selected for increased body-weight gain (line W), for increased food consumption (line F), for improved efficiency of food utilisation (line E) or at random (line C), after 12 generations of selection. 2. The use of N tau-methylhistidine as an index of myofibrillar protein breakdown was validated in male and female chickens of lines E and F by following the fate of injected N tau-(14CH3)methylhistidine. Most of the radioactivity (79.3 +/- 1.1%) was excreted in 4 d, with the remainder retained in the carcase. In excreta, 94 +/- 2% of the radioactivity was associated with free N tau-methylhistidine and for the carcase, this value was 88 +/- 3%. 3. In the main experiment, final body weights averaged 497, 651, 588 and 537 g and food: gain ratio averaged 2.47, 2.21, 3.14 and 2.06 for lines C, W, F and E respectively, Carcase protein content (g/100 g body weight) was not different between the lines. 4. N tau-methylhistidine excretion was 5.86, 5.48, 6.43 and 4.99 mumoles/mole carcase-N/d for lines C, W, F and E, respectively. The rate for line F was significantly higher than for lines W and C and that for line E was significantly less than for the control line. 5. The N tau-methylhistidine excretion rate was positively correlated with food: gain ratio. 6. Selection for rapid growth, high food consumption or improved food utilisation results in changes in N tau-methylhistidine excretion which suggest proportionate changes in muscle protein turnover.     

Skeletal muscle protein metabolism and serum growth hormone, insulin, and cortisol concentrations in growing steers implanted with estradiol-17 beta, trenbolone acetate, or estradiol-17 beta plus trenbolone acetate.

Skeletal muscle protein degradation, measured by urinary N tau-methylhistidine excretion, and circulating concentrations of growth hormone (GH), insulin (INS), and cortisol (CT) were monitored in steers before and after implantation with estradiol-17 beta (E2; 24 mg) and trenbolone acetate (TBA; 300 mg). Yearling crossbred steers (n = 43) were randomly assigned to four treatment groups in a 2 x 2 factorial arrangement: nonimplanted controls (C); TBA; E2; and TBA plus E2 (TBA+E2). A subgroup (Block 1) of 16 steers was bled on d -12, 31, and 72 after implanting. Deposition of skeletal muscle protein was markedly increased (P less than .001) by E2 and TBA+E2 treatment. This response occurred mainly within the first 40 d after implantation and declined (P less than .001) in concert with decreasing (P less than .01) concentration of serum E2. Anabolic steroid treatment did not affect the rate of skeletal muscle protein breakdown. There was no apparent relationship between reduced serum CT concentration (linear effect; P less than .01) in TBA-treated steers and skeletal muscle protein degradation rate. Blood concentration and pulse activity of INS were not affected by anabolic steroid administration. Both TBA- and TBA+E2-implanted steers displayed a linear decrease (P less than .05) in serum GH concentration over time, which was similar to C. Lowered mean GH concentration resulted from a reduction (TBA main effect; P less than .05) in pulse amplitude of GH. Unlike TBA, TBA+E2, and C, only E2 maintained serum GH concentrations over time. Although increased muscle protein deposition was evident in TBA+E2-treated steers, an obvious causal relationship between this response and circulating GH, INS, and CT was not revealed. These results do not support the concept that combined androgenic agent and estrogen administration effectively reduce bovine muscle protein degradation by static modulation of circulating endogenous anabolic and antianabolic hormones.     

Effects of food intake and food withholding on plasma ghrelin concentrations in healthy dogs

OBJECTIVE: To investigate the physiologic endocrine effects of food intake and food withholding via measurement of the circulating concentrations of acylated ghrelin, growth hormone (GH), insulin-like growth factor-I (IGF-I), glucose, and insulin when food was administered at the usual time, after 1 day's withholding, after 3 days' withholding and after refeeding the next day in healthy Beagles. ANIMALS: 9 healthy Beagles. PROCEDURES: Blood samples were collected from 8:30 AM to 5 PM from Beagles when food was administered as usual at 10 AM, after 1 day's withholding, after 3 days' withholding, and after refeeding at 10 AM the next day. RESULTS: Overall mean plasma ghrelin concentrations were significantly lower when food was administered than after food withholding. Overall mean plasma GH and IGF-I concentrations did not differ significantly among the 4 periods. Circulating overall mean glucose and insulin concentrations were significantly higher after refeeding, compared with the 3 other periods. CONCLUSIONS AND CLINICAL RELEVANCE: In dogs, food withholding and food intake were associated with higher and lower circulating ghrelin concentrations, respectively, suggesting that, in dogs, ghrelin participates in the control of feeding behavior and energy homeostasis. Changes in plasma ghrelin concentrations were not associated with similar changes in plasma GH concentrations, whereas insulin and glucose concentrations appeared to change reciprocally with the ghrelin concentrations.     

Effects of somatostatin immunoneutralization on growth and endocrine parameters in chickens

The effects of somatostatin immunoneutralization on growth rate, growth hormone (GH) secretion and circulating insulin-like growth factor I (IGF-I) concentrations were investigated in chickens through the use of passive and active immunization techniques. Intravenous bolus injection of goat-antisomatostatin stimulated a significant (P less than .05) increase in plasma GH levels for one hour post-injection in four and six week old male broiler chickens. The GH response to an intravenous bolus injection of hGRF44NH2 was similar in the antisomatostatin treated chicks and normal goat serum treated controls. Despite the presence of circulating somatostatin antisera after 28 hours, plasma GH levels were not different between control and antisomatostatin-treated chicks at that time. Continuous administration of somatostatin antisera by Alzet pump over a two-week period resulted in significant (P less than .05) elevations in plasma GH levels at one week post-implantation and in circulating IGF-I concentrations after two weeks of administration. Chicks which developed antibodies against somatostatin following active immunization exhibited a 7.1% increase in growth rate which was associated with a significant decrease in abdominal fat. However, neither GH nor IGF-I concentrations were elevated in the chicks which developed somatostatin antibodies. Thus, the benefits gained from somatostatin immunoneutralization may be exerted through mechanisms other than GH.     

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.     

Effect of intracerebroventricular injection of IGF-I on circulating growth hormone concentrations in the sheep

The effect of intracerebroventricular administration of IGF-1 on circulating growth hormone (GH) concentrations has been studied in sheep. Twenty sheep were fitted with jugular vein catheters and with indwelling cerebroventricular cannulae. IGF-I was injected into a lateral cerebral ventricle and changes in the circulating concentrations of GH were measured in jugular vein blood samples. Administration of saline had no effect on circulating GH concentrations over a 3-hr period, and administration of IGF-I (at 1, 3 and 10 micrograms/sheep) also had no significant effect on circulating GH concentrations. From these data we surmise that centrally administered IGF-I does not influence GH secretion and it seems probable that cerebrospinal fluid concentrations of IGF-I do not have a role in regulating GH release in sheep.     

Circulating IGF-I in plasma of growing male and female turkeys of medium and heavy weight lines

Plasma concentrations of insulin-like growth factor-I (IGF-I) were determined in male and female turkeys from a medium weight (RBC2) and a related heavy weight line (F) from 1 to 28 wks of age. At hatch, the concentrations of IGF-I were relatively low and not different between lines or sexes. During the neonatal period (1 to 7 wks), the concentrations of IGF-I increased and were higher in the faster growing F line and in males. During the juvenile period (8 to 15 wks) the concentrations of IGF-I were higher in males but not different between lines. During the preadolescent period (16 to 21 wks), the concentrations of IGF-I were higher in males but was not different between lines in males while the females of the RBC2 line had higher concentrations than females of the F line. During the adolescent period (22 to 28 wks) the concentrations of IGF-I were higher in males but was not different between lines in males while the females of the RBC2 line had higher concentrations than females of the F line. A phenotypic correlation (+.25) between plasma IGF-I and growth rate was present after statistical absorption of model effects during the neonatal period but not at the later ages. We conclude that IGF-I concentration was positively correlated with growth rate during the neonatal period, but that this relationship changed during the preadolescent and adolescent periods so that IGF-I concentrations were not related to growth rate in males but were negatively related to growth rate in females.     

Interactions of amino acids and hormones regulate the balance between growth and milk protein synthesis in lactating rats fed diets differing in protein content

Insulin-like growth factor-I (IGF-I), growth hormone (GH), and prolactin (PRL) play important roles in milk protein synthesis, and their plasma concentrations were reported to be affected by dietary protein intake. To investigate the relationship between circulating amino acid (AA) and concentrations of these hormones, 18 Wistar rats aged 14 wk were assigned to a low (LP; 9% protein), standard (SP; 21% protein), or high-protein (HP; 35% protein) diet from parturition through day 15 of lactation. Plasma, liver, pituitary gland, skeletal muscle, and mammary gland samples were collected at the end of treatment. Circulating and hepatic IGF-I concentrations increased linearly with elevated dietary protein concentrations (P < 0.0001). Rats receiving the HP diet had higher circulating GH (P < 0.01) and pituitary PRL concentrations (P < 0.0001) but lower pituitary GH concentration (P < 0.0001) relative to those in rats receiving the LP and SP diets. Pearson correlation test performed on composed data across treatments showed that several circulating AAs were correlated with circulating and tissue concentrations of IGF-I, GH, and PRL. Multiple linear regression analyses identified Leu, Gln, Ala, Gly, and Arg as the main AAs associated with hormone responses (R² = 0.37 ~ 0.80; P < 0.05). Rats fed the LP and HP diets had greater Igf1 and Ghr gene expression in skeletal muscle than those fed the SP diets (P < 0.01). However, LP treatment decreased Prlr mRNA abundance in mammary glands as compared with the SP and HP treatments (P < 0.05). The HP diets increased AA transporter expression (P < 0.01) but decreased mammalian target of rapamycin (P < 0.05) and 70 kDa ribosomal protein S6 kinase 1 (P < 0.01) phosphorylation in mammary glands as compared with the LP and SP diets. The results of the present study suggested that several circulating AAs mediated the effects of dietary protein supply on concentrations of IGF-I, GH, and PRL, which in turn altered the metabolism status in peripheral tissues including the lactating mammary glands.     

Concentrations of hormones, glucose, triglycerides and free fatty acids in the plasma of broiler chickens selected for weight gain or food conversion

1. The concentrations of growth hormone (GH), insulin-like growth factor I (IGF-I), thyroxine (T4), triiodothyronine (T3), reverse-triiodothyronine (rT3), triglycerides (Tri), free fatty acids (FFA) and glucose (Glu) were determined at 2, 4 and 6 weeks of age in blood plasma of male and female chickens of broiler lines selected for body weight (GL) or food conversion (FC). 2. Plasma concentrations measured in the same animal over a 24 h or a 2 week interval were not significantly correlated with each other. For different traits measured in the same plasma sample only the correlation between T4 and rT3 differed significantly from zero. 3. All traits were dependent on age. Line and sex effects were significant (P less than 0.05) for GH, T4, Tri, FFA and Glu. Additionally, line significantly influenced the plasma T3/T4 ratio and sex influenced plasma rT3. Interactions between line, sex and/or age were seldom significant. 4. Within line and sex, GH (at 6 weeks of age) and T3 (at 4 weeks of age) were negatively, and IGF-I and Tri (both at 6 weeks of age) positively correlated with the amount of abdominal fat at 6 weeks of age. No significant correlation between body weight at 2, 4 or 6 weeks of age and any of the plasma traits was found.     

Mechanisms of reduced and compensatory growth

Growth is an integrated process, resulting from the response of cells dependent on the endocrine status and nutrient availability. During feed restriction, the production and secretion of growth hormone (GH) by the pituitary gland are enhanced, but the number of GH receptors decreases. Changes of GH binding proteins induce GH resistance and are followed by reduced insulin-like growth factor-I (IGF-I) secretion. On the other hand, high circulating levels of GH enhance the mobilization of fatty acids, which are used to support energy requirements. Thus, when feed restriction in growing animals is moderate, there is mainly protein but barely fat accretion. By contrast, a severe feed restriction enhances the release of catabolic hormones and stimulates, from muscle cells, the liberation of amino acids, which are used by hepatocytes for gluconeogenesis. During refeeding and compensatory growth, the secretion of insulin is sharply enhanced and plasma GH concentrations remain high. This situation probably allows more nutrients to be used for growth processes. The role of plasma IGF-I during compensatory growth is not clear and must be explained in connection with changes of its binding proteins. Thyroxin and 3,5,3'-triiodothyronine seem to have a permissive effect on growth. The simultaneous occurrence of puberty with refeeding can exert a synergistic effect on growth. Initially, compensatory growth is characterized by the deposition of very lean tissue, similar as during feed restriction. This lasts for some weeks. Then, protein synthesis decreases and high feed intake leads to increased fat deposition.     

Energy and nitrogen metabolism of broilers selected over ten generations for increased growth rate,food consumption and conversion of food to gain

1. Energy and nitrogen (N) metabolism were studied in 6‐week‐old male birds taken from 4 lines of chickens selected for 10 generations for increased weight gain (line W), increased food consumption (line F), increased conversion of food to gain (line E) or at random (controls, line C).

2. Calorimetric measurements were made 8 times on each line while fed ad libitum in large open‐circuit respiration chambers for 3 d, and 11 to 13 times without food in smaller closed‐circuit respiration chambers for 24 h.

3. The F line ate 60% more food, produced 90% more excreta and 34% more heat and retained 80% more energy and 35% more N in their bodies than lines E and C. Line W was intermediate.

4. When differences in body weight were taken into account, the E and W lines had lower heat production than the C line, while the F line ate 40% more food, produced 30% more heat and retained 70% more energy and 30% more N than the E line.

5. In lines W, F, E and C respectively, the mean metabolisability of dietary energy (%) was 69·4, 62·9, 70·1 and 67·8 ; the fasting heat production (mean ± SE) was 481 ±9, 569 ± 10, 485 ± 9, and 508 ± 9 kJ/kgW d; the net availability of metabolisable energy (NAME) was 0·68 ± 0·05, 0·76 ± 0·04, 0·85 ± 0·06 and 0·73 ± 0·04; the estimated daily maintenance energy requirements were 671 + 15, 866 ± 14, 701 ± 13, and 742 ± 11 kJ ME/ kgW; and the proportion of N retained per unit increase in N intake was 0·38 ± 0·08, 0·50 ± 0·06, 0·56 ± 0·10 and 0·53 ± 0·06. 6. The contribution of line differences in the above traits to large line differences in efficiency of food utilisation is discussed.

     

Injection of porcine growth hormone releasing hormone gene plasmid in skeletal muscle increases piglets' growth and whole body protein turnover

The aim of the current study was to investigate the effects of a porcine growth hormone releasing hormone (pGHRH) gene plasmid injection in piglets on growth performance and whole body protein turnover. Sixty male Canadian Landrace × Chinese Taihu piglets were assigned to an intramuscular injection of 0 (control), 0.25, 0.5, 1 and 2 mg. All pigs were fed with the same diet (crude protein: 239.8 g/kg, digestible energy: 14.28 MJ/kg) at ad libitum intake. Protein turnover was determined on the 22nd day with a three-pool model by using a single-dosage, end-product analysis method with ¹⁵ N-glycine as a tracer. Injection of the pGHRH gene plasmid increased the piglets' growth rate, altered feed intake and decreased feed conversion ratio. It increased plasma growth hormone releasing hormone (GHRH), growth hormone (GH), insulin-like growth factor-I (IGF-I) and somatostatin but reduced serum urea and triglyceride. It reduced the urinary nitrogen excretion and led to higher nitrogen retention as well as the efficiencies of nitrogen retention and digestible N utilization. It increased the rates of protein synthesis, protein breakdown and net protein gain. Excretion of endogenous urinary nitrogen was reduced and nitrogen reutilization rate was improved. Conclusions: Injection of the pGHRH gene plasmid in skeletal muscle stimulated GHRH, GH and IGF-I excretion in piglets. Protein deposition was increased by an increase in protein synthesis and a smaller increase in protein breakdown, which was accompanied by reducing amino acid oxidation and increasing nitrogen reutilization.     

A comparison of the energy and nitrogen metabolism of broilers selected for increased growth rate,food consumption and conversion of food to gain

1. Calorimetric measurements were made on 5‐week‐old male chickens sampled from the third generation of three lines selected for either increased live‐weight gain (W), food consumption (F), or food conversion efficiency (E). A control line (C) was also measured.

2. Food intake and food conversion ratio were greater (P<0.05) in the F line than in the E and C lines.

3. Metabolisability of the diet was 0.8% higher in the E line than in the other lines.

4. Metabolisable energy (ME) intake and heat production were greater (P<0.05) in the F line than in the E and C lines, and energy balance was greater (P<0.05) in the F than in the W and E lines.

5. During starvation, excreta energy and heat production were greater (P < 0.05) in the F than the other lines.

6. Availability of ME (net energy) was the same (85%) for all lines but calculated daily maintenance energy requirements (kJ ME/kgW) were W, 860; F, 937; E, 796 and C, 810.

7. By 9 weeks the F line contained more fat and less water than lines E and C.     

Temporal secretory patterns of growth hormone in male meat-type chickens of lines selected for body weight gain or food conversion.

1. The pronounced episodic release of chicken growth hormone (cGH) in male broiler chicks, as well as its peak interval of about 90 min was confirmed in this study. 2. Lines selected for 6-week body weight (GL line) or for food conversion between 3 and 6 weeks of age (FC line) not only differ in the selection criteria but also in percentage abdominal fat. 3. The overall mean plasma GH concentration was higher in the FC line in both experiments, and this was only caused by peak amplitudes which were almost twice as high as in the FC line. Peak intervals were identical in both selected lines. 4. These observations suggest that decreased abdominal fat and increased protein efficiency may be genetically linked to increased plasma GH peak amplitude. This, and earlier observations on the effectiveness of pulsatile administration of cGH on the growth of broilers suggest that the biological function of GH may depend on its secretory pattern rather than its mean concentration in plasma.     

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.     

Reduction in channel catfish hepatic growth hormone receptor expression in response to food deprivation and exogenous cortisol

The objective of this study was to assess the effects of food deprivation and exogenous cortisol administration on somatic growth of channel catfish, Ictalurus punctatus, and examine the resultant changes in circulating insulin-like growth factor-I (IGF-I) concentrations and growth hormone receptor (GHR) gene expression. Integral to this objective, we report the isolation, sequence, and characterization of channel catfish GHR. Sequence analysis and characterization results indicate sequence identity and tissue distribution similar to GHRs in other teleost fish and several functional characteristics conserved in known vertebrate GHRs. The effects of food deprivation and dietary exogenous cortisol administration were assessed as part of a 4-week study. Growth was significantly reduced after 4 weeks in cortisol-fed fish compared to fed-control fish, and fasting resulted in weight loss. At the end of the 4-week study, both IGF-I plasma concentrations and hepatic GHR mRNA abundance were significantly reduced in fasted and cortisol-fed catfish. Levels of hepatic GHR mRNA were positively correlated to circulating IGF-I levels. These results suggest that a reduction in hepatic GHR gene expression might serve as a mechanism for the reduction of circulating IGF-I and growth in channel catfish during periods of food deprivation and stress.     

The effect of dietary calcium upon growth rate, food utilisation and plasma constituents in lines of chickens selected for aspects of growth or body composition

1. The effect of increasing dietary calcium from 10.3 to 20 g/kg on 5- to 17-day growth performance and plasma minerals, electrolytes, total protein, albumin and glucose in chickens from 4 lines selected for: high 8-week body weight (W), low abdominal fat (L), high abdominal fat (F) or at random (C) was studied in two experiments. 2. High dietary calcium significantly reduced weight gain and plasma phosphate and potassium but increased food:gain ratio, plasma total calcium, glucose and albumin. 3. Significant correlations were found between plasma total calcium and plasma phosphate (r = -0.5, P less than 0.01), plasma total calcium and protein (r = 0.4, P less than 0.01) and between plasma total protein and albumin (r = 0.55, P less than 0.01). 4. Genotypes differed in their response to dietary calcium content. There was a substantial response in line F but little effect in line L. 5. In contrast to the three other lines, in line F high dietary calcium significantly increased plasma ionised calcium without altering plasma phosphate or total calcium concentration. 6. It was concluded that genetic selection has produced lines which vary in their tolerance to high dietary concentrations of calcium. Birds selected for increased fatness were less tolerant to high dietary calcium than their lean-selected counterparts.     

Use of trenbolone acetate and estradiol in intact and castrate male cattle: effects on growth, serum hormones, and carcass characteristics

The effects of anabolic implant on growth, carcass characteristics, and serum hormones were examined in 30 young bulls and steers fed a growing diet then a finishing diet. In a 2 X 3 factorial arrangement, steers and bulls received an implant of trenbolone acetate (TBA), TBA and estradiol-17 beta (E2), or no implant. Blood samples were taken serially (every 20 min for 6 h) at intervals during the growing and finishing phases. Percentage of DM, fat, protein, and ash and Warner-Bratzler shear test were measured and taste panel evaluations of the 9-10-11 rib section were obtained. Treatment with TBA and E2 increased weight gain in steers but not in bulls. There were no differences in feed efficiency, serum growth hormone (GH), and cortisol concentrations between bulls and steers or between treated groups and controls in bulls or steers, although during the finishing phase mean GH concentrations in treated steers were twofold higher than in controls and were similar to those in the bull groups. Serum insulin-like growth factor-I (IGF-I) increased twofold during the growing phase, then remained at that level. Steers implanted with TBA and E2, which had the highest gains among the steer groups, had the highest serum GH and IGF-I. Longissimus steaks from bulls treated with TBA alone or TBA and E2 were comparable to steaks from steers in the shear test. Taste panelists found steaks from TBA- and E2-treated bulls to be similar in tenderness and connective tissue to steaks from steers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth hormone stimulates protein synthesis in bovine skeletal muscle cells without altering insulin-like growth factor-I mRNA expression

Growth hormone is a major stimulator of skeletal muscle growth in animals, including cattle. In this study, we determined whether GH stimulates skeletal muscle growth in cattle by direct stimulation of proliferation or fusion of myoblasts, by direct stimulation of protein synthesis, or by direct inhibition of protein degradation in myotubes. We also determined whether these direct effects of GH are mediated by IGF-I produced by myoblasts or myotubes. Satellite cells were isolated from cattle skeletal muscle and were allowed to proliferate as myoblasts or induced to fuse into myotubes in culture. Growth hormone at 10 and 100 ng/mL increased protein synthesis in myotubes (P < 0.05), but had no effect on protein degradation in myotubes or proliferation of myoblasts (P > 0.05). Insulin-like growth factor-I at 50 and 500 ng/mL stimulated protein synthesis (P < 0.01), and this effect of IGF-I was much greater than that of GH (P < 0.05). Besides stimulating protein synthesis, IGF-I at 50 and 500 ng/mL also inhibited protein degradation in myotubes (P < 0.01), and IGF-I at 500 ng/mL stimulated proliferation of myoblasts (P < 0.05). Neither GH nor IGF-I had effects on fusion of myoblasts into myotubes (P > 0.1). These data indicate that GH and IGF-I have largely different direct effects on bovine muscle cells. Growth hormone at 10 and 100 ng/mL had no effect on IGF-I mRNA expression in either myoblasts or myotubes (P > 0.1). This lack of effect was not because the cultured myoblasts or myotubes were not responsive to GH; GH receptor mRNA was detectable in them and the expression of the cytokine-inducible SH2-containing protein (CISH) gene, a well-established GH target gene, was increased by GH in bovine myoblasts (P < 0.05). Overall, the data suggest that GH stimulates skeletal muscle growth in cattle in part through stimulation of protein synthesis in the muscle and that this stimulation is not mediated through increased IGF-I mRNA expression in the muscle.