Chronic control of the beta- and alpha 2-adrenergic systems of sheep adipose tissue by growth hormone and insulin

Bovine Somatotropin Alters Myosin Heavy Chains and Beta Receptors in Skeletal Muscle of Feedlot Heifers with Little Impact on Live or Carcass Performance

The objective was to determine whether recombinant bovine somatotropin (rbST) enhanced live performance,skeletal muscle biological activity, and beta-adrenergic receptor expression of feedlot heifers during the finishing phase. Heifers (n = 16; initial body weight = 457 ± 3 kg) were randomly assigned to pens (4 pens/treatment; 2 heads/pen) and treatment: (1) no rbST (Control); (2) 500 mg/hd of sometribove zinc at day 0 and 14 (rbST; Posilac®; Elanco AnimalHealth, Greenfield, IN). Longissimus muscle biopsies for muscle chemistry were collected on day 0, 14, 28, 42, and 56. The rbST heifers had increased expression of AMP-activated protein kinase alpha and beta 3 adrenergic receptor (P < 0.05). Day of the study affected the expression of myosin heavy chain-IIA (MHC-IIA), MHC-IIX, beta 2 adrenergic receptor, peroxisome proliferator-activated receptor gamma, and stearoyl-CoA desaturase (P < 0.05). Day had a significant effect on muscle fiber cross-sectional area and proportion (P < 0.05). As days on feed increased, the area of MHC-I fibers decreased whereas MHC-IIA and IIX area increased (P < 0.05). The rbST heifers had decreased proportions of MHC-I fibers and increased proportions of MHC-IIX fibers (P < 0.05). The greatest density of Paired Box 7-positive cells was on day 0, 28, and 42 (P < 0.05), and the greatest density of Myogenic factor 5-positive cells was on day 42 and 56 (P < 0.05). Also, the greatest density of cells positive for Paired Box 7:Myogenic factor 5 was measured on day 28 (P < 0.05). These data indicate that, as days on feed increase, the effects of skeletal muscle biological activity are not dependent on rbST administration but may be more due to physiological changes occurring as the animal reaches physio-logical maturity.

Carcass and meat characteristics of sheep with an additional growth hormone gene

Effects of high growth hormone (GH) activity on body composition and some aspects of meat quality were examined in sheep transgenic for an additional copy of the ovine GH gene, as a tool to explore the biological importance of the GH axis in sheep selected commercially for meat production. Carcasses of 16 GH and 25 control mixed-sex sheep aged 45 months, and 6 GH and 6 control ewes aged 20 months, were measured. The dressing percentage was lower in the GH sheep (P < 0.001). The GH sheep had similar muscle mass to controls, but the weight of their fat depots was reduced (P < 0.001) to approximately 40% of controls, whereas limb-bone mass was 43% greater (P < 0.001) than controls. Fore and hind limbs were equally affected. Skin and most internal organs were heavier, particularly the pancreas, kidney, alimentary canal, and the liver. The concentration of intramuscular fat in the GH sheep was only 27% that of controls (P < 0.001), whereas the average pH of muscle 24 h after slaughter and the melting point of subcutaneous fat were both increased (P < 0.05). Similar changes in organ weights and body composition have been observed in sheep selectively bred to enhance lamb growth rate or to decrease fatness, suggesting that relative GH activity contributed to the outcomes of those experiments. This study indicates the importance of a multi-trait breeding objective to ensure that mechanisms associated with GH do not impair meat quality.

Effects of growth hormone on the function of beta-adrenoceptor subtypes in rat adipocytes

OBJECTIVE

The influence of growth hormone (GH) on the regulation of lipolytic response to specific agonists to beta-adrenoceptors and several post-receptor steps in the lipolytic cascade were investigated.

RESEARCH METHODS AND PROCEDURES

Adipose tissues from rats were incubated with or without GH (1.38 nM). After a 24-hour incubation, isolated adipocytes were prepared for different assays. Rats were hypophysectomized. One week after operation, L-thyroxine and hydrocortisone acetate was given to hypophysectomized rats. One group of rats was treated with GH (1.33 mg/kg, daily). After 1 week of hormonal treatment, adipose tissues were removed for different studies.

RESULTS

GH treatment increased both basal lipolysis and lipolytic sensitivity to dobutamine and CGP 12177 in adipocytes. The lipolytic sensitivity to terbutaline was not influenced by GH treatment. GH treatment increased the maximal lipolytic response to dobutamine and CGP 12177, but not to terbutaline as determined with absolute values of lipolysis. Forskolin-induced lipolysis was increased by addition of GH to tissues. Moreover, GH treatment resulted in enhanced expression of hormone-sensitive lipase. GH treatment in hypophysectomized rats influenced neither the expressions of G alpha s protein and cholera toxin-catalyzed adenosine diphosphate-ribosylation of G alpha s protein, nor cholera toxin-induced 3',5'-cyclic adenosine monophosphate accumulation. However, the expression of G alpha i protein was decreased after GH treatment.

DISCUSSION

These and previous results suggest that GH increases lipolysis in rat adipocytes partly through the beta-adrenergic system, including increases in both beta(1)- and beta(3)-adrenergic receptor function, and partly through enhanced adenylate cyclase function, and expression of hormone-sensitive lipase, perhaps via a decrease in G alpha i protein expression.

Technologies for the control of fat and lean deposition in livestock

When the ratio of lean to fat deposition is improved, so is feed conversion efficiency. Net benefits may include lower production costs, better product quality, less excretion of nitrogenous wastes into the environment, decreased grazing pressure on fragile landscapes, and reduced pressure on world feed supplies. However, finding a way to achieve these goals that is reliable, affordable, and acceptable to the majority of consumers has proved to be a major challenge. Since the European Union banned hormonal growth promoters (HGPs) 15 years ago, countries such as Australia and the United States have licensed new products for livestock production, including bovine growth hormone (GH), porcine and equine GH, and the beta-agonist ractopamine. There has also been considerable research into refining these products, as well as developing new technologies. Opportunities to improve beta-agonists include lessening their effects on meat toughness, reducing adverse effects on treated animals, and prolonging their duration of action. In the last regard, the combined use of a beta-agonist with GH, which upregulates beta-adrenoceptors, can produce an outstanding improvement in carcass composition and feed efficiency. Insulin-like growth factor-1 (IGF-1) mediates many of the actions of GH, but has proved to be of more use as a growth reporter/selection marker in pigs, than as a viable treatment. However, a niche for this product could exist in the manipulation of neonatal growth, causing a life-long change in lean:fat ratio. Another significant advance in endocrinology is the discovery of hormones secreted by muscle and fat cells, that regulate feed intake, energy metabolism, and body composition. Leptin, adiponectin and myostatin were discovered through the study of genetically obese, or double-muscled animals. Through genetic manipulation, there is potential to exploit these findings in a range of livestock species, although the production of transgenic animals is still hampered by the poor level of control over gene expression, and faces an uphill battle over consumer acceptance. There are several alternatives to HGPs and transgenics, that are more likely to gain world-wide acceptance. Genetic selection can be enhanced by using markers for polymorphic genes that control fat and lean, such as thyroglobulin, or the callipyge gene. Feed additives of natural origin, such as betaine, chromium and conjugated linoleic acid, can improve the fat:lean ratio under specific circumstances. Additionally, 'production vaccines' have been developed, which alter the neuro-endocrine system by causing an auto-immune response. Thus, antibodies have been used to neutralise growth-limiting factors, prolong the half-life of anabolic hormones, or activate hormone receptors directly. Unfortunately, none of these technologies is sufficiently well advanced yet to rival the use of exogenous HGPs in terms of efficacy and reliability. Therefore, further research is needed to find ways to control fat and lean deposition with due consideration of industry needs, animal welfare and consumer requirements.

Blunted lipolytic response to fasting in abdominally obese women: evidence for involvement of hyposomatotropism

BACKGROUND

Abdominal obesity is associated with a blunted lipolytic response to fasting that may contribute to the preservation of adipose tissue mass.

OBJECTIVE

To further explore the pathophysiology of blunted lipolysis during fasting in obesity, we simultaneously measured lipolysis and distinct neuroendocrine regulatory hormones in abdominally obese and normal-weight (NW) women.

DESIGN

Eight abdominally obese [x +/- SD body mass index (BMI; in kg/m(2)): 32.1 +/- 2.6] and 6 NW (BMI: 22.7 +/- 1.5) women were studied during the last 8 h of a 20-h fast. The glycerol appearance rate and the serum and plasma concentrations of insulin, leptin, cortisol, and growth hormone were measured regularly.

RESULTS

At 13 h of fasting, the mean (+/-SD) glycerol appearance rate corrected for fat mass was greater in NW women than in obese women (7.2 +/- 1.0 and 5.1 +/- 0.6 micro mol.kg(-1).min(-1), respectively; P = 0.001). After a 20-h fast, lipolysis increased to 8.9 +/- 1.5 mmol.kg(-1).min(-1) in NW women (23%), whereas it did not change significantly in obese women (-2%). Fasting decreased insulin concentrations by approximately 30% in both groups, but it did not induce significant changes in leptin concentrations. Mean cortisol concentrations and urinary catecholamine excretion were comparable in both groups. However, mean plasma growth hormone concentrations were higher in NW women than in obese women (1.81 +/- 0.98 compared with 0.74 +/- 0.52 mU/L; P = 0.046). The relative change in lipolysis tended to correlate with mean plasma growth hormone concentrations (r = 0.515, P = 0.059).

CONCLUSION

Abdominal obesity-associated hyposomatotropism may be involved in the blunted increase in lipolysis during fasting.

Preferential stimulation of abdominal subcutaneous lipolysis after prednisolone exposure in humans

OBJECTIVE

The role of cortisol in the regulation of lipolysis is not clear. This study was undertaken to explore whether a standard dose of prednisolone for 1 week would influence lipolysis in abdominal and femoral tissue.

RESEARCH METHODS AND PROCEDURES

We used the microdialysis technique, the forearm technique, and indirect calorimetry, in the fasting state, after 1 week of treatment with prednisolone (30 mg daily) or placebo. Eight healthy young men (age: 25 +/- 3 years; height: 181 +/- 1 cm; body mass index [BMI]: 23.3 +/- 0.7 kg/m(2)) were studied.

RESULTS

Treatment with prednisolone induced insulin resistance (Homeostasis Model Assessment index: placebo vs. prednisolone: 7.15 +/- 1.63 vs. 17.00 +/- 14.26, p = 0.03), hyperinsulinemia (p = 0.01), and hyperglucagonemia (p = 0.001), whereas growth hormone concentrations were unaffected. Abdominal adipose tissue interstitial glycerol was increased during treatment with prednisolone in the face of significant hyperinsulinemia, although it barely reached statistical significance (p = 0.06). At the femoral adipose tissue depot, no difference in lipolysis was found. Arterial and venous free fatty acids (FFA) were comparable in the two situations, whereas the arteriovenous difference across the forearm was significantly decreased during treatment with prednisolone, indicating increased uptake, or decreased release of FFA. Energy expenditure (p = 0.3), respiratory quotient (p = 0.9), glucose oxidation (p = 0.9), lipid oxidation (p = 1.0), and protein oxidation (p = 0.1) were unaltered on the 2 study days.

DISCUSSION

Short-term treatment with a standard dose of corticosteroids induces increased abdominal adipose tissue lipolysis, as well as hyperinsulinemia, hyperglucagonemia, and insulin resistance.

Manipulation of growth in pigs through treatment of the neonate with clenbuterol and somatotropin

Neonatal pigs were treated with lipolytic agents to determine whether this would cause a long-term decrease in their ability to deposit fat, with a consequent increase in muscle growth and feed efficiency. Groups of 25 female piglets were given clenbuterol (100 microg/kg BW), porcine somatotropin (pST; 100 microg/kg BW), pST plus clenbuterol, or saline injections from 3 d to 40 d of age. Five piglets from each group were then slaughtered to determine body composition. Clenbuterol and pST both increased ADG up to weaning when given separately (24%, P < 0.05; 20%, P < 0.1 respectively) but did not reduce fat deposition. In contrast, pigs given clenbuterol plus pST showed no increase in ADG and a 41% reduction in carcass fat (P < 0.05). Clenbuterol caused a marked decrease in beta2-adrenoceptor density in porcine adipose tissue (P < 0.001) and skeletal muscle (P < 0.01). This effect was attenuated by concurrent pST treatment, which helps to explain the synergistic effect of these drugs on fat deposition. Once the drugs were withdrawn at 40 d, the anabolic effect of pST gradually disappeared, so that the live weight of pST-treated and control pigs was identical at 168 d. Clenbuterol withdrawal caused the rapid loss of extra weight gained, plus an additional 4 to 5 kg live weight that was never recovered. During the 4-wk finishing period there was an increase in feed intake in pigs that had previously undergone treatment with pST (23%, P < 0.1), with no increase in ADG, and so feed efficiency was impaired (P < 0.05). Pigs that were treated with pST plus clenbuterol showed no marked increase in feed intake during this period. Carcasses from clenbuterol-treated pigs tended to be leaner at 168 d, but there was no long-term effect of pST or the combined treatment on carcass composition. Overall, the treatment of neonatal pigs with repartitioning agents was counter-productive, due to the withdrawal effects of the beta-adrenefgic agonist and the delayed long-term effect of pST on feed intake.

Alpha2 -adrenoceptor function in arterial hypertension associated with obesity in dogs fed a high-fat diet

OBJECTIVE

To investigate the status of alpha2-adrenoceptors in a model of obesity-related arterial hypertension.

DESIGN

A parallel study in dogs randomly assigned to a high-fat diet (HFD group, n = 6) or normal canine food (controls, n = 6) for 9 weeks.

METHODS

Postsynaptic vascular alpha2-adrenoceptors were assessed through analysis of dose-pressor responses to clonidine [2.5, 5.0 and 15.0 microg/kg intravenously (i.v.)] after muscarinic, beta- and alpha1-adrenergic receptor blockade. Presynaptic and central alpha2-adrenoceptors were studied through measurement of changes in plasma concentrations of catecholamine induced by yohimbine (0.05 mg/kg i.v.). The number of platelet alpha2-adrenoceptors (expressed as fmol/mg protein) and the percentage in a state of high affinity were measured using [3H]RX821002.

RESULTS

Clonidine, when administered to dogs that were under autonomic blockade, elicited a dose-dependent increase in blood pressure. The doses of clonidine required to induce a 50% maximum increase in systolic and diastolic blood pressures remained unchanged after 9 weeks of a high-fat diet (systolic: 6.0 +/- 0.3 microg/kg at baseline and 5.6 +/- 0.2 microg/kg after 9 weeks; diastolic: 4.2 +/- 0.2 microg/kg at baseline and 3.9 +/- 0.2 microg/kg after 9 weeks). After 9 weeks of the regimen, plasma concentrations of noradrenaline were significantly greater in the HFD group than in controls (337 +/- 22 pg/ml compared with 212 +/- 37 pg/ml). The increment in plasma concentrations of noradrenaline elicited by yohimbine after 9 weeks was smaller in the HFD group than in controls (93 +/- 44% compared with 181 +/- 46%; P = 0.024). In the HFD group, the number of platelet alpha2-adrenoceptors and the percentage that were in a state of high affinity were significantly lower after 9 weeks, compared with baseline (number: 239 +/- 21 fmol/mg protein at baseline and 95 +/- 7 fmol/mg protein after 9 weeks; high-affinity: 30 +/- 3% at baseline and 21 +/- 4% after 9 weeks; P < 0.05).

CONCLUSIONS

These results suggest that presynaptic or central alpha2-adrenoceptor function, or both, is specifically impaired after 9 weeks of a high-fat diet. These modifications may account for the development of arterial hypertension in this model.

The effects of human GH and its lipolytic fragment (AOD9604) on lipid metabolism following chronic treatment in obese mice and beta(3)-AR knock-out mice

Both human GH (hGH) and a lipolytic fragment (AOD9604) synthesized from its C-terminus are capable of inducing weight loss and increasing lipolytic sensitivity following long-term treatment in mice. One mechanism by which this may occur is through an interaction with the beta-adrenergic pathway, particularly with the beta(3)-adrenergic receptors (beta(3)-AR). Here we describe how hGH and AOD9604 can reduce body weight and body fat in obese mice following 14 d of chronic ip administration. These results correlate with increases in the level of expression of beta(3)-AR RNA, the major lipolytic receptor found in fat cells. Importantly, both hGH and AOD9604 are capable of increasing the repressed levels of beta(3)-AR RNA in obese mice to levels comparable with those in lean mice. The importance of beta(3)-AR was verified when long-term treatment with hGH and AOD9604 in beta(3)-AR knock-out mice failed to produce the change in body weight and increase in lipolysis that was observed in wild-type control mice. However, in an acute experiment, AOD9604 was capable of increasing energy expenditure and fat oxidation in the beta(3)-AR knock-out mice. In conclusion, this study demonstrates that the lipolytic actions of both hGH and AOD9604 are not mediated directly through the beta(3)-AR although both compounds increase beta(3)-AR expression, which may subsequently contribute to enhanced lipolytic sensitivity.

Increase of fat oxidation and weight loss in obese mice caused by chronic treatment with human growth hormone or a modified C-terminal fragment

OBJECTIVE

To observe the chronic effects of human growth hormone (hGH) and AOD9604 (a C-terminal fragment of hGH) on body weight, energy balance, and substrate oxidation rates in obese (ob/ob) and lean C57BL/6Jmice. In vitro assays were used to confirm whether the effects of AOD9604 are mediated through the hGH receptor, and if this peptide is capable of cell proliferation via the hGH receptor.

METHOD

Obese and lean mice were treated with hGH, AOD or saline for 14 days using mini-osmotic pumps. Body weight, caloric intake, resting energy expenditure, fat oxidation, glucose oxidation, and plasma glucose, insulin and glycerol were measured before and after treatment. BaF-BO3 cells transfected with the hGH receptor were used to measure in vitro 125I-hGH receptor binding and cell proliferation.

RESULTS

Both hGH and AOD significantly reduced body weight gain in obese mice. This was associated with increased in vivo fat oxidation and increased plasma glycerol levels (an index of lipolysis). Unlike hGH, however, AOD9604 did not induce hyperglycaemia or reduce insulin secretion. AOD9604 does not compete for the hGH receptor and nor does it induce cell proliferation, unlike hGH.

CONCLUSIONS

Both hGH and its C-terminal fragment reduce body weight gain, increase fat oxidation, and stimulate lipolysis in obese mice, yet AOD9604 does not interact with the hGH receptor. Thus, the concept of hGH behaving as a pro-hormone is further confirmed. This data shows that fragments of hGH can act in a manner novel to traditional hGH-stimulated pathways.

Effects of gastric inhibitory polypeptide, somatostatin and epidermal growth factor on lipogenesis in ovine adipose explants

Feeding raises the plasma concentrations of a number of gut-related hormones that may, in turn, influence the metabolism of peripheral tissues. This study investigated the effects of gut-related hormones on lipogenesis in explants from three differing adipose depots in lambs (aged 4-9 months). Incorporation of [14C]-acetate into lipid was measured over a 2-h period, following 24 h pre-incubation in the presence of hormone combinations. In perirenal fat explants, gastric inhibitory polypeptide (GIP) in the concentration range 0.01-10 nM stimulated lipogenesis. Maximal effects were seen at 1 nM (an average increase of 64% over basal values). In contrast, in the presence of insulin (0.1 nM), a dose-dependent decrease in lipogenesis was seen with increasing GIP concentration (P < 0.001 for the insulin x GIP interaction). Epidermal growth factor (EGF) and somatostatin in the same concentration range each inhibited lipogenesis. both in the presence and the absence of insulin (P < 0.001 in each case). Subcutaneous (back) fat and intermuscular (popliteal) fat responded similarly to each other, but significantly differently from the perirenal depot (P < 0.001). Here GIP, somatostatin or EGF (each at 1 nM) all separately stimulated lipogenesis.

The mechanism of effect of growth hormone on preadipocyte and adipocyte function

Growth hormone (GH) is not only the major regulator of postnatal somatic growth but also exerts profound effects on body composition through a combination of anabolic, lipolytic and antinatriuretic actions. GH enhancement of the lipolytic activity of adipose tissue in combination with a reduction of triglyceride accumulation via inhibition of lipoprotein lipase activity appears to be the major mechanism by which GH results in a reduction of the total fat mass. Recently, much progress has been made in understanding the molecular mechanism by which GH affects cellular function. This review provides a brief discourse and summary of the mechanism of effects of GH on preadipocyte/adipocyte function. It is intended to provide a functional understanding of the mechanism of action of GH as it relates to adipogenesis and adipocyte function.

Effects of oral administration of a synthetic fragment of human growth hormone on lipid metabolism

A small synthetic peptide sequence of human growth hormone (hGH), AOD-9401, has lipolytic and antilipogenic activity similar to that of the intact hormone. Here we report its effect on lipid metabolism in rodent models of obesity and in human adipose tissue to assess its potential as a pharmacological agent for the treatment of human obesity. C57BL/6J (ob/ob) mice were orally treated with either saline (n = 8) or AOD-9401 (n = 10) for 30 days. From day 16 onward, body weight gain in AOD-9401-treated animals was significantly lower than that of saline-treated controls. Food consumption did not differ between the two groups. Analyses of adipose tissue ex vivo revealed that AOD-9401 significantly reduced lipogenic activity and increased lipolytic activity in this tissue. Increased catabolism was also reflected in an acute increase in energy expenditure and glucose and fat oxidation in ob/ob mice treated with AOD-9401. In addition, AOD-9401 increased in vitro lipolytic activity and decreased lipogenic activity in isolated adipose tissue from obese rodents and humans. Together, these findings indicate that oral administration of AOD-9401 alters lipid metabolism in adipose tissue, resulting in a reduction of weight gain in obese animals. The marked lipolytic and antilipogenic actions of AOD-9401 in human adipose tissues suggest that this small synthetic hGH peptide has potential in the treatment of human obesity.

Effects of growth hormone on body composition and bone metabolism

Physiologic effects of growth hormone (GH) extend beyond the stimulation of linear growth during childhood and adolescence. These effects include building and sustaining lean body mass, facilitating the utilization of fat mass for energy needs, and maintaining bone mineral density. These nongrowth effects of GH appear to be important throughout life. Children and adults with severe GHD demonstrate marked reductions in lean body mass, increases in percent body fat, and subnormal bone mineral density. Replacement of GH attenuates these abnormalities, though it remains unknown whether it does so completely. Children with body composition abnormalities resembling the GHD state (e.g., Prader-Willi syndrome) also appear to respond favorably to administration of GH treatment, and demonstrate concomitant improvements in strength and agility. Long-term body composition benefits of GH supplementation in these and other non-GHD individuals remain unproven.

Effects of a physiological GH pulse on interstitial glycerol in abdominal and femoral adipose tissue

Physiologically, growth hormone (GH) is secreted in pulses with episodic bursts shortly after the onset of sleep and postprandially. Such pulses increase circulating levels of free fatty acid and glycerol. We tested whether small GH pulses have detectable effects on intercellular glycerol concentrations in adipose tissue, and whether there would be regional differences between femoral and abdominal subcutaneous fat, by employing microdialysis for 6 h after administration of GH (200 microgram) or saline intravenously. Subcutaneous adipose tissue blood flow (ATBF) was measured by the local Xenon washout method. Baseline of interstitial glycerol was higher in adipose tissue than in blood [220 +/- 12 (abdominal) vs. 38 +/- 2 (blood) micromol/l, P < 0.0005; 149 +/- 9 (femoral) vs. 38 +/- 2 (blood) micromol/l, P < 0.0005] and higher in abdominal adipose tissue compared with femoral adipose tissue (P < 0.0005). Administration of GH induced an increase in interstitial glycerol in both abdominal and femoral adipose tissue (ANOVA: abdominal, P = 0. 04; femoral, P = 0.03). There was no overall difference in the response to GH in the two regions during the study period as a whole (ANOVA: P = 0.5), but during peak stimulation of lipolysis abdominal adipose tissue was, in absolute but not in relative terms, stimulated more markedly than femoral adipose tissue (ANOVA: P = 0. 03 from 45 to 225 min). Peak interstitial glycerol values of 253 +/- 37 and 336 +/- 74 micromol/l were seen after 135 and 165 min in femoral and abdominal adipose tissue, respectively. ATBF was not statistically different in the two situations (ANOVA: P = 0.7). In conclusion, we have shown that a physiological pulse of GH increases interstitial glycerol concentrations in both femoral and abdominal adipose tissue, indicating activated lipolysis. The peak glycerol increments after GH were higher in abdominal adipose tissue, perhaps due to a higher basal rate of lipolysis in this region.

Growth hormone and dexamethasone stimulate lipolysis and activate adenylyl cyclase in rat adipocytes by selectively shifting Gi alpha2 to lower density membrane fractions

GH, in the presence of glucocorticoid, produces a delayed increase in lipolysis in rat adipose tissue, but the biochemical mechanisms that account for this action have not been established. Other lipolytic agents rapidly activate adenylyl cyclase (AC) and the resulting production of cAMP initiates a chain of reactions that culminates in the activation of hormone-sensitive lipase. We compared responses of segments of rat epididymal fat or isolated adipocytes to 30 ng/ml GH and 0.1 microg/ml dexamethasone (Dex) with 0.1 ng/ml isoproterenol (ISO), which evoked a similar increase in lipolysis. All measurements were made during the fourth hour after the addition of GH+Dex or immediately after the addition of ISO to cells or tissues that had been preincubated for 3 h without hormone. Although no significant increases in cAMP were discernible in homogenates of GH+Dex-treated tissues, Rp-cAMPS (Rp-adenosine 3'5'-phosphothioate), a competitive inhibitor of cAMP, was equally effective in decreasing lipolysis induced by GH+Dex or ISO. The proportion of PKA that was present in the active form was determined by measuring the incorporation of 32P from [gamma-32P]ATP into kemptide in the absence and presence of saturating amounts of cAMP. GH+Dex and ISO produced similar increases in protein kinase A activity in tissue extracts. Treatment with GH+Dex did not change the total forskolin-stimulated AC present in either a crude membrane pellet sedimented at 16K x g or a less dense membrane pellet sedimented at 100K x g, but doubled the AC activity in the 16K pellet when assayed in the absence of forskolin. To evaluate possible effects on G proteins, pellets obtained from centrifugation of adipocyte homogenates at 16K x g and 100K x g were solubilized and subjected to PAGE and Western analysis. GH+Dex decreased Gi alpha2 by 44% (P < 0.02) in the 16K pellets and increased it by 52% (P < 0.01) in the 100K pellets. Gs alpha in the 16K pellet was unaffected by GH+Dex and was decreased (P < 0.05) in the 100K pellet. Sucrose density fractionation of the 16K pellets revealed a similar GH+Dex-dependent shift of Gi alpha2 to less dense fractions as determined by both Western analysis and [32P]NAD ribosylation catalyzed by pertussis toxin. No such changes were seen in the distribution of Gs alpha or 5'-nucleotidase. Colchicine (100 microM) blocked the GH+Dex-dependent shift of Gi alpha2 from the 16K to the 100K pellet and blocked the lipolytic effects of GH+Dex, but not those of ISO. We conclude that by modifying the relationship between AC and Gi alpha2, GH+Dex relieves some inhibition of cAMP production and consequently increases lipolysis.

Effect of somatotropin, insulin, and glucocorticoid on lipolysis in chronic cultures of adipose tissue from lactating cows

In vitro effects of bovine somatotropin (bST) and insulin plus dexamethasone on lipolysis were evaluated using chronic cultures (48 h) of adipose tissue from lactating cows. Treatments were control (culture medium alone), bST (100 ng/ml), insulin (100 ng/ml) plus dexamethasome (10 nM), and insulin plus dexamethasone plus bST. Following the 48-h cultures, rates of lipolysis were measured in 3-h incubations with isoproterenol (10 microM), adenosine deaminase (0.75 U/ml), and various concentrations of a nonhydrolyzable adenosine analog. The addition of bST to cultures did not alter basal or isoproterenol-stimulated lipolysis. However, the ability of adenosine to inhibit rates of lipolysis was reduced by bST. When measured in the presence of maximal concentrations of adenosine analog, isoproterenol caused an increase in lipolysis above basal, which was twofold greater for explants cultured with insulin plus dexamethasone plus bST than for explants cultured with insulin plus dexamethasone. Dose-response curves for adenosine inhibition of isoproterenol-stimulated lipolysis demonstrated that chronic culture with bST decreased adipose tissue responsiveness and sensitivity to adenosine. Overall, results demonstrated that an in vitro chronic culture system can be used to examine factors that regulate lipolysis. The addition of insulin plus dexamethasone to chronic cultures better maintained the intracellular signaling system, including sensitivity and responsiveness to adenosine inhibition of lipolysis. Results also confirm that bST alters the antilipolytic response to adenosine. Thus, bST effects are in large part due to a relief in the tonic inhibition of lipolysis via changes in the inhibitory G-protein signaling system.

Biology of somatotropin in growth and lactation of domestic animals

Impressive progress has been made during the past 15 years in our understanding of the biology of somatotropin (ST) in domestic animals. In part, this progress was sparked by advances in biotechnology that made feasible the production of large quantities of recombinant bovine ST (bST) and porcine ST (pST). The availability of recombinant bST and pST resulted in an exponential increase in investigations that explored their role in growth and lactation biology, as well as evaluated their potential for commercial use. Collectively, these studies established that administration of bST to lactating dairy cows increased milk yield, and treatment of growing pigs with pST markedly stimulated muscle growth and reduced fat deposition. In addition to these "efficacy" studies, a substantial number of investigations examined the mechanisms by which ST affects lactation and growth of domestic animals. This review summarizes the diverse physiological effects ST has on growth and lactation and discusses the underlying mechanisms that mediate these effects in domestic animals.

Insulin selectively downregulates alpha2-adrenoceptors in the arcuate and dorsomedial nucleus

Intracerebroventricular infusion of insulin (2 mU/ day) produced selective downregulation of 3H-paraminoclonidine binding to alpha2-adrenoceptors in the hypothalamic arcuate (14%) and dorsomedial (19%) nuclei out of 16 forebrain areas in Wistar rats. Binding of 3H-prazosin to alpha1-adrenoceptors was unaffected. This is in keeping with the known effect of insulin on catecholamine and neuropeptide Y metabolism in these brain regions that play an important role in energy homeostasis.

Modification of feed intake response to a beta 2-agonist by bovine somatotropin in lactating or dry dairy cows

The aim of this experiment was to show whether growth hormone could increase the effect of the beta 2-adrenergic agonist clenbuterol on feed intake. Two groups of Holstein cows [lactating (n = 4) and dry (n = 4)] were used to rule out the possible effects of increased energy requirements on feed intake. Treatments were administered according to a crossover experimental design with two 13-d periods and an 8-d readjustment interval. Treatments consisted of daily injections of 40 mg of bST or a placebo. Moreover, clenbuterol (3.16 micrograms/kg of BW) or saline challenges were infused intravenously for 4 h either on d 8 or on d 11 of each period. The cows were offered a dehydrated total mixed diet for ad libitum intake for 4 h twice daily. For both groups of cows, clenbuterol reduced DMI on the experimental day only, and bST intensified the effects of clenbuterol on DMI, causing a sharper decrease in DMI over 2 d. This enhancement of clenbuterol activity modified certain metabolic parameters including higher lipid mobilization during infusion and higher glycogenolysis. For dry cows, the initial rate of eating was reduced under bST treatment alone. During early lactation, bST, either by itself or through increased beta 2-adrenergic stimulation may restrict the development of intake by dairy cows over a few weeks.

Growth hormone treatment of hypophysectomized rats increases catecholamine-induced lipolysis and the number of beta-adrenergic receptors in adipocytes: no differences in the effects of growth hormone on different fat depots

Growth hormone (GH) has a lipolytic effect in adipose tissue but this effect may differ in adipose tissue from various fat depots. This latter possibility was investigated in the present study, in which the effects of GH in vivo on catecholamine-induced lipolysis and the number of beta-adrenergic receptors in isolated adipocytes from different fat depots of hypophysectomized rats were investigated. Female and male Sprague-Dawley rats were hypophysectomized or sham-operated at 45 days of age. One week after the operation, hormonal replacement therapy with L-thyroxine and hydrocortisone acetate was given. In addition, groups of rats were treated with GH (1.33 mg/kg per day, given as two daily subcutaneous injections). After 1 week of hormonal treatment, adipocytes were isolated from the parametrial, epididymal and inguinal fat pads, and glycerol release after catecholamine-stimulation and 125I-cyanopindolol binding were measured. Hypophysectomy resulted in a marked decrease in the lipolytic response to catecholamines. GH treatment significantly increased catecholamine-induced lipolysis with similar effects in adipocytes from parametrial or epididymal and inguinal fat depots in both female and male rats. There were no differences between norepinephrine compared with isoproterenol-induced responses. 125I-cyanopindolol binding was reduced after hypophysectomy and normalized by GH treatment, without differences between parametrial and inguinal adipose tissue regions. We conclude that the lipolytic effects of GH in the rat may partly be mediated by a stimulatory effect on beta-adrenergic receptors in adipocytes. In addition, GH exerted similar effect on catecholamine induced lipolysis and beta-adrenergic receptors in adipocytes from parametrial, epididymal and inguinal fat depots.

Effects of breed and adipose depot location on responsiveness and sensitivity to adrenergic stimulation in ovine adipose tissue

Karakul tail adipose tissue had the smallest adipocytes, and this tissue was also the least lipolytically responsive. However, lipolytic responsiveness did not vary with breed or depot when expressed per gram of tissue. Sensitivity to isoproterenol and epinephrine was higher in tissues of the Karakul than of the Outaouais breed of sheep. As well, there was evidence for alpha-antilipolytic action in Karakul but not Outaouais adipose tissue. The Karakul breed is a unique model for the study of adipocyte metabolism in that a wide range of adipocyte volumes exist within an individual, and the Karakul adipose tissue appears to be particularly sensitive to adrenergic regulation.

Effect of bovine somatotropin and food deprivation on beta-adrenergic and A1 adenosine receptor binding in adipose tissue of lactating cows

Lactating Holstein cows were used to assess the effect of bovine somatotropin (bST; n = 8) and fasting (FAST; n = 4) on ligand binding to beta-adrenergic (BAR) and Type-1 adenosine (A1R) receptors in adipose tissue. Cows received exogenous bST (sometribove; 40 mg/d) or no hormone (control) for 4 d in a single-reversal design with a 7-d interval between treatment periods. Subcutaneous adipose tissue biopsies were taken on day 4 of each treatment. Eight d after the bST regimen, 4 cows were fasted for 3 d and adipose biopsies were taken. Ligand binding was quantified with a postnuclear, total adipose tissue membrane preparation (100,000 x g pellet). Binding to BAR and A1R was assessed with the antagonists [125I]iodocyanopindolol (ICP) and [3H]8-cyclopentyl-1,3-dipropylxanthine (DCPCX), respectively. The binding affinity (Kd) of BAR for ICP was not affected by bST but was enhanced by FAST; maximal binding (Bmax) was increased with bST treatment (P < 0.06) and reduced by FAST (61%, P < 0.01). Kd values for DCPCX binding to A1R were not changed by bST or FAST. bST did not affect Bmax for A1R; however, FAST reduced the Bmax by 38%. Data highlight the differential regulation of BAR and A1R by bST and FAST.

Effect of somatotropin treatment on lipogenesis, lipolysis, and related cellular mechanisms in adipose tissue of lactating cows

The effect of bST on the metabolism of lipid in adipose tissue was studied using tissue biopsies from lactating cows treated with bST for 8 d. Cows responded to treatment by increasing daily milk yield by 10.9 kg, although net energy intake was not changed. Thus, net energy balance was changed from highly positive to slightly negative (+7.7 to -1.1 Mcal/d). Consistent with these changes in net energy balance, lipogenesis rates were dramatically reduced (97%) in adipose tissue from bST-treated cows. Activities of acetyl-coenzyme A carboxylase (initial and total) and fatty acid synthase were also dramatically decreased. Therefore, for cows in positive energy balance, reduced lipid synthesis in adipose tissue represents a major mechanism whereby bST alters nutrient partitioning to support greater milk synthesis. Treatment with bST had no effect on beta-adrenergic-stimulated lipolysis in adipose tissue explants incubated in vitro with adenosine deaminase. However, bST treatment reduced the ability of adenosine to inhibit lipolysis in adipose tissue, which involved changes in both sensitivity and responsiveness to adenosine. Therefore, the enhanced lipolytic response to catecholamine in vivo with bST treatment relates to relief in the adenosine inhibitory signaling cascade rather than to a direct effect on the stimulatory signaling pathway.

Sexual dimorphism in the growth response of entire and gonadectomized rats to clenbuterol

This study compared the anabolic effects of clenbuterol in male and female rats and determined the relative contribution of testicular and ovarian hormones to any observed gender difference. Seventy-two 12-wk-old rats were used in a 2 x 2 x 2 factorial design in which animals were either male or female, entire or gonadectomized at 3 wk of age, and fed either a control diet or a diet containing 4 mg clenbuterol/kg feed for 8 days. Compared with entire male rats, entire females gained 64% less weight, had lighter carcasses (-36%) and gastrocnemius muscles (-62%), and had higher plasma concentrations of the catabolic hormone corticosterone (P < 0.05). Castration had a negative effect on growth in male rats, and ovariectomy had a positive effect in females, but there was still a gender difference in body weight between gonadectomized males and females, which amounted to 34% of the gender difference observed in intact rats. The density of beta 2-adrenoceptors in skeletal muscle was not different between males and females, nor was it affected by gonadectomy. Clenbuterol increased both weight gain and gastrocnemius muscle weight, with the latter response in entire and castrated male rats (+ 1.31 and + 1.17 g) being more than double that seen in entire and ovariectomized females (+ 0.58 and + 0.55 g). The downregulation response of beta 2-adrenoceptors in this muscle was remarkably consistent in all treated groups (-50% to -53%).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of lactation on the signal transduction systems regulating lipolysis in sheep subcutaneous and omental adipose tissue

The effect of lactation on the regulation of lipolysis by beta- and alpha 2-adrenergic agents and by adenosine has been investigated. When changes in adipocyte mean cell volume (which decreases with lactation) are allowed for, lactation increased the maximum response both to beta-adrenergic agents and to the adenosine analogue N6-phenylisopropyladenosine, but had no apparent effect on the responsiveness of the alpha 2-adrenergic system in both subcutaneous and omental adipocytes. For subcutaneous adipocytes, there was no significant change in the number of beta-adrenergic or alpha 2-adrenergic receptors, but the amount of Gs and the maximum (forskolin-stimulated) adenylate cyclase activity were increased by lactation. In contrast, in omental adipocytes, the number of beta- (but not alpha 2-) adrenergic receptors and the amount of Gs were increased, whereas forskolin-stimulated adenylate cyclase activity was unchanged by lactation. In both types of adipocyte, cyclic AMP phosphodiesterase and total protein kinase A activities were unchanged. Lactation had no effect on the number of adenosine receptors but increased the amounts of the Gi isoforms expressed in both types of adipocyte. These various adaptations differ markedly in a number of respects from those described previously in the rat. Lactation, then, while having a similar overall effect on the response to beta-adrenergic agonists of adipocytes, achieves this by depot-specific mechanisms. In contrast, changes in response to adenosine appear to involve the same mechanism in the two depots investigated.

Effects of prolonged treatment of lactating goats with bovine somatotropin on aspects of adipose tissue and liver metabolism

The effects of prolonged (22 weeks) treatment of lactating goats with bovine somatotropin on the metabolism of adipose tissue and liver has been investigated. Somatotropin treatment resulted in smaller adipocytes, decreased rate of fatty acid synthesis and decreased total acetyl-CoA carboxylase activity of adipocytes, but with no change in the proportion of this enzyme in the active state. The rate of acylglycerol glycerol synthesis from glucose of adipocytes tended to decrease as did total glucose utilization by the tissue. Glucose conversion to lactate was unchanged by somatotropin treatment but glucose conversion to other products was decreased. Maximum response of adipose tissue to insulin was unchanged but the sensitivity to insulin decreased on somatotropin treatment. Treatment with somatotropin had no effect on basal lipolysis and decreased maximum response to the beta-agonist isoproterenol, but this probably reflects the rate of isoproterenol-stimulated lipolysis varying with cell volume in adipocytes. No apparent change in response either to alpha 2-adrenergic agonists or to adenosine was apparent. The number of beta-adrenergic receptors was unchanged in adipocyte membranes but the number of alpha 2-adrenergic receptors increased. The rate of hepatic gluconeogenesis in vitro, the activity of key gluconeogenic enzymes and the modulation of the rate of gluconeogenesis by butyrate were unchanged except for the effect of this latter agent on gluconeogenesis from propionate. Hepatic ketogenic activity, as indicated by the activity of carnitine palmitoyl-CoA-transferase-1 and the concentrations of carnitine and acyl carnitines, was unchanged by treatment. Thus at the end of a prolonged period of treatment with somatotropin in lactating goats, lipid synthesis in adipose tissue is still decreased but no effects on liver lipid and carbohydrate metabolism were apparent.

Glucocorticoids and insulin but not growth hormone modulate insulin binding to adipocyte membranes from sheep

Mechanisms by which the glucocorticoid analogue dexamethasone and growth hormone modulate insulin action in sheep adipose tissue have been investigated. Maintenance of sheep adipose tissue in culture for 48 hr in the absence of exogenous hormones resulted in a decrease in insulin binding to adipocyte membranes; this was prevented by the inclusion of 10 nM dexamethasone during culture. Tissue culture for 48 hr with insulin itself decreased insulin binding to adipocyte membranes, whereas tissue culture with growth hormone had no effect on insulin binding. Neither dexamethasone nor growth hormone altered the ability of insulin to decrease insulin binding to its receptor. The study shows that the insulin-antagonistic effects of growth hormone on sheep adipose tissue metabolism are due to an effect subsequent to insulin binding to its receptor, whereas some of the effects of dexamethasone may be due to an increase in the insulin-binding capacity of membranes.

Effect of somatotropin on adipose tissue metabolism: ontogeny of the enhanced response to adrenergic challenge in the lactating cow

Seven multiparous Holstein cows (> 150 d postpartum) were used to evaluate the time course of the chronic adaptation in lipolytic response to adrenergic challenge with bovine somatotropin (bST) treatment. Cows received daily bST (sometribove; 40 mg/d) or excipient injections for 7 d (single reversal design) with a 7-d interim between periods. Epinephrine challenges (1.4 microgram/kg body weight intravenously) were administered on Days -2, -1, 1, 2, 3, 5, and 7 of treatment at 10:00 a.m. (15 hr after bST or excipient injection). Frequent blood samples were collected, and concentrations of plasma glycerol (GLY) and nonesterified fatty acids (NEFA) were determined. Treatment with bST increased milk yield 23% (P < 0.05) and milk fat content 33% (P < 0.001) compared with controls. Somatotropin-treated cows entered negative energy balance by Day 3 and had higher basal plasma concentrations of GLY and NEFA than did controls by Day 2 and Day 3, respectively. Response to epinephrine, expressed as area under the response curve corrected for basal, was enhanced by bST treatment, regardless of energy balance. GLY response was greater than control by Day 1 of bST treatment (P < 0.01), and had plateaued by Day 2 (P < 0.001). The NEFA response area was higher than control and had plateaued by Day 1 of bST treatment (P < 0.001). Day 1 represented 15 hr after the first bST injection. Results illustrate that bST treatment results in enhanced in vivo lipolytic response to catecholamine challenge, and the metabolic adaptation is in place by 15 hr after the first bST injection.

Influence of adenosine or adrenergic agonists on growth hormone stimulated lipolysis by chicken adipose tissue in vitro

In vitro lipolysis by chicken adipose explants was stimulated by growth hormone (GH) or glucagon. Adenosine or the adenosine agonist, N6-phenylisopropyladenosine (PIA), inhibited GH stimulated lipolysis, the effect of adenosine not being observed in the presence or adenosine deaminase. Glucagon induced lipolysis was also reduced by PIA. It is suggested that adenosine may act by Gi linked to either adenylate cyclase (for glucagon) or the signal transduction mechanism for GH. Lipolysis was not stimulated by GH in the presence of phenylephrine (alpha 1 adrenergic agonist), isoproterenol (beta adrenergic agonist), adrenaline or glucagon. Although the presence of p-amino clonidine (alpha 2 adrenergic agonist) depressed basal lipolysis, a response to GH was still present. Either glucagon or beta-adrenergic agonists (isoproterenol, adrenaline) stimulated lipolysis. In both cases, GH attenuated the lipolytic response to these hormones, which act via a cyclic adenosine monophosphate signal transduction mechanism.

Growth hormone decreases the response to anti-lipolytic agonists and decreases the levels of Gi2 in rat adipocytes

The effect of growth hormone (GH) in vivo on the Gi-mediated anti-lipolytic signalling system of rat adipocytes has been investigated. Lowering of serum GH levels, by treatment of rats with an antiserum (anti-rGH) specific for rat GH, increased the sensitivity of adipocytes to the anti-lipolytic agonists N6-phenylisopropyladenosine (PIA) and prostaglandin E1. This occurred in the absence of any change in PIA binding to adipocyte membranes. Immunoblot analysis of adipocyte membranes revealed that lowering of serum GH resulted in at least a 3-fold increase in the levels of alpha-subunit of Gi2, but had no effect on the alpha-subunits of Gi1 and Gi3 nor on the 42 and 45 kDa forms of the alpha-subunit of Gs. Replenishment of serum GH, by concurrent administration of ovine GH to rats, prevented all of these effects of anti-rGH. It is concluded that GH down-regulates the amount of Gi2 alpha-subunit in adipocyte membranes, resulting in a decrease in the sensitivity of the cells to anti-lipolytic agonists.

Mechanisms involved in the adaptations of the adipocyte adrenergic signal-transduction system and their modulation by growth hormone during the lactation cycle in the rat

The mechanisms responsible for the diminished lipolytic response of adipocytes to catecholamines after litter removal from lactating rats and their modulation by growth hormone have been investigated. Lactation, litter removal and growth-hormone treatment did not alter the ability of noradrenaline to activate protein kinase A (A-kinase), showing that the defect in signal transduction in rats after litter removal is after A-kinase. Litter removal had no effect on hormone-sensitive lipase activity itself, but the proportion of the lipase associated with the fat droplet was decreased; growth-hormone treatment increased hormone-sensitive lipase activity and the proportion associated with the fat droplet. In addition, a number of other adaptations in the beta-adrenergic signal-transduction system occur during the lactation cycle and in response to growth hormone treatment, including changes in receptor number, adenylate cyclase activity and cyclic AMP phosphodiesterase activity, but a defect in the ability of hormone-sensitive lipase to associate with the lipid droplet appears to be the major reason for the diminished response to catecholamines on litter removal.

Effect of long-term rhGH administration in GH-deficient adults on fat cell epinephrine response

Besides exerting its own lipolytic effect, growth hormone (GH) has been reported to potentiate the lipolytic response of adipose tissue to epinephrine. It was thought interesting to find out whether long-term recombinant human growth hormone (rhGH) administration modifies epinephrine-induced lipolysis in isolated adipocytes of GH-deficient adults. In a double-blind protocol, GH-deficient subjects received either 6 mo placebo (controls, n = 5) or 6 mo rhGH (treated, n = 5). Biopsies of fat were obtained from the periumbilical region before and after placebo or rhGH administration. The response of the collagenase-isolated fat cells to various concentrations of epinephrine was assessed by glycerol release, measured by bioluminescence. Epinephrine-induced lipolysis was not altered by 6 mo placebo, while it was significantly increased by 6 mo rhGH. A similar response was obtained with isoproterenol, but no significant differences occurred in either group with UK 14304, an alpha 2-adrenoreceptor agonist. Thus, in GH-deficient adults, long-term rhGH administration improves the lipolytic response of isolated adipocytes to epinephrine, essentially by increasing the efficiency of the beta-adrenergic pathway.