Effects of the cannabinoid CB1 receptor antagonist SR141716 on oxygen consumption and soleus muscle glucose uptake in Lep(ob)/Lep(ob) mice

OBJECTIVE

To investigate the effect of SR141716, a selective CB1 receptor antagonist, on energy expenditure and on glucose uptake in isolated soleus muscle of Lep(ob)/Lep(ob) mice.

DESIGN

Female Lep(ob)/Lep(ob) mice (8-10 weeks old) were treated with SR141716 (10 mg/kg, i.p. once daily) or vehicle for 7 days.

MEASUREMENTS

Oxygen consumption, daily food and water intake, body weight and glucose uptake in isolated soleus muscle.

RESULTS

SR141716 (10 mg/kg, i.p. once daily) resulted in a significant reduction of daily food intake (P<0.01) and body weight (P<0.05) 5 days after daily treatment. Body weight continued to be lower for the rest of the treatment period (P<0.05). There was no significant difference in body weight between the pair-fed and vehicle-treated animals. A 7-day treatment with SR141716 (10 mg/kg, i.p. once daily) caused 37% increase in basal oxygen consumption compared to that of vehicle-treated (90 min mean; P<0.01), and a significant 68% increase in glucose uptake in isolated soleus muscle preparations.

CONCLUSION

It is concluded that SR141716 has a direct effect on energy expenditure suggesting that the antiobesity effect of SR141716 is due to activation of thermogenesis in addition to the initial hypophagia. The increase in soleus muscle glucose uptake with SR141716 treatment may contribute to the improved glycaemia seen in the previous studies.

Leptin expression in the fetus and placenta during mouse pregnancy

During pregnancy, leptin concentrations in the maternal circulation are elevated in both humans and rodents but decrease to pre-pregnancy levels at birth, suggesting a role for leptin in the maintenance of pregnancy. Synthesis of leptin by the human placenta is established but whether the murine placenta synthesizes leptin remains controversial. The aims of this study were to determine (a) if the mouse wild-type placenta expresses the ob gene using Reverse Transcription-Polymerase Chain Reaction (RT-PCR) and (b) whether the mouse fetus and placenta contribute to the significant increase of leptin in the maternal circulation during pregnancy. The mouse placenta did not express the ob gene at a level that could be readily detected using RT-PCR. Moreover, both maternal gain in weight and undetectable concentrations of leptin in sera in leptin-deficient ob/ob mothers bearing heterozygote (ob/+) fetuses suggested that the mouse fetus and placenta do not make a significant contribution to the dramatic increase in maternal plasma concentrations of leptin during late gestation. It is therefore concluded that neither fetal- nor placental-derived leptin modulates maternal weight gain during pregnancy.

Chronic CNS administration of Agouti-related protein (Agrp) reduces energy expenditure

OBJECTIVE

To investigate whether the Agouti-related protein (Agrp), the melanocortin receptor antagonist, alters oxygen consumption, as a measure of energy expenditure.

DESIGN

A 7-day intracerebroventricular administration of Agrp (1 nmol/day) in rats.

MEASUREMENTS

Oxygen consumption was determined in closed-circuit respirometers on days 1 and 8. BRL-35135, a beta3-adrenoreceptor agonist known to activate the brown adipose tissue (BAT) thermogenesis directly and increase core temperature, was administered i.p. (40 microg/kg) on day 9 to challenge functionally the BAT.

RESULTS

Agrp treatment caused a 54% increase in daily food intake and a 12% increase in body weight. An 8% decrease in VO(2) measurements was observed following ICV Agrp treatment on day 1. A similar decrease (7%) was observed on day 8. BRL-35135 stimulated colonic temperature in control rats. However, in the rats that had previously been treated with Agrp this effect was significantly blunted.

CONCLUSION

Chronic CNS administration of Agrp decreases oxygen consumption and decreases the capacity of BAT to expend energy. The obesity observed following CNS administration of Agrp is the result of decreased energy expenditure and increased food intake.

Immunohistochemical identification of the beta(3)-adrenoceptor in intact human adipocytes and ventricular myocardium: effect of obesity and treatment with ephedrine and caffeine

OBJECTIVES

To investigate whether the beta(3)-adrenoceptor could be identified by immunohistochemistry in intact human white and brown adipocytes and other human tissues, and to investigate the influence of obesity and its treatment with ephedrine and caffeine on the expression of the beta(3)-adrenoceptor in adipocytes.

METHODS

Morbidly obese patients were given a hypoenergetic diet (70% of energy expenditure) and some were also treated with ephedrine and caffeine (20/200 mg, three times daily) for 4 weeks. Adipose tissue and other tissues were taken during surgery. Immunohistochemistry was carried out using a monoclonal antibody raised against the human beta(3)-adrenoceptor.

RESULTS

Staining was localized to the periphery of cells. All white adipocytes were stained. Those from lean subjects and obese subjects treated with ephedrine and caffeine showed more intense staining than those from untreated obese subjects. Staining was more intense in brown than in white adipocytes in perirenal adipose tissue from phaeochromocytoma patients. Staining was also seen in ventricular myocardium, and in smooth muscle of the prostate, ileum, colon and gall bladder.

DISCUSSION

The tissue and subcellular distribution of staining was consistent with it being due to binding of the antibody to the human beta(3)-adrenoceptor. The presence of the beta(3)-adrenoceptor in human white adipocytes is consistent with evidence that it can mediate lipolysis in human white adipocytes. The increased expression of the beta(3)-adrenoceptor in obese subjects treated with caffeine and ephedrine supports the potential of beta(3)-adrenoceptor agonists in the treatment of obesity and type 2 diabetes. Its expression in ventricular myocardium is consistent with evidence that the beta(3)-adrenoceptor mediates a negative inotropic effect in this tissue.

Mechanism of the thermogenic effect of Metabolite 2 (BTS 54 505), a major pharmacologically active metabolite of the novel anti-obesity drug, sibutramine

OBJECTIVE

To investigate the pharmacological mechanisms underlying the induction of thermogenesis by Metabolite 2 (M2; BTS 54 505), a major pharmacologically active metabolite of the anti-obesity drug, sibutramine.

DESIGN

Adult female Wistar rats were treated with M2 or vehicle, with or without various monoamine receptor antagonists, prazosin, RS79948, metergoline, propranolol and (+)butaclamol.

MEASUREMENTS

Colonic temperature and food intake at room temperature (21+/-1 degrees C), thermoregulatory behavioural response, operant responding for exogenous heat at -8 degrees C and oxygen consumption at thermoneutrality (29 degrees C).

RESULTS

M2 (10 mg/kg, p.o.) significantly increased colonic temperature during the 4.5 h period following drug administration. This effect was abolished by the non-selective 5-HT receptor antagonist, metergoline (1 mg/kg, p.o.), and alpha(1)-adrenoceptor antagonist, prazosin (1 mg/kg, p.o.), measured at 1.5-2.5 h post-M2 administration, and was partially antagonized by each antagonist at 3.5-4.5 h. The non-selective beta-adrenoceptor antagonist, propranolol (1 mg/kg, p.o.), had no effect on the M2-induced increase in colonic temperature, whereas at 20 mg/kg (p.o.), propranolol partially inhibited the effect of M2 on colonic temperature. By contrast, the selective alpha(2)-adrenoceptor antagonist, RS79948 (1 mg/kg, p.o.), and the D2/D1 receptor antagonist, (+)butaclamol (200 micro g/kg, p.o.), did not alter the effect of M2 on colonic temperature. In the thermoregulatory study, M2 (10 mg/kg, i.p.)-treated rats required significantly less radiant heat at -8 degrees C to maintain body temperature, and this effect was not affected by the D2/D1 receptor antagonist (+)butaclamol (100 micro g/kg(-1), i.p.). The hypophagia induced by M2 (10 mg/kg) measured up to 24 h was partially antagonized by the alpha(1)-adrenoceptor antagonist, prazosin, whereas metergoline, RS79948, propranolol and (+)butaclamol had no effect on M2-induced hypophagia.

CONCLUSION

It is concluded that 5-HT, alpha(1)- and beta(3)-adrenoceptors are involved in the induction of thermogenesis by M2, whereas the hypophagic effect is mainly mediated via alpha(1)-adrenoceptors. These findings are consistent with M2 increasing 5-HT and noradrenaline tone via potent reuptake inhibition which subsequently results in increased efferent sympathetic activity to brown adipose tissue (BAT).

Leptin requirement for conception, implantation, and gestation in the mouse

The ob/ob mouse has a complete absence of circulating leptin, resulting in obesity and infertility. Using the minimum daily dose of leptin required to maintain normal body weight and sexual maturation (5 mg/kg, ip), leptin-treated ob/ob females were mated with either wild-type (+/+) or leptin-treated ob/ob males. The leptin treatment continued throughout pregnancy until weaning or was withdrawn at 0.5, 3.5, 6.5, or 14.5 d post coitum (dpc). Normal pregnancy and parturition with pups of normal weight resulted when ob/ob females were mated with +/+ males and leptin treatment was continued throughout pregnancy (6 of 8 pregnancies), to 14.5 dpc (6 of 8 pregnancies), or to 6.5 dpc (9 of 12 pregnancies). Pregnancy did not result when treatment was stopped at 3.5 dpc (1 of 7 pregnancies) or 0.5 dpc (0 of 6 pregnancies). Similar results were obtained when leptin-treated ob/ob females were mated with leptin-treated ob/ob males. The newborn pups failed to survive after birth in groups treated with leptin up to 14.5 and 6.5 dpc despite reinstating leptin at birth. This appeared to be due to a lack of development of the mammary glands. In conclusion, we have shown that leptin is essential for normal preimplantation and/or implantation processes. It is also essential for normal development of the mammary glands, but is not required for pregnancy and parturition once implantation is established.

The effect of leptin on luteinizing hormone release is exerted in the zona incerta and mediated by melanin-concentrating hormone

The adipose hormone, leptin, not only restrains appetite, but also influences energy expenditure. One such influence is to promote sexual maturation and fertility. The neuromodulatory circuits that mediate this effect are not well known but the present study suggests that one mediator could be melanin-concentrating hormone (MCH). We show that the long-form receptor (Ob-Rb) is expressed in the zona incerta of the rat and that administration of leptin (both 0.5 microg and 1.0 microg/side) into this area of ovariectomized, oestrogen-primed rats stimulated the release of luteinizing hormone (LH) within 1 h, the effect enduring for a further 1 h. Injections of leptin into the arcuate nucleus induced a smaller, transient rise in LH while injections into the paraventricular and ventromedial nuclei were without effect. MCH neurones are present in the zona incerta and administration of this hormone into the medial preoptic area (mPOA) stimulates LH release, therefore we investigated the possibility that MCH might mediate this effect of leptin. An injection of MCH antiserum into mPOA prevented the rise in LH normally induced by leptin injected into the zona incerta. In addition, melanocortin receptor antagonists ([D-Arg8]ACTH(4-10) and [Ala6]ACTH(4-10)), previously shown to inhibit the stimulatory effect of MCH on LH release, also inhibited the effect of leptin. We propose that one route by which leptin may promote reproductive activity is by enhancing MCH release from fibres within the mPOA. Speculative mechanisms for the action of MCH include the following possibilities: MCH may be acting on the specific MCH receptor which in turn interacts with a melanocortin or melanocortin-like receptor; MCH may bind directly to one of the melanocortin receptors; or melanocortin antagonists may interact with the MCH receptor.

Gluttony and thermogenesis revisited

The evolutionary and biological significance of adaptive, homeostatic forms of heat production (thermogenesis) is reviewed. After summarizing the role and selective value of thermogenesis in body temperature regulation (shivering and non-shivering thermogenesis) and the febrile response to infection (fever), the review concentrates on diet-induced thermogenesis (DIT). Animal studies indicate that DIT evolved mainly to deal with nutrient-deficient or unbalanced diets, and re-analysis of twelve overfeeding studies carried out between 1967 and 1999 suggests the same may be so for humans, particularly when dietary protein concentration is varied. This implies that the role of DIT in the regulation of energy balance is secondary to its function in regulating the metabolic supply of essential nutrients. However, individual differences in DIT are much more marked when high- or low-protein diets are overfed, and this could provide a very sensitive method for discriminating between those who are, in metabolic terms, resistant and those who are susceptible to obesity.

The effect of sibutramine on energy expenditure and appetite during chronic treatment without dietary restriction

OBJECTIVE

To assess the contribution of a thermogenic effect to weight loss induced by eight weeks treatment with sibutramine (15mg/d) vs placebo in obese subjects.

DESIGN

Randomised, placebo controlled, double blind study.

SUBJECTS

Thirty-two (7 male, 25 female) healthy obese body mass index (BMI) 33.9+/-0.5 kg/m2 subjects completed the trial.

MEASUREMENTS

Energy expenditure (EE) was measured by indirect calorimetry during a 32 h stay in a respiration chamber before and after 8 weeks treatment. Visual analogue scales were completed for assessment of appetite sensation. No dietary restriction was given.

RESULTS

Sibutramine caused a significant weight loss compared with placebo (-2.4 kg vs+0.3 kg, P<0.001). Despite the larger weight loss after 8 weeks, 24-h EE did not decrease more in the sibutramine than in the placebo group (-2. 6% vs -2.5%, P=ns). When the changes in 24-h EE were adjusted for changes in body weight, 24-h EE decreased significantly less in the sibutramine group than in the placebo group (0.8% vs 3.8%, P<0.02). Sibutramine significantly decreased both hunger and anticipated food consumption, and increased satiety scores.

CONCLUSIONS

The weight reducing effect of sibutramine in humans is caused by a dual mechanism: reduction of energy intake by increasing satiety and decreasing hunger and prevention of the decline in EE that follows weight loss.

The tissue distribution of the human beta3-adrenoceptor studied using a monoclonal antibody: direct evidence of the beta3-adrenoceptor in human adipose tissue, atrium and skeletal muscle

OBJECTIVE

To develop a monoclonal antibody that recognises an epitope of the native beta3-adrenoceptor expressed on the extracellular surface of human cells and tissues.

DESIGN

A high affinity monoclonal antibody, Mab72c, was raised against the human beta3-adrenoceptor expressed on a transfected mammalian cell line.

RESULTS

In CHO (Chinese hamster ovary) cells transfected with beta3-adrenoceptor cDNA, antibody labelling was found to be proportional to receptor density measured by the binding of the radiolabelled beta-adrenoceptor antagonist, [125I]-iodocyanopindolol. The use of Mab 72c has demonstrated the expression of the beta3-adrenoceptor in a variety of human tissues, including gall bladder, prostate and colon, where a mRNA signal had been detected previously. This study also provides the first direct demonstration of the expression of beta3-adrenoceptors in human skeletal muscle, atrium and adipose tissue.

CONCLUSION

The development of this antibody represents an important addition to the armentarium of reagents that are available to study the localisation of beta3-adrenoceptors in human tissues.

Thermal preference behavior following clonidine, norepinephrine, isoproterenol, and ephedrine

A thermal gradient (temperature range 7-45 degrees C) was used to assess ambient temperature (Ta) preferences of rats following treatment with clonidine (25 microg/kg), norepinephrine (NE, 250 microg/kg), isoproterenol (ISO, 50 microg/kg), and ephedrine (EPH, 10 mg/kg). Clonidine produced a preference for a temperature (31.5 degrees C) slightly warmer than that preferred after saline (28.3 degrees C), but this resulted in no significant change in posttest colonic temperature (Tc). NE, ISO and EPH produced a preference for a colder region of the gradient (20-22 degrees C) compared to saline (24.5-28.9 degrees C). Posttest Tc was reduced significantly from 37.7-37.9 degrees C after saline to 37.2 degrees C (NE), 37.3 degrees C (ISO), and 36.8 degrees C (EPH). Thus, given the opportunity to select an environmental temperature, the animals selected a Ta that resulted in significantly lower body temperatures after NE, ISO, and EPH. This suggests that paradoxical thermoregulatory effects of these thermogenic adrenergic agonists are due, at least in part, to a preference for a lower body temperature.

Role of sympathetic activity in controlling the expression of vascular endothelial growth factor in brown fat cells of lean and genetically obese rats

The thermogenic activity of brown adipose tissue (BAT) is heavily dependent on high perfusion, through its dense vascular system. Angiogenesis must go hand-in-hand with BAT functions, but little is known about the factors controlling it. In the present study we demonstrate that: (a) vascular endothelial growth factor (VEGF) is synthesised and released in brown adipocytes in culture; (b) VEGF mRNA isoforms and protein appear in dispersed mature brown adipocytes and whole tissue; (c) VEGF expression is increased in BAT from cold-exposed rats, and in cultured brown adipocytes exposed to noradrenaline and the beta3-adrenoceptor agonists; (e) BAT from genetically obese (falfa) rats exhibits reduced expression of VEGF as well as a change in the ratio of mRNA isoforms. It is concluded that sympathetic control of VEGF expression via noradrenaline acting on beta3-adrenoceptors plays a major role in developmental and adaptive angiogenesis, and defects in this contribute to the reduced thermogenic capacity of BAT in genetic obesity.

Thermogenic effects of sibutramine in humans

BACKGROUND

Sibutramine is an effective compound for the treatment of obesity, acting both on serotonergic and noradrenergic pathways. Animal studies have shown that sibutramine exerts its effect by enhancing satiety as well as by increasing thermogenesis.

OBJECTIVE

We tried to compare the acute thermogenic effect of a single 30-mg dose of sibutramine with placebo on basal energy expenditure (EE) and diet-induced thermogenesis.

DESIGN

The study was randomized, double-blind, and placebo controlled. Eleven healthy, normal-weight men underwent 4 distinct treatment regimens separated by washout periods of 6-10 d. EE was measured by indirect calorimetry before and for 5.5 h after sibutramine or placebo administration with or without a 2.1-MJ breakfast. Visual analogue scales for assessment of appetite were completed hourly.

RESULTS

Sibutramine caused a significant increase in EE above that for placebo (over 5.5 h) during both the fed (34%, 0.15 kJ/min) and fasted (183%, 0.20 kJ/min) states (P < 0.02) as well as during the last 3.5 h of this 5.5-h period and in the fed (87%, 0.26 kJ/min) and fasted (152%, 0.22 kJ/min) states, respectively (P < 0.01). The sibutramine-induced increase in EE was accompanied by an increase in plasma epinephrine (P < 0.01), heart rate (P < 0.001), blood pressure (P < 0.05), and plasma glucose (P < 0.02). About 25% of the increased heart rate with sibutramine could be explained by increased thermogenesis. Sibutramine increased satiety more than did placebo (5-h area under the curve, P < 0.05).

CONCLUSIONS

Sibutramine caused a significant increase in both EE and satiety, which may both contribute to its weight-reducing properties.

The contribution of classical (beta1/2-) and atypical beta-adrenoceptors to the stimulation of human white adipocyte lipolysis and right atrial appendage contraction by novel beta3-adrenoceptor agonists of differing selectivities

The role of beta3- and other putative atypical beta-adrenoceptors in human white adipocytes and right atrial appendage has been investigated using CGP 12177 and novel phenylethanolamine and aryloxypropanolamine beta3-adrenoceptor (beta3AR) agonists with varying intrinsic activities and selectivities for human cloned betaAR subtypes. The ability to demonstrate beta1/2AR antagonist-insensitive (beta3 or other atypical betaAR-mediated) responses to CGP 12177 was critically dependent on the albumin batch used to prepare and incubate the adipocytes. Four aryloxypropanolamine selective beta3AR agonists (SB-226552, SB-229432, SB-236923, SB-246982) consistently elicited beta1/2AR antagonist-insensitive lipolysis. However, a phenylethanolamine (SB-220646) that was a selective full beta3AR agonist elicited full lipolytic and inotropic responses that were sensitive to beta1/2AR antagonism, despite it having very low efficacies at cloned beta1- and beta2ARs. A component of the response to another phenylethanolamine selective beta3AR agonist (SB-215691) was insensitive to beta1/2AR antagonism in some experiments. Because no [corrected] novel aryloxypropanolamine had a beta1/2AR antagonist-insensitive inotropic effect, these results establish more firmly that beta3ARs mediate lipolysis in human white adipocytes, and suggest that putative 'beta4ARs' mediate inotropic responses to CGP 12177. The results also illustrate the difficulty of predicting from studies on cloned betaARs which betaARs will mediate responses to agonists in tissues that have a high number of beta1- and beta2ARs or a low number of beta3ARs.

Effect of clenbuterol on endocrine status and nitrogen and energy balance in food-restricted rats

We administered clenbuterol as a dietary admixture (4 mg/kg diet) to three groups of male Wistar rats (n = 8) housed individually in metabolism cages and fed for 15 d at 110, 160, and 235% (ad libitum) of the estimated requirement for energy maintenance. Untreated groups at each level of energy intake and a baseline group were also included. In the diet-restricted rats, clenbuterol induced greater and more persistent increases in nitrogen balance, biological value, and net protein utilization than it did in the ad libitum-fed rats. Energy balance was increased by clenbuterol treatment in the diet-restricted rats, with no significant changes occurring in the ad libitum fed rats. Compared with untreated rats, clenbuterol reduced blood glucose in all diet groups and serum insulin in the ad libitum and the moderately restricted (160%) rats. Serum IGF-I was increased in the highly restricted (110%) rats. Corticosterone levels were increased by clenbuterol treatment in all diet groups. These results are consistent with previous results showing that clenbuterol can help improve growth, but they also show that clenbuterol can offset the effects of food restriction on protein and energy metabolism.

Diet-induced loss of cyclic ovarian function at normal body weight in a rodent model for bulimia nervosa

A bulimic rat model was used to test whether type and frequency of food intake mimicking that in human bulimia nervosa could disrupt oestrous cyclicity, induce an effect on glycoprotein (LH) structure, or affect both processes and if so, to determine whether any such effects were acute, or persisted after return to normal eating patterns. Voluntary hyperphagia was induced by offering female rats a varied and palatable choice of human food items--the 'cafeteria diet'. There were four groups: control (normal chow), obese (continuous cafeteria diet), post-obese (cafeteria diet, then fasted to reduce weight to that of controls) and binge (cafeteria alternated with normal diet every few days). Animals were maintained on these diets for 60 days (phase I). They were then given a GnRH challenge on day 2 of dioestrus of the cycle. Twenty-four hours later, half of the animals in each group were killed for assessment of effects on their reproductive organs. The remaining animals were returned to normal diets and kept for a further 40 days, when the GnRH challenge was repeated and the animals were killed 24 h later (phase II). All animals on the cafeteria diet in phase I exhibited significant disruption of oestrous cyclicity irrespective of body weight. LH released in response to the first GnRH challenge showed a prolonged half-life, and/or increased rate of secretion in the obese and post-obese groups but in the binge group the secretory/clearance properties resembled those of control animals. After the second GnRH challenge at the end of phase II, however, the LH of the binge group appeared to have different secretory or clearance characteristics, whereas that of the previously obese animals had returned to normal. These data show ovarian cyclicity was disrupted by hyperphagia and irregular eating, even at normal body weight. Relating ovarian function to pituitary output in terms of LH, the effects of the continuous cafeteria diet did not appear to persist in the animals that returned to normal diets, but in the binge group the effect, presumably of the diet manipulation, was manifested after return to a normal eating pattern. This finding suggests that irregular eating habits may exert a direct (and acute) effect on the ovary, but that effects on the pituitary (and LH glycoforms) take longer to be expressed, explaining many features of bulimia nervosa.

Hyperphagia in cold-exposed rats is accompanied by decreased plasma leptin but unchanged hypothalamic NPY

Chronic cold exposure stimulates sympathetically driven thermogenesis in brown adipose tissue (BAT), resulting in fat mobilization, weight loss, and compensatory hyperphagia. Hypothalamic neuropeptide Y (NPY) neurons are implicated in stimulating food intake in starvation, but may also suppress sympathetic outflow to BAT. This study investigated whether the NPY neurons drive hyperphagia in rats that have lost weight through cold exposure. Rats exposed to 4 degrees C for 21 days weighed 14% less than controls maintained at 22 degrees C (P < 0.001). Food intake increased after 3 days and remained 10% higher thereafter (P < 0.001). Increase BAT activity was confirmed by 64, 96, and 335% increases in uncoupling protein-1 mRNA at 2, 8, and 21 days. Plasma leptin decreased during prolonged cold exposure. Cold-exposed rats showed no significant changes in NPY concentrations in any hypothalamic regions or in hypothalamic NPY mRNA at any time. We conclude that the NPY neurons are not activated during cold exposure. This is in contrast with starvation-induced hyperphagia, but is biologically appropriate since enhanced NPY release would inhibit thermogenesis causing potentially lethal hypothermia. Other neuronal pathways must therefore mediate hyperphagia in chronic cold exposure.

A role for brown adipose tissue in diet-induced thermogenesis

Measurement of energy balance during voluntary overeating in rats unequivocally establishes the quantitative importance of diet-induced thermogenesis in energy balance. Like cold-induced thermogenesis, this form of heat production involves changes in the activity of the sympathetic nervous system and brown adipose tissue which suggest that this tissue may determine metabolic efficiency and resistance to obesity.

Effects of alpha 1-adrenoceptor and Ca2+ channel inhibition on norepinephrine-induced thermoregulatory behavior in the cold

This experiment examined whether paradoxical temperature-dependent effects of norepinephrine (NE) can be blocked by the alpha 1-adrenoceptor antagonist WB 4101 (WB) and the Ca(2+)-channel blocker nifedipine. An operant lever-pressing task was used to measure the demand for heat in a cold environment. As noted previously, NE alone (250 micrograms/kg) produced a substantial and significant increase in the demand for heat, and yet post-test colonic temperature (Tc) fell. When tested alone, WB and nifedipine also increased the demand for heat, but this was sufficient to maintain Tc. When combined with NE, WB and nifedipine reduced the demand for heat and the fall in Tc such that there were no differences between the effects of the blockers given alone or with NE. These results indicate that paradoxical thermoregulatory effects of NE in the cold can be antagonized effectively by either an alpha 1-adrenoceptor antagonist or a Ca(2+)-channel blocker.

Sibutramine: a review of the pharmacology of a novel anti-obesity agent

Sibutramine is a noradrenaline and 5-hydroxytryptamine reuptake inhibitor which causes weight loss in laboratory rodents via effects on both food intake and metabolic rate. Sibutramine's effects are predominantly mediated by two pharmacologically-active metabolites (its primary and secondary amines). Sibutramine and its active metabolites do not cause the release of monoamine neurotransmitters and do not have affinity for their receptors. Sibutramine dose-dependently inhibits 24 h food intake in rats by enhancing the natural physiological process of satiety. Sibutramine also stimulates thermogenesis in rats, producing sustained (> 6 h) increases in oxygen consumption of up to 30%. The thermogenic effect of sibutramine results from central activation of efferent sympathetic activity which, in turn, involves activation of beta 3-adrenoceptors. Sympathetic stimulation of brown adipose tissue via beta 3-adrenoceptors is thought to be the cause of the large, 18 fold increase in brown adipose tissue glucose utilization induced by sibutramine. These dual effects of sibutramine on food intake and thermogenesis explain its anti-obesity effect in animals.

Temperature-dependent effects of ephedrine in the cold

This experiment examined the effects of intraperitoneal ephedrine (5-25 mg/kg) on operant thermoregulatory behavior of rats in the cold (-8 degrees C), and of 10 mg/kg on metabolic rate at a thermoneutral (22 degrees C) ambient temperature (Ta) and in the cold (5 degrees C). Posttest colonic temperature (Tc) decreased dose-dependently in the behavioral tests, yet the demand for heat varied little with respect to saline except for a significant reduction at the lowest dose tested (i.e., there was no compensation for the reduced Tc induced by ephedrine). Ephedrine had a potent thermogenic effect at a Ta of 22 degrees C, increasing both metabolic rate and Tc. In the cold, ephedrine reduced Tc, but this effect could not be accounted for by a reduction in metabolism, which was not significantly different from saline.

Effect of clenbuterol on growth and body composition during food restriction in rats

Clenbuterol was administered as a dietary admixture (4 mg/kg diet) to three groups of male Wistar rats (n = 8) housed individually in metabolism cages and fed for 15 d at 110, 160, and 235% (ad libitum) of estimated requirement for energy maintenance. Untreated groups at each level of energy intake were also included. There was no effect of clenbuterol on food intake in the ad libitum group, but the drug produced significant increases in body weight, feed efficiency, and carcass weight, dressing and protein content at all three levels of energy intake. This effect of clenbuterol was particularly noticeable in the restricted animals. Clenbuterol caused changes in body composition (increased percentage of water and protein, decreased percentage of fat) in the ad libitum rats but had no effect in the restricted groups. The reduction in the growth of the viscera caused by energy restriction was not affected by clenbuterol, apart from in the 110% restricted group, where the gastrointestinal tract was 26% heavier in the clenbuterol-treated rats. The results show that the growth anabolic actions of clenbuterol can be sustained and may be even more marked in rats fed restrictively than in those given ad libitum access to feed.

Relationship between feeding pattern and body mass index in 220 free-living people in four age groups

OBJECTIVE

To assess the relationship between feeding pattern and body mass index in free-living humans.

DESIGN AND SUBJECTS

Feeding pattern was assessed from 220 7-day weighed dietary records. 187 records were obtained from three separate existing studies, and reanalysed. These studies contained data on three age groups in the British population; Elderly group (n = 88), Middle-aged group (n = 40), Working age group (n = 59). A separate study of 13-14 year olds living in Croydon was conducted from which 33 usable diet records were collected to produce a fourth, Adolescent group.

RESULTS

'Nibbling' and greater energy intakes at breakfast were associated with a lower body mass index (BMI) in the Adolescent group. In the Middle-aged group, greater energy intakes at breakfast and lower energy intakes during the evening were associated with a lower BMI. However, when diet records which produced unreasonably low energy intakes were removed from the analysis, these relationships disappeared except for energy intakes at breakfast and BMI in the Adolescent group.

CONCLUSION

It is suggested that the relationship between feeding pattern and BMI observed in the Adolescent and Middle-aged groups was caused by underestimation of 'habitual' energy intake from snacks and the omission of breakfast by females and those who were overweight. The lack of relationship in the Working age group was attributed to the fact that more individuals in this group appeared to report valid diet records. Reported energy intake was directly related to BMI in the Working age group, but was not related to BMI in the other three age groups. It is concluded that feeding pattern is not a major factor in determining BMI in humans. Also, since snacks have a relatively high sugar and low fat composition compared with meals, it is suggested that biased under-reporting of snacks by the obese could produce spurious results from free-living studies which show that obesity is related to the proportion of energy from fat in the diet.

Effect of selective beta-adrenoceptor stimulation on UCP synthesis in primary cultures of brown adipocytes

Given the co-existence of the three beta-adrenoceptor (beta AR) subtypes (beta 1AR, beta 2AR and beta 3AR) in brown adipocytes, the present study was undertaken to determine the relative importance of these in the induction of UCP synthesis in mouse BAT precursor cells in primary culture. Cells at different stages of differentiation were exposed to different beta AR agonists: prenalterol (a selective beta 1AR agonist), salbutamol or clenbuterol (selective beta 2AR agonists), or BRL 37344 (a selective beta 3AR agonist). As with the endogenous agonist, noradrenaline, and the non-selective beta AR agonist, isoprenaline, all four beta AR agonists induced UCP in the confluent stage of the cells, but with different potencies, and with the highest induction being seen after clenbuterol or BRL 37344 treatment. Cells in the confluent stage of development were the most sensitive to the effects of the agonists, although clenbuterol and BRL 37344 induced a weak UCP synthesis in pre-confluent cells. None of these beta AR agonists were able to induce UCP synthesis in the post-confluent period. The responses to prenalterol and salbutamol were inhibited by propranolol at relatively low concentrations, suggesting their effects were mediated by beta 1AR and beta 2AR, respectively. However, propranolol was a particularly weak antagonist of BRL 37344 and, unexpectedly, of the clenbuterol UCP responses, which suggests that both induce UCP synthesis via the beta 3AR. In summary, the beta 3AR is the most important adrenoceptor coupled to the induction of UCP synthesis, although both beta 1AR and beta 2AR activation may make a contribution. However, all three beta AR subtypes do not become fully functional until cultured cells become confluent.

Biphasic effects of the beta-adrenoceptor agonist, BRL 37344, on glucose utilization in rat isolated skeletal muscle

1. The effects of the selective beta 3-adrenoceptor agonist, BRL 37344 (BRL) on glucose uptake and phosphorylation (i.e. glucose utilization; GU) and glycogen synthesis in rat isolated soleus and extensor digitorium longus (EDL) muscle preparations in vitro were investigated by use of 2-deoxy-[3H]-glucose (GU) and [U-14C]-glucose (glycogen synthesis). 2. Low concentrations of BRL (10(-11)-10(-9) M) significantly increased GU, with maximal increases of 30% in soleus and 24% in EDL at 10(-11) M. Neither the selective beta 1-adrenoceptor antagonist, atenolol (10(-8)-10(-6) M), nor the selective beta 2-adrenoceptor antagonist, ICI 118551 (10(-8)-10(-6) M) had any effect on the stimulation of GU induced by 10(-11) M BRL. 3. High concentrations of BRL (10(-6)-10(-5) M) caused significant inhibition (up to 30%) of GU in both soleus and EDL muscles. The inhibition of 10(-6) M BRL was blocked completely by 10(-6) and 10(-7) M ICI 118551 in soleus, and by 10(-6)-10(-8) M ICI 118551 in EDL; atenolol (10(-8)-10(6) M) had no effect. 4. Another selective beta 3-adrenoceptor agonist, CL 316,243, also caused a significant stimulation of muscle GU, with maximal increases of 43% at 10(-9) M in soleus and 45% at 10(-10) M in EDL. The stimulation of GU declined with further increases in the concentration of CL 316,243, but no inhibition of GU was seen, even at the highest concentration (10(-5) M) tested. 5. BRL at 10(-5) M inhibited completely insulin-stimulated glycogen synthesis in both soleus and EDL, but this inhibitory effect of BRL was abolished by 10(-6) M ICI 118551. BRL at 10(-11) M (with or without 10(-6) M ICI 118551) had no effect on insulin-stimulated glycogen synthesis. 6. It is concluded that: (i) low (< nM) concentrations of BRL stimulate GU via an atypical beta-adrenoceptor that is resistant to conventional beta 1-adrenoceptor and beta 2-adrenoceptor antagonists; (ii) the stimulation of GU is negated by the activation of beta 2-adrenoceptors that occurs at higher (> nM) concentrations of BRL; (iii) inhibition of GU via beta 2-adrenoceptor activation is associated with inhibition of glycogen synthesis, possibly due to activation of glycogenolysis; (iv) the opposing effects of beta 2-adrenoceptor and atypical beta-adrenoceptor activation on GU suggest that in skeletal muscle these adrenoceptors are linked to different post-receptor pathways.

Inhibition of brown fat activity during hypothalamic stimulation in the rat

Previous work has shown that low-level electrical stimulation of the ventromedial hypothalamus (VMH) in anesthetized rats produces a sustained decrease (phase 1) in interscapular brown adipose tissue (IBAT) temperature followed by a rise (phase 2) after the stimulus has stopped [Woods, A. J., and M. J. Stock. Am. J. Physiol. 266 (Regulatory, Integrative Comp. Physiol. 35): R328-R337, 1994]. In this study, rat oxygen consumption was found to decrease (24%) and then increase (74%) during phase 1 and 2, respectively. The effect of norepinephrine, alpha-adrenoceptor antagonists, substance P, and neuropeptide Y, with and/or without VMH stimulation, suggested that vasoconstriction was unlikely to account for the phase 1 decreases in thermogenesis and temperature. However, measurement with radio-labeled microspheres showed that IBAT capillary blood flow was reduced by 70% during phase 1, and this, plus a 50% decrease in blood oxygen extraction, indicated that phase 1 could be due to vasodilatation of arteriovenous anastomoses. It was postulated that phase 1 resulted from release of neuropeptides, such as substance P, causing diversion of arterial blood away from IBAT capillaries, thereby increasing convective heat loss and inhibiting heat production during phase 1.

Contribution of beta 3-adrenoceptor activation to ephedrine-induced thermogenesis in humans

OBJECTIVE

To investigate the contribution of beta 3-adrenoceptor activation to sympathetic stimulation of thermogenesis in humans using a sympathomimetic (ephedrine) in combination with a non-selective beta-adrenoceptor antagonist (nadolol).

DESIGN

Three doses (2.5, 5 and 10 mg) of nadolol were used to estimate what fraction of the thermogenic response to ephedrine (30 mg) remained after inhibition of beta 1- and beta 2-adrenoceptor mediated responses.

SUBJECTS

Nine healthy, young male volunteers at rest after an overnight fast.

MEASUREMENTS

Energy expenditure, respiratory quotient, heart rate, blood pressure and plasma potassium, glucose, lactate, glycerol, NEFA and triglycerides were measured before, and for 3 h after treatment with placebo, ephedrine and ephedrine plus three doses of nadolol.

RESULTS

Ephedrine produced significant increases in energy expenditure (thermogenesis), heart rate, systolic blood pressure and plasma glucose; the other parameters measured did not change significantly. Nadolol caused significant inhibition of all responses, but 43% of the thermogenic response to ephedrine remained after the 2.5 mg dose of nadolol, whereas the same dose completely inhibited the heart rate and plasma glucose responses.

CONCLUSION

All three beta-adrenoceptor subtypes (beta 1, beta 2 and beta 3) may be involved in ephedrine-induced thermogenesis, but the resistance to complete inhibition by the non-selective antagonist nadolol indicates that at least 40% of the response is mediated by an atypical receptor, which is presumed to be the beta 3-adrenoceptor.

Temperature-dependent effects of alpha-adrenergic agonists and antagonists in the cold

This series of experiments examined whether temperature-dependent effects of the alpha-antagonists prazosin and yohimbine compromised their use as blockers of alpha-adrenergic agonist responses in cold-exposed rats. An operant leverpressing task was used to measure the demand for heat in a cold environment. The alpha 1-antagonist prazosin had modest effects, but the alpha 2-antagonist yohimbine was thermolytic in that it dose dependently increased operant responding but decreased posttest colonic temperature (Tc). These potent effects of the alpha 2-antagonist led to tests of the alpha 2-agonist clonidine. Clonidine increased operant responding for heat to an extraordinary degree, resulting in significant increases in posttest Tc. However, clonidine was found to be a hypothermic agent when tested in rats at 5 degrees C but denied the opportunity to increase body temperature by operant lever pressing, suggesting a central effect on the control of thermal balance. Measurement of changes in metabolic rate at 5 and 23 degrees C showed that yohimbine increased metabolism at 23 degrees C but decreased it in the cold. Prazosin had little effect on metabolism or Tc at either temperature. Prazosin inhibited the decrease in Tc induced by norepinephrine (NE), but had little effect on the lever-pressing response. Yohimbine had no significant antagonistic effect on NE-induced changes in lever-pressing behavior or posttest Tc, but neither did the thermolytic effects of yohimbine exacerbate those of NE. These results show that alpha-antagonist interactions with agonists can be complicated by temperature-dependent effects of each.

Effects of epinephrine on thermoregulatory behavior in lean and obese Zucker rats in the cold

This series of experiments examined whether epinephrine (EPI) produces the same thermoregulatory effects in the cold that have been reported for norepinephrine and isoproterenol. Lean and obese Zucker rats were trained to press a lever to activate infrared heat lamps in a cold (-8 degrees C) environment. Operant thermoregulatory behavior increased dose-dependently following EPI (0-100 micrograms/kg), but posttest colonic temperature (Tc) fell. Thermal balance calculations showed a substantial increase in net heat loss, more so in obese than lean animals. EPI is therefore thermolytic--i.e., disrupts thermal balance. A low dose (100 micrograms/kg) of the alpha-antagonist phentolamine produced a marked improvement in operant behavior, Tc, and thermal balance, whereas a comparable dose of the beta-antagonist propranolol had no beneficial effect. Increasing the dose of phentolamine worsened the responses with respect to the 100-micrograms/kg dose. The selective alpha 1-antagonist prazosin ameliorated the decrease in Tc induced by EPI but had little effect on operant behavior or thermal balance; the selective alpha 2-antagonist yohimbine had no effect on any parameter compared to EPI alone. These results suggest that the paradoxical effects of EPI in the cold are mediated by alpha-adrenoceptors, but definitive identification of the subclass of receptor involved cannot be determined.

Acute effects of the beta 3-adrenoceptor agonist, BRL 35135, on tissue glucose utilisation

1. The acute effects of BRL 35135 (BRL) on tissue glucose utilisation index (GUI) in vivo were investigated in anaesthetized rats by use of 2-deoxy-[3H]-glucose. 2. Intravenous injection of BRL caused a dose-dependent increase in GUI in skeletal muscle, and white and brown adipose tissue; plasma insulin and fatty acid concentrations were also increased. Chronic treatment with BRL added to the diet caused a 34 fold increase in basal GUI of brown adipose tissue (BAT), but had no effect on GUI in other tissues. After chronic treatment, the acute tissue response to an intravenous maximal dose of BRL had disappeared completely in all tissues apart from the soleus muscle. 3. A high dose (20 mg kg-1) of the non-selective beta-antagonist, propranolol, inhibited the acute effect of BRL on GUI in BAT, but failed to affect GUI in muscle. A lower dose (1 mg kg-1) of the antagonist also inhibited the BAT response, but had little or no effect on the response in Type I (working) muscles such as soleus and adductor longus (ADL), and potentiated the response in Type II (non-working) muscles such as tibialis and extensor digitorium longus (EDL). 4. A low dose (1 mg kg-1) of the selective beta 1-antagonist, atenolol, had no effect on the BRL response but the same dose of the selective beta 2-antagonist, ICI 118551, potentiated significantly the effect of BRL on GUI in most muscles without altering plasma insulin levels. 5. It is concluded that: (i) the heterogeneous tissue responses of different muscle fibre types in the presence of P-antagonists indicates that BRL affects muscle GUI directly, in addition to effects mediated by increases in plasma insulin concentration; (ii) the resistance of the BRL response to conventional P-adrenoceptor antagonists implicates an atypical adrenoceptor mediating the GUI response in skeletal muscle, but this may not be identical to the adipose tissue P3-adrenoceptor; (iii) the potentiation of BRL responses by ICI 118551 indicates an inhibitory P2-adrenoceptor-mediated component in the muscle GUI response to BRL.

Differences in conicity in young adults of European and south Asian descent

A survey of 320 (175 male, 155 female) 19 year-old medical students showed that male students of South Asian origin in the top tertile for body weight or body mass index had a significantly greater conicity index than European males in these top tertiles. This difference in conicity was not significant in the group as a whole, or when ethnic pairs were matched for body weight or body mass index. However, females of South Asian descent had a significantly higher conicity index than females of European descent irrespective of how the groups were compared. The trend towards higher conicity (i.e. abdominal obesity) in young Asians may help explain the higher incidence of diabetes and cardiovascular disease seen in elderly Asians living in the United Kingdom.

Effects of clenbuterol and salbutamol on tissue rubidium uptake in vivo

In anesthetized rats, injection of the beta 2-adrenoceptor (beta 2-AR) agonist clenbuterol (0.45 mumol/kg) caused a marked stimulation of 86RbCl (Rb) uptake by skeletal muscle, but had no effect on other tissues; soleus muscle showed the largest (144% increase) response. Injection of another beta 2-AR agonist (salbutamol 0.45 mumol/kg) had no effect on Rb uptake by any tissue except soleus muscle (83%). Both agonists increased body (colonic) temperature to the same extent. A 3-day treatment with salbutamol as a dietary admixture had no effect on body weight, muscle mass, or tissue Rb uptake, whereas the same treatment using clenbuterol produced significant increases in body weight and muscle mass and significant decreases in Rb uptake in three of the four muscle groups studied; Rb uptake in soleus was not affected. In another experiment, the short-term effect of clenbuterol injection on muscle Rb uptake was found to be resistant to a high dose (20 mg/kg) of the selective beta 2-AR antagonist ICI 118551. It was concluded that the selective effects of short-term administration of clenbuterol on muscle Rb uptake, coupled with its effects over 3 days on Rb uptake and muscle hypertrophy, implicate beta-AR modulation of cation transport (possibly via Na,K-adenosine triphosphatase [ATPase] activity) in the anabolic effects of clenbuterol on muscle protein deposition. Since the stimulation of Rb uptake by clenbuterol was resistant to high doses of a selective beta 2-AR antagonist and since salbutamol had little or no effect on muscle hypertrophy or Rb uptake, it is suggested that clenbuterol may exert its effects via an atypical beta-AR.

Sources of energy from meals versus snacks in 220 people in four age groups

OBJECTIVES

To assess meals versus snacks in terms of their contribution to total daily energy intake (TDI), macronutrient composition, and food commodity profile.

DESIGN AND SUBJECTS

Meals and snacks were assessed from 220 7-day weighed dietary records. 187 records were obtained from three separate existing studies, and reanalysed. These studies contained data on three different age groups in the British population; elderly group (n = 88), middle-aged group (n = 40), young adult group (n = 59). A separate study of 13-14-year-olds living in Croydon was conducted from which 33 usable diet records were collected (adolescent group).

RESULTS

Boys in the adolescent group consumed more of their TDI as snacks (29.0%) compared with men in the young adult (18.9%) and elderly groups (16.6%), but not the middle-aged group (25.8%). Females consumed about the same percentage of their TDI as snacks; adolescent group 23.6%, young adult group 19.4%, middle-aged group 21.4%, elderly group 17.9%. Meals were higher in protein and fat, and lower in total sugars, compared with snacks. Chocolate confectionery, crisps and fizzy drinks and squashes were popular snack foods in the adolescent group. Unlike snacks, the food commodity profiles of meals were similar in all age groups.

CONCLUSIONS

This study shows that foods and drinks consumed as snacks by the British public, including the elderly, have a relatively high total sugar composition. These results add to the concern relating snack foods with dental caries.

Thermoregulatory effects of resiniferatoxin in the rat

When administered acutely, the vanilloid (capsaicin) receptor agonist resiniferatoxin induces marked hypothermia in the ferret, rat and mouse. The aim of this study was to further characterise the thermoregulatory effects of resiniferatoxin in the rat in an attempt to understand the mechanism by which resiniferatoxin induces this hypothermic effect. Three doses of resiniferatoxin were administered (50, 100, 200 micrograms/kg s.c.) in separate animals at an ambient temperature (Ta) of 20 degrees C but there was no apparent dose-related effect on the decrease in colonic temperature over this range. Resiniferatoxin (50 micrograms/kg s.c.) decreased whole body oxygen consumption when measured below thermoneutrality (Ta = 20 degrees C) but not at thermoneutrality (Ta = 29 degrees C); likewise there was no hypothermic response to resiniferatoxin when measured at a Ta of 29 degrees C. Operant responding for radiant heat in a cold environment (-8 degrees C) was also measured in resiniferatoxin-treated (50 micrograms/kg s.c.) rats. These experiments showed that resiniferatoxin-treated rats attempted to defend body temperature by lever pressing for more radiant heat. However, this was not sufficient to reverse the hypothermia. Two repeat doses, 1 week apart, had little or no effect on colonic temperature, oxygen consumption or operant responding in the cold. Resiniferatoxin (50 micrograms/kg s.c.) also produced hypothermia (Ta = 20 degrees C) in neonatally capsaicinized adult rats. The exact site and mode of action is still under investigation, but it is postulated that resiniferatoxin activates, and then destroys or desensitizes warm thermoreceptors.

Anabolic effects of clenbuterol after long-term treatment and withdrawal in t the rat

Injection of rats with the beta 2-adrenoceptor agonist clenbuterol (1 mg/kg/d for 15 days) stimulated an increase in body weight (9%) and protein (8%) and water (7%) content, but reduced food intake (4%) and epididymal fat pad mass (39%). Nine days after termination of treatment, ex-clenbuterol rats were heavier (5%) and had a greater protein (7%) and water (6%) content and lower fat pad mass (32%) than controls. Clenbuterol-fed rats (2 mg/kg diet for 10 days, providing an average of 0.04 mg clenbuterol/kg/d) increased body weight (7%), muscle mass (15% to 21%), and muscle protein content (9% to 26%), whereas epididymal fat pad weight and muscle glycogen content were reduced. During the withdrawal period, the greater body weight of ex-clenbuterol rats was sustained overall (ANOVA, P < .00005), but by day 10 this difference was no longer significant. At this point, gastrocnemius muscle mass was still higher (11%) when compared with that of control animals, but soleus muscle mass, muscle glycogen concentration, and epididymal fat pad weight had reverted to control values. These results were corroborated in a subsequent experiment using older rats. It was concluded that, unlike other beta-adrenoceptor-mediated effects, muscle protein accumulated during clenbuterol treatment can be maintained in certain muscles after removal of the drug for a period of time that is at least equivalent to the duration of treatment. This could have implications for the potential therapeutic use of this class of compound, and differences in the response observed between muscle types may help to elucidate the mechanisms responsible for the muscle protein deposition induced by clenbuterol.

Effects of digoxin on the anabolic response to clenbuterol

The possible involvement of increased cation exchange in the anabolic response to the beta 2-selective adrenergic agonist clenbuterol was investigated using dietary admixtures of clenbuterol and the Na,K-adenosine triphosphatase (ATPase) inhibitor digoxin. In a rat feeding trial to assess the effects on body composition, it was found that the higher of two levels (5 and 30 mg/kg diet) of digoxin had an inhibitory effect on the repartitioning effects (ie, increased body weight and fat-free mass) of clenbuterol (2 mg/kg diet). In two further experiments using 30 and 60 mg digoxin/kg diet, it was found that the anabolic effects of clenbuterol on gastrocnemius muscle protein deposition were inhibited by digoxin, but the effects of clenbuterol on soleus muscle protein were more resistant to inhibition. Given the observed dose-dependent inhibition by digoxin of gastrocnemius muscle protein deposition in the three experiments, it was concluded that at least part of clenbuterol's anabolic actions on skeletal muscle may depend on increased Na,K-ATPase activity. However, different mechanisms or a different time course of Na,K-ATPase activation may occur in different muscle fiber types.

Biphasic brown fat temperature responses to hypothalamic stimulation in rats

Low-level electrical stimulation (monophasic square-wave pulses: 15 Hz, 7.0 microA, 0.5 ms) of the ventromedial hypothalamus (VMH) in anesthetized rats produced a decrease (phase 1) in interscapular brown adipose tissue (IBAT) temperature that was sustained for as long as the stimulus was applied (2-45 min). A rise in IBAT temperature (phase 2) occurred only after the stimulation had stopped. VMH stimulations ipsilateral and contralateral to a lateral hypothalamic (LH) lesion indicated that the phase 1 response required an intact LH, and denervation of IBAT showed that both phases required an intact sympathetic innervation. Central intracerebroventricular injections of amphetamine and dopamine produced decreases in IBAT temperature similar in magnitude to the phase 1 response to electrical stimulation of the VMH. This, as well as the observation that pimozide blocked phase 1, suggested that dopaminergic pathways were responsible for mediating the phase 1 decrease in IBAT temperature. The peripheral mechanisms responsible for phase 1 are unknown, but a vascular component might explain the unexpected decrease in IBAT temperature seen during sustained VMH stimulation.

Effects of beta-adrenoceptor agonists and antagonists on thermoregulation in the cold in lean and obese Zucker rats

This experiment examines whether the thermoregulatory ability of obese Zucker rats is comparable to that of lean rats following treatment with beta-adrenoceptor agonists and antagonists in a cold (-8 degrees C) environment. Half-maximal doses of the nonselective beta-adrenoceptor agonist isoproterenol (ISO) produced net thermolytic (heat loss) effects in both obese and lean rats in an operant lever pressing for radiant heat task. ISO increased the demand for heat, but posttest colonic temperature (Tc) decreased. A low dose of propranolol (100 micrograms/kg) normalized thermoregulatory behavior, Tc, and thermal balance when coadministered with ISO. Activation of thermogenesis with the selective beta 3-agonist BRL 35135 (BRL) reduced heat influx by both obese and lean rats at doses between 2 and 10 micrograms/kg, but no dose-response effects were evident within this range. Posttest Tc and thermal balance indicated no thermolytic effects. No evidence was found for a beta 2-component in the BRL response when a supramaximal dose (40 micrograms/kg) was tested with the selective beta 2-antagonist ICI 118551 (1 mg/kg). These data show that, despite a higher baseline demand for heat, the obese Zucker rat responds to the thermogenic effects of BRL and the thermolytic effects of ISO as does the lean rat.

Thermoregulatory responses to beta-adrenergic agonists at low ambient temperatures in the rat

Dose-response effects on heat production (HP) and dry heat loss (DHL) following injection with the non-selective (beta 1/beta 2) adrenergic agonist isoprenaline (ISO) and the atypical B3 agonist BRL 35135 (BRL) were established at an ambient temperature of 25 degrees C in rats. Subsequently, the effects of HP and DHL of a maximal thermogenic dose of ISO (75 micrograms/kg) and a supramaximal dose of BRL (40 micrograms/kg) were tested at ambient temperatures of 5, 10 and 15 degrees C. In terms of heat production, BRL was no different from saline at 5 degrees C, but its thermogenic activity became increasingly evident as ambient temperature increased. For ISO, HP was lower than, or no different from, saline at 5 and 10 degrees C, respectively, but DHL exceeded HP at both temperatures, and colonic temperature fell significantly; ISO and BRL responses were similar at 15 degrees C. ISO was also capable of producing a decrease in HP at 10 degrees C if the rats were shaven. Substitution for endogenous, sympathetically mediated thermogenesis would explain the attenuation of the BRL and ISO effects at cool ambient temperatures, whereas the hypothermic effects of ISO in the cold appeared to be due to an inappropriate increase in DHL, which was exacerbated at 5 degrees C by a reduction in HP below saline values. The increase in DHL was consistent with beta 2-mediated effects of ISO on peripheral blood flow, but the mechanism responsible for the reduction in HP in the cold is unknown, although reduced vascular thermogenesis has been offered as a putative explanation.

Thermoregulatory effects of beta adrenoceptors: effects of selective agonists and the interaction of antagonists with isoproterenol and BRL-35135 in the cold

Dose-dependent effects of the selective beta 1 adrenergic antagonist atenolol and the beta 2 antagonist erythro-dl-1-(7-methylinden-4-yloxy)-3-isopropylamino-2-ol were tested on the thermoregulatory responses elicited by half-maximal thermogenic doses of the nonselective beta agonist isoproterenol (ISO) in a cold environment. ISO alone increased operant responding for exogenous heat but decreased body temperature and increased net heat loss. These effects of ISO were blocked by erythro-dl-1-(7-methylinden-4-yloxy)3-isopropylamino-2-ol in a dose-dependent manner, whereas atenolol at the highest dose tested (2 mg/kg) exacerbated the effects of ISO. The beta 3 agonist, (R*,R*-(+/-)-methyl 4-[2-[2-hydroxy-2-(3-chlorophenyl)ethylamino]propylphenoxyacetate hydrobromide, reduced operant responding for heat and net heat loss in the cold; these effects were sustained in the presence of both beta 1 and beta 2 antagonists. The beta 2 adrenoceptor agonist, fenoterol, produced a dose-dependent increase in operant behavior but colonic temperature fell and thermal balance reflected the characteristic effects of a thermolytic agent. The beta 1 agonist, prenalterol, had no apparent effect on thermoregulatory behavior, colonic temperature or thermal balance but it blocked the thermolytic effects of fenoterol when both agonists were coadministered. Fenoterol had no significant effect on metabolic rate or colonic temperature when tested in a warm ambient temperature of 23 degrees C but it decreased colonic temperature without a significant effect on metabolic rate at an ambient temperature of 5 degrees C, suggesting an effect on heat loss mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)