Discrepancies in lipolytic activities induced by beta-adrenoceptor agonists in human and rat adipocytes

A new era of understanding in vivo metabolic flux in thermogenic adipocytes

Nonshivering thermogenesis by brown adipose tissue (BAT) is an adaptive mechanism for maintaining body temperature in cold environments. BAT is critical in rodents and human infants and has substantial influence on adult human metabolism. Stimulating BAT therapeutically is also being investigated as a strategy against metabolic diseases because of its ability to function as a catabolic sink. Thus, understanding how brown adipocytes and the related brite/beige adipocytes use nutrients to fuel their demanding metabolism has both basic and translational implications. Recent advances in mass spectrometry and isotope tracing are improving the ability to study metabolic flux in vivo. Here, we review how such strategies are advancing our understanding of adipocyte thermogenesis and conclude with key future questions.

Is the β3-Adrenoceptor a Valid Target for the Treatment of Obesity and/or Type 2 Diabetes?

β3-Adrenoceptors mediate several functions in rodents that could be beneficial for the treatment of obesity and type 2 diabetes. This includes promotion of insulin release from the pancreas, cellular glucose uptake, lipolysis, and thermogenesis in brown adipose tissue. In combination, they lead to a reduction of body weight in several rodent models including ob/ob mice and Zucker diabetic fatty rats. These findings stimulated drug development programs in various pharmaceutical companies, and at least nine β3-adrenoceptor agonists have been tested in clinical trials. However, all of these projects were discontinued due to the lack of clinically relevant changes in body weight. Following a concise historical account of discoveries leading to such drug development programs we discuss species differences that explain why β3-adrenoceptors are not a meaningful drug target for the treatment of obesity and type 2 diabetes in humans.

Acylated and unacylated ghrelin directly regulate ß-3 stimulated lipid turnover in rodent subcutaneous and visceral adipose tissue ex vivo but not in vivo

Ghrelin has garnered interest as a gut-derived regulator of lipid metabolism, beyond its classical roles in driving appetite and growth hormone release. Ghrelin’s circulating concentrations follow an ultradian rhythm, peak immediately before a meal and point towards a potential metabolic role in reducing the mobilization of fatty acid stores in preparation for the storage of ingested food. Here, we demonstrate that both acylated and unacylated ghrelin have physiological roles in attenuating lipolysis in mature subcutaneous and visceral adipose tissue depots of rats. Ghrelin blunted the ß3-induction (CL 316, 243) of glycerol release (index of lipolysis) which coincided with a reduced activation of the key lipid hydrolase HSL at two of its serine residues (Ser⁵⁶³/⁶⁶⁰). Furthermore, ghrelin appeared to inhibit fatty acid reesterification in the presence of CL such that fatty acid concentrations in the surrounding media were maintained in spite of a reduction in lipolysis. Importantly, these aforementioned effects were not observed following ghrelin injection in vivo, as there was no attenuation of CL-induced glycerol release. This highlights the importance of exercising caution when interpreting the effects of administering ghrelin in vivo, and the necessity for uncovering the elusive mechanisms by which ghrelin regulates lipolysis and fatty acid reesterification. We conclude that both acylated and unacylated ghrelin can exert direct inhibitory effects on lipolysis and fatty acid reesterification in adipose tissue from rats. However, these effects are not observed in vivo and outline the complexity of studying ghrelin’s effects on fatty acid metabolism in the living animal.

Negative Affect-Related Autonomic Arousal Mediates the Association between Baroreflex Dysfunction and Insulin Resistance in Non-Diabetic Young Adults

Autonomic dysfunction, in particular under-regulation of heart rate (HR) by the baroreflex, is implicated in development of insulin resistance (IR). According to reactivity hypothesis, sympathetic response to stressors may be more sensitive at predicting IR than baroreceptor sensitivity (BRS), a baseline measure of baroreflex functioning. Using ecological momentary assessment (EMA) of negative affect coupled with minute-to-minute HR and heart-rate variability (HRV) monitoring, we examined whether negative affect (NA)-related autonomic arousal mediates the association of BRS with IR. At baseline, BRS was measured, and fasting serum glucose and insulin levels were collected from 178 young adults (18-39 years old), from which homeostasis model assessment of IR (HOMA-IR) and beta-cell functioning (HOMA %B) were derived. Participants subsequently underwent one day of Holter HR and HRV monitoring while reporting negative affect levels via EMA. Multilevel modeling was used to assess the associations of momentary negative affect with HR and low- (LF) and high-frequency (HF) HRV during the 5-minute intervals following each EMA reading. Structural equation modeling was then used to determine whether individual differences in these associations mediated the association of BRS with IR, measured by HOMA-IR, HOMA %B, and insulin levels. As predicted, BRS was negatively associated with the IR (β = -.17, p = .024). However, NA-related autonomic arousal mediated their association, accounting for 56% of the covariance between BRS and IR. Not only do these results provide support for reactivity hypothesis, they reveal a potential point of intervention in the treatment of affective dysregulation.

Contrasts between the effects of zinc-α2-glycoprotein, a putative β3/2-adrenoceptor agonist and the β3/2-adrenoceptor agonist BRL35135 in C57Bl/6 (ob/ob) mice

Previous studies by Tisdale et al. have reported that zinc-α(2)-glycoprotein (ZAG (AZGP1)) reduces body fat content and improves glucose homeostasis and the plasma lipid profile in Aston (ob/ob) mice. It has been suggested that this might be mediated via agonism of β(3)- and possibly β(2)-adrenoceptors. We compared the effects of dosing recombinant human ZAG (100 μg, i.v.) and BRL35135 (0.5 mg/kg, i.p.), which is in rodents a 20-fold selective β(3)- relative to β(2)-adrenoceptor agonist, given once daily for 10 days to male C57Bl/6 Lep(ob)/Lep(ob) mice. ZAG, but not BRL35135, reduced food intake. BRL35135, but not ZAG, increased energy expenditure acutely and after sub-chronic administration. Only BRL35135 increased plasma concentrations of glycerol and non-esterified fatty acid. Sub-chronic treatment with both ZAG and BRL35135 reduced fasting blood glucose and improved glucose tolerance, but the plasma insulin concentration 30 min after administration of glucose was lowered only by BRL35135. Both ZAG and BRL35135 reduced β(1)-adrenoceptor mRNA levels in white adipose tissue, but only BRL35135 reduced β(2)-adrenoceptor mRNA. Both ZAG and BRL35135 reduced β(1)-adrenoceptor mRNA levels in brown adipose tissue, but neither influenced β(2)-adrenoceptor mRNA, and only BRL35135 increased β(3)-adrenoceptor and uncoupling protein-1 (UCP1) mRNA levels in brown adipose tissue. Thus, ZAG and BRL35135 had similar effects on glycaemic control and shared some effects on β-adrenoceptor gene expression in adipose tissue, but ZAG did not display the thermogenic effects of the β-adrenoceptor agonist, nor did it increase β(3)-adrenoceptor or UCP1 gene expression in brown adipose tissue. ZAG does not behave as a typical β(3/2)-adrenoceptor agonist.

Antiobesity and Antidiabetic β-agonists: Lessons Learned and Questions to be Answered

In several species of obese animals, a group of phenethanolamine beta-agonists stimulates lipolysis and thermogenesis, resulting in the loss of body fat and weight. Brown adipose tissue is considered to be the major target tissue for the antiobesity activity of these compounds. Independent of this antiobesity activity, some of these compounds are also antidiabetic and increase muscle mass. Based on the pharmacological profile of these compounds, a beta3-receptor was proposed and characterized in mouse, rat, and humans. The beta3-receptor in brown adipose tissue has been suggested to mediate the antiobesity activity of these beta-agonists. Whether this receptor is responsible for the antidiabetic activity and whether there is a linkage between the antiobesity/antidiabetic activity and the nutrient partitioning activity is not clear. Clinical trials with these mixed beta-agonists showed marginal antiobesity effects when caloric intake of subjects was restricted. Insulin sensitivity was also improved in some of the trials designed to test the antidiabetic activity of these compounds. Side effects included tachycardia and tremor. To eliminate these side effects, a second generation of compounds was selected for its agonist activity on rat beta3-receptors. Clinical trials with these compounds have shown little increase of energy expenditure even at high doses. Successful development of an antiobesity and antidiabetic drug from this class of compounds will require the elucidation of the physiological role of the human beta3-receptor and the regulatory mechanism between fuel efficiency and feeding behavior.

β3-adrenergic agonists: potential therapeutics for obesity

During the last decade a number of beta3-adrenergic receptor agonists have been advanced to clinical trials. The results of human studies to date have been disappointing with respect to sustained increases in metabolic rate and weight loss. Cloning of beta3-adrenoceptors in a number of species and subsequent pharmacological evaluations revealed these early investigational drugs to be weakly active against the human receptor, suggesting an explanation for the poor performance of these compounds in clinical trials. This information has been integrated into subsequent research efforts resulting in the discovery of agents with activities optimised for the human receptor. This new generation of compounds is in late preclinical/early clinical development and are poised to address the role beta3-adrenoceptor signalling plays in the obese state. Issues related to the potential for beta3-adrenergic agonists to positively impact metabolic parameters in humans are also discussed.

Response to intra- and extracellular lipolytic agents and hormone-sensitive lipase translocation are impaired in adipocytes from rats adapted to a high-protein, carbohydrate-free diet

We showed previously that rats adapted to a high-protein (70%), carbohydrate-free (HP) diet have reduced lipolytic activity. To clarify the underlying biochemical mechanisms, several metabolic processes involved in adipose tissue lipolysis were investigated. The experiments were performed in rats adapted for 15 d to an HP or a balanced diet. In agreement with previous results, microdialysis experiments showed that the concentrations of adipose tissue interstitial and arterial plasma glycerol were lower in rats adapted to the HP diet. Under nonstimulated conditions, rates of lipolysis, estimated by glycerol release to the incubation medium, were reduced in adipocytes from HP rats. Under the same conditions, there was a small, but significant (17%) reduction in the activity of hormone sensitive lipase (HSL), with no change in the content of the enzyme. Upon stimulation with isoproterenol, the percentage of the enzyme in the adipocyte cytosol translocated to the fat droplet was 20-25%in HP rats and 40-50% in rats fed the balanced diet. Adipocytes from HP diet-adapted rats had a significantly reduced response (approximately 40%) to the lipolytic action of nonspecific (norepinephrine, epinephrine, isoproterenol) and specific (CL316,243, BRL37,344, dobutamine, clenbuterol) beta-adrenergic agonists. Adipocytes from HP rats also had a reduced lipolytic response to the intracellular agents, dibutyryl cAMP (44%), forskolin (46%), and isobutyl-methylxanthine (29%). The data suggest that the main mechanism responsible for the reduced basal and stimulated lipolysis in HP diet-adapted rats is an impairment in the intracellular process of lipolysis activation, with a deficient translocation of HSL to the fat droplet.

Genetic vulnerability to diet-induced obesity in the C57BL/6J mouse: physiological and molecular characteristics

The development of the metabolic syndrome in an increasing percentage of the populations of Western societies, particularly in the United States, requires valid models for establishing basic biochemical changes and performing preclinical studies on potential drug targets. The C57BL/6J mouse has become an important model for understanding the interplay between genetic background and environmental challenges such as high-fat/high-calorie diets that predispose to the development of the metabolic syndrome. This review highlights metabolic and signal transduction features that are altered during the course of disease progression, many of which mirror the human situation.

First-phase insulin secretion: does it exist in real life? Considerations on shape and function

To fulfill its preeminent function of regulating glucose metabolism, insulin secretion must not only be quantitatively appropriate but also have qualitative, dynamic properties that optimize insulin action on target tissues. This review focuses on the importance of the first-phase insulin secretion to glucose metabolism and attempts to illustrate the relationships between the first-phase insulin response to an intravenous glucose challenge and the early insulin response following glucose ingestion. A clear-cut first phase occurs only when the beta-cell is exposed to a rapidly changing glucose stimulus, like the one induced by a brisk intravenous glucose administration. In contrast, peripheral insulin concentration following glucose ingestion does not bear any clear sign of biphasic shape. Coupling data from the literature with the results of a beta-cell model simulation, a close relationship between the first-phase insulin response to intravenous glucose and the early insulin response to glucose ingestion emerges. It appears that the same ability of the beta-cell to produce a pronounced first phase in response to an intravenous glucose challenge can generate a rapidly increasing early phase in response to the blood glucose profile following glucose ingestion. This early insulin response to glucose is enhanced by the concomitant action of incretins and neural responses to nutrient ingestion. Thus, under physiological circumstances, the key feature of the early insulin response seems to be the ability to generate a rapidly increasing insulin profile. This notion is corroborated by recent experimental evidence that the early insulin response, when assessed at the portal level with a frequent sampling, displays a pulsatile nature. Thus, even though the classical first phase does not exist under physiological conditions, the oscillatory behavior identified at the portal level does serve the purpose of rapidly exposing the liver to elevated insulin levels that, also in virtue of their up-and-down pattern, are particularly effective in restraining hepatic glucose production.

Beta-adrenoceptors mediate inhibition of lipolysis in adipocytes of tilapia (Oreochromis mossambicus)

The regulation of triglyceride mobilization by catecholamines was investigated in the teleost fish Oreochromis mossambicus (tilapia) in vivo and in vitro. In vitro experiments were carried out with adipocytes that were isolated for the first time from fish adipose tissue. For the in vivo experiments, cannulated tilapia were exposed to stepwise decreasing oxygen levels (20, 10, and 5% air saturation; 3.9, 1.9, and 1.0 kPa PO(2), respectively), each level being maintained for 2 h. Blood samples were taken at timed intervals and analyzed for plasma lactate, glucose, free fatty acids, epinephrine, norepinephrine, and cortisol. Hypoxia exposure did not change plasma epinephrine levels. In contrast, the plasma norepinephrine concentration markedly increased at all hypoxia levels. Over the same period, plasma free fatty acid levels showed a significant continuous decrease, suggesting that norepinephrine is responsible for the reduced plasma free fatty acid concentration, presumably through inhibition of lipolysis in adipose tissue. To elucidate the mechanism, adipocytes were isolated from mesenteric adipose tissue of tilapia and incubated with 1) norepinephrine, 2) norepinephrine + phentolamine (alpha(1),alpha(2)-antagonist), 3) isoproterenol (nonselective beta-agonist), 4) isoproterenol + timolol (beta(1),beta(2)-antagonist), 5) norepinephrine + timolol, and 6) BRL-35135A (beta(3)-agonist). The results demonstrate for the first time that norepinephrine and isoproterenol suppress lipolysis in isolated adipocytes of tilapia. The effect of norepinephrine is not mediated through alpha(2)-adrenoceptors but, like isoproterenol, via beta-adrenoceptors. Furthermore, this study provides strong indications that beta(3)-adrenoceptors are involved.

Lipolytic effect of BRL 35 135, a beta3 agonist, and its interaction with dietary lipids on the accumulation of fats in rat body

The type of intaked fat and fat uptake mechanisms such as adrenergic-induced lipolysis affect patterns of fat accumulation in animal body. In this study, in vitro lipolytic effect of BRL 35135, a selectivebeta3 agonist, and its interaction with different dietary fats on fat accumulation in animal body (in vivo) were studied. For in vitro study, adipocytes isolated from epididymal fat were incubated with 10(-5) M -10(-9) M of either BRL 35135 or isoproterenol, a non-selectivebeta-agonist. In animal study, two groups of SD-rats, i.e., BRL35135-intaked (dosed at 0.5 mg/kg/day in diet) and control, were divided into 4 sub-groups and fed diets containing 12% of either beef tallow (BT), canola oil (CO), olive oil (OO) or safflower oil(SO) for 6 weeks. In vitro study showed that BRL 35135 was 10 times more potent than isoproterenol in increasing the lipolysis in rat adipocytes. In animal study, inclusion of BRL35135 reduced daily weight gain in CO and SO groups (P < 0.05). Abdominal fat weight in BRL35135-intaked group was significantly lower than control in all dietary sub-groups (CO, OO and SO) except BT (P < 0.05). In BT group, abdominal fat contained significantly higher amount of total saturated fatty acids (SFAs) compared to CO, OO or SO. It was concluded that, although BRL 35135 was very potent in increasing lipolysis in the isolated adipocytes of rat, its preventive effect on lipid accumulation in animal body through the lipolysis could be affected by the type of dietary fat and was lesser when rats fed fats rich in SFAs.

(+/-)-Pindolol acts as a partial agonist at atypical beta-adrenoceptors in the guinea pig duodenum

The agonistic and antagonistic effects of (+/-)-pindolol (1-(1H-indol-4-yloxy)-3-[(1-methylethyl)amino]-2-propanol) were estimated to clarify whether (+/-)-pindolol acts as a partial agonist on atypical beta-adrenoceptors in the guinea pig duodenum. (+/-)-Pindolol induced concentration-dependent relaxation with a pD2 value of 5.10 +/- 0.03 and an intrinsic activity of 0.83 +/- 0.03. However, the relaxations to (+/-)-pindolol were not antagonized by the non-selective beta1- and beta2-adrenoceptor antagonist (+/-)-propranolol (1 microM). In the presence of (+/-)-propranolol (1 microM), the non-selective beta1-, beta2- and beta3-adrenoceptor antagonist (+/-)-bupranolol (30 microM) induced a rightward shift of the concentration-response curves for (+/-)-pindolol (apparent pA2 = 5.41 +/- 0.06). In the presence of (+/-)-propranolol, (+/-)-pindolol (10 microM) weakly but significantly antagonized the relaxant effects to catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline), a selective beta3-adrenoceptor agonist BRL37344 ((R*,R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl) amino]propyl]phenoxyacetic acid sodium salt) and a non-conventional partial beta3-adrenoceptor agonist (+/-)-CGP12177A([4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one] hydrochloride). These results demonstrate that (+/-)-pindolol possesses both agonistic and antagonistic effects on atypical beta-adrenoceptors in the guinea pig duodenum.

Sustained improvement in glucose homeostasis in lean and obese mice following chronic administration of the beta 3 agonist SR 58611A

1. Acute SR 58611A (0.25 mg kg-1), was effective in reducing the blood glucose response to a glucose tolerance test (GTT) in normal lean (control) and spontaneously obese/diabetic CBA/Ca mice and to be equipotent to 1.25 mg kg-1 glibenclamide in lean mice. 2. Neither brown (BAT) nor white (WAT) adipose tissue lipogenesis was altered by acute SR 58611A (2 - 8 mg kg-1) in lean mice, but both increased significantly at the higher doses in the obese mice. 3. Acute SR 58611A produced a hypoglycaemia 40 min after dosing in lean and obese animals, the duration and potency of which was less than that of glibenclamide. Plasma insulin levels increased 20 min after acute SR 58611A and glibenclamide in lean and obese mice. 4. Chronic treatment (0.25 mg kg-1, 15 days) with SR 58611A increased its effectiveness in improving glucose tolerance, but did not affect the body weight (BW) or food intake of either lean or obese mice. 5. Acute and chronic SR 58611A prolonged the hypoglycaemic effect of exogenous insulin in lean but not obese mice. 6. In fed and fasted lean mice and in fasted obese mice chronic SR 58611A produced an acute hypoglycaemia 30 min post administration which was greater than after a single dose. 7. SR 58611A maintained its effectiveness in improving glucose tolerance in lean and obese mice over a dosing period of 15 days. The improvement in glucose tolerance was achieved at a dose less than that required to stimulate adipose tissue lipogenesis and which did not affect food intake or body weight.

Differential regulation of beta3-adrenoceptors in gut and adipose tissue of genetically obese (ob/ob) C57BL/6J-mice

1. Levels of beta3-adrenoceptor (AR) mRNA were compared using reverse transcription-polymerase chain reaction (RT PCR) in white adipose tissue (WAT), brown adipose tissue (BAT), ileum and colon from genetically obese (ob/ob) and lean (+/+) C57BL/6J mice. Functional responses to the beta3-AR agonist CL 316243 were also characterized in ileal longitudinal smooth muscle from obese and lean mice. 2. Beta3-AR mRNA levels were significantly higher in WAT (100+/-16%) and BAT (100+/-13%) from lean compared to WAT (21.0+/-0.9%; n=4; P<0.005) and BAT (14.1+/-2.2%; n=5; P<0.01) from obese mice. In contrast, beta3-mRNA levels were not significantly different in ileum (100+/-15%) and colon (100+/-22%) from lean mice, compared to ileum (78+/-13%; n=4; P= 0.31) or colon (82+/-15%; n =4; P=0.52) from obese mice. 3. Concentration-response curves to CL 316243 did not differ significantly in slope or position in ileal longitudinal smooth muscle from obese or lean mice. pEC50 (+/-s.e.mean) values were not significantly different (P= 0.59) between obese (7.90+/-0.13, n = 7) and lean (7.77+/-0.20, n = 7) mice. 4. pKB values for the beta1-AR and beta2-AR selective antagonist propranolol or the beta3-AR selective antagonist SR 58894 against relaxations to CL 316243 were similar in ileum of genetically obese (propranolol 6.31+/-0.22 and 6.13+/-0.12; SR 58894 8.22+/-0.06) and lean mice (propranolol 6.40+/-0.08 and 6.60+/-0.13; SR 58894 8.27+/-0.12) and were consistent with values previously found at beta3-AR. 5. Treatment of lean C57BL/6J mice with dexamethasone (1 mg kg(-1), i.p.) significantly reduced beta3-AR mRNA levels after 4 h in WAT (100+/-6.1 to 41.4+/-4.3; n= 16 18; P<0.0001) and BAT (100+/-8.0 to 35.1+/-5.8; n= 17; P<0.0001), but caused no change in ileum (100+/-6.1 to 101+/-17; n= 10-11; P=0.95) or colon (100+/-11 to 101+/-11; n= 11; P= 0.94). Beta3-mRNA levels in ileum and colon also did not change significantly when examined over 24 h or after the administration of a higher dose of dexamethasone (5 mg kg(-1)). 6. In summary, beta3-AR mRNA levels were considerably lower in WAT and BAT of obese compared to lean mice whereas the levels in ileum and colon were not significantly different. The similar beta3-mRNA levels in ileum of obese and lean mice were associated with indistinguishable responses of carbachol-contracted ileum to a beta3-agonist and similar affinity for beta-antagonists. Administration of glucocorticoids to lean mice reduced beta3-AR mRNA levels in WAT and BAT but not in ileum or colon. These studies show that in mice, beta3-ARs are differentially regulated in ileum and colon compared to adipose tissues.

Beta3-adrenoceptor mechanisms in guinea-pig taenia caecum

Beta-adrenoceptor-mediated relaxation of guinea-pig taenia caecum was investigated by studying the effects of BRL37344, CGP12177 and norepinephrine. These drugs caused graded relaxation of the guinea-pig taenia caecum. The concentration-response curves for these drugs were unaffected by propranolol, atenolol, butoxamine, prazosin, yohimbine and phentolamine. Bupranolol produced shifts of the concentration-response curves for these drugs. Schild regression analyses carried out for bupranolol against BRL37344, CGP12177 and norepinephrine gave pA2 values of 5.79, 5.61 and 5.53, respectively. CGP12177 and norepinephrine significantly increased cyclic AMP levels in this preparation. Bupranolol significantly decreased cyclic AMP levels elicited by CGP12177 and norepinephrine, whereas propranolol produced no effect. These results suggest that the relaxant responses to BRL37344, CGP12177 and norepinephrine in the guinea-pig taenia caecum are mediated by beta3-adrenoceptors.

Comparison of atypical beta3-adrenoceptor agonists with their respective metabolic activities in rat white adipocytes

The metabolic activities of four types of beta3-adrenoceptor (AR) agonists, BRL35135A, BRL28410, ICI215001 and CL316243, were compared with those of other beta1- and beta2-AR agonists in rat white adipocytes. All the beta3-AR agonists caused cAMP formation, free fatty acid release and 2-deoxyglucose uptake; the maximum activity levels were similar except for ICI215001, which was lower. However, the magnitude of potency and selectivity of these agonists differed. The most potent and selective beta3-agonist was CL316243. Metabolic activities and Northern blotting showed that there were three beta-AR subtypes that were coupled to adenylyl cyclase and contributed to the induction of lipolysis and glucose uptake. The rank order of the amounts of beta-AR subtypes was beta3 >>beta1> beta2. However, the physiological functions of beta-AR subtypes were essentially similar in rat white adipocytes. On the other hand, cAMP accumulation and Northern blotting showed that human adipocytes predominantly contained beta2-AR, with far lower levels of beta1- and beta3-ARs. These findings suggested that the beta3-AR plays an important role in energy metabolism and thermogenesis in which cross talk exists between beta1- and beta3-ARs in rat adipocytes, while beta2-AR is the most important for the lipolysis regulation in human subcutaneous adipocytes.

Hyperinsulinemia and cardiovascular disease in elderly men: the Honolulu Heart Program

Hyperinsulinemia has been associated with cardiovascular disease (CVD), but whether this relation is independent of other CVD risk factors is uncertain. Most studies have focused on coronary heart disease (CHD), but few have included peripheral vascular disease (PVD) and stroke. Moreover, evidence in elderly and minority populations is limited. Between 1991 and 1993, 3562 elderly (71 to 93 years) Japanese-American men from the Honolulu Heart Program were examined and had fasting insulin levels measured. Hyperinsulinemia, defined as a fasting insulin > or =95th percentile among nonobese men with normal glucose tolerance and no diabetic history or medication use, was observed in 22% of the population. Subjects with hyperinsulinemia had a more adverse CVD risk factor profile and had higher age-adjusted prevalences of CHD, angina, PVD, thromboembolic stroke, and hemorrhagic stroke compared with those without hyperinsulinemia. Age-adjusted fasting insulin levels but not 2-hour levels were also significantly elevated (P<.01) in those with prevalent CVD compared with those without. In logistic regression analyses, adjustment for multiple CVD risk factors attenuated the relations of hyperinsulinemia with CHD, angina, and PVD to nonsignificant levels, whereas those involving thromboembolic and hemorrhagic stroke were strengthened and remained significant (odds ratios=2.27 and 7.53, 95% confidence intervals=1.25 to 4.13 and 1.65 to 34.25, respectively). When multivariate analyses were restricted to nondiabetic subjects, associations were slightly weaker and in general nonsignificant. Nondiabetic men with thromboembolic stroke were twice as likely to have hyperinsulinemia as those who were stroke-free, although this association was of borderline significance (odds ratio= 1.99, 95% confidence interval=0.95 to 4.17, P=.069). In subjects with elevated total cholesterol levels, somewhat stronger associations were observed for PVD and stroke but not for CHD. Although further prospective studies are indicated, particularly for PVD and stroke, these cross-sectional results are consistent with an indirect role for insulin in CVD, wherein hyperinsulinemia or an underlying insulin-resistant state may adversely affect other CVD risk factors or serve as a marker for an atherogenic or thrombogenic state.

Lipolytic effects of conventional beta 3-adrenoceptor agonists and of CGP 12,177 in rat and human fat cells: preliminary pharmacological evidence for a putative beta 4-adrenoceptor

1. The nature of rat and human fat cell beta 3-adrenoceptors was investigated by studying the effects of the new beta 3-adrenoceptor selective antagonist, SR 59,230A, on lipolysis induced by the conventional beta 3-adrenoceptor agonists, CL 316,243 and SR 58,611A, and by the non-conventional partial beta 3-adrenoceptor agonist CGP 12,177 (a potent beta 1- and beta 2-adrenoceptor antagonist with partial beta 3-adrenoceptor agonist property). 2. In rat fat cells, the rank order of potency of agonists was: CL 316,243 > isoprenaline > SR 58,611A > CGP 12,177. The three former agents were full agonists whereas CGP 12,177 was a partial agonist (intrinsic activity of 0.70). In human fat cells, the lipolytic effect of CGP 12,177 reached 25% of isoprenaline effect. CL 316,243 was a poor inducer of lipolysis and SR 58,611A was ineffective. 3. In rat fat cells, lipolysis induced by CL 316,243 and SR 58,611A was competitively antagonized by SR 59,230A. Schild plots were linear with pA2 value of 6.89 and 6.37, respectively. Conversely, 0.1, 0.5 and 1 microM SR 59,230A did not modify the concentration-response curve of CGP 12,177. A rightward shift of the curve was however observed with 10 and 100 microM of SR 59,230A. The apparent pA2 value was 5.65. The non-selective beta-adrenergic antagonist, bupranolol, competitively displaced the concentration-response curve of CGP 12,177 and CL 316,243. Schild plots were linear with pA2 values of 6.70 and 7.59, respectively. CL316,243-mediated lipolytic effect was not antagonized by CGP 20,712A. In human fat cells, CGP 12,177-mediated lipolytic effect was antagonized by bupranolol and CGP 20,712A. SR 59,230A (0.1, 1 and 10 microM) did not modify the concentration-response curve of CGP 12,177. A rightward shift was however observed at 100 microM leading to an apparent pA2 value of 4.32. 4. The results suggest that the non-conventional partial agonist CGP 12,177 can activate lipolysis in fat cells through the interaction with a beta-adrenoceptor pharmacologically distinct from the beta 3-adrenoceptor, i.e. through a putative beta 4-adrenoceptor. They suggest that the two subtypes coexist in rat fat cells whereas only the putative beta 4-adrenoceptor mediates lipolytic effect of CGP12,177 in human fat cells.

Beta3-adrenoceptors mediate relaxation of guinea pig taenia caecum by BRL37344A and BRL35135A

Beta-adrenoceptor-mediated relaxation of guinea pig taenia caecum was investigated by studying the effects of the beta3-adrenoceptor agonists, BRL37344A [(R*,R*)-(+/-)-4-[2'-[2-hydroxy-2-(3-chlorophenyl) ethylamino] propyl] phenoxyacetic acid sodium salt sesquihydrate] and BRL35135A [(R*,R*)-(+/-)-methyl-4-[2-[2-hydroxy-2-(3-chlorophenyl) ethylamine] propyl] phenoxyacetate hydrobromide]. BRL37344A and BRL35135A caused dose-dependent relaxation of the guinea pig taenia caecum. The concentration-response curves for BRL37344A and BRL35135A were unaffected by propranolol, ICI118551 [erythro-1-(7-methylindan-4-yloxy)-3-(isopropylamine)-but an-2-ol], atenolol, butoxamine, prazosin, yohimbine and phentolamine. Bupranolol produced shifts of the concentration-response curves for BRL37344A and BRL35135A. Schild regression analyses carried out for bupranolol against BRL37344A and BRL35135A gave pA2 values of 5.79 and 5.84, respectively. These results suggest that the relaxant response to BRL37344A and BRL35135A of the guinea pig taenia caecum is mediated by beta3-adrenoceptors.

Beta-adrenoceptor subtype expression and function in rat white adipocytes

1. The pharmacological features of rat white adipocyte beta-adrenoceptor subtypes were investigated by saturation and beta-agonist competition studies with [3H]-CGP 12177 and by lipolysis induced by beta-agonists as well as their inhibition by CGP 20712A (selective beta 1-antagonist) and ICI 118551 (selective beta 2-antagonist) in an attempt to establish a relationship between the functionality and binding capacity of beta-adrenoceptor subtypes. 2. Two populations of binding sites were identified on adipocyte membranes, one with high affinity (0.22 +/- 0.07 nM) and the other with low affinity (23 +/- 7 nM). The low affinity binding sites constituted 90% of the total binding sites. 3. The competition curves, with 15 nM [3H]-CGP 12177, for the beta-agonists, isoprenaline (Iso), noradrenaline (NA) and adrenaline (Ad), and the selective beta 3-agonist, BRL 37344 (BRL), were clearly biphasic (P < 0.001). The rank orders of agonist potency (pKi) in competing for [3H]-CGP 12177 high affinity and low affinity binding sites, respectively, were Iso (9.28 +/- 0.24) > NA (8.90 +/- 0.12) > Ad (8.65 +/- 0.12) > > BRL (4.53 +/- 0.17) and BRL (7.38 +/- 0.19) > > Iso (2.96 +/- 0.26) > or = NA (2.80 +/- 0.17) > Ad (2.10 +/- 0.11) indicating the expression of beta 1- and beta 3-adrenoceptor subtypes on rat white adipocytes, respectively. Inversely, competition studies with the selective beta 1-agonist, xamoterol (Xam), provided evidence for a single homogeneous population of binding sites with low density (81 +/- 9 fmol mg-1) and high pKi value (7.23 +/- 0.26) confirming the presence of beta 1-adrenoceptors. 4. To assess a possible contribution of the beta 2-subtype, procaterol (Proc), a selective beta 2-agonist, was used to compete with 2 nM [3H]-CGP 12177. A single low affinity (4.61 +/- 0.07) population of binding sites was identified. The density of these sites (71 +/- 12 fmol mg-1) was similar to the one obtained with Xam, suggesting that Proc displaced [3H]-CGP 12177 from the beta 1-subtype. 5. The functional potency (pD2) order with BRL (9.07 +/- 0.20) and catecholamines (Iso: 7.26 +/- 0.06, NA: 6.89 +/- 0.02 and Ad: 6.32 +/- 0.07) was the same as that found for the low affinity binding sites in competition studies. Xam induced lipolysis with greater potency than dobutamine (Dob), 6.31 +/- 0.06 and 5.66 +/- 0.10, respectively. Proc stimulated lipolysis with a low potency (5.59 +/- 0.21). 6. The lipolytic response to 0.001 microM BRL was inhibited by both, selective beta 1- and beta 2-antagonist, in a monophasic manner with low potencies (CGP 20712A pKi: < 4.5 and ICI 118551 pKi: 5.57 +/- 0.13). Similar monophasic profiles were obtained for inhibition of Xam- and Dob-induced lipolysis. In this case, CGP 20712A was more potent (> 10 times) than ICI 118551. The monophasic inhibition was also observed with ICI 118551 in the presence of 0.05 microM Iso or 0.13 microM NA. In contrast, two populations of sites were identified with CGP 20712A in the presence of Iso as well as NA. The pKi values for the first sites were 8.41 +/- 0.09 and 8.58 +/- 0.17, respectively, and for the second population of sites 4.73 +/- 0.22 and 4.27 +/- 0.27, respectively. The proportion of the first sites was low: 19 +/- 4 and 22 +/- 5%, respectively. Biphasic curves were obtained with both antagonists using 2.5 microM Proc (CGP 20712A: pKi1: 8.17 +/- 0.08, site1: 23 +/- 6%, pKi2: 4.77 +/- 0.14; ICI 118551: pKi1: 7.78 +/- 0.03, site1: 37 +/- 2%, pKi2: 5.35 +/- 0.25). 7. Our results show that the radioligand [3H]-CGP 12177 allows the characterization of beta 1- and beta 3-adrenoceptor subtypes on rat white adipocytes. Lipolysis is highly dependent on beta 1- and beta 3-adrenoceptors. Finally, binding and functional studies confirm that lipolysis is mainly driven by the beta 3-subtype.

Agonist and antagonist properties of beta 3-adrenoceptors in human omental and mouse 3T3-L1 adipocytes

The pharmacological properties of the native human beta 3-adrenoceptor are poorly defined. In the present study, the agonist and antagonist properties of beta 3-adrenoceptors in human omental and mouse 3T3-L1 adipocytes were compared by measuring lipolysis in the absence or presence of adrenoceptor blockers. Methodological experiments revealed that all three beta-adrenoceptors were functionally expressed in both types of adipocytes. This makes the human and the mouse cells directly comparable in pharmacological studies. CGP 12177 was a selective partial beta 3-adrenoceptor agonist in both cell types with a pD(2) of about 7.5. The order of potency of classical non-selective adrenoceptor agonists, when determined during blockade of beta 1-, beta 2- and alpha 2-adrenoceptors, was isoprenaline>noradrenaline>adrenaline in both human and 3T3-L1 adipocytes. This is different from the order of potency of the same agonists at the beta 1- or beta 2-adrenoceptors. The sensitivity of the beta 3-adrenoceptor to these catecholamines, expressed as pD(2) values, were virtually identical in both adipocyte types. Isoprenaline, noradrenaline, and adrenaline were almost full agonists in both cell types (intrinsic activity from 74% or 95%) during combined beta 1, beta 2- and alpha 2-adrenoceptor blockade. Antagonist potencies (expressed as pA(2) and using CGP 12177 as agonist) at the alpha 3-adrenoceptor were similar in both adipocyte types: bupranolol>propranolol>metoprolol. The corresponding pA(2) values for bupranolol, propanolol and metoprolol were about 7, 6 and 5, respectively in both species. In conclusion, the pharmacological properties of classical catecholamines, beta-adrenoceptor blockers and CGP 12177 are almost identical at the beta 3-adrenoceptors of human omental adipocytes and 3T3-L1 adipocytes.

In situ assessment of the role of the beta 1-, beta 2- and beta 3-adrenoceptors in the control of lipolysis and nutritive blood flow in human subcutaneous adipose tissue

1. The involvement of beta 1-, beta 2- and beta 3-adrenoceptors in the control of lipolysis and nutritive blood flow was investigated in abdominal subcutaneous adipose tissue of healthy young adults by use of an in situ microdialysis technique. 2. Dialysis probes were infused either with isoprenaline (non-selective beta-adrenoceptor agonist), CGP 12,177 (selective beta 3-adrenoceptor agonist having beta 1-/beta 2-antagonist properties), dobutamine (selective beta 1-adrenoceptor agonist) or terbutaline (selective beta 2-adrenoceptor agonist). The recovery of each probe used for perfusion was calculated by an in vivo calibration method. The local blood flow was estimated through the measurement of the escape of ethanol infused simultaneously with the drugs included in the probe. 3. Isoprenaline infusion at 0.01 microM had a weak effect while higher concentrations of isoprenaline (0.1 and 1 microM) caused a rapid, sustained and concentration-dependent increase of glycerol outflow; the maximum increase was 306 +/- 34% with 1 microM. Isoprenaline also increased the nutritive blood flow in adipose tissue; a significant effect appeared at 0.1 microM isoprenaline and was greater at 1 microM. 4. CGP 12,177 (10 and 100 microM) increased the glycerol concentration in the dialysate (128 +/- 8 and 149 +/- 12%, respectively) and nutritive blood flow. Terbutaline and dobutamine (100 microM) both provoked rapid and similar increases in glycerol outflow (252 +/- 18 and 249 +/- 18%, respectively). Both, terbutaline and dobutamine increased nutritive blood flow. 5. It is concluded that beta 1- and beta 2-adrenoceptor subtypes are both mainly involved in the mobilization of lipids and in the control of nutritive blood flow. beta 3-Adrenoceptors play a weaker role in the control of lipolysis and nutritive blood flow in human subcutaneous abdominal adipose tissue.

Adrenoceptor heterogeneity in human white adipocytes differentiated in culture as assessed by cytosolic free calcium measurements

Changes in intracellular calcium concentration [Ca2+]i in response to norepinephrine (NE) and to various adrenergic agonists were monitored by dual excitation microfluorimetry in single human adipocytes differentiated in culture and loaded with fura-2 acetoxymethyl ester (fura-2 AM). The addition of NE elicited increases in [Ca2+]i that were depending on the cell, (1) either rapid (time to peak: 9 +/- 3 s), large, and transient; or (2) slow (time to peak: 125 +/- 8 s), small, and sustained. The rapid and large [Ca+]i response, which was inhibited by 90% by the alpha 1-antagonist prazosin and only by 20% by the non-specific beta antagonist (-)-propranolol, was considered to be mediated by the alpha 1-adrenoceptor. In fact, an alpha 1A-adrenoceptor was found to be expressed in human white adipose tissue. Consecutive additions of beta-agonists specific for each subtype of alpha-adrenoceptor enabled the characterization of four cell populations with different response patterns: 47% of the cells had alpha 1- and beta 1-, beta 2- and beta 3-induced [Ca2+]i responses; 29% had only beta 1-, beta 2-, beta 3-responses; 14% had alpha 1- and beta 3-responses, and 10% had only an alpha 1-response. Taken together, these results show that in differentiated human adipocytes: (1) alpha 1- and beta-adrenergic stimulations induce [Ca2+]i increases with different kinetics and amplitudes; (2) there is a beta 3-adrenergic response similar to the beta 1- or beta 2-adrenergic responses; and (3) there is a marked adrenoceptor heterogeneity.

The acid metabolite of ZD7114 is a partial agonist of lipolysis mediated by the rat beta 3-adrenoceptor

Experiments were performed to characterise the lipolytic effects of the acid metabolite, ZM215001, ((S)-4-[2-hydroxy-3-phenoxy-propylamino-ethoxy] phenoxyacetic acid) of the putative beta 3-adrenoceptor agonist, ZD7114 ((S)-4-[2-hydroxy-3-phenoxy-propylamino-ethoxy]-N-(2-methoxyethyl) phenoxyacetamide) on isolated rat white adipocytes. ZM215001 was used for these studies since it is the predominant moiety after in vivo administration of ZD7114. The agonist properties of ZM215001 were assessed in comparison to the standard nonselective beta-adrenoceptor agonist (+/-)-isoprenaline and the beta 3-adrenoceptor-selective agonist BRL 37344. Isoprenaline, BRL 37344 and ZM215001 all stimulated the rate of free fatty acid release from isolated adipocytes with the order of potency being BRL > isoprenaline > ZM215001. The maximum effect of BRL 37344 was equivalent to that of isoprenaline, but ZM215001 achieved only 30% of the maximum isoprenaline response. ZM215001 competitively antagonised the lipolytic response to BRL 37344 (pA2 = 7.26), whereas the agonist effects of BRL 37344 were not antagonised competitively by the selective antagonists ICI 118551 and CGP 20712A, at concentrations which would be expected to block beta 1- and beta 2-adrenoceptors respectively. These results indicate that ZM215001 has low intrinsic activity at the rat adipocyte beta 3-adrenoceptor, and is a partial agonist of lipolysis in rat white adipocytes.

Adrenergic receptors and fat cells: differential recruitment by physiological amines and homologous regulation

The control of fat cell lipolysis by the catecholamines involves at least four different adrenoceptor subtypes; three beta (beta 1-, beta 2-, and beta 3-ARs) and one alpha 2-adrenoceptor (alpha 2-AR). The physiological importance of the beta- and alpha 2A-ARs varies according to the species, the sex, the age, the anatomical location of fat deposits and the degree of obesity in humans and animals. The physiological amines operate through differential recruitment of these sites on the basis of their relative affinities. This point has been assessed by in vitro studies and has partly been confirmed in in vivo experiments using selected alpha/beta-AR antagonists and in situ microdialysis. The affinity of the beta 3-AR for catecholamines is less than that of the classical beta 1- and beta 2-ARs in the various species investigated. Conversely, it is the alpha 2-AR which exhibit the highest affinity for the physiological amines in all fat cells. The relative order of affinity of the various fat cell ARs for the physiological amines defined in binding studies and in vitro assays is alpha 2 > beta 1 > or = beta 2 > beta 3 for norepinephrine and alpha 2 > beta 2 > beta 1 > beta 3 for epinephrine. When considering differential beta-AR recruitment by catecholamines, it is the beta 1-AR which is always activated at the lowest norepinephrine levels, whatever the species, while the activation of the beta 3-AR requires higher norepinephrine levels. In addition to the differential recruitment, differential regulation by hormones could also occur for each fat cell AR subtype. The alpha 2-and beta 3-ARs are less prone to desensitization and down-regulation by comparison with the beta 1- and beta 2-AR.

A novel adrenaline derivative, AZ002, and its hypoglycemic action in yellow KK mice

AZ002 (L-threo-(3,4-dihydroxy phenyl)-N-methyl serine methyl ester) is a newly synthesized adrenaline derivative. AZ002 caused relaxation of rat jejunum (beta 3-receptors) (ED50 = 18 microM), but did not affect the atrial rate (beta 1) or tracheal relaxation (beta 2) at a concentration of 0.3 mM. The pA2 values for propranolol in inhibiting the isoproterenol- and AZ002-stimulated relaxation of rat jejunum were 6.27 and 6.33, respectively. Thus, AZ002 is a selective agonist for beta 3-adrenoceptor. AZ002 stimulated lipolysis (ED50 = 10 microM) and glucose uptake (ED50 = 1 microM) in rat adipocytes. In both cases, stimulation was antagonized by high concentrations of the beta-adrenoceptor antagonist propranolol, but not by the alpha-adrenoceptor antagonist phentolamine. The effect of AZ002 on glucose uptake was synergistic with that of insulin. AZ002 was also assessed in vivo by using genetically obese mice (KK/Ay strain) with hyperglycemia. Administration of AZ002 in the diet for a week decreased blood glucose and non-esterified fatty acids.

Demonstration of an in vivo functional beta 3-adrenoceptor in man

Although it is well established in several mammalian species that beta 3-adrenoceptors play a major role in regulating lipolysis and thermogenesis in adipose tissue, the functional existence and role of this receptor subtype in man has been controversial. We investigated whether the beta 3-adrenoceptor functionally co-exists with beta 1- and beta 2-adrenoceptors in vivo in human adipose tissue. Subcutaneous abdominal adipose tissue of healthy non-obese subjects was microdialyzed with equimolar concentrations of dobutamine (selective beta 1-adrenoceptor agonist), terbutaline (selective beta 2-adrenoceptor agonist), or CGP 12177 (selective beta 3-adrenoceptor agonist). All three agents caused a rapid, sustained, concentration-dependent and significant elevation of the glycerol level in the microdialysate (lipolysis index). However, only terbutaline stimulated the nutritive blood flow in adipose tissue, as measured by an ethanol escape technique. Dobutamine and CGP 12177 was equally effective in elevating the glycerol level (maximum effect 150% above baseline). Terbutaline was significantly more effective than the other two beta-agonists (maximum effect 200% above baseline). When adipose tissue was pretreated with the beta 1/beta 2-selective adrenoceptor blocker propranolol the glycerol increasing effect of dobutamine or terbutaline was inhibited by 80-85% but the glycerol response to CGP 12177 was not influenced. It is concluded that a functional beta 3-adrenoceptor is present in vivo in man. It co-exists with beta 1- and beta 2-adrenoceptors in adipose tissue and may therefore play a role in lipolysis regulation. It appears, however, that the beta 2-adrenoceptor is the most important beta-adrenoceptor subtype for the mobilization of lipids from abdominal subcutaneous adipose tissue because of its concomitant stimulatory effect on lipolysis and blood flow.

Desensitization of beta-adrenergic responses in adipocytes involves receptor subtypes and cAMP phosphodiesterase

Acute exposure of isolated adipocytes to isoproterenol induces the desensitization of lipolytic responses to norepinephrine and selective beta 1-, beta 2- and beta 3-adrenoceptor agonists, as well as the adrenocorticotropic hormone 1-24 fragment (ACTH). Forskolin and 8-bromo-cAMP responses are also desensitized. When lipolysis was measured in the presence of OPC 3911 [N-cyclohexyl-N-2-hydroxyethyl-4(6-(1,2-dihydro-2- oxoquinolyloxy))butyramide], a specific inhibitor of the cAMP phosphodiesterase of adipocytes, the desensitization of all lipolytic agents--except the beta 2-adrenoceptor agonist--was abolished. Isoproterenol induced a similar loss (35%) of both membrane beta 1- and beta 2-adrenoceptors and an uncoupling of beta 1-adrenoceptors, but did not modify the weak coupling of control beta 2-adrenoceptors. These data suggest that isoproterenol induced (i) an activation of the cAMP phosphodiesterase, which is solely responsible for the desensitization of norepinephrine response as well as beta 1- and beta 3-adrenoceptor mediated responses and (ii) an additional desensitization of the sole beta 2-adrenergic signaling system which suggests a subtype-selective pattern of regulating processes.

A pathogenic role of visceral fat beta 3-adrenoceptors in obesity

Increased release of free fatty acids (FFA) from visceral fat cells to the portal venous system may cause several metabolic disturbances in obesity. However, this hypothesis and the underlying mechanism remain to be demonstrated. In this study catecholamine-induced lipid mobilization through lipolysis in omental adipose tissue was investigated in vitro in 25 markedly obese subjects (body mass index range 35-56 kg/m2) undergoing weight reduction surgery and in 19 nonobese subjects (body mass index range 20-28 kg/m2) undergoing cholecystectomy. Release of FFA and glycerol, induced by norepinephrine or adrenergic receptor subtype-specific agonists, were determined in isolated omental fat cells. The obese subjects had higher fat cell volume, blood pressure, plasma insulin levels, blood glucose, plasma triglycerides, and plasma cholesterol than the controls. There was evidence of upper-body fat distribution in the obese group. The rate of FFA and glycerol response to norepinephrine was increased twofold in the cells of obese subjects; no significant reutilization of FFA during catecholamine-induced lipolysis was observed in any of the groups (glycerol/FFA ratio near 1:3). There were no differences in the lipolytic sensitivity to beta 3- or beta 2-adrenoceptor specific agonists between the two groups. However, beta 3-adrenoceptor sensitivity was approximately 50 times enhanced (P = 0.0001), and the coupling efficiency of these receptors was increased from 37 to 56% (P = 0.01) in obesity. Furthermore, the obese subjects demonstrated a sixfold lower alpha 2-adrenoceptor sensitivity (P = 0.04). beta 3-Adrenoceptor sensitivity, but not alpha 2-, beta 1-, or beta 2-adrenoceptor sensitivity, correlated with norepinephrine-induced lipolysis (r = -0.67, P = 0.0001) and fat cell volume (r = -0.71, P = 0.0001). In conclusion, catecholamine-induced rate of FFA mobilization from omental fat cells is accelerated due to elevated rate of lipolysis in obesity, mainly because of an increased beta 3-adrenoceptor function, but partly also because of a decreased alpha 2-adrenoceptor function. This promotes an increased release of FFA to the portal system, which may contribute to the parallel metabolic disturbances observed in upper-body obesity.

Potentiation of the anti-obesity effect of the selective beta 3-adrenoceptor agonist BRL 35135 in obese Zucker rats by exercise

1. The effects of chronic treatments with a selective beta 3-adrenoceptor agonist and a selective alpha 2-adrenoceptor antagonist and their interactions with physical exercise training were studied in experimental obesity. 2. BRL 35135 (beta 3-agonist, 0.5 mg kg-1 day-1 p.o.), atipamezole (alpha 2-antagonist, 4.0 mg kg-1 day-1 p.o.) and placebo were given to genetically obese male Zucker rats. Half of the rats were kept sedentary whereas the other half were subjected to moderate treadmill exercise training. Body weight gain, cumulative food intake, the neuropeptide Y content of the hypothalamic paraventricular nucleus, brown adipose tissue thermogenic activity (measured as GDP binding), plasma insulin and glucose levels were measured after 3 weeks' treatment and exercise. 3. Treatment with BRL 35135 reduced weight gain by 19%, increased brown adipose tissue thermogenic activity 45-fold and reduced plasma insulin by 50%. Atipamezole slightly increased food intake and neuropeptide Y content in the paraventricular hypothalamic nucleus but had no effect on the other measured parameters. Exercise alone had no effect on weight gain, food intake or thermogenic activity, whereas it reduced plasma insulin and glucose levels. 4. The effect of BRL 35135 on weight gain and thermogenic activity was significantly potentiated by exercise; the reduction in weight gain was 56% in comparison with 19% in sedentary animals. Food intake was significantly reduced in the BRL 35135-treated-exercise-trained animals, although neither beta 3-agonist nor exercise alone affected it. 5. Based on the present results in genetically obese Zucker rats, combination of 03-agonist treatment with a moderate physical training may offer a new feasible approach to the therapy of obesity.-

KEYWORDS

BRL 35135; atipamezole; P3-adrenoceptor agonism; M2-adrenoceptor antagonism; brown adipose tissue; thermogenesis;genetic obesity; Zucker rat; exercise; neuropeptide Y

Mediation of most atypical effects by species homologues of the beta 3-adrenoceptor

1. A wide panel of compounds acting on beta-adrenoceptors active either in mammalian heart or in rodent digestive tract and adipose tissues, were investigated for their effects on Chinese hamster ovary cells transfected with the human or murine beta 3-adrenoceptor gene. 2. The beta 3-agonists, bucindolol, CGP 12177A and pindolol exhibited the highest binding affinities; BRL 37344, LY 79771, ICI 201651 and SR 58611A presented high potencies in stimulating adenylyl cyclase; bupranolol appeared as the most efficient beta 3-antagonist. 3. This pharmacological analysis further established that the beta 3-adrenoceptor is the prototype of the adipose tissue atypical beta-adrenoceptor, since these receptors share a number of pharmacological properties which differ strikingly from those of beta 1- and beta 2-adrenoceptors: low affinities for conventional beta-adrenoceptor agonists and antagonists, high potencies for novel compounds active in adipose tissues, partial agonistic activities for several beta 1/beta 2-antagonists. 4. Although the pharmacological profiles of the human and murine beta 3-receptor were very similar, some quantitative or even qualitative differences were observed for particular compounds such as propranolol, which exhibited weak and partial agonistic effects at the human beta 3-receptors and antagonistic effects at the murine beta 3-receptors. These differences may result from key amino-acid substitutions between the human and the murine beta 3-receptor sequences, which may alter the binding site or signal processing. 5. Compounds active on atypical beta-sites of other tissues such as heart and digestive tract were also potent on the beta 3-adrenoceptor expressed in Chinese hamster ovary cells, suggesting that this receptor mediates most of the atypical properties described in various tissues, and that differences in ligand effects may result from tissue-related specificities.

Cardiac effects of the beta 3-adrenoceptor agonist BRL35135 in man

The aim of the present study was to evaluate the cardiac effects of the beta 3-adrenoceptor agonist BRL35135, and determine whether beta 3-receptors are involved in mediating chronotropic or inotropic responses in man. Eight normal males received single oral doses of BRL35135 8 mg (BRL) or the selective beta 2-adrenoceptor agonist salbutamol 8 mg (SAL), after pretreatment with either placebo (PL), bisoprolol 5 mg (B5) as a selective beta 1-adrenoceptor antagonist, or nadolol 20 mg (N20) to block beta 1- and beta 2- but not beta 3-receptors. Both BRL and SAL produced a significant increase in postural finger tremor in keeping with beta 2-adrenoceptor stimulation, and this response was totally abolished by pretreatment with N20. Significant increases in systolic blood pressure and Doppler stroke distance occurred with BRL and SAL which were unaffected by pretreatment with B5 and completely blocked by N20, in keeping with beta 2-mediated effects. BRL and SAL produced significant chronotropic and minute distance responses which were unaffected by beta 1-adrenoceptor blockade. However, whereas N20 blocked these responses to SAL, a small but significant response occurred with BRL in comparison with placebo despite complete blockade of co-existing beta 2-mediated effects. Compared with PL, the mean responses to N20/BRL, and the 95% confidence interval for the differences between the means were 7.4 beats min-1 [3.2 to 11.6] (P = 0.002) for heart rate, and 208.8 cm [38.3 to 379.3] (P = 0.02) for minute distance responses.(ABSTRACT TRUNCATED AT 250 WORDS)

No functional atypical beta-adrenergic receptors in human omental adipocytes

Isoproterenol stimulates lipolysis in human omental adipocytes with an EC50 (concentration at which an agonist produces half-maximal stimulation) of 120 nM. CGP12177 (dl-4-3[(1,1-dimethylethyl) amino]-2-hydroxylpropoxy]1,3-dihydro-2H-benzimidazol-2-one hydrochloride), a potent beta 1-/beta 2- adrenergic receptor (AR) antagonist but being an agonist for atypical beta-AR, fails to stimulate lipolysis in these cells, even at a concentration as high as 0.1 mM. Since CGP12177 is a partial agonist, its failure to stimulate lipolysis may result from a poor stimulus-response coupling, so that it can not be excluded that atypical beta-AR are actually present and even functional in these cells. To evaluate this hypothesis, we estimated the potency of CGP12177 to inhibit the isoproterenol-stimulated lipolysis. This inhibition curve reflects a single class of sites and the IC50-value (concentration at which an antagonist produces half-maximal inhibition) of CGP12177 (3.8 nM) is in good agreement with what should be expected for beta 1-AR/beta 2-AR. Moreover, metoprolol and atenolol, two beta 1- AR- selective antagonists, shift the isoproterenol dose-response curve to the right with high potency as well. These potencies are similar to the ones found for beta 1-AR in the human heart but appreciably higher than those which should be expected for atypical beta-AR. The present study suggest that atypical beta-AR are not functional in human omental adipocytes.

Anti-obesity and anti-diabetic effects of CL 316,243, a highly specific beta 3-adrenoceptor agonist, in yellow KK mice

The anti-obesity and anti-diabetic effects of CL 316,243, a highly specific beta 3-adrenoceptor agonist (beta 1: beta 2: beta 3 = 0:1:100,000), were evaluated in obese diabetic yellow KK mice and C57Bl control mice. The study compound was fed through a gastric tube at a rate of 0.1 mg/kg/day for 2 weeks. The following parameters were compared in the treated and control animals given distilled water: brown adipose tissue thermogenesis, resting metabolic rate, insulin receptors in adipocytes, and blood glucose and serum insulin levels during a glucose overloading test. CL 316,243 significantly increased brown adipose tissue thermogenesis and resting metabolic rate in both yellow KK mice and C57Bl controls. The amount of white adipose tissue decreased, although food intake was not affected. The effects contributed to the mitigation of obesity in yellow KK mice. CL 316,243 also increased the concentration of insulin receptors and decreased the levels of serum insulin and blood glucose during the glucose overloading test in yellow KK mice. These observations suggest that CL 316,243 possesses anti-obesity and anti-diabetic effects and consequently may be useful for treating obesity as well as non-insulin-dependent diabetes mellitus in obese persons, without causing excessive side effects.

Beta-adrenoceptor subtypes mediating the airways response to BRL 35135 in man

1 The purpose of the study was to assess the bronchorelaxant effects of the beta3-adrenoceptor agonist BRL 35135 in normal human airways. 2 Eight healthy male subjects were studied, having previously demonstrated airways responsiveness to inhaled salbutamol 200 microg, with a group mean (+/- s.e. mean) fall in airways resistance (Raw), from baseline, of 37 +/- 5%. 3 Subjects attended the laboratory on 3 separate days, having fasted and taken placebo (PL) or nadolol 20 mg (N20), 2 h previously. 4 After 30 min rest, baseline measurements of Raw, serum potassium, glucose and free fatty acid were performed before subjects were given single oral doses of BRL 35135 8 mg (BRL) or placebo BRL. Measurements were repeated 60 min after the BRL or placebo BRL were given. 5 There was a significant (P < 0.05) fall in Raw (% change from baseline, as means and 95% CI) with PL/BRL: -32(-18, -46), compared with either PL/PL: -8(5, -21), or N20/BRL: -11(2, -24). There was no significant difference between PL/PL and N20/BRL. 6 A similar pattern to Raw was observed for both of the beta2-mediated metabolic responses which were also antagonised by nadolol. For the potassium response (mmol l(-1)), there was a significant (P < 0.05) difference between PL/BRL: -0.50(-0.31, -0.69), in comparison with either PL/PL: 0.08(-0.11, 0.27) or N20/BRL: 0.09(-0.10, 0.28), but values for PL/PL and N20/BRL were not significantly different. In contrast, with the free fatty acid response (nmol 1(-1)), the increase seen with N20/BRL: 85(1.0, 171.0) was significantly (P < 0.05) different from PL/PL: 3.7(-82.3, 89.8), but was not different from PL/BRL: 132.5(46.5, 218.5). 7 In conclusion, BRL 35135 produced airways, potassium and glucose responses which were antagonised by nadolol, whereas the lipolysis response was not. This suggests that there are not functional beta3-adrenoceptors in human airways.

Zeneca ZD7114 acts as an antagonist at beta 3-adrenoceptors in rat isolated ileum

1. The relaxant effects of Zeneca ZD7114, BRL37344 (putative beta 3-adrenoceptor agonists) and various phenylethylamine-based agonists were studied in isolated ileum of the rat where tone was increased with carbachol (0.5 microM). Agonist-induced relaxation.was measured under equilibrium conditions with alpha-, beta 1- and beta 2-adrenoceptors inhibited. 2. Relaxant responses were obtained to isoprenaline, noradrenaline, and BRL37344, although, the efficacy of this latter agent was significantly.lower than that of isoprenaline. Salbutamol caused weak relaxation (< 20%) at high concentrations (10 microM) and ZD7114 was without significant relaxant effect even at high concentrations (10 microM). 3. Relaxant responses to isoprenaline and BRL37344 were weakly antagonized by high concentrations of (+/-)-propranolol (10 and 100 microM) yielding pKB values of 5.7 with isoprenaline as the agonist and 5.5 with BRL37344 as the agonist. 4. The non-selective beta-adrenoceptor antagonist, (+/-)-alprenolol (1-100 microM) caused competitive antagonism of the relaxant responses to isoprenaline (pA2 value = 6.5). A similar pKB value was obtained when BRL37344 was used as the agonist (6.4). 5. Relaxant effects of isoprenaline and BRL37344 were also antagonized by ZD7114 (1-100 microM) yielding pA2 and pKB values of 6.3 and 6.7 respectively. 6. The low potencies of (+/-)-propranolol and (+/-)-alprenolol as antagonists of the relaxant responses to isoprenaline and BRL37344 indicate that both the agonists and antagonists employed in the current study may interact with beta 3-adrenoceptors in the rat isolated ileum. Contrary to the previous findings in guinea-pig ileum, where BRL37344 and ZD7114 were full agonists, in the current study, BRL37344 was a partial agonist and ZD7114 an antagonist at the beta 3-adrenoceptor in rat ileum.

Evidence for a functional beta 3-adrenoceptor in man

1. The existence of a functional beta 3-adrenoceptor in man was investigated by studying the lipolytic action of selective beta-adrenoceptor agents in isolated white omental and subcutaneous fat cells. 2. The non-selective beta 1/beta 2-adrenoceptor antagonist, CGP 12177 was lipolytic in both omental and subcutaneous fat cells. The intrinsic activity relative to isoprenaline was greater in omental than in subcutaneous cells. 3. Addition of the beta 2-adrenoceptor antagonist, ICI 118,551 and the beta 1-adrenoceptor antagonist CGP 20712A in combination or the non-selective beta-adrenoceptor antagonist propranolol alone (all 10(-7) M), induced a rightward shift of the dose-response curves for isoprenaline- and BRL37344-stimulated lipolysis of about 4 and 2 log-units, respectively. However, the antagonists did not alter lipolysis induced by CGP12177. 4. Several concentrations of beta-adrenoceptor antagonists were used to determine the pA2 values by Schild analysis. The values for CGP 20712A and ICI 118,551 (6.63 +/- 0.20 and 6.25 +/- 0.12) as antagonists of the lipolytic effects of CGP 12177 were over 2 units lower than the pA2 value for CGP 20712A against the response to the selective beta 1-agonist dobutamine (8.58 +/- 0.23) and the pA2 value for ICI 118,551 against the response to the selective beta 2-agonist terbutaline (9.15 +/- 0.26). 5. beta 3-Adrenoceptor mRNA expression, investigated with a polymerase chain reaction assay, was demonstrated in both types of adipocytes in the same cell preparations that had a lipolytic response to CGP 12177. 6. In conclusion, human white fat cells express an atypical beta-adrenoceptor in addition to beta 1- and beta 2-adrenoceptors. This receptor is stimulated more selectively by the beta1/beta2-antagonist CGP 12177 than by BRL 37344 and is poorly sensitive to blockade by selective beta1- and beta2-antagonists. On the basis of the pharmacological properties and the mRNA analyses, we suggest that this atypical receptor corresponds to the beta 3-adrenoceptor subtype.

In vivo beta-adrenergic induction of the unmasking of the uncoupling protein in rat brown fat

1. In 28 degrees C adapted rats (WA) both cold stress and norepinephrine (NE) led to a 4-fold increase of uncoupling protein dependent proton conductance which was abolished by propranolol (PRO). 2. In 4-day warm re-exposed rats (after 10 days at 5 degrees C) (WR) the same uncoupling by cold stress was observed but the NE effect was lower. Uncoupling by cold stress was not abolished by PRO. 3. In WR rats, uncoupling was not due to the involvement of an alpha-adrenergic pathway. 4. Both beta-agonist isoproterenol and beta 3-agonists BRL 35135A and ICI D7114 led to high levels of unmasking. 5. Interscapular brown adipose tissue surgical denervation, which abolished cold stress unmasking both in WA and, WR rats, indicates a mediation by direct sympathetic innervation. 6. Depending on the thermal history of the rat, the possibility that unmasking by cold stress could be mediated by different types of beta-receptors is discussed.

The beta-adrenoceptors mediating relaxation of rat oesophageal muscularis mucosae are predominantly of the beta 3-, but also of the beta 2-subtype

1. beta-Adrenoceptor-mediated relaxation of rat oesophageal smooth muscle was investigated by studying the effects of beta 1- and beta 2-selective antagonists on the relaxation induced by (-)-isoprenaline, the beta 2-selective agonists fenoterol and clenbuterol and the beta 3-agonist, BRL 37344. 2. The highly beta 1-selective antagonist CGP 20721A did not antagonize (-)-isoprenaline- or BRL 37344-induced relaxations in concentrations up to 10 microM. Only at 100 microM of CGP 20712A were clear rightward shifts of the agonist concentration-response curves (CRCs) observed, with pA2 values of 4.70 and 4.97 against (-)-isoprenaline and BRL 37344, respectively. 3. ICI 118,551, a potent and selective beta 2-antagonist, at 100 nM caused moderate rightward shifts of the CRCs of (-)-isoprenaline, fenoterol and clenbuterol; with fenoterol and clenbuterol, this was accompanied by a clear steepening of the curve. Only at the highest concentration (100 microM ICI 118,551) did the shifts to the right further increase substantially. Resulting Schild-plots were clearly biphasic. BRL 37344-induced relaxations were only antagonized at 100 microM ICI 118,551, yielding a pA2 value of 5.48. 4. These results clearly demonstrate that the BRL 37344-induced relaxation of rat oesophageal muscularis mucosae is mediated solely through beta 3-adrenoceptors, whereas (-)-isoprenaline-, fenoterol- and clenbuterol-induced relaxations were shown to involve both beta 2- and, predominantly, beta 3-adrenoceptors.

Evaluation of ICI D7114, a putative stimulant of brown adipocytes, on histamine-contracted guinea-pig ileum

1. Experiments were performed to characterize the effects of the novel brown adipocyte stimulant, ICI D7114, in the guinea-pig isolated ileum, right atrium and tracheal chain. In the ileum, agonist-induced inhibition of the contractile response to either histamine or prostaglandin E2 (PGE2) was assessed, along with effects on resting rate in the atrium and resting tone in the tracheal chain. In the latter two preparations, antagonism of isoprenaline-induced responses by ICI D7114 was also assessed. 2. Inhibitory responses were obtained in the ileum to ICI D7114, isoprenaline, BRL37344, and noradrenaline. The responses to ICI D7114, isoprenaline and BRL37344 were resistant to blockade with propranolol (5 microM), naloxone (1 microM), methysergide (0.1 microM), cimetidine, indomethacin and 8-phenyltheophylline (all 10 microM). These responses to isoprenaline, in the presence of propranolol (5 microM), were competitively antagonized by alprenolol (1-100 microM) with a pA2 value of 6.44. The responses to ICI D7114 and BRL37344 were antagonized by single concentrations of alprenolol (1 microM) with apparent pKB values of 6.53 and 6.57 respectively. These data indicate an effect of ICI D7114 at the atypical beta-adrenoceptor in the guinea-pig ileum. 3. The order and relative potency of agonists at the atypical beta-adrenoceptor was BRL37344 (4) < isoprenaline (1) = ICI D7114 (1.1) > noradrenaline (0.5). 4. ICI D7114 (1 nM - 10 microM) caused no significant change in the rate of beating or the resting tone of the guinea-pig right atrium or tracheal chain respectively. It did, however, cause selective blockade of the responses to isoprenaline in these tissues (apparent pKB values 7.63 and 5.85 in atrium and tracheal chain respectively). Responses to histamine (atrium) and aminophylline (tracheal chain) were not significantly affected by 10 microM ICI D7114.5. These results demonstrate that ICI D7114 possesses selective agonist activity at atypical beta-adrenoceptors in the guinea-pig ileum and its use as a tool may help to establish a role for the atypical beta-adrenoceptor in the control of gastrointestinal motility.

Differences in functional cyclic AMP compartments mediating lipolysis by isoprenaline and BRL 37344 in four adipocyte types

Triglyceride mobilization and adenylyl cyclase activation in adipocytes from Wistar rats, lean Zucker (Fa/?) rats, obese Zucker (fa/fa) rats and humans were investigated in concentration-response studies with (-)-isoprenaline and the atypical beta 3-adrenoceptor selective agonist BRL 37344. Maximum FFA production by both agonists was identical in Wistar rat and lean Zucker rat adipocytes, while obese Zucker rat adipocytes and human adipocytes produced significantly less FFA, especially with BRL 37344. Maximum adenylyl cyclase activation by (-)-isoprenaline was similar for all types of adipocyte ghosts, whereas BRL 37344 was a partial agonist in all cases with the lowest intrinsic activity in human adipocytes. For (-)-isoprenaline the relationship between cAMP and lipolysis was steepest with Wistar rat adipocytes, followed by human and lean Zucker rat adipocytes, while obese Zucker rat cells showed a shallow relationship. For BRL 37344, the relationship was very steep and similar for all four adipocyte types, despite the marked differences in maximal lipolysis and cyclic AMP production. The results strongly argue in favour of cyclic AMP compartmentalization, the activity ratio between the functional and the non-functional compartment being least favourable in obese Zucker rat adipocytes. The atypical beta 3-adrenoceptor agonist BRL 37344 very efficiently directs the generated cyclic AMP into the functional compartment in all four adipocytes types investigated.