beta-Adrenoceptor studies. 6. Further investigations on the hybrid nature of the rat adipocyte beta-adrenoceptor

Dietary zilpaterol hydrochloride. I. Feedlot performance and carcass traits of steers and heifers

Experiments were conducted at 3 US locations (CA, ID, and TX) to determine the effects of dietary zilpaterol hydrochloride (Zilmax, Intervet Inc., Millsboro, DE) and duration of zilpaterol feeding on performance and carcass merit of finishing steers and heifers. At each site, 160 steers and 160 heifers were stratified within sex by initial BW (study d -1) and assigned randomly within BW strata to 1 of 4 treatments in a randomized complete block design (4 blocks/treatment for each sex). The 4 treatments were arranged in a 2 (no zilpaterol vs. zilpaterol) x 2 (20 or 40 d duration of zilpaterol feeding) factorial arrangement of treatments. When included in the diet, zilpaterol was supplemented at 8.3 mg/kg of DM. Each pen consisted of 10 animals. Each animal was individually weighed unshrunk on d 1, 21 or 41, and 66 of the experiment. Following d 66, cattle were slaughtered and carcass data collected. Feeding zilpaterol increased (P<0.01) final BW of steers and heifers by 11.6 and 6.7 kg, respectively. In addition, feeding zilpaterol hydrochloride increased (P <or= 0.001) ADG 36 and 18%, and increased (P<0.001) G:F 28 and 21% for steers and heifers, respectively. For heifers, DMI was decreased (P<0.001) 6.2% when zilpaterol was fed, whereas in steers DMI tended (P=0.09) to be decreased 2%. For steers and heifers, feeding zilpaterol increased (P<0.001) HCW 16.4 and 12.1 kg, dressing percentage 1.5 percentage units for each sex, and LM area 8.23 and 6.37 cm(2), respectively. Twelfth-rib fat (P >or= 0.12) and KPH (P >or= 0.70) were not affected by feeding zilpaterol to steers or heifers. Feeding zilpaterol decreased (i.e., improved; P=0.02) calculated yield grade of steer and heifer carcasses. Marbling score (P=0.002) and quality grade (P=0.002) were decreased when zilpaterol hydrochloride was fed to steers, and the decrease in marbling score and quality grade tended to be greater when zilpaterol was fed for 40 compared with 20 d (zilpaterol x duration interaction, P=0.07). For heifers, marbling score tended (P=0.07) to be decreased and quality grade was decreased (P=0.05) when zilpaterol hydrochloride was fed. In general, it appears from these data that zilpaterol hydrochloride fed for 20 to 40 d at the end of the finishing period enhances growth performance and carcass muscle deposition for steers and heifers.

The discovery of drugs for obesity, the metabolic effects of leptin and variable receptor pharmacology: perspectives from beta3-adrenoceptor agonists

Although beta3-adrenoceptor (beta3AR) agonists have not become drugs for the treatment of obesity or diabetes, they offer perspectives on obesity drug discovery, the physiology of energy expenditure and receptor pharmacology. beta3AR agonists, some of which also stimulate other betaARs in humans, selectively stimulate fat oxidation in rodents and humans. This appears to be why they improve insulin sensitivity and reduce body fat whilst preserving lean body mass. Regulatory authorities ask that novel anti-obesity drugs improve insulin sensitivity and reduce mainly body fat. Drugs that act on different targets to stimulate fat oxidation may also offer these benefits. Stimulation of energy expenditure may be easy to detect only when the sympathetic nervous system is activated. Leptin resembles beta3AR agonists in that it increases fat oxidation, energy expenditure and insulin sensitivity. This is partly because it raises sympathetic activity, but it may also promote fat oxidation by directly stimulating muscle leptin receptors. The beta1AR and beta2AR can, like the beta3AR, display atypical pharmacologies. Moreover, the beta3AR can display variable pharmacologies of its own, depending on the radioligand used in binding studies or the functional response measured. Studies on the beta3AR demonstrate both the difficulties of predicting the in vivo effects of agonist drugs from in vitro data and that there may be opportunities for identifying drugs that act at a single receptor but have different profiles in vivo.

Function and distribution of beta3-adrenoceptors in rat, rabbit and human urinary bladder and external urethral sphincter

1. Activation of beta-adrenoceptors causes relaxation of the urinary bladder and contraction of the external urethral sphincter, which consists of fast-contracting skeletal muscles. A beta2-adrenoceptor agonist, clenbuterol, recently has been developed as a therapeutic drug for the treatment of urinary incontinence, however beta2-adrenoceptor agonists have undesirable effects on cardiac and striated muscle function. 2. In this study, we compared the effects of the beta2-adrenoceptor agonist, clenbuterol and of a novel beta3-adrenoceptor agonist, GS332, on urinary bladder and external urethral sphincter function in rat, rabbit and human. We also determined the distribution of beta3-adrenoceptors in human urinary bladder and external urethral sphincter, using radioligand-binding techniques. 3. Clenbuterol induced marked relaxations in rat, rabbit and human urinary bladder smooth muscles and also induced marked contractions in rat periurethral striated muscles (external urethral sphincter), while GS332 induced marked relaxations in rat and human, but not in rabbit, urinary bladder smooth muscles and induced small contractions in rat periurethral striated muscles. 4. The radioligand binding studies showed presence of beta2- and beta3-adrenoceptors in human urinary bladder, external urethral sphincter and abdominal rectus muscles. The affinities of GS332 were the highest in urinary bladder and the lowest in the skeletal (abdominal rectus) muscles, while the affinities of clenbuterol were similar in urinary bladder, external urethral sphincter and the skeletal (abdominal rectus) muscles. 5. These results suggest that GS332 could, similarly clenbuterol, have a role in the treatment of urinary frequency and urinary incontinence.

Stimulatory effect of octopamine on beta 3-adrenoceptors to lower the uptake of [14C]-deoxy-D-glucose into rat adipocytes in vitro

1. The effect of octopamine on beta 3-adrenoceptors has been studied in isolated adipocytes of Wistar rats using uptake of [14C]-deoxy-D-glucose as the indicator. 2. Octopamine (0.1-1 nmol 1-1) induced a concentration-dependent decrease of [14C]-deoxy-D-glucose uptake into the adipocytes and this inhibition was not influenced by haloperidol at concentrations sufficient to block dopaminergic receptors. 3. Pindolol and propranolol reversed this inhibition of octopamine in a concentration-dependent manner. The effect of octopamine was reduced in the presence of Rp-cyclic AMPS triethylamine, the membrane-permeable antagonist of cyclic AMP (cAMP), indicating the mediation of cAMP in this inhibition. 4. A direct effect of octopamine on beta 3-adrenoceptors was proved using the application of antibodies. In the presence of an antibody for beta 3-adrenoceptors, the actions of octopamine were concentration-dependently reduced in a manner similar to the decrease of BRL37344-induced inhibitions. 5. The same degree of diminished activities for octopamine as that of BRL37344, the well-known specific agonist of beta 3-adrenoceptors, was also obtained in isoprenline-desensitized adipocytes. Insulin-stimulated uptake of [14C]-deoxy-D-glucose into adipocytes was not modified by isoprenaline induced desensitization. 6. These results suggest that octopamine can activate beta 3-adrenoceptors to lower the glucose uptake through an increase of cAMP in rat white adipocytes.

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.

Differential activation of beta 1-, beta 2- and beta 3-adrenoceptors by catecholamines in white and brown adipocytes

This review summarizes the experiments performed by various groups to determine how the activation of the different beta-adrenoceptors (beta-ARs) is ordinated when they are present in the same fat cell and involved in the same biological event. When expressed after the transfection of their genes in Chinese hamster ovary cell (CHO cells), beta 1- and beta 2-ARs present a higher affinity for catecholamines than beta 3-ARs. In vitro, the lipolytic effect induced by low concentrations of catecholamines in dog and rat white fat cells is due to the selective activation of beta 1- and/or beta 2-ARs. Higher concentrations only are able to activate beta 3-ARs. Similar results have been obtained in rat brown adipocytes. On the other hand, the lipolytic effect of catecholamines in human and primate adipocytes does not involve a beta 3-AR component whatever the concentration used. In vivo experiments in the dog have also shown that lipomobilization induced by low doses of isoprenaline only involved beta 1- and beta 2-AR activation, this effect being blocked by beta 1-/beta 2-antagonist pretreatment. However, in the same blockade conditions, perfusion of a 10-fold higher dose of isoprenaline revealed a beta 3-AR contribution in the lipomobilizing effect. These data showed that brown and white adipocyte beta 3-ARs possess a lower affinity for catecholamines than beta 1- and beta 2-ARs and are only recruited by high concentrations of the amines.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between lipolysis and cyclic AMP generation mediated by atypical beta-adrenoceptors in rat adipocytes

1. The nature of the beta-adrenoceptor(s) mediating adenylyl cyclase activation in rat adipocyte ghosts by (-)-isoprenaline and the lipolytically selective beta-adrenoceptor agonist, BRL 37344, was investigated by use of the beta 1-selective antagonist, CGP 20712A. The results were compared with lipolysis in adipocytes. 2. While in lipolysis BRL 37344 was a full and 10 times more potent agonist than (-)-isoprenaline, in adenylyl cyclase activation similar pD2 values for both agonists were found. BRL 37344 was only a partial agonist on rat adipocyte adenylyl cyclase, with an intrinsic activity of 0.62. 3. With CGP 20712A small rightward shifts of the (-)-isoprenaline concentration-response curve (CRC) were observed at concentrations up to 10 microM, while at 100 microM and 1 mM clear rightward shifts occurred. The BRL 37344 CRC was not shifted with antagonist concentrations up to 10 microM. Only at 100 microM and 1 mM CGP 20712A were rightward shifts observed. 4. CGP 20712A concentrations of 10 microM and 100 microM depressed the maximum of the (-)-isoprenaline CRC to 89 and 60%, while the BRL 37344 CRCs retained the control maximum effect (62% of (-)-isoprenaline). Only at 1 mM CGP 20712A, was the CRC of BRL 37344 depressed, while the (-)-isoprenaline maximum was diminished further. 5. It was concluded that as with lipolysis, (-)-isoprenaline acts both through typical beta 1- and atypical beta 3-adrenoceptors for activation of adenylyl cyclase, while BRL 37344 acts solely through atypical beta 3-adrenoceptors. 6. The results also demonstrate that the relationship between adenosine 3':5'-cyclic monophosphate (cyclic AMP) and lipolysis is different for BRL 37344 and (-)-isoprenaline. Although the maximum activation of adenylyl cyclase by BRL 37344 is only 62% of that by (-)-isoprenaline, the distance between the lipolysis and adenylyl cyclase CRCs is much larger in the case of BRL 37344, indicating a larger transduction reserve for this agonist.

The beta-adrenoceptor selectivity profile of BRL 37344 in the pithed rat

1. Rats were pithed in order to disrupt baroreflex pathways. Heart rate was used as a measure of beta 1-adrenoceptor activity, blood pressure as a measure of beta 2-adrenoceptor activity and oxygen consumption and brown adipose tissue temperature as measures of beta 3-adrenoceptor activity. 2. The effects of the selective beta 3-adrenoceptor agonist BRL 37344 were compared with those of isoprenaline, a non-selective beta-adrenoceptor agonist, and denopamine and salbutamol, which are respectively beta 1 and beta 2-adrenoceptor agonists. 3. Denopamine was 10-fold more potent on heart rate than blood pressure, whilst salbutamol was 18-fold more potent on blood pressure than heart rate. These findings confirm that in this preparation increases in heart rate are predominantly beta 1 adrenoceptor-mediated, whilst blood pressure is beta 2-adrenoceptor-mediated. Further confirmation is provided by the blockade with atenolol, of the chronotropic effect, but not the blood pressure effect, of isoprenaline. 4. BRL 37344 was the most potent beta-adrenoceptor agonist on both oxygen consumption and brown adipose tissue temperature, revealing the beta 3-nature of these responses. Dose-response curves for oxygen consumption and brown adipose tissue temperature were identical, whichever of the beta-adrenoceptor agonists was used. Both systems may be considered equally effective as indicators of beta 3-adrenoceptor agonist activity.

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)

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.

Non-homogeneous distribution of beta 1- and beta 2-adrenoceptors in various human tissues

The distribution of beta 1- and beta 2-adrenoceptor subtypes was determined in membranes obtained from the right atrium, abdominal fat pad, kidney, lung and liver of each of three human subjects. The number of each beta-adrenoceptor subtype was calculated by nonlinear least-squares computer analysis of the competition curve of (-) [125I]iodocyanopindolol (125I-CYP) binding, using beta 1-selective antagonist CGP 20712A and beta 2-selective antagonist ICI 118,551. Competition studies using CGP 20712A indicated that both beta 1- and beta 2-adrenoceptor subtypes coexist in all five tissues, with marked difference in the relative proportion of beta 1- and beta 2-adrenoceptors among these tissues. Most notably, the existence of beta 1-subtypes could be clearly shown in the lung (27%) and liver (18%); according to previous reports, beta 2-subtypes are found almost exclusively in the lung and liver in humans. Regarding the kidney, the present results (beta 2-subtypes: 68%) also differed from previous human data in that beta 1-subtypes predominated. Consistent results were obtained in experiments with ICI 118,551. These results provide a new insight into the mechanisms by which catecholamines regulate a particular beta-adrenergic function in these tissues.

In vitro study of a novel atypical beta-adrenoceptor agonist, SM-11044

SM-11044 (L-threo-3-(3,4-dihydroxyphenyl)-N-[3-(4-fluorophenyl) propyl] serine pyrrolidine amide hydrobromide) stimulated the relaxation of guinea pig ileum (EC50: 3.0 x 10(-8) M), trachea (EC50: 1.3 x 10(-7) M), lung parenchyma (EC50: 2.1 x 10(-6) M) and the increase in right atrial rate (EC50: 6.9 x 10(-6) M). It also induced lipolysis of rat white adipocytes (EC50: 1.2 x 10(-6) M). Both the relaxant response of ileum and the lipolytic response of adipocytes to SM-11044 were resistant to inhibition by propranolol (10(-6) M), but were antagonized by cyanopindolol (10(-6) M). In contrast, the responses to SM-11044 in trachea, lung parenchyma and right atrium were almost completely abolished by propranolol (10(-6) M). Furthermore, the selectivity of SM-11044 relative to isoproterenol was ileum greater than adipocytes greater than trachea (beta 2) greater than lung (beta 2) greater than atrium (beta 1). These results suggest that SM-11044 is a selective agonist of atypical beta-adrenoceptors that are resistant to antagonism by propranolol but sensitive to cyanopindolol. The receptor binding and adenylate cyclase stimulating activity of SM-11044 were also examined on transfected Chinese hamster ovary cells expressing human beta 1-, beta 2- or beta 3-adrenoceptors. SM-11044 inhibited the binding of [125I]iodocyanopindolol to the three types of receptors in a concentration-dependent manner. The selectivity in terms of Ki values was beta 3 (Ki: 1.3 x 10(-6) M) greater than beta 2 (Ki: 4.1 x 10(-6) M) greater than beta 1 (Ki: 18.1 x 10(-6) M)-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of BRL-35135 on LTB4-induced guinea pig eosinophil chemotaxis

The effect of an atypical beta-adrenoceptor agonist, BRL-35135 on leukotriene B4-induced-guinea pig eosinophil chemotaxis was studied. BRL-35135 and SC-41930 (leukotriene B4-antagonist) inhibited the chemotaxis in a concentration-dependent manner (IC50 = 9.0 x 10(-6) and 2.6 x 10(-7) M, respectively). However, isoproterenol, fenoterol and another atypical beta-agonist, BRL-37344 had no effects. The inhibitory effect of BRL-35135 was not affected by (+/-)-propranolol (10(-4) M). In contrast, the nonselective beta-adrenoceptor antagonist, (-)-alprenolol (10(-4) M) dextrally shifted the inhibitory curve of BRL-35135. The response to BRL-35135 was antagonized in a competitive manner by (-)-alprenolol, with the slope of the Schild plot close to unity, and a pA2 value of 5.62. These findings suggest that guinea pig eosinophils possess an "atypical receptor", which differs from either beta 1-, beta 2- or atypical beta-adrenoceptor on guinea pig ileum, and through which eosinophil chemotaxis can be modulated by BRL-35135.

Coexistence of three beta-adrenoceptor subtypes in white fat cells of various mammalian species

The nature of the beta-adrenoceptors (beta-ARs) of the white fat cells of five mammalian species (rat, hamster, rabbit, dog and humans) was reassessed. The coexistence of at least three beta-ARs on the fat cell (except human) was demonstrated. Comparative binding and lipolysis studies were performed, using recently synthesized compounds selective for the atypical beta-AR of the rat brown fat cell and of the rat colon. beta 1- and beta 2-ARs have previously been identified in all the mammalian white fat cells using [125I]cyanopindolol ([125]CYP) or [3H]dihydroalprenolol. In addition to these receptors, we now demonstrated the existence of a third beta-AR directly involved in adrenergic-mediated lipolysis, and identified it in the white fat cells of the most commonly studied animal species, except humans. This receptor is not detected by the classically used beta-antagonist radioligands, explaining the discrepancies in reports on the nature of the beta-ARs of the adipose tissue. Pharmacological delineation of the third type of beta-AR-induced lipolysis showed this receptor to be rather similar to the previously proposed atypical beta-AR of brown and white rat fat cells. Its pharmacological properties were clarified, using new selective full agonists and partial agonists also acting as non-selective beta 1/beta 2-antagonists. The limits of [125]CYP as a radioligand were reported and the usefulness of BRL 37344, (+/-)-CGP 12177 and phenylethanolaminotralines derivatives (having an atypical beta-activity on intestinal motility) as major tools usable for atypical beta-AR activation was demonstrated. Moreover, confirming our previous results about the nature of the beta-ARs (beta 1- and beta 2-ARs) located in the fat cells of women (Mauriège et al., J. Lipid Res., 1987, 17, 156), no atypical beta-AR-mediated lipolysis was identified in abdominal adipose tissue from healthy women. The possible differences and similarities between this receptor and the recently cloned beta 3-AR are discussed.

Biotechnology of beta-adrenergic receptors

The emergence of Biotechnology has provided pharmacologists with a variety of methods for investigating the structure, the function, and the regulation of membrane-bound receptors with a precision that was not imagined even five years ago. These new tools have been developed and used to analyze the known catecholamine beta 1- and beta 2-adrenergic receptors and to discover and study a new subtype, the beta 3 receptor. We review here the salient features of each of these three receptors, compare their structural and functional properties, and propose models to explain their differential regulation in time and space. A whole family of proteins has now been found to share with the beta-adrenergic receptors their most prominent features, including seven transmembrane domains and coupling with GTP-binding "G" proteins. We therefore propose that the biotechnology-based procedures developed for the beta-adrenergic receptors will be well applicable to the other members of this "R7G" family of receptors.

Direct evidence for the atypical nature of functional beta-adrenoceptors in rat adipocytes

1. The nature of the rat epididymal adipocyte beta-adrenoceptor was investigated by studying the effects of beta 1- and beta 2-selective antagonists on lipolysis induced by (-)-isoprenaline and the lipolytically selective agonist BRL 37344. 2. From 10 nM to 10 microM, the potent and highly selective beta 1-adrenoceptor antagonist CGP 20712A did not influence the concentration-response curve (CRC) of BRL 37344 whereas small but consistent shifts to the right of the (-)-isoprenaline-induced CRC were observed. Clear rightward shifts of the CRCs induced by both (-)-isoprenaline and BRL 37344 were produced only at 100 microM CGP 20712A with the corresponding pA2 values being 4.80 and 4.61, respectively. 3. When the beta 2-selective antagonist ICI 118,551 was used at 10 microM and higher, clear and concentration-dependent shifts to the right of the CRCs of both agonists were observed. The slopes of the Schild plots did not deviate significantly from unity, the pA2 values being 5.49 and 5.33 against (-)-isoprenaline and BRL 37344, respectively. 4. The results demonstrate that (-)-isoprenaline-induced lipolysis in rat white adipocytes is mediated predominantly by atypical beta-adrenoceptors, whereas the typical beta 1-adrenoceptors play a small, subordinate role. The lipolytically selective agonist BRL 37344 acts solely through atypical beta-adrenoceptors.

Molecular characterization of the human beta 3-adrenergic receptor

Since the classification of beta-adrenergic receptors (beta-ARs) into beta 1 and beta 2 subtypes, additional beta-ARs have been implicated in the control of various metabolic processes by catecholamines. A human gene has been isolated that encodes a third beta-AR, here referred to as the "beta 3-adrenergic receptor." Exposure of eukaryotic cells transfected with this gene to adrenaline or noradrenaline promotes the accumulation of adenosine 3',5'-monophosphate; only 2 of 11 classical beta-AR blockers efficiently inhibited this effect, whereas two others behaved as beta 3-AR agonists. The potency order of beta-AR agonists for the beta 3-AR correlates with their rank order for stimulating various metabolic processes in tissues where atypical adrenergic sites are thought to exist. In particular, novel beta-AR agonists having high thermogenic, antiobesity, and antidiabetic activities in animal models are among the most potent stimulators of the beta 3-AR.

Hormone-sensitive adenylyl cyclase in preadipocytes cultured from adipose tissue: comparison with 3T3-L1 cells and adipocytes

The adenylyl cyclase system of preadipocytes derived from the stromal vascular fraction of perirenal rat fat pads was characterized. Unlike mature adipocytes, preadipocyte adenylyl cyclase was only weakly stimulated by catecholamines and adrenocorticotrophic hormone, but was stimulated by guanine nucleotides. Parathyroid hormone and 2-chloroadenosine also stimulated preadipocyte adenylyl cyclase. The adenylyl cyclase system of preadipocytes resembled that of undifferentiated 3T3-L1 cells. However, agents which induced the differentiation of the 3T3-L1 cell adenylyl cyclase system did not have a similar effect on preadipocytes. A medium (CDM6) which induced some differentiation of preadipocyte adenylyl cyclase was developed. The observations that the adenylyl cyclase system of preadipocytes and undifferentiated 3T3-L1 cells are similar, that preadipocyte adenylyl cyclase can be induced to develop along lines similar to early differentiation of 3T3-L1 cells, and that the adenylyl cyclase system of fully-differentiated 3T3-L1 cells has characteristics intermediate between preadipocytes and adipocytes, suggest that the differentiation of preadipocyte and 3T3-L1 adenyly cyclase in vitro mimics adipose adenylyl cyclase development in vivo. The increased catecholamine and ACTH stimulation, and reduced GTP and adenosine sensitivities of adipocytes compared to preadipocytes suggest that a number of genes affecting adenylyl cyclase-associated regulatory and receptor proteins are coordinately repressed and derepressed during development.

Beta-adrenoceptor sensitivity of brown and white adipocytes after chronic pretreatment of rats with reserpine

The effect of pretreatment with reserpine (1.0 mg/kg i.p. daily for 7 days) on beta-adrenoceptor-mediated responses has been measured in epididymal white and interscapular brown adipocytes, left atria and vas deferens of rats in order to investigate the classification of the receptors and whether they are innervated. Lipolysis was measured in adipocytes, and from the same rats, the beta 1-adrenoceptor-mediated positive inotropic responses of isolated paced left atria and beta 2-adrenoceptor-mediated inhibition of field stimulation-induced contraction of the vas deferens were examined. The agonists used were isoprenaline, oxyfedrine (atria only) and (except in brown adipocytes) ritodrine, which was a partial agonist in white adipocytes, atria and vas deferens. Atria and brown adipocytes exhibited beta-adrenoceptor supersensitivity after reserpine pretreatment, whereas vas deferens and white adipocytes did not. Reserpine-induced reductions in food intake and body weight did not appear to influence beta-adrenoceptor-mediated lipolysis, since restriction of the diet equivalent to that of reserpine-treated rats produced no change in white adipocyte sensitivity. Responses mediated via beta 1-, but not beta 2-adrenoceptors, display supersensitivity after chronic depletion of neuronal catecholamines with reserpine and this is evidence for innervation of this receptor subtype. Thus, atrial beta 1-adrenoceptors are assumed to be innervated, whereas vas deferens beta 2-adrenoceptors are not. The present results are consistent with histochemical evidence that brown, but not white, adipocyte beta-adrenoceptors are innervated. However, they are not compatible with conventional receptor classification studies, which suggest that rat brown and white beta-adrenoceptors are similar--either both beta 1 or both atypical.

The beta-adrenoceptor-adenylate cyclase complex. From model to biochemical reality

Developments in the receptor concept have greatly influenced our current knowledge of the beta-adrenoceptor. The triad of pharmacology, organic chemistry and studies in structure-activity relationships is discussed along historical lines, as it has been and still is an impetus for progress in the biochemistry of ligand-receptor interactions. With respect to the beta-adrenoceptor complex these advances which have led to a model in which three protein structures are functionally interacting within the frame of the cell wall: the beta-adrenoceptor, the regulatory guanine nucleotide binding protein, and the enzyme adenylate cyclase, are reviewed.

The antinociceptive action of some beta-adrenoceptor agonists in mice

The antinociceptive actions of several beta-adrenoceptor agonist drugs have been studied in mice by use of a modified abdominal constriction test. All the drugs studied had high antinociceptive activity, with ID50 values in the nmol kg-1 range. (-)-Isoprenaline and (+/-)-isoxsuprine were the most potent, being about ten times more active than salbutamol, the least potent drug studied. All these drugs produced their action very rapidly and appear to act within the peritoneum. (-)-Isoprenaline had about six times the potency of the (+)-isomer. (+/-)-Propranolol caused rightward shifts, usually parallel, of the dose-response curves for (-)-isoprenaline. (+)-Propranolol was more than ten times less potent than the racemic drug. Practolol also caused parallel, rightward shifts of the dose-response curves for (-)-isoprenaline, and was about twice as potent as (+/-)-propranolol, whether given by subcutaneous or intraperitoneal injection. Atenolol and ICI 118551 had intermediate potencies. Propranolol, practolol and ICI 118551 were all considerably less potent in antagonizing the antinociceptive actions of fenoterol and RO363, than (-)-isoprenaline. None of these antagonist drugs showed more than a slight ability to discriminate between the beta 1- and beta 2-selective agonist drugs. No evidence was found for the involvement of opioid, dopamine, or alpha-adrenoceptors in the antinociceptive action of the beta-adrenoceptor agonist drugs. Evidence for and against the involvement of beta-adrenoceptors is discussed, and it is concluded that if these receptors do mediate the antinociceptive action they appear to be atypical.

Acute change in the cyclic AMP content of rat mammary acini in vitro. Influence of physiological and pharmacological agents

The cyclic AMP content of acini, freshly prepared from mammary tissue of lactating rats, was measured during incubation in vitro. Neither adrenergic agonists nor cyclic AMP phosphodiesterase inhibitors alone caused a change of more than 2-fold in the basal cyclic AMP content of acini. Together, however, these agents provoked increases of around 20-fold in acini cyclic AMP content. Forskolin caused similar effects. The relative potency of adrenergic agonists in increasing cyclic AMP in acini, together with the ability of selective antagonists to oppose such rises, indicated that beta 2-adrenergic receptors were involved in mediating the effects. Receptor-binding experiments using [3H]dihydroalprenolol and selective beta-antagonists confirmed the predominant presence of beta 2-adrenergic receptors on acini membranes and on membranes prepared from purified mammary secretory epithelial cells. These results elucidate some previous findings [Robson, Clegg & Zammit (1984) Biochem. J. 217, 743-749; Williamson, Munday, Jones, Roberts & Ramsey (1983) Adv. Enzyme Regul. 21, 135-145], questioning the role of cyclic AMP in the regulation of lipogenesis in mammary acini.

Direct assessment of beta-adrenergic receptors in intact rat adipocytes by binding of [3H]CGP 12177. Evidence for agonist high-affinity binding complex and for beta 1 and beta 2 receptor subtypes

The characteristics of the binding of the hydrophilic beta-adrenergic antagonist [3H]CGP 12177 to intact rat adipocytes were studied at 37 degrees C and 6 degrees C. At both temperatures and at 90% saturation, the non-specific binding was less than 30% of the total binding. At 37 degrees C, specific [3H]CGP 12177 binding was rapid, reversible of high affinity (1.8 +/- 0.4 nM) and saturable. The number of specific binding sites per adipocyte increased with the fat cell size (about 35 000 and 115 000 sites per cell in adipocytes with diameters of 60 microns and 88 microns, respectively) but remained constant when expressed per unit fat cell surface area. Displacement of [3H]CGP 12177 bound to adipocytes by unselective and selective beta-antagonists was stereospecific, had the same characteristics as those found in adipocyte membranes and showed a heterogeneous specificity for beta 1 and beta 2 adrenergic subtypes. In contrast, beta agonist competition curves, which modeled to two affinity-states of binding, showed high-affinity-state Kd values for agonists 10-25-times higher than those found in membranes under the same experimental conditions. At 6 degrees C, although the number and affinity of the specific binding sites for [3H]CGP 12177 were the same as those found at 37 degrees C, the Kd value for (-)-isoproterenol binding to the high affinity state of these sites (3.0 +/- 0.5 nM) was 25-times lower than at 37 degrees C and similar to the value found in membrane preparations (1.5-4 nM). These results show that the [3H]CGP 12177 specific binding sites detected on intact adipocytes represent the physiological beta-adrenergic receptors. Moreover, this study extends to the adipocyte the validity of the model recently proposed for other cell lines, according to which in intact cells, but not in membranes, agonist-binding promotes a rapid and temperature-dependent conformational change of the beta-adrenergic receptors, leading to a progressive loss of capacity of agonists to form a high-affinity complex.

Atypical beta-adrenoceptor on brown adipocytes as target for anti-obesity drugs

Recent studies suggest that thermogenesis in brown adipose tissue has an important role in the regulation of energy balance. Thermogenesis is effected by noradrenaline released from sympathetic nerve endings; the noradrenaline stimulates beta-adrenoceptors, causing lipolysis, and the released fatty acids then promote the uncoupling of oxidative phosphorylation from electron transport. It has been widely accepted that mammalian beta-adrenoceptors exist as two subtypes, beta 1 and beta 2, and rat brown adipocyte beta-adrenoceptors have been classed as beta 1 or as a mixed beta 1/beta 2 population. The beta 1 subtype predominates in atria, whereas the beta 2 subtype predominates in trachea. However, we have now found a novel group of beta-adrenoceptor agonists that selectively stimulate lipolysis in brown adipocytes. In contrast, isoprenaline, fenoterol and salbutamol are less potent as stimulants of lipolysis than as stimulants of atrial rate or tracheal relaxation. Therefore, beta-adrenoceptors in rat brown adipocytes are of neither the beta 1 nor beta 2 subtypes. Compounds that selectively stimulate brown adipocyte beta-adrenoceptors should have potential as thermogenic anti-obesity agents and this has been demonstrated with BRL 26830A , BRL 33725A and BRL 35135A .

The rat lipolytic beta-adrenoceptor: studies using novel beta-adrenoceptor agonists

EC50 and relative intrinsic activity values were obtained for isoprenaline, fenoterol, salbutamol, prenalterol and three new beta-adrenoceptor agonists, BRL 28410, BRL 35113 and BRL 35135 on rat white adipocyte lipolysis, rat atrial rate and tension, rat uterus tension and guinea-pig tracheal tension. Fenoterol and salbutamol were selective for tracheal and uterine responses, prenalterol was selective for atrial responses, but BRL 28410, BRL 35113 and BRL 35135 were selective for the adipocyte lipolytic response. pA2 values for propranolol, practolol, ICI 118,551 and sotalol were obtained on adipocytes, atria and trachea. pA2 values for propranolol and sotalol were much lower on adipocytes than on atria or trachea. The pA2 value for practolol was lower on adipocytes than on atria and the pA2 value for ICI 118,551 was lower on adipocytes than on trachea. Both agonist and antagonist studies therefore suggest that the rat adipocyte lipolytic receptor does not fit into the current beta 1/beta 2-adrenoceptor classification.

Adrenergic control of lipolysis in swine adipose tissue

Most potential adrenergic compounds did not stimulate lipolysis in swine adipose tissue slices. Most of the sympatholytic agents antagonized lipolysis. Most beta 1- and beta 2-adrenergic agonists were not active but many were active with rat adipose tissue. Catecholamines (epinephrine, norepinephrine and isoproterenol), the resorcinol containing beta 2-agonists (terbutaline, metaproterenol and fenoterol) and the beta 1-agonist, dobutamine were active. The beta 1-antagonists were generally more potent and efficacious than the beta 2-antagonists. The swine adipose tissue adrenoceptor was not readily classified as either beta 1- or beta 2-specific.

Specificity of beta-adrenergic control of lipolysis in swine adipose tissue

Fenoterol (beta 2-agonist) and several analogs stimulated swine adipose tissue lipolysis in addition to epinephrine, norepinephrine and isoproterenol. Lipolytic activity was inhibited by propranolol (beta 1 + beta 2) greater than metoprolol (beta 1) greater than ICI 118,551 (beta 2) greater than practolol (beta 1) much greater than butoxamine (beta 2) regardless of the agonist used in the test system. The swine adipose adrenoceptor is not readily classified into a beta 1 or beta 2 subtype. It may be unique or the classification systems devised in laboratory animals may not be applicable across species.

Identification and subclassification of rat adipocyte beta-adrenoceptors using (+/-)-[125I]cyanopindolol

The ligand binding characteristics of beta-adrenoceptors on membranes derived from rat whole fat pads and isolated adipocytes have been compared to those present on lung membranes using the specific ligand (+/-)-[125I]cyanopindolol [(125I]CYP). The equilibrium dissociation constant (KD) of [125I]CYP was similar in all three preparations, whilst the degree of stereospecificity displayed by the isomers of propranolol varied between lung and isolated adipocyte membranes. The rank order of potency in displacing [125I]CYP binding was isoprenaline greater than adrenaline greater than noradrenaline in lung and whole fat pads, suggesting an overall beta 2-receptor subtype, and isoprenaline greater than noradrenaline greater than adrenaline in isolated adipocytes, suggesting a beta 1-receptor. Pharmacological characterisation of receptor subtype with selective beta-adrenoceptor agents indicated that all three preparations contained a heterogeneous receptor population (lung 80% beta 2, 20% beta 1; whole fat pad 62% beta 2, 38% beta 1; isolated adipocyte 15% beta 2, 85% beta 1). In view of the high proportion of beta 2-receptors on whole fat pad membranes, the overall amount of beta 2-receptors on isolated adipocytes may reflect contamination from other cell types. The nature of the beta 1-receptors present on fat cells is discussed in relation to the apparently atypical beta-receptor involved in mediating the functional lipolytic response.

Beta-adrenergic receptors in white fat cells of cold-acclimated rats

Beta-adrenergic receptors were studied in membranes prepared from isolated fat cells obtained from both control and cold-acclimated rats. Beta-adrenergic receptors were identified using beta-adrenergic antagonist, (-) - [3H] dihydroalprenolol. Scatchard plots of specific binding suggest the existence of two binding sites for (-) - [3H] dihydroalprenolol with different affinities. Beta-adrenergic agonists competed for specific binding sites with typical beta 1-adrenergic specificity. There appears to be approximately 5,000 sites of high affinity per fat cell. Cold-acclimation (4 weeks at 4 degrees C) reduced adipocyte size; it did not appear to alter either the maximal number of binding sites of high and low affinity, or their affinity for beta-antagonists or agonists; however the density of binding sites in the fat cell was increased in cold-acclimated rats. It is concluded that the cold-adaptative process which induces in vivo the enhancement of calorigenic effect of norepinephrine is not found at the level of beta-receptors in white fat cells. The increased lipolytic response to norepinephrine observed in vitro in epididymal fat may be explained by cold-induced modifications in cell size and number.

Characteristics of the lipolytic beta-adrenergic receptors in hamster adipocytes

Hamster adipocyte beta-adrenergic receptors were characterized by the effect on the lipolysis rate of several selective beta-adrenergic agonists and antagonists. Prenalterol and procaterol, selective beta 1- and beta 2-agonists, respectively, both stimulated the lipolysis rate half-maximally at 1-2 microM, a concentration approximately 100-fold higher than that needed for half-maximal stimulation by isoprenaline. The maximal procaterol effect was similar to that of isoprenaline, while the effect of prenalterol was 40% lower. Submaximally isoprenaline-stimulated lipolysis was half-maximally inhibited by propranolol at 0.2 microM and by atenolol and H 35/25, selective beta 1- and beta 2-antagonists, at 23 and 6 microM concentration, respectively. Highly specific beta 1 and beta 2 stimulation was induced by incubation with prenalterol or procaterol, with simultaneous maximal selective beta 2 or beta 1 inhibition, respectively. The maximal beta 2 stimulation was approximately twice the corresponding beta 1 effect under these conditions, the sum of both effects closely approaching that of maximal nonselective beta-adrenergic stimulation with isoprenaline. Similar results were obtained with the selective beta 2-antagonist, ICI 118,551, or the beta 1-antagonists, pamatolol and practolol. The findings are most easily explained by the existence of a heterogeneous beta 1- and beta 2-adrenergic receptor population on hamster adipocytes.