Is the adipocyte beta-adrenoceptor a prototype for the recently cloned atypical 'beta 3-adrenoceptor'?

Long-term agonist exposure induces upregulation of beta 3-adrenergic receptor expression via multiple cAMP response elements

During continuous stimulation by agonist, beta 1- and beta 2-adrenergic receptors (ARs) undergo processes that lead to decreases in receptor expression. This receptor down-regulation serves to limit the cellular cAMP response during chronic agonist exposure. In the recently described third subtype of the beta AR, denoted beta 3AR, we found four potential cAMP response elements in the 5' flanking region, suggesting that expression of this receptor might be positively regulated by agonists. These elements were cloned into the vector pA10CAT2, which contains a chloramphenicol acetyltransferase reporter gene, and transiently expressed in VERO cells. Three of these elements, TGACTCCA, TGAGGTCT, and CGAGGTCA (located 518, 622, and 1125 bases upstream of the beta 3AR coding block, respectively) were found to increase transcription of the chloramphenicol acetyltransferase gene in response to cAMP analogues and agents that increase intracellular cAMP. 3T3-F442A cells, when differentiated into the adipocyte phenotype by insulin, expressed beta 3AR, and nuclear runoff studies from such cells confirmed cAMP enhancement of beta 3AR mRNA transcription. In these cells, beta 3AR mRNA increased in response to exposure to the beta 3AR agonist isoproterenol and remained elevated during exposures of up to 24-30 hr. During prolonged exposure to agonist, no downregulation of beta 3AR expression in 3T3-F442A cells occurred. Indeed, beta 3AR expression increased during agonist exposure to approximately 165% of basal expression. In marked contrast, beta 1AR expression declined by approximately 70% in response to chronic agonist exposure. These studies reveal a subtype-specific prolonged transcriptional regulation of a beta AR gene by the end product of its signal transduction pathway. Thus, the beta 3AR undergoes a paradoxical increase in receptor expression during chronic agonist exposure.

Localization and characterization of two propranolol resistant (-) [125I]cyanopindolol binding sites in rat skeletal muscle

Autoradiographic studies were performed in sections of rat gastrocnemius, plantaris and soleus muscle bundles with (-)-[125I]cyanopindolol (59-69 pM) in the presence of (-)-propranolol (1 microM) to block beta 1- and beta 2-adrenoceptors. Two distinct populations of binding sites remained, one evenly distributed over the muscle bundles and the other localized in discrete patches. Evenly distributed binding was highest in the soleus muscle and inhibited by (+/-)-, (-)- and (+)-alprenolol (20 microM), tertatolol (1 microM), BRL 37344 (2-20 microM), (-)-isoprenaline (100 microM), phentolamine (10 microM) and haloperidol (250 microM) but not ICI 118,551 (70 nM), CGP 20712A (100 nM), (+)-isoprenaline (100 microM), pindolol (2 microM), cimaterol (100 microM) or serotonin (10 microM). Stereoselectivity for the optical isomers of alprenolol was displayed in the soleus muscle only. Highly localized binding was inhibited by serotonin (10 microM), (-)- and (+)-isoprenaline (100 microM) and phentolamine (10 microM).

Cardiac beta 2-adrenoceptors and the inotropic response to exercise in man

To find out what role, if any, beta 2-adrenoceptors play in cardiac contractility, the heart rate, stroke volume and cardiac output of twelve healthy male and female volunteers (aged 18-28 years) were studied at rest (standing) and during two stages of treadmill exercise, 2 h after ingestion of propranolol (100 mg) or atenolol (100 mg) or a placebo, on different occasions, in a double-blind crossover manner. Cardiac output was measured by a carbon dioxide-rebreathing method. Atenolol and propranolol caused equal reductions in heart rate at rest, and in heart rate and cardiac output during exercise (p less than 0.001, two-way analysis of variance). Neither atenolol nor propranolol had any significant effect on resting cardiac output, resting stroke volume or stroke volume during exercise. Since atenolol (100 mg) has been shown to be beta 1-adrenoceptor-selective, we conclude that cardiac inotropic function during exercise is largely beta 1-adrenoceptor-mediated with little or no beta 2-adrenoceptor involvement.

Salmeterol, a novel, long-acting beta 2-adrenoceptor agonist: characterization of pharmacological activity in vitro and in vivo

1. Salmeterol, a novel, long-acting beta-adrenoceptor agonist, has been compared with isoprenaline and salbutamol for activity in vitro on a range of beta-adrenoceptor containing preparations from laboratory animals and man, and in vivo for bronchodilator activity in the conscious guinea-pig. 2. Salmeterol, like isoprenaline and salbutamol, relaxed preparations of both guinea-pig trachea (contracted by prostaglandin (PG)F2alpha or electrical stimulation) and human bronchus (contracted by PGF 2 alpha) in a concentration-related fashion. Salmeterol was of similar potency to isoprenaline and more potent than salbutamol on both airway preparations. 3. Relaxant responses of superfused guinea-pig trachea and human bronchus to isoprenaline and salbutamol declined rapidly when the agonists were washed from the tissues, with complete recovery within 10 min, whereas responses to salmeterol were more persistent. In electrically-stimulated guinea-pig trachea preparations, inhibition by salmeterol persisted for periods of up to 12h, despite continuous superfusion with agonist-free medium. However, these persistent responses were rapidly and fully reversed by the beta-adrenoceptor blocking drug, propranolol (0.1 microM). In further studies on guinea-pig trachea, propranolol caused concentration-related parallel, rightward shifts of salmeterol concentration-effect curves, yielding a pA2 of 9.0. The slope of the Schild plot was 1.02. 4. Another beta-adrenoceptor blocking drug, sotalol (10 microM), also fully and rapidly reversed established submaximal responses to salmeterol in superfused guinea-pig trachea. However, after administration of sotalol was stopped, the antagonism waned, and salmeterol responses were reasserted without the addition of further agonist. 5. In the beta 1-adrenoceptor containing preparation, rat left atria, isoprenaline exhibited potent, concentration-related, positive inotropic activity, whereas salbutamol and salmeterol were at least 2000-5000 fold less potent, and appeared to be partial agonists. At a concentration of 72 microM, salmeterol exhibited weak antagonism of isoprenaline-induced increases in atrial force of contraction. This antagonism was less marked than that caused by salbutamol (42 microM).6. On the guinea-pig isolated gastric fundus strip, a putative beta3-adrenoceptor containing preparation, salbutamol and salmeterol had only modest agonist activity, being 20-30 fold less potent than isoprenaline and the selective ,beta3-adrenoceptor agonist, BRL 35135.7. In conscious guinea-pigs, inhaled salmeterol and salbutamol were approximately equipotent in causing dose-related bronchodilatation. Whereas the duration of action of salbutamol at its threshold-effective dose was less than 90min, the responses to a similarly effective dose of salmeterol were well-maintained for at least 6 h.8. Salmeterol is therefore a potent and selective beta2-adrenoceptor agonist with a remarkably long duration of action in isolated superfused airways smooth muscle. It also causes persistent bronchodilatation in vivo, in the guinea-pig, when administered by the inhaled route.

Effect of conventional (mixed beta 1/beta 2) and novel (beta 3) adrenergic agonists on thermoregulatory behavior

The effects of submaximal and maximal thermogenic doses of isoproterenol (ISO) on operant thermoregulatory responses in a cold (-8 degrees C) environment were tested in lean (+/?) Zucker rats trained to barpress for radiant heat. Contrary to expectations, ISO rats pressed for twice as much exogenous heat as controls, but showed a smaller rise in colonic temperature. Conversely, a beta 3-selective adrenergic agonist (RO40-2148) decreased the requirement for exogenous heat and produced larger rises in colonic temperature. RO40-2148 and another beta 3-agonist (ICI D7114) produced similar responses in obese (fa/fa) Zucker rats, but tests with ISO were terminated because it caused profound, and lethal hypothermia. The hypothermic effects of ISO on colonic temperature were also observed in Sprague-Dawley rats at room temperature (22 degrees C), whereas RO40-2148 produced hyperthermia. These results provide behavioral evidence for the high thermogenic selectivity of these novel adrenergic agonists and support the existence of an atypical beta 3-adrenoceptor. The hypothermic effects of ISO are presumed to be due to actions on beta 1- and/or beta 2-adrenoceptors.

ICI D7114 a novel selective beta-adrenoceptor agonist selectively stimulates brown fat and increases whole-body oxygen consumption

1. ICI D7114 is a novel, beta-adrenoceptor agonist which stimulates whole body oxygen consumption in conscious rats, cats and dogs and brown adipose tissue (BAT) activity in conscious rats. Treatment of rats with ICI D7114 stimulated oxygen consumption (ED50, 0.04 mg kg-1, p.o.) and BAT mitochondrial guanosine diphosphate (GDP)-binding (ED50, 0.15 mg kg-1, p.o.) with no chronotropic effects on the heart at these doses. 2. Reference beta-adrenoceptor agonists, isoprenaline and clenbuterol, also stimulated oxygen consumption and BAT activity but were less selective because they also produced effects on heart rate at these doses. 3. Treatment of conscious rats with ICI D7114 did not attenuate the chronotropic effects on the heart of a subsequent isoprenaline challenge. 4. Administration of ICI D7114 or of its acid metabolite had no effect in a cat soleus muscle model of tremor or on blood potassium levels in the conscious dog, indicating lack of effects at beta 2-adrenoceptors. 5. The results indicate that ICI D7114 may have activity at atypical beta-adrenoceptors in brown adipose tissue leading to increased whole body oxygen consumption.

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.

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.

Characterization of catecholamine-mediated relaxations in rat isolated gastric fundus: evidence for an atypical beta-adrenoceptor

1. Experiments were carried out in order to characterize the receptors mediating relaxant responses to catecholamines in the rat gastric fundus. The effects of noradrenaline, isoprenaline and the 'atypical' or beta 3-adrenoceptor agonist, BRL 37344, on methacholine-induced tone were measured. Prazosin, propranolol and cyanopindolol were used as antagonists. 2. Relaxant responses to noradrenaline, in the presence of propranolol (1 microM) were antagonized in a concentration-dependent manner by prazosin (0.01 to 1 microM), although this antagonism was weak and non-competitive in nature. Relaxant responses to isoprenaline, in the presence of prazosin (0.1 microM), were antagonized only by the highest concentration of propranolol (1 microM) giving a pKB of 6.3 BRL 37344 also relaxed the rat gastric fundus in the presence of prazosin (0.1 microM), and the responses to BRL 37344 were unaffected by propranolol (1 microM). 3. Tachyphylaxis to BRL 37344 was observed, a second concentration-response curve being significantly shifted to the right. Exposure of tissues to BRL 37344 (1 microM) between concentration-response curves also caused an 11 fold rightward shift in the response to isoprenaline. 4. In the presence of prazosin (0.1 microM) and propranolol (1 microM), the rank order of potency of the agonists was: (-)-isoprenaline (1.0) greater than (-)-noradrenaline (0.39) greater than BRL 37344 (0.10). 5. Responses to BRL 37344 in the presence of prazosin (0.1 microM) and propranolol (1 microM) were antagonized by (+/-)-cyanopindolol (1 microM), with a pKB of 6.56. Responses to isoprenaline, under the same conditions, were antagonized in a competitive manner by (+/-)-cyanopindolol (0.1-1 microM), with the slope of a Schild plot close to unity and a pA2 value of 7.44. 6. The resistance to blockade by prazosin and propranolol and the antagonism by cyanopindolol of the responses mediated by isoprenaline and BRL 37344 suggest that atypical beta-adrenoceptors similar to 'atypical',beta-adrenoceptors in rat adipocytes and other tissues are present in the rat gastric fundus.

Autoradiography of adrenoceptors in rat and human brain: alpha-adrenoceptor and idazoxan binding sites

This chapter reviews the current classification of adrenoceptors, and notes the difficulties of combining the molecular biological and pharmacological classifications of adrenoceptors. Possibilities for mapping the distribution of the proposed subtypes of adrenoceptors using currently available ligands are discussed, and the autoradiographic visualisation of the broad subtypes of alpha 1-, alpha 2-, beta 1-, and beta 2-adrenoceptors in the rat, monkey and human brain described and illustrated. The non-selectivity of ligands currently being used to label alpha-adrenoceptors is shown; we compare the distribution of [3H]idazoxan binding sites with the distribution of alpha 2-adrenoceptors visualised using other ligands. Resolution limitations of current autoradiographic approaches are considered and we shown how in situ hybridisation can complement data from receptor labelling studies used to localise receptors to pre- or postsynaptic sites.

Multiple potential regulatory elements in the 5' flanking region of the beta 3-adrenergic receptor

Since the promoter regions of several genes which modulate intracellular cAMP contain cAMP responsive elements which participate in the regulation of transcription, we assessed whether similar elements were present in the 5' flanking region of the recently cloned human beta 3-adrenergic receptor. This receptor is coupled to cAMP formation, and may play a key role in adrenergic receptor-mediated lipolysis and thermogenesis. Therefore, from a human genomic library we cloned the beta 3-AR coding block and a 1,320 base pair region immediately 5' upstream from the initiation of translation. Based on the classic consensus sequence, we found four potential cAMP responsive elements in this 5' flanking region: CGAGGTCA, TCACATCA, TGAGGTCT and TGACTCCA.

Evidence for the existence of 'atypical' beta-adrenoceptors (beta 3-adrenoceptors) mediating relaxation in the rat distal colon in vitro

1. Experiments were carried out to characterize the adrenoceptors mediating relaxant responses in the rat distal colon. Three agonists were used: noradrenaline, isoprenaline and the beta 3-adrenoceptor agonist BRL 37344. Phentolamine, propranolol and (+/- )-cyanopindolol were tested as antagonists. Tone in the rat distal colon was induced with KCl (30-40 mM) as a spasmogen, and relaxations of this KCl-induced tone produced by the agonists were measured. 2. Relaxant responses to noradrenaline that were obtained in the presence of propranolol (1 microM) were not antagonized by phentolamine (0.01 to 1 microM). Relaxant responses to isoprenaline that were obtained in the presence of phentolamine (1 microM) were antagonized in a concentration-dependent manner by propranolol (0.01 to 3 microM), although this antagonism was weak and non-competitive. Relaxant responses to BRL 37344 that were obtained in the presence of phentolamine (1 microM) were only weakly antagonized by high (1 microM) concentrations of propranolol. 3. Tachyphylaxis to BRL 37344 was observed, a second concentration-response curve being shifted to the right by 15 fold. Exposure of the tissues to BRL 37344 (1 microM) between concentration-response curves also caused rightward shifts in the responses to noradrenaline (18 fold) and isoprenaline (19 fold) but not to papaverine. 4. In the presence of phentolamine (1 microM) and propranolol (1 microM), the rank order of potency of the agonists was: (-)-isoprenaline (1.0) greater than or equal to BRL 37344 (0.93) greater than (-)-noradrenaline (0.3). 5. Responses to BRL 37344 in the presence of phentolamine (1 microM) and propranolol (1 microM) were antagonized by (+/-)-cyanopindolol (I microM), with an apparent pA2 value of 6.67. Responses to isoprenaline, under the same conditions, were antagonized in a competitive manner by (+/-)-cyanopindolol (0.1 to 10 microM), with the slope of the Schild plot close to unity and a pA2 value of 7.12. 6. The resistance of the relaxant responses to antagonism by phentolamine and propranolol, along with the relatively high potency of the beta 3-adrenoceptor agonist BRL 37344 and the antagonism of 'resistant' responses by (+/-)-cyanopindolol would suggest that 'atypical' beta 6-adrenoceptors, similar to the beta 3-adrenoceptors of rat adipocytes and other tissues, exist in the rat distal colon.

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.

In vitro inhibition of intestinal motility by phenylethanolaminotetralines: evidence of atypical beta-adrenoceptors in rat colon

1. The new compounds phenylethanolaminotetralines (PEAT), unlike the reference beta-adrenoceptor agonists isoprenaline (Iso), ritodrine (Ri) and salbutamol (Sal), produced half-maximal inhibition of spontaneous motility of rat isolated proximal colon at substantially lower concentrations (EC50 2.7-30 nM) than those inducing beta 2-adrenoceptor-mediated responses (relaxation of guinea-pig isolated trachea and rat uterus) and had virtually no chronotropic action (EC50 greater than 3 x 10(5) M) on the guinea-pig isolated atrium (a beta 1-adrenoceptor-mediated response). 2. The nonselective beta-adrenoceptor antagonists alprenolol and propranolol prevented the inhibition of rat colon motility by the PEAT with low and different potencies (pA2 values around 7.5 and 6.5 respectively). Conversely alprenolol and propranolol had a higher and similar potency (pA2 values around 9.0) in preventing typical beta 1 or beta 2-responses (increase in atrial frequency by Iso or tracheal relaxation by Ri or Sal). 3. The selective beta-adrenoceptor antagonists CGP 20712A (beta 1) and ICI 118,551 (beta 2) either alone or in combination, did not prevent rat colon motility inhibition by the representative PEAT SR 58611A, which was also fully resistant to alpha-adrenoceptor, acetylcholine, dopamine, histamine, opioid and 5-hydroxytryptamine antagonists. 4. These results indicate that the PEAT are a new class of beta-adrenoceptor agonists and suggest that their preferential intestinal action may be accounted for by selectivity for atypical beta-adrenoceptors, abundant in the rat colon and distinct from the currently recognized beta 1 and beta 2 subtypes.

Characterization of beta adrenoceptors on cultured endothelial cells by radioligand binding

The properties of beta-adrenoceptors present on the cultured bovine pulmonary artery endothelial cells were studied by radioligand binding. These cell contain a high density of beta-adrenoceptors (approximately 16,000 receptors/cell) having high affinity (Kd 18 pM) for 125I-iodocyanopindolol (ICYP). Competition binding experiments suggested the presence of more than two subtype of beta-adrenoceptors. 25% of the total population of receptors was found to be of beta 1-type. The remaining 75% represented a mixed population containing what is suggested to be a mixture of beta 2- and atypical beta-adrenoceptors.