Adrenergic effects on plasma levels of glucagon, insulin, glucose and free fatty acids in rabbits

Adrenergic effects on plasma levels of glucagon, insulin, glucose and free fatty acids were studied in fasted rabbits by infusing epinephrine, norepinephrine, isoproterenol, phentolamine (an adrenergic alpha-receptor blocking drug) and propranolol (an adrenergic beta-receptor blocking drug). The adrenergic effects on the plasma levels of insulin, glucose and free fatty acids were similar to those found in other species. The plasma levels of insulin were increased by beta-receptor stimulation (isoproterenol, phentolamine + epinephrine) and decreased by alpha-receptor stimulation (epinephrine, norepinephrine, propranolol + epinephrine). The plasma levels of glucose were increased by both alpha- and beta-receptor stimulation, and the epinephrine-induced hyperglycaemia was only blocked by combined infusions with phentolamine and propranolol. The plasma levels of free fatty acids were increased by saline and further increased by beta-receptor stimulation (isoproterenol), while epinephrine and norepinephrine gave variable results. Alpha-receptor stimulation (propranolol + epinephrine) slightly decreased the plasma levels of free fatty acids. The plasma levels of glucagon, however, were mainly increased by alpha-receptor stimulation (epinephrine, norepinephrine, propranolol + epinephrine) and increased only to a minor extent by beta-receptor stimulation (isoproterenol, phentolamine + epinephrine) in rabbits. This is in contrast to results reported for humans, where beta-receptor stimulation seems to be most important in stimulating glucagon release.     

Characterization of beta-adrenergic binding sites on rodent Leydig cells

A radioligand binding technique was used to study beta-adrenergic binding sites on rodent Leydig cells. Beta-Adrenergic binding sites were found on Leydig cells in both the rat and mouse. Binding of [3H]CGP-12177 [4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]benzimidazole-2-one] to purified rat Leydig cells was found to be saturable, temperature and time dependent, stereospecific, and readily reversible by the beta-adrenergic antagonist propranolol. Scatchard analysis revealed the presence of high-affinity sites with an apparent dissociation constant (Kd) of 0.79 +/- 0.22 nM and maximal binding capacity (Bmax) of 1716 +/- 245 sites per rat Leydig cell. Competition of various beta-adrenergic agonists and antagonists with [3H]CGP indicates an order of potency of L-isoproterenol greater than epinephrine = salbutamol greater than norepinephrine greater than D-isoproterenol and dl-propranolol = ICI 118,551 much greater than atenolol, respectively. These observations suggest that the binding sites are predominantly of the beta 2-receptor subtype. Incubation of freshly isolated rat Leydig cells with luteinizing hormone (100 ng/ml) caused consistent stimulation of androgen production, but only occasional stimulation by the beta-agonist isoproterenol (10 microM) was observed. However, these cells consistently responded to the beta-agonist after 3 h in primary cultures. These findings indicate that rodent Leydig cells possess beta-adrenergic binding sites and point out a possible dissociation between receptor recognition and physiologic response.     

Coexistence of beta 1- and beta 2-adrenoceptors in the melanophore of the goby Tridentiger obscurus

The effects of beta-adrenergic agonists and antagonists on the pigmentary state of denervated melanophores in isolated, split, caudal fins of the goby Tridentiger obscurus were examined to investigate the function and the subtype of the beta-adrenoceptors of the melanophores. Salbutamol, terbutaline, and dobutamine partially inhibited the pigment-aggregating response of melanophores to norepinephrine. The effects of these beta-agonists were inhibited by propranolol. It was confirmed that the melanophores possess both alpha- and beta-adrenoceptors, and that the activation of the beta-adrenoceptors induces the dispersion of pigment in the melanophores. Norepinephrine, epinephrine, isoproterenol, dobutamine, salbutamol, and terbutaline evoked the dispersion of pigment in the melanophores in which pigment had previously been aggregated by treatment with verapamil in the presence of phentolamine. The pigment-dispersing effects of two beta 1-selective agonists, norepinephrine and dobutamine, were effectively inhibited by metoprolol, a selective antagonist of beta 1-receptors. By contrast, the pigment-dispersing effects of two beta 2-selective agonists, salbutamol and terbutaline, were not inhibited by metoprolol. Both the effects of nonselective agonists, epinephrine and isoproterenol, were partially inhibited by metoprolol. The actions of all of the beta-agonists used were effectively inhibited by propranolol, and they were partially inhibited by butoxamine. These results suggest co-existence of beta 1- and beta 2-adrenoceptors in the melanophores. The relative numbers of beta 1- and beta 2-adrenoreceptors as a percentage of the total population of beta-adrenoceptors were estimated to be 18.6% and 81.4%, respectively, from analyses of Hofstee plots of the effects of the beta-agonists on the melanophores in the presence of butoxamine or metoprolol.     

Absence of alpha-2 adrenergic effects on cAMP production in a genital tract smooth muscle cell line

This study sought to evaluate alpha-2 and beta adrenergic modulation of cAMP production in the DDT1 MF-2 transformed smooth muscle myocyte. After stimulation with forskolin or adrenergic agonists with or without subtype specific antagonists, cAMP production was determined. These experiments confirmed an increase of cAMP in response to forskolin, isoproterenol, epinephrine, and norepinephrine; the adrenergic stimulation was inhibited by propranolol. On the other hand, the alpha-2 agonist clonidine did not inhibit cAMP production. Likewise, alpha-2 receptor blockade did not increase cAMP production in response to epinephrine. These studies, therefore, suggest that the DDT1 MF-2 myocyte does not contain a significant population of functional alpha-2 adrenergic receptors.     

Adrenergic control of lipogenesis and lipolysis in the chicken in vitro

The adrenergic inhibition of lipogenesis and stimulation of lipolysis in the avian has been examined using chicken hepatocytes and adipose tissue explants in vitro. Lipogenesis was inhibited by adrenergic agonists: epinephrine (alpha + beta) greater than isoproterenol (beta 1/beta 2) greater than norepinephrine (alpha 1/alpha 2, beta 1) greater than metaproterenol (beta 2), phenylephrine (alpha 1). Dobutamine (beta 1 agonist) and dopamine (dopaminergic agonist) did not significantly affect [14C]acetate incorporation into lipid, while clonidine and para-aminoclonidine (alpha 2 agonists) were slightly stimulatory. Lipolysis in young and adult chicken adipose tissue was stimulated by epinephrine, isoproterenol, phenylephrine, dobutamine and metaproterenol, but was inhibited by clonidine and para-aminoclonidine. Both the antilipogenic and lipolytic effects of epinephrine were partially blocked by phentolamine (alpha 1 = alpha 2 antagonist) or propranolol (beta 1 = beta 2 antagonist), but completely inhibited by phentolamine and propranolol administered together.     

Effect of adrenergic agonists on human chorionic gonadotropin release by human trophoblast cells obtained from first trimester placenta

The cultured syncytiotrophoblast cells from human first trimester placenta were used to determine the effect of adrenergic agonists on human chorionic gonadotropin (hCG) production in vitro. Beta-adrenergic agonists isoproterenol, ritodrine and isoxsuprine increased the hCG release during the 2 h incubation period, however, alpha-agonists norepinephrine and phenylephrine and a beta 1-agonist dobutamine had no effect. The effect of isoproterenol was blocked by propranolol and butoxamine, but less efficiently by phentolamine and atenolol. These results indicate that placental hCG production can be modulated by stimulation of beta-, possibly beta 2-adrenoceptors but not by alpha-adrenoceptors.     

Adrenergic receptors mediating prostaglandin production in the rabbit vas deferens

Contractile and prostaglandin E (PGE)-producing effects of adrenergic agonists were compared in the rabbit isolated vas deferens to determine which adrenergic receptor(s) potentially could mediate neural responses. Additionally, interactions among receptors were elucidated by comparing responses to norepinephrine, phenylephrine and isoproterenol to those in the presence of selective adrenergic agonists or antagonists. Norepinephrine increased the force of muscle contraction and the immunoassayable PGE concentrations in a concentration-dependent manner with EC50's of 55 +/- 8 and 112 +/- 39 microM, respectively. Propranolol (10 microM) enhanced the contractile effects of norepinephrine (p less than 0.01) whereas yohimbine (100 microM) or prazosin (1 microM) reduced norepinephrine-induced contractions and PGE production (p less than 0.01). Propranolol did not alter the PGE production induced by norepinephrine. Metoprolol (100 microM) also enhanced contractile effects of norepinephrine (p less than 0.05). The beta adrenergic agonist, isoproterenol (100 nM), decreased the contractile, but not the PGE-producing, effects of phenylephrine (p less than 0.001). Isoproterenol, given alone, increased PGE concentrations and inhibited electrically-induced force generation in a concentration-dependent manner. These results are consistent with the presence of alpha receptors on the vas deferens which mediate smooth muscle contraction and PGE generation. Beta receptors which mediate relaxation and PGE production also are present. Tentative identification of the beta receptor subtype revealed the presence of a beta 1 receptor.     

Identification of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian palate

A direct radioligand binding technique utilizing a beta-adrenergic antagonist [3H]Dihydroalprenolol [( 3H]DHA) was employed in the identification and characterization of fetal palatal beta-adrenergic receptors. [3H]DHA binding was saturable (Bmax 16 fmol/mg protein) with high affinity and an apparent equilibrium dissociation constant (KD) of 1.5 nM. Binding of [3H]DHA was displaced by the competitive beta-adrenergic antagonist propranolol in a concentration-dependent manner. Dissociation kinetic studies demonstrated almost complete reversibility of radioligand binding within 60 min. The functionality of these beta-adrenergic receptors was demonstrated by showing that fetal palatal mesenchymal cells responded to catecholamine agonists with dose-dependent accumulations of intracellular cAMP. This effect could be entirely blocked by the beta-antagonist, propranolol. The relative potency order of catecholamines in eliciting an elevation of cellular cAMP was characteristic of a beta 2-adrenergic receptor-mediated response: (-) isoproterenol greater than (-) epinephrine greater than (-) norepinephrine. In addition, this response was found to be stereospecific with (-) isoproterenol being significantly more potent than (+) isoproterenol. Both the [3H]DHA binding characteristics and the catecholamine sensitivity of fetal palatal tissue support the presence of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian secondary palate.     

Beta 2-receptors in the rat anterior pituitary mediate adrenergic stimulation of prolactin release

As previously shown, the beta-adrenergic agonists isoproterenol, epinephrine and norepinephrine stimulate prolactin (PRL) release from superfused rat anterior pituitary cell aggregates. In order to further characterize the beta-adrenergic response in this tissue preparation, the effects of various beta-adrenergic agents were investigated. The beta 2-agonist, zinterol, stimulated PRL release at concentrations more than 4 orders of magnitude lower than prenalterol, a beta 1-agonist with high potency in rat heart. The order of potency of the antagonists IPS 339 (beta 2), ICI 118.551 (beta 2), propranolol, sotalol, practolol (beta 1), metoprolol (beta 1) and H 35/25 for inhibition of beta-agonist-stimulated PRL release provided additional support for a beta 2-stimulatory effect. beta-Agonists were also capable of stimulating PRL release from superfused intact pituitaries. The beta-adrenergic response desensitized rapidly during prolonged exposure of the aggregates to beta-agonists.     

Differential response of adenylate cyclase to beta-adrenergic agonists in white adipocyte membranes from lean and obese (ob/ob) mice

The quality of the beta-adrenergic response, as measured by the activation of adenylate cyclase, was found to differ in adipocyte membranes from lean and obese mice. In the tissue from lean mice, the response was similar to that in rat adipose tissue and could, by analogy, be classified as beta 1-receptor response. In the tissue preparations from the obese mice, the rank order of potency of the three classical agonists (isoproterenol, epinephrine, and norepinephrine) was more typical of a beta 2-receptor response.     

Characterization of the beta receptor responsible for enhanced histamine release from bovine leukocytes

The present study was undertaken to determine the beta-receptor subtype responsible for the enhanced anaphylactic histamine release from bovine leukocytes produced by isoprenaline. The selective beta 1 antagonist practolol competitively inhibited the enhancement of histamine release produced by isoprenaline. The selective beta 2 agonists salbutamol and terbutaline produce no significant effect on antigen-induced histamine release. This indicates that the receptor responsible for enhancement of histamine release from bovine granulocytes is of the beta 1 subtype. The question is raised as to whether this is a normal physiological response or an abnormal condition produced by the sensitization procedure.     

The (--)[3H]dihydroalprenolol binding to rat adipocyte membranes: an explanation of curvilinear Scatchard plots and implications for quantitation of beta-adrenergic sites

In rat adipocyte membranes, both beta-adrenergic agonists and beta-adrenergic antagonists competed with (--)[3H]dihydroalprenolol for high affinity (KD 2-4 nM) and low capacity binding sites. The antagonists but not the agonists competed with (--)[3H]dihydroalprenolol for lower affinity and higher capacity sites. The present studies were performed in order to characterize the adipocyte beta-adrenergic receptor and distinguish it from low affinity, higher capacity sites which were heat-labile and not stereoselective. When isoproterenol was used to define the nonspecific binding, saturation studies showed a single binding site with a capacity of approximately 100 fmol/mg membrane protein (corresponding to approximately 50,000 sites/adipocyte). Binding was saturated by 10 nM (--)[3H]dihydroalprenolol. Approximate KD's of 204 nM were observed. Kinetic analysis of (--)[3H]dihydroalprenolol binding provided an independent measurement of KD between 0.75 and 1.1 nM. This binding site had the characteristics of a beta 1-adrenergic receptor with the potency of isoproterenol greater than norepinephrine greater than or equal to epinephrine as competitors of binding. Furthermore, the KD of inhibition of (--)[3H]dihydroalprenolol binding correlated with the Ki of inhibition by antagonists or Ka of activation by agonists of glycerol release in isolated adipocytes (r = 0.968, P less than 0.001). These results suggest that beta-adrenergic agonists compete with (--)[3H]dihydroalprenolol for the high affinity binding site which represents the physiological site. Furthermore, the use of antagonists (propranolol, alprenolol) to define specific beta-binding includes nonspecific site(s) as well as the beta-adrenergic site. Previous characterization and quantitation of beta receptors in rat fat cell membranes may have been in error by incorporating both types of binding in their measurement.     

Mediation of Norepinephrine-Stimulated Cyclic AMP Accumulation by Adrenergic Receptors in Hypothalamic and Preoptic Area Slices: Effects of Estradiol

Adrenergic receptor agonists and antagonists were employed to establish (a) which receptor subtypes mediate the cyclic AMP response to norepinephrine in hypothalamic and preoptic area slices from gonadectomized female rats and (b) which receptor subtypes might be modulated by the steroid hormone estradiol. Slice cyclic AMP levels were elevated by the beta receptor agonist isoproterenol, but not by alpha 1 (phenylephrine, methoxamine) or alpha 2 (clonidine) agonists. However, the alpha agonist phenylephrine potentiated the effect of the beta agonist isoproterenol on slice cyclic AMP accumulation. In slices from rats given no hormone treatment, the beta antagonist propranolol inhibited norepinephrine-stimulated cyclic AMP production, while the alpha 1 antagonist prazosin was without effect. In contrast, the cyclic AMP response to norepinephrine in slices from estradiol-treated rats was blocked more effectively by prazosin than by propranolol. Estradiol treatment also attenuated the production of cyclic AMP by the beta agonist isoproterenol. The data suggest (a) that norepinephrine induction of cyclic AMP accumulation in hypothalamic and preoptic area slices is mediated by beta receptors and potentiated by alpha receptor activation and (b) that estradiol depresses beta and increases alpha 1 receptor function in slices from brain regions associated with reproductive physiology.     

Catecholamine effects on testicular testosterone production in the gonadally active and the gonadally regressed adult golden hamster

Several lines of evidence support a role of testicular innervation and peripheral catecholamines in the control of male gonadal function, particularly before puberty. It was therefore of interest to compare the effects of catecholamines on androgen production during the periods of gonadal activity and quiescence in a seasonally breeding species. We have examined direct effects of epinephrine (EPI), norepinephrine (NE), the beta-adrenergic agonist isoproterenol (ISO), and the alpha-adrenergic agonist phenylephrine (PHE) on testicular testosterone (T) production in hamsters with gonadal regression induced by 12 wk exposure to short photoperiod (SD) and in gonadally active hamsters maintained in long photoperiod (LD). Fragments of decapsulated testes were incubated with various combinations of these catecholamines (10(-5)-10(-9) M), human chorionic gonadotropin (hCG; 3.1 mIU/ml), the beta-receptor antagonist propranolol (10(-5) M) and the alpha-l-receptor antagonist prazosin (10(-5) M), for 6 h. In the incubations of testes from LD hamsters, the accumulation of T in the medium was stimulated by hCG but not affected by either catecholamine. However, EPI, NE, and PHE at 10(-5) M, but not ISO, augmented the stimulation of T by hCG. In sharp contrast to these findings, T production by the regressed testes of SD animals was stimulated by EPI (at 10(-8)-10(-5) M), NE (at 10(-6)-10(-5) M), and PHE (at 10(-6)-10(-5) M) in a dose-related manner, but unaffected by ISO. These stimulatory effects were prevented by prazosin, but not by propranolol. Moreover, 10(-5) M of EPI, NE, and PHE augmented the stimulatory effect of hCG on T production. We conclude that the seasonal transition from gonadal activity to quiescence in the adult golden hamster is accompanied by a major increase in the responsiveness of testicular steroidogenesis to catecholamines acting via the alpha-1-adrenoreceptor and that catecholamines can modulate Leydig cell response to gonadotropins in this species. These findings could be related to up-regulation of the alpha-1-receptor in the testis of the SD animal and suggest that catecholamines may be involved in the regulation of the testis during physiological suppression of gonadotropin release and during stress.     

Mechanisms of leptin secretion from white adipocytes

The mechanisms regulating leptin secretion were investigated in isolated rat white adipocytes. Insulin (1-100 nM) linearly stimulated leptin secretion from incubated adipocytes for at least 2 h. The adrenergic agonists norepinephrine, isoproterenol (two nonselective beta-agonists), or CL-316243 (potent beta3) all inhibited insulin (10 nM)-stimulated leptin release. The inhibitory effects of norepinephrine and isoproterenol could be reversed not only by the nonselective antagonist propranolol but also by the selective antagonists ICI-89406 (beta1) or ICI-118551 (beta2), the beta2-antagonist being less effective than the beta1. Insulin-stimulated leptin secretion could also be inhibited by a series of agents increasing intracellular cAMP levels, such as lipolytic hormones (ACTH and thyrotropin-stimulating hormone), various nonhydrolyzable cAMP analogs, pertussis toxin, forskolin, methylxanthines (caffeine, theophylline, IBMX), and specific inhibitors of phosphodiesterase III (imazodan, milrinone, and amrinone). Significantly, antilipolytic agents other than insulin (adenosine, nicotinic acid, acipimox, and orthovanadate) did not mimic the acute stimulatory effects of insulin on leptin secretion under these conditions. We conclude that norepinephrine specifically inhibits insulin-stimulated leptin secretion not only via the low-affinity beta3-adrenoceptors but also via the high-affinity beta1/beta2-adrenoceptors. Moreover, it is suggested that 1) activation of phosphodiesterase III by insulin represents an important metabolic step in stimulation of leptin secretion, and 2) lipolytic hormones competitively counterregulate the stimulatory effects of insulin by activating the adenylate cyclase system.     

Beta-adrenergic receptors and stimulatory effects of (-) isoproterenol on testosterone production in fetal mouse Leydig cells

Beta-adrenergic receptors were characterized in freshly excised fetal mouse testis using the radioiodinated antagonist iodocyanopindolol (ICYP). [125I]-CYP bound to a single class of high affinity sites with a KD value of 42.2 +/- 7.0 pM. Adrenergic agonists competed for ICYP binding sites with the following order of potency: (-)isoproterenol greater than (-)epinephrine much greater than (-)norepinephrine which is typical for a beta 2-adrenergic receptor. A selective beta 2-receptor antagonist ICI 118-551 showed an approximately 200 fold higher affinity than the beta 1-selective compound, betaxolol. The beta-adrenergic agonist (-)isoproterenol did not or slightly affect testosterone production by freshly isolated fetal Leydig cells. The ability of fetal Leydig cells to respond to (-)isoproterenol increased during culture. This change in responsiveness was not accompanied either by modification of the number of binding sites or by change in the binding affinity. Taken together these data suggest that i) the stimulatory effect of (-)isoproterenol on testosterone production by cultured fetal Leydig cells is mediated through beta 2-adrenergic receptors ii), the inability of freshly Leydig cells to respond to catecholamines is probably due to post receptor events.     

Roles of alpha and beta adrenergic receptors in control of glucose oxidation in hamster epididymal adipocytes.

Oxidation of [14C] glucose in isolated epididymal adipocytes from Golden hamsters was stimulated by isoproterenol, epinephrine and norepinephrine, which all interact with beta-adrenergic receptors and by adrenocorticotrophic hormone. In contrast alpha-receptor agonists, such as phenylephrine, methoxamine or clonidine did not increase basal glucose oxidation. The beta-adrenergic blocking drug propranolol inhibited both lipolysis and glucose oxidation when these had been stimulated by isoproterenol, epinephrine or norepinephrine. Conversely, the alpha-adrenergic blocking drugs phentolamine and phenoxybenzamine did not influence lipolysis or glucose oxidation when isoproterenol provided the stimulus and increased both lipolysis and glucose metabolism in the present of either epinephrine or norepinephrine. All alpha-adrenergic agonists tested (phenylephrine, methoxamine and clonidine) lowered lipolysis and glucose oxidation isolated adipocytes exposed to isoproterenol. However, when adrenocorticotropin provided the stimulus for glucose oxidation and lipolysis, only clonidine produced a significant reduction in lipolysis and glucose oxidation. None of the alpha-agonists influenced glucose metabolism which had been increased by insulin. These data confirm the presence of both alpha and beta adrenergic receptors on hamster epididymal adipocytes and suggest that they exert antagonistic influences on lipolysis and glucose oxidation. These data are also consistent with the view that adrenergic stimulation of glucose oxidation and lipolysis in adipocytes are both mediated through beta receptors.     

Coexistence of β1- and β2-Adrenoceptors in the Melanophore of the Goby Tridentiger obscurus*

The effects of β-adrenergic agonists and antagonists on the pigmentary state of denervated melanophores in isolated, split, caudal fins of the goby Tridentiger obscurus were examined to investigate the function and the subtype of the β-adrenoceptors of the melanophores. Salbutamol, terbutaline, and dobutamine partially inhibited the pigment-aggregating response of melanophores to norepinephrine. The effects of these β-agonists were inhibited by propranolol. It was confirmed that the melanophores possess both α-and β-adrenoceptors, and that the activation of the β-adrenoceptors induces the dispersion of pigment in the melanophores. Norepinephrine, epinephrine, isoproterenol, dobutamine, salbutamol, and terbutaline evoked the dispersion of pigment in the melanophores in which pigment had previously been aggregated by treatment with verapamil in the presence of phentolamine. The pigment-dispersing effects of two β₁-selective agonists, norepinephrine and dobutamine, were effectively inhibited by metoprolol, a selective antagonist of β₁-receptors. By contrast, the pigment-dispersing effects of two β₂-selective agonists, salbutamol and terbutaline, were not inhibited by metoprolol. Both the effects of nonselective agonists, epinephrine and isoproterenol, were partially inhibited by metoprolol. The actions of all of the β-agonists used were effectively inhibited by propranolol, and they were partially inhibited by butoxamine. These results suggest coexistence of β₁- and β₂-adrenoceptors in the melanophores. The relative numbers of β₁- and β₂-adrenoreceptors as a percentage of the total population of β-adrenoceptors were estimated to be 18.6% and 81.4%, respectively, from analyses of Hofstee plots of the effects of the β-agonists on the melanophores in the presence of butoxamine or metoprolol.     

Inhibition of cholinergic neurotransmission in human airways by beta 2-adrenoceptors

The purpose of the study was to determine whether catecholamines modulate cholinergic neurotransmission in isolated human airway smooth muscle. Bronchial rings were suspended in organ baths for isometric measurement of tension, and contractions were induced by either electrical field stimulation (EFS) or exogenous acetylcholine (ACh). Isoproterenol, epinephrine, and norepinephrine in that order of potency produced concentration-dependent inhibition of comparable responses to EFS and ACh. However a potency difference of 100-fold for isoproterenol (IC50 = 4.80 X 10(-8) M for EFS and 3.70 X 10(-6) M for ACh) and 10-fold for both epinephrine and norepinephrine was observed for inhibition of responses to EFS compared with responses to ACh. The inhibitory effects of isoproterenol on responses to EFS were prevented by propranolol and ICI 118551 (a beta 2-antagonist) but not by betaxolol (a beta 1-antagonist). Tyramine had no effect on contractions elicited by EFS. These experiments demonstrate that beta-agonists inhibit cholinergic nerve-induced contractions of human bronchi more potently than contractions induced by exogenous ACh, suggesting modulation of cholinergic neurotransmission by prejunctional beta 2-receptors.     

A preadipocyte clonal line from mouse brown adipose tissue. Short- and long-term responses to insulin and beta-adrenergics

A clonal cell line has been established from the interscapular brown adipose tissue (BAT) of the C57 BL/6J +/+ mouse. The line, designated BFC-1, is aneuploid and exhibits both morphological and biochemical properties characteristics of mature adipocytes. Adipose conversion begins after confluence and is accompanied by an early emergence of lipoprotein lipase; a later emergence of glycerol-3-phosphate dehydrogenase and acid: CoA ligase; an increase in the average triglyceride content. Adipose conversion, estimated by activities of enzyme markers, is enhanced at any given time by the continuous presence in the culture medium of insulin and triiodothyronine, both within their physiological range of concentrations. In addition to both hormones, chronic exposure of confluent cells to beta-adrenergics brings similar long-term effects on adipose conversion. The uptake of labelled 2-deoxyglucose by differentiated BFC-1 cells is stimulated by insulin; the half-maximum effect is observed at 1 nM insulin. Differentiated BFC-1 cells, in which endogenous triglycerides have been prelabelled on the fatty acid moiety, do respond to beta-adrenergics by releasing radioactive fatty acids. The agonist potency order and the EC50 value for each agonist are BRL 37344 (0.5 nM) greater than isoproterenol (1.5 nM) greater than norepinephrine (3 nM) greater than epinephrine (7 nM) greater than salbutamol (15 nM). The half-maximally and maximally effective concentrations of corticotropin to stimulate lipolysis are found to be 4 and 100 nM, respectively. The lipolytic response to isoproterenol is counteracted by prior addition of insulin or simultaneous addition of propranolol. Parallel studies performed on Ob17 cells, a clonal line established from mouse white adipose tissue (Négrel et al., Proc natl acad sci US 75 (1978) 6054), show that the agonist potency order and the EC50 value for each agonist are BRL 37344 (3 nM) greater than isoproterenol (10 nM) greater than norepinephrine (20 nM) greater than epinephrine (40 nM). Thus both BFC-1 cells and Ob17 cells show an atypical beta-adrenoreceptor similar to that described in rat adipocytes (Arch et al., Nature 309 (1984) 163), but the sensitivity of BFC-1 cells toward beta-agonists is found to be 6-fold higher than that of Ob17 cells. Thus the BFC-1 line represents a useful model for the study of short- and long-term responses to beta-adrenergics.