Regulation of beta-adrenoceptors in the guinea pig left ventricle and skeletal muscle following chronic agonist treatment

Pretreatment of guinea pigs with adrenaline, isoprenaline or terbutaline for 7 days significantly reduced the Bmax for the radioligand [125I]cyanopindolol (ICYP) in the gastrocnemius muscle (beta 2-adrenoceptors). Pretreatment of guinea pigs with terbutaline reduced the responsiveness of gastrocnemius muscle slices adenylate cyclase to isoprenaline (10(-4) M). In the left ventricle (predominantly beta 1-adrenoceptors) pretreatment of guinea pigs with isoprenaline or adrenaline for 7 days did not alter the Bmax for ICYP. The responsiveness of adenylate cyclase to isoprenaline (10(-4) M) in left ventricle slices was significantly reduced following isoprenaline pretreatment of the guinea pigs. Thus desensitisation of beta-adrenoceptors in left ventricle and skeletal muscle developed following chronic agonist pretreatment. Reduction of beta 2-adrenoceptors in the skeletal muscle could be responsible for the desensitisation of adenylate cyclase. In the left ventricle the receptors were resistant to agonist induced down-regulation. It is proposed that other mechanisms which are tissue- and species-specific independent of the receptor subtype can be responsible for agonist-induced desensitisation in the left ventricle of the guinea pig in vivo.     

The use of the guinea-pig lung parenchyma preparation in studies of the beta-adrenoceptor adenylate cyclase system.

The binding, adenylate cyclase activation, and functional effect of four beta-adrenoceptor agonists were studied in the guinea-pig lung parenchyma preparation and the results were compared with those obtained earlier in guinea-pig left-heart ventricle (beta 1-adrenoceptors) and soleus muscle (beta 2-adrenoceptors) preparations. The pKi-values of the unselective compounds, isoprenaline and orciprenaline, were in good agreement with those obtained in the heart and soleus muscle. The beta 2-adrenoceptor selective compounds KWD 2026 and terbutaline were bound to two sites, one corresponding to the beta 1-adrenoceptors and the other to the beta 2-adrenoceptors. The pKi-value of isoprenaline was in good agreement with its pKact-value indicating that maximum adenylate cyclase activity is obtained when the occupancy of the receptors is maximal. Further, the relative intrinsic efficacy calculated from the functional effect and receptor occupancy agreed well with the relative maximum adenylate cyclase activation by the agonists which was also found earlier for the guinea-pig heart ventricle and soleus muscle preparations. Relative effects were obtained from both functional experiments and from affinity and adenylate cyclase activating studies. There was good agreement between relative effects obtained in these two ways. It is concluded that the guinea-pig lung parenchyma preparation may be useful for the study of the beta-adrenoceptor adenylate cyclase system.     

Effect of pretreatment with bronchodilator drugs on in vitro responsiveness of guinea pig lung adenylate cyclase.

Pretreatment of guinea pigs with 5 microgram/kg isoprenaline or 10 microgram/kg salbutamol s.c. thrice daily for 7 days reduced the responsiveness of lung slice and tracheal ring adenylate cyclase to isoprenaline, but not to prostaglandin E1. Pretreatment of guinea pigs with isoprenaline also reduced the sensitivity of tracheal smooth muscle strip adenylate cyclase to isoprenaline. Cross-tolerance developed to noradrenaline in lung slices obtained from guinea pigs pretreated with isoprenaline. Propranolol blocked the response of lung slice adenylate cyclase of control and isoprenaline-pretreated animals to approximately the same degree. The presence of phentolamine in the incubation medium did not affect the reduced sensitivity to isoprenaline. Possible mechanisms of development of tolerance to sympathomimetic bronchodilator drugs are discussed.     

Human cardiac beta-adrenoceptors: both beta 1- and beta 2-adrenoceptors are functionally coupled to the adenylate cyclase in right atrium

We investigated properties of catecholamine-sensitive adenylate cyclase in membranes from human right atria. Basal adenylate cyclase was activated by Mg2+-ions and guanyl nucleotides [Gpp(NH)p, GTP] in a concentration-dependent manner; guanyl nucleotide activation was strongly dependent on the presence of Mg2+-ions. Catecholamines stimulated adenylate cyclase activity in the following order of potency: (-)-isoprenaline greater than (-)-adrenaline = (-)-noradrenaline greater than phenylephrine, indicating that, in human right atrium, beta 1-adrenoceptors predominate. The beta 1-agonist dobutamine and the beta 2-agonists fenoterol and procaterol activated adenylate cyclase with an intrinsic activity of 0.5-0.7 (isoprenaline = 1.0). Adenylate cyclase activation by dobutamine or procaterol was not additive with the activation induced by isoprenaline. On the contrary, combination of dobutamine (100 microM) and procaterol (10 microM) resulted in activation of adenylate cyclase which was not different from that evoked by saturating concentration of isoprenaline (10 microM), indicating that dobutamine (beta 1) and procaterol (beta 2) produce adenylate cyclase activation through stimulation of different beta-adrenoceptor subtypes. The beta 2-selective antagonist ICI 118,551 was much more potent in inhibiting procaterol-than isoprenaline-stimulated adenylate cyclase activity, whereas the beta 1-selective antagonist betaxolol inhibited isoprenaline-stimulated activity more potently. We conclude that in human right atrium, both beta 1- and beta 2-adrenoceptors are functionally coupled to the adenylate cyclase system.     

Lymphocyte beta 2-adrenoceptors and adenosine 3':5'-cyclic monophosphate during and after normal pregnancy.

1. The beta 2-sympathomimetics, used to inhibit preterm labour, bind predominantly to beta 2-adrenoceptors, activating adenylate cyclase to form adenosine 3':5'-cyclic monophosphate (cyclic AMP), a messenger substance which inhibits the enzyme cascade triggering smooth muscle contraction. beta 2-Adrenoceptor density and cyclic AMP formation can be used as markers of beta 2-adrenergic effect. 2. The present study addresses the influence of pregnancy on the beta-adrenoceptor system. beta 2-Adrenoceptor density and cyclic AMP concentrations (basal and evoked by isoprenaline) in circulating lymphocytes were determined at three points in gestation (16, 29 and 37 weeks) and 9 weeks post partum in 22 normal pregnancies. (-)-[125Iodo]-cyanopindolol was used as the ligand to identify a homogeneous population of beta 2-adrenoceptors on lymphocytes. B- and T-cell fractions were estimated from the same samples. 3. beta 2-Adrenoceptor density decreased significantly during gestation until week 37 (P < 0.01), then increased post partum (P < 0.005). Cyclic AMP concentrations (basal and evoked by isoprenaline) were significantly lower after 16 weeks of gestation than post partum (P < 0.05). 4. The results, which cannot be explained in terms of a shift in the lymphocyte (B- and T-cell) ratio, indicate that beta-adrenoceptor density and function are reduced in normal pregnancy and only return to normal post partum. These findings may be of significance in devising future tocolytic therapy with beta 2-adrenoceptor agonists.     

Beta 1- and beta 2-adrenoceptor antagonist activities of ICI-215001, a putative beta 3-adrenoceptor agonist.

1. The present study was undertaken to characterize the beta 3-adrenoceptor agonist activity of ICI-215001 and to determine whether it exhibits additional activities on beta 1- and beta 2-adrenoceptors in isolated spontaneously beating atrium, trachea and ileum of guinea-pig. 2. In guinea-pig atrium, isoprenaline, a non-selective beta-adrenoceptor agonist, caused concentration-dependent, positive chronotropic effects that were inhibited by atenolol, a selective beta 1-antagonist. ICI-215001 also competitively antagonized the increase in heart rate caused by isoprenaline. 3. ICI-215001 exhibited low intrinsic activity at increasing the beating rate of atrium and no activity on resting or induced tone of tracheal strips. 4. In strips of guinea-pig trachea, contracted submaximally with carbachol, isoprenaline, caused concentration-dependent relaxations. Both ICI-118551, a selective beta 2-adrenoceptor antagonist, and ICI-215001 competitively inhibited the relaxations caused by isoprenaline. 5. In isolated strips of guinea-pig ileum longitudinal smooth muscle contracted with histamine, isoprenaline and ICI-215001 caused relaxations which were inhibited by alprenolol, a beta-adrenoceptor antagonist with modest affinity for beta 3-adrenoceptors, but were resistant to ICI-118551 and atenolol. 6. These results indicate that ICI-215001 exhibits beta 3-adrenoceptor agonist activity as demonstrated by relaxations mediated via atypical beta-adrenoceptors in the longitudinal smooth muscle of guinea-pig ileum. Further, the studies demonstrate that ICI-215001 can act as an antagonist at beta 1- and beta 2-adrenoceptors in situations where its intrinsic agonist activity is low.     

Mechanism of isoprenaline induced desensitization of rabbit aortic smooth muscle beta-adrenoceptor responses

The effects of short term (1 h) as well as prolonged (16 h) activation of beta-adrenoceptors by isoprenaline on the functioning of the beta-adrenoceptor linked adenylate cyclase system of rabbit aortic smooth muscle cells in the contractile phenotype in primary culture was examined. Smooth muscle cells responded to 50 mumol/l isoprenaline with a rapid increase in intracellular cAMP. Continued exposure of these cells to this concentration of isoprenaline results in a rapid time dependent reduction in maximum beta-adrenoceptor responsiveness. In vitro analyses of adenylate cyclase activities, phosphodiesterase activity and 125I-iodocyanopindolol specific (beta-adrenoceptor) binding sites suggest that the decrease in the cells' ability to increase intracellular cAMP may be due to a reduction in beta-adrenoceptor binding sites. The loss in cell receptor responsiveness and membrane receptor concentration was only very slowly reversible. These results suggest that following prolonged exposure of vascular smooth muscle cells to beta-adrenoceptor agonists, beta-adrenoceptors are rapidly internalized and degraded. This mechanism may account for the haemodynamic tolerance observed to chronic therapy with beta-adrenoceptor agonists.     

Difference of tolerances to isoprenaline between tracheal and vascular smooth muscles and cardiac conduction system in guinea pigs

Pretreatment to guinea pigs with sc isoprenaline (Iso) 10 micrograms/kg tid x 7 d reduced the effect of Iso on protecting histamine-induced asthma and decreased its pD2 values in relaxing isolated tracheal strip. This treatment did not change the asthmatic effect induced by histamine and the effect of Iso on positive chronotropic action, but elevated the blood pressure. These results suggest that it is easier to develop the tolerance of beta 2-adrenoceptors of respiratory smooth muscles than that of beta 1-adrenoceptors of heart. Radioligand binding assay showed that the treatment decreased the number of binding sites of beta-adrenoceptors on lungs of guinea pigs but did not change the binding affinity.     

Pseudomonas exotoxin A prevents beta-adrenoceptor-induced upregulation of Gi protein alpha-subunits and adenylyl cyclase desensitization in rat heart muscle cells

Exposure of rat heart muscle cells to noradrenaline (1 microM) for 48 hr led to a decrease in the number of beta 1-adrenoceptors of 50% and a concomitant decrease in adenylyl cyclase stimulation by isoprenaline and forskolin of about 60 and 30%, respectively. In addition, the levels of two inhibitory guanine nucleotide-binding protein (Gi protein) alpha-subunits (Gi alpha 40 and Gi alpha 41) were increased in membranes of noradrenaline-treated cells. Evidence is presented that noradrenaline induces this increase by activation of beta-adrenoceptors. First, the noradrenaline action was mimicked by the beta-adrenoceptor agonist isoprenaline. Second, beta-adrenoceptor blockade by timolol but not alpha-adrenoceptor blockade by prazosin prevented the noradrenaline-induced up-regulation of Gi alpha proteins. Furthermore, timolol but not prazosin abolished the noradrenaline-induced down-regulation of beta 1-adrenoceptors and the decreases in receptor-dependent (isoprenaline) and -independent (forskolin) adenylyl cyclase stimulation. The specific protein synthesis inhibitor Pseudomonas exotoxin A was used to study whether the noradrenaline-induced up-regulation of Gi alpha subunits depends on increased synthesis of these proteins. This toxin inhibits peptide chain elongation by ADP-ribosylating elongation factor 2. Treatment of rat heart muscle cells with Pseudomonas exotoxin A (1 ng/ml) completely prevented the noradrenaline-induced increase in Gi alpha proteins, measured by both pertussis toxin-catalyzed ADP-ribosylation and immunoblotting with anti-Gi alpha antibodies. Most importantly, Pseudomonas exotoxin A also completely prevented the noradrenaline-induced decrease in forskolin-stimulated adenylyl cyclase activity. Furthermore, the noradrenaline-induced decrease in isoprenaline-stimulated adenylyl cyclase activity was significantly attenuated by the toxin, although the down-regulation of beta 1-adrenoceptors caused by noradrenaline treatment was not affected. The data presented suggest that prolonged activation of beta-adrenoceptors in rat heart muscle cells, in addition to causing a receptor down-regulation, induces the synthesis of Gi alpha proteins, which then apparently mediate a decreased adenylyl cyclase responsiveness. The data, additionally, suggest that the synthesis of Gi alpha proteins is under control of the activity of the adenylyl cyclase system and that altered levels of these proteins may play a major role in long term regulation of signal transduction by this enzyme.     

A different desensitization pattern of cardiac beta-adrenoceptor subtypes by prolonged in vivo infusion of isoprenaline

(-)Isoprenaline was continuously administered to rats at a rate of 0.4 mg/kg/h for 7 days via subcutaneously (s.c.) implanted osmotic minipumps. This treatment induced cardiac hypertrophy and a marked decrease in basal as well as catecholamine-stimulated adenylate cyclase activity in a ventricular plasma membrane fraction. The total number of beta-adrenoceptors was downregulated by one-half the amount of the receptor sites obtained in a control group. However, in the isoprenaline-treated group, the beta 2-adrenoceptors constituted a significantly smaller proportion of the total beta-adrenoceptor population (28%) than in the control group (50%). Transformation of these relative into absolute values indicates that prolonged isoprenaline treatment induced a significantly higher downregulation of beta 2- than of beta 1-adrenoceptors. The fact of a different beta-adrenoceptor desensitization pattern in response to in vivo administration of nonselective beta-adrenergic agonists therefore must be taken into consideration when desensitization is used as a method for determination of subtype selectivity of an agonist per se. However, we were unable to detect the "lost" beta-adrenoceptors in a light vesicular fraction. In our study, this fraction was not separable from plasma membranes, as substantiated by levels of plasma membrane markers as high as in the plasma membrane fraction and by a guanine nucleotide-dependent adenylate cyclase activity.     

Coexistence of beta 1- and beta 2-adrenoceptors in the rabbit heart: quantitative analysis of the regional distribution by (-)-3H-dihydroalprenolol binding

We determined the amount of beta 1- and beta 2-adrenoceptors in right and left atria and ventricles of rabbits. For this purpose inhibition of specific (-)-3H-dihydroalprenolol [(-)-3H-DHA] binding (5 nM) by beta 1-selective (practolol, metoprolol) and beta 2-selective (zinterol, IPS 339) adrenergic drugs was determined and analyzed by pseudo-Scatchard (Hofstee) plots. For both atria, inhibition of binding by the four selective beta-adrenergic drugs resulted in non-linear Hofstee plots, suggesting the coexistence of both beta-adrenoceptor subtypes. From these plots we calculated a beta 1:beta 2-adrenoceptor ratio of 72:28 for the right atrium and of 82:18 for the left. In contrast, only a very small amount of beta 2-adrenoceptors (approximately 5-7% of the total beta-adrenoceptor population) could be detected in the ventricles. For comparison we analyzed the inhibition of specific (-)-3H-DHA binding in tissues with homogeneous population of beta-adrenoceptors (beta 1:guinea pig left ventricle; beta 2: cerebellum of mature rats). For both tissues the four selective beta-adrenergic drugs showed linear Hofstee plots, demonstrating that in tissues with homogeneous beta-receptor population interaction of each drug with the receptor followed simple mass-action kinetics. We conclude that beta 1- and beta 2-adrenoceptors coexist in rabbit atria while the ventricles are predominantly endowed the beta 1-adrenoceptors.     

An analysis of the beta 2-adrenoceptor selectivity in three series of beta-adrenoceptor agonists

The aim of the study was to analyse the beta 2-adrenoceptor selectivity earlier found in two series of catecholamines and one series of resorcinolamines (Johansson et al. 1986). The affinity of the compounds was assessed in binding studies in preparations from the guinea-pig left heart ventricle (beta 1-adrenoceptors) and the soleus muscle (beta 2-adrenoceptors) using 3H-CGP-12177 as radioligand. Further, the activation of the adenylate cyclase by the compounds was studied in the same preparations. Selectivity quotients were obtained from both functional effects and from affinity and adenylate cyclase activating studies. There was a good correlation between the selectivity quotients obtained in these two ways. Tertiary butyl substitution on the amino nitrogen gave the highest beta 2-adrenoceptor selectivity in both the catechol and resorcinol series. In comparison with their isopropyl substituted analogues the beta 2-adrenoceptor selectivity of these compounds (KWD 2026 and terbutaline) was mainly due to a change in affinity for the beta 1- and beta 2-adrenoceptors and, to a lesser degree, a change in intrinsic efficacy.     

Blockade of beta 1- and desensitization of beta 2-adrenoceptors reduce isoprenaline-induced cardiac fibrosis

The aim of the present study was to analyse the role of beta(1)- and beta(2)-adrenoceptors in the catecholamine-induced myocardial remodeling, especially the interstitial fibrosis. Wistar rats were subjected to a 2-week chronic isoprenaline administration (30 microg/kg/h). Rats received a concomitant treatment with the selective beta(1)-adrenoceptor antagonist, bisoprolol (50 mg/kg/day p.o.) or were chronically pretreated with the selective beta(2)-adrenoceptor agonist salbutamol (40 microg/kg/h) for 1 week to induce beta(2)-adrenoceptor desensitization. The pretreatment with salbutamol induced a 59% down-regulation of left ventricular beta(2)-adrenoceptors compared to control. The extent of the isoprenaline-induced left ventricular fibrosis was significantly reduced in both the bisoprolol and salbutamol groups compared with the control isoprenaline-treated group especially in the apical region (1.7+/-0.6% and 1.4+/-0.3% versus 6.0+/-1.3%, respectively, P<0.005). beta(1)-adrenoceptor blockade and beta(2)-adrenoceptors down-regulation provided similar protection against isoprenaline-induced cardiac interstitial fibrosis suggesting that both beta-adrenoceptors are involved in such cardiac remodeling process.     

Regional distribution of beta-adrenoceptors in the human heart: coexistence of functional beta 1- and beta 2-adrenoceptors in both atria and ventricles in severe congestive cardiomyopathy

We evaluated the amount of beta 1- and beta 2-adrenoceptors in human right and left atrium as well as in right and left ventricular wall obtained from heart transplant recipients who suffered from end-stage congestive cardiomyopathy. The total number of myocardial beta-adrenoceptors was assessed with the nonsubtype selective beta-adrenoceptor radioligand (-)[125I]iodocyanopindolol (ICYP); concomitantly, the number of beta 1-adrenoceptors was determined with the selective beta 1-adrenoceptor radioligand (-)[3H]bisoprolol. The number of beta 2-adrenoceptors was calculated by subtracting (-)[3H]bisoprolol binding sites from ICYP binding sites. With this technique, a beta 1/beta 2-ratio of approximately 65/35% for both atria and of approximately 75/25% for both ventricles was found. Identical results were obtained when the beta 1/beta 2-ratio was calculated indirectly by nonlinear regression analysis of competition curves of the selective beta 1-adrenoceptor antagonist bisoprolol and the selective beta 2-adrenoceptor antagonist ICI 118,551 with ICYP binding. In addition, on atria and on ventricles, adenylate cyclase was activated by norepinephrine (presumably by beta 1- and beta 2-adrenoceptor stimulation) and by procaterol (by beta 2-adrenoceptor stimulation). It is concluded that in the human heart functional beta 1- and beta 2-adrenoceptors coexist on both atria and both ventricles. In end-stage congestive cardiomyopathy, there appears to be a selective down-regulation of cardiac beta 1-adrenoceptors, whereas beta 2-adrenoceptors are obviously not affected. This may explain the beneficial effects of beta 2-adrenoceptor agonists in severe heart failure.     

Characterization of beta-adrenoceptor subtypes in rat kidney with new highly selective beta 1 blockers and their role in renin release

Highly selective beta-adrenoceptor blocking agents with a beta 1: beta 2-selectivity ratio of 0.015 to 3400 were used to characterize the beta-adrenoceptors present in rat kidney and to identify those mediating renin release. The results obtained with ICYP binding to kidney membranes revealed the presence of both beta 1- and beta 2-adrenoceptors in a ratio of 1:1. The pKD beta 1- and pKD beta 2-values of selective beta-antagonists obtained in rat kidney membranes correlated well with those found in guinea pig left ventricle (beta 1) and lung (beta 2), indicating that kidney receptor subtypes are pharmacologically identical with those in the ventricle and lung, respectively. In the isolated perfused rat kidney, the apparent pA2 values of beta 1-selective blockers for inhibition of isoprenaline-stimulated renin release correlated well with pKD beta 1, but not with pKD beta 2 values. These results clearly show that the beta 1-adrenoceptor subtype mediates renin release in the rat kidney.     

ICI 118,551: an effective ocular hypotensive agent with selectivity for the ciliary process beta 2-adrenoceptor and with minimal cardiac side effects

Prior biochemical studies have shown that the ciliary process epithelium, which is involved in the secretion of aqueous humour, is rich in beta-adrenoceptors with pharmacological characteristics similar to those of the beta 2 subclass. The present experiments demonstrate that the beta-adrenoceptor antagonist, ICI 118,551, is a potent inhibitor of isoprenaline-stimulated adenylate cyclase activity measured in broken cell preparations of rabbit ciliary process. In rabbit cardiac muscle, however, ICI 118,551 is a relatively weak antagonist of isoprenaline-stimulated adenylate cyclase, being approximately 100 fold less potent than the non-selective beta-adrenoceptor antagonist, timolol. ICI 118,551 is also less potent than timolol in inhibiting isoprenaline-sensitive adenylate cyclase of rabbit lung. ICI 118,551 applied topically to eyes of unanaesthetized rabbits causes a dose-dependent decrease in intraocular pressure. Furthermore, in a blind crossover study in rabbits, topically applied ICI 118,551 decreased intraocular pressure for more than 6 h and was more effective than an identical dose of the clinically effective anti-glaucoma agent, timolol. Systemic absorption from topically-applied timolol, but not ICI 118,551, is sufficient to alter cardiac response to subcutaneous administration of isoprenaline. Furthermore, dose-response studies, using direct systemic administration of the two beta-adrenoceptor antagonists, revealed that ICI 118,551 is about 60 times less potent than timolol in blocking isoprenaline-induced cardio-acceleration. ICI 118,551, applied to one eye, causes a decrease in intraocular pressure in the contralateral eye, and systemic administration of ICI 118,551 results in decreased intraocular pressure in both eyes, data indicating that at least part of the ocular hypotensive effect of topical ICI 118,551 is mediated through systemic absorption.(ABSTRACT TRUNCATED AT 250 WORDS)     

Contribution of beta 1- and beta 2-adrenoceptors of human atrium and ventricle to the effects of noradrenaline and adrenaline as assessed with (-)-atenolol.

1. (-)-Atenolol was used as a tool to assess the function of beta 1- and beta 2-adrenoceptors in human heart. Right atrial and left ventricular preparations from patients undergoing open heart surgery were set up to contract isometrically. Membrane particles were prepared for beta-adrenoceptor labelling with [3H]-(-)-bupranolol and adenylate cyclase assays. 2. The positive inotropic effects of (-)-noradrenaline were antagonized to a similar extent by (-)-atenolol in atrial and ventricular preparations. (-)-Atenolol consistently antagonized the effects of (-)-adrenaline to a lesser extent than those of (-)-noradrenaline in atrial preparations. In ventricular preparations (-)-atenolol antagonized the effects of low concentrations of (-)-adrenaline to a lesser extent than those of high concentrations. 3. pKB values (M) of (-)-atenolol, estimated with non-linear analysis from the blockade of the positive inotropic effects of the catecholamines, were 7.4 for beta 1-adrenoceptors and 6.0 for beta 2-adrenoceptors. 4. (-)-Atenolol inhibited the binding of [3H]-(-)-bupranolol to ventricular beta 1-adrenoceptors with a pKD (M) of 5.9 and to ventricular beta 2-adrenoceptors with a pKD of 4.6. 5. (-)-Atenolol inhibited the catecholamine-induced adenylate cyclase stimulation in the atrium and ventricle with pKB values of 5.8-6.4 for beta 1- and pKB values of 4.7-5.7 for beta 2-adrenoceptors. The binding and cyclase assays suggest a partial affinity loss for (-)-atenolol inherent to membrane preparations. 6. beta 1-Adrenoceptors mediate the maximum positive inotropic effects of (-)-noradrenaline in both the atrium and ventricle of man. beta 2-Adrenoceptors appear to be capable of mediating maximal positive inotropic effects of (-)-adrenaline in atrium. In contrast, ventricular beta 2-adrenoceptors mediated only submaximal effects of (-)-adrenaline.     

Alterations in blood levels of carbohydrate and lipid metabolites and of cyclic AMP mediated by beta1- and beta2-adrenoceptors in beagle dogs: effects of procaterol, a new selective beta2-adrenoceptor agonist.

The intravenous injection of isoprenaline (10 nmole/kg) into conscious beagle dogs caused significant increases in the blood level of lactate, glucose, FFA, insulin and cyclic AMP. These metabolic alterations induced by isoprenaline were blocked completely by pretreatment of the dog with propranolol (1 mg/kg). Butoxamine (10 mg/kg) antagonized isoprenaline-induced increases in glucose, lactate and insulin, but not the increases in FFA. Practolol (10 mg/kg) diminished the increase in blood FFA very strongly. Salbutamol, which is known to be an agonist of the beta 2-subtype in its bronchomotor and cardiovascular actions, produced marked increases in the blood concentrations of lactate, glucose and insulin but were without effect on the FFA level. Thus metabolic responses of conscious beagle dogs to beta-adrenoceptor agonists appeared to depend differentially on two types of beta-adrenoceptors: beta1-adrenoceptors are largely involved in lipolysis while beta2-adrenoceptors are involved in the regulation of blood glucose metabolism and insulin secretion. A new beta-adrenoceptor agonist, 5-(1-hydroxy-2-isopropylaminobutyl)-8-hydroxycarbostyril hydrochloride hemihydrate (Procaterol), was classified as a beta 2-agonist, because it markedly increased plasma concentrations of glucose, lactate and insulin but increased the plasma level of FFA to a lesser degree. The order of potency of beta2-agonists was procaterol greater than salbutamol greater than trimetoquinol. Metabolic responses of beagle dogs would be useful for appreciating the selectivity and potency of beta-adrenoceptor agonists and antagonists.     

Discrepancies between inotropic responses and beta-adrenoceptor characteristics after global ischemia in isolated hearts.

The influence of global ischemia on cardiac beta-adrenoceptors was studied in rat and guinea pig Langendorff hearts (LH), both by functional and binding experiments using the specific beta-adrenoceptor ligand (-)-[125I]-iodocyanopindolol. Neither ischemia (30 or 60 min) nor postischemic reperfusion caused any change in beta-adrenoceptor density, affinity or in the beta 1/beta 2 ratio in LH of normal rats or in LH of rats pretreated with reserpine or 6-hydroxydopamine (6-OHDA), or in guinea pig LH, whereas perfusion of rat LH with 10(-5) M isoprenaline (15 min) caused the expected decrease in beta-adrenoceptor density. After ischemia, isoprenaline was no longer able to influence beta-adrenoceptor density, suggesting that the internalization mechanism is impaired. In functional studies, perfusion of the rat LH with 10(-5) M isoprenaline (15 min) shifted the concentration-response curve for isoprenaline to the right. Thirty-minute global ischemia virtually abolished the inotropic but not the chronotropic response to isoprenaline. Ischemia did not impair the inotropic response to ouabain or to calcium, indicating that the contractile apparatus itself was still largely intact. Our results suggest that the contractile failure after ischemia is not caused by a decrease in beta-adrenoceptor density or by a defect in the contractile apparatus but by an impaired second-messenger system.     

Pharmacological analysis of atypical beta-adrenoceptors in the guinea pig gastrointestinal tissue systems

The purpose of the present study was to characterize the atypical beta-adrenoceptors involved in relaxant responses in guinea pig gastric fundus, duodenum and ileum in functional experiments with catecholamines (isoprenaline, noradrenaline and adrenaline), beta 3-adrenoceptor agonists (BRL37344 and CGP12177A) and a non-selective beta 1-, beta 2- and beta 3-adrenoceptor antagonist bupranolol, and to obtain further evidence to clarify whether there is a tissue difference in atypical beta-adrenoceptors in the guinea pig gastrointestinal tissue systems. The atypical beta-adrenoceptors are present in gastric fundus, duodenum and ileum of guinea pig. In the presence of propranolol (1 microM) or atenolol (100 microM) plus butoxamine (100 microM), bupranolol caused a concentration-dependent rightward shift of the concentration-response curves for catecholamines and beta 3-adrenoceptor agonists. There was not a significant difference of pA2 values for bupranolol against these agonists between gastric fundus, duodenum and ileum of guinea pig. These results suggest that guinea pig gastric fundus, duodenum and ileum relaxation are mediated predominantly by an atypical beta-adrenoceptor population whereas the classical beta 1- or/and beta 2-adrenoceptors play a subordinate function role and that the receptors of three tissues are pharmacological identified by functional approaches. There is not a tissue difference in atypical beta-adrenoceptors in the guinea pig gastrointestinal tissue systems between stomach and ileum.