Beta-adrenoceptor density in chronic infarcted myocardium: a subtype specific decrease of beta1-adrenoceptor density

Beta-adrenoceptor density is altered in different cardiac diseases. In heart failure beta-adrenoceptor density is down regulated but in acute myocardial ischemia beta-adrenoceptor density is up regulated. In hearts with myocardial infarction total beta-adrenoceptor density is decreased shortly after myocardial infarction. AIMS AND METHODS: To investigate whether total beta-adrenoceptor number is altered in the chronic phase after myocardial infarction, and to identify the specificity of alteration, we studied male Wistar rats (n = 18) which underwent a ligation of the left coronary artery or a sham operation. Twelve weeks after coronary ligation, rats were sacrificed and hearts were excised, perfused to obtain blood-free myocardium and frozen in liquid nitrogen. Infarcted myocardium was identified visually and separated from non-infarcted myocardium. Total beta-adrenoceptor number was calculated in fmol (-)-[125I]iodocyanopindolol specifically bound/mg protein and the relative amount of beta1- and beta2-adrenoceptor density was measured by inhibition of (-)-[125I]iodocyanopindolol binding with CGP 20712 A. RESULTS: Total beta-adrenoceptor number in infarcted myocardium was significantly decreased (25.7+/-1.4 vs. 24.9+/-2.2 vs. 20.1+/-3.2 fmol/mg protein (P=0.03) resp. Sham vs. Non-infarcted vs. Infarcted myocardium), due to a decrease of only beta1-adrenoceptor density (14.7+/-0.61 vs. 12.7+/-1.09 vs. 4.84+/-0.96 fmol/mg protein (P=0.004) resp.), whereas the beta2-adrenoceptor density and the dissociation constant (Kd) were not significantly decreased. CONCLUSION: In the infarcted myocardium total beta-adrenoceptor density is decreased due to a decreased beta1-adrenoceptor density at 12 weeks after myocardial infarction.     

Decrease in beta 1- and increase in beta 2-adrenoceptors in long-term follow-up after orthotopic cardiac transplantation.

Total beta 1- and beta 2- subtype distribution were examined in right ventricular biopsies taken from 100 patients 1-60 months after orthotopic cardiac transplantation and from eight prospective transplant donor hearts serving as controls. The patients were classified into eight groups depending on the time after transplantation that the biopsies were taken: 1-3 (n = 15), 4-7 (n = 15), 8-11 (n = 6), 12 (n = 15), 24 (n = 15), 36 (n = 12), 48 (n = 12) and 60 months (n = 10). The non-selective beta-adrenoceptor antagonist (-)-[125I]-iodocyanopindolol (ICYP) was used as a radioligand to assess total beta-adrenoceptor density. The beta 1- and beta 2-subtype distribution was determined with a beta 1-adrenoceptor saturating concentration of the selective beta 1-adrenoceptor antagonist CGP 20712A (300 nmol/l). In transplant donor hearts the total beta-adrenoceptor density was found to be 70.8 +/- 7.1 fmol/mg protein including a beta 1:beta 2-adrenoceptor ratio of about 80:20%. Until 36 months after cardiac transplantation the total number of beta-adrenoceptors showed no significant change. A slight but insignificant decrease was observed after 48 (16.2%) and 60 (21.2%) months. In contrast, from 12 to 60 months after cardiac transplantation the beta 1:beta 2-adrenoceptor ratio was shifted significantly (66:33% to 61:39%) as compared with transplant donor hearts which was due to an increase in beta 2- and a decrease in beta 1-adrenoceptor number. Thus, the surgically denervated, transplanted human heart exhibits a beta 2-adrenoceptor up-regulation during long-term follow-up. It is suggested that this up-regulation of the beta 2-adrenoceptor subtype could be owing to an increased importance of circulating catecholamines in modulating positive chronotropic and inotropic effects.     

Beta 1- and beta 2-adrenoceptors in sheep cardiac ventricular muscle.

The presence of beta 2-adrenoceptors in the sheep ventricular myocardium was assessed by the radioligand binding technique and functional studies. In membrane preparations, the competition curve between [3H]-dihydroalprenolol and the selective beta 1-antagonist CGP 20712A (0.1 nM-1 mM) was clearly biphasic, and revealed the presence of two different binding sites showing an affinity (pKD) for CGP 20712A of 9.5 +/- 0.9 and 4.5 +/- 0.4, respectively. The relative proportion of beta 1:beta 2 adrenoceptors was about 70:30 in both the right and left ventricle. In ventricular trabeculae driven at 1Hz, isoprenaline (1-300 nM) caused a dose-dependent increase in the force of contraction, the maximum effect being 298 +/- 26 mg, associated with reduction of time to peak tension (t1, clinotropic effect) and relaxation time (t2, 298 +/- 26 mg, associated with reduction of time to peak tension (t1, clinotropic effect) and relaxation time (t2, lusitropic effect). The inotropic dose-response curve for isoprenaline was significantly shifted to the right by pretreatment of the preparations with 0.1 microM CGP 20712A or with the selective beta 2-antagonist ICI 118551 (50 nM). In the presence of CGP 20712A (0.1 microM), isoprenaline, up to a concentration of 10 microM, did not affect either t1 or t2; on the other hand, pretreatment of the preparations with ICI 118551 (50 nM) fully antagonized the clinotropic but not the lusitropic effect of isoprenaline. In the presence of CGP 20712A procaterol (0.01-10 microM), a beta 2-adrenoceptor agonist, induced a positive intropic effect which was not associated with any significant modifications in t1 or t2. This effect was completely abolished by ICI 118551 (50 nM). The positive inotropic action of isoprenaline (1 microM) was associated with a significant decrease in action potential duration measured at -60 mV (220 +/- 8 and 193 +/- 10 ms in the absence and presence of isoprenaline, respectively; P less than 0.05). In the presence of CGP 20712A (0.1 microM) alone, isoprenaline (1 microM) still induced a significant increase in contractility but the action potential profile was only slightly affected. The effects of isoprenaline were fully antagonized by the simultaneous presence of CGP 20712A and ICI 118551 (10 nM). It is concluded that both beta 1- and beta 2-adrenoceptors appear to coexist in sheep ventricular myocardium where their stimulation mediates a positive inotropic effect. However, their functional role on the relaxation phase of the twitch may be different.     

Electrophysiological characterization of cardiac beta 2-adrenoceptors in sheep Purkinje fibers

The electrophysiological effects mediated by beta 2-adrenoceptor stimulation were studied in sheep cardiac Purkinje fibers. beta 2-Adrenoceptor stimulation was achieved by using isoproterenol (ISO) in the presence of the highly selective beta 1-antagonist CGP 20712A. ICI 118,551 was used as a selective beta 2-antagonist. In driven preparations, ISO (0.1 to 1 microM) caused a positive inotropic effect which was associated with the shortening of action potential duration and was completely abolished only by the simultaneous presence of CGP 20712A (0.3 microM) and ICI 118,551 (50 nM). In previously quiescent preparations, ISO (0.1 to 1 microM) induced spontaneous activity in 15 out of 24 preparations. In the presence of CGP 20712A (0.3 microM) only five preparations out of 24 became automatic when exposed to ISO, and their rate of firing was significantly reduced (13 +/- 4 vs. 43 +/- 6 beats/min, P less than 0.05) with respect to ISO alone. CGP 20712A completely abolished the steepening of diastolic depolarization and the increase of the pacemaker current caused by ISO (0.1 to 1 microM). The beta 2-antagonist ICI 118,551 (50 nM) completely failed to modify the effect of ISO on the rate of spontaneous firing, the slope of diastolic depolarization and the pacemaker current. On the other hand, the increase of oscillatory afterpotential (OAP) amplitude caused by ISO in strophanthidin-treated preparations was significantly reduced only by the beta 1-antagonist CGP 20712A, but not by the beta 2-antagonist ICI 118,551. These results demonstrate that beta 2-adrenoceptors are functionally present in sheep Purkinje fibers where their stimulation consistently causes a positive inotropic effect. However, beta 2-adrenoceptors do not appear to affect the processes of normal automaticity, and do not greatly contribute to triggered activity due to OAPs.     

Beta-adrenergic control of motility in the rat colon. II. Proportions of beta 1- and beta 2-adrenoceptors identified with 125I-(-)pindolol binding

The specific binding of 125I-(-)pindolol to membranes prepared from rat colon displayed high affinity (Kd = 75 +/- 3 pM) and relatively low capacity (Bmax = 48 +/- 3.8 fmol/mg protein). Furthermore, the binding was reversible and possessed properties expected of a beta-adrenoceptor. Thus, for example, the potency of agonists in displacing 125I-(-)pindolol was, in decreasing order, isoproterenol greater than epinephrine greater than norepinephrine, and the negative isomer of propranolol was two orders of magnitude more potent than the positive isomer. Displacement of specific binding by the beta 1-adrenoceptor antagonist pafenolol and the beta 2-adrenoceptor antagonist ICI 118.551 revealed that the colon preparation possessed a heterogenous beta-adrenoceptor population. Analysis of the inhibition curves using computer-assisted curve fitting suggested that the sites consisted of a small (14%-21%) beta 1-adrenoceptor population and a large (79%-86%) beta 2-adrenoceptor population. The coexistence of beta 1- and beta 2-adrenoceptors is discussed in relation to evidence of a functional linkage of both subtypes with colon motility.     

Characterization of beta 1- and beta 2-adrenoceptor subtypes in the rat atrioventricular node by quantitative autoradiography

We characterized beta-adrenoceptor subtypes in the atrioventricular node of the rat heart by quantitative autoradiography. Consecutive 16-microns-thick sections from single rat hearts containing the atrioventricular node were incubated with increasing concentrations of [125I]iodocyanopindolol. After exposure to [3H]Ultrofilm, optical densities corresponding to the atrioventricular node were determined by computerized densitometry after comparison with [125I]standards. The computer program LIGAND was used for analysis of receptor subtypes. Delineation of beta-adrenoceptor subtypes was achieved by incubating consecutive tissue sections with 50 pM [125I]iodocyanopindolol in the presence of increasing concentrations of the beta 1-selective antagonist atenolol or the beta 2-selective antagonist ICI 118,551. The atrioventricular node contains a higher concentration of beta-adrenoceptors than the adjacent interventricular septum. We estimated that the proportions of beta 1- and beta 2-adrenoceptors in the atrioventricular node were about 56% and 44% of the total binding capacity respectively.     

Agonist-induced desensitization of beta-adrenoceptor function in humans. Subtype-selective reduction in beta 1- or beta 2-adrenoceptor-mediated physiological effects by xamoterol or procaterol

We investigated the effects of beta 2- (procaterol 2 x 50 micrograms/day for 9 days) and beta 1- (xamoterol 2 x 200 mg/day for 14 days) adrenoceptor agonists on lymphocyte beta 2-adrenoceptor density and beta 1- and beta 2-adrenoceptor in vivo function (assessed as isoprenaline-infusion-evoked hemodynamic effects and exercise-induced tachycardia) in healthy volunteers. Procaterol decreased lymphocyte beta 2-adrenoceptor density and all beta 2-adrenoceptor-mediated in vivo effects but did not affect beta 1-adrenoceptor-mediated in vivo effects. In contrast, xamoterol neither affected lymphocyte beta 2-adrenoceptors nor beta 2-adrenoceptor-mediated in vivo effects but decreased beta 1-adrenoceptor-mediated in vivo effects. It is concluded that in humans, generally long-term application of beta 1- or beta 2-adrenoceptor agonists causes desensitization of beta-adrenoceptor function but in a beta-adrenoceptor subtype-selective fashion.     

Beta-adrenoceptor stimulation attenuates the hypertrophic effect of alpha-adrenoceptor stimulation in adult rat ventricular cardiomyocytes

OBJECTIVES: The study investigated whether beta-adrenoceptor antagonists augment the hypertrophic response of cardiomyocytes evoked by norepinephrine. BACKGROUND: In adult ventricular cardiomyocytes, stimulation of alpha- but not beta-adrenoceptors induces myocardial hypertrophy. Natural catecholamines, like norepinephrine, stimulate simultaneously alpha- and beta-adrenoceptors. We investigated whether beta-adrenoceptor stimulation interferes with the hypertrophic response caused by alpha-adrenoceptor stimulation. METHODS: Adult ventricular cardiomyocytes isolated from rats were used as an experimental model. Hypertrophic parameters under investigation were stimulation of phenylalanine incorporation and protein mass, stimulation of 14C-uridine incorporation and RNA mass, and increases in cell shape. RESULTS: Norepinephrine (0.01 to 10 micromol/liter) increased concentration-dependent phenylalanine incorporation; pEC50 value was 5.9 +/- 0.1 (n = 8). The alpha1-adrenoceptor antagonist prazosin (0.1 micromol/liter) suppressed norepinephrine-induced increase in rate of protein synthesis. Conversely, propranolol (1 micromol/liter) and the beta1-adrenoceptor selective antagonists CPG 20712A (300 nmol/liter) or atenolol (1 micromol/liter) augmented increases in phenylalanine incorporation caused by norepinephrine. Addition of the beta2-adrenoceptor antagonist ICI 118,551 (55 nmol/liter) did not influence the hypertrophic effect of norepinephrine. Atenolol augmented the norepinephrine-induced increases of all hypertrophic parameters investigated (i.e., protein mass, uridine incorporation, RNA mass, cell volume, and cross-sectional area). In the presence of norepinephrine, inhibition of beta1-adrenoceptors increased the amount of protein kinase C-alpha and -delta isoforms translocated into the particulate fraction. The effect of pharmacological inhibition of beta1-adrenoceptors could be mimicked by Rp-cAMPS (adenosine-3', 5'-cyclic phosphorothiolate-Rp). The inhibitory effect of beta1-adrenoceptor stimulation on the alpha-adrenoceptor-mediated effect persisted in cardiomyocytes isolated from hypertrophic hearts of rats submitted to aortic banding. CONCLUSIONS: In isolated ventricular cardiomyocytes from rats, beta1-adrenoceptor stimulation attenuates the hypertrophic response evoked by alpha1-adrenoceptor stimulation.     

Increased beta 2-adrenoceptor number in peripheral sympathetic ganglia of spontaneously hypertensive rats

We determined beta-adrenoceptor concentrations in sympathetic ganglia from adult Spontaneously Hypertensive Rats (SHR) and Wistar-Kyoto (WKY) control rats by quantitative autoradiography. Adjacent tissue sections were incubated with [125I]iodocyanopindolol, with or without excess of unlabeled (-)-propranolol, the beta 1-antagonist CGP 20712A or the beta 2-antagonist ICI 118,551. Most beta-adrenoceptors were of the beta 2-type. Their concentration was higher in the superior cervical and stellate ganglia of SHR when compared to normotensive WKY. Our results indicate that beta 2-adrenoceptor stimulation may be enhanced in sympathetic ganglia of SHR, and could play a role in the maintenance of their increased sympathetic activity.     

Carvedilol blocks beta2- more than beta1-adrenoceptors in human heart

OBJECTIVE: To understand the basis of the effectiveness of carvedilol in heart failure by determining its specific properties at human heart beta1- and beta2-adrenoceptors. METHODS: The positive inotropic effects of noradrenaline (in the presence of the beta2-selective antagonist ICI118551) and adrenaline (in the presence of the beta1-selective antagonist CGP20712), mediated through beta1- and beta2-adrenoceptors, respectively, were investigated in atrial and ventricular trabeculae. The patch-clamp technique was used to investigate effects of noradrenaline and adrenaline on L-type Ca2+ current in human atrial myocytes. RESULTS: Carvedilol was a 13-fold more potent competitive antagonist of the effects of adrenaline at beta2-adrenoceptors (-logKB=10.13+/-0.08) than of noradrenaline at beta1-adrenoceptors (-logKB=9.02+/-0.07) in human right atrium. Chronic carvedilol treatment of patients with non-terminal heart failure reduced the inotropic sensitivity of atrial trabeculae to noradrenaline and adrenaline 5.6-fold and 91.2-fold, respectively, compared to beta1-blocker-treated patients, consistent with persistent preferential blockade of beta2-adrenoceptors. In terminal heart failure carvedilol treatment reduced 1.8-fold and 25.1-fold the sensitivity of right ventricular trabeculae to noradrenaline and adrenaline, respectively, but metoprolol treatment did not reduce the sensitivity to the catecholamines. Increases of current (ICa,L) produced by noradrenaline and adrenaline were not different in atrial myocytes obtained from non-terminal heart failure patients treated with metoprolol or carvedilol, consistent with dissociation of both beta-blockers from the receptors. CONCLUSIONS: Carvedilol blocks human cardiac beta2-adrenoceptors more than beta1-adrenoceptors, thereby conceivably contributing to the beneficial effects in heart failure. The persistent blockade of beta-adrenoceptors is attributed to accumulation of carvedilol in cardiac tissue.     

Age- and sex-associated changes in cardiac beta(1)-adrenoceptors from the muscular dystrophy (mdx) mouse

Variations in beta(1)-adrenoceptor function due to age or sex were examined in the mouse model of Duchenne muscular dystrophy. Positive chronotropic and inotropic responses to (-)-isoprenaline and antagonist effects of CGP20712A were determined in isolated right and left atria from young (12 week) and old (12 month) male and old (12 month) female mdx mice and their age- and sex-matched C57BL/10ScSn (C57) mice. There was no difference in efficacy to (-)-isoprenaline when expressed as an increase in the rate of contraction or force of contraction as a percentage of Ca(2+)-induced increase respectively in right or left atria from age- and sex-matched mdx and C57. Old mdx males showed a decreased sensitivity to (-)-isoprenaline (P<0.05) and a reduced affinity to CGP 20712A (P<0.05) in both right and left atria compared with old C57 males. These same changes were also observed in left atria between old and young mdx males. A reduced efficacy to (-)-isoprenaline was also evident when young and old mdx males were compared. In contrast, in old females, mdx showed an increased affinity to CGP20712A in left and right atria (P<0.05), and an enhanced sensitivity to (-)-isoprenaline in right atria. Finally, in left atria, the maximum Ca(2+)-induced increase in force of contraction was lower in all mdx compared to their age- and sex-matched C57 (P<0.05). In conclusion, age- and sex-associated changes in beta(1)-adrenoceptor function and responses to calcium were demonstrated in cardiac muscle from mdx mice, with a marked deterioration in beta(1)-adrenoceptor function occurring with aging in male mdx being particularly evident.     

Beta1-adrenoceptors in rat anterior pituitary may be constitutively active. Inverse agonism of CGP 20712A on basal 3',5'-cyclic adenosine 5'-monophosphate levels

Catecholamines directly stimulate GH, ACTH, and prolactin secretion from rat anterior pituitary through the beta(2)-adrenoceptor (AR). We recently showed that gonadotrophs express the beta(1)-AR and that glucocorticoids drastically increase its mRNA expression level. The present investigation explores whether beta(1)-ARs are functionally coupled to adenylate cyclase. In anterior pituitary cell aggregates, the highly selective beta(1)-AR antagonists CGP 20712A and ICI 89,406-8a attenuated isoproterenol-stimulated cAMP accumulation, but no agonist action of norepinephrine could be detected. Remarkably, CGP 20712A inhibited basal cAMP levels by its own for at least 50%, an action that tended to be more effective in dexamethasone-supplemented medium. The latter effect was abolished by the beta-AR antagonist carvedilol, but not by other beta-AR antagonists. Pretreatment with pertussis toxin abolished the action of CGP 20712A on basal cAMP. CGP 20712A also attenuated isoproterenol-induced cAMP accumulation in the gonadotroph cell lines alphaT3-1 and LbetaT2, but not in the somatotroph precursor cell line GHFT and the folliculo-stellate cell line TtT/GF. However, in LbetaT2 cells CGP 20712A did not inhibit basal cAMP levels by its own. The present data suggest that beta(1)-AR in the anterior pituitary is positively coupled to adenylyl cyclase but is constitutively active in a pertussis toxin-sensitive manner. CGP 20712A may act as an inverse agonist with approximately 50% negative intrinsic activity, suggesting that the beta(1)-AR significantly contributes to basal adenylate cyclase activity in the pituitary.     

Lack of an effect of age on beta-adrenoceptor-mediated lipolysis in isolated human adipocytes

We examined the effect of age on the beta-adrenoceptor response in adipocytes that contain both beta 1- and beta 2-adrenoceptors. Twelve healthy young and 12 old subjects on a 150 mEq/24 h sodium diet underwent gluteal fat biopsies. Isolated adipocytes from all the subjects were stimulated with increasing concentrations of isoproterenol for glycerol release. In 13 of the subjects (7 young and 6 old), we also performed beta-adrenoceptor binding studies on adipocyte membranes. In addition in eight subjects (four young and four old), we also utilized a competitive binding assay to calculate the percent of beta-adrenoceptors that were of the beta 1 subtype. Our data showed that old subjects, when treated under identical conditions as the young subjects prior to fat biopsy, did not demonstrate any differences in the beta-adrenoceptor stimulated lipolysis. The Vmax of lipolysis was 10.6 +/- 1.4 nmoles glycerol/mg lipid/2 h in the young group and 9.9 +/- 1.1 in the old group. The concentrations of isoproterenol that resulted in Vmax and 1/2 Vmax were also the same in the two age groups. The addition of 8-cyclopentyl-1,3-dimethylxanthine (CPT), a specific A1-adenosine receptor antagonist, resulted in mild but equivalent enhancement of glycerol release in both age groups. The beta-adrenoceptor numbers and affinities from adipocyte membranes did not demonstrate age-related differences either (Bmax 106 +/- 17 fmol/mg of protein in the young, and 137 +/- 27 in the old; Kd 79.6 +/- 21.3 pM in the young and 81.9 +/- 16.6 in the old). The percent of beta 1-adrenoceptors on the adipocyte membranes was also similar in the two age groups (35.2 +/- 2.6% in the young; 37.1 +/- 4.5% in the old). In conclusion, we could not demonstrate any differences in the beta-adrenoceptor responses from peripheral adipocytes that contain both beta 1- and beta 2-adrenoceptors, in a group of healthy elderly and young subjects who were subjected to identical dietary and orthostatic conditions prior to the biopsy. These data suggest that neither beta 1- nor beta 2-adrenoceptor responses in human adipocytes show significant changes due to aging.     

Functional assessment of beta adrenoceptor subtypes in human colonic circular and longitudinal (taenia coli) smooth muscle

BACKGROUND AND AIMS: The subtype and species related heterogeneity of beta adrenoceptors prompted a functional reappraisal of these molecular targets of motility inhibition in the human colon. METHODS: Relaxation of muscle strips was measured in vitro. RESULTS: The following agonists had decreasing relaxing potency (effective concentration range 10(-8)-10(-4) mol/l): (-)isoprenaline (non-selective), terbutaline (beta(2) selective), CGP 12177 (beta(3) selective, also beta(1), beta(2) antagonist), and SR 58611A (beta(3) selective). Isoprenaline and terbutaline were more potent on circular than taenia strips; CGP 12177 and SR 58611A weakly and partially relaxed taenia but had little effect on circular strips. The potency of isoprenaline on circular strips was greatly reduced by the beta(1) selective antagonist CGP 20712 (10(-7) mol/l), and less so by ICI 118551 (10(-7) mol/l, beta(2) selective). CGP 20712 and ICI 118551 together (both 3 x 10(-6) mol/l) had no effect on taenia relaxation by SR 58611A and rendered isoprenaline and terbutaline virtually inactive on circular strips, although not on taenia, which was relaxed at higher than control concentrations and maximally by isoprenaline. Propranolol, a beta(1), beta(2) non-selective antagonist, at high concentrations (10(-5) mol/l) prevented taenia relaxation by CGP 12177 and SR 58611A; its quantitative antagonism of isoprenaline (in common with that of CGP 12177 used as an antagonist) was competitive in circular strips but not on taenia. CONCLUSIONS: beta(1), beta(2), and beta(3) adrenoceptors are functionally detectable in the human colon; agonist stimulation of any one type relaxed taenia but only isoprenaline was fully effective at the beta(3) subtype.     

Characteristics of adrenoceptors and [3H]nitrendipine receptors of porcine vascular smooth muscle: differences between coronary artery and aorta

Characteristics of the bindings of [3H](-)dihydroalprenolol, [125I](-)iodocyanopindolol, [3H]prazosin, [3H]yohimbine, and [3H]nitrendipine to porcine coronary membranes were investigated and the results compared with studies of porcine aortic membranes. In the equilibrium binding study carried out in sarcolemma-enriched fractions, there were no major differences in the Kd values of these radioligands between coronary artery and aorta. However, the densities of beta-, alpha 1-, and alpha 2-adrenoceptors and [3H]nitrendipine receptors of coronary artery were 258, 12, 12, and 561 fmol/mg protein, respectively, while those of aorta were 37, 525, 1,000, and 215 fmol/mg protein. beta-Adrenergic agonists competed with [3H](-)dihydroalprenolol binding sites in coronary artery, the order of potency being (-)isoproterenol greater than (-)norepinephrine greater than (-)epinephrine greater than (+)isoproterenol. In case of aorta, the order was (-)isoproterenol greater than (-)epinephrine greater than (-)norepinephrine. The competition by (+/-)bisoprolol (beta 1-selective antagonist) and ICI 118,551 (beta 2-selective antagonist) for [125I](-)iodocyanopindolol binding sites in coronary artery resulted in nonlinear Hofstee plots (beta 1:beta 2 = 90%:10%). In case of aorta, linear Hofstee plots were obtained. From these results, we conclude that coronary beta-receptors in pigs are predominantly of beta 1-type, while those of aorta are of beta 2-type; regarding the relative population of adrenoceptors, coronary artery is beta-dominant (beta/alpha = 11), while aorta is alpha-dominant (beta/alpha = 0.02); compared with alpha-adrenoceptors, coronary artery has a greater number of [3H]nitrendipine binding sites (nitrendipine/alpha-adrenoceptor = 23) than aorta (nitrendipine/alpha-adrenoceptor = 0.14).     

Myocardial beta-adrenoceptor changes in heart failure: concomitant reduction in beta 1- and beta 2-adrenoceptor function related to the degree of heart failure in patients with mitral valve disease

In patients suffering from end-stage congestive cardiomyopathy, cardiac beta 1-adrenoceptor function is markedly reduced, whereas cardiac beta 2-adrenoceptor function is nearly normal. To determine whether beta 1-adrenoceptor function is impaired in heart failure selectively, beta 1- and beta 2-adrenoceptor density and functional responsiveness in the right and left atria and the left papillary muscles from patients with mitral valve disease (functional class III to IV) were studied. In all three tissues concomitantly beta 1- and beta 2-adrenoceptor density gradually declined when the degree of heart failure increased from functional class III to IV. This decrease in beta 1- and beta 2-adrenoceptor density was accompanied by similar decreases in the contractile response of isolated electrically driven right atrial and left ventricular papillary muscles to beta-adrenergic agonists. It is concluded that a decrease in cardiac beta-adrenoceptor function is a general phenomenon in heart failure, and its extent is related to the degree of heart failure. However, in contrast to congestive cardiomyopathy, in mitral valve disease the decrease in cardiac beta-adrenoceptor function is due to a concomitant decrease in beta 1- and beta 2-adrenoceptors.     

Hypothalamic beta 2-adrenoceptor control of renal sympathetic nerve activity and urinary sodium excretion in conscious, spontaneously hypertensive rats

The contributions of beta 1-, B2-, and alpha 2-adrenoceptors in the posterior hypothalamus to the increased renal sympathetic nerve activity and decreased urinary sodium excretion resulting from environmental stress (air jet) in conscious spontaneously hypertensive rats were examined. Air stress increased mean arterial pressure and renal sympathetic nerve activity (54% from 7.0 +/- 0.7 integrator resets/min), and decreased urinary sodium excretion (44% from 2.7 +/- 0.4 microEq/min per 100 g body weight). After bilateral injection of ICI 118,551 (beta 2-adrenoceptor antagonist) into the posterior hypothalamus of the same spontaneously hypertensive rats, air stress had no effect on renal sympathetic nerve activity (8% from 4.8 +/- 0.7 integrator resets/min) or urinary sodium excretion (2% from 5.2 +/- 0.8 microEq/min per 100 g body weight), but still increased mean arterial pressure. Bilateral injection of isoproterenolol (beta-adrenoceptor agonist) into the posterior hypothalamus enhanced the renal sympathetic nerve activity and urinary sodium excretion (but not mean arterial pressure) responses to air stress. Air stress had no effect on renal sympathetic nerve activity or urinary sodium excretion when ICI 118,551 was given into the posterior hypothalamus before isoproterenol. Atenolol (beta 1-adrenoceptor antagonist) had no effect on the renal responses to air stress when given alone or before isoproterenol. Similarly, ICI 118,551 administered into the lateral hypothalamus or lateral cerebral ventricle, or guanabenz (alpha 2-adrenoceptor agonist) given into the posterior hypothalamus, had no effects on the renal or mean arterial pressure responses to air stress. Thus, beta 2-adrenoceptors in the posterior hypothalamus mediate the increased renal sympathetic nerve activity and antinatriuresis resulting from environmental stress in conscious spontaneous hypertensive rats.     

Positive inotropic effects of the beta2-adrenoceptor agonist terbutaline in the human heart: Effects of long-term beta1-adrenoceptor antagonist treatment

Objectives. This study was conducted to determine whether activation of cardiac beta₂-adrenoceptors increases contractility in humans and whether this is affected by long-term beta₁-adrenoceptor antagonist treatment.Background. Coexistence of beta₁- and beta₂-adrenoceptors in the human heart is generally accepted. The functional importance of cardiac beta₂-adrenoceptors for increases in contractility in humans, however, has not been completely established.Methods. We studied 1) the beta-adrenoceptor subtype mediating positive inotropic effects of the beta₂-adrenoceptor agonist terbutaline in vitro (on right atrial and left ventricular preparations from nonfailing human hearts) and increases in contractility (by measurement of systolic time intervals) in vivo in seven healthy male volunteers; and 2) in vivo whether long-term treatment of volunteers with the beta₁-adrenoceptor antagonist bisoprolol affects terbutaline-induced increases in contractility.Results. In vitro terbutaline caused a concentration-dependent increase in atrial and ventricular adenylate cyclase activity and force of contraction. Terbutaline effects were antagonized only by the beta₂-adrenoceptor antagonist ICI 118,551, indicating that they were mediated by beta₂-adrenoceptor stimulation. In vivo intravenous infusions of terbutaline (dose range 25 to 300 ng/kg body weight per min for 15 min) dose dependently increased heart rate and shortened the pre-ejection period and heart rate-corrected electromechanical systole (QS₂) time. These effects are mediated predominantly by beta₂-adrenoceptor stimulation because they were only marginally affected by the beta₁-adrenoceptor antagonist bisoprolol (1 × 10 mg orally), either given 2 h before infusion or long term for 3 weeks.Conclusions. Stimulation of cardiac beta₂-adrenoceptors in humans causes not only in vitro but also in vivo positve inotropic effects. Long-term beta₁-adrenoceptor antagonist treatment does not considerably affect beta₂-adrenoceptor-mediated in vivo increases in contractility. Thus, it may be possible to treat patients with chronic heart failure and long-term beta₁-adrenoceptor antagonist therapy with beta₂-adrenoceptor agonists if immediate inotropic support is needed.     

Coexistence of beta 1- and beta 2-adrenoceptors in human right atrium. Direct identification by (+/-)-[125I]iodocyanopindolol binding

The highly specific beta-adrenoceptor radioligand, (+/-)-[125I]iodocyanopindolol, has been used to subclassify beta-adrenoceptors in membranes from human right atrial appendage obtained during open heart surgery. Binding of (+/-)-[125I]iodocyanopindolol was saturable (Bmax = 86.4 +/- 7.4 fmol (+/-)-[125I]iodocyanopindolol bound/mg protein, n = 4), of high affinity (KD = 53 +/- 6 pM, n = 4), rapid, reversible, and stereospecific. The relative potencies of isoprenaline, adrenaline, and noradrenaline for inhibition of (+/-)-[125I]iodocyanopindolol binding and activation of adenylate cyclase were 1:10:10, indicating a population composed mainly of beta 1-adrenoceptors. Inhibition of (+/-)-[125I]iodocyanopindolol binding by beta 1- (practolol, metoprolol, betaxolol) and beta 2- (IPS 339, ICI 118,551, zinterol, procaterol) selective drugs, however, resulted in biphasic displacement curves with slope factors (nH, pseudo Hill coefficients) significantly less than 1.0. Nonlinear regression analysis of these curves revealed a beta 1: beta 2 ratio of 80:20 in human right atrial appendage. Nonselective beta-adrenergic drugs (propranolol, isoprenaline, and adrenaline), on the contrary, inhibited binding with monophasic displacement curves and nH = 1.0. Binding of agonists to the beta-adrenoceptors in human right atrial appendage seems to be regulated by guanyl nucleotides. In the absence of GTP, isoprenaline binds to high and low affinity state of the beta-adrenoceptors. GTP (10(-4) M) converts this heterogeneous binding into a homogeneous one of low affinity. It is concluded that, in human right atria, beta 1- and beta 2-adrenoceptors coexist; however, beta 1-adrenoceptors predominate. The physiological function of beta 2-adrenoceptors in human right atrium remains to be elucidated.     

Impairment of atypical beta-adrenergic-mediated relaxation in spontaneously hypertensive rats before and during the development of arterial hypertension

The aims of this study was to characterize the functional response of atypical beta-adrenoceptors (beta-AR) in rat aorta and to investigate whether this relaxation was altered before and during the development of hypertension. Aortic rings from 4 or 12 weeks old Wistar Kyoto (WKY) rats or spontaneously hypertensive rats (SHR) were placed in organ baths and constricted with phenylephrine. Then, cumulative concentration-relaxation curves to the beta-AR agonists were constructed. In intact aortic rings from 12 weeks old WKY rats, CGP 12,177 (CGP) and cyanopindolol (partial beta 3-AR agonists and atypical beta-AR agonists with beta 1/beta 2-AR antagonist properties) produced concentration-dependent relaxation (pD2 = 5.09 +/- 0.03; Emax = 60.4 +/- 2.5%; n = 9; pD2 = 6.17 +/- 0.05; Emax = 95.9 +/- 1%; n = 5 respectively). The endothelium removal did not modify this relaxation. In 12 weeks old WKY rats, the endothelium-independent relaxation to CGP was not modified in the presence of nadolol (beta 1/beta 2-AR antagonist) or L-748 337 (beta 3-AR antagonist) excluding the participation of beta 1, beta 2 et beta 3-AR in this effect. By contrast, this relaxation was significantly inhibited by CGP 20712A or bupranolol, atypical beta-AR antagonists at high concentrations. In 12 weeks old SHR, endothelium-independent relaxation to CGP or cyanopindolol was greatly inhibited. In order to sought out whether impairment of atypical beta-AR-mediated relaxation was due to hypertension, experiments were performed in 4 weeks old SHR. At this age, CGP-induced relaxation was greatly inhibited compared to that obtained in age-matched WKY rats. In 12 weeks old SHR pretreated with pertussis toxin (10 micrograms/kg i.p./3 days), the relaxant effect to CGP was partly restored. We conclude that the atypical beta-AR were functionally expressed in aortic vascular smooth muscle cells of rat aorta. In 4 or 12 weeks old SHR rats, atypical beta-AR-mediated relaxation was impaired, suggesting that this dysfunction occurs before the establishment of hypertension. Gi proteins may be one of the factors that contributes to this impairment.