Beta-adrenoceptor antagonists (non-selective as well as beta 1-selective) with partial agonistic activity decrease beta 2-adrenoceptor density in human lymphocytes. Evidence for a beta 2-agonistic component of the partial agonistic activity

?2-Adrenoceptor-mediated intrinsic sympathomimetic activity of carteolol: an in vivo study

The intrinsic sympathomimetic activity (ISA) of a beta-adrenoceptor blocker can be mediated by beta(1)- or beta(2)-adrenoceptors. The aim of this study was to characterize the ISA of the beta-adrenoceptor blocker carteolol in healthy volunteers. Two approaches were employed. First, we assessed the effects of carteolol (20, 40 or 80 mg p.o.) on blood pressure, heart rate and heart-rate corrected duration of electromechanical systole (QS(2)c, a measure of cardiac contractility) in the volunteers. Carteolol dose-dependently increased systolic blood pressure, heart rate and contractility and decreased diastolic blood pressure. The beta(1)-adrenoceptor blocker bisoprolol did not attenuate these carteolol effects, but rather enhanced the effects on heart rate and systolic blood pressure. Second, we treated volunteers for 7 days with 1 x 20 mg/day carteolol and assessed lymphocyte beta(2)-adrenoceptor density (by (-)-[(125)I]-iodocyanopindolol binding) and functional responsiveness (by 10 muM isoprenaline-induced increase in lymphocyte cyclic AMP content). Carteolol significantly reduced lymphocyte beta(2)-adrenoceptor density and function. After withdrawal of carteolol lymphocyte beta(2)-adrenoceptor density and function recovered only very slowly and had not returned to control levels 11 days after carteolol withdrawal. In conclusion, the fact that, on the one hand, the cardiovascular effects of carteolol were not attenuated by the beta(1)-adrenoceptor blocker bisoprolol and, on the other, carteolol significantly decreased lymphocyte beta(2)-adrenoceptor density and function is in favour of the idea that the ISA of carteolol is mediated by beta(2)-adrenoceptors. Involvement of an additional receptor site (e.g. the propranolol-resistant state of the beta(1)-adrenoceptor), however, cannot be excluded.

Vasodilating beta-adrenoceptor blockers as cardiovascular therapeutics

beta-Adrenoceptor blocking agents (beta-blockers) have been established as therapeutics for treatment of patients with hypertension, ischemic heart diseases, chronic heart failure, arrhythmias, and glaucoma. However, their clinical use is limited because some patients are adversely affected by their side effects. The discovery of cardioselective (beta(1)-selective) blockers has overcome some of the problems. Current retrospective studies have revealed that vasodilating beta-blockers (so-called beta-blockers of the third generation) have advantages over the conventional type of beta-blockers in terms of minimizing the adverse effects and improving the disease-derived dysfunction, thus enhancing the quality of life variables. Some of the possible advantages include improvement of insulin resistance, decrease in low-density lipoprotein cholesterol in association with increase in high-density lipoprotein cholesterol, attenuation of bronchial asthma attack and respiratory dysfunction, alleviation of coronary vasospasm provocation, peripheral circulatory disturbances, and erectile dysfunction, and better patient compliance. Release of nitric oxide, antioxidant action, beta(2)-adrenoceptor activation, Ca(2+) entry blockade, and other mechanisms underlying the vasodilating action may be responsible for the beneficial therapeutic effects of these agents.

Interactions of sildenafil with various coronary vasodilators in isolated porcine coronary artery

There are reports of serious hypotension or circulatory shock when sildenafil citrate, a selective cyclic nucleotide phosphodiesterase type 5 inhibitor, which was developed for the treatment of erectile dysfunction, is given to patients taking certain coronary vasodilators. We thus examined the interaction of sildenafil with various coronary vasodilators including nitric oxide (NO) donors in isolated porcine coronary artery. Sildenafil caused concentration-dependent relaxations of the artery precontracted with U46619 (9,11-dideoxy-9 alpha,11 alpha-methanoepoxy-prostaglandin F(2alpha)). Incubation with the NO synthase inhibitor NG-nitro-L-arginine or the soluble guanylate cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one) significantly shifted the concentration-response curve for sildenafil to the right without affecting the maximum response, indicating that some part of the relaxant response to sildenafil may be the result of the inhibition of phosphodiestrase type 5-induced degradation of cyclic GMP (cGMP) that is produced through guanylate cyclase activation by NO released spontaneously. The relaxant effects of the vasodilators with an NO donor property, isosorbide dinitrate, sodium nitroprusside, nicorandil and nipradilol, were significantly enhanced by sildenafil, as shown by a significant leftward shift of their concentration-response curves. In contrast, the relaxant responses to the drugs without a property as an NO donor, diltiazem, celiprolol and pinacidil, were not affected by sildenafil. The cGMP level of the tissue was elevated after adding sildenafil, and the cGMP-generating effect of a combination of sildenafil and sodium nitroprusside was higher than that of each drug alone. The cyclic AMP level determined simultaneously was not changed by sildenafil. These results suggest that sildenafil potentiates specifically the relaxant responses of porcine coronary artery to the drugs which behave as an NO donor, providing basic evidence that the benefit of sildenafil in the treatment of erectile dysfunction can be limited by a risk of marked vasodilation when used together with NO-related coronary vasodilators.

Bopindolol: pharmacological basis and clinical implications

Bopindolol, a non-selective antagonist of beta 1- and beta 2-adrenoceptors (ARs), has been found by pharmacological, molecular biological techniques and molecular modeling to have several unique properties. Bopindolol produces sustained blockade of beta 1- and beta 2-ARs, has intrinsic sympathomimetic as well as membrane stabilizing actions, inhibits renin secretion, and interacts with 5-HT receptors. Also, our recent molecular modeling studies identified possible interaction sites between bopindolol and beta-AR subtypes. The reviewed studies support our findings that bopindolol is non-selective for beta 1- and beta 2-ARs, has low affinity for beta 3-AR subtype and has pharmacological properties that are likely to be beneficial in the treatment of cardiovascular diseases.

THE EFFECTS OF BISOPROLOLA SELECTIVE β1-BLOCKERON GLUCOSE METABOLISM BY LONG-TERM ADMINISTRATION IN ESSENTIAL HYPERTENSION

Bisoprolol, a beta1-selective beta-blocker, was administered to 13 patients with mild to moderate essential hypertension once daily at doses of 5-10 mg for 24 weeks, and its long-term effects on blood pressure and glucose metabolism were investigated. The systolic, diastolic and mean blood pressures were significantly reduced. At the end of the treatment period, the blood glucose level and hemoglobin A1c were not significantly different from those at baseline. In the glucose tolerance test at the end of the treatment period, the blood glucose and plasma insulin levels after a glucose load of 75 g were not significantly different at any time point, and the sums of each were not significantly different from their baseline levels. Based on these results, bisoprolol appears to be a beta1-selective beta-blocker possessing a satisfactory hypotensive effect without any adverse effects on glucose metabolism for long-term use, and is therefore a safe and useful drug for the treatment of essential hypertension.

Cardiac effects of beta-adrenoceptor antagonists with intrinsic sympathomimetic activity in humans: beta1- and/or beta2-adrenoceptor mediated?

The aim of this study was to find out whether cardiac responses to the beta-adrenoceptor antagonists with intrinsic sympathomimetic activity (ISA) xamoterol and celiprolol are mediated by cardiac beta1- or beta2-adrenoceptors or both. For this purpose we assessed, in six healthy male volunteers, the effects of xamoterol (100 and 200 mg, p.o.) and celiprolol (200, 600, and 1,200 mg, p.o.) on blood pressure, heart rate, and heart rate-corrected duration of the electromechanical systole (QS2c, as a measure of inotropism). Xamoterol, in both doses, increased systolic blood pressure and heart rate, transiently decreased diastolic blood pressure, and shortened QS2c; all these effects were attenuated after pretreatment of the volunteers with the beta1-adrenoceptor antagonist bisoprolol. Celiprolol, in all three doses, increased heart rate, decreased diastolic blood pressure, and shortened QS2c but only marginally increased systolic blood pressure. Bisoprolol did not attenuate these celiprolol effects but rather enhanced celiprolol effects on systolic blood pressure and heart rate. In a further set of experiments, we studied cardiovascular effects of celiprolol in six healthy volunteers whose beta2-adrenoceptors had been desensitized by a 2-week treatment with 3x5 mg/day terbutaline. Under these conditions, celiprolol failed to increase heart rate or to shorten QS2c. We conclude that, under resting conditions, in healthy volunteers, beta-adrenoceptor antagonists with ISA can exert increases in heart rate and contractility that are mediated by either cardiac beta1-adrenoceptor (xamoterol) or cardiac beta2-adrenoceptor (celiprolol) stimulation. Thus in the human heart, the ISA of beta-adrenoceptor antagonists can be a beta1- or beta2-adrenoceptor agonistic component.

Different effects of the beta-adrenoceptor antagonists celiprolol and metoprolol on vascular structure and function in long-term type I diabetic rats

An intriguing problem of diabetes mellitus is the development of generalized angiopathy and concomitant hypertension. However, there is still a controversy whether beta-adrenoceptor antagonists can be used as antihypertensive agents in diabetes. Four groups of rats were investigated: nondiabetic controls, diabetes mellitus, diabetes + celiprolol (250 mg/kg body weight/day), diabetes + metoprolol (125 mg/kg body weight/day) after 6 months. Diabetes was induced by i.v. streptozotocin injection. We examined vascular structure and function histologically and by an in vitro microvideoangiometry of isolated perfused mesenterium. Additionally, we investigated the effects of hyperglycemia and celiprolol on NO release in cultivated aortic endothelial cells and the effect of celiprolol on transendothelial paracellular permeability. Diabetes resulted in endothelial dysfunction, characterized by a reduced response to acetylcholine and L-N(G)-nitro-arginine and an unchanged response to sodium nitroprusside (SNP). These effects were significantly antagonized by celiprolol but were not influenced by metoprolol treatment. This was supported by the finding of typical vascular changes associated with diabetes like media thickening, reduced cardiac capillary/muscle fiber ratio, and glomerulosclerosis, which were significantly reduced by celiprolol but not influenced by metoprolol treatment. Ketonuria improved after celiprolol treatment, whereas blood glucose, lipids, and body weight were not different between the diabetic groups. In cultured cells, celiprolol did not induce direct NO release but reversed the impairment of stimulated NO release caused by hyperglycemia. Furthermore, celiprolol reduced endothelial paracellular permeability. We conclude that celiprolol can exert antiangiopathic effects in diabetic rats and that both beta-adrenoceptor antagonists did not aggravate diabetic angiopathy and metabolic derangement.

Selectivity of antagonist and partial agonist activity of celiprolol in normal subjects

1. The aims of this study were to assess the relative beta 1/beta 2 selectivity of the antagonist and partial agonist activity (PAA) of celiprolol in man. 2. Eight normal males received single oral doses of celiprolol 200 mg (C200), 400 mg (C400) and 800 mg (C800); atenolol 50 mg (A50), 100 mg (A100) and 200 mg (A200); nadolol 40 mg (N40) and placebo (PL), administered in a single-blind, randomised crossover design. 3. At rest, in the presence of low levels of circulating adrenaline and noradrenergic tone, a low dose of celiprolol (C200) showed evidence of beta 1-PAA by significant increases in systolic blood pressure and resting heart rate. At higher doses (C400, C800), beta 2-PAA became evident by a significant increase in postural finger tremor, whereas C200 had no effect. 4. In the presence of a beta 1-adrenoceptor agonist, as assessed by reduction of exercise tachycardia, increasing doses of celiprolol produced significantly less beta 1-adrenoceptor blockade compared with atenolol. Furthermore, there was no increase in beta 1-adrenoceptor blockade beyond C400. 5. In the presence of a beta 2-adrenoceptor agonist, as assessed by blunting of terbutaline-induced chronotropic, hypokalaemic and finger tremor responses, celiprolol exhibited less beta 2-adrenoceptor blockade than comparable doses of atenolol used in clinical practice. 6. Exercise hyperkalaemia was blunted significantly by C400 and C800 in comparison with all doses of atenolol and nadolol.(ABSTRACT TRUNCATED AT 250 WORDS)

Single and repeated doses of the vasodilator/β-adrenergic antagonist, carvedilol, block cirazoline- and isoproterenol-mediated hemodynamic responses in the conscious rat

The purpose of this study was to evaluate the effects of carvedilol, a beta 1&2-adrenergic blocker and vasodilator, on cirazoline-mediated changes in arterial blood pressure and isoproterenol-mediated changes in heart rate after acute and chronic administration. Conscious, chronically instrumented male Sprague-Dawley rats were injected with carvedilol (1 mg/kg, IV), prazosin (0.3 mg/kg, IV), or propranolol (1 mg/kg, twice daily for 8 days. After administration of the first dose of carvedilol on day 1, the vasopressor response to cirazoline (60 +/- 3 mmHg predrug) and the isoproterenol-induced tachycardia (152 +/- 13 beats/min predrug) were blocked (e.g., 7 +/- 4 mmHg postdrug and 11 +/- 3 beats/min postdrug, respectively). After the administration of carvedilol on day 8, the cirazoline vasopressor response was 2 +/- 1 mmHg and the isoproterenol-induced tachycardia was 4 +/- 3 beats/min, indicating effective alpha 1- and beta-adrenergic blockade after chronic dosing with carvedilol. Prazosin blocked the cirazoline-induced vasopressor response on both days 1 and 8 but had no effect on the isoproterenol-induced tachycardia. Propranolol blocked the isoproterenol-induced tachycardia on both days 1 and 8 but had no effect on the cirazoline vasopressor response. These data indicate that only carvedilol effectively blocked both alpha- and beta-adrenergic hemodynamic responses and that the antagonism of these responses with carvedilol was not diminished after chronic dosing of twice-a-day treatment for 8 days.

Celiprolol exerts microvascular dilatation by activation of ?2-adrenoceptors

In order to clarify the question whether the beta 1-selective adrenoceptor antagonist celiprolol possesses vasodilating properties, isolated vascular networks were perfused with increasing concentrations of celiprolol (in a cumulative manner) ranging from 10(-8) to 10(-4) mol/l. The study was carried out using the isolated mesenteric vascular bed of the guinea pig mesenterium coli. Vascular diameters of four different vascular regions [vessels classified as G1 (585 +/- 30 microns), G2 (403 +/- 25 microns), G3 (282 +/- 27 microns) and G4 (197 +/- 13 microns)] were assessed by means of microscopic videoangiometry. Perfusion with celiprolol resulted in concentration dependent vasodilation which was more pronounced in G3 and G4 vessels. In addition, cumulative concentration-response curves were determined from responses obtained in the presence of 10(-8), 10(-7), 10(-6) and 10(-4) mol/l ICI 118,551 (a highly selective adrenoceptor antagonist). In the presence of ICI 118,551 at concentrations greater than or equal to 10(-6) mol/l, no celiprolol response could be observed. Lower concentrations of ICI 118,551 shifted the celiprolol concentration-response curve to the right in a concentration-dependent manner. Therefore, it is concluded (a) that celiprolol has a vasodilating effect, (b) that this vasodilation is produced by stimulation of beta 2-adrenoceptors and (c) that the vasodilating effect is more pronounced in smaller than in larger vessels (G3, G4 vs G1, G2).

Hemodynamic interactions of a new beta blocker, celiprolol, with nifedipine in angina pectoris

The hemodynamic consequences of blockade at both beta-adrenoceptors and slow calcium channels is of therapeutic importance for patients with angina pectoris. The hemodynamic interaction of a new cardioselective beta blocker, celiprolol, and nifedipine was examined in an acute hemodynamic study using three prospectively matched groups with angiographically confirmed coronary artery disease (n = 10/group). Patients were randomly allocated to intravenous celiprolol (8 mg), sublingual nifedipine (20 mg), or their combination. Rest and exercise (supine bicycle) hemodynamics were determined before and following each therapy. At rest, celiprolol did not alter pumping function; nifedipine reduced diastolic blood pressure and systemic vascular resistance index (SVRI), with a small increase in heart rate. Combination therapy reduced systemic arterial pressure and SVRI; heart rate and cardiac stroke volume index increased. During exercise celiprolol tended to reduce heart rate and cardiac index; nifedipine reduced exercise SVR and cardiac stroke work indices. Combination therapy reduced all components of blood pressure; cardiac stroke work and SVR indices fell. These hemodynamic data suggest that beta blockade with celiprolol may result in a slight depression of cardiac pumping during exercise; however, such effects are offset by the vasodilating actions of nifedipine (reflex sympathetic action offsetting cardiodepression). Thus the acute hemodynamic effects of this combination were seemingly safe in these patients; the longer term effects during maintained therapy should be further assessed.

Beta receptor antagonists in the treatment of heart failure

The use of beta-receptor antagonists in the treatment of heart failure is controversial. Available data do not allow general recommendations regarding their use. In dilated cardiomyopathy, several studies suggest that long-term treatment in individual patients reduces symptoms and increases exercise capacity. Short-term treatment is usually not beneficial, except in patients with ischemically induced left ventricular dysfunction. In heart failure, post myocardial infarction and in chronic ischemic heart disease, no proper long-term study has been performed to evaluate its effects. However, patients with acute myocardial infarction tolerate beta blockers, despite the presence of left ventricular dysfunction and long-term prognosis is improved. Newer agents, some with ancillary properties, such as intrinsic activity and vasodilatation, may have advantages. In the future we need a better description of the cardiac status in our patients in order to be able to select those that will respond favorably to beta-receptor antagonists. The mechanisms by which some patients improve are still obscure. Protection against receptor downregulation, restoration of receptor density, protection against cardiotoxicity of catecholamines, and improvement in ischemic systolic and diastolic left ventricular function are all possible. The fear that beta-receptor antagonists are dangerous in heart failure is in most instances not warranted, but an initial deterioration may have to be accepted in order to gain long-term beneficial effects. Ongoing studies in both idiopathic cardiomyopathy and in postinfarction failure will hopefully help us to define the use of beta-adrenoceptor antagonists in the future.

Clinical safety and efficacy of celiprolol

The management of essential hypertension requires therapeutic selections that are not only effective in reducing diastolic blood pressure but are also tailored to the individual patient, with minimal effect on patient demographics, concurrent illnesses, and cardiovascular risk factors. Celiprolol hydrochloride is a new highly cardioselective vasodilating beta-adrenoceptor antagonist that has been proven effective and safe for the treatment of essential hypertension. It is comparable to other therapies in blood pressure control while demonstrating an excellent safety profile, favorable hemodynamic activity, and minimal effects on other cardiovascular risk factors. Celiprolol may offer the physician a unique therapeutic alternative.

Preclinical pharmacology of celiprolol: a cardioselective beta-adrenergic antagonist and mild vasodilator

Celiprolol is a new antihypertensive agent that represents a new generation of beta-blockers. It combines cardioselective beta-adrenergic antagonism (beta 1) with a mild vasodilation via vasoselective beta-adrenergic agonism (beta 2). Results of animal studies show that celiprolol has beta 1-antagonist potency similar to that of propranolol and atenolol, and cardioselectivity slightly greater than that of atenolol. Celiprolol does not produce bronchoconstriction but has mild propranolol-resistant bronchodilatory properties in cats. The compound also relaxes vascular smooth muscle in a propranolol-sensitive fashion, suggesting a mechanism of beta 2-agonism. The beta 2-agonism results in a selective downregulation in beta 2-receptor number and response in tissue culture, as well as in peripheral tissue from celiprolol-treated volunteers. The decreases in beta 2-receptors are blocked by concomitant treatment with propranolol. Celiprolol is devoid of cardiac depressant activity and in fact has mild cardiostimulatory actions. The cardiostimulation is not via beta 1-stimulation, since it is not abolished by beta-blocking doses of propranolol. In a model of severe myocardial ischemia, celiprolol attenuates the ischemia-induced myocardial acidosis and improves the regional segment function. These results are suggestive of myocardial protection. In summary, celiprolol distinguishes itself from other beta-blockers by virtue of its cardioselectivity, vasorelaxation via beta 2-agonism, and the lack of bronchoconstriction and cardiodepression. These properties observed in animal studies have also been documented in clinical trials.

A dose-ranging study to evaluate the beta-adrenoceptor selectivity of single doses of betaxolol

1. Six normal subjects were given single oral doses of betaxolol 10 mg (B10), 20 mg (B20), 40 mg (B40), 80 mg (B80), propranolol 40 mg (P40), or placebo (PL) in a single-blind randomised cross-over design. 2. beta 1-adrenoceptor blockade was assessed by reductions in exercise heart rate. Betaxolol produced dose-related reductions in exercise heart rate (beats min-1) up to a ceiling at B40, after which B80 showed a lesser effect: (158 +/- 8 PL, 128 +/- 3 B10, 123 +/- 2 B20, 116 +/- 4 B40, 136 +/- 10 B80, 135 +/- 4 P40). All doses of betaxolol (except B80) produced greater reductions compared with P40: (B10 P less than 0.001, B20 P less than 0.005, B40 P less than 0.001). 3. beta 2-adrenoceptor blockade was assessed by attenuation of finger tremor and cardiovascular responses to graded infusions of i.v. isoprenaline. Dose-response curves were constructed and the doses required to increase heart rate by 25 beats min-1, finger tremor by 200%, calf blood flow by 0.5 ml dl-1 min-1, and decrease diastolic blood pressure by 10 mm Hg, after each treatment were calculated. These were then compared with placebo responses and expressed as dose-ratios. 4. Dose-ratios for finger tremor showed significant attenuation by all doses of betaxolol (compared with PL): B10 1.5 +/- 0.18 (P less than 0.05), B20 2.62 +/- 0.45 (P less than 0.005), B40 2.55 +/- 0.33 (P less than 0.001), B80 2.48 +/- 0.48 (P less than 0.01); and by P40 6.49 +/- 1.12 (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization pattern of cardiac beta-adrenoceptor subtypes by prolonged in vivo infusion of pindolol and celiprolol in rats

The regulatory effects of pindolol and celiprolol on cardiac beta-adrenoceptor density were studied in vivo in order to assess the subtype selectivity of their partial agonistic activity (PAA). The substances were continuously administered to rats for 1 week by means of implanted osmotic minipumps. The density of beta-adrenoceptor subtypes were estimated from ICYP saturation binding experiments performed on cardiac ventricular plasma membranes in the presence of a highly selective antagonist (CGP 20172 A or ICI 118,551). Both antagonists were employed at concentrations as high as to block one subtype only without affecting the complementary subtype. For control purposes, rats were also treated with isoprenaline (0.4 mg/kg/h) and propranolol (0.15 mg/kg/h), or vehicle. Pindolol (0.036 mg/kg/h) and celiprolol (0.36 mg/kg/h) reduced the density of ventricular beta 2-adrenoceptors by 46% and 23%, respectively, which--in the case of pindolol--was significant when compared to the non-treated controls. Both compounds, however, produced a small, but distinct increase in the number of beta 1-adrenoceptors by approximately 26%. This finding is in contrast to the propranolol--induced up-regulation of both beta 1- and beta 2-adrenoceptors by approximately 80%. Since supramaximal doses of each drug were administered, a significant smaller increase of beta 1-adrenoceptors by pindolol and celiprolol--as compared to the increase produced by propranolol--can be interpreted as evidence for a PAA of pindolol and celiprolol on beta 1-adrenoceptors as well. Isoprenaline as a full agonist caused a marked loss of of both beta-adrenoceptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of beta 1- and beta 2-adrenoceptors following chronic treatment with beta-adrenoceptor antagonists

The effect of chronic pretreatment of guinea pigs with various beta-adrenoceptor antagonists on the binding characteristics of the radioligand 125I-cyanopindolol (ICYP) and responsiveness of adenylate cyclase to isoprenaline in the gastrocnemius muscle (beta 2-adrenoceptors) and the left ventricle (beta 1-adrenoceptors) were compared. Pretreatment of guinea pigs with propranolol or ICI 118,551 for one week significantly increased the density of the beta 2-adrenoceptors in the gastrocnemius muscle. Atenolol pretreatment for one week did not affect the density of the receptors. Pretreatment of the animals with pindolol for one week reduced the density of beta 2-adrenoceptors in skeletal muscle. In the left ventricle pretreatment of the guinea pigs with any of the antagonists did not alter either the density or the KD of beta 1-adrenoceptors. The responsiveness of adenylate cyclase to isoprenaline (10(-4) M) in the left ventricle or skeletal muscle was not affected when the guinea pigs were pretreated with propranolol. Pretreatment of the guinea pigs with reserpine (0.5 mg.kg-1) intraperitoneally for one week, to deplete catecholamines did not affect beta-adrenoceptor density or KD in the left ventricle or skeletal muscle. We conclude that the regulation of beta-adrenoceptors by the antagonist may not be caused by the prevention of the access of endogenous agonists to beta-adrenoceptors and it is dependent on the selectivity of the antagonist and on the susceptibility of the receptors to regulation.

Intrinsic sympathomimetic activity of beta-adrenoceptor antagonists: down-regulation of cardiac beta 1- and beta 2-adrenoceptors

Prolonged treatment of cultured rat heart muscle cells containing beta 1- and non-muscle cells containing beta 2-adrenoceptors with beta-adrenoceptor antagonists devoid of intrinsic sympathomimetic activity had no effect on beta-adrenoceptor density. In contrast, antagonists with intrinsic sympathomimetic activity decreased beta-adrenoceptor density and response (adenylate cyclase stimulation) in both heart muscle (beta 1) and non-muscle cells (beta 2) by a maximum of about 50%. An even larger down-regulation of beta-adrenoceptors and loss of receptor-stimulated adenylate cyclase activity was induced by the full endogenous agonist, noradrenaline, with the beta-adrenoceptors of heart muscle cells (beta 1) being much more sensitive to the beta 1-selective noradrenaline than the heart non-muscle cell beta 2-adrenoceptors. When combined with noradrenaline, the antagonists with intrinsic sympathomimetic activity prevented the action of noradrenaline at both beta 1- and beta 2-adrenoceptors, thereby leading to an apparent up-regulation of receptor density and response. This apparent reversal from an agonist to an antagonist action was observed at much lower concentrations of noradrenaline at beta 1- than at beta 2-adrenoceptors. The data presented indicate that the beta-adrenoceptor antagonists with intrinsic sympathomimetic activity, but not those without, upon prolonged treatment decrease the density and responsiveness of both beta 1- and beta 2-adrenoceptors in cultured rat heart cells. This suggests that the intrinsic sympathomimetic activity of these agents is not a subtype-selective component. Furthermore, the agonist and antagonist activity of these agents apparently depends on the concomitant presence of an endogenous full agonist and an its own affinity and that of the partial agonist for the beta-adrenoceptor subtype.

Hemodynamic and beta-adrenergic receptor adaptations during long-term beta-adrenoceptor blockade. Studies with acebutolol, atenolol, pindolol, and propranolol in hypertensive patients

In an attempt to further clarify the mechanism of the maintenance of the antihypertensive effect of beta-adrenoceptor antagonists, the effects of four antagonists with different ancillary properties (acebutolol, atenolol, pindolol, and propranolol) on systemic and renal hemodynamics, body fluid volumes, hormones, and lymphocyte beta-adrenoceptor density were studied in four groups of 10 hypertensive patients. The patients were observed for 3 weeks during active treatment and for 2 weeks after withdrawal of treatment. At the end of the 3-week treatment period, the four drugs had an equal antihypertensive effect (fall in mean arterial pressure, 10-13%). Although renin activity was suppressed (60-70%) by all four drugs, changes in renin or pretreatment values of renin levels were not correlated with the fall in blood pressure. The drugs had no effect on plasma catecholamine concentrations or body fluid volumes. Despite similar antihypertensive effects among the four drugs, the changes in flow and resistance underlying the fall in blood pressure differed considerably. With pindolol, the fall in blood pressure was associated with a fall in vascular resistance (26 +/- 6%), whereas with propranolol, it was predominantly associated with a fall in cardiac output (11 +/- 7%). No significant changes in vascular resistance or cardiac output occurred with atenolol or acebutolol. The changes in renal blood flow and renal vascular resistance occurred in parallel with the changes in cardiac output and systemic vascular resistance. Plasma epinephrine concentration and pretreatment cardiac chronotropic responsiveness to isoproterenol appeared to be inversely correlated with lymphocyte beta-adrenoceptor density (Bmax) (r = -0.41 and -0.43, respectively). With pindolol, Bmax decreased maximally by 39 +/- 6%, and with propranolol, it increased by 51 +/- 17%. With both drugs, significant changes in Bmax were already present 24 hours after treatment. Furthermore, 1 week after withdrawal of treatment with pindolol, Bmax was still down-regulated, and cardiac chronotropic responsiveness was still decreased, whereas 1 week after withdrawal of propranolol, Bmax was still up-regulated, and cardiac chronotropic responsiveness was still increased. No changes in Bmax occurred with the beta 1-selective antagonists acebutolol and atenolol. Thus, despite an equal antihypertensive effect, the four beta-adrenoceptor antagonists appear to have dissimilar effects on cardiac output, renal blood flow, and lymphocyte beta-adrenoceptors. Changes in cardiac output, the circulating blood volume, or angiotensin-mediated vasoconstriction are factors unlikely to be crucial for the antihypertensive effect of beta-adrenoceptor antagonists. Therefore, interference with vasoconstrictor nerve activity through blockade of either central or peripheral prejunctional beta-adrenoceptors could be an alternative explanation of their blood pressure-lowering potential.

Drug- or hormone-induced adaptation: model of adrenergic hypersensitivity

A pharmacokinetic/pharmacodynamic model of hypersensitivity to adrenergic stimulation following abrupt withdrawal of chronic beta blockade was developed. The model employs the Hill equation, a term which describes the competition between isoproterenol and l-propranolol for beta receptors, and a kinetic term which characterizes the appearance and disappearance rates of up-regulated beta receptors. The model predicted peak chronotropic hyperresponsiveness to isoproterenol 48 hr following abrupt withdrawal of chronic treatment with daily propranolol doses of 160 mg, and a drug half-life of 3.5 hr. The model also predicted that increasing the dose rate and prolonging the half-life of propranolol delayed and decreased the extent of adrenergic hypersensitivity. The time-course of adrenergic hypersensitivity simulated by our model was in excellent agreement with that observed in studies which were published earlier by our laboratory. The model underestimated the extent of adrenergic hypersensitivity. The results of our simulation are consistent with a beta agonist-receptor-effector system, which involves spare receptors, amplification of response by second and third messengers, and beta agonist- antagonist-induced receptor regulation.

The functional importance of beta 1 and beta 2 adrenoceptors in the human heart

Radioligand binding studies have demonstrated convincingly the coexistence of beta 1 and beta 2 adrenoceptors in the human heart. Both subtypes are involved in the increase in tissue levels of cyclic adenosine monophosphate in isolated, electrically driven, human right atria and in the activation of adenylate cyclase in human cardiac membrane preparations. In isolated, electrically driven strips of human right atria, isoproterenol increased contractile force through stimulation of both beta 1 and beta 2 adrenoceptors, while the selective beta 2-adrenoceptor agonist, procaterol, caused its positive inotropic effect predominantly through beta 2-adrenoceptor stimulation. Norepinephrine, however, increased contractile force solely via beta 1-adrenoceptor stimulation. In this preparation, dobutamine also acted as a full agonist, producing a positive inotropic effect through stimulation of both beta-adrenoceptor subtypes. Dopexamine hydrochloride, on the other hand, having an approximately 10-fold greater affinity for right atrial beta 2 than for beta 1 adrenoceptors, acted as a partial agonist (maximal positive inotropic effect: about 30% that of isoproterenol). Similar effects have been obtained in human right and left ventricular strips; thus, there can be no doubt that cardiac beta 2 adrenoceptors can contribute to the positive inotropic effects of beta-adrenoceptor agonists in the human heart. Besides mediating positive inotropic effects, right atrial beta 2 adrenoceptors may be involved in the regulation of heart rate since, in healthy volunteers, the selective beta 2-adrenoceptor antagonist, ICI 118,551, was more potent than the selective beta 1-adrenoceptor antagonist, bisoprolol, in antagonizing isoproterenol-induced tachycardia, when both antagonists were administered in doses that selectively occupied more than 90% of beta 2 and beta 1 adrenoceptors, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Certain beta-blockers can decrease beta-adrenergic receptor number: I. Acute reduction in receptor number by tertatolol and bopindolol

We have previously reported that a potent new beta-blocker, tertatolol, when given at therapeutic doses to healthy volunteers, rapidly reduced the number of human mononuclear leukocyte beta-receptors. In the present study, the mechanism of receptor regulation by beta-antagonists incubated with target cells in vitro was investigated. Two different cell types (human mononuclear leukocytes and S49 murine lymphoma cells) were used, and beta-adrenergic receptors were measured using either the hydrophilic ligand 3H-CGP 12177 (specific for surface receptors) or lipophilic 125I-pindolol (which measures total receptors). In a comparison between beta-blockers, tertatolol and bopindolol, but not propranolol and pindolol, were found to rapidly (1 hour at 37 degrees C) reduce the number of beta-adrenergic receptors. This was paralleled by a reduction in isoproterenol-stimulated cyclic AMP accumulation. The reduction in receptors was the same whether surface or total receptors were measured; thus, it was not due to receptor sequestration. This effect was not caused by partial agonist activity (bopindolol is a weak partial agonist); in parallel experiments, tertatolol and bopindolol, but not pindolol (potent partial agonist) and isoproterenol (full agonist), reduced beta-adrenergic receptors. Finally, this effect was not due to irreversible binding: the receptor reduction induced by the irreversible blocker bromo-acetyl-alprenolol-methane (BAAM) was stable for several hours, while the effect of tertatolol and bopindolol was slowly reversed over the same time course. We suggest that tertatolol and bopindolol have two effects on beta-adrenergic receptors: they bind competitively, and then they modify the receptors so that they are no longer available for binding by ligands or catecholamines.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective regulation of beta 1- and beta 2-adrenoceptors in the human heart by chronic beta-adrenoceptor antagonist treatment

1. In 44 patients undergoing coronary artery bypass grafting, the effect of chronic administration of the beta-adrenoceptor antagonists sotalol, propranolol, pindolol, metoprolol and atenolol on beta-adrenoceptor density in right atria (containing 70% beta 1- and 30% beta 2-adrenoceptors) and in lymphocytes (having only beta 2-adrenoceptors) was studied. 2. beta-Adrenoceptor density in right atrial membranes and in intact lymphocytes was assessed by (-)-[125I]-iodocyanopindolol (ICYP) binding; the relative amount of right atrial beta 1- and beta 2-adrenoceptors was determined by inhibition of ICYP binding by the selective beta 2-adrenoceptor antagonist ICI 118,551 and analysis of the resulting competition curves by the iterative curve fitting programme LIGAND. 3. With the exception of pindolol, all beta-adrenoceptor antagonists increased right atrial beta-adrenoceptor density compared to that observed in atria from patients not treated with beta-adrenoceptor antagonists. 4. All beta-adrenoceptor antagonists increased right atrial beta 1-adrenoceptor density; on the other hand, only sotalol and propranolol also increased right atrial beta 2-adrenoceptor density, whereas metoprolol and atenolol did not affect it and pindolol decreased it. 5. Similarly, in corresponding lymphocytes, only sotalol or propranolol increased beta 2-adrenoceptor density, while metoprolol and atenolol did not affect it and pindolol decreased it. 6. It is concluded that beta-adrenoceptor antagonists subtype-selectively regulate cardiac and lymphocyte beta-adrenoceptor subtypes. The selective increase in cardiac beta 1-adrenoceptor density evoked by metoprolol and atenolol may be one of the reasons for the beneficial effects observed in patients with end-stage congestive cardiomyopathy following intermittent treatment with low doses of selective beta 1-adrenoceptor antagonists.