Effects of different left ventricular load conditions on myocardial β-adrenoceptor density in patients with rheumatic heart valvular disease

• 1.1. The possibility that different left ventricular load conditions may influence myocardial β-adrenoceptor function in various ways was evaluated by determining the receptor density in all four chambers of 69 patients with rheumatic heart valvular disease.
• 2.2. The left ventricular β-adrenoceptor density was reduced by 44% in patients with left ventricular pressure overload (LVP), 66% in left ventricular volume overload (LVV), 56% in mixed volume and pressure overload (MOL), and 60% in those with no left ventricular pressure overload (NOL). Similarly, the right ventricular receptor density decreased significantly by 46%, 54%, 43%, and 46%, left atrial by 15%, 29%, 14%, and 21%; and right atrial by 27%, 30%, 28%, and 12% in LVP, LVV, MOL, and NOL, respectively. Thus, the general trend in the decrease in receptor density was LVV>MOL=NOL>LVP.
• 3.3. Furthermore, the LVV patients with the largest decrease in receptor density in all four chambers, similarly exhibited the largest ejection fractions (EF) and left ventricular internal diastolic and systolic diameters.
• 4.4. The results show that left ventricular volume overload is a major cause of attenuation in myocardial β-adrenoceptor density, compared to other forms of ventricular overload in heart valvular disease.
• 5.5. Since elevated EF in volume overload patients is an indication of the severity of the disease, the decrease in their myocardial receptor density may be a reflection of the degree of influence of the disease on their sympathetic activity.

     

Myocardial oxidative stress contributes to transgenic β₂-adrenoceptor activation-induced cardiomyopathy and heart failure

BACKGROUND AND PURPOSE

While maintaining cardiac performance, chronic β-adrenoceptor activation eventually exacerbates the progression of cardiac remodelling and failure. We examined the adverse signalling pathways mediated by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and reactive oxygen species (ROS) after chronic β₂-adrenoceptor activation.

EXPERIMENTAL APPROACH

Mice with transgenic β₂-adrenoceptor overexpression (β₂-TG) and non-transgenic littermates were either untreated or treated with an antioxidant (N-acetylcysteine, NAC) or NADPH oxidase inhibitors (apocynin, diphenyliodonium). Levels of ROS, phosphorylated p38 mitogen-activated protein kinase (MAPK), pro-inflammatory cytokines and collagen content in the left ventricle (LV) and LV function were measured and compared.

KEY RESULTS

β₂-TG mice showed increased ROS production, phosphorylation of p38 MAPK and heat shock protein 27 (HSP27), expression of pro-inflammatory cytokines and collagen, and progressive ventricular dysfunction. β₂-adrenoceptor stimulation similarly increased ROS production and phosphorylation of p38 MAPK and HSP27 in cultured cardiomyocytes. Treatment with apocynin, diphenyliodonium or NAC reduced phosphorylation of p38 MAPK and HSP27 in both cultured cardiomyocytes and the LV of β₂-TG mice. NAC treatment (500 mg·kg⁻¹·day⁻¹) for 2 weeks eliminated the up-regulated expression of pro-inflammatory cytokines and collagen in the LV of β₂-TG mice. Chronic NAC treatment to β₂-TG mice from 7 to 10 months of age largely prevented progression of ventricular dilatation, preserved contractile function (fractional shortening 37 ± 5% vs. 25 ± 3%, ejection fraction 52 ± 5% vs. 32 ± 4%, both P < 0.05), reduced cardiac fibrosis and suppressed matrix metalloproteinase activity.

CONCLUSION AND IMPLICATIONS

β₂-adrenoceptor stimulation provoked NADPH oxidase-derived ROS production in the heart. Elevated ROS activated p38 MAPK and contributed significantly to cardiac inflammation, remodelling and failure.

LINKED ARTICLE

This article is commented on by Di Lisa et al., pp. 1009–1011 of this issue. To view this commentary visit http://dx.doi.org/10.1111/j.1476-5381.2010.01130.x     

Effect of verapamil withdrawal on cardiac β1-adrenoceptor density

Summary Ischaemia imposes a progression of damage on the myocardium, starting with a loss of adenosine triphosphate, creatine phosphate, potassium and active tension-generating capacity. These changes progress until the tissue is incapable of maintaining ionic homeostasis, is depleted of purine precursors and shows evidence of structural disorganization. Upon reperfusion the ischaemia-induced damage is exaggerated, primarily because of the accompanying uncontrolled gain in calcium, increasing tissue osmolarity and release of endogenous noradrenaline. When used prophylactically, calcium antagonists attenuate many of the deleterious effects of ischaemia and reperfusion. We have previously shown that long-term administration of verapamil to rats (50 mg/kg daily, orally) depletes their cardiac stores of noradrenaline (NA) (3.9 ± 0.3 g/g dry wt in controls vs 0.9 ± 0.1 g/g dry wt NA after 6 weeks of therapy). This loss of NA was not accompanied by a change in ₁-adrenoceptor density (35.5 ± 1.9 fmol/mg protein for controls vs 31.2 ± 2.3 fmol/mg protein after 6 weeks of therapy). Verapamil withdrawal after 6 weeks of therapy resulted in a restoration of ventricular NA levels; within 2 days they had recovered to 75% of their original values. The density of the ₁-adrenoceptor was unaltered. Withdrawal of verapamil results in rapid repletion of cardiac NA, with an initial but transient reduction in ₁-adrenoceptor density. The absence of ₁-adrenoceptor up-regulation under these conditions probably contributes to the absence of withdrawal problems upon cessation of verapamil therapy.     

Predominantβ-adrenoceptor blocking effect of xamoterol averaged over the day in patients with mild to moderate heart failure: insight into the mechanism of its long-term clinical efficacy

Summary Xamoterol acts as a ₁-adrenoceptor agonist at low sympathetic activity and as an antagonist at high activity. Although its long-term efficacy has been proven in patients with mild to moderate heart failure, it remains unclear which effect, agonism or antagonism, accounts for its long-term activity.To clarify the effect of xamoterol on cardiac sympathetic activity in daily life, 24-h R-R interval histograms were obtained during administration of xamoterol 100 mg b. d. for 1 week to 10 patients with mild to moderate heart failure. Eight normal subjects were also studied as controls. To examine the relation between the effect of xamoterol and sympathetic activity, plasma noradrenaline (NA) levels were measured under 5 graded conditions simulating daily living.Xamoterol administration significantly decreased the standard deviation of the R-R interval, both in patients with heart failure and in normal subjects. The mean R-R interval, however, was increased in patients with heart failure, relative to normal subjects.In both groups, the R-R interval histograms had two peaks, i. e. a short daytime peak and a long night-time peak. Xamoterol decreased the median of the night-time peak without changing the daytime peak in normal subjects. In contrast, it increased the median of the daytime peak without producing a significant change in the nighttime peak in patients with heart failure. Levels of plasma NA were significantly higher in patients than in normal subjects under all conditions.Thus, in normal subjects xamoterol predominantly increased the slower heart rate at night with only a minor effect on the higher heart rate in the daytime, whereas it predominantly attenuated the daytime tachycardia induced by sympathetic stimulation in patients with heart failure.It is concluded that xamoterol tends overall to act as a-adrenoceptor antagonist during the day, especially in the daytime in patients with mild to moderate heart failure. Its antagonist rather than its agonist effect may account for the long-term efficacy of xamoterol in patients with mild to moderate heart failure.     

Effect of left ventricular pressure and volume overload on α-adrenoceptor activity in patients with rheumatic heart valvular disease

We have investigated the possibility that the various left ventricular load conditions may exert different effects on the sympathetic function by comparing the influence of volume (VOL) and pressure (POL) overload on platelet α-adrenoceptor activity, plasma catecholamines and cAMP in 44 patients with rheumatic heart valvular disease. Receptor activity was determined by radioligand binding methods, catecholamines by HPLC using an electrochemical detector, and cAMP by radioimmunoassay. The mean a-adrenoceptor density (B_max) of the control group (n = 29) was 4.71±0.41 fmol per 10⁷ platelets and the corresponding dissociation constant (K_d) was 2.47±0.15 nM. In VOL patients, the density was elevated by 70% (P<0.001), but it remained unchanged in the POL patients. In contrast to the B_max the K_d of the VOL group was not changed, and it increased by 34% (P<0.01) in the POL group. Norepinephrine was elevated by 91% (P<0.05) in POL, and epinephrine increased by 65% (P<0.05) in POL and 71% (P<0.05) in VOL. These results suggest that the sympathetic nervous system responds to left ventricular volume overload by increasing α-adrenoceptor density with no apparent change in receptor affinity toward [³H]-yohimbine binding, and to left ventricular pressure overload by decreasing their binding affinity without a parallel decrease in receptor density. The increase in receptor density in VOL is accompanied by an increase in plasma epinephrine, and the decrease in binding affinity in POL is associated with increased plasma norepinephrine and epinephrine levels     

Effect of β2-adrenoceptor agonists on plasma potassium and cardiopulmonary responses on exercise in patients with chronic obstructive pulmonary disease

Objective: The effect of ₂-adrenoceptor agonist-induced hypokalaemia on cardiac arrhythmias might be exacerbated during exercise, especially in patients with more compromised airway function. Methods: To evaluate the effect of ₂-adrenoceptor agonists on plasma potassium and cardiopulmonary function during exercise, two identical submaximal treadmill exercise tests were performed, at least 48 h apart, by 13 patients with moderate to severe COPD (11 men and 2 women, mean age 66 y, mean FEV₁/FVC ratio 48.9 (2.8)%) 30 min after they had received nebulised fenoterol or salbutamol (2 mg). The experiment was done as a randomised, double-blind, crossover trial after an initial baseline study with vehicle (0.45% saline). Plasma potassium concentration, spirometry and the degree of breathlessness (Borg scale) were measured before treatment and immediately after exercise; oxygen saturation, QTc interval and cardiac rhythm were monitored continuously before, during and for 30 min after exercise. Results: After the saline control, exercise caused an increase in Borg rating (of 4.9), a premature ventricular contractions (VPC) (2.8 beats/min), and a fall in oxygen saturation (-6.7%), but no significant change in plasma potassium (+0.04 mEq·dl^–1), FEV₁ or QTc interval. Inhalation of fenoterol and salbutamol did not affect QTc interval, Borg scale or VPC frequency at rest, but significantly increased the duration of exercise undertaken to reach the submaximal levels (786 s, versus 783 s) compared to the vehicle control. Following exercise, plasma potassium fell after fenoterol by 0.2 mEq·dl^–1 and it increased after salbutamol by 0.1 mEq·dl^–1 compared to baseline levels. Plasma potassium after exercise was significantly lower after fenoterol (3.2 mEq·dl^–1) compared to the saline control (3.7 mEq · dl^–1) and salbutamol (3.6 mEq · dl^–1). Neither fenoterol nor salbutamol had any significant effect on the change in FEV₁, oxygen saturation, Borg scale, frequency of VPCs or QTc interval during or after exercise compared to the saline control. Conclusion: When compared to salbutamol 2 mg, fenoterol 2 mg caused more marked hypokalaemia but no significant difference in cardiopulmonary response in patients with COPD during exercise.     

Is initial dose titration of amlodipine worthwhile in patients with mild to moderate hypertension?

Amlodipine, a dihydropyrimidine calcium antagonist, is effective in the treatment of patients with mild to moderate hypertension at doses of 5-10 mg daily. The aim of the study reported here was to determine whether an early increase in dosage of amlodipine provided an advantage in terms of antihypertensive effect. This was a single-blind, randomised study in 115 patients with mild to moderate hypertension (diastolic blood pressure 95-114 mmHg) conducted at 10 centres with two parallel groups. Group I received amlodipine 5 mg once daily for the entire 10-week treatment period, while group II received amlodipine 5 mg once daily for two weeks, with the option to increase the dose to 10 mg once daily were the diastolic blood pressure to exceed 90 mmHg. The dose was increased in 40% of group II patients (20/50). Diastolic and systolic blood pressure decreased steadily until the end of the sixth week of treatment in both groups, with no statistically significant difference between the groups. The response rate (diastolic blood pressure < or = 90 mmHg) at the end of treatment was 84% in both groups. Because there is no advantage in an early increase in dosage of amlodipine in terms of antihypertensive effect, a dose increase should not be considered until after six weeks of treatment at 5 mg once daily.     

Dose-response relationship of the β-adrenoceptor antagonist bisoprolol in patients with coronary heart disease and chronic obstructive bronchitis

Summary The effects of single, consecutively increased, oral doses of the -adrenoceptor antagonist bisoprolol (5, 10, 15, 20, 30 and 40 mg) on blood pressure, heart rate and bronchomotor tone were investigated in an open acute trial with 16 patients suffering from angina pectoris due to coronary heart disease and reversible chronic obstructive bronchitis. Even the lowest dose of bisoprolol (5 mg) caused a marked, long-lasting reduction in blood pressure and heart rate. After doses exceeding 20 mg, the incidence of an exaggerated pharmacodynamic effect on heart rate ( ₁-blockade) increased with dose. At doses above 30 mg, bisoprolol showed incipient impairment of bronchomotor function ( ₂-blockade) in individual patients. It is concluded that bisoprolol exhibits high ₁-selectivity, i.e. a wide ₁/ ₂ split, since blockade of bronchial ₂-receptors only occurred at doses well above the therapeutically relevant dose range. The results may not be applicable to chronic treatment.     

The effect of β-adrenoceptor blocking agents on evolving myocardial necrosis in coronary ligated rats with and without reperfusion

Summary The left coronary artery of rats was ligated either permanently, or for a period of 40 or 60 min, with subsequent reperfusion. In experiments with permanent occlusion, the hearts were removed and investigated 5 h after the coronary ligation, or immediately after death in animals which died earlier. The hearts from the reperfusion experiments were investigated 60 min after reopening the occluded artery.The extent of the ischaemic and necrotic areas of the hearts was determined. A quantitative photometric method was developed, for this purpose, using negative staining with Evans blue for the ischaemic area, and negative staining with triphenyltetrazolium chloride for the necrotic area.In experiments in which ligation was permanent, the percentage of the ischaemic area which underwent necrosis increased with the time after coronary occlusion. In reperfusion experiments, myocardial necrosis was detected earlier than in experiments with permanent coronary ligation.The -adrenoceptor blocking agents pindolol, propranolol, and metoprolol significantly decreased the percentage of necrosis in experiments with permanent ligation of the coronary artery. The most selective of the -adrenoceptor blockers, i.e. metoprolol was tested in the reperfusion experiments. In these experiments, the amount of necrosis was also significantly decreased.An abstract was given at the Joint Meeting of the French and German Pharmacological and Toxicological Societies in Freiburg, 19–22 September 1983; W. Bernauer, Naunyn-Schmiedeberg's Arch Pharmacol, Supplement to Vol 324, R 9 (1983)     

Effects of insulin on mononuclear leukocyte β-adrenoceptor density and adenylate cyclase coupling

The effects of insulin on human β-adrenoceptor density and isoproterenol-induced cyclic AMP (cAMP) accumulation were characterized in mononuclear leukocytes from healthy subjects. In cells equilibrated with theophylline (4 mM) at 37°C, insulin (4 μU/ml) was present in periods from 1 to 35 min prior to stimulation. The basal cAMP levels were not influenced. After 1 min pretreatment with insulin, the (−)-isoproterenol concentration necessary to cause half-maximal stimulation (EC₅₀) decreased from 260 to 170 nM (P < 0.025) and the maximal (−)-isoproterenol response above basal increased from 44 to 63 pmol/10⁶ cells (P < 0.01). The short exposure to insulin caused an increase in the number of functional β-adrenoceptors from 1420 to 2160 receptors/cell (P < 0.01). The increased (−)-isoproterenol responsiveness showed a time-dependent decline. When insulin had been present for 35 min before stimulation, the EC₅₀ value had increased to 600 nM (P < 0.01 vs. control) and the maximal (−)-isoproterenol response above basal was reduced to 29 pmol/10⁶ cells (P < 0.01 vs. control). The receptor density decreased to the pretreatment value (1480 receptors/cell) after 35 min exposure to insulin. The present study shows that insulin modifies the β-adrenoceptor density as well as the β-adrenoceptor coupling to adenylate cyclase, dependent on the duration of exposure.     

The influence of animal age on β-adrenoceptor density and function in tracheal airway smooth muscle

We examined the influence of animal age on the functional response of guinea-pig (0-156 weeks) and rat (4-136 weeks) isolated tracheal tissue to beta-adrenoceptor agonists. In addition, the binding density and affinity of [125I]iodocyanopindolol ([125I]CYP) binding to tracheal tissue was examined with respect to animal age. Significant age-related changes in isoprenaline potency were observed in tracheal ring preparations taken from animals during the early maturation phase of animal growth in the guinea-pig and rat. In addition, in rat isolated tracheal tissue, age-related decreases in fenoterol potency were observed during senescence, but not maturation. The changes in the functional responsiveness of tracheal tissue were not reflected by changes in the binding density or affinity for [125I]cyanopindolol ([125I]CYP) of beta-adrenoceptors, or in changes in specific autoradiographic grain density over smooth muscle tissue. In both guinea-pig and rat, no significant age-related changes in the influence of catechol-O-methyl transferase (COMT) or of extraneuronal uptake inhibition were detected. This study has demonstrated significant age-related changes in the responsiveness of guinea-pig and rat isolated tracheal tissue to beta-adrenoceptor agonists that were not related to changes in the density or affinity of the beta-adrenoceptor population or in the activity of COMT or extraneuronal uptake. The possibility of age-related changes in receptor-signal transduction coupling should be explored.     

Interaction between simvastatin and metoprolol with respect to cardiac β-adrenoceptor density,catecholamine levels and perioperative catecholamine requirements in cardiac surgery patients

β-Blockade is a standard cardiovascular therapy known to induce the up-regulation of β-adrenoceptor density. Upon ligand-binding, β-adrenoceptors are normally internalised via the arrestin pathway, and after dissociation they are re-inserted into the membrane. This means that at high catecholamine levels the adrenoceptor density is low and under β-blockade it is high. The insertion of receptors into the membrane is often dependent on farnesylation processes that can be inhibited by statins. We carried out a prospective, controlled, observational study to determine whether β-blockade-induced up-regulation of β-adrenoceptor density is attenuated by statin therapy and whether this would subsequently affect catecholamine consumption during surgery. We obtained pre-operative blood samples and intra-operative biopsies of the right atrial appendage from 39 patients (age: 65±5 years; BMI: 28±1) undergoing coronary bypass surgery with or without simvastatin (20 mg/day) therapy and with or without concomitant metoprolol therapy (50 mg/day).. The atrial tissue was used for radioligand-binding studies with (−)-[¹²⁵I]-iodocyanopindolol (ICYP) and for assessment of the β-adrenoceptor subtype distribution following standard protocols. In the blood samples, plasma adrenaline and noradrenaline concentrations were determined using HPLC. In all tissue samples, we found a total β-adrenoceptor density of 38±4 fmol/mg protein in untreated controls; this which was up-regulated to 55±5 fmol/mg protein in patients receiving metoprolol. This increase in receptor number was nearly prevented completely by simvastatin therapy (42±5 fmol/mg protein). The up-regulation could be attributed to increases in the β₁-adrenoceptor subtype. In contrast, simvastatin alone had no effect on β-adrenoceptor density. Pre-operative adrenaline levels were slightly reduced in all drug therapy groups (nonsignificant differences), while the levels noradrenaline were not significantly different among the groups. With respect to the perioperative catecholamine requirements, patients on metoprolol needed significantly less dopamine than control patients, while patients undergoing simvastatin/metoprolol therapy needed as much as the controls. The post-operative total catecholamine requirements were not different among the four groups of patients. There were no differences in plasma metoprolol concentration between patients receiving metoprolol alone and those receiving a combination of metoprolol and simvastatin. In conclusion: Simvastatin therapy seems to counter-regulate the up-regulation of β-adrenoceptor density. In the up-regulated state induced by metoprolol therapy, the patients seemed to need less catecholamines during cardiac surgery, which may be due to the higher number of β-adrenoceptors. Additional simvastatin therapy did not reduce post-operative catecholamine consumption.     

Differential effects of β-adrenoceptor partial agonists in patients with postural hypotension

Summary The central haemodynamic effects of pindolol and xamoterol have been investigated in patients with postural hypotension. Pindolol is a non-selective beta-adrenoceptor partial agonist, whereas xamoterol is ₁-selective and possesses a higher degree of agonist activity.The study comprised 16 patients with postural hypotension of different aetiologies. Blood pressure, heart rate and stroke volume were measured in the supine and head-up tilted positions. Left ventricular ejection fraction (LVEF) was measured in the supine position, and vascular resistance, left ventricular volume, and left ventricular contractility were derived. Pindolol and xamoterol were administered intravenously in incremental doses to reach total doses of 0.02 and 0.20 mg · kg^–1, respectively.Pindolol showed -adrenoceptor antagonistic effects in the supine position through decrements in heart rate from 70 to 66 beats · min^–1 and LVEF from 0.57 to 0.52, and reduced mean arterial blood pressure from 103 mm Hg to 93 mm Hg. Xamoterol showed beta-adrenoceptor agonistic effects in the supine position through increments in heart rate from 72 to 90 beats · min^–1 and LVEF from 0.58 to 0.66, and raised mean arterial blood pressure from 108 to 123 mm Hg.It is concluded that the degree of agonist activity of a beta-adrenergic agent is of importance if it is given to a patient with postural hypotension.     

Effect of pretreatment with the selective β1-adrenoceptor antagonist bisoprolol on the subsequent cardiovascular actions and β-adrenoceptor subtype specific occupancy of celiprolol in healthy man

Summary The cardiovascular effects at rest and during exercise and ₁- and ₂-adrenoceptor occupancy following a single dose of 1200 mg celiprolol p. o. were investigated in 8 healthy subjects with or without pretreatment with a single dose of 20 mg bisoprolol p. o., using a place-bo-controlled, 2-way cross-over design.The ergometric responses of heart rate (HR) and systolic blood pressure (SBP) after celiprolol were reduced to a similar extent as after bisoprolol, but the cardiovascular function at rest was affected in a different way: there was a rise in HR, clear enhancement of cardiac systolic performance, and a considerable drop in the estimated total peripheral vascular resistance, associated with median ₁-RRA and ₂-RRA occupancies of 88 and 34%, respectively. The cardiovascular effects of celiprolol were not affected by pretreatment with bisoprolol. Celiprolol thus binds extensively to ₁-adrenoceptors, moderately to ₂-adrenoceptors, acts as ₁-adrenergic antagonist (exemplified by the ergometric effects) but has vasodilator, positive chronotropic and cardiac systolic performance enhancing properties, which do not involve either direct or indirect ₁-adrenergic agonism, but which might reflect ₂-adrenergic agonism.     

Effect of losartan and captopril on expression of cardiac angiotensin Ⅱ AT1 receptor mRNA in rats following myocardial infarction

目的：探讨氯沙坦（Ｌｏｓ）和卡托普利（Ｃａｐ）对大鼠心肌梗死后血管紧张素Ⅱ（Ａｎｇ）ＡＴ１受体ｍＲＮＡ表达的影响．方法：２４只梗死大鼠随机分为四组并分别用Ｃａｐ（２ｇ·Ｌ－１加水中饮用）、Ｌｏｓ（１０ｍｇ·ｋｇ－１·ｄ－１和３０ｍｇ·ｋｇ－１·ｄ－１，灌胃）及安慰剂治疗６周，假扎鼠作对照，用地高辛标记的ｃＤＮＡ探针，进行点杂交反应，检测ＡｎｇＡＴ１受体ｍＲＮＡ表达．结果：三个药物治疗组的ＡｎｇＡＴ１受体ｍＲＮＡ表达水平分别与安慰剂组比较，均显著降低（Ｐ＜００１）．三个治疗组之间的ＡｎｇＡＴ１受体ｍＲＮＡ表达水平及分别与假扎组比较，均无显著差异（Ｐ＞００５）．结论：Ｃａｐ和Ｌｏｓ均可逆转大鼠心肌梗死后ＡｎｇＡＴ１受体ｍＲＮＡ表达水平的增高．     

Effect of inhibition of extracellular signal-regulated kinase on relaxations to β-adrenoceptor agonists in porcine isolated blood vessels

Background and purpose:

Stimulation of vascular β-adrenoceptors causes vasodilatation through activation of adenylyl cyclase (AC) and plasma membrane potassium channels, and β-adrenoceptors have been linked to activation of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase in various cell lines. However, how these findings relate to functional responses in intact tissues is largely unknown. The aim of this study, therefore, was to investigate the role of ERK in β-adrenoceptor-induced vasodilatation.

Experimental approach:

Segments of porcine coronary artery were mounted in a Mulvany wire myograph and bathed in Krebs–Henseleit buffer gassed with 95% O₂/5% CO₂ and maintained at 37°C. Tissues were pre-contracted with the thromboxane mimetic U46619, endothelin-1 or KCl. Cumulative concentration–response curves to β-adrenoceptor agonists or forskolin were then carried out in the absence or presence of the mitogen-activated protein kinase kinase (MEK) inhibitors PD98059 (10 or 50 µM) or U0126 (10 µM).

Key results:

PD98059 caused a concentration-dependent leftward shift in response to isoprenaline (pEC₅₀ control, 7.5 ± 0.1; 50 µM PD98059, 8.1 ± 0.1: P < 0.05). Inhibition of MEK also enhanced the maximum relaxation seen with salbutamol, but not the responses to the β₁-adrenoceptor selective agonist xamoterol or the AC activator forskolin. There was no enhancement of the relaxations to β-adrenoceptor agonists after inhibition of ERK activation in tissues pre-contracted with KCl or treated with the K⁺ channel blocker tetraethylammonium.

Conclusions and implications:

These data indicate that ERK inhibits β₂-adrenoceptor-mediated vasodilatation through a mechanism which may involve inactivation of plasma membrane potassium channels.     

Effects ofβ-adrenoceptor antagonist administration on β2-adrenoceptors density in human lymphocytes

Abrupt withdrawal of beta-adrenoceptor antagonists may lead to "rebound-effects". To study the mechanism underlying this phenomenon, the effects of the nonselective beta-adrenoceptor antagonists propranolol [no intrinsic sympathomimetic activity (ISA)], alprenolol (weak ISA) and mepindolol (strong ISA) on lymphocyte beta 2-adrenoceptor density--assessed by (+/-)-[125I]-iodocyanopindolol (ICYP) binding--and plasma renin activity (PRA) were investigated in male healthy volunteers aged 23-35 years. Propranolol treatment (4 X 40 mg/day) increased the density of beta 2-adrenoceptors by 25% after 2 days; concomitantly PRA and heart rate were reduced. During treatment beta 2-adrenoceptor density remained elevated. After withdrawal of propranolol PRA reached pre-drug levels rapidly, while heart rate was significantly enhanced. Beta 2-Adrenoceptor density, however, declined slowly being still significantly increased after 3 days, although propranolol was not detectable in plasma after 24 h. The affinity of ICYP to beta 2-adrenoceptors was not changed during or after treatment. Mepindolol treatment (2 X 5 mg/day) caused a 30% decrease of beta 2-adrenoceptor density and PRA after 2 days; both parameters remained reduced during treatment. After withdrawal, PRA reached rapidly pre-drug levels, whereas beta 2-adrenoceptor density was still after 4 days significantly diminished. The KD-values for ICYP, however, were not changed. During and after treatment heart rate was not affected. Alprenolol treatment (4 X 100 mg/day) led to a rapid fall in PRA, but did not significantly affect beta 2-adrenoceptor density. It is concluded, that the ISA may play an important role in modulating beta 2-adrenoceptor density and hence tissue responsiveness to beta-adrenoceptor stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

The Arg16Gly-β2-adrenoceptor single nucleotide polymorphism: exercise capacity and survival in patients with end-stage heart failure

Heart failure (HF) is characterized by impaired myocardial β-adrenergic signal transduction. Single nucleotide polymorphisms (SNPs) within the β₁- (Ser49Gly, Arg389Gly) and β₂-adrenoceptor (Arg16Gly, Gln27Glu, Thr164Ile) have been associated with alterations in adrenoceptor (AR) function sensitivity in vitro and in vivo and possibly contribute to HF progression. The present study evaluated the relation of those SNPs to morbidity and mortality in patients with end-stage HF. A total of 226 patients with end-stage HF (ejection fraction ≤35%) were genotyped for the two β₁AR SNPs and the three β₂AR SNPs. Outcome (death, heart transplantation (HTX)) was determined from May 2003 to June 2004. Heart rate, systolic and diastolic blood pressure, and peak oxygen uptake were measured during graded treadmill exercise. Left ventricular end-diastolic and end-systolic diameters, ejection fraction, and fractional shortening at rest were measured using two-dimensional echocardiography. Minor allele frequencies were 0.12 for Gly49 and 0.27 for Gly389 (β₁AR) and 0.37 for Arg16, 0.43 for Glu27 and 0.01 for Ile164 (β₂AR). During follow-up, 45 patients died (20%), and 27 patients underwent HTX (12%). No significant differences in the incidence or in the time-to-endpoint of death and HTX between genotypes of the different SNPs within the β₁- and β₂AR were detected. However, patients carrying the Arg16-β₂AR tended to have lower exercise capacity and a higher probability for death/HTX within 45 months (survival proportion 46%) than patients carrying the Gly16Gly-β₂AR (survival proportion 64%). In conclusion, the Arg16Gly-β₂AR might impact on exercise capacity and outcome in end-stage heart failure.