Alpha-adrenergic effects of dopamine and dobutamine on the coronary circulation

After beta-adrenergic blockade, dopamine causes coronary vasoconstriction that is blocked by non-selective alpha-adrenergic antagonists. This study was carried out to determine the relative importance of alpha 1- and alpha 2-adrenoceptors in mediating coronary vasoconstriction in response to dopamine. Because dobutamine has been reported to cause alpha-adrenergic stimulation, the response to dobutamine was also examined. The circumflex coronary artery was cannulated and perfused at a constant blood flow rate in 14 dogs; coronary vasomotor responses were assessed from changes in perfusion pressure. Central effects were eliminated by vagotomy and stellate ganglionectomy; propranolol (1 mg/kg i.v.) was administered to block beta-adrenergic effects. The coronary responses to intracoronary bolus doses of dopamine and dobutamine were determined; the effects of selective alpha 1-blockade with prazosin (600 micrograms/kg i.v.) and selective alpha 2-blockade with idazoxan or rauwolscine (1-5 micrograms/kg per min intracoronary for 10 min) were examined. Dopamine produced dose-related coronary vasoconstriction; this response was not significantly altered by alpha 1-blockade with prazosin, but was abolished by the addition of alpha 2-adrenergic blockade with idazoxan or rauwolscine. Dobutamine did not produce coronary vasoconstriction at any dose tested. These data demonstrate that coronary vasoconstriction produced by dopamine is mediated through postjunctional alpha 2-adrenergic receptors.     

Ketotifen and cardiovascular effects of xamoterol following single and chronic dosing in healthy volunteers

AIMS: To study whether desensitization occurs after long-term administration of the 1-adrenoceptor partial agonist xamoterol and, if so, whether this can be influenced by ketotifen. METHODS: In a double-blind, randomized design 10 young, healthy males received ketotifen (2 x 1 mg day(-1) p.o.) or placebo for 3 weeks with xamoterol (2 x 200 mg day(-1) p.o.) administered concomitantly during the last 2 weeks. 'l1-adrenoceptor mediated responses were assessed as exercise-induced tachycardia and isoprenaline-induced shortening of heart rate corrected electromechanical systole (QS2c); isoprenaline-induced tachycardia was measured as a mixed beta1-/beta2-adrenoceptor-mediated effect. RESULTS: The first dose of xamoterol significantly increased resting heart rate and systolic blood pressure and significantly shortened QS2c. The last dose of xamoterol after 2 weeks of treatment still produced the same responses. Ketotifen did not influence these effects of xamoterol on resting haemodynamics. The first dose of xamoterol caused a rightward shift of the exercise- and isoprenaline-induced tachycardia (mean dose ratios+/-s.e.mean: 1.20+/-0.05 and 2.46+/-0.23) and the isoprenaline-evoked shortening of QS2c (dose ratio 3.59+/-0.68). This rightward shift was even more pronounced after 2 weeks xamoterol treatment. This additional rightward shift after 2 weeks of xamoterol was not affected by ketotifen (mean difference (95% CI) of log transformed dose ratios between placebo and ketotifen: exercise tachycardia 0.001 (-0.03; 0.04); isoprenaline tachycardia 0.03 (-0.15; 0.21); isoprenaline induced shortening of QS2c 0.13 (-0.22; 0.48)). CONCLUSIONS: In humans xamoterol is a partial beta1-adrenoceptor agonist with positive chrono- and inotropic effects at rest and antagonistic properties under conditions of beta-adrenoceptor stimulation. These effects were well maintained after chronic dosing with no signs of beta1-adrenoceptor desensitization. Ketotifen does not change the beta-adrenoceptor mediated responses of xamoterol after chronic dosing.     

Pharmacological and hemodynamic profile of nebivolol, a chemically novel, potent, and selective beta 1-adrenergic antagonist

The pharmacological profile of nebivolol (N), a chemically novel beta-adrenergic antagonist, was assessed in investigations on isolated tissues, awake spontaneously hypertensive rats (SHR), closed-chest anesthetized dogs, and humans. In vitro, N was found to be a potent antagonist of beta 1-adrenergic receptors (A2 value, 5.8 X 10(-9) M) and only a weak beta 2-adrenergic antagonist (A2 value, 1.7 X 10(-6) M). The selectivity for the beta 1-adrenergic receptor was higher for N than for any of the reference compounds. In dogs--similarly with atenolol--N was more potent in blocking the isoprenaline (I)-induced increases in left ventricular performance than the I-induced decrease in arterial pressure. In dogs, as compared with propranolol, N (0.025 and 0.01 mg.kg-1 i.v.) increased cardiac output and stroke volume, lowered systemic vascular resistance, and had no significant effect on the variables related to left ventricular contraction. In contrast to other beta-adrenergic antagonists, N acutely lowered arterial blood pressure in SHR (1.25 mg.kg-1 i.p.) and in hypertensive patients (1 oral dose of 5 mg) for several hours. In healthy volunteers N (5 mg) lowered systemic vascular resistance during daily oral treatment and did not negatively affect left ventricular function. In conclusion, N is a potent and selective beta 1-adrenergic blocking agent with an interesting hemodynamic profile. In hypertensive subjects and SHR, a single dose lowers arterial blood pressure for substantial periods of time.     

Dose difference in beta-adrenergic blockers with intrinsic sympathomimetic action to block beta-adrenoceptors and induce sympathomimetic action

1. Isoprenaline, a beta-adrenergic full agonist, and carteolol, befunolol and BFE-55, beta-adrenergic blockers with an intrinsic sympathomimetic action, induced dose-related increases in heart rate when administered intravenously (i.v.). 2. The beta-partial agonists administered i.v. shifted a dose-heart rate increased response curve of isoprenaline, suggesting a competitive antagonism. 3. In carteolol, doses to block beta-adrenoceptors were found to be 400-1000 times lower than that to induce agonistic action. In BFE-55, doses to block beta-adrenoceptors were equal to that to induce sympathomimetic action. The difference between doses of the befunolol to produce both actions was intermediate. 4. These results suggest that there may be a difference between doses blocking beta-adrenoceptors and inducing sympathomimetic action in some beta-adrenergic partial agonists and, therefore, that there may exist beta-partial agonists which do not induce sympathomimetic action in doses blocking beta-adrenoceptors.     

The effects of nitroglycerin on regional myocardial contractile dysfunction produced by treadmill exercise or isoprenaline stimulation in dogs.

1. To compare different methods of cardiac stress testing that are clinically applied in the management of coronary heart disease, 2 groups of dogs each were chronically instrumented and subjected to treadmill exercise or isoprenaline infusion in the presence of coronary stenosis. 2. It was of interest to determine differences in haemodynamic and regional myocardial contractile parameters, the response to antianginal therapy (nitroglycerin 15 micrograms kg-1 15 min-1, i.v.), and, in particular, whether this response differed according to the mode of cardiac stimulation, i.e. treadmill exercise or isoprenaline infusion. 3. After stenosis of the circumflex branch of the left coronary artery which affected resting myocardial function only minimally, treadmill exercise or isoprenaline infusion induced transient regional contractile dysfunction. Heart rate, arterial blood pressure, left ventricular end-diastolic pressure and left ventricular dp/dtmax were registered and myocardial oxygen demand was calculated. Regional contractile performance was assessed by ultrasonic distance measurement in the underperfused and in a normally perfused area. 4. Treadmill exercise led to an increase in systolic arterial and left ventricular end-diastolic pressure. In contrast, isoprenaline-induced stimulation led to a decrease in diastolic arterial and left ventricular end-diastolic pressure. Regional contractile function in the critically underperfused area showed a deterioration during both modes of stress. Nitroglycerin completely abolished stress-induced contractile dysfunction only in the group where treadmill exercise was employed for stimulation. 5. The inability of nitroglycerin to prevent myocardial dysfunction in the isoprenaline group may be due to exhaustion of the arterial and/or venous vasodilator potency of nitroglycerin in the presence of adrenoceptor vasodilatation induced by isoprenaline. 6. These findings indicate that clinical antianginal drug testing and the evaluation of the course of disease in patients with coronary heart disease may be highly dependent on the test method chosen.     

The Arg389Gly beta1-adrenoceptor gene polymorphism determines contractile response to catecholamines

OBJECTIVES: Recently, the Arg389Gly beta1-adrenoceptor (beta1AR) gene polymorphism has been detected. The Arg variant exhibited increased responsiveness to agonist-induced stimulation in vitro. Functional studies in isolated human atrial muscle strips and in-vivo studies revealed contradictory results regarding the functional relevance of this polymorphism. We sought to characterize the functional consequences of the Arg389Gly beta1-AR polymorphism in 30 consecutive healthy male volunteers in vivo. METHODS: beta1-AR genotype was determined by PCR and restriction analysis, which was confirmed by DNA sequencing. We compared heart rate, blood pressure, and contractile response of the various genotype carriers with a modified dobutamine stress echocardiography protocol. RESULTS: Subjects homozygous for the Arg389 beta1AR showed a significantly higher increase in fractional shortening upon cumulative doses of dobutamine as compared to subjects carrying one or two copies of the Gly389 allele. A statistically significant difference was observed at a dobutamine dose of 10 microg/kg/min (46.5 +/- 1.3 vs. 41.8 +/- 1.0 %; P = 0.023) and was maximal at 40 microg/kg/min (61.9 +/- 1.4 vs. 52.8 +/- 1.6; P = 0.001). As a result, the systolic blood pressure response to dobutamine was significantly enhanced in individuals homozygous for the Arg389 allele, whereas the effect on heart rate did not differ between the two groups. Normalization for changing afterload conditions by calculating the pressure-dimension ratio revealed similar effects, indicating that the beta1AR-mediated effects are mainly a result of increased myocardial inotropy. CONCLUSION: These data indicate that the Arg389Gly beta1AR polymorphism is functionally relevant in vivo and determines contractile responsiveness to catecholamines in humans.     

Synthesis,beta-adrenergic blocking activity and beta-receptor binding affinities of 1-substituted-3-(2-isopropyl-5-methyl-phenoxy)-propan-2-ol oxalates

The compounds 1-isopropylamino-3-(2-isopropyl-5-methyl-phenoxy)-propan-2-ol oxalate (5) and 1-tert-butylamino-3-(2-isopropyl-5-methyl-phenoxy)-propan-2-ol oxalate (6) were synthesized from thymol (1), a naturally occurring agent in Thymus vulgaris L. Pharmacological evaluation of 5 and 6 were carried out using mouse ECG and isolated rat uterus models. Pretreatment of 5 (100 microg/kg, i.v.) and 6 (50 microg/kg, i.v.) antagonized isoprenaline (2 microg/kg, i.v.) induced tachycardia, similar to that of atenolol (CAS 29122-68-7, 20 microg/kg, i.v.) pretreatment in mouse ECG experiments as measured by R-R interval. Pretreatment of 5 and 6 blocked isoprenaline and adrenaline induced relaxation of isolated rat uterus (unprimed). Also the compounds 5 and 6 were subjected to in vitro beta1- and beta2-adrenergic receptor binding assay using turkey erythrocyte membrane (beta1) and lung homogenate of rats (beta2). Both 5 and 6 showed beta-adrenergic receptor affinity comparable with that of propranolol (propranolol hydrochloride, CAS 318-98-9) with out selectivity to any one beta-adrenergic receptor. These results suggest that both the compounds possess non-selective beta-adrenergic blocking activity, with the tert-butyl derivative 6 being more active than the isopropyl derivative 5.     

Downregulation of propranolol-sensitive beta-adrenoceptor signaling after inhibition of nitric oxide synthesis

The beta-adrenoceptor agonist, isoprenaline, elicits vasodilation and tachycardia in anesthetized rats via activation of propranolol-sensitive beta1- and beta2-adrenoceptors and also by propranolol-insensitive beta1- and beta3-adrenoceptors. The aim of this study was to determine whether the relative contribution of propranolol-sensitive and -insensitive beta-adrenoceptors to the changes in heart rate (HR) and vascular resistances elicited by isoprenaline is altered after blockade of nitric oxide (NO) synthase, in pentobarbital-anesthetized rats. The hemodynamic responses elicited by isoprenaline (0.1 and 0.5 microg kg(-1), i.v.) were determined before and after injection of saline or the NO synthase inhibitor, N(G)-nitro-L-arginine methylester (L-NAME, 50 micromol kg(-1), i.v.), and again after injection of the beta1- and beta2-adrenoceptor antagonist, propranolol (1 mg kg(-1), i.v.). The responses elicited by the above doses of isoprenaline were also determined before and during infusion of the alpha1-adrenoceptor agonist, phenylephrine (3 microg kg(-1) min(-1), i.v.), and again 15-20 min after injection of propranolol (1.0 mg kg(-1), i.v.). Both doses of isoprenaline elicited tachycardia and reductions in vascular resistances. Propranolol eliminated the responses elicited by the lower dose of isoprenaline and substantially diminished the responses elicited by the higher dose of the beta1-, beta2- and beta3-adrenoceptor agonist. The maximal vasodilator responses elicited by both doses of isoprenaline were not diminished whereas the maximal increases in HR were higher after injection of L-NAME. The ability of propranolol to diminish the hemodynamic actions of isoprenaline was substantially diminished in L-NAME-treated rats, whereas propranolol retained its potency in rats that received an equi-pressor infusion of the alpha1-adrenoceptor agonist, phenylephrine. The finding that the maximal vasodilator responses elicited by isoprenaline were not diminished by L-NAME suggests that the vasodilation elicited by this drug was due to direct activation of beta-adrenoceptors on vascular smooth muscle and that the full compliment of isoprenaline-sensitive receptors was not changed after inhibition of NO synthesis. However, these results suggest that the activities of propranolol-sensitive beta-adrenoceptors are downregulated, whereas propranolol-insensitive beta-adrenoceptors are upregulated upon the loss of exposure to endothelial nitrosyl factors.     

The Gln27Glu beta2-adrenoceptor polymorphism slows the onset of desensitization of cardiac functional responses in vivo

Studies performed have shown that the Arg16Gly allele in beta-adrenoceptors (beta2AR) enhances susceptibility to agonist-induced down-regulation, while the Gln27Glu polymorphism diminishes it. In this study, we tested whether similar phenotypes occur in vivo. We assessed 32 volunteers (mean age 25 +/- 2 years) with different genotypes (group A: wild-type beta2AR, n = 16; group B: homozygous Glu27, n = 10; group C: homozygous Gly16, n = 6) for the effect of 2 weeks treatment with 3 x 5 mg/day oral terbutaline on terbutaline infusion-induced increases in heart rate and contractility (i.e. shortening of heart rate-corrected duration of electromechanical systole, QS2c). At baseline, terbutaline infusion increased heart rate and contractility similarly among subjects in the three groups. Treatment with oral terbutaline for 14 days reduced the ability of intravenous (i.v.) terbutaline to increase heart rate and contractility. The extent of this reduction was similar but the time course of desensitization differed among the three groups. While in groups A and C terbutaline infusion-induced increases in heart rate and contractility were reduced within 24 h after oral ingestion of terbutaline, a significant effect on response to terbutaline infusion was not evident for the first 3 days of terbutaline treatment in group B. The Arg16Gly and the Gln27Glu variants of the beta2AR do not alter the extent of agonist-induced beta2AR desensitization in vivo but Glu27 homozygotes develop desensitization more slowly. This result may have implications for cardiac side-effects in patients who are Glu27 homozygotes and who receive beta2AR agonist therapy.     

Comparative effects of dobutamine and corwin, a beta 1-adrenergic partial agonist, in experimental left ventricular failure

Corwin is a new, long-acting beta 1-adrenergic partial agonist for oral and intravenous (i.v.) use. The effects of corwin were compared with those of dobutamine in acute ischemic left ventricular failure in dogs. Failure was produced by embolization of the left main coronary artery with 50 micron plastic microspheres. This induced severe depression in left ventricular function, as evidenced by a marked increase in left ventricular end-diastolic pressure, reduction in left ventricular dP/dtmax, and cardiac output. After 45 min was allowed for stabilization, the 27 dogs were randomly assigned to three groups: control (n = 9), dobutamine-treated (5-10 micrograms/kg/min i.v., n = 9), and corwin-treated (0.025-0.10 mg/kg i.v., n = 9). The doses of dobutamine and corwin were adjusted to give an increase in left ventricular dP/dtmax of 50%. Both drugs similarly increased cardiac output (p less than 0.01), lowered left ventricular end-diastolic pressure (p less than 0.01) and total peripheral vascular resistance (p less than 0.01), but did not affect the heart rate. Only dobutamine increased the mean arterial pressure (p less than 0.01). Both drugs also increased the arterial concentrations and myocardial uptake of fatty acids (p less than 0.05) but caused only a small and nonsignificant increase in myocardial oxygen consumption. Our findings indicate that the hemodynamic and metabolic profiles of corwin and dobutamine are similar, and both drugs should be of special value in the treatment of congestive heart failure. Since corwin can be given orally and has a longer duration of action, it is potentially useful in the long-term treatment of heart failure.     

Effect of adrenoceptor agonists on striated muscle strips of the canine oesophagus.

1. Acute psychological stress, which could be related to the release of a large amount of catecholamines, may cause oesophageal motility disorders. Therefore, the aim of our study was to elucidate the influence of adrenoceptor agonists on the striated muscle portion of the oesophagus by use of isolated strips from dogs. 2. Contractions were evoked in isolated striated muscle strips by electrical field stimulation (1 pulse min-1, 1 ms/pulse, submaximal voltage). The effects induced by administration of adrenoceptor agonists alone or in the presence of antagonists were tested to determine the nature of the adrenoceptors on this muscle preparation. 3. The administration of both the natural adrenoceptor agonists, adrenaline and noradrenaline, and the synthetic beta-adrenoceptor agonists, isoprenaline (beta 1 + beta 2), dobutamine (beta 1) or ritodrine (beta 2), enhanced the amplitude of the contractions induced by electrical stimulation in a concentration-dependent manner. The maximum responses were 82.6 (adrenaline), 66.2 (noradrenaline), 86.2 (isoprenaline), 34.6 (dobutamine) and 80.8% (ritodrine). The EC20 values obtained were respectively 2 nM, 0.2 microM, 0.91 nM, 3 microM and 80 nM. The administration of the alpha 1-adrenoceptor agonist, phenylephrine, also enhanced the contractile response in a concentration-dependent manner (EC20 value = 0.3 microM) and the maximum response was 64.6%, but the administration of the alpha 2-adrenoceptor agonist, clonidine, did not influence the contractile response. These data suggest the involvement of beta 2- and possibly alpha 1-adrenoceptors in the responses of these adrenoceptor agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of the effects of dobutamine with dopamine and isoproterenol on inotropism and chronotropism in the mammalian heart (author's transl)

The effects of dobutamine on inotropism and chronotropism of the heart were studied in vivo and in vitro and were compared with those of dopamine and isoproterenol. These compounds increased epicardial contractile force and the heart rate of the open-chest, bilaterally vagotomized dog under pentobarbital anesthesia. The dose-ratio for the contraction was [dobutamine: dopamine: isoproterenol = 1:0.8:40] and for the heart rate, [= 1:1:300]. Both drugs augmented the twitch contraction of the isolated dog ventricular papillary muscle with the dose-ratio of [dobutamine: dopamine: isoproterenol = 1:0.7:11]. This mechanical response was associated with an elevation of the plateu voltage and an increase in repolarization of the action potential, but with no alteration of the maximum rate of rise of the action potential, the resting potential and the input membrane resistance. The discharge frequency of the rabbit S-A node pacemaker potential was accelerated chiefly due to an increase in the slope of the diastolic slow depolarization. With concentration of these catecholamines for the equivalent positive inotropic potency on the papillary muscle, this effect of isoproterenol was more potent than the effects of dobutamine and dopamine. These positive inotropic and chronotropic actions of the catecholamines were abolished by a beta-receptor antagonist. Those actions of dopamine were markedly reduced by reserpine pretreatment. In addition, dobutamine had little vascular effect. These results indicatte that dobutamine has a positive inotropic effect and a less positive chronotropic effect and that such is due to the direct action on the ventricular myocardium and the S-A node through beta 1-adrenergic receptors.     

Comparative peripheral and coronary haemodynamic effects of rimiterol and isoprenaline.

1. Rimiterol and isoprenaline produced significant dose-related increases in cardiac output. 2. These changes in cardiac output were accompanied by increases in heart rate and myocardial oxygen consumption which were similar for each drug and dose-related. 3. Isoprenaline in contrast with rimiterol produced direct coronary vasodilation, i.e. coronary vasodilation in excess of that required to meet increases in myocardial oxygen demands. 4. It is suggested that the beta-adrenergic receptors in the human coronary vasculature are mainly of the beta1 type. 5. Rimiterol, because it does not produce direct coronary vasodilation may be preferable to isoprenaline in the treatment of low-cardiac output syndrome where there is regional myocardial ischaemia, since it would be less likely to produce a "coronary steal" effect.     

Functional analysis of G protein-coupled receptor kinase (beta ARK1) by intracellular immunization

G protein-coupled receptor kinases (GRKs) are believed to involve in desensitization of the G protein-coupled receptors. So far, cDNAs of six GRKs were cloned from several species including human and rat. However, it is unknown whether single GRK phosphorylates various receptors and desensitizes them in the cells. To determine whether GRK2 (also called beta ARK1) involves desensitization of the beta 1-adrenergic receptor-mediated response in heart, we tried to apply monoclonal antibody which could recognize only beta ARK1 and inhibit its phosphorylating activity to the heart cells. Monoclonal antibody was obtained by immunization of carboxyl terminus of beta ARK1 as fusion protein of glutathione-S-transferase (GST). The resulting monoclonal antibody specifically reacted with beta ARK1, and inhibited the binding of purified beta gamma subunit to the carboxyl terminus. Monoclonal antibody completely inhibited phosphorylation of the m2 muscarinic acetylcholine receptor as well as phosphorylation of GST-intracellular third loop fusion protein of the m2 receptor. When monoclonal antibody was applied to myocyte prepared from guinea pig heart, the desensitization of the beta 1-adrenergic receptor was partially inhibited as measured by Ca2+ channel activation. Thus intracellular application of monoclonal antibody is promising approach to analyze function of GRKs.     

Comparison of the beta2-adrenoceptor selectivity of rimiterol, salbutamol and isoprenaline by the intravenous route in man.

1 The bronchodilating efficacy and the degree of beta2-adrenoceptor selectivity of rimiterol, salbutamol and isoprenaline were determined in seven subjects who exhibited histamine-induced bronchoconstriction. 2 Rimiterol, 0.5 (high dose) and 0.05 (low dose) mug kg-1 min-1, salbutamol, 0.3 and 0.03 mug kg-1 min-1, isoprenaline, 0.05 and 0.005 mug kg-1 min-1 and placebo were administered by a single intravenous injection over 6 min, and the protection against histamine-induced bronchoconstriction, changes in heart rate, pulse pressure and skeletal muscle tremor were measured. 3 Rimiterol (98%), salbutamol (96%) and isoprenaline (69%) protected against histamine-induced bronchoconstriction. For these ventilatory responses, there was a heart rate increase of 31.9, 24.7 and 44.3 beats/min for rimiterol, salbutamol and isoprenaline respectively. The three drugs produced similar increases in pulse pressure and tremor. 4 Significant dose-responses were obtained for all the parameters with each drug. 5 Isoprenaline was approximately 7 and 5 times as potent as rimiterol and salbutamol respectively in bronchodilator action when equimolar doses were compared. Similarly, isoprenaline was approximately 14 and 10 times as potent in increasing the heart rate as rimiterol and salbutamol respectively. 6 Rimiterol, a new beta-adrenoceptor stimulating drug, is an effective bronchodilator and has similar beta2-adrenoceptor selectivity to salbutamol when administered intravenously. The relative potencies and degrees of beta2-adrenoceptor selectivity of these drugs depend partly on their route of administration.     

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.     

Reduced sensitivity to beta-adrenoceptor stimulation and blockade in insulin dependent diabetic patients with hypoglycaemia unawareness.

Nine IDDM-patients with hypoglycaemia unawareness, seven IDDM-patients with hypoglycemia awareness and a control group of nine healthy persons were included in this study. The patients were recruited from the medical out-patients' department of the University Hospital of Tromsø. The pathophysiological changes which cause hypoglycaemia unawareness are today not clear. Reduced peripheral tissue sensitivity to catecholamines is suggested as one of several mechanisms which may contribute. For further investigation of beta-adrenergic sensitivity an isoprenaline/metoprolol sensitivity test was performed. Isoprenaline and metoprolol were administered intravenously, and the effects on heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP) and plasma levels of adrenaline (ADR) and noradrenaline (NA) were measured. All subjects were given the same doses of isoprenaline (0.25-8 micrograms) and metoprolol (0.5-8 mg). Metoprolol was given together with the dose of isoprenaline which increased heart rate by 25 beats min-1. The dose/response curves of both isoprenaline/HR and metoprolol/HR were significantly shifted to the right in IDDM-patients with hypoglycaemia unawareness compared with controls and IDDM-patients with hypoglycaemia awareness (P < 0.05). Reduced sensitivity of isoprenaline stimulation has also been shown before, whereas reduced sensitivity of a blocking agent has not earlier been shown. These findings support the hypothesis of reduced beta-adrenergic sensitivity as one pathophysiological component in hypoglycaemia unawareness.     

Constitutive activity of the human beta(1)-adrenergic receptor in beta(1)-receptor transgenic mice

We tested the hypothesis that the human beta(1)-adrenergic receptor displays constitutive activity and that beta-adrenergic antagonists differ in their ability to modulate this constitutive activity. Transfection of the cDNAs of the human beta(1)- and beta(2)-adrenergic receptors into COS-7 cells caused increases in basal cAMP that were proportional to the receptor levels, thus demonstrating constitutive activity for both subtypes. At comparable receptor levels, the increase in basal cAMP was about 5-fold higher for the beta(2)- than for the beta(1)-subtype. As a model for enhanced beta-adrenergic signaling at the whole-organ level, we used transgenic mice with heart-specific overexpression of the human beta(1)-adrenergic receptor. In this model, the beta(1)-adrenergic receptor displayed constitutive activity as evidenced by a higher spontaneous beating rate of isolated right atria from beta(1)-transgenic versus wild-type mice. This difference was abolished by the addition of CGP20712A, demonstrating inverse agonist properties of this compound. We then tested whether various beta-adrenergic antagonists currently in clinical use for the treatment of heart failure differ in their ability to modulate constitutive activity of the cardiac beta(1)-adrenergic receptor. The beta(1)-selective antagonists metoprolol and bisoprolol showed significant inverse agonist activity at the beta(1)-adrenergic receptor. Carvedilol behaved as a neutral antagonist and xamoterol displayed marked partial agonist activity. We conclude that the human beta(1)-adrenergic receptor displays constitutive activity that is considerably lower than that of the beta(2)-subtype. beta-Adrenergic antagonists currently in clinical use differ in their ability to exert inverse agonist activity at the human beta(1)-adrenergic receptor, which may contribute to their therapeutic effects.     

The relative potencies and beta2-selectivities of intravenous rimiterol, salbutamol and isoprenaline in asthmatic patients.

The bronchodilating efficacy and the degree of beta2-selectivity of rimiterol, salbutamol and isoprenaline were determined in seven asthmatic patients. Rimiterol, 0.5 (high dose) and 0.05 mug/kg/min (low dose), salbutamol, 0.3 and 0.03 mug/kg/min, isoprenaline, 0.05 and 0.005 mug/kg/min, and placebo were administered by a single intravenous injection over 6 minutes in a double-blind trial. Airway resistance, heart rate, blood pressure and skeletal muscle tremor were measured before and at various times for 2 hours after each injection. The high doses of rimiterol (37%), salbutamol (37%) and isoprenaline (32%) produced immediate and effective bronchodilatation. The duration of action of rimiterol and isoprenaline was similar and shorter than that of salbutamol. For these ventilatory responses there were heart rate increases of 32, 20 and 40 beats/min for rimiterol, salbutamol and isoprenaline, respectively. The three drugs produced similar increases in pulse pressure and tremor. Dose-responses were obtained for each drug with all parameters measured and significant differences at various times found. Isoprenaline was approximately 8 and 5 times as potent as rimiterol and salbutamol, respectively, in bronchodilator action, when equimolar doses were compared. Similarly, isoprenaline was approximately 16 and 12 times as potent in increasing the heart rate as rimiterol and salbutamol, respectively. For an equal bronchodilator action, isoprenaline increased the heart rate 2 and 2.5 times more than rimiterol and salbutamol, respectively. Rimiterol is an effective, short-acting bronchodilator, with similar beta2-selectivity to salbutamol, when administered intravenously to asthmatic patients. The relative potencies and degrees of beta2-selectivity of these drugs depend partly on their route of administration.     

Two weeks of intermittent dobutamine therapy restores cardiac performance and inotropic responsiveness in conscious rats with heart failure.

Dobutamine is frequently used for acute therapy in heart failure. In the present study, the hemodynamic effects of long-term intermittent dobutamine therapy were investigated in conscious rats with heart failure. Rats with healed myocardial infarctions received two i.p. injections of dobutamine per day for 2 weeks. Hemodynamic measurements were performed 90-180 min after the last injection. Two weeks of intermittent dobutamine significantly restored all hemodynamic changes induced by infarction. The maximal cardiac output during volume loading was depressed due to infarction and dose-dependently restored by 2 weeks of intermittent dobutamine. An increased stroke volume accounted for this improvement since the heart rate was not altered. In order to investigate changes in adrenergic responsiveness, the effects of acute dobutamine in nontreated and 2 weeks of dobutamine-treated infarcted rats were compared to those in control rats. Whereas chronotropic responses to acute dobutamine were comparable for all experimental groups, the inotropic response was reduced in nontreated infarcted rats but dose-dependently restored after 2 weeks of intermittent dobutamine therapy. From the data, we conclude that 2 weeks of intermittent dobutamine therapy in conscious rats with healed myocardial infarcts improved cardiac performance and restored the inotropic response to acute dobutamine administration. Data indicate that dobutamine has a long-term effect on cardiac function, which differs from the acute inotropic effect.