Beta-adrenergic regulation of amine precursor amino acid transport across the blood-brain barrier.

Beta-Adrenoceptor agonists were administered i.p. into rats and amino acid levels in brain and plasma were then determined to assess the effects on transport across the blood-brain barrier. Isoproterenol (10 mumol/kg) caused significant increases in aromatic amino acid (tyrosine, phenylalanine and tryptophan) levels in cerebral cortex and decreases in almost all amino acid concentrations in plasma. This effect of isoproterenol on brain tyrosine level was dose-dependent with an ED50 of 0.25 mumol/kg. Salbutamol (beta 2-adrenoceptor agonist, 10 mumol/kg) showed similar effects, but dobutamine (beta 1-adrenoceptor agonist, 50 mumol/kg) failed to increase brain amino acid levels. When 1-threo-3,4-dihydroxyphenylalanine (L-DOPA, 100 mumol/kg) was i.p. loaded, beta-adrenoceptor agonists promoted the transport of L-DOPA into brain without increasing the clearance rate of plasma L-DOPA. Moreover, significant increases in dopamine and its metabolites were observed in rat brain. These findings suggest that the transport of aromatic amino acids across the blood-brain barrier may be regulated through beta 2-adrenoceptors and that co-administration of beta 2-adrenoceptor agonists with L-DOPA may enhance the therapeutic efficacy of L-DOPA.     

The beta-adrenoceptors mediating relaxation of rat oesophageal muscularis mucosae are predominantly of the beta 3-, but also of the beta 2-subtype.

1. beta-Adrenoceptor-mediated relaxation of rat oesophageal smooth muscle was investigated by studying the effects of beta 1- and beta 2-selective antagonists on the relaxation induced by (-)-isoprenaline, the beta 2-selective agonists fenoterol and clenbuterol and the beta 3-agonist, BRL 37344. 2. The highly beta 1-selective antagonist CGP 20721A did not antagonize (-)-isoprenaline- or BRL 37344-induced relaxations in concentrations up to 10 microM. Only at 100 microM of CGP 20712A were clear rightward shifts of the agonist concentration-response curves (CRCs) observed, with pA2 values of 4.70 and 4.97 against (-)-isoprenaline and BRL 37344, respectively. 3. ICI 118,551, a potent and selective beta 2-antagonist, at 100 nM caused moderate rightward shifts of the CRCs of (-)-isoprenaline, fenoterol and clenbuterol; with fenoterol and clenbuterol, this was accompanied by a clear steepening of the curve. Only at the highest concentration (100 microM ICI 118,551) did the shifts to the right further increase substantially. Resulting Schild-plots were clearly biphasic. BRL 37344-induced relaxations were only antagonized at 100 microM ICI 118,551, yielding a pA2 value of 5.48. 4. These results clearly demonstrate that the BRL 37344-induced relaxation of rat oesophageal muscularis mucosae is mediated solely through beta 3-adrenoceptors, whereas (-)-isoprenaline-, fenoterol- and clenbuterol-induced relaxations were shown to involve both beta 2- and, predominantly, beta 3-adrenoceptors.     

Studies on the mechanisms by which clenbuterol, a beta-adrenoceptor agonist, enhances 5-HT-mediated behaviour and increases metabolism of 5-HT in the brain of the rat

The head twitch response in mice produced by injection of 5-hydroxytryptophan (100 mg/kg i.p.) and carbidopa (25 mg/kg i.p.) was enhanced by administration of clenbuterol (0.5 mg/kg i.p.), a beta-adrenoceptor agonist. Clenbuterol also enhanced the hyperactivity syndrome in rats produced by quipazine (25 mg/kg i.p.), a 5-hydroxytryptamine (5-HT) agonist. This enhancement was not prevented by depletion of 5-HT in brain with p-chlorophenylalanine or after pretreatment with prazosin. The behavioural responses of the rats to administration of the alpha 2-adrenoceptor agonist, clonidine, was unaltered by acute or longer-term administration of clenbuterol. Following chronic administration of clenbuterol (5 mg/kg daily for 14 days), a procedure resulting in down-regulation of central beta-adrenoceptors, a larger dose of clenbuterol was necessary to enhance the quipazine-induced hyperactivity, suggesting that the mechanism of enhancement involved central post-synaptic beta-adrenoceptors. Further evidence for this conclusion was that a lesion of central noradrenaline pathways produced by 6-hydroxydopamine did not abolish the clenbuterol-induced enhancement of the quipazine-mediated behaviour. The binding characteristics of 5-HT2-receptors were unchanged by acute or chronic administration of clenbuterol. Clenbuterol (5 mg/kg) increased the percentage of plasma free (non-albumin bound) tryptophan, plasma free fatty acid concentration and the concentration of tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) in the brain. The increase in 5-HT turnover in brain was prevented by pretreatment with the beta 1-adrenoceptor antagonist atenolol, which enters the brain poorly. It is therefore suggested that the clenbuterol-induced increase in 5-HT metabolism results from the increase in the concentration of plasma free fatty acid which increases plasma free tryptophan and thus increases the concentration of tryptophan in brain and 5-HT synthesis in brain. The clenbuterol-induced enhancement of 5-HT-mediated behaviour is therefore not associated with its effect on 5-HT metabolism. The data are discussed in relation to that obtained after administration of antidepressant drugs.     

Heart-rate variability effects of beta-adrenoceptor agonists (xamoterol, prenalterol, and salbutamol) assessed nonlinearly with scatterplots and sequence methods.

Full antagonists of the cardiac beta-adrenoceptor improve heart-rate variability (HRV) in humans; however, partial agonism at the beta2-adrenoceptor has been suggested to decrease HRV. We therefore studied the HRV effects of some partial agonists of the beta1- and beta2-adrenoceptors in normal volunteers. Under double-blind and randomised conditions (Latin square design), eight healthy volunteers received placebo; xamoterol, 200 mg (beta1-adrenoceptor partial agonist); prenalterol, 50 mg (beta1- and beta2-adrenoceptor partial agonist); salbutamol, 8 mg (beta2-adrenoceptor partial agonist); ICI 118,551, 25 mg (selective beta2-adrenoceptor antagonist); and combinations of each partial agonist with ICI 118,551. Single oral doses of medication (at weekly intervals) were administered at 22:30 h with HRV assessed from the overnight sleeping heart rates. HRV was determined by using standard time-domain summary statistics and two nonlinear methods, the Poincaré plot (scatterplot) and cardiac sequence analysis. On placebo, the sleeping heart rate decreased significantly, between 2 and 8 h after dosing. The heart rate with ICI 118,551 was unaltered. Xamoterol, prenalterol, and salbutamol increased the sleeping heart rate. ICI 118,551 blocked the heart-rate effects of salbutamol, attenuated those of prenalterol, but did not influence the xamoterol heart rate. The scatterplot (Poincaré) area was reduced by beta1-adrenoceptor (xamoterol), beta2-adrenoceptor (salbutamol), and combined beta1- and beta2-adrenoceptor (prenalterol) agonism. A reduction in scatterplot length followed salbutamol, prenalterol alone, and prenalterol in combination with ICI 118,551. The geometric analysis of the scatterplots allowed width assessment (i.e., dispersion) at fixed RR intervals. At higher heart rates (i.e., 25 and 50% of RR scatterplot length), dispersion was decreased after xamoterol, prenalterol, and prenalterol/ICI 118,551. Cardiac sequence analysis (differences between three adjacent beats; deltaRR vs. deltaRRn+1) assessed the short-term patterns of cardiac acceleration and deceleration; four patterns were identified: +/+ (a lengthening sequencing), +/- or -/+ (balanced sequences), and finally -/- (a shortening sequence). Cardiac acceleration or deceleration episodes (i.e., number of times deltaRR and deltaRRn+1 were altered in the same direction) were increased after salbutamol and prenalterol. In conclusion, partial agonism at either the cardiac beta1-adrenoceptor (xamoterol), beta2-adrenoceptor (salbutamol), and beta1- plus beta2-adrenoceptors (prenalterol) altered the autonomic balance toward sympathetic dominance in healthy volunteers; blockade of the beta2-adrenoceptor with the highly selective beta2-antagonist ICI 118,551 prevented the effects of salbutamol on HRV, attenuated the HRV effects of prenalterol, but had no effect on the actions of xamoterol. Agonism at both the beta1- and beta2-adrenoceptor reduced HRV in healthy subjects; the implications for the preventive use of the beta-adrenoceptor compounds in cardiovascular disease warrant further investigation.     

Increased insulin is not required for beta2-adrenoceptor-induced increases in mouse brain tryptophan

The current study tested the hypothesis that beta(2)-adrenoceptor-mediated increases in brain tryptophan are caused by increased insulin secretion. Male mice were treated with streptozotocin (40 mg/kg) for 5 days to induce experimental diabetes. Control and diabetic mice were treated with the beta(2)-adrenoceptor agonist, clenbuterol (0.1 mg/kg), 1 h before selected brain regions were dissected for analysis by high performance liquid chromatography (HPLC) with electrochemical detection for tryptophan content, and plasma was collected for analysis of total and free tryptophan and glucose concentrations. Clenbuterol increased brain tryptophan and plasma glucose and decreased plasma total tryptophan but did not alter plasma free tryptophan. There were no significant differences in brain or plasma tryptophan between control and streptozotocin-treated mice. In a separate experiment, pretreatment of the mice with an insulin antibody did not prevent the clenbuterol-induced increases in brain tryptophan. These results suggest that beta(2)-adrenoceptor agonists increase brain tryptophan by a mechanism that does not involve changes in insulin.     

β2-Adrenoceptor antagonism in anxiety

The relative role of beta 1- and beta 2-adrenoceptor antagonism in the management of anxiety symptoms is not clear. We studied the effect of ICI 118,551, a selective beta 2-antagonist, in 51 patients presenting with acute anxiety symptoms and fulfilling DSM-III criteria for anxiety disorder. All patients received placebo during the first week of treatment followed by thrice daily diazepam (2 mg) or ICI 118,551 (50 mg) or placebo for 4 weeks with double-blind, random allocation. Hamilton anxiety scale scores improved on all treatments but there was no significant difference between treatments. Beta 2-adrenoceptor antagonism does not appear to be effective in acute anxiety neurosis. Some earlier literature suggests that beta 1-antagonism may be more important.     

Reduced brain serotonergic activity after repeated treatment with β-adrenoceptor antagonists

Rats were treated subchronically (14 days) or acutely (single dose) with the ₁-selective adrenoceptor antagonist metoprolol or the ₂-selective adrenoceptor antagonist ICI 118,551. Metoprolol (350 mg/kg/day for 14 days, orally) significantly reduced the 5-hydroxytryptophan (5-HTP) accumulation when measured 30 min after inhibition of L-amino acid decarboxylase by NSD 1015 (100 mg/kg IP) in the limbic forebrain, the corpus striatum, the cerebral cortex, the brain stem, and in the cerebellum. ICI 118,551 (0.5 mg/kg, twice daily for 14 days, SC) also significantly reduced the 5-HTP accumulation in the same brain regions except in the corpus striatum and the brain stem. Simultaneously assayed tryptophan levels were largely unaffected. Thus sustained -adrenoceptor blockade causes a decrease in the in vivo rate of tryptophan hydroxylation in various rat brain regions. The subchronic treatments with metoprolol or ICI 118,551 also significantly reduced the endogenous levels of 5-hydroxytryptamine (5-HT) in the various rat brain regions studied. Acute treatment with either metoprolol (2 mg/kg SC) or ICI 118,551 (0.5 mg/kg SC) did not affect the 5-HTP accumulation or the endogenous 5-HT levels in the brain regions studied. This inhibitory effect on brain 5-HT systems produced by sustained -adrenoceptor blockade may be of significance both for the long-term cardiovascular action and for occasional neuropsychiatric side effects during -blocking therapy.     

Evidence for beta 2-adrenoceptor-mediated facilitation of 3H-noradrenaline release from isolated guinea pig papillary muscles

The effects of beta-adrenoceptor agonists and antagonists on field-stimulated release of radioactivity from superfused guinea pig papillary muscles preincubated with 3H-noradrenaline were studied. Stimulation-evoked overflow of tritium was abolished in the absence of Ca2+ or the presence of tetrodotoxin. Isoprenaline (1 mumol/L) caused a slight facilitation of evoked overflow, whereas phentolamine (1 mumol/L) exerted a strong facilitatory action. However, when phentolamine (1 mumol/L) was present throughout superfusion, isoprenaline and the selective beta 2-adrenoceptor agonist, zinterol, caused concentration-dependent increases (half-maximal effects at 1 nmol/L). The effects of the agonists were inversely related to stimulation frequency. Furthermore, the concentration-response curve of isoprenaline was shifted to the right by the selective beta 2-adrenoceptor antagonist, ICI 118,551, but not by the selective beta 1-adrenoceptor antagonist, ICI 89,406. Schild-plot analysis revealed competitive antagonism and a pA2 value of 9.04 for ICI 118,551. Both ICI 118,551 and ICI 89,406, as well as beta-adrenoceptor antagonists with intrinsic sympathomimetic activity (pindolol and celiprolol; 1 mumol/L), had no effect on stimulation-evoked overflow of tritium (phentolamine present). It is concluded that guinea pig papillary muscles are endowed with prejunctional beta 2 adrenoceptors facilitating impulse-evoked noradrenaline release. The facilitation is markedly promoted by blockade of prejunctional alpha adrenoceptors.     

A pharmacodynamic study on clenbuterol-induced toxicity: beta1- and beta2-adrenoceptors involvement in guinea-pig tachycardia in an in vitro model.

Beta(2)-receptor adrenergic agonists as clenbuterol and analogues are illegally used as growth promoters in cattle, in Europe, as well as in other countries. Following consumption of meat or liver, intoxication cases were described, and cardiovascular toxic effects (tachycardia, hypertension) were of clinical relevance. Therefore, we investigated whether heart rate increase induced by clenbuterol could depend upon stimulation of beta(1)- and/or beta(2)-adrenergic receptors, and in which ratio. We used in vitro guinea-pig atria, a model in which beta(1)-/beta(2)-receptors ratio is similar to that found in men. In our experiments both beta(1)- and beta(2)-receptors contributed to clenbuterol-induced heart rate increase, but with a different potency. The selective beta(2)-antagonist ICI-118,551 competitively antagonized responses to clenbuterol with high affinity (pA(2) 9.47+/-0.28, SchildSlope 0.98+/-0.20 not significantly different from unity, K(B) 0.34 nM). The selective beta(1)-antagonist atenolol antagonized clenbuterol with a relatively lower affinity (pA(2)=7.59+/-0.14), the SchildSlope=1.97+/-0.33 was significantly different from unity (P<0.05). Results show that clenbuterol stimulates guinea-pig heart rate by acting chiefly on beta(2)-adrenoceptor, although responses to clenbuterol apparently are mediated by an inter-play between both beta-adrenoceptors. Further experiments are necessary to understand which beta-adrenergic antagonists are of effectiveness to counteract cardiovascular effects in case of intoxication following clenbuterol, or other beta-adrenergic stimulants.     

Effects of beta-adrenoceptor ligands in the elevated X-maze 'anxiety' model and antagonism of the 'anxiogenic' response to 8-OH-DPAT

Effects of β-adrenoceptor agonists and antagonists have been examined on open/total arm entry ratio (OTR) over a wide range of doses in the rat elevated X-maze 'anxiety' model. For clenbuterol, terbutaline and adrenaline, OTR was reduced only at doses at or above those reducing total entries; dobutamine was inactive. Neither the β(1)-adrenoceptor antagonist metoprolol nor the β( 2)-adrenoceptor antagonist ICI 118,551 affected OTR or total entries. The peripherally acting β(1)-antagonist practolol was also inactive. The elevated X-maze therefore does not appear to detect β-adrenoceptor-mediated changes in 'anxiety'. Among the β-adrenoceptor antagonists which also bind to 5-HT(1) receptors, sotalol and timolol were inactive but restricted doses of alprenolol (0.1 mg/kg) and pindolol (0.1-0.25 mg/kg) caused an anxiolytic-like increase in OTR while propranolol (5-10 mg/kg) and pindolol (1.0 mg/kg) reduced OTR without affecting total entries. These effects may be attributable to the activity of these agents at 5-HT( 1) receptors. Since metoprolol (3.0 mg/kg) and ICI 118,551 (1.0 mg/kg) did not alter the fall in OTR caused by the selective 5-HT(1A) agonist 8-OH-DPAT, the antagonism of this fall by alprenolol (0.5 mg/kg), pindolol (0.5 mg/kg), propranolol (3.0 mg/kg) and timolol (3.0 mg/kg) is most likely to represent a 5-HT(1) receptor antagonist effect of these agents.     

kappa-opioid receptor stimulation inhibits cardiac hypertrophy induced by beta1-adrenoceptor stimulation in the rat

To test the hypothesis that kappa-opioid receptor stimulation inhibits cardiac hypertrophy induced by beta1-adrenoceptor stimulation, we determined the effects of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate salt (U50,488H), a selective kappa-opioid receptor agonist, on cardiac hypertrophy induced by isoprenaline, a selective beta-adrenoceptor agonist, in neonatal ventricular myocytes upon blockade of beta2-adrenoceptor. Hypertrophy of cardiomyocytes was determined by increases in (i) total protein content; (ii) [3H]leucine incorporation; and iii) cell size. 10 micromol/l isoprenaline increased all three parameters. The effects were abolished by 2 micromol/l propranolol, a beta-adrenergic receptor antagonist, or 300 nmol/l CGP20712A, a beta1-adrenoceptor antagonist, but not by 100 nmol/l ICI118,551, a beta2-adrenoceptor antagonist. The effects were also abolished by Rp-cAMPs 100 micromol/l, a protein kinase A inhibitor and not by pertussis toxin 5 mg/l. The effects of isoprenaline in the presence or absence of ICI118,551 were also abolished by 1 micromol/l U50,488H. The inhibitory effects of U50,488H were abolished by 1 micromol/l nor-binaltorphimine, a selective kappa-opioid receptor antagonist. U50,488H also abolished the increases in the amplitude and frequency of the spontaneous intracellular Ca2+ transient induced by 10 micromol/l isoprenaline in the presence or absence of ICI118,551, an effect also abolished by nor-binaltorphimine. In conclusion the results show that kappa-opioid receptor stimulation abolished both the cardiac hypertrophy and enhanced amplitude and frequency of the spontaneous intracellular Ca2+ transient induced by beta1-adrenoceptor stimulation.     

Alleviation of myocardial dysfunction and abnormal lactate metabolism during coronary stenosis in dogs by ICI 118,551

Hemodynamic parameters, segment shortening in the ischemic myocardium and cardiac lactate extraction were estimated in the presence of a critical coronary stenosis, before and after administration of the selective beta 2-adrenoceptor antagonist, ICI 118,551, or the beta 1-adrenoceptor antagonist, atenolol, to anesthetized dogs. ICI 118,551 (0.2 and 0.5 mg/kg i.v.) and atenolol (0.2 mg/kg i.v.) produced significant decreases in both heart rate (by 6, 14 and 20% of the predrug value, respectively) and maxLVdP/dt (by 15, 26 and 24% of the predrug value, respectively). ICI 118,551 (0.5 mg/kg) and atenolol significantly improved the impaired shortening of the myocardial segment when compared with the change seen after saline administration. ICI 118,551 at both doses and atenolol significantly increased depressed cardiac lactate extraction while saline did not. Increasing heart rate by pacing abolished the beneficial effects of ICI 118,551 and atenolol on ischemic myocardial segment shortening and lactate metabolism. The data suggest that not only beta 1- but also beta 2-adrenoceptor blockade may contribute to the amelioration of myocardial ischemia in a model of coronary stenosis.     

Reserpine-induced hypothermia: participation of beta 1 and beta 2 adrenergic receptors

In mice, reserpine-induced hypothermia is partly antagonized by clenbuterol, a beta-adrenergic agonist specific for beta 2 receptors, and completely antagonized by dobutamine, a beta-adrenergic agonist specific for beta 1 receptors. In addition, the effects of dobutamine and of clenbuterol are impaired by betaxolol (1 and 4 mg/kg) and unchanged by ICI 118,551 (1 and 4 mg/kg) beta-blocking drugs respectively selective for beta 1 and beta 2 receptors. These results indicate that reserpine-induced hypothermia depends on the beta 1 receptors and lend support to an indirect effect of clenbuterol. After a chronic treatment, clenbuterol-induced antagonism of reserpine hypothermia is facilitated. This facilitation is impaired by ICI 118,551 and by betaxolol but, in this last case, with high doses only. So the facilitation involves beta 2 adrenergic receptors and implies an increase in the sensitivity of beta 1 receptors.     

Subclassification of beta-adrenoceptors responsible for steroidogenesis in primary cultures of bovine adrenocortical zona fasciculata/reticularis cells

1. Forty eight hour primary cultures of purified bovine adrenocortical zona fasciculata/reticularis cells secreted hydrocortisone in response to stimulation with beta-adrenoceptor agonists. The observed order of potency was isoprenaline greater than noradrenaline greater than dobutamine greater than salbutamol greater than BRL37344. 2. Salbutamol acted as a partial agonist on these cells hence suggesting the presence of a beta 1-adrenoceptor. 3. Schild analysis of the hydrocortisone response to isoprenaline showed that the selective beta 1-antagonist practolol and the selective beta 2-antagonist ICI118,551 gave pA2 values of 6.85 and 7.17, respectively. These values were in close agreement with corresponding pA2 values previously obtained for the beta 1-adrenoceptor. 4. We conclude that beta 1-adrenoceptors are responsible for mediating catecholamine-stimulated hydrocortisone secretion from primary cultures of bovine zona fasciculata/reticularis cells.     

Stimulation of beta-adrenoceptors activates astrocytes and provides neuroprotection

Our previous studies established that induction of growth factor synthesis and neuroprotection by the beta(2)-adrenoceptor agonist clenbuterol in vitro and in vivo was associated with the activation of astrocytes, the major source of trophic factors in the brain. In the present study, we further investigated the specificity of beta(2)-adrenoceptor-mediated effects on astrocyte activation and neuroprotection. In mixed hippocampal cultures neuroprotection against glutamate-induced cell death by clenbuterol (1 microM) was blocked by the beta(1/2)-adrenoceptor antagonist propranolol and the specific beta(2)-adrenoceptor antagonists 1-[2,3-(Dihydro-7-methyl-1H-inden-4-yl)-oxy]-3-[(1-methylethyl)-amino]-2-butanol (ICI 118,551, 10 microM) and butoxamine (10 microM), while the beta(1)-adrenoceptor-selective antagonist metoprolol (10 microM) showed no effect. The beta(2)-adrenoceptor agonists clenbuterol (1-100 microM) and salmeterol (0.01-1 microM) induced profound morphological changes of cultured astrocytes which transformed into activated astroglia with pronounced dendrite-like processes. This phenomenon was blocked by butoxamine (1 mM) and propranolol (10 microM), but not by metoprolol (10 microM). However, similar morphological changes in astrocytes were also observed after stimulation of beta(1)-adrenoceptors by dobutamine (1-10 microM) and norepinephrine (1-10 microM). This effect was blocked by propranolol (10 microM) and metoprolol (10 microM) but not by butoxamine (1 mM), suggesting that stimulation of either beta(1)- or beta(2)-adrenoceptors was sufficient to induce activation of astrocytes. In addition, beta(1)-adrenoceptor stimulation by dobutamine (1-10 microM) protected hippocampal neurons against glutamate toxicity. In a model of focal cerebral ischemia in mice the cerebroprotective effect of clenbuterol (0.3 mg/kg) was blocked by propranolol (5 mg/kg) and butoxamine (5 mg/kg). Interestingly, the infarct size was reduced after co-treatment with clenbuterol (0.3 mg/kg) and metoprolol (5 mg/kg) as compared to clenbuterol treatment (0.3 mg/kg) alone. In conclusion, activation of astrocytes and neuroprotection can be achieved by stimulation of either beta(1)- or beta(2)-adrenoceptors in vitro, whereas in vivo neuroprotection is preferentially mediated through beta(2)-adrenoceptors.     

Heterogenic contractile response of rat left ventricular myocytes to beta1-adrenoceptor stimulation

We measured the contractile response of left ventricular cardiac myocytes from female rats to selective beta(1)-adrenoceptor stimulation (isoprenaline, 10(-8) M and 10(-7) M in the presence of 10(-7) M ICI 118,551 a beta2-adrenoceptor inverse agonist). A heterogenic response to stimulation, inversely related to the extent of cell shortening prior to adrenergic stimulation, was observed. Challenge of cardiac myocytes with a selective beta1-antagonist, atenolol (10(-7) M), suggests the heterogenic response is not caused by basal beta1-adrenoceptor activity. Thus, basal myocyte contractility determines the response to beta1-adrenoceptor stimulation, this should be taken into account when experimental conditions are designed.     

Influence of beta 1- versus beta 2-adrenoceptor blockade on left ventricular function in humans

Pharmacological and radio-ligand binding studies have recently indicated the existence of beta 2-adrenoceptors in the human heart. Their physiological role, however, remains to be elucidated. The present study investigated in 17 normal, young volunteers the effect on resting left ventricular (LV) function of two types of beta-blockers; a predominant beta 1-adrenoceptor antagonist (atenolol, 50 mg once daily) and ICI 118,551 (20 mg t.i.d.), a new, predominant beta 2-antagonist. LV function was assessed using M-mode echocardiograms and systolic time intervals. Atenolol, ICI 118,551, and placebo were given according to a randomized, double-blind, cross-over protocol. As compared with placebo, both drugs caused a decrease in resting heart rate, but the reduction by ICI 118,551 was less pronounced. Systolic blood pressure was only reduced by atenolol 8 mm Hg on average. Cardiac output was decreased to the same extent following treatment with atenolol (-20%) as after ICI 118,551 (-17%). These decreases in cardiac output were related to the beta-blocker-induced bradycardia, since stroke volumes were not affected during either selective beta 1- or beta 2-blockade. In addition, all other echocardiographic variables reflecting LV pump function, such as fractional shortening, velocity of diameter change and of displacements, pre-ejection period, and the ratio of PEP/LVET, were not different from placebo. We conclude that in normal young subjects, global LV pump function is not affected by beta 1- or by beta 2-blockade, despite the negative chronotropic effect of both drugs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of the effect on heart rate variability of a beta2-adrenoceptor agonist and antagonist using non-linear scatterplot and sequence methods

AIMS: To examine the impact on heart rate variability (HRV), of agonism or antagonism at the cardiac beta2-adrenoceptor in healthy volunteers, using standard time-domain summary statistics and non-linear methods (scatterplot and quadrant analysis). METHODS: Under double-blind and randomised conditions (Latin square design), 17 normal volunteers received placebo, salbutamol (beta2-adrenoceptor partial agonist), ICI 118,551 (specific beta2-adrenoceptor antagonist), or salbutamol plus ICI 118,551. Single oral doses of medication (at weekly intervals) were administered at 22.30 h, with HRV assessed from the sleeping heart rates. RESULTS: Salbutamol reduced the long-term (SDNN: 135 ms [120, 156], SDANN: 107 ms [89, 124]) time-domain indicators of HRV compared with placebo (SDNN: 39 [24, 55], SDANN 42 [29, 56], [mean difference [95% confidence intervals of difference]]). Alone, ICI 118,551 did not effect HRV, but in combination blocked the actions of salbutamol. Scatterplot length (944 ms [869, 1019]) and area (222*10(3) ms2 [191, 253]) were reduced by salbutamol compared with placebo; (length difference (164 [98, 230]) and area difference 59 [36, 83]). Scatterplot width (dispersion) was lower at both low (width RR-1 25% salbutamol 277 ms [261, 293]: salbutamol minus placebo 14 ms [0, 28]) and high (width 75% salbutamol 417 [391, 443]: salbutamol minus placebo 41 [20, 62]) heart rates. ICI 118,551 alone did not alter scatterplot parameters but in combination blocked the effect of salbutamol. Cardiac acceleration episodes (i.e. consecutive deltaRR and deltaRRn+1 shorten) were increased following salbutamol 7288 [6089, 8486] compared with placebo -1890 [-2600, -1179]; the beat-to beat difference (deltaRRn+1) was reduced after salbutamol compared with the other treatments. ICI 118,551 did not effect acceleration episodes but reduced the effect of salbutamol when used in combination. CONCLUSIONS: Agonism at the cardiac beta2-adrenoceptor in healthy volunteers with salbutamol altered autonomic balance towards sympathetic dominance; this re-balancing was blocked by ICI 118,551 given in combination with salbutamol. However antagonism at the beta2-adrenoceptor with ICI 118,551 alone did not significantly alter the HRV. The beta2-adrenoceptor modulates HRV in healthy volunteers; the implications of agonism and antagonism at the beta2-adrenoceptor in cardiovascular disease states warrants further investigation.     

Pindolol increases plasma norepinephrine concentration by stimulation of beta 2-adrenoceptors in conscious spontaneously hypertensive rats.

The beta-adrenoceptor antagonist pindolol [10-1,000 micrograms/kg subcutaneously (s.c.)] caused dose-related decreases in mean arterial pressure (MAP) and increased heart rate (HR) in conscious spontaneously hypertensive rats (SHR). The lowest dose of pindolol (10 micrograms/kg) decreased MAP by 25 mm Hg (-16%) without affecting plasma norepinephrine (NE) or plasma renin concentration (PRC). However, higher doses of pindolol elicited dose-related increases in plasma NE concentration and PRC. Plasma epinephrine concentration was not altered by pindolol. The selective beta 2-adrenoceptor antagonist ICI 118,551 (3 mg/kg, s.c.) prevented the tachycardia but not the increase in PRC caused by 100 micrograms/kg pindolol. Treatment with ICI 118,551 completely eliminated the 45% increase in plasma NE elicited by 100 micrograms/kg of pindolol even though the decrease in MAP caused by this dose of pindolol was the same in the presence (-33 mm Hg) and absence (-34 mm Hg) of beta 2-adrenoceptor blockade. These results indicate that the vasodepressor action of pindolol in SHR does not result from an agonistic effect at postjunctional beta 2-adrenoceptors in the vasculature. In addition, the increases in plasma NE concentration produced by pindolol result from stimulation of beta 2-adrenoceptors. These beta 2-adrenoceptors may be located prejunctionally on sympathetic neurons.     

Selective stimulation of glucagon secretion by beta 2-adrenoceptors in isolated islets of Langerhans of the rat.

1. In rat isolated islets of Langerhans the selective beta 2-adrenoceptor agonist, clenbuterol (1 to 20 microM), significantly increased the level of adenosine 3':5'-cyclic monophosphate (cyclic AMP) within 2 min of incubation. 2. The cyclic AMP response to clenbuterol was inhibited in the presence of the selective beta 2 adrenoceptor antagonist, ICI 118551 (0.1 or 10 microM) but remained unchanged when the beta 1-antagonist, atenolol (0.1 microM) was administered. 3. Despite causing an elevation in cyclic AMP, clenbuterol (up to 20 microM) failed to influence insulin secretion at any glucose concentration tested, even in the presence of a phosphodiesterase inhibitor. 4. By contrast, clenbuterol elicited a dose-dependent rise in the rate of glucagon secretion; the maximal agonist-induced increase in secretion was two fold, a response equivalent to that observed with 20 mM L-arginine. 5. ICI 118551 significantly inhibited the rise in glucagon secretion induced by clenbuterol (up to 20 microM). 6. The results indicate that the rat islet A cell population is equipped with functional beta 2-adrenoceptors which influence glucagon secretion via the second messenger cyclic AMP, but that the B cells are deficient in functional beta-receptors.