Tommy Abrahamsson

The β1 and β2-adrenoceptor affinity and β1-blocking potency of S- and R-metoprolol

1 The β‐adrenoceptor affinity and blocking potency of the two enantiomers and the racemate of metoprolol were investigated in vitro, by use of a receptor‐binding technique, and in vivo in the anaesthetized cat. 2 The enantiomeric purity of the S‐ and R‐form was: >99.2% and >99.9%, respectively. 3 The β1‐ and β2‐adrenoceptor affinity (–log equilibrium dissociation constant) of the enantiomers was determined from competition binding experiments (radioligand: [125I]‐(S)‐pindolol) performed in membranes prepared from the guinea‐pig left ventricular free wall (predominantly β1) and soleus muscle (β2). The β1‐adrenoceptor affinity was (means ± s.d.): 7.73 ± 0.10 and 5.00 ± 0.06 for the S‐ and R‐form of metoprolol, respectively. The corresponding values for β2‐adrenoceptors were 6.28 ± 0.06 (S) and 4.52 ± 0.09 (R). Thus, the difference in affinity for the two enantiomers was greater on β1 (about 500) than on β2‐adrenoceptors (about 50). The β1‐adrenoceptor selectivity of the S‐form (about 30) was similar to that of the racemic metoprolol, while the R‐form was almost non‐selective (3 fold β1‐selective). 4 In the anaesthetized cat, the (–log) intravenous doses (μmol kg−1) of S‐ and R‐metoprolol causing a 50% reduction (ED50) in the heart rate response to sympathetic nerve stimulation were determined. The doses inducing a 25% depression (DD25) of the basal myocardial contractility were also estimated. For the two enantiomers, the β1‐blocking potency (‐log ED50) was 7.04 ± 0.16 (S) and 4.65 ± 0.16 (R). A significant cardiodepressive effect was observed at high doses (‐log DD25): 4.18 ± 0.20 (S) and 4.08 ± 0.10 (R). 5 It is concluded that the binding of metoprolol to β1‐adrenoceptors has a stricter steric requirement than that for the binding of this β‐blocker to β2‐adrenoceptors. Furthermore, the non‐specific cardiodepressive effect of metoprolol was observed at equally high doses for the two enantiomers.

The β1- and β2-adrenoceptor affinity of atenolol and metoprolol: A receptor-binding study performed with different radioligands in tissues from the rat, the guinea pig and man

The radioligand binding technique was used to perform a systematic investigation of the beta 1- and beta 2-adrenoceptor affinity of atenolol and metoprolol in tissues from the rat, the guinea pig and man. Radioligands, [125I](+/-)hydroxybenzylpindolol, [125I](-)pindolol, [3H](-)dihydroalprenolol and [3H](-)CGP12177, with different degrees of lipophilicity were used in the binding experiments. In membrane preparations of rat ventricular myocardium and uterus, the number of specific binding sites was similar when comparing experiments performed with the different radioligands. The percentage of the beta 1- and beta 2-adrenoceptor subpopulations in the tissues studied was not dependent on the radioligand or displacing compound used. Furthermore, the affinity of metoprolol and atenolol for beta 1- and beta 2-adrenoceptors was independent of the radioligand used or the tissue studied. The beta 1-adrenoceptor affinity of metoprolol was about 6-7 times higher than that of atenolol, while the beta 1-adrenoceptor selectivity was similar (about 30-fold) for the two beta-blockers. It is concluded that the physical-chemical properties of the radioactive ligands and beta-blockers studied do not affect the results obtained from beta-adrenoceptor-binding experiments in cellular membrane fractions. The beta 1- and beta 2-adrenoceptor affinities did not change in any experiments performed in tissues from the rat, the guinea pig and man for either atenolol or metoprolol.

Endogenous noradrenaline masks beta-adrenergic receptors in rat heart membranes via tight agonist binding

The tight binding of noradrenaline (NA) to beta-adrenergic receptors was studied in membranes from the left ventricular myocardium of the rat. Addition of GTP (0.1 mM) to membrane preparations from control rats (NA concentration 8.5 +/- 2.1 nM) caused a 4-35% (N = 8) increase (P less than 0.01) in the number of specific binding sites of [125I](-)pindolol. In contrast, addition of GTP did not cause any changes in the number of beta-adrenergic receptors in heart membranes from reserpinized animals (NA concentration less than 0.1 nM). In heart membranes from reserpinized animals, preincubation with NA (followed by washing) revealed a time- and concentration-dependent decrease with a maximum of 35-40% in the [125I](-)pindolol-binding sites. This agonist-mediated decrease in the number of receptors was prevented if GTP was also present in the NA-preincubation medium. It is concluded that NA can undergo tight binding to beta-adrenergic receptors in rat heart membranes. The heart-membrane preparations contain endogenous NA which, via tight agonist binding, is responsible for masking part of the beta-adrenergic receptor population.

Cardiac Antiischemic Effect of Metoprolol: Role of β-Blockade within the Ischemic Region

Summary: The distribution of metoprolol and atenolol into ischemic and nonischemic myocardium was studied in anesthetized dogs, pigs, and cats. The &bgr;‐blockers were administered intravenously after coronary artery occlusion. Metoprolol was found to be significantly more efficiently distributed to the ischemic myocardium than atenolol in all three species. To investigate the functional implications of this difference in tissue distribution, the antiischemic effects of the two &bgr;‐blockers were studied in the 2‐h period following coronary artery occlusion in anesthetized cats, in which heart rate was kept at a constant level. In this model, metoprolol (0.3 mg • kg−1 + 0.15 mg • kg−1 • h−1) was found to attenuate or delay the developing ischemic process. This is shown by its significant reduction of (a) the decline of CK activity in ischemic myocardium, (b) the ST elevation in a precordial ECG lead, and (c) the decrease of arterial pressure and cardiac output. In contrast to metoprolol, atenolol (0.3 mg • kg−1 + 0.15 mg • kg−1 • h−1) caused no significant antiischemic effect in this cat model. The difference in the effectiveness of the two drugs can most probably be explained by their differential distribution in the ischemic heart. Furthermore, the anti‐ischemic effect of metoprolol shows that the presence of a &bgr;‐blocker in ischemic left ventricular myocardium can favorably affect the early phase of developing infarction.