Stereoisomers Of Propranolol Research Articles

An enantioselective voltammetric sensor for the recognition of propranolol stereoisomers

An enantioselective voltammetric sensor for the recognition of propranolol stereoisomers is developed based on a carbon-paste electrode modified with uracil supramolecular structures and the use of chemometric methods. The conditions for the formation of an enantioselective selector on the surface of graphite (graphene) particles and the selective registration of voltammograms of enantiomers, as well as the possibility of the recognition thereof, are studied by methods of molecular dynamics simulation and voltammetry.

Influence of alpha- and beta-adrenoceptor antagonists on ventricular fibrillation in ischemic rat hearts.

Although it is generally accepted that adrenergic influences contribute to arrhythmias during myocardial ischemia, it is still a matter of debate whether these arrhythmogenic effects are mediated via alpha- or beta-adrenergic receptors and which particular receptor subtype is involved. Controversial results may be due to ancillary properties of the adrenergic receptor antagonists used to resolve this question. Therefore, we compared the influence of various, structurally different alpha- and beta-adrenoceptor blocking agents on the occurrence of ventricular fibrillation in rat isolated hearts. Regional ischemia was induced by ligature of the left coronary artery and ECG was monitored over 30 min of coronary occlusion. Myocardial ischemia precipitated ventricular fibrillation in a well reproducible manner with an incidence of about 80% in control hearts. Racemic propranolol (0.1-1 micromol/l) concentration-relatedly reduced the incidence of ventricular fibrillation from 71% in controls to 10%, whereas the beta-adrenoceptor blocking agents atenolol (10 micromol/l) and timolol (1 micromol/l) did not influence the occurrence of arrhythmias. Moreover, both stereoisomers of propranolol were equipotent in suppressing ischemia-induced ventricular fibrillation, indicating an action of propranolol independent of its beta-adrenoceptor blocking properties. Unselective antagonism of alpha-adrenoceptors by phentolamine decreased the incidence of ventricular fibrillation from 90% to 58% at 0.1 micromol/l and totally suppressed ventricular fibrillation at 1 micromol/l. The alpha1-selective antagonists prazosin and HEAT concentration-dependently (0.1-10 micromol/l) reduced the incidence of ventricular fibrillation from 83% to 0%, whereas the alpha2-selective antagonist RX 821002 revealed no antiarrhythmic effect. Furthermore, subtype specific antagonism of alpha1A-adrenoceptors by WB 4101 clearly reduced the occurrence of ventricular fibrillation in a concentration-dependent manner (0.01-1 micromol/l) from 66% to 17%. Conversely, CEC, known to block the alpha1B-adrenoceptor subtype, possessed no antifibrillatory effect. In conclusion, the contribution of catecholamines to ischemia-induced arrhythmias in rat isolated heart is primarily mediated via WB 4101-sensitive alpha1-adrenergic activation. Beta- and alpha2-adrenoceptor blockade did not affect ventricular fibrillation in this model. The antifibrillatory action of propranolol was rather due to ancillary properties of this agent.

Beta-adrenoceptor antagonists suppress elevation in body temperature and increase in plasma IL-6 in rats exposed to open field.

The purpose of these studies was to assess the involvement of beta-adrenoceptors in the development of psychological stress-induced elevation in body temperature (Tb) and rise in circulating interleukin-6 (IL-6). We selected three drugs to attempt to block the rise in body temperature and plasma IL-6; L-propranolol, D-propranolol and nadolol. Both stereoisomers of propranolol have "local anesthetic' membrane-stabilizing activity and are capable of penetrating into the brain. However, D-propranolol has significantly lower beta-blocking activity than L-propranolol. Nadolol has beta-blocking activity similar to L-propranolol without membrane-stabilizing activity. Furthermore, nadolol does not cross the blood-brain barrier. All beta-blockers were injected intraperitoneally (i.p. 7.5 mg/kg) or into the third cerebral ventricle (i.c.v., 5 or 50 micrograms/animal), 20 min or just before exposure of rats to an open field, respectively. Blood samples for measurement of plasma IL-6 activity (IL-6-dependent B9 cell bioassay) were taken from rats immediately following exposure to the open field. After exposure to the open field, rats not treated with beta-blockers responded with a rapid rise in Tb measured by biotelemetry as well as with an increase in plasma IL-6 activity. The increase in Tb of open field-exposed rats was significantly suppressed by L-propranolol injected i.p. (delta Tmax = 0.14 +/- 0.15 degrees C for L-propranolol vs. 0.78 +/- 0.15 degrees C for vehicle-treated rats). Neither i.p. injection of D-propranolol nor nadolol had any effect on the increase in Tb induced by exposure to the open field. Both i.c.v. doses of L-propranolol and nadolol markedly attenuated the open field-induced rise in Tb. The large i.c.v. dose of D-propranolol (50 micrograms) did, whereas the lower dose (5 micrograms) did not suppress the elevation in Tb in open field exposed rats. The open field-exposed rats injected with L-propranolol (both i.p. or i.c.v.) had lower plasma IL-6 activity than that of open field-exposed rats injected with vehicle (for i.p. injection: 5.2 +/- 1.3 U/ml for L-propranolol vs. 17.4 +/- 3.8 U/ml for vehicle; for i.c.v. injection: 3.5 +/- 2.3 U/ml for L-propranolol vs. 24.4 +/- 7.2 U/ml for vehicle). Nadolol blocked the open field-induced rise in plasma IL-6 only when injected i.c.v. but no i.p. Neither i.p. nor i.c.v. D-propranolol injection had an effect on plasma IL-6 activity in open field-exposed rats. These data show that beta-adrenoceptors in the central nervous system are involved in the psychological stress-induced elevation in Tb and rise in plasma IL-6 activity caused by exposure to an open field.

Beta-adrenergic receptors in human anterior optic nerve: an autoradiographic study.

Sections of human anterior optic nerve and nerve head were incubated in a physiological solution containing a radiolabelled beta blocker at a low concentration. The beta blocker used was (-)-(125iodo)-cyanopindolol, which has a high affinity and specificity for beta-adrenergic receptors. Concurrent incubations were performed with a great excess of unlabelled beta blocker added to demonstrate non-specific binding. Following incubation the sections were washed and dried. They were then apposed to photographic film for 5 days and developed. Incubations were performed with the stereoisomers of propranolol and an alpha blocker as well as specific beta-one and beta-two blockers. Beta-adrenergic receptors were demonstrated in anterior optic nerve and optic nerve head. The majority were of the beta-two subtype.

Role of phospholipase D-derived diradylglycerol in the activation of the human neutrophil respiratory burst oxidase. Inhibition by phosphatidic acid phosphohydrolase inhibitors.

An agonist-activated phospholipase D/phosphatidic acid phosphohydrolase (PAH) pathway was recently demonstrated in human neutrophils, and evidence suggests that phosphatidic acid (PA) and/or diradylglycerol (DG) generated from this pathway participates in activation of the O2(-)-generating respiratory burst. We have used a series of cationic amphiphilic compounds (sphingosine, propranolol, chlorpromazine, and desipramine) and antibiotics (clindamycin, trimethoprim, and roxithromycin) all of which inhibit the respiratory burst, to investigate the role of the phospholipase D/PAH pathway in neutrophil activation. The phosphatidylcholine (PC) pool in intact cells was first labeled using [3H]-1-O-alkyl-lysoPC; released [3H]-PA and [3H]-DG were then quantified after the addition of either chemo-attractant or PMA. Using either agonist, all compounds showed a dose-dependent inhibition of [3H]-DG generation which correlated with inhibition of O2- generation, but compounds failed to inhibit directly the NADPH oxidase in a cell-free system. For either activator, a plot of the ID50 values for O2- generation vs those for DG generation was linear over four orders of magnitude. In many cases, inhibition of [3H]-DG generation corresponded to an increase in [3H]-PA, implicating PAH as the locus of inhibition. Superoxide generation was inhibited under conditions where PA was either elevated or minimally affected. Neither O2- release nor DG generation showed any selectivity for stereoisomers of propranolol, suggesting that this inhibition does not act via a specific binding site on PAH. No evidence was obtained for an effect of the inhibitors on PA mobility as monitored by electron spin resonance studies of spin-labeled PA in a model membrane system. Data are consistent with an effect of the inhibitors at the level of the interaction of PAH with the membrane and/or its substrate. These data imply that DG produced via the phospholipase D/PAH pathway functions in the activation or maintenance of the respiratory burst.

Effects of calcium channel blockers on the pharmacokinetics of propranolol stereoisomers

Diltiazem and verapamil inhibit oxidative drug metabolism both in vivo and in vitro. We compared their effects on the stereoselective pharmacokinetics and protein binding of propranolol in 12 subjects. After 6 days of coadministration with racemic propranolol, diltiazem caused decreases of 27% and 24% in d-propranolol and 1-propranolol oral clearances, respectively (p less than 0.05 versus control). With verapamil, d-propranolol oral clearance decreased 32% (p less than 0.05), and 1-propranolol oral clearance decreased 26% (p less than 0.05). The unbound fraction of d-propranolol was higher than that of 1-propranolol (p less than 0.05), but the protein binding was not altered by diltiazem or verapamil. Both drugs therefore decreased the unbound oral clearance of each propranolol enantiomer (p less than 0.05). Verapamil caused a stereoselective effect and increased the d/l ratio of propranolol serum concentrations (p less than 0.05) and decreased the d/l ratio of oral clearance (p less than 0.05).

Effects of age on the protein binding and disposition of propranolol stereoisomers

Previous studies of the effects of age on the disposition of propranolol have produced variable results. We evaluated the stereoselective disposition and protein binding of propranolol enantiomers in 10 young (mean age, 28 years) and 10 older (mean age, 64 years) healthy subjects. After receiving racemic propranolol orally for 6 days, the oral clearances of d-propranolol and l-propranolol were lower by 13% and 17% in the older group compared to the young group, but these differences were not statistically significant. The older subjects had higher alpha 1-acid glycoprotein concentrations (p less than 0.05) and lower unbound fractions of l-propranolol (p less than 0.05). After protein binding was accounted for, the unbound oral clearance of each enantiomer was similar in both groups. l-Propranolol was more highly protein bound than d-propranolol (p less than 0.05) in both young and older subjects. The unbound oral clearance d/l ratio was not different from unity in either group, indicating that the stereoselective differences in oral clearance were largely attributable to the stereoselective differences in protein binding.

Propranolol inhibits nonexocytotic noradrenaline release in myocardial ischemia

Ischemic induces a nonexocytotic noradrenaline release in the heart, which leads to high and potentially harmful interstitial noradrenaline concentrations. The effect of beta-adrenoceptor antagonists on noradrenaline release in ischemia has been investigated in the present study. DL-Propranolol (1-100 mumol/l) concentration-dependently reduced noradrenaline release during 20 min of global and total ischemia in the perfused rat heart. Other beta-adrenoceptor blocking agents such as atenolol, metoprolol, and timolol (10 mumol/l each), however, did not share this effect. Moreover, both stereoisomers of propranolol were equipotent in suppression of ischemia-induced noradrenaline release, indicating a property of propranolol independent from interaction with beta-adrenoceptors. The well known local anesthetic action of propranolol was not likely to cause its inhibitory effect on ischemia-induced noradrenaline release, as lidocaine (10 mumol/l) did not affect noradrenaline overflow in ischemia. The effect of propranolol was further examined in cyanide intoxication, an experimental model of energy depletion. In this experimental setting the release of dihydroxyphenylethyleneglycol--the major neuronal metabolite of noradrenaline--served as indicator of increased axoplasmic noradrenaline levels which are present during nonexocytotic noradrenaline release. In cyanide intoxication DL-propranolol also reduced noradrenaline overflow but did not affect release of dihydroxyphenylethylene glycol. The latter finding suggests an interaction of propranolol with the neuronal membrane transport of noradrenaline. In ischemia and cyanide intoxication, transport of noradrenaline across the plasma membrane is known to be driven by the noradrenaline carrier (uptake1) working in reverse of its normal direction--from inside to outside. Consequently, inhibitors of the noradrenaline carrier like desipramine were shown to suppress nonexocytotic noradrenaline release in ischemia and cyanide intoxication.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and .beta.-adrenergic antagonist activity of stereoisomeric practolol and propranolol derivatives

A series of stereoisomeric practolol and propranolol derivatives has been synthesized in which the N-isopropyl group of the drug was replaced by an asymmetric heptanoic acid terminated by a substituted p-toluidide or p-(trifluoromethyl)anilide. The asymmetric epoxide, 3-(p-acetamidophenoxy)-1,2-epoxypropane, was allowed to react with a preformed enantiomeric 6-aminoheptanoic acid amide to yield the stereoisomeric practolol congener derivatives. An asymmetric drug precursor epoxide was prepared from p-acetamidophenol and enantiomeric 3-(tosyloxy)-1,2-epoxypropane 6-aminoheptanoic acid amides were allowed to react with one of the enantiomers of 3-(1-naphthyloxy)-1,2-epoxypropane. This drug precursor epoxide was prepared either by combining 1-naphthol with enantiomeric 3-(tosyloxy)-1,2-epoxypropane (the Sharpless epoxide) or by combining 1-naphthol with enantiomeric 3-(tosyloxy)-1,2-propanediol followed by epoxidation. Pharmacological studies carried out for the practolol derivatives demonstrated a significant dependence of enhanced potency and tissue/subreceptor specificity on both the configuration of the drug asymmetric carbon and the configuration of the spacer asymmetric carbon. The compounds containing the S configuration at the drug asymmetric center and the R configuration at the spacer asymmetric carbon exhibited an increase in potency over the other stereoisomeric congener derivatives and the progenitor drug. For the propranolol congener derivatives, a large decrease in potency was observed for all of the stereoisomers over the progenitor drug. The propranolol stereoisomers containing the S configuration at the drug asymmetric center were more active than those containing the R configuration at that center.

The distribution of β-adrenoceptors in dog kidney: an autoradiographic analysis

The distribution of β-adrenoceptors in slide-mounted dog kidney sections was determined using the radioligand (-)-[ 125I]cyanopindolol ((-)-[ 125I]CYP) and autoradiography. Using conditions designed to prevent (-)-[ 125I]CYP binding to non-β-adrenoceptor sites, biochemical studies revealed that (-)-[ 125I]CYP binding equilibrated within 150 min (K 1 = 3.2 × 10 8 M −1 min −1), was saturable (K D = 30.72 ± 2.96 pM; B max = 0.57 ± 0.03 fmol/section, n = 4) and stereoselective with respect to the stereoisomers of propranolol and pindolol. Delineation of β-adrenoceptor subtypes with the selective β 1-adrenoceptor antagonist betaxolol and β 2-adrenoceptor antagonist ICI 118,551 demonstrated that the proportions of β 1-: β 2-adrenoceptors was between 1:6 and 1:11. Autoradiographic studies showed that β 1-adrenoceptors were localized on the juxtaglomerular apparatus and glomwruli, while β 2-adrenoceptors were localized on medullary rays. The distribution of β-adrenoceptors with respect to renal function in the dog kidney is discussed.

Beta-adrenergic receptors and cyclic adenosine monophosphate generation in human fetal lung.

We designed experiments to determine whether beta-adrenergic receptors are present and functional in human fetal lung during the 2nd trimester of gestation. To determine the presence of beta receptors, characterize their binding sites, and assess changes in receptor with gestational age, we performed radioligand binding assays with the specific, high-affinity beta antagonist, 125I-iodocyanopindolol, in membrane particulates from the lungs of 2nd trimester abortuses (15-23 wk). Binding of 125I-iodocyanopindolol was saturable and of high affinity (dissociation constant = 40 pM). Binding was stereoselective as determined by competition studies with (-) and (+) stereoisomers of propranolol. Agonist affinities (isoproterenol greater than epinephrine much greater than norepinephrine) were consistent with a predominance of beta-2 receptors; this predominance was confirmed by competition studies with the specific beta-2 receptor antagonist ICI 118-551 (75% beta-2, 25% beta-1). The concentration of beta-adrenergic receptors increased with gestational age. To assess the functional coupling of the beta receptors, we tested the ability of receptor occupancy to activate adenylate cyclase. For this assay, we incubated minced human fetal lung with beta agonists and determined the amount of cAMP generated. beta Agonists stimulated cAMP generation more than 2-fold. We conclude that beta-adrenergic receptors are present and functional in human fetal lung as early as the 2nd trimester.

Nonstereoselective aspects of propranolol pharmacodynamics.

Twenty years after its discovery, the beta-adrenergic blocking agent propranolol continues to interest pharmacologists and clinicians. Its therapeutic profile has extended to areas beyond the purview of the cardiovascular system, and its ocular and central nervous system effects have been well documented. In addition, it still remains a very good pharmacological tool to map out the adrenergic beta-receptors in the body, and stereoisomers of propranolol and other beta-blockers serve as valuable agents to distinguish between the effects related to beta-adrenoceptors and those which are not. The primary purpose of this review is to summarize the evidence indicating that beta-adrenergic blocking agents lack stereoselectivity in some of their effects, including several of considerable therapeutic importance. Because many pharmacological actions of propranolol followed a nonsteroselective pattern, the involvement of beta-adrenoceptors in them was questioned and this led to the search for alternate mechanisms to explain these effects. Studies with propranolol and some related drugs indicated the involvement of a cholinergic mechanism in their antiarrhythmic, ocular hypotensive and some central effects. Also, a presynaptic inhibitory effect at the skeletal neuromuscular junction has been suggested to explain the benefical effect of propranolol and other beta-blockers in tremor. Biochemical studies with these drugs revealed their inhibitory action on the cholinesterase enzyme in blood and other tissues like myocardium and brain. It is thus hypothesized that modulation of cholinergic neurotransmission by propranolol could explain some of its nonstereoselective actions and open new vistas in propranolol pharmacodynamics.

Studies on the stereoisomers of beta-adrenoceptor antagonists in conscious A-V blocked dogs.

Atrial and ventricular chronotropic effects of the individual stereoisomers of propranolol, pindolol, metoprolol and penbutolol were studied in conscious dogs with chronic atrio-ventricular (A-V) block. Ventricular beta-adrenoceptor blocking activity was assessed for all drugs against isoprenaline under the same experimental conditions. At low doses, the stereoisomers of propranolol and penbutolol decreased atrial rate, whereas those of pindolol and metoprolol produced an increase. At higher doses, all drugs increased atrial rate. All drugs decreased ventricular rate dose-dependently except (+)-pindolol. Relative ventricular beta-blocking potencies of the (-)-isomers of propranolol, pindolol, metoprolol and penbutolol were respectively 38, 21, greater than 43 and 31 times higher than those of their corresponding (+)-isomers. In addition, beta-blocking potencies of (-)- and (+)-pindolol were respectively 60 and 120 times higher, those of (-)- and (+)-penbutolol 7 and 8 times higher and those of (-)- and (+)-metoprolol 4 and greater than 4 times weaker than those of (-)- and (+)-propranolol. At comparable levels of ventricular beta-adrenoceptor blockade, (-)-pindolol and (-)-metoprolol were more potent in producing ventricular bradycardia than their respective (+)-isomers, whereas (-)- and (+)-propranolol and (-)- and (+)-penbutolol were equiactive. In addition, regardless of which isomer was being studied, the order of ventricular bradycardiac potencies, at comparable levels of beta-adrenoceptor blockade, was metoprolol greater than propranolol greater than penbutolol greater than pindolol. In addition, regardless of which isomer was being studied, the order of ventricular bradycardiac potencies, at comparable levels of beta-adrenoceptor blockade, was metoprolol > propranolol > penbutolol >pindolol. 5 These results show that antagonism of beta-adrenoceptors in the ventricle is at least partly responsible for the ventricular bradycardiac effect produced by these drugs, but also that some other factor, apparently distinct from the membrane stabilizing activity, is involved, suggesting the existence of some other as yet unknown pharmacological property of the beta-adrenoceptor blocking drugs, especially evident in metoprolol. Finally, these results demonstrate that the intrinsic sympathomimetic activity exhibited by some of these drugs attenuate their bradycardiac effect.

Hyponeophagia in the Roman rat strains: Effects of 5-methoxy-N,N-dimethyltryptamine, diazepam, methysergide and the stereoisomers of propranolol

The effects of 5-MeODMT (2.5 mg/kg), diazepam (1 mg/kg), methysergide and the stereoisomers of propranolol (6 mg/kg) on hyponeophagia were studied in both sexes of the Roman strains of rats, selectively bred for acquisition of a two-way conditioned avoidance response. Diazepam, methysergide and 1-propranolol increased feeding in a novel environment whilst 5-MeODMT decreased it and d-propranolol was inactive. Several strain differences in drug responsiveness occurred, the Roman Low Avoidance subjects being most sensitive to all drugs tested as well as being most neophobic. A sex difference in 5-MeODMT sensitivity was also found, female rats of the Roman High and Control Avoidance strains being more sensitive than males. The findings are discussed in connection with differences in arousal and biochemical parameters between these strains.

Digitalis-Sensitive Na+,K+-ATPase

It has been documented that biogenic amines can stimulate Na+, K+-ATPase from various tissue preparations. However, it is unclear whether or not this stimulation occurs in myocardial tissues. We have evaluated possible catecholamine stimulation of purified Na+, K+- ATPase preparations utilizing a bovine ventricular microsomal preparation. We have studied the dose response of the enzyme to ouabain and digitoxigenin in the presence and absence of propranolol and norepinephrine. Our results indicate that propranolol increases the sensitivity of Na+, K+-ATPase to inhibition by digitalis, and that stimulation of Na+, K+-ATPase in bovine myocardium is not mediated via an adrenergic mechanism. In addition, our results indicate that in myocardial tissue, both stereoisomers of propranolol produce a direct nonspecific membrane effect which can modify Na+, K+-ATPase activity.

Pharmacokinetics and pharmacodynamics of propranolol stereoisomers in hyperthyroid patients.

The disposition of propranolol stereoisomers after administration of a single oral dose of the racemic drug was investigated in seven hyperthyroid patients before and after antithyroid drug therapy. The possibility of hypersensitivity to propranolol in the patients was evaluated by constructing plasma propranolol concentration -- beta-blocking effect curves. There was no statistically significant difference in elimination half-life (t1/2) between (+/-)- and (-)-propranolol before and after antithyroid drug therapy. However, the plasma clearance (Vp) of (-)-propranolol was smaller than that of (+/-)-propranolol, and the difference was statistically significant after antithyroid drug therapy. Vp decreased or did not change in young patients after therapy. No significant difference was observed in the relationship between the tilt-induced pulse rate response and plasma propranolol concentration when treated patients became euthyroid compared to their response in the hyperthyroid state.

Effects of centrally administered propranlol on plasma renin activity, plasma norepinephrine and arterial pressure

Single doses of d,l-propranolol (0.1–0.5 mg/kg) administered intarcisternally (IC) to anesthetized dogs produced dose-dependent decreases in plasma renin activity and mean arterial pressure whereas identical doses given intravenously had no significant effect on these parameters. Intracisternal d,l-propranolol also significantly reduced circulating norepinephrine levels concomitant with the renin suppressing and hypotensive effects. Acute renal denervation abolished the renin suppressing, but not the hypotensive effect of intracisternal d,l-propranolol. Both d- and l-propranolol administered intracisternally significantly lowered plasma renin activity and arterial pressure to a similar degree, but intracisternal lidocaine had no effect on these parameters. These results indicate that propranolol can suppress plasma renin activity through an action within the central nervous system and suggest that both the renin suppression and hypotension produced by intracisternal propranolol resulted from a centrally mediated reduction in peripheral sympathetic activity. The equal potency of the stereoisomers of propranolol and the observed differences between the effect of propranolol and lidocaine suggest that these central actions involve a mechanism that may be independent of beta receptor blockade or the local anesthetic properties of drug.

Differentiation of β-adrenoceptors in right atrium, diaphragm and adipose tissue of the rat, using stereoisomers of propranolol, alprenolol, nifenalol and practolol

In order to characterize the β-adrenoceptors in rat adipocytes in more detail, the β-adrenoceptor blocking potencies of the stereoisomers of propranolol, alprenolol, nifenalol and practolol were determined, using isoproterenol as the agonist, on β-adrenoceptors in isolated adipocytes, right atrium and hemidiaphragm of the rat. Marked differences in stereoselectivity of the four stereoisomer pairs were found in the three preparations and it is suggested that the β-adrenoceptors in the three preparations are not identical.

Stereochemical aspects of β-adrenoceptor antagonist-receptor interaction in adipocytes. Differentiation of β-adrenoceptors in human and rat adipocytes

In order to define further the possibilities and limitations of the use of rat adipocytes as a model for the study of β-adrenoceptor antagonism in human adipose tissue, the potencies against isoprenaline induced lipolysis of the stereoisomers of propranolol, alprenolol, nifenalol and practolol on human and rat adipocytes were determined. All four compounds showed higher stereoselectivity on human than on rat adipocyte β-adrenoceptors, the dextrorotatory isomers being approximately equipotent in both species, in contrast to the levorotatory isomers which were considerably more potent on human adipocyte β-adrenoceptors. It is suggested that human and rat adipocyte β-adrenoceptors are very similar, except for the site that interacts with the β-hydroxy-group which is present in the sidechain of most β-adrenoceptor stimulating and blocking agents.

Binding of (125I)iodohydroxybenzylpindolol to putative beta-adrenergic receptors of rat glioma cells and other cell clones.

[125I]Iodohydroxybenzylpindolol, an extremely potent beta-adrenergic antagonist, has been purified to theoretical specific activity (2200 Ci/mmol) and used as a ligand to characterize the beta-adrenergic receptors of cultured rat glioma cells and other cultured cell clones. Appropriate receptor sites were identified by stereoselective competition for binding by a number of adrenergic agonists and antagonists, by correlation between the potency of these compounds to inhibit binding and to affect adenylate cyclase activity, and by correlation of binding with the presence or absence of response to catecholamines (stimulation of adenylate cyclase) in various cell clones. In equilibrium experiments, the dissociation constant for binding of the iodinated ligand to the beta-adrenergic receptor of a clone of rat glioma cells (C6TG1A) was 250 pM; the corresponding value for a clone of human fibroblasts (VA2) was 15 pM. For C6TG1A, KD was verified by analysis of the kinetics of binding: k1 = 10(8) 1/mol/min: k-1 = 0.017/min. This rate of dissociation of ligand from the receptor was also established by study of the rate of activation of adenylate cyclase by isoproterenol after prior equilibration with iodohydroxybenzylpindolol. For VA2 cells, where affinity was higher, the rate of reversal of binding was only 0.0035/min. C6TG1A contained approximately 4000 receptor sites/cell (75 fmol/mg of protein), and these sites appeared to be coupled to adenylate cyclase in a stoichiometric manner. A second site with equal affinity for iodohydroxybenzylpindolol (KD = 250 pM) was also identified in C6TG1A by both kinetic analysis and equilibrium binding studies. While most compounds that interacted with the beta-adrenergic receptor also influenced binding to the second site, the latter did not distinguish between stereoisomers of propranolol, and its affinity for the other compounds tested was poorer.