Comparison of beta adrenergic receptor binding characteristics and coupling to adenylate cyclase in rat pulmonary artery versus aorta

In an effort to define the mechanisms regulating pulmonary vasodilatation and explain the greater in vitro response to iso-proterenol in the pulmonary artery (PA) vs. aorta (AO), we compared beta adrenergic receptor binding characteristics and coupling to adenylate cyclase in PA and AO obtained from adult male rats. Beta adrenergic receptor binding characteristics and affinity for agonists were determined with [125I]-iodocyanopindolol. Agonist displacement studies were characteristic of a beta-2 adrenergic receptor subtype. Receptor density (44.7 +/- 7.3 vs. 39.6 +/- 0.8 fmol/mg of protein means +/- S.E.M., PA vs. AO) and the dissociation constant for the radioligand (10.3 +/- 2.6 vs. 13.4 +/- 3.5 pM) were similar in the two arteries. However, affinity for l-isoproterenol was greater (the inhibition constant was lower) in PA compared to AO (0.08 +/- 0.03 vs. 1.20 +/- 0.18 microM, P less than .05), as was affinity for l-epinephrine (0.89 +/- 0.20 vs. 3.87 +/- 0.62 microM, P less than .05). Affinity was similar for l-norepinephrine (18.93 +/- 3.63 vs. 13.49 +/- 3.12 microM). Base-line cyclic AMP (cAMP) content, basal adenylate cyclase activity and adenylate cyclase activity stimulated by GTP, isoproterenol plus GTP and forskolin were measured by radioimmunoassay for cAMP. Base-line cAMP content was greater in PA than in AO (513.5 +/- 46.9 vs. 125.5 +/- 19.1 pmol of cAMP per mg of protein, P less than .001), as was basal adenylate cyclase activity (10.8 +/- 1.2 vs. 5.7 +/- 1.3 pmol of cAMP per mg of protein per min, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Selective regulation of beta-1 and beta-2 adrenergic receptors by atypical agonists

The interactions of the atypical agonists pindolol and celiprolol with beta adrenergic receptors were compared with those of the full agonist, isoproterenol. Studies were carried out using intact cells as well as membranes prepared from C6 glioma cells. Computer-assisted analysis of dose-response curves resulting from the inhibition of the binding of [125I]iodopindolol by the beta-1 and beta-2 selective compounds ICI 89,406 and ICI 118,551 revealed that approximately one-third of the beta adrenergic receptors on these cells were beta-1 receptors. Addition of GTP to the binding assay simplified the dose-response curve for inhibition of the binding of [125I]iodopindolol by isoproterenol and diminished the potency of the agonist. GTP had no effect on the binding of pindolol or celiprolol, suggesting that these drugs do not induce the formation of a ternary complex with the receptor and the guanine nucleotide-binding protein for stimulation of adenylate cyclase activity. When added to the growth medium of intact C6 cells, isoproterenol induced a 40-fold increase in cyclic AMP accumulation. Pindolol and celiprolol, however, caused no elevation of enzyme activity. Addition of isoproterenol to the growth medium of intact cells resulted in an 80% decrease in the density of both beta-1 and beta-2 adrenergic receptors within 8 hr. Growing cells in the presence of pindolol or celiprolol induced a 50% decrease in the density of beta-2 receptors, which was inhibited by beta adrenergic antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of clenbuterol on central beta-1 and beta-2 adrenergic receptors of the rat

Repeated administration of the centrally acting beta adrenoceptor agonist, clenbuterol, to rats reduced the ability of isoproterenol to increase the concentration of cyclic AMP (cAMP) in slices of cerebellum. This reduced responsiveness to isoproterenol was accompanied by a marked reduction in the density of beta adrenoceptors as measured by the binding of the beta adrenoceptor antagonist [125I]iodopindolol. In addition, the agonist-binding properties of remaining cerebellar beta adrenoceptors were altered after clenbuterol treatment. The clenbuterol-induced reduction in the density of beta adrenoceptors in the cerebellum is in marked contrast to its inability to do this in cerebral cortex. Comparison of the ability of clenbuterol to that of isoproterenol to increase levels of cAMP in slices of cerebral cortex or cerebellum showed that clenbuterol is a weakly potent agonist in both brain regions. The increase in cAMP induced by isoproterenol in the cortex was significantly reduced in the presence of the selective beta-1 adrenoceptor antagonist, ICI 89,406. In contrast, the clenbuterol-induced increase in cortical cAMP was unchanged by ICI 89,406 but was reduced significantly by the beta-2 adrenoceptor antagonist, ICI 118,551. In cerebellum, both isoproterenol- and clenbuterol-stimulated accumulation of cAMP were antagonized much more potently by ICI 118,551 than by ICI 89,406. Furthermore, clenbuterol antagonized the cAMP response induced by isoproterenol in the presence of ICI 118,551 in a concentration-dependent manner. In terms of measurement of cAMP in brain slices, clenbuterol is weakly potent as an agonist at beta-2 adrenoceptors and has antagonist properties at beta-1 adrenoceptors.     

Coexistence of beta-1 and beta-2 adrenergic receptors in the human heart: effects of treatment with receptor antagonists or calcium entry blockers

The properties of the binding of [125I]iodopindolol ([125I]IPIN) to beta adrenergic receptors on plasma membranes prepared from right atrial tissue removed during cardiac bypass surgery were investigated. Some of the patients from whom the tissue was removed had been treated before surgery with either a beta adrenergic receptor antagonist or a calcium entry blocker or both. The specific binding of [125I]IPIN to beta adrenergic receptors was saturable, stereoselective and rapidly reversible. Studies of the inhibition of the specific binding of [125I]IPIN by drugs selective for beta-1 or beta-2 adrenergic receptors suggested that both beta-1 and beta-2 adrenergic receptors are present in the tissue, with approximately 55% of the receptors having the properties of beta-2 adrenergic receptors. The density of receptors in patients not treated with beta adrenergic receptor antagonists or calcium entry blockers was approximately 80 fmol/mg of protein, whereas the density of beta adrenergic receptors in treated patients was increased by approximately 50%. The relative proportion of beta-1 to beta-2 adrenergic receptors in subjects treated with beta adrenergic receptor antagonists and/or calcium entry blockers was not significantly different from that in untreated subjects. Studies were also carried out with a limited number of samples of human ventricular muscle obtained from untreated subjects at the time of surgery. The density of receptors was lower than that observed in studies with atrial tissue. However, as with atrial tissue, approximately half of the receptors appeared to be beta-2 adrenergic receptors.     

Coupling of subtypes of the muscarinic receptor to adenylate cyclase in the corpus striatum and heart

The binding properties of a series of muscarinic antagonists were compared with their ability to antagonize muscarinic receptor mediated inhibition of adenylate cyclase activity in homogenates of the corpus striatum and heart of rats. When measured by the competitive inhibition of the binding of the muscarinic antagonist N-[3H]methylscopolamine, the binding properties of selective muscarinic antagonists in the corpus stratum and cerebral cortex were consistent with a model incorporating a minimum of three populations of muscarinic receptors, a high affinity site for pirenzepine (M1), a high affinity site for AF-DX 116 [11] [2-[ (diethylamino)methyl]-1-piperidinyl] acetyl] -5, 11-dihydro-6H-pyrido [2,3-b] 1,4] benzodiazepine-6-one (M2) and a third population (non-Ml, non-M2 sites) displaying low affinity for the latter antagonists. The results of similar experiments on the heart showed that this tissue contained a uniform population of M2 muscarinic receptors. The binding properties of the M2 receptor in cerebral cortex and corpus stratum were also investigated directly in antagonist [3H] AF-DX 116 competition experiments and, although the high affinity AF-DX 116 site in brain (M2) exhibited selectivity for the cardioselective antagonists AF-DX 116 and gallamine, some differences were noted between M2 sites in brain and heart. The muscarinic adenylate cyclase response in the corpus striatum was relatively insensitive to the M2 selective antagonists AF-DX 116 and gallamine as well as the M1 selective antagonist pirenzepine, suggesting that non-M1, non-M2 sites inhibit adenylate cyclase activity in the corpus striatum. In contrast, the effects of muscarinic antagonists on the muscarinic adenylate cyclase response in the heart were consistent with the postulate that M2 receptors inhibit adenylate cyclase activity in this tissue.     

Coupling of muscarinic receptors to adenylate cyclase in the rabbit myocardium: effects of receptor inactivation

The relationship between muscarinic receptor occupancy and adenylate cyclase inhibition was investigated in homogenates of the rabbit myocardium. The highly efficacious muscarinic agonist oxotremorine-M caused half-maximal inhibition of adenylate cyclase activity at a concentration (Ki) that was 10-fold smaller than that required for half-maximal receptor occupancy in the presence of 0.1 mM GTP (D50-GTP) as measured by competitive displacement of the binding [3H]N-methylscopolamine. In contrast, there was much closer agreement between the Ki and D50-GTP of the less efficacious oxotremorine analog BM5 [N-methyl-N-(1-methyl-4-pyrrolidino-2-butynyl)acetamide]. By comparing equal levels of adenylate cyclase inhibition before and after partial inactivation of muscarinic receptors with benzilycholine mustard, it was possible to estimate the dissociation constants (KA) of the oxotremorine analogs. There was good agreement between KA and D50-GTP and also between the degree of receptor inactivation determined pharmacologyically and that estimated by measurements of the binding of [3H]N-methylscopolamine.     

Pharmacological evidence for distinct muscarinic receptor subtypes coupled to the inhibition of adenylate cyclase and to the increased generation of inositol phosphates in the guinea pig myometrium

In the guinea pig myometrium, muscarinic receptor activation leads to contraction and elicits two biochemical responses viz. an increased formation of inositol phosphates (via a guanine nucleotide regulatory protein, distinct from the stimulatory and inhibitory G proteins of the adenylate cyclase system and a decreased synthesis of cyclic AMP involving inhibitory G protein activation. We now describe two major differences in the effects of muscarinic agonists. First, the greater potency of carbachol in inhibiting cyclic AMP formation (EC50 = 8 nM) than in stimulating the accumulation of inositol phosphates and tension (EC50 = 15 and 2 microM, respectively). Second, carbachol, oxotremorine and pilocarpine were equally effective in eliciting cyclic AMP inhibition but the order of potency for inositol phosphate formation was carbachol greater than oxotremorine and pilocarpine was without effect. The partial agonists, pilocarpine and oxotremorine, inhibited carbachol-mediated inositol phosphate formation. Pirenzepine, selective for muscarinic M1 receptor subtype, displayed a low affinity for antagonizing cyclic AMP inhibition, inositol phosphate generation and tension due to carbachol (Ki = 286, 92 and 110 nM, respectively). AF-DX116 (11-[( 2-[(diethylamino)methyl]-1- piperidinyl]acetyl)-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepine- 6-one), selective for cardiac M2 receptors blocked cyclic AMP inhibition with high affinity (Ki = 1.14 nM) while it antagonized inositol phosphate formation with low affinity (Ki = 346 nM). Both high (Ki = 1 nM) and low (Ki = 100 nM) affinities were displayed by AF-DX116 in antagonizing contractions due to carbachol (24 and 76% inhibition, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Perinatal changes in the coupling of beta 1- and beta 3 adrenergic receptors to brown fat adenylyl cyclase.

The stimulation of adenylyl cyclase by catecholamines in neonatal brown adipose tissue (BAT) is markedly biphasic, suggesting the existence of receptors that have both high and low affinities for catecholamines. The identities of these receptors were examined by comparing responses in neonatal BAT membranes to those of Chinese hamster ovary cells which had been transfected to express the cloned rat beta 1 and beta 3 receptors. The results from these experiments indicate that high-affinity stimulation of adenylyl cyclase by catecholamines in BAT is mediated by beta 1 receptors, as evidenced by the potencies of norepinephrine and isoproterenol at this receptor and the potent blockade of the receptor by alprenolol. The low-affinity catecholamine receptor appears to be the beta 3 receptor, as indicated by the low potency of catecholamine agonists and the inability of low concentrations of alprenolol to block this activity. Furthermore, this receptor, like the cloned rat beta 3 receptor, was antagonized by (-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one (CGP 12177) and was stimulated by (R',R')-4-(2-[(2[(3-chlorophenyl)-2- hydroxyethyl]amino)propyl]phenyl)phenoxyacetic acid (BRL 37344). These results indicate that both beta 1 and beta 3 receptors couple to adenylyl cyclase in BAT and that activation of adenylyl cyclase in neonatal BAT is mediated primarily by beta 3 receptors. Beta 3 receptors were also clearly detected in weanling BAT with the beta 3-selective agonist BRL 37344. However, when catecholamines were used to stimulate activity, the activation of adenylyl cyclase by beta 1 receptors, which occurred at low concentrations of catecholamines, obscured the activation of adenylyl cyclase by beta 3 receptors, which occurred only at high concentrations.     

Selective regulation of beta-2 adrenergic receptors by the chronic administration of the lipophilic beta adrenergic receptor agonist clenbuterol: an autoradiographic study

Many antidepressant drugs, when administered chronically to rats, have been shown to produce decreases in the density of beta adrenergic receptors in the central nervous system. The centrally active beta adrenergic receptor agonist clenbuterol is currently being evaluated clinically as an antidepressant. The chronic administration of this drug to rats resulted in a large decrease in the density of beta adrenergic receptors in some areas of the rat brain but not in others. Thus, autoradiographic studies revealed that the total density of beta adrenergic receptors in the molecular layer of the cerebellum, but not in layers 1 to 3 or layer 4 of the cerebral cortex, was decreased. To examine whether this regional selectivity occurred because of differences in plasticity of cerebellum and cortex or because cerebellum contains mainly beta-2 adrenergic receptors and cortex contains mainly beta-1 adrenergic receptors, separate analyses of the subtypes of beta adrenergic receptors were performed in each area. These experiments indicated that the decrease in receptor density was entirely specific for beta-2 adrenergic receptors, whereas the density of beta-1 receptors was unchanged. Thus, even in layers 1 to 3 and layer 4 of the cerebral cortex, beta-2 receptor density was decreased, with no change in beta-1 receptor density. Using the autoradiographic assay for ligand binding, it was shown that clenbuterol has equal affinity for beta-1 and beta-2 adrenergic receptors, indicating that the selective effect of this drug was not due to a selective affinity for beta-2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Physiological approach of beta receptor coupling to adenylate cyclase in rat airways: ontogenical modification and functional antagonism

The relaxant effects of isoproterenol, forskolin and sodium nitroprusside were studied on tracheal pieces and lung parenchymal strips of Sprague-Dawley and Wistar rats according to age and functional antagonism with carbachol applied previously to induce the contraction. The beta receptor-related maximal relaxant effect of isoproterenol decreased from 4 to 11 weeks in Sprague-Dawley rat airways contracted with 10(-6) M carbachol. This maximal relaxant effect did not change with age in the Wistar strain. When lower carbachol concentrations were applied to Wistar trachea, the maximal relaxant effect of isoproterenol raised with a large decrease of the EC50 values. In the Sprague-Dawley strain, a similar diminution of carbachol concentration also allowed to increase the maximal amplitude of relaxation, but a smaller decrease of EC50 was observed as referred to the Wistar strain. These results suggest that the decrease with age of the maximal relaxation of Sprague-Dawley airways by isoproterenol might be linked to impaired functional antagonism between beta adrenergic and muscarinic stimulation in this rat strain. This hypothesis was strengthened by the observation of the effects of forskolin, an activator of adenylate cyclase, and sodium nitroprusside, a cyclic GMP-related relaxant drug, that did not show any modified effect in function of age in both rat strains. A modified regulation of adenylate cyclase complex with ontogenesis and with rat strain is suggested.     

Effects of agonist exposure on the coupling of beta 1 and beta 3 adrenergic receptors to adenylyl cyclase in isolated adipocytes.

The coupling of beta 1 and beta 3 adrenergic receptors to adenylyl cyclase was examined in membranes of isolated white adipocytes. The activation of adenylyl cyclase by isoproterenol (ISO) was biphasic. The high-affinity activation of adenylyl cyclase, which occurred at submicromolar concentrations of ISO, was mediated by beta 1 receptors, whereas low-affinity activation was mediated by beta 3 receptors. The relative activation of adenylyl cyclase by beta 3 receptors was less when ISO was used to stimulate activity, compared to when the beta 3-selective agonist BRL 37344 was used. These data indicate that both receptor subtypes can stimulate the same adenylyl cyclase in membranes of control cells, and that the activation of adenylyl cyclase by beta 1 receptors by low concentrations of catecholamines can obscure the activation by beta 3 receptors by high concentrations of catecholamines. Exposure of adipocytes to ISO at concentrations that either selectively stimulate beta 1 receptors or nonselectively stimulate both beta receptor subtypes greatly decreased the ability of beta 1, but not beta 3, receptors to activate adenylyl cyclase. In contrast, the exposure of cells to the beta 3-selective agonist BRL only slightly desensitized beta 1 receptors and did not affect beta 3 receptor activation of adenylyl cyclase. These data indicate that acute agonist exposure desensitizes beta 1, but not beta 3, receptors.     

Involvement of alpha-2 adrenergic receptor subtypes in hyperglycemia

Alpha-2 adrenoceptor stimulation induces in the mouse a hyperglycemic response which is accompanied by a concomitant inhibition of insulin secretion. To test the possibility that one of the postulated subtypes of alpha-2 adrenoceptors is preferentially implicated in this response, we compared the interaction of several drugs with known selectivity toward alpha-2A or alpha-2B adrenoceptor subtypes in our model. The alpha-2A preferential agonist oxymetazoline induced in the mouse a hyperglycemic response similar to that of the nonselective alpha-2 adrenoceptor agonist UK 14.304. This hyperglycemic response to oxymetazoline was accompanied by a concomitant inhibition of insulin release. Both the effect on glycemic level and the inhibition of insulin release by oxymetazoline were antagonized by the alpha-2 adrenoceptor antagonist idazoxan. The alpha-2B preferential antagonists ARC-239, prazosin or chlorpromazine failed to block the modifications in both glycemic and insulin levels induced by alpha-2 adrenoceptor stimulation. The nonselective antagonists rauwolscine, yohimbine, WY 26703, phentolamine and corynanthine, as well as the receptor antagonists with alpha-2A selectivity like WB 4101, idazoxan and tolazoline, dose-dependently antagonized both the glycemic and the insulin responses to UK 14.304. A positive correlation was obtained between the potencies of these drugs in antagonizing the hyperglycemic response to UK 14.304 and their affinities for alpha-2A adrenergic receptors (r = 0.918, P less than .001) but no correlation was obtained with their affinities for alpha-2B adrenergic receptors (r = 0.048, P = N.S.)(ABSTRACT TRUNCATED AT 250 WORDS)     

Defective dopamine-1 receptor adenylate cyclase coupling in the proximal convoluted tubule from the spontaneously hypertensive rat.

The natriuretic effect of DA-1 agonists is less in the spontaneously hypertensive rat (SHR) than its normotensive control, the Wistar-Kyoto rat (WKY). To determine a mechanism of the decreased effect of DA-1 agonists on sodium transport, DA-1 receptors in renal proximal convoluted tubule (PCT) were studied by radioligand binding and by adenylate cyclase (AC) determinations. Specific binding of 125I-SCH 23982 (defined by 10 microM SCH 23390, a DA-1 antagonist) was concentration dependent, saturable, and stereoselective. The dissociation constant, maximum receptor density, and DA-1 antagonist inhibition constant were similar in SHR and WKY. The apparent molecular weight of the DA-1 receptor determined by the photoaffinity D1 probe 125I-MAB was also similar in WKY and SHR. However, DA-1 agonists competed more effectively for specific 125I-SCH 23982 binding sites in WKY than in SHR. Basal as well as forskolin, parathyroid hormone, GTP and Gpp(NH)p-stimulated-AC activities were similar. In contrast DA-1 agonists (fenoldopam, SKF 38393, SND 911C12) stimulated AC activity to a lesser extent in SHR. GTP and Gpp(NH)p enhanced the ability of DA-1 agonists to stimulate AC activity in WKY but not in SHR. These data suggest a defect in the DA-1 receptor-second messenger coupling mechanism in the PCT of the SHR.     

Thermodynamic properties of agonist interactions with the beta adrenergic receptor-coupled adenylate cyclase system. II. Agonist binding to soluble beta adrenergic receptors

The thermodynamic parameters associated with the interactions of agonists and antagonists with digitonin-solubilized beta adrenergic receptors were determined. A rapid method for measuring the binding of [125I]iodopindolol to soluble receptors using glass-fiber filters was developed. The binding of [125I]iodopindolol, an antagonist with intrinsic sympathomimetic activity, to soluble receptors was temperature-sensitive as is the binding of the ligand to membrane-bound receptors. The interactions of propranolol and timolol with soluble receptors were independent of temperature. In contrast, the binding of agonists to soluble receptors was sensitive to temperature, although insensitive to GTP. Thermodynamically, the interactions of the antagonists timolol and propranolol with soluble beta adrenergic receptors were entropy-driven, with little contribution from changes in enthalpy. This is consistent with a hydrophobic interaction between the receptor and the antagonist. The binding of [125I]iodopindolol was enthalpy-driven. The binding of full agonists with soluble receptors was described thermodynamically by changes in enthalpy and entropy that were negative relative to the values for propranolol and timolol, suggesting that the guanine nucleotide-binding protein required for stimulation of adenylate cyclase activity and an intact lipid environment are not involved in the thermodynamics of formation of the low-affinity component of agonist binding. These results are consistent with an agonist-induced change in the conformation of the receptor.     

Pineal beta adrenergic receptor: correlation of binding of 3H-l-alprenolol with stimulation of adenylate cyclase.

3H-l-Alprenolol, a potent competitive beta adrenergic antagonist, binds to sites in rat pineal gland membranes. The properties of these binding sites were compared to those of the receptors which mediate the beta adrenergic activation of pineal adenylate cyclase. Both sites are highly stereospecific. The l-stereoisomers of alprenolol and propranolol were at least two orders of magnitude more potent than the d-stereoisomers in inhibiting isoproterenol-stimulated adenylate cyclase or 3H-l-alprenolol binding. The dissociation constants (Kd) of the l-stereoisomers of both alprenolol and propranolol were 10 to 22 nM as determined by competition for binding sites or by inhibition of isoproternol-stimulated adenylate cyclase. Beta adrenergic agonists which stimulated adenylate cyclase also competitively inhibited the binding of 3H-l-alprenolol. They showed the same order of potency (isoproterenol greater than norepinephrine greater than or equal to epinephrine) and the same individual affinities in the two systems. Alpha adrenergic blockers were ineffective in inhibiting either adenylate cyclase stimulation or 3H-l-alprenolol binding. Isoproternol stimulation of adenylate cyclase acrivity, and 3H-l-alprenolol binding, were rapid and rapidly reversible. The 3H-l-alprenolol binding sites were saturable and bound 0.6 pmol of ligand per mg of added protein. The data suggest that the binding of 3H-l-alprenolol occurs at sites indistinguishable from the pineal beta adrenergic receptor.     

Desensitization of adenylate cyclase and down regulation of beta adrenergic receptors after in vivo administration of beta agonist

In vitro incubation of cells with catecholamines leads to both down regulation of beta adrenergic receptor number and desensitization of agonist-stimulated adenylate cyclase activity. These same parameters, down regulation of beta adrenergic receptor number and desensitization of adenylate cyclase activity were assessed in rat lung membranes after in vivo administration of metaproterenol, a beta-2 selective agonist. In vivo treatment with metaproterenol leads to: 1) reduced beta adrenergic receptor number; 2) reduced isoproterenol-stimulated adenylate cyclase activity; 3) unaffected NaF or 5'-guanylylimidodiphosphate-stimulated adenylate cyclase activity; and 4) reduced affinity of the receptor for isoproterenol similar to the affinity observed in the presence of 5'-guanylylimidodiphosphate. The date suggest that in vivo metaproterenol administration results in an uncoupled receptor-adenylate cyclase complex. The effects of in vivo administration of the glucocorticoid, methylprednisolone, to metaproterenol-pretreated animals were also assessed. Glucocorticoid treatment was associated with 1) increased beta adrenergic receptor number in rats in which the receptors have been down regulated, 2) increased isoproterenol responsiveness in agonist-desensitized rats and 3) no effect on agonist affinity in desensitized animals. These data suggest that the restoration of agonist responsiveness by glucocorticoids in the catecholamine refractive state is not simply a reversal of receptor down regulation or adenylate cyclase desensitization.     

Stereospecificity of beta adrenergic antagonists: R-enantiomers show increased selectivity for beta-2 receptors in ciliary process

The (+)-stereoisomers of arylethanolamine beta adrenergic agonists and antagonists are usually much less active in biological systems than their corresponding (-)-forms. In the eye, however, prior physiological studies have shown that these (+)-stereoisomers are unexpectedly potent in altering intraocular pressure, results which could be due to a difference in distribution and metabolism or to a difference in receptor interaction. The present experiments evaluated six stereoisomeric pairs of beta adrenergic antagonists for their ability to block rabbit ciliary process and cardiac beta adrenergic receptors activating adenylate cyclase, in vitro, under conditions in which the effects of drug metabolism, distribution and membrane lipid solubility were minimized. In the heart, all six pairs of antagonists demonstrated the expected increased potency of (-)-forms, with isomeric activity ratios of: 33 for metoprolol, 44 for timolol; 48 for bunitrolol; 76 for t-butyl-betaxolol; 100 for t-butyl-didesmethyl-ICI-118,551; and 530 for betaxolol. Under identical assay conditions in the ciliary process, (+)-enantiomers were much more potent relative to (-)-forms, with isomeric activity ratios of: 0.82 for timolol; 3.3 for bunitrolol; 7.4 for t-butyl-didesmethyl-ICI-118,551; 10 for metoprolol; 16 for t-butyl-betaxolol; and 190 for betaxolol. With the exception of metoprolol, all (+)-enantiomers demonstrated a substantially higher absolute affinity for ciliary process receptors (known to be almost exclusively of the beta-2 subtype) than for cardiac receptors.(ABSTRACT TRUNCATED AT 250 WORDS)