Effect of phencyclidine and two monohydroxy metabolites on 3H QNB binding in vivo in rats

The purpose of these investigations was to determine if phencyclidine (PCP) and/or two of its major monohydroxy metabolites [1-(1-phenylcyclohexyl)4-hydroxy piperidine (4-OH-pip PCP) and 1-(1-phenyl-4-hydroxycyclohexyl) piperidine (4-OH-cyclo PCP)] influence 3H quinuclidinyl benzilate (QNB) binding in rat brain after in vivo administration. PCP and 4-OH-pip PCP but not 4-OH-cyclo PCP enhanced QNB binding. The effect was blocked by atropine. Since the dose of 4-OH-pip PCP necessary to alter QNB binding in rat brain is substantially higher than that required for PCP, this metabolite probably does not play a major role in the PCP effect on QNB binding in rat brain.

Influence of veratridine on [3H]-L-quinuclidinyl benzilate ([3H]QNB) binding in mouse hindbrain

The neurotoxin veratridine is well known for its ability to open sodium channels in neuronal and muscle tissues in micromolar concentrations. It has also been shown that veratridine is an inhibitor of the potent muscarinic receptor antagonist L-quinuclidinyl benzilate (QNB) at these concentrations. These findings prompted us to examine the relationships between action potential sodium channels and muscarinic receptors in a glass-fiber filtration assay for [3H]QNB binding to mouse hindbrain membranes using agents known to affect interconversion of the affinity states in some muscarinic receptor populations, i.e. guanosine triphosphate (GTP) and magnesium (Mg2+). The actions of the sodium channel antagonist tetrodotoxin (TTX) were also examined. Veratridine inhibited [3H]QNB binding with a Ki value of approximately 2.5 microM. This inhibition exhibited a competitive mechanism at higher concentrations (5-10 microM), while showing an apparent non-competitive action at low concentrations (1 microM). Magnesium caused a parallel shift to the right in the inhibition curve with a 32% increase in the veratridine Ki. GTP caused a non-parallel shift to the left with the greatest displacement occurring at lower veratridine concentrations (2-5 microM). The addition of magnesium to GTP did not alter the action of GTP significantly. TTX (5 microM) caused a parallel shift of the veratridine inhibition curve to the right. In addition, TTX alone inhibited the binding of [3H]QNB. Therefore, it appears that there may be more than one binding site for veratridine which may be linked to the muscarinic system and that these may be action potential sodium channels.

Stereospecific 3H-QNB binding to human erythrocyte membranes associated with muscarinic cholinergic receptors

Specific binding of the muscarinic antagonist 3H-QNB can be demonstrated on human erythrocyte membranes. Specific 3H-QNB binding is stereospecific and can be inhibited by a variety of cholinergic agonists and antagonist with affinities similar to their affinities for muscarinic cholinergic receptor binding in other tissues. Accordingly, the data indicate the presence of muscarinic cholinergic receptors on human erythrocyte membranes. Their density is very low, varies between healthy persons, but seems to be an individual characteristic. The data reported are consistent with the assumption that the muscarinic cholinergic receptors on human erythrocyte membranes are mainly of the M1 receptor subtype.

Effects of acetylethylcholine mustard on [3H]quinuclidinyl benzilate binding and acetylcholine release in rat brain synaptosomes

The effects of acetylethylcholine mustard and its aziridinium derivative (AMMA) on acetylcholine (ACh) release and [3H]quinuclidinyl benzilate (QNB) binding were studied in rat cortical synaptosomes. After incubation for 5 min at 37 degrees C, AMMA reduced [3H]QNB binding with an IC50 of 9 microM. Following incubation for 5 min with 50 microM AMMA and washing, there was a 62% reduction in the [3H]QNB binding capacity with no change in the KD value for the remaining receptors, a result indicating the irreversibility of the AMMA binding. AMMA and oxotremorine both reduced the basal and 30 mM K+-induced release of newly synthesized [3H]ACh in dose-dependent manners over a 2.5-min period. At identical 50 microM concentrations, AMMA produced a much longer inhibition of basal [3H]ACh release than oxotremorine did. The inhibition of basal and 30 mM K+-induced [3H]ACh release by AMMA (10-250 microM) was blocked by 2 microM atropine during a 2.5-min release incubation, but not during a 30-min release incubation. After synaptosomes were treated with 50 microM AMMA for 5 min and the unbound drug was washed out from the tissue, [3H]ACh release (basal and K+-induced) was reduced. AMMA (50 microM) reduced high-affinity choline uptake and ACh synthesis by greater than 90% in this tissue, but these effects did not account for the [3H]ACh release inhibition, because they were not atropine sensitive and hemicholinium-3 had no effect on [3H]ACh release under the conditions used in these studies, i.e., after extracellular [3H]choline was washed out. Taken together, these results suggest that AMMA may be an irreversible agonist at presynaptic muscarinic autoreceptors.

[Solubilization by various detergents, of the muscarinic cholinoreceptor and its complex with quinuclidinyl benzilate]

The possibilities to solubilize the rat brain cortex muscarinic acetylcholine receptor and its complex with [3H]-L-quinuclidinyl benzilate (QNB) were studied, using 14 detergents. It was shown that the native muscarinic cholinoreceptor was solubilized in addition to digitonin, also by CHAPS, with a 6% yield. Besides, the receptor-QNB complex was solubilized with the detergents Triton X-100, -102, -114, -165 (with 30% and 50% yields) and within a narrow concentration range with sodium dodecyl sulfate (50% yield). Some detergents of the Tween series, e.g., Triton X-45 and -305, as well as sodium deoxycholate and sodium oxycholate, did not solubilize the native receptor and its complex with QNB. It was found that yield of receptor solubilization did not exceed half of the total number of the receptor sites in the membranes, despite the fact that different concentrations of detergents were applied. The solubilization yield did not increase, when different mixtures of detergents were used. It was assumed that incomplete solubilization of the receptor protein reflects its heterogeneity in the membrane structure.

Characterization of high affinity [3H]pirenzepine and (-)-[3H] quinuclidinyl benzilate binding to muscarinic cholinergic receptors in rabbit peripheral lung

We have characterized the binding of the selective muscarinic antagonist [3H]pirenzepine ([3H])PZ) and the classical muscarinic antagonist (-)-[3H]quinuclidinyl benzilate ((-)-[3H]QNB) to muscarinic cholinergic sites in rabbit peripheral lung membranes. For both radioligands, high affinity binding with pharmacologic specificity was demonstrated. The high affinity Kd for [3H]PZ binding determined from saturation isotherms was 4.5 nM and the Kd for (-)-[3H]QNB binding was 6.2 pM. Comparison of the total binding capacity values determined by saturation experiments with [3H] PZ and (-)-[3H]QNB demonstrates that approximately 78% of the total muscarinic binding sites in rabbit peripheral lung bind [3H]PZ with high affinity. There was no significant effect of the guanine nucleotide, guanyl-5'-yl imidodiphosphate, on the inhibition of (-)-[3H]QNB binding by the muscarinic agonist carbachol in peripheral lung membranes. If the pulmonary muscarinic receptor with high affinity for PZ proves to have an important role in bronchoconstriction, its characterization could result in the development of more selective bronchodilators.

Use of 3H-QNB in the isolation of plasma membrane from smooth muscle of the urinary bladder: effect of oxalate on calcium uptake by the membrane fractions

Specific binding of tritiated quinuclidinyl benzilate (3H-QNB) to surface membrane muscarinic receptors was utilized to identify plasma membrane (PM) fractions from smooth muscle of the rabbit urinary bladder. Accumulation of 3H-QNB in the PM fraction was 4-5-fold higher than that in fractions of endoplasmic reticulum (EM) or mitochondria (M). A similar pattern of distribution was found for 5'-nucleotidase. 3H-QNB binding therefore appears to be a suitable marker for plasma membrane of the urinary bladder. Data on ATP-dependent calcium uptake by PM and ER fractions showed that oxalate highly potentiated calcium uptake by both fractions and consequently this feature cannot be used to identify ER fractions specifically.

Effects of deoxyribonuclease I and neuraminidase treatments on the specific binding of 3H-prazosin and 3H-quinuclidinyl benzilate (3H-QNB) to alpha-adrenergic and muscarinic receptors in rat myocardial membranes

The effects of neuraminidase and deoxyribonuclease I (DNase) treatments on the specific binding of 3H-prazosin and 3H-quinuclidinyl benzilate (3H-QNB) to alpha-adrenoceptors and muscarinic cholinergic receptors in the membrane of the rat myocardium was examined, and the dissociation constant (Kd) and the maximum number of binding sites (Bmax) was analyzed using the method of Scatchard analysis. Although no changes in Kd values were observed when DNA or sialic acid was removed from cardiac muscles by treatments with DNase or neuraminidase, the Bmax values of alpha-adrenergic and muscarinic receptors were markedly decreased after treatment with DNase, while neuraminidase treatment induced an increase in the Bmax values of the alpha-adrenoceptors. The possibility that these results provide the basis for elucidation of the characteristics of these receptors in the rat myocardium was discussed.

[3H]pirenzepine and (-)-[3H]quinuclidinyl benzilate binding to rat cerebral cortical and cardiac muscarinic cholinergic sites. II. Characterization and regulation of antagonist binding to putative muscarinic subtypes

Studies show [3H]PZ identified selectively a subpopulation of muscarinic binding sites compared to classical antagonists like (-)-[3H]QNB in many central and peripheral tissues. We characterized the binding and regulation of selected antagonists to high-affinity [3H]PZ (putative M1) and low-affinity PZ (putative M2) sites in rat cerebral cortex (predominantly M1) and heart (predominantly M2). Saturation isotherms of [3H]PZ and (-)-[3H]QNB were performed under various conditions. Guanyl-5'-yl-imidodiphosphate (30 microM) showed little effect on Kd (dissociation constant) or total binding capacity (total receptor density) values. Higher ionic strength buffers yielded lower affinity values for [3H]PZ and (-)-[3H]QNB. Kinetic studies confirmed high affinity Kd values seen in steady-state assays. We conducted inhibition studies of selected muscarinic antagonists including the reportedly cardioselective (putative M2) drug, AF-DX 116 (11-[(2-(diethylamino)methyl-1-piperidinyl)-acetyl]-5, 11-dihydro-6H-pyrido(2,3-b)(1,4)-benzodiazepine-6-one], the reportedly M1 selective compound, PZ, and the classical antagonist (-)QNB, using [3H]PZ and (-)-[3H]QNB-labeled cerebral cortical and cardiac homogenates. Assays were done with and without guanyl-5'-yl-imidophosphate at 25 degrees C in 10 mM Na-K-phosphate, 50 mM Na-K-phosphate and modified Krebs-phosphate buffer. Studies showed antagonists generally had higher affinity in 10 mM Na-K-phosphate buffer, were insensitive to guanyl-5'-yl imidodiphosphate and had Hill values (nH) nearly equal to one. Cardiac PZ/[3H]QNB curves were steep.(ABSTRACT TRUNCATED AT 250 WORDS)

[3H]pirenzepine and (-)-[3H]quinuclidinyl benzilate binding to rat cerebral cortical and cardiac muscarinic cholinergic sites. I. Characterization and regulation of agonist binding to putative muscarinic subtypes

The binding and regulation of selected muscarinic agonists to putative subtypes in rat cerebral cortex and heart were studied. Parallel inhibition studies of [3H]pirenzepine ([3H]PZ) and (-)-[3H]quinuclidinylbenzilate [(-)-[3H]QNB]-labeled membranes were done with and without 30 microM guanyl-5'-yl imidodiphosphate [Gpp(NH)p] at 25 degrees C in 10 mM Na-K-phosphate buffer which enhances PZ binding affinity and in modified Krebs-phosphate buffer, which mimics physiological conditions. Classical agonists such as carbachol, oxotremorine and acetylcholine inhibited (-)-[3H]QNB binding to membranes with shallow Hill values (nH less than 1), were better fit to a 2-state model, were Gpp(NH)p-regulated and showed lower affinity in modified Krebs-phosphate buffer than in 10 mM Na-K-phosphate buffer. Some agonists were not significantly better fit to a 2-state model in [3H]PZ-labeled cortical membranes, especially in 10 mM Na-K-phosphate buffer. Whereas putative M1 and M2 binding sites distinguished by PZ possessed multiple agonist affinity states, as judged by carbachol, and agonist binding to [3H]PZ-labeled sites were Gpp(NH)p modulated, the partial agonist pilocarpine and nonclassical agonist McN-A-343 [3-(m-chlorophenylcarbamoyloxy)-2-butynyl trimethylammonium chloride] showed little Gpp(NH)p-induced shift in [3H]PZ-labeled cortical membranes in physiological conditions. Agonist binding to (-)-[3H]QNB-labeled putative M2 cardiac sites was more sensitive to Gpp(NH)p than (-)-[3H]QNB-labeled cortical sites. Carbachol and acetylcholine showed significant selectivity for putative M2 sites.(ABSTRACT TRUNCATED AT 250 WORDS)

A muscarinic receptor type in human lymphocytes: a comparison of 3H-QNB binding to intact lymphocytes and lysed lymphocyte membranes

Human blood lymphocytes from normal blood donors exhibited specific binding of the muscarinic antagonist 3H-quinuclidinyl benzilate (3H-QNB). The 3H-QNB binding to intact viable lymphocytes as well as to lysed lymphocyte membranes "P2" was saturable and displaceable by both muscarinic agonists and antagonists. For the lysed lymphocyte membranes "P2" a single binding site with a Bmax of 109 pmol/g protein and a Kd of 15 nM was obtained. Intact viable lymphocytes also showed one binding site with a Kd of 24 nM and a Bmax of 1556 pmol/g protein. The higher Bmax value might be explained in terms of uptake of the ligand when using intact cells or through loss of binding sites when using lysed lymphocyte membranes "P2". IC50 values were lower by a factor of 10(2) for atropine and scopolamine and by 10(4) for pirenzepine when lysed lymphocyte membranes "P2" were used instead of intact viable lymphocytes.

Biochemical characteristics of muscarinic cholinoreceptors in swine tracheal smooth muscle

The tritiated muscarinic cholinoreceptor antagonist quinuclidinyl benzilate, [3H]QNB, was used to characterize the muscarinic receptors associated with homogenized membrane of the smooth muscle from swine trachea. Based on receptor binding assays, the homogenate had specific, saturable, high-affinity receptors for [3H]QNB. Specific binding was time- and temperature-dependent. The association of [3H]QNB with the muscarinic receptor reached equilibrium much sooner at 37 degrees C than 25 degrees C at a [3H]QNB concentration of 180 pM (30 min and 2 h, respectively). Equilibrium at both temperatures was attained within 5 min at a [3H]QNB concentration of 1800 pM. All remaining experiments were performed at 37 degrees C. Binding was saturable with respect to [3H]QNB and tissue concentrations. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 51 +/- 20 pM and a maximum receptor density (Bmax) of 2.17 +/- 0.27 pmole/mg protein. The Hill coefficient for [3H]QNB binding was 1.07 +/- 0.16. The association (K1) and dissociation (K-1) rate constants were determined to be (5.51 +/- 0.16) X 10(8) M-1 min-1 and (1.41 +/- 0.18) X 10(-2) min-1, respectively. KD calculated from the ratio of K1 and K-1 was 26.3 +/- 3.8 pM; this value is close to the value of KD calculated from Scatchard plots of binding isotherms. The density of muscarinic receptor binding sites was 10-fold greater in tracheal smooth muscle than in tracheal epithelium (0.20 +/- 0.03 pmole/mg protein). There is no difference between weanling and young adult swine in the density of muscarinic receptors in tracheal smooth muscle. The nonselective muscarinic antagonists atropine, scopolamine and quinuclidinyl benzilate (QNB) competitively inhibited [3H]QNB binding to the homogenate with Hill coefficients of 0.9-1.0 and inhibition constants (Ki) of nanomolar range. Competition with selective muscarinic antagonists pirenzepine and 3-quinuclidinyl xanthene-9-carboxylate (QNX) gave Ki values, 0.26 M and 0.78 nM, respectively, and Hill coefficients of approximately 1. There was a single population of [3H]QNB binding sites of the M2 subtype for all tested muscarinic antagonists. Competition with selective muscarinic agonists pilocarpine and carbachol yielded Ki values of micromolar range, Hill coefficients of less than 1, and revealed the existence of two binding sites (P less than 0.01).

Effects of Pro-Leu-Gly-NH2 and cyclo (Leu-Gly) on the binding of 3H-quinuclidinyl benzilate to striatal cholinergic muscarinic receptors

The effects of Pro-Leu-Gly-NH2 (melanotropin release inhibiting factor, MIF) and its analog, cyclo (Leu-Gly) on the mouse and rat striatal cholinergic muscarinic receptors labeled with 3H-quinuclidinyl benzilate (QNB) were investigated. 3H-QNB bound to the rat striatal muscarinic receptors at a single high affinity site with receptor density (Bmax value) of 1200 fmol per mg protein and an apparent dissociation constant (Kd value) of 53.5 pM. At 140 pM concentration of 3H-QNB, the specific binding to the receptors was 724 fmol per mg protein. MIF in a concentration range of 10(-9) to 10(-4) M did not alter the binding of 3H-QNB but at 10(-3) M decreased the binding by 25%. Cyclo (Leu-Gly), on the other hand, in the concentration range of 10(-9) to 10(-3) M had no effect on the binding of 3H-QNB. A single injection of MIF (3 or 10 mg/kg IP) to rats did not alter the Bmax or the Kd value of 3H-QNB to bind to the striatal membranes. 3H-QNB bound to the mouse striatal muscarinic receptors at a single high affinity site with a Bmax value of 991 fmol/per mg protein and a Kd value of 21 pM. Neither acute administration of MIF (3 or 10 mg/kg IP) nor chronic treatment of the peptide (2, 8 or 32 mg/kg IP, daily for 5 days) to mice could influence the binding of 3H-QNB to the striatal muscarinic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereoselective L-[3H]quinuclidinyl benzilate-binding sites in nervous tissue of Aplysia californica: evidence for muscarinic receptors

The muscarinic antagonist L-[3H]quinuclidinyl benzilate (L-[3H]QNB) binds with a high affinity (Kd = 0.77 nM) to a single population of specific sites (Bmax = 47 fmol/mg of protein) in nervous tissue of the gastropod mollusc, Aplysia. The specific L-[3H]QNB binding is displaced stereoselectively by the enantiomers of benzetimide, dexetimide, and levetimide. The pharmacologically active enantiomer, dexetimide, is more potent than levetimide as an inhibitor of L-[3H]QNB binding. Moreover, the muscarinic cholinergic ligands, scopolamine, atropine, oxotremorine, and pilocarpine are effective inhibitors of the specific L-[3H]QNB binding, whereas nicotinic receptor antagonists, decamethonium and d-tubocurarine, are considerably less effective. These pharmacological characteristics of the L-[3H]QNB-binding site provide evidence for classical muscarinic receptors in Aplysia nervous tissue. The physiological relevance of the dexetimide-displaceable L-[3H]QNB-binding site was supported by the demonstration of the sensitivity of the specific binding to thermal denaturation. Specific binding of L-[3H]QNB was also detected in nervous tissue of another marine gastropod, Pleurobranchaea californica. The characteristics of the Aplysia L-[3H]QNB-binding site are in accordance with studies of numerous vertebrate and invertebrate tissues indicating that the muscarinic cholinergic receptor site has been highly conserved through evolution.

Cholinergic muscarinic receptors in human fetal brain: ontogeny of [3H]quinuclidinyl benzilate binding sites in corpus striatum, brainstem, and cerebellum

The ontogeny of muscarinic receptors was studied in human fetal striatum, brainstem, and cerebellum to investigate general principles of synaptogenesis as well as the physiological balance between various chemical synapses during development in a given region of the brain. [3H]Quinuclidinyl benzilate ([3H]QNB) binding was assayed in total particulate fraction (TPF) from various parts of brain. In the corpus striatum, QNB binding sites are present at 16 weeks of gestation (average concentration 180 fmol/mg protein of TPF), slowly increase up to 24 weeks (average concentration 217 fmol/mg protein), and rapidly increase during the third trimester to 480 fmol/mg protein of TPF. In contrast, dopaminergic receptors exist as two subpopulations, one with low affinity and the other with high affinity up to the 24th week of gestation; all of them acquire the high-affinity characteristic during the third trimester. In brainstem, the muscarinic receptors show maximum concentration by 16 weeks of age (360 fmol/mg protein of TPF). Subsequently the muscarinic receptor concentration shows a gradual decline in the brainstem. In cerebellum, except for a slight increase at 24 weeks (average concentration 90 fmol/mg protein of TPF), the receptor concentration remained nearly constant at about 60-70 fmol/mg protein of TPF throughout fetal life. This study demonstrates that the ontogeny of muscarinic receptors varies among the different regions, and the patterns observed suggest that receptor formation occurs principally in the third trimester. Also noteworthy is the finding that the QNB binding sites decreased in all regions of the human brain during adult life.

The binding of 3H-acetylcholine to cholinergic receptors in bovine cerebral arteries

Cholinergic receptor sites in bovine cerebral arteries were analyzed using radioligand binding techniques with the cholinergic agonist, 3H-acetylcholine (ACh), as the ligand. Specific binding of 3H-ACh to membrane preparations of bovine cerebral arteries was saturable, of two binding sites, with dissociation constant (KD) values of 0.32 and 23.7 nM, and maximum binding capacity (Bmax) values of 67 and 252 fmol/mg protein, respectively. Specific binding of 3H-ACh was displaced effectively by muscarinic cholinergic agents and less effectively by nicotinic cholinergic agents. IC50 values of cholinergic drugs for 3H-ACh binding were as follows: atropine, 38.5 nM; ACh, 59.8 nM; oxotremorine, 293 nM; scopolamine 474 nM; carbamylcholine, 990 nM. IC50 values of nicotinic cholinergic agents such as nicotine, cytisine and alpha-bungarotoxin exceeded 50 microM. Choline acetyltransferase activity was 1.09 nmol/mg protein/hour in the cerebral arteries. These findings suggest that the cholinergic nerves innervate the bovine cerebral arteries and that there are at least two classes of ACh binding sites of different affinities on muscarinic receptors in these arteries.

Degradation of rat brain cholinergic muscarinic receptors in vitro: enhancement by agonists and inhibition by antagonists

Cholinergic muscarinic receptors undergo proteolytic degradation in vitro under physiological conditions as shown by a loss in [3H]quinuclidinylbenzilate binding activity. The serine protease inhibitor phenylmethylsulfonyl fluoride was very effective in diminishing the receptor loss. Soybean trypsin inhibitor was less effective. Both EDTA and EGTA were also effective in abolishing receptor degradation, suggesting the involvement of metallopeptidases in the process. Calcium-dependent neutral proteases requiring sulfhydryl reducing agents did not seem to be involved in receptor degradation. Dithiothreitol failed to enhance receptor degradation and iodoacetamide, leupeptin, and antipain, inhibitors of this enzyme class, failed to alter receptor loss as measured by radioligand binding. Most of the proteolytic activity occurred in the cytosol and was readily resolved from the receptor in the membrane fraction. We found that [3H]quinuclidinylbenzilate, an antagonist, inhibited the rate of receptor loss. On the other hand, agonists (acetylcholine, methacholine, and muscarine) appeared to enhance the rate of receptor loss. We postulate that these opposite effects are due to differences in receptor conformation in response to ligand binding. Susceptibility to proteolysis may therefore serve as a probe for receptor conformation.

Relations between muscimol, quinuclidinyl benzilate and nicotine binding sites in brain after very long treatment with ethanol in rats

Rats were treated with ethanol in the drinking fluid 2 X 1 h daily for 83 weeks. [3H]Muscimol, [3H]quinuclidinyl benzilate ( [3H]QNB) and [3H]nicotine binding was measured in selected brain areas 7, 14 and 21 days after withdrawal of ethanol. A significant increase (P less than 0.05) when compared to controls was found on day 7 in both [3H]-QNB binding in the cortex and high affinity [3H]muscimol binding in the cerebellum. Furthermore, on day 7 of abstinence there was a positive correlation in number of binding sites between [3H]muscimol (high affinity sites; cerebellum) and [3H]QNB (cortex) while a negative correlation was found between [3H]muscimol (low affinity sites; cerebellum) and [3H]nicotine (cortex). The first correlation might indicate a relation between increased excitation (QNB) and increased inhibition (muscimol) in two brain areas.

Ascorbate induced lipid peroxidation results in loss of receptor binding in tris, but not in phosphate, buffer. Implications for the involvement of metal ions

Rat brain homogenate was incubated with various concentrations of ascorbate in a Tris-HC1 buffer. Thiobarbituric acid-reactive substances (TBARS) were measured and 3H-QNB (quinuclidinyl benzilate) binding was also assayed on the same homogenate. Good parallelism between TBARS formation and loss of 3H-QNB binding activity confirmed that loss of 3H-QNB binding resulted from ascorbate-induced lipid peroxidation. However, neither formation of TBARS nor loss of 3H-QNB binding occurred in phosphate buffer or in the Tris-HC1 system in the presence of metal-chelating reagents. This indicates that phosphate addition prevents the ascorbate effects due to complete chelation of intrinsic metal ions.

Comparison of the affinity constant of some muscarinic receptor antagonists with their displacement of [3H]quinuclidinyl benzilate binding in atrial and ileal longitudinal muscle of the guinea-pig

The ability of the muscarinic receptor antagonists fenipramide, 4-diphenylacetoxy-N-methyl piperidine methiodide (4-DAMP) and secoverine to displace [3H]QNB binding was correlated with the inhibition of responses of cholinomimetics at muscarinic receptors in the atria and ileal longitudinal muscle of the guinea-pig. Fenipramide and 4-DAMP exhibited a 2-4 fold higher affinity for muscarinic receptors in ileal longitudinal muscle in both types of experiments. Secoverine exhibited no difference in affinity in the two tissues.

Use of 3-quinuclidinyl 4-iodobenzilate as a receptor binding radiotracer

3-Quinuclidinyl 4-iodobenzilate was shown to bind to the muscarinic acetylcholine receptor (mAChR) by testing the saturability and the stereoselectivity in the corpus striatum, cerebellum, and the heart. But the ratio of radioactivity in tissues containing different concentrations of mAChR was less than the ratio of mAChR concentrations determined by in vitro saturation assay. As a result, the sensitivity to change in receptor concentration by external imaging will be reduced for this receptor binding radiotracer.

Failure of gallamine and pancuronium to inhibit selectively (-)-[3H]quinuclidinyl benzilate binding in guinea-pig atria

Binding of (-)-[3H]quinuclidinyl benzilate (QNB) to muscarinic sites in guinea-pig atrial and ileal longitudinal muscle homogenates showed the presence of a single population of binding sites in atria (KD = 41 (32-53) (95% confidence limits) pM; Bmax = 0.225 +/- 0.02 pmol/mg protein (3)) and two binding sites in the ileum (KD = 20.9 (8.8-49) pM and 11.3 nM; Bmax = 0.436 +/- 0.09 and 11.85 +/- 2.63 pmol/mg protein (4), respectively). Atropine, gallamine, and pancuronium displaced (-)-[3H]QNB binding from the high affinity binding sites in the two tissues in a dose-dependent manner with -log Ki values of 8.6, 6.4, and 6.9, respectively, in atria and 8.7, 6.8, and 6.9, respectively, in ileal longitudinal muscle. The lack of selectivity of gallamine and pancuronium in binding experiments differed from results obtained in isolated tissue experiments where these antagonists showed a marked difference in their ability to antagonize cholinomimetics in the two tissues. In addition, the Ki values for gallamine and pancuronium in ileal homogenates were ca. 130- and 16-fold lower, respectively, than their KB values determined from isolated tissue experiments. Attempts to correlate data from binding experiments and isolated tissue experiments using combinations of antagonists led to variable results attributed to differences in the rates of dissociation of the antagonists from muscarinic receptors. It is concluded that the interaction of gallamine or pancuronium with agonists or antagonists at muscarinic receptors is not a simple bimolecular interaction.

Binding of N-[propionyl-3H]propionylated alpha-bungarotoxin and L-[benzilic-4,4'-3H] quinuclidinyl benzilate to CNS extracts of the cockroach Periplaneta americana

The nerve cord of the cockroach (Periplaneta americana) contains distinct saturable components of specific binding for the ligands N-[propionyl-3H]propionylated alpha-bungarotoxin and L-[benzilic-4,4'-3H]quinuclidinyl benzilate. N-[Propionyl-3H]propionylated alpha-bungarotoxin bound reversibly to homogenates with a Kd of 4.8 nM and Bmax of 910 fmol mg-1. The association rate constant (1.9 X 10(5) M-1 s-1) and dissociation rate constant (1.2 X 10(-4) s-1) yielded a Kd of 0.6 nM. Nicotinic ligands were found to displace toxin binding most effectively. The binding sites characterized in this way showed many similarities with the properties of the vertebrate neuronal alpha-bungarotoxin binding site. For a range of cholinergic ligands, inhibition constants calculated from toxin binding studies closely corresponded to their effectiveness in blocking the depolarizing response to acetylcholine recorded by electrophysiological methods from an identified cockroach motoneurone. The N-[propionyl-3H]propionylated alpha-bungarotoxin binding component therefore appears to be a constituent of a functional CNS acetylcholine receptor. Binding of L-[benzilic-4,4'-3H]quinuclidinyl benzilate was reversible with a Kd of 8 nM and Bmax of 138 fmol mg-1, determined from equilibrium binding experiments. The Kd calculated from the association rate constant (2.4 X 10(5) M-1 s-1) and dissociation rate constant (1.3 X 10(-4) s-1) was 1.9 nM. Muscarinic ligands were the most potent inhibitors of quinuclidinyl benzilate binding. The characteristics of this binding site resembled those of vertebrate CNS muscarinic cholinergic receptors. In contrast with vertebrate CNS, the nerve cord of Periplaneta americana contains more (approximately X 7) alpha-bungarotoxin binding sites than quinuclidinyl benzilate binding sites.

Muscarinic cholinergic receptors in human foetal brain: characterization and ontogeny of [3H]quinuclidinyl benzilate binding sites in frontal cortex

Twenty-two frontal cortices from normal human foetal brains of gestational ages ranging from 16 to 40 weeks and five postnatal brains ranging from 5 to 50 years were analysed for the ontogeny of muscarinic receptors using [3H]quinuclidinyl benzilate (QNB) as the ligand. QNB binding sites were shown to be stable up to 4 1/2 months of storage at -70 degrees C. QNB binding was characterized in frontal cortices of 28-week-old foetal brains as muscarinic receptors by the following criteria: (1) it was localised mainly in particulate fraction; (2) binding was saturable at a concentration of 1.5 nM; (3) the cholinergic antagonists atropine and scopolamine competed for the binding, with IC50 values of 1 and 0.8 nM, respectively. The agonists oxotremorine, carbachol, and pilocarpine gave IC50 values of 1, 15 and 18 microM, respectively. Nicotinic receptor ligands and noncholinergic drugs could not compete for the binding. Bimolecular association and dissociation rate constants for the reversible binding are 6.23 X 10(8) M-1 X min-1 and 2.0 X 10(-2) X min-1, respectively. The equilibrium dissociation constant is 33 pM. The KD obtained by saturation binding data is 103 pM. Ontogeny of muscarinic receptors showed three distinct phases: In phase I, they appear between 16 and 18 weeks [average concentration 109 fmol/mg protein of total particulate fraction (TPF)] and slowly increase up to 20 weeks (average concentration 147 fmol/mg protein TPF). Phase II is a lag period between 20 and 24 weeks at which time receptor concentration does not change perceptibly (average concentration (67 fmol/mg protein TPF).(ABSTRACT TRUNCATED AT 250 WORDS)

Computer-assisted image analysis to quantify regional and specific receptor ligand binding: upregulation of [3H]QNB and [3H]WB4101 binding in denervated hippocampus

Quantitative regional analysis of receptor autoradiographs using the Nikon Magiscan image analysis system permits resolution of regional variations in specific binding in non-homogeneous CNS structures, such as the hippocampus. Cholinergic denervation, produced by fimbrial transections, elicits a 24% increase in atropine-displaceable [3H]QNB binding in whole coronal sections of the hippocampal formation, which is greatest in the dorsal subiculum, CA3 and dentate gyrus. This lesion also elicits a 69% increase in lower affinity [3H]WB4101 binding which is displaceable by phentolamine, but not by prazosin. This represents a sum of increases and decreases in binding in several subregions. Taken together, these findings serve to emphasize the need for normalized regional evaluation of subtracted images which have been calibrated, and linearized or transformed, to reveal binding specific to a single site.

Receptor binding profile of quinupramine, a new tricyclic antidepressant

The receptor binding profile, composed of the Ki-values measured in eight different receptor binding models using rat brain membranes, is reported for the new tricyclic antidepressant quinupramine, 10,11-dihydro-5-(3-quinuclidinyl)-5H-dibenz[b, f]azepine, and three reference compounds with a tertiaryamine side chain. Quinupramine was found to possess high affinity for muscarinic cholinergic and histamine H1 receptor binding sites in rat brain, whereas its affinity for imipramine binding sites was only one seventieth that of imipramine. Receptor binding profiles of the reference compounds were almost similar to that of quinupramine, except in the case of imipramine binding sites.

Choline acetyltransferase activity and [3H]quinuclidinylbenzilate binding in brains of scrapie-infected hamsters

Choline acetyltransferase (ChAT) activity and [3H]quinuclidinylbenzilate binding were studied in the brain of scrapie-infected hamsters and sham inoculated controls. Although scrapie-infected hamsters showed no reduction of ChAT activity compared to the controls, they showed a decrease in the affinity and maximum number of post-synaptic muscarinic receptors. Scrapie virus thus alters the cholinergic system at the post-synaptic rather than at the pre-synaptic level.

[3H]Pirenzepine and [3H]quinuclidinyl benzilate binding to brain muscarinic cholinergic receptors. Differences in measured receptor density are not explained by differences in receptor isomerization

Muscarinic receptor densities were measured in membranes prepared from rat cerebral cortex using [3H]pirenzepine and [3H]quinuclidinyl benzilate. Isotherms of equilibrium binding data modeled to a single apparent binding site for both ligands. However, as has been reported previously, [3H]pirenzepine labeled only a small fraction of the binding sites that were labeled by [3H]quinuclidinyl benzilate. This observation has been used to support the hypothesis that subtypes of muscarinic receptors exist. Several investigators have previously suggested that antagonist binding to muscarinic receptors involves an isomerization of the receptor-antagonist complex, and it is only the isomerized form of the receptor that is identified by radioligand binding studies. To examine the possibility that the difference in the density of binding sites identified by [3H]pirenzepine and [3H]quinuclidinyl benzilate is due to differences in the degree of isomerization of the receptor associated with the binding of each ligand, the kinetics of the binding of [3H]pirenzepine and [3H]quinuclidinyl benzilate to membranes prepared from rat cerebral cortex were examined. The pseudo-first-order rate constant of association for both ligands showed a nonlinear (hyperbolic) dependence on ligand concentration. These results suggested that a rapidly equilibrating initial binding step was followed by a more slowly equilibrating isomerization of the initially formed ligand-receptor complex. The kinetic data were computer-modeled to obtain estimates of the equilibrium constants for both reaction steps. The equilibrium constants for the isomerization step were 0.1 and 0.004 for [3H]pirenzepine and [3H]quinuclidinyl benzilate, respectively. Our measurements, in agreement with others, suggested that only the fraction of receptors which isomerized were measurable using filtration binding assays. Although essentially all (99.6%) of the [3H]quinuclidinyl benzilate binding sites appeared to isomerize, only 90% of the [3H]pirenzepine binding sites isomerized, and thus only 90% were measured in our assay. It therefore appears that differences in receptor isomerization can partially, but not wholly, account for the differences between [3H]pirenzepine and [3H]quinuclidinyl benzilate binding in rat cerebral cortex.

Kinetics of in vivo binding of antagonist to muscarinic cholinergic receptor in the human heart studied by positron emission tomography

Positron Emission Tomography (PET) was used to analyse in vivo antagonist binding to human myocardial muscarinic cholinergic receptor. The methiodide salt of the muscarinic antagonist, quinuclidinyl benzilate (MQNB), was labeled with the positron emitter, Carbon-11, and injected intravenously to 8 normal subjects. 11C-MQNB concentration was determined in vivo in the ventricular septum from 40 cross-sectional images acquired at the same transverse level over a period of 70 minutes. In 4 subjects, various amounts of unlabeled atropine were rapidly injected at 20 minutes to study whether atropine competitively inhibited MQNB. The kinetics of binding of 11C-MQNB were not the same in vivo and in vitro. The apparent dissociation rate of 11C-MQNB in vivo was much slower (by 1 to 2 orders of magnitude) than that observed in vitro with 3H-QNB. After atropine injection, 11C-MQNB dissociated from its binding sites at a rate that apparently depended on the amount of atropine present. 11C-MQNB kinetics were analysed with a mathematical model which assumes the existence of a boundary layer containing free ligand in the vicinity of the binding sites. The dissociation rate of the radioligand depends on the probability of its rebinding to a free receptor site.

Light microscopic autoradiographic localization of [3H]pirenzepine and [3H](−)quinuclidinyl benzilate binding in human stellate ganglia

We have utilized the LKB Ultrofilm method of autoradiography to anatomically localize putative M1 and M2 muscarinic receptor subtypes in human stellate ganglia. Ten micron sections were labeled in vitro with either 1 nM of the classical antagonist [3H](-)quinuclidinyl benzilate ([3H](-)QNB) or 20 nM of the non-classical antagonist [3H]pirenzepine ([3H]PZ), using 1 microM atropine sulfate to define non-specific binding for both ligands. Our results indicate that [3H](-)QNB and [3H]PZ binding sites are distributed within the principal ganglion cells and nerve bundles.

[125I] 3-quinuclidinyl 4-iodobenzilate: a high affinity, high specific activity radioligand for the M1 and M2-acetylcholine receptors

We have prepared a radioiodinated ligand which binds with high affinity to the muscarinic acetylcholine receptor (m-AChR). A derivative of 3-quinuclidinyl benzilate, [125I] labeled (R) 1-aza-bicyclo(2.2.2)oct-3-yl (R,S)-alpha-hydroxy-alpha-(4-[125I]iodophenyl)phenyl acetate (4- IQNB ) exhibits an affinity for the m-AChR from corpus striatum higher than that of (R) [3H] QNB. Additionally, [125I] 4- IQNB exhibits receptor selectivity for the M1 receptor since the affinity for the receptor from dog and rat heart is lower than that using dog or rat corpus striatum.

Differentiation between ligand trapping into intact cells and binding on muscarinic receptors

Binding properties of [3H] dexetimide , L-quinuclidinyl[phenyl-4-3H] benzilate and [3H]methylscopolamine were compared with intact 108 CC 15 cells and membrane preparations of those. The ability of the three ligands to label specifically muscarinic receptors on membrane fractions was quite similar. By contrast, when performed with intact cells, [3H] dexetimide and L-quinuclidinyl [phenyl-4-3H]benzilate revealed higher nonspecific binding which was prevented by methylamine, suggesting a trapping of the ligands within the cells presumably in the lysosomes. To the contrary, such nonspecific 'binding' or trapping was not detectable when [3H]methylscopolamine was used as ligand, a fact which makes this ligand particularly appropriate for labelling cell surface muscarinic receptors. It is concluded that more caution is needed in binding studies when performed with intact cells; indeed, besides specific binding on receptor sites, [3H]ligand can be entrapped within the cell and can even sometimes give the illusion of specific binding. The use of lysosomal agents which do not interfere with specific receptors on membrane preparations should allow one, in most cases, to discard the possibility of a trapping phenomenon in intact cells.

Simultaneous assay of muscarinic and beta-adrenergic receptors using a double isotope technique

Muscarinic receptor and beta-adrenergic receptor binding were measured simultaneously in a membrane fraction of bovine tracheal smooth muscle using [3H]-L-quinuclidinyl benzilate and [125I]-(-) iodocyanopindolol. The binding characteristics, affinity and receptor density, obtained in the double receptor assay and in the control experiments were the same within experimental error. Moreover, there appears to be neither a significant influence of an excess of d,1-propranolol on [3H]-L-quinuclidinyl benzilate binding nor a significant influence of an excess of 1-quinuclidinyl benzilate on [125I]-(-)iodocyanopindolol binding. The method is advantageous where both receptors have to be assayed and where limited amounts of biological material, like in biopsy specimen, are available.

Quantitative autoradiography of muscarinic cholinergic receptors in the rat brain

Using the in vitro autoradiographic technique with tritium-sensitive LKB sheet film and the liquid scintillation counting method, the distribution and the binding parameters of the muscarinic cholinergic receptors (MChR) were determined in various discrete regions of the rat brain. The results obtained in the present study were as follows: (1) Specific binding of [3H]QNB to the slide-mounted tissue sections increased slowly when incubated at room temperature; saturation occurred 2 h after incubation. Only 23% of [3H]QNB bound to the tissue section was dissociated 5 h after the addition of 20 microM atropine to the medium. These findings were very different from those obtained in the study using the tissue homogenates. (2) The regional distribution of MChR was determined using both autoradiographic and liquid scintillation counting methods. The distribution of MChR was heterogeneous, with highest densities in the striatum and nucleus accumbens and lowest in the globus pallidus, nucleus interpeduncularis and nucleus septi. Moreover, MChR were unevenly distributed within the subfields of each region. (3) In saturation binding studies using the slide-mounted tissue sections of 20 micron thickness the (Kd)app-values were similar but not exactly identical in 5 discrete regions, i.e. the striatum, somatosensory cortex, hippocampus (the subiculum + CA1 field), nucleus accumbens and gyrus dentatus, determined in the present study. The (Kd)app-value of each region was about 700 pM which was about 20 times higher than that obtained in the study using the tissue homogenates. However (Kd)app-values obtained with 5 and 10 micron tissue sections were approximately 3-fold lower.

In vivo competition studies with analogues of 3-quinuclidinyl benzilate

Among ligands that bind to the alpha- and beta-adrenoceptors and to the muscarinic acetylcholine receptor (m-AChR), those that bind to the latter have the best properties for external detection of receptor sites by gamma-camera imaging. To develop the optimal radiotracer, nonradioactive analogues of 3-quinuclidinyl benzilate (I) were tested in in vivo displacement studies with (-)-[3H]I to determine their ability to compete with (-)-[3H]I for the muscarinic acetylcholine receptor. There is a linear correlation between the ability to compete with (-)-[3H]I for the m-AChR and the affinity constant of the analogue as determined by in vitro assay, suggesting that the test is a valid indicator of in vivo distribution. One radioiodinated analogue, 3-quinuclidinyl p- iodobenzilate , bound to m-AChR in the heart and brain of rats.

Comparison of [3H]pirenzepine and [3H]quinuclidinylbenzilate binding to muscarinic cholinergic receptors in rat brain

The properties of [3H]quinuclidinylbenzilate ( [3H]QNB) binding and [3H]pirenzepine ( [3H]PZ) binding to various regions of rat brain were compared. [3H]PZ appeared to bind with high affinity to a single site, with a Kd value of approximately 15 nM in the cerebral cortex. The rank order of potencies of muscarinic drugs to inhibit binding of either [3H]QNB or [3H]PZ was QNB greater than atropine = scopolamine greater than pirenzepine greater than oxotremorine greater than bethanechol. Muscarinic antagonists (except PZ) inhibited both [3H]PZ and [3H]QNB binding with Hill coefficients of approximately 1. PZ inhibited [3H]QNB binding in cortex with a Hill coefficient of 0.7, but inhibited [3H]PZ binding with a Hill coefficient of 1.0. Hill coefficients for agonists were less than 1. The density of [3H]PZ binding sites was approximately half the density of [3H]QNB binding sites in cortex, striatum and hippocampus. In pons-medulla and cerebellum, the densities of [3H]PZ binding sites were 20 and 0%, respectively, relative to the densities of [3H]QNB binding sites. When unlabeled PZ was used to compete for [3H]QNB binding, the relative number of high-affinity PZ binding sites in cortex, pons and cerebellum agreed with the relative number of [3H]PZ binding sites in those regions. The binding of [3H]PZ and [3H]QNB was nonadditive in cortex. GTP inhibited high-affinity oxotremorine binding, but not PZ binding. Together, these data suggest that [3H]PZ binds to a subset of [3H]QNB binding sites. Whether this subset reflects the existence of subtypes of muscarinic receptors or is a consequence of coupling to another membrane protein remains to be seen.

The differential loss of [3H]pirenzepine vs [3H](−)Quinuclidinylbenzilate binding to soluble rat brain muscarinic receptors indicates that pirenzepine binds to an allosteric state of the muscarinic receptor

[3H]Pirenzepine [( 3H]PZ) and [3H] (-)Quinuclidinylbenzilate [( 3H] (-)QNB) specific binding to soluble rat brain muscarinic cholinergic receptors was assessed as a function of time subsequent to receptor solubilization. The soluble brain muscarinic receptor is stable at 4 degrees C when assayed by [3H] (-)QNB binding (t 1/2 = 80 hrs). In contrast the pirenzepine state of the receptor decays rapidly (t 1/2 = 3.0 hrs). Prior occupation of the receptor with [3H] (-)QNB or [3H]PZ increases the receptor stability by two to five fold (t 1/2 QNB greater than 1,000 hrs; t 1/2 PZ = 6.5 hrs). These data indicate that pirenzepine binds to an allosteric state of the muscarinic receptor and that caution should be employed in the assignment of receptor subtypes based solely upon the binding of ligands which recognize unique conformational states.

The characteristics of I-125 4-IQNB and H-3 QNB in vivo and in vitro

The accumulation of (R)-(H-3)-3-quinuclidinyl benzilate (H-3 QNB) and (R,S)-1-azabicyclo(2.2.2)oct-3-yl (R,S)-alpha-hydroxy-(4-[I-125]iodophenyl) benzeneacetate (I-125 4- IQNB ) in heart, caudate/putamen, and cerebellum of rats was determined at intervals from 15 min to 4 hr after injection. The behavior of the two radiotracers in the heart is consistent with in vitro results with respect to affinities and specificities. In the brain, however, the compounds differ in tissue selectivity. At high specific activity, neither compound provides localization that is consistent with the concentration of receptor in the tissues. The results of this study do not indicate quantification of receptor concentration by means of single external images.

External imaging of cerebral muscarinic acetylcholine receptors

A radioiodinated ligand that binds to muscarinic acetylcholine receptors was shown to distribute in the brain by a receptor-mediated process. With single-photon-emission imaging techniques, radioactivity was detected in the cerebrum but not in the cerebellum, whereas with a flow-limited radiotracer, radioactivity was detected in cerebrum and cerebellum. Single-photon-emission computed tomography showed good definition of the caudate putamen and cortex in man.

Effects of lithium on [3H](-)quinuclidinyl benzilate [( 3H](-)QNB) binding to rat brain muscarinic cholinergic receptors

Addition of lithium in vitro inhibited the binding of [3H](-)QNB to muscarinic cholinergic receptors of homogenates of tissue prepared from the striatum, cortex, and hippocampus of rat brain. Chronic in vivo exposure of rats to lithium in their food produced serum levels of lithium comparable to therapeutic levels. After in vivo exposure, the tissue homogenates prepared from these rats had an apparent decrease in receptor density in the three brain areas. However, if the tissue homogenates were washed twice, before addition to the assays, no differences in binding were detectable. Similar effects on unwashed and washed tissue homogenates can be demonstrated after in vivo exposure to lithium. Therefore, the apparent decrease in binding after in vivo lithium treatment is probably due to lithium retained in the tissue. No permanent alterations in the muscarinic receptor characteristics were measurable after the removal of the lithium. Nevertheless, in vivo interactions at muscarinic receptors may be important under conditions when therapeutic levels of lithium are present.

Muscarinic receptors on intact, cultured neurons. Characterization by [3H]quinuclidinylbenzilate binding

[3H]Quinuclidinylbenzilate (QNB) was used to identify muscarinic cholinergic receptors on intact, cultured neurons from fetal rat brains. Scatchard analysis revealed a single binding site with a dissociation constant Kd = approximately or equal to 170 pM. The rank order of potency of cholinergic drugs to displace [3H]QNB from intact neurons was similar to that observed using isolated membranes of brain homogenates. No difference in the rank order was observed with cultures of neurons from different brain regions which vary in their neuronal composition.

Contraction and [3H]QNB binding in collagenase isolated fundic smooth muscle cells

Smooth muscle cells from the guinea pig gastric fundus were isolated by successive collagenase digestions. Tritiated quinuclidinyl benzilate [( 3H]QNB) was used to study the binding characteristics of the muscarinic cholinergic receptors on these cells. Each cell bound 8.3 X 10(-19) mol of QNB, and a concentration of QNB of 0.19 nM was required to label one-half of the binding sites. This suggests a concentration of about 500,000 muscarinic cholinergic receptors per smooth muscle cell. The muscarinic cholinergic receptor antagonists atropine and scopolamine inhibited QNB binding with a 50% inhibiting concentration (IC50) in the nanomolar range, whereas the agonists acetylcholine (ACh), oxotremorine, and carbamylcholine had IC50S in the micromolar range. Hill coefficients (nH) for antagonists approached unity, but agonists displayed fractional nH. Exposure of cells to cholinergic muscarinic agonists resulted in dose-dependent decreases in cell length. The concentration of agonist required to induce half-maximal contractions (ED50) was 8.3 X 10(-12) M for ACh and 6.3 X 10(-13) M for oxotremorine. Atropine (10(-9) M) decreased the sensitivity to ACh, increasing the ED50 for ACh-induced contractions to 1.2 X 10(-10) M. These results suggest the existence of muscarinic receptor heterogeneity for cholinergic agonists but not for antagonists.

Differences in affinities of muscarinic acetylcholine receptor antagonists for brain and heart receptors

The affinities of atropine, scopolamine, 3-quinuclidinyl benzilate and twelve analogues of 3-quinuclidinyl benzilate were determined for the muscarinic acetylcholine receptor (m-AChR) using membrane preparations from caudate/putamen. The affinity constants thus obtained were compared with affinities previously reported for the m-AChR obtained from ventricular muscle. The affinities differed significantly for six of the compounds, the largest difference being 16-fold. Neither solubilization nor variation of physiologically significant salts led to a significant change in the affinity of that compound. These results are interpreted as supporting the subclassification of the muscarinic acetylcholine receptor.

Obligatory role of a Tris/choline allosteric site in guanine nucleotide regulation of [3H]-L-QNB binding to muscarinic acetylcholine receptors

Tris and choline reduce the maximal binding capacity (RT) of the muscarinic cholinergic antagonist [3H]-L-quinuclidinyl benzilate ([3H]-L-QNB) to atrial membranes, when compared to control values in physiological salt solution (PBS) or NaPi buffer. Addition of guanine nucleotides (GN) to incubations containing choline or Tris reverses the effect of choline and Tris on RT and restores it to levels determined in NaPi or PBS alone. GN addition fails to alter RT or KD values determined in NaPi or PBS in the absence of choline and Tris. This GN effect follows a nucleotide specificity similar to that of the GN regulatory proteins coupled to adenylate cyclase. Tris or choline are required for the expression of GN regulation of [3H]-L-QNB binding to muscarinic acetylcholine receptors (mAChR). An allosteric site recognizing choline and Tris appears involved in the interaction between the guanine nucleotide regulatory protein and antagonist binding to mAChR.