himbacine and methoctramine

1. Muscarinic cholinoceptors (MChR) in freshly dispersed rat salivary gland (RSG) cells were characterized using microphysiometry to measure changes in acidification rates. Several non-selective and selective muscarinic antagonists were used to elucidate the nature of the subtypes mediating the response to carbachol. 2. The effects of carbachol (pEC(50) = 5.74 +/- 0.02 s.e.mean; n = 53) were highly reproducible and most antagonists acted in a surmountable, reversible fashion. The following antagonist rank order, with apparent affinity constants in parentheses, was noted: 4-DAMP (8.9)= atropine (8.9) > tolterodine (8.5) > oxybutynin (7.9) > S-secoverine (7.2) > pirenzepine (6.9) > himbacine (6.8) > AQ-RA 741 (6.6) > methoctramine (5.9). 3. These studies validate the use of primary isolated RSG cells in microphysiometry for pharmacological analysis. These data are consistent with, and extend, previous studies using alternative functional methods, which reported a lack of differential receptor pharmacology between bladder and salivary gland tissue. 4. The antagonist affinity profile significantly correlated with the profile at human recombinant muscarinic M(3) and M(5) receptors. Given a lack of antagonists that discriminate between M(3) and M(5), definitive conclusion of which subtype(s) is present within RSG cells cannot be determined.

Topics: Alkaloids; Animals; Atropine; Benzhydryl Compounds; Benzodiazepinones; Binding, Competitive; Biosensing Techniques; Carbachol; Cholinergic Agonists; Cresols; Diamines; Dose-Response Relationship, Drug; Furans; Male; Mandelic Acids; Muscarinic Antagonists; Naphthalenes; Phenethylamines; Phenylpropanolamine; Piperidines; Pirenzepine; Rats; Rats, Sprague-Dawley; Receptors, Muscarinic; Submandibular Gland; Tolterodine Tartrate

1. The characteristics of muscarinic receptors mediating relaxation and/or contraction in the rat iris dilator muscle were examined. 2. Relaxation was induced in a dilator muscle by application of acetylcholine (ACh) at low doses (3 microM or less) and contraction was induced by high doses. Methacholine and carbachol also showed biphasic effects similar to those of ACh; in contrast, bethanechol, arecoline, pilocarpine and McN-A-343 induced mainly relaxation but no substantial contraction. 3. After parasympathetic denervation by ciliary ganglionectomy, the relaxant response to muscarinic agonists disappeared upon nerve stimulation. Application of McN-A-343 and pilocarpine induced only small contractions in denervated dilator muscles, indicating that these are partial agonists for contraction. 4. pA2 values of pirenzepine, methoctramine, AF-DX 116, himbacine, and 4-DAMP for antagonism to pilocarpine-induced relaxation in normal dilator muscles and those for antagonism to ACh-induced contraction in denervated dilator muscles were determined. The pA2 values for antagonism to relaxation of all these antagonists were most similar to those for M3-type muscarinic receptors. 5. Although pA2 values for contraction of these antagonists, except for methoctramine, were very close to those for relaxation, contraction was not significantly antagonized by methoctramine. Contraction might be mediated by M3-like receptors which have a very low affinity for methoctramine. 6. In conclusion, ACh-induced biphasic responses in rat iris dilator muscles were clearly distinguished from each other by specific muscarinic agonists and parasympathetic denervation, whereas muscarinic receptors could not be subclassified according to the pA2 values of 5 specific antagonists only.

Topics: (4-(m-Chlorophenylcarbamoyloxy)-2-butynyl)trimethylammonium Chloride; Acetylcholine; Alkaloids; Animals; Arecoline; Bethanechol; Carbachol; Diamines; Dose-Response Relationship, Drug; Furans; Ganglia, Parasympathetic; Ganglionectomy; Iris; Male; Methacholine Chloride; Muscarinic Agonists; Muscle Contraction; Muscle Relaxation; Muscle, Smooth; Naphthalenes; Parasympatholytics; Pilocarpine; Piperidines; Pirenzepine; Rats; Rats, Wistar; Receptors, Muscarinic

1. The effects of pertussis toxin (PTX) on contraction and/or relaxation induced by agonists or transmural nerve stimulation (TNS) were examined in the rat iris dilator and sphincter muscles. 2. TNS in the presence of phentolamine induced an atropine-sensitive biphasic response: initial contraction followed by relaxation in dilator muscles. Exogenously applied acetylcholine (ACh) elicited a large relaxation at low doses (3 microM or less) and a concentration at high doses. 3. Only the ACh-induced relaxation was affected by injection of PTX (10 ng) into the anterior eye chamber. Relaxation was decreased 12 h after injection and had completely disappeared after 24 h. Relaxation recovered in part 3 weeks and almost completely 8 weeks after PTX treatment. A gradual decrease in muscarinic relaxation in a dilator muscle was also observed in vitro after addition of PTX to the bathing solution. 4. The pA2 values of muscarinic blockers, pirenzepine, AF-DX 116, 4-DAMP, and himbacine for competitive antagonism to ACh-induced contraction were 7.14, 6.53, 9.03, and 6.80, respectively, in PTX-pretreated dilator muscles. These values are comparable to those obtained in parasympathectomized dilator muscles and may indicate involvement of M3 or M3-like receptors in muscle contraction. 5. Pretreatment with PTX did not significantly affect contraction induced by noradrenaline or 5-hydroxytryptamine or the relaxation induced by isoprenaline in dilator muscles. 6. In conclusion, among several agonist-induced responses in the rat iris dilator and sphincter muscles, only muscarinic relaxation in dilator muscle occurs via activation of PTX-sensitive GTP binding proteins.

Topics: Acetylcholine; Alkaloids; Animals; Atropine; Diamines; Dose-Response Relationship, Drug; Furans; GTP-Binding Proteins; In Vitro Techniques; Iris; Male; Microinjections; Muscarinic Antagonists; Muscle Contraction; Muscle Relaxation; Muscle, Smooth; Naphthalenes; Parasympatholytics; Pertussis Toxin; Phentolamine; Piperidines; Pirenzepine; Rats; Rats, Wistar; Receptors, Muscarinic; Virulence Factors, Bordetella

The possibility of an allosteric interaction by himbacine, a cardioselective antagonist, with rat cardiac muscarinic receptors was studied. Himbacine allosterically decelerated the dissociation of bound [3H]N-methylscopolamine [( 3H]NMS) in a concentration-dependent manner with an IC50 value of 103.7 microM. When compared to the IC50 values of other cardioselective antagonists, the rank order of potencies was: methoctramine greater than gallamine greater than himbacine greater than AF-DX 116. In contrast, the potencies of these compounds to displace [3H]NMS binding were: himbacine greater than methoctramine greater than AF-DX 116 greater than gallamine. The allosteric potencies were found not to be correlated with binding potencies (correlation coefficient = -0.15). A striking common feature of the cardioselective antagonists is their ability to bind to an allosteric site on cardiac muscarinic receptors.

Topics: Alkaloids; Allosteric Regulation; Animals; Atropine; Binding, Competitive; Diamines; Dose-Response Relationship, Drug; Furans; Gallamine Triethiodide; Male; Myocardium; N-Methylscopolamine; Naphthalenes; Piperidines; Rats; Rats, Inbred Strains; Receptors, Muscarinic; Scopolamine Derivatives

To compare the proportions of four muscarinic receptors in different rat brain regions, we used competition curves with four selective antagonists, at 1-[N-methyl-3H]scopolamine methyl chloride [( 3H]NMS) binding equilibrium and after allowing [3H]NMS dissociation for 35 min. Himbacine and methoctramine were shown to discriminate two muscarinic receptor subtypes having a high affinity for 4-diphenylacetoxy-N-methylpiperidine methiodide and hexahydrosiladifenidol, intermediate affinity for pirenzepine, and low affinity for AF-DX 116. One M4 subtype had a high affinity for himbacine and methoctramine; it was found predominantly in homogenates from rat striatum (46% of total [3H]NMS receptors) and in lower proportion in cortex (33% of [3H]NMS receptors) and hippocampus (16% of [3H]NMS receptors). Its binding properties were identical to those of muscarinic receptors in the neuroblastoma x glioma NG 108-15 hybrid, suggesting that it was encoded by m4 mRNA. The M3 subtype (typically found in rat pancreas, a tissue expressing the m3 mRNA) had a low affinity for himbacine and methoctramine and represented about 10% of all [3H]NMS receptors in rat brain cortex, hippocampus, striatum, and cerebellum. M1 and M2 receptors were identified in rat brain by their high affinity for pirenzepine and AF-DX 116, respectively.

Topics: Alkaloids; Animals; Brain; Diamines; Furans; Humans; Hybrid Cells; N-Methylscopolamine; Naphthalenes; Parasympatholytics; Piperidines; Rats; Receptors, Muscarinic; Scopolamine Derivatives; Tumor Cells, Cultured

1. The action of methoctramine and himbacine at muscarinic receptors has been studied using guinea-pig isolated trachea, oesophageal muscularis mucosae, paced left atria, and rat aortic preparations. 2. Methoctramine (1 x 10(-6)-3.2 x 10(-4) M), but not himbacine, elicited positive inotropic responses. These responses were enhanced by pretreating the animals with reserpine. The responses in reserpine-treated animals were not antagonized by phentolamine (1 x 10(-6) M) but were antagonized by propranolol (1 x 10(-6) M). 3. Methoctramine, but not himbacine, exhibited allosteric inhibitory effects at cardiac muscarinic receptors, resulting in a curvilinear Schild plot. Deviations from competitive antagonism were also observed in combination dose-ratio experiments using atropine and methoctramine. At 1 x 10(-6) M, the pKB value for methoctramine was 7.88 +/- 0.15 (mean +/- s.e.mean, n = 5). The pA2 value for himbacine at cardiac muscarinic receptors was 8.52 +/- 0.06 (n = 3). 4. At tracheal and oesophageal muscularis mucosal smooth muscle receptors, the Schild plots for both antagonists were linear. The pA2 values for methoctramine at receptors in these two preparations were similar (6.08 +/- 0.05 and 6.03 +/- 0.09 respectively, n = 4) and were approximately 60 fold less than those values observed at atrial receptors. Himbacine, also exhibited similar values at muscarinic receptors in the trachea and oesophageal muscularis mucosae (7.61 +/- 0.05 and 7.57 +/- 0.04 respectively, n = 4). 5. Muscarinic receptors mediating relaxation of the rat aortic endothelium exhibited pA2 values for methoctramine (5.87 +/- 0.12, n = 6) which were similar to those observed in the smooth muscle, but not the atria. The pA2 values for himbacine at endothelial muscarinic receptors were approximately 0.5 pA2 units lower than those observed at muscarinic receptors in smooth muscle (6.92 + 0.80, n = 6). In addition, the Schild slopes for methoctramine and himbacine at these receptors were significantly (P < 0.05) less than unity. 6. Methoctramine, and to a lesser extent himbacine, are potent and selective antagonists for cardiac muscarinic receptors. However, caution should be used in interpretation of the data with methoctramine in view of the inhibitory allosteric properties and direct inotropic actions of this compound.

Topics: Alkaloids; Animals; Diamines; Drug Interactions; Endothelium; Furans; Guinea Pigs; Heart Atria; In Vitro Techniques; Male; Muscle, Smooth; Naphthalenes; Piperidines; Rats; Rats, Inbred Strains; Receptors, Muscarinic