A comparison of the affinities of antagonists for acetylcholine receptors in the ileum, bronchial muscle and iris of the guinea-pig

Myorelaxant Activity of essential oil from Origanum majorana L. on rat and rabbit

ETHNOPHARMACOLOGICAL RELEVANCE

Origanum majorana L. (Lamiaceae) was usually used in Moroccan folk medicine to treat infantile colic and abdominal discomfort.

MATERIALS AND METHODS

The essential oil from the aerial part of the dry Origanum majorana L. (EOOM) was obtained through hydro distillation and analyzed by gas chromatography/mass spectrometry (GC/MS). The effect of EOOM on muscle relaxation was measured on rabbit and rat intestinal smooth muscle mounted in an isotonic transducer.

RESULTS

1) The main compounds obtained from the aqueous extract of this plant were alpha Terpineol, L-terpinen-4-ol and Beta.-Linalool. 2) EOOM inhibited in a concentration-dependent manner spontaneous contraction of rabbit jejunum, with an IC₅₀ = 64.08 ± 2.42 μg/mL. 3) In rat intestine, EOOM induced the relaxation of the tissue in concentration-dependent manner with an IC₅₀ = 39.70 ± 2.29 μg/mL when the tissue was pre-contracted with CCh 10^-6 M, and 48.70 ± 2.26 μg/mL when the tissue was pre-contracted with 25 mM KCl. 4) The relaxation effect induced by EOOM was more important than that obtained in the presence of atropine, hexamethonium, Nifedipine, L-NAME and Blue of methylene.

CONCLUSION

the present result indicates that essential oil of Origanum majorana L. exhibit an effect on intestinal relaxation in vitro. This effect further validates the traditional use of Origanum majorana L. to treat infantile colic and abdominal discomfort.

Spasmogenic and spasmolytic activities of Agastache mexicana ssp. mexicana and A. mexicana ssp. xolocotziana methanolic extracts on the guinea pig ileum

ETHNOPHARMACOLOGICAL RELEVANCE

Agastache mexicana has been used in traditional medicine for relief of abdominal pain and treatment of other diseases. Two subspecies have been identified: A. mexicana ssp. mexicana (AMM) and A. mexicana ssp. xolocotziana (AMX) and both are used traditionally without distinction or in combination.

AIM OF THE STUDY

To determine the effect of methanol extracts of A. mexicana ssp. mexicana and A. mexicana ssp. xolocotziana on gut motility and their possible mechanism of action.

MATERIALS AND METHODS

The effect of AMM and AMX methanol extracts were tested on the spontaneous activity in the isolated guinea pig ileum and on tissues pre-contracted with KCl, electrical field stimulation (EFS) or ACh. In addition, the possible mechanism of action of each subspecies on gut motility was analyzed in the presence of hexametonium, indomethacin, L-NAME, verapamil, atropine or pyrylamine. A comparative chromatographic profile of these extracts was also done to indicate the most abundant flavonoids presents in methanol extracts of both subspecies.

RESULTS

AMM, but not AMX, induced a contractile effect in the guinea pig ileum. This spasmogenic effect was partially inhibited by atropine, antagonist of muscarinic receptors; and pyrilamine, antagonist of H₁ receptors. In contrast, AMX, but not AMM, diminished the contractions induced by KCl, EFS or ACh. The spasmolytic activity of AMX was partially inhibited by hexamethonium, ganglionic blocker; and indomethacin, inhibitor of the synthesis of prostaglandins; but not by L-NAME, inhibitor of nitric oxide synthase. In addition, AMX diminished the maximal contraction induced by CaCl₂ in a calcium-free medium. Chromatographic analyses of these methanol extracts showed the presence of acacetin and tilanin in both.

CONCLUSIONS

These results suggest that in folk medicine only AMX should be used as spasmolytic, and not in combination with AMM as traditionally occurs, due to the spasmogenic effects of the latter. In addition, activation of nicotinic receptors, prostaglandins and calcium channels, but not nitric oxide mechanisms, could be responsible for the spasmolytic activity of AMX. On the other hand, release of ACh and histamine could be involved in the spasmogenic effect induced by AMM. Acacetin and tilanin are present in methanol extracts of both subspecies and both flavonoids were more abundant in AMX than AMM. Our findings contribute to the validation of the traditional use of Agastache mexicana in relieving gastrointestinal disorders, but indicate that the subspecie that should be used for this effect is A. mexicana ssp. xolocotziana.

Dose-response relationships in tropicamide-induced mydriasis and cycloplegia

1 Mydriatic and cycloplegic effects of tropicamide in the normal and in the guanethidine-treated human eye have been recorded following prolonged exposure of the eye to drug solutions. 2 Marked effects and satisfactory dose-response relationships were observed at drug concentrations within the range 1.25-40 μg/ml, by comparison with concentrations of 5-10 mg/ml usually applied as eye-drops. 3 The findings are discussed in relation to drug concentrations which produce cholinergic antagonism in vitro.

Spasmolytic activity of Rosmarinus officinalis L. involves calcium channels in the guinea pig ileum

ETHNOPHARMACOLOGICAL RELEVANCE

Rosmarinus officinalis L. is a plant used around the world for its properties to cure pain in several conditions, such as arthritic and abdominal pain or as an antispasmodic; however, there are no scientific studies demonstrating its spasmolytic activity. Therefore, the aim of the present study was to investigate the effect of an ethanol extract from Rosmarinus officinalis aerial parts and the possible mechanism involved by using rings from the isolated guinea pig ileum (IGPI).

MATERIALS AND METHODS

The IGPI rings were pre-contracted with potassium chloride (KCl; 60 mM), acetylcholine (ACh; 1 × 10(-9) to 1 × 10(-5)M) or electrical field stimulation (EFS; 0.3 Hz of frequency, 3.0 ms of duration and 14 V intensity) and tested in the presence of the Rosmarinus officinalis ethanol extract (150, 300, 600 and 1 200 μg/mL) or a referenced smooth muscle relaxant (papaverine, 30 μM). In addition, the possible mechanism of action was analyzed in the presence of hexametonium (a ganglionic blocker), indomethacine (an inhibitor of prostaglandins), l-NAME (a selective inhibitor of the nitric oxide synthase) and nifedipine (a calcium channel blocker).

RESULTS

Rosmarinus officinalis ethanol extract exhibited a significant and concentration-dependent spasmolytic activity on the contractions induced by KCl (CI(50) = 661.06 ± 155.91 μg/mL); ACh (CI(50) = 464.05 ± 16.85 μg/mL) and EFS (CI(50) = 513.72 ± 34.13 μg/mL). Spasmolytic response of Rosmarinus officinalis (600 μg/mL) was reverted in the presence of nifedipine 1 μM, but not in the presence of hexamethonium 0.5mM, indomethacine 1 μM or L-NAME 100 μM.

CONCLUSION

The present results reinforce the use of Rosmarinus officinalis as antispasmodic in folk medicine. Moreover, it is demonstrated the involvement of calcium channels in this activity, but not the participation of nicotinic receptors, prostaglandins or nitric oxide.

Muscarinic stimulation of airway smooth muscle cells

1. Acetylcholine, the principal neurotransmitter of the parasympathetic nervous system, is released at both ganglionic synapses and postganglionic neuroeffector junctions and acts by activation of nicotinic and muscarinic cholinoceptors. This review focuses on the effects of postjunctional muscarinic stimulation of airway smooth muscle. 2. On pharmacological criteria, four distinct subtypes of muscarinic cholinoceptor, denoted M1, M2, M3 and M4 receptors, have been identified by use of selective antagonists. Cloned muscarinic cholinoceptors are members of the family of GTP-binding protein-coupled receptors, which are characterized by seven transmembrane (TM) regions connected by intra- and extracellular loops. Between the fifth and the sixth TM regions, muscarinic receptors possess a large intracytoplasmic loop that is considered to be responsible for G-protein-coupling selectivity and exhibits high divergence between the different subtypes. 3. At the site of the smooth muscle itself, both binding and Northern blot studies have demonstrated, in a variety of species, that muscarinic receptor subtypes present are M2 and M3. M2 receptors are coupled to Gi proteins and adenylyl cyclase inhibition and thus to cAMP signaling. M3 receptors are coupled to Gq/11 protein and phosphoinositide hydrolysis and thus to calcium signaling. 4. Muscarinic-induced contraction of airway smooth muscle is mediated by M3 receptors. M2-mediated inhibition of adenylyl cyclase contributes to the prevention of bronchodilation. Cross-talk between muscarinic and beta2 adrenoceptors is likely to be present in airway smooth muscle. The pathophysiological role of this cross-talk requires further investigation.

The use of 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine (4-DAMP mustard) for estimating the apparent affinities of some agonists acting at muscarinic receptors in guinea-pig ileum

1. 4-Diphenylacetoxy-N-(2-chloroethyl)-piperidine (4-DAMP mustard), which is known to block muscarinic M3 receptors in preference to muscarinic M2 receptors, was used to estimate the apparent affinity constants of some agonists acting at muscarinic receptors in guinea-pig ileum. Estimates for carbachol and n-pentyl-trimethyl ammonium iodide were similar to published values obtained in similar conditions: those for n-hexyl-trimethyl ammonium iodide were slightly lower. 2. The results for the agonists, n-pentyl- and n-hexyl-trimethyl ammonium iodides and for the partial agonist, n-heptyl-trimethyl ammonium iodide were not as regular as was suggested by Stephenson, though there is an overall increase in apparent affinity with chain length. 3. Estimates of apparent affinity may be affected by hexamethonium, usually present in experiments on ileum. Its absence had little effect on the results with carbachol but reduced the estimates obtained with n-pentyl trimethyl ammonium, which has strong nicotinic effects compared with its muscarinic effects. On ileum treated with tetrodotoxin the values for n-pentyl trimethyl ammonium were similar to those obtained in the presence of hexamethonium (0.28 mM): slightly higher estimates of affinity were obtained in the presence of indomethacin (2.8 microM). The nicotinic effects of n-pentyl ammonium may involve the release of prostaglandins. 4. The estimates of apparent affinity did not depend on the method used to calculate them as the 'null' method and the 'operational' method give similar answers. Estimates of the transducer-ratio for the partial agonist, n-heptyl-trimethyl ammonium iodide, were numerically the same as those of its efficacy. 5. This work illustrates the use of 4-DAMP mustard as a tool for measuring the apparent affinity of agonists acting at muscarinic M3 receptors.

Functional consequences of prejunctional receptor activation or blockade in the iris

The iris is innervated by nerves of the sympathetic, parasympathetic, and sensory nervous systems. The terminal nerve fibres are endowed with prejunctional receptors which modulate neurotransmitter release. Activation or blockade of prejunctional receptors by drugs may have an influence on iris smooth muscle tone. Several findings are in favour of the hypothesis that prejunctional receptors may be involved in regulation of iris smooth muscle tone and/or pathophysiological events. (i). Release of acetylcholine from parasympathetic nerves of guinea-pig iris sphincter evoked by electrical stimulation is subject to autoinhibition via prejunctional M2 muscarinic receptors, and the release can be enhanced by M2 selective antagonists such as methoctramine or gallamine. Concomitantly with the increased neurotransmitter release, the sphincter contraction is enhanced in the presence of M2 antagonists, since the postjunctional muscarinic receptors (presumably M3, or at least not M2) are not simultaneously blocked. Unlike the non-selective blocker atropine, M2 antagonists are not expected to cause mydriasis but rather miosis. (ii). Sensory nerves are involved in pathophysiological events following ocular irritation. Release of substance P and/or neurokinin A from sensory nerves of rabbit iris is followed by a non-adrenergic-non-cholinergic iris sphincter contraction (mediated by NK1 and NK3 receptors) which can be used to estimate sensory neurotransmitter release. Exocytotic release of the sensory neurotransmitters is inhibited by activation of alpha 2B-adrenoceptors and probably also via putative prejunctional imidazoline receptors. Alpha-adrenoceptors are stimulated by oxymetazoline and other imidazoline derivatives (which are agonists at imidazoline receptors) leading to a reduction of sensory neurotransmitter release, as evident from a decrease in evoked sphincter contraction. Imidazolines in eye drops may not only cause relief in ocular inflammation due to postjunctional vasoconstriction but also possibly due to a prejunctional effect, a reduction of sensory neurotransmitter release. Reinforcement of inflammation due to release of sensory neurotransmitters may thus be prevented.

Reactivity of human iris-sphincter to muscarinic drugs in vitro

From tissue to tissue the contractile response of human irides to carbachol varied by 40 fold. The mean EC50 value of carbachol in tissues remaining in an in vitro environment for 24-37 h was equal, however, to that obtained from tissues examined during 79-161 h. The maximum response of the tissue to the highest concentration of carbachol increased up to 24 h, then a gradual decline in the maximum occurred. In 38 observations, the average decline after 72 h was approximately 30%. A plot of negative log EC50 values (n = 38) of carbachol exhibited normal Gaussian curve. The geometric mean EC50 value of carbachol was 0.38 mumol/l (0.28-0.51 mumol/l, 95% C.L.). Based on EC50 values, the rank order of potency of cholinergic agonists is as follows: Muscarine = carbachol, 1 > pilocarpine, 1/5 > methacholine, 1/23 > bethanechol, 1/29 > acetylcholine, 1/1310. The percent maximum contraction of irides to muscarine, carbachol, pilocarpine, methacholine and bethanechol were 100, 100, 80, 76 and 95, respectively. Acetylcholine at the highest concentration tested produced 71% of the maximum produced by carbachol. Within a concentration range of 1 to 100 mumol/l, physostigmine consistently contracted isolated irides. The mean EC50 value was 6.73 mumol/l. The effect was sensitive to blockade by atropine. When the temperature of the bathing medium was lowered from 37.5 degrees C to 27.5 degrees C or 17.5 degrees C the magnitude and the duration of the response of the iris to carbachol was increased, the EC50 value, however, was not changed significantly. The response to pilocarpine was similarly altered by the lower temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for prejunctional inhibitory muscarinic receptors on sympathetic nerves innervating guinea-pig trachealis muscle

1 Relaxation responses induced by stimulation of the postganglionic sympathetic nerve trunk were studied in the isolated, fluid-filled, innervated tracheal tube preparation of the guinea-pig. 2 The thromboxane-mimetic U46619, prostaglandin F2 alpha and histamine each caused concentration-dependent increases in the intraluminal pressure (ILP) of the fluid-filled tracheal tube, reflecting contraction of the trachealis muscle. Sympathetic nerve stimulation in the presence of the spasmogens caused relaxations which increased with increasing ILP. Relaxant responses evoked in the presence of these three spasmogens were comparable at any given ILP. 3 Muscarinic agonists caused concentration-dependent increases in ILP, pilocarpine being more potent than acetylcholine. Sympathetic nerve-induced relaxations were reduced in the presence of pilocarpine and acetylcholine when compared to those obtained at the same ILP in the presence of U46619. This inhibitory effect of muscarinic agonists on sympathetic nerve-induced responses was concentration-dependent. 4 Exogenously applied noradrenaline opposed the contractile effect of U46619 and acetylcholine to a similar extent, indicating that a comparable degree of postjunctional functional antagonism exists between the sympathetic neurotransmitter noradrenaline and both spasmogens. 5 The selective M2 muscarinic antagonists, gallamine and methoctramine, altered neither the postjunctional contractile action of acetylcholine nor its inhibitory effect on sympathetic nerve-induced relaxations. In addition, the inhibitory effect of acetylcholine was not modified by concentrations of pirenzepine known to block M1 muscarinic receptors. 6 The postjunctional contractile action of acetylcholine and its inhibitory effect on sympathetic neuro-transmission were antagonized by atropine, by the M3 muscarinic antagonist hexahydrosiladiphenidol and by higher concentration of pirenzepine. 7. These results suggest that in the guinea-pig trachea, muscarinic cholinoreceptor agonists inhibit sympathetic neurotransmission via activation of muscarinic receptors located on the sympathetic nerve endings. These inhibitory prejunctional muscarinic heteroreceptors are of the M3 subtype.

Identification of three muscarinic receptor subtypes in rat lung using binding studies with selective antagonists

Heterogeneity of the muscarinic receptor population in the rat central and peripheral lung was found in competition binding experiments against [3H]quinuclidinyl benzilate [( 3H]QNB) using the selective antagonists pirenzepine, AF-DX 116 and hexahydrosiladifenidol (HHSiD). Pirenzepine displaced [3H]QNB with low affinity from preparations of central airways indicating the absence of M1 receptors in the trachea and bronchi. Muscarinic receptors in the central airways are comprised of both M2 and M3 receptors since AF-DX 116, an M2-selective antagonist, bound with high affinity to 70% of the available sites while HHSiD, an M3-selective antagonist bound with high affinity to the remaining binding sites. In the peripheral lung, pirenzepine bound with high affinity to 14% of the receptor population, AF-DX 116 bound with high affinity to 79% of the binding sites while HHSiD bound with high affinity to 18% of the binding sites. The presence of M1 receptors in the peripheral airways but not in the central airways was confirmed using [3H]telenzepine, an M1 receptor ligand. [3H]Telenzepine showed specific saturable binding to 8% of [3H]QNB labeled binding sites in homogenates of rat peripheral lung, while there was no detectable specific binding in homogenates of rat trachea or heart. The results presented here demonstrate that there are three muscarinic receptor subtypes in rat lungs, and that the distribution of the different subtypes varies within the lungs. Throughout the airways, the dominant muscarinic receptor subtype is M2. In the trachea and bronchi the remaining receptors are M3, while in the peripheral lungs, the remaining receptors are both M1 and M3.

Different muscarine receptors mediate the prejunctional inhibition of [3H]-noradrenaline release in rat or guinea-pig iris and the contraction of the rabbit iris sphincter muscle

To investigate the muscarine receptor type mediating inhibition of [3H]-noradrenaline release from the isolated rat and guinea-pig iris we have determined the potency of antimuscarinic drugs to antagonize the methacholine-induced inhibition of [3H]-noradrenaline overflow evoked by field stimulation (3 Hz, 2 min). The prejunctional apparent affinities were compared with those obtained for postjunctional muscarine receptors mediating the methacholine-induced contraction of the isolated rabbit iris sphincter muscle. Prejunctional apparent affinity constants of pirenzepine (6.67), himbacine (8.51), methoctramine (7.92), 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, 8.00), hexahydro-difenidol enantiomers (6.92, (R); 5.77, (S)) in the rat iris and methoctramine (7.58) in the guinea-pig iris indicate the presence of M2 receptors. Although the postjunctional affinity constants in the rabbit iris sphincter of methoctramine (5.93), gallamine (3.92), and 4-DAMP (9.07) confirm our previous suggestions of the presence of M3-like receptors, the results obtained with the hexahydro-difenidol enantiomers do not agree with that concept. The postjunctional affinity constants of the hexahydro-difenidol enantiomers were not different from the prejunctional values (6.86, (R); 5.55, (S)), indicating a similar and low degree of stereoselectivity for these stereoisomers at both receptor sites (14 and 17, (R)/(S)-ratios, respectively). Hence, the postjunctional muscarine receptor in the rabbit iris sphincter fails to exhibit the high degree of stereoselectivity observed for hexahydro-difenidol enantiomers at M3 receptors on other smooth muscles.

The interaction of hexamethonium with muscarinic receptor subtypes in vitro

1. The action of hexamethonium has been studied at a range of muscarinic receptors in vitro by use of both functional and radioligand binding studies. 2. In functional studies, hexamethonium exhibited little or no significant (P less than 0.05) antagonism of contractile responses to carbachol at muscarinic receptors in the guinea-pig ileum, oesophageal muscularis mucosae, urinary bladder and trachea. However, antagonism was observed at muscarinic receptors in the guinea-pig left atria mediating negative inotropic responses and the calculated pKB value was 3.80. Hexamethonium also antagonized contractile responses to carbachol in the canine saphenous vein. The pKB value at these receptors was 3.75. 3. In the presence of 3.2 mM hexamethonium, the pA2 value for methoctramine at atrial muscarinic receptors was reduced by approximately 10 fold (control pA2 value was 7.81 +/- 0.05; pA2 value in hexamethonium was 6.73 +/- 0.04). In contrast at tracheal muscarinic receptors, the pA2 values for methoctramine were unaffected in the presence of 3.2 mM hexamethonium (control pA2 = 5.58 +/- 0.07; pA2 value in hexamethonium was 5.63 +/- 0.12). All values quoted are mean +/- s.e. mean, n = 8. 4. In competition radioligand binding studies, hexamethonium exhibited a higher affinity for cardiac M2 receptors (pKi = 3.68) than for cerebrocortical M1 receptors (pKi = 3.28) or for submaxillary gland M3 receptors (pKi = 2.61). At M2 receptors hexamethonium at concentrations of 0.1-10 mM, increased the half life of the dissociation rate of [3H]-N-methylscopolamine 1.6-4.3 fold. This was observed at M3 receptors only at 10 mM, when the half life was increased 1.7 fold. 5. We conclude that hexamethonium, in addition to its well characterized nicotinic antagonist properties, can act as a weak muscarinic antagonist and differentiates between cardiac M2 receptors and glandular/smooth muscle M3 receptors. However, hexamethonium differentiates less clearly between M1 and M2 receptors. The selectivity between M2 and M3 receptors observed in the present study with hexamethonium is comparable to other M2 selective antagonists such as AF-DX 116 and himbacine. 6. Caution should be exercised with regard to the inclusion of hexamethonium in functionsal studies of M2 muscarinic receptor subtypes at concentrations of 0.1 mm and above.

Identification of M1 muscarinic receptors in pulmonary sympathetic nerves in the guinea-pig by use of pirenzepine

1. The effect of pirenzepine, a muscarinic antagonist considered to be selective for M1 receptors, was studied on bronchoconstriction and bradycardia elicited by preganglionic stimulation of the parasympathetic vagal nerves and by i.v. injections of acetylcholine (ACh) in anaesthetized guinea-pigs. 2. Pirenzepine was equipotent in the heart and lung as an antagonist of the effects of i.v. ACh at postjunctional muscarinic receptors. Doses of pirenzepine in excess of 1 mumol kg-1 abolished all muscarinic responses consistent with non-selective blockade of M3 receptors on airway smooth muscle and M2 receptors on atrial cells. 3. In the lung, low doses of pirenzepine (1-100 nmol kg-1) increased vagally-induced bronchoconstriction despite concurrent partial blockade of the postjunctional receptors. This suggests blockade of neuronal muscarinic receptors. 4. Propranolol (1 mg kg-1) increased control bronchoconstrictor responses elicited by ACh and vagal stimulation but did not alter the potency of pirenzepine for postjunctional receptors in heart or lung. However, pirenzepine-induced enhancement of vagally-induced bronchoconstriction was abolished by propranolol, suggesting that pirenzepine may be an antagonist for muscarinic receptors located in the sympathetic nerves innervating airway smooth muscle. 5. These results confirm that bronchoconstrictor stimuli indirectly initiate activation of an opposing sympathetic reflex in the guinea-pig lung. This response is facilitated by muscarinic receptors located in the sympathetic nervous pathway. 6. The high potency of pirenzepine for the neuronal receptors in the sympathetic nerves suggests that these are M1 receptors. In contrast, the parasympathetic nerves innervating airway smooth muscle in this species contain M2 receptors which inhibit neurotransmission.

M2 muscarinic receptors on the iris sphincter muscle differ from those on iris noradrenergic nerves

The pre- and postjunctional affinity constants of a series of muscarinic antagonists were determined in guinea pig and rabbit irises. Field stimulation-evoked [3H]noradrenaline release from superfused isolated irises was concentration dependently inhibited by (+/-)-methacholine, confirming the presence on the iris noradrenergic nerves of prejunctional inhibitory muscarinic receptors. The affinity constants of the antagonists at the pre- and postjunctional receptors are compatible with the coexistence in the iris of two different M2 receptors: the cardiac (M2 alpha) subtype on the noradrenergic nerves and the smooth muscle (M2 beta) subtype on the iris sphincter muscle. The rank order of potency of the antagonists studied at the prejunctional site was: atropine greater than himbacine greater than AF-DX 116 greater than pirenzepine greater than hexahydrosiladifenidol. The order of potency at the postjunctional receptors mediating the methacholine-induced isotonic contraction of the isolated rabbit iris sphincter was: atropine greater than hexahydrosiladifenidol greater than pirenzepine greater than himbacine greater than AF-DX 116.

Heterogeneity of presynaptic muscarinic receptors involved in modulation of transmitter release

In order to extend the characterization of muscarinic receptors at presynaptic sites their inhibitory effect on the stimulation-evoked release of [3H]noradrenaline and [3H]acetylcholine from different axon terminals was studied and the dissociation constants and potencies of different antagonists were estimated, in guinea-pig and rat. While oxotremorine reduced the release of [3H]acetylcholine and [3H]-noradrenaline in a concentration-dependent manner from different release sites (Auerbach plexus, noradrenergic neurons in the right atrium, cerebral cortex), McN-A 343, an M1 receptor agonist, enhanced their release evoked by field stimulation. When the inhibitory effect of oxotremorine on transmitter release was studied, pancuronium, pirenzepine and atropine were competitive antagonists of presynaptic muscarinic receptors located on the noradrenergic axon terminals of the atrium. While atropine and pirenzepine inhibited the muscarinic receptors of cholinergic axon terminals in the Auerbach plexus, pancuronium and gallamine had a very low affinity. Significant differences were found in the affinity constants of antagonists for muscarinic receptors located in the cholinergic axon terminals of Auerbach plexus and cerebral cortex, and noradrenergic axon terminals of the atrium. While atropine and pirenzepine exerted similar effects on these presynaptic sites, pancuronium, gallamine and (11-(2-[diethylamino)-methyl)-1-piperidinyl)acetyl)-5, 11-dihydro-6(1-pyrido(2,3-b)(1,4)-benzodiazepin-6-on) were much more effective on muscarinic receptors controlling acetylcholine release from the cerebral cortex and noradrenaline release from the heart. There was more than 100-fold (2.0 pA2 units) difference in affinities of these antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitters and intraocular pressure

The role of the ocular autonomic nervous system in IOP regulation has been well established. Pharmacological and autohistoradiographic studies confirmed the high density of beta 2 and alpha 2 receptors on ciliary processes and iris epithelium. Their respective pharmacological activation or blockade is discussed. The role of other ocular neurotransmitters is also complex, as shown by the paradoxical similar action of dopamine agonists and antagonists on IOP. Concerning the cholinergic system, ocular muscarinic receptors are pharmacologically not well documented. Numerous other neurotransmitters may modulate IOP without necessarily leading to the development of new drugs. Drugs of the future will probably concentrate on dopaminergic agonists, cAMP-stimulators such as forskolin, prostaglandins, and cannabinoids.

Subtypes of muscarinic receptors in vagal inhibitory pathway to the lower esophageal sphincter of the opossum

We assessed the characteristics of muscarinic neural transmission in the vagal inhibitory pathway to the lower esophageal sphincter (LES) of anesthetized opossums. LES relaxation was induced by electrical stimulation of the cervical vagus. Measurements were made of LES relaxation before and after intravenous administration of nicotinic (hexamethonium), serotonergic (5-Meo-DMT), nonselective muscarinic (atropine), and selective muscarinic (pirenzepine-M1 and 4-DAMP-M2) antagonists. The latency of LES relaxation was increased substantially by pirenzepine and atropine, increased slightly by hexamethonium, but was not affected by 4-DAMP or 5-Meo-DMT. Given as concurrent intravenous infusions, hexamethonium, 5-Meo-DMT and 4-DAMP added to pirenzepine or atropine did not significantly increase LES relaxation latency above that caused by pirenzepine or atropine alone. None of the antagonists alone had a significant effect on percent LES relaxation. The combination of pirenzepine or 4-DAMP with hexamethonium and 5-Meo-DMT did not affect percent LES relaxation. The combination of atropine with hexamethonium and 5-Meo-DMT reduced LES relaxation to 18%. The combination of pirenzepine and 4-DAMP with hexamethonium and 5-Meo-DMT, however, had no effect on percent LES relaxation. We conclude that muscarinic participation in vagally induced LES relaxation exhibits two functional receptor subtypes: (1) M1 receptors that determine LES relaxation latency and are antagonized by pirenzepine or atropine, and (2) non-M1, non-M2 receptors (Mx receptors) that contribute to the magnitude of LES relaxation and are antagonized by atropine, but not by pirenzepine or 4-DAMP.

Pancuronium and gallamine are antagonists for pre- and post-junctional muscarinic receptors in the guinea-pig lung

The effects of atropine, pancuronium and gallamine were tested on pre- and post-junctional muscarinic receptors in the lung. Inhibition of bronchoconstriction induced by intravenous injection of acetylcholine (ACh) was used as a measure of post-junctional receptor blockade. All three antagonists reduced ACh-induced bronchoconstriction. The effects were dose-related for atropine and pancuronium and complete inhibition was obtained with 0.01 mg/kg and 10 mg/kg respectively. Gallamine was much less potent than the other two drugs; the inhibitory effect was not dose-related and never exceeded 50% even at a dose of 10 mg/kg. In contrast, blockade of pre-junctional inhibitory muscarinic receptors in pulmonary parasympathetic nerves by these three antagonists, produced potentiation of bronchoconstriction induced by vagal-nerve stimulation. Consequently, the effect of the three antagonists on vagally-induced bronchoconstriction is dependent on the balance between their pre- and post-junctional blocking activity. Gallamine was the most effective and atropine the least effective antagonist for potentiating nerve-induced bronchoconstriction. At doses which produce 100% neuromuscular blockade, both pancuronium (0.04 mg/kg) and gallamine (4 mg/kg) potentiated vagally-induced bronchoconstriction. At these doses, pancuronium doubled and gallamine caused a four-fold increase in vagally-induced bronchoconstriction, despite partial concurrent blockade of muscarinic receptors in the smooth muscle of the airways.

Action of agonists and antagonists at muscarinic receptors present on ileum and atria in vitro

The action of 'selective' agonists and antagonists at muscarinic receptors mediating ileal contractions, and the rate and force of atrial contractions has been assessed. The effect of nicotinic receptor stimulation, catecholamine release and acetylcholinesterase (AChE) action on muscarinic activity has also been assessed. The nicotinic actions of carbachol did not affect its agonist potency nor the antagonist affinity data obtained when this agonist was used in atrial and ileal preparations. Antagonist data indicated that muscarinic receptors mediating the rate and force of atrial contractions did not differ. Differences in agonist potencies at these two muscarinic receptors were attributable to either differences in intrinsic efficacy or susceptibility to the action of acetylcholinesterase. The small differences in agonist potency observed between atrial and ileal muscarinic receptors were considered not sufficient to indicate receptor heterogeneity. The pirenzepine affinity data indicated that all three receptors are of the M2 type. Affinity data using secoverine and 4-diphenyl-acetoxy-N-methyl piperidine methiodide indicated that ileal and atrial muscarinic receptors differ. Data obtained using gallamine, pancuronium and stercuronium cannot be regarded as indicative of receptor affinity since the antagonism is not competitive; it did nonetheless corroborate the conclusion that ileal and atrial muscarinic receptors are different.

A search for selective antagonists at M2 muscarinic receptors

Isolated preparations of guinea-pig ileum and atria have been used to estimate the dose-ratios produced by antagonists at muscarinic receptors. Experiments with 4-diphenyl-acetoxy-N-methylpiperidine (4DAMP) metho-salts and with its isomer, 3DAMP methiodide, indicate that these are only slightly affected by the choice of physiological salt solution, the choice of agonist and the presence or absence of hexamethonium. Methyl or chloro groups in the p-position of the two benzene rings in 4DAMP metho-salts markedly reduce affinity and selectivity. When the two benzene rings are linked together, as in the fluorene-9-carboxylic ester, the affinity for the receptors in the atria is comparable with that of 4DAMP methobromide but that for the ileum is about half, so the selectivity is reduced. When the rings are linked as in the xanthene-9-carboxylic ester, the affinity for receptors in both tissues is greater than that of 4DAMP methobromide but there is less selectivity. When two molecules of 4DAMP are linked together by a polymethylene chain of from 4 to 12 carbon atoms the effects on affinity for muscarinic receptors in the guinea-pig ileum are different from those on affinity for muscarinic receptors in guinea-pig atria. The pentamethylene compound is the most selective: compared with 4DAMP methobromide it has slightly less affinity for receptors in the ileum but much less affinity for receptors in the atria. The effects of the compounds in antagonizing the actions of carbachol on atrial rate are not markedly different from their effects in antagonizing its actions on the force of the atrial contractions.

The relative potencies of some agonists at M2 muscarinic receptors in guinea-pig ileum, atria and bronchi

The effects of some agonists on isolated preparations of guinea-pig ileum, atria and bronchial muscle have been compared with those of carbachol. The concentrations producing comparable responses were used to estimate the equipotent molar ratio relative to carbachol. Arecaidine propargyl ester was 4 to 5 times as active as carbachol on the ileum but more than 10 times as active as carbachol on atrial rate or atrial force, so the results confirm that this compound has a 2 to 3 fold selectivity for receptors in atria. Ethoxyethyltrimethylammonium iodide was one-quarter to one-third as active as carbachol on ileum but only one-tenth as active as carbachol on atrial rate or atrial force and so shows a 3 to 4 fold selectivity for receptors in ileum. The other compounds tested, which included acetylcholine, methacholine, n-pentyltrimethyl-ammonium iodide and bethanechol showed less selectivity. There were no obvious differences between effects on atrial rate and effects on atrial force, though with esters it was often difficult to obtain effects on atrial rate in the absence of an inhibitor of cholinesterase. Activity on bronchial muscle was generally similar to activity on ileum.

Cholinergic systems and multiple cholinergic receptors in ocular tissues

Acetylcholine (ACh), choline acetyltransferases and cholinesterases occur in cornea, iris-ciliary body complex and retina of several vertebrates. In cornea, ACh may serve as a sensory transmitter as well as a local hormone, the function of which is not delineated. The function of ACh as the parasympathetic neurotransmitter at the iris and ciliary body is well established. The muscarinic receptors on the iris smooth muscle are similar to the muscarinic receptors (M2 type in two way classification) at other smooth muscles towards their interaction with agonists and antagonists. Binding studies using radiolabeled antagonists and their displacement by agonists indicate that muscarinic receptors in membranes of iris-ciliary body complex are heterogeneous indicating more than one subtype of muscarinic receptors. A subtype other than M2 receptors may occur at the presynaptic sites of parasympathetic nerves, which have yet to be investigated using specific agonists and antagonists. Cholinergic markers, choline acetyltransferase and acetylcholinesterase, differ quantitatively and qualitatively in retinas of different species. However, amacrine cells are cholinergic in all vertebrate species. Although they make up 1% of retinal neurons, they influence the activity of a majority of ganglion cells. Cholinergic effects in ganglia are mediated through nicotinic and muscarinic receptors. Both of these types of cholinergic receptors are heterogeneous. They have yet to be investigated for their subtypes using specific agonists and antagonists. Although the role of cholinergic retinal neurons in the processing of visual information is not known, their input to ganglion cells generally increases the rate of spontaneous activity or the number of action potentials in light-evoked responses. Thus, the cholinergic input seems to modify the overall neuronal input to the ganglion cells from the receptive fields. Endothelial cells of blood vessels contain muscarinic receptors, which are activated by ACh to cause relaxation. Although retinal blood vessels provide recognizable characteristic signs in diabetes mellitus and hypertensive disease, no information is available on the muscarinic receptors of these vessels.

Pharmacological evidence for cardiac muscarinic receptor subtypes

The chronotropic and inotropic effects of muscarinic receptor agonists (Acetylcholine, Arecoline, Carbachol, Furtrethonium) and antagonists (Atropine, N-methyl and N-butyl scopolammonium, pirenzepine) on isolated guinea-pig atria were studied. All had a greater affinity constants for muscarinic receptors as assessed in terms of inotropic effects than in terms of chronotropic effects. This difference, well correlated with the pharmacological effect, suggests the occurrence of cardiac muscarinic receptor subtypes, one mediating heart rate and the other contractile force. The ratio of chronotropic to inotropic potencies for each agent shows that the physiological mediator. Acetylcholine, differentiates best between the two subtypes, while atropine is the least discriminatory.

Cholinergic effects of cimetidine and ranitidine

Cimetidine and ranitidine were submitted to eight in vitro assays: their intrinsic activity was evaluated on guinea-pig auricles and ileum, acetylcholine synergism on guinea-pig tracheal muscle, frog rectus abdominis, guinea-pig vas and rat phrenic nerve diaphragm, and antagonism on guinea-pig auricles and rat jejunum estimated at a range of concentrations. Ranitidine alone opposed 1-hyoscyamine antagonism in guinea-pig ileum when acetylcholine was the agonist, but not when the agonist was bethanechol.

Modification by hexamethonium of the muscarinic receptors blocking activity of pancuronium and homatropine in isolated tissues of the guinea-pig

In the guinea-pig left atrium, hexamethonium (C6) (0.1-3 mM) caused a parallel rightward shift of concentration-response curves for the negative inotropic response to carbachol (CCh) and oxotremorine (Oxo), the dose ratios obtained with the latter agonist being significantly greater than those for CCh at all concentrations of C6 investigated. In the presence of C6, the muscarinic receptor blocking activity of homatropine (20 microM) or of pancuronium (0.3-2.7 microM) appears to be reduced but if the effect of hexamethonium on concentration-response curves to the agonists is taken into consideration, the dose ratios produced by the combination of C6 with either homatropine or pancuronium were essentially as predicted for the combination of 2 competitive antagonists. The difference in the affinity of pancuronium for cardiac muscarinic receptors and ileal muscarinic receptors was also reduced in the presence of hexamethonium (0.3 mM).

In vitro and in vivo activities of anticholinergics on the cholinoceptors of the cardiac pacemaker cells

The tachycardia-inducing effects of atropine, N-methylscopolammonium and N-butylscopolammonium were studied in the conscious dog, both in the presence and absence of sympathetic innervation. These effects were compared with the anticholinergic effects of the drugs on the muscarinic cholinoceptors of guinea-pig isolated atria. The order of potency for inhibition of the effects of acetylcholine on the isolated atria was N-methylscopolammonium greater than atropine greater than N-butylscopolammonium. This order was the same for the tachycardia-inducing potencies. Maximal tachycardia with these three drugs was less marked in the absence of sympathetic innervation than in its presence. However, the doses of atropine and of N-butylscopolammonium required to block the effects of vagal influence were greater. Sympathectomy had no effect on the vagolytic potency of N-methylscopolammonium. In the case of atropine and N-methyl-scopolammonium there was good correlation between anticholinergic and tachycardia-inducing potencies. The tachycardiac effects of these compounds can be explained by their anticholinergic effect. The tachycardia-inducing potency of N-butylscopolammonium was greater than its anticholinergic potency.

The affinity of atropine for muscarine receptors in human sphincter pupillae

Concentration-dependent contractions in response to carbachol were determined on isolated pieces of human iris. Atropine competivtively antagonized the effect of carbachol. An apparent dissociation equilibrium constant of 0.4--0.7 nM was estimated for the muscarine receptor-atropine complex.

Peripheral and central muscarinic receptor affinity of psychotropic drugs

The muscarinic receptor affinity of 27 psychotropic and 5 anticholinergic substances was examined in 2 in-vivo and 2 in-vitro models. A highly significant correlation was obtained between the effect of all compounds examined on the atropine sensitive binding of 3H-PrBCM and the effect in the conventional guinea-pig ileum preparation. Antagonism of oxotremorine induced tremors in mice by anticholinergics and neuroleptics was also significantly correlated to the corresponding data obtained in the in-vitro tests. Due to very low potency in the physostigmine induced mortality test in mice too few ED50 values were obtained to perform statistical comparisons. It is concluded, that the conventional guinea-pig ileum model and the 3h-prBCM binding model are equally predictive as tests for antimuscarinic properties. When in-vivo anticholinergic data for neuroleptics are used it must be considered that a possible dopamine receptor blockade may diminish the antimuscarinic effect of the substance.

A comparison of affinity constants for muscarine-sensitive acetylcholine receptors in guinea-pig atrial pacemaker cells at 29 degrees C and in ileum at 29 degrees C and 37 degrees C

1 The affinity of 17 compounds for muscarine-sensitive acetylcholine receptors in atrial pacemaker cells and ileum of the guinea-pig has been measured at 29 degrees C in Ringer-Locke solution. Measurements were also made at 37 degrees C with 7 of them. 2 Some of the compounds had much higher affinity for the receptors in the ileum than for those in the atria. For the most selective compound, 4-diphenylacetoxy-N-methylpiperidine methiodide, the difference was approximately 20-fold. The receptors in the atria are therefore different the structure from those in the ileum. 3 The effect of temperature on affinity are not the same for all the compounds, tested indicating different enthalpies and entropies of adsorption and accounting for some of the difficulty experienced in predicting the affinity of new compounds.

Factors determining the potency of mydriatic drugs in man

1 The mydriasis resulting from topical application of five atropine-like drugs was measured photographically in man. Drug potency was obtained from log dose-response curves. 2 The in vitro potency of eight cholinoceptor blocking drugs, including those studied in man, was obtained by measuring their affinity constants for binding to the receptors of an isolated preparation of the rabbit sphincter pupillae. Values agreed closely with those obtained for the muscarinic receptors of guinea-pig ileum. 3 In vitro and in vivo potency was compared to obtain a quantitative measure of the relative ease with which drugs gain access to the receptors after topical application. 4 The large differences that occur in the intensity and duration of the mydriatic response to atropine-like drugs is primarily the result of differences in their ability to blcok the receptors. Only with tropicamide does its relatively high accessibility affect its potency in man.

A new probe for heterogeneity in muscarinic receptors: 2-methyl-spiro-(1, 3-dioxolane-4, 3')-quinuclidine

The title compound (MSDQ) is a new muscarinic agonist related to 3-acetoxyquinuclidine (3-AcQ) but also of a highly rigid structure. In view of this, it may constitute a probe for the detection of heterogeneity among muscarinic receptors. Indeed, equipotent molar ratios (EPMR) for ACh, 3-AcQ and MSDQ were as follows: guinea pig ileum, 1 : 14 : 240; vasodepressor effect in the cat, 1 : 6 : 188. But EPMR for 3-AcQ and MSDQ as stimulants of the superior cervical ganglion in the cat were 1 : 1 and for the induction of tremors in mice, 9 : 5. No such subtle differences in receptor specificity were detected when the probe used was the 2, 2-diphenyl analogue (DiPSDQ) of MSDQ which was a powerful competitive antagonist in all systems, more potent than atropine, but with a CNS/PNS activity of 1.1 compared to 26 for atropine. In view of this, the use of potent antagonists as probes for muscarinic receptor heterogeneity is questionable.

Quantitative correlations between chemical structure and affinity for acetylcholine receptors

The affinity constants (log K, pA2) of 128 quaternary ammonium compounds belonging to several different series have been correlated linearly with the hydrophobicity (piR) constant, the dipole moment (muR), and the number of hydroxyl groups (nOH) of the side chain; the dependence on the hydrophobicity constant of the quaternary ammonium head (pi-N identical to) is shown to be parabolic. A correlation coefficient of 0.96 is obtained for all the compounds using only 4 independent variables (6 terms). Based on the quantitative correlation obtained, intermolecular forces involved in the drug-receptor interaction are discussed. Further molecular modifications to enhance the affinity to cholinergic receptors are suggested.

Are muscarinic receptors in the central and peripheral nervous system different?

The concentration-dependent binding of atropine-3 H to membrane fractions from bovine tracheal muscle, parotid gland and caudate nucleus, measured by equilibrium dialysis, revealed the presence of virtually identical high affinity binding sites in all three tissues. Sch 1000 and Sch 1178 geometrical isomers of N-isopropylatropine bromide with a large potency ratio as antimuscarinics, inhibited atropine binding identically in all three tissues. Differences in properties of muscarinic receptors in these tissues are either non-existent or too small to be detected by the applied techniques.

Conditions affecting the interaction of cholinergic agents on the longitudinal muscle of the guinea-pig ileum--unexpected effects with hexamethonium

Affinity constants were determined for atropine and N-methyl atropine, using both acetylbetamethylcholine and carbaminoylcholine as agonists, on plexus-containing and plexus-free preparations of the guinea-pig ileum. Hexamethonium (1 times 10−4M) decreased the apparent affinity constant of atropine on the plexus-free preparation when carbaminoylcholine but not acetylbetamethylcholine was the agonist. With the latter agonist both antiacetylcholine drugs were equiactive. Hexamethonium’s selective action was associated with a significant shift to the left of the log dose-response curve to carbaminoylcholine on the plexus-free preparation. While others have observed that hexamethonium may be atropine-like, samples of hexamethonium have been reported as being contaminated with a “depolarizing compound”, which may account for the results presented in this study. Nevertheless, the present results are consistent with an allosteric rather than a direct binding mechanism in the blocking action of atropine. The observation that the agonist, acetylbetamethylcholine, was not similarly affected by hexamethonium (or any possible contaminant) suggests that agents with primarily muscarinic activity may not be so affected. This study is another example emphasizing the care needed in interpreting data obtained with the aid of “pharmacological tools”.

A comparison of phenylalkyl- and phenoxyalkyl- trimethylammonium and triethylammonium salts; their apparent molal volumes at infinite dilution and effects on the frog rectus and guinea-pig ileum preparations

1. The effects of phenoxyalkyltrimethylammonium and triethylammonium bromides on the frog rectus preparation and on the isolated guinea-pig ileum in the presence of hexamethonium have been compared with those of analogous phenylalkyltrimethylammonium and triethylammonium bromides. Affinity constants have been measured when possible.2. The apparent molal volumes at infinite dilution of some of the compounds have been measured from estimates of density made with an Anton Paar precision density meter.3. An ether oxygen occupies at infinite dilution in water only about one-third of the volume occupied by a methylene group.4. The replacement of methylene by ether oxygen reduces nicotine-like activity and affinity for nicotine-sensitive receptors. The reduction in affinity may be partly due to the decrease in size.5. The replacement of methylene by ether oxygen reduces activity and affinity at muscarine-sensitive receptors but in some compounds there is a bigger reduction in affinity than would be expected simply from the reduction in size.6. It is suggested that the affinity of these compounds largely depends on hydrophobic bonding and that effects on water structure in the environment of the receptor may also be involved in the actions of agonists.

The central and peripheral activities of anti-acetylcholine drugs. Some concepts of practical relevance

Five procedures have been used to compare the pharmacological effects and time-activity profiles of anti-acetylcholine drugs in the central and peripheral nervous system (ens, pns). In particular, the potencies of optically pure enantiomers of anti-acetylcholine drugs which contain an asymmetric centre have been determined. The following four observations are made which are of relevance to all studies of anti-acetylcholine drugs. (1) Times to onset of activity of anti-acetylcholine drugs in three in vivo tests and the duration of action in one in vivo test are shown to increase as affinity constants, determined in vitro, increase. (2) Below a dose of ca 0.03 μmol kg−1no anti-acetylcholine drug produces maximum mydriatic effects and all anti-acetylcholine drugs which have log affinity constant values >9.49 produce maximal effects at approximately this dose. (3) The receptor with which anti-acetylcholine drugs interact is essentially the same in the eye, salivary gland and ens of the mouse, the guinea-pig ileum, and in the cat ens. (4) For any anti-acetylcholine drug the time to onset of effects in the ens is similar to the time of onset of effects in the pns and appears to depend on the affinity constant rather than on the partition properties of the drug. The practical and theoretical significance of these and other observations are discussed.