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

Muscarinic receptor agonists and antagonists: effects on ocular function

Muscarinic agonists act mainly via muscarinic M₃ cholinoceptors to cause contraction of the iris sphincter, ciliary muscle and trabecular meshwork as well as increase outflow facility of aqueous humour. In the iris dilator, the effect of muscarinic agonists is species dependent but is predominantly relaxation via muscarinic M₃ receptors. In the conjunctiva, muscarinic agonists stimulate goblet cell secretion which contributes to the protective tear film. Muscarinic M₂ and M₃ receptors appear mainly involved. In the lens muscarinic agonists act via muscarinic M₁ receptors to produce depolarization and increase [Ca(2+)](i). All five subtypes of muscarinic receptor are present in the retina. In the developing retina, acetylcholine appears to limit purinergic stimulation of retinal development and decrease cell proliferation. In the adult retina acetylcholine and other muscarinic agonists may have complex effects, for example, enhancing light-evoked neuronal firing in transient ON retinal ganglion cells and inhibiting firing in OFF retinal ganglion cells. In the lacrimal gland, muscarinic agonists activate M₃ receptors on secretory globular acinar cells to stimulate tear secretion and also cause contraction of myoepithelial cells. In Sjögren's syndrome, antibodies to the muscarinic M₃ receptor disrupt normal gland function leading to xerophthalmia although the mechanism of action of the antibody is still not clear. Atropine and pirenzepine are useful in limiting the development of myopia in children probably by an action on muscarinic receptors in the sclera, although many other muscarinic receptor antagonists are not effective.

3H-noradrenaline release from mouse iris?ciliary body: role of presynaptic muscarinic heteroreceptors

Sympathetic neurotransmitter release and its modulation by presynaptic muscarinic heteroreceptors were studied in mouse iris-ciliary bodies. Tissue preparations were preincubated with (3)H-noradrenaline and then superfused and stimulated electrically. Firstly, experimental conditions were defined, allowing study of presynaptic sympathetic inhibition in mouse iris-ciliary body. If tissue was stimulated four times with 36 pulses/3 Hz, tritium overflow peaks were reliably and reproducibly measured. As expected, these stimulation conditions led to marked alpha(2)-autoinhibition as indicated by the release-enhancing effect of the alpha(2)-antagonists phentolamine and rauwolscine. To ensure autoinhibition-free (3)H-noradrenaline release, which is optimal for studying presynaptic sympathetic inhibition, alpha(2)-receptors were blocked in all subsequent experiments. Under these conditions, evoked tritium overflow was almost completely abolished in the presence of the sodium channel blocker tetrodotoxin, indicating a neuronal origin of (3)H-noradrenaline release. Secondly, muscarinic inhibition of (3)H-noradrenaline release was characterized using the conditions described above (36 pulses/3 Hz; phentolamine 1 muM and rauwolscine 1 muM throughout). The muscarinic receptor agonist oxotremorine M decreased evoked tritium overflow in a concentration-dependent manner with an IC(50) of 0.33 muM and maximal inhibition of 51%. The concentration-response curve of oxotremorine M was shifted to the right by the muscarinic antagonists ipratropium and methoctramine, whereas pirenzepine was ineffective. The observed rank order of antagonist potencies, ipratropium > methoctramine > pirenzepine, which is typical for the M(2) subtype, indicates that presynaptic muscarinic receptors on sympathetic axons of mouse iris-ciliary bodies are predominantly M(2). Finally, inhibition of (3)H-noradrenaline release by endogenously secreted acetylcholine was investigated. Longer pulse trains, 120 pulses/3 Hz and 600 pulses/5 Hz, were used and the cholinesterase inhibitor physostigmine was added to the superfusion medium to increase synaptic levels of endogenous acetylcholine. Under these conditions, ipratropium approximately doubled the evoked overflow of tritium, indicating that endogenously released acetylcholine can activate presynaptic muscarinic heteroreceptors. In conclusion, the present experiments establish measurement of the electrically induced release of (3)H-noradrenaline from mouse iris-ciliary bodies. As in other species, noradrenaline release in this preparation was subject to presynaptic muscarinic inhibition. Our results also indicate that the presynaptic muscarinic receptors on sympathetic axons in mouse iris-ciliary body are predominantly M(2). Moreover, these receptors can be activated by both exogenous agonists and endogenously released acetylcholine and, hence, may operate physiologically in the interplay between the parasympathetic and sympathetic nervous system.

Innervation of rat iris by trigeminal and ciliary neurons expressing pChAT, a novel splice variant of choline acetyltransferase

We have recently discovered a splice variant of choline acetyltransferase (ChAT) mRNA and designated the variant protein pChAT because of its preferential expression in peripheral neuronal structures. In this study, the presence of pChAT in rat iris was examined by immunohistochemistry and Western blot using a pChAT antiserum, in combination with RT-PCR analysis and ChAT enzyme assay. For comparison, the conventional ChAT (cChAT) was studied in parallel. By pChAT immunohistochemistry, intense labeling was found to occur in nerve fibers of the iris and in neurons of the ciliary and trigeminal ganglia. Denervation studies, analyzed by semiquantitative morphometry, indicated that these iridial pChAT fibers originated about half from the ciliary ganglion and the other half from the trigeminal ganglion. The presence of pChAT protein in the iris and trigeminal ganglion was confirmed by Western blot. The expression of pChAT mRNA in the ciliary and trigeminal ganglia was proved by RT-PCR. Although cChAT protein and mRNA were detected in the ciliary ganglion, neither was detectable in the trigeminal ganglion. The contributions of the ciliary and trigeminal ganglia to the iridial ChAT enzyme activity were verified by the present ChAT assay. Here, we provide evidence that iridial pChAT nerves are composed of postganglionic parasympathetic efferents from the ciliary ganglion and, more interestingly, somatic sensory afferents of the trigeminal ophthalmic nerve.

Anticholinergic effects of desloratadine, the major metabolite of loratadine, in rabbit and guinea-pig iris smooth muscle

Allergic conjunctivitis is the most common ocular allergic disease. Although very symptomatic it does not endanger vision, and topical antihistamines or chromones are the first choice treatment in clinical practice. Recently, equivalent nanomolar affinities for histamine H and muscarinic M 1 and M3 cloned human receptors have been reported for desloratadine, the active metabolite of loratadine, a widely prescribed antihistamine. This property might enhance its utility in the treatment of asthma, but could induce adverse anticholinergic effects after topical administration. In the present study, we compare the anticholinergic activity of desloratadine with other known muscarinic antagonists and antihistamines on rabbit and guinea-pig iris smooth muscle. Desloratadine was found to be a competitive antagonist (pA2 = 6.67+/-0.09) of carbachol-induced contractions in isolated rabbit iris smooth muscle. Atropine (pA2 = 9.44+/-0.02) and NPC-14695 (pA2 = 9.18+/-0.03) also behaved as competitive antagonists, whereas tiotropium bromide (pD'2 = 9.06+/-0.02) exhibited a non-competitive behaviour in this tissue. Carebastine (pA2 = 5.64+/-0.04) and fexofenadine (pA2 < 4.0) were also studied. After topical administration on the guinea-pig eye conjunctiva, desloratadine produced a potent (ED50 = 2.3 mg/ml) and long lasting mydriasis (> 120 min at the ED50) in conscious animals. Fexofenadine and carebastine were inactive even at the highest concentration tested (10 mg/ml). Atropine (ED50 = 30 microg/ml) and tiotropium bromide (ED50 = 10 microg/ml) were much more potent than desloratadine or pirenzepine (ED50 = 3 mg/ml) in this model. The competitive muscarinic antagonism of desloratadine in vitro, and its potency and duration of action in vivo, suggest that topical treatment of allergic conjunctivitis and rhinitis with desloratadine could produce undesirable peripheral anticholinergic side effects such as mydriasis and xerostomia.

Synthesis and antimuscarinic activity of a series of 4-(1-Imidazolyl)-2,2-diphenylbutyramides: discovery of potent and subtype-selective antimuscarinic agents

In a study directed toward the development of new, selective agents with potential utility in the treatment of altered smooth muscle contractility and tone, for example, as seen in urinary incontinence associated with bladder muscle instability, a series of 4-(1-imidazolyl)-2,2-diphenylbutyramide derivatives was prepared. These compounds were examined for M1, M2, and M3 muscarinic receptor subtype selectivity in isolated tissue assays. The compounds that showed potency and/or selectivity in these tests were further evaluated for in vivo anticholinergic effects on various organs and tissues, including urinary bladder, salivary gland, and eye in rats. The structure activity relationships for the series of 4-(1-imidazolyl)-2,2-diphenylbutyramide derivatives are also discussed. This study led to the identification of 4-(2-methyl-1-imidazolyl)-2,2-diphenylbutyramide (KRP-197) as a candidate drug for the treatment of urinary bladder dysfunction.

Comparative pharmacology of recombinant human M3 and M5 muscarinic receptors expressed in CHO-K1 cells

1. Affinity estimates were obtained for several muscarinic antagonists against carbachol-stimulated [3H]-inositol phosphates accumulation in Chinese hamster ovary (CHO-KI) cells stably expressing either human muscarinic M3 or M5 receptor subtypes. The rationale for these studies was to generate a functional antagonist affinity profile for the M5 receptor subtype and compare this with that of the M3 receptor, in order to identify compounds which discriminate between these two subtypes. 2. The rank order of antagonist apparent affinities (pK(B)) at the muscarinic M5 receptor was atropine (8.7) > or =tolterodine (8.6) = 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 8.6)> darifenacin (7.7) > or =zamifenacin (7.6)>oxybutynin (6.6)= para-fluorohexahydrosiladifenidol (p-F-HHSiD, 6.6)>pirenzepine (6.4) > or = methoctramine (6.3)=himbacine (6.3)>AQ-RA 741 (6.1). 3. Antagonist apparent affinities for both receptor subtypes compare well with published binding affinity estimates. No antagonist displayed greater selectivity for the muscarinic M5 subtype over the M3 subtype, but himbacine, AQ-RA 741, p-F-HHSiD, darifenacin and oxybutynin displayed between 9- and 60 fold greater selectivity for the muscarinic M3 over the M5 subtype. 4. This study highlights the similarity in pharmacological profiles of M3 and M5 receptor subtypes and identifies five antagonists that may represent useful tools for discriminating between these two subtypes. Collectively, these data show that in the absence of a high affinity M5 selective antagonist, affinity data for a large range of antagonists is critical to define operationally the M5 receptor subtype.

Pharmacological characterization of muscarinic receptors in rabbit isolated iris sphincter muscle and urinary bladder smooth muscle

1. The pharmacological characteristics of muscarinic receptors in the rabbit iris sphincter muscle were studied and compared to M3 receptors in rabbit urinary bladder smooth muscle. 2. (+/-)-Cis-dioxolane induced concentration-dependent contractions of the iris sphincter muscle (pEC50 = 6.41+/-0.10, Emax = 181+/-17 mg, n = 38) and urinary bladder smooth muscle (pEC50 = 6.97+/-0.04, Emax = 4.28+/-0.25 g, n = 54). These contractions were competitively antagonized by a range of muscarinic receptor antagonists (pK(B) values are given for the iris sphincter muscle and the bladder smooth muscle, respectively): atropine (9.30+/-0.07 and 9.40+/-0.04), AQ-RA 741 (6.35+/-0.04 and 6.88+/-0.03), darifenacin (9.56+/-0.05 and 9.12+/-0.05), methoctramine (5.75+/-0.07 and 5.81+/-0.06), oxybutynin (8.10+/-0.09 and 8.59+/-0.06), pirenzepine (6.79+/-0.05 and 6.89+/-0.04), secoverine (7.54+/-0.05 and 7.66+/-0.05), p-F-HHSiD (7.55+/-0.09 and 7.50+/-0.05) and zamifenacin (8.69+/-0.10 and 8.36+/-0.06). A significant correlation between the pK(B) values in the bladder and the pK(B) values in the iris was obtained. 3. In both tissues, the pK(B) values correlated most favorably with pKi values for these compounds at human recombinant muscarinic m3 receptors. A reasonable correlation was also noted at human recombinant muscarinic m5 receptors given the poor discriminative ability of ligands between m3 and m5 receptors. 4. Overall, the data from this study suggest that the muscarinic receptors mediating contraction of the rabbit iris sphincter muscle and urinary bladder smooth muscle are similar and equate most closely with the pharmacologically-defined muscarinic M3 receptor.

Characterization of muscarinic receptors mediating relaxation and contraction in the rat iris dilator muscle

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.

Effects of human recombinant interleukins on stimulation-evoked noradrenaline overflow from the rat perfused spleen

Experiments were carried out in the isolated spleen of the rat to study in a lymphoid organ the influence of interleukins (ILs) on noradrenaline release. Spleens were perfused with Tyrode's solution and the overflow of endogenous noradrenaline was determined by HPLC with electrochemical detection. Perivascular electrical stimulation (4 or 10 Hz, 20-28 mA, 2 min) caused an increase in noradrenaline overflow and in perfusion pressure, both of which were markedly reduced by perfusion with Ca(2+)-free solution, abolished by tetrodotoxin, unaffected by hexamethonium, and subject to alpha 2-adrenoceptor- and muscarinic receptor-mediated modulation as shown by the effects of rauwolscine and methacholine. Human recombinant IL-1 beta and IL-2 and mouse recombinant IL-2 10 ng/ml failed to affect the evoked overflow of noradrenaline after an exposure time of 15 min. In contrast, human recombinant IL-1 beta and IL-2 0.1 ng/ml reduced the evoked overflow after exposure for 80 min; the inhibition tended to increase 30 min later despite washout. Murine recombinant IL-2 1.2 ng/ml caused no change after contact with the tissue for 80 min but there was an inhibition 30 min later after washout. Human recombinant IL-6 (0.1 ng/ml) caused no significant change. The inhibitory effect of low concentrations of IL-1 beta and IL-2 supports the idea that locally produced mediators of the immune system may affect neuronal function.

Characterization of the muscarine receptor subtype on sympathetic nerve endings in the rat caudal artery

The prejunctional muscarine receptor on sympathetic nerves in the rat caudal artery was characterized using several selective antagonists. The inhibitory response of carbachol on vasoconstriction elicited by sympathetic nerve stimulation was antagonized by benzhexol (trihexyphenidyl; pKB 7.1), heptane-1,7-bis(dimethyl-3'-phthalimidopropyl ammonium bromide) (C7/3-phth; pKB 6.5) and hexahydrosiladifenidol (HHSiD; apparent pKB 6.0). These pKB values suggest that the receptor most closely resembles the muscarine M2 receptor subtype rather than the muscarine M1, M3 or M4 receptor subtypes.

Characterization of the prejunctional muscarinic receptors mediating inhibition of evoked release of endogenous noradrenaline in rabbit isolated vas deferens

The aim of the present study was to characterize the prejunctional modulation of evoked release of endogenous noradrenaline in rabbit vas deferens by the use of muscarinic receptor agonists and subtype-preferring antagonists. Vasa deferentia of the rabbit were stimulated electrically by trains of 120 pulses delivered at 4 Hz or trains of 30 pulses at 1 Hz. The inhibition by muscarinic agonists of the stimulation-evoked overflow of endogenous noradrenaline in the absence and presence of antagonists was used to determine affinity constants for antagonists. These values were compared with those observed at putative M1 receptors inhibiting neurogenic twitch contractions in the rabbit vas deferens and with affinity data obtained at M1(m1)-M4(m5) receptors in functional studies and binding experiments. The evoked overflow of noradrenaline from sympathetic nerves was enhanced by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) and indomethacin, indicating a tonic inhibition by endogenous A1 and P2 purinoceptor agonists and prostanoids, respectively. The stimulation-evoked overflow at 4 Hz was not sensitive to inhibition by the muscarinic agonists methacholine or 4-(4-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium iodide (4-Cl-McN-A-343). In contrast, at a stimulation frequency of 1 Hz the evoked noradrenaline release was decreased by muscarinic agonists (EC50): arecaidine propargyl ester (0.062 microM), 4-Cl-McN-A-343 (0.32 microM), 4-(4-fluorophenylcarbamoyloxy)-2-butynyl-N-methyl-pyrrolidinium tosylate (4-F-PyMcN+; 0.48 microM) and methacholine (0.86 microM). The affinity constants of most of the muscarinic antagonists [atropine: pKB = 9.47; (R)-trihexyphenidyl: pKB = 9.18; pirenzepine: pA2 = 7.68; methoctramine: pKB = 6.90] are consistent with estimates of these antagonists at M1(m1) receptors determined in various functional and binding studies. The high antagonistic potency of pirenzepine and (R)-trihexyphenidyl and the agonistic activity of 4-F-PyMcN+ argue for the involvement of M1, and against that of M2 and M3 receptors in the inhibition of evoked noradrenaline overflow. However, the high apparent pKB of 8.30 for himbacine is not in accordance with an M1 receptor; by contrast, it would be compatible with the presence of M2 or M4 receptors. The potencies of the tested muscarinic agonists and antagonists largely agree with those obtained for the inhibition of neurogenic twitch responses (0.05 Hz) in the rabbit vas deferens.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

A muscarinic receptor different from the M1, M2, M3 and M4 subtypes mediates the contraction of the rabbit iris sphincter

In order to analyse the subtype of muscarinic receptors involved in the methacholine-induced contraction of the rabbit iris sphincter we have determined equilibrium dissociation constants (KB) of various antagonists in the sphincter muscle. The values were compared with those observed at M1 (rabbit vas deferens), M2 (heteroreceptors in rat iris) and M3 receptors (guinea-pig ileum), or at the muscarinic receptors in the guinea-pig uterus. The methacholine-induced contraction of the uterus from immature guinea-pigs was competitively antagonized by pirenzepine (6.64, -log KB), 4-DAMP (8.39), hexahydrodifenidol (HHD; 7.00 for the (R)- and 5.40 for the (S)-enantiomer), p-fluoro-hexahydrosiladifenidol (p-F-HHSiD; 6.25) and valethamate bromide (8.04). The affinity of the antagonists is consistent with the presence of an M2 receptor. The -log KB values of the antagonists in the rabbit iris sphincter (6.43, p-F-HHSiD; 6.22, AQ-RA 741; 7.23 and 5.34, (R)- and (S)-trihexyphenidyl) were lower than, or within the lowest range of, estimates in the other experimental models, irrespective of the subtype selectivity of the antagonist. This excludes the presence of an M1, M2, M3 or M4 receptor in this smooth muscle. The affinity of UH-AH 37 in the iris was intermediate between that for M1 or M3, and M2 receptors. The low affinity of AQ-RA 741 and the low enantiomeric ratio of trihexyphenidyl (THP) in the iris (77.6) would be compatible with a presumed M5 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic muscarinic receptor subtypes involved in the inhibition of acetylcholine and noradrenaline release in bovine cerebral arteries

Experiments were performed in bovine cerebral arteries preincubated with [3H]-choline or [3H]-noradrenaline to analyze the presynaptic muscarinic receptors involved in inhibition of acetylcholine and noradrenaline release induced by electrical stimulation (4 Hz, 200 mA, 0.3 ms, 1 min). For this purpose, the actions of several muscarinic receptor antagonists on the 3H overflow and on the carbachol-induced inhibition of this overflow were assessed. The evoked [3H]-acetylcholine release and [3H]-noradrenaline release were markedly reduced by the presence of tetrodotoxin, Ca(2+)-free medium, and the inhibitor of both choline transport and choline acetyltransferase, AF64A. Chemical sympathetic denervation with 6-hydroxydopamine (6-OHDA) decreased the uptake of [3H]-noradrenaline, and AF64A reduced mainly the uptake of [3H]-choline, but also of [3H]-noradrenaline. Carbachol reduced the evoked [3H]-noradrenaline and [3H]-acetylcholine release; the IC50 values were 0.37 and 0.43 mumol/l, respectively. Atropine and 4-DAMP, but not AF-DX 116, methoctramine or pirenzepine, increased the evoked [3H]-acetylcholine release. However, these muscarinic antagonists failed to modify the evoked [3H]-noradrenaline release. Carbachol inhibited the release of both acetylcholine and noradrenaline. The inhibition was blocked by the antagonists. The rank orders of potency (based on plC50 values) were, in the case of [3H]-acetylcholine release, atropine greater than 4-DAMP greater than AF-DX 116 greater than or equal to pirenzepine greater than or equal to methoctramine, and, in the case of [3H]-noradrenaline release, atropine greater than 4-DAMP greater than AF-DX 116 greater than or equal to methoctramine greater than or equal to pirenzepine.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic M2-muscarinic receptors on noradrenergic nerve endings and endothelium-derived M3 receptors in cat cerebral arteries

The muscarinic (M) receptors involved in the vasodilation elicited by acetylcholine (ACh) and in the carbachol inhibition in electrically induced [3H]noradrenaline (NA) release in cat cerebral arteries was investigated. For this, atropine, pirenzepine, AF-DX 116, 4-DAMP, non-specific, M1, M2 and M3 receptor antagonists, respectively, were used. ACh elicited concentration-dependent relaxations up to 10(-6) M which were attenuated by these antagonists; the order of potency (pA2 values) to inhibit the ACh-induced relaxation was: atropine (10.1) 4-DAMP (8.9) greater than pirenzepine (7.6) greater than AF-DX 116 (5.9). The electrical stimulation (200 mA, 0.3 ms, 2 Hz, during 1 min) of these arteries preincubated with [3H]NA caused tritium release which was inhibited by carbachol (10(-6) M). The 4 antagonists attenuated the action of the M agonist; the order of potency (pIC50 values) was: atropine (8.7) greater than 4-DAMP (8.1) greater than AF-DX 116 (7.9) greater than pirenzepine (5.8). The action of McN-A-343, a putative M1 agonist, was also investigated. This agent produced small vasodilator responses and elevated concentrations (5 x 10(-5) M) inhibited the stimulated NA release, which was partially antagonized by atropine (10(-7) M) and pirenzepine (10(-8) and 10(-7) M). These results suggest the existence of M3 and M2 receptors mediating the relaxation induced by ACh and the NA release inhibition evoked by carbachol, respectively.

Direct effects of phosphamidon on isolated working rat heart electrical and mechanical function

Phosphamidon (2-chloro-3-(diethylamino)-1-methyl-3-oxopropanyldimethyl phosphate) is widely used in agriculture as an organophosphate insecticide. It is a water-soluble cholinesterase inhibitor whose estimated rat LD50 is 9.2 mg/kg, ip. Mechanical and electrophysiologic direct effects of phosphamidon were studied in an isolated working rat heart model. Phosphamidon caused a positive inotropic effect, as indicated by increased maximum time derivative of left ventricular pressure and increased cardiac output. Stroke work and total pressure-volume area increased in a dose-dependent manner following perfusion with phosphamidon. Phosphamidon had in this preparation a biphasic effect on heart rate: at 10(-6) M concentration, heart rate increased, but at higher concentrations (10(-4)-10(-3) M) heart rate decreased significantly. High concentrations of phosphamidon caused a significant prolongation of the Q-T interval. We conclude that phosphamidon has potent cardiotoxic effects, both mechanical and electrophysiologic, on the isolated rat heart.

M3-muscarinic receptor mediates prejunctional inhibition of noradrenaline release and the relaxation in cat femoral artery

The aim of the present study was to analyse the muscarinic receptors involved in the vasodilation elicited by acetylcholine (ACh) and the carbachol inhibition of electrically-evoked [3H]noradrenaline (NA) release in cat femoral artery. For this purpose, the following receptor antagonists were used, atropine, pirenzepine (M1-antagonist), AF-DX 116 (M2-antagonist) and 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP; M3-antagonist). The order of potency (pA2 values) of these drugs at postjunctional level was: atropine (9.7) greater than or equal to 4-DAMP (9.6) greater than pirenzepine (7.2) greater than AF-DX 116 (6.0), and at prejunctional level (pIC50 values) was: 4-DAMP (9.3) greater than atropine (8.5) greater than AF-DX 116 (7.1) greater than pirenzepine (5.9). These findings indicate that the muscarinic receptors mediating the vasodilation induced by ACh and the carbachol inhibition of NA release are of the M3-subtype.

Selective inhibition of responses to carbachol and McN-A-343 in the rabbit vas deferens

1. The effect of several selective muscarine receptor antagonists were evaluated on the responses of carbachol (CCh) and McN-A-343 (McN) during sympathetic nerve stimulation in the rabbit vas deferens. 2. The muscarine M1 receptor antagonist pirenzepine exhibited similar apparent pKB values for antagonism of the prejunctional inhibitory response of either CCh (pKB, 8.2) or McN (pKB, 8.5) on sympathetic nerve stimulation. 3. The muscarine M2 receptor antagonists, pancuronium and the bisalkyl ammonium compound 'C7/3-phth' were selective inhibitors of the postjunctional facilitatory response produced by CCh on sympathetic nerve stimulation. They were also 17- and three-fold, respectively, less potent against the inhibitory responses of McN on sympathetic nerve stimulation. The apparent pKB value of pancuronium on the inhibitory response produced by CCh did not differ significantly (P greater than 0.05) from that using McN. A similar finding was made for C7/3-phth. 4. Selective blockade of the inhibitory response to CCh with pirenzepine (0.03 or 0.5 mumol/L) did not significantly (P greater than 0.05) modify the apparent pKB value for pancuronium on the facilitatory response of CCh. 5. Selective blockade of the facilitatory response to CCh with a low concentration of pancuronium (0.5 mumol/L) did not significantly (P greater than 0.05) modify the apparent pKB value for pancuronium (30 mumol/L) on the inhibitory response of CCh. 6. It is suggested that CCh and McN activate the same prejunctional M1 muscarine receptor and that pancuronium is the most selective of the muscarine M2 receptor antagonists presently tested in this preparation for distinguishing between muscarine M1 and M2 receptors.

Putative M2 muscarinic receptors of rat heart have high affinity for organophosphorus anticholinesterases

The M2 subtype of muscarinic receptor is predominant in heart, and such receptors were reported to be located in muscles as well as in presynaptic cholinergic and adrenergic nerve terminals. Muscarinic receptors of rat heart were identified by the high affinity binding of the agonist (+)-[3H]cis-methyldioxolane ([3H]CD), which has been used to label a high affinity population of M2 receptors. A single population of sites (KD 2.74 nM; Bmax of 82 fmol/mg protein) was detected and [3H]CD binding was sensitive to the M2 antagonist himbacine but much less so to pirenzepine, the M1 antagonist. These cardiac receptors had different sensitivities to NiCl2 and N-ethylmaleimide from brain muscarinic receptors, that were also labeled with [3H]CD and considered to be of the M2 subtype. Up to 70% of the [3H]CD-labeled cardiac receptors had high affinities for several organophosphate (OP) anticholinesterases. [3H]CD binding was inhibited by the nerve agents soman, VX, sarin, and tabun, with K0.5 values of 0.8, 2, 20, and 50 nM, respectively. It was also inhibited by echothiophate and paraoxon with K0.5 values of 100 and 300 nM, respectively. The apparent competitive nature of inhibition of [3H]CD binding by both sarin and paraoxon suggests that the OPs bind to the acetylcholine binding site of the muscarinic receptor. Other OP insecticides had lower potencies, inhibiting less than 50% of 5 nM [3H]CD binding by 1 microM of EPN, coumaphos, dioxathion, dichlorvos, or chlorpyriphos. There was poor correlation between the potencies of the OPs in reversibly inhibiting [3H]CD binding, and their anticholinesterase activities and toxicities. Acetylcholinesterases are the primary targets for these OP compounds because of the irreversible nature of their inhibition, which results in building of acetylcholine concentrations that activate muscarinic and nicotinic receptors and desensitize them, thereby inhibiting respiration. Nevertheless, the high affinities that cardiac muscarinic receptors have for these toxicants point to their extra vulnerability. It is suggested that the success of iv administration of the muscarinic receptor inhibitor atropine in initial therapy of poisoning by OP anticholinesterases may be related in part to the extra sensitivity of M2 receptors to certain OPs.

Muscarine receptor types mediating autoinhibition of acetylcholine release and sphincter contraction in the guinea-pig iris

The potencies of several muscarine receptor antagonists in blocking either the autoinhibition of acetylcholine release or the muscarinic contraction of the sphincter muscle upon acetylcholine release were investigated in the guinea-pig iris. The agonist at pre- or postjunctional muscarine receptors was acetylcholine released upon field stimulation (5.5 Hz, 2 min) of the irides preloaded with 14C-choline. The stimulation-evoked 14C-overflow was doubled in the presence of atropine 0.1 mumol/l but unaffected by the agonist (+/-)-methacholine (50 mumol/l). Thus, under the present stimulation conditions, the autoinhibition of acetylcholine release on the guinea-pig iris cholinergic nerves was nearly maximally activated. Isotonic contractions of the irides upon field stimulation consisted of a rapid, atropine (0.1 mumol/l)-sensitive peak phase followed by a sustained contraction which involved a cholinergic and a non-cholinergic stimulation of the sphincter muscle. The M2-selective antagonists methoctramine (10 mumol/l) and gallamine (100 mumol/l) increased both the 14C-overflow and the peak contractions evoked by field stimulation. In contrast, the M3-selective antagonist hexahydrosiladifenidol (0.1-10 mumol/l) failed to affect the evoked 14C-release but concentration-dependently (1-10 mumol/l) reduced the iris contractions. Pirenzepine (10 mumol/l) enhanced the evoked 14C-overflow and inhibited the peak contractions (0.1-10 mumol/l; maximal effect at 10 mumol/l). The low potency of the antagonist at both receptor sites indicates that an M1 muscarine receptor is not involved. The results are consistent with the idea of M2 muscarine receptors mediating autoinhibition of acetylcholine release in the guinea-pig iris and M3-like receptors inducing the contraction of the sphincter muscle.

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.