Characterization of muscarinic receptors in salivary and lacrimal glands of the rat

Using Ipratropium Bromide Nasal Spray Response as a Screening Tool in the Diagnostic Workup of Cerebrospinal Fluid Rhinorrhea

OBJECTIVES

Unilateral clear thin rhinorrhea (UCTR) can be concerning for a nasal cerebrospinal fluid (CSF) leak. Beta-2 transferrin electrophoresis has been the gold standard for initial non-invasive confirmatory testing for CSF rhinorrhea, but there can be issues with fluid collection and testing errors. Ipratropium bromide nasal spray (IBNS) is highly effective at reducing rhinitis-related rhinorrhea, and should presumably not resolve CSF rhinorrhea. This study assessed whether different clinical features and IBNS response helped predict presence or absence of CSF rhinorrhea.

METHODS

A prospective cohort study was conducted where all patients with UCTR had nasal fluid tested for beta-2 transferrin, and were prescribed 0.06% IBNS. Patients were diagnosed with CSF rhinorrhea or other rhinologic conditions. Clinical variables like IBNS response (rhinorrhea reduction), positional worsening, salty taste, postoperative state, female gender, and body-mass index were assessed for their ability to predict CSF rhinorrhea. Sensitivity, specificity, and predictive values and odds ratios were calculated for all clinical variables.

RESULTS

Twenty patients had CSF rhinorrhea, and 53 had non-CSF etiologies. Amongst clinical variables assessed for predicting CSF absence or presence, significant associations were shown for IBNS response (OR = 844.66, p = 0.001), positional rhinorrhea worsening (OR = 8.22, p = 0.049), and body-mass index ≥30 (OR = 2.92, p = 0.048). IBNS response demonstrated 96% sensitivity and 100% specificity, and 100% positive and 91% negative predictive values for predicting CSF rhinorrhea.

CONCLUSIONS

In patients with UCTR, 0.06% IBNS response is an excellent screening tool for excluding CSF rhinorrhea, and should be considered in the diagnostic workup of CSF rhinorrhea.

LEVEL OF EVIDENCE

2 Laryngoscope, 134:56-61, 2024.

Treatment of Clozapine-Associated Sialorrhea: The Role of Benzamide Derivatives

PURPOSE

Hypersalivation is one of the most prevalent and distressing adverse effects associated with clozapine treatment. Currently, there is no standard therapeutic approach toward how to overcome it. Clinicians use various medications for managing this adverse effect. However, some of the agents are not effective enough, whereas others can induce other adverse effects. Recently, several reviews have been published on the treatment of clozapine-associated hypersalivation, in which the focus was on drugs from various pharmacological groups, and little attention was paid to drugs from the group of substituted benzamides. The intention of this brief narrative review is to draw the attention of clinicians to the use of the benzamide group for the treatment of this unpleasant adverse effect.

METHODS

A MEDLINE search was conducted to identify published treatment studies and case reports in the literature from 2000 to September 2021, concerning a treatment of clozapine-associated hypersalivation, mainly substituted benzamides.

RESULTS

Accumulating evidence during the last 2 decades indicates that agents derived from the benzamide group may be effective and safe agents for treatment of clozapine-associated hypersalivation. Whether with a psychotropic effect or without, medications from this group may produce a beneficial response.

CONCLUSIONS

Substitute benzamide derivatives have emerged as effective and well-tolerated agents for treatment clozapine-associated hypersalivation.

Double-Blind, Randomized, Placebo-Controlled Trial of Metoclopramide for Hypersalivation Associated With Clozapine

Hypersalivation is a frequent, disturbing, and uncomfortable adverse effect of clozapine therapy that frequently leads to noncompliance. The aim of this study was to examine the efficacy of metoclopramide (dopamine D2 antagonist, antiemetic medication) as an option for management of hypersalivation associated with clozapine (HAC). A 3-week, double-blind, placebo-controlled trial was conducted in university-based research clinics from January 2012 to May 2014, on 58 inpatients treated with clozapine who were experiencing hypersalivation. The subjects were randomly divided into placebo and metoclopramide groups. The starting dose was 10 mg/d. Participants who did not respond were up-titrated 10 mg/d weekly to a total of 30 mg/d during the third week. The number of placebo capsules was increased accordingly up to 3 capsules per day. Primary outcome was the change from baseline to the end of study in the severity of hypersalivation as measured with the Nocturnal Hypersalivation Rating Scale and the Drooling Severity Scale. Secondary outcomes included Clinical Global Impression of Improvement scale and adverse effect scales. Significant improvement on the Nocturnal Hypersalivation Rating Scale was demonstrated in the metoclopramide group from the end of the second week (P < 0.004), and on the Drooling Severity Scale (P < 0.02) in the third week. Clinical Global Impression-Improvement scale scores revealed major improvement. Twenty subjects (66.7%) treated with metoclopramide reported significant decline or total disappearance of HAC in comparison to 8 patients (28.6%) who received placebo (P = 0.031). No adverse effects to metoclopramide were reported. Metoclopramide was found to be safe and effective for the treatment of HAC.

Expression of muscarinic receptor subtypes in salivary glands of rats, sheep and man

In rat parotid, submandibular and sublingual glands and in ovine parotid and in human labial glands, the expression of muscarinic receptor subtypes was examined by immunoblotting and immunohistochemistry. Functional correlates were searched for in rat salivary glands. In the rat submandibular and sublingual glandular tissues clear signals of muscarinic M1 and M5 receptors could be detected in the immunoblotting and vague bands for muscarinic M3 and, in particular for, M4 receptors. The rat parotid gland differed. In this gland, the signal was less obvious for the muscarinic M1 receptor, and further, muscarinic M4 receptors appeared more strongly marked than in the submandibular glands. The results from the immunohistochemistry could be interpreted as the muscarinic M4 receptors are located on nerve fibres, since the outer layer of lobuli were densely stained. Intraglandular vessels in the rat submandibular and parotid glands showed expression of M3 receptors. In contrast to the parotid gland, the submandibular vessels also expressed M1 and M2 receptors. Occasionally M5 receptors appeared in the arteries and veins also. The functional studies in the rat confirmed muscarinic M1 receptor mediated secretion in the submandibular gland. Since the M1 receptor blockade did not affect submandibular blood flow, indirect vascular effects could not in total explain the secretory inhibition. Also in the human labial glands, muscarinic M1, M3 and M5 receptors occurred. No or low amounts of muscarinic M2 and M4 receptors could be detected. In patients with Sjögren-like symptoms an up-regulation of M3, M4 and M5 receptors was apparent in the labial glands. In ovine parotid glands all receptors could be detected, but constantly with vague bands for muscarinic M2 receptors. In conclusion, muscarinic M1 receptors seem to be expressed in seromucous/mucous glands. A secretory effect by muscarinic M5 receptors is not to be excluded, since they were expressed in all the glands examined. However, other functions, such as promotion of inflammation, cell growth and proliferation are possible as well.

Segment-dependent expression of muscarinic acetylcholine receptors and G-protein coupling in the equine respiratory tract

Muscarinic receptors are considered to be of comparable clinical importance in chronic obstructive pulmonary disease (COPD) in equines and in humans. At present, data are scarce on the expression and distribution of probable subtypes of these receptors and their signalling pathways in airway segments, including lung parenchyma and bronchial and tracheal epithelium with the underlying smooth muscle in horses. Specific [N-methyl-3H]scopolamine chloride ([3H]NMS) binding to all three tissues was saturable and of high affinity, with KD values ranging between 1.6+/-0.7 and 1.9+/-0.3 nmol/L. [3H]NMS binding identified a higher density of total muscarinic receptors (fmol/mg protein) in the trachea (720+/-59 nmol/L) than in bronchi (438+/-48 nmol/L) or lung (22 +/- 3 nmol/L). Competitive binding studies using [3H]NMS and the unlabelled subtype-selective antagonists pirenzepine and telenzepine (M1), methoctramine and himbacine (M2), 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) (M3), tropicamide (M4) and mamba toxin (MT-3) (M4) indicated the presence of at least three muscarinic receptor subtypes in peripheral lung tissue (50:40:24-28%: M2>M3>M1), whereas in bronchus and trachea M2 subtypes (87-90%) predominated over M3 (14-22%), and M1 subtypes were lacking. No differences were found between tissues in high-affinity binding sites for carbachol in the absence (31-36%) or presence of guanosine 5'-triphosphate (GTP) (approximately 100%). Western blotting for G-protein alpha-subunits showed a much more robust expression of G(alphai1/2) in the trachea (with highest receptor density) than in the lung or bronchi, whereas G(alphas)-protein was dominantly expressed in bronchus. Concomitantly, carbachol inhibited isoproterenol- and GTP-stimulated adenylyl cyclase activity with increasing muscarinic receptor expression (trachea > bronchi > lung). We conclude that the expression and signalling pathways of muscarinic receptors in the equine respiratory tract are segment-dependent. These receptors might contribute to the pathogenesis of COPD in the horse and could provide potential drug targets for the therapeutic use of anticholinergics in this species.

Distribution and function of muscarinic receptor subtypes in the ovine submandibular gland

The effects of muscarinic receptor antagonists on responses to electrical stimulation of the chorda-lingual nerve were determined in pentobarbitone-anesthetized sheep and correlated to the morphology of tissue specimens. Stimulation at 2 Hz continuously, or in bursts of 1 s at 20 Hz every 10 s, for 10 min induced similar submandibular fluid responses (19 ± 3 vs. 21 ± 3 μl·min−1·g gland−1), whereas vasodilatation was greater during stimulation in bursts (−52 ± 4 vs. −43 ± 5%; P < 0.01). Continuous stimulation at 8 Hz induced substantially greater responses (66 ± 9 μl·min−1·g gland−1 and −77 ± 3%). While atropine (0.5 mg/kg iv) abolished the secretory response at 2 and 20 Hz (1:10 s), a small response persisted at 8 Hz (<5%). The “M1-selective” antagonist pirenzepine (40 μg/kg iv) reduced the fluid response at all frequencies tested ( P < 0.05–0.01), most conspicuously at 2 Hz (reduced by 69%). Methoctramine (“M2/M4-selective”; 100 μg/kg iv; n = 5) had no effect on fluid or the vascular responses but increased the protein output at 2 (+90%, P < 0.05) and 8 Hz (+45%, P < 0.05). The immunoblotting showed distinct bands for muscarinic M1, M3, M4, and M5 receptors, and immunohistochemistry showed muscarinic M1 and M3 receptors to occur in the parenchyma. Thus muscarinic M1 receptors contribute to the secretory response to parasympathetic stimulation but have little effect on the vasodilatation in the ovine submandibular gland. Increased transmitter release caused by blockade of neuronal inhibitory receptors of the M4 subtype would explain the increase in protein output.

Studies of muscarinic receptor subtypes in salivary gland function in anaesthetized rats

The in vivo study aimed to examine whether muscarinic receptor subtypes other than muscarinic M3 receptors exert exocrine functional roles in the rat salivary glands. The effects of pirenzepine, methoctramine and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) were examined on secretion from the major salivary glands evoked by acetylcholine (0.001-10 micromol kg(-1) i.v.) in pentobarbitone-anaesthetized rats. Observations were occasionally made on glandular blood flow. 4-DAMP (0.1-100 nmol kg(-1) i.v.) markedly and equipotently inhibited the acetylcholine-evoked fluid responses in all glands. Pirenzepine (0.1 micromol kg(-1) i.v.-10 mmol kg(-1) i.v.) showed significantly lower inhibitory potency than 4-DAMP, most conspicuously in the parotid, while methoctramine (0.1 micromol kg(-1) i.v.-10 mmol kg(-1) i.v.) exerted an even lesser inhibitory effect. Also against acetylcholine-evoked blood flow increases, 4-DAMP showed a conspicuous potency. At 1 and 10 micromol kg(-1) i.v. of pirenzepine, the antagonist reduced the protein concentration in the submandibular saliva, but not in the parotid saliva. While 4-DAMP (1 and 10 nmol kg(-1) i.v.) significantly inhibited acetylcholine-evoked protein secretory responses in the submandibular glands, methoctramine (below 10 micromol kg(-1) i.v.) affected the responses in neither gland. The reduction of the protein concentration in submandibular saliva caused by 4-DAMP and pirenzepine was inhibited by N(omega)-nitro-L-arginine methyl ester (L-NAME; 30 mg kg(-1) i.p.), while L-NAME had no or only minute effects on the parotid protein secretion. Thus, in addition to muscarinic M3 receptors, other muscarinic receptors contribute to in vivo functional responses in rat submandibular and sublingual glands. While these other receptors are muscarinic M1 receptors in the sublingual gland, they may be a different subtype, possibly muscarinic M5 receptors, in the submandibular gland. However, muscarinic M1 receptors may induce indirect effects via nitric oxide in the submandibular gland.

M3 muscarinic receptor mediates regulation of protein secretion in rabbit lacrimal gland

PURPOSE

To identify which muscarinic receptor subtypes mediate protein secretion in isolated rabbit lacrimal glands. To compare protein secretory profiles in vitro with those in rabbit tears.

METHODS

Rabbit lacrimal gland slices were incubated with carbachol in the presence and absence of different muscarinic antagonists. Protein secretion into the incubation medium was characterized with a Bio-Rad protein assay dye reagent in conjunction with Laemmli's method of SDS-PAGE. The medium protein profile was compared to that in tear samples.

RESULTS

Dose dependent increases in protein secretion were elicited by carbachol between 10(-7) and 10(-4) M. During the first 20 min period, a maximal increase of 64% above the basal level was seen at the highest concentration. With 10(-4) M carbachol, the response was transient because after 100 min it decreased to its basal level. The increases in protein secretion caused by 10(-4) M carbachol were completely suppressed in the presence of 10(-5) M atropine. On the other hand, the relatively selective M1 antagonist, pirenzepine, at concentrations from 10(-6) M to 10(-4) M, had no effect on either the basal levels or the stimulatory effects of carbachol. Similarly, the relatively selective M2 antagonist, gallamine, at concentrations from 10(-6) M to 10(-4) M, had no effect on either of these levels. In contrast, the relatively selective M3 antagonist, 4-DAMP, at concentrations from 10(-6) M to 10(-4) M, progressively suppressed the stimulated level of protein secretion elicited by 10(-4) M carbachol without affecting the basal level. The protein profiles found in the tears and in the incubation medium were similar to one another.

CONCLUSION

In vitro rabbit lacrimal gland protein secretion is comparable to that in vivo. Cholinergic-mediated control of rabbit lacrimal gland protein secretion occurs through stimulation of the M3 muscarinic receptor subtype.

Identification of muscarinic receptor subtypes in mouse parotid gland

Immunoprecipitation of muscarinic receptors from mouse parotid membranes by specific subtype antisera showed that M3 and M1 receptors represented 75 and 15% of the total number of precipitable receptors, respectively. [N-methyl-3H]methylscopolamine (NMS) labeled a single class of high-affinity binding sites in membranes from parotid glands with a dissociation constant of 0.67 +/- 0.02 nM and a maximum binding capacity of 176 +/- 15 fmol/mg protein. Competition curves for NMS, atropine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and para-fluoro-hexahydro-sila-difenidol fit best to a one-site binding model, whereas pirenzepine and methoctramine fit best to a two-site binding model, indicating 76-90% M3 receptors. Results from the use of pirenzepine indicated that the second mouse parotid receptor subtype, unlike that of the submandibular gland, has atypical characteristics for an M1 receptor. The rank order of potency of muscarinic antagonists in inhibiting phosphoinositide turnover and biphasic effects of carbachol on isoproterenol-stimulated adenosine 3',5'-cyclic monophosphate (cAMP) accumulation was atropine > or = 4-DAMP >> pirenzepine > AF-DX 116. A specific M1 antagonist, m1-toxin, had no effect on carbachol augmentation or inhibition of isoproterenol responses. Results suggest that M3 receptors couple to both augmentation and inhibition of stimulated cAMP levels.

Muscarinic receptors in developing rat colon

Muscarinic receptor subtypes were characterized in fetal (21 day), newborn (3 day), and adult (3 month) rat colon smooth muscle. Saturation binding of the nonselective muscarinic antagonist radioligand [3H]quinuclidinyl benzilate revealed a single class of binding sites in all three age groups. The binding affinities of [3H]quinuclidinyl benzilate were not significantly different among three age groups (KD: 0.19-->0.27 nM). In contrast, the receptor densities (Bmax, fmol/mg protein) showed a significant age-related decrease with fetus (518.9 +/- 7.4) > newborn (480.3 +/- 45.6) >> adult (192.4 +/- 32.8). In both newborn and adult tissues, the muscarinic agonist carbachol bound to two sites with high and low affinities. Although the agonist binding affinities in the newborn tissue were not significantly different from those in the adult tissue, the high-affinity binding sites for carbachol were significantly increased in the later (41%-->61%). Addition of guanosine-5'-O-(3-thio)triphosphate (100 microM) abolished apparent high-affinity binding sites in both newborn and adult tissues. Antagonist competition binding in the newborn tissue indicated a homogeneous population of muscarinic M2 receptors. Unlike in newborn tissues, the heterogeneous binding of pirenzepine and 4-diphenylacetoxy-N-methylpiperidine methobromide in adult tissues revealed coexistence of muscarinic M3 (45%) and M2 (55%) receptors. In accordance, activation of muscarinic receptors in the adult tissue stimulated synthesis of inositol 1,4,5-trisphosphate. These results suggest maturational changes of muscarinic receptor subtypes and their coupling to G proteins in rat colonic smooth muscle. These changes may account, at least in part, for developmental alterations of functional responses in colonic smooth muscle.

Muscarinic receptor subtypes in the submandibular gland and the urinary bladder of the rabbit: in vivo and in vitro functional comparisons of receptor antagonists

1. In pentobarbitone-anaesthetized rabbits, the inhibitory effects of muscarinic receptor antagonists with different selectivity profiles were examined on carbachol-evoked submandibular secretion and urinary bladder contractions, and on parasympathetically nerve-evoked secretion. On isolated submandibular gland fragments, the inhibitory effects of the antagonists were studied on carbachol-evoked release of potassium and on the overflow of tritium in response to electrical field stimulation. 2. In vivo, 4-DAMP equipotently inhibited simultaneously carbachol-evoked submandibular secretory and contractile responses of the urinary bladder, while pirenzepine was found to be four times as potent in inhibiting the secretory response compared with the contractile response. 3. The inhibition of carbachol-evoked salivation caused by atropine, 4-DAMP and pirenzepine was as great as their inhibition of parasympathetic nerve-evoked salivation. Methoctramine exerted less inhibitory effect on nerve-evoked salivation than on carbachol-evoked, thus seemingly causing greater presynaptic inhibition. 4. In vitro, pirenzepine was only 30 times less potent in inhibiting carbachol-evoked potassium release than 4-DAMP (pA2, 9.58 vs 8.10). Whereas atropine, 4-DAMP and pirenzepine abolished the overflow of tritium from isolated glands in response to electrical field stimulation, methoctramine increased it. 5. It is concluded that the muscarinic secretory response in the rabbit submandibular gland is exerted via both muscarinic M1 and M3 receptors, while the contractile response of the urinary bladder to muscarinic agonists is exerted via muscarinic M3 receptors. The release of acetylcholine from nerve terminals in the gland can be inhibited via M2 autoreceptors in rabbits.

Retrotrapezoid nucleus muscarinic receptor subtypes localized by autoradiography

Microinjection of muscarinic receptor subtype antagonists into the region of the cat retrotrapezoid nucleus (RTN) decreases blood pressure (greatest efficacy; M2 subtype) and both baseline phrenic activity and CO2 sensitivity (greatest efficacy; M3/M1 subtype). Here we examine, in cat medullary sections at the level of the RTN, the effects of the same antagonists on binding of the high affinity muscarinic agonist quinuclidinyl benzilate (QNB). 3H-QNB binding was saturated and highly specific at 1 nM concentration and stable over 30 to 120 min (Kd, 0.49 nM; Bmax, 136 fm/mg protein). Studied biochemically, we found IC50 values for whole sections of 4.9 x 10(-6) M (M1 antagonist pirenzepine); 1.0 x 10(-6) M (M2 antagonist AFDX); and 0.64 x 10(-7) M (M3 antagonist DAMP; P < 0.03 vs PZ). Densitometric analysis of whole medullary cross section autoradiograms resulted in similar IC50 values as in the biochemical approach. Specific analysis of the RTN region demonstrated the presence of 3H-QNB binding and similar competition by the antagonists. Average IC50 values determined by densitometry were 14 x 10(-6) M (pirenzepine); 1.3 x 10(-6) M (AFDX; P < 0.01 vs PZ); and 0.53 x 10(-7) M (DAMP; P < 0.01 vs PZ). All three subtypes of muscarinic receptors identifiable via pharmacological antagonists appear to be present in the RTN region but we could not distinguish a subtype-specific pattern of receptor distribution.

Muscarinic cholinergic receptor subtypes in rat sublingual glands

Mucin glycoprotein secretion by rat sublingual glands is regulated primarily by muscarinic cholinergic receptors. Studies were conducted to identify muscarinic receptor subtypes in whole glands as well as in isolated acinar structures. In radioligand binding studies, we used subtype-selective antagonists in competition studies to initially determine receptor subtype heterogeneity. In membranes from whole glands, both pirenzepine and methoctramine displayed two affinity sites (M1 and M3) of nearly equal proportions. In contrast, acinar membranes contained a 1:2 and 2:1 ratio of M1 to M3 sites for pirenzepine and methoctramine, respectively. In all cases, p-fluoro-hexahydro-siladifenidol and 4-diphenylacetoxy-N-methylpiperidine each bound to a single class of binding sites. Northern analysis using oligonucleotide probes specific for the 5' ends of the translated regions of m1 through m5 receptors detected only m1 and m3 subtypes in poly(A)+ RNA from whole glands. We also used antisera specific for each receptor subtype to immunoprecipitate solubilized receptors from membrane preparations. Only m1 (51.7 and 64.9%) and m3 (48.3 and 34.7%) subtypes were found consistently in membranes from whole sublingual glands and isolated acini, respectively. Studies with other exocrine glands generally described the predominance of m3 receptors, and m1 receptors, if present, were presumably associated with contaminating neural structures. Our results therefore demonstrate that mucous acini from rat sublingual glands contain abundant amounts of both m1 and m3 receptors.

M3 muscarinic acetylcholine receptor coupling to PLC in rat exorbital lacrimal acinar cells

This study was designed to characterize the muscarinic acetylcholine receptor (mAChR) subtype present in rat exorbital lacrimal gland as well as its biochemical coupling. The nonselective muscarinic antagonist [N-methyl-3H]scopolamine ([3H]NMS) binds with high affinity to a homogeneous population of binding sites in both membranes [dissociation constant (Kd) = 82.3 +/- 3.2 pM] and acinar cell (Kd = 170.3 +/- 20 pM) preparations. Muscarinic antagonist inhibition of [3H]NMS binding is homogeneous with the following order of potency: atropine > or = 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) > pirenzepine > 11-([2-(diethylamino)-ethyl]-1-piperidinyl)-acetyl- 5,11-dihydro-6H-pirido[2,3-b]1,4,benzo diazepine-6-one (AFDX 116). Both the affinity of the selective antagonists 4-DAMP, pirenzepine, and AFDX 116 and Northern blot analysis of lacrimal gland mRNAs show a single mAChR population of the M3 subtype. Muscarinic agonist inhibition of [3H]NMS binding displays both high (approximately 20%)- and low-affinity sites (approximately 80%). Both the receptor occupancy and the stimulation by agonists or the inhibition by antagonists of the accumulation of [3H]inositol phosphate were examined under identical conditions with respect to tissue preparations (acinar cells) and buffer (Krebs-Ringer). Results demonstrate 1) the efficient coupling of the M3 mAChR subtype with the phosphatidylinositol (4,5))bisphosphate-specific phospholipase C activity and 2) that the efficacy of a muscarinic agonist is dependent on its structure. Lastly, comparison of the agonists affinity and potency to trigger the [3H]inositol phosphate accumulation suggests that the occupation of the high-affinity agonist binding state of the M3 mAChR was involved in the cellular response.

Expression of muscarinic and nicotinic receptor mRNA in the salivary gland of rats: a study by in situ hybridization histochemistry

Expression of muscarinic receptor mRNA subtypes (m1-5) and nicotinic receptor subunits (alpha 2-4, and beta 2) was examined in the rat submandibular gland by in situ hybridization histochemistry, using oligonucleotide probes for the muscarinic receptor and RNA probes for the nicotinic receptors. m2, alpha 3, and beta 2 mRNA were strongly expressed in the submandibular ganglion, and m3, alpha 2, alpha 3, alpha 4, and beta 2 were expressed in the striated and interlobular duct cells. Both muscarinic and nicotinic receptors were coexpressed in the same ganglion neurons, while none of these mRNA were detected in the terminal secretory units.

Characterization of muscarinic receptors in rat kidney

Muscarinic receptors in mammalian kidney seem to be involved in diuresis. In this study we give a detailed characterization of receptors in rat kidney. Specific binding of [3H](-)-quinuclidinylbenzilate ([3H]QNB) to membranes of rat kidney cortex was saturable and of high affinity. A dissociation constant of 0.063 +/- 0.003 nM and a receptor density of 1.46 +/- 0.07 pmol/g wet weight were obtained. The dissociation kinetics could be best described by assuming a mono-exponential function (k-1 = (0.52 +/- 0.1) x 10(-4) s-1). The binding of [3H]QNB reached a maximum in 60 min at 0.6 nM at 37 degrees C. Competition experiments with the enantiomers of benzetimide confirmed the muscarinic nature of the [3H]QNB binding sites. The inhibition constants of pirenzepine (0.23 +/- 0.02 microM), (+-)-hexahydrosiladifenidol (0.040 +/- 0.002 microM), AF-DX 116 (1.45 +/- 0.07 microM), methoctramine (1.67 +/- 0.02 microM) and gallamine (78 +/- 3 microM) classified this receptor as an M3 receptor. Inhibition of [3H]QNB binding by the agonists methylfurtrethonium, arecoline, isoarecoline methiodide, arecaidine propargyl ester and McN-A-343 displayed monophasic inhibition curves. With (+/-)-cis-2-methyl-4-dimethylaminomethyl-1,3- dioxolane methiodide in two out of four experiments a small (11%) population of high affinity agonist sites could be detected. The potassium sparing diuretic amiloride inhibited [3H]QNB binding (36 +/- 3 microM). Although in a way related to the amiloride binding site, the muscarinic receptors in rat kidney are unlikely to be the primary target of diuretic action of this drug.

Characterization of muscarinic receptor subtype of rat eccrine sweat gland by autoradiography

The muscarinic cholinergic receptor of rat eccrine sweat gland was characterized using quantitative autoradiography and [3H]QNB as radioligand. The distribution of radioligand was maximal in the secretory coil. Autoradiographic competition binding studies were performed using selective antagonists to M1 (pirenzepine), M2 (AF-DX 116), and M3 (4-DAMP) and the classical nonselective antagonist atropine. pKi for pirenzepine, AF-DX 116, 4-DAMP, and atropine was 6.58, 5.47, 8.50, and 8.66 respectively indicating that the eccrine sweat gland muscarinic receptor was predominantly M3.

Characterization of the muscarinic cholinergic receptor in the opossum (Didelphis virginiana, Kerr) submandibular gland: differences in receptor density and subtype compared with higher mammalian species

1. Kinetic, saturation and inhibition radioligand binding experiments with [3H]-N-methylscopolamine and [3H]quinuclidinyl benzilate were used to characterize the muscarinic cholinergic receptor in opossum (Didelphis virginiana, Kerr) submandibular salivary gland membranes. 2. The receptor density in opossum submandibular gland was found to be more than 3-fold higher than in rat, and 22-fold higher than in human, submandibular glands. 3. Inhibitor equilibrium dissociation constants for the antagonists pirenzepine, dicyclomine, atropine, N-methylscopolamine and AF-DX 116 revealed that the muscarinic receptor present in opossum submandibular gland appears to be the M1 subtype rather than the M3 subtype found in human and rat.

Muscarinic receptor differentiation

1. Several selective antagonists are available to differentiate between muscarinic receptors. 2. Further subdivision of M1 and M2 muscarinic receptors appears possible and is supported by studies with cloned receptors. 3. Reasons for differences between affinity constants determined in functional and binding studies and whether receptor subtypes couple exclusively with a particular cellular mechanism are still to be determined.

Characterization of the antimuscarinic effect of heptane-1,7-bis-(dimethyl-3'-phthalimidopropyl ammonium bromide)

The effect of heptane-1,7-bis-(dimethyl-3'-phthalimidopropyl ammonium bromide) (C7/3'-phthalimidopropyl), an alkane bisquaternary compound with muscarinic receptor blocking activity was studied in guinea-pig atria and ileal longitudinal muscle. C7/3'-phthalimidopropyl was a more potent inhibitor of atrial muscarinic receptors, the cardioselectivity being ca. 32-fold. Previous studies in guinea-pig atria have shown that its antimuscarinic effect was of an allosteric nature. In ileal longitudinal muscle C7/3'-phthalimidopropyl (3 to 100 microM) appeared to behave in a competitive manner towards carbachol but the combination of atropine or homatropine with C7/3'-phthalimidopropyl produced a supra-additive inhibitory effect on the responses to carbachol. In both atria and ileal longitudinal muscle homogenates, C7/3'-phthalimidopropyl also slowed the dissociation rate of [3H]QNB suggesting an allosteric mechanism. In binding studies using either [3H]QNB or [3H]oxo-M, C7/3'-phthalimidopropyl recognized two binding sites in atria and ileum. In both tissues, C7/3'-phthalimidopropyl bound with high affinity (ca. 30-70 nM) to 60-85% of the sites and with low affinity (ca. 1-9 microM) to the remaining sites. Correlation of these affinity constants with the dissociation constants obtained in functional studies in the two tissues is discussed.

Ipratropium bromide treatment of experimental rhinovirus infection

The importance of parasympathetic-cholinergic mechanisms in the production of common cold symptoms is not clear. The quaternary ammonium anticholinergic antagonist ipratropium bromide was intranasally administered under double-blind, randomized, placebo-controlled conditions to assess its tolerance and efficacy in reducing nasal hypersecretion in adult volunteers with experimental rhinovirus colds. Ipratropium was sprayed intranasally three times daily (80 micrograms per treatment) for 5 days beginning 24 h after intranasal inoculation of rhinovirus type 39. Clinical colds occurred in 50% of 30 infected ipratropium recipients and in 76% of 33 infected placebo recipients (P = 0.04). The nasal mucus weights tended to be lower for ipratropium-treated persons (mean +/- standard deviation, 14.7 +/- 15.1 g/5 days) than for placebo-treated recipients (24.7 +/- 28.0 g/5 days; P = 0.076). Whereas total nasal symptom scores were similar between the two groups, the rhinorrhea score analyzed for each day of treatment showed nonsignificant trends favoring the ipratropium group over the last 4 days of treatment. Ipratropium was generally well tolerated. The results suggest that cholinergic mechanisms are at least partially responsible for nasal mucus production in rhinovirus colds but that the effect of anticholinergic compounds alone is insufficient to be of practical use in treatment, although they may have value as components of multi-ingredient preparations.

Cloning and expression of the human and rat m5 muscarinic acetylcholine receptor genes

The human and rat genes for a fifth muscarinic receptor have been cloned and expressed in mammalian cells. The 532 amino acid human protein has 89% sequence identity to the 531 amino acid rat protein and is most closely related to the m3 receptor. Both proteins are encoded by single exons. The receptor has intermediate affinity for pirenzepine and low affinity for AF-DX 116, and it increases metabolism of phosphatidylinositol when stimulated with carbachol. Expression of mRNA has yet to be observed in brain or selected peripheral tissues, suggesting that either it is substantially less abundant than m1-m4 or its distribution is quite different.

Characterization of muscarinic receptor subtypes in human tissues

The affinities of selective, pirenzepine and AF-DX 116, and classical, N-methylscopolamine and atropine, muscarinic cholinergic receptor antagonists were investigated in displacement binding experiments with [3H]Pirenzepine and [3H]N-methylscopolamine in membranes from human autoptic tissues (forebrain, cerebellum, atria, ventricle and submaxillary salivary glands). Affinity estimates of N-methylscopolamine and atropine indicated a non-selective profile. Pirenzepine showed differentiation between the M1 neuronal receptor of the forebrain and the receptors in other tissues while AF-DX 116 clearly discriminated between muscarinic receptors of heart and glands. The results in human tissues confirm the previously described selectivity profiles of pirenzepine and AF-DX 116 in rat tissues. These findings thus reveal the presence also in man of three distinct muscarinic receptor subtypes: the neuronal M1, the cardiac M2 and the glandular M3.