Neuromuscular blocking agents' detrimental airway effects may occur as a result of interactions with muscarinic receptors, allergic reactions, or histamine release. Rapacuronium, a nondepolarizing muscle relaxant, was withdrawn from clinical use because of its association with fatal bronchospasm. Despite its withdrawal from clinical use, it is imperative that the mechanism by which bronchospasm occurred is understood so that new muscle relaxants introduced to clinical practice do not share these same detrimental airway effects.
Airway smooth muscle force was measured in guinea pig tracheal rings in organ baths exposed to muscle relaxants with or without subthreshold concentrations of acetylcholine. Antagonism of muscarinic, histamine, neurokinin, leukotriene receptors, or blockade of L-type calcium channels or depletion of nonadrenergic, noncholinergic neurotransmitters was performed. Muscle relaxants' potentiation of acetylcholine-stimulated inositol phosphate synthesis and allosteric interactions on the kinetics of atropine-induced [3H]N-methylscopolamine dissociation were measured in cells expressing recombinant human M3 muscarinic receptors.
Rapacuronium, within clinically achieved concentrations, contracted tracheal rings in the presence but not in the absence of subthreshold concentrations of acetylcholine. This effect was prevented or reversed only by atropine. The allosteric action of rapacuronium was demonstrated by the slowing of atropine-induced dissociation of [3H]N-methylscopolamine, and positive cooperativity was demonstrated by potentiation of acetylcholine-induced inositol phosphate synthesis.
Many muscle relaxants have allosteric properties at muscarinic receptors; however, positive cooperativity at the M3 muscarinic receptor within clinically relevant concentrations is unique to rapacuronium. These findings establish novel parameters that should be considered in the evaluation of airway safety of any newly synthesized neuromuscular blocking agents considered for clinical practice.
NEUROMUSCULAR blocking agents are widely used in clinical anesthesia to rapidly achieve muscle relaxation for airway intubation and to facilitate some surgical procedures. Succinylcholine continues to be the drug of choice to rapidly achieve optimal intubation conditions, and its short duration of action is advantageous in the event that intubation of the airway is difficult. However, numerous clinical side effects of succinylcholine have stimulated the search for a nondepolarizing muscle relaxant with similar clinical properties. One such muscle relaxant was rapacuronium, which was used clinically for only a short period of time because of the development of severe bronchospasm that accompanied its use in some patients.1,,2,,3,,4,,5 Despite its withdrawal from clinical use, it is imperative to understand the mechanism by which rapacuronium contributed to fatal bronchospasm so that new nondepolarizing muscle relaxants currently in development6,,7,,8 do not share these same airway detrimental effects.
The intended clinical target of neuromuscular blocking agents is the nicotinic receptor on skeletal muscle. However, neuromuscular blocking agents also interact with muscarinic receptors9,,10,,11,,12,,13,,14,,15,,16 because nicotinic and muscarinic receptor share the same endogenous ligand (i.e. , acetylcholine). Muscarinic receptors are found on numerous cells within the airway, including postganglionic parasympathetic nerves and the airway smooth muscle, which these nerves innervate. Muscarinic receptors on the parasympathetic nerves are of the M2 subtype and normally function in an inhibitory autofeedback mode to prevent the further release of acetylcholine. Muscarinic receptors on the airway smooth muscle are of the M2 and M3 subtypes, which function to inhibit relaxation and facilitate contraction, respectively. Irritation of the upper trachea (e.g. , by the introduction of an endotracheal tube) initiates a neural reflex that results in the release of acetylcholine from parasympathetic nerves. The acetylcholine binds to M2 and M3 muscarinic receptors in airway smooth muscle, resulting in bronchoconstriction. Normally the release of acetylcholine is terminated by activation of presynaptic M2 muscarinic inhibitory autofeedback receptors on the parasympathetic nerves terminals.
We have previously proposed a scenario that could explain the detrimental airway effects of rapacuronium: selective inhibition of the M2 muscarinic receptor during a period of heightened parasympathetic nerve stimulation (i.e. , intubation). Therefore, if large intubating doses of rapacuronium were administered, selective blockade of the M2 muscarinic receptor on the nerve would allow for enhanced acetylcholine release to act on relatively unopposed M3 muscarinic receptors on airway smooth muscle, resulting in enhanced bronchoconstriction. Indeed, we have shown that rapacuronium displays greater blockade of the M2 muscarinic receptor relative to the M3 muscarinic receptor in both radioligand binding and airway functional assays. However, several other muscle relaxants block M2 muscarinic receptors (e.g. , pancuronium, gallamine) within clinically relevant concentrations but have not elicited bronchospasm with the severity and prevalence that has been observed with rapacuronium. This suggests that rapacuronium may have detrimental effects on the airway in addition to M2 muscarinic receptor blockade. One possibility is that rapacuronium, like many other neuromuscular blocking agents (e.g. , pancuronium, gallamine, alcuronium), is an allosteric modulator at muscarinic receptors. Positive cooperativity with acetylcholine at the airway smooth muscle M3 muscarinic receptors would promote bronchoconstriction.10,,19,,20,,21
It is well established that muscarinic receptors are susceptible to allosteric modulation by various pharmacologic compounds.20,,21 An allosteric effect is defined by the ability of a substance to bind to a site on the receptor (allosteric site) other than the classic ligand binding site (orthosteric site) and thereby effect the binding of the traditional ligand (i.e. , acetylcholine) at the orthosteric site. An allosteric substance can demonstrate positive (facilitating the binding of ligand to the orthosteric site), negative (inhibiting the binding of ligand to the orthosteric site), or neutral (not effecting binding of ligand to orthosteric site but occupying the allosteric site instead of another allosteric substance) cooperativity. Indeed, two nondepolarizing muscle relaxants no longer used clinically, gallamine and alcuronium, are used as classic allosteric modulators for certain muscarinic receptor subtypes.20,,21 Conceptually, allosteric interactions of muscle relaxants with muscarinic receptors in the airways could be of immense clinical importance because these drugs could modulate the effects of acetylcholine in either a beneficial or a detrimental manner.
Therefore, in the current study, we used intact guinea pig tracheal rings and recombinantly expressed human M3 muscarinic receptors to determine whether rapacuronium exhibited an allosteric effect with positive cooperativity at M3 muscarinic receptors.
Materials and Methods
Clinical formulations of rapacuronium and vecuronium (Organon, West Orange, NJ) were used in these studies. Control vehicle solutions were prepared according to the manufacturer’s product insert. Methoctramine, tetrodotoxin, ranitidine, nifedipine, pyrilamine, N-vanillylnonanamide (capsaicin analog), and atropine were purchased from Sigma (St. Louis, MO). The peptide MEN10376 (NK2 antagonist), sendide (NK1 antagonist), BAY-u9773 (nonselective leukotriene antagonist), and MK571 (D4 leukotriene antagonist) were purchased from Biomol Research Laboratories (Plymouth Meeting, PA). [3H]N-methylscopolamine (3H-NMS; 81 Ci/mmol) was purchased from PerkinElmer (Boston, MA). [3H]-myo -inositol (20 Ci/mmol) was purchased from MP Biomedicals (Irvine, CA). All drugs were dissolved in distilled water except for nifedipine, which was dissolved in dimethyl sulfoxide such that the final concentration of dimethyl sulfoxide in the baths was 0.01%.
Characterization of the Effect of Rapacuronium on Baseline Resting Tone of Tracheal Rings
All studies were approved by the Institutional Animal Care and Use Committee (College of Physicians and Surgeons, Columbia University, New York, New York). Force measurements were performed on closed guinea pig tracheal rings suspended in organ baths as previously described. Briefly, Hartley male guinea pigs (approximately 400 g) were anesthetized with 50 mg intraperitoneal pentobarbital, and the tracheas were removed promptly and dissected into closed rings comprised of two cartilaginous rings from which mucosa, connective tissue, and epithelium were removed. Trachea were attached using silk threads to a fixed tissue hook in a 2-ml bath (Radnoti Glass Technology, Inc., Monrovia, CA) and a Grass (Grass Telefactor, West Warwick, RI) force transducer coupled to a computer via BioPac hardware and Acqknowledge 7.3.3 software (Biopac Systems, Inc., Goleta, CA) for continuous digital recording of muscle force. Rings were equilibrated at 1 g isotonic force for 1 h with new Krebs-Henseleit buffer (including 10 μm indomethacin) (pH 7.4, 37°C) added every 15 min while gassed with 95% oxygen and 5% carbon dioxide.
Rings were subjected to (1) two cycles of acetylcholine-induced contractions (increasing cumulative concentrations of 0.1 μm–1.0 mm acetylcholine) followed by two washing protocols: washed only three times or washed nine times with Krebs-Henseleit buffer; (2) a single acetylcholine concentration (0.1 μm); or (3) no pretreatment. Cumulatively increasing concentrations of rapacuronium (1 nm–100 μm), vecuronium (10 nm–100 μm), rocuronium (10 nm–200 μm), or pancuronium (100 nm–100 μm) were then added to the organ baths. In the studies where rapacuronium caused an increase in baseline force, antagonists were added at the peak of contraction in an attempt to reverse the increase in baseline tone. The antagonists included atropine (1 μm), pyrilamine (10 μm; H1 histamine receptor antagonist), nifedipine (10 μm; calcium channel blocker), sendide (30 μm; NK1 receptor antagonist), MEN10376 (30 μm; NK2 receptor antagonist), MK571 (10 μm; leukotriene D4 antagonist), or BAY-u9773 (10 μm; nonselective leukotriene antagonist) and were added to individual baths in an attempt to antagonize this rapacuronium-induced increase in baseline tone.
In a separate series of experiments, these same antagonists were added after the acetylcholine precontractions and washing but 30 min before the addition of cumulatively increasing concentrations of rapacuronium.
To determine whether rapacuronium was increasing baseline tone by activation of nonadrenergic, noncholinergic (NANC) nerves, N-vanillylnonanamide (10 μm; capsaicin analog) was added first to activate and deplete NANC contractile tachykinins. After N-vanillylnonanamide–induced force had returned to baseline (60–90 min), rapacuronium was added to determine whether force increased despite depletion of NANC neurotransmitters. To confirm adequate depletion of contractile NANC neurotransmitters, control tissues received a repeated dose of N-vanillylnonanamide.
Inositol Phosphate Assays
The M3 muscarinic receptor classically couples to the stimulation of inositol phosphate synthesis, and this assay was thus used as a measure of M3 receptor function in response to acetylcholine in the absence and presence of rapacuronium. Synthesis of total 3H-inositol phosphates was measured in Chinese hamster ovary (CHO) cells stably expressing the human M3 muscarinic receptor (a kind gift from Tom I. Bonner, Ph.D., Senior Investigator, Laboratory of Genetics, National Institutes of Health, Bethesda, Maryland)23,,24 grown to confluence in 24-well tissue culture plates. Briefly, after overnight loading with 3H-myo -inositol (10 μCi/ml, 20 Ci/mmol) in inositol-free and serum-free Dulbecco’s modified Eagle’s medium, plates were washed three times (37°C, 500 μl Hanks balanced salt solution with 10 mm LiCl). Incubation of cells in a final volume of 300 μl at 37°C for 30 min was performed in the absence and presence of 1 μm acetylcholine. Parallel wells were incubated with 1 μm acetylcholine plus a range of concentrations (0.1–10 μm) of rapacuronium. Reactions were terminated, and total [3H]inositol phosphates recovered by chromatography as described. To confirm that the effects of rapacuronium (and acetylcholine) on inositol phosphate synthesis was via the M3 muscarinic receptor, two types of control experiments were performed. The M3 muscarinic receptor expressed in CHO-M3 cells was inactivated by alkylation with 1 μm 4-DAMP mustard for 1 h before exposure to acetylcholine or rapacuronium. In a separate cell line that expresses the human M2 but not the M3 muscarinic receptor (CHO-M2; a kind gift from Tom I. Bonner, Ph.D.). cells were exposed to acetylcholine and rapacuronium as above before the measurement of inositol phosphate synthesis.
Dissociation Kinetic Studies
Allosteric effects are defined by the ability of a substance (e.g. , muscle relaxant) to delay the dissociation of a ligand from a receptor.26,,27,,28 We determined whether clinically relevant concentrations of several muscle relaxants (rapacuronium, vecuronium, rocuronium, and gallamine) had allosteric interactions at the M3 muscarinic receptor by slowing the atropine-induced dissociation of 3H-NMS from M3 muscarinic receptors. CHO-M3 membranes (0.2–0.3 mg/ml) were incubated in triplicate in buffer (20 mm HEPES, 100 mm NaCl, 10 mm MgCl2, pH 7.4) with 0.88 nm 3H-NMS (two times the Kd for these membranes) for 70 min at 32°C. Dissociation of 3H-NMS was induced at time 0 min by the simultaneous addition of 1 μm atropine with or without muscle relaxants. The reaction was terminated at 5 min (3 ml cold wash buffer [50 mm Tris-HCl in 0.9% NaCl, pH 7.4]) and collected on glass fiber filters (GF/B) presoaked in 0.1% polyethylenimine using a cell harvester (Brandell, Gaithersburg, MD). Filters were washed three times with 3 ml cold wash buffer. Filters were immersed in 5 ml Econo scintillation fluid, stored overnight, and counted (Beckman Liquid Scintillation 5000 TD; Beckman, Fullerton, CA).16,,20,,29,,30
Smooth muscle force was continuously recorded using Acqknowledge 3.7.3 software. Statistical analysis was determined using GraphPad Instat 3.01 software (GraphPad Software, Inc., San Diego, CA) by two-way analysis of variance with Bonferroni multiple comparison post test. Statistical significance was defined by a P value less than 0.05. In all experiments, n represents the number of individual tracheal rings studied or the independent number of experiments for radioligand dissociation studies or inositol phosphate assays.
Organ Bath Studies
illustrates a representative force recording of a tracheal ring exposed to two cycles of cumulatively increasing acetylcholine concentrations followed by three washings and then exposed to 1–100 μm rapacuronium, resulting in increased force that returned to baseline after extensive washing (nine times). After extensive washing (i.e. , removing residual acetylcholine), this tracheal ring was unresponsive to another rapacuronium dose response. illustrates the failure of rapacuronium (1–100 μm) to induce contraction in the absence of a precontraction with acetylcholine; however, the contractile effect of rapacuronium subsequently occurs after the same ring is subjected to two cycles of cumulatively increasing acetylcholine concentrations followed by three washes. To further confirm that subthreshold acetylcholine concentrations were necessary for the effect of rapacuronium, we added a low concentration of acetylcholine (0.1 μm) to the baths. This concentration alone had no effect on muscle force but allowed for the dose-dependent potentiation of muscle force of rapacuronium (). Although we did not measure the concentration of acetylcholine in the organ baths, both methods resulted in approximately the same percent increase in baseline force when rapacuronium was added. Rapacuronium (10–100 μm) significantly increased baseline muscle force in the presence (but not in the absence) of subthreshold acetylcholine (P < 0.001; n = 5; ). Similarly, vecuronium (5–100 μm) significantly increased baseline force in the presence of subthreshold acetylcholine but only at vecuronium concentrations above those achieved clinically (P < 0.001; n = 5; ). Conversely, pancuronium (2–100 μm) significantly decreased baseline force within clinically achieved concentrations in the presence of subthreshold acetylcholine concentrations (P < 0.001; n = 4; ). High concentrations of rocuronium (50–100 μm) also increased baseline force in the presence of subthreshold concentrations of acetylcholine, but this contraction was qualitatively different from the other relaxants studied in that it spontaneously and rapidly returned to baseline (data not shown). Taken together, these results suggest that rapacuronium, vecuronium, and rocuronium can increase baseline force under conditions where subthreshold concentrations of acetylcholine are present and are consistent with a positive allosteric effect at M3 muscarinic receptors. Most significantly, however, rapacuronium was the only muscle relaxant studied to increase force within clinically relevant concentrations.
In an attempt to confirm that this rapacuronium-induced increase in resting force in the presence of low acetylcholine concentrations involved muscarinic receptors and did not involve other classic contractile agonists in airway smooth muscle, a series of inhibitors was used either before the addition of rapacuronium to the baths or after the rapacuronium-induced force had reached a peak. 3,,4 illustrate representative force recordings of tracheal rings prepared as above with two acetylcholine dose responses followed by three washes, pretreatment with an inhibitor, and then the addition of rapacuronium. Only atropine prevented () or reversed () the rapacuronium-induced increase in baseline force. Pretreatment of tracheal rings with pyrilamine (H1 histamine receptor antagonist; ), sendide plus MEN10376 (neurokinin receptor 1 and 2 antagonists; ), MK-571 (leukotriene LTD4 antagonist; ), BAY-υ9773 (nonselective leukotriene antagonist; ), or nifedipine (L-type calcium channel blocker; ) had no effect on the subsequent muscle contraction induced by rapacuronium. Furthermore, preactivation and depletion of NANC neurotransmitters by a capsaicin analog did not prevent the subsequent increase in baseline tone elicited by rapacuronium (data not shown). The functional studies in tracheal rings suggesting a positive allosteric effect of rapacuronium at muscarinic receptors led us to perform biochemical (inositol phosphate) and radioligand (dissociation rates) assays in vitro using human recombinant M3 muscarinic receptors expressed in CHO cells.
Inositol Phosphate Assays
Rapacuronium (0.1–1 μm) accentuated the acetylcholine-induced increase in inositol phosphate synthesis in CHO cells expressing the human M3 muscarinic receptor consistent with a positive allosteric effect (P < 0.05 compared with acetylcholine alone; n = 6; ). Rapacuronium alone had no effect on inositol phosphate synthesis. Higher concentrations of rapacuronium (5–10 μm) decreased inositol phosphate synthesis likely due to either an apparent classic orthosteric antagonist effect or steric hindrance of the orthosteric site by the allosteric binding of rapacuronium on the M3 muscarinic receptor (). Inactivation of the M3 muscarinic receptor by alkylation (1 h of 1 μm 4-DAMP mustard) blocked the ability of acetylcholine or acetylcholine plus rapacuronium to increase inositol phosphate synthesis (). To further confirm that rapacuronium’s potentiation of acetylcholine-induced inositol phosphate synthesis was mediated via the effect of both acetylcholine and rapacuronium at the M3 muscarinic receptor, these assays were repeated in a CHO cell line devoid of M3 muscarinic receptors (but expressing human M2 muscarinic receptors, which do not couple to inositol phosphate synthesis). Neither rapacuronium nor acetylcholine increased inositol phosphate synthesis in this human M2 muscarinic receptor–expressing cell line (0.5 μm rapacuronium, 93.8 ± 5.3% of control; 1 μm acetylcholine, 99.2 ± 5.1% of control).
Radioligand Binding Dissociation Kinetic Assays
Further evidence for the allosteric effects of muscle relaxants at M3 muscarinic receptors was provided by dissociation kinetic radioligand binding studies. Rapacuronium (50–100 μm) significantly inhibited atropine induced 3H-NMS dissociation from the CHO-M3 muscarinic receptors (P < 0.01; n = 6; ) within clinically relevant concentrations. Gallamine, a muscle relaxant with known allosteric interactions with muscarinic receptors (positive control), significantly inhibited 3H-NMS dissociation at 100 μm (P < 0.001; n = 6; ). Similarly, vecuronium (50–100 μm) (P < 0.001; n = 6; ) and rocuronium (P < 0.05; n = 5; ) significantly inhibited atropine-induced 3H-NMS dissociation; however, the concentrations required are above those concentrations achieved with routine clinical use of these relaxants (peak serum concentrations: vecuronium, 3.2 μm; rocuronium, 4.7 μm).
These data, along with previously published data, demonstrate that rapacuronium has at least two detrimental effects on airway muscarinic receptors leading to bronchospasm. Rapacuronium exhibits a positive allosteric effect at M3 muscarinic receptors, which potentiated the contractile effect of acetylcholine at airway muscarinic receptors. This detrimental airway effect is in addition to and is additive to the previously described preferential antagonism of presynaptic M2 muscarinic receptors described for rapacuronium.
Muscarinic receptors on airway parasympathetic nerves are of the M2 subtype and normally function in an inhibitory autofeedback mode to prevent the further release of acetylcholine. Muscarinic receptors on the airway smooth muscle are of the M2 and M3 subtypes, which function to inhibit relaxation and facilitate contraction, respectively. Irritation of the well-innervated upper trachea (e.g. , by the introduction of an endotracheal tube) initiates a neural reflex that results in the release of acetylcholine from parasympathetic nerves that acts on M2 and M3 muscarinic receptors in airway smooth muscle, resulting in bronchoconstriction. Normally, the release of acetylcholine is terminated by activation of the inhibitory autofeedback receptors on the parasympathetic nerves. The introduction of a large intubating dose of a muscle relaxant with the properties of rapacuronium during this period of parasympathetic nerve activation creates a scenario where negative feedback inhibition of acetylcholine release is blocked (antagonism of neural M2 muscarinic receptor) coincident with augmentation of the contractile effect of acetylcholine at the muscle M3 muscarinic receptor (positive allosteric effect at M3 muscarinic receptor). This dual detrimental effect of rapacuronium at airway muscarinic receptors within clinically relevant concentrations is unique among the muscle relaxants we have studied.10,,11
Increased baseline force of tracheal rings seen when rapacuronium was added in the presence of subthreshold acetylcholine (i.e. , after an acetylcholine dose response with minimal washing or after a single small dose of acetylcholine) suggests that rapacuronium increases airway tone by a positive allosteric effect at the M3 muscarinic receptor. Two clinical studies are also consistent with this mechanism. Increased peak airway inflating pressures or a decrease in maximal expiratory flow occurred with the administration of rapacuronium during steady state anesthesia, a period not associated with heightened activity of airway parasympathetic nerves.
The increase in muscle force by rapacuronium in the presence of subthreshold acetylcholine in the organ baths was inhibited by atropine but was unaffected by antagonists of histamine, neurokinin, or leukotriene receptors or by inhibition of calcium channels or by previous depletion of NANC neurotransmitters. Taken together, these findings suggest that the mechanism of the effect of rapacuronium on baseline force required acetylcholine and involved muscarinic receptors. These findings again are consistent with a positive allosteric effect of rapacuronium on M3 muscarinic receptors that allows acetylcholine to have an increased effect as an agonist at its orthosteric site.
Neuromuscular blocking agents are known to have the potential for both orthosteric9,,11 and allosteric interactions with muscarinic receptors.15,,16,,33,,34 Gallamine and alcuronium are the most extensively characterized muscle relaxants in terms of muscarinic allosteric interactions, and gallamine serves as the prototypical muscarinic allosteric ligand. Similarly, alcuronium has been shown to enhance binding of muscarinic antagonists , but neither gallamine nor alcuronium has been shown to enhance the binding of agonists at the muscarinic receptor.37,,38 In our previous studies, gallamine displayed antagonist affinities for the M2 versus M3 muscarinic receptors that were similar to the antagonist affinity profile seen with rapacuronium (i.e. , M2 > M3), but clinical bronchospasm was not a widely reported problem with gallamine. One interpretation of these findings that may explain the different clinical experiences with these two drugs is that positive allosteric effects at the M3 muscarinic receptor (by rapacuronium) has more deleterious effects in the airway than antagonism of prejunctional M2 muscarinic receptors (by gallamine or rapacuronium). Alternatively, the combination of prejunctional M2 antagonism and postjunctional M3 positive allosterism may synergize to account for the detrimental airway effects seen with rapacuronium. Indeed, rapacuronium is the only muscle relaxant studied that within clinically relevant concentrations displays both M2 muscarinic receptor antagonism and M3 muscarinic receptor positive allosteric effects. Although M1 muscarinic receptors have been described in the parasympathetic ganglia of some species, they were not relevant to the current study, which was performed without activation of the cholinergic nerves in the tracheal rings (i.e. , without electrical field stimulation).
Interestingly vecuronium, another commonly used muscle relaxant, had a similar increase in baseline force in the organ baths (), but this effect occurred at much greater concentrations than those achieved clinically. Likewise, rocuronium led to an increase in baseline force; however, it was not sustained, and it only occurred at concentrations of rocuronium above those achieved clinically. Conversely, pancuronium potently decreased the baseline force, demonstrating either classic orthosteric antagonism or strong negative cooperativity at the airway smooth muscle M3 muscarinic receptor.
These functional studies in the organ baths suggesting allosteric interactions led us to perform biochemical (i.e. , inositol phosphate) and radioligand (i.e. , dissociation kinetic) studies to further characterize the interaction of rapacuronium on the allosteric site on recombinantly overexpressed human M3 muscarinic receptors in CHO cells. Low concentrations of rapacuronium alone had no effect on inositol phosphate synthesis but enhanced acetylcholine-induced increases in inositol phosphate synthesis via the M3 muscarinic receptor, whereas higher concentrations of rapacuronium decreased acetylcholine-induced inositol phosphate synthesis. This biphasic effect of rapacuronium on acetylcholine-induced inositol phosphate synthesis mimicked the effects on force in intact airway smooth muscle, suggesting that at low concentrations (but still within clinically achieved ranges), rapacuronium has a positive allosteric effect at M3 muscarinic receptors, whereas at high concentrations (above those achieved clinically), rapacuronium can antagonize M3 muscarinic receptors via blockade of the orthosteric site. The selectivity of this effect for the M3 muscarinic receptor was further confirmed by inactivation of the M3 muscarinic receptor by alkylation with 4-DAMP mustard, which resulted in complete attenuation of the increase in inositol phosphate synthesis seen with acetylcholine with or without rapacuronium. Acetylcholine or rapacuronium also had no effect on inositol phosphate synthesis in another cell line (CHO-M2) that expresses the human M2 muscarinic receptor.
A radioligand dissociation kinetic assay evaluates whether an agent has an allosteric interaction with a receptor. Allosteric agents inhibit the rate of dissociation of a radioligand induced by an orthosteric competitive ligand (e.g. , atropine or acetylcholine for the M3 muscarinic receptor). Rapacuronium at clinically significant doses inhibited atropine-induced dissociation of 3H-NMS, confirming that rapacuronium interacts with the M3 muscarinic receptor in an allosteric manner. Vecuronium and rocuronium also exhibited allosteric interactions with the M3 muscarinic receptor; however, this only occurred at concentrations of vecuronium or rocuronium above those achieved with clinical use.
Another common mechanism associated with the potential of muscle relaxants for bronchoconstriction is histamine release. Histamine release is described after administration of several nondepolarizing muscle relaxants, including curare, atracurium, and mivacurium.39,,40,,41,,42 However, this seems to be an unlikely mechanism for rapacuronium because two studies of increased airway tone during the maintenance phase of general anesthesia attributed their results to selective M2 muscarinic receptor antagonism,31,,32 and in a study of seven adult patients who had development of bronchospasm, histamine concentrations did not increase while the patients were receiving rapacuronium. Our current study is consistent with this in that pyrilamine pretreatment had no effect on rapacuronium-induced increases in muscle force, ruling out release of endogenous histamine from cells within the tracheal rings.
The search for a muscle relaxant with rapid onset and offset has led to the clinical introduction of relaxants with low potency, requiring larger doses to achieve a targeted clinical effect. In the current study, rapacuronium and vecuronium both demonstrated similar increases in baseline force at similar concentrations. However, because rapacuronium is a much less potent agent, its clinically recommended intubation dose was 10–20 times greater (1.5–2.5 mg/kg) than the clinically recommended dose of vecuronium (0.1 mg/kg). Therefore, this allosteric effect at M3 muscarinic receptors is not unique to rapacuronium, because many muscle relaxants act allosterically with muscarinic receptors. The detrimental consequences of rapacuronium at airway muscarinic receptors are most likely due to the requirement of large clinical doses and the achievement of clinical concentrations that allow for positive cooperativity at the airway smooth muscle M3 muscarinic receptor and antagonism of the M2 muscarinic receptor.
In summary, rapacuronium has dual detrimental and unique effects at airway muscarinic receptors. Not only does rapacuronium block M2 muscarinic receptors, potentiating vagal induced acetylcholine release,10,,11 but rapacuronium also binds to an allosteric site on the M3 muscarinic receptor, resulting in an enhancement of acetylcholine activation of this receptor. Both of these mechanisms lead to increased smooth muscle tone and would be additive in their ability to provoke bronchospasm. Despite the removal of rapacuronium from clinical practice, it is important to understand the mechanism by which rapacuronium apparently contributed to fatal bronchospasm. All muscle relaxants developed to date have affinity for muscarinic receptors, and it is likely that the next generation of muscle relaxants that are developed to attempt to replace succinylcholine will also have affinities for muscarinic receptors. We have defined two muscarinic receptor–mediated effects of rapacuronium that likely contribute to its potential to induce bronchospasm. It is therefore prudent that all newly designed muscle relaxants should be evaluated for these properties at concentrations likely to be achieved clinically.