Roles of amino acids and subunits in determining the inhibition of nicotinic acetylcholine receptors by competitive antagonists

Rocuronium action can be affected by hyperventilation: a case report and computational simulation

The neuromuscular blocking potency of rocuronium varies with respiratory pH changes, increasing at lower pH and decreasing at higher pH; thus, hyperventilation-induced respiratory alkalosis is expected to decrease the potency of rocuronium. We report a case of anesthetic management of modified electroconvulsive therapy (m-ECT) for a patient monitored with electromyography-based neuromuscular monitoring during two patterns of ventilation to elucidate their relationship and propose the possible mechanisms underlying the effects by computational simulations. Case presentation: The patient was a 25-year-old man with schizophrenia. In m-ECT, hyperventilation may be used to produce longer seizures. We compared the neuromuscular monitoring data recorded during hyperventilation and during normal ventilation while receiving the same dose of rocuronium. Despite receiving the same dose of rocuronium, the time required for the first twitch to decrease to 80% of the control value was delayed in hyperventilation compared to normal ventilation. Conclusions: This case report and computational simulation suggest that respiratory alkalosis might delay the action of rocuronium. It is necessary to consider the delayed action of rocuronium when hyperventilation is performed.

Simulation of the kinetics of neuromuscular block: implications for speed of onset

BACKGROUND

The onset time for paralysis varies 3-fold among nondepolarizing muscle relaxants. Possible explanations include: (a) pharmacokinetic differences among drugs and (b) buffering of drug molecules by acetylcholine receptors as they diffuse into the neuromuscular junction. Although some pharmacokinetic models consider buffered diffusion, these models do not account for either the high density of receptors or synapse geometry. Here, I used computer simulations to calculate the kinetics of buffered diffusion. The goal was to determine the conditions under which buffered diffusion could account for differences in onset time among nondepolarizing muscle relaxants.

METHODS

Monte Carlo simulation was used along with a realistic 3-dimensional model of the rat neuromuscular junction. Simulations determined the time dependence of the number of drug-bound receptors. A 1000-fold range of drug potency was examined. In some simulations, the drug concentration outside the junction was changed instantaneously. In other simulations, the concentration changed according to predictions of pharmacokinetic models assuming time-dependent changes in plasma drug concentration. The rate constant for equilibration of drug between plasma and muscle, keo, was varied between 0.15 and 0.6 min(-1). Twitch amplitude was calculated from receptor occupancy assuming a high safety margin for neuromuscular transmission. Some simulations used a synaptic model with an increased nerve-muscle contact width.

RESULTS

Simulations with instantaneous changes in drug concentration at the synapse, indicated that the time to 50% twitch depression (onset time) was 0.1 to 30 seconds and was proportional to drug potency. This corresponds to iontophoretic application of drug to isolated neuromuscular junctions, but is too fast to explain onset times in humans. When pharmacokinetic models were used to calculate the drug concentration outside the synapse, buffered diffusion increased onset times of potent drugs (drugs for which the effective concentration at 50% twitch height is <600 nM). Simulations using keo = 0.6 min(-1) and a model with a 2- to 3-fold wider nerve-muscle contact width indicated that buffered diffusion could account for the differences in clinical onset times among the nondepolarizing muscle relaxants.

CONCLUSION

Monte Carlo simulation provides a biophysically appropriate way to incorporate buffered diffusion into pharmacokinetic modeling. The simulations indicated that buffered diffusion could account for differences in onset time among drugs. However, a better understanding of the geometry of the human neuromuscular junction is needed before the magnitude of the effect of buffered diffusion can be quantified.

Three arginines in the GABAA receptor binding pocket have distinct roles in the formation and stability of agonist- versus antagonist-bound complexes

Binding of the agonist GABA to the GABA(A) receptor causes channel gating, whereas competitive antagonists that bind at the same site do not. The details of ligand binding are not well understood, including which residues interact directly with ligands, maintain the structure of the binding pocket, or transduce the action of binding into opening of the ion channel gate. Recent work suggests that the amine group of the GABA molecule may form a cation-π bond with residues in a highly conserved "aromatic box" within the binding pocket. Although interactions with the carboxyl group of GABA remain unknown, three positively charged arginines (α(1)Arg67, α(1)Arg132, and β(2)Arg207) just outside of the aromatic box are likely candidates. To explore their roles in ligand binding, we individually mutated these arginines to alanine and measured the effects on microscopic ligand binding/unbinding rates and channel gating. The mutations α(1)R67A or β(2)R207A slowed agonist binding and sped unbinding with little effect on gating, demonstrating that these arginines are critical for both formation and stability of the agonist-bound complex. In addition, α(1)R67A sped binding of the antagonist 2-(3-carboxypropyl)-3-amino-6-(4 methoxyphenyl)pyridazinium bromide (SR-95531), indicating that this arginine poses a barrier to formation of the antagonist-bound complex. In contrast, β(2)R207A and α(1)R132A sped antagonist unbinding, indicating that these arginines stabilize the antagonist-bound state. α(1)R132A also conferred a new long-lived open state, indicating that this arginine influences the channel gate. Thus, each of these arginines plays a unique role in determining interactions with agonists versus antagonists and with the channel gate.

Site selectivity of competitive antagonists for the mouse adult muscle nicotinic acetylcholine receptor

The muscle-type nicotinic acetylcholine receptor has two nonidentical binding sites for ligands. The selectivity of acetylcholine and the competitive antagonists (+)-tubocurarine and metocurine for adult mouse receptors is known. Here, we examine the site selectivity for four other competitive antagonists: cisatracurium, pancuronium, vecuronium, and rocuronium. We rapidly applied acetylcholine to outside-out patches from transfected BOSC23 cells and measured macroscopic currents. We have reported the IC(50) of the antagonists individually in prior publications. Here, we determined inhibition by pairs of competitive antagonists. At least one antagonist was present at a concentration producing > or =67% receptor inhibition. Metocurine shifted the apparent IC(50) of (+)-tubocurarine in quantitative agreement with complete competitive antagonism. The same was observed for pancuronium competing with vecuronium. However, pancuronium and vecuronium each shifted the apparent IC(50) of (+)-tubocurarine less than expected for complete competition but more than expected for independent binding. The situation was similar for cisatracurium and (+)-tubocurarine or metocurine. Cisatracurium did not shift the apparent IC(50) of pancuronium or vecuronium, indicating independent binding of these two pairs. The data were fit to a two-site, two-antagonist model to determine the antagonist binding constants for each site, L(alphaepsilon) and L(alphadelta). We found L(alphaepsilon)/L(alphadelta) = 0.22 (range, 0.14-0.34), 20 (9-29), 21 (4-36), and 1.5 (0.3-2.9) for cisatracurium, pancuronium, vecuronium, and rocuronium, respectively. The wide range of L(alphaepsilon)/L(alphadelta) for some antagonists may reflect experimental uncertainties in the low affinity site, relatively poor selectivity (rocuronium), or possibly that the binding of an antagonist at one site affects the affinity of the second site.

Synergy between pairs of competitive antagonists at adult human muscle acetylcholine receptors

BACKGROUND

Synergistic neuromuscular blocking effects have been observed clinically with certain pairs of nicotinic acetylcholine receptor (nAChR) competitive antagonists. The mechanism for synergy has not been elucidated. We tested the hypothesis that synergy arises from a differential selectivity of antagonists for the two ligand binding sites on adult human nAChR.

METHODS

We expressed nAChR in BOSC23 cells. We applied ACh with or without antagonists to outside-out patches and measured macroscopic currents at room temperature. We determined the IC(90) for (+)-tubocurarine, metocurine, pancuronium, vecuronium, cisatracurium, rocuronium, and atracurium. For 15 combinations of two antagonists, we determined the IC(90) for one antagonist in the presence of the IC(70) of a second antagonist. We constructed isobolograms for 90% inhibition. For single antagonists, we measured inhibition of receptors containing mutations in the epsilon- and delta-subunits to determine site selectivity.

RESULTS

Two pairs of antagonists, metocurine+cisatracurium and cisatracurium+ atracurium exhibited additive inhibition. Ten combinations, including (+)-tubocurarine+ pancuronium and pancuronium+vecuronium, were highly synergistic such that the combination was two to three times more effective than expected for additivity. Three combinations were 1.5-1.6 times more effective than expected for additivity. Inhibition by (+)-tubocurarine and metocurine was sensitive to mutations in the epsilon-subunit only. Vecuronium was affected by the delta-subunit mutation only. Inhibition by other antagonists was decreased by mutations in either subunit.

CONCLUSIONS

Many combinations of antagonists exhibited synergistic effects on adult human nAChR. Synergy was observed with structurally similar and dissimilar antagonists. The degree of synergy did not always correlate well with site specificity assayed with mutants. In some, but not all cases, the synergy at the receptor level correlated with clinical determinations of synergy. We conclude that the synergistic actions of muscle relaxants can be partially explained by direct interactions with adult human nAChR.

The kinetics of competitive antagonism of nicotinic acetylcholine receptors at physiological temperature

Detailed information about the ligand-binding site of nicotinic acetylcholine receptors has emerged from structural and mutagenesis experiments. However, these approaches provide only static images of ligand-receptor interactions. Kinetic measurements of changes in protein function are needed to develop a more dynamic picture. Previously, we measured association and dissociation rate constants for competitive inhibition of current through embryonic muscle acetylcholine receptor channels at 25 degrees C. Little is known about competitive antagonism at physiological temperatures. Here, we performed measurements at 37 degrees C and used thermodynamics to estimate the energetics of antagonism. We used rapid solution exchange protocols to determine equilibrium and kinetics of inhibition of acetylcholine-activated currents in outside-out patches by (+)-tubocurarine, pancuronium and cisatracurium. Kinetic rates as high as 600 s(-1) were resolved by this technique. Binding was primarily enthalpy driven. The 12 degrees C increase in temperature decreased equilibrium antagonist binding by 1.7- to 1.9-fold. In contrast, association and dissociation rate constants increased 1.9- to 6.0-fold. Activation energies for dissociation were 90 +/- 6, 106 +/- 8 and 116 +/- 10 kJ mol(-1) for cisatracurium, (+)-tubocurarine and pancuronium, respectively. The corresponding apparent activation energies for association were 38 +/- 6, 85 +/- 6 and 107 +/- 13 kJ mol(-1). The higher activation energy for association of (+)-tubocurarine and pancuronium compared with cisatracurium is notable. This may arise from either a more superficial binding site for the large antagonist cisatracurium compared to the other ligands, or from a change in receptor conformation upon binding of (+)-tubocurarine and pancuronium but not cisatracurium. Differences in ligand desolvation and ligand conformation are not likely to be important.