[New aspects of common antagonists used in the perioperative period: preface and comments]

Anesthesia is a state made by multiple pharmacological agents that affect the functions of central, peripheral and autonomic nervous systems. Antagonists are often used to reverse the effects of anesthetic agents, muscle relaxants, and so forth. Antagonists often have side effects other than their specific antagonistic effect. Some side effects may be clinically useful. Ketamine was found to be a NMDA receptor antagonist. Ketamine potentially has neuroprotective, and anti-tumor effect. Local anesthetic agent also has been known to have anti-NMDA receptor effect. Although the mechanisms of local anesthetics to prevent or treat chronic pain have not been clearly elucidated, local anesthetics may have a place in the treatment of chronic pain. Naloxone may have spinal protective effects when used during thoracic aortic aneurysm surgery and stent placement. Peripherally and centrally active anticholinesterase agents have unique effects. Centrally active anticholinesterase agents such as donepezil and revastigmine, and galantamine have been used to treat patients with Alzheimer's disease. These agents may antagonize the effects of non-depolarizing muscle relaxants. Flumazenil may have adverse effects in patients on chronic benzodiazepines and tricyclic or tetracyclic antidepressants. Because sugammadex has little adverse effects, it has become a popular agent to reverse muscle relaxation by rocuronium in Japan.

TETRAHYDROAMINACRIN AS A DECURARISING AGENT

Tetrahydroaminacrin has marked anticholinesterase activity. It is a mild antagonist of tubocurarine, and this may be shown on the rat phrenic nerve diaphragm preparation. This antagonism is more marked in rats and dogs. Other pharmacological properties are described. THA has been used successfully to decurarise patients who have received tubocurarine or gallamine. Usually the administration of atropine is not necessary and recurarisation does not occur.

Effect of novel anti-inflammatory ethanolamine derivatives with antioxidant properties on drug metabolising enzymes

The influence of four ethanolamine derivatives with anti-inflammatory and antioxidant activity on the in vitro aminopyrine N-demethylation was studied. It was found that these compounds inhibit the N-demethylation of aminopyrine. 1-Cyclohexyl-5-(2-hydroxy-ethylamino)-pentan-2-one (compound 4), possessing the highest inhibitory activity and found earlier to be a potent anti-inflammatory agent, is further tested in vivo on zoxazolamine-induced paralysis, after a single administration to rats, and on aminopyrine N-demethylation, rat hepatic total cytochrome P450 and protein (postmitochondrial and microsomal) content, after a prolonged treatment. It was found that the examined compound had no significant influence on the above biotransformations, however, it could decrease the catalytically active hepatic cytochrome P450 content. These results, considered together with some structural and physicochemical properties of the compound, indicate that this compound may act as a CYP2D6 substrate.

Reversal of benzodiazepine-induced renal vasculature relaxation with flumazenil

The effects of the central-type benzodiazepine receptor antagonist flumazenil on renal vascular tone and its ability to reverse the benzodiazepine-induced vasodilation were investigated. The isolated and perfused rat kidney model was used. Flumazenil was unable to modify renal vascular resistance under basal conditions and in noradrenaline-pretreated kidneys. Relaxation induced by diazepam or clonazepam of noradrenaline-preconstricted renal vasculature was blunted by 10 microM flumazenil. These results suggest that central-type benzodiazepine receptors could be involved in benzodiazepine-induced renal vasodilation.

A comparison between the effects of edrophonium and choline in the skeletal muscles of the cat

The effects of edrophonium and choline have been compared with those of the depolarizing substances acetylcholine, decamethonium, and suxamethonium, in both innervated and chronically denervated tibialis anterior muscles of cats under chloralose anaesthesia. Both edrophonium and choline were more potent antagonists to paralysis by tubocurarine than could be accounted for by their ability to stimulate the motor end-plates directly. It appeared likely that direct depolarization of the end-plate played no part in the anti-curare action of edrophonium and only some part in the anti-curare action of choline. A paralysis produced by the neuromuscular blocking agent, benzoquinonium, was more readily antagonized by a tetanus or by acetylcholine, suxamethonium, and decamethonium than a similar paralysis produced by tubocurarine. The tetraethyl ammonium ion was also slightly more effective against a paralysis by benzoquinonium. On the other hand, edrophonium was about 300 times and choline about five times less potent as an antagonist to benzoquinonium than to tubocurarine. Furthermore, the previous administration of benzoquinonium abolished the antagonistic action to tubocurarine of normally effective doses of edrophonium and reduced that of choline. These results were similar to those previously obtained with neostigmine, physostigmine and ethyl pyrophosphate and suggested that there was some similarity in the mechanism of action of all of these substances. Benzoquinonium, therefore, showed promise as a useful pharmacological tool for distinguishing compounds with this particular type of action. These anti-curare compounds did not appear to act by cholinesterase inhibition, not by an increase in the sensitivity of the motor end-plates. In common with other workers, we suggest that there is a pre-synaptic mechanism of action.

Interaction of taurine on baclofen intestinal absorption: a nonlinear mathematical treatment using differential equations to describe kinetic inhibition models

Previous studies showed that the in situ absorption of baclofen in rat jejunum was inhibited by beta-alanine, a nonessential amino acid, and therefore mediated, at least in part, by some beta-amino acid carrier. In this paper a similar study was undertaken using taurine, a sulfonic beta-amino acid, in order to evaluate its effect and to establish a general inhibition model. To achieve this goal, remaining concentrations of inhibitor were also measured and incorporated into the model. Previously, kinetic absorption in situ parameters for taurine in free solution were obtained: Vm = 27.73 +/- 9.99 mM h-1, K(m) = 8.06 +/- 2.82 mM, Ka (passive difussion component) = 0.40 +/- 0.28 h-1. Isotonic solutions containing 0.5 mM baclofen with starting taurine concentrations ranging from 0 to 100 mM were perfused in rat jejunum, and the remaining concentrations of both compounds were measured. The apparent rate pseudoconstant of the drug clearly decreased as the remaining taurine concentration increased. The interaction can be described as a complete competitive inhibition plus a second component, K, noninhibited, K = 0.58 (+/- 0.03) h-1, Ki = 20.62 (+/- 4.04) mM, Vmi = 28.12 (+/- 6.12) mM h-1, Kmi = 11.71 (+/- 2.53) mM, Kai = 0.47 (+/- 0.10) h-1. A residual absorption of baclofen in the presence of high taurine concentrations was observed, which should be attributed to another transport system not associated with the taurine carrier. In order to elucidate whether or not taurine and beta-alanine carriers are two separate entities that baclofen can use for absorption, further experiments using beta-alanine and taurine together as inhibitors (baclofen, 0.5 mM; beta-alanine, 50 mM, and taurine, 50 mM) were developed. Results indicated that baclofen and both amino acids share the same carrier in the intestinal absorption process. We have completed studies using leucine, taurine, and GABA together as inhibitors of drug absorption. An isotonic perfusion solution of 0.5 mM baclofen in the presence of 50 mM leucine, 25 mM taurine, and 25 mM GABA was perfused. Under these conditions the absorption rate pseudoconstant of baclofen decreases until 0.080 h-1 (+/- 0.069). Practical implications of these phenomena are briefly discussed.

Flumazenil: an unreliable antagonist in baclofen overdose

We report a case of inadvertent overdose of baclofen given intrathecally resulting in coma. This was unresponsive to flumazenil and required supportive intensive therapy. With the increasing use of baclofen intrathecally for spasticity and its wide interpatient dose variability, there is a need to find a safe antagonist to baclofen for routine medical use.

Effect of the centrally acting muscle relaxant tizanidine on spinal reflexes: involvement of descending noradrenergic systems

Experiments were performed on intact and spinalized rats anesthetized with urethane and alpha-chloralose. In intact rats, administration of tizanidine (0.1 mg/kg, i.v.) decreased the mono- (MSR) and polysynaptic reflex potentials (PSR). Blood pressure was initially elevated and then lowered by tizanidine. Although pretreatments with hexamethonium and phentolamine prevented the tizanidine-induced decrease in blood pressure, the depressant effects of tizanidine on the reflexes remained. The alpha 2-antagonist idazoxan inhibited the tizanidine-induced decrease in spinal reflexes, suggesting that central alpha 2-adrenoceptors are involved in the depression of the reflexes. In spinalized rats, tizanidine transiently increased the MSR and gradually decreased the PSR. Blood pressure was elevated transiently by tizanidine. Although the hypertensive effect of tizanidine was inhibited by phentolamine, the effect of tizanidine on the PSR did not change. Prazosin blocked the stimulatory effect of tizanidine on the MSR and caused a rapid decrease of the PSR, suggesting that spinal alpha 1-adrenoceptors are involved in the enhancement of the reflexes. These results suggest that the depressant effects of tizanidine on spinal reflexes are due to the supraspinal and spinal effects of the drug, and not to changes in blood pressure.

Oxytocin attenuates tolerance not only to the hypothermic but also to the myorelaxant and akinesic effects of ethanol in mice

Inhibition of ethanol tolerance by oxytocin has been demonstrated previously using the hypothermic effect only. The purpose of the present experiment was to investigate the effect of oxytocin on the development of tolerance to ethanol-induced hypothermia, myorelaxation and akinesia in mice. Four groups of mice received daily intraperitoneal injections of saline or oxytocin (0.005 mg) plus saline or ethanol (2 g/kg). The peptide was administered 2 hours before ethanol. For five consecutive days, temperature measurements were performed 20 minutes before and after ethanol injection. Myorelaxation and akinesia were evaluated following the second temperature measure. Oxytocin pretreatment, which had no intrinsic effects, resulted in a robust selective attenuation of tolerance to ethanol-induced hypothermia, myorelaxation and akinesia. These results suggest that the mechanisms for peptide modulation are common to these three typical effects of ethanol.

Glibenclamide blocks the relaxant action of pinacidil and cromakalim in airway smooth muscle

Concentration-relaxation curves for pinacidil and cromakalim were obtained in isolated guinea-pig tracheas contracted by histamine. Pinacidil produced complete relaxation with an EC50 value of 2.8 microM. The antidiabetic sulfonylurea, glibenclamide (0.1, 1, 10 microM), caused concentration-dependent rightward shifts (3- to 16-fold) of the concentration-relaxation curve for pinacidil without changing the maximal relaxant response. Cromakalim produced 85% relaxation with an EC50 value of 1.1 microM. Glibenclamide (0.1 microM) displaced the concentration-relaxation curve to the right and at higher concentrations (1-10 microM) caused nearly complete suppression of the maximal relaxant response to cromakalim. Glibenclamide not only prevented the effects of pinacidil and cromakalim but also produced a concentration-dependent and complete reversal of submaximal relaxations produced by these drugs. Glibenclamide was a selective antagonist of the relaxation of airway smooth muscle induced by pinacidil and cromakalim. Concentration-relaxation curves for theophylline, terbutaline and verapamil were unaffected by glibenclamide.

Phaclofen antagonizes the antinociceptive but not the sedative effects of (-)-baclofen

1. Intraperitoneal (ip) injection of (-)-baclofen induced long-lasting antinociceptive and sedative effects in rats. 2. Phaclofen, the phosphonic derivative of baclofen, fully antagonized the antinociceptive effect of (-)-baclofen. When injected intracerebroventricularly (icv), but not ip, phaclofen antagonized in a dose-dependent fashion (50-200 micrograms) the delays in behavioral response induced by (-)-baclofen (2.5-10 mg/kg ip) in both hot plate and tail flick tests. 3. In addition phaclofen (100 micrograms icv) counteracted the loss of the righting reflex induced by (-)-baclofen (7.5-15 mg/kg ip). 4. In contrast, phaclofen (100-200 micrograms icv) counteracted only in part the sedative effect of (-)-baclofen. In rats pretreated with the antagonist (200 micrograms icv), the electrocorticographic hypersynchrony due to (-)-baclofen (5 mg/kg ip) is replaced by a synchronized pattern associated with behavioral sedation. 5. These data are consistent with the reported antagonism by phaclofen on the effects of (-)-baclofen. They also seem to indicate that in rats phaclofen-sensitive GABA-B receptors play an important role in the analgesic effects of baclofen, but only a minor role in the sedative effects of this drug.

Pertussis toxin reduces the antiadrenergic effect of 2-chloroadenosine on papillary muscle and the direct negative inotropic effect of 2-chloroadenosine on atrium

2-Chloroadenosine reduced the contractile tension of guinea-pig atria directly, and inhibited the increase in tension produced by beta-adrenergic stimulation of guinea-pig papillary muscle. Both effects were reduced by 8-phenyltheophylline, a competitive antagonist at extracellular P1-purinoceptors. Treatment of guinea-pigs with pertussis toxin reduced the sensitivity of both atria and ventricles to 2-chloroadenosine. Atria were significantly affected after treatment with 125 micrograms/kg toxin, but not 100 micrograms/kg. 60 micrograms/kg toxin had no effect on the sensitivity of the ventricles, but 100 and 125 micrograms/kg significantly decreased the antiadrenergic effect of 2-chloroadenosine. We conclude that both the direct and antiadrenergic effects are mediated by an inhibitory guanine nucleotide binding protein.

Reversal of the muscle relaxant effect of diazepam but not of progabide by a specific benzodiazepine antagonist: Ro 15-1788

The muscle relaxant effects of diazepam and progabide were studied in mutant Han-Wistar rats which exhibit spontaneous tonic activity in the electromyogram of the gastrocnemius-soleus muscle. Treatment with diazepam or progabide produced a dose-related reduction of the spontaneous activity measured in the electromyogram. The depressant effect of diazepam, but not that of progabide was antagonized by the specific benzodiazepine antagonist Ro 15-1788. From these data it is apparent that the muscle relaxant effect of progabide is not mediated through an alteration of benzodiazepine receptors.

[Decurarization using the cation-anion molecular interaction of a myorelaxant and its antagonist]

In experiments on the cat neuromuscular preparation the authors investigated the anticurare action of the new compound IEM-931 having sulphonate-anionic groups 20 A apart. The new compound administered in a dose of 184 mkM/kg prevents and removes the curare effect of rythetronium which is its structural analogue. Regarding tubocurarine the antagonism is displayed, however, only from the dose of 384 mkM/kg. This indicates that efficacy of the anticurare action of IEM-931 is directly related to complimentarity of the interacting molecule structure. The experiment with regional relaxation of the cat muscles shows that the anticurare action of IEM-931 is realized both in the blood and on the cholinoreceptor. Moreover, as a result of molecular interaction the inactive complex is formed.