Effects of corticosteroids on neuromuscular blocking actions of d-tubocurarine

Impact of intravenous dexamethasone on the initiation and recovery of atracurium in children: A double-blinded randomized controlled trial

BACKGROUND

Chronic steroid intake has been associated with attenuation of neuromuscular block. Despite some promising animal and adult studies, the effect of a single dose of intravenous dexamethasone on neuromuscular blockers is not well established. Thus, the present study aimed to demonstrate the effect of dexamethasone given at the time of induction for the prevention of PONV on the action of neuromuscular blockers in children undergoing elective surgery.

METHOD

After obtaining approval from the Institute Ethics Committee and written informed parental consent, 100 ASA I and II children aged 4-15 years undergoing elective surgery randomized to receive either: 0.15 mg/kg (maximum of 5 mg) of dexamethasone diluted to a total volume of 2 ml with 0.9% saline (n = 50) or 2 ml of 0.9% saline (n = 50) at the time of induction. The time interval between application of atracurium and maximum T1 depression, 25% twitch height recovery of T1, amid 25% and 75% twitch height recovery of T1, amid the 25% twitch height recovery of T1 and recovery of the neuromuscular block to a TOF ratio of 0.9, and in between the initiation of atracurium injection till the recovery of the neuromuscular block to a TOF ratio of 0.9 was defined as onset time, clinical duration, recovery index, recovery time, and total recovery period, respectively, and recorded.

RESULTS

The onset time and recovery index time were lower (1.96 ± 0.39, 8.04 ± 2.14, respectively) with dexamethasone in comparison with saline (2.01 ± 0.51, 8.9 ± 3.4, respectively) but not statistically significant. The clinical duration, recovery time, and total recovery period were similar.

CONCLUSION

Application of a single bolus dose (0.15 mg/kg) of dexamethasone during induction does not attenuate atracurium-induced neuromuscular blockade in children.

The Safety Factor for Neuromuscular Transmission: Effects of Dimethylsulphoxide, Cannabinoids and Synaptic Homeostasis

BackgroundIn myasthenia gravis, impaired postsynaptic sensitivity to acetylcholine results in failure of neuromuscular transmission and fatiguing muscle weakness.ObjectiveDevelop an ex vivo muscle contraction assay to test cannabinoids and other substances that might act on the myasthenic neuromuscular junction to restore control of the muscle.MethodsTubocurarine was added to an ex vivo, mouse phrenic nerve-hemidiaphragm muscle preparation to reduce acetylcholine sensitivity. This produced a myasthenia-like decrement in twitch force during a train of 10 nerve impulses (3 / sec). Endplate potential (EPP) recordings were used to confirm and extend the findings.ResultsSurprisingly, addition to the bath of dimethylsulphoxide (DMSO), at concentrations as low as 0.1%(v/v), partially reversed the decrement in nerve-evoked force. Intracellular electrophysiology, conducted in the presence of tubocurarine, showed that DMSO increased the amplitudes of both the spontaneous miniature EPP (MEPP) and the (nerve-evoked) EPP. In the absence of tubocurarine (synaptic potentials at physiological levels), an adaptive fall in quantal content negated the DMSO-induced rise in EPP amplitude. The effects of cannabinoid receptor agonists (solubilized with DMSO) in the contraction assay do not support their further exploration as useful therapeutic agents for myasthenia gravis. CP 55,940 (a dual agonist for cannabinoid receptor types 1 and 2) reversed the beneficial effects of DMSO.Conclusions:We demonstrate a powerful effect of DMSO upon quantal amplitude that might mislead pharmacological studies of synaptic function wherever DMSO is used as a drug vehicle. Our results also show that compounds targeting impaired neuromuscular transmission should be tested under myasthenic-like conditions, so as to avoid confounding effects of synaptic homeostasis.

Effect of protracted dexamethasone exposure and its withdrawal on rocuronium-induced neuromuscular blockade and sugammadex reversal: an ex vivo rat study

Studies have reported that protracted dexamethasone treatment induces resistance to nondepolarizing neuromuscular blocking agents (NMBAs) and the association with nicotinic acetylcholine receptors in the diaphragm of rats. Here, we investigated the effect of protracted dexamethasone administration on the sensitivity to rocuronium and the recovery profile when reversed by sugammadex; additionally, we observed the recovery period of pharmacodynamic change after withdrawal. Sprague-Dawley rats received daily intraperitoneal injections of dexamethasone or saline for 14 days. On days 1, 3, and 7 after the last dexamethasone treatment (Dexa1, Dexa3, and Dexa7, respectively) or 1 day after saline (control group), the phrenic nerve-hemidiaphragm preparation was dissected for assay. The dose-response curve of rocuronium in Dexa1 was shifted to the right compared to controls, but curves in Dexa3 and Dexa7 were not significantly different. Groups were not significantly different in attaining the train-of-four ratio ≥ 0.9, but the recovery index in Dexa7 was shorter than that in control and Dexa1. Recovery profiles (period of sugammadex reversal) were not correlated with resistance properties but rather with total administered drugs (binding capacity of NMBAs and sugammadex). Protracted dexamethasone exposure induced resistance to rocuronium but seemed to have no effect on sugammadex reversal in the rat diaphragm.

Effects of methylprednisolone on the duration of rocuronium-induced neuromuscular block: A randomized double-blind trial

BACKGROUND

We aim to investigate whether intraoperative use of methylprednisolone could affect the duration of rocuronium-induced neuromuscular blockade.

METHODS

A double blind, randomized, placebo-controlled trial was conducted. A total of 136 patients underwent gynecologic laparoscopic surgery were randomly divided into 3 groups: pregroup, receiving intravenous injection of methylprednisolone (40 mg) 30 minutes before induction of anesthesia; postgroup, receiving intravenous injection of methylprednisolone (40 mg) immediately after induction of anesthesia and intubation; and control group, receiving intravenous injection of normal saline. Patients were intravenously administrated with rocuronium 0.6 mg/kg, and changes in adductor policies evoked twitch responses were measured by ulnar nerve stimulator.

RESULTS

We found that all patients achieved maximum blockade effects, and there was no difference in onset time among the 2 groups. For time required to achieve train-of-four ratio (TOFR) 90%, pregroup (64.50 ± 10.52 minutes) and postgroup (65.29 ± 11.64 minutes) were significantly shorter than that of the control group (71.04 ± 10.55 minutes, P = .027), whereas clinical duration and total duration were significantly shorter in the 2 groups received methylprednisolone than the control group. However, there was no significant difference between the 2 treatment groups either in clinical duration and total duration of muscle relaxants, or time required to achieve TOFR 90%. No significant difference was found in recovery index among the 3 groups.

CONCLUSION

Our findings suggest that a single intravenous injection of methylprednisolone, no matter preoperatively or intraoperatively, could shorten the duration of rocuronium-induced neuromuscular blockade.

Effect of dexamethasone on the onset time and recovery profiles of cisatracurium

BACKGROUND

The effect of dexamethasone injection on cisatracurium-induced neuromuscular block was compared according to different injection time points.

METHODS

One hundred seventeen patients were randomly assigned to three groups: 8 mg of dexamethasone injected intravenously 2-3 h before anesthesia (group A), just before anesthesia induction (group B), and at the end of surgery (control group). Three minutes after anesthesia induction, intubation was performed without neuromuscular blockers, and acceleromyography was initiated. All patients received 0.05 mg/kg cisatracurium; the onset time and recovery profiles were recorded.

RESULTS

Eighty patients were finally enrolled. The onset time (median [interquartile range], seconds) was significantly hastened in group A (520.0 [500.0-560.0], n = 30) compared to that in group B (562.5 [514.0-589.0], n = 22) (P = 0.008) and control group (586.5 [575.0-642.5], n = 28) (P < 0.001). The onset time in group B was faster than the control group (P = 0.015). The recovery time [mean (95% CI) minutes] was significantly hastened in group A [28.5 (27.3-29.6)] compared to that in group B [32.3 (31.0-33.6)] (P < 0.001) and control group [30.9 (29.9-31.8)] (P = 0.015). The total recovery time was significantly hastened more in group A [47.1 (45.5-48.6)] than group B [52.8 (51.6-54.0) minutes] (P < 0.001) and control group [50.5 (48.7-52.3) minutes] (P = 0.008).

CONCLUSIONS

A single dose of 8 mg of dexamethasone hastened the onset and total recovery times of cisatracurium-induced block by approximately 15 and 9%, respectively if administered 2-3 h prior to surgery.

Dexamethasone decreases the duration of rocuronium-induced neuromuscular block: a randomised controlled study

BACKGROUND

Several drugs influence the time course of neuromuscular block during general anaesthesia.

OBJECTIVE

To evaluate the effect of a single dose of dexamethasone 8 mg on the time course of a rocuronium-induced neuromuscular block.

DESIGN

A randomised controlled, unblinded, monocentre trial.

SETTING

Kreiskrankenhaus Dormagen, Dormagen, Germany.

PATIENTS

One hundred and eight adult patients scheduled for elective gynaecological laparoscopic surgery allocated to three groups.

INTERVENTIONS

Patients received dexamethasone 8 mg intravenously 2 to 3 h prior to surgery (Group A), during induction of anaesthesia (Group B) or after recovery of the neuromuscular block (Group C, control).

MAIN OUTCOME MEASURES

The time course of the neuromuscular block of rocuronium 0.3 mg kg was assessed using acceleromyography. The primary end point was the time from start of injection of rocuronium until recovery to a train-of-four ratio of 0.9.

RESULTS

The clinical duration was decreased in Group A (15.8 ± 4.5 min) compared with Group B (18.7 ± 5.8 min; P = 0.031). The recovery index was reduced in Group A (6.8 ± 1.8 min) compared with Group B (8.1 ± 2.6 min; P = 0.018) and Group C (8.3 ± 2.8 min; P = 0.01). The recovery to a train-of-four ratio of 0.9 was shorter in Group A (30.4 ± 6.9 min) than in Groups B (36.3 ± 10.7 min; P = 0.031) and C (36.8 ± 11.3 min; P = 0.02).

CONCLUSION

A single dose of dexamethasone 8 mg attenuated rocuronium-induced block by 15 to 20% if administered 2 to 3 h prior to induction of anaesthesia. However, the administration of dexamethasone during induction of anaesthesia did not influence the time course of the neuromuscular block.

TRIAL REGISTRATION

ClinicalTrial.gov Identifier: NCT01782820.

Dexamethasone‑induced hyposensitivity to rocuronium in rat diaphragm associated with muscle‑fiber transformation

The aim of the current study was to investigate the effect of chronic dexamethasone (Dex) administration on rat diaphragm function and sensitivity to rocuronium and muscle‑fiber transformation. Adult male Sprague‑Dawley rats were randomized to receive a daily intraperitoneal injection of Dex to evaluate whether alterations in diaphragm function and susceptibility to rocuronium would be induced. In addition, diaphragm contractile properties, histopathology and isometric twitch tensions of nerve‑hemidiaphragm preparations were evaluated. Dex administration led to impaired diaphragm force generation, increased fatigue resistance and a prolonged half‑relaxation time, as well as time‑to‑peak tension. Dex treatment led to desensitization of the rat diaphragm to rocuronium, as demonstrated by a shift of the rocuronium concentration‑twitch tension curves to the right. Histochemical analysis of adenosine triphosphatase revealed that the distribution and cross‑sectional area of type II fibers were decreased in rats exposed to Dex. The present study indicates that chronic Dex treatment induced alterations in muscle function and that susceptibility to rocuronium is associated with muscle fiber‑type transformation, which may aid in directing future administration of muscle relaxants.

Attenuation of a rocuronium-induced neuromuscular block in patients receiving prednisolone

BACKGROUND

This study tested the influence of continuous medication (more than 4 weeks) with prednisolone on a rocuronium-induced neuromuscular block.

METHODS

The time course of a rocuronium-induced neuromuscular blockade (0.3 mg/kg) was investigated in 40 patients with chronic inflammatory bowel disease undergoing elective abdominal surgery. The primary end point was the time from the start of injection of rocuronium until recovery of the TOF ratio to 0.9. Twenty patients received continuous medication with prednisolone (group A), and 20 were without glucocorticoid medication (group B). Additionally, another 20 patients without inflammatory bowel disease and without glucocorticoid medication served as control (group C).

RESULTS

The onset time was prolonged in group A [253 (51.2) s] compared with group B [187 (61.3) s]. Twitch height at the onset of the block was higher in group A [16.5 (0-61)%] than that in group B [5.0 (0-33)%]. The duration to 25% twitch height was shorter in group A [12.6 (0-20.7) min] compared with group B [16.7 (0-25.3) min] and group C [16.9 (0-29.3) min]. The recovery to a train-of-four ratio of 0.9 was reduced in group A [25.7 (23-34.3) min] compared with group B [34.7 (32.7-44.2) min] and group C [36.5 (31.7-42.3) min].

CONCLUSIONS

Prednisolone treatment in patients with inflammatory bowel disease is associated with a delayed onset and a shorter duration of action of rocuronium. The presence of an inflammatory bowel disease did not influence the neuromuscular block.

Influence of a continuous prednisolone medication on the time course of neuromuscular block of atracurium in patients with chronic inflammatory bowel disease

BACKGROUND

Corticosteroids interact with neuromuscular blocking agents. However, experimental data are contradictory: enhancement and attenuation of the neuromuscular block has been observed. This study tested the influence of long-term medication with prednisolone on atracurium-induced neuromuscular block.

METHODS

Sixty patients with chronic inflammatory bowel disease undergoing elective abdominal surgery were investigated. Thirty patients received a long-term medication with prednisolone (Group A) and 30 were without corticoid medication (Group B). Additionally, another 30 patients without inflammatory bowel disease and without corticoid medication served as control (Group C). The following parameters of an atracurium-induced neuromuscular block (0.25 mg kg(-1)) were measured: onset time, maximum block, recovery to 25% first twitch height, recovery index (time from 25% until 75% recovery of first twitch), duration to recovery to a train-of-four (TOF) rate of 0.7 and 0.9.

RESULTS

The groups did not differ with regard to onset time, maximum block, and recovery index. The duration to 25% twitch height was significantly lower in Group A [18.1 (0-30.7) min] compared with Group B [23.5 (0-36.7) min; P<0.05]. Duration to a TOF rate of 0.7 and 0.9, respectively, were significantly reduced in Group A [36.1 (7.9) and 40.9 (9.0 min)] compared with Group B [47.9 (7.6) and 53.4 (9.2) min; P<0.001].

CONCLUSIONS

Long-term medication with prednisolone resulted in a shorter duration of an atracurium-induced neuromuscular block in patients with Crohn's disease or ulcerative colitis. The presence of the inflammatory bowel disease did not influence the time course of the neuromuscular block.

Approach to neuromuscular disorders in the intensive care unit

Neuromuscular disorders increasingly are recognized as a complication in patients in the intensive care unit (ICU) and represent a common cause of prolonged ventilator dependency. The distinct syndromes of critical illness myopathy, prolonged neuromuscular blockade, and critical illness polyneuropathy (CIP) may arise as a consequence of sepsis, multi-organ failure, and exposure to various medications--notably, intravenous corticosteroids and neuromuscular blocking agents--but the pathophysiology of these disorders remains poorly understood. More than one syndrome may occur simultaneously, and the distinctions may be difficult in a particular patient, but a specific diagnosis usually can be established after careful clinical, electrodiagnostic, and, when necessary, histological evaluation. For example, asthmatics requiring treatment with corticosteroids and neuromuscular blocking agents may develop an acute myopathy characterized by generalized weakness, preserved eye movements, elevated creatine kinase levels, and myopathic motor units on electromyography (EMG). Muscle biopsy demonstrates distinctive features of thick (myosin) filament loss on ultrastructural studies. Conversely, those with a prolonged ICU course that is complicated by episodes of sepsis with failure to wean from the ventilator, distal or generalized flaccid limb weakness, and areflexia probably have CIP. EMG in these patients demonstrates reduced or absent motor and sensory potentials with neurogenic motor units. Prolonged neuromuscular blockade most commonly occurs in patients with renal failure who have received prolonged infusions of neuromuscular blockers. There is severe flaccid, areflexic paralysis with normal sensation, facial weakness, and ophthalmoparesis that persists for days or weeks after the neuromuscular blockers have been discontinued. Repetitive nerve stimulation shows a decrement of the compound muscle action potential and, in most cases, establishes a disorder of neuromuscular transmission. With the recent epidemic of West Nile virus infection, a clinical syndrome of acute flaccid paralysis with several features indistinguishable from poliomyelitis has emerged. This article critically examines the clinical, electrophysiological, and pathological features of these and other acute neuromuscular syndromes that arise in the context of ICU care and summarizes the current understanding of the pathophysiology and treatment of these disorders.

Acute toxicity of gymnodimine to mice

The acute toxicity of the phycotoxin gymnodimine to female Swiss mice by intraperitoneal injection and by oral administration has been determined. Gymnodimine was highly toxic by injection, the LD50 being only 96 microg/kg. Animals either died within 10 min of injection or made a full recovery with no perceptible long-term effects. Gymnodimine was also toxic after oral administration by gavage (LD50 755 microg/kg), but was much less toxic when administered with food. No signs of toxicity were seen in mice voluntarily ingesting food containing gymnodimine at a level sufficient to give a dose of approximately 7500 microg/kg. Pre-treatment with physostigmine or neostigmine protected against injected gymnodimine, suggesting that the latter exerts its toxic effects via blockade of nicotinic receptors at the neuromuscular junction. The low toxicity of gymnodimine when ingested with food suggests that this compound is of low risk to humans, a conclusion that is consonant with anecdotal evidence for the absence of harmful effects in individuals consuming shellfish contaminated with gymnodimine.

New evidence for a presynaptic action of prednisolone at neuromuscular junctions

The action of prednisolone at the neuromuscular junction was studied in mouse isolated phrenic nerve-diaphragm and rat external popliteal/sciatic nerve-tibialis anterior muscle preparations. Prednisolone (0.03 mM and 0.3 mM) did not alter the twitch-tension in phrenic nerve-diaphragm preparations after 120 min, but increased the frequency (170 +/- 4%) and amplitude (200 +/- 13%) of miniature end-plate potentials. Quantal content was not influenced by the glucocorticoid treatment. Prednisolone (400 microg/kg) did not change the twitch-tension in rat external popliteal/sciatic nerve-tibialis anterior muscle preparations. However, this steroid (0.3 mM) prevented the neuromuscular blockade by d-tubocurarine (1.45 microM) in mouse preparations by 70 +/- 10% (P < 0.05). A similar effect (82 +/- 6% protection, P < 0.05) occurred in rats treated with prednisolone (400 microg/kg) before d-tubocurarine (225 microg/kg). In phrenic nerve-diaphragm preparations, prednisolone (0.3 mM) increased (13 +/- 4%, p < 0.05) the twitch-tension in the presence of beta-bungarotoxin (1 microM), and prevented the blockade produced by this toxin (0.15 microM) in its third phase of action. This presynaptic facilitatory effect may contribute to the usefulness of prednisolone in myasthenia gravis.

Prednisolone-induced muscle dysfunction is caused more by atrophy than by altered acetylcholine receptor expression

Large doses of glucocorticoids can alter muscle physiology and susceptibility to neuromuscular blocking drugs by mechanisms not clearly understood. We investigated the effects of moderate and large doses of prednisolone on muscle function and pharmacology, and their relationship to changes in muscle size and acetylcholine receptor (AChR) expression. With institutional approval, 35 Sprague-Dawley rats were randomly allocated to receive daily subcutaneous doses of 10 mg/kg prednisolone (P10 group), 100 mg/kg prednisolone (P100 group), or an equal volume of saline (S group) for 7 days. A fourth group of rats was pair fed (food restricted) with the P100 rats for 7 days (FR group). On Day 8, the nerve-evoked peak twitch tensions, tetanic tensions, and fatigability, and the dose-response curves of d-tubocurarine in the tibialis cranialis muscle were measured in vivo and related to muscle mass or expression of AChRs. Rate of body weight gain was depressed in the P100, FR, and P10 groups compared with the S group. Tibialis muscle mass was smaller in the P100 group than in the P10 or S groups. The evoked peak twitch and tetanic tensions were less in the P100 group than in the P10 or S groups, however, tension per milligram of muscle mass was greater in the P100 group than in the S group. The 50% effective dose of d-tubocurarine (microg/kg) in the tibialis muscle was smaller in the P10 (33.6 +/- 5.4) than in the S (61.9 +/- 5.0) or the P100 (71.3 +/- 9.6) groups. AChR expression was less in the P10 group than in the S group. The evoked tensions correlated with muscle mass (r(2) = 0.32, P < 0.001), however, not with expression of AChR. The 50% effective dose of d-tubocurarine did not correlate with muscle mass or AChR expression. Our results suggest that the neuromuscular dysfunction after prednisolone is dose-dependent, and derives primarily from muscle atrophy and derives less so from changes in AChR expression.

IMPLICATIONS

The mechanisms by which chronic glucocorticoid therapy alters neuromuscular physiology and pharmacology are unclear. We suggest that the observed effects are dose-dependent and derive primarily from muscle atrophy and derive less from changes in acetylcholine receptor expression.

Generalized Paralysis in the Intensive Care Unit: Emphasis on the Complications of Neuromuscular Blocking Agents and Corticosteroids

Generalized weakness in intensive care unit (ICU) patients is increasingly recognized as a frequent complication and a common cause of prolonged ventilator dependency. Intravenous corticosteroids and neuromuscular blocking agents, sepsis, and multiorgan failure have been strongly implicated in the ICU paralysis syndromes, but the pathophysiology of these disorders is poorly understood. The combination of neuromuscular blocking agents and corticosteroids may induce three distinct syndromes of generalized weakness in ICU patients: acute myopathy, prolonged neuromuscular blockade, and critical illness polyneuropathy. More than one syndrome may occur simultaneously, and the distinctions may be difficult in a particular patient, but a specific diagnosis usually can be established after careful clinical, electrodi-agnostic, and histological evaluation. Acute myopathy with generalized weakness, preserved eye movements, elevated creatine kinase levels, and myopathic motor units on electromyography (EMG) have developed in asthmatics requiring neuromuscular blockers and steroids. Muscle biopsy has shown distinctive changes, with fiber atrophy, scattered necrosis, and thick (myosin) filament depletion on ultrastructural studies. Patients who have had a prolonged ICU stay or sepsis with failure to wean from the ventilator, distal weakness, and areflexia probably have critical illness polyneuropathy. EMG in these patients has demonstrated reduced or absent motor and sensory potentials with neurogenic motor units. Prolonged neuromuscular blockade most commonly has occurred in patients with renal failure who received prolonged infusions of neuromuscular blockers. Severe flaccid, areflexic paralysis with normal sensation, facial weakness, and ophthalmoparesis persists for days or weeks after the neuromuscular blockers have been discontinued. Repetitive nerve stimulation has shown a decrement of the compound muscle action potential, and it establishes a disorder of neuromuscular transmission in most patients. We critically examine the clinical, electrophysiological, and pathological features of each of these syndromes, and we summarize current understanding of the pathophysiology of these disorders and the relationship to neuromuscular blocking agents and corticosteroids.

Inhibitory action of nicotinic antagonists on transmitter release at the neuromuscular junction of the rat

The effects of two nicotinic antagonists, d-tubocurarine (TC) and hexamethonium (HEX) were tested on the rat diaphragm neuromuscular junction during train-of-six stimuli to determine if a second action of these antagonists on evoked release could be demonstrated, in addition to its known impact of blocking the autoreceptor pathway. To minimize the autoreceptor pathway, the preparations were examined under low transmitter release conditions. It was observed that both compounds significantly depressed the end-plate potential amplitudes more than the miniature end-plate potential amplitudes, while also significantly depressing quantal release output. This inhibitory action is contrary to what is observed when transmitter release is high, where feedback regulation via the autoreceptors serves a prominent role. It is concluded that this depressive action on transmitter output contributes to onset of tetanic fade and that when higher concentrations of these antagonists are used this inhibitory action of TC and HEX may override autoreceptor feedback regulation.

Nicotinic receptors on the rat phrenic nerve: evidence for negative feedback

The effects of low concentrations (nanomolar) of d-tubocurarine (TC) on end-plate potential (EPP) and miniature end-plate potential (MEPP) amplitude, and quantal transmitter release were examined at the rat neuromuscular junction in an attempt to identify the functional role of nicotinic receptors on the nerve terminal. TC (50 and 75 nM) significantly depressed the MEPP amplitude but not the amplitude of the initial EPPs during a train-of-six stimulation at 50 Hz. The lack of depression in EPP amplitude by TC was due to an increase in quantal release. The nearly equipotent response of the pre- and post-synaptic effects of TC suggests that the autoreceptors on the nerve terminal are very similar to the nicotinic receptors on the end-plate. These results suggest that nicotinic autoreceptors are functional even with a single action potential. The results support the hypothesis that ACh released from the nerve terminal normally has a negative feedback effect by depressing transmitter release.

Presynaptic, facilitatory effects of the corticosteroid dexamethasone in rat diaphragm: modulation by beta-bungarotoxin

Low concentrations of dexamethasone (up to 200 nM) increase the accumulation of choline (Ch) and its incorporation into acetylcholine (ACh) in the endplate rich area (EPA) of stimulated and unstimulated diaphragms in the presence of 10 microM Ch. Tissue ACh is not significantly altered, even after 140 min incubation. The specific radioactivity of the ACh in the EPA is thus increased by dexamethasone (Dex). The corticosteroid has no effects on acetylcholinesterase or choline acetyltransferase in diaphragm extracts. In the same medium, the amplitudes of the MEPPs, MEPCs and EPCs are also increased by Dex. Neither the quantal content of the EPCs nor the MEPP frequency, nor the half decay time of the MEPCs are altered. Therefore Dex (200 nM) increases both the resting and evoked output, and turnover of ACh in rat diaphragm. Beta-bungarotoxin (beta-BuTx) antagonizes the Dex-induced increase in Ch accumulation and its incorporation into ACh, and abolishes the increases in MEPC- and EPC-amplitudes, providing further argument for a presynaptic effect of Dex. In continuously-stimulated diaphragms, beta-BuTx causes an accumulation of ACh which is much greater than in unstimulated tissue. This accumulation of ACh is less in the presence of Dex, provided that Dex is added before beta-BuTx. The interaction of Dex and beta-BuTx is discussed in terms of their possible presynaptic sites of action.

Effects of corticosteroids on the myoneural junction. A morphometric and electrophysiological study

The mean size of the synaptic vesicles in the nerve endings of the isolated rat diaphragm was significantly increased after incubation with the glucocorticoids prednisolone (10(-5) mol/l) and dexamethasone (2 X 10(-7) mol/l). The shape of the vesicles was also changed, i.e. they became rounder. Similar increases in the mean size of the diaphragm synaptic vesicles were seen when the rats were injected with prednisolone (i.p., 2 mg/kg) and dexamethasone (i.p., 50 micrograms/kg) 2-4 h previously. The amplitude of the miniature endplate potentials was also significantly increased in diaphragms isolated from rats which had been injected with prednisolone or with dexamethasone. Changes in frequency of the MEPPs were less marked after injection of prednisolone, but a significant increase was seen after injection of dexamethasone. It is concluded that relatively low concentrations of glucocorticoids have direct effects on the motor endplate in rat diaphragm both in vitro and in vivo. We tentatively suggest that presynaptic effects may contribute to the beneficial effect of corticosteroids in deficient neuromuscular transmission, e.g. in myasthenia gravis.