Contractile and phosphatidylinositol responses of rat trachea to anticholinesterase drugs

PURPOSE: Some anticholinesterases (anti-ChE) such as neostigmine and pyridostigmine but not edrophonium, stimulate phosphatidylinositol (PI) response. Although a direct relationship was suggested between the increase in PI response and airway smooth muscle contraction, there are no data regarding the effects of anti-ChE drugs on airway smooth muscle. Thus, we examined the contractile properties and PI responses produced by anti-ChE drugs. METHODS: Contractile response. Rat tracheal ring was suspended between two stainless hooks in Krebs-Henseleit (K-H) solution. (1) Carbachol (CCh), anti-ChE drugs (neostigmine, pyridostigmine, edrophonium) or DMPP (a selective ganglionic nicotinic agonist) were added to induce active contraction. (2) The effects of 4-diphenylacetoxy-N-methyl-piperidine methobromide (4-DAMP), an M3 muscarinic receptor antagonist, on neostigmine- or pyridostigmine-induced contraction of rat tracheal ring were examined. (3) Tetrodotoxin (TTX) was tested on the anti-ChE drugs-induced responses. PI response. The tracheal slices were incubated in K-H solution containing LiCl and 3[H]myo-inositol in the presence of neostigmine or pyridostigmine with or without 4-DAMP, an M3 muscarinic receptor antagonist. 3[H]inositol monophosphate (IP1) formed was counted with a liquid scintillation counter. RESULTS: Carbachol (0.1 microM), neostigmine (1 microM), pyridostigmine (10 microM) but not edrophonium or DMPP, caused tracheal ring contraction. 4-DAMP, but not tetrodotoxin, inhibited neostigmine and pyridostigmine-induced contraction. Neostigmine- or pyridostigmine-induced IP1, accumulation was inhibited by 4-DAMP. CONCLUSIONS: The data suggest that anti-ChE drugs activate the M3 receptors at the tracheal effector site.     

Interactions of edrophonium with neostigmine in the rat trachea

The muscarinic M(3) receptor of airway smooth muscle has both an orthosteric binding site and an allosteric binding site. Edrophonium may bind to the allosteric site, resulting in the inhibition of the action of the orthosteric site. Therefore, we examined the effects of edrophonium on neostigmine-induced contractile and phosphatidylinositol responses of rat trachea. Neostigmine (100 micro M in final concentration) was added, and ring tension was examined by the addition of edrophonium. After the completion of the experiment, Krebs-Henseleit (K-H) solution containing both edrophonium and neostigmine was changed three times with fresh K-H solution, and the tension was recorded. Tracheal slices were incubated with [(3)H]myo-inositol and 100 micro M neostigmine in the presence or absence of edrophonium. The [(3)H]inositol monophosphate (IP(1)) was measured. Data were expressed as mean +/- SE. Statistical significance (P < 0.05) was determined with analysis of variance. Neostigmine-induced tension and IP(1) accumulation were attenuated by edrophonium at concentrations of 100 micro M or more. This attenuation was reversed to more than 80% of control levels by washing with fresh K-H solution. The results suggest that edrophonium would bind to the allosteric site, resulting in the inhibition of the action of the orthosteric site of muscarinic M(3) receptors of rat trachea. IMPLICATIONS: We examined the effects of edrophonium on neostigmine-induced contractile and phosphatidylinositol responses of rat trachea. Neostigmine-induced tension and inositol monophosphate accumulation were attenuated by edrophonium. This attenuation was reversed by washing. The results suggest that edrophonium would bind to the allosteric site.     

Anticholinesterase drugs stimulate smooth muscle contraction of the rat trachea through the Rho-kinase pathway

We performed this study to determine the effects of Rho-kinase inhibitors, Y-27632 and fasudil, on the anticholinesterase (anti-ChE)-induced contractile and phosphatidylinositol responses of the rat trachea. In vitro measurements of isometric tension and [3H] inositol monophosphate (IP1) that was formed were conducted by using rat tracheal rings or slices. Neostigmine- and pyridostigmine-induced contractions were almost completely inhibited by Y-27632 and fasudil at 30 microM each, whereas acetylcholine-induced contraction was inhibited incompletely, i.e., by 56% by Y-27632 and by 51% by fasudil, at 100 microM for each, respectively. The inhibitory effects of fasudil on neostigmine- and acetylcholine-induced contractions were completely reversed by calyculin-A, a myosin phosphatase inhibitor. Neostigmine-induced IP1 accumulation was attenuated by fasudil at 100 microM. The results suggest that anti-ChEs cause airway smooth muscle contraction, in part, through activation of the Rho-kinase pathway.     

Succinylcholine potentiates acetylcholine-induced contractile and phosphatidylinositol responses of rat trachea

Purpose Although succinylcholine (SCh) is often used as a muscle relaxant in electroconvulsive therapy, its influence on airway reactivity has not been fully investigated. We examined the effects of SCh on acetylcholine (ACh)-, carbachol (CCh)-, and electrical field stimulation (EFS)-induced contractions, and on the ACh-induced phosphatidylinositol (PI) response of rat trachea. Methods Thirty-two male Wistar rats weighing 250–350 g were used. The trachea was rapidly isolated and cut into 3-mm-wide rings. The resting tension was adjusted periodically to 1.0 g during the equilibration period. ACh, 1 μM; carbachol (CCh), 0.05 μM; or neither of them, was added, and SCh was then added at 1–300 μM final concentrations, and ring tension was examined. Contractions were elicited by EFS in the presence or absence of 100 μM SCh. Tracheal slices were incubated with [³H] myo-inositol, 1 μM ACh, and various concentrations of SCh. The accumulation of [³H] inositol monophosphate (IP₁) was measured. Results SCh did not affect the tension by itself without ACh, or with CCh, but SCh potentiated the ACh-induced contraction of rat trachea at concentrations of 10 μM or more (50% effective concentration [EC₅₀]; 43.6 μM). SCh produced a significant increase in the amplitude and duration of EFS-induced contractions. SCh, at concentrations of 10 μM and 100 μM, potentiated ACh-induced IP₁ accumulation. Conclusion SCh potentiated ACh-induced, but not CCh-induced, contractile and PI responses, and enhanced EFS-induced contraction of rat trachea, suggesting that competition for butyrylcholinesterase (BChE) in airway smooth muscle could be involved in the potentiation by SCh of ACh-induced airway smooth muscle contraction.     

Metoclopramide causes airway smooth muscle relaxation through inhibition of muscarinic M3 receptor in the rat trachea

Although metoclopramide, often used as an antiemetic, is reported to have an anticholinesterase action, the effect on airway smooth muscle remains unclear. We investigated the effect of metoclopramide on the contraction, phosphatidylinositol response, and binding affinity of muscarinic M(3) receptors in rat trachea preparations. Male Wistar rats were anesthetized and their tracheas excised and chopped into 3-mm-wide rings, 1-mm-wide slices, or frozen 10- microm-thick sections. Contraction was induced with 0.55 microM carbachol (CCh) and, 30 min later, metoclopramide (10 microM to 1 mM) was added. The slices were incubated with (3)[H]myo-inositol, 0.55 microM CCh, and metoclopramide, and the formation of (3)[H] inositol monophosphate was measured. A radioligand binding study was conducted to examine the effects of metoclopramide using [(3)H] 4-diphenylacetoxy-N-methyl-piperidine methobromide (4-DAMP), a muscarinic M(3) receptor antagonist, in sections of the trachea. Metoclopramide concentration dependently attenuated CCh-induced contraction and inositol monophosphate accumulation, and also attenuated the binding affinity of 4-DAMP to muscarinic M(3) receptors. The 50% inhibitory concentration of metoclopramide against the binding affinity of 4-DAMP to muscarinic M(3) receptors of rat trachea was 24 micro M. These findings suggest that the attenuation by metoclopramide of CCh-induced contraction and phosphatidylinositol response may be mediated through the muscarinic M(3) receptors. IMPLICATIONS: We investigated the effect of metoclopramide on the contraction, phosphatidylinositol response, and binding affinity of muscarinic M(3) receptors in rat trachea preparations. Our findings suggest that the attenuation by metoclopramide of carbachol-induced contraction and phosphatidylinositol response may be mediated through the muscarinic M(3) receptors.     

Incomplete reversal of pancuronium neuromuscular blockade by neostigmine, pyridostigmine, and edrophonium

Three clinically used anticholinesterases--neostigmine, pyridostigmine, and edrophonium--were tested for their ability to reverse two levels (60% and 95%) of neuromuscular blockade produced by pancuronium. A controlled in vitro environment of the rat diaphragm-phrenic nerve system was used for the studies. Concentrations of anticholinesterases spanned the clinical range and were extended beyond to establish dose-response curves. Neostigmine was the most potent reversal drug (ED50 for 95% block 5.5 +/- 4 nM), followed by pyridostigmine (0.27 +/- 0.06 microM) and edrophonium (2.1 +/- 0.05 microM). The three drugs were equally effective at reversal of block and fade as measured by train-of-four stimulation. The dose-response curves for all three drugs showed a ceiling effect for reversal of tension and fade. Supraclinical concentrations of drug did not effect complete reversal, especially at 95% block. High concentrations of anticholinesterase led to randomly appearing hyperactivity manifested by spontaneous twitching and repetitive firing with severe fade on stimulation.     

Myasthenia gravis and intestinal resection: is dehiscence likely to occur?

Experimental and clinical data have proved that anticholinesterase drugs are responsible for vigorous peristaltic contractions and for an increase of the intraluminal pressure, because they determine muscarinic effects on the smooth muscle of the intestine both in small and large intestine. Therefore, a greater incidence of intestinal anastomotic disruption has been supposed when anticholinesterases are used both in the early postoperative period, to reverse curarization, and in myasthenic patients. The authors report a case of a patient with myasthenia gravis who received maximal doses of pyridostigmine and underwent left hemicolectomy and small intestine resection and afterwards total colectomy in order to treat a sigmoid perforated diverticulitis. In the postoperative course an anastomotic leak developed after both surgical operations. The authors believe that pyridostigmine could have had an important role in the pathogenesis of the leak and assert that, when an intestinal resection has to be performed in a myasthenic patient, it could be useful to reduce in the preoperative period the administration of anticholinesterase drugs and always perform a protective ileostomy.     

Effects of ketamine on neostigmine-induced contractile and phosphatidylinositol responses of the rat trachea

Purpose. Neostigmine causes airway smooth muscle contraction through the direct stimulation of muscarinic receptors and the activation of phosphatidylinositol (PI) responses. Ketamine attenuates airway smooth muscle contraction. It is not clear whether ketamine attenuates neostigmine-induced airway smooth muscle contraction by inhibiting the PI response. This study was designed to examine the effects of ketamine on neostigmine-induced contractile and PI responses of the rat trachea. Methods. Thirty male Wistar rats weighing 250–350 g were used. In the experiment on the contractile response, active contraction was induced with 1 μM neostigmine in the presence or absence of ketamine. In the experiment on the phosphatidylinositol response, the trachea slices were incubated with [³H]myo-inositol, 1 μM neostigmine, or 100 μM aluminum fluoride, and ketamine. The formation of [³H]inositol monophosphate (IP₁), a degradation product of the phosphatidylinositol response, was measured with a liquid scintillation counter. Statistical significance (P < 0.05) was determined by analysis of variance. Results. Neostigmine 1 μM caused tracheal ring contraction. This contraction was attenuated by ketamine dose-dependently and reached resting tension at 100 μM. Neostigmine- and aluminum fluoride-induced IP₁ accumulation was also attenuated by ketamine. Conclusion. The results suggest that ketamine attenuates neostigmine-induced contractile responses, at least in part, through the inhibition of phospholipase C coupled with G protein in the PI response. Received: December 12, 2002 / Accepted: February 8, 2003 Acknowledgments. This study was supported in part by Grant-in-Aid for Scientific Research C, No. 10671421, from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Address correspondence to: M. Saito     

Comparison of anticholinesterases and their effects on acetylcholine-activated ion channels

Single-channel recording techniques have been used to examine interactions between anticholinesterases and ion channels activated by acetylcholine. Single-channel currents activated by 200 nM acetylcholine were recorded from cell-attached patches of BC3H1 mouse tumor cells grown in culture. Channels were recorded in the absence and presence of edrophonium (1-20 microM), neostigmine (2-20 microM), or pyridostigmine (10-200 microM). All three drugs shortened channel open time but did not alter single-channel current amplitude. Effects on channel open time were not secondary to inhibition of cholinesterase but appeared to involve direct interactions between anticholinesterase drugs and acetylcholine-activated channels. Drug concentrations calculated to reduce the time constant of open time distributions by 50% were 3.8 microM edrophonium, 4.6 microM neostigmine, or 97 microM pyridostigmine. Channel open time was decreased by edrophonium at concentrations comparable to those occurring during reversal of neuromuscular block, but it was reduced by neostigmine and pyridostigmine only at levels higher than those encountered clinically. Differences in interactions between anticholinesterases and acetylcholine-activated channels at the end plate may possibly account for some of the clinical differences between these drugs.     

High concentrations of landiolol,a β<Subscript>1</Subscript>-adrenoceptor antagonist,stimulate smooth muscle contraction of the rat trachea through the Rho-kinase pathway

Purpose Gradually progressing contraction of airway smooth muscle is suggested to be due to the Rho-kinase signaling pathway. In our preliminary study in rat tracheas, landiolol, a β₁-adrenoceptor antagonist, at high doses caused gradually progressing contraction, and this contraction reached a plateau after 20 min. Therefore, this study was carried out to clarify whether landiolol could stimulate the Rho-kinase pathway or the phosphatidylinositol (PI) response in the rat trachea. Methods Seventy-eight male Wistar rats weighing 250–350 g were used for the experiments. Their tracheas were cut into 3-mm-wide ring segments or 1-mm-wide slices. Measurements of isometric tension and [³H] inositol monophosphate (IP₁) production were conducted, using these tracheal rings or slices. Data values are expressed as means ± SD, and statistical significance (P < 0.05) was determined using analysis of variance (ANOVA). Results Landiolol (700 μM)-induced contraction was completely inhibited by fasudil at 30 μM, while the landiolol-induced contraction was not inhibited by 4-diphenylacetoxy-N-methyl-piperidine methobromide (4-DAMP), ketanserin, or nicardipine. Landiolol did not stimulate IP₁ production. Conclusion These results suggest that high concentrations of landiolol could cause airway smooth muscle contraction through the Rho-kinase pathway, but not through the PI response coupled with muscarinic M₃ receptors, 5-HT receptors or the activation of L-type Ca²⁺ channels.     

Mechanisms underlying the inhibitory effect of propofol on the contraction of canine airway smooth muscle.

BACKGROUND: Propofol has been shown to produce relaxation of preconstricted airway smooth muscle. Although the inhibition of calcium mobilization is supposed to be the major mechanism of action, the whole picture of the mechanisms is not completely clear. METHODS: Contractile response was performed using canine tracheal rings. The effects of propofol on carbachol-induced mobilization of intracellular Ca2+ and phosphoinositide hydrolysis were measured using cultured canine tracheal smooth muscle cells by monitoring fura-2 signal and assessing the accumulation of [3H]-inositol phosphates. To detect the effect of propofol on muscarinic receptor density and affinity, [3H]N-methyl-scopolamine was used as a radioligand for receptor binding assay. RESULTS: Pretreatment with propofol shifts the concentration-response curves of carbachol-induced smooth muscle contraction to the right in a concentration-dependent manner without changing the maximal response. Propofol not only decreased the release of Ca2+ from internal stores but also inhibited the calcium influx induced by carbachol. In addition, carbachol-induced inositol phosphate accumulation was attenuated by propofol; the inhibitory pattern was similar to the contractile response. Moreover, propofol did not alter the density of muscarinic receptors. The dissociation constant value was not altered by pretreatment with 100 microM propofol but was significantly increased by 300 microM (propofol, 952+/-229 pM; control, 588+/-98 pM; P<0.05). CONCLUSIONS: Propofol attenuates the muscarinic receptor-mediated airway muscle contraction. The mechanism underlying these effects was attenuation of inositol phosphate generation and inhibition of Ca2+ mobilization through the inhibition of the receptor-coupled signal-transduction pathway.     

Neostigmine,pyridostigmine and edrophonium as antagonists of deep pancuronium blockade

To compare the ability of equipotent doses of neostigmine, pyridostigmine and edrophonium to antagonize intense pancuronium neuromuscular blockade, one hundred and twenty ASA physical status I or II patients scheduled for elective surgery received 0.06 mg.kg-1 pancuronium during a thiopentone nitrous oxide-enflurane anaesthetic. Train-of-four stimulation was applied every 12 s and the force of contraction of the adductor pollicis muscle was recorded. In the first 60 patients, spontaneous recovery was allowed until ten per cent of initial first twitch height. Then neostigmine (0.005, 0.01, 0.02 or 0.05 mg.kg-1), pyridostigmine (0.02, 0.04, 0.1 or 0.2 mg.kg-1), or edrophonium (0.1, 0.2, 0.4 or 1 mg.kg-1) was injected by random allocation. Dose-response relationships were established from the measurement of first twitch height (T1) ten minutes later. From these, neostigmine, 0.04 and 0.08 mg.kg-1 was found to be equipotent to pyridostigmine, 0.2 and 0.38 mg.kg-1, and edrophonium, 0.54 and 1.15 mg.kg-1, respectively. These doses were given by random allocation to the next 60 patients, but at one per cent spontaneous recovery. Neostigmine, 0.04 mg.kg-1, produced a T1 of 73 +/- 4 per cent (mean +/- SEM), and a train-of-four ratio (TOF) of 39 +/- 3 per cent. This was significantly greater than with pyridostigmine, 0.2 mg.kg-1 (T1 = 50 +/- 6 per cent; TOF = 25 +/- 3 per cent), and edrophonium, 0.54 mg.kg-1 (T1 = 54 +/- 3 per cent; TOF = 17 +/- 2 per cent).(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparative effects of plasma exchange and pyridostigmine on respiratory muscle strength and breathing pattern in patients with myasthenia gravis.

BACKGROUND--Pyridostigmine, an acetylcholinesterase antagonist, is useful in improving respiratory function in patients with myasthenia gravis. More recently, plasma exchange has been employed in myasthenia gravis because it acts presumably by removal of circulating antibodies against acetylcholine receptors. Surprisingly, comparative data on the effects of pyridostigmine and plasma exchange on lung volumes, respiratory muscle strength, and ventilatory control system in patients with myasthenia gravis are lacking. METHODS--Nine consecutive patients with grade IIb myasthenia gravis were studied under control conditions and after a therapeutic dose of pyridostigmine. In a second study the patients were re-evaluated a few days after a cycle of plasma exchange, before taking pyridostigmine. In each subject pulmonary volumes, inspiratory (MIP) and expiratory (MEP) muscle force, and respiratory muscle strength, calculated as average MIP and MEP as percentages of their predicted values, were measured. The ventilatory control system was evaluated in terms of volume (tidal volume, VT) and time (inspiratory time, TI, and total time, TTOT) components of the respiratory cycle. Mean inspiratory flow (VT/TI)--that is, the "driving"--and TI/TTOT--that is, the "timing"--components of ventilation were also measured. RESULTS--In each patient treatment relieved weakness and tiredness, and dyspnoea grade was reduced with plasma exchange. Following treatment, vital capacity (VC) increased on average by 9.7% with pyridostigmine and by 14% with plasma exchange, and MIP increased by 18% and 26%, respectively. In addition, with plasma exchange but not with pyridostigmine forced expiratory volume in one second (FEV1) increased by 16% and MEP increased by 24.5%, while functional residual capacity (FRC) decreased a little (6.8%). The change in respiratory muscle strength was related to change in VC (r2 = 0.48). With plasma exchange, VT increased by 18.6% and VT/TI increased by 13.5%, while neither TI nor TI/TTOT changed. CONCLUSIONS--Plasma exchange can be used in patients with myasthenia gravis when symptoms are not adequately controlled by anticholinesterase agents. Plasma exchange increases respiratory muscle force and tidal volume due to changes in "driving" but not "timing" of the respiratory cycle.     

Carbachol, Norepinephrine, and Hypocapnia Stimulate Phosphatidylinositol Turnover in Rat Tracheal Slices

Background: The intracellular mechanisms involved in the alpha-adrenoceptor- or hyperventilation-induced bronchoconstriction remain unknown. Because there is a direct relationship between phosphatidylinositol (PI) metabolism and airway smooth muscle contraction induced by muscarinic agonists, the authors examined the effects of carbachol (CCh), norepinephrine (NE), and hypocapnia on PI turnover in the airway smooth muscle.

Methods: Rat tracheal slices were incubated in Krebs-Henseleit solution containing LiCl and [sup 3 Hydrogen]myo-inositol in the presence of NE, CCh, or neither. The PCO2 in the solution was 36 plus/minus 3 mmHg (normocapnia), 19 plus/minus 2 mmHg (moderate hypocapnia), or 5 plus/minus 2 mmHg (severe hypocapnia), respectively. [sup 3 Hydrogen]inositol monophosphate (IP1) formed was counted with a liquid scintillation counter.

Results: Basal IP1 formed was greater at severe hypocapnia than at normocapnia. Norepinephrine- and CCh-induced IP1 formation were also greater at hypocapnia than at normocapnia.     

The role of cholinesterases in rat urinary bladder contractility

This study examines the effects of inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) on acetylcholine (ACh)-induced contraction in rat urinary bladder smooth muscle. Neostigmine, a non-selective ChE inhibitor, caused concentration-dependent contractions in rat urinary bladder strips, whereas tetraisopropylpyrophosphoramide (iso-OMPA; a BuChE inhibitor) failed to affect the resting tone of the preparations. Neostigmine (1 microM) markedly augmented the contractile responses to ACh. Although iso-OMPA (10 microM) also potentiated ACh-induced contraction, the effect was less than that evoked by neostigmine. The activities of AChE in rat urinary bladder strips were significantly (P<0.05) higher than those of BuChE. These results indicated that AChE, rather than BuChE, plays an important role in controlling ACh-induced contractions of rat urinary bladder.     

Minimally Invasive Gastric Bypass in a Morbidly Obese Patient with Myasthenia Gravis

Associated or rare diseases, such as myasthenia gravis, introduce a challenge to the perioperative management of severely obese patients undergoing bariatric surgery. We report the surgical management and unique anesthetic approach to a 55-year-old morbidly obese woman with a complex past medical history that included myasthenia gravis, who underwent laparoscopic gastric bypass. Her myasthenia was controlled on pyridostigmine and her greatest concern was the potential need for postoperative mechanical ventilation. While the laparoscopic surgical approach was ideal to reduce pain and the adverse effects on ventilatory mechanics associated with open upper abdominal surgery, a combined inhalational and intravenous anesthetic without muscle relaxants resulted in satisfactory surgical conditions, and allowed for immediate postoperative extubation followed by an uneventful postoperative course. Continued perioperative anticholinesterase administration may have facilitated this successful outcome. We conclude that a diagnosis of myasthenia gravis does not mandate postoperative mechanical ventilation following laparoscopic gastric bypass.     

Different properties of the bradycardia produced by neostigmine and edrophonium in the cat

Purpose

The bradycardia produced by neostigmine and edrophonium was examined according to its relation to cholinesterase inhibition and to its sensitivity to block by muscarinic receptor antagonists. For comparison, the ability of muscdrinic antagonists to block the bradycardia produced by electrical stimulation of the vagus nerve was determined.

Methods

Cats were anaesthetized, vagotomized and propranolol-treated. Heart rate was continuously recorded. Erythrocyte cholinesterase activity of arterial blood was measured using a radiometric technique. The right vagus nerve was isolated for electrical stimulation. The muscarinic antagonists used were atropine, glycopyrrolate, pancuronium, gallamine, and AFDX-116.

Results

Neostigmine produced a dose-dependent decrease in cholinesterase activity which, reached a plateau at a cumulative dose of 0.16 mg · kg^?1 (ED₅₀ 0.009 ± 0.003 mg · kg^?1). Neostigmine produced a dose-dependent decrease in heart rate with the. dose-response relationship (ED₅₀ 0.1 ± 0.01 mg · kg^?1; P = 0.0006) shifted to the right of that for the inhibition of cholinesterase activity. In contrast to the anticholinesterase effect, the bradycardic effect did not reach a plateau and continued to increase even at doses at which the cholinesterase inhibition was maximal. The maximal decrease in heart rate when the heart was still in sinus rhythm was by 81 ± 13 bpm (49 ± 7% of baseline), which was produced by a dose of 0.32 mg · kg^?1. Edrophonium produced dose-dependent decreases in cholinesterase activity and heart rate, which were highly correlated (correlation coefficient r = 0.99, P < 0.0001).The ED₅₀ of the reduction in heart rate (0.9 ± 0.75 mg · kg^?1) and cholinesterase activity (0.89 ± 0.12 mg · kg^?1) produced by edrophonium were similar. Moreover, the reduction in heart rate and cholinesterase activity produced by edrophonium reached a plateau at the same dose (6.4 mg · kg^?1). At this dose, heart rate decreased by 22 ± 2 bpm (14.6 ± 0.9% of baseline). Compared to the bradycardia produced by stimulation of the vagus nerve, that produced by neostigmine was blocked by muscarinic antagonists at significantly lower doses while that produced by edrophonium was blocked at similar doses.

Conclusions

The neostigmine-induced bradycardia is poorly correlated with cholinesterase inhibition compared to that produced by edrophonium, and has a higher sensitivity to muscarinic receptor antagonists compared to that produced by edrophonium or vagus nerve stimulation. These results are consistent with the hypothesis that the neostigmine-induced bradycardia is, in pari, the result of neostigmine directly activating cholinergic receptors within the cardiac parasympathetic pathway. The bradycardia produced by edrophonium may be accounted for solely by an anticholinesterase action.     

Myasthenia gravis

Opinion statement The treatment of patients with myasthenia gravis should be individualized according to the extent (ocular versus generalized) and severity (mild to severe) of disease, the presence or absence of concomitant disease (including but not limited to other autoimmune diseases and thymoma), and, to a lesser degree, the age of the patient. Thymectomy should be performed in patients with generalized disease, especially those who have detectable levels of circulating antibodies to acetylcholine receptor (anti-AChR), as it should be in all patients thought to have an operable thymoma (observed on imaging studies of the chest). Symptomatic therapy consists of anticholinesterase drugs (usually pyridostigmine); occasionally, other drugs are required to reduce the muscarinic side effects. At times, patients need immunosuppressive or immunomodulatory therapy with glucocorticoids, azathioprine, cyclosporine or cyclophosphamide, plasma exchange, and intravenous immunoglobulin. Remission, whether spontaneous or pharmacologically induced, or significant improvement can be achieved in most patients, but some treatments entail significant side effects and considerable cost.     

Role of Intracellular Ca2+ Stores in the Inhibitory Effect of Halothane on Airway Smooth Muscle Contraction

Background: Halothane directly inhibits contraction of airway smooth muscle, mainly by decreasing the intracellular concentration of free Ca2+ ([Ca2+]i). The role of intracellular Ca2+ stores, sarcoplasmic reticulum, is still unclear. We investigated the role of sarcoplasmic reticulum in the inhibitory effect of halothane on contraction of airway smooth muscle by measuring [Ca2+]i and intracellular concentration of inositol 1,4,5-triphosphate ([IP3]i), a second messenger for release of Ca2+ from sarcoplasmic reticulum.

Methods: [Ca2+]i was monitored by measuring the 500-nm light emission ratio (F340/F380) of a Ca2+ indicator fura-2 with isometric tension of canine tracheal smooth muscle strip. During Ca2+-free conditions, carbachol (10-5 M) was introduced with pretreatment of halothane (0-3%). During Ca2+-free conditions, 20 mM caffeine, a Ca (2+-induced) Ca2+ release channel opener, was introduced with or without halothane. We measured [IP3]i during exposure to carbachol and halothane by radioimmunoassay technique.

Results: Pretreatment with halothane significantly diminished carbachol-induced increases in [Ca2+]i by 77% and muscle tension by 83% in a dose-dependent manner. Simultaneous administration of halothane significantly enhanced caffeine-induced transient increases in [Ca2+] (i) and muscle tension in a dose-dependent manner, by 97% and 69%, respectively. Pretreatment with halothane abolished these responses. Rapid increase in [IP3]i produced by carbachol was significantly inhibited by 32% by halothane in a dose-dependent manner.     

Mechanisms of Direct Inhibitory Action of Propofol on Uterine Smooth Muscle Contraction in Pregnant Rats

Background : Although propofol directly inhibits uterine smooth muscle contraction, the mechanisms of this effect are still unknown. The current study aimed to clarify the mechanisms of the inhibitory effect of propofol on oxytocin-induced uterine smooth muscle contraction by measuring (1) the concentration of intracellular free Ca2+ ([Ca2+]i) simultaneously with muscle tension, (2) the amount of intracellular inositol 1,4,5-triphosphate ([IP3]i), and (3) voltage-dependent Ca2+ channel (VDCC) activity.

Methods : Uterine smooth muscle tissues were obtained from pregnant rats (in late gestation). [Ca2+]i with isometric tension was monitored by the 500-nm light emission ratio of preloaded Ca2+ indicator fura-2. [IP3]i and VDCC activity were measured by radioimmunoassay and patch clamp techniques, respectively. The uterine smooth muscle was stimulated by 20 nm oxytocin and exposed to propofol (10-7 ~ 10-4 m).

Results : Propofol had significant inhibitory effects on oxytocin-induced uterine smooth muscle contraction and increased [Ca2+]i in pregnant rats in a dose-dependent manner, without affecting the agonist-receptor binding affinity. Propofol inhibited the increase in [IP3]i induced by oxytocin. Propofol also inhibited VDCC activity in both activated and inactivated states. The solvent Intralipid(R) had no effects on these parameters.