Extracellular calcium-dependent and -independent effects of methylmercury on spontaneous and potassium-evoked release of acetylcholine at the neuromuscular junction

Acute bath administration of methylmercury (MeHg) to the murine neuromuscular junction causes an initial surge in the frequency of occurrence of miniature end-plate potentials (MEPPs), followed by a complete suppression of asynchronous spontaneous release. The goals of the present study were to determine: whether the MeHg-induced in MEPP frequency was dependent upon extracellular Ca++, whether MeHg produced this effect by actions within the motor nerve terminal and whether the secondary suppression of release was due to transmitter depletion. Conventional intracellular microelectrode recording measurements of MEPPs were made from myofibers of the isolated hemidiaphragm of the rat. Increasing the bath concentration of Ca++ from 1 to 2 or 4 mM decreased the time period required by 100 microM MeHg to produce a peak increase of spontaneous release from 52 to 39 to 28 min, respectively. Further increasing bath Ca++ to 8 mM actually increased this period back to 49 min. Increasing [Ca++]o had no consistent effect on the magnitude of the MeHg-induced increase in MEPP frequency. After depolarization of the nerve terminal with elevated extracellular K+ (15 mM) the time to peak increases in MEPP frequency was shortened from approximately 40 min to 1 to 2 min. The time required for MeHg to cause complete cessation of MEPPs was also shortened. In experiments conducted in K+-depolarized preparations to which no Ca++ was added, MeHg still increased MEPP frequency, although not as rapidly, or to the same extent as in solutions containing Ca++.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of activation of sodium and calcium entry on spontaneous release of acetylcholine induced by methylmercury

The effect of ionophores and channel activators for Ca and Na on the time course and magnitude of methylmercury (MeHg)-induced increase in spontaneous release of neurotransmitter was studied at the murine neuromuscular junction using intracellular microelectrode recording techniques. The goal was to test whether chemicals that increase entry of Na+ or Ca++ into nerve terminals would shorten the latent period that precedes the onset of MeHg-induced increase in MEPP frequency. Administration of MeHg (100 microM) with A23187 (25 microM), a calcium ionophore, caused a more rapid time to peak induced increase in MEPP frequency than "control" MeHg preparations. This effect also occurred in solutions to which no extracellular Ca++ was added. Use of monensin, a Na+ ionophore (25-100 microM), did not shorten the time to peak increase of MEPP frequency. The dihydropyridine Ca++ channel agonist Bay K 8644 (750 nM) produced the most marked shortening of the time to peak MEPP frequency for MeHg. This effect also occurred in solutions deficient in extracellular Ca++. Veratridine (20 microM), a sodium channel activator, decreased the time to peak MEPP frequency when used in conjunction with MeHg in both Ca++-containing and Ca++-deficient solutions. Replacement of sodium in the extracellular perfusion solution with methylamine, which does not penetrate axon sodium channels, did not prevent the MeHg-induced increase in MEPP frequency although it did prolong the time to peak increase and decreased the maximal MEPP frequency induced by MeHg compared with experiments conducted in sodium-containing solutions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Alterations in cardiac alpha and beta adrenoceptors during the development of spontaneous hypertension

To study potential cardiac receptor alterations during the development of spontaneous hypertension, specific binding of [3H]-2-N(2,6-dimethoxyphenoxyethyl)amino-methyl-1,4-benzodioxane, (-)-[3H]dihydroalprenolol and (-)-[3H]quinuclidinyl benzilate in ventricles of Wistar Kyoto rats (WKY), spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP) at different ages was determined. The Kd and maximal binding for specific binding of [3H]-2-N(2,6-dimethoxyphenoxyethyl)amino-methyl-1,4-benzodioxane and (-)-[3H]dihydroalprenolol in ventricular homogenates of SHR and SHRSP at prehypertensive ages were similar to those of age-matched WKY. With the development of spontaneous hypertension in SHR and SHRSP, there was a significant decrease in the maximal binding for both ligands without a change in Kd. The decrease in maximal binding in SHR and SHRSP at 10 weeks of age was 29 to 38%, compared with age-matched WKY. There was no difference in ventricular (-)-[3H]quinuclidinyl benzilate binding between WKY and SHRSP. Hofstee analysis of the inhibition of ventricular (-)-[3H]dihydroalprenolol binding by practolol demonstrated a specific 51% decrease in ventricular beta-1 receptor density in 10-week-old SHRSP. In addition, the inotropic response to isoproterenol in isolated papillary muscles from SHRSP was significantly smaller than that in WKY. Thus, it is concluded that during the development of spontaneous hypertension in SHR and SHRSP, there is a specific loss in number of cardiac alpha and beta-1 adrenoceptors with a consequently reduced responsiveness of isolated papillary muscles to isoproterenol in SHRSP. These results are compatible with the reported increase in sympathetic outflow to the cardiovascular system in spontaneous hypertension.     

Influence of different release times on spontaneous breathing pattern during airway pressure release ventilation

OBJECTIVE: Airway pressure release ventilation (APRV) is a ventilatory mode with a time cycled change between an upper (P(high)) and lower (P(low)) airway pressure level. APRV is unique because it allows unrestricted spontaneous breathing throughout the ventilatory cycle. We studied the influence of different release times (time of P(low)) on breathing pattern and gas exchange in patients during partial mechanical ventilation. SETTING: Mixed intensive care unit in a university hospital. PATIENTS: Twenty-eight patients were included in the study. Nine patients suffering from acute lung injury (ALI), 7 patients with a history of chronic obstructive pulmonary disease (COPD) and 12 patients with nearly normal lung function, ventilated for non-respiratory reasons (postoperatively), were studied prior to extubation. INTERVENTIONS: At constant pressure levels and a pre-set airway pressure release rate of 12/min, P(low) was diminished and P(high) was prolonged in four steps of 0.5 s. Each respiratory setting was studied for 20 min after a steady state period had been achieved. MEASUREMENTS AND MAIN RESULTS: We measured gas exchange and respiratory mechanics. The different time intervals of P(high) and P(low) had only minor effects on the actual spontaneous inspiration and expiration times, but the proportion of spontaneous breathing on total ventilation increased when the duration of P(low) was decreased. Gas exchange was almost unaffected by the interventions despite a significant increase in mean airway pressure. However, when P(low) was set to only 0.5 s an increase in PaCO(2) occurred in patients with COPD and ALI, probably due to a decrease in mechanical ventilatory support. CONCLUSIONS: Airway pressure release ventilation is an open system which allows patients to maintain the "time control" over the respiratory cycle independent of the chosen duration for P(high) and P(low).     

Severe weakness complicating status asthmaticus despite minimal duration of neuromuscular paralysis

Objective  Mechanically ventilated patients with status asthmaticus who undergo prolonged paralysis are at risk for severe weakness due to myopathy. In the mid-1990s, we changed our usual method of achieving tolerance of ventilatory support in asthmatic patients from continuous paralysis to deep sedation. This study examines the impact of this change in practice on the development of clinically significant weakness in status asthmaticus. Design and setting  Retrospective cohort study in university-affiliated county hospital. Patients  Mechanically ventilated asthmatic patients seen before (n = 96) and after (n = 74) a clinical practice change in 1995 that markedly restricted use of paralytics. Results  The duration of neuromuscular paralysis declined sharply after 1995 (23.7 ± 42.2 vs. 1.8 ± 4.0 h, P < 0.001), but this was not associated with a significant difference in the incidence of weakness (21 vs. 14%, P = 0.23). Within the post-1995 cohort, there was no significant difference in the duration of paralysis for weak and non-weak patients (3.5 ± 6.2 vs. 1.5 ± 3.5 h, P = 0.10). However, weak patients had a much longer duration of mechanical ventilation than did patients without weakness (11.9 ± 3.6 vs. 1.9 ± 1.8 days, P < 0.001). Conclusion  Mechanically ventilated patients with status asthmaticus who are immobilized for prolonged periods of time by deep sedation remain at risk for clinically significant weakness.     

Intervention effect of neuromuscular electrical stimulation on ICU acquired weakness: A meta-analysis

ObjectiveThe early use of neuromuscular electrical stimulation (NMES) to prevent intensive care unit-acquired weakness (ICU-AW) in critical patients is still a controversial topic. We conducted a systematic review to clarify the effectiveness of NMES in preventing ICU-AW.MethodsThe Cochrane Library, PubMed, EMBASE, MEDLINE, Web of Science, Ovid, CNKI, Wanfang, VIP, China Biology Medicine disc (CBMdisc) and other databases were searched for randomized controlled trials on the influence of NMES on ICU-AW. The studies were selected according to the inclusion and exclusion criteria. After data and quality were evaluated, a meta-analysis was performed by RevMan 5.3 software.ResultsA total of 11 randomized controlled trials with 576 patients were included. The meta-analysis results showed that NMES can improve muscle strength [MD = 1.78, 95% CI (0.44, 3.12, P = 0.009); shorten the mechanical ventilation (MV) time [SMD = −0.65, 95% CI (−1.03, −0.27, P = 0.001], ICU length of stay [MD = −3.41, 95% CI (−4.58, −4.24), P < 0.001], and total length of stay [MD = −3.97, 95% CI (−6.89, −1.06, P = 0.008]; improve the ability of patients to perform activities of daily living [SMD = 0.9, 95% CI (0.45, 1.35), P = 0.001]; and increase walking distance [MD = 239.03, 95% CI (179.22298.85), P < 0.001]. However, there is no evidence indicating that NMES can improve the functional status of ICU patients during hospitalization, promote the early awakening of patients or reduce mortality (P > 0.05).ConclusionEarly implementation of the NMES intervention in ICU patients can prevent ICU-AW and improve their quality of life by enhancing their muscle strength and shortening the MV duration, length of stay in the ICU and total length of stay in the hospital.     

Alterations in neuromuscular patterns between pre and one-year post-total knee arthroplasty

Background

Total knee arthroplasty is a common treatment for severe knee osteoarthritis. Objective measures are needed to evaluate the effect of arthroplasty surgery on function and joint loading, in particular given the rise in younger adults receiving this intervention. The objective was to compare neuromuscular activation patterns of the knee musculature during level walking one-week prior to and one-year following total knee arthroplasty.

Methods

Surface electromyograms from seven periarticular muscles were recorded from 43 patients with severe medial compartment knee osteoarthritis during walking one-week prior to and one-year following total knee arthroplasty. Principal component analysis extracted patterns from the electromyographic waveforms and assigned scores for these patterns, which were statistically compared between test times and between medial and lateral sites within a muscle group.

Findings

Significantly lower overall activation amplitudes were found for the quadriceps and hamstrings, with decreased activity during mid-late stance following surgery. Significant increases in gastrocnemius activity were found late stance, along with altered waveform shapes.

Interpretation

In general, the post-surgical changes moved toward more typical asymptomatic patterns, supporting improved neuromuscular strategies during walking. Given that improvements would not be expected to occur naturally in severe osteoarthritic knees the positive changes in neuromuscular characteristics during specific phases of the gait cycle can be explained in part by the altered mechanical environment and reduction in pain from the surgical intervention. These objective findings are directly relevant to the joint loading environment and can be valuable for evaluating surgical techniques, different prostheses and pre–post surgical management.     

Supine fall in lung volumes in the assessment of diaphragmatic weakness in neuromuscular disorders

OBJECTIVE: To determine whether diaphragmatic function can be determined by noninvasive respiratory indices in neuromuscular disease. DESIGN: Vital capacity (VC) and mouth pressure generated during a maximal static inspiratory effort (Pi max) were measured with patients in both sitting and supine positions. SETTING: Rehabilitation hospital. PATIENTS: Twenty-four patients with generalized neuromuscular disease. MAIN OUTCOME MEASURES: Changes in indices from sitting to supine position were compared with invasive diaphragmatic function indices consisting of transdiaphragmatic pressures during maximal sniff (Pdi sniff) and the ratio of gastric pressure (Pga) increases over transdiaphragmatic pressure (DeltaPga/DeltaPdi) during quiet breathing. RESULTS: The fall in VC in the supine position was greater in the 15 patients who had spontaneous paradoxical diaphragmatic motion (DeltaPga/DeltaPdi < 0) than in the 9 patients who did not. Specificity and sensitivity of a greater than 25% supine fall in VC for the diagnosis of diaphragmatic weakness (DeltaPga/DeltaPdi < 0 and/or Pdi sniff < 30cmH2O) were 90% and 79%, respectively. Stepwise multiple regression analysis of Pdi sniff showed that both the supine fall in VC and Pi max were associated with diaphragmatic weakness (R(2) =.66; p <.0001). These factors contributed 52% and 14% of the Pdi sniff variance, respectively. CONCLUSIONS: Simple VC measurement in the sitting and supine positions may be helpful in detecting severe or predominant diaphragmatic weakness.     

Alterations in thyroid iodine release and the peripheral metabolism of thyroxine during acute falciparum malaria in man

Previous studies of thyroid function during various infections have yielded conflicting results, but most have suggested an acceleration of peripheral thyroxine (T₄) turnover during the acute infectious illness. In the present studies, thyroid function was examined by a method allowing simultaneous analysis of both endogenous thyroidal release and peripheral T₄ disposal in normal volunteers after induction of acute falciparum malaria. Subjects received iodide-¹²⁵I, followed in 5-7 days by ¹³¹I-T₄ intravenously. 4 days later, infection was induced by the injection of parasitized red blood cells. Bidaily measurements of serum protein-bound ¹²⁵I and protein-bound ¹³¹I, and urinary ¹²⁵I and ¹³¹I, together with frequent estimates of serum ¹²⁷I-T₄ (Murphy-Pattee) and free T₄ (FT₄), were made during a control period, during acute illness, and during convalescence. Alterations in the peripheral metabolism of ¹³¹I-T₄ during infection included significant decreases in the fractional disappearance rate for T₄ [(k)], and in the clearance and daily disposal of T₄, all of which returned to control values during convalescence. Total serum ¹²⁷I-T₄ increased late in the infected period to become greater during convalescence than either before or during infection, while FT₄ did not increase significantly until convalescence. An analysis of serum ¹³¹I-T₄/¹²⁷I-T₄ and ¹³¹I-T₄/PB¹²⁵I ratios confirmed these observations. The slope with time of ratios for urinary ¹²⁵I/¹³¹I, a reflection of thyroidal iodine release, was decreased during infection, but rebounded to control values during the convalescent period. The observed increments in serum ¹²⁷I-T₄ concentration in the convalescent phase may reflect in part the slowing of (k), but together with the rising ratios of urine ¹²⁵I/¹³¹I suggests enhanced thyroidal T₄ secretion immediately after the acute illness. Thus, with malarial infection, there appears to be an initial depression followed by a rebound in rates of thyroidal iodine release. In contradistinction to other infections, fractional turnover and daily disposal of hormone is decreased in malaria, perhaps due to hepatic dysfunction and the consequent impairment in cellular deiodinative processes.     

Relationships between choline uptake, acetylcholine synthesis and acetylcholine release in isolated rat atria

Isolated rat atria take up [3H]choline and synthesize [3H]acetylcholine (ACh). The uptake of [3H]choline has a high-affinity component with a Km of approximately 0.2 microM and a Vmax of approximately 6 fmol/min/mg wet wt. This high-affinity component of choline uptake is difficult to measure directly because it represents only a small portion of total [3H]choline uptake. However, the rate of synthesis of [3H] ACh from [3H]choline appears to reflect the activity of the high-affinity choline uptake system. Thus, [3H]ACh synthesis is most efficient at low choline concentrations and is inhibited in the presence of hemicholinium-3 and low NaCl medium. The neuronal localization of the [3H]Ach synthesized from [3H]choline is demonstrated by the finding that [3H]ACh is released from the atria by depolarization with 57 mM K+ medium. The release is Ca++ -dependent and there is a compensatory increase in the synthesis of [3H]ACh after depolarization-induced ACh release. These data suggest that [3H]choline can be specifically incorporated into a releasable pool of [3H]ACh localized in cardiac parasympathetic neurons. The synthesis of [3H]ACh is inhibited by blockade of high-affinity choline uptake and is regulated in response to neuronal activity. The application of these methods will provide a means for directly examining the physiological and pharmacological control of ACh synthesis and release from cardiac parasympathetic neurons.     

肌筋膜放松训练对正常人腰部神经肌肉功能影响的研究

摘要 目的：观察肌筋膜放松训练对正常人腰部神经肌肉功能的影响,为肌筋膜放松训练的临床应用提供依据。 方法：试验采用随机双盲设计,共23例男性健康受试者,随机分为空白组（7例）、安慰剂组（8例）、试验组（8例）。试验组进行腰部的肌筋膜放松训练,安慰剂组给予模拟电刺激,空白组不做任何干预。每位受试者在干预后即刻（0 min）、10 min、20 min进行躯干的屈曲-伸展测试,并同时采集L3、L5水平左右双侧竖脊肌和多裂肌的表面肌电信号（surface electromyography, sEMG）以及躯干和骨盆在矢状面上的运动加速度信号。计算并分析三组的腰椎屈曲关节活动度、竖脊肌和多裂肌在屈曲—放松现象中肌电静息开始（EMG-off）和终止（EMG-on）时的腰椎角度,以及屈曲—放松比（flexion-relaxation ratio, FRR）。 结果：空白组、安慰剂组、试验组的腰椎关节活动度、竖脊肌和多裂肌EMG-off、EMG-on时的腰椎角度无显著性差异（P>0.05）,在三个不同时间点内试验组的FRR值与空白组和安慰剂组比较,大部分具有显著性差异（P<0.05）。 结论：肌筋膜放松训练后腰部竖脊肌和多裂肌的主动活动功能提高,肌肉激活模式得到改善。放松后不影响腰部的神经肌肉功能,对腰椎稳定性具有强化作用。     

神经肌肉电刺激与早期被动活动对机械通气患者ICU获得性虚弱的影响

目的:探讨早期被动活动与神经肌肉电刺激(NMES)对机械通气患者ICU获得性虚弱(ICU-AW)的影响。方法:前瞻性随机对照的研究方法,共纳入144例机械通气患者,按随机数字法分到对照组、活动组、NEMS组及联合组,监测机械通气后患者第1、3、5、7d肌力水平,并记录机械通气、入住ICU时间及总住院时间。结果:相比对照组,其余三组患者肌力均有所改善,联合组肌力增加水平更显著且时间更早,第3天MRC(Medical Research Council score,MRC-score)评分联合组即高于对照组(P0.05);而NMES组至第5天仅高于对照组(P0.05)。各组患者ICU-AW患病率均呈上升趋势,但第7天时联合组患病率明显低于其余三组(患病率分别为55.26%、45.71%、33.33%、22.86%),第5、7天时的对照组与NMES组、联合组以及活动组与联合组的患病率有差异(P0.05)。四组机械通气及入住ICU时间比较,联合组比对照组、活动组和NMES组比对照组均明显减少(P0.05);NMES组与联合组仅在机械通气时间中有差异(P0.05);各组总住院时间均无差异(P0.05)。结论:NMES与早期被动活动均可维持或改善患者肌力,使患者ICU-AW的患病率降低、机械通气时间和入住ICU时间减少,NMES的疗效优于手动被动干预,而联合组效果更加显著     

The role of future longitudinal studies in ICU survivors: understanding determinants and pathophysiology of weakness and neuromuscular dysfunction

PURPOSE OF REVIEW: The goals of this review are to discuss the pathophysiology and determinants of muscle weakness and neuromuscular dysfunction after critical illness, and to offer thoughts regarding the role of future longitudinal studies in this area. RECENT FINDINGS: While recent studies support the finding that neuromuscular dysfunction is common and important after critical illness, reversible risk factors and approaches to prevention and treatment remain unproven. Pathophysiologic studies implicate disease and treatment associated factors in the development of nerve and muscle damage during critical illness; these factors may provide targets for future studies. SUMMARY: Additional studies with improved methodology that address epidemiology and that test interventions are needed to understand and to improve neuromuscular function after critical illness.     

Resting and evoked spinal substance P release during chronic intrathecal morphine infusion: parallels with tolerance and dependence

Spinal opiate analgesia is associated with presynaptic inhibition of release of excitatory neurotransmitters/neuromodulators, e.g., substance P (SP), from primary afferent terminals. Chronic intrathecal (i.t.) administration of opiates such as morphine results in an initial analgesia followed by tolerance and a state of dependence. In this study, we examined the resting and evoked neurokinin 1 receptor (NK1r) internalization, indicative of endogenous SP release, in dorsal horn neurons of the lumbar spinal cord by immunocytochemistry during chronic i.t. infusion of morphine in rats. Noxious mechanical stimulation (compression) applied to unilateral hind paw evoked a significant increase in NK1r internalization in lamina I neurons in the ipsilateral dorsal horn. Intrathecal morphine infusion (40 nmol/microl/h) for 1 day possessed similar analgesic efficacy as acute morphine and blocked compression-induced spinal NK1r internalization. After 5 days of morphine infusion, thermal escape latencies were the same as in preinfusion animals or saline-infused controls, and compression-evoked NK1r internalization was no longer suppressed. Systemic administration of naloxone to rats on day 6 of morphine infusion resulted in prominent withdrawal behaviors and a concomitant increase in NK1r internalization in dorsal horn. The naloxone-induced internalization was blocked by NK1r antagonist L-703,606 [cis-2-(diphenylmethyl)-N-[(2-iodophenyl)methyl]-1 azabicyclo[2.2.2]octan-3-amine] or pretreatment with capsaicin, confirming that the internalization is due to the endogenous SP release from the primary afferents. We conclude that inability to suppress release of excitatory neurotransmitters/neuromodulators from primary afferents by morphine after chronic exposure is an important component in spinal morphine tolerance, and excessive release from these afferents contributes to the spinal morphine withdrawal syndrome.