Chloride channel inhibition improves neuromuscular function under conditions mimicking neuromuscular disorders.

AIM: The skeletal muscle Cl- channels, the ClC-1 channels, stabilize the resting membrane potential and dampen muscle fibre excitability. This study explored whether ClC-1 inhibition can recover nerve-stimulated force in isolated muscle under conditions of compromised neuromuscular transmission akin to disorders of myasthenia gravis and Lambert-Eaton syndrome. METHODS: Nerve-muscle preparations were isolated from rats. Preparations were exposed to pre-or post-synaptic inhibitors (ω-agatoxin, elevated extracellular Mg2+ , α-bungarotoxin or tubocurarine). The potential of ClC-1 inhibition (9-AC or reduced extracellular Cl- ) to recover nerve-stimulated force under these conditions was assessed. RESULTS: ClC-1 inhibition recovered force in both slow-twitch soleus and fast-twitch EDL muscles exposed to 0.2 µmol/L tubocurarine or 3.5 mmol/L Mg2+ . Similarly, ClC-1 inhibition recovered force in soleus muscles exposed to α-bungarotoxin or ω-agatoxin. Moreover, the concentrations of tubocurarine and Mg2+ required for reducing force to 50% rose from 0.14 ± 0.02 µmol/L and 4.2 ± 0.2 mmol/L in control muscles to 0.45 ± 0.03 µmol/L and 4.7 ± 0.3 mmol/L in muscles with 9-AC respectively (P < .05, paired T test). Inhibition of acetylcholinesterase (neostigmine) and inhibition of voltage-gated K+ channels (4-AP) relieve symptoms in myasthenia gravis and Lambert-Eaton syndrome, respectively. Neostigmine and 9-AC additively increased the tubocurarine concentration required to reduce nerve-stimulated force to 50% (0.56 ± 0.05 µmol/L with 9-AC and neostigmine) and, similarly, 4-AP and 9-AC additively increased the Mg2+ concentration required to reduce nerve-stimulated force to 50% (6.5 ± 0.2 mmol/L with 9-AC and 4-AP). CONCLUSION: This study shows that ClC-1 inhibition can improve neuromuscular function in pharmacological models of compromised neuromuscular transmission.

Exploring fetal fibronectin testing as a predictor of labour onset: In parturient women from isolated communities.

OBJECTIVE: To investigate whether the fetal fibronectin assay would be useful for determining if a woman was close to a term delivery. If effective, this test would allow parturient women to stay in their communities longer. DESIGN: This feasibility study used a prospective cohort design to examine the negative predictive value of the fetal fibronectin test at term. SETTING: Iqaluit, NU. PARTICIPANTS: A total of 30 parturient women from rural and isolated communities in Nunavut. INTERVENTION: Starting at 36 weeks' gestation, women were tested every 2 days, and after 39 weeks this increased to every day until labour. MAIN OUTCOME MEASURES: The negative predictive value of the fetal fibronectin test was assessed. RESULTS: Women were no more likely to give birth at 7 or more days after their last negative fetal fibronectin test result relative to their likelihood of giving birth at 6 or fewer days after their last negative test result. Hence, the presence of fetal fibronectin in cervical secretion did not predict term delivery. CONCLUSION: This project indicated that the fetal fibronectin test did not have adequate sensitivity or specificity as a diagnostic measure to predict a delay of labour at term.

Comparison of regulated passive membrane conductance in action potential-firing fast- and slow-twitch muscle.

In several pathological and experimental conditions, the passive membrane conductance of muscle fibers (G(m)) and their excitability are inversely related. Despite this capacity of G(m) to determine muscle excitability, its regulation in active muscle fibers is largely unexplored. In this issue, our previous study (Pedersen et al. 2009. J. Gen. Physiol. doi:10.1085/jgp.200910291) established a technique with which biphasic regulation of G(m) in action potential (AP)-firing fast-twitch fibers of rat extensor digitorum longus muscles was identified and characterized with temporal resolution of seconds. This showed that AP firing initially reduced G(m) via ClC-1 channel inhibition but after approximately 1,800 APs, G(m) rose substantially, causing AP excitation failure. This late increase of G(m) reflected activation of ClC-1 and K(ATP) channels. The present study has explored regulation of G(m) in AP-firing slow-twitch fibers of soleus muscle and compared it to G(m) dynamics in fast-twitch fibers. It further explored aspects of the cellular signaling that conveyed regulation of G(m) in AP-firing fibers. Thus, in both fiber types, AP firing first triggered protein kinase C (PKC)-dependent ClC-1 channel inhibition that reduced G(m) by approximately 50%. Experiments with dantrolene showed that AP-triggered SR Ca(2+) release activated this PKC-mediated ClC-1 channel inhibition that was associated with reduced rheobase current and improved function of depolarized muscles, indicating that the reduced G(m) enhanced muscle fiber excitability. In fast-twitch fibers, the late rise in G(m) was accelerated by glucose-free conditions, whereas it was postponed when intermittent resting periods were introduced during AP firing. Remarkably, elevation of G(m) was never encountered in AP-firing slow-twitch fibers, even after 15,000 APs. These observations implicate metabolic depression in the elevation of G(m) in AP-firing fast-twitch fibers. It is concluded that regulation of G(m) is a general phenomenon in AP-firing muscle, and that differences in G(m) regulation may contribute to the different phenotypes of fast- and slow-twitch muscle.