Chronic Nicotine Modifies Skeletal Muscle Na,K-ATPase Activity through Its Interaction with the Nicotinic Acetylcholine Receptor and Phospholemman

Our previous finding that the muscle nicotinic acetylcholine receptor (nAChR) and the Na,K-ATPase interact as a regulatory complex to modulate Na,K-ATPase activity suggested that chronic, circulating nicotine may alter this interaction, with long-term changes in the membrane potential. To test this hypothesis, we chronically exposed rats to nicotine delivered orally for 21–31 days. Chronic nicotine produced a steady membrane depolarization of ∼3 mV in the diaphragm muscle, which resulted from a net change in electrogenic transport by the Na,K-ATPase α2 and α1 isoforms. Electrogenic transport by the α2 isoform increased (+1.8 mV) while the activity of the α1 isoform decreased (−4.4 mV). Protein expression of Na,K-ATPase α1 or α2 isoforms and the nAChR did not change; however, the content of α2 subunit in the plasma membrane decreased by 25%, indicating that its stimulated electrogenic transport is due to an increase in specific activity. The physical association between the nAChR, the Na,K-ATPase α1 or α2 subunits, and the regulatory subunit of the Na,K-ATPase, phospholemman (PLM), measured by co-immuno precipitation, was stable and unchanged. Chronic nicotine treatment activated PKCα/β2 and PKCδ and was accompanied by parallel increases in PLM phosphorylation at Ser63 and Ser68. Collectively, these results demonstrate that nicotine at chronic doses, acting through the nAChR-Na,K-ATPase complex, is able to modulate Na,K-ATPase activity in an isoform-specific manner and that the regulatory range includes both stimulation and inhibition of enzyme activity. Cholinergic modulation of Na,K-ATPase activity is achieved, in part, through activation of PKC and phosphorylation of PLM

Effects of membrane depolarization and changes in extracellular [K+] on the Ca2+ transients of fast skeletal muscle fibers. Implications for muscle fatigue

Repetitive activation of skeletal muscle fibers leads to a reduced transmembrane K+ gradient. The resulting membrane depolarization has been proposed to play a major role in the onset of muscle fatigue. Nevertheless, raising the extracellular K+ (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document}) concentration (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document}) to 10 mM potentiates twitch force of rested amphibian and mammalian fibers. We used a double Vaseline gap method to simultaneously record action potentials (AP) and Ca2+ transients from rested frog fibers activated by single and tetanic stimulation (10 pulses, 100 Hz) at various \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document} and membrane potentials. Depolarization resulting from current injection or raised \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document} produced an increase in the resting [Ca2+]. Ca2+ transients elicited by single stimulation were potentiated by depolarization from −80 to −60 mV but markedly depressed by further depolarization. Potentiation was inversely correlated with a reduction in the amplitude, overshoot and duration of APs. Similar effects were found for the Ca2+ transients elicited by the first pulse of 100 Hz trains. Depression or block of Ca2+ transient in response to the 2nd to 10th pulses of 100 Hz trains was observed at smaller depolarizations as compared to that seen when using single stimulation. Changes in Ca2+ transients along the trains were associated with impaired or abortive APs. Raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document} to 10 mM potentiated Ca2+ transients elicited by single and tetanic stimulation, while raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document} to 15 mM markedly depressed both responses. The effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document} on Ca2+ transients, but not those of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document}, could be fully reversed by hyperpolarization. The results suggests that the force potentiating effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document} might be mediated by depolarization dependent changes in resting [Ca2+] and Ca2+ release, and that additional mechanisms might be involved in the effects of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document} on force generation

Controlled Trial of Transfusions for Silent Cerebral Infarcts in Sickle Cell Anemia

BACKGROUND Silent cerebral infarcts are the most common neurologic injury in children with sickle cell anemia and are associated with the recurrence of an infarct (stroke or silent cerebral infarct). We tested the hypothesis that the incidence of the recurrence of an infarct would be lower among children who underwent regular bloodtransfusion therapy than among those who received standard care. METHODS In this randomized, single-blind clinical trial, we randomly assigned children with sickle cell anemia to receive regular blood transfusions (transfusion group) or standard care (observation group). Participants were between 5 and 15 years of age, with no history of stroke and with one or more silent cerebral infarcts on magnetic resonance imaging and a neurologic examination showing no abnormalities corresponding to these lesions. The primary end point was the recurrence of an infarct, defined as a stroke or a new or enlarged silent cerebral infarct. RESULTS A total of 196 children (mean age, 10 years) were randomly assigned to the observation or transfusion group and were followed for a median of 3 years. In the transfusion group, 6 of 99 children (6%) had an end-point event (1 had a stroke, and 5 had new or enlarged silent cerebral infarcts). In the observation group, 14 of 97 children (14%) had an end-point event (7 had strokes, and 7 had new or enlarged silent cerebral infarcts). The incidence of the primary end point in the transfusion and observation groups was 2.0 and 4.8 events, respectively, per 100 years at risk, corresponding to an incidence rate ratio of 0.41 (95% confidence interval, 0.12 to 0.99; P=0.04). CONCLUSIONS Regular blood-transfusion therapy significantly reduced the incidence of the recurrence of cerebral infarct in children with sickle cell anemia. (Funded by the National Institute of Neurological Disorders and Stroke and others; Silent Cerebral Infarct Multi-Center Clinical Trial ClinicalTrials.gov number, NCT00072761, and Current Controlled Trials number, ISRCTN52713285.

Inward rectification in the transverse tubular system of frog skeletal muscle studied with potentiometric dyes.

The non-penetrating potentiometric dyes NK2367 and WW375 were used to investigate the effect of inward rectification on the weighted-average tubular membrane potential in single frog muscle fibres voltage clamped using a three-Vaseline-gap method. In 100 mM-K solution, when inward rectification was activated by hyperpolarization the steady-state amplitude of the transverse tubular system (T-system) optical signal was reduced, and its rise time was faster than that recorded for an equivalent depolarization. The voltage dependence of the optical attenuation followed that of inward rectification, increasing with increasing hyperpolarization. For a voltage-clamp step of -140 mV the optical attenuation was 0.72 which corresponds to a weighted-average T-system potential change of 100 mV. When inward rectification was blocked in a Cs, TEA solution the optical attenuation was also abolished. The voltage dependence of the block of the inward currents in solutions containing low concentrations of Cs was also reflected in the T-system optical signals. Our results were satisfactorily predicted by a radial cable model of the T-system, assuming the same specific inward rectifier conductance in surface and tubular membranes. This analysis predicts that the measured optical attenuation corresponds to a decrease in the tubular space constant, lambda T, from 120 micron under passive conditions to about 40 micron when inward rectification is fully, activated. The voltage dependence of inward rectification measured at the surface membrane was reasonably well predicted by assuming that the specific conductance obeyed a Boltzmann type of voltage dependence; the major effect of tubular decrements was to reduce the steepness of the total (surface + T-system) conductance-voltage relation

Enantiomeric effects on excitation-contraction coupling in frog skeletal muscle by a chiral phenoxy carboxylic acid

Aromatic monocarboxylic acids are known to significantly potentiate the mechanical response of skeletal muscle fibers. In this study we investigated the effects of enantiomers of 2-(4-chlorophenoxy)propionic acid, chemically one of the simplest aromatic monocarboxylic acids with chiral properties, on mechanical threshold and charge movement in frog skeletal muscle. The R(+), but not the S(-), enantiomer lowered rheobase mechanical threshold and shifted charge movement to more negative potentials. The R(+) enantiomer also significantly slowed charge movement kinetics, with pronounced delays of the OFF charge transitions. These effects required high temperature for their production. The stereospecific actions of the R(+) enantiomer are interpreted in terms of a specific interaction of this compound at an anion-sensitive site involved in excitation-contraction coupling, most likely on the dihydropyridine-sensitive voltage sensor in the T-syste