Diana Conte-Camerino

Chloride channel regulation in the skeletal muscle of normal and myotonic goats

External intercostal muscle biopsies from normal and congenitally myotonic goats were studied in vitro at 30° C using a two-microelectrode square-pulse cable analysis assisted by computer. The resting chloride conductance (Gcl) was estimated from the difference between the mean membrane conductance in chloride-containing and chloride-free bathing media. The protein kinase C (PKC) activator, 4-β-phorbol-12,13-dibutyrate, (0.1–2.0 μM) blocks a maximum of 76% of Gcl in normal goat fibers and induces myotonic hyperexcitability similar to that of congenitally myotonic goat fibers. The Gcl block was partially antagonized by pretreatment with the PKC inhibitor, staurosporine (10 μM). The “inactive” 4-αphorbol-12, 13,didecanoate had no effect at 50 μM, whereas the “active” 4-β isomer blocked 41% Gcl at 1 μM. The nearly absent Gcl of congenitally myotonic goat fibers was not restored by treatment with high concentrations of the PKC inhibitors staurosporine, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7), or tetrahydropapaveralone (THP). Also, forskolin and cholera toxin, which may increase cyclic adenosine monophosphate (cAMP) levels, or the R(+) clofibric acid enantiomers and taurine, which increase Gcl in normal fibers, were also unable to restore Gcl in myotonic goat fibers. The data suggest that PKC may be a chloride channel regulator in normal goat skeletal muscle fibers, however the molecular defect of congenitally myotonic fibers does not appear to be due to excessive activity of PKC.

Enantiomers of clofibric acid analogs have opposite actions on rat skeletal muscle chloride channels

The S-(−) isomers of a series of clofibric acid analogs produced only a block of chloride conductance of rat skeletal muscle fibers with increasing concentrations until block was nearly complete. The R-(+) isomers, on the other hand, at low concentrations increased chloride conductance by as much as 9% to 39% and at higher concentrations decreased chloride conductance, but never by more than 27% of the control value. The actions of the enantiomeric pairs to either produce or inhibit myotonic excitability paralleled their ability to block or increase chloride conductance, respectively.

The Toxic Effects of Clofibrate and Its Metabolite on Mammalian Skeletal Muscle: An Electrophysiologigal Study

Membrane electrical parameters of rat extensor digitorum longus muscles were analysed in vitro, with intracellular microelectrodes, after chronic treatment with clofibrate or after in vitro application of chlorophenoxyisobutyric acid, the in vivo metabolite of clofibrate. Clofibrate caused only a 38% decrease in the membrane chloride conductance while its metabolite induced a large decrease of chloride conductance and drastic changes in the excitability characteristics of the muscles sampled. The capability of clofibrate to induce two types of myotonia is discussed.

Carboxylic acids and skeletal muscle chloride channel conductance: Effects on the biological activity induced by the introduction of an aryloxyalkyl group α to the carboxylic function of 4-chloro-phenoxyacetic acid

2-(4-Chloro-phenoxy)propanoic and 2-(4-chloro-phenoxy)butanoic acids are compounds known to block chloride membrane conductance in rat striated muscle by interaction with a specific receptor. In the present study, a series of chiral analogues has been prepared and tested to evaluate the influence of a second aryloxy moiety introduced in the side-chain at a variable distance from the stereogenic centre. The results show that this chemical modification is detrimental for biological activity which, however, is increased by lengthening the alkyl chain up to three methylenic groups, then decreases to remain constant in the next analogues of the series. A possible explanation for this is proposed on the basis of steric effects and/or different approach of the molecules to the receptor.

Dantrolene sodium: Stimulatory and depressant effects on the contractility of guinea-pig uterus in vitro

The effects of dantrolene sodium on the spontaneous and oxytocin-induced contraction of guinea-pig uterus were studied in vitro. Dantrolene reversibly increased the spontaneous contraction frequency of guinea-pig uterus. The effect was not dose-related and was not antagonized by atropine, methysergide or indomethacin. Dantrolene dose dependently and reversibly reduced the contractile responses of guinea-pig uterus to oxytocin. The varying effects of dantrolene on the uterus probably involve inhibition of calcium movement at more than one site of the muscle.

Electrical properties of rat extensor digitorum longus muscle after chronic application of emetine to the motor nerve

Abstract Muscle weakness is a side effect of the chronic administration of emetine [(−)emetine] in the treatment of amoebiasis. The precise nature of the weakness is still controversial. In this study we were interested in whether or not emetine can alter the electrical behavior of muscle fibers when applied to the nerve at some distance from the muscle. To test this idea, silastic cuffs containing 0.1 to 0.2% emetine were placed on the peroneal nerve of adult rats and the electrical properties of the extensor digitorum longus (EDL) muscle fibers were monitored in vitro at 4 to 40 days after application of the cuff. Action potential generation was affected by emetine treatment: the rheobasic current was significantly lowered (from a control value of 78 to 40 nA), and a small increase in current in the treated fibers produced seven or more action potentials. In the emetine-treated fibers, mean chloride conductance G Cl decreased from a control value of 3737 to 2453 μS/cm 2 while mean potassium conductance G K showed an insignificant increase from 304 to 408 μS/cm 2 . We conclude that emetine interferes with specific materials involved in the maintenance of the high resting G Cl of mammalian skeletal muscle fibers.

The effects of the calcium channel agonist, Bay K-8644, on guinea-pig ileum and rat uterine horn

Bay K‐8644 (methyl 1,4‐dihydro‐2,6‐dimethyl‐3‐nitro‐4‐(2‐trifluoromethylphenyl)pyridine‐5‐carboxylate) concentration‐dependently caused contractions of the partly depolarized ileum, but at higher concentrations (10−6 to 10−5 M) produced relaxation. 10−8 to 4 times 10−7M nifedipine antagonized while 10−9 and 4 times 10−9 M potentiated Bay K‐8644. On the partly depolarized uterus, Bay K‐8644 (10−9 to 10−6 M) had only a spasmogenic effect whereas nifedipine (4 times 10−9 to 10−7 M) was spasmolytic. Verapamil and diltiazem (each at 10−4 M) both reduced the maximal response to Bay K‐8644, while other spasmolytics were ineffective. Thus, Bay K‐8644 activates ileum and uterus by opening voltage‐operated Ca2+ channels, but its relaxant action at high concentration and its potentiation by nifedipine is not seen in both organs. Such differences probably depend on the concentration of the compounds used and the polarization state of the cell membranes.

Carboxylic acids and skeletal muscle chloride channel conductance: effects on the biological activity induced by the introduction of methyl groups on the aromatic ring of chiral α-(4-chloro-phenoxy)alkanoic acids

One or two methyl groups have been introduced on the aromatic ring of two chiral clofibric acid analogs, 2-(4-chloro-phenoxy)propanoic and 2-(4-chloro-phenoxy)butanoic acids. The biological activity of the derivatives obtained (3-6) has been evaluated on the skeletal muscle chloride conductance (gCl). The results confirm the hypothesis of two different sites modulating chloride channel function, an excitatory site that increases channel activity and an inhibitory site that produces a channel block. In fact, this chemical modification strongly reduces the blocking activity of the (R)- and (S)-enantiomers in comparison with the parent compounds, but does not markedly affect the ability of the (R)-enantiomers to increase chloride channel conductance.

Effect of Taurine on Chloride Conductance and Excitability of Rat Skeletal Muscle Fibers

Taurine is found in mammalian skeletal muscle fibers and all other excitable tissues (17), but its function is not well understood (16). Abnormal taurine contents whether pathologically or experimentally induced are often associated with changes of excitability in heart (9), nerve (8,21), and retina (4). The taurine content is increased in skeletal muscle from the dystrophic chicken (23).

Effects of different dosages of vincristine on the component conductances of rat skeletal muscle fibers.

Summary Membrane potentials, cable parameters and component resting conductances, were measured in extensor digitorum longus (EDL) muscle fibers from adult rats, in vitro at 25°–28°C, at 6 to 33 days following chronic application of 0.05% and 0.1% vincristine sulfate to the peroneal nerve. All the vincristine treated fibers were depolarized; from 6 to 14 days of treatment tetrodotoxin resistant action potentials, were present. Membrane resistance in normal solution in the 0.05% vincristine treated fibers did not change significantly until the 23th day of t reatment; for longer period of 0;05% and in the 0;1% vincristine treated preparations it increased significantly. Membrane resistance in chloride free medium showed a large decrease in all the preparations. In the 0.05% vincristine treated fibers, mean chloride conductance G C1 did not change significantly, until the 23th day of treatment, while mean potassium conductances G k increased significantly from a mean control value of 163±17 to 454±68 μmhos/cm 2 ; at 33 day of treatment G C1 dropped significantly while G k was still high. In the 0.1% vincristine treated fibers, mean G k showed a tendency to rise, while G C1 decreased significantly from a control value of 3427±114 to 1699±108 μmhos/cm 2 . In accordance with the literature it was concluded that factors transported by the microtubular and may be by the microfilament system are important for the mainteinance of the high resting G Cl and the low resting G k of mammalian skeletal muscle fibers and therefore for the integrity of the neuromuscular function.