Mechanical, electrical, and morphological characteristics of skeletal muscle fibers from Xenopus and other species of frogs

Hydrogen peroxide decelerates recovery of action potential after high-frequency fatigue in skeletal muscle

Effects of reactive oxygen species (ROS), especially hydrogen peroxide (H(2)O(2)), on recovery of action potential by resting for 30 min after high-frequency fatigue were studied using frog skeletal muscle fibers. After stimulation at a frequency of 50 HZ for 2 min, the action potential amplitude was decreased by 14.5 mV from controls, and resting membrane was depolarized by 15.4 mV. Action potential duration was also prolonged by high-frequency stimulation (1.5 ms in controls to 2.6 ms). The high-frequency stimulation used here caused no muscle damage. The action potential was partially improved after a 30-min rest. Addition of catalase at 500 units/ml or H(2)O(2) at 0.5 mM to sartorius muscle did not alter any of the parameters of the action potential after high-frequency stimulation. Treatment with catalase accelerated post-fatigue recovery of the action potential. Application of H(2)O(2) delayed post-fatigue recovery of resting and action potentials. When added to detubulated toe muscle fibers, catalase no longer improved the attenuation of action potential induced by high-frequency stimulation, even after a 30-min rest. These findings suggest that removal of H(2)O(2) from transverse tubules is effective for post-fatigue recovery of action potential in skeletal muscle.

A novel phasic contraction induced by dithiothreitol in frog skeletal muscle

1. Dithiothreitol (DTT), at 50-100 mM, induced a phasic reversible contraction of frog skeletal muscle. 2. Exposure of single fibers to nifedipine (20 microM), an L-type Ca2+ antagonist, blocked the twitch and tetanus tensions but never affected the DTT-induced contraction. 3. DTT also produced a phasic contraction in fibers where voltage sensors were inactivated in the presence of high K+ concentration (190 mM). 4. A fiber was mechanically skinned after observation of DTT-induced contraction. The skinned fiber contracted in response to a DTT concentration similar to that required to produce contraction in intact fibers before skinning. 5. In skinned fibers, DTT, at 100 or 200 mM, inhibited the accumulation of Ca2+ by SR, but not Ca2+ ATPase activity. 6. These results suggest that a high concentration of DTT triggers Ca2+ efflux from the SR through action on the Ca2+ release channel and/or closely associated proteins, such as triadin and FK-506 binding protein.

H2O2 modulates twitch tension and increases Po of Ca2+ release channel in frog skeletal muscle

The effect of H2O2 was examined to elucidate the basis of muscle injury after exercise. Exposure of single fibers to 1.5-6 mM H2O2 led to twitch potentiation followed by a marked decrease. Then, fibers contracted spontaneously. BAY K 8644 augmented twitch potentiation and slowed the decay of twitches. In 5 mM dithiothreitol (DTT), twitch potentiation and spontaneous contraction were not observed on H2O2 addition. Cytoplasmic application of 1.5-3 mM H2O2 to heavy sarcoplasmic reticulum (SR) vesicles incorporated into planar lipid bilayers increased the open probability of Ca2+ release channels, an effect reversed by DTT. We investigated oxidation of sulfhydryl groups on proteins in SR membrane by H2O2 with N-(7-dimethylamino-4-methyl-3-coumarinyl)maleimide. Pretreatment of light and heavy SR membranes with 1.5 mM H2O2 exponentially increased fluorescence intensity. The time constant of the intensity increase was increased markedly only in heavy SR in solution containing 50 microM cytoplasmic Ca2+, so Ca2+ release was associated with protein oxidation by H2O2. Thus extracellular H2O2 probably acts by oxidizing sulfhydryls of proteins at two distinct sites: the dihydropyridine receptors, oxidation of which elicits potentiation and subsequent inhibition of twitches, and Ca2+ release channels, whose oxidation elicits spontaneous contraction, resulting in muscle dysfunction.

Silver ion-induced tension development and membrane depolarization in frog skeletal muscle fibres

Silver ions elicit dose-dependently a transient contracture in single fibres of bull-frog toe muscle placed in 0-Ca2+, Cl- -free MOPS solution containing 3 mM Mg2+ and NO3-. To elucidate the mechanisms involved, changes in membrane potential and in tension development were continuously measured following exposure to Ag+. The effect of Ag+ on contraction in fibres in which the membrane had been depolarized by elevating the external K+ concentration was also examined. The major findings of this investigation are as follows. (1) The mechanical threshold was shifted towards more negative potentials by 5 mV (-51 to -56 mV), when Ca2+ and Cl- in the Ringer's solution were replaced with Mg2+ and NO3-, respectively. (2) On the exposure of the fibres to 5 microM Ag+, the membrane potential decreased by 1.6 mV from -87.8 mV and tension was developed. (3) In fibres soaked in a solution containing 10 mM K+ (corresponding to a membrane potential of -69.5 mV), 5 microM Ag+ produced a large contracture similar to that seen in the control solution. (4) The Ag+-induced contracture was inactivated when more than 20 mM K+ was used. (5) The membrane depolarization evoked by either 20 or 50 microM Hg2+ did not produce contraction. (6) Muscle fibres which had been exposed to 20 microM Hg2+ for 5 min responded to 5 microM Ag+ by a transient tension development. These findings strongly suggest that Ag+-induced tension development is not associated with depolarization of the surface membrane but rather is caused by specific actions of Ag+ on membrane proteins in the T-tubules.