Fatigue-induced changes in muscle fiber action potentials estimated by wavelet analysis.

We aimed to investigate fatigue-induced changes in the spectral parameters of slow (SMF) and fast fatigable muscle fiber (FMF) action potentials using discrete wavelet (DWT) and fast Fourier (FFT) transforms. Intracellular potentials were recorded during repetitive stimulation of isolated muscle fibers immersed in Ca(2+)-enriched medium, while extracellular potentials were obtained from muscle fibers pre-exposed to electromagnetic microwaves (MMW, 2.45 GHz, 20 mW/cm(2)). The changes in the frequency distribution of the action potentials during the period of uninterrupted fiber activity were used as criteria for fatigue assessment. The wavelet coefficients' changes in the calculated frequency scales demonstrated a contribution of the increased [Ca(2+)](0) to an earlier compression of the frequency spectrum towards lower ranges. Root mean square (RMS) analysis of the wavelet coefficients calculated from SMF potentials showed a reduction of the higher frequencies (scale 1) by 90% in elevated [Ca(2+)](0) vs. 55% in controls and an increase of low frequencies (scale 5) by 323% vs. 187%, respectively. For FMF potentials a decrease of 71% vs. 59% for high frequencies (scale 1, elevated [Ca(2+)](0) vs. control) and an increase of 386% vs. 295% in scale 5, respectively, were observed. MMW pre-exposure resulted in increased muscle fiber resistance to fatigue. The fatigue-induced decrease of potential high frequencies (SMF: 59% vs. 96%, MMW vs. control; FMF: 30% vs. 92%, respectively), and the increase of low frequencies (SMF: 200% vs. 207%, MMW vs. control; FMF: 93% vs. 314%, respectively) were significantly smaller and delayed in exposed muscle fibers. Data from RMS analysis indicate that DWT provides a reliable method for estimation of muscle fatigue onset and progression.

Differential changes in myoelectric characteristics of slow and fast fatigable frog muscle fibres during long-lasting activity.

The electrical activity of different muscle fibre types during fatigue at varying stimulation frequency and fibre stretch was studied. Extracellular action potentials (ECAPs) were recorded from isolated frog muscle fibres at initial length and stretched by 15%, 25% and 35% and stimulated for 180 s by suprathreshold pulses with frequencies of 5, 6.7 and 10Hz. The changes in ECAP negative phase duration (T(0)), propagation velocity of excitation (PV), potential power spectrum and its median frequency (MDF) were analysed for the period of uninterrupted activity (endurance time, ET). Slow (SMF) and fast (FMF) fatigable muscle fibre types were distinguished by the rate of PV decrease during ET. With the increase of stimulation frequency and fibre stretch, the rate of ECAP parameter changes increased and was larger in FMF, but this proportion was reversed with stretching over 25% and 10Hz stimulation. In both fibre types the power spectrum shift to lower frequencies during continuous activity was more pronounced with higher stimulation frequency. In FMFs the rates of MDF changes were positively and more strongly correlated with the rates of PV changes, whilst in SMFs the inverse correlation between the rates of changes of MDF and T(0) was stronger. The results indicate specific adaptation of slow and fast fatigable muscle fibres to stretch and activation frequency due to the differences in their membrane processes.

Slow and fast fatigable frog muscle fibres: electrophysiological and histochemical characteristics.

Continuous activity of isolated frog gastrocnemius muscle fibres provoked by repetitive stimulation of 5 Hz was used as an experimental model for fatigue development in different fibre types. Parameter changes of the elicited intracellular action potentials and mechanical twitches during the period of uninterrupted activity were used as criteria for fatigue evaluation. Slow fatigable muscle fibre (SMF) and fast fatigable muscle fibre (FMF) types were distinguished depending on the duration of their uninterrupted activity, which was significantly longer in SMFs than in FMFs. The normalized changes of action potential amplitude and duration were significantly smaller in FMFs than in SMFs. The average twitch force and velocity of contraction and relaxation were significantly higher in FMFs than in SMFs. Myosin ATPase (mATPase) and succinate dehydrogenase activity were studied by histochemical assessment in order to validate the fibre type classification based on their electrophysiological characteristics. Based on the relative mATPase reactivity, the fibres of the studied muscle were classified as one of five different types (1-2, 2, 2-3, 3 and tonic). Smaller sized fibres (tonic and type 3) expressed higher succinate dehydrogenase activity than larger sized fibres (type 1-2, 2), which is related to the fatigue resistance. The differences between fatigue development in SMFs and FMFs during continuous activity were associated with fibre-type specific mATPase and succinate dehydrogenase activity.

About the biological effects of high and extremely high frequency electromagnetic fields.

This paper deals with the effects of high (microwave) and extremely high (millimetre waves, MMW) frequency electromagnetic fields on the membrane processes and ion channels, molecular complexes, excitable and other structures. Microwaves as well as millimetre waves are widely used in medical practice and in everyday life. The existence of interaction between the exogenous and endogenous electromagnetic fields with biological systems is now a subject of intense discussion. The most contentious question is the existence of a possible specific (non-thermal) effect of microwaves, unrelated to that caused by increased temperature. Although numerous data have been published on the possible non-thermal effects of the studied electromagnetic fields on different kinds of living systems, only little understanding is gained about the modes of microwave action. Here we review data, which provide evidence that non-thermal microwave effects do exist and may play a significant role. This evidence is based on research at all biological levels, from cell-free systems through cells, tissues and organs, to animal and human organisms.

Effect of microwave electromagnetic field on skeletal muscle fibre activity.

The aim of the present study was to investigate the influence of microwave irradiation on fatiguing activity of isolated frog skeletal muscle fibres. The changes in the electrical and mechanical activity were used as criteria for the exposure effects. Repetitive suprathreshold stimulation with interstimulus interval of 200 ms for 3 min was applied. Intracellular (ICAP) and extracellular (ECAP) action potentials and twitch contractions (Tw) of muscle fibres after 1 hour microwave exposure (2.45 GHz, 20 mW/cm( 2) power density) were compared with those recorded after one hour sham exposure (control). The duration of uninterrupted activity in the trial (endurance time; ET) was not significantly affected by microwave field exposure. After microwave irradiation, the ICAP amplitude was higher, the rising time was shorter, and the resting membrane potential was more negative compared to controls. There was a slower rate of parameters changes during ET in potentials obtained from irradiated fibres. Microwave exposure increased the propagation velocity of excitation, the ECAP and Tw amplitudes, as well as shortened their time parameters. We concluded that a 2.45 GHz microwave field possesses a stimulating effect on muscle fibre activity, which is in part due to its specific, non-thermal properties. The microwave induced-changes in muscle fibre activity may reduce development of skeletal muscle fatigue.

Long-lasting (fatiguing) activity of isolated muscle fibres influenced by microwave electromagnetic field.

The study aims to clarify the effect of exposure to microwave electromagnetic field (MMW) on muscle fibre fatigue. Repetitive stimulation with interstimulus interval of 200 ms was applied on isolated frog muscle fibre to evoke intracellular action potentials and twitch contractions. After their recording muscle fibre preparation was moved in a Petri dish with radius of 28 mm on open air for one hour exposure to continuous MMW with frequency of 2.45 GHz and power density of 20 mW/cm2. Then it was again moved in the chamber with non irradiated Ringer's solution at controlled temperature for the repeated records. After MMW exposure the changes in amplitude and time parameters characterizing fatigue were attenuated and delayed vs. controls. The twitch amplitude curve described an drastic fall in the first 5 sec followed by an increase and next decrease. MMW (2.45 GHz) have a specific, non-thermal influence on muscle fibre activity resulting in some resistance to fatigue.

Influence of a protein hydrolysate from green algae on the activity of some ATPase systems in frog skeletal muscle.

The present study investigated the effect of a protein hydrolysate from green algae cultured in the Bulgarian region of Rupy, on the enzyme activity of frog skeletal muscle. The activity of pure Mg(2+)-ATPase, Mg2+,Ca(2+)-ATPase, NaHCO3-stimulated Mg(2+)-ATPase and the latter in the presence of the inhibitors NaSCN and NaN3 in mitochondrial (B-3) and membrane (B-12) fractions were determined before and after treatment with the protein hydrolysate from green algae (30 and 300 micrograms/ml). The differences between ATPase activity of mitochondrial and membrane fractions were described and it was established that in the B-3 fraction, the activity of the NaHCO3-stimulated Mg(2+)-ATPase and Ca(2+)-dependent Mg(2+)-ATPase were accelerated by increasing concentrations of the algae protein hydrolysate. Irrespective of the different (equal or inverse) dose-dependent effects, the protein hydrolysate stimulated Mg(2+)-ATPase and that inhibited by NaSCN an NaN3 bicarbonate-stimulated Mg(2+)-ATPase activity. In most of the probes, the protein hydrolysate produced some increase in enzyme activity of NaHCO3-stimulated Mg(2+)-ATPase and Ca(2+)-dependent Mg(2+)-ATPase in B-12 fractions. The observed properties of the algae protein hydrolysate suggest that it is capable of stimulating enzyme processes in addition to having some antitoxic effect in skeletal muscle.

Further studies on Nivalin P-induced changes in muscle fiber membrane processes.

Nivalin P, composed of Nivalin (galanthamine hydrobromide) and Pymadin (4-aminopyridine hydrochloride), was applied extracellularly to isolated skeletal muscle fibers during prolonged activity (fatiguing) to better understand the effects of the drug on membrane ionic processes. Changes in intracellular action potential (ICAP) and twitch (Tw) parameters were monitored from treated and untreated fibers during uninterrupted activity (endurance time, ET) produced by repetitive stimulation every 200 msec for 3 min. Nivalin P-induced a shortening of the ET, drastic changes in repolarization of the ICAP corresponding to changes in negative afterpotential and falling area and an initial increase of the Tw amplitude and duration. These results suggest that Nivalin P: (i) inhibits the Na+, K(+)-pump due to nonspecific reduction of Na+ influx, stimulates the Na(+)-Ca2+ exchanger and inhibits K+ conductance; (ii) increases Ca2+ release and delays Ca2+ uptake under sufficient depolarization. It was concluded that fatigue develops faster in the presence of Nivalin P.

Spectral and time domain characteristics of single muscle fibre action potentials during continuous activity extracted from model considerations.

A model of the muscle fibre extracellular action potentials (ECAPs) calculation using experimentally recorded intracellular action potentials (ICAPs) has been applied to investigate the effect of repetitive stimulation on the electrical activity of isolated frog muscle fibres. The ECAPs were calculated both at small (0.01 mm) and at large (5 mm) radial distances to the fibre axis, and their relationship with the original ICAP parameters has been inferred. Fourier transformation of the calculated ECAPs in order to obtain the spectral characteristics and to trace out their behaviour during continuous fibre activity was performed. Stimulation frequency dependence on the ECAP time characteristics and on the shift of the maximum spectral density towards low frequencies at small and large radial distance were observed. The spectral density peak frequency is propagation velocity (PV)-dependent. The advantage of the presented method over the available experimental extracellular recording techniques from isolated muscle fibers is the possibility to show the effect of continuous muscle fibre activity on the parameters of the ECAPs and their spectral characteristics at large radial distance, which is not experimentally accessible. Our results are in agreement with those experimentally obtained. The results from the model prove the role of changes in PV of excitation along the muscle fibres (representing the last link in the complex organized motor system) in the development of fatigue.

Discharge rate of selected motor units in human biceps brachii at different muscle lengths.

Action potentials of selected motor units (MUs) from biceps brachii muscle were recorded and analysed at three different elbow angles: 90, 120 and 150 degrees, corresponding to short, control and long muscle length, respectively. Using branched and conventional bipolar wire electrodes, superficial and deep-situated MUs were selectively recorded at relatively equal torques (the torque values were normalized to the corresponding maximal torque for a given muscle length). A total of 138 MUs (74 superficial and 64 deep) were investigated. The mean interspike intervals were significantly shorter at 90 degrees for the majority (52.2%) of the investigated MUs than at the other two angles. This increased discharge rate compensates for the reduction of twitch duration of evoked contraction at short muscle length. The other MUs were divided into three almost equal groups: two with significantly higher discharge rates at 120 and 150 degrees and one with discharge rates unaffected by the joint angle. No significant difference in the discharge rate of superficial and deep MUs at a fixed joint angle was found.

Pattern of continuous muscle fibre activity depending on fibre stretch and stimulation frequency.

Isolated frog muscle fibres of four different lengths (L0, initial; L1, 15%; L2, 25%; and L3, 35% stretched vs. L0) were stimulated continuously (3 min) by suprathreshold rectangular pulses at three different frequencies (f1 = 5 Hz, f2 = 6.7 Hz and f3 = 10 Hz). Using a pair of electrodes mounted at a fixed distance, the action potential at two different sites was recorded extracellularly and the propagation velocity (PV) of excitation was calculated. Throughout the trials two kinds of activity (continuous and intermittent) were observed. Two types of muscle fibres, slow (SMFs) and fast (FMFs), were distinguished depending on the rate of decrease in the PV during the period of continuous fibre activity. The duration of this period decreased with the increase of L and f. The continuous activity was followed by intermittent activity. The variety of alternations of rest periods (i.e. failure of potentials and periods of activity) characterized the pattern of SMF and FMF activity evoked by the protocol used. A coefficient of activity (k), i.e. the ratio between the number of action potentials and the number of stimulus pulses delivered for a determined time interval, was used to estimate the specificity of this pattern. A decrease in k was observed with an increase of L and f, and was differently expressed for the two fibre types. The stretch of SMFs contributed more to the shortening of the period of continuous activity and the decrease in k than did the increase of stimulation frequency. For FMFs the effect of the two factors (L and f) was identical. It is suggested that the disturbances of sarcolemmal and t-system membrane excitability during repetitive stimulation and stretch are fibre-type dependent and underlie the pattern of muscle fibre activity.

Stretch- and stimulation frequency-induced changes in extracellular action potentials of muscle fibres during continuous activity.

The present investigation aimed to use the extracellular action potentials (ECAPs) of muscle fibres for evaluation of the membrane functional state during long-lasting activity. Repetitive stimulation during 3 min trials at three different frequencies (5, 6.7 and 10 Hz) was applied to isolated frog muscle fibres at four different rates of stretch (up to 35% of the initial length). The evoked changes in the ECAPs were estimated by alterations in: the peak-to-peak amplitude (A); the time intervals between the positive and the negative maxima (T1) and between cross-points of the potential rise and fall with the base line (i.e. the negative phase duration) (T0); as well as in the propagation velocity (PV) of excitation. Due to the significantly shorter duration of continuous activity of the fast muscle fibres (FMFs), at the time-point of potential failure the decrease of PV and the increase in T1 and T0 were more pronounced in the slow muscle fibre (SMF) potentials than in FMF potentials. The amplitude decrease in most of the trials for both fibre types was similar. Up until the end of the trials, the activity of both fibre types was intermittent, and in the majority of trials the percentage changes in the potential parameters held the values reached during the continuous activity. Only the time parameters gradually increased throughout the trial at maximal stretch and 5 Hz stimulation frequency, as for the FMF potentials they were more prolonged than those of the SMF potentials. T1 and T0 reflect the slowing of the depolarization phase and rapid repolarization of the intracellular potential. Hence the duration of the ECAPs was the parameter most affected by the maximal stretch. The changes in the ECAP parameters and PV induced by repetitive stimulation and fibre stretch reflect changes in ionic currents and muscle fibre membrane conductivity.

Frog muscle fibre action potential and different extracellular calcium concentration at lowered pH in the medium.

This article is mainly concerned with the influence of Ca2+[o in acidified extracellular medium on the intracellular action potentials (ICAPs) and total ionic current (Ii) during ICAP of isolated skeletal muscle fibre. The bundles of frog muscle fibres were bathed in Ringer's solution with standard Ca2+[o at pH 6.5 after which the fibres were exposed for 30 min to Ca(2+)-free solution and Ca(2+)-enriched solution at pH 6.5. The ICAPs in standard Ca2+[o solution (control) and after exposure for 30 min to Ringer's solutions with different Ca2+[o at pH 6.5 were recorded and the Ii during ICAP was calculated. The ICAP amplitude from the fibres in Ca(2+)-free solutions at pH 6.5 showed a significant increase vs. control, while the time characteristics if the ICAPs in different Ca2+[o decreased except for the ICAP depolarization phase duration in Ca(2+)-enriched solution. The Ii alterations reflect ICAP changes. It was suggested that the changed Ca2+[o at pH 6.5 compensated to some extent the observed inhibitory effect of lowered pH on ICAP parameters in solution with standard Ca2+[o.

Nivalin P-induced changes in muscle fiber membrane processes.

Nivalin P. composed of Nivalin (galanthamine hydrobromide) and Pymadin (4-aminopyridine hydrochloride), acts as an enhancer of cholinergic function and is currently of interest in the treatment of diseases associated with disorders in the transmission of impulses in the central and peripheral nervous system. The purpose of this study was to elucidate the effects of direct application of Nivalin P on muscle fiber membrane processes. The effects of the two components Nivalin and Pymadin on electrical and mechanical activity of treated isolated frog muscle fibers were also studied separately. Nivalin caused a decrease in the amplitude and an increase in the duration of intracellular (ICAP) and extracellular (ECAP) action potentials and total ionic current (li), probably acting to modulate nonspecific Na+ conductance, thereby reducing Na+ influx. Pymadin blocked K+ conductance in the cell membrane, prolonging the ICAP repolarization phase and decreasing the outward phase of the li. The Ca2+ channel kinetics and Ca2+ release were also affected, and as a result, the twitch amplitude (TwA) of muscle fibers treated with both Nivalin and Pymadin was potentiated. Nivalin P, therefore, combines the effects of its two components on muscle fiber membrane properties, the most favorable of which is the increase in muscle fiber contractility.

Extracellular action potentials of skeletal muscle fibre affected by 4-aminopyridine: a model study.

A new specially designed analytical function approximating the intracellular action potentials (ICAPs) for calculation of the extracellular potentials (ECAPs) at various radial and axial distances from the active fibre is proposed. 4-Aminopyridine (4-AP) was used to obtain ICAPs with a prolonged repolarization phase in order to investigate the influence of changes in ICAP shape on the ECAPs. From the experimentally recorded ICAPs before and after treatment of frog skeletal muscle fibres with 4-AP, approximated by the new function, the ECAPs were calculated applying the line-source model in a finite fibre. Using this function allowed calculation of the ECAPs at distances not accessible for the experimental recordings. The total ionic current (Ii) during the action potential was calculated using the cable equation. Our results showed that the ratio of the first positive to the negative phases of the ECAPs of treated fibres increased at large radial distances (3000 microns and more) and the terminal positive phase was asymmetric with an abrupt initial deflection followed by a slow inverse deflection. The calculated ECAPs at various axial distances from the fibre end (cylindrical and conical part) and at radial distances from the fibre membrane ranging from 0 to 5000 microns, corresponded in shape to the experimentally recorded potentials of untreated and 4-AP-treated muscle fibres.

Effects of piperazine derivatives on the activity of frog skeletal muscle fibers.

1. This study was undertaken to characterize the effects of some piperazine derivatives on excitable cell membranes. Three original Bulgarian compounds with favorable effects on cardiovascular and nervous system--piperazine derivatives with code names P-11 (N1-[3-oxo-3-phenyl-2-methyl-propyl]-N4-[trans-3-hydroxy-1,2,3,4- tetrahydro-2-naphthyl]-piperazine dihydrochloride), AS2 (N1-benzhydryl-N4-allyl piperazine dihydrochloride) and 35-M (Schiff's base of N1-benzhydryl-N4-aminopiperazine with triacetonamine, dioxalate salt) were tested in experiments with conventional microelectrode technique on isolated frog muscle fibers. 2. After 30-min treatment with tested drugs at concentrations of 10-100 microM the recorded intra-(ICAP) and extracellular action potentials (ECAPs) showed an amplitude decrease and duration increase. The total ionic current (Ii) decreased as the outward phase was almost abolished by P-11. The propagation velocity (PV) of excitation and the twitch amplitude also decreased. These changes were agent- and concentration-dependent. 3. The effect potency of the agents diminished in the following order: P-11 > AS2 > 35-M. 4. Concentrations higher than 100 microM for all agents completely, but reversibly, inhibited membrane excitability. 5. The results demonstrate compound- and concentration-induced modulation of Ca2+ current with blockade of Ca(2+)-dependent K+ and Cl- membrane channels of muscle fiber treated with the compound tested.

Influence of intracellular potential and conduction velocity on extracellular muscle fibre potential.

The influence of alterations in intracellular action potential (ICAP) duration and in muscle fibre conduction velocity (CV) on the extracellular action potential (ECAP) duration was studied. The experiments were carried out on frog isolated skeletal muscle fibres. The ECAPs were recorded at different radial distances (from 0 µm to 3000 µm) to the muscle fibre membrane. Alterations in the CV were induced by changes in the temperature of the medium. The CV decrease led to increase in both ICAP duration and duration of the ECAPs recorded at different radial distances. The ECAP duration reflected changes in the ICAP duration and increased together with increase in the radial distances. The different extent of alteration in ICAP duration and CV following potassium conductivity blockage was used as a model for separately studying the effects of these two parameters on the ECAP duration. The duration of the ECAP in the vicinity of the fibre reflected changes in the ICAP rising time (Rt), and were practically independent of alterations of the CV. The ECAP duration at large radial distance depended on CV alterations and on radial distance.

4-Aminopyridine and tetraethylammonium-induced changes in action potentials of unmyelinated axons.

The effects of different concentrations of 4-aminopyridine (4-AP) and tetraethylammonium (TEA) on the intra- and extracellular action potentials (ICAPs, ECAPs) of unmyelinated axons of Lumbricus terrestris were studied. The results showed different sensitivity of the axons to both potassium current blockers (4-AP and TEA), added to the medium. 4-AP led to spontaneous and single stimulus- evoked repetitive activity, manifested as a slow "burst" action potential propagation like "plateau" ICAP with oscillations. The ICAP duration of the TEA-treated axons increased mainly at the expense of the repolarization phase which reflected the increased duration of the ECAP recorded at long radial distances. The amplitude of the ICAPs after treatment with both blockers was decreased to 20% as compared to the controls (untreated axons) and ECAPs decreased to 40% in the TEA-treated axons. The conduction velocity (CV) of the action potentials was not significantly changed. The calculated total ionic current during the action potential upon TEA treatment was decreased and the duration of the outward phase was prolonged.

Effect of chloramine-T and N-bromsuccinimide on intracellular and extracellular action potentials from unmyelinated axons.

The change in intracellular (ICAPs) and extracellular (ECAPs) potentials of unmyelinated axons of Lumbricus terrestris under the effect of Chloramine-T (CT) and N-Bromsuccinimide (NBS), applied extracellularly, is studied. Delay of the repolarization phase of ICAPs was found under the effect of NBS, as well as slight decrease of the amplitude of ICAPs and ECAPs, with an insignificant decrease in the propagation velocity of the excitation. The first (V') and second (V") time derivatives and the total ionic current (Ii) are calculated from the intracellular action potential in norm and under the effect of NBS. A delay of the inactivation phase of the inward sodium current and a decrease of the amplitude of the total ionic current, calculated from ICAP, were found. Under the effect of CT there was a decrease in the amplitude of the action potentials, without a delay in the repolarization phase and the propagation velocity of the excitation decreases. CT leads to lasting inhibition of the excitability of the axon under prolonged impact (20 min).

[Relation between inter- and intracellular action potentials of frog isolates muscle fiber at various temperatures]. / Sviaz' mezhdu vne- i vnutrikletochnymi potentsialami deistviia izolirovannogo myshechnogo volokna liagushki pri raznykh temperaturakh.

Extra- and intracellular action potentials (AP) of isolated muscle fibres plunged into volume conductor were studied at different temperatures. Changes of the first and second AP derivatives and their temperature dependence were described. The changes are explained by temperature effect on the density of input and output ion currents. Changes of the shape of extracellular potential with temperature increase were described. They were concerned with the changes of the first and second AP derivatives and are due to the peculiar distribution of the potential field in the volume conductor around an excitable fibre of the finite length.