Biomedical subjects

N Radicheva

Publications and source records attributed to N Radicheva.

At least 19 recordsLinked to original sources

PubMed2003

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.

Animals↗

PubMed2002

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.

Action Potentials↗

PubMed2001

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.

Action Potentials↗

PubMed1999

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.

Adenosine Triphosphatases↗

PubMed1998

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.

Action Potentials↗

PubMed1998

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.

Action Potentials↗

PubMed1998

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.

Action Potentials↗

PubMed1998

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.

Action Potentials↗

PubMed1998

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.

Action Potentials↗

PubMed1996

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.

4-Aminopyridine↗

PubMed1996

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.

4-Aminopyridine↗

PubMed1995

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.

Action Potentials↗

PubMed1988

Dependence between intra- and extracellular action potentials of isolated frog muscle fibres at different temperatures.

The intra- and extracellular action potentials of isolated frog muscle fibre immersed in a volume conductor at different temperatures are described. It was found that upon increasing the temperature the amplitude of the first time derivative of the intracellular action potentials increased linearly, whereas the amplitude of the second time derivative increased nonlinearly. The duration of the separate phases of the time derivatives shortened upon heating, as the velocity of spreading of the excitation increased. The length of the separate phases of the space derivatives of the action potential shortened when increasing the temperature. The amplitudes of the space derivatives were calculated. The changes in the derivatives of the action potential were explained by the influence of the temperature on the peak inward and outward transmembrane current. The changes in the extracellular action potentials produced by the temperature near the membrane and at longer radial distance at points far and near the end of the fibre are described. They were explained by the changes in the space derivatives of the intracellular action potential as well as by the features of the distribution of the extracellular potential field in the volume conductor around the finite-in-length excitable fibre.

Action Potentials↗

PubMed1987

Extracellular potential field of unmyelinated active axons.

Experimental and model investigations were carried out on the extracellular action potentials and potential fields of some unmyelinated axons: the medial and the lateral giant axons of Lumbriscus terrestris and small unmyelinated axons (nerve fibres of group C). Close to the membrane the extracellular action potentials are similar in shape to the second space derivative of the intracellular action potentials--they are triphasic in case of intracellular action potential with depolarization after-potential, having four phases in case of intracellular action potentials with hyperpolarization after-potential. Upon increasing the radial distance, the amplitudes of the different phases of the extracellular action potentials decrease at a different rate, while their maxima are shifted, which results in considerable changes in their shape.

Action Potentials↗

PubMed1986

Influence of the muscle fibre end geometry on the extracellular potentials.

Intra- and extracellular action potentials of isolated frog muscle fibres were recorded at different distances to the end of the fibre. The first and second time derivatives of the intracellular action potentials were also recorded. The intracellular action potentials and their first and second time derivatives were almost the same regardless of the place of recording. With the decrease in the axial distance to the end the extracellular action potentials changed gradually in a complicated manner from a shape similar to the second time derivative into a shape similar to the first time derivative. Extracellular potentials, having two negative maxima, were recorded over the terminal taper part of the fibres. These alterations were simulated by a mathematical model. It was shown that the changes in the shape of the extracellular action potentials around the end of the fibres were mainly due to the existence of the fibre end though a better correspondence of the experimentally recorded and the calculated extracellular action potentials was obtained when the morphology of the fibre end was taken into consideration.

Action Potentials↗

PubMed1986

Changes in the muscle fibre extracellular action potentials in long-lasting (fatiguing) activity.

The dependence of extracellular action potentials (ECAPs) of single frog muscle fibres on intracellular action potentials (ICAPs) was studied during long-lasting (fatiguing) activity. The conduction velocity, peak-to-peak amplitude and amplitudes of the separate phases of the first and second ICAP time derivatives decreased during long-lasting activity. The phases of the first and second ICAP space derivatives also decreased in amplitude and lengthened. ECAPs near the membrane were similar in shape and proportional in amplitude to (formula; see text) when recording at a distance from both the end of the fibre and the point of stimulation. At long radial distances, the amplitudes of the separate ECAP phases depended on the amplitude and length of the corresponding phases of (formula; see text). Thus the decrease in ECAP amplitude during long-lasting activity at long radial distances was less than at points close to the muscle fibre membrane. The consequences of these findings for the changes in electromyograms recorded by needle or superficial electrodes during long-lasting (fatiguing) activity are discussed.

Action Potentials↗

PubMed1986

Intracellular and extracellular action potentials in frog muscle fibre upon blocking the potassium conductivity.

The dependence between intra- and extracellular action potentials of single muscle fibre of Rana ridibunda when 4-aminopyridine is applied for blocking the potassium conductivity of the membrane was investigated. The changes in the intracellular action potential reflect the degree of the blocker's effect. The initial part of this potential remains practically unchanged. A considerable prolongation of the repolarization phase is observed until a plateau-type potential is established. The extracellular action potentials recorded close to the fibre do not manifest substantial changes. Conclusions are reached about the importance of K-currents for the shape of the extracellular action potential shape.

4-Aminopyridine↗

PubMed1986

Effect of the blocked inactivation of sodium channels on intracellular and extracellular action potentials from isolated frog muscle fibres.

The intra- and extracellular action potentials of isolated frog muscle fibres, treated with ATX II for blocking sodium inactivation, are investigated. In concentrations of 0.1 to 0.5 X 10(-6) M, ATX II leads to a considerable change of the repolarization phase of the intracellular potential, with a tendency toward achieving a plateau-type potential. In addition to the intracellular action potentials, extracellular action potentials were also recorded from the same fibres. Immediately next to the fibre membrane and at a small radial distance the extracellular potentials did not manifest essential differences compared with recordings under normal physiological conditions. Conclusions are reached about the influence of the blocked Na-inactivation on the intra- and extracellular action potentials of muscle fibres.

Action Potentials↗