[Mobbing and violence at school. Trends from 2002 to 2010]

The purpose of this study was to undertake an assessment and differentiated examination of the development of bullying and violence in schools between 2002 and 2010 in Germany.We examined the national German data of Health Behaviour in School-aged Children (HBSC) study in 2002, 2006 and 2010. A paper-pencil questionnaire was distributed to a representative sample (N=17 929) of 11-, 13- and 15-year-old school children. The evaluation of the data was done by descriptive statistics and logistic regression analyses, controlled by age, gender, family affluence, school type and survey year.A clear positive trend could be identified: from 2002 to 2010 the number of bullies and bully victims decreased whereas the group of the uninvolved pupils increased. There was a delay in this trend for children with low family affluence.The obvious success in the prevention of violence is shown by the decreasing rate of bullies. The paper discusses whether future prevention should focus more on victims and children with educationally deprived background.

[The HBSC Study in Germany--study design and methodology]

The aim of the HBSC-Study is to collect data on the physical and mental health and health behaviour of children and adolescents and to gain a deeper insight into their situation and the specific environment they grow up in. The HBSC-study is an international school-based cross-sectional survey conducted in collaboration with the World Health Organization (WHO). The survey takes place every 4 years since 1982 and is based on a standardised protocol. In Germany the survey was first conducted in 1994 as a pilot study in North Rhine-Westphalia. The German sample is based on a random sample of classes in all public schools in Germany. 11-, 13-, and 15-year-old pupils are surveyed by means of a paper and pencil questionnaire. The questionnaire comprises a broad selection of -topics, including sociodemographics, health and risk behaviours, family, school and peers. The reported trends in the supplement are based on the data from surveys in 2002 (N=5.650), 2006 (N=7.274) and 2010 (N=5.005). The representative samples for each of the survey years are defined as follows: in 2002 the data is based on information collected in 4 Federal States (Berlin, Hesse, North Rhine-Westphalia, Saxony); in 2006 5 states define the German data file (Berlin, Hamburg, Hesse, North Rhine-Westphalia, Saxony). The data from the 2010 survey comprises data from 15 Federal States. The HBSC-data contributes towards a better understanding of the relationship between health and living conditions of young people. The papers in this supplement deliver important insights into the living context of young people and in doing this they provide important information about their health and the long-term effectiveness of public-health-measures.

[Bullying, psychosocial health and risk behaviour in adolescence]

BACKGROUND

Bullying as a subform of aggressive behaviour has not received much attention as a specific risk behaviour in adolescence. Especially the adverse health effects in relation to bullying have been barely discussed in Germany. The objective of this study is to present age- and gender-specific prevalences in bullying and to analyse the association between the different bullying roles and subjective health as well as risk behaviour.

METHODS

Data were obtained from the German part of the international WHO collaborative study "Health Behaviour in School-aged Children (HBSC)" in 2002. Overall, 5,650 school children aged 11-15 years were interviewed with a standardised questionnaire. Multivariate logistic regression models were used to analyse the association between bullying, psychosocial health and risk behaviour separately for girls and boys.

RESULTS

About 17% of the boys and 10% of the girls aged 11-15 years were classified as repeated bullying perpetrators. About 10% of the school children are victims of being bullied several times a month. Another 3-5% of the adolescents belonged to the group of simultaneous victims and perpetrators (bully-victims). Perpetrators as well as victims showed strong associations with psychosocial health and risk behaviour. Independently of gender, victims were significantly more likely to report repeated psychosomatic complaints, adverse mental health and negative self-reported health (boys only), than uninvolved students. Especially for male perpetrators, strong associations with regular tobacco and alcohol use and repeated drunkenness were found, while these behaviour types were significantly less prevalent among victims. The bully-victim group is characterised by high rates of psychosomatic complaints and mental health problems (boys only).

CONCLUSIONS

Bullying also seems to be widespread in schools in Germany and is strongly associated with subjective health and substance-related risk behaviour. The results suggest that bullying is a critical issue that requires increasing attention in health research. The unique health problems of victims and perpetrators suggest different intervention strategies for both groups.

A possible role of the junctional face protein JP-45 in modulating Ca2+ release in skeletal muscle

We investigated the functional role of JP-45, a recently discovered protein of the junctional face membrane (JFM) of skeletal muscle. For this purpose, we expressed JP-45 C-terminally tagged with the fluorescent protein DsRed2 by nuclear microinjection in myotubes derived from the C2C12 skeletal muscle cell line and performed whole-cell voltage-clamp experiments. We recorded in parallel cell membrane currents and Ca(2+) signals using fura-2 during step depolarization. It was found that properties of the voltage-activated Ca(2+) current were not significantly changed in JP-45-DsRed2-expressing C2C12 myotubes whereas the amplitude of depolarization-induced Ca(2+) transient was decreased compared to control myotubes expressing only DsRed2. Converting Ca(2+) transients to Ca(2+) input flux using a model fit approach to quantify Ca(2+) removal, the change could be attributed to an alteration in voltage-activated Ca(2+) permeability rather than to altered removal properties or a lower Ca(2+) content of the sarcoplasmic reticulum (SR). Determining non-linear capacitive currents revealed a reduction of Ca(2+) permeability per voltage-sensor charge. The results may be explained by a modulatory effect of JP-45 related to its reported in vitro interaction with the dihydropyridine receptor and the SR Ca(2+) binding protein calsequestrin (CSQ).

Voltage-controlled Ca2+ release and entry flux in isolated adult muscle fibres of the mouse

The voltage-activated fluxes of Ca(2+) from the sarcoplasmic reticulum (SR) and from the extracellular space were studied in skeletal muscle fibres of adult mice. Single fibres of the interosseus muscle were enzymatically isolated and voltage clamped using a two-electrode technique. The fibres were perfused from the current-passing micropipette with a solution containing 15 mm EGTA and 0.2 mm of either fura-2 or the faster, lower affinity indicator fura-FF. Electrical recordings in parallel with the fluorescence measurements allowed the estimation of intramembrane gating charge movements and transmembrane Ca(2+) inward current exhibiting half-maximal activation at -7.60 +/- 1.29 and 3.0 +/- 1.44 mV, respectively. The rate of Ca(2+) release from the SR was calculated after fitting the relaxation phases of fluorescence ratio signals with a kinetic model to quantify overall Ca(2+) removal. Results obtained with the two indicators were similar. Ca(2+) release was 2-3 orders of magnitude larger than the flux carried by the L-type Ca(2+) current. At maximal depolarization (+50 mV), release flux peaked at about 3 ms after the onset of the voltage pulse and then decayed in two distinct phases. The slower phase, most likely resulting from SR depletion, indicated a decrease in lumenal Ca(2+) content by about 80% within 100 ms. Unlike in frog fibres, the kinetics of the rapid phase of decay showed no dependence on the filling state of the SR and the results provide little evidence for a substantial increase of SR permeability on depletion. The approach described here promises insight into excitation-contraction coupling in future studies of genetically altered mice.

Voltage-dependent Ca2+ fluxes in skeletal myotubes determined using a removal model analysis

The purpose of this study was to quantify the Ca2+ fluxes underlying Ca2+ transients and their voltage dependence in myotubes by using the "removal model fit" approach. Myotubes obtained from the mouse C2C12 muscle cell line were voltage-clamped and loaded with a solution containing the fluorescent indicator dye fura-2 (200 microM) and a high concentration of EGTA (15 mM). Ca2+ inward currents and intracellular ratiometric fluorescence transients were recorded in parallel. The decaying phases of Ca2+-dependent fluorescence signals after repolarization were fitted by theoretical curves obtained from a model that included the indicator dye, a slow Ca2+ buffer (to represent EGTA), and a sequestration mechanism as Ca2+ removal components. For each cell, the rate constants of slow buffer and transport and the off rate constant of fura-2 were determined in the fit. The resulting characterization of the removal properties was used to extract the Ca2+ input fluxes from the measured Ca2+ transients during depolarizing pulses. In most experiments, intracellular Ca2+ release dominated the Ca2+ input flux. In these experiments, the Ca2+ flux was characterized by an initial peak followed by a lower tonic phase. The voltage dependence of peak and tonic phase could be described by sigmoidal curves that reached half-maximal activation at -16 and -20 mV, respectively, compared with -2 mV for the activation of Ca2+ conductance. The ratio of the peak to tonic phase (flux ratio) showed a gradual increase with voltage as in rat muscle fibers indicating the similarity to EC coupling in mature mammalian muscle. In a subgroup of myotubes exhibiting small fluorescence signals and in cells treated with 30 microM of the SERCA pump inhibitor cyclopiazonic acid (CPA) and 10 mM caffeine, the calculated Ca2+ input flux closely resembled the L-type Ca2+ current, consistent with the absence of SR Ca2+ release under these conditions and in support of a valid determination of the time course of myoplasmic Ca2+ input flux based on the optical indicator measurements.

Voltage-activated calcium signals in myotubes loaded with high concentrations of EGTA

In the present study we describe the analysis of optically recorded whole cell Ca(2+) transients elicited by depolarization in cultured skeletal myotubes. Myotubes were obtained from the mouse muscle-derived cell line C2C12 and from mouse satellite cells. The cells were voltage-clamped and perfused with an artificial intracellular solution containing 15 mM EGTA to ensure that the bulk of the Ca(2+) mobilized by depolarization is bound to this extrinsic buffer. The apparent on- and off-rate constants of EGTA and the dissociation rate constant of fura-2 in the cell were estimated by investigating the Ca(2+)-dependence of kinetic components of the fluorescence decay after repolarization. These parameters were used to calculate the time course of the total voltage-controlled flux of Ca(2+) to the myoplasmic space (Ca(2+) input flux). The validity of the procedure was confirmed by model simulations using artificial Ca(2+) input fluxes. Both C2C12 and primary-cultured myotubes showed a very similar phasic-tonic time course of the Ca(2+) input flux. In most measurements, the input flux was considerably larger and showed a different time course than the estimated Ca(2+) flux carried by the L-type Ca(2+) channels, indicating that it consists mainly of voltage-controlled Ca(2+) release from the sarcoplasmic reticulum. In cells with extremely small fluorescence transients, the calculated input fluxes matched the kinetic characteristics of the Ca(2+) inward current, indicating that Ca(2+) release was absent. These measurements served as a control for the fidelity of the fluorimetric flux analysis. The procedures promise a deeper insight into alterations of Ca(2+) release gating in studies employing myotube expression systems for mutant or chimeric protein components of excitation-contraction coupling.

Excitation-contraction coupling in skeletal muscle of a mouse lacking the dihydropyridine receptor subunit gamma1

1. In skeletal muscle, dihydropyridine (DHP) receptors control both Ca(2+) entry (L-type current) and internal Ca(2+) release in a voltage-dependent manner. Here we investigated the question of whether elimination of the skeletal muscle-specific DHP receptor subunit gamma1 affects excitation-contraction (E-C) coupling. We studied intracellular Ca(2+) release and force production in muscle preparations of a mouse deficient in the gamma1 subunit (gamma-/-). 2. The rate of internal Ca(2+) release at large depolarization (+20 mV) was determined in voltage-clamped primary-cultured myotubes derived from satellite cells of adult mice by analysing fura-2 fluorescence signals and estimating the concentration of free and bound Ca(2+). On average, gamma-/- cells showed an increase in release of about one-third of the control value and no alterations in the time course. 3. Voltage of half-maximal activation (V(1/2)) and voltage sensitivity (k) were not significantly different in gamma-/- myotubes, either for internal Ca(2+) release activation or for the simultaneously measured L-type Ca(2+) conductance. The same was true for maximal Ca(2+) inward current and conductance. 4. Contractions evoked by electrical stimuli were recorded in isolated extensor digitorum longus (EDL; fast, glycolytic) and soleus (slow, oxidative) muscles under normal conditions and during fatigue induced by repetitive tetanic stimulation. Neither time course nor amplitudes of twitches and tetani nor force-frequency relations showed significant alterations in the gamma1-deficient muscles. 5. In conclusion, the overall results show that the gamma1 subunit is not essential for voltage-controlled Ca(2+) release and force production.

Malignant hyperthermia and excitation-contraction coupling

Malignant hyperthermia (MH) is a state of elevated skeletal muscle metabolism that may occur during general anaesthesia in genetically pre-disposed individuals. Malignant hyperthermia results from altered control of sarcoplasmic reticulum (SR) Ca2+ release. Mutations have been identified in MH-susceptible (MHS) individuals in two key proteins of excitation-contraction (EC) coupling, the Ca2+ release channel of the SR, ryanodine receptor type 1 (RyR1) and the alpha1-subunit of the dihydropyridine receptor (DHPR, L-type Ca2+ channel). During EC coupling, the DHPR senses the plasma membrane depolarization and transmits the information to the ryanodine receptor (RyR). As a consequence, Ca2+ is released from the terminal cisternae of the SR. One of the human MH-mutations of RyR1 (Arg614Cys) is also found at the homologous location in the RyR of swine (Arg615Cys). This animal model permits the investigation of physiological consequences of the homozygously expressed mutant release channel. Of particular interest is the question of whether voltage-controlled release of Ca2+ is altered by MH-mutations in the absence of MH-triggering substances. This question has recently been addressed in this laboratory by studying Ca2+ release under voltage clamp conditions in both isolated human skeletal muscle fibres and porcine myotubes.

Malignant hyperthermia mutation Arg615Cys in the porcine ryanodine receptor alters voltage dependence of Ca2+ release

Ca2+ inward current and fura-2 Ca2+ transients were simultaneously recorded in porcine myotubes. Myotubes from normal pigs and cells from specimens homozygous for the Arg615Cys (malignant hyperthermia) mutation of the skeletal muscle ryanodine receptor RyR1 were investigated. We addressed the question whether this mutation alters the voltage dependence of Ca2+ release from the sarcoplasmic reticulum. The time course of the total flux of Ca2+ into the myoplasm was estimated. Analysis showed that the largest input Ca2+ flux occurred immediately after depolarization. Amplitude and time course of the Ca2+ flux at large depolarizations were not significantly different in the Arg615Cys myotubes. Ca2+ release from the sarcoplasmic reticulum was activated at more negative potentials than the L-type Ca2+ conductance. In the controls, the potentials for half-maximal activation V 1/2 were -9.0mV and 16.5 mV, respectively. In myotubes expressing the Arg615Cys mutation, Ca2+ release was activated at significantly lower depolarizing potentials (V = -23.5 mV) than in control myotubes. In contrast, V of conductance activation (13.5 mV) was not significantly different from controls. The specific shift in the voltage dependence of Ca2+ release caused by this mutation can be well described by altering a voltage-independent reaction of the ryanodine receptor that is coupled to the voltage-dependent transitions of the L-type Ca2+ channel.

Effects of the dihydropyridine receptor subunits gamma and alpha2delta on the kinetics of heterologously expressed L-type Ca2+ channels

Ba2+ currents through L-type Ca2+ channels were measured in tsA201 cells transiently transfected with expression vectors encoding the dihydropyridine (DHP) receptor subunits alpha1C, beta1a-GFP, alpha2delta and gamma. The subunit effect on channel function was studied by omitting either alpha2delta or gamma from the transfection mixture and analyzing the voltage dependence and kinetics of activation, inactivation and recovery from inactivation. Activation could be described by a single exponential function while the time course of inactivation of the Ba2+ current followed a double exponential function. Progressively longer depolarization led to increasingly slower recovery, indicating the successive occupancy of several inactive states. Activation parameters remained largely unaffected in y-deficient cells whereas the voltage dependence of inactivation was shifted by 16 mV to more positive potentials and the larger one of the two inactivation time constants was increased by one-third. On the other hand, alpha2delta-deficient cells showed decreased current density and slowed activation and inactivation. Recovery from inactivation was significantly slowed by gamma coexpression. This and the effect of the gamma subunit on steady-state inactivation were independent of the presence of alpha2delta. We conclude that y stabilizes L-type Ca2+ channel inactivation in a way similar to certain Ca(2+)-antagonistic drugs. Alpha2delta is not needed for this effect.

Tools for flash-photolysis experiments on voltage-clamped muscle fibre segments

An experimental set-up is described that allows the combination of rapid transmembrane voltage changes and photometric calcium recording with the fast photochemical turnover of substances applied externally or intracellularly to cut skeletal muscle fibres. It consists of a double-vaseline-gap system, designed for use with a xenon-flash-lamp device and a dual-wavelength microscope photometer. The pools of the vaseline gap chamber that contain the solutions surrounding the cut ends and the voltage-clamped segment of the muscle fibre are closed and have volumes of 20-50 microl. Thin tubes allow rapid solution change or continuous perfusion in the chamber compartments. Accessory tools were constructed to simplify focussing and measuring the flash-light intensity. A pilot light delivered from a red laser diode is used as a guide beam to target the ultraviolet (UV) flash to the preparation. The light distribution in the focal region and the relative changes in flash intensity with increasing numbers of flashes were quantified with an instrument that integrates the photo-current of a UV-sensitive silicon diode. The function of the set-up was demonstrated by measuring the efficiency of Ca2+ release from DM-nitrophen in quartz capillaries using the Ca(2+)-sensitive dye antipyrylazo III and by recording the flash-induced recovery of L-type calcium currents in muscle fibres blocked by the light-sensitive dihydropyridine drug nifedipine.

[Violence in the school--an analysis and prevention]

The "Forschungsgruppe Schulevaluation" (Research Group for School Evaluation, Technical University of Dresden) conducted several empirical investigations which led to rich knowledge concerning the amount of violence in schools, the different forms of violence, roles that actors of and sufferers from violent action play and the causes for violence. Besides the wellknown socialisatory influence of families, media consumption and peers for the formation of violent behaviour of pupils and of behaviour that has an affinity to violence a causal influence of schools was detected. Questions concerning the prevention of violence, the conducting of pilot studies on the latter point in one single school as well as the evaluation of the preventory measures taken were important points in our work. In the present study we will present the most important empirical results and our experience with the prevention of violence.

Kinetics of inactivation and restoration from inactivation of the L-type calcium current in human myotubes

1. Inactivation and recovery kinetics of L-type calcium currents were measured in myotubes derived from satellite cells of human skeletal muscle using the whole cell patch clamp technique. 2. The time course of inactivation at potentials above the activation threshold was obtained from the decay of the current during 15 s depolarizing pulses. At subthreshold potentials, prepulses of different durations, followed by +20 mV test pulses, were used. The time course could be well described by single exponential functions of time. The time constant decreased from 17.8 +/- 7.5 s at -30 mV to 1.78 +/- 0.15 s at +50 mV. 3. Restoration from inactivation caused by 15 s depolarization to +20 mV was slowed by depolarization in the restoration interval. The time constant increased from 1.11 +/- 0.17 s at -90 mV to 7.57 +/- 2.54 s at -10 mV. 4. Restoration showed different kinetics depending on the duration of the conditioning depolarization. While the time constant was similar at restoration potentials of -90 and -50 mV after a 1 s conditioning prepulse, it increased with increasing prepulse duration at -50 mV and decreased at -90 mV. 5. The experiments showed that the rates of inactivation and restoration of the L-type calcium current in human myotubes were not identical when observed at the same potential. The results indicate the presence of more than one inactivated state and point to different voltage-dependent pathways for inactivation and restoration.

Modification of excitation-contraction coupling by 4-chloro-m-cresol in voltage-clamped cut muscle fibres of the frog (R. pipiens)

1. The effect of 5 microM 4-chloro-m-cresol (4-CmC) on voltage-controlled Ca2+ release was studied in cut muscle fibres of the frog loaded with internal solutions containing 15 mM EGTA. Fibres were voltage clamped using a double Vaseline gap system, and Ca2+ signals were recorded with the fluorescent indicator dye fura-2 2. Resting intracellular free Ca2+ concentration increased from 61 to 100 nM upon application of 4-CmC. 3. Both peak rate of release of intracellularly stored Ca2+ and the steady level attained after 50 ms of depolarization increased, but the potentiation of the latter was more pronounced (by a factor of 1.7 versus 1.3). The voltage of half-maximal activation remained unchanged. 4. Non-linear intramembranous charge movements showed no significant change in voltage dependence while the maximal charge displaced by depolarization increased by 25 %. 5. The dependence of peak release flux on total intramembranous charge was not different in 4-CmC, but for the steady level of release the steepness of the relation increased by a factor of 1.3. 6. The stimulating effect of 5 microM 4-CmC on depolarization-induced Ca2+ release resembled the potentiation by 0.5 mM caffeine. However, 0.5 mM caffeine increased the peak and steady levels of the release rate by a similar factor and caused no increase in the resting free calcium concentration, indicating different modes of action of the two substances. 7. Neither 5 microM 4-CmC nor 0.5 mM caffeine led to a loss of voltage control of Ca2+ release during repolarization after short depolarizations, as has been reported previously for caffeine. Potentiated Ca2+ release could be terminated by repolarization as fast as under control conditions both with 15 mM and 0.1 mM internal EGTA. 8. The effects of 4-CmC may result from a direct opening of the release channel combined with an enhancement of the transduction mechanism that couples channel opening to displacement of voltage sensor charges.

Voltage-controlled Ca2+ release in normal and ryanodine receptor type 3 (RyR3)-deficient mouse myotubes

1. Primary cultured myotubes were derived from satellite cells of the diaphragm obtained from both normal mice (RyR3+/+) and mice with a targeted mutation eliminating expression of the type 3 isoform of the ryanodine receptor (RyR3-/-). Using the whole-cell patch clamp technique, L-type Ca2+ currents were measured during step depolarizations. Simultaneously, intracellular Ca2+ transients were recorded with the fluorescent indicator dye fura-2. 2. After correction for non-instantaneous binding of Ca2+ to the indicator dye and taking into account the dynamics of Ca2+ binding to intracellular constituents, an estimate of the time course of the Ca2+ release rate from the sarcoplasmic reticulum (SR) was obtained. 3. The calculated SR Ca2+ release flux exhibited a marked peak within less than 12 ms after the onset of the voltage-clamp depolarization and fell rapidly thereafter to a five times lower, almost steady level. It declined rapidly after termination of the depolarization. 4. Signals in normal and RyR3-deficient myotubes showed no significant difference in the activation of Ca2+ conductance and in amplitude, time course and voltage dependence of the Ca2+ efflux from the SR. 5. In conclusion, the characteristics of voltage-controlled Ca2+ release reported here are similar to those of mature mammalian muscle fibres. In contrast to differences observed in the contractile properties of RyR3-deficient muscle fibres, a contribution of RyR3 to excitation-contraction coupling could not be detected in myotubes.

Voltage-dependent calcium release in human malignant hyperthermia muscle fibers

Malignant hyperthermia (MH) results from a defect of calcium release control in skeletal muscle that is often caused by point mutations in the ryanodine receptor gene (RYR1). In malignant hyperthermia-susceptible (MHS) muscle, calcium release responds more sensitively to drugs such as halothane and caffeine. In addition, experiments on the porcine homolog of malignant hyperthermia (mutation Arg615Cys in RYR1) indicated a higher sensitivity to membrane depolarization. Here, we investigated depolarization-dependent calcium release under voltage clamp conditions in human MHS muscle. Segments of muscle fibers dissected from biopsies of the vastus lateralis muscle of MHN (malignant hyperthermia negative) and MHS subjects were voltage-clamped in a double vaseline gap system. Free calcium was determined with the fluorescent indicator fura-2 and converted to an estimate of the rate of SR calcium release. Both MHN and MHS fibers showed an initial peak of the release rate, a subsequent decline, and rapid turn-off after repolarization. Neither the kinetics nor the voltage dependence of calcium release showed significant deviations from controls, but the average maximal peak rate of release was about threefold larger in MHS fibers.

Numerical methods to determine calcium release flux from calcium transients in muscle cells

Several methods are currently in use to estimate the rate of depolarization-induced calcium release in muscle cells from measured calcium transients. One approach first characterizes calcium removal of the cell. This is done by determining parameters of a reaction scheme from a fit to the decay of elevated calcium after the depolarizing stimulus. In a second step, the release rate during depolarization is estimated based on the fitted model. Using simulated calcium transients with known underlying release rates, we tested the fidelity of this analysis in determining the time course of calcium release under different conditions. The analysis reproduced in a satisfactory way the characteristics of the input release rate, even when the assumption that release had ended before the start of the fitting interval was severely violated. Equally good reconstructions of the release rate time course could be obtained when the model used for the analysis differed in structure from the one used for simulating the data. We tested the application of a new strategy (multiple shooting) for fitting parameters in nonlinear differential equation systems. This procedure rendered the analysis less sensitive to ill-chosen initial guesses of the parameters and to noise. A locally adaptive kernel estimator for calculating numerical derivatives allowed good reconstructions of the original release rate time course from noisy calcium transients when other methods failed.

Calcium currents and transients of native and heterologously expressed mutant skeletal muscle DHP receptor alpha1 subunits (R528H)

Rabbit cDNA of the alpha1 subunit of the skeletal muscle dihydropyridine (DHP) receptor was functionally expressed in a muscular dysgenesis mouse (mdg) cell line, GLT. L-type calcium currents and transients were recorded for the wild type and a mutant alpha1 subunit carrying an R528H substitution in the supposed voltage sensor of the second channel domain that is linked to a human disease, hypokalemic periodic paralysis. L-type channels expressed in GLT myotubes exhibited currents similar to those described for primary cultured mdg cells injected with rabbit wild type cDNA, indicating this system to be useful for functional studies of heterologous DHP receptors. Voltage dependence and kinetics of activation and inactivation of L-type calcium currents from mutant and wild type channels did not differ significantly. Intracellular calcium release activation measured by fura-2 microfluorimetry was not grossly altered by the mutation either. Analogous measurements on myotubes of three human R528H carriers revealed calcium transients comparable to controls while the voltage dependence of both activation and inactivation of the L-type current showed a shift to more negative potentials of approximately 6 mV. Similar effects on the voltage dependence of the fast T-type current and changes in the expression level of the third-type calcium current point to factors not primarily associated with the mutation perhaps participating in disease pathogenesis.

Fura-2 calcium signals in skeletal muscle fibres loaded with high concentrations of EGTA

Fura-2 is one of the most frequently used fluorescent Ca indicator dyes; yet it has limitations in tracking large intracellular Ca transients due to its high affinity for Ca. Since high affinity is of advantage when small Ca changes are to be detected, we tried the application of Fura-2 in skeletal muscle fibres which had been loaded with 15 mM internal EGTA to eliminate contractile artifacts. Under these conditions, the free Ca transients are considerably reduced in amplitude and strong saturation of Fura-2 is avoided. Cut segments of isolated muscle fibres were voltage-clamped in a double vaseline gap set-up. In the presence of high internal EGTA, free Ca (as measured with the rapid metallochromic indicator antipyrylazo III) drops rapidly from one value to a lower quasi steady-state value at the end of a depolarizing voltage pulse. This property allowed inspection of the dissociation kinetics of Ca from Fura-2 in the myoplasmic environment. The dissociation rate constant koff in the fibre was determined from the time constant of the exponential decay of the Fura-2 signal as a function of the final level of free Ca. We obtained a value of 26 s-1 at the experimental temperature of 12 degrees C. Knowledge of koff in the cell is essential for reconstructing the time course of free Ca from indicator bound Ca and for estimating the time course of the rate of release from the sarcoplasmic reticulum. The described combination of high EGTA buffering with Fura-2 fluorescence recording may be particularly useful for the determination of Ca release in small muscle cells.

L-type calcium current activation in cultured human myotubes

The time course of activation of the skeletal muscle L-type calcium channel was studied in voltage-clamped myotubes derived from human satellite cells. The slow L-type current was isolated by inactivating faster calcium current components using appropriate prepulses or by subtracting the currents not blocked by 5 microM nifedipine. The L-type current exhibited a single exponential activation and time constants which showed little voltage dependence in the range +10 to +50mV. Currents blocked by nifedipine could be partially restored by UV-light flash photolysis. When a flash of light was applied during a depolarizing step, the activation time course of the resulting inward current contained a rapid, almost instantaneous component followed by a slower component. The amplitude of the rapid component was different when the flash was applied at different times during the depolarizing step: depolarization first increased and then decreased the fraction of channels which could rapidly be restored from the block by photolysis. Plotted versus time after the onset of the depolarization this fraction closely matched the time course of the L-type current obtained before the block by nifedipine. This indicates that the slow gating recations of the Ca2+ channel remain functional in the nifedipine-blocked state. Large conditioning depolarizations which had been shown to enhance the speed of L-type current activation in frog muscle fibres showed no effect in human myotubes. Numerical simulations using a gating scheme proposed for frog muscle demonstrate that such differences can be caused by changing just a single kinetic parameter.

Expression and functional characterization of the cardiac L-type calcium channel carrying a skeletal muscle DHP-receptor mutation causing hypokalaemic periodic paralysis

A histidine substitution for the outermost arginine in II/S4 of the alpha1 subunit of the human skeletal muscle dihydropyridine (DHP) receptor has been reported to cause hypokalaemic periodic paralysis (HypoPP). This mutation shifts the voltage dependence of L-type Ca curent inactivation in myotubes from HypoPP patients by -40 mV without affecting activation. Based on the strong homology of II/S4 in cardiac and skeletal muscle alpha1, we introduced the corresponding mutation into the rabbit cardiac alpha1 subunit (R650H). Wild type (WT) and mutant constructs were transiently transfected in HEK cells together with beta and alpha2delta subunits and Ca and Ba currents were studied using the whole-cell patch-clamp technique. In contrast to the results obtained from human myotubes, R650H produced a small (-5 mV) but significant shift of both the steady-state activation and inactivation curves. When external pH was increased from 7.4 to 8.4 in order to favour deprotonization of H650, the only difference between WT and mutant channels was a slightly reduced steepness of the inactivation curve. Additional cotransfection of the gamma subunit which is only found in skeletal but not in heart muscle, shifted the inactivation curves of both WT and R650H by -20 mV. We conclude that R650 plays a different role in voltage-dependent gating of the cardiac L-type Ca channel than the corresponding residue in the human skeletal muscle L-type channel, since a distinct and selective effect on the midpoint voltage of steady-state inactivation could not be found for R650H.

Applications of SNMS in archaeometry

The recently developed High Frequency Mode HFM of electron gas SNMS allows investigations on insulating samples with the well known advantages of the SNMS Direct Bombardment Mode DBM for the analysis of conducting samples. HFM has been applied to analyses of different historic ceramic and glass samples in order to demonstrate the possibilities of SNMS in this field. It is shown that manufacturing places of ceramic samples could be distinguished by SNMS mass spectra ("fingerprints"). Furthermore questions of the constituents of colour remains on a painted ceramic ("Kaisermedaillon") could be answered by our SNMS analyses. SNMS investigations have been also applied to corrosion phenomena on different glass samples.

Calcium current reactivation after flash photolysis of nifedipine in skeletal muscle fibres of the frog

1. L-type calcium currents were activated by depolarization of cut muscle fibres of the frog. The current was blocked by the dihydropyridine compound nifedipine (5-10 microM) and reactivated by flash photolysis of the drug. 2. In the presence of nifedipine, increasing the time interval between the onset of depolarization and the flash resulted in progressively faster kinetics of the flash-induced current. This change developed with a slow time course similar to that of normal current activation. 3. A fast gating mode of the normally slow L-type channel was induced by conditioning activation (500 ms prepulses) applied 80 ms before a test step to the same potential. After block by nifedipine, flash-photolysis was carried out 40 ms before the end of the long conditioning pulse. The flash-induced current had the same rapid time course as the current activated by the subsequent test voltage step. 4. Similarly, the time course of current activation was comparable for the voltage-induced fast mode activation (flash applied 5 ms before the test step) and the flash-induced activation 40 ms after the onset of the test depolarization. 5. Our data suggest that in frog skeletal muscle nifedipine inhibits calcium current activation by blocking a rapid channel gating step while the slow conformational change that normally limits the rate of activation of the L-type calcium channel remains unaffected. UV flash illumination results in a fast reactivation indicating that the channels need not be inactivated to be blocked by nifedipine.

Skeletal muscle DHP receptor mutations alter calcium currents in human hypokalaemic periodic paralysis myotubes

1. Mutations in the gene encoding the alpha 1-subunit of the skeletal muscle dihydropyridine (DHP) receptor are responsible for familial hypokalaemic periodic paralysis (HypoPP), an autosomal dominant muscle disease. We investigated myotubes cultured from muscle of patients with arginine-to-histidine substitutions in putative voltage sensors, IIS4 (R528H) and IVS4 (R1239H), of the DHP receptor alpha 1-subunit. 2. Analysis of the messenger ribonucleic acid (mRNA) in the myotubes from such patients indicated transcription from both the normal and mutant genes. 3. In control myotubes, the existence of the slow L-type current and of two rapidly activating and inactivating calcium current components (T-type with a maximum at about -20 mV and 'third type' with a maximum at +10 to +20 mV) was confirmed. In the myotubes from patients with either mutation, the third-type current component was seen more frequently and, on average, with larger amplitude. 4. In myotubes with the IVS4 mutation (R1239H) the maximum L-type current density was smaller than control (-0.53 +/- 0.31 vs. -1.41 +/- 0.71 pA pF-1). The voltage dependence of activation was normal, and hyperpolarizing prepulses to -120 mV for 20 s did not increase the reduced current amplitude during test pulses. 5. In myotubes with the IIS4 mutation (R528H) the L-type current-voltage relation, determined at a holding potential of -90 mV, was normal. However, the voltage dependence of inactivation was shifted by about 40 mV to more negative potentials (voltage at half-maximum inactivation, V1/2 = -41.5 +/- 8.2 vs. -4.9 +/- 4.3 mV in normal controls).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of extracellular metal cations in the potential dependence of force inactivation in skeletal muscle fibres

The present experiments were designed to further characterize a metal ion binding site at the voltage sensor in the T-tubular (TT) membrane which controls the release of Ca2+ from the sarcoplasmic reticulum. For this purpose the potential dependence of force inactivation was measured under voltage clamp control in short toe muscle fibres of the frog. External solutions contained in each case one species of metal ion (Ca2+, Ba2+, Na+ and Li+, respectively). Assuming that the metal ion binds with different affinities to the resting and active state of the sensor and that the metal ion free sensor is inactivated, we estimated the dissociation constants by using the inactivation midpoint voltages (V) at different concentrations of one species of metal ion. For Ca2+ the analysis resulted in a low apparent dissociation constant KD1 (binding to the resting state) of approximately 5 x 10(-8) M and a high apparent dissociation constant KD2 > 23 mM (binding to the active state). The corresponding values for Ba2+ were: KD1 = 5 x 10(-5) M and KD2 > 125 mM. For different reasons, the data for Na+ and Li+ proved to be inconclusive.

A possible role of sarcoplasmic Ca2+ release in modulating the slow Ca2+ current of skeletal muscle

Ca2+ channels are regulated in a variety of different ways, one of which is modulation by the Ca2+ ion itself. In skeletal muscle, Ca2+ release sites are presumably located in the vicinity of the dihydropyridine-sensitive Ca2+ channel. In this study, we have tried to investigate the effects of Ca2+ release from the sarcoplasmic reticulum on the L-type Ca2+ channel in frog skeletal muscle, using the double Vaseline gap technique. We found an increase in Ca2+ current amplitude on application of caffeine, a well-known potentiator of Ca2+ release. Addition of the fast Ca2+ buffer BAPTA to the intracellular solution led to a gradual decline in Ca2+ current amplitude and eventually caused complete inhibition. Similar observations were made when the muscle fibre was perfused internally with the Ca2+ release channel blocker ruthenium red. The time course of Ca2+ current decline followed closely the increase in ruthenium red concentration. This suggests that Ca2+ release from the sarcoplasmic reticulum is involved in the regulation of L-type Ca2+ channels in frog skeletal muscle.

Modulation of calcium current gating in frog skeletal muscle by conditioning depolarization

1. Ca2+ inward currents were measured by voltage clamping cut skeletal muscle fibres of the frog (Rana esculenta) in a double-Vaseline-gap system. 2. In order to study the basis of the previously described fast gating mode induced in the Ca2+ inward current by a conditioning depolarization we quantitatively analysed the response to differing features of the conditioning prepulse. 3. The faster activation seen during the second of two depolarizations was confined to the component of the inward current which could be blocked by 5 to 10 microM nifedipine. 4. By applying depolarizing conditioning pulses of gradually increasing length the time course of the transition to the fast gating mode could be determined. 5. Both the transition to the fast gating mode (point 4) caused by a depolarization and the slow inward current activated during the same depolarization showed similar voltage-dependent kinetics. 6. The kinetic change of the test current appeared to be equal when the same fractional activation was achieved at the end of the conditioning pulse independent of its duration or amplitude. 7. Flash photolysis of nifedipine in the interval between conditioning and test pulse showed that the predepolarization causes a rate-enhancing effect even though the slow channels were blocked by nifedipine during the conditioning pulse. 8. We conclude that the transition of the calcium channel from its slow to its fast gating mode is determined by the slow voltage-dependent reaction which limits the rate of channel opening under control conditions. This reaction is apparently not prevented by the binding of nifedipine and the block of current flow through the channel.

Excitation-contraction coupling in a pre-vertebrate twitch muscle: the myotomes of Branchiostoma lanceolatum

The segmented trunk muscle (myotome muscle) of the lancelet (Branchiostoma lanceolatum), a pre-vertebrate chordate, was studied in order to gain information regarding the evolution of excitation-contraction (EC) coupling. Myotome membrane vesicles could be separated on isopycnic sucrose gradients into two main fractions, probably comprising solitary microsomes and diads of plasma membrane and sarcoplasmic reticulum, respectively. Both fractions bound the dihydropyridine PN 200/110 and the phenylalkylamine (-)D888 (devapamil) while specific ryanodine binding was observed in the diad preparation only. Pharmacological effects on Ca2+ currents measured under voltage-clamp conditions in single myotome fibers included a weak block by the dihydropyridine nifedipine and a shift of the voltage dependences of inactivation and restoration to more negative potentials by (-)D888. After blocking the Ca2+ current by cadmium in voltage-clamped single fibers, the contractile response persisted and a rapid intramembrane charge movement could be demonstrated. Both responses exhibited a voltage sensitivity very similar to the one of the voltage-activated Ca2+ channels. Our biochemical and electrophysiological results indicate that the EC coupling mechanism of the protochordate myotome cell is similar to that of the vertebrate skeletal muscle fiber: Intracellular Ca2+ release, presumably taking place via the ryanodine receptor complex, is under control of the cell membrane potential. The sarcolemmal Ca2+ channels might serve as voltage sensors for this process.

Ca2+ current in myotome cells of the lancelet (Branchiostoma lanceolatum)

1. By using the whole-cell patch clamp method Ca2+ and Ba2+ currents were measured in the extremely thin twitch muscle cells of the protochordate Branchiostoma lanceolatum whose Ca2+ channels are likely to resemble the evolutionary ancestors of those found in vertebrate skeletal muscle. 2. When using 10 mM-Ca2+ in the artificial external solution and 1 mM-EGTA in the internal solution two kinetically different Ca2+ inward current components could be observed, showing very similar voltage dependence of activation and inactivation. 3. In solutions containing 10 mM-Ba2+ as an external charge carrier the biphasic inward current turned into a single rapidly activated and slowly inactivating current. 4. Inspecting peak currents, the voltage dependence of fractional activation and inactivation was nearly the same in Ca2+ and in Ba2+. 5. A transformation into a single component of the Ca2+ current could also be observed after perfusing the intracellular lumen with 10 mM of either EGTA or BAPTA. In the case of EGTA this transformation required considerably more time. Probably a higher internal concentration of EGTA is necessary since it binds Ca2+ more slowly than BAPTA. 6. Soon after establishing the whole-cell configuration a gradual increase of the second, slow inward current phase was observed relative to the fast component, indicating an enhancement of the slow component by intermediate intracellular buffer concentrations. 7. We conclude that the Branchiostoma myotome cells have only one Ca2+ channel system. The biphasic appearance of the inward current is caused by an unusually rapid inactivation due to Ca2+ ions, which enter the myoplasm during the current and temporarily bind to an inactivation site at the channel. The second phase probably reflects reactivation from the inactivated state upon dissociation of Ca2+ from the binding site.

Fast gating kinetics of the slow Ca2+ current in cut skeletal muscle fibres of the frog

1. Calcium currents and intramembrane charge movements were measured in cut twitch muscle fibres of the frog and the time course of activation of the current was studied using various conditioning pulse protocols. 2. When a conditioning activation was produced by a depolarizing pulse which ended before inactivation occurred, a subsequent depolarization led to a faster onset of activation, indicating that the system had not completely returned to the initial state during the interval between the two pulses. 3. The interval between conditioning and test pulse was varied at different subthreshold potentials to study the time course of restoring the steady-state conditions. Complete restoration required a waiting period of about 1 min at the holding potential of -80 mV due to a very slow process but partial recovery was reached within 100 ms. This initial recovery process was strongly voltage dependent and became considerably slower when the interval potential approached the threshold for current activation. 4. Stepping to a roughly 10 mV subthreshold potential without applying a conditioning activation caused no change in the time course of the current produced by a subsequent test depolarization. Depolarizing just to the current threshold caused a slowly progressing acceleration of test current activation. 5. The peak current-voltage relation in the fast gating regime caused by a conditioning activation coincided with the current-voltage relation measured under steady-state conditions, indicating not that a new channel population had become activated but that the same channels showed a different gating behaviour. 6. Intramembrane charge movements measured in 2 mM-Cd2+ and tested at potentials between -40 and +40 mV showed negligible changes when preceded by a strong depolarization. 7. We discuss several possible models which can explain the fact that the current is speeded up by a conditioning activation while the charge movements remain unchanged. It is possible that the fast voltage-dependent transition which becomes visible after conditioning pulses reflects a rapid conformational change of the Ca2+ channel molecule which also occurs during its normal gating mode but remains undetectable in terms of conductance. In view of the hypothesis that the Ca2+ channel molecule forms a voltage sensor for excitation-contraction coupling this fast transition could be coupled to the control of Ca2+ release from the sarcoplasmic reticulum.

Effects of gallopamil on calcium release and intramembrane charge movements in frog skeletal muscle fibres

1. Intramembrane charge movements and changes in intracellular Ca2+ concentration were studied in voltage clamp experiments on cut twitch muscle fibres of the frog. The restoration from inactivation caused by steady depolarization and its modification by the phenylalkylamine Ca2+ channel antagonist gallopamil (D600, 10-30 microM) were investigated. 2. D600 prevented the restoration from inactivation of Ca2+ release which normally occurred at -80 mV. In D600 Ca2+ release recovered from inactivation at -120 mV. 3. D600 did not alter the characteristics of intramembrane charge movements in the depolarized fibre (charge 2) but the increase in the amount of mobile charge in the test voltage range above -60 mV, which normally occurs after changing the holding potential to -80 mV, was suppressed. The charge movement characteristics of D600-paralysed fibres, which were held at -80 mV, equalled those of normal depolarized and inactivated fibres. 4. Control records for the charge movement analysis were always obtained by voltage steps above 0 mV. Using the 'conventional' control in the potential range between -80 and -160 mV led to an underestimation and a kinetic deformation of charge movements in D600-treated fibres, which was due to various amounts of nonlinear charge in the control. 5. Like the restoration of Ca2+ release at -80 mV in normal fibres the recovery from paralysis at -120 mV in D600-treated fibres was accompanied by a significant increase in mobile charge in the potential range positive of -60 mV. Both Ca2+ release and charge movement at test potentials above -60 mV recovered with almost identical time course. 6. Restoration of Ca2+ release at a holding potential of -80 mV in normal fibres or at -120 mV in D600-treated fibres could not be clearly correlated to charge movement changes in the voltage range negative of -60 mV (charge 2). 7. Our results are consistent with a voltage-dependent inhibitory effect of D600 on the charge displacement that controls Ca2+ release from the sarcoplasmic reticulum but provide little evidence for a conversion of charge 2 into the charge that is involved in the control of Ca2+ release.

A general procedure for determining the rate of calcium release from the sarcoplasmic reticulum in skeletal muscle fibers

A general procedure for using myoplasmic calcium transients measured with a metallochromic indicator dye to calculate the time course of calcium release from the sarcoplasmic reticulum in voltage-clamped skeletal muscle fibers is described and analyzed. Explicit properties are first assigned to all relatively rapidly equilibrating calcium binding sites in the myoplasm so that the calcium content (CaF) in this pool of "fast" calcium can be calculated from the calcium transient. The overall properties of the transport systems and relatively slowly equilibrating binding sites that remove calcium from CaF are then characterized experimentally from the decay of CaF following fiber repolarization. The rate of calcium release can then be calculated as dCaF/dt plus the rate of removal of calcium from CaF. Two alternatives are assumed for the component of CaF that is due to fast binding sites intrinsic to the fiber: a linear instantaneous buffer or a set of binding sites having properties similar to thin filament troponin. Both assumptions yielded similar calcium release wave forms. Three alternative methods for characterizing the removal system are presented. The choice among these or other methods for characterizing removal can be based entirely on convenience since any method that reproduces the decay of CaF following fiber repolarization will give the same release wave form. The calculated release wave form will be accurate provided that the properties assumed for CaF are correct, that release turns off within a relatively short time after fiber repolarization, that the properties of the slow removal system are the same during and after fiber depolarization, and that possible spatial nonuniformities of free or bound calcium do not introduce major errors.

Intramembrane charge movement and calcium release in frog skeletal muscle

Intramembrane charge movement and myoplasmic free calcium transients (delta[Ca2+]) were monitored in voltage-clamped segments of isolated frog muscle fibres cut at both ends and mounted in a double Vaseline-gap chamber. The fibres were stretched to sarcomere lengths of 3.5-4.6 micron to minimize mechanical movement and the related optical artifacts. The over-all calcium removal capability of each fibre was characterized by analysing the decay of delta[Ca2+] following pulses of several different amplitudes and durations. The rate of sarcoplasmic reticulum (s.r.) calcium release was then calculated for each delta[Ca2+] using the calcium removal properties determined for that fibre. The calculated calcium release wave form reached a relatively early peak and then declined appreciably during a 100-150 ms depolarizing pulse. The voltage dependence of the peak rate of calcium release was steeper and was centred at more positive membrane potentials than the steady-state voltage dependence of charge movement in the same fibres. A considerable fraction of the total intramembrane charge was moved at potentials at which delta[Ca2+] and calcium release were only a few per cent of maximum. This 'subthreshold' charge may correspond to charge moved in preliminary transitions that precede a final charge transition that activates release. A 'stepped on' pulse protocol was used to experimentally separate the subthreshold charge movement from the charge movement of the final transitions that may control calcium release. The stepped on pulse consisted of a set 50 ms pre-pulse to a potential just at or below the potential for detectable delta[Ca2+] followed immediately by a test pulse of varying amplitude and duration. For a wide range of test pulse amplitudes and durations in the stepped on protocol the peak rate of calcium release was linearly related to the charge movement during the test pulse. This result points to a tight control of activation of s.r. calcium release by intramembrane charge movement. The voltage dependence of both charge movement and of the rate of calcium release could be fitted simultaneously with a three-state, two-transition sequential model in which charge moves in both transitions but only the final transition activates s.r. calcium release. A model with three identical and independent charged gating particles per channel gave an equally good fit to the data. Both models closely fit the charge movement and release data except within about 10 mV of the voltage at which release became detectable, where release varied more steeply with membrane potential than predicted by either model.(ABSTRACT TRUNCATED AT 400 WORDS)

The removal of myoplasmic free calcium following calcium release in frog skeletal muscle

Transient changes in intracellular free calcium concentration (delta [Ca2+]) in response to pulse depolarizations were monitored in isolated segments of single frog skeletal muscle fibres cut at both ends and voltage clamped at a holding potential of -90 mV in a double-Vaseline-gap chamber. Calcium transients were monitored optically using the metallochromic indicator dye Antipyrylazo III (APIII), which entered the fibre by diffusion from the solution applied to the cut ends. Optical artifacts due to fibre movement were minimized or eliminated by stretching the fibres to sarcomere lengths at which there was little or no overlap of thick and thin contractile filaments. Remaining movement-independent optical changes intrinsic to the fibre and unrelated to the dye were monitored at 850 nm, where free and dye-bound APIII have no absorbance. These 850 nm signals scaled by lambda -1.2 were used to remove intrinsic components from the signals at 700 or 720 nm, wave-lengths at which the APIII absorbance increases when calcium is bound. The corrected 700 or 720 nm signals were used to calculate delta [Ca2+]. The decay of delta [Ca2+] following fibre repolarization at the termination of a depolarizing pulse was well described by a single exponential plus a constant. The exponential rate constant for the decay of delta [Ca2+] decreased and the final 'steady' level that delta [Ca2+] appeared to be approaching increased with increasing amplitude and/or duration of the depolarizing pulse. Both the decreasing decay rate and the build up of the 'steady' level can be accounted for using a two-component model for the removal of free calcium from the myoplasm. One component consists of a set number of a single type of saturable calcium binding site in the myoplasm. The second component is a non-saturable, first-order uptake mechanism operating in parallel with the saturable binding sites. The removal model parameter values were adjusted to fit simultaneously the decay of delta [Ca2+] after pulses of various amplitudes and durations in a given fibre. The basic procedure was to track delta [Ca2+] during each pulse when an undetermined calcium release was occurring, but to calculate the decay of delta [Ca2+] starting 14 ms after repolarization when release was assumed to be negligible. After appropriate selection of parameter values, the model reproduced most aspects of the decay of delta [Ca2+].(ABSTRACT TRUNCATED AT 400 WORDS)

Use of a metallochromic indicator to study intracellular calcium movements in skeletal muscle

The transient increase in free myoplasmic calcium concentration due to depolarization of a skeletal muscle fibre is the net result of the release of calcium from the sarcoplasmic reticulum (SR) and its simultaneous removal by binding to various sites and by reuptake into the SR. We review here procedures recently developed in this laboratory for empirically characterizing the calcium removal processes in voltage-clamped fibres and for using such characterization to determine the time course of SR calcium release during a depolarizing pulse.

Time course of calcium release and removal in skeletal muscle fibers

The transient increase in free myoplasmic calcium concentration due to depolarization of a skeletal muscle fiber is the net result of the release of calcium from the sarcoplasmic reticulum (SR) and its simultaneous removal by binding to various sites and by reuptake into the SR. We present a procedure for empirically characterizing the calcium removal processes in voltage-clamped fibers and for using such characterization to determine the time course of SR calcium release during a depolarizing pulse. Our results reveal a decline of the SR calcium release rate during depolarization that was not anticipated from simple inspection of the calcium transients.

Twitch activation in Ca2+ -free solutions in the myotomes of the lancelet (Branchiostoma lanceolatum)

The question of whether a Ca2+ influx is necessary to activate twitches in the very thin myotome cells of the lancelet was reinvestigated. Though twitches were blocked in EGTA containing Ca2+- free bathing solutions, they reappeared under certain conditions: a) when the Mg2+ level was lowered, b) when small amounts of caffeine were added, and c) when Cl- in the bathing solution was partly replaced by SCN-. In Ca2+ -containing solutions the changes a) to c) increased the twitch height. The results suggest that a Ca2+ release from the intracellular stores is involved in twitch activation and that external Ca2+ modifies the coupling between excitation and Ca2+ release rather than initiating contractile activation.

Electrical membrane properties of the muscle lamellae in Branchiostoma myotomes

The membrane characteristics of amphioxus myotome cells were investigated using the chopped-current clamp method. The extremely thin myotome lamellae had a membrane resistance of 2.2 to 7.7 k omega cm2 and a capacity of 0.4 to 1.3 microF/micron2. The steady state current-voltage relation showed outward rectification. In Cl- - and Na+ -free solutions containing 400 mM TEA the input resistance and the time constant increased threefold and Ca2+ -dependent action potentials occurred, indicating the presence of a Ca2+ component in membrane current. It is discussed whether Ca2+ ions entering the cell during the normal action potential may significantly increase the intracellular free Ca2+ concentration.