[Epidemiology of stress fractures from an occupational health viewpoint]. / Zur Epidemiologie von Ermüdungsbrüchen aus arbeitsmedizinischer Sicht.

Stress fractures may occur after extremely high exercises in sports, military training, and occupational activities. The pathogenetic mechanisms include processes of material fatigue as well as repair capacities of the bone. By means of szintigraphic technique a sensitive tool for diagnostics of stress fractures is available. Recently, stress fractures of the vertebral processes only are acknowledged as a professional disease in Germany. As a rule, in orthopaedic and surgical practice fatigue fractures of other bones are considered as cases of accidental injuries.

The effects of calcium channel modulators on contractions of tonic frog muscle fibres.

Effects of adrenaline (ADR) and the dihydropyridine Ca channel agonist CGP-IOS were studied on twitch and tetanic contractions of isolated tonic muscle fibres or small muscle fibre bundles containing tonic fibres of the frog Rana temporaria. Tetanization caused a gradual increase of tension between 10 and 70 Hz. CGP-IOS produced an increase in twitch amplitude. After CGP-IOS administration (2 x 10(-6) - 10(-5) mmol/l), the twitch amplitude increased and the high frequency tetanus developed more rapidly. However, during the phase of high tension level a breakdown of tetanic tension appeared and this occurred earlier at higher stimulation frequencies. After the end of tetanization the contraction curve did not return to the initial level, so that the remaining contraction (contracture) lasted 2 to 5 min. Sometimes just after replacement of CGP-IOS by Ringer, a large enhancement of tetanus amplitude was observed followed by long lasting contractures. ADR (10(-5) mmol/l) increased the tetanic tension without changing the shape of tetanus but did not affect twitch amplitude. Adrenergic modulation of tension requires extracellular Ca2+. Combined administration of ADR and CGP-IOS had an integrative effect, so that independent action of each of them can be assumed. It is to suggest that direct and indirect Ca channel modulators, DHP derivatives and ADR, have different targets. The sites responsible for adrenergic modulation of the contraction may be Ca channels not identical to DHP-sensitive channels, i.e. Ca-releasing channels in SR or the DHP-insensitive Ca channels in muscle membrane.

External potassium and action potential propagation in rat fast and slow twitch muscles.

The role of extracellular K+ concentration in the propagation velocity of action potential was tested in isolated rat skeletal muscles. Different K+ concentrations were produced by KCl additions to extracellular solution. Action potentials were measured extracellularly by means of two annular platinum electrodes. Fibre bundles of m. soleus (SOL), m. extensor digitorum longus (EDL), red (SMR) and white (SMW) part of m. sternomastoideus were maximum stimulated. The conduction velocity (c.v.) was calculated from the distance between the electrodes and the time delay of the potentials measured at 22 degrees C. In Tyrode solution containing 5 mmol/l K+, the c.v. was close to 1 m.s-1. Bundles of the fast muscle type seemed to have a somewhat higher c.v. The differences observed in these studies were not significant. At higher temperatures, the c.v. increased (Q10 of approx. 2) and a dissociation between SMR and SMW muscles appeared. An elevation of K+ concentration to 10 mmol/l induced a drop of the c.v. by approx. 25% and 15% in EDL and SOL muscles, respectively. After return to normal solution, the recovery was not complete within 30 min. In K+ free solution the c.v. of EDL and SM muscles rose by a factor of 1.5, but less in SOL muscles. The weaker response of SOL to K+ modification was related to the higher resistance of this muscle to fatigue. This suggestion was supported by experiments on fatigued fibre bundles. Immediately after a tetanic stimulation producing fatigue, the c.v. of EDL and SOL muscles dropped similarly as in 10 mmol/l K+; again, the drop was less for SOL muscles. Adrenaline (0.5-10.0 mumol/l) enhanced both the c.v. and the twitch amplitude. The results support the suggestion that extracellular K+ accumulation during activity is an essential factor of muscle fatigue.

Trichloroethylene interactions with muscle cells.

The toxic effect of trichlorethylene (TCE) was investigated on isolated muscles prepared from frog and rats. Twitch and tetanic contractions as well as caffeine-induced contractures, were recorded. Trichloroethylene at a concentration of 0.25-4.0 mM depressed the force development of both twitch and tetanic tension in a dose-dependent manner. This effect was not influenced by the type of muscle. As TCE shortened the time to peak of twitch contractions, it may alter the Ca2+ binding kinetics. Subthreshold caffeine concentrations applied after pre-exposure to TCE (1 or 2mM) induced contractures. The same TCE exposure enhanced regular caffeine contractures through increasing the speed of tension development and the absolute force. Exposure to 5 or 10 mM TCE did not affect the first caffeine-induced contracture but enhanced the potency of the second caffeine dose given 15 min after the first. The results suggest that the interaction of TCE with membrane sites is responsible for Ca2+ release for contractile processes.

[Problems of muscular fatigue--relationship to stimulation conduction velocity and K(+) concentration]. / Probleme der Muskelermüdung--Beziehung zur Erregungsleitungsgeschwindigkeit und K(+)-Konzentration.

Muscle fatigue is accompanied by a series of biochemical correlations as substrate depletion, lactate accumulation, shifts of pH, increase of phosphate (Pi), arise of free radicals or disturbances of ionic balances. In last time high interest has been directed to the increase of extracellular potassium during extensive muscle activity. It was suggested that high K+ concentration in the interstitium may alter propagation of action potential along the T-tubules or induces membrane depolarization with physiological consequences. In order to elucidate the role of potassium accumulation, experiments were performed on isolated rat muscles. An elevation from 5 to 10 mmol K+ of the bath solution causes a significant decrease of the conduction velocity of the action potential. This effect is more pronounced on fatigue-sensitive fast twitch EDL muscles than on fatigue-resistant slow twitch SOL muscles. Moreover, after tetanic stimulations of these muscles in normal solution, the conduction velocity dropped by the same amount as in high K+ solution but, again, differently in both muscle types. Therefore it is supposed that K+ accumulation during intensive muscle activity contributes to fatigue.

Changes of the conduction velocity of isolated muscles induced by altered external potassium concentration.

The propagation of action potentials along fibre bundles of fast and slow twitch rat muscles was tested by means of two separated different electrodes. In spite of marked differences in contractile properties between fast and slow twitch bundles, the conduction velocity (c.v.) was found to be similar in all preparations at room temperature. At 35 degrees C the c.v. was 2.76 +/- 0.36 m.s-1 in SOL and 3.0 +/- 0.77 m.s-1 in EDL muscles, respectively. An elevation of the extracellular K+ from 5 to 10 mM caused a reduction of the c.v. by 20 - 40% in limb and sternomastoid muscles. The effect increased with higher K+ concentrations. Repeated tetanic stimulation also induced a decrease of the c.v. The opposite effect occurred in solution with reduced external K+. In O K+ solution the c.v. of fast twitch fibres increased by almost 50%. The results show that extracellular K+ interacts with processes of the regeneration of the action potential along the fibres.

Twitch and tetanic contraction of lamprey muscle exposed to fatty acid at different temperatures.

The effect of Na-octanoate (NaC8) on the development of twitch and tetanic tension of a striated muscle (m. longitudinalis linguae) of Lampetra fluviatilis was tested at different temperatures. The muscle exhibited posttetanic potentiation as well as cold potentiation similar to other poikilotherme animals. The sensitivity to NaC8 was higher than that of striated frog or rat muscles but similar to that of mammalian smooth or cardiac muscle preparations. A decrease of temperature remarkably reduced the effect of NaC8 on the tension development. Also the fatigue of muscles during tetanic stimulation was smaller if the temperature was lower. It seems that both the process of fatigue and the action of NaC8 are delayed by the cold.

Isometric twitch and tetanic contraction of frog skeletal muscles at temperatures between 0 to 30 degrees C.

Maximum twitch and tetanic tension development, time to peak, and half relaxation time were studied on isolated frog sartorius muscles stimulated directly in Ringer's solution at different temperatures. Cooling from 20 degrees C to 10 degrees C decreased the tetanic tension (Q10 = 1.3-1.4). At temperatures above 25 (30) degrees C the tension output was reduced. The response to cooling of the twitch contraction was a prolongation of the time to peak (Q10 = 2.4) and of the relaxation time (Q10 = 2.7) independently of the amplitude which increased in most muscles. Between 20 and 10 degrees C the tension output rose by a factor of 1.2-1.3. The failure of this response showed no relation to season. The increase of the twitch tension but the decrease of tetanic tension in parallel with the temperature drop shifted the ratio twitch/tetanus to higher values (0.5 to 0.8). The results suggest that cooling effects both the Ca2+ release and and the Ca2+ re-uptake but the latter one with a higher Q10. This causes a prolongation of the active state and a cold potentiation if further facilitating conditions are present. In contrast, the response to temperature of the tetanic tension seems to be due to the temperature dependent force generation per cross-bridge.

Contractile properties of fast and slow twitch muscles of the rat at temperatures between 6 and 42 degrees C.

The effect of a wide range of temperature on the development of twitch and tetanic tension was investigated in directly stimulated rat fast (EDL) and slow (SOL) twitch muscle preparations. When increasing the temperature from 6 to 30 degrees C the maximum tetanic tension rose steadily. The Q10 was 2.3 (EDL) and 2.7 (SOL) for temperatures between 12 and 22 degrees C. The twitch tension output of SOL muscle increased up to 36-38 degrees C, whereas the EDL muscle exhibited a distinct maximum at 22 degrees C followed by a 50% decrease at 34 degrees C. Post-tetanic potentiation was observed in EDL muscle at temperatures higher than 20 degrees C. In SOL muscle neither posttetanic potentiation nor cold potentiation could be observed. The twitch/tetanus ratio was 0.2-0.3 at 35 degrees C but 0.7-0.8 at 6 degrees C. In both muscle types the most characteristic effect of temperature was the prolongation of the time to peak and the relaxation time in parallel to cooling. The tension rise of fast twitch rat muscle during cooling from 35 degrees C downwards can be compared to the cold potentiation of frog sartorius muscle. It is suggested that the main effect of temperature on muscle function concerns the process of Ca2+ release and of Ca2+ uptake. The different response of SOL muscle may be related to the less developed sarcoplasmic reticulum and the lower Ca2+ ATPase activity.

Effects of Na-octanoate on potassium contractures in normal and denervated frog tonic fibres.

In frog twitch muscle fibres, Na-octanoate (NaC8) shifted the relation between potassium induced tension and membrane potential to the right. The present study has been carried out to investigate the effect of this fatty acid on frog tonic fibres. Potassium contractures measured on bundles of 30-40 fibres of ileofibularis muscles were less decreased by NaC8 (2.5-10 mmol/l) than those of twitch fibre bundles. In denervated muscles the sensitivity to NaC8 was increased, probably due to the development of sodium channels in the membranes. Experiments with mixed fibre bundles also showed a lower influence of NaC8 on potassium contracture of tonic fibres. On the other hand, tonic fibres showed a lower threshold of the potassium induced tension as well as a lower K+ concentration for maximal activation. This lower threshold was further lowered by NaC8, corresponding to a shift of the relation between potassium concentration and tension to the left. The membrane resting potentials were -58 +/- 9 mV in tonic fibres and -83 +/- 5 mV in twitch fibres. Five mmol/l NaC8 only induced depolarization of the membrane of tonic fibres. This depolarization (by about 20 mV) may be responsible for the threshold shift to lower K+ concentration in NaC8-exposed tonic fibres. In addition to the effects of NaC8 on sodium channels, interactions with Ca2+ binding sites are discussed.

Depressive effects of free fatty acids on the development of tension in different muscle types.

The depression of twitch and tetanic tensions, contractures, and spontaneous activities of various muscle preparations by a selected free fatty acid (Na-octanoate, NaC8) depends on the concentration, the temperature, the time of exposure, and the type of muscle. Among skeletal muscles, there is neither a significant difference between frog and rat twitch muscles nor between fast- and slow-twitch rat muscles at room temperature. Small differences seem to occur between frog tonic and phasic fibres as well as between fast- and slow-twitch rat muscles at 32 degrees C. Smooth muscles are more sensitive than skeletal muscles but less sensitive than papillary muscles in which 0.5 mmol/l NaC8 decrease the contractility by about 50%. The different sensitivity of the various muscle types might be caused by unequal alterations of membrane properties responsible for excitation-contraction coupling. It is supposed that free fatty acids interact with the calcium binding sites of the membranes.

Effects of free fatty acids on the muscle cell membrane and their possible relations to the depression of contraction.

High concentrations of free fatty acids (FFA) depolarize the resting muscle cell membrane and decrease the membrane resistance. In contrast, low concentrations have a "membrane stabilizing effect" which decreases action potential (AP) and voltage-dependent ionic currents. The changes of AP may result in a decrease of excitation-induced Ca++ release from cellular stores. This may be a causal factor for depression of contraction by FFA.

Contractile properties of isolated frog skeletal muscles under the influence of Na-octanoate.

Na-octanoate (2-10 mM) altered the characteristics of the isometric twitch of isolated frog skeletal muscles. The peak tension, the maximum rate of tension rose, the time to peak tension, and the half relaxation time decreased. The tetanus tension was reduced by the same amount in percent as a single isometric twitch so that no changes occurred in the twitch/tetanus relationship. The fusion frequency of tetanic contractions increased. The isotonic contractions had a reduced shortening period. The shortening velocity was not affected. The K+ induced contractures were diminished. The S-shaped curve which relates peak tension to potassium concentration was shifted to more positive potentials. The maximum contractile strength was not attained even by complete depolarization. 1-2 mM caffeine removed the octanoate effects described above. The contracture induced by 7.5 mM caffeine produced the same maximum tension as that obtained in Ringer's solution, but the contracture began later and the rise of tension was retarded. The tension development of glycerol-extracted muscles was not affected by octanoate. The results are consistent with the hypothesis that the octanoate-induced changes of muscle contraction are due to a reduced Ca++ release from cellular stores.

Octanol and octanoate induced inhibition of the mechanical activity of isolated frog skeletal muscles.

Octanol (1 mM) or octanoate (10 mM) almost totally depress the contraction amplitude of directly stimulated muscles in a few minutes. Octanoate in a concentration of 2 mM/l decreases the contraction amplitude by 20% and retards the caffeine contracture. The ratio between twitch and tetanus is affected by octanol only. The results suggest that octanol and octanoate alter binding or releasing properties for Ca2+ of skeletal muscle cells.

The influence of free fatty acids on functional properties of isolated skeletal muscles. Octanoate action on membrane resistance.

1. Na-octanoate (in Ringer's solution) affects the membrane resistance of isolated frog skeletal muscle fibres in a biphasic way. Initially there is an increase followed by a more slowly developing decrease. The effect depends on both the concentration of the fatty acid (1-20 mM) and the time of exposure (2-20 min). 2. In muscles exposed either to isotonic K2SO4 or to Ringer's solution with Cl- substituted by methylsulphage, the octanoate-induced resistance drop is the same as in normal Ringer's solution whereas an initial increase is not observed. 3. In Na-free (Tris-) Ringer's solution, only an increase in membrane resistance is caused by octanoate. 4. The results suggest that Na-octanoate decreases the conductance of the resting muscle cell membrane for anions (Cl-1) whereas the permeability for cations (Na+, K+) is increased.

[Effect of homologous n-alkanoic acids on the function of isolated skeletal muscles. V. Relations between twitch, tetanus and contracture]. / Einfluss homologer n-Alkansäuren auf funktionelle Eigenschaften isolierter Skeletmuskeln. V. Beziehungen zwischen Einzelzuckung, Tetanus und Kontraktur.

Twitch, tetanus, and contractures induced by potassium ions (K+) or caffeine were investigated on isolated bundles of frog muscles. Using supramaximal stimuli the twitch amplitude amounts to about 50% of the tetanic tension (22 degrees C). Usually the contracture in 190 mM K+ is smaller than the tetanic tension, in 20 mM caffeine the contracture maximum corresponds to the tetanus amplitude. All of the mechanical answers are diminished if the fibres were bathed in solutions containing 2.5 to 10 mM Na-octanoate or 0.5 mM decanoic acid. Twitch and tetanic tensions decrease nearly in the same degree. A sequence of tetanic stimulations in a distance of 2 s induces a depression of the amplitudes. Fatty acids increase this depression and delay restoration between the series. Contractures in 190 mM K+ are diminished by a preceding bath in 10 mM Na-octanoate. Contractures with submaximum concentrations of K+ were decreased by lower concentrations of octanoate. The caffeine induced contractures are delayed by fatty acids, but the maximum tension is not reduced. It is supposed that the mobilisation of calcium ions is alterated by fatty acids. The decreasing effect of fatty acids on the mechanical activity, induced by different ways of the muscles' stimulation suggests that the main action may take place in the process of mechanical coupling, probably during the activation of calcium ions.

[Effect of omega-H-perfluorinated carbonic acids on the wet weight, membrane potential and contractility of isolated skeletal muscle]. / Zur Wirkung omega-H-perfluorierter Karbonsäuren auf Frischgewicht, Membranpotential und Kontraktionsverhalten isolierter Skeletmuskeln.

To investigate the effect of fluoralkanes on cellular functions, isolated frog muscles were exposed to octafluoro-pentanoic acid (OP) and hexadecafluoro-nonanoic acid (HN). 20 MM of OP induced a small depression of the contraction amplitude and a loss of wet weight of 10% which was similar to the osmotic effect of pentanoic acid or sucrose. The membrane resting potential was not changed up to 50 mM. HN was much more effective: 0,5 mM decreased the contraction amplitude in a few minutes and depolarized the membrane by 20 to 30 mV in 2 h. 10 mM HN induced a weight gain of 20% connected with a contracture. Since the caffeine contracture was also depressed by 1 mM it is assumed that HN interact with cellular membranes and alter their Ca-binding properties.

[Influence of homologous n-alcanoic acids on the function properties of isolated skeletal muscles. III. Contractures by fatty acids and relations to the effects of caffeine]. / Einfluss homologer n-Alkansäuren auf funktionelle Eigenschaften isolierter Skeletmuskeln. III. Fettsäurekontraktur und Beziehungen zur Koffeinwirkung.

The influences of octanoic, decanoic, and hexadencanoic acid were tested on the contracture capability of isolated skeletal muscle of frogs and rats. 1. 100 mM octanoic or 10mM decanoic acid induce contractures in skeletal mucles after 20-30 min of exposure. 2. The time of exposure necessary for induction of contractures is shortened by an increase of bath temperature, electrical stimulation or KCl-depolarization of muscles. 3. Simultaneous addition of fatty acid and caffeine (10 mM) effects a depression and a delay of the caffeine contracture. The contractures evoked by 5 mM caffeine are inhibited by lower concentrations of fatty acids (1 mM octaonoic acid, 0,1 mM hexadecanoic acid). 4. After the complete development of a caffeine (or fatty acid) contracture the muscle is not able to develop an identical contracture by a second application of the same drug, even after intermediate treatment during one or two hours in Ringer solution. If the contracture is interrupted one minute after the caffeine application by changing the solution, the tension returns quickly to the resting level. A subsequent addition of caffeine (10 mM) after about 10 minutes effects an identical contracture. Thus the effect of fatty acids on caffeine contracture may be studied on the same muscle which served as its own control. 5. As mechanisms involved in the development of fatty acid contractures and in the inhibition of caffeine contractures, interactions of free fatty acids and lipids of biological membranes are disucssed. Especially, there may be changes of the calcium affinity of cellular membranes.