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.

[Modern labor and stress on skeletal muscles]. / Moderne Arbeit und Beanspruchung der Skelettmuskulatur.

Introductorily the actual state in analysis and evaluation of physical work is characterized as a problem of changing chemical into mechanical energy in the skeletal muscle. Modern work is characterized in the view of the physiologist. Considering experimental results and epidemiological studies, problems of the K+ homeostasis during long lasting low level muscle activity are discussed as reasons of limiting performance. Changes in the potassium distribution between intra- and extracellular space have enormous consequences for excitability, contractili, metabolism, and arterial blood pressure. Finally references are given for evaluation and designing of modern work.

Muscle elasticity: effect of muscle length and temperature.

The present study describes a new method which allows the determination of muscle elasticity by applying quick releases at one end of a muscle and measuring the resulting tension drops at both ends, and simultaneously the propagation time of the mechanical impulse. The elasticity determined by both methods was examined on resting as well as on tetanized frog muscles (Rana esculenta, M. flexor hallucis brevis) in relation to muscle length and at two different temperatures 1 degree C and 20 degrees C. The average propagation velocity of the mechanical impulse of resting muscle was 55.7 +/- 4.7 m/s and of the contracting muscle 104.8 +/- 24.7 m/s at 1 degree C (L0, n = 6), which corresponds to elasticities of 3.3 +/- 0.5 N/mm2 and 14.4 +/- 6.2 N/mm2, respectively. The elasticity modulus calculated from the tension drop was for the resting muscle 2.3 +/- 0.5 N/mm2 and for the contracting muscle 11.1 +/- 2.1 N/mm2 (L0, 1 degree C, n = 6). When the muscle length is varied, the elasticity modulus corresponds to the length-tension relation of the resting and the isometrically contracting muscle. There is a strong correlation between the elasticity moduli which was determined by both methods for measurement of resting (r = 0.99, n = 19, p less than 0.05) and contracting muscle (r = 0.97, n = 19, p less than 0.05). This relation between elasticity and tetanic tension, i.e. filament overlap at constant temperature, can be interpreted in accordance with the sliding filament theory. But if the temperature is increased from 1 degree C to 20 degrees C, an increase of the tetanic tension (Q10 = 1.56) and a decrease of the elasticity measured by both methods (Q10 of 0.86 and 0.84) were obtained. This suggests that the increased tetanic force is generated by a smaller number of attached cross-bridges, but with a higher amount of force generated by each cross-bridge.

[The effect of increased environmental temperature on the behavior of physiologic parameters in bicycle ergometry within the scope of the physical endurance limit]. / Einfluss erhöhter Umgebungstemperatur auf das Verhalten physiologischer Parameter bei Fahrradergometerarbeit im Bereich der physischen Dauerleistungsgrenze.

For determination of the endurance limit range, 8 healthy male probands aged between 20 and 26 years loads had carried out for one hour on a bicycle ergometer at an ambient temperature of +20 degrees C, +28 degrees C and +35 degrees C. Staying in an ambient temperature of 35 degrees C leads to a decrease in the physical endurance limit in an order of magnitude of 10 W. At 20 degrees C, in the endurance range the following values result (mean +/- s): load 68+/- 10.3 W, heart rate 113.6 +/- 5.8 beats per min., net heart rate 31.8 +/- 2.4 beats per min., oxygen uptake 1340 +/- 170 ml per min., auditory canal temperature 36.5 +/- 0.24 degrees C, sweat rate 275 +/- 143 g. At 35 degrees C, the endurance limit is attained at the following values: 57 +/- 10.6 W, heart rate 125 +/- 11.8 beats per min., net heart rate 27.4 +/- 3.6 beats per min., oxygen uptake 1210 +/- 130 ml per min., auditory canal temperature 37.2 +/- 0.3 degrees C, sweat rate 577 +/- 96 g.

Temperature influence on phases of the tension response to sudden reduction of muscle length.

Phasic tension transients following a quick release in muscle length were studied to obtain information about the mechanism of force generation at different temperatures. The isometric tetanic tension increased (Q10 = 1.5) in the temperature range of 1.4 degrees C to 11.4 degrees C. The instantaneous stiffness (tension drop during the applied quick release in relation to the percentage of change of length) is slightly reduced by increasing temperature (Q10 = 0.9). The rate constant of the early tension recovery (phase 2) increased with increasing temperature (Q10 = 1.8). The results indicate that the increase of tetanic tension with increasing temperature is due to an increasing amount of force, generated by each cross bridge and a higher speed of rotation of the myosin head. The number of force generating cross bridges defined by the instantaneous elasticity is nearly independent of temperature range studied.

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.

Force, endurance time, and cardiovascular responses in voluntary isometric contractions of different muscle groups.

A study was made to reinvestigate the general validity of the force/endurance relation of isometric contractions. Relative force development, mean bioelectrical muscle activity and circulatory responses (blood pressure, interbeat interval) during isometric contractions of different muscle groups were compared. 6 men performed maximum isometric contractions of foot plantar- and dorsalflexors and hand flexors lasting 10 minutes. The hydrostatic blood pressure was varied by placing subject's arms and legs in two different positions, high and low. 9 Women performed plantarflexion and dorsalflexion with maximum strength and at constant load of 60% MVC to exhaustion. The results confirm that the force/endurance relationship during isometric exercise is not a biological constant, applicable for all muscles. The fibre composition of the muscle and the blood pressure varied by different hydrostatic components are discussed as factors causing differences in force/endurance relationship.

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.

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.

Quick release by shortening both ends of the muscle fibre.

A method is described allowing in quick release experiments the sensing of tension transients as well as the release on each of both ends of the muscle simultaneously. Preliminary results indicate a delay and differences in the time course of the tension transients between the released end and the fixed end. The delay corresponds to the propagation velocity of the mechanical wave. The elasticity of the muscle computed from the propagation velocity is compared with the elasticity determined by Young's modules.

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.