Slowing Of Relaxation Research Articles

Multiple Causes of Fatigue during Shortening Contractions in Rat Slow Twitch Skeletal Muscle

Fatigue in muscles that shorten might have other causes than fatigue during isometric contractions, since both cross-bridge cycling and energy demand are different in the two exercise modes. While isometric contractions are extensively studied, the causes of fatigue in shortening contractions are poorly mapped. Here, we investigate fatigue mechanisms during shortening contractions in slow twitch skeletal muscle in near physiological conditions. Fatigue was induced in rat soleus muscles with maintained blood supply by in situ shortening contractions at 37°C. Muscles were stimulated repeatedly (1 s on/off at 30 Hz) for 15 min against a constant load, allowing the muscle to shorten and perform work. Fatigue and subsequent recovery was examined at 20 s, 100 s and 15 min exercise. The effects of prior exercise were investigated in a second exercise bout. Fatigue developed in three distinct phases. During the first 20 s the regulatory protein Myosin Light Chain-2 (slow isoform, MLC-2s) was rapidly dephosphorylated in parallel with reduced rate of force development and reduced shortening. In the second phase there was degradation of high-energy phosphates and accumulation of lactate, and these changes were related to slowing of muscle relengthening and relaxation, culminating at 100 s exercise. Slowing of relaxation was also associated with increased leak of calcium from the SR. During the third phase of exercise there was restoration of high-energy phosphates and elimination of lactate, and the slowing of relaxation disappeared, whereas dephosphorylation of MLC-2s and reduced shortening prevailed. Prior exercise improved relaxation parameters in a subsequent exercise bout, and we propose that this effect is a result of less accumulation of lactate due to more rapid onset of oxidative metabolism. The correlation between dephosphorylation of MLC-2s and reduced shortening was confirmed in various experimental settings, and we suggest MLC-2s as an important regulator of muscle shortening.

Lactic acid accumulation is an advantage/disadvantage during muscle activity

Lactic acid accumulation is an advantage/disadvantage during muscle activity

Slowed relaxation in fatigued skeletal muscle fibers of Xenopus and Mouse. Contribution of [Ca2+]i and cross-bridges.

Slowing of relaxation is an important characteristic of skeletal muscle fatigue. The aim of the present study was to quantify the relative contribution of altered Ca2+ handling (calcium component) and factors downstream to Ca2+ (cross-bridge component) to the slowing of relaxation in fatigued fibers of Xenopus and mouse. Two types of Xenopus fibers were used: easily fatigued, type 1 fibers and fatigue resistant, type 2 fibers. In these Xenopus fibers the free myoplasmic [Ca2+] ([Ca2+]i) was measured with indo-1, and the relaxation of Ca2(+)-derived force, constructed from tetanic [Ca2+]i records and in vivo [Ca2+]i-force curves, was analyzed. An alternative method was used in both Xenopus and mouse fibers: fibers were rapidly shortened during the initial phase of relaxation, and the time to the peak of force redevelopment was measured. These two methods gave similar results and showed proportional slowing of the calcium and cross-bridge components of relaxation in both fatigued type 1 and type 2 Xenopus fibers, whereas only the cross-bridge component was slowed in fatigued mouse fibers. Ca2+ removal from the myoplasm during relaxation was markedly less effective in Xenopus fibers as compared to mouse fibers. Fatigued Xenopus fibers displayed a reduced rate of sarcoplasmic reticulum Ca2+ uptake and increased sarcoplasmic reticulum Ca2+ leak. Some fibers were stretched at various times during relaxation. The resistance to these stretches was increased during fatigue, especially in Xenopus fibers, which indicates that longitudinal movements during relaxation had become less pronounced and this might contribute to the increased cross-bridge component of relaxation in fatigue. In conclusion, slowing of relaxation in fatigued Xenopus fibers is caused by impaired Ca2+ handling and altered cross-bridge kinetics, whereas the slowing in mouse fibers is only due to altered cross-bridge kinetics.

Slowing of relaxation and [Ca2+]i during prolonged tetanic stimulation of single fibres from Xenopus skeletal muscle.

1. Parvalbumin (PA) has been proposed to take up Ca2+ and enhance skeletal muscle relaxation in brief contractions; as the duration of the contraction is increased, PA will become saturated with Ca2+ and no longer contribute to relaxation which therefore will be slowed. The rate of Ca2+ loading of PA is determined by the Mg2+ off rate (about 4 s-1 at 22 degrees C). In the present study we produced prolonged tetani in intact, single fibres of Xenopus frogs while measuring force and the free myoplasmic [Ca2+] ([Ca2+]i) with indo-1. 2. Mean rate constants of slowing of force relaxation with increasing tetanus duration ranged between 3.2 and 4.8 s-1, thus, similar to the Mg2+ off rate of PA. 3. The amplitude of the tail of [Ca2+]i after tetani increased with tetanus duration. This increase developed with a rate constant similar to the Mg2+ off rate of PA 4. Steady-state force-[Ca2+]i curves were produced from tetani of various frequencies and tetani produced when force was depressed after fatiguing stimulation. These curves were used to convert [Ca2+]i records into Ca(2+)-derived force. Relaxation of Ca(2+)-derived force was slowed following a time course similar to that of real force. The lag between Ca(2+)-derived and real force during relaxation was not affected by tetanus duration. 5. Tails of elevated [Ca2+]i after tetani were used to analyse the function of the SR Ca2+ pumps. This analysis showed a marked decline in the rate of Ca2+ uptake with prolonged tetani. 6. In conclusion, in Xenopus fibres the slowing of relaxation with increasing tetanus duration can be explained by altered Ca2+ handling due to PA Ca2+ loading and impaired SR Ca2+ uptake. This contrasts to our previous results in mouse fibres and the difference can be explained by a markedly lower rate of SR Ca2+ uptake resulting in higher tetanic [Ca2+]i in Xenopus fibres.

Slowing of relaxation during fatigue in single mouse muscle fibres.

1. In the preceding paper we analysed the force decline in fatigue of isolated mouse muscle fibres. Using the same preparation and stimulation scheme we have now examined another aspect of muscle fatigue, namely slowing of relaxation. 2. Relaxation at the end of a tetanic contraction can usually be divided into two phases, an initial nearly linear force decline, followed by an exponential phase. We have analysed the initial phase in terms of decline rate and duration. In rested fibres the decline rate after a 350 ms tetanus was not affected by a 30% reduction of tetanic tension (obtained by decreasing the stimulation frequency). 3. Relaxation became gradually slower during fatiguing stimulation with a maximum reduction at the time when tetanic tension was reduced to about 75% of the original (end of phase 2, see preceding paper). At this time the decline rate of the linear phase had fallen to 55.2 +/- 2.9% (mean +/- S.E.M., n = 25) and its duration had increased to 151.3 +/- 14.2% of the control (unfatigued 350 ms, 70 Hz tetanus). 4. Short rest periods (duration = 10s), given at various times during fatiguing stimulation, resulted in a clear, but partial, normalization of the relaxation parameters; at the time of maximum slowing the mean decline rate increased from 50.3 to 58.7% and the duration decreased from 167.9 to 144.0% of the control (n = 14). 5. The influence of intracellular acidosis on relaxation was studied by exposing rested fibres to 30% CO2 instead of the normal 5%. This resulted in a decline rate of 43.0 +/- 2.6% and a duration of 221.1 +/- 13.1% of the control (n = 7). 6. In amphibian muscle relaxation is known to become gradually slower during a single, prolonged tetanus. The existence of such an effect also in the present preparation was studied by giving 'interrupted' tetani with a total duration of about 2 s. In rested fibres the mean rate of relaxation was found to fall from 140.9 to 71.8% (n = 11) of the control (end of 350 ms stimulation) with a time constant of about 0.5 s. Thus, a marked slowing during a long tetanus occurs also in mammalian muscle. 7. A distinct slowing of relaxation during prolonged tetani was observed also in the fatigued and in the acidified state. Under these two conditions the mean rate of relaxation fell from 87.0 to 34.0% (n = 3) and from 74.0 to 23.0% (n = 5) of the control, respectively, with time constants similar to that in the rested state.(ABSTRACT TRUNCATED AT 400 WORDS)

Slowing of relaxation in the fatiguing hamster diaphragm is enhanced by theophylline

Hamster diaphragm muscle strips were treated with theophylline (100 mg/l) or caffeine (100 mg/l) to study the effect on the time constant of relaxation (tau) during repeated contractions and with recovery. Two stimulation protocols were used: a high-tension time index (TTI, 60 Hz, 160 ms, 2/s) and a low TTI (25 Hz, 160 ms, 1/s). In the high TTI protocol an early increase in the tau was noted in theophylline but not in caffeine or control. In the low TTI protocol there was no difference in tau with theophylline. The combination of theophylline (100 mg/l) and verapamil (5 microM) was also studied. Verapamil decreased force in contractions of 300-ms duration but not in those lasting 160 ms and had no effect on tau. It did not block the prolongation of tau seen with theophylline. These studies suggest that theophylline has a direct effect on relaxation of skeletal muscle, which is not prevented by verapamil, and also that external calcium may be important for sustained contractions of skeletal muscle.

Slowing of relaxation in the fatiguing hamster diaphragm is enhanced by theophylline

Hamster diaphragm muscle strips were treated with theophylline (100 mg/l) or caffeine (100 mg/l) to study the effect on the time constant of relaxation (tau) during repeated contractions and with recovery. Two stimulation protocols were used: a high-tension time index (TTI, 60 Hz, 160 ms, 2/s) and a low TTI (25 Hz, 160 ms, 1/s). In the high TTI protocol an early increase in the tau was noted in theophylline but not in caffeine or control. In the low TTI protocol there was no difference in tau with theophylline. The combination of theophylline (100 mg/l) and verapamil (5 microM) was also studied. Verapamil decreased force in contractions of 300-ms duration but not in those lasting 160 ms and had no effect on tau. It did not block the prolongation of tau seen with theophylline. These studies suggest that theophylline has a direct effect on relaxation of skeletal muscle, which is not prevented by verapamil, and also that external calcium may be important for sustained contractions of skeletal muscle.

Muscle fatigue due to changes beyond the neuromuscular junction.

A number of changes in function occur beyond the neuromuscular junction during activity; three main types are described. (a) During high frequency stimulation there is a rapid loss of force accompanied by a slowing of the action potential waveform and an increase in the excitation threshold of the muscle. It is suggested that accumulation of K+ in the extracellular spaces of the muscle may be responsible for these changes. (b) Slowing of relaxation is a feature of fatigued muscle. The slowing allows a reduction in activation frequency (minimizing high frequency fatigue) without resulting in an appreciable loss of force. (c) Changes in shape and amplitude of the twitch have considerable effects on the force generated by low frequencies of stimulation. After a brief tetanus there is a reduction in the width of the twitch which increases the fusion frequency of the muscle and may account for the "sag' seen at the start of low frequency contractions. After a prolonged series of contractions the twitch amplitude is reduced and remains so for several hours. This may be the result of some structural damage to the sarcoplasmic reticulum or transverse tubular system.

Metabolic changes associated with the slowing of relaxation in fatigued mouse muscle.

1. The biochemical basis of the slowing of relaxation seen in fatigue has been examined using an isolated mouse soleus preparation. 2. Slowing of relaxation occurred during prolonged tetani under anaerobic conditions when ATP and PC fell and lactate accumulated. 3. Slowing of relaxation was also demonstrated with muscles poisoned with cyanide and iodoacetic acid when there was a fall in ATP and PC but no accumulation of lactate. During a period of anaerobic recovery following a fatiguing tetanus, relaxation became faster at a time when lactate was accumulating in the muscle. 4. It is concluded that the slowing of relaxation in fatigue is not a consequence of lactate accumulation, and a relationship is demonstrated between the ATP content of the muscle and the rate of relaxation in muscles fatigued by prolonged stimulation, 5. Rates of ATP turn-over in fresh muscle, and at intervals throughout a tetanus are consistent with the suggestion that the rate limiting step for myofibrillar ATPase may be directly related to the rate limiting step for the decay of tension during relaxation.