SUMMARY1. Alechanical factor(s) associated with the initiation of eccentric contractioninduced muscle injury were investigated in isolated rat soleus muscles (n = 180; 42 protoc ols with 4-6 muscles per protocol). Five eccentric contractions were performed with 4 min between contractions. Three levels of peak eccentric contraction force (100, 125 and 150% of pre-injury maximal isometric tetanic tension. P0), length change (041. 0 2 and 0 3 muscle length, Lo) and lengthening velocity (0(5, 1P0 and 1P5 Lo/s) were utilized. Force was varied with stimulation frequency (10-150 Hz). The eccentric contractions were initiated at muscle lengths of 0(85 or 0(90 Lo.Following the fifth eccentric contraction, the muscle was incubated in Krebs-Ringer buffer for 60 min. Peak isometric twitch tension (PT), PO, maximal rate of tension development (+dPI/dt), maximal rate of relaxation (-dP/dt), and creatine kinase (CK) release were measured prior to the five eccentric contractions and at 15 min intervals during the incubation period. Total muscle [Ca2+] was measured after 60 min incubation.2. The mean (+S.E.M.) initial decline in P0 for the muscles performing the most injurious protocol was 13-6 + 4-8 % (n = 6); P0 in control muscles immediately following performance of five isometric contractions was elevated 1P2 + 1 0 % (n = 8). These means were different at probability. p = 0(005. Mean [ATP] G. L. WARREN AND OTHERS control muscles but the elevation was unrelated to any of the four mechanical factors. 4. These data support the hypothesis that eccentric contraction-induced injury is initiated by mechanical factors, with muscle tension playing the dominant role. They also demonstrate that specific mechanical factors differentially affect the various injury criteria, i.e. reductions in contractile performance were most related to produced forces, and CK release was most related to lengthening velocity.

SUMMARY1. Histological evidence suggests that the force deficit associated with eccentric contraction-induced muscle injury is due to structural damage to contractile elements within the muscle fibre. Alternatively, the force deficit could be explained by an inability to activate the contractile proteins. It was the objective of this study to investigate the latter possibility.2. Mouse soleus muscles were isolated, placed in an oxygenated Krebs-Ringer buffer at 37°C, and baseline measurements were made. The muscle then performed one of three contraction protocols: (1) twenty eccentric (n = 10 muscles); (2) ten eccentric (n = 12); or (3) twenty isometric (n = 10) contractions. At the end of the injury protocol, measurements were made during performance of a passive stretch, twitch and tetanus. Next, force was recorded during exposure of the muscle to buffer containing 50 mm caffeine.3. Decrements in maximal isometric tetanic force (P0) observed for muscles in the twenty eccentric, ten eccentric, and twenty isometric contraction protocols were 42-6 + 4-2, 20-0 + 2-3 and 3.9 + 2-4 %, respectively. However, the caffeine-elicited forces in muscles from the three protocols were not different when corrected for initial differences in P0 (64X9 + 1X3, 64-2 + 2-1 and 68-9 + 2-5 % of pre-injury P0). The peak caffeine-elicited force was 118-4+8-6% of post-injury Po for the muscles in the twenty eccentric contraction protocol, which was significantly different from that observed for the other protocols (71-8-80-2 % post-injury P0). These findings indicate that the force deficit in this muscle injury model results from a failure of the excitation process at some step prior to calcium (Ca2+) release by the sarcoplasmic reticulum.4. In an attempt to locate the site of failure, intracellular measurements were made in injured muscles to test whether injury to the sarcolemma might have resulted in a shift of the resting membrane potential of the muscle fibre. However, microelectrode measurements of resting membrane potential for muscles in the twenty eccentric contraction protocol (-74-4 + 0-6 mV) were not different from muscles in the twenty isometric contraction protocol (-73-4+ 1-0 mV). These data suggest that membrane resting conductances were normal and are compatible with MS 1641 G. L. WARREN AND OTHERS the idea that the ability of the injured fibres to conduct action potentials was probably not impaired.

The primary objective of this study was to compare the magnitude of injury in mouse extensor digitorum longus (EDL) and soleus muscles induced by high-force eccentric contractions. A second objective was to study the effect of altering the daily loading of the muscles through hindlimb suspension (HS) on the injury. One of two protocols was performed in vitro: 1) 15 eccentric contractions (n = 20: 10 EDL and 10 soleus muscles) or 2) 15 isometric contractions (n = 20: 10 EDL and 10 soleus muscles). After the protocol, the decrements in contractile performance and lactate dehydrogenase (LDH) release were measured at 15-min intervals over 1 h. Immediately after the eccentric contraction protocol, markedly greater decrements in maximal isometric tetanic force (Po) occurred in the normal EDL than in the normal soleus muscles (60.7 +/- 4.2 vs. 7.6 +/- 2.1%, P < or = 0.0001). LDH release immediately after the eccentric contraction protocol was 2.7-fold greater in the normal EDL than in the normal soleus muscles. To investigate the role of recent loading of the muscles in the injury, EDL (n = 9) and soleus (n = 10) muscles from mice subjected to HS for 14 days performed the eccentric contraction protocol. HS resulted in greater decrements in contractile performance for the soleus muscles (decreases in Po immediately after the protocol for HS and normal soleus muscles were 31.0 +/- 1.8 and 7.6 +/- 2.1%, respectively; P < or = 0.0001) but not for the EDL muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

The purpose of this study was to evaluate the relationship between mitochondrial Ca2+ concentration (MCC) and the extent of muscle injury in rats that have performed prolonged downhill walking (eccentric exercise). MCC was used as an indicator of elevated [Ca2+] in the muscles, and injury was estimated from histochemical analysis of muscle cross sections by determining the numbers of intact fibers per unit area in the muscles. Elevations in MCC in the soleus and vastus intermedius muscles over time postexercise were inversely related (P less than 0.05) to the number of intact fibers per square millimeter in the respective muscles after downhill walking. Verapamil administration attenuated the elevation in MCC and injury in histochemical sections resulting from the downhill walking in soleus muscle, but intraperitoneal injection of the chelators EDTA or ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'- tetraacetic acid significantly attenuated the increases in MCC and injury to both the vastus intermedius and soleus muscles in the downhill walkers. The chelators appear to exert their "protective" effects within the specific muscles that show the injury and do not significantly affect serum [Ca2+]. It is concluded that increases in MCC occur during exercise-induced fiber injury and that elevations in cellular Ca2+ may have a role in the etiology of the injury process.

Previous work has demonstrated that muscular injury in rat soleus muscles resulting from eccentric contractions (downhill walking) is accompanied by elevations in mitochondrial [Ca2+] (MCC). Muscles are stretched during eccentric contractions, and there is evidence in the literature that stretch of the cell membrane induces Ca2+ influx in various tissues, including skeletal muscle. The purpose of this study was to determine if passive stretch of rat soleus muscles will induce increases in total muscle [Ca2+] (TCC) and MCC. Soleus muscles from female rats (51-122 g) were isolated and incubated in vitro for 2 h at resting length (Lo) or at the maximal in situ length (S). TCC (+62%) and MCC (+56%) were elevated in the S muscles. Also, there was a 63% reduction in maximal twitch tension in the S muscles. ATP concentration, phosphocreatine concentration, and lactate release between Lo and S muscles were the same, indicating that impaired metabolism was not responsible for the observed differences in [Ca2+] and force production between Lo and S muscles. Increases in TCC in the S condition indicate that stretch results in Ca2+ influx from the extracellular space, which is supported by the observation that when S muscles were incubated in Ca(2+)-free buffer, TCC and MCC did not increase. High concentrations of verapamil (0.25-0.75 mM) blocked the elevations in TCC and MCC in the S muscles, but the magnitude of the drug concentration required makes it questionable whether the effect resulted from specific blockade of slow voltage-sensitive Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)