Eccentric contraction-induced injury in mammalian skeletal muscle
Abstract of thesis entitled
?ccentric contraction-induced injury in mammalian skeletal muscle ?submitted by Ella Wai YEUNG
for the degree of Doctor of Philosophy
at the University of Hong Kong in February, 2003
Eccentric contractions , in which muscles are lengthened during contraction, may injure skeletal muscle but the mechanism(s) for this remain uncertain. The hypothesis tested is that alterations in intracellular concentrations of ions such as Na+ or H+ may underlie some of the functional impairment. The initial phase of eccentric contraction-induced injury was investigated: the influence of eccentric contraction on developed force, intracellular pH, Na+ homeostasis and T-tubule morphology was examined; the roles of these changes in the development of muscle damage are discussed.
Single fibres from the flexor brevis muscle of mice or small bundles of fibres from the soleus or extensor digitorum longus muscle of rats were dissected. Muscles underwent either 10 isometric tetani (controls) or 10 eccentric tetani, during which a 30 or 40 % stretch of the optimal length (Lo) was applied. Eccentrically-contracted muscles showed 3 characteristic features of stretch-induced damage: (i) reduced maximal force, (ii) greater reduction of force at low stimulation frequencies, (iii) shift in L o to a longer muscle length. Ten isometric tetani or stretches of resting fibres reproduced none of these features.
Intracellular pH ( pHi) was determined in rat soleus muscle with the fluorescent indicator BCECF. The resting pHi was more acidic after eccentric contractions (6.80 ?0.06) than after isometric contractions (6.97 ?0.04) . The rate of pH i recovery following an acid load was reduced from 0.022 + 0.003 units
min- 1 following isometric contractions to 0.013 + 0.002 units min-1 following eccentric contractions. The results suggested that the ability of the muscle to regulate pHi was impaired after eccentric contractions, which may partially explain the reduction in force.
T-tubule morphology and function were studied in single mouse muscle fibres with confocal microscopy. Following eccentric contractions, vacuoles connected to the T-tubules appeared, and the diffusion of an extracellular marker (sulforhodamine B) from the T-tubules was slowed to a half time 6.3 ?2.4 min, compared to 18 ?1 s in isometric controls.
[Na+]i measurements were performed with the fluorescent indicator SBFI or sodium green. Isometric tetani had no detectable effect on [Na+]i (7.2 ?0.5 mM), whereas eccentric contraction increased [Na+]i to 16.3 ?1.6 m M. Confocal images showed a uniform increase in [Na+]i after eccentric tetani with no localized elevations of [Na+]i. Gadolinium, a blocker of stretch-sensitive channels prevented the rise of [Na+]i and reduced the force deficit after eccentric damage.
The slow extrusion of intracellular protons following eccentric contractions may be explained by the rise in [Na+]i which would be expected to reduce the inward Na+ gradient and hence slow proton efflux. The Na+ may enter by very small and widely distributed membrane tears, or alternatively through stretch-sensitive channels which remain open for many minutes after eccentric contractions. The vacuoles may result from osmotic stresses involved in pumping out the excess Na+.
The functional consequences of damage to the surface membrane or the T-system warrant further investigation. (Word count: 476)
School:The University of Hong Kong
School Location:China - Hong Kong SAR
Source Type:Master's Thesis
Keywords:muscle contraction muscles wounds and injuries contractility biology
Date of Publication:01/01/2003