APPS November 2002 Meeting Abstract 1363

Ella W. Yeung², David G. Allen¹, 2 Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, 1 Department of Physiology and Institute for Biomedical Science, University of Sydney, NSW 2006.

Muscles which are stretched during contraction (eccentric contraction) are liable to damage¹. We recently showed that following eccentric contractions vacuoles developed attached to the T-tubules and the rate of diffusion of an extracellular marker (sulforhodamine B) from the T-tubules was slowed². We postulated that Na⁺ leaked into the fibres through membrane tears causing activation of the Na⁺ pump. The vacuoles were thought to develop because the osmotic load of Na⁺ and H₂0 pumped out of the cell exceeded the capacity of the T-tubules. In the present study we tested several aspects of this hypothesis; (i) does [Na⁺]_i rise following eccentric contraction and (ii) does the Na⁺ enter through tears in the membrane.

Single fibres or small bundles of fibres were dissected from the flexor brevis muscle of mice. Intracellular sodium was measured either with SBFI or sodium green. The muscles were stimulated either with 10 isometric tetani or 10 eccentric tetani (40% L_o stretch imposed over 100 ms). Eccentric tetani reduced force to 34 + 4 % while isometric tetani had no effect. Isometric tetani had no detectable effect on [Na⁺]_i (resting level 7.3 + 0.2 mM) where after eccentric contraction [Na⁺]_i rose slowly reaching a peak of 16.3 + 1.6 mM after 10 min. To test with the increased [Na⁺]_i was via membrane tears we imaged [Na⁺]_i with a confocal microscope. [Na⁺]_i increased uniformly after eccentric tetani; no localised elevations of [Na⁺]_i were detected which might be expected if discrete membrane tears caused Na⁺ entry.

These results confirm that a large rise in [Na⁺]_i occurs following eccentric contractions. We could not detect localised elevations of [Na⁺]_i and this results is consistent with Na⁺ entering by very small and widely distributed membrane tears or, alternatively, through a class of Na⁺ permeable channels which are open for many minutes after eccentric contractions³.