Intracellular sodium in mammalian muscle fibers after eccentric contractions

The effect of eccentric contractions on intracellular Na+concentration ([Na+]i) and its distribution were examined in isolated rat and mouse muscle fiber bundles. [Na+]iwas measured with either Na+-binding benzofuran isophthalate or sodium green. Ten isometric contractions had no significant effect on force (measured after 5 min of recovery) and caused no significant change in the resting [Na+]i(7.2 ± 0.5 mM). In contrast 10 eccentric contractions (40% stretch at 4 muscle lengths/s) reduced developed force at 100 Hz to 45 ± 3% of control and increased [Na+]ito 16.3 ± 1.6 mM (n = 6; P < 0.001). The rise of [Na+]ioccurred over 1-2 min and showed only minimal recovery after 30 min. Confocal images of the distribution of [Na+]ishowed a spatially uniform distribution both at rest and after eccentric contractions. Gd3+(20 μM) had no effect on resting [Na+]ior control tetanic force but prevented the rise of [Na+]iand reduced the force deficit after eccentric damage. These data suggest that Na+entry after eccentric contractions may occur principally through stretch-sensitive channels.