Changes in the t-tubular system following eccentric muscle damage

Abstract

PURPOSE:To examine the morphology and functional status of the T-tubular system following an eccentric contraction protocol. METHODS:Single fibers were dissected from mouse flexor digitorum brevis muscles. The fibers were stretched by 40% of their optimal length for a series of ten tetani. The T-tubules were examined using confocal microscopy and an extracellular fluorescent dye, sulforhodamine B. The time course of the dye washout was used as an estimate of the accessibility of the T-system to the extracellular space. RESULTS:Following eccentric damage, the fibers showed a reduction in force to 31 ± 2.7% of control, a shift in the peak length-tension by 1.24 ± 0.02 and a steepening of the force-frequency relation. This damage was associated with the development of vacuoles connected to the T-tubules. The rate of diffusion of the dye in and out of the T-system was significantly slowed (P < 0.01) with a half-time of 6.32 ± 2.4 min. Ouabain, a sodium pump inhibitor, inhibited the formation of vacuoles suggesting that the sodium pump is essential for their development. CONCLUSION:We propose that in eccentric contractions, T-tubules suffer shearing damage caused by sarcomere inhomogeneities. This leads to a localized increase in intracellular sodium ions. This sodium, accompanied by water, is pumped out of the cell by the sodium pump. the resulting volume load exceeds the capacity of the T-tubules and results in vacuole production.