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This study used Rana temporaria sartorius muscles to examine the effect of fatiguing electrical stimulation on the gap between the T-tubular and sarcoplasmic reticular membranes (T-SR distance) and the T-tubule diameter and compared this with corresponding effects on resting fibres exposed to a range of extracellular conditions that each replicate one of the major changes associated with muscular activity: membrane depolarisation, isotonic volume increase, acidification and intracellular lactate accumulation. Following each treatment, muscles were immersed in isotonic fixative solution and processed for transmission electron microscopy (TEM). Mean T-SR distances were estimated from orthogonal intercepts to provide estimates of diffusion distances between T and SR membranes and T-tubule diameter was estimated by measuring its shortest axis in the sampled J-SR complexes. Measurements from muscles fatigued by low frequency intermittent stimulation showed significant (P &lt;&lt; 0.05) reversible increases in both T-SR distance and T-tubule diameter from 15.97 +/- 0.37 nm to 20.15 +/- 0.56 nm and from 15.44 +/- 0.60 nm to 22.26 +/- 0.84 nm (n = 40, 30) respectively. Exposure to increasing concentrations of extracellular [K(+)] in the absence of Cl(-) to produce membrane depolarisation without accompanying cell swelling reduced T-SR distance and increased T-tubule diameter, whilst comparable increases in [K(+)](e) in the presence of Cl(-) suggested that isotonic cell swelling has the opposite effect. Acidification alone, produced by NH(4)Cl addition and withdrawal, also decreased T-SR distance and T-tubule diameter. A similar reduction in T-SR distance occurred following exposure to extracellular Na-lactate where such acidification was accompanied by elevations of intracellular lactate, but these conditions produced a significant swelling of T-tubules attributable to movement of lactate from the cell into the T-tubules. This study thus confirms previous reports of significant increases in T-SR distance and T-tubule diameter following stimulation. However, of membrane depolarisation, isotonic cell swelling, intracellular acidification and lactate accumulation, only isotonic cell swelling increases T-SR distance whilst membrane depolarisation and intracellular lactate likely contribute to the observed increases in T-tubule diameter.

Alterations in triad ultrastructure following repetitive stimulation and intracellular changes associated with exercise in amphibian skeletal muscle