Glycogen content and contractile responsiveness to T-system depolarization in skinned muscle fibres of the rat.

1. Glycogen content (determined microfluorometrically), response capacity to transverse tubular (T) system depolarization and the relationship between these two parameters were examined in single, mechanically skinned fibres from rat extensor digitorum longus (EDL) muscle in the presence of high and constant concentrations of ATP and creatine phosphate. 2. The mean total glycogen content (tGlyc) in freshly dissected fibres was 58.1 +/- 4.2 mmol glucosyl units/L fibre (n = 53). 3. A large proportion of tGlyc was retained in the skinned fibres (SFGlyc) after 2 and 30 min exposure to an aqueous relaxing solution (73.1 +/- 2.8 and 64 +/- 12.3%, respectively). 4. When fibres were incubated for 30 min in a high (30 micromol/L)-Ca2+ solution, the proportion of SFGlyc was markedly lower (approximately 28%), which suggests that rat skinned fibres contain a Ca2+-sensitive glycogenolytic system. 5. In rat skinned fibres, T-system depolarization-induced Ca2+ release was not accompanied by a detectable loss of fibre glycogen and there was no correlation between response capacity and initial SFGlyc, indicating that other factors, unrelated to glycogen depletion, ultimately limited the capacity of rat skinned fibres to respond to T-system depolarization. 6. It is concluded that rat mechanically skinned fibre preparations are well suited for studies of glycogenolysis at a cellular level and that, with further refinement of the depolarization protocol, they may be suitable for studies of the non-metabolic role of glycogen in mammalian skeletal muscle contractility.