Speed of repolarization and morphology of glycerol‐treated frog muscle fibre

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Abstract
1. Single muscle fibres from frog semitendinosus were subjected to sudden changes in [K]o, while recording membrane potential.2. In agreement with Hodgkin & Horowicz (1960), a sudden increase in [K]o in normal fibres produced a rapid depolarization (half‐time 0·3 sec), whereas a sudden decrease in [K]o produced a slower repolarization (half‐time 2‐3 sec).3. Fibres were subjected to ‘glycerol‐treatment’, a procedure which was supposed to produce a functional disconnexion of the T‐system from the surface. In these glycerol‐treated fibres both depolarization and repolarization induced by changes of [K]o took place rapidly.4. The results suggest that the slowness of the repolarization in normal fibres is due to a retention of K ions inside the T‐tubules.5. Electron microscopical observation of single fibres or bundles of fibres, which have been soaked in a Ringer containing ferritin, revealed that normal fibres contained ferritin particles in the T‐system, while glycerol‐treated fibres showed no ferritin. Except for the presence of some large vacuoles and some swelling of the T‐system, glycerol‐treated fibres appeared morphologically normal.6. Prolonged soaking in a high potassium solution produced electrical effects suggesting that K ions can enter the tubules of treated fibres very slowly, in spite of their inaccessibility to ferritin.7. The main effect of glycerol‐treatment does not seem to be a total disconnexion of the T‐system from the fibre surface, but rather constriction of the T‐tubules near their openings to the exterior.

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