Delayed rectification in the transverse tubules: origin of the late after-potential in frog skeletal muscle.

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Abstract
Tetanic stimulation of [frog] skeletal muscle fibers elicited a train of spikes followed by a long-lasting depolarization called the late after-potential (LAP). Isolated single muscle fibers were treated with a high K solution (5 mM or 10 mM K) followed by a sudden reduction of K concentration to 2.5 mM. This procedure produced a slow repolarization (K repolarization), which reflected a diffusional outflow of K from inside the lumen of the transverse tubular system (T system). Tetanic stimulation was then applied to the same fiber and the LAP was recorded. The time courses of K repolarization and LAP decay were compared and were roughly the same. This approximate equality held under various conditions that changed the time courses of both events over a wide range. Both K repolarization and the LAP became slower as fiber radius increased. LAP decay and K repolarization represent the same process. The LAP was caused by K accumulation in the T system. Delayed rectification channels existed in the T system. The density of delayed rectification channels was less in the T system than in the surface membrane.

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