Potassium concentration changes in the transverse tubules of vertebrate skeletal muscle

Vertebrate skeletal muscle fibers have evolved a network of narrow tube-like invaginations of the cell membrane. This 'transverse tubular system' (TTS) provides a pathway for radial impulse propagation from the cell surface to the interior. As a consequence of electrical activity in the TTS, impulses are followed by 'early' and 'late' afterdepolarizations. The late afterdepolarization is now believed to be due mainly to K⁺-accumulation in the TTS. Excessively large afterdepolarizations of this type may be the cause of the 'myotonic discharge' observed in mammals suffering from pathologically low muscle membrane permeability to chloride. Potassium concentration changes in the transverse tubular system can also be induced artificially under voltage-clamp conditions. Analysis of K⁺-depletion under voltage clamp allows conclusions about the localization of K⁺-permeability as well as speed of K⁺-diffusion in the transverse tubules.