K-contractures and membrane potential in mammalian skeletal muscle

Abstract

A FUNDAMENTAL property of skeletal muscle fibres is the ability to develop tension in response to surface membrane depolarisation. The relation between membrane potential (V_m) and tension has been measured in amphibian muscle using external potassium concentration (K₀) to clamp V_m ^1–4. A K-contracture follows exposure to high K₀ and tension is graded with K₀ between 20 mM (V_m = −50 mV) and 50 mM (V_m= −40 mV). Human intercostal muscle, however, either does not respond mechanically to K₀ of 80 mM or gives a very small contracture⁵. It is not clear whether this is a result of diffusion delays in the large bundles of fibres or whether there is a basic species difference in excitation–contraction coupling. I report here that, in bundles of fewer than 20 fibres, the relationship between K-contracture tension and K₀ is shifted to higher K₀ in rat muscle because changes in K₀ are less effective in altering V_m.