Change in intracellular calcium ion concentration induced by caffeine and rapid cooling in frog skeletal muscle fibres

Abstract

In a single skeletal muscle fibre treated with concentrations of caffeine below threshold for caffeine contracture, rapid lowering of the temperature of the bathing solution from 18 degrees C to below 5 degrees C induced a contracture (rapid cooling contracture). Intracellular Ca2+ concentration ([Ca2+]i) was recorded during rapid cooling contracture using aequorin. Low concentrations of caffeine often caused a slight elevation of the light signal in resting muscle without detectable tension. During rapid cooling contracture, the change in light signal occurred in three phases. The first phase was a transient change of [Ca2+]i accompanying slight tension. During the second phase, the light signal slowly increased as cooling produced maximum tension development. The third phase was an additional light signal induced after the second phase, even though the tension was saturated. The second and third phases were more sensitive to low concentrations of procaine (0.2-0.5 mM) than the first phase. Synchronous oscillations of light and tension were often observed during the second phase. The light signal during rapid cooling contracture was only slightly affected by long incubation in Ca-free or Ca-rich solutions. These results are interpreted as follows. A low concentration of caffeine elevates cytoplasmic resting Ca2+ level without tension development. The oscillations of light and tension often observed in the second phase might represent a cyclic release of Ca2+ from the sarcoplasmic reticulum (s.r.). The third phase is considered to be due to a massive Ca2+ release by a Ca-induced Ca-release mechanism which might be similar to that in skinned fibres. The second phase is probably essential for generation of rapid cooling contracture tension and the third phase represents an excess Ca2+ for tension development.