Generation of tension by glycerol-extracted vertebrate skeletal muscle fibres in the absence of calcium.

When a small bundle of glycerol-extracted fibres from either frog, tortoise or rabbit skeletal muscle was first exposed to high MgATP (5 mM) in the absence of Ca2+ (less than 1 nM) and at low ionic strength (less than 0.11) at 20 degrees C, it produced a single sharp transient contraction followed by a lower maintained isometric tension. The maintained tension was investigated further in rabbit psoas fibres. Ca2+-free tension was dependent on the ionic strength in the range 0.04-0.10, on the temperature in the range 6-20 degrees C and the free Mg2+ in the range 0-6 mM. It was promoted by low ionic strength, low Mg2+ and high temperature, and was unaffected by varying the MgATP2- in the range 0.4-4 mM and by adding ATP regenerating components. A separate regime of tension generation was detected at MgATP2- concentrations of less than 0.1 mM, in which MgATP2- concentration was critical. The results are interpreted on the assumption that binding of Mg2+ to some component of the regulatory system is necessary to maintain its inhibitory effect in the absence of Ca2+. Ionic strength and temperature, on the other hand, may affect actomyosin directly.