Lattice structure and force development in skinned striated muscle

Recently it was suggested that the length dependence of Ca-sensitivity curves (developed tension against pCa, the negative logarithm of the Ca concentration) might be located in the lattice structure of the actin-myosin matrix or more specifically the radial spacing of the filaments. To test this hypothesis the authors changed the volume of skinned fibre preparations (to achieve different fibre diameters at the same sarcomere length) by osmotic compression. The authors used freeze-dried skinned single skeletal (rabbit M. gracilis) fibres and thin strips of papillary muscle taken from the right ventricle of the rabbit. It was shown that for sarcomere lengths up to 2.8 mu m the Ca-sensitivity curves shifted to higher pCa-values. The same effect is observed when the preparation is simply lengthened. since in both cases the fibre diameter is reduced (in the first case by osmotic compression, in the second case because of constant volume behaviour of the fibre) this indicates that the length dependence of force development is at least partially determined by radial filament spacing.