The Location of Muscle Calcium with Respect to the Myofibrils

Autoradiographs have been prepared from frog toe muscles soaked in Ca⁴⁵ and fixed with an osmium-oxalate solution. A majority of the grains over the A bands were over the myofibrils. The grain density over the I bands was greatest over the space between the myofibrils. The significance of this distribution is discussed in the light of previous information about the longitudinal distribution of Ca⁴⁵ in skeletal muscle.