Mounding phenomenon: an experimental study in vitro

"Mounding" is a local contracture of skeletal muscle that is induced by tapping the muscle directly. We studied the mounding phenomenon in vitro on rat muscles that were percussed by a rotating hammer. In the bulged portion of muscle, there were scattered areas where the sarcomere length was shortened. After muscle was depolarized by the application of KCl-Ringer solution or relaxing solution, the mounding phenomenon was preserved. Agents that abolish the function of T-tubule or T-SR junction did not alter the phenomenon, either. However, after the function of intracellular membrane system such as the sarcoplasmic reticulum (SR) was destroyed by Brij 58, the mounding disappeared. Thus, the mounding is a local contracture that is probably induced by calcium ion liberated from the SR by percussion.