Potassium adsorption sites in frog muscle visualized by cesium and thallium under the transmission electron microscope.

Frog muscles in which a major portion of the intracellular K+ was reversibly replaced by Cs+ or Tl+ were frozen, freeze-dried, and embedded in Spurr medium. Electron microscopic observation revealed a well-stained striation pattern of myofibrils, indicating that in the living state heavy tracers of K+ are mainly located in the A bands and Z lines. The findings contradict the membrane theory but are in accordance with the association-induction hypothesis.