The suppression of the late after-potential in rubidium-containing frog muscle fibers.

The late after-potential that follows trains of impulses in frog muscle fibers is virtually absent when most of the intracellular potassium is replaced by rubidium and the muscle is immersed in rubidium-containing Ringer's fluid. Its amplitude is also reduced in freshly dissected, potassium-containing muscle fibers that are immersed directly in Rb-Ringer's fluid. These findings are discussed in terms of the model for muscle membrane of Adrian and Freygang (1962 a, b) and in relation to the report of Adrian (1964) that Rb-containing muscle fibers do not exhibit the variations in potassium permeability as a function of membrane potential that are found in fibers with normal intracellular potassium concentration immersed in Ringer's fluid.