Stimulation of the Na+ pump by hypotonic solutions in skeletal muscle.

The fractional loss of 22 Na+ from frog sartorius muscle is increased when the tonicity of the external solution is reduced. The effect, which is larger the lower the osmolarity, exhibits the following characteristics: (1) quick onset and reversibility, (2) is not reduced in the absence of external Na+, (3) is completely abolished by strophanthidin (3. 10-5 M), (4) is neither the result of membrane depolarization nor K+ accumulation in the extracellular space.