A dichtomy of the membrane potential response of rat soleus muscle fibers to low extracellular potassium concentrations.

The effects of different extracellular potassium concentrations [K+]o on the resting membrane potential (EM) of rat soleus muscle fibres was assessed in the absence and presence of 10(-3) M ouabain. At concentrations of 3 mM K and below, the fibres could be divided into two significantly different and normally distributed populations on the basis of EM response: One group responded to a lowering of the [K]o with a graded hyperpolarization to between -80 and -103 mM. The second fibre population had a less negative EM (-70 mV) which did not respond to changes in [K]o between 0 mM and 3 mM. In the [K]o range, 0 mM to 5.9 mM, the mean EM of fibres treated with ouabain was only slightly less negative than the EM of the second fibre population. We conclude that this dichtomy of the mean EM at low [K]o reflects the presence of two fibre types with different electrochemical properties.