Change of intracellular K+ activity in rat soleus muscle during hypokalemia

The relationship between intracellular total K+ concentration [( K]i) as determined by a flame spectrophotometer and intracellular K+ activity (aKi) as determined by an ion-selective microelectrode was studied in soleus muscle of rats on a diet deficient in K+ for 40 days. [K]i began to fall immediately from the initial stage of hypokalemia, while aKi was well-maintained for 15 days. Then, aKi decreased gradually. The measured resting potential (Em) hyperpolarized beyond the EK was calculated from aKi in hypokalemic rat muscle from day 20 to 40. A rapid increase in aKi occurred over 3 hours in soleus muscle of hypokalemic rats for 5 to 6 weeks. It was concluded that the bound intracellular K+ acts as a buffer for aKi in hypokalemic rat muscle, that Em exceeds EK because the Na+-K+ pump is stimulated by increased [Na]i and that the increase in aKi after denervation is due to the removal of a Na+-K+ pump inhibitor normally released from nerve ending.