Activities of Sodium and Potassium Ions in Epithelial Cells of Small Intestine

Abstract

Intracellular molar Na⁺ activity (a_Na), measured with cation-selective glass microelectrodes, in epithelial cells of isolated bullfrog small intestine immersed at 26°C in a sodium sulfate Ringer solution containing mannitol was 0.0144 ± 0.0031 (average value plus or minus standard deviation). The corresponding K⁺ activity (a_K) was 0.0854 ± 0.0060. Combination of these values with previous estimates of intracellular Na⁺ and K⁺ concentrations under identical conditions indicated that a substantial fraction of the cellular Na⁺ is in an osmotically inactive state. When the cells were exposed to a Ringer solution in which 26 millimoles of mannitol per liter were replaced by 3-O-methyl glucose, highly significant decreases in a_K and a_Na were observed. The decrease in a_K was proportionately larger than the corresponding decrease in a_Na.