Activities of sodium and potassium ions in epithelial cells of small intestine.

Intracellular molar Na(+) activity (a(Na)), measured with cation-selective glass microelectrodes, in epithelial cells of isolated bullfrog small intestine immersed at 26 degrees 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).