The measurement of sodium concentration in human red blood cells.

Experiments are described which indicate that iodinated human serum albumin underestimates the amount of extracellular sodium trapped in the packed layer of red blood cells, when cells and plasma are separated by centrifugation. Sucrose-(14)C also underestimates the amount of trapped extracellular sodium, but the difference between the percentages of sucrose-(14)C and extracellular sodium trapped is constant and independent of mean relative centrifugal force. It is concluded that human red blood cell sodium concentration can be measured with accuracy (a) if trapped plasma sodium is estimated with radioisotopes of sodium and a correction made for entry of sodium into the cells, providing cells and plasma can be separated rapidly; (b) by the use of sucrose as a standard plasma marker to derive the amount of trapped plasma sodium; (c) by washing the cells with sodium-free solutions. Reported values for red blood cell sodium concentration in healthy adults are critically reviewed.