Red cells of newborn calves contain 105–110 mmole K+ and 1–5 mmole Na+ per liter of cells. As the animals age the K+ content decreases to a value of 25–30 mmole/liter of cells after about 60 days. At approximately the same time, the sodium content reaches a value of 60–70 mmole/liter. The time required for half change (t½) is 35–37 days for both Na+ and K+. The activity of (Na + K)-adenosine triphosphatase (ATPase) and the influx of K42 and Rb86 into the red cells are high at birth and are reduced to 5 and 15% of their original values, respectively, in mature animals. t½ for both is of the order of 30–35 days. The membrane Mg-ATPase activity is also high at birth and is reduced with a t½ of 28–32 days to a final value of about 20% of its activity at birth. Separation of red cells according to their age showed that, in animals at the age of transition, newly formed red cells contain a higher K/Na ratio and a higher active transport capacity than older red cells of the same animal. It is suggested that the changes observed are a reflection of the average age of the red cell population as the animal grows.

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