Oocytes of Rana pipiens were incubated in 22Na-Ringer's solution and 22Na-23Na exchange in the nucleus and cytoplasm followed by low-temperature microdissection. Cytoplasmic sodium consists of 2 kinetic fractions: the larger (88%) slowly exchanging (t½ ∼ 2 days), and the smaller (12%) rapidly exchanging (t½ ∼ 53 min). Nuclear sodium consists of a single fraction whose concentration is similar and whose rate constant is identical with that of the rapidly exchanging cytoplasmic fraction. Our findings are consistent with a model of compartmentalization in which rapidly exchanging sodium is nearly uniformly distributed in the water of the nucleus and cytoplasm and its exchange limited by movement through the cell surface. On the other hand, slowly exchanging sodium is restricted to the cytoplasm, sorbed to or sequestered in some as yet unidentified cytoplasmic constituent.
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