Nuclear sodium and potassium

Na + and K + influence gene expression 1–3 , and it is important to know the concentrations and chemical activities of these cations in the cell nucleus and to understand their control. However, measurements are hampered by the small size of nuclei, and also because (1) the steep Na + and K + activity gradients across cell membranes can cause experimental manipulations to provoke rapid (∼0.1 s half-time for a 10 µm cell) artefactual cation redistributions; (2) intracellular activities cannot be directly inferred from concentrations because Na + , K + and water exist partly in bound form. We circumvent these difficulties by using a giant cell (amphibian oocyte), cryogenic methods to limit diffusion, and an artificial intracellular reference phase 4,5 . Century et al. 6 found K + more concentrated in the oocyte nucleus than in cytoplasm, and the reverse for Na + . Our data for the Desmognathus o. ochrophaeus (salamander) oocyte are similar: the nucleus contains 30±3 µequiv Na + and 144±3µequiv K + per ml H 2 O; the cytoplasm contains 87±2 µequiv Na + and 87±2 µequiv K + per ml H 2 O (means±s.e.m.). These data could imply that the nucleus actively transports Na + and K + , but strong evidence suggests otherwise: the nuclear envelope, with pores of effective radius 45 Å (ref. 7), is too permeable to maintain small solute concentration gradients 8–10 , and microelectrodes measure equal nuclear and cytoplasmic cation activities 11–13 . Here, we confirm and extend these findings, showing that nucleus/cytoplasm cation concentration differences are due to (1) partial exclusion of diffusive Na + and K + by cytoplasm and (2) differential Na + and K + binding by nucleus and cytoplasm.