Distribution of Na+, K+ and Cl− between nucleus and cytoplasm inChironomus salivary gland cells

Summary Previous studies of the electrochemical activity coefficients of intracellular Na⁺ and K⁺ have suggested that the free form of these ions may be unevenly distributed within the intracellular fluids. One possible site of such subcellular compartmentalization is the cell nucleus. In order to examine this possibility, the cells ofChironomus salivary glands were studied with conventional liquid ion-exchange microelectrodes sensitive to K⁺ and Cl^–, with a new liquid ion-exchange microelectrode sensitive to Na⁺, and with open-tipped micropipets. Both the electrochemical activities for Na⁺, K⁺ and Cl^–, and the electrical potential were the same on both sides of the nuclear membrane. The possibility was considered that a difference in the junction potentials within the nucleoplasm and cytoplasm might have masked a real difference in electrical potential between these two phases. To study that possibility, changes were induced in the junction potentials by altering the composition of the fluid filling the exploring micropipets. The results have suggested that the magnitudes of the junction potentials are the same on both sides of the nuclear envelope. The simplest interpretation of the data is that the chemical activities of Na⁺, K⁺ and Cl^– are the same within the nucleus and cytoplasm. This suggests that other subcellular structures, possibly the endoplasmic reticulum and mitochondria, are responsible for subcellular compartmentalization.