Intracellular Na+ and Ca2+ in leech Retzius neurones during inhibition of the Na+?K+ pump

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Abstract
The intracellular Na activity,aNai, and the intracellular Ca activity,aCai, were measured with double-barrelled neutral carrier Na+- and Ca2+-sensitive microelectrodes in Retzius neurones in the central nervous system of the leechHirudo medicinalis. TheaNai was measured to be 8.0 mM (corrected for Ca interference), which corresponds to a cytoplasmic Na+ concentration of 10.7 mM, assuming a Na activity coefficient of 0.75. The calculated Na+ equilibrium potential was 59 mV, giving a total Na+ electrochemical gradient of approximately 102 mV. TheaCai was found to range between 1 and 5×10−7 M, from which a Ca2+ equilibrium potential near +120 mV was estimated. When the Na+−K+ pump was inhibited by lowering the external K+ concentration or by adding the glycoside ouabain (5×10−4 M), theaNai reversibly increased severalfold. WhenaNai increased to high levels following complete pump inhibition, theaCai increased above 10−6 M, and the membrane input resistance decreased. After removal of ouabain,aNai,aCai and the membrane resistance recovered within 30 min after a delay of 20–40 min. Our results suggest that a large increase ofaNai produces a rise inaCai, possibly by means of a Na+−Ca2+ exchange across the cell membrane. The elevation of theaCai may be responsible for the decrease in membrane resistance, and may also be related to the uncoupling of the paired Retzius neurones observed in the presence of Na+−K+ pump inhibitors.