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

Abstract

The intracellular Na activity,aNa_i, and the intracellular Ca activity,aCa_i, were measured with double-barrelled neutral carrier Na⁺- and Ca²⁺-sensitive microelectrodes in Retzius neurones in the central nervous system of the leechHirudo medicinalis. TheaNa_i 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. TheaCa_i 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), theaNa_i reversibly increased severalfold. WhenaNa_i increased to high levels following complete pump inhibition, theaCa_i increased above 10⁻⁶ M, and the membrane input resistance decreased. After removal of ouabain,aNa_i,aCa_i and the membrane resistance recovered within 30 min after a delay of 20–40 min. Our results suggest that a large increase ofaNa_i produces a rise inaCa_i, possibly by means of a Na⁺−Ca²⁺ exchange across the cell membrane. The elevation of theaCa_i 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.