Electrical responses of Retzius cells of the leech to inhibition of active ionic transport by ouabain

Effects of tetraethylammonium-chloride and divalent cations on the afterhyperpolarization following repetitive firing in leech neurons

In leech Retzius cells, repetitive activity evoked a prolonged Ca2+-dependent after-hyperpolarization (PAH) (30-60 s) accompanied by an increase in input conductance. PAH persisted in Retzius cells, as well as in nociceptive (N) cells, when Sr2+ but not Mg2+ was substituted for Ca2+. In the presence of tetraethylammonium-chloride (TEA) or Ba2+, PAH was replaced by a Ca2+-dependent, Mg2+-blockable depolarization which was present in the order N greater than R. Careful study of the differences in such phenomena in identified cells may improve our understanding of the differential susceptibility of various neurons to hyperexcitability.

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

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 leech Hirudo medicinalis. The aNai 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. The aCai was found to range between 1 and 5 X 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 X 10(-4) M), the aNai reversibly increased severalfold. When aNai increased to high levels following complete pump inhibition, the aCai 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 of aNai produces a rise in aCai, possibly by means of a Na+-Ca2+ exchange across the cell membrane. The elevation of the aCai 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.