Membrane potential dependence of intracellular pH regulation by identified glial cells in the leech central nervous system

Abstract

1. We have measured the intracellular pH (pHi) and membrane potential of identified glial cells in the central nervous system of the leech, Hirudo medicinalis, using double-barrelled pH-sensitive microelectrodes. 2. When extracellular K+ concentration was increased, the glial membrane potential decreased and pHi increased; lowering the extracellular K+ concentration hyperpolarized the glial membrane and decreased pHi. These pHi changes were largely dependent upon the presence of CO2-HCO3-; in nominally CO2-HCO3(-)-free saline solution, they were 50-80% smaller. 3. The steady-state pHi of the glial cells in CO2-HCO3(-)-buffered saline solution strongly correlated with the membrane potential between -40 and -90 mV. The slope of this relationship was 60 mV/pH unit. 4. The neurotransmitter 5-hydroxytryptamine (50 microM), which hyperpolarizes the glial membrane, also produced a large, CO2-HCO3(-)-dependent decrease in pHi. The size of the pHi change depended upon the amplitude of the membrane hyperpolarization. 5. The increase in pHi produced by the membrane depolarization in 20 mM-K+ was abolished in Na(+)-free saline. Removal of external Na+ in the presence of 20 mM-K+ reversed the pHi increase. 6. The pHi increase in 20 mM-K+ was also inhibited by the stilbene 4,4-diisothiocyanostilbene-2'-disulphonic acid (DIDS, 0.5 mM). In a DIDS-poisoned preparation a small decrease of pHi was observed in 20 mM-K+ both in the presence and nominal absence of CO2-HCO3-. 7. In neurones, neither CO2-HCO3- nor 20 mM-K+ produced an intracellular alkanization. The steady-state pHi of several identified neurones was not correlated with the membrane potential. 8. We conclude that in glial cells, but not in neurones, the pHi is dependent upon the membrane potential. This membrane potential dependence is due to the activity of the electrogenic Na(+)-HCO3- co-transporter in the glial cell membrane.