Intracellular chloride activity in glial cells of the leech central nervous system.

1. Chloride-sensitive double-barrelled microelectrodes were used to measure the intracellular Cl- activity (aicl) and membrane potential (Em) in neuropile glial cells of the leech, Hirudo medicinalis. 2. A close relation between the equilibrium potential for Cl- (ECl = -66.1 +/- 4.9 mV; mean +/- S.D.) and the resting potential (Em = -67.8 +/- 5.2 mV; n = 19) was observed in nominally CO2-HCO3(-)-free, HEPES-buffered solutions. A saline buffered with 2% CO2, 11 mM-HCO3- elicited a membrane hyperpolarization and a concomitant decrease of aCl. 3. Changes in ECl followed these of Em with a lag of less than 30 s in response to various extracellular K+ concentrations [( K+]o) or due to bath-application of carbachol or serotonin. 4. Introduction of a Cl(-)-free solution resulted in a transient depolarization indicating a substantial Cl- conductance and a rapid decrease of aiCl to an apparent value of 0.5-0.9 mM. 5. The loop diuretics furosemide (1 mM) and bumetanide (0.2 mM) did not affect the K(+)-induced changes of aiCl. 6. The results indicate a passive Cl- distribution across the membrane of leech neuropile glial cells as a result of a high Cl- conductance.