REGULATION OF INTRACELLULAR PH IN PYRAMIDAL NEURONS FROM THE RAT HIPPOCAMPUS BY NA+-DEPENDENT CL--HCO3- EXCHANGE

1. We have measured intracellular pH (pH(i)) in freshly isolated pyramidal neurones from the CA1. region of the rat hippocampus using the fluorescent indicator 2',7'-bis(carboxyethyl)-5(and-6)-carboxyfluorescein (BCECB). 2. The neurones selected by our isolation procedure, when studied in the nominal absence of CO2-HCO3-, had a mean steady-state pH(i) of 6.81 +/- 0.02 (n = 163). The recovery of pH(i) from acid loads was very slow. The rate of recovery from acid loads was reduced by Na+ removal, but only very slightly inhibited by 1 mM amiloride. 3. The addition of 5% CO2-25 mM HCO3- caused steady-state pH(i) to increase from 6.74 +/- 0.05 to 7.03 +/- 0.03 (n = 28). In the presence of 5% CO2-25 mM HCO3-, the rate of pH, recovery from acid loads was much faster than in its absence. 4. The HCO3--induced alkalinization was reversible, and did not occur in the absence of extracellular Na+ or in the presence of DIDS (4,4'-diisothyocyanatostilbene-2,2'-disulphonic acid). 5. In the absence of external Cl-, successive exposures to CO2-HCO3- elicited alkalinizations that were progressively reduced in rate and amplitude. This effect, presumably due to gradual depletion of internal Cl-, was rapidly reversed by returning Cl- to the external medium. 6. We conclude that the major acid-extrusion mechanism in pyramidal CA1 neurones is the Na+-dependent Cl--HCO3- exchanger. The Na+-dependent mechanism that operates the nominal absence of HCO3- is far less active.