Intracellular pH regulation of CA1 neurons in Na+/H+ isoform 1 mutant mice

To understand the role of Na+/H+ exchanger 1 (NHE1) in intracellular pH (pH (i)) regulation and neuronal function, we took advantage of natural knockou t mice lacking NHE1, the most ubiquitously and densely expressed NHE isofor m in the central nervous system (CNS). CA1 neurons from both wildtype (WT) and NHE1 mutant mice were studied by continuous monitoring of pH(i), using the fluorescent indicator carboxy-seminaphthorhodafluor-1 (SNARF-1) and con focal microscopy. In the nominal absence of CO2/HCO3-, steady-state pH(i) w as higher in WT neurons than in mutant neurons. Using the NH4Cl prepulse te chnique, we also show that H+ flux in WT neurons was much greater than in m utant neurons. The recovery from acid load was blocked in WT neurons, but n ot in mutant neurons, by removal of Na+ from the extracellular solution or by using 100 mu M 3-(methylsulfonyl-4-piperidinobenzoyl)-guanidine methanes ulfonate (HOE 694) in HEPES buffer. Surprisingly, in the presence of CO2/HC O3-, the difference in H+ flux between WT and mutant mice was even more exa ggerated, with a difference of more than 250 mu M/s between them at pH 6.6. H+ flux in CO2/HCO3- was responsive to diisothiocyanato-stilbene-2,2'-disu lfonate (DIDS) in the WT but not in the mutant. We conclude that (a) the ab sence of NHE1 in the mutant neurons tended to cause lower steady-state pH(i ) and, perhaps more importantly, markedly reduced the rate of recovery from an acid load; and (b) this difference in the rate of recovery between muta nt and WT neurons was surprisingly larger in the presence, rather than in t he absence, of HCO3-, indicating that the presence of NHE1 is essential for the regulation and/or functional expression of both HCO3--dependent and -i ndependent transporters in neurons.