pH Regulating Transporters in Neurons from Various Chemosensitive Brainstem Regions in Neonatal Rats

We studied the membrane transporters that mediate intracellular pH (pH _i ) recovery from acidification in brainstem neurons from chemosensitive regions of neonatal rats. Individual neurons within brainstem slices from the retrotrapezoid nucleus (RTN), the nucleus tractus solitarii (NTS), and the locus coeruleus (LC) were studied using a pH-sensitive fluorescent dye and fluorescence imaging microscopy. The rate of pH _i recovery from an NH ₄ Cl-induced acidification was measured, and the effects of inhibitors of various pH-regulating transporters determined. Hypercapnia (15% CO ₂ ) resulted in a maintained acidification in neurons from all three regions. Recovery in RTN neurons was nearly entirely eliminated by amiloride, an inhibitor of Na ⁺ /H ⁺ exchange (NHE). Recovery in RTN neurons was blocked ∼50% by inhibitors of isoform 1 of NHE (NHE-1) but very little by an inhibitor of NHE-3 or by DIDS (an inhibitor of HCO ₃ -dependent transport). In NTS neurons, amiloride blocked over 80% of the recovery, which was also blocked ∼65% by inhibitors of NHE-1 and 26% blocked by an inhibitor of NHE-3. Recovery in LC neurons, in contrast, was unaffected by amiloride or blockers of NHE isoforms but was dependent on Na ⁺ and increased by external HCO ₃ ⁻ . On the basis of these findings, pH _i recovery from acidification appears to be largely mediated by NHE-1 in RTN neurons, by NHE-1 and NHE-3 in NTS neurons, and by a Na- and HCO ₃ -dependent transporter in LC neurons. Thus, pH _i recovery is mediated by different pH-regulating transporters in neurons from different chemosensitive regions, but recovery is suppressed by hypercapnia in all of the neurons.