Respiratory response to systemic inhibition of the Na+/H+ exchanger type 3 in intact rats

Intracellular pH regulation by acid-base transporters in mammalian neurons

Intracellular pH (pH_i) regulation in the brain is important in both physiological and physiopathological conditions because changes in pH_i generally result in altered neuronal excitability. In this review, we will cover 4 major areas: (1) The effect of pH_i on cellular processes in the brain, including channel activity and neuronal excitability. (2) pH_i homeostasis and how it is determined by the balance between rates of acid loading (J_L) and extrusion (J_E). The balance between J_E and J_L determine steady-state pH_i, as well as the ability of the cell to defend pH_i in the face of extracellular acid-base disturbances (e.g., metabolic acidosis). (3) The properties and importance of members of the SLC4 and SLC9 families of acid-base transporters expressed in the brain that contribute to J_L (namely the Cl-HCO₃ exchanger AE3) and J_E (the Na-H exchangers NHE1, NHE3, and NHE5 as well as the Na⁺- coupled HCO₃⁻ transporters NBCe1, NBCn1, NDCBE, and NBCn2). (4) The effect of acid-base disturbances on neuronal function and the roles of acid-base transporters in defending neuronal pH_i under physiopathologic conditions.

CO2 chemoreception in cardiorespiratory control

Considerable progress has been made elucidating the cellular signals and ion channel targets involved in the response to increased CO2/H+ of brain stem neurons from chemosensitive regions. Intracellular pH (pHi) does not exhibit recovery from an acid load when extracellular pH (pHo) is also acid. This lack of pHi recovery is an essential but not unique feature of all chemosensitive neurons. These neurons have pH-regulating transporters, especially Na+/H+ exchangers, but some may also contain HCO3--dependent transporters as well. Studies in locus ceruleus (LC) neurons have shown that firing rate will increase in response to decreased pHi or pHo but not in response to increased CO2 alone. A number of K+ channels, as well as other channels, have been suggested to be targets of these pH changes with a fall of pH inhibiting these channels. In neurons from some regions it appears that multiple signals and multiple channels are involved in their chemosensitive response while in neurons from other regions a single signal and/or channel may be involved. Despite the progress, a number of key issues remain to be studied. A detailed study of chemosensitive signaling needs to be done in neurons from more brain stem regions. Fully describing the chemosensitive signaling pathways in brain stem neurons will offer new targets for therapies to alter the strength of central chemosensitivity and will yield new insights into the reason why there are multiple central chemoreceptive sites.

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 approximately 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 approximately 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.