Regulation of intracellular pH in pyramidal neurones from the rat hippocampus by Na(+)‐dependent Cl(‐)‐HCO3‐ exchange.

Abstract

1. We have measured intracellular pH (pHi) in freshly isolated pyramidal neurones from the CAI region of the rat hippocampus using the fluorescent indicator 2',7'-bis(carboxyethyl)-5(and-6)-carboxyfluorescein (BCECF). 2. The neurones selected by our isolation procedure, when studied in the nominal absence of C02-HCO3-, had a mean steady-state pHi of 6-81 + 0-02 (n = 163). The recovery of pH1 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 % C02-25mm HC03-caused steady-state pHi to increase from 6-74 + 0 05 to 7 03 + 0 03 (n = 28). In the presence of 5 % C02-25 mm HC03-, the rate of pH1 recovery from acid loads was much faster than in its absence.4. The HC03--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 CAL neurones is the Na+-dependent Cl--HC03-exchanger. The Na+-dependent mechanism that operates in the nominal absence of HC03-is far less active.

Characterization of Human SLC4A10 as an Electroneutral Na/HCO3 Cotransporter (NBCn2) with Cl– Self-exchange Activity

“…The SLC4A10 gene product has been named "NCBE" for Na ϩ -driven Cl-bicarbonate exchanger. However, the data presented in support of this nomenclature, 1) NCBE-mediated 22 Na and 36 Cl influx, 2) HCO 3 Ϫ -stimulated 36 Cl efflux, and 3) bath Cl Ϫ dependence of NCBE-mediated HCO 3 Ϫ influx (20), do not prove NDCBE activity. In the present study, we present evidence that the Na/HCO 3 cotransport activity of SLC4A10 under physiological conditions is independent of any Cl Ϫ countertransport and thus that NCBE in fact normally functions as an electroneutral Na/HCO 3 cotransporter.…”

“…The importance of Na ϩ -driven Cl-HCO 3 exchange activity for regulating pH i has long been established by physiological means in squid giant axons (16,17), snail neurons (18), giant barnacle muscle fibers (19), and mammalian neurons (3). NDCBE is markedly different from NBCn1 as it has an absolute dependence on Cl Ϫ countertransport for its Na/HCO 3 cotransport activity.…”

“…1 and 2). Electroneutral NCBTs play critical roles in regulating the intracellular pH (pH i ) of neurons (3) and in secreting HCO 3 Ϫ across the choroid plexus (4,5). These activities modulate pH in the surrounding brain extracellular fluid (BECF).…”

The Na+-driven Cl−/HCO3−Exchanger

“…NCBE mRNA is also expressed in the brain at high levels. Based on physiological studies (12,27), NCBE may be present in hippocampal neurons and astrocytes, but its physiological significance in such cells is not known at present. Further investigation is necessary to clarify the structure and function relationships of NCBE and its physiological roles in various tissues.…”

“…Three AEs (3) and three NBCs (6 -10) have been cloned and functionally characterized, but the molecular structure of the K ϩ -HCO 3 Ϫ cotransporter and the Na ϩ -driven Cl Ϫ / HCO 3 Ϫ exchanger has remained unknown. The Na ϩ -driven Cl Ϫ /HCO 3 Ϫ exchanger was first discovered in invertebrate neurons (11) and was found later in vertebrate neurons and non-neuronal cells, including the brain (12), vascular endothelial cells (13), sperm (14), kidney (15), and pancreatic ␤-cells (16). The Na ϩ -driven Cl Ϫ /HCO 3 Ϫ exchanger is an intracellular pH regulator that transports extracellular Na ϩ and HCO 3 Ϫ into cells in exchange for intracellular Cl Ϫ and H ϩ playing an important role in cellular alkalinization (5,11).…”