Proton conductance and intracellular pH recovery from an acid load in chicken enterocytes

Potential contribution of a voltage-activated proton conductance to acid extrusion from rat hippocampal neurons

We examined the potential contribution of a voltage-gated proton conductance (gH+) to acid extrusion from cultured postnatal rat hippocampal neurons. In neurons loaded with Ca2+- and/or pH-sensitive fluorophores, transient exposures to 25-139.5 mM external K+ (K+o) or 20 microM veratridine in the presence of 2 mM Ca2+o (extracellular pH (pHo) constant at 7.35) caused reversible increases and decreases in intracellular free calcium concentration ([Ca2+]i) and intracellular pH (pHi), respectively. In contrast, under external Ca2+-free conditions, the same stimuli failed to affect [Ca2+]i but caused an increase in pHi, the magnitude of which was related to the [K+]o applied and the change in membrane potential. Consistent with the properties of gH+s in other cell types, the magnitude of the rise in pHi observed in the absence of external Ca2+ was not affected by the removal of external Na+ but was sensitive to external Zn2+ and temperature and was dependent on the measured transmembrane pH gradient (DeltapHmemb). Increasing DeltapH(memb) by pretreatment with carbonylcyanide-p-trifluoromethoxyphenylhydrazone augmented both the high-[K+]o-evoked rise in pHi and the Zn2+-sensitive component of the rise in pHi, suggestive of increased acid extrusion via a gH+. The inhibitory effect of Zn2+ at a given DeltapHmemb was further enhanced by increasing pHo from 7.35-7.8, consistent with a pHo-dependent inhibition of the putative gH+ by Zn2+. Under conditions designed to isolate H+ currents, a voltage-dependent outward current was recorded from whole-cell patch-clamped neurons. Although the outward current appeared to show some selectivity for protons, it was not sensitive to Zn2+ or temperature and the H+-selective component could not be separated from a larger conductance of unknown selectivity. Nonetheless, taken together, the results suggest that a Zn2+-sensitive proton conductive pathway is present in rat hippocampal neurons and contributes to H+ efflux under depolarizing conditions.

Modulation of H+ Transport Mechanisms by Interleukin-1 in Isolated Bovine Articular Chondrocytes

The proinflammatory cytokine interleukin-1 (IL-1) promotes the degradation of articular cartilage by inhibiting matrix synthesis and stimulating degradative enzyme activity. Generation of nitric oxide (NO) in response to IL-1 is implicated in these actions. The catabolic actions of IL-1 can be inhibited by manoeuvres which are predicted to dissipate H+ gradients across the chondrocyte plasma membrane. In the present study, the effects of IL-1 on H+ extrusion from bovine articular chondrocytes were investigated. pH was measured using the H+-sensitive fluorescent dye BCECF. Cells were acidified by ammonium rebound and the contribution of the Na+-H+ exchanger (NHE) and of the vacuolar H+-ATPase to acid extrusion was characterised by ion substitution and inhibitor studies. Overnight (18 h) exposure to IL-1 stimulated acid extrusion in a dose-dependent fashion. This effect represented stimulation of both NHE and the ATPase. Characterisation of the timecourse of this response indicated that, while stimulation of acid extrusion was rapid, effects on the ATPase were only apparent after greater than 8h incubation with the cytokine. In keeping with this observation, the protein synthesis inhibitor cycloheximide abolished the stimulatory effect of IL-1 on ATPase-mediated extrusion. The upregulation of ATPase activity by IL-1 was inhibited by the NOS inhibitor L-NAME and by the NO scavenger PTIO. In cells which had not been exposed to IL-1, treatment with the NO donor SNAP also stimulated acid extrusion by the ATPase. In contrast, NHE activity was not altered by any of these compounds. Taken together, these results imply that IL-1 can stimulate acid extrusion in chondrocytes and that this reflects rapid upregulation of NHE with slower induction of H+-ATPase activity which requires elevated levels of NO. While ATPase induction involves protein synthesis, this process may not constitute synthesis of ATPase proteins per se, but rather of some associated regulatory process.

Intracellular pH homeostasis in cultured human placental syncytiotrophoblast cells: recovery from acidification

Resting or basal intracellular pH (pHi) measured in cultured human syncytiotrophoblast cells was 7.26 ± 0.04 (without HCO3−) or 7.24 ± 0.03 (with HCO3−). Ion substitution and inhibitor experiments were performed to determine whether common H+-transporting species were operating to maintain basal pHi. Removal of extracellular Na+or Cl−or addition of amiloride or dihydro-4,4′-diisothiocyanatostilbene-2,2′-disulfonate (H2DIDS) had no effect. Acidification with the K+/H+exchanger nigericin reduced pHito 6.25 ± 0.15 (without HCO3−) or 6.53 ± 0.10 (with HCO3−). In the presence of extracellular Na+, recovery to basal pHiwas prompt and occurred at similar rates in the absence and presence of HCO3−. Ion substitution and inhibition experiments were also used to identify the species mediating the return to basal pHiafter acidification. Recovery was inhibited by removal of Na+or addition of amiloride, whereas removal of Cl−and addition of H2DIDS were ineffective. Addition of the Na+/H+exchanger monensin to cells that had returned to basal pHielicited a further increase in pHito 7.48 ± 0.07. Analysis of recovery data showed that there was a progressive decrease in ΔpH per minute as pHiapproached the basal level, despite the continued presence of a driving force for H+extrusion. These data show that in cultured syncytial cells, in the absence of perturbation, basal pHiis preserved despite the absence of active, mediated pH maintenance. They also demonstrate that an Na+/H+antiporter acts to defend the cells against acidification and that it is the sole transporter necessary for recovery from an intracellular acid load.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

Arachidonic acid activatable electrogenic H+ transport in the absence of cytochrome b558 in human T lymphocytes

To test the suggested structural relationship between the electrogenic H+ transporting system and the NADPH oxidase of phagocytes, the existence of the enzyme and the transport process was investigated in human tonsillar T lymphocytes. It is shown that tonsillar T cells possess an arachidonic acid activatable, Cd(2+)- and Zn(2+)-sensitive electrogenic H+ efflux pathway with similar properties as reported earlier in various phagocytic cells. The presence of cytochrome b558, the membrane component of the oxidase, could not be detected in tonsillar T lymphocytes either by immunoblot or by flow cytometric analysis. It is suggested that the electrogenic H+ transporting pathway is structurally independent of the NADPH oxidase complex.

PKC activators stimulate H+ conductance in chicken enterocytes

Chicken enterocytes present a H+-conducting pathway involved in the recovery of intracellular pH (pHi) from an acid load. In the current study we have tested the effect of protein kinase C (PKC) activators on the rate of proton efflux through the H+-conducting pathway. The rate of proton efflux was increased by the addition of 1,2-dioctanoyl-rac-glycerol (DOG) or phorbol 12-myristate 13-acetate (PMA), but it was not affected by the addition of the inactive phorbol ester analogue, 4alpha-phorbol 12, 13-didecanoate. DOG stimulated the process in a dose-dependent manner with a half-maximal effect at 45 microM. Staurosporine and Zn2+ prevented the DOG-dependent increase in the rate of proton efflux. The rate of proton efflux was affected by the pH, and DOG shifted this relationship upward and to the right. These results suggest that the proton-conducting pathway is regulated by PKC.