pH recovery from intracellular alkalinization in Retzius neurones of the leech central nervous system

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.

Mechanism of the kainate-induced intracellular acidification in leech Retzius neurons

We examined the effect of the glutamatergic agonist kainate on the membrane potential, the intracellular Na+ concentration ([Na+]i), the intracellular-free Ca2+ concentration, and on the intracellular pH of Retzius neurons of the medicinal leech, Hirudo medicinalis, in order to investigate the mechanism responsible for the intracellular acidification caused by glutamatergic stimulation. The recordings were made with Na+- and pH-sensitive microelectrodes and iontophoretically injected Fura-2. Bath application of kainate evoked a marked membrane depolarization, a [Na+]i increase, and an intracellular acidification. The intracellular acidification was unaffected by reversal of the electromotive force for H+, suggesting that an influx of H+ from the interstitial space does not contribute to the acidification. While the Ca2+ channel blockers La3+ and Co2+ had no effect on the kainate-induced intracellular acidification, suggesting that a Ca2+ influx via voltage-dependent Ca2+ channels was not relevant, the acidification was reduced in Ca2+-free saline solution. In Na+-free saline solution the kainate-induced intracellular acidification was absent, suggesting the involvement of Na+ influx in generating the acidification. When injected iontophoretically Na+ induced an intracellular acidification but Li+, K+, Rb+ or Cs+ did not. Furthermore, a [Na+]i increase induced by blocking the Na+/K+ pump also led to an intracellular acidification. We conclude that the [Na+]i increase is the crucial event underlying the kainate-induced intracellular acidification. Possible mechanisms linking the [Na+]i increase to the intracellular acidification are discussed.

pH regulation in horizontal cells of the skate retina

To examine the mechanisms by which horizontal cells regulate intracellular pH (pHi), measurements were recorded from isolated cells enzymatically dissociated from the skate retina utilizing the pH-sensitive dye BCECF. In a HCO3--containing Ringer solution, steady-state pHi was 7.32+/-0.13 (mean+/-S.D., n=70). Recovery from acidification was examined using the NH4+ prepulse technique. When NH4+ was removed from the extracellular solution, pHi dropped rapidly to approximately 0.3 pH units below the initial baseline, and then recovered at an initial rate of approximately 0.072 pH units/min. During recovery of pHi after the acid load, the removal of Na+ or the addition of amiloride from a HCO3--free extracellular solution reduced the rate of recovery by 79%+/-11% and 69%+/-14%, respectively. In the presence of DIDS, which inhibits primarily anion transport, or during the removal of Na+, the recovery from acidification was reduced by 83%+/-10% and 70%+/-11%, respectively, as compared to the control value in HCO3--containing solution. These results suggest that the skate horizontal cell possesses a Na/H exchanger as well as a Na+-and HCO3--dependent mechanism for removal of excess acid. Removal of HCO3- or Cl- from the extracellular solution had little effect on pHi, but removing external Na+ induced a marked decrease in pHi that fell at an initial rate of approximately 0.3 pH units min-1. This rate of acidification was decreased by 58%+/-19% in the presence of DIDS (500 micron) and reduced by 28%+/-13% with the addition of amiloride (2 mm). Thus, Na- and HCO3-dependent transport was about 2-fold more active than Na/H exchange during low Na+-induced acidification. The intrinsic pH-buffer capacity, determined from the pHi change induced by incremental reductions in the [NH4+] of the extracellular solution, was 24.2 mm/pH unit at the horizontal cell's resting pHi. Moreover, pHi was relatively insensitive to changes in membrane potential; in experiments under whole-cell voltage clamp (-70 mV), intracellular pH remained constant during depolarizing voltage swings to -30 mV or +30 mV, as well as during hyperpolarizing pulses to -90 or -110 mV.

Ionic basis of the membrane potential responses of rat dorsal vagal motoneurones to HEPES buffer

The effects of 10 mM HEPES (N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid) buffered artificial cerebrospinal fluid (aCSF) on membrane potential and the action potential were studied in 93 dorsal vagal motoneurones (DVMs) using an in vitro slice preparation of the rat medulla. Changing from bicarbonate/CO2 aCSF to HEPES aCSF resulted in a depolarisation of 6.0 +/- 0.6 mV and an increase in input resistance (RIn; n = 61). In the presence of 5 mM 4-AP, HEPES either had little effect (n = 9) or hyperpolarised the membrane (n = 10). Mn2+ (3 mM) or Ni2+ (200 microM) abolished the hyperpolarisation and its associated increase in RIn. In voltage-clamp studies 5 mM 4-AP eliminated a transient outward current and Ni2+ blocked an inactivating inward current. It is concluded that HEPES buffer reduces the contribution of the A current to resting membrane potential and also reduces a Ni(2+)-sensitive transient ICa.

pH regulation and proton signalling by glial cells

The regulation of H+ in nervous systems is a function of several processes, including H+ buffering, intracellular H+ sequestering, CO2 diffusion, carbonic anhydrase activity and membrane transport of acid/base equivalents across the cell membrane. Glial cells participate in all these processes and therefore play a prominent role in shaping acid/base shifts in nervous systems. Apart from a homeostatic function of H(+)-regulating mechanisms, pH transients occur in all three compartments of nervous tissue, neurones, glial cells and extracellular spaces (ECS), in response to neuronal stimulation, to neurotransmitters and hormones as well as secondary to metabolic activity and ionic membrane transport. A pivotal role for H+ regulation and shaping these pH transients must be assigned to the electrogenic and reversible Na(+)-HCO3-membrane cotransport, which appears to be unique to glial cells in nervous systems. Activation of this cotransporter results in the release and uptake of base equivalents by glial cells, processes which are dependent on the glial membrane potential. Na+/H+ and Cl-/HCO3-exchange, and possibly other membrane carriers, accomplish the set of tools in both glial cells and neurones to regulate their intracellular pH. Due to the pH dependence of a great variety of processes, including ion channel gating and conductances, synaptic transmission, intercellular communication via gap junctions, metabolite exchange and neuronal excitability, rapid and local pH transients may have signalling character for the information processing in nervous tissue. The impact of H+ signalling under both physiological and pathophysiological conditions will be discussed for a variety of nervous system functions.

Intracellular chloride activity of leech neurones and glial cells in physiological, low chloride saline

Leech blood apparently contains considerably less chloride than generally used in physiological experiments. Instead of 85-130 mM Cl- used in experimental salines, leech blood contains around 40 mM Cl- and up to 45 mM organic anions, in particular malate. We have reinvestigated the distribution of Cl- across the cell membrane of identified glial cells and neurones in the central nervous system of the leech Hirudo medicinalis L., using double-barrelled Cl(-)- and pH-selective microelectrodes, in a conventional leech saline, and in a saline with a low Cl- concentration (40 mM), containing 40 mM malate. The interference of anions other than Cl- to the response of the ion-selective microelectrodes was estimated in Cl(-)-free salines (Cl- replaced by malate and/or gluconate). The results show that the absolute intracellular Cl- activities (aCli) in glial cells and neurones, but not the electrochemical gradients of Cl- across the glial and the neuronal cell membranes, are altered in the low Cl-, malate-based saline. In Retzius neurones, aCli is lower than expected from electrochemical equilibrium, while in pressure neurones and in neuropil glial cells, aCli is distributed close to its equilibrium in both salines, respectively. The steady-state intracellular pH values in the glial cells and Retzius neurones are little affected (< or = 0.1 pH units) in the low Cl-, malate-based saline.

Neuron-glial interaction during injury and edema of the CNS

During injury and ischemia of the CNS mediator compounds are released or activated which cause secondary swelling and damage of nerve cells. Such mediators are glutamate, acidosis, free fatty acids, or high extracellular potassium. Glial homeostatic mechanisms are activated to prevent the secondary injury from these mediators. The glial clearance mechanisms have been studied in detail using in vitro systems allowing for a close control of the glial environment. Current evidence suggests glial swelling to occur together with glutamate uptake or in response to extracellular acidosis. Glial swelling, therefore, is rather the result of homeostatic mechanisms than an indication of glial demise.

Potential, pH, and arachidonate gate hydrogen ion currents in human neutrophils

Indirect evidence indicates that a proton-selective conductance is activated during the respiratory burst in neutrophils. A voltage- and time-dependent H(+)-selective conductance, gH, in human neutrophils is demonstrated here directly by the whole-cell patch-clamp technique. The gH is extremely low at large negative potentials, increases slowly upon membrane depolarization, and does not inactivate. It is enhanced at high external pH or low internal pH and is inhibited by Cd2+ and Zn2+. Arachidonic acid, which plays a pivotal role in inflammatory reactions, amplifies the gH. The properties of the gH described here are compatible with its activation during the respiratory burst in stimulated neutrophils, in which it may facilitate sustained superoxide anion release by dissipating metabolically generated acid.