The effect of extracellular weak acids and bases on the intracellular buffering power of snail neurones

pH-dependent effects of procaine on equine gamete activation†

Procaine directly triggers pH-dependent cytokinesis in equine oocytes and induces hypermotility in stallion spermatozoa, an important event during capacitation. However, procaine-induced hyperactivated motility is abolished when sperm is washed to remove the procaine prior to sperm-oocyte co-incubation. To understand how procaine exerts its effects, the external Ca2+ and Na+ and weak base activity dependency of procaine-induced hyperactivation in stallion spermatozoa was assessed using computer-assisted sperm analysis. Percoll-washed stallion spermatozoa exposed to Ca2+-depleted (+2 mM EGTA) procaine-supplemented capacitating medium (CM) still demonstrated hyperactivated motility, whereas CM without NaCl or Na+ did not. Both procaine and NH4Cl, another weak base, were shown to trigger a cytoplasmic pH increase (BCECF-acetoxymethyl (AM)), which is primarily induced by a pH rise in acidic cell organelles (Lysosensor green dnd-189), accompanied by hypermotility in stallion sperm. As for procaine, 25 mM NH4Cl also induced oocyte cytokinesis. Interestingly, hyperactivated motility was reliably induced by 2.5-10 mM procaine, whereas a significant cytoplasmic cAMP increase and tail-associated protein tyrosine phosphorylation were only observed at 10 mM. Moreover, 25 mM NH4Cl did not support the latter capacitation characteristics. Additionally, cAMP levels were more than 10× higher in boar than stallion sperm incubated under similar capacitating conditions. Finally, stallion sperm preincubated with 10 mM procaine did not fertilize equine oocytes. In conclusion, 10 mM procaine causes a cytoplasmic and acidic sperm cell organelle pH rise that simultaneously induces hyperactivated motility, increased levels of cAMP and tail-associated protein tyrosine phosphorylation in stallion spermatozoa. However, procaine-induced hypermotility is independent of the cAMP/protein tyrosine phosphorylation pathway.

Choline and NMDG directly reduce outward currents: reduced outward current when these substances replace Na+ is alone not evidence of Na+-activated K+ currents

Choline chloride is often, and N-methyl-d-glucamine (NMDG) sometimes, used to replace sodium chloride in studies of sodium-activated potassium channels. Given the high concentrations used in sodium replacement protocols, it is essential to test that it is not the replacement substances themselves, as opposed to the lack of sodium, that cause any observed effects. We therefore compared, in lobster stomatogastric neurons and leech Retzius cells, the effects of applying salines in which choline chloride replaced sodium chloride, and in which choline hydroxide or sucrose was added to normal saline. We also tested, in stomatogastric neurons, the effect of adding NMDG to normal saline. These protocols allowed us to measure the direct effects (i.e., effects not due to changes in sodium concentration or saline osmolarity or ionic strength) of choline on stomatogastric and leech currents, and of NMDG on stomatogastric currents. Choline directly reduced transient and sustained depolarization-activated outward currents in both species, and NMDG directly reduced transient depolarization-activated outward currents in stomatogastric neurons. Experiments with lower choline concentrations showed that adding as little as 150 mM (stomatogastric) or 5 mM (leech) choline reduced at least some depolarization-activated outward currents. Reductions in outward current with choline chloride or NMDG replacement alone are thus not evidence of sodium-activated potassium currents. NEW & NOTEWORTHY We show that choline or N-methyl-d-glucamine (NMDG) directly (i.e., not due to changes in extracellular sodium) decrease outward currents. Prior work studying sodium-activated potassium channels in which sodium was replaced with choline or NMDG without an addition control may therefore be artifactual.

Lidocaine increases intracellular sodium concentration through a Na+-H+ exchanger in an identified Lymnaea neuron

BACKGROUND

The intracellular sodium concentration ([Na(+)]in) is related to neuron excitability. For [Na(+)]in, a Na(+)-H(+) exchanger plays an important role, which is affected by intracellular pH ([pH]in). However, the effect of lidocaine on [pH]in and a Na(+)-H(+) exchanger is unclear. We used neuron from Lymnaea stagnalis to determine how lidocaine affects [pH]in, Na(+)-H(+) exchanger, and [Na(+)]in.

METHODS

Intracellular sodium imaging by sodium-binding benzofuran isophthalate and intracellular pH imaging by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein were used to measure [Na(+)]in and [pH]in. Measurements for [Na(+)]in were made in normal, Na(+) free saline, with modified extracellular pH, and a Na(+)-H(+) exchanger antagonist [(5-N-ethyl-N-isopropyl amiloride, N-methylisopropylamiloride, and 5-(N,N-hexamethylene)-amiloride) pretreatment trials. Furthermore, [Na(+)]in and [pH]in were recorded simultaneously. From 0.1 to 10 mM, lidocaine, mepivacaine, bupivacaine, prilocaine, and QX-314 were evaluated.

RESULTS

Lidocaine, mepivacaine, and prilocaine increased the [Na(+)]in in a dose-dependent manner. In contrast, QX-314 did not change the [Na(+)]in at each dose. In the Na(+) free saline or in the presence of each Na(+)-H(+) exchanger antagonist, lidocaine failed to increase [Na(+)]in. Lidocaine, mepivacaine, and prilocaine induced a significant decrease in [pH]in below baseline with an increase in [Na(+)]in. In contrast, QX-314 did not change the [pH]in. These results demonstrated that lidocaine increases [Na(+)]in through Na(+)-H(+) exchanger activated by intracellular acidification, which is induced by the proton trapping of lidocaine. This [Na(+)]in increase and [pH]in change induces cell toxicity.

CONCLUSION

Lidocaine increases the [Na(+)] through a Na(+)-H(+) exchanger by proton trapping.

The weak bases NH3 and trimethylamine inhibit the medium and slow afterhyperpolarizations in rat CA1 pyramidal neurons

The weak bases NH(3) and trimethylamine (TMeA), applied externally, are widely used to investigate the effects of increasing intracellular pH (pH(i)) on neuronal function. However, potential effects of the compounds independent from increases in pH(i) are not usually considered. In whole-cell patch-clamp recordings from rat CA1 pyramidal neurons, bath application of 1-40 mM NH(4)Cl or TMeA HCl reduced resting membrane potential and input resistance, inhibited the medium and slow afterhyperpolarizations (AHPs) and their respective underlying currents, mI(ahp) and sI(ahp), and led to the development of depolarizing current-evoked burst firing. Examined in the presence of 1 microM TTX and 5 mM TEA with 10 mM Hepes in the recording pipette, NH(3) and TMeA increased pH(i) and the magnitudes of depolarization-evoked intracellular [Ca(2+)] transients, Ca(2+)-dependent depolarizing potentials, and inward Ca(2+) currents but reduced the slow AHP and sI(ahp). When internal H(+) buffering power was raised by including 100 mM tricine in the patch pipette, the effects of NH(3) and TMeA to increase pH(i) and augment Ca(2+) influx were attenuated whereas the reductions in the slow AHP and sI(ahp) (as well as membrane potential and input resistance) were maintained. The findings indicate that increases in pH(i) contribute to the increases in Ca(2+) influx observed in the presence of NH(3) and TMeA but not to the reductions in membrane potential, input resistance or the magnitudes of AHPs. The results have implications for the interpretation of data from experiments in which pH(i) is manipulated by the external application of NH(3) or TMeA.

The Second Intracellular Loop of the Glycine Transporter 2 Contains Crucial Residues for Glycine Transport and Phorbol Ester-induced Regulation

Na+ and Cl(-)-coupled glycine transporters control the availability of glycine neurotransmitter in the synaptic cleft of inhibitory glycinergic pathways. In this report, we have investigated the involvement of the second intracellular loop of the neuronal glycine transporter 2 (GLYT2) on the protein conformational equilibrium and the regulation by 4alpha-phorbol 12 myristate 13-acetate (PMA). By substituting several charged (Lys-415, Lys-418, and Lys-422) and polar (Thr-419 and Ser-420) residues for different amino acids and monitoring plasma membrane expression and kinetic behavior, we found that residue Lys-422 is crucial for glycine transport. The introduction of a negative charge in 422, and to a lower extent in neighboring N-terminal residues, dramatically increases transporter voltage dependence as assessed by response to high potassium depolarizing conditions. In addition, [2-(trimethylammonium)ethyl] methanethiosulfonate accessibility revealed a conformational connection between Lys-422 and the glycine binding/permeation site. Finally, we show that the mutation of positions Thr-419, Ser-420, and mainly Lys-422 to acidic residues abolishes the PMA-induced inhibition of transport activity and the plasma membrane transporter internalization. Our results establish a new structural basis for the action of PMA on GLYT2 and suggest a complex nature of the PMA action on this glycine transporter.

Intracellular pH regulation in isolated hepatopancreas cells from the Roman snail (Helix pomatia)

The mechanisms of intracellular pH (pHi) regulation were studied in isolated hepatopancreas cells from the Roman snail, Helix pomatia. The relationship between intracellular and extracellular pH indicated that pHi is actively regulated in these cells. At least three pHi-regulatory ion transporters were found to be present in these cells and to be responsible for the maintenance of pHi: an amiloride-sensitive Na+/H+ exchanger, a 4-acetamido-4'-isothiocyanostilbene-2,2'disulfonic acid (SITS)-sensitive, presumably Na(+)-dependent, Cl-/HCO3-exchanger, and a bafilomycin-sensitive H(+)-pump. Inhibition of one of these transporters alone did not affect steady state pHi, whereas incubation with amiloride and SITS in combination resulted in a significant intracellular acidification. Following the induction of intracellular acidosis by addition of the weak acid Na+propionate, the Na+/H+ exchanger was immediately activated leading to a rapid recovery of pHi towards the baseline level. Both the SITS-sensitive mechanism and the H(+)-pump responded more slowly, but were of similar importance for pHi recovery. Measurement of pHi recovery from acidification in the three discernible types of hepatopancreas cells with a video fluorescence image system revealed slightly differing response patterns, the physiological significance of which remains to be determined.

Brain bioenergetics in murine models of scrapie using in vivo 31P magnetic resonance spectroscopy

The bioenergetic status of the brain in scrapie mouse models was investigated during the late, clinical phase of the disease, by in vivo phosphorus magnetic resonance spectroscopy (MRS). The only significant change observed in the scrapie-infected mice compared with controls was an increase in intracellular brain pH (7.20+/-0.06 vs 7.10+/-0.05). No other changes in energetic metabolism were observed in the infected mice beside a trend in the decrease of phosphomonoester (PME) level, possibly associated with an alteration in glycolytic intermediates. This study showed that even in the presence of severe cellular vacuolation and microglia infiltrate, cerebral bioenergetic is maintained.

Ionic mechanism of 4-aminopyridine action on leech neuropile glial cells

Extracellular 4-aminopyridine (4-AP), tetraethylammonium chloride (TEA) and quinine depolarized the neuropile glial cell membrane and decreased its input resistance. As 4-AP induced the most pronounced effects, we focused on the action of 4-AP and clarified the ionic mechanisms involved. 4-AP did not only block glial K+ channels, but also induced Na+ and Ca2+ influx via other than voltage-gated channels. The reversal potential of the 4-AP-induced current was -5 mV. Application of 5 mM Ni2+ or 0.1 mM d-tubocurarine reduced the 4-AP-induced depolarization and the associated decrease in input resistance. We therefore suggest that 4-AP mediates neuronal acetylcholine release, apparently by a presynaptic mechanism. Activation of glial nicotinic acetylcholine receptors contributes to the depolarization, the decrease in input resistance, and the 4-AP-induced inward current. Furthermore, the 4-AP-induced depolarization activates additional voltage-sensitive K+ and Cl- channels and 4-AP-induced Ca2+ influx could activate Ca2+-sensitive K+ and Cl- channels. Together these effects compensate and even exceed the 4-AP-mediated reduction in K+ conductance. Therefore, the 4-AP-induced depolarization was paralleled by a decreasing input resistance.

Influence of extracellular pH, sodium propionate and trimethylamine on excitation-contraction coupling in the rat tail artery

1. The effects of extracellular or intracellular pH changes on agonist- or depolarization-induced contractions of the rat tail artery were investigated. 2. Vessels were perfused initially (25 min) with physiological salt solution (PSS) at a pressure of 30 mmHg. Perfusion was then continued with calcium-free PSS containing either 3.0 micromol/L noradrenaline (NA) or 100 mmol/L K+, which had been made either acidotic or alkalotic. Contractile responses to graded concentrations of calcium were assessed. 3. A reduction in the intracellular or extracellular pH was induced by the addition of a weak acid (30 mmol/L sodium propionate) or reduction of the concentration of HCO3- in the PSS, respectively; an elevation of the intracellular or extracellular pH was produced by the addition of a weak base (10 mmol/L trimethylamine) or by increasing HCO3-, respectively. The PSS was bubbled with 5% CO2/95% O2. 4. Lowered intracellular pH did not alter NA- or K+-stimulated contractions. During lowered extracellular pH, contractile responsiveness and peak response were significantly reduced in K+-stimulated arteries, but were not affected in NA-stimulated arteries. 5. Elevated intracellular pH did not alter NA-induced contraction, but reduced the sensitivity to K+-stimulated contractions. Elevated extracellular pH had little effect on the magnitude of K+-induced contractions, but slightly enhanced (although not significantly) NA-induced contractions. 6. It is concluded that reduced contractile responses to K+ during extracellular acidosis are due to the modulation of potential-operated calcium channels (POC). Alkalotic vasodilatation is mediated by intracellular events and is POC-modulated, whereas alkalotic vasoconstriction appears to be due to extracellular events and is modulated by receptor-operated calcium channels (ROC).

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.

Chloride-dependent pH regulation in connective glial cells of the leech nervous system

We used double-barreled pH-sensitive microelectrodes to study the mechanisms by which the intracellular pH is regulated in the connective glial cells of the medicinal leech. The experiments indicate that a Cl(-)- and HCO3(-)-dependent mechanism mediates some recovery from intracellular alkalosis even in the absence of external Na+. This suggests the presence of Na(+)-independent Cl-/HCO3- exchange in the connective glial membrane. At alkaline pHi, this exchange most likely operates in the direction of net acid loading (i.e. HCO3- efflux).

Flow cytometric studies of bicarbonate-mediated Ca2+ influx in boar sperm populations

Boar spermatozoa loaded with the Ca2+ probe fluo-3 were incubated in various Tyrode's-based media similar to those used for in vitro fertilization (IVF), and samples were then analysed by two-colour flow cytometry; propidium iodide was included in the media to detect membrane-damaged ("dead") cells. If media contained bicarbonate/CO2 (a component thought to promote capacitation), part of the live sperm population experienced a considerable influx of Ca2+ into both head and tail compartments. The percentage of responding cells reached a maximum after about 30 min, but both during and after this period there was also a steady increase in the number of dead cells. This bicarbonate-mediated increase in cell death took place in the absence of external Ca2+. Evidence was obtained that the entry of propidium iodide was preceded by a change in permeability of the plasma membrane, detectable by leakage of carboxydichlorofluorescein, and it was therefore deduced that the Ca2+ influx detected by fluo-3 was due to destabilization of the plasma membrane. A similar response could be produced by both caffeine and papaverine (best known as phosphodiesterase inhibitors), but neither cyclic AMP nor activators of adenylate cyclase had any effect. There was no influence of substrate on the process, but, in comparison to poly(vinyl alcohol), serum albumin enhanced it. The precise relevance of this destabilization to capacitation is not yet clear, but it seems significant that the process is mediated or enhanced by components often specifically included in IVF media, and that different individual cells respond after different times.

Membrane conductances involved in amplification of small signals by sodium channels in photoreceptors of drone honey bee

1. Voltage signals of about 1 mV evoked in photoreceptors of the drone honey bee by shallow modulation of a background illumination of an intensity useful for behaviour are thought to be amplified by voltage-dependent Na+ channels. To elucidate the roles of the various membrane conductances in this amplification we have studied the effects of the Na+ channel blocker tetrodotoxin (TTX) and various putative K+ channel blockers on the membrane potential, Vm. 2. Superfusion of a slice of retina with 0.5-10 mM-4-aminopyridine (4-AP) depolarized the membrane and, in fifty of sixty-three cells induced repetitive action potentials. Ionophoretic injection of tetraethylammonium produced similar effects. 3. In order to measure the depolarization caused by 4-AP, action potentials were prevented by application of TTX: 4-AP was applied when the membrane was depolarized to different levels by light. 4-AP induced an additional depolarization at all membrane potentials tested (-64 to -27 mV). We conclude that there are 4-AP-sensitive K+ channels that are open at constant voltage over this range. 4. 4-AP slowed down the recovery phase of the action potential induced by a light flash by a factor that ranged from 0.51 to 0.16. This reduction could be accounted for by the reduction in a voltage-independent K+ conductance estimated from the steady-state depolarization. 5. After the voltage-gated Na+ channels had been blocked by TTX, exposure to 4-AP further changed the amplitude of the response to a small (approximately 10%) decremental light stimulus. The change was an increase when the background illumination brought Vm to potentials more negative than about -40 mV; it was a decrease when Vm > -40 mV. The data could be fitted by a circuit representation of the membrane with a light-activated conductance and a K+ conductance (EK = -66 mV) that was partly blocked by 4-AP. The voltage range studied was from -52 to -27 mV; neither conductance in the model was voltage dependent. 6. The responses to small changes in light intensity in the absence of TTX were mimicked by a model. We conclude that a voltage-dependent Na+ conductance described by the Hodgkin-Huxley equations can amplify small voltage changes in a cell membrane that is also capable of generating action potentials; the magnitude of the K+ conductance is critical for optimization of signals while avoiding membrane instability.

Mechanisms of pH recovery from intracellular acid loads in the leech connective glial cell

We used double-barrelled, neutral carrier, pH-sensitive microelectrodes to study the mechanisms by which the intracellular pH (pHi) is regulated in the connective glial cells of the medicinal leech. In HEPES-buffered, nominally CO2/HCO3(-)-free solutions the recovery of pHi from intracellular acidosis is Na(+)-dependent and reduced by at least half in the presence of amiloride, suggesting the action of Na+:H+ exchange. The rate of pHi recovery by this mechanism can be increased by raising the extracellular buffering power or by increasing extracellular pH. The presence of CO2/HCO3(-)-greatly increases the rate of pHi recovery from intracellular acidosis. This CO2/HCO3(-)-stimulated recovery is also dependent on external Na+, largely Cl(-)-independent, inhibited by DIDS, and accompanied by membrane hyperpolarization. This is consistent with it being mediated by the electrogenic cotransport of Na+ and HCO3- into the cells. A Cl(-)-dependent component to Na(+)- and HCO3(-)-dependent regulation is most easily explained by the added presence of a Na(+)-dependent exchange of HCO3- and Cl-.

Extracellular MgATP activates the Cl-/HCO3- exchanger in single rat cardiac cells

1. The effect of extracellular MgATP on cytosolic pH (pHi) was investigated in single rat cardiac cells loaded with the pH-sensitive probe Snarf-1. 2. Basal pHi in HEPES-buffered solution (containing 4.4 mM-NaHCO3) was 7.08. MgATP induced a transient acidification followed by an alkalinization. The latter is prevented by ethylisopropylamiloride (EIPA) and has been attributed to the activation of the Na+/H+ antiport. The MgATP-induced acidification reached a maximal value of 0.42 +/- 0.03 pH units (U pH). It was concentration dependent with a K0.5 of 2.6 microM-MgATP. This acidification was also observed with the same magnitude in the presence of the more physiological Krebs-bicarbonate buffer but was greatly reduced in nominally HCO3-free HEPES. 3. The MgATP-induced acidification was prevented by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), probenecid and ethacrynic acid but not by bumetanide. It was dependent upon the external chloride concentration. The K0.5[Cl-] was 9 mM and the maximal acidification required 60 mM-Cl-. 4. MgATP accelerated the recovery from an alkalinization triggered by a pulse of NH4Cl. The nucleotide also facilitated the efflux of HCO3- when the cell was switched from a Krebs-bicarbonate buffer gassed with 5% CO2 to an HEPES buffer. 5. The acidification was only evoked by MgATP and its poorly hydrolysable analogues but not by the other nucleotides (ADP, GTP (guanosine triphosphate), CTP (cytidine triphosphate) UTP (urodine triphosphate), ITP (inositol triphosphate) nor by adenosine. It required the presence of Mg2+ ions. 6. These results provide evidence that MgATP activates the Cl-/HCO3- exchanger and that this activation accounts for the acidification. Such an activation could not be related to the P1- or the P2-purinergic receptors since it requires triphosphate adenylic compounds and Mg2+ ions. This leads us to suggest the existence of a putative P3-type of purinergic receptor.

Glucose availability and sensitivity to anoxia of isolated rat peroneal nerve

The contrast between resistance to ischemia and ischemic lesions in peripheral nerves of diabetic patients was explored by in vitro experiments. Isolated and desheathed rat peroneal nerves were incubated in the following solutions with different glucose availability: 1) 25 mM glucose, 2) 2.5 mM glucose, and 3) 2.5 mM glucose plus 10 mM 2-deoxy-D-glucose. Additionally, the buffering power of all of these solutions was modified. Compound nerve action potential (CNAP), extracellular pH, and extracellular potassium activity (aKe) were measured simultaneously before, during, and after a period of 30 min of anoxia. An increase in glucose availability led to a slower decline in CNAP and to a smaller rise in aKe during anoxia. This resistance to anoxia was accompanied by an enhanced extracellular acidosis. Postanoxic recovery of CNAP was always complete in 25 mM HCO3(-)-buffered solutions. In 5 mM HCO3- and in HCO3(-)-free solutions, however, nerves incubated in 25 mM glucose did not recover functionally after anoxia, whereas nerves bathed in solutions 2 or 3 showed a complete restitution of CNAP. We conclude that high glucose availability and low PO2 in the combination with decreased buffering power and/or inhibition of HCO3(-)-dependent pH regulation mechanisms may damage peripheral mammalian nerves due to a pronounced intracellular acidosis.

Intracellular H+ buffering power and its dependency on intracellular pH

Intracellular hydrogen ion (H+) buffering power, conventionally defined as the amount of acid or base that would have to be introduced into the cell cytosol to decrease or increase ipH by one pH unit, is generally said to increase as intracellular pH (ipH) decreases. This implies that the cell has a lesser capability to resist acute acid or base perturbations at its steady state ipH than at any lower ipH. We re-examined this notion, reasoning that the logarithmic nature of the pH unit could limit the validity of the conventional expression of buffering power in imparting physiologic insight into the mechanisms of cellular H+ homeostasis. The mathematical derivation of the formula, delta i[NH4+]/delta ipH, conventionally used to estimate buffering power using the NH4Cl technique, revealed that this parameter is, by design, inversely proportional to the exponential of ipH. This a priori dependence on pH dictates an increase in buffering power with decreasing ipH, and thereby interferes with the assessment of the physiologic capability of the intracellular milieu to buffer protons at different ipH levels. To circumvent this problem, buffering power was defined as the amount of hydrogen ions that would have to be added to or removed from the cell to effect a change in the concentration of H+ in the cell cytosol of 1 mM (a term heretofore referred to as the cell H+ buffering coefficient). The mathematical derivation of the formula used to calculate the cell H+ buffering coefficient, delta i[NH4+]/delta[H+]i, does not suffer from an a priori dependence on ipH.(ABSTRACT TRUNCATED AT 250 WORDS)