Fall in intracellular pH and increase in resting tension induced by a mitochondrial uncoupling agent in crayfish muscle

The K+-H+ exchanger, nigericin, modulates taste cell pH and chorda tympani taste nerve responses to acidic stimuli

The relationship between acidic pH, taste cell pH(i), and chorda tympani (CT) nerve responses was investigated before and after incorporating the K(+)-H(+) exchanger, nigericin, in the apical membrane of taste cells. CT responses were recorded in anesthetized rats in vivo, and changes in pH(i) were monitored in polarized fungiform taste cells in vitro. Under control conditions, stimulating the tongue with 0.15 M potassium phosphate (KP) or 0.15 M sodium phosphate (NaP) buffers of pHs between 8.0 and 4.6, KP or NaP buffers did not elicit a CT response. Post-nigericin (500 × 10(-6) M), KP buffers, but not NaP buffers, induced CT responses at pHs ≤ 6.6. The effect of nigericin was reversed by the topical lingual application of carbonyl cyanide 3-chloro-phenylhydrazone, a protonophore. Post-nigericin (150 × 10(-6) M), KP buffers induced a greater decrease in taste cell pH(i) relative to NaP buffers and to NaP and KP buffers under control conditions. A decrease in pH(i) to about 6.9 induced by KP buffers was sufficient to elicit a CT response. The results suggest that facilitating apical H(+) entry via nigericin decreases taste cell pH(i) and demonstrates directly a strong correlation between pH(i) and the magnitude of the CT response.

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.

Optical and pharmacological tools to investigate the role of mitochondria during oxidative stress and neurodegeneration

Mitochondria are critical for cellular adenosine triphosphate (ATP) production; however, recent studies suggest that these organelles fulfill a much broader range of tasks. For example, they are involved in the regulation of cytosolic Ca(2+) levels, intracellular pH and apoptosis, and are the major source of reactive oxygen species (ROS). Various reactive molecules that originate from mitochondria, such as ROS, are critical in pathological events, such as ischemia, as well as in physiological events such as long-term potentiation, neuronal-vascular coupling and neuronal-glial interactions. Due to their key roles in the regulation of several cellular functions, the dysfunction of mitochondria may be critical in various brain disorders. There has been increasing interest in the development of tools that modulate mitochondrial function, and the refinement of techniques that allow for real time monitoring of mitochondria, particularly within their intact cellular environment. Innovative imaging techniques are especially powerful since they allow for mitochondrial visualization at high resolution, tracking of mitochondrial structures and optical real time monitoring of parameters of mitochondrial function. The techniques discussed include classic imaging techniques, such as rhodamine-123, the highly advanced semi-conductor nanoparticles (quantum dots), and wide field microscopy as well as high-resolution multiphoton imaging. We have highlighted the use of these techniques to study mitochondrial function in brain tissue and have included studies from our laboratories in which these techniques have been successfully applied.

Mitochondrial inhibition prior to oxygen-withdrawal facilitates the occurrence of hypoxia-induced spreading depression in rat hippocampal slices

Oxygen withdrawal blocks mitochondrial respiration. In rat hippocampal slices, this triggers a massive depolarization of CA1 neurons and a negative shift of the extracellular DC potential, the characteristic sign of hypoxia-induced spreading depression (HSD). To unveil the contribution of mitochondria to the sensing of hypoxia and the ignition of HSD, we modified mitochondrial function. Mitochondrial uncoupling by carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP, 1 microM) prior to hypoxia hastened the onset and shortened the duration of HSD. Blocking mitochondrial ATP synthesis by oligomycin (10 microg/ml) was without effect. Inhibition of mitochondrial respiration by rotenone (20 microM), diphenyleneiodonium (25 microM), or antimycin A (20 microM) also hastened HSD onset and shortened HSD duration. 3-nitropropionic acid (1 mM) increased HSD duration. Cyanide (100 microM) hastened HSD onset and increased HSD duration. At higher concentrations, cyanide (1 mM), azide (2 mM), and FCCP (10 microM) triggered SD episodes on their own. Compared with control HSD, the spatial extent of the intrinsic optical signals of cyanide- and azide-induced SDs was more pronounced. Monitoring NADH (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) autofluorescence and mitochondrial membrane potential verified the mitochondrial targeting by the drugs used. Except 1 mM cyanide, no treatment reduced cellular ATP levels severely and no correlation was found between ATP, NADH, or FAD levels and the time to HSD onset. Therefore ATP depletion or a cytosolic reducing shift due to NADH/FADH2 accumulation cannot serve as a general explanation for the hastening of HSD onset on mitochondrial inhibition. Additional redox couples (glutathione) or events downstream of the mitochondrial depolarization need to be considered.

ATP from glycolysis is required for normal sodium homeostasis in resting fast-twitch rodent skeletal muscle

Myocellular sodium homeostasis is commonly disrupted during critical illness for unknown reasons. Recent data suggest that changes in intracellular sodium content and the amount of ATP provided by glycolysis are closely related. The role of glycolysis and oxidative phosphorylation in providing fuel to the Na(+)-K(+) pump was investigated in resting rat extensor digitorum longus muscles incubated at 30 degrees C for 1 h. Oxidative inhibition with carbonyl cyanide m-chlorophenylhydrazone, known as CCCP (0.2 microM), or by hypooxygenation did not alter myocellular sodium or potassium content ([Na(+)](i), [K(+)](i), respectively), whereas treatment with iodoacetic acid (0.3 mM), which effectively blocked glycolysis, dramatically increased [Na(+)](i) and the [Na(+)](i)/[K(+)](i) ratio. Experiments using ouabain and measurements of myocellular high-energy phosphates indicate that Na(+)-K(+)-ATPase activity is only impaired when glycolysis is inhibited. The data suggest that normal glycolysis is required to regulate intracellular sodium in fast-twitch skeletal muscles, because it is the predominant source of the fuel for the Na(+)-K(+)-ATPase.

Real-time measurements of cellular oxygen consumption, pH, and energy metabolism using nuclear magnetic resonance spectroscopy

Changes in molecular expression or apoptotic behavior, induced by malignant transformation or anticancer treatment, are frequently reflected in cellular metabolism and oxygen consumption. A technique to monitor oxygen consumption, cell physiology, and metabolism noninvasively would provide a better understanding of interactions between molecular changes and metabolism in malignant transformation and following cancer treatment. Such a system was developed in this study by adapting multinuclear MRI and spectroscopic techniques to an isolated cell perfusion system. The system was evaluated by studying the effects of two agents, carbonyl cyanide m-chlorophenylhydrazone (CCCP) which is an uncoupler of oxidative phosphorylation, and antimycin, an inhibitor of oxidative phosphorylation, on the oxygen consumption and metabolism of MCF-7 and MatLyLu cancer cell lines.

Effects of 2,4-dinitrophenol (DNP) on the relationship between the chemosensory activities of the rat carotid body and the intracellular calcium of glomus cells

To test the hypothesis that the uncoupler 2,4-dinitrophenol (DNP) increases [Ca2+]i equally well, independent of pHi, we studied the effects of 250 microM DNP on [Ca2+]i and carotid sinus nerve (CSN) activity of rat carotid body (CB). CSN activity was measured in CB perfused and superfused with hypocapnic (pHo 7.80) and normocapnic (pHo 7.42) Tyrode solutions. [Ca2+]i of glomus (type I cells) was assessed by superfusion techniques under identical conditions as for CSN recording experiments. The results indicate that 250 microM DNP increased [Ca2+]i of type I cells as well as CSN activity at both pHos, although alkalosis diminished these responses. Given that pHi will change with pHo, DNP did not make any additional pHi change, although [Ca2+]i changed. We conclude that DNP effects were due to [Ca2+]i change alone, and the relationship between [Ca2+]i and CSN activity are internally consistent.

Mechanisms mediating the vasorelaxing action of eugenol, a pungent oil, on rabbit arterial tissue

The inhibitory actions of eugenol on intracellular Ca2+ concentration ([Ca2+]i) and the contractions induced by excess extracellular K+ concentration ([K+]o) in rabbit thoracic aorta were investigated. Application of excess [K+]o solution (30-90 mM) produced contraction and increased the intensity of the Ca2+ fluorescence signal. Pretreatment with eugenol (> or =0.1 mM) reduced both the amplitude of contraction and the intensity of the Ca2+ fluorescence signal, but the contraction was more strongly affected than the [Ca2+]i. Application of eugenol (0.3 mM) to tissue precontracted by 90 mM [K+]o solution (immediately after the removal of the 90 mM [K+]o solution) slowed the decay of the [Ca2+]i signal, but it did not change the rate of relaxation. Carbonyl cyanide m-chlorophenylhydrozone (10 microM), a mitochondrial metabolic inhibitor, produced a reduction in tension despite a slight increase in [Ca2+]i when applied to muscle precontracted by 90 mM [K+]o solution. These results indicate that eugenol relaxes the rabbit thoracic aorta while suppressing the Ca2+-sensitivity and both the uptake and extrusion mechanisms for Ca2+. To judge from the similarities between its actions and those of metabolic inhibitors, eugenol may produce its actions at least partly through metabolic inhibition.

Effects of mitochondrial uncouplers on intracellular calcium, pH and membrane potential in rat carotid body type I cells

1. Mitochondrial uncouplers are potent stimulants of the carotid body. We have therefore investigated their effects upon isolated type I cells. Both 2,4-dinitrophenol (DNP) and carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP) caused an increase in [Ca2+]i which was largely inhibited by removal of extracellular Ca2+ or Na+, or by the addition of 2 mM Ni2+. Methoxyverapamil (D600) also partially inhibited the [Ca2+]i response. 2. In perforated-patch recordings, the rise in [Ca2+]i coincided with membrane depolarization and was greatly reduced by voltage clamping the cell to -70 mV. Uncouplers also inhibited a background K+ current and induced a small inward current. 3. Uncouplers reduced pHi by 0.1 unit. Alkaline media diminished this acidification but had no effect on the [Ca2+]i response. 4. FCCP and DNP also depolarized type I cell mitochondria. The onset of mitochondrial depolarization preceded changes in cell membrane conductance by 3-4 s. 5. We conclude that uncouplers excite the carotid body by inhibiting a background K+ conductance and inducing a small inward current, both of which lead to membrane depolarization and voltage-gated Ca2+ entry. These effects are unlikely to be caused by cell acidification. The inhibition of background K+ current may be related to the uncoupling of oxidative phosphorylation.

Influence of extracellular and intracellular pH on GABA-gated chloride conductance in crayfish muscle fibres

The effect of intracellular and extracellular pH on GABA-gated Cl- conductance was studied using H(+)-selective microelectrodes and a three-microelectrode voltage clamp in crayfish leg opener muscle fibres in bicarbonate-free solutions. Experimental variation of intracellular pH in the range 6.4-8.0 did not affect the GABA-gated conductance. In contrast to this, the GABA-gated conductance was sensitive to changes in external pH. Raising the external pH from 7.4 to 8.4 decreased the GABA-gated peak conductance observed immediately following application of GABA by 30%, and a change from 7.4 to 6.4 produced an increase of 26%. The effect of extracellular pH on the GABA-gated peak conductance was approximately linear in the pH range 6.4-8.9. A slight decrease in the slope of the pH-conductance relationship was evident in the pH range 5.4-6.4. The desensitization of the GABA-gated conductance was also affected by external pH. At pH 6.9 the conductance produced by 1 mM GABA showed a desensitization of about 15%, and at pH 8.9 this value was 34%. Raising the external pH in the presence of GABA decreased the GABA-gated peak conductance and increased the fractional desensitization, while lowering the external pH produced opposite effects, and was capable of repriming the conductance from a desensitized state to the non-desensitized state. The above results show that the GABA-gated conductance is sensitive to changes in external pH in the physiological range, and suggest that pH-dependent changes in the postsynaptic efficacy of GABA-mediated inhibition may contribute to H+ modulation of neuronal excitability.

Inhibitory effect of dicumarol on the excitability of the sarcolemma of skeletal muscle

Dicumarol (7.5 x 10(-6) M -7.5 x 10(-5) M) inhibited the twitches and the simultaneously recorded electromyogram of the indirectly stimulated rat diaphragm preparation. The directly stimulated curarized preparation was inhibited in a similar way. Accordingly, the excitability of the sarcolemma was inhibited by dicumarol. The inhibitory effect was irreversible upon washing. Intracellular microelectrode recordings disclosed that the inhibitory effect was accompanied by a depolarization, and the inhibition was potentiated in high K+ (9.5 mM) solution. In addition, the ability of the preparations to produce a depolarization contracture with KCl (100 mM) was inhibited by dicumarol. Therefore, a part of the inhibitory effect might be caused by the depolarization. On the other hand, subthreshold direct stimulation disclosed that the inhibitory effect might be due to an increase of the threshold. However, a lack of synergistic interaction with lidocaine (0.2 mM) showed that this effect of dicumarol was probably not a membrane stabilizing or local anaesthetic effect.

Mechanism of action of GABA on intracellular pH and on surface pH in crayfish muscle fibres

1. The mode of action of gamma-aminobutyric acid (GABA) on intracellular pH (pHi) and surface pH (pHs) was studied in crayfish muscle fibres using H(+)-selective microelectrodes. The extracellular HCO3- concentration was varied (0-30 mM) at constant pH (7.4). 2. GABA (5 x 10(-6)-10(-3) M) produced a reversible fall in pHi which showed a dependence on the concentrations of both GABA and HCO3-. The fall in pHi was associated with a transient increase in pHs and it was inhibited by a K(+)-induced depolarization. 3. In the presence of 30 mM-HCO3-, a near-saturating concentration of GABA (0.5 mM) produced a mean fall in pHi of 0.43 units. This change in pHi accounted for about two-thirds of the GABA-induced decrease (from -66 to -29 mV) in the sarcolemmal H+ driving force, while the rest was due to the simultaneous depolarization. 4. The apparent net efflux of HCO3- (JHCO3e) produced by a given concentration of GABA was estimated on the basis of the instantaneous rate of change of pHi. In the presence of 30 mM-HCO3-, JHCO3e following exposure to 0.5 mM-GABA had a mean value of 8.0 mmol l-1 min-1. Under steady-state conditions (at plateau acidosis), the intracellular acid load produced by 0.5 mM-GABA was about 25% of that seen at the onset of the application. 5. The GABA-induced HCO3- permeability, calculated on the basis of the flux data, showed a concentration dependence similar to that of the GABA-activated conductance described in previous work. 6. The GABA-induced increase in pHs was immediately blocked by both a membrane-permeant inhibitor of carbonic anhydrase (acetazolamide, 10(-6) M) and by a poorly permeant inhibitor (benzolamide, 10(-6) M). 7. Application of acetazolamide (10(-4) M) for 5 min or more produced a decrease of up to 60% in the maximum rate of fall of pHi at GABA concentrations higher than 20 microM. 8. The recovery of the GABA-induced acidosis was associated with a fall in pHs. The recovery was completely blocked in solutions devoid of Na+ or of Cl-, as well as by DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid, 10(-5) M). This indicates that the maintenance of a non-equilibrium H+ gradient at plateau acidosis and the recovery of pHi are attributable to Na(+)-dependent Cl(-)-HCO3- exchange. 9. We conclude that the effects of GABA on pHi and pHs are due to electrodiffusion of HCO3- across postsynaptic anion channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Dependence of intracellular free calcium and tension on membrane potential and intracellular pH in single crayfish muscle fibres

The dependence of intracellular free calcium ([Ca2+]i) and tension on membrane potential and intracellular pH (pHi) was studied in single isolated fibres of the crayfish claw-opener muscle using ion-selective microelectrodes. Tension (T) was quantified as a percentage of the maximum force, or as force per cross-sectional area (N/cm2). In resting fibres, pHi had a mean value of 7.06. Contractions evoked by an increase extracellular potassium [( K+]0) produced a fall in pHi of 0.01-0.05 units. The lowest measured levels of resting [Ca2+]i corresponded to a pCai (= -log [Ca2+]i) of 6.8. Intracellular Ca2+ transients recorded during K(+)-induced contractions did not reveal any distinct threshold for force development. Both the resting [Ca2+]i and resting tension were decreased by an intracellular alkalosis and increased by an acidosis. The sensitivity of resting tension to a change in pHi (quantified as -dT/dpHi) showed a progressive increase during a fall in pHi within the range examined (pHi 6.2-7.5). The pHi/[Ca2+]i and pHi/tension relationships were monotonic throughout the multiphasic pHi change caused by NH4Cl. A fall of 0.5-0.6 units in pHi did not produce a detectable shift in the pCai/tension relationship at low levels of force development. The results indicate that resting [Ca2+]i is slightly higher than the level required for contractile activation. They also show that the dependence of tension on pHi in crayfish muscle fibres is attributable to a direct H+ and Ca2+ interaction at the level of Ca2+ sequestration and/or transport. Finally, the results suggest that in situ, the effect of pH on the Ca2+ sensitivity of the myofibrillar system is not as large as could be expected on the basis of previous work on skinned crustacean muscle fibres.

Postsynaptic fall in intracellular pH and increase in surface pH caused by efflux of formate and acetate anions through GABA-gated channels in crayfish muscle fibres

H(+)-selective microelectrodes and a two- or three-microelectrode voltage clamp were used to examine the influence of weak-acid, carboxylate anions on the actions of GABA on postsynaptic intracellular pH, surface pH and on membrane potential in fibres of the crayfish leg opener muscle. Substitution of 30 mM Cl- by formate or acetate promoted a GABA-induced decrease in intracellular pH, which was coupled to an increase in surface pH and to a depolarization. Such effects were not seen in the presence of an equivalent amount of lactate, methanesulphonate or glucuronate. Both the GABA-induced depolarization and the fall in internal pH promoted by formate and acetate were blocked by picrotoxin, and the fall in pH was reversibly inhibited by a K(+)-induced depolarization. The rate of the fall in intracellular pH produced by GABA (0.2 mM) was about 0.02 pH units/min in the presence of formate and 0.03 pH units/min in the presence of acetate. Under steady-state conditions, both 30 mM formate and acetate (but not lactate) induced a positive shift in the reversal potential of GABA-activated current, which was accounted for by a relative permeability vs Cl- of formate and acetate of 0.5 and 0.15, respectively. The conductance sequence of the anions was identical to the permeability sequence, i.e. Cl- greater than formate greater than acetate greater than lactate approximately equal to 0. This sequence is strictly correlated to the Stokes diameter of the anions. The relative permeabilities of the anions indicate that the effective diameter of the GABA-gated channel is about 0.5 nm. The fact that the GABA-induced acidosis was slower in the presence of formate than in the presence of acetate suggests that, in the former case, the rate-limiting step in the fall in internal pH is the entry of non-dissociated formic acid. All the above results are consistent with a scheme where GABA induces a channel-mediated efflux of permeant weak-acid anions, which gives rise to an inward (depolarizing) current and to an intracellular acidosis. A comparison of the permeability properties of crayfish and vertebrate GABA-gated channels suggests that effects similar to those seen in this work are likely to occur in mammalian and other vertebrate neurons in the presence of permeant weak-acid anions.

Influence of GABA-gated bicarbonate conductance on potential, current and intracellular chloride in crayfish muscle fibres

1. The effects of gamma-aminobutyric acid (GABA) on membrane potential and conductance as well as on the intracellular Cl- activity (aiCl) and intracellular pH (pHi) were studied in crayfish muscle fibres using a three-microelectrode voltage clamp and ion-selective microelectrodes. In the presence of CO2-HCO3-, the intracellular HCO3- activity (aiHCO3) was estimated from pHi. 2. In a nominally HCO3(-)-free solution, a near-saturating concentration of GABA (0.2 mM) produced a marked increase in membrane conductance but little change in potential. In a solution containing 30 mM-HCO3- (equilibrated with 5% CO2 + 95% air; pH 7.4), the GABA-induced increase in conductance was associated with a depolarization of about 15 mV, with an increase in aiCl and with a decrease in aiHCO3. All these effects were blocked by picrotoxin (PTX). The depolarizing action of GABA was augmented following depletion of extracellular and intracellular Cl-. 3. The GABA-induced increase in aiCl which took place in the presence of HCO3- was blocked by clamping the membrane potential at its resting level. This indicates that the increase in aiCl was due to passive redistribution of Cl-. In both the presence and absence of HCO3-, the GABA-activated transmembrane flux of Cl- showed reversal at the level of the resting potential, which indicates that under steady-state conditions the Cl- equilibrium potential (ECl) is identical to the resting potential. 4. In a Cl(-)-free, 30 mM-HCO3(-)-containing solution, 0.5 mM-GABA produced a PTX-sensitive increase in conductance which amounted to 15% of the conductance activated in the presence of Cl-. In the absence of both Cl- and HCO3-, the respective figure was 2.8%. Assuming constant-field conditions, the conductance data yielded a permeability ratio PHCO3/PCl of 0.42 for the GABA-activated channels. 5. In a Cl(-)-containing, HCO3(-)-free solution, the reversal potential of the GABA-activated current (EGABA) was, by about 1 mV, less negative than the resting membrane potential (RP). In a solution containing Cl- and 30 mM-HCO3-, EGABA-RP was 12 mV. Simultaneous measurements of EGABA, aiCl and aiHCO3 (pHi) gave a PHCO3/PCl value of 0.33. 6. In a Cl(-)-free, HCO3(-)-containing solution EGABA was close to the HCO3- equilibrium potential (EHCO3) and an experimental acidosis which produced a negative shift in EHCO3 was associated with a similar shift in EGABA.(ABSTRACT TRUNCATED AT 400 WORDS)