Intracellular chloride activity and the effects of acetylcholine in snail neurones

The role of SLC12A family of cation-chloride cotransporters and drug discovery methodologies

The solute carrier family 12 (SLC12) of cation-chloride cotransporters (CCCs) comprises potassium chloride cotransporters (KCCs, e.g. KCC1, KCC2, KCC3, and KCC4)-mediated Cl- extrusion, and sodium potassium chloride cotransporters (N[K]CCs, NKCC1, NKCC2, and NCC)-mediated Cl- loading. The CCCs play vital roles in cell volume regulation and ion homeostasis. Gain-of-function or loss-of-function of these ion transporters can cause diseases in many tissues. In recent years, there have been considerable advances in our understanding of CCCs' control mechanisms in cell volume regulations, with many techniques developed in studying the functions and activities of CCCs. Classic approaches to directly measure CCC activity involve assays that measure the transport of potassium substitutes through the CCCs. These techniques include the ammonium pulse technique, radioactive or nonradioactive rubidium ion uptake-assay, and thallium ion-uptake assay. CCCs' activity can also be indirectly observed by measuring γ-aminobutyric acid (GABA) activity with patch-clamp electrophysiology and intracellular chloride concentration with sensitive microelectrodes, radiotracer 36Cl-, and fluorescent dyes. Other techniques include directly looking at kinase regulatory sites phosphorylation, flame photometry, 22Na+ uptake assay, structural biology, molecular modeling, and high-throughput drug screening. This review summarizes the role of CCCs in genetic disorders and cell volume regulation, current methods applied in studying CCCs biology, and compounds developed that directly or indirectly target the CCCs for disease treatments.

Chloride flux in phagocytes

Phagocytes, such as neutrophils and macrophages, engulf microbes into phagosomes and launch chemical attacks to kill and degrade them. Such a critical innate immune function necessitates ion participation. Chloride, the most abundant anion in the human body, is an indispensable constituent of the myeloperoxidase (MPO)-H2 O2 -halide system that produces the potent microbicide hypochlorous acid (HOCl). It also serves as a balancing ion to set membrane potentials, optimize cytosolic and phagosomal pH, and regulate phagosomal enzymatic activities. Deficient supply of this anion to or defective attainment of this anion by phagocytes is linked to innate immune defects. However, how phagocytes acquire chloride from their residing environment especially when they are deployed to epithelium-lined lumens, and how chloride is intracellularly transported to phagosomes remain largely unknown. This review article will provide an overview of chloride protein carriers, potential mechanisms for phagocytic chloride preservation and acquisition, intracellular chloride supply to phagosomes for oxidant production, and methods to measure chloride levels in phagocytes and their phagosomes.

Chloride distribution in Aplysia neurones

1. The intracellular Cl(-) concentration (Cl(i)) and the membrane potential (E(m)) were measured in the medial pleural neurones of Aplysia under various experimental conditions designed to determine the Cl(-) conductance of the neurones and investigate the possibility of an active Cl(-) transport.2. The magnitude of the Cl(-) conductance of the cell depends on the experimental conditions.3. In normal sea water, large changes of E(m) produced by passing current across the cell membrane caused no change of Cl(i), suggesting that the Cl(-) conductance was low. Similarly, moderate changes of E(Cl) produced by decreasing Cl(o) or increasing Cl(i) had little or no effect on E(m).4. A high Cl(-) conductance was observed in high K(o) or very low Cl(o). It was greatly reduced if the external Ca(2+) was replaced by Co(2+), or in the presence of tubocurarine, or if the experiment was performed on an isolated cell soma. The high Cl(-) conductance is therefore attributed to the release of ACh and perhaps other transmitters from synaptic terminals.5. High concentrations of tetraethylammonium ions or procaine induced a depolarization of the cell, but a decrease of Cl(i). The rate of fall of Cl(i) was increased by lowering external K(+) or raising external Ca(2+), and was decreased by replacing external Ca(2+) by Co(2+).6. NH(4) (+) ions applied externally had effects similar to those of K(+) ions. In situations in which intracellular NH(4) (+) might be increased a fall in Cl(i) was observed.7. The changes of Cl(i) caused by TEA, procaine, or internal NH(4) (+) occur against the driving force for passive Cl(-) movements. They are still observed in isolated cell bodies, and cannot be attributed to the activation of synaptic channels.8. Some interpretations of these anomalous Cl(-) movements are discussed which could also account for the difference between E(Cl) and E(m) observed under normal conditions.

Initial studies on the direct and modulatory effects of nitric oxide on an identified central Helix aspersa neuron

The generation of the novel messenger molecule nitric oxide (NO) has been demonstrated in many tissues across phyla including nervous systems. It is produced on demand by the enzyme nitric oxide synthase often stimulated by intracellular calcium and typically affecting guanylate cyclase thought to be its principal target in an auto and/or paracrine fashion. This results in the generation of the secondary messenger cyclic guanosine monophosphate (cGMP). Nitric oxide synthase has been demonstrated in various mollusk brains and manipulation of NO levels has been shown to affect behavior in mollusks. Apart from modulation of the effect of the peptide GSPYFVamide, there appears little published on direct or modulatory effects of NO on Helix aspersa central neurons. We present here initial results to show that NO can be generated in the region around F1 in the right parietal ganglion and that NO and cGMP directly hyperpolarize this neuron. For example, application of the NO-donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP; 200 µM) can cause a mean hyperpolarization of 41.7 mV, while 2 mM 8-bromo-cyclic guanosine monophosphate (8-bromo-cGMP) produced a mean hyperpolarization of 33.4 mV. Additionally, pre-exposure to NO-donors or cGMP appears to significantly reduce or even eliminates the normal hyperpolarizing K(+)-mediated response to dopamine (DA) by this neuron; 200 µM SNAP abolishes a standard response to 0.5 µM DA while 1 mM 8-bromo-cGMP reduces it 62%.

Genetically encoded optical sensors for monitoring of intracellular chloride and chloride-selective channel activity

This review briefly discusses the main approaches for monitoring chloride (Cl⁻), the most abundant physiological anion. Noninvasive monitoring of intracellular Cl⁻ ([Cl⁻]i) is a challenging task owing to two main difficulties: (i) the low transmembrane ratio for Cl⁻, approximately 10:1; and (ii) the small driving force for Cl⁻, as the Cl⁻ reversal potential (E_Cl) is usually close to the resting potential of the cells. Thus, for reliable monitoring of intracellular Cl⁻, one has to use highly sensitive probes. From several methods for intracellular Cl⁻ analysis, genetically encoded chloride indicators represent the most promising tools. Recent achievements in the development of genetically encoded chloride probes are based on the fact that yellow fluorescent protein (YFP) exhibits Cl⁻-sensitivity. YFP-based probes have been successfully used for quantitative analysis of Cl⁻ transport in different cells and for high-throughput screening of modulators of Cl⁻-selective channels. Development of a ratiometric genetically encoded probe, Clomeleon, has provided a tool for noninvasive estimation of intracellular Cl⁻ concentrations. While the sensitivity of this protein to Cl⁻ is low (EC₅₀ about 160 mM), it has been successfully used for monitoring intracellular Cl⁻ in different cell types. Recently a CFP–YFP-based probe with a relatively high sensitivity to Cl⁻ (EC₅₀ about 30 mM) has been developed. This construct, termed Cl-Sensor, allows ratiometric monitoring using the fluorescence excitation ratio. Of particular interest are genetically encoded probes for monitoring of ion channel distribution and activity. A new molecular probe has been constructed by introducing into the cytoplasmic domain of the Cl⁻-selective glycine receptor (GlyR) channel the CFP–YFP-based Cl-Sensor. This construct, termed BioSensor-GlyR, has been successfully expressed in cell lines. The new genetically encoded chloride probes offer means of screening pharmacological agents, analysis of Cl⁻ homeostasis and functions of Cl⁻-selective channels under different physiological and pathological conditions.

Genetically encoded chloride indicator with improved sensitivity

Chloride (Cl) is the most abundant physiological anion. Abnormalities in Cl regulation are instrumental in the development of several important diseases including motor disorders and epilepsy. Because of difficulties in the spectroscopic measurement of Cl in live tissues there is little knowledge available regarding the mechanisms of regulation of intracellular Cl concentration. Several years ago, a CFP-YFP based ratiometric Cl indicator (Clomeleon) was introduced [Kuner, T., Augustine, G.J. A genetically encoded ratiometric indicator for chloride: capturing chloride transients in cultured hippocampal neurons. Neuron 2000; 27: 447-59]. This construct with relatively low sensitivity to Cl (K(app) approximately 160 mM) allows ratiometric monitoring of Cl using fluorescence emission ratio. Here, we propose a new CFP-YFP-based construct (Cl-sensor) with relatively high sensitivity to Cl (K(app) approximately 30 mM) due to triple YFP mutant. The construct also exhibits good pH sensitivity with pK(alpha) ranging from 7.1 to 8.0 pH units at different Cl concentrations. Using Cl-sensor we determined non-invasively the distribution of [Cl](i) in cultured CHO cells, in neurons of primary hippocampal cultures and in photoreceptors of rat retina. This genetically encoded indicator offers a means for monitoring Cl and pH under different physiological conditions and high-throughput screening of pharmacological agents.

Intracellular chloride activity and the effect of 5-hydroxytryptamine on the chloride conductance of leech Retzius neurons

Intracellular Cl- activity (aCli) and 5-hydroxytryptamine (5-HT)-induced membrane currents of Retzius neurons in the central nervous system of the medicinal leech were measured using Cl- sensitive microelectrodes and a two-microelectrode voltage-clamp technique. At the membrane of Retzius neurons Cl- ions were not passively distributed. Under different conditions the chloride equilibrium potential (ECl, -60.1 mV for isotonic saline and -57.8 mV for a hypertonic saline) was negative with respect to the membrane potential (Em, -55 +/- 3.8 and -47 +/- 3.4 mV respectively). The endogenous neurohormone 5-HT always polarized the membrane of Retzius neurons in the direction of ECl. When voltage-clamping the membrane of Retzius neurons near the resting potential both in situ and in primary culture, application of 5-HT produced an outward current (I5-HT) and an increase in membrane conductance. Current-voltage relationships for I5-HT showed a slight outward rectification and reversal potentials of -61.6 +/- 3.1 mV in situ and -66 +/- 3.1 mV in primary culture, both values being comparable to the ECl of Retzius neurons as measured in situ. The results indicate that 5-HT increases the Cl- conductance of Retzius neurons, thereby hyperpolarizing the cell membrane and affecting both the excitability of the neuron and 5-HT release from it. This could affect the feeding and swimming behaviour of the leech.

Different types of glutamate receptors in isolated and identified neurones of the mollusc Planorbarius corneus

1. The membrane currents evoked by glutamate agonists on isolated and identified neurones of molluscan pedal ganglia were investigated using the voltage clamp technique. 2. The fast chloride current (Er (reversal potential) = -41 mV) evoked in a Ped-9 neurone by application of glutamate, quisqualate and ibotenic acid could be blocked by furosemide (0.1 mM). The slow potassium current (Er = -85 mV) evoked in Ped-8 and Ped-9 neurones by glutamate, quisqualate and kainate could be blocked by tetraethylammonium (50 microM). 3. N-Methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA) failed to induce a response in neurones studies. 4. The spider venoms argiopine and argiopinine III (50-500 nM) selectively inhibited quisqualate-induced potassium current, but had no influence on glutamate-, ibotenate- or quisqualate-induced chloride and kainate-induced potassium currents. Glutamate-induced potassium current was partially inhibited by argiopine and argiopinine III. 5. The existence of several types of distinct glutamate receptors was confirmed in cross-desensitization experiments, and a lack of interaction was observed between quisqualate and kainate. 6. Potassium currents induced both by quisqualate and kainate strongly depended on temperature and could be blocked by pertussis toxin. Intracellular injection of the calcium chelator, EGTA, did not affect quisqualate and kainate responses. 7. In neurones loaded with non-hydrolysable GTP analogues, GTP-gamma-S (guanosine-5'-O-(3-thio)triphosphate) or GppNHp (5'-guanylylimidodiphosphate), the potassium current was gradually induced in the absence of agonists. As this current progressed, the magnitude of the glutamate- or kainate-evoked current transients became smaller and finally negligible. The GTP-gamma-S-induced current was not inhibited by argiopine. 8. These data indicate that in the molluscan neurones studied there are at least three pharmacologically distinct glutamate receptors: (1) a receptor of quisqualate-ibotenate type which directly controls chloride channel; (2) quisqualate and (3) kainate receptors which control in calcium-independent manner the common potassium channel by activation of GTP-binding protein.

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)

The ionic mechanism associated with the action of putative transmitters on identified neurons of the snail, Helix aspersa

Intracellular recordings were made from identified neurons in the suboesophageal ganglionic mass of the snail, Helix aspersa. The ionic mechanisms associated with acetylcholine excitation and inhibition, dopamine excitation and inhibition, gamma-aminobutyric acid (GABA) excitation and inhibition and serotonin excitation were investigated. Acetylcholine excitation was found to involve an initial increase in sodium conductance while acetylcholine inhibition was a pure chloride event which reversed at membrane potentials more negative than the chloride equilibrium potential. Dopamine excitation appeared to involve only an increase in sodium conductance while serotonin excitation involved an increase in conductance to both sodium and calcium ions. Dopamine inhibition was associated with an increase in potassium conductance but failed to reverse at membrane potentials more negative than the potassium equilibrium potential. GABA excitation involved conductance increases to both sodium and chloride ions while GABA inhibition was a pure chloride event. An attempt was made to estimate the degree of co-operativity of the putative transmitters with their receptors using log-log and Hill plots. The slopes of the line for the log-log plots for acetylcholine excitation and inhibition were 0.88 and 1.1, respectively, suggesting the interaction of one molecule of acetylcholine with the receptor. The slope of the log-log plot for dopamine inhibition was 0.46 while that for serotonin excitation was 0.75. The Hill plots for GABA excitation and inhibition were 1.64 and 1.42, respectively, suggesting that two molecules of GABA are required for receptor activation.

Acid influx into snail neurones caused by reversal of the normal pHi-regulating system

Intracellular pH (pHi), and Na+ and Cl- activities were measured with ion-sensitive micro-electrodes in Helix aspersa neurones, and the effects of reducing external pH (pHo) were investigated. When pHo was changed from 7.5 to 6.5 keeping CO2 constant, there was a slow fall in pHi, a rise in internal Cl and a fall in internal Na. These ionic changes are opposite to those caused by normal operation of the pHi-regulating system. These effects of external acidification were inhibited by the application of SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) or by the removal of external Cl. Raising intracellular Na activity by inhibiting the Na pump increased the rate of fall of pHi in acid solutions. In acid solutions the average rate of acid uptake attributable to reversed pHi-regulation was about three times the rate of loss of internal Na, and about twice the rate of Cl uptake. We conclude that these intracellular ion changes in acid solutions were largely due to a reversal of the pHi-regulating mechanism, so that it carried acid into, rather than out of, the cell interior.

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic mechanisms of the rapid (nicotinic) phase of acetylcholine response in identified Planobarius corneus neurones

Current responses to acetylcholine (ACh) and to suberyldicholine (D-6) applied from the double-barrelled ionophoretic micropipette were studied in two identified neurones (LPed-2 and LPed-3) isolated from the left ganglion of pulmonate mollusc, Planorbarius corneus. Experiments made with K2SO4-filled microelectrodes show that in LPed-2 neurone two kinds of cholinoreceptors are involved in the rapid phase of ACh response one of which induces chloride conductance and the other, sodium conductance. The Cl-dependent component can be separated from the cationic one by C-6 whereas the cationic component can be separated from the Cl--dependent one by furosemide. Cl- conductance can be induced selectively by D-6. In the LPed-3 neurone only Cl- conductance increases during rapid phase of ACh response. The reversal potential of Cl--dependent responses was found to be more negative than the resting potential in experiments made with K2SO4-filled microelectrodes but less negative than the resting potential in the case of KCl-filled microelectrodes. This difference seems to be due to the artificial increase of intracellular chloride concentration.

Measurement of intracellular chloride in guinea-pig vas deferens by ion analysis, 36chloride efflux and micro-electrodes

1. Cl-sensitive micro-electrodes were used to measure the intracellular Cl activity (a(Cl) (i)) in smooth muscle cells of the guinea-pig vas deferens. The values obtained were compared with those of intracellular Cl (Cl(i)) found by both ion analysis and (36)Cl efflux.2. Various combinations of filling solution for recording membrane potential (E(m)), and type of micro-electrode were tested. The most successful, which allowed continuous recording of a(Cl) (i) for several hours, was a double-barrelled electrode using the reference liquid ion exchanger (RLIE; Thomas & Cohen, 1981). However, a(Cl) (i) measured both by simultaneous impalements of separate cells with Cl-sensitive and conventional micro-electrodes, and by double-barrelled micro-electrodes, was about 42 mM in normal Krebs solution. This is five times higher than the value from a passive distribution. E(Cl) was about -24 mV, more than 40 mV positive to E(m).3. On complete removal of extracellular Cl (Cl(o)), a(Cl) (i) fell to an apparent level of about 3 mM. If this represents interference from other anions, the maximum error in E(Cl) measured in normal Krebs solution is 2.5 mV. Replacement of Cl(o) caused a rapid increase in a(Cl) (i). This must be caused by an active transport of Cl(-) ions into the cell against their electrochemical gradient.4. The stabilized values of a(Cl) (i) measured at different levels of Cl(o) agree surprisingly well with a(Cl) (i) estimated from ion analysis and (36)Cl efflux, assuming that the intracellular activity coefficient was the same as measured in the normal Krebs solution. The relationship of a(Cl) (i) to Cl(o) was hyperbolic.5. It is concluded that Cl-sensitive micro-electrodes accurately measure a(Cl) (i) in smooth muscle cells. The remarkable agreement between the direct and indirect methods of measuring Cl(i) suggests that Cl(-) ions are not bound to a significant extent and that the compartment seen by the micro-electrodes is probably representative of the whole cell.

The role of intracellular chloride in hyperpolarizing post-synaptic inhibition of crayfish stretch receptor neurones

1. The intracellular Cl(-) activity (a(Cl) (i)) of isolated crayfish stretch receptor neurones was measured using liquid ion exchanger Cl(-)-selective micro-electrodes. The potential developed due to the difference between the normal extracellular Cl(-) activity (a(Cl) (o)) and a(Cl) (i) (V(Cl)) was compared with the simultaneously measured reversal potential of the inhibitory post-synaptic potential (E(i.p.s.p.)) to further clarify the ionic basis of the i.p.s.p..2. In normal Ringer solution, V(Cl) (63.3 +/- 2.3 mV) was found to be close to the resting membrane potential (E(m), 62.6 +/- 3.9 mV) while E(i.p.s.p.) (74.5 +/- 1.9 mV) was more negative than either. The V(Cl) value corresponds to an apparent a(Cl) (i) of 12.7 +/- 1.3 mM, which is about 4 mM more than required for a Cl(-) governed E(i.p.s.p.) of 74.5 mV.3. Reducing a(Cl) (o) caused smaller changes in V(Cl) than predicted for passive Cl(-) re-distributions. On complete removal of extracellular Cl(-) (Cl(o) (-)), V(Cl) increased to 84.6 +/- 2.7 mV, equivalent to an apparent a(Cl) (i) of about 5 mM-Cl(-). This value can be used as an estimate of the level of intracellular interference on the Cl(-)-selective micro-electrode.4. Increasing extracellular K(+) (K(0) (+)) decreased both V(Cl) and E(i.p.s.p.). Decreasing K(o) (+) had the converse effect. The time course of the changes in V(Cl) and E(i.p.s.p.) was much the same. The difference between V(Cl) and E(i.p.s.p.) decreased to about 3 mV in high K(o) (+), and increased to about 30 mV in low K(o) (+). This variation in the difference between E(i.p.s.p.) and V(Cl) is consistent with the assumption that anions other than Cl(-) contribute to the recorded V(Cl) rather than another ion contributes to the inhibitory current.5. Application of 5 mM-NH(4) (+) or of frusemide (6 x 10(-4) M) decreased V(Cl) and E(i.p.s.p.). The difference between V(Cl) and E(i.p.s.p.) was also decreased.6. We conclude that a(Cl) (i) is lower than predicted from a passive distribution and thus the chloride equilibrium potential (E(Cl)) is more negative than E(m). If a constant intracellular interference equivalent to about 4 mM-Cl(-) is assumed to contribute to the recorded V(Cl), E(Cl) was approximately equal to E(i.p.s.p.) in all the experimental conditions. Therefore we suggest that the i.p.s.p. is solely generated by Cl(-) ions.

Intracellular chloride activity in rabbit papillary muscle: effect of ouabain

Intracellular chloride activity (aiCl) and membrane potential (Vm) were measured in rabbit papillary muscle under in vitro conditions. The cellular chloride concentration (Cli) was estimated from measurements of total water content, extracellular space, and total chloride concentration. The effects of therapeutic (10(-8) M) and toxic (10(-6) M) concentrations of ouabain on these parameters were tested. The chloride-sensitive microelectrodes were of the liquid-ion exchanger type. Selectivity for HCO-3 was taken into account in the calculation of aiCl. In 11 control experiments made with two different protocols aiCl was determined in subendocardial and in deeper cells. The mean membrane potentials were -78.7 and -78.0 mV and the mean cytoplasmic chloride activities were 17.5 and 17.7 mM, respectively. The chloride equilibrium potentials were -43.5 and -43.2 mV. These results indicated that chloride is not passively distributed in rabbit papillary muscle. Ouabain (10(-8) M) did not change Vm or aiCl. At a toxic concentration of ouabain, Vm fell to -68.0 mV in superficial cells and to -67.8 mV in deeper cells, but aiCl remained unchanged . These results suggested that under in vitro conditions intracellular chloride is distributed within more than one cellular compartment.

Non-passive chloride distribution in mammalian heart muscle: micro-electrode measurement of the intracellular chloride activity

1. Liquid ion-exchanger Cl- -sensitive micro-electrodes were used to make continuous measurements of the intracellular Cl activity, aCli, of quiscent sheep cardiac Purkinje fibres in vitro. 2. aCli was higher than that expected from a passive distribution, (which would have been about 5 mM). It was 3--4 times hiable; EC1 was about 35 mV positive to Em. It was over twice as high in the nominal absence of bicarbonate/CO2 (when the buffer-system was HEPES/O2) but was not always so stable, and ECl was about 20 mV positive to Em. 3. Experiments designed to assess the maximum possible error likely to occur in the measurement of aCli showed that this could not be large and that the estimates of ECl were accurate to within 8 mV. 4. The ability of Cl to move down both concentration and potential gradients was established by demonstrating a loss of aCli in Cl-free solutions and a gain when Em was depolarized positive to ECl in high-K solutions. In both cases, the changes were complete within about 100--160 min. 5. The decline of aCli in Cl-free solutions (glucuronate-substituted) was not significantly affected by changes of [Ca]o from 0 to 12 mM or by the depolarizations of Em of up to 60 mV that sometimes occurred in low or zero [Ca]o. 6. Only 2--3 mM-aClo was sufficient to impede substantially the ready loss of aCli in HEPES-buffered solutions. 7. In high-K solutions (45 mM), Cl appeared to be passively distributed since, at equilibrium, Em and ECl differed by less than 2 mV. 8. In HEPES-buffered Tyrode, ECl of quiescent papillary muscle of the guinea-pig was, on average, 39 mV positive to Em. 9. It is concluded that liquid ion-exchanger Cl- -sensitive micro-electrodes are suitable for studying the Cl regulation of sheep Prukinje fibres, and probably of other cardiac tissues. The measurements of resting aCli are quite accurate when using either HEPES or bicarbonate-buffered Tyrode. The results are discussed in relation to estimates of the apparent membrane Cl permeability under various conditions and the possible existence of an inwardly directed 'Cl pump'.

Receptors for gamma-aminobutyric acid (GABA) on Aplysia neurons

Aplysia neurons show 5 different types of response (three excitatory and two inhibitory) to iontophoretic application of gamma-aminobutyric acid (GABA). Four of these are associated with a membrane conductance increase, but one is associated with a conductance decrease. The most common response is a fast hyperpolarization which reverses at about--58 mV and is sensitive to manipulation of external Cl- concentration, and thus is due to a specific increase in Cl- conductance. There is an infrequent, slower hyperpolarizing response which does not reverse above about--80 mV and is insensitive to external Cl-. This response appears to result from a conductance increase to K+. Two types of depolarizing responses are associated with conductance increases. These responses differ in their latency, duration and sensitivity to curare. The more frequent is relatively rapid (peak at 1-2 sec) and is depressed by curare at high concentrations. In other neurons, GABA causes a slower response, peaking at 6-10 sec, which is not curare-sensitive. Usually for both types of response, the voltage and conductance changes are completely abolished by perfusion with Na+-free seawater, and the responses cannot be reversed with depolarization. In other neurons such as L11, the response can be reversed with depolarization, and appears to result from a conductance increase to both Na+ and Cl-. In neuron R15, GABA causes a slow depolarizing response (peak at about 9 sec) which is associated with a decreased membrane conductance, probably to K+. The classical GABA antagonists, picrotoxin and bicuculline, block Cl- responses but no others, while the fast Na+ and Cl- responses are depressed by curare. Strychnine does not affect any GABA response. The multiplicity of GABA responses, the specificity of their organization and the fact that only some neurons have receptors for GABA, argue that GABA may have a role as a neurotransmitter in Aplysia. Furthermore, the existence of several types of excitatory GABA response suggests that GABA may function both as an inhibitory and excitatory neurotransmitter.

Liquid and solid-state Cl- -sensitive microelectrodes. Characteristics and application to intracellular Cl- activity in Balanus photoreceptor

When intracellular chloride activity (aiCl) was monitored with chloride-sensitive liquid ion exchanges (CLIX) microelectrodes in Balanus photoreceptors, replacement of extracellular chloride (Cl0) by methanesulfonate or glutamate was followed by a rapid but incomplete loss of aiCl. When propionate was used as the extracellular anion substitute, CLIX electrodes detected an apparent gain in aiCl, while a newly designed Ag-AgCl wire-in glass microelectrode showed a loss of aiCl under the same conditions. This discrepancy in Cl- washout when propionate replaced Cl0 is explained by the differences in selectivity of CLIX and Ag-AgCl electrodes for native intracellular anions and for the extracellular anion substitute which also replaces Cli and interferes in the determination of aiCl. Both electrodes indicate that ECl approximately Em when the cells are bathed in normal barnacle saline, and both electrodes showed the rate of Cl washout (tau approximately 5 min) to be independent of Cli when Cl0 was replaced by glutamate. Details of Ag-AgCl microelectrode construction are presented. These electrodes were tested and found to be insensitive to the organic anion substitutes used in this study. Selectivity data of CLIX electrodes for several anions of biological interest are described.

Continuous direct measurement of intracellular chloride and pH in frog skeletal muscle

1. Ion-sensitive electrodes (made with a chloride-sensitive ion-exchange resin) were used to measure the internal chloride activity (a(i) (Cl)) of frog sartorius fibres at 25 degrees C.2. The internal pH (pH(i)) of other sartorius fibres was measured with a recessed tip pH-sensitive electrode (made with pH-sensitive glass).3. In normal bicarbonate-free solution (containing 2.5 mM potassium), the average chloride equilibrium potential, E(Cl) (calculated from a(i) (Cl) and the measured chloride activity of the external solution (a(o) (Cl)) was 87.7 +/- 1.7 mV (mean +/- S.E.; n = 16) in fibres where the average membrane potential, E(m), was 88.3 +/- 1.5 mV (mean +/- S.E.; n = 16). In experiments where a(i) (Cl) was varied between about 1 and 10 mM (which corresponds to values of E(m) between about -105 and -50 mV) E(Cl) was within 1-3 mV of E(m) at equilibrium. These measurements of a(i) (Cl) were obtained from the potential difference between the chloride-sensitive electrode and an intracellular indifferent micro-electrode filled with potassium chloride. If a potassium sulphate-filled indifferent micro-electrode was used, then values of a(i) (Cl) below about 5 mM were erroneously high, probably due to interference from other sarcoplasmic ions at the indifferent electrode.4. In solutions containing 15 mM bicarbonate and gassed with 5% CO(2), pH(i) was 6.9, corresponding to an internal bicarbonate concentration of 7.6 mM. E(Cl) measured in this solution was some 4 mV positive to E(m). Most of the difference between E(Cl) and E(m) could be ascribed to interference by sarcoplasmic bicarbonate on the basis of selectivity measurements of chloride against bicarbonate made on the ion-exchange resin in the relevant range of a(Cl).5. If bicarbonate/CO(2) in the external solution was replaced by HEPES/pure O(2) at constant pH, then pH(i) rose from 6.88 +/- 0.02 (mean +/- S.E.) to 7.05 +/- 0.02. A change in external pH of 1 unit caused pH(i) to change by about 0.02 unit and the intracellular buffering power was calculated to be about 35.6. In solution made hypertonic by the addition of sucrose, E(m) changed little or depolarized and E(Cl) and E(m) remained close. In contrast, in solution made hypertonic by the addition of solid sodium chloride (high-chloride solution) E(Cl) became negative to E(m). Conversely in low chloride solution E(Cl) became positive to E(m).7. When the chloride permeability (P(Cl)) was reduced by the use of acid solution, E(Cl) moved positive to E(m) indicating an accumulation of internal chloride. When P(Cl) was increased again by returning to more alkaline solution, E(m) depolarized to E(Cl).8. The results are consistent with the existence of a small, active movement of chloride, the effects of which are normally obscured by large passive movements of chloride when P(Cl) is large.

Pancreatic acinar cells: localization of acetylcholine receptors and the importance of chloride and calcium for acetylcholine-evoked depolarization

1. Intracellular micro-electrode recordings of acinar cell membrane potential and resistance were made from the mouse pancreas superfused in vitro. The acinar cells under investigation were stimulated by micro-iontophoretic ACh application from an extracellular AChCl-filled micro-electrode.2. Passing short-lasting ejecting current pulses through the AChCl-electrode caused acinar cell depolarization when the electrode was in an extracellular position not far (< 50 mum) from an acinus impaled by a KCl micro-electrode. After insertion of the AChCl electrode into a neighbouring acinar cell, electrically coupled to the acinar cell already impaled by the KCl-electrode, ejecting ACh current pulses only affected the membrane potential in a direct electrical manner whereas there was no sign of an effect of ACh on the membrane potential.3. Replacing extracellular chloride by sulphate caused a marked increase in the amplitude of the ACh-evoked depolarization. If the membrane potential was recorded with a KCl electrode ACh continued to evoke very large depolarizations even after more than 1 hr exposure to Cl-free solution. If the membrane potential was recorded with a K-citrate electrode the effect of Cl-removal was only transient. Removal of Na(+) during exposure to Cl-free solution reduced the amplitude of the ACh-evoked depolarization somewhat. Readmission of Cl after more than 1 hr of Cl deprivation caused an immediate reversal of the ACh effect into a hyperpolarization.4. Removal of extracellular Ca(2+) caused a marked reduction in the amplitude of small depolarizations evoked by just suprathreshold doses of ACh, whereas there was very little effect on larger depolarizations evoked by maximal or supramaximal ACh ejections. The effect of Ca removal was fully reversible. Addition of Mn after Ca-deprivation was as efficient as Ca in restoring normal electrophysiological responses to small doses of ACh.5. The acinar cell membrane seems only to be responsive to ACh added to the extracellular side and ACh probably causes an increase in membrane Cl permeability in addition to the previously described effects on Na and K permeability. Ca may be important in determining ACh receptor sensitivity.

A new solid-state microelectrode for measuring intracellular chloride activities

Solid-state microelectrodes from measuring intracellular Cl activity (alphaiCl) were made by sealing the tips of tapered glass capillaries (tip diameter 0.3 mum), coating them under vacuum with a 0.2-0.3 mum thick layer of spectrscopic grade silver, and sealing them (except for the terminal 2-5 mum of the tip) inside tapered glass shields. 106 microelectrodes had an average slope of 55.0+/- 0.6 m V (S,E,) per decade c hange in alphaCl. Tip resistance was (77.1+/- 3.1) x 10(9) omega(n=30). Electrode response was rapid (10-20 s), was unaffected by HCO3, H2PO4, HPO42 or protein, and remained essentially unchanged over a 24-h period. AlphaiCl in frog sartorius muscle fibers and epithelial cells of bullfrog small intestine was measured in vitro. In both tissues, alphaiCl significantly exceeded the value corresponding to equlibrium ditribution of Cl across the cell membrane.

The effect of carbon dioxide on the intracellular pH and buffering power of snail neurones

1. Intracellular pH (pHi) was measured using pH-sensitive glass micro-electrodes. The effects on pHi of CO2 applied externally and HCO3-, H+ and NH4+ injected iontophoretically, were investigated. 2. The transport numbers for iontophoretic injection into aqueous micro-droples were found by potentiometric titration to be 0-3 for HCO3- and 0-94 for H+. 3. Exposure to Ringer, pH 7-5, equilibrated with 2-2% CO2 caused a rapid, but only transient, fall in pHi. Within 1 or 2 min pHi began to return exponentially to normal, with a time constant of about 5 min. 4. When external CO2 was removed, pHi rapidly increased, and then slowly returned to normal. The pHi changes with CO2 application or removal gave a calculated intracellular buffer value of about 30 m-equiv H+/pH unit per litre. 5. Injection of HCO3- caused a rise in pHi very similar to that seen on removal of external CO2. 6. The pHi responses to CO2 application, CO2 removal and HCO3- injection were slowed by the carbonic anhydrase inhibitor acetazolamide. 7. H+ injection caused a transient fall in pHi. In CO2 Ringer pHi fell less and recovered faster than in CO2-free Ringer. Calculation of the internal buffer value from the pHi responses to H+ and HCO3- injection gave very similar values. 8. The internal buffer value (measured by H+ injection) was greatly increased by exposure to CO2 Ringer. Acetazolamide reduced this effect of CO2, suggesting that the function of intracellular carbonic anhydrase may be to maximize the internal buffering power in CO2. 9. It was concluded that the internal HCO3- was determined primarily by the CO2 level and pHi, that internal HCO3- made a large contribution to the buffering power, and that after internal acidfication pHi was restored to normal by active transport of H+, OH- or HCO3- across the cell membrane. The active transport was much faster in CO2 than in CO2-free Ringer.

The effects of lithium and sodium on the potassium conductance of snail neurones

1. The iontophoretic injection of lithium into snail neurones reversibly increased the resting relative potassium permeability (PK). 2. Long exposures to snail Ringer containing 25 mM-Li and correspondingly reduced Na also caused an increase in PK. This did not occur with Ringer in which the same reduction of Na was made by replacing it with Tris. 3. Replacement of part of the Ringer Na by either Li or Tris led to proportional decreases in internal Na. 4. Injecting large quantities of Na into ouabain-treated cells caused effects similar to those of Li injection. Without ouabain, Na injection stimulated the electrogenic Na pump. 5. A number of tests failed to produce any clear evidence that intracellular Ca was involved in the response to Li.