Intracellular free sodium and potassium, post-carbachol hyperpolarization, and extracellular potassium-undershoot in rat sympathetic neurones

Quantitative determination of cellular [Na+] by fluorescence lifetime imaging with CoroNaGreen

Meyer et al. establish the suitability of the sodium-sensitive indicator dye CoroNaGreen for fluorescence lifetime imaging inside cells. This approach represents a valuable tool for quantitative and dynamic determination of intracellular sodium concentrations independent of dye concentration.

Fluorescence lifetime imaging microscopy (FLIM) with fluorescent ion sensors enables the measurement of ion concentrations based on the detection of photon emission events after brief excitation with a pulsed laser source. In contrast to intensity-based imaging, it is independent of dye concentration, photobleaching, or focus drift and has thus been successfully employed for quantitative analysis of, e.g., calcium levels in different cell types and cellular microdomains. Here, we tested the suitability of CoroNaGreen for FLIM-based determination of sodium concentration ([Na⁺]) inside cells. In vitro measurements confirmed that fluorescence lifetimes of CoroNaGreen (CoroNaFL) increased with increasing [Na⁺]. Moreover, CoroNaFL was largely independent of changes in potassium concentration or viscosity. Changes in pH slightly affected FL in the acidic range (pH ≤ 5.5). For intracellular determination of [Na⁺], HEK293T cells were loaded with the membrane-permeable form of CoroNaGreen. Fluorescence decay curves of CoroNaGreen, derived from time-correlated single-photon counting, were approximated by a bi-exponential decay. In situ calibrations revealed a sigmoidal dependence of CoroNaFL on [Na⁺] between 0 and 150 mM, exhibiting an apparent K_d of ∼80 mM. Based on these calibrations, a [Na⁺] of 17.6 mM was determined in the cytosol. Cellular nuclei showed a significantly lower [Na⁺] of 13.0 mM, whereas [Na⁺] in perinuclear regions was significantly higher (26.5 mM). Metabolic inhibition or blocking the Na⁺/K⁺-ATPase by removal of extracellular K⁺ caused significant [Na⁺] increases in all cellular subcompartments. Using an alternative approach for data analysis (“Ratio FLIM”) increased the temporal resolution and revealed a sequential response to K⁺ removal, with cytosolic [Na⁺] increasing first, followed by the nucleus and finally the perinuclear regions. Taken together, our results show that CoroNaGreen is suitable for dynamic, FLIM-based determination of intracellular [Na⁺]. This approach thus represents a valuable tool for quantitative determination of [Na⁺] and changes thereof in different subcellular compartments.

Modeling activity-dependent changes of axonal spike conduction in primary afferent C-nociceptors

Action potential initiation and conduction along peripheral axons is a dynamic process that displays pronounced activity dependence. In patients with neuropathic pain, differences in the modulation of axonal conduction velocity by activity suggest that this property may provide insight into some of the pathomechanisms. To date, direct recordings of axonal membrane potential have been hampered by the small diameter of the fibers. We have therefore adopted an alternative approach to examine the basis of activity-dependent changes in axonal conduction by constructing a comprehensive mathematical model of human cutaneous C-fibers. Our model reproduced axonal spike propagation at a velocity of 0.69 m/s commensurate with recordings from human C-nociceptors. Activity-dependent slowing (ADS) of axonal propagation velocity was adequately simulated by the model. Interestingly, the property most readily associated with ADS was an increase in the concentration of intra-axonal sodium. This affected the driving potential of sodium currents, thereby producing latency changes comparable to those observed for experimental ADS. The model also adequately reproduced post-action potential excitability changes (i.e., recovery cycles) observed in vivo. We performed a series of control experiments replicating blockade of particular ion channels as well as changing temperature and extracellular ion concentrations. In the absence of direct experimental approaches, the model allows specific hypotheses to be formulated regarding the mechanisms underlying activity-dependent changes in C-fiber conduction. Because ADS might functionally act as a negative feedback to limit trains of nociceptor activity, we envisage that identifying its mechanisms may also direct efforts aimed at alleviating neuronal hyperexcitability in pain patients.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Regulation of the subthreshold chloride conductance in the rat sympathetic neuron

The mechanisms that control chloride conductance (gCl) in the rat sympathetic neuron have been studied by the two-electrode voltage-clamp technique in mature, intact superior cervical ganglia in vitro. In addition to voltage dependence in the membrane potential range -120/-50 mV, gCl displays time- and activity-dependent regulation (sensitization). The resting membrane potential is governed by voltage-dependent gK and gCl, which determine values of cell input conductance ranging from 7 to 18 nS (full deactivation) to an upper value of about 130 nS (full activation and maximal gCl sensitization). The quiescent neuron, held at constant membrane potential, spontaneously and gradually moved from a low- to a high-conductance status. An increase (about 40 nS) in gCl accounted for this phenomenon, which could be prevented by imposing intermittent hyperpolarizing episodes. Following spike firing, gCl increased by 20-33 nS, independent of the cell conductance value preceding tetanization, and thereafter decayed to the pre-stimulus level within 5 min. Intracellular sodium depletion and its successive ionophoretic restoration moved the neuron from a stable low-conductance state to maximum gCl sensitization, pointing to a link between gCl sensitization and [Na+]i. The dependence of gCl build-up on [Na+]i and the time-course of such Na+-related modulation have been examined: gCl sensitization was absent at 0 [Na+]i, was well developed (20 nS) at 15 mM and tended towards a saturating value of 60 nS for higher [Na+]i. Sensitization was transient in response to neuron activity. In the silent neuron, sensitization of gCl shifted membrane potential over a range of about 15 mV.

G protein activation inhibits gating charge movement in rat sympathetic neurons

G protein-coupled receptors (GPCRs) control neuronal functions via ion channel modulation. For voltage-gated ion channels, gating charge movement precedes and underlies channel opening. Therefore, we sought to investigate the effects of G protein activation on gating charge movement. Nonlinear capacitive currents were recorded using the whole cell patch-clamp technique in cultured rat sympathetic neurons. Our results show that gating charge movement depends on voltage with average Boltzmann parameters: maximum charge per unit of linear capacitance (Q(max)) = 6.1 +/- 0.6 nC/microF, midpoint (V(h)) = -29.2 +/- 0.5 mV, and measure of steepness (k) = 8.4 +/- 0.4 mV. Intracellular dialysis with GTPgammaS produces a nonreversible approximately 34% decrease in Q(max), a approximately 10 mV shift in V(h), and a approximately 63% increase in k with respect to the control. Norepinephrine induces a approximately 7 mV shift in V(h) and approximately 40% increase in k. Overexpression of G protein beta(1)gamma(4) subunits produces a approximately 13% decrease in Q(max), a approximately 9 mV shift in V(h), and a approximately 28% increase in k. We correlate charge movement modulation with the modulated behavior of voltage-gated channels. Concurrently, G protein activation by transmitters and GTPgammaS also inhibit both Na(+) and N-type Ca(2+) channels. These results reveal an inhibition of gating charge movement by G protein activation that parallels the inhibition of both Na(+) and N-type Ca(2+) currents. We propose that gating charge movement decrement may precede or accompany some forms of GPCR-mediated channel current inhibition or downregulation. This may be a common step in the GPCR-mediated inhibition of distinct populations of voltage-gated ion channels.

Na+ signals at central synapses

A basic characteristic of animal cells is the maintenance of a steep inwardly directed electrochemical gradient for sodium ions. In vertebrate neurons, this Na+ gradient energizes intracellular ion regulation and enables influx of Na+ during action potentials and excitatory postsynaptic currents. Several studies suggested that Na+ ions could also play a role in activity-dependent synaptic plasticity. This review focuses on recent studies that demonstrated the presence of substantial intracellular Na+ transients during action potential firing or excitatory synaptic transmission in postsynaptic dendrites and dendritic spines. The large amplitudes of these activity-induced Na+ transients suggest that this signal will significantly alter electrical and biochemical properties of spines and dendrites and might influence the properties of synaptic transmission.

The identification of the sympathetic neurons innervating the hamster submandibular gland and their electrophysiological membrane properties

The neuron innervating the hamster submandibular (SM) gland was identified in the superior cervical ganglion (SCG) in vitro by recording the antidromic response using the intracellular recording technique. After the cellular response was recorded, methylene blue was injected iontophoretically into the neuron from the recording electrode, and the location of the cell soma was determined. The salivatory neurons of the SM gland were in the small- to medium-sized group of the entire cell population of the SCG. The cell size was 36.3 x 24.4 microm (mean, n=45). The postganglionic fibers were entirely unmyelinated (mean: 0.34 m/sec at 28-30 degrees C, n=141). Eighty-seven percent of the cells were distributed in the central one-third of area between the external carotid nerve origin and the caudal pole in the SCG. The resting membrane potential, membrane input resistance, membrane time constant and membrane input capacitance of the salivatory neuron were as follows: -49.2+/-7.6 mV (n=102), 52.9+/-23.6 Mohms (n=71), 8.0+/-3.4 msec (n=71) and 147+/-50 pF (n=71). Fast- and slow-excitatory postsynaptic potentials (EPSPs) were evoked, but not slow-inhibitory postsynaptic potentials (IPSPs). The fast EPSP was 13.1+/-5.7 mV in amplitude and 46.2+/-17.1 msec in duration (n=35). The slow EPSP (20 Hz, 5 sec) was 6.9+/-11 .9 mV in amplitude and 101+/-43 sec in duration (n=16). The directly-evoked spike was 63.0+/-11.9 mV in amplitude and 5.9+/-1.3 msec in duration (n=54). The spike after-hyperpolarization (AHP) was 12.5+/-3.5 mV in amplitude and 353+/-161 msec in duration. Na+ and Ca+ channels were involved in the spike generation. The voltage-dependent K+ channels (delayed rectifier), A channels and rapidly Ca2+-activated K+ channels (BK channels) regulated the spike-falling phase. The delayed rectifiers, A channels, and BK and SK (slowly Ca2+-activated) channels were involved in generation of spike-AHP. Muscarine suppressed the Ca2+ component of spike via muscarinic receptors.

A novel voltage-dependent cation current in rat neocortical neurones

1. Using the whole-cell configuration of the patch-clamp technique, an unexpected voltage-dependent cation current (Icat) was recorded from acutely isolated rat neocortical neurones, the Na+, K+ and Ca2+ currents of which were pharmacologically suppressed. 2. Icat was activated at potentials more positive than -45 mV, displayed outward rectification, and deactivated with a slow voltage-dependent time course causing prominent inward tail currents. 3. Activation of Icat was not dependent on Ca2+ influx or increases in cytosolic Ca2+, since it was not abolished by inorganic Ca2+ channel blockers or by internal Ca2+ chelators. 4. Icat was reduced by tetraethylammonium at high concentrations, but not by 4-amino-pyridine, and proved to be insensitive to cation channel blockers such as Cs+, amiloride or gadolinium. 5. Ion substitution experiments revealed that the channel producing Icat was permeable to a number of monovalent cations, including K+, Cs+, Na+ and choline+, but not to the Cl-anion. 6. The features of Icat suggest that, in electrically active neurones, it should play a role in both the initial repolarization of membrane potential after strong depolarization and the generation of depolarizing after-potential.

A five-conductance model of the action potential in the rat sympathetic neurone

The origin of the action potential in neurones has yet to be answered satisfactorily for most cells. We present here a five-conductance model of the somatic membrane of the mature and intact sympathetic neurone studied in situ in the isolated rat superior cervical ganglion under two-electrode voltage-clamp conditions. The neural membrane hosts five separate types of voltage-dependent ionic conductances, which have been isolated at 37 degrees C by using simple manipulations such as conditioning-test protocols and external ionic pharmacological treatments. The total current could be separated into two distinct inward components: (1) the sodium current, INa, and (2) the calcium current, ICa; and three outward components: (1) the delayed rectifier, IKV, (2) the transient IA, and (3) the calcium-dependent IKCa. Each current has been kinetically characterized in the framework of the Hodgkin-Huxley scheme used for the squid giant axon. Continuous mathematical functions are now available for the activation and inactivation (where present) gating mechanisms of each current which, together with the maximum conductance values measured in the experiments, allow for a satisfactory reconstruction of the individual current tracings over a wide range of membrane voltage. The results obtained are integrated in a full mathematical model which, by describing the electrical behaviour of the neurone under current-clamp conditions, leads to a quantitative understanding of the physiological firing pattern. While, as expected, the fast inward current carried by Na+ contributes to the depolarizing phase of the action potential, the spike falling phase is more complex than previous explanations. IKCa, with a minor contribution from IKV, repolarizes the neurone only under conditions of low cell internal negativity. Their role becomes less pronounced in the voltage range negative to -60 mV, where membrane repolarization allows IA to deinactivate. In the spike arising from these voltage levels the membrane repolarization is mainly sustained by IA, which proves to be the only current sufficiently fast and large enough to recharge the membrane capacitor at the speed observed during activity. Different modes of firing coexist in the same neurone and the switching from one to another is fast and governed by the membrane potential level, which makes the selection between the different voltage-dependent channel systems. The neurone thus seems to be prepared to operate within a wide voltage range; the results presented indicate the basic factors underlying the different discrete behaviours.

Is intracellular sodium involved in the mechanism of tolerance to opioid drugs?

Recent studies indicate that one of the more likely mechanisms of opioid tolerance could involve a decrease in the efficiency with which agonists can induce coupling of their specific binding sites in neuronal membranes to the activation (or deactivation) of an effector system. Reports of sodium-induced decreases in opioid receptor agonist binding and in the size of ligand/receptor complexes, as well as modulation of opioid activity by manipulation of sodium in vivo, indicate that sodium might play a physiological role in modulating opioid receptor function. Reports of morphine-induced systemic sodium retention in animals, as well as morphine-induced increases in brain intracellular sodium and decreases in brain Na+, K(+)-ATPase activity, indicate that the development of tolerance may be accompanied by changes in the disposition of sodium. The direction of these sodium- and morphine-induced changes is consistent with the hypothesis that an increase in intracellular sodium could participate in the mechanism(s) of opioid tolerance development.

A quantitative description of the sodium current in the rat sympathetic neurone

The somata of rat sympathetic neurones were voltage-clamped in vitro at 27 degrees C using separate intracellular voltage and current micro-electrodes. Na currents were isolated from other current contributions by using: Cd to block the Ca current (ICa) and the related Ca-dependent K current (IK(Ca)), and external tetraethylammonium to suppress the delayed rectifier current (IK(V) ). The holding potential was maintained at -50 mV to inactivate the fast transient K current (IA) when the IA contamination was unacceptable. Step depolarizations beyond -30 mV activated a fast, transient inward current carried by Na ions; it was suppressed by tetrodotoxin and was absent in Na-free solution. Once activated, INa declined exponentially to zero with a voltage-dependent time constant. The underlying conductance, gNa, showed a sigmoidal activation between -30 and +10 mV, with half-activation at -21.1 mV and a maximal value (mean gNa) of 4.44 microS per neurone. The steady-state inactivation level, h infinity, varied with membrane potential, ranging from complete inactivation at -30 mV to minimal inactivation at about -90 mV with a midpoint at -56.2 mV. Double-pulse experiments showed that development and removal of inactivation followed a single-exponential time course; tau h was markedly voltage-dependent and ranged from 46 ms at -50 mV to 2.5 ms at -100 mV. Besides the fast inactivation, the Na conductance showed a slow component of inactivation. The steady-state value, s infinity, was maximal at -80 mV and minimal at -40 mV. The removal of slow inactivation is a two-time-constant process, the first with a time constant in the order of hundreds of milliseconds and the second with a time constant of seconds. Slow inactivation onset appeared to be a faster process than its removal. When slow inactivation was fully removed the peak INa increased by a factor of 1.8. INa was well described by assuming it to be proportional to m3h. The temperature dependence of peak INa, tau m and tau h was studied in the temperature range 17-27 degrees C and found similar to that reported for other preparations. The Q10 of these parameters allowed the reconstruction of the INa kinetic properties at 37 degrees C.

Changes in extracellular pH during electrical stimulation of isolated rat vagus nerve

Double-barrelled pH-sensitive micro-electrodes were used to record changes of extracellular pH during repetitive stimulation of isolated rat vagus nerves. It was found that a small initial alkaline shift was followed by a prolonged acidification. The acidification was correlated in time with the poststimulus undershoot of the extracellular K+ activity and with the recovery phase of the nerve conduction velocity. In the presence of ouabain, the acid component of the pH change was completely abolished (indicating a metabolic origin), whereas the alkaline component remained unaltered. These pH changes were too small to make a significant contribution to the activity-related changes in conduction velocity of the vagal C-fibres.

Acetylcholine responses in snail neurons: increase and decrease in potassium conductance succeeding inward currents

In the identified neurons B1 and B3 of the buccal ganglion of Helix pomatia, the initial acetylcholine (ACh) inward current was succeeded by two types of secondary responses. The secondary responses consisted either in an outward current or in a long-lasting inward current or in a combination of both. The secondary outward current was decreased with membrane hyperpolarization, associated with a decrease of membrane resistance and abolished in Ca2+-free Co2+ solution. It is assumed to be a K+ current activated by an influx of Ca2+. The secondary inward current also decreased with membrane hyperpolarization, but was associated with an increase of the membrane resistance and could be mimicked by an injection of Na+ into the cells. It is suggested to be due to a block of K+ channels by intracellular Na+. When the secondary responses appeared combined, the outward current preceded the inward current.

Activity-dependent excitability changes in normal and demyelinated rat spinal root axons

Myelinated nerve fibres with a reduced safety factor for conduction due to demyelination are easily blocked by trains of impulses. To find out why, in vivo recordings from rat ventral root fibres demyelinated with diphtheria toxin have been supplemented with in vivo and in vitro recordings from normal fibres. Despite a small rise in extracellular potassium activity, normal fibres were invariably hyperpolarized by intermittent trains of impulses. This hyperpolarization resulted in an increase in threshold and also in an enhancement of the depolarizing after-potential and the superexcitable period. Replacement of NaCl in the extracellular solution by LiCl completely blocked both the membrane hyperpolarization and the threshold increase which were normally observed during intermittent trains of impulses. At demyelinated nodes which were blocked by trains of impulses (10-50 Hz), conduction block was preceded by a rise in threshold current and in an increase in internodal conduction time, but by no detectable reduction in the outward current generated by the preceding node. It was found possible to prevent the threshold from changing during a train by automatic adjustment of a d.c. polarizing current. This 'threshold clamp' prevented the conduction failure and virtually abolished the changes in internodal conduction time. The threshold changes were attributed to hyperpolarization, as in normal fibres, since (a) the polarizing current required to prevent them was always a depolarizing current, and (b) they were accompanied by an increase in superexcitability. The post-tetanic depression that can follow continuous trains of impulses was attributed to the combination of increased threshold and enhanced superexcitable period due to hyperpolarization. It is concluded that the susceptibility of these demyelinated fibres to impulse trains is not due to a membrane depolarization induced by extracellular potassium accumulation but to a membrane hyperpolarization as a consequence of electrogenic sodium pumping.

Two components of muscarine-sensitive membrane current in rat sympathetic neurones

Membrane currents induced by muscarine (Imus) were recorded in voltage-clamped neurones in isolated rat superior cervical ganglia. Two components of Imus were regularly recorded: an inward current resulting from inhibition of the outward K+ current, IM; and an outward current attributable to the reduction of a steady inward current. The presence of these two components caused a 'cross-over' in the current-voltage curves at -50 +/- 3 mV in neurones impaled with KCl-filled micro-electrodes or at -63 +/- 4 mV in neurones impaled with K-acetate-filled electrodes. Both components of Imus were prevented by atropine. Both persisted in Krebs solution containing tetrodotoxin (1 microM), Cd2+ (200 microM) or 0 Ca2+. When IM was inhibited by external Ba2+ or internal Cs+ only the outward component of Imus could be detected. This component reversed at +3 +/- 2 mV in cells impaled with CsCl-filled electrodes or at -20 +/- 3 mV in cells impaled with Cs-acetate-filled electrodes. The reversal potentials agreed with those for the currents induced by gamma-aminobutyric acid (+4 +/- 2 mV and -16 +/- 3 mV with CsCl and Cs acetate electrodes respectively). Replacement of external NaCl with Na acetate (so reducing external Cl- concentration ( [Cl-]o) from 155 to 22 mM) shifted the reversal potential for Imus by +25 and +14.5 mV in two cells impaled with CsCl-filled electrodes. A tenfold reduction of external [Na+] (by glucosamine replacement) did not significantly alter the reversal potential for Imus in KCl or CsCl-impaled cells. Under conditions where IM is already inhibited, the residual outward component of Imus can lead to hyperpolarization and inhibition of neuronal activity in unclamped cells. We conclude that both inward and outward components of Imus result from direct activation of muscarinic receptors on the ganglion cells. The inward component results from IM inhibition. We suggest that the outward component results from inhibition of another, voltage-independent current IX which largely comprises a Cl- current. The inward component induces membrane depolarization and an increased excitability; the outward component can lead to hyperpolarization and reduced excitability.

Reaccumulation of [K+]o in the toad retina during maintained illumination

Using K+-selective microelectrodes, [K+]o was measured in the subretinal space of the isolated retina of the toad, Bufo marinus. During maintained illumination, [K+]o fell to a minimum and then recovered to a steady level that was approximately 0.1 mM below its dark level. Spatial buffering of [K+]o by Müller (glial) cells could contribute to this reaccumulation of K+. However, superfusion with substances that might be expected to block glial transport of K+ had no significant effect upon the reaccumulation of K+. These substances included blockers of gK (TEA+, Cs+, Rb+, 4-AP) and a gliotoxin (alpha AAA). Progressive slowing of the rods' Na+/K+ pump (perhaps caused by a light-evoked decrease in [Na+]i) also could contribute to this reaccumulation of K+ by reducing the uptake of K+ from the subretinal space. As evidence for a major contribution by this mechanism, treatments designed to prevent such slowing of the pump reversibly blocked reaccumulation. These treatments included superfusion with 2 microM ouabain, or lowering [K+]o, PO2, or temperature. It is likely that such treatments inhibit the pump, increase [Na+]i, and attenuate any light-evoked decrease in [Na+]i. The results are consistent with the following hypothesis. At light onset, the decrease in rod gNa will reduce the Na+ influx and the resulting rod hyperpolarization will reduce the K+ efflux. In combination with these reduced passive fluxes, the continuing active fluxes will lower both [K+]o and [Na+]i, which in turn will inhibit the pump. In support of this hypothesis, the solutions to a pair of coupled differential equations that model changes in both [K+]o and [Na+]i match quantitatively the time course of the observed changes in [K+]o during and after maintained illumination for all stimuli examined.

Different types of potassium transport linked to carbachol and gamma-aminobutyric acid actions in rat sympathetic neurons

Carbachol and gamma-aminobutyric acid depolarize mammalian sympathetic neurons and increase the free extracellular K+-concentration. We have used double-barrelled ion-sensitive microelectrodes to determine changes of the membrane potential and of the free intracellular Na+-, K+- and Cl- -concentrations ( [Na+]i, [K+]i and [Cl-]i) during neurotransmitter application. Experiments were performed on isolated, desheathed superior cervical ganglia of the rat, maintained in Krebs solution at 30 degrees C. Application of carbachol resulted in a membrane depolarization accompanied by an increase of [Na+]i, a decrease of [K+]i and no change in [Cl-]i. Application of gamma-aminobutyric acid also induced a membrane depolarization which, however, was accompanied by a decrease of [K+]i and [Cl-]i, whereas [Na+]i remained constant. Blockade of the Na+/K+-pump by ouabain completely inhibited both the reuptake of K+ and the extrusion of Na+ after the action of carbachol, and also the post-carbachol undershoot of the free extracellular K+-concentration. On the other hand, in the presence of ouabain, no changes in the kinetics of the reuptake of K+ released during the action of gamma-aminobutyric acid could be observed. Furosemide, a blocker of K+/Cl- -cotransport, inhibited the reuptake of Cl- and K+ after the action of gamma-aminobutyric acid. In summary, the data reveal that rat sympathetic neurons possess, in addition to the Na+/K+-pump, another transport system to regulate free intracellular K+-concentration. This system is possibly a K+/Cl- -cotransport.

Intracellular electrolyte concentrations in rat sympathetic neurones measured with an electron microprobe

Intracellular element concentrations were measured in rat sympathetic neurones using energy dispersive electron microprobe analysis. The resting intracellular concentrations of sodium potassium and chloride measured in ganglia maintained for about 90 min in vitro at 25 degrees C were 3, 155 and 25 mmol/kg total tissue wet weight respectively. Recalculated in mmol/l cell water, these values are 5, 196 and 32 respectively. There were no significant differences between the nuclear and cytoplasmic values of these ions. Incubation in either carbachol (180 mumol/l, 4 min) or ouabain (1 mmol/1, 60 min) significantly increased the intracellular sodium and decreased the intracellular potassium concentrations. Neither substance materially altered the intracellular chloride concentration. The data obtained are compared and contrasted to those obtained in mammalian sympathetic neurones using chemical analysis and ion-sensitive microelectrodes.