AN ELECTROGENIC SODIUM PUMP IN SNAIL NERVE CELLS

Multipurpose Na+ ions mediate excitation and cellular homeostasis: Evolution of the concept of Na+ pumps and Na+/Ca2+ exchangers

Ionic signalling is the most ancient form of regulation of cellular functions in response to environmental challenges. Signals, mediated by Na⁺ fluxes and spatio-temporal fluctuations of Na⁺ concentration in cellular organelles and cellular compartments contribute to the most fundamental cellular processes such as membrane excitability and energy production. At the very core of ionic signalling lies the Na⁺-K⁺ ATP-driven pump (or NKA) which creates trans-plasmalemmal ion gradients that sustain ionic fluxes through ion channels and numerous Na⁺-dependent transporters that maintain cellular and tissue homeostasis. Here we present a brief account of the history of research into NKA, Na⁺ -dependent transporters and Na⁺ signalling.

Proposal of a new mechanism for the directional propagation of the action potential using a mimicking system

A nerve conduction model is constructed by using some liquid-membrane cells that mimic the function of the K⁺ and Na⁺ channels. By imitating two types of Na⁺ channels (ligand-gated Na⁺ channels and voltage-gated Na⁺ channels), a new mechanism for the directional propagation of the action potential along the axon toward the axon terminal is proposed. When the nerve cell is excited by an external (outer) stimulus, it can be presumed that the ligand-gated channels work as power sources at the synapse to propagate the change in the membrane potential, and then the voltage-gated channels locally assist the propagation at each site of the axon (nodes of Ranvier).

Na(+)/K(+) pump interacts with the h-current to control bursting activity in central pattern generator neurons of leeches

The dynamics of different ionic currents shape the bursting activity of neurons and networks that control motor output. Despite being ubiquitous in all animal cells, the contribution of the Na(+)/K(+) pump current to such bursting activity has not been well studied. We used monensin, a Na(+)/H(+) antiporter, to examine the role of the pump on the bursting activity of oscillator heart interneurons in leeches. When we stimulated the pump with monensin, the period of these neurons decreased significantly, an effect that was prevented or reversed when the h-current was blocked by Cs(+). The decreased period could also occur if the pump was inhibited with strophanthidin or K(+)-free saline. Our monensin results were reproduced in model, which explains the pump's contributions to bursting activity based on Na(+) dynamics. Our results indicate that a dynamically oscillating pump current that interacts with the h-current can regulate the bursting activity of neurons and networks.

Mechanical compression insults induce nanoscale changes of membrane-skeleton arrangement which could cause apoptosis and necrosis in dorsal root ganglion neurons

In a primary spinal cord injury, the amount of mechanical compression insult that the neurons experience is one of the most critical factors in determining the extent of the injury. The ultrastructural changes that neurons undergo when subjected to mechanical compression are largely unknown. In the present study, using a compression-driven instrument that can simulate mechanical compression insult, we applied mechanical compression stimulation at 0.3, 0.5, and 0.7 MPa to dorsal root ganglion (DRG) neurons for 10 min. Combined with atomic force microscopy, we investigated nanoscale changes in the membrane-skeleton, cytoskeleton alterations, and apoptosis induced by mechanical compression injury. The results indicated that mechanical compression injury leads to rearrangement of the membrane-skeleton compared with the control group. In addition, mechanical compression stimulation induced apoptosis and necrosis and also changed the distribution of the cytoskeleton in DRG neurons. Thus, the membrane-skeleton may play an important role in the response to mechanical insults in DRG neurons. Moreover, sudden insults caused by high mechanical compression, which is most likely conducted by the membrane-skeleton, may induce necrosis, apoptosis, and cytoskeletal alterations.

Some properties of the external activation site of the sodium pump in crab nerve

1. Methods are described for using the changes in respiration of intact Libinia nerve to follow the rate of energy utilization by the sodium pump in this tissue.2. Short tetani in 10 K(Na)ASW (artificial sea water in which Na is the major cation and the potassium concentration is 10 mM) increased the oxygen uptake which then declined exponentially. From the net influx of Na during the tetanus and the associated oxygen uptake, values between 1.9 and 3.4 were calculated for the Na: approximately P ratio. After longer tetani, the recovery curve was S-shaped.3. The pump was activated by potassium ions in the external medium and this activation was competitively inhibited by external sodium ions. The data are consistent with a Michaelis constant (K(m)) for external potassium of 1 mM and an inhibitor constant (K(i)) for external sodium of 60 mM.4. In activating the pump, K could be replaced by Tl(+), Rb, NH(4) and Cs ions; but, of the monovalent ions tested, sodium seemed to be unique in its inhibitory action.5. In sea waters containing 460 mM-Na, ouabain behaved like a mixed inhibitor of the pump, reducing both the maximum velocity and the apparent affinity for external potassium. At a given ouabain concentration, reducing the sodium content of the medium was without effect on the maximum rate of pumping; but the apparent affinity for potassium increased more steeply than in a ouabain-free solution.6. The rate of energy utilization associated with pumping was unaffected by inclusion of quite high concentrations of sulphydryl-blocking agents in the external medium.

An in vitro model of neurotrauma in organotypic spinal cord cultures from adult mice

Cellular degeneration after spinal cord injury (SCI) involves numerous pathways. It is essential to use appropriate experimental models in order to understand the complex processes, which evolve after the initial trauma. The purpose of this study was to develop and assess an in vitro model of neurotrauma using organotypic slice culture of adult mice spinal cord. This model will facilitate the investigation of primary and secondary mechanisms of cell death that occurs after SCI. We modified previously described methods for generating organotypic cultures of murine spinal cord. The viability of organotypic cultures was assessed by observing the outgrowth of neurites and by using a mitochondria dependent dye for live cells (tetrazolium dye; MTT). The morphological integrity of cultures was examined histologically by hematoxylin and eosin (H&E) staining for general morphology and with luxol fast blue (LFB) for myelin. Neuronal and glial (GFAP; CNPase) markers were used to identify neurons, astrocytes and oligodendroglia, respectively. Primary injury was achieved by using a weight drop (0.2 g) model of injury. Cell death after primary injury was attenuated by pre-treatment with two known neuroprotective agents: the AMPA/KA blocker CNQX and methylprednisolone. The nuclear markers Propidium iodide and Sytox-green, as well as the TUNEL (in situ terminal deoxytransferase-mediated dUTP nick end labeling) technique, were used as a quantitative indicators of cell death at 24, 48 and 72 h post-injury using a confocal microscope and image analysis software. This novel in vitro model of SCI is easy to reproduce, will facilitate the examination of post-trauma cell death mechanisms and the neuroprotective effects of pharmacological agents and aid in the study of transgenic murine models.

Effects of l-amphetamine on the central neurons of the snail

The effects of l-amphetamine on the spontaneous firing of central neurons of African snails (Achatina fulica Ferussac) were studied electrophysiologically. The effects of dopamine, noradrenaline, d-amphetamine, and methamphetamine on the central neurons also were tested. The l- and d-amphetamines (0.3 mM) elicited bursting firing of action potentials in the RP4 neuron of the snail, whereas dopamine (0.3 mM), noradrenaline (NE, 0.3 mM), and methamphetamine (2 mM) did not. The bursting firing of action potentials elicited by l-amphetamine was decreased if potassium-free solution, sodium-free solution, or solution containing oubain (0.1 mM), a sodium pump inhibitor, was perfused. The results suggested that l-amphetamine did, and methamphetamine did not, elicit a sodium-dependent bursting firing of action potentials of the neuron.

Bursting firing of action potentials in central snail neurons elicited by d-amphetamine: role of the electrogenic sodium pump

The effects of d-amphetamine on central neurons were studied electrophysiologically in the identifiable RP4 neuron of the African snail, Achatina fulica Ferussac. d-Amphetamine elicited bursting activity from the central RP4 neuron in a concentration-dependent manner. The bursting activity was not blocked in a high magnesium (30 mM) medium, or after a continuous perfusion of propranolol, prazosin, haloperidol, phenobarbital, hexamethonium, d-tubocurarine, atropine, or calcium-free solution containing EDTA or verapamil. These results suggested that the bursting activity elicited by d-amphetamine was not due to: (1) the synaptic effects of neurotransmitters; or (2) the cholinergic or adrenergic receptors of the excitable membrane. However, the bursting activity elicited by d-amphetamine was blocked in the presence of ouabain or in the medium containing potassium-free, low sodium solutions. d-Amphetamine did not elicit the bursting activity of the LP4 neuron in the same ganglia preparation, and did not alter the GABA-elicited currents of the snail neuron. It is concluded, therefore, that d-amphetamine induced a potassium- and sodium-dependent bursting activity of central neurons. The bursting activity of the central neuron may be associated with the sodium pump of the neuron.

Cyanide-induced alteration of cytosolic pH: involvement of cellular hydrogen ion handling processes

Neuronal cells exposed to cyanide rapidly lose the capacity to regulate internal Ca2+ homeostasis, thereby accumulating an excess cytosolic Ca2+ load. The present study was undertaken to examine the effects of KCN on another important ion: hydrogen ion. KCN (1-10 mM) rapidly decreased intracellular pH (pHi) of cultured pheochromocytoma (PC12) cells as indicated by the pH-sensitive fluorescent dye 2',7-bis(carboxyethyl)-5(6)-carboxyfluorescein. Removal of Ca2+ from the media or pretreating the cells with diltiazem (10(-5) M), a calcium channel blocker, delayed the onset and reduced the magnitude of the drop in pHi. Lowering the pH of the incubation medium (pHo) to 6.9 exaggerated the drop in pHi, while raising it to 7.9 attenuated the change in pHi. Removal of Na+ from the media enhanced the cyanide effect. Reintroduction of Na+ or substitution with Li+ reversed the cytosolic acidification, suggesting involvement of the Na+/H+ exchanger in the cyanide action. Pretreatment of cells with amiloride, 0.2 mM, blunted the cytosolic acidification induced by KCN, possibly by decreasing intracellular Na+ accumulation and disrupting H+ efflux. Cyanide thus produces a rapid dysfunction of hydrogen ion handling mechanisms and this may play a role in cyanide neurotoxicity.

Sodium-azide-evoked noradrenaline and catecholamine release from peripheral sympathetic nerves and chromaffin cells

1. The spontaneous release of [3H]noradrenaline [( 3H]NA) has been measured from rabbit pulmonary arteries and bovine chromaffin cells in the presence of neuronal uptake blocker cocaine (3 x 10(-5) M). 2. The Na+-pump inhibitor sodium-azide (NaN3, 2mM) produced a moderate increase of [3H]NA release from both preparations and relaxed the arteries. The [3H]releasing action of NaN3 was accompanied by a 30% inhibition of 86Rb-uptake into chromaffin cells. 3. In both preparations, ouabain (10(-4) M) markedly increased the release of [3H], contracted the arteries and inhibited the 86Rb-uptake of chromaffin cells by about 75%. A combined application of NaN3 and ouabain produced a similar inhibition of 86Rb-uptake of chromaffin cells and failed to increase further the release of [3H] in comparison to that found in response to ouabain alone. 4. Removal of K+ from the external medium increased both the release of [3H]NA and the tone of pulmonary arteries. NaN3 further increased the transmitter release in "K+-free" solution but relaxed the muscle. In the absence of external K+ and in the presence of azide, ouabain further enhanced the transmitter release but failed to produce significant contraction. 5. Reactivation of the Na+-pump by readmission of K+ (5.9 mM) to the external medium abolished the transmitter releasing action of NaN3 in arteries. 6. It is concluded that in peripheral sympathetic nerves and chromaffin cells, NaN3 inhibits the Na+-pump producing NA and CA release respectively and in nerves even if NA release had already been increased by K+-removal.(ABSTRACT TRUNCATED AT 250 WORDS)

Studying the isolated central nervous system; a report on 35 years: more inquisitive than acquisitive

1. The CNS from invertebrate animals such as slugs, snails, leeches, and cockroaches, can be isolated and kept alive for many hours. 2. The electrical and pharmacological properties of invertebrate CNS neurons have many similarities and it is probable that the basic rules governing the CNS evolved more than 600 million years ago. 3. The nerve cells can show sodium action potentials, calcium action potentials, EPSP, IPSP, biphasic potentials, electrogenic sodium pump potentials, and a variety of potassium, sodium, calcium and chloride currents. 4. Invertebrate CNS ganglia contain identifiable individual nerve cells whose properties and responses to neurotransmitters and drugs are constant and repeatable from preparation to preparation. 5. It was possible to set up an isolated CNS-nerve trunk-muscle preparation and study the transport of radioactive material from the CNS to the muscle and from muscle to CNS. This has provided information about axoplasmic transport in both invertebrate and vertebrate preparations. 6. The methods developed from studies of invertebrate isolated CNS preparations have been applied to vertebrate isolated CNS preparations. 7. In addition to thin slices of the mammalian brain, it is possible to keep 5 cm lengths of the whole mammalian spinal cord and brain stem alive for many hours. 8. The isolated mammalian spinal cord has functional ipsilateral and contralateral reflexes, ascending and descending pathways, extensive sensory integrative local area networks, and inhibitory interneuron circuits. Much of the in vivo circuitry is functional in vitro. 9. The isolated mammalian spinal cord and brain stem can be developed to include functional higher brain circuits that will provide increased understanding of the control and integrative action of the mammalian central nervous system.

A-23187 evoked transmitter release from rabbit pulmonary artery and its inhibition by reactivation of sodium-pump

1. The spontaneous [3H]-release has been measured from the isolated main pulmonary artery of the rabbit preloaded with [3H]noradrenaline in the presence of uptake blockers (cocaine, 3 x 10(-5) M; corticosterone, 5 x 10(-5) M). 2. The Ca-ionophore A-23187 (3 x 10(-7)-3 x 10(-5) M) increased the outflow of [3H] by a concentration dependent manner. 3. Inhibition of Na+-pump by removal of K+ from the external medium also increased the release of labelled noradrenaline. 4. In the absence of external K+, the applied A-23187 (3 x 10(-6) M; EC50) further increased the release of [3H]. 5. Reactivation of Na+-pump by readmission of K+ (5.9 mM) to the external medium abolished the [3H]-release which had previously been increased in "K+-free" solution. 6. The reactivated Na+-pump significantly inhibited the transmitter releasing action of A-23187. 7. This latter was antagonized by an increase of external Ca2+ (7.5 mM). 8. It is concluded that the reactivated Na+-pump caused re-establishment of Na+-gradient is capable to counteract the Ca-ionophore facilitated Ca2+-influx and release from internal stores, which can be antagonized by excess Ca2+.

Role of ion conductance changes and of the sodium-pump in adrenaline-induced hyperpolarization of rat diaphragm muscle fibres

The ionic mechanism of membrane hyperpolarization induced by adrenaline in rat diaphragm muscle fibres was studied. Removal of the extracellular K+ ([K+]o) from Krebs-Ringer solution initially increased the resting membrane potential and then caused an increase in the intracellular Na+ activity ([Na+]i) and a decrease in the intracellular K+ activity ([K+]i). All the changes were maintained for more than 3 h. Application of ouabain (0.1 mM) or lowering the temperature rapidly reduced the resting potential by about 10 mV in the K+-free solution. It then produced further progressive decreases in resting potential and in [K+]i and a progressive increase in [Na+]i. These observations indicate that an electrogenic Na-pump operates in the K+-free solution. Removal of most of the Cl- in the K+-free solution did not affect the resting potential or the magnitude of the initial decrease produced by ouabain, despite an increased input resistance; this result implies a passive distribution of Cl-. Adrenaline (30-60 microM) either added to the bathing solution or applied to the membrane by ionophoresis produced a hyperpolarization (3-10 mV: adrenaline hyperpolarization), the amplitude of which was decreased with a rise in [K+]o and increased with a reduction in [K+]o, but unaffected by the removal of Cl-. Adrenaline produced an increase in input resistance, the relative magnitude (17-18%) of which was constant whether external K+ or Cl- was removed. In contrast, a conditioning membrane hyperpolarization hardly affected the resistance. Ouabain (0.1 mM) or low temperature (8-10 degrees C) abolished both the hyperpolarization and the increased input resistance induced by adrenaline. The [K+]i, [Na+]i and the peak of the action potential remained unchanged after a 20 min exposure to adrenaline (30 microM). The hyperpolarization induced by the replacement of all Na+ with Tris (Tris-hyperpolarization) in the K+-free solution was depressed by 39% during the early period (4-31 min) of exposure to adrenaline (30 microM), while it was enhanced by 26% during the later period (80-130 min). The initial depression suggested a decrease in the ratio of the membrane permeability for Na+ (PNa) to that for K+ (PK). These results suggest that the adrenaline hyperpolarization is generated largely by a decrease in PNa/PK, which is associated with the activity of the Na-pump.

Chloride conductance and extracellular potassium concentration interact to modify the excitability of rat optic nerve fibres

The excitability of developing rat optic nerves has been studied under conditions in which extracellular Cl- was replaced with other anions. In nerves younger than 3 days old, replacing Cl- with propionate or SO4(2-) usually led to spontaneous and repetitive cycling of extracellular K+ concentration ([K+]o). [K+]o reached peaks of 8-12 mM and then fell transiently below the base-line level of 5 mM before increasing again. This cycling behaviour continued, with a wave-length of 1-2 min, for as long as 2 h. Nerves older than 5 days either did not cycle or did so only transiently. Substitution of ten different anions for Cl- indicated that a minimum hydrated radius, between that of BrO3- and HCO3-, was necessary to induce cycling behaviour. Cycling behaviour was abolished by the Na+-channel blocker tetrodotoxin. Reduction of the bath [K+] to 2.5 mM slowed the frequency of spontaneous cycles; a bath [K+] of 1 mM abolished them. When the temperature was lowered, cycle frequency slowed. Substitution of large anions for Cl- enhanced axonal excitability. This was inferred from the prevalence of spontaneous action potentials during cycling behaviour, and from the generation of relatively large evoked increases of [K+]o. Cycling behaviour is hypothesized to result from a repetition of the following three processes: (i) spontaneous axonal firing elicits a gradual increase in [K+]o which increases axonal excitability and facilitates further K+ release, (ii) axonal firing and K+ release are eventually halted by a combination of depolarization block, intracellular Na+ accumulation and hyperpolarization from electrogenic pumping, (iii) recovery of [K+]o to its minimal value depends on active K+ reuptake mediated by a highly stimulated axonal Na+-K+-ATPase. We conclude that a large proportion of the resting membrane conductance of optic nerve fibres is Cl- specific. A high Cl- conductance may stabilize fine central axons against the depolarizing effects of [K+]o increases.

Intracellular free magnesium in neurones of Helix aspersa measured with ion-selective micro-electrodes

Cytoplasmic free Mg2+ concentration, [Mg2+]i, was measured in identified neuronal cell bodies of the suboesophageal ganglia of Helix aspersa, using Mg2+-selective micro-electrodes. In calibration solutions, the electrodes showed significant interference from K+, but not from Na+, or Ca2+, at concentrations found intracellularly. Therefore, in order to calibrate the electrodes properly, it was necessary first to obtain an accurate value for intracellular free K+ concentration [( K+]i). The mean value for [K+]i was 91 mM (S.E. of the mean +/- 2.2 mM, n = 8), measured with K+-sensitive 'liquid ion exchanger micro-electrodes'. In seven experiments, which met stringent criteria for satisfactory impalement and electrode calibration, the mean [Mg2+]i was 0.66 mM (S.E. of the mean +/- 0.05 mM). The mean [Mg2+]i in cells that had spontaneous spike activity was not significantly different from that in quiescent cells. If Mg2+ was in electrochemical equilibrium, the ratio [Mg2+]i/[Mg2+]o would be about 55. Mg2+ is therefore not passively distributed across the neuronal membrane and an outwardly directed extrusion mechanism must exist to keep [Mg2+]i low and constant, even in cells undergoing spike activity.

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)

An identification of the K activated Na pump current in sheep Purkinje fibres

The cell membrane of sheep Purkinje fibers hyperpolarizes transiently on returning to K containing media after several minutes in K free solution. To analyse this 'K activated response' voltage clamp experiments and measurements of the internal Na activity (aiNa) are performed in fibres bathed in solution containing 0.2-2 mM BaCl2. Compared to the response in Tyrode solution the transient hyperpolarization beyond the resting potential is increased in Ba containing media. The response is blocked by 10(-4) M dihydroouabain. During the response, in Ba treated fibres, aiNa and a transient outward current decline with the same time constant at all clamp potentials tested. The transient outward current is most probably due to a temporary increase in electrogenic Na pumping. Both the amplitude and the time constant of the pump current show little voltage dependence. The electrogenic fraction of the active Na efflux is estimated to be about 39% and is independent of aiNa. Ba ions facilitate the analysis of the pump current in voltage clamped fibres because K depletion is reduced and changes of the I-V relationship by K depletion are minimized. It is concluded that activation of the electrogenic Na pump is mainly responsible for the K activated response of fibres in Ba containing media.

Effects of monensin on the response to low potassium in a molluscan muscle, the radular protractor of Busycon canaliculatum

1. A sucrose gap technique was used to study the effect of a sodium ionophore on the potential changes which occur during superfusion with potassium-free solution. Crucial values were checked with a microelectrode technique. 2. Potassium-free solution induces a complex response consisting of a hyperpolarizing phase (HP) and then a depolarization phase (DP) during exposure to zero K+, followed by a transient extra hyperpolarizing phase (EHP) on readmission of K+. 3. The sodium ionophore, monensin, has the effect of increasing the amplitude of the DP, in perfusion medium of normal Na+ content. This is similar to the effect of treatment with neurohumors and to the after-effect of direct electrical stimulation. The effects of ACh and monensin are additive. 5. These effects are consistent with an action of monensin in increasing Na+ flux in the direction of the concentration gradient and support the hypothesis that neurohumors stimulate the sodium pump by increasing Na+ influx.

Electrogenic Na pump evidenced by injecting various Na salts into the isolated A-V node cells of rabbit heart

Electrogenicity of the Na pump was confirmed by injecting Na salts into a small cluster of A-V node cells. Injection of Na glutamate or Na acetate induced marked hyperpolarization, accompanying with cessation of spontaneous activity. The hyperpolarization exceeded EK in 27 mM K Tyrode solution and was inhibited by 10(-5) M strophanthidin. Injection of NaCl or NaI depolarized the membrane. These data showed that inward-going current carried by the injected anion antagonized the outward-going pump current and thus determined the net effect of the injection.

Excitatory and inhibitory effects of histamine on molluscan neurons

Histamine elicited depolarization (excitation) in some neurons and hyperpolarization (inhibition) in other neurons of the central nervous system of the marine mollusc, Onchidium verruculatum. The histamine sensitive region was along the axon at some distance from the soma. H1-receptor blockers (SA-97 and mepyramine) suppressed the excitatory (H1) response without affecting the inhibitory (H2) response, while H2-receptor blockers (burimamide and metiamide) suppressed the H2-response without affecting the H1-response. The H1-response was associated with a marked increase in membrane conductance and was blocked by removal of the external Na. The H2-response consisted of a hyperpolarization without much change in conductance, compared with the hyperpolarization of same amplitude produced by glutamate in the same neuron. Passive polarization of the membrane and reduction of Cl concentrations to 1/5-1/25 caused no significant change in H2-response. The H2-response was slightly suppressed in K-free saline. Thus, it seems difficult to account for the hyperpolarization only by an increase in K or Cl conductance. Complete removal of Na and addition of ouabain blocked the H2-response, suggesting a contribution of an electrogenic Na-pump to the hyperpolarization. However, in 20 mM Na saline with or without K, histamine still caused clear hyperpolarization. In this solution, the histamine response was not affected by ouabain. Although it is difficult to exclude the possibility that an increase in K conductance may be responsible for the hyperpolarization, it is tentatively proposed as a hypothesis that the H2-response involved some active transport mechanism, different from a ouabain-sensitive electrogenic Na-pump.

Barium ions block the acetylcholine evoked slow H-response in the neurones of Helix pomatia L

The effects of barium ions on slow acetylcholine (Ach) H receptor activated hyperpolarization has been studied. The results provide evidence of a sensitive and reversible blocking action of Ba2+ on the involved potassium conductance. 3-Aminopyridine (3-AP) also attenuated the slow H-response by 50%, showing partial effectiveness. TEA+ sensitively inhibited the Ach evoked hyperpolarization interacting with the receptors.

Metabolic component in the epithelial intracellular potential of rabbit cornea

Effects of the change of external ionic composition and the addition of metabolic inhibitors on rabbit cornea were studied by recording the epithelial intracellular potential. High K and Li Ringer's solutions, applied to the corneal endothelial side, caused a marked depolarization of the epithelial cells, but no potential change was seen when applied to the epithelial side. Ouabain, MIAA and NaCN applied to the endothelial side reduced the epithelial potential, while those applied to the epithelial side did not change the potential. DNP and FDNB also had no effect when applied to the epithelial side only. The thermal dependence of the epithelial intracellular potentials of whole eye (0.85 mV/degrees C) and excised cornea (2.01 mV/degrees C) preparations were greater than about 0.2 mV/degrees C predicted by the Nernst equation. It is concluded that the epithelial cell layer of rabbit cornea act as a tight barrier against diffusion of K ion and metabolic inhibitors from the tear side to the epithelial basal cell. A high thermal dependence of the epithelial intracellular potential may depend greatly on the pump ingibition.

[Potassium activity in the cat cortex: experimental epilepsy]

Two mechanisms are discussed which link extracellular potassium accumulation and epileptogenic neuronal hyperactivity in the cortex. The potassium concentration (aK) of the environment of a repetitively discharging membrane can increase sufficiently for a supra-threshold depolarization at afferent erminals. This can explain the finding of ectopic spike generation and the antidromic breakthrough in thalamo-cortical projections after a primary cortical discharge. Spread and recurrent enhancement of excitatory drives may be the result of this mechanism. Initiation and termination of seizure is not explained by potassium accumulation. There is a ceiling level in potassium of about 10 mequ/1 which is strictly maintained during normal as well as epileptiform activity. This level is probably not high enough for depolarizing inactivation of neuronal membranes. Stimulation of cortical afferents can have a dual effect on aK. After a primary shortlasting increase, aK can reach subnormal values. This is possibly brought about by a stimulated re-uptake of K+. Seizures can be initiated at these subnormal levels. The effect of the re-uptake e.g. hyperpolarization of terminal afferents and increase of evoked transmitter release is discussed for the initiation for paroxysmal activity.