The K+ liquid ion exchange electrode system: responses to drugs and neurotransmitters

Ion dynamics during seizures

Changes in membrane voltage brought about by ion fluxes through voltage and transmitter-gated channels represent the basis of neural activity. As such, electrochemical gradients across the membrane determine the direction and driving force for the flow of ions and are therefore crucial in setting the properties of synaptic transmission and signal propagation. Ion concentration gradients are established by a variety of mechanisms, including specialized transporter proteins. However, transmembrane gradients can be affected by ionic fluxes through channels during periods of elevated neural activity, which in turn are predicted to influence the properties of on-going synaptic transmission. Such activity-induced changes to ion concentration gradients are a feature of both physiological and pathological neural processes. An epileptic seizure is an example of severely perturbed neural activity, which is accompanied by pronounced changes in intracellular and extracellular ion concentrations. Appreciating the factors that contribute to these ion dynamics is critical if we are to understand how a seizure event evolves and is sustained and terminated by neural tissue. Indeed, this issue is of significant clinical importance as status epilepticus—a type of seizure that does not stop of its own accord—is a life-threatening medical emergency. In this review we explore how the transmembrane concentration gradient of the six major ions (K⁺, Na⁺, Cl⁻, Ca²⁺, H⁺and HCO3−) is altered during an epileptic seizure. We will first examine each ion individually, before describing how multiple interacting mechanisms between ions might contribute to concentration changes and whether these act to prolong or terminate epileptic activity. In doing so, we will consider how the availability of experimental techniques has both advanced and restricted our ability to study these phenomena.

Potassium dynamics in the epileptic cortex: new insights on an old topic

The role of changes in the extracellular potassium concentration [K(+)](o) in epilepsy has remained unclear. Historically, it was hypothesized that [K(+)]( o) is the causal factor for epileptic seizures. This so-called potassium accumulation hypothesis led to substantial debate but subsequently failed to find wide acceptance. However, recent studies on the pathophysiology of tissue from epileptic human patients and animal epilepsy models revealed aberrations in [K(+)](o) regulation. Computational models of cortical circuits that include ion concentration dynamics have catalyzed a renewed interest in the role of [K(+)](o) in epilepsy. The authors here connect classical and more recent insights on [K(+)]( o) dynamics in the cortex with the goal of providing starting points for a next generation of [K(+)](o) research. Such research may ultimately lead to an entirely new class of antiepileptic drugs that act on the [K(+)](o) regulation system.

Sensitivity of valinomycin-based K(+)-selective micro-electrodes to inhibitors of K+ transport

The sensitivies of double-barrelled K(+)-selective micro-electrodes (KSMs) employing the low-impedance membrane cocktail based on the neutral K(+)-selective ion carrier valinomycin (Fluka, Cocktail B 60398) to the following 3 different classes of inhibitors of K+ transport were measured: (1) general metabolic inhibitors (dinitrophenol, potassium cyanide, sodium azide, rotenone, dicyclohexylcarbodiimide, salicylhydroxamic acid); (2) P-type ATPase inhibitors (vanadate, ouabain, amiloride, SCH 28080); and (3) anion-dependent K+ transport inhibitors (bumetanide, 4-acetamide-4-isothiocyanostilbene-2,2-disulphonic acid). Of the 12 inhibitors tested, only dinitrophenol had any significant effect on the response of KSMs to K+ activity. Comparison of the calibrations in solutions with and without 0.1 mM dinitrophenol showed that this inhibitor behaved as a 'classical' interferent whereby its contribution to the K+ activity signal was statistically significant at K+ activities of 36.0 mM and less. However, at higher K+ activities (97.0 mM), dinitrophenol interference was not significant. It was possible to correct for the DNP interference and to obtain measurements of intracellular K+ activity in insect muscles.

Evidence for the uptake of neuronally derived choline by glial cells in the leech central nervous system

1. With ion-sensitive microelectrodes based on the Corning exchanger 477317, the accumulation of an unidentified interfering substance was monitored in leech neuropile glial cells but not in neurons after a 10-fold increase in extracellular K+ concentration. Evidence is presented which shows that this substance may be choline. 2. The accumulation of interfering ions was not observed in Ca2(+)-free saline and was substantially reduced in the presence of eserine (a blocker of acetylcholinesterase). 3. In neuropile (and also packet) glial cells, extracellularly applied choline (10(-4) M) caused a steady increase in ion signal. This increase was not affected by removal of extracellular calcium, by hemicholinium-3 (a blocker of high-affinity choline uptake) or eserine. Shortly after the removal of choline from the saline the increase in ion signal stopped and the ion signal then decreased slowly to its original level. 4. Extracellular acetylcholine (10(-4) M) caused a similar increase in intracellular ion signal of neuropile glial cells to that caused by choline. This increase was blocked by eserine. 5. Extracellular choline caused a comparatively small increase in ion signal of Retzius neurones which was blocked by hemicholinium-3. In pressure neurones, choline or hemicholinium-3 had no effect on intracellular ion signal. 6. Autoradiographic analysis of [3H]choline uptake showed that most of the choline was taken up by glial cells in a time- and dose-dependent manner. Small but significant amounts of choline were taken up by neurones and connective tissue. 7. It is concluded that the neuropile and packet glial cells possess an effective choline uptake system which is activated by exogenous choline but also by choline that stems from enzymatic inactivation of acetylcholine released by neurones.

Bias current modifies the selectivity of liquid membrane ion-selective microelectrodes

A negative bias potential of up to -80 mV applied to the back of a liquid membrane ion-selective microelectrode containing classical "K+" ion-exchanger was found to make it more selective for millimolar concentrations of K+ over micromolar concentrations of choline, tetramethylammonium, tetraethylammonium and 5-hydroxytryptamine. Conversely, positive bias potential increased severalfold the sensitivity to micromolar concentrations of these ions while decreasing the sensitivity to K+. An increase in response amplitude for millimolar changes of ion concentration was also observed in neutral carrier electrodes for Na+, K+ and Ca2+ with negative bias potential. The various ions caused the resistances of the electrodes to change; these resistance changes contributed to the changes in response amplitude, but there were additional, unexplained, factors. The phenomenon was used to test if the signal from a K+ ion-exchanger microelectrode in extracellular space in bee retina was contaminated by substances other than K+.

Diffusion in slice preparations bathed in unstirred solutions

A diffusion model is described here, which allows for the estimations of drug concentration changes in porous media, such as in slice tissues of the central nervous system (CNS) bathed in unstirred solutions following abrupt changes of drug concentration. This model may be used for the interpretation of data obtained in neuropharmacological studies if (i) the diffusion coefficient of the molecules under investigation is constant within the excised tissue, (ii) drug molecules are diffusing only in the extracellular space (ECS) and are not bound by the tissue, (iii) drug molecules diffuse mainly within one dimension, (iv) the drug concentration in the bath is changed within 5 s, and (v) the bathing solutions at the surfaces of the slices are stagnant during the period of diffusion. To test this model, estimated tetramethylammonium (TMA) ion concentrations within a tissue slice were compared to actual TMA concentration changes measured at the same depth in the tissue of hippocampal slices by means of TMA-sensitive microelectrodes. A statistically significant correlation (P less than 0.05) was observed between the estimated and measured TMA concentrations which indicates that the model is valid under the defined conditions.

Continuous measurement of pentylenetetrazol concentration by a liquid ion exchanger microelectrode

A double-barrelled microelectrode is described, which permits the continuous measurement of the concentration of the epileptogenic agent pentylenetetrazol (PTZ). The electrode is based on the liquid potassium exchanger (Corning No. 477 317) and enables measurements of PTZ concentration in physiological salines down to 1 mM. The electromotive behaviour of the liquid membrane against PTZ cannot directly be described by the Nicolsky-Eisenman formalism. It is suggested that specific interactions of the PTZ molecule with the Corning ligand are involved in the potential generating mechanisms.

Intracellular ion changes of astrocytes in response to extracellular potassium

Intracellular changes of K+, Na+, and Cl- were investigated by the aid of radiotracers in primary cultures of astrocytes when extracellular K+ was (1) increased from 3 to 12 mM and subsequently again decreased to 3 mM; and (2) increased from 5.4 to 54 mM with subsequent decrease to 5.4 mM. In both situations the K+ content increased by 50% within seconds, and it doubled within 1-2 min. The increase must be carrier mediated, because keeping the K X Cl product (Donnan equilibrium) constant did not lower the K+ accumulation rates. The Na+ content decreased when K+ was increased to 12 mM, but the decrease corresponded only to 10% of the accumulated K+. When K+ was increased to 54 mM, the Na+ content increased transiently. Cl- increased by about 15-25% of the accumulated K+. Return of extracellular K+ to original levels evoked a very fast K+ release, reversing all ion changes. The Na+ content increased transiently during the release process. For an interpretation of these observations, it is necessary to postulate endogenous production of an anion and of H+, which in turn is partly exchanged with Na+.

Changes of intracellular sodium and potassium ion concentrations in frog spinal motoneurons induced by repetitive synaptic stimulation

A post-tetanic membrane hyperpolarization following repetitive neuronal activity is a commonly observed phenomenon in the isolated frog spinal cord as well as in neurons of other nervous tissues. We have now used double-barrelled Na+- and K+-ion-sensitive microelectrodes to measure the intracellular Na+- and K+-concentrations and also the extracellular K+-concentration of lumbar spinal motoneurons during and after repetitive stimulation of a dorsal root. The results show that the post-tetanic membrane hyperpolarization occurred at a time when the intracellular [Na+] reached its maximal value, intracellular [K+] had its lowest level and extracellular [K+] was still elevated. The hyperpolarization was blocked by ouabain and reduced by Li+. These data support the previous suggestion that an electrogenic Na+/K+ pump mode may be the mechanism underlying the post-tetanic membrane hyperpolarization.