Changes In Extracellular Potassium Research Articles

Activity-dependent extracellular potassium changes in unmyelinated versus myelinated areas in olfactory regions of the isolated female guinea-pig brain

The potassium released in the extracellular space during neuronal activity is rapidly removed by glia and neurons to maintain tissue homeostasis. Oligodendrocyte-derived myelin axonal coating contributes to potassium buffering and is therefore crucial to control brain excitability.We studied activity-dependent extracellular potassium ([K+]o) changes in the piriform cortex (PC), a region that features highly segregated bundles of myelinated and unmyelinated fibers. Four-aminopyridine (4AP; 50 μM) treatment or patterned high-frequency stimulations (hfST) were utilized to generate [K+]o changes measured with potassium-sensitive electrodes in the myelinated lateral olfactory tract (LOT), in the unmyelinated PC layer I and in the myelinated deep PC layers in the ex vivo isolated guinea-pig brain.Seizure-like events induced by 4AP are initiated by the abrupt [K+]o rise in the layer I formed by unmyelinated fibers (Uva et al., 2017). Larger [K+]o shifts occurred in unmyelinated layers compared to the myelinated LOT. LOT hfST that mimicks pre-seizure discharges also generated higher [K+]o changes in unmyelinated PC layer I than in LOT and deep PC layers. The treatment with the Kir4.1 potassium channel blocker BaCl2 (100 μM) enhanced the [K+]o changes generated by hfST in myelinated structures.Our data show that activity-dependent [K+]o changes are intrinsically different in myelinated vs unmyelinated cortical regions. The larger [K+]o shifts generated in unmyelinated structures may represent a vehicle for seizure generation.

Development of Mechanistic Neural Mass (mNM) Models that Link Physiology to Mean-Field Dynamics.

Brain rhythms emerge from the mean-field activity of networks of neurons. There have been many efforts to build mathematical and computational embodiments in the form of discrete cell-group activities-termed neural masses-to understand in particular the origins of evoked potentials, intrinsic patterns of activities such as theta, regulation of sleep, Parkinson's disease related dynamics, and mimic seizure dynamics. As originally utilized, standard neural masses convert input through a sigmoidal function to a firing rate, and firing rate through a synaptic alpha function to other masses. Here we define a process to build mechanistic neural masses (mNMs) as mean-field models of microscopic membrane-type (Hodgkin Huxley type) models of different neuron types that duplicate the stability, firing rate, and associated bifurcations as function of relevant slow variables - such as extracellular potassium - and synaptic current; and whose output is both firing rate and impact on the slow variables - such as transmembrane potassium flux. Small networks composed of just excitatory and inhibitory mNMs demonstrate expected dynamical states including firing, runaway excitation and depolarization block, and these transitions change in biologically observed ways with changes in extracellular potassium and excitatory-inhibitory balance.

TNF\u03b1-mediated necroptosis in brain endothelial cells as a potential mechanism of increased seizure susceptibility in mice following systemic inflammation

BackgroundSystemic inflammation is a potent contributor to increased seizure susceptibility. However, information regarding the effects of systemic inflammation on cerebral vascular integrity that influence neuron excitability is scarce. Necroptosis is closely associated with inflammation in various neurological diseases. In this study, necroptosis was hypothesized to be involved in the mechanism underlying sepsis-associated neuronal excitability in the cerebrovascular components (e.g., endothelia cells).MethodsLipopolysaccharide (LPS) was used to induce systemic inflammation. Kainic acid intraperitoneal injection was used to measure the susceptibility of the mice to seizure. The pharmacological inhibitors C87 and GSK872 were used to block the signaling of TNFα receptors and necroptosis. In order to determine the features of the sepsis-associated response in the cerebral vasculature and CNS, brain tissues of mice were obtained for assays of the necroptosis-related protein expression, and for immunofluorescence staining to identify morphological changes in the endothelia and glia. In addition, microdialysis assay was used to assess the changes in extracellular potassium and glutamate levels in the brain.ResultsSome noteworthy findings, such as increased seizure susceptibility and brain endothelial necroptosis, Kir4.1 dysfunction, and microglia activation were observed in mice following LPS injection. C87 treatment, a TNFα receptor inhibitor, showed considerable attenuation of increased kainic acid-induced seizure susceptibility, endothelial cell necroptosis, microglia activation and restoration of Kir4.1 protein expression in LPS-treated mice. Treatment with GSK872, a RIP3 inhibitor, such as C87, showed similar effects on these changes following LPS injection.ConclusionsThe findings of this study showed that TNFα-mediated necroptosis induced cerebrovascular endothelial damage, neuroinflammation and astrocyte Kir4.1 dysregulation, which may coalesce to contribute to the increased seizure susceptibility in LPS-treated mice. Pharmacologic inhibition targeting this necroptosis pathway may provide a promising therapeutic approach to the reduction of sepsis-associated brain endothelia cell injury, astrocyte ion channel dysfunction, and subsequent neuronal excitability.

WNKs are potassium-sensitive kinases.

With no lysine (K) (WNK) kinases regulate epithelial ion transport in the kidney to maintain homeostasis of electrolyte concentrations and blood pressure. Chloride ion directly binds WNK kinases to inhibit autophosphorylation and activation. Changes in extracellular potassium are thought to regulate WNKs through changes in intracellular chloride. Prior studies demonstrate that in some distal nephron epithelial cells, intracellular potassium changes with chronic low- or high-potassium diet. We, therefore, investigated whether potassium regulates WNK activity independent of chloride. We found decreased activity of Drosophila WNK and mammalian WNK3 and WNK4 in fly Malpighian (renal) tubules bathed in high extracellular potassium, even when intracellular chloride was kept constant at either ∼13 mM or 26 mM. High extracellular potassium also inhibited chloride-insensitive mutants of WNK3 and WNK4. High extracellular rubidium was also inhibitory and increased tubule rubidium. The Na+/K+-ATPase inhibitor, ouabain, which is expected to lower intracellular potassium, increased tubule Drosophila WNK activity. In vitro, potassium increased the melting temperature of Drosophila WNK, WNK1, and WNK3 kinase domains, indicating ion binding to the kinase. Potassium inhibited in vitro autophosphorylation of Drosophila WNK and WNK3, and also inhibited WNK3 and WNK4 phosphorylation of their substrate, Ste20-related proline/alanine-rich kinase (SPAK). The greatest sensitivity of WNK4 to potassium occurred in the range of 80-180 mM, encompassing physiological intracellular potassium concentrations. Together, these data indicate chloride-independent potassium inhibition of Drosophila and mammalian WNK kinases through direct effects of potassium ion on the kinase.

Serotonin, norepinephrine, and acetylcholine differentially affect astrocytic potassium clearance to modulate somatosensory signaling in male mice.

Changes in extracellular potassium ([K+ ]e ) modulate neuronal networks via changes in membrane potential, voltage-gated channel activity, and alteration to transmission at the synapse. Given the limited extracellular space in the central nervous system, potassium clearance is crucial. As activity-induced potassium transients are rapidly managed by astrocytic Kir4.1 and astrocyte-specific Na+ /K+ -ATPase, any neurotransmitter/neuromodulator that can regulate their function may have indirect influence on network activity. Neuromodulators differentially affect cortical/thalamic networks to align sensory processing with differing behavioral states. Given serotonin (5HT), norepinephrine (NE), and acetylcholine (ACh) differentially affect spike frequency adaptation and signal fidelity ("signal-to-noise") in somatosensory cortex, we hypothesize that [K+ ]e may be differentially regulated by the different neuromodulators to exert their individual effects on network function. This study aimed to compare effects of individually applied 5HT, NE, and ACh on regulating [K+ ]e in connection to effects on cortical-evoked response amplitude and adaptation in male mice. Using extracellular field and K+ ion-selective recordings of somatosensory stimulation, we found that differential effects of 5HT, NE, and ACh on [K+ ]e regulation mirrored differential effects on amplitude and adaptation. 5HT effects on transient K+ recovery, adaptation, and field post-synaptic potential amplitude were disrupted by barium (200µM), whereas NE and ACh effects were disrupted by ouabain (1µM) or iodoacetate (100µM). Considering the impact [K+ ]e can have on many network functions; it seems highly efficient that neuromodulators regulate [K+ ]e to exert their many effects. This study provides functional significance for astrocyte-mediated buffering of [K+ ]e in neuromodulator-mediated shaping of cortical network activity.

A Novel Focal Seizure Pattern Generated in Superficial Layers of the Olfactory Cortex.

Seizure patterns identified in focal epilepsies caused by diverse etiologies are likely due to different pathogenic mechanisms. We describe here a novel, region-specific focal seizure pattern that mimics seizure activity observed in a subpopulation of patients submitted to presurgical monitoring with intracerebral electrodes. Distinctive seizure-like events (SLEs) are induced in the olfactory regions by acute treatment of both tangential brain slices and the isolated guinea pig brain with the potassium channel blocker 4-aminopyridine. Analysis of field potentials, intracellular activities, and extracellular potassium changes demonstrates that SLEs in the piriform cortex initiate in the superficial layer 1 lacking principal neurons with an activity-dependent increase of extracellular potassium. SLE progression (but not onset) does not require the participation of synaptic transmission and is mediated by diffusion of potassium to deep cortical layers. The novel seizure pattern here described is not observed in other cortical regions; it is proposed to rely on the peculiar organization of the superficial piriform cortex layers, which are characterized by unmyelinated axons and perisynaptic astroglial envelopes. This study reveals a sequence of ictogenic events in the olfactory cortex that were never described before in other cortical structures and supports the notion that altered potassium homeostasis and unmyelinated fibers may represent a potential vehicle for focal ictogenesis.SIGNIFICANCE STATEMENT We describe a novel seizure pattern peculiar of the olfactory cortex that resembles focal seizures with low-voltage fast activity at onset observed in humans. The findings suggest that network mechanisms responsible for seizure onset can be region specific.

Heart Rate and Extracellular Sodium and Potassium Modulation of Gap Junction Mediated Conduction in Guinea Pigs.

Background: Recent studies suggested that cardiac conduction in murine hearts with narrow perinexi and 50% reduced connexin43 (Cx43) expression is more sensitive to relatively physiological changes of extracellular potassium ([K+]o) and sodium ([Na+]o).Purpose: Determine whether similar [K+]o and [Na+]o changes alter conduction velocity (CV) sensitivity to pharmacologic gap junction (GJ) uncoupling in guinea pigs.Methods: [K+]o and [Na+]o were varied in Langendorff perfused guinea pig ventricles (Solution A: [K+]o = 4.56 and [Na+]o = 153.3 mM. Solution B: [K+]o = 6.95 and [Na+]o = 145.5 mM). Gap junctions were inhibited with carbenoxolone (CBX) (15 and 30 μM). Epicardial CV was quantified by optical mapping. Perinexal width was measured with transmission electron microscopy. Total and phosphorylated Cx43 were evaluated by western blotting.Results: Solution composition did not alter CV under control conditions or with 15μM CBX. Decreasing the basic cycle length (BCL) of pacing from 300 to 160 ms decreased CV uniformly with both solutions. At 30 μM CBX, a change in solution did not alter CV either longitudinally or transversely at BCL = 300 ms. However, reducing BCL to 160 ms caused CV to decrease more in hearts perfused with Solution B than A. Solution composition did not alter perinexal width, nor did it change total or phosphorylated serine 368 Cx43 expression. These data suggest that the solution dependent CV changes were independent of altered perinexal width or GJ coupling. Action potential duration was always shorter in hearts perfused with Solution B than A, independent of pacing rate and/or CBX concentration.Conclusions: Increased heart rate and GJ uncoupling can unmask small CV differences caused by changing [K+]o and [Na+]o. These data suggest that modulating extracellular ionic composition may be a novel anti-arrhythmic target in diseases with abnormal GJ coupling, particularly when heart rate cannot be controlled.

Is Potassium a Ubiquitous Mediator of Vasodilation in the Central Nervous System?

Is Potassium a Ubiquitous Mediator of Vasodilation in the Central Nervous System?

Energy Demand of Synaptic Transmission at the Hippocampal Schaffer-Collateral Synapse

Neuroenergetic models of synaptic transmission predicted that energy demand is highest for action potentials (APs) and postsynaptic ion fluxes, whereas the presynaptic contribution is rather small. Here, we addressed the question of energy consumption at Schaffer-collateral synapses. We monitored stimulus-induced changes in extracellular potassium, sodium, and calcium concentration while recording partial oxygen pressure (pO(2)) and NAD(P)H fluorescence. Blockade of postsynaptic receptors reduced ion fluxes as well as pO(2) and NAD(P)H transients by ∼50%. Additional blockade of transmitter release further reduced Na(+), K(+), and pO(2) transients by ∼30% without altering presynaptic APs, indicating considerable contribution of Ca(2+)-removal, transmitter and vesicle turnover to energy consumption.

Extracellular potassium accumulation may contribute to long-term potentiation induced with long 100 Hz stimulus trains

Long-term potentiation is often induced by high-frequency stimulation (100 Hz) for 1 second. However, in hippocampal CA1 pyramidal cells we observe that EPSPs quickly become depressed in such trains and it is rare to see action potentials in the pyramidal cell after 200 ms into the train. Because LTP induction depends on postsynaptic depolarization and there appeared to be little depolarization after 200 ms, we predicted that lengthening the train from 200 ms to 1600 ms should produce little additional LTP. However we found that LTP magnitude progressively increased with train length. Further investigation revealed that fiber volley amplitude decreased and latency increased during a long train and that these changes could be reproduced by raising extracellular potassium concentration in the bath. In addition NMDA receptor mediated field EPSPs were increased with elevated extracellular potassium. These experimental findings strongly suggest that decreased afferent excitability and decreased glutamate release during a train are more than compensated by increases in extracellular potassium which enhances the NMDA receptor response and allows long trains to produce additional LTP. We have constructed computational models to test this idea and gain insight into how elevated extracellular potassium might be produced by long high-frequency trains and how this might affect LTP. If extracellular potassium concentration is raised from 3.4 mM to 13 mM, we note that: i) the NMDA reversal potential changes only 1.5 mV, ii) the potassium reversal potential depolarizes by 35 mV, and iii) resting potential depolarizes by 20 mV. The effect of these changes is to double the amount of calcium coming into a dendritic spine through NMDA receptor channels, but these changes also make the neuron fire action potentials even when input is depressed, in contradiction to the experimental results. This suggests that changes in extracellular potassium are likely to be restricted to regions around the afferent axons and the sites of synaptic contact. We are currently investigating this hypothesis and are seeking parameter conditions that reproduce the experimental findings.

A tale from the Crypt: splice variants of BK channels in colonic potassium secretion

A tale from the Crypt: splice variants of BK channels in colonic potassium secretion

Heterogeneous ventricular chamber response to hypokalemia and inward rectifier potassium channel blockade underlies bifurcated T wave in guinea pig

It was previously demonstrated that transmural electrophysiological heterogeneities can inscribe the ECG T wave. However, the bifurcated T wave caused by loss of inward rectifier potassium current (I(K1)) function is not fully explained by transmural heterogeneities. Since right ventricular (RV) guinea pig myocytes have significantly lower I(K1) than left ventricular (LV) myocytes, we hypothesized that the complex ECG can be inscribed by heterogeneous chamber-specific responses to hypokalemia and partial I(K1) blockade. Ratiometric optical action potentials were recorded from the epicardial surface of the RV and LV. BaCl(2) (10 micromol/l) was perfused to partially block I(K1) in isolated guinea pig whole heart preparations. BaCl(2) or hypokalemia alone significantly increased RV basal (RV(B)) action potential duration (APD) by approximately 30% above control compared with LV apical (LV(A)) APD (14%, P<0.05). In the presence of BaCl(2), 2 mmol/l extracellular potassium (hypokalemia) further increased RV(B) APD to a greater extent (31%) than LV(A) APD (19%, P<0.05) compared with BaCl(2) perfusion alone. Maximal dispersion between RV(B) and LV(A) APD increased by 105% (P<0.05), and the QT interval prolonged by 55% (P<0.05) during hypokalemia and BaCl(2). Hypokalemia and BaCl(2) produced an ECG with a double repolarization wave. The first wave (QT1) corresponded to selective depression of apical LV plateau potentials, while the second wave (QT2) corresponded to the latest repolarizing RV(B) myocytes. These data suggest that final repolarization is more sensitive to extracellular potassium changes in regions with reduced I(K1), particularly when I(K1) availability is reduced. Furthermore, underlying I(K1) heterogeneities can potentially contribute to the complex ECG during I(K1) loss of function and hypokalemia.

Mechanism of reactive oxygen species generation after opening of mitochondrial KATP channels

opening of the ATP-sensitive K+ (KATP) channel mediates the cardioprotective effect induced by pathophysiological stressors such as ischemic preconditioning (IPC) ([15][1], [29][2]), heat shock ([19][3]), and pharmacological agents, including adenosine ([5][4]), ACh ([26][5]), opioids ([12][6]),

Electrocardiographic manifestations: electrolyte abnormalities

Because myocyte depolarization and repolarization depend on intra- and extracellular shifts in ion gradients, abnormal serum electrolyte levels can have profound effects on cardiac conduction and the electrocardiogram (EKG). Changes in extracellular potassium, calcium, and magnesium levels can change myocyte membrane potential gradients and alter the cardiac action potential. These changes can result in incidental findings on the 12-lead EKG or precipitate potentially life-threatening dysrhythmias. We will review the major electrocardiographic findings associated with abnormalities of the major cationic contributors to cardiac conduction—potassium, calcium and magnesium.

Stimulus-evoked modulation of sensorimotor pyramidal neuron EPSPs.

Sensory cortical neurons display substantial receptive field dynamics during and after persistent sensory drive. Because a cell's response properties are determined by the inputs it receives, receptive field dynamics are likely to involve changes in the relative efficacy of different inputs to the cell. To test this hypothesis, we have investigated if brief repetitive stimulus drive in vitro alters the efficacy of two types of corticocortical inputs to layer V pyramidal cells. Specifically, we have used whole cell recordings to measure the effect of repetitive electrical stimulation at the layer VI/white matter (WM) border on the synaptic response of layer V pyramidal cells to corticocortical input evoked by electrical stimulation of layer I or layer II/III and emulated by local application of glutamate. Repetitive stimulation (10 Hz for 3 s) at the layer VI/WM border transiently potentiated excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of layer II/III by 97 +/- 12% (mean +/- SE). The recovery of EPSP amplitude to its preconditioning value was well-described by a single-term decaying exponential with a time constant of 7.2 s. The same layer VI/WM conditioning train that evoked layer II/III EPSP potentiation frequently caused an attenuation of layer I EPSPs. Similarly, subthreshold postsynaptic responses to local glutamate application in layers II/III and I were potentiated and attenuated, respectively, by the conditioning stimulus. Potentiation and attenuation could be evoked in the same cell by repositioning the glutamate puffer pipette in the appropriate layer. The conditioning stimulus that led to the transient modification of upper layer EPSP efficacy also evoked a slow depolarization in glial cells. The membrane potential of glial cells recovered with a time course similar to the dissipation of the potentiation effect, suggesting that stimulus-evoked changes in extracellular potassium (ECK) play a role in layer II/III EPSP potentiation. Consistent with this proposal, increasing the bath concentration of ECK caused a substantial increase of layer II/III EPSP amplitude. EPSP potentiation was sensitive to postsynaptic membrane potential and, more importantly, was significantly weaker for synaptic currents than for synaptic potentials, suggesting that it involves the recruitment of a postsynaptic voltage-dependent mechanism. Two observations suggest that layer II/III EPSP potentiation may involve the recruitment of postsynaptic sodium channels: EPSP potentiation was strongly reduced by intracellular application of N-(2,6-dimethyl-phenylcarbamoylmethyl) triethylammonium bromide (QX-314) and responses to local glutamate application were potentiated by high ECK in the presence of cadmium but not in the presence of tetrodotoxin. The results demonstrate a novel way in which brief periods of repetitive stimulus drive are accompanied by rapid, transient, and specific alterations in the functional connectivity and information processing characteristics of sensorimotor cortex.

Disorders of mineralocorticoid synthesis.

Abnormalities of mineralocorticoid synthesis and/or metabolism profoundly affect the regulation of electrolyte and water balance and of blood pressure. Characteristic changes in extracellular potassium, sodium and hydrogen ion concentrations are usually diagnostic. Serious deficiency may be acquired, for example in Addison's disease, or inherited. In most of the inherited syndromes, the precise molecular changes in specific steroidogenic enzymes have been identified. Mineralocorticoid excess may be caused by aldosterone or 11-deoxycorticosterone by inadequate conversion of cortisol to cortisone by 11beta-hydroxysteroid dehydrogenase type 2 in target tissues (see Chapter 4), by glucocorticoid receptor deficiency or by constitutive activation of renal sodium channels. Changes in electrolyte balance and renin as well as the abnormal pattern of corticosteroid metabolism are usually diagnostic. Where these abnormalities are inherited (e.g. 11beta- or l7alpha-hydroxylase deficiencies, glucocorticoid remediable hyperaldosteronism (GRA), receptor defects, Liddle's syndrome), the molecular basis is again usually known and, in some cases, may provide the simplest diagnostic tests. Primary aldosteronism, although readily identifiable, presents problems of differential diagnosis, important because optimal treatment is different for each variant. Moreover, the mechanisms by which the variants develop are poorly understood. Finally, a significant proportion of patients with essential hypertension show characteristics of mild mineralocorticoid excess, for example low renin levels. Is this relevant to pathophysiology and, if so, is the effect induced via classic mechanisms of action or through newly discovered direct actions on the brain, heart and blood vessels? These questions are the subject of current research.

Potassium signalling in the brain: its role in behaviour

This paper examines evidence that glial cells respond to changes in extracellular potassium ([K +] e) in ways that contribute to modulation of neuronal activity and thereby behaviour. Glial cells spatially (and probably directionally) redistribute potassium from regions of increasing concentration to those with a lesser concentration. This redistribution is largely responsible for slow potential shifts associated with behavioural responses of animals. These slow shifts are related in amplitude to the level of ‘arousal’ of an animal, and its motivational state. In addition, glia, especially astrocytes, respond to changes in [K +] e, the presence of transmitters like nor-adrenaline and glutamate and at least some hormones with changes in their metabolism and/or the morphological characteristics of the cell. The ionic, metabolic and morphological responses of glia to changes in extracellular potassium after neuronal activity have been associated with at least some forms of learning, including habituation, one trial passive avoidance learning and changes associated with enriched environments. The implication of these effects of potassium signalling in the brain is that there is considerable involvement of glia in a number of processes crucial to neuronal activity. Glia may also form another route for information distribution in the brain that is at least bi-directional, though less specific than its neuronal counterparts. It is evident that the Neuroscience of the future will have to incorporate much more study of neuron-glial interactions than hitherto.

Ionic changes accompanying astrocytic intercellular calcium waves triggered by mechanical cell damaging stimulation.

Mechanically poking or damaging a single cell within a confluent astrocyte culture produces the so-called intercellular calcium (Ca(2+)) waves, that is, cell-to-cell propagating changes of intracellular free Ca(2+). We were interested whether intercellular Ca(2+) waves are also associated with changes in other intra- or extracellular ions. To that purpose, we investigated spatiotemporal changes of intracellular Ca(2+) (Ca(i)2+), sodium (Na(i)+) and protons (H(i)+) in primary cultures of rat cortical astrocytes using microfluorescence imaging with fura-2, SBFI and BCECF, respectively; changes of extracellular potassium (K(e)+) were monitored with K(+)-sensitive microelectrodes. Mechanical damage to a single cell by stimulation with a piezo-electrically driven micropipette initiated intercellular Ca(2+) waves that propagated to about 160 microm away from the stimulation point. Na(i)(+) increases could be detected in cells located 2-3 cell diameters from the stimulated cell, acidification was observed 1-2 cell diameters away and Ke(+) increases were measured up to 75 microm away. Kinetic analysis suggests that the Na(i)(+) and H(i)(+) changes occur after, and thus secondary to the Ca(i)(2+) changes. In contrast, K(e)(+) changes occurred very fast, even before the Ca(i)(2+) changes, but their propagation speed was too fast to implicate them as a trigger of Ca(i)(2+) changes. As Na(i)(+) is an important regulator of glycolysis in astrocytes, we hypothesize that astrocytic Na(i)(+) changes in cells located remotely from a damaged cell might be a signal that activates glycolysis thereby producing more lactate that is transferred to the neurons and increases their energy potential to survive the inflicted damage.

Vagal control of heart rate is modulated by extracellular potassium

Heart rate (HR) recovery from heavy exercise is associated with a shift in cardiac sympatho-vagal balance and a transient hypokalaemia. Since changes in extracellular potassium ([K +] 0) affect membrane currents in the sino-atrial node, in particular the acetylcholine-activated potassium current ( I K,ACh), the hyperpolarization-activated current ( I f) and the L-type calcium current ( I Ca,L), we investigated whether mimicking [K +] 0 concentrations seen during and immediately after exercise could directly modulate the HR response to vagal nerve stimulation (VNS) in the isolated guinea-pig atria preparation pre-stimulated with noradrenaline (NA, 1 μM). Lowering [K +] 0 from 4 to 3 mM significantly enhanced the HR response to VNS (5 Hz, 5 V, 30 s, ΔHR 84.5±14.1 bpm and 119.3±18.2 bpm, respectively). Increasing [K +] 0 to 8 or 10 mM significantly decreased the drop in HR with VNS in comparison to the response to 3 mM K + Tyrode (ΔHR 56.4±9.1 bpm and 52.1±8.7 bpm, respectively). These results could be simulated using the OXSOFT heart sino-atrial node computer model by activating I K,ACh during changes in [K +] 0. However, changing [K +] 0 in the model had no significant effect on the decrease in beating frequency brought about by decreasing I f or I Ca,L. We conclude that the magnitude of the decrease in HR with VNS is enhanced in low [K +] 0 and reduced in high [K +] 0. The increased efficacy of cardiac vagal activation in low [K +] 0 might therefore facilitate the drop in HR after heavy exercise where there is a transient hypokalaemia. Modelling suggests this result may be explained by the effects of changes in [K +] 0 on the current–voltage relationship for I K,ACh.

Role of apical H-K exchange and basolateral K channel in the regulation of intracellular pH in rat distal colon crypt cells.

An apical membrane ouabain-sensitive H-K exchange and a barium-sensitive basolateral membrane potassium channel are present in colonic crypt cells and may play a role in both K absorption and intracellular pH (pHi) regulation. To examine the possible interrelationship between apical membrane H-K exchange and basolateral membrane K movement in rat distal colon in the regulation of pHi, experiments were designed to assess whether changes in extracellular potassium can alter pHi. pHi in isolated rat crypts was determined using microspectrofluorimetric measurements of the pH-sensitive dye BCECF-AM (2', 7'-bis(carboxyethyl-5(6)-carboxy-fluorescein acetoxy methylester). After loading with the dye, crypts were superfused with a Na-free solution which resulted in a rapid and reversible fall in pHi (7.36 +/- 0.02 to 6.98 +/- 0.03). Following an increase in extracellular [K] to 20 mm, in the continued absence of Na, there was a further decrease in pHi (0.20 +/- 0.02, P < 0.01). K-induced acidification was blocked both by 2 mm bath barium, a K channel blocker, and by 0. 5 mm lumen ouabain. K-induced acidification was also observed when intracellular acidification was induced by a NH4Cl prepulse. These observations suggest that increased basolateral K movement increases intracellular [K] resulting in a decrease in pHi that is mediated by a ouabain-sensitive apical membrane H,K-ATPase. Our results demonstrate an interrelationship between basolateral K movement and apical H-K exchange in the regulation of pHi and apical K entry in rat distal colon.