Changes In Potassium Currents Research Articles

Modeling the effects of extracellular potassium on bursting properties in pre-Bötzinger complex neurons

There are many types of neurons that intrinsically generate rhythmic bursting activity, even when isolated, and these neurons underlie several specific motor behaviors. Rhythmic neurons that drive the inspiratory phase of respiration are located in the medullary pre-Bötzinger Complex (pre-BötC). However, it is not known if their rhythmic bursting is the result of intrinsic mechanisms or synaptic interactions. In many cases, for bursting to occur, the excitability of these neurons needs to be elevated. This excitation is provided in vitro (e.g. in slices), by increasing extracellular potassium concentration (K out) well beyond physiologic levels. Elevated K out shifts the reversal potentials for all potassium currents including the potassium component of leakage to higher values. However, how an increase in K out , and the resultant changes in potassium currents, induce bursting activity, have yet to be established. Moreover, it is not known if the endogenous bursting induced in vitro is representative of neural behavior in vivo. Our modeling study examines the interplay between K out, excitability, and selected currents, as they relate to endogenous rhythmic bursting. Starting with a Hodgkin-Huxley formalization of a pre-BötC neuron, a potassium ion component was incorporated into the leakage current, and model behaviors were investigated at varying concentrations of K out. Our simulations show that endogenous bursting activity, evoked in vitro by elevation of K out , is the result of a specific relationship between the leakage and voltage-dependent, delayed rectifier potassium currents, which may not be observed at physiological levels of extracellular potassium.

The effect of pressure on potassium currents in the inner hair cells isolated from guinea-pig cochlea.

An increase in hydrostatic pressure in the endolymphatic system causes hydrops-related inner ear diseases such as Meniere's disease or low tone sensorineural hearing loss. In the present study, we investigated the effects of pressure exerted on potassium currents in acutely isolated inner hair cells of the guinea-pig cochlea using whole-cell voltage-clamp techniques. By applying negative or positive pressure via the patch pipette using a syringe, intracellular hydropressure was changed between -40 cm H2O to +20 cm H2O. Negative pressure potentiated the amplitude of potassium currents, whereas positive pressure suppressed the amplitude of potassium currents. Gadolinium, a blocker of stretch-activated cation channels, did not influence pressure-dependent changes in potassium currents; however, cinnarizine blocked pressure-dependent changes in potassium currents. The current changes were not dependent on the sign of the pressure change, that is, similar increases in negative pressures (between -10 cm H2O and -40 cm H2O) and similar decreases in positive pressures (between +10 cm H2O and +20 cm H2O) were observed.

Prolonged Action Potential and After depolarizations Are Not due to Changes in Potassium Currents in NOS3 Knockout Ventricular Myocytes

Ventricular myocytes deficient in endothelial nitric oxide synthase (NOS3−/−) exhibit prolonged action potential (AP) duration and enhanced spontaneous activity (early and delayed afterdepolarizations) during β-adrenergic (β-AR) stimulation. Studies have shown that nitric oxide is able to regulate various K+ channels. Our objective was to examine if NOS3−/− myocytes had altered K+ currents. APs, transient outward (I to), sustained (I Ksus), and inward rectifier (I K1) K+ currents were measured in NOS3−/− and wild-type (WT) myocytes. During β-AR stimulation, AP duration (measured as 90% repolarization-APD90) was prolonged in NOS3−/− compared to WT myocytes. Nevertheless, we did not observe differences in I to, I Ksus, or I K1 between WT and NOS3−/− myocytes. Our previous work showed that NOS3−/− myocytes had a greater Ca2+ influx via L-type Ca2+ channels with β-AR stimulation. Thus, we measured β-AR-stimulated SR Ca2+ load and found a greater increase in NOS3−/− versus WT myocytes. Hence, our data suggest that the prolonged AP in NOS3−/− myocytes is not due to changes in I to, I Ksus, or I K1. Furthermore, the increase in spontaneous activity in NOS3−/− myocytes may be due to a greater increase in SR Ca2+ load. This may have important implications for heart failure patients, where arrhythmias are increased and NOS3 expression is decreased.

Increased coupling and altered glutamate transport currents in astrocytes following kainic-acid-induced status epilepticus

Profound astrogliosis coincident with neuronal cell loss is universally described in human and animal models of temporal lobe epilepsy (TLE). In the kainic acid-induced status epilepticus (SE) model of TLE, astrocytes in the hippocampus become reactive soon after SE and before the onset of spontaneous seizures. To determine if astrocytes in the hippocampus exhibit changes in function soon after SE, we recorded from SR101-labeled astrocytes using the whole-cell patch technique in hippocampal brain slices prepared from control and kainic-acid-treated rats. Glutamate transporter-dependent currents were found to have significantly faster decay time kinetics and in addition, dye coupling between astrocytes was substantially increased. Consistent with an increase in dye coupling in reactive astrocytes, immunoblot experiments demonstrated a significant increase in both glial fibrillary acidic protein (GFAP) and connexin 43, a major gap junction protein expressed by astrocytes. In contrast to what has been observed in resected tissue from patients with refractory epilepsy, changes in potassium currents were not observed shortly after KA-induced SE. While many changes in neuronal function have been identified during the initial period of low seizure probability in this model of TLE, the present study contributes to the growing body of literature suggesting a role for astrocytes in the process of epileptogenesis.

Controlling auditory excitability: the benefits of a cultured environment

Controlling auditory excitability: the benefits of a cultured environment

Shared requirement for dynein function and intact microtubule cytoskeleton for normal surface expression of cardiac potassium channels

Potassium channels at the cardiomyocyte surface must eventually be internalized and degraded, and changes in cardiac potassium channel expression are known to occur during myocardial disease. It is not known which trafficking pathways are involved in the control of cardiac potassium channel surface expression, and it is not clear whether all cardiac potassium channels follow a common pathway or many pathways. In the present study we have surveyed the role of retrograde microtubule-dependent transport in modulating the surface expression of several cardiac potassium channels in ventricular myocytes and heterologous cells. The disruption of microtubule transport in rat ventricular myocytes with nocodazole resulted in significant changes in potassium currents. A-type currents were enhanced 1.6-fold at +90 mV, rising from control densities of 20.9 +/- 2.8 to 34.0 +/- 5.4 pA/pF in the nocodazole-treated cells, whereas inward rectifier currents were reduced by one-third, perhaps due to a higher nocodazole sensitivity of Kir channel forward trafficking. These changes in potassium currents were associated with a significant decrease in action potential duration. When expressed in heterologous human embryonic kidney (HEK-293) cells, surface expression of Kv4.2, known to substantially underlie A-type currents in rat myocytes, was increased by nocodazole, by the dynein inhibitor erythro-9-(2-hydroxy-3-nonyl) adenine hydrochloride, and by p50 overexpression, which specifically interferes with dynein motor function. Peak current density was 360 +/- 61.0 pA/pF in control cells and 658 +/- 94.5 pA/pF in cells overexpressing p50. The expression levels of Kv2.1, Kv3.1, human ether-a-go-go-related gene, and Kir2.1 were similarly increased by p50 overexpression in this system. Thus the regulation of potassium channel expression involves a common dynein-dependent process operating similarly on the various channels.

Developmental changes in potassium currents at the calyx of Held presynaptic terminal

Developmental changes in potassium currents at the calyx of Held presynaptic terminal

Developmental changes in potassium currents at the rat calyx of Held presynaptic terminal

During early postnatal development, the calyx of Held synapse in the auditory brainstem of rodents undergoes a variety of morphological and functional changes. Among ionic channels expressed in the calyx, voltage-dependent K+ channels regulate transmitter release by repolarizing the nerve terminal. Here we asked whether voltage-dependent K+ channels in calyceal terminals undergo developmental changes, and whether they contribute to functional maturation of this auditory synapse. From postnatal day (P) 7 to P14, K+ currents became larger and faster in activation kinetics, but did not change any further to P21. Likewise, presynaptic action potentials became shorter in duration from P7 to P14 and remained stable thereafter. The density of presynaptic K+ currents, assessed from excised patch recording and whole-cell recordings with reduced [K+]i, increased by 2-3-fold during the second postnatal week. Pharmacological isolation of K+ current subtypes using tetraethylammonium (1 mM) and margatoxin (10 nM) revealed that the density of Kv3 and Kv1 currents underwent a parallel increase, and their activation kinetics became accelerated by 2-3-fold. In contrast, BK currents, isolated using iberiotoxin (100 nM), showed no significant change during the second postnatal week. Pharmacological block of Kv3 or Kv1 channels at P7 and P14 calyceal terminals indicated that the developmental changes of Kv3 channels contribute to the establishment of reliable action potential generation at high frequency, whereas those of Kv1 channels contribute to stabilizing the nerve terminal. We conclude that developmental changes in K+ currents in the nerve terminal contribute to maturation of high-fidelity fast synaptic transmission at this auditory relay synapse.

Serotonin-Induced Inhibition of K V Current

See related article, pages 931–938 Pulmonary vascular tone is largely determined by potassium and calcium currents in the smooth muscle cells (SMCs) of the small resistance arteries. Under normoxic circumstances an outward potassium current ( I K) passes through voltage-gated (Kv), calcium-activated (KCa), and background tandem-pore domain (K2P) potassium channels. The latter (coded by the KCNK family of genes) include the twik-related acid sensitive K+ channel TASK and TASK-like channels. This current keeps the resting membrane potential in the range of −50 to −60 mV and inhibits calcium entry through L-type calcium channels.1 Vasodilator substances such as nitric oxide and prostacyclin, released from the endothelium, increase I K, one of several actions that lead to further vasodilatation.2,3 Vasoconstrictor substances, such as endothelin, can inhibit I K4 and/or release calcium from the sarcoplasmic reticulum.5 In addition to vasoactive effects, an increase in cytosolic potassium inhibits smooth muscle cell apoptosis,6 and an increase in cytosolic calcium promotes cellular proliferation.7 Consequently, the inhibition of I K is important not only in causing membrane depolarization and calcium entry but also in stimulating vascular remodeling and in the development of chronic pulmonary hypertension. Agents that can cause pulmonary hypertension, such as the anorectic drugs and hypoxia, inhibit I K8,9 and also enhance calcium release from the sarcoplasmic reticulum, leading to subsequent repletion of calcium through store-operated channels.10,11,12 The smooth muscle cells in the resistance pulmonary arteries of patients with pulmonary arterial hypertension (PAH) exhibit both reduced I K and decreased expression of Kv channels,13 and also increased expression of the TRPC ( t ransient r eceptor p otential, c anonical) genes that code for store-operated and receptor-operated calcium channels.14 The decreased expression and function of the Kv channels may …

Mechanisms of ventricular arrhythmias in heart failure

Congestive heart failure continues to be a leading cause of mortality and morbidity worldwide. In approximately 50% of these patients, the mode of death is sudden. Ventricular tachycardia and fibrillation represent the majority of arrhythmias; the mechanisms responsible are heterogeneous and complex. Myocardial scar, a potent environment for reentry, is likely to contribute to many of the ventricular arrhythmias in ischemic heart failure. Altered calcium handling and changes in potassium currents may contribute to the increase in early and delayed afterdepolarizations seen in the failing heart. In addition, compensatory mechanisms may become deleterious and potentially arrhythmogenic via a variety of mechanisms. This article provides a general overview of the mechanisms thought to be responsible for ventricular arrhythmias in chronic heart failure.

Alterations of potassium currents in ischemia-vulnerable and ischemia-resistant neurons in the hippocampus after ischemia

CA1 pyramidal neurons in the hippocampus die 2–3 days following transient forebrain ischemia, whereas CA3 pyramidal neurons and granule cells in the dentate gyrus remain viable. Excitotoxicity is the major cause of ischemic cell death, and potassium currents play important roles in regulating the neuronal excitability. The present study compared the changes of potassium currents in acutely dissociated hippocampal neurons at different intervals after ischemia. In CA1 neurons, the amplitude of rapid inactivating potassium currents ( I A) was significantly increased at 14 h and returned to control levels at 38 h after ischemia; the rising slope and decay time constant of I A were accordingly increased after ischemia. The activation curve of I A in CA1 neurons shifted to the depolarizing direction at 38 h after ischemia. In granule cells, the amplitude and rising slope of I A were significantly increased at 38 h after ischemia; the inactivation curves of I A shifted toward the depolarizing direction accordingly at 38 h after ischemia. The I A remained unchanged in CA3 neurons after ischemia. The amplitudes of delayed rectifier potassium currents ( I Kd) in CA1 neurons were progressively increased after ischemia. No significant difference in I Kd was detected in CA3 and granule cells at any time points after reperfusion. These results indicated that the voltage dependent potassium currents in hippocampal neurons were differentially altered after cerebral ischemia. The up-regulation of I A in dentate granule cells might have protective effects. The increase of I Kd in CA1 neurons might be associated with the neuronal damage after ischemia.

Calcium-dependent changes in potassium currents in guinea-pig coronary artery smooth muscle cells after acute cobalt loading in vivo.

The aim of the present study was to determine whether cobalt poisoning induces haem oxidase isoenzyme-1 (HO-1) in coronary artery smooth muscle, or accounts for any changes in coronary smooth muscle cell (SMCs) membrane ionic currents that could result from this type of heavy metal poisoning. In SMCs isolated from cobalt-treated guinea-pig coronaries, K+ channel currents (IK) were much smaller than those in cells isolated from non-treated animals. Haemin (HO substrate) increased IK concentration dependently. This effect was mimicked by 1% CO and was abolished by pretreatment of cells with a competitive HO inhibitor, by inhibitors of guanylyl cyclase, protein kinase G or phospholipase C, as well as by blocking inositol trisphosphate-dependent Ca release, or sarcoplasmic reticulum Ca-ATPase, or by bathing cells in Ca-free external solution. Expression of the Na/Ca exchanger-1 (NCX-1) protein was reduced substantially in SMCs from coronary arteries of cobalt-treated animals. No expression of HO-1 was detected. It is concluded that acute cobalt poisoning in vivo depresses Ca-sensitive K currents via CO-dependent modulation of intracellular calcium availability, most probably by suppressing the expression of NCX-1 protein.

The changes of potassium currents in rabbit ventricle with healed myocardial infarction.

To elucidate the mechanism of arrhythmia in healed myocardial infarction (HMI), the changes of action potential duration (APD), transient outward potassium current (Ito), delayed rectifier potassium current (IK) and inward rectifier potassium current (IK1) of left ventricular myocytes in non-infarcted zone of HMI were investigated. Rabbits were randomly assigned into two groups: HMI group, in which animals were subjected to thoracotomy and ligation of the circumflex coronary and sham-operated group, in which rabbits underwent thoracotomy but no conorary ligation. 3 months after the operation, the whole myocyte patch clamp technique was used to record APD, Ito, IK, and IK1 of ventricular myocytes in non-infarcted zone. Our results showed that the membrane capacitance was larger in HMI group than in sham-operated group. Action potential duration was significantly lengthened in HMI group and early afterdepolarization (EAD) appeared in HMI group. The densities of Ito, I(K, tail), and IK1 were reduced significantly in HMI group, from 6.72 +/- 0.42 pA/pF, 1.54 +/- 0.13 pA/pF and 25.6 +/- 2.6 pA/pF in sham-operated group to 4.03 +/- 0.33 pA/pF, 1.14 +/- 0.11 pA/pF and 17.6 +/- 2.3 pA/pF, respectively. It is concluded that the reduced densities of Ito, I(K, tail) and IK1 in ventricular myocytes of non-infarcted zone in HMI were responsible for the prolongation of APD and the presentation of EAD which played important roles in the development of malignant arrhythmia in HMI.

Properties and postnatal changes of potassium currents in rat median raphe neurons

Properties and postnatal changes of potassium currents in rat median raphe neurons

Alterations of single potassium channel activity in CA1 pyramidal neurons after transient forebrain ischemia

Selective neuronal injury in the CA1 zone of hippocampus following transient cerebral ischemia has been well documented ( Pulsinelli et al., 1982; Kirino, 1982). Extracellular potassium concentration markedly increases during ischemia/hypoxia ( Hansen, 1985; Schmidt-Kastener and Freund, 1991). Accumulating evidence has indicated that the outward potassium currents, including delayed rectifier potassium current, not only influence membrane excitability but also mediate apoptosis ( Bortner et al., 1997; Hughes et al., 1997; Yu et al., 1997). It has been shown that the amplitude of delayed rectifier potassium current in CA1 neurons significantly increased after cerebral ischemia ( Chi and Xu, 2000b). To elucidate the mechanisms underlying the changes of potassium currents following ischemia, single potassium channel activities of rat CA1 neurons were compared before and after transient forebrain ischemia. Using cell-attached configuration, depolarizing voltage steps activated outward single channel events. The channel properties, the kinetics and pharmacology of these events resemble the delayed rectifier potassium current. After ischemia, the unitary amplitude of single channels significantly increased, the open probability, mean open time and open time constant also significantly increased while the conductance remained unchanged. These data indicate that the increase of single channel activity is responsible, at least in part, for the increase of delayed rectifier potassium current in CA1 neurons after cerebral ischemia.

Tonotopic map of potassium currents in chick auditory hair cells using an intact basilar papilla

The avian basilar papilla is tonotopically organized such that hair cells along the sensory epithelium respond best to acoustic stimulation at differing frequencies. This specificity arises due to the mechanics of the cochlea itself and intrinsic electrical properties of the hair cells. Tall hair cells show membrane voltage oscillations in response to step current injection that may allow cells to act as electrical resonators, boosting the response at the resonant frequency. These oscillations and the underlying currents have been studied in enzymatically isolated cells. This study uses a whole chick ( Gallus domesticus) basilar papilla preparation where the entire epithelium and its afferent connections are intact. With this preparation, a map of changes in potassium currents of tall hair cells was produced. All cells recorded from expressed two K + currents, a calcium-activated K + current, I K(Ca), and a voltage-activated K + current, I K. Also, apical cells expressed an inward rectifier K + current, I IR. The amplitude of total outward current increases in a gradient along the tonotopic axis. Pharmacological blockers were used to separate the outward K + currents. These experiments showed that both currents individually increase in magnitude along a gradient from apex to base. Finally, measurements of oscillation frequency in response to current steps suggest a discontinuous change in the electrical resonances at about 33% from the apex. This study demonstrates a new preparation to study the electrical properties of hair cells in more detail along the tonotopic axis of the chick basilar papilla.

Postnatal development of spike generation in rat medial vestibular nucleus neurons.

Image stability during self motion depends on the combined actions of the vestibuloocular and optokinetic reflexes (VOR and OKR, respectively). Neurons in the medial vestibular nucleus (MVN) participate in the VOR and OKR by firing in response to both head and image motion. Their intrinsic spike-generating properties enable MVN neurons to modulate firing rates linearly over a broad range of input amplitudes and frequencies such as those that occur during natural head and image motion. This study examines the postnatal development of the intrinsic spike-generating properties of rat MVN neurons with respect to maturation of peripheral vestibular and visual function. Spike generation was studied in a brain stem slice preparation by recording firing responses to current injected intracellularly through whole cell patch electrodes. MVN neurons fired spontaneously and modulated their firing rate in response to injected current at all postnatal ages. However, the input-output properties of the spike generator changed dramatically during the first two postnatal weeks. Neurons younger than postnatal day 10 could not fire faster than 80 spikes/s, modulated their firing rates over a limited range of input amplitudes, and tended to exhibit a nonlinear relationship between input current and mean evoked firing rate. In response to sustained depolarization, firing rates declined significantly in young neurons. Response gains tended to be highest in the first few postnatal days but varied widely across neurons and were not correlated with age. By about the beginning of the third postnatal week, MVN neurons could fire faster than 100 spikes/s in response to a broad range of input amplitudes, exhibited predominantly linear current-firing rate relationships, and adapted little in response to sustained depolarization. Concomitant decreases in action potential width and the time course of the afterhyperpolarization suggest that changes in potassium currents contribute to the maturation of the MVN neuronal spike generator. The results demonstrate that developmental changes in intrinsic membrane properties enable MVN neurons to fire linearly in response to a broad range of stimuli in time for the onset of visual function at the beginning of the third postnatal week.

Differential changes of potassium currents in CA1 pyramidal neurons after transient forebrain ischemia.

CA1 pyramidal neurons are highly vulnerable to transient cerebral ischemia. In vivo studies have shown that the excitability of CA1 neurons progressively decreased following reperfusion. To reveal the mechanisms underlying the postischemic excitability change, total potassium current, transient potassium current, and delayed rectifier potassium current in CA1 neurons were studied in hippocampal slices prepared before ischemia and at different time points following reperfusion. Consistent with previous in vivo studies, the excitability of CA1 neurons decreased in brain slices prepared at 14 h following transient forebrain ischemia. The amplitude of total potassium current in CA1 neurons increased approximately 30% following reperfusion. The steady-state activation curve of total potassium current progressively shifted in the hyperpolarizing direction with a transient recovery at 18 h after ischemia. For transient potassium current, the amplitude was transiently increased approximately 30% at approximately 12 h after reperfusion and returned to control levels at later time points. The steady-state activation curve also shifted approximately 20 mV in the hyperpolarizing direction, and the time constant of removal of inactivation markedly increased at 12 h after reperfusion. For delayed rectifier potassium current, the amplitude significantly increased and the steady-state activation curve shifted in the hyperpolarizing direction at 36 h after reperfusion. No significant change in inactivation kinetics was observed in the above potassium currents following reperfusion. The present study demonstrates the differential changes of potassium currents in CA1 neurons after reperfusion. The increase of transient potassium current in the early phase of reperfusion may be responsible for the decrease of excitability, while the increase of delayed rectifier potassium current in the late phase of reperfusion may be associated with the postischemic cell death.

Oita International Electrocardiology Symposium 2000 “Electrophysiology and Management of Lethal Arrhythmias in the New Millennium: From Genes to Bedside”

Torsades de Pointes arrhythmias can be triggered by early after-depolarizations (EAD) . Mechanisms of generation of EAD's attributable to genetic mutations in the sodium channel, to changes in potassium currents, and to multiple changes in gene expression levels are reconstructed using computer models. One of these mechanisms has already been incorporated into whole organ models. Work is in progress to incorporate the other cellular mechanisms. Biophysically detailed models of this kind can identify combinations of receptor actions that would be expected to be therapeutic.

Developmental changes of potassium currents of embryonic leech ganglion cells in primary culture.

The expression of calcium-activated potassium currents (IK(Ca)), delayed outward rectifier potassium currents (IK(slow)), and transient outward currents (IA) was studied during the development of the nervous system of the leech using the whole-cell patch-clamp recording technique. Dissociated cells were isolated from leech embryos between stage E7 and E16 and maintained in primary culture. K+ currents were recorded at E7, when only few anterior ganglia had formed beneath the primordial mouth. IK(slow) was present in all cells tested, while IK(Ca) was expressed in only 67% of the cells studied. Even as early as E7, different types of IK(Ca) have been found. Neither frequency of occurrence nor the charge density of IK(Ca) showed significant changes between E7 and E16. The density of IK(slow), however, increased by a factor of two between E7 and E8, which resulted in a significant increase in the total K+ current of these cells. This rise in potassium outward current developed in parallel with the appearance of Na+ and Ca2+ inward currents (Schirrmacher and Deitmer: J Exp Biol 155:435-453, 1991) during early development, shaping the electrical excitability in embryonic leech neurones. I(A) could be separated by its voltage-dependence and pharmacological properties. The current was detected at stage E9, when all 32 ganglia are formed in the embryo. The frequency of occurrence of I(A) increased from 16% at E9 to 70% at E15. The channel density, steady state inactivation, and kinetics showed no significant changes during development.