Large-conductance Calcium-dependent Potassium Channels Research Articles

Antioxidant and Vasodilator Activity of Ugni molinae Turcz. (Murtilla) and Its Modulatory Mechanism in Hypotensive Response.

Hypertension is a systemic condition with high morbidity and mortality rates worldwide, which poses an increased risk for cardiovascular diseases. In this study, we demonstrated the antioxidant and vasodilator activity of Ugni molinae Turcz. (Murtilla) fruit, a berry native to Chile and proposed models to explain its modulatory mechanism in hypotensive response. Murtilla fruits were cultivated in a germplasm bank and submitted to chemical and biological analyses. The phenolic compounds gallic acid, Catechin, Quercetin-3-β-D-glucoside, Myricetin, Quercetin, and Kaempferol were identified. Murtilla extract did not generate toxic effects on human endothelial cells and had significant antioxidant activity against ROS production, lipid peroxidation, and superoxide anion production. Furthermore, it showed dose-dependent vasodilator activity in aortic rings in the presence of endothelium, whose hypotensive mechanism is partially mediated by nitric oxide synthase/guanylate cyclase and large-conductance calcium-dependent potassium channels. Murtilla fruits might potentially have beneficial effects on the management of cardiovascular diseases.

Large-conductance Ca2+-activated potassium channel in mitochondria of endothelial EA.hy926 cells

In the present study, we describe the existence of a large-conductance Ca²⁺-activated potassium (BKCa) channel in the mitochondria of the human endothelial cell line EA.hy926. A single-channel current was recorded from endothelial mitoplasts (i.e., inner mitochondrial membrane) using the patch-clamp technique in the mitoplast-attached mode. A potassium-selective current was recorded with a mean conductance equal to 270 ± 10 pS in a symmetrical 150/150 mM KCl isotonic solution. The channel activity, which was determined as the open probability, increased with the addition of calcium ions and the potassium channel opener NS1619. Conversely, the activity of the channel was irreversibly blocked by paxilline and iberiotoxin, BKCa channel inhibitors. The open-state probability was found to be voltage dependent. The substances known to modulate BKCa channel activity influenced the bioenergetics of mitochondria isolated from human endothelial EA.hy926 cells. In isolated mitochondria, 100 μM Ca²⁺, 10 μM NS1619, and 0.5 μM NS11021 depolarized the mitochondrial membrane potential and stimulated nonphosphorylating respiration. These effects were blocked by iberiotoxin and paxilline in a potassium-dependent manner. Under phosphorylating conditions, NS1619-induced, iberiotoxin-sensitive uncoupling diverted energy from ATP synthesis during the phosphorylating respiration of the endothelial mitochondria. Immunological analysis with antibodies raised against proteins of the plasma membrane BKCa channel identified a pore-forming α-subunit and an auxiliary β₂-subunit of the channel in the endothelial mitochondrial inner membrane. In conclusion, we show for the first time that the inner mitochondrial membrane in human endothelial EA.hy926 cells contains a large-conductance calcium-dependent potassium channel with properties similar to those of the surface membrane BKCa channel.

Changes in Calcium-Dependent Potassium Conductance of Isolated Smooth Muscle Cells of the Rat Bladder in Experimental Diabetes

Activation of large conductance calcium-dependent potassium channels (BK channels) affects the membrane repolarization at action potential generation and the resting potential of detrusor smooth muscle cells (SMC). Overactive bladder syndrome (OBS) is one of the complications of diabetes mellitus. Using patch-clamp technique, we have found a significant decrease in potassium current (IBK) through BK channels of isolated detrusor SMC, and a decrease in the amplitude and frequency of BK-mediated spontaneous transient outward currents (STOCs) in rats with streptozotocin-induced diabetes mellitus compared with control animals. Under the diabetes, IBK density at step depolarization to +50 mV averaged (10.0 ± 0.5) pA / pF (the control values were (15.0 ± 0.4) pA / pF), whereas the mean values of the STOC frequency and amplitude at holding potential of -20 mV decreased from (12.0 ± 1.5) Hz to (2.4 ± 0.6) Hz and (0.9 ± 0.1) to 0.5 pA / pF ± 0.1 pA / pF, respectively. Using real time polymerase chain reaction (RT-PCR), it was found that mRNA expression of primary pore-forming KCa1.1-subunit of the BK channel increased in diabetes, whilst that for the auxiliary BKCaβ1-subunit decreased. It has been concluded that the observed changes in BK-channel currents in diabetes may enhance excitability of detrusor SMC, thereby promoting myogenic OBS. However, more studies are needed to determine how a decrease in BKCa β1 expression affects the functional properties of BK channels in diabetes, and to identify the type and direction of calcium signal changes modulating channel activation.

The role of the large-conductance calcium-dependent potassium channel, BK/Slowpoke, in shaping motor neuron firing during rhythmic activity

Rhythmic muscle contractions underlie a number of crucial motor behaviors, such as respiration and locomotion. The timing of contractions is determined by the intrinsic activity and synaptic interactions of neurons within what are called central pattern generating (CPG) networks [1,2]. In many systems, motor neurons (MNs) are not part of the classically-defined CPG. However, research suggests that ionic currents in MNs may shape the timing of the final motor output [3,4]. A lot of work has focused particularly on the role of potassium currents in shaping responsiveness and firing of MNs [3,5]. Large-conductance calcium-dependent potassium (BK) currents, encoded by members of the Slowpoke (Slo) gene family, can contribute to action potential repolarization, regulation of firing frequency and interspike interval, repetitive firing, and burst termination [6]. Mutations of Slo genes also lead to a variety of motor disturbances [6]. We developed a biophysical model of bursting activity in MNs to explore the circumstances under which a BK/Slo current expressed in MNs can shape the timing of motor output underlying locomotion. We identify mechanisms by which the BK/Slo current changes the bursting output of MNs, and describe the different behaviors that are observed for varying membrane densities of the underlying channel. We also present preliminary data consisting of electrophysiological recordings from larval Drosophila showing that the changes in motor output predicted by the model are indeed observed when genetic manipulations of Slo channel density (RNA interference constructs) are targeted to MNs [7]. Our results not only further understanding of the specific role of BK/Slo channels in MNs, but contribute more generally to the growing knowledge on the role intrinsic MN properties play in shaping rhythmic motor output.

Different anticonvulsive effects of hesperidin and its aglycone hesperetin on electrical activity in the rat hippocampus in-vitro

This study investigated the possibility that hesperidin or hesperetin might interact directly with brain matter in a physiological manner. The effects of both compounds were followed in the in-vitro hippocampus preparation by continuous superfusion in a concentration-dependent manner in the presence of single stimuli and theta-burst stimulation of the Schaffer Collaterals. Hesperidin increased the population spike response at a concentration up to 10 microM, especially after induction of long-term potentiation, but attenuated it significantly at higher concentrations of up to 60 microM. Hesperetin only attenuated the response within the same concentration range. Modulation of the pyramidal cell response in the presence of tetraethylammonium (TEA) and pentylentetrazol on one hand and 4-aminopyridine (4-AP) and bicuculline on the other was influenced in a different way. Whereas hesperidin attenuated the response to 4-AP and bicuculline but not to TEA or pentylentetrazol, hesperetin was able to attenuate the response to TEA and pentylentetrazol, but not to 4AP or bicuculline. This feature was reproduced and confirmed ex-vivo after repetitive administration of hesperidin and hesperetin in-vivo for one week (500 mg kg(-1) orally) before in-vitro testing against the challenging effects of 4-AP and TEA. Since the action of hesperidin was sensitive to the presence of iberiotoxin, the involvement of a large conductance calcium-dependent potassium channel might be assumed. In summary, the results provide the possibility for use of both compounds to control pathophysiological disturbances of brain excitability in drug abuse, migraine and epilepsy.

Electrophysiological and contractile evidence of the ability of human mesenchymal stromal cells to correct vascular malfunction in rats after ionizing irradiation

The effect of intravenous administration of human mesenchymal stromal stem cells (hMSC) has been evaluated by means of large-conductance calcium-dependent potassium channel (BK(Ca)) activity measurements in thoracic aorta smooth muscle cells (SMC) obtained from non-fatal whole-body irradiated rats, using the patch clamp technique in whole-cell modification, and the standard acetylcholine (ACh) test to evaluate functional endothelium integrity using SM contractile recordings. Myofilament calcium sensitivity was estimated using simultaneous contractile recordings versus [Ca(2+)](i). Arterial blood was measured in intact and irradiated rats before and after hMSC administration. Stimulation of isolated SMC from the control group of animals with depolarizing voltage steps showed that outward K(+) currents sensitive to the BK(Ca) inhibitor paxilline were expressed. Outward currents in SMC obtained from irradiated animals were significantly reduced on the 30th day of post-irradiation. Irradiation led to a significant elevation in arterial blood pressure and reduced ACh-induced relaxation responses in irradiated rats as compared with the control group. Simultaneous measurements of contractile force and [Ca(2+)](i) showed that myofilament Ca(2+) sensitivity had increased following irradiation. Intravenously injected hMSC effectively restored BK(Ca) current and the amplitude of ACh-induced endothelium-dependent vasodilatation in vascular tissues obtained from post-irradiated rats. SMC obtained from irradiated rats treated with hMSC demonstrated a significantly increased paxilline-sensitive component of outward potassium currents, indicating that BK(Ca) activity had been restored. hMSC administration normalized increased blood pressure and myofilament Ca(2+) sensitivity in irradiated animals. When administered to healthy rats, hMSC were without effects on either of these. This study does not provide any immunohistochemical proof of hMSC engraftment in the host rats. PCR analysis showed that hMSCs were negative for hematopoietic cell markers and positive for hMSC markers. There were no clinical signs of graft-versus-host disease throughout the experimental period of 30 days. The data obtained suggest that hMSC demonstrate a clearly expressed ability to normalize vascular function damaged following irradiation, i.e. to reduce an elevated arterial blood pressure and myofilament Ca(2+) sensitivity, and to repair BK(Ca) function and endothelium-dependent relaxation in vascular tissues obtained from irradiated animals. Thus, hMSC seem to be worthwhile therapeutic approach in cases of ionizing irradiation accident or radiation beam therapy.

Large-conductance calcium-dependent potassium channels prevent dendritic excitability in neocortical pyramidal neurons

Large-conductance calcium-dependent potassium channels (BK channels) are homogeneously distributed along the somatodendritic axis of layer 5 pyramidal neurons of the rat somatosensory cortex. The relevance of this conductance for dendritic calcium electrogenesis was studied in acute brain slices using somatodendritic patch clamp recordings and calcium imaging. BK channel activation reduces the occurrence of dendritic calcium spikes. This is reflected in an increased critical frequency of somatic spikes necessary to activate the distal initiation zone. Whilst BK channels repolarise the somatic spike, they dampen it only in the distal dendrite. Their activation reduces dendritic calcium influx via glutamate receptors. Furthermore, they prevent dendritic calcium electrogenesis and subsequent somatic burst discharges. However, the time window for coincident somatic action potential and dendritic input to elicit dendritic calcium events is not influenced by BK channels. Thus, BK channel activation in layer 5 pyramidal neurons affects cellular excitability primarily by establishing a high threshold at the distal action potential initiation zone.

Functional coupling of TRPV4 cationic channel and large conductance, calcium-dependent potassium channel in human bronchial epithelial cell lines

Calcium-dependent potassium channels are implicated in electrolyte transport, cell volume regulation and mechanical responses in epithelia, although the pathways for calcium entry and their coupling to the activation of potassium channels are not fully understood. We now show molecular evidence for the presence of TRPV4, a calcium permeable channel sensitive to osmotic and mechanical stress, and its functional coupling to the large conductance calcium-dependent potassium channel (BK(Ca)) in a human bronchial epithelial cell line (HBE). Reverse transcriptase polymerase chain reaction, intracellular calcium imaging and whole-cell patch-clamp experiments using HBE cells demonstrated the presence of TRPV4 messenger and Ca(2+) entry, and outwardly rectifying cationic currents elicited by the TRPV4 specific activator 4alpha-phorbol 12,13-didecanoate (4alphaPDD). Cell-attached and whole-cell patch-clamp of HBE cells exposed to 4alphaPDD, and hypotonic and high-viscosity solutions (related to mechanical stress) revealed the activation of BK(Ca) channels subsequent to extracellular Ca(2+) influx via TRPV4, an effect lost upon antisense-mediated knock-down of TRPV4. Further analysis of BK(Ca) modulation after TRPV4 activation showed that the Ca(2+) signal can be generated away from the BK(Ca) location at the plasma membrane, and it is not mediated by intracellular Ca(2+) release via ryanodine receptors. Finally, we have shown that, unlike the reported disengagement of TRPV4 and BK(Ca) in response to hypotonic solutions, cystic fibrosis bronchial epithelial cells (CFBE) preserve the functional coupling of TRPV4 and BK(Ca) in response to high-viscous solutions.

Trans-Arachidonic acids induce a heme oxygenase-dependent vasorelaxation of cerebral microvasculature

Nitrative stress is an important regulator of vascular tone. We have recently described that trans-arachidonic acids ( TAA) are major products of NO 2 ·–mediated isomerization of arachidonic acid in cell membranes and that nitrative stress increases TAA levels leading to neural microvascular degeneration. In the present study, we explored whether TAA exert acute effects on neuromicrovascular tone and investigated potential mechanisms thereof. TAA induced an endothelium-dependent vasorelaxation of rat brain pial microvasculature. This vasorelaxation was independent of nitric oxide, prostanoids, lipoxygenase products, and CYP 450 metabolite trans-hydroxyeicosatetraenoic acids. However, inhibition of heme oxygenase (using zinc protoporphyrin IX) and of dependent soluble guanylate cyclase (sGC; using ODQ) significantly diminished (by ∼ 70%) the TAA-induced vasorelaxation. Consistent with these findings, TAA stimulated heme oxygenase (HO)-2-dependent bilirubin (using siRNA HO-2) and cGMP formation, and the HO product carbon monoxide (using CO-releasing CORM-2) reproduced the sGC-dependent cGMP formation and vasorelaxation. Further exploration revealed that TAA-induced vasorelaxation and bilirubin formation (HO activation) were nearly abrogated by large-conductance calcium-dependent potassium channels (BK Ca) (using TEA and iberiotoxin). Opening of BK Ca with the selective activator NS1619 induced a concentration-dependent vasorelaxation, which was inhibited by HO and sGC inhibitors. Coimmunoprecipitation suggested a molecular complex interaction between BK Ca and HO-2 (but not HO-1). Collectively, these findings identify new properties of TAA, specifically cerebral vasorelaxation through interactive activation of BK Ca with HO-2 and, in turn, sGC. Our findings provide new insights into the characterization of nitrative stress-derived TAA products, by showing they can act as acute mediators of nitrative stress on neurovascular tone.

Ca(2+)-activated K(+) channels modulate basal and E(2)beta-induced rises in uterine blood flow in ovine pregnancy.

Uterine blood flow (UBF) increases >30-fold during ovine pregnancy. During the last trimester, this reflects vasodilation, which may be due to placentally derived estrogens. In nonpregnant ewes, estradiol-17 beta (E(2)beta) increases UBF >10-fold by activating nitric oxide synthase and large conductance calcium-dependent potassium channels (BK(Ca)). To determine whether BK(Ca) channels modulate basal and E(2)beta-induced increases in UBF, studies were performed in near-term pregnant ewes with uterine artery flow probes and catheters for intra-arterial infusions of tetraethylammonium (TEA), a selective BK(Ca) channel antagonist at <1 mM, in the absence or presence of E(2)beta (1 microg/kg iv). Uterine arteries were collected to measure BK(Ca) channel mRNA. TEA (0.15 mM) decreased basal UBF (P < 0.0001) 40 +/- 8% and 55 +/- 7% (n = 11) at 60 and 90 min, respectively, and increased resistance 175 +/- 48% without affecting (P > 0.1) mean arterial pressure (MAP), heart rate, or contralateral UBF. Systemic E(2)beta increased UBF 30 +/- 6% and heart rate 13 +/- 1% (P < or = 0.0001, n = 13) without altering MAP. Local TEA (0.15 mM) inhibited E(2)beta-induced increases in UBF without affecting increases in heart rate (10 +/- 4%; P = 0.006). BK(Ca) channel mRNA was present in uterine artery myocytes from pregnant and nonpregnant ewes. Exponential increases in ovine UBF in late pregnancy may reflect BK(Ca) channel activation, which may be mediated by placentally derived estrogens.

A Novel Nervous System β Subunit that Downregulates Human Large Conductance Calcium-Dependent Potassium Channels

The pore-forming alpha subunits of many ion channels are associated with auxiliary subunits that influence channel expression, targeting, and function. Several different auxiliary (beta) subunits for large conductance calcium-dependent potassium channels of the Slowpoke family have been reported, but none of these beta subunits is expressed extensively in the nervous system. We describe here the cloning and functional characterization of a novel Slowpoke beta4 auxiliary subunit in human and mouse, which exhibits only limited sequence homology with other beta subunits. This beta4 subunit coimmunoprecipitates with human and mouse Slowpoke. beta4 is expressed highly in human and monkey brain in a pattern that overlaps strikingly with Slowpoke alpha subunit, but in contrast to other Slowpoke beta subunits, it is expressed little (if at all) outside the nervous system. Also in contrast to other beta subunits, beta4 downregulates Slowpoke channel activity by shifting its activation range to more depolarized voltages and slowing its activation kinetics. beta4 may be important for the critical roles played by Slowpoke channels in the regulation of neuronal excitability and neurotransmitter release.

Simultaneous Binding of Two Protein Kinases to a Calcium-Dependent Potassium Channel

Large-conductance calcium-dependent potassium channels are subject to modulation by protein kinases, phosphatases, and other signaling proteins, and it has been inferred from electrophysiological experiments that signaling proteins sometimes can be intimately associated with these channels in a regulatory complex. We show here that endogenous protein kinase activity coimmunoprecipitates with both native and recombinant Drosophila Slowpoke (dSlo) calcium-dependent potassium channels. Coimmunoprecipitation experiments using antibodies against several protein kinases demonstrate that dSlo can bind simultaneously to the Src tyrosine kinase and to the catalytic subunit of the cAMP-dependent protein kinase (PKAc). Both kinases can phosphorylate the channel in Drosophila heads and in heterologous host cells. The PKAc binds directly to a 172-amino acid region in the C-terminal domain of dSlo, without the intervention of regulatory subunits or anchoring proteins, and channel phosphorylation by PKAc is not required for this binding interaction. In contrast, several phosphorylatable tyrosine residues in dSlo are important for Src binding. The results are consistent with the idea that an ion channel can act as a scaffold for its own specific set of modulatory enzymes.

Does potassium channel opening contribute to endothelium-dependent relaxation in human internal thoracic artery?

Opening of potassium channels can cause hyperpolarization and relaxation of vascular smooth muscle cells. The aim of this work was to investigate the contribution of potassium channel activation to vasorelaxation in internal thoracic artery taken from patients undergoing coronary artery bypass graft surgery. Relaxations to carbachol and sodium nitroprusside were studied in isolated rings of internal thoracic artery in the absence and presence of nitric oxide synthase inhibitors and potassium channel blockers. The nitric oxide synthase inhibitors Nomega-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine abolished relaxations to carbachol. Relaxations to both carbachol and sodium nitroprusside were attenuated in the presence of raised extracellular potassium and the potassium channel blockers charybdotoxin, iberiotoxin and tetraethylammonium. Neither apamin nor glibenclamide modified relaxation. ODQ (1H-[1,2,4]oxadiazolol-[4,3a] quinoxalin-1-one), an inhibitor of soluble guanylate cyclase, abolished relaxation to carbachol in rings from some but not all subjects. These results suggest that potassium channel opening may make a small contribution to endothelium-dependent vasorelaxation in internal thoracic artery. The potassium channels had characteristics consistent with those of large-conductance calcium-dependent potassium channels.

Characteristics of single, large-conductance calcium-dependent potassium channels (BKCa) from smooth muscle cells isolated from the rabbit mesenteric artery.

Smooth muscle cells isolated from the secondary and tertiary branches of the rabbit mesenteric artery contain large Ca2+-dependent channels. In excised patches with symmetrical (140 mm) K+ solutions, these channels had an average slope conductance of 235 +/- 3 pS, and reversed in direction at -6.1 +/- 0.4 mV. The channel showed K+ selectivity and its open probability (Po) was voltage-dependent. Iberiotoxin (50 nm) reversibly decreased Po, whereas tetraethylammonium (TEA, at 1 mm) reduced the unitary current amplitude. Apamin (200 nm) had no effect. The channel displayed sublevels around 1/3 and 1/2 of the mainstate level. The effect of [Ca2+] on Po was studied and data fitted to Boltzmann relationships. In 0.1, 0.3, 1.0 and 10 microM Ca2+, V1/2 was 77.1 +/- 5.3 (n = 18), 71.2 +/- 4.8 (n = 16), 47.3 +/- 10.1 (n = 11) and -14.9 +/- 10.1 mV (n = 6), respectively. Values of k obtained in 1 and 10 microM [Ca2+] were significantly larger than that observed in 0.1 microM [Ca2+]. With 30 microM NS 1619 (a BKCa channel activator), V1/2 values were shifted by 39 mV to the left (hyperpolarizing direction) and k values were not affected. TEA applied intracellularly, reduced the unitary current amplitude with a Kd of 59 mm. In summary, BKCa channels show a particularly weak sensitivity to intracellular TEA and they also display large variation in V1/2 and k. These findings suggest the possibility that different types (isoforms) of BKCa channels may exist in this vascular tissue.

Activation of large-conductance potassium channels in pregnant human myometrium by pinacidil

Objective: The aim was to investigate the effects of the potassium-channel opener pinacidil on single uterine potassium channels and the contribution of the latter to pinacidil-induced myometrial relaxation. Study Design: Myometrial strips and freshly dispersed uterine myocytes were prepared from the myometrial biopsy samples of women undergoing elective, nonlabor caesarean section at term gestation. Results: In isometric tension experiments pinacidil potently relaxed pregnant nonlabor human myometrial strips, with an agonist concentration yielding the half maximal response of 0.4 ± 0.1 μmol/L. This effect was antagonized by 500 nmol/L charybdotoxin. Application of 10 μmol/L glibenclamide also inhibited the pinacidil-induced relaxation. Coapplication of charybdotoxin (500 nmol/L) and glibenclamide (10 μmol/L) produced a biphasic curve, which was fitted to a two-site model with values for agonist concentration yielding the half maximal response of 0.6 ± 0.2 μmol/L and 189.7 ± 0.8 μmol/L. Large-conductance calcium-dependent potassium channel activity was dramatically increased after application of pinacidil (between 10 and 100 μmol/L) to both inside-out and outside-out patches. The activation required the presence of calcium ions at the intracellular aspect of the membrane. Charybdotoxin but not glibenclamide blocked pinacidil-induced unitary large-conductance calcium-dependent potassium channel activity. Conclusion: Pinacidil-mediated relaxation of human pregnant myometrial strips may be partially attributable to the opening of uterine large-conductance calcium-dependent potassium channels in addition to adenosine triphosphate potassium channel activation. Drugs with specific potassium channel–activating properties may have important clinical application as novel tocolytics in the treatment of preterm labor. (Am J Obstet Gynecol 1998;178:1027-34.)

Properties of inward and outward potassium currents in cultured mouse motoneurons

Inward rectifier potassium currents and calcium-dependent potassium currents have been studied in cultured embryonic mouse motoneurons. Sustained unitary inward rectifier potassium currents were recorded from cell-attached patches and the channel conductance was dependent on external K+ concentration with a value of 25 pS when external K+ was 140 mM. The channel open probability exhibited a sigmoidal dependence on potential with the largest values (near 0.7) at depolarizing patch potentials. Inactivating inward rectifier potassium currents were also recorded in some cell-attached patches following voltage steps to hyperpolarizing potentials with the rate of inactivation faster with larger hyperpolarizing steps. Whole-cell inward rectifier potassium currents increased from an initial level to a steady-state level with hyperpolarizing steps to -120 mV from a holding potential of -60 mV; with larger hyperpolarizing commands the peak currents decayed to the steady-state. The steady-state current-voltage relation exhibited a region of negative slope resistance. External Cs+ (0.5-1 mM) reduced the amplitudes of macroscopic currents and diminished the open times of unitary currents consistent with block of open rectifying channels with an estimated KD for channel block of 1 mM. A large conductance calcium-dependent potassium channel was isolated in inside-out patches with a conductance of 240 pS with symmetrical 140 mM K+ across the patches and a conductance of 110 pS when the external K+ was reduced to 5 mM. With symmetrical K+ the channel open probability exhibited a sigmoid dependence on potential with the largest values, in excess of 0.8, associated with patch depolarization. The dependence of open probability on potential was dependent on the concentrations of internal Ca2+ and external K+. Properties of inward rectifier and calcium-dependent K+ channels, such as the voltage dependence of open probability, are involved in the establishment of cellular excitability in motoneurons. Future studies will be useful to investigate whether channel properties of motoneurons are altered after cell treatment with neurotoxic agents including oxygen radicals or excitotoxic amino acids.