Potassium Permeability Research Articles

The use of nicorandil in cardioplegia solution

EDITOR: We would like to comment on the article by Chinnan and co-workers [1] written on myocardial protection by nicorandil during open-heart surgery under cardiopulmonary bypass (CPB). In relation to cardioplegia, coronary artery spasm (CAS) is frequently underestimated during CPB. Until now, CAS was not detectable with current monitoring techniques during CPB and its impact on cardiac surgery has not been elucidated in previous studies. During CAS the blood flow to the myocardium decreases depending on the degree of coronary vasoconstriction, thereby increasing anaerobic metabolites and myocardial ischaemia. Consequently, CAS could be responsible for right and left ventricular failure after leaving CPB. Therefore, apart from being a potassium channel opening drug preventing intracellular Ca2+ loading [2], the fact that nicorandil is a coronary vasodilator might have contributed to favourable results in your study. The link between vasospastic angina and vulnerability for ventricular tachycardia or ventricular fibrillation outside the setting of cardiac surgery has already been described in several case reports [3-5]. Iida and colleagues [5] reported that ventricular fibrillation improved and disappeared with the start of treatment with nitrates and suggested that CAS should be considered as a differential diagnosis in the presence of ST-segment changes and/or intractable ventricular arrhythmias. Extending this conclusion into CPB surgery, this could imply that rather than giving an anti-arrhythmic drug, a coronary vasodilator should be used to prevent and to treat arrhythmias secondary to ischaemic episodes caused by CAS. As your results show, a link between CAS and ventricular fibrillation seems to be apparent. After cross-clamp removal in coronary artery bypass grafting (CABG) patients, two patients in the nicorandil group developed significant cardiac dysrythmias vs. six patients in the placebo group. Unfortunately, the number of your patients is limited and it would be interesting to pursue your study as a multicentre trial on a larger scale. The relationship between nicorandil and coronary vasodilatation also explains the enhanced distribution of cardioplegia solution, a faster time until electromechanical arrest in the nicorandil group and a faster return of electromechanical activity after aortic cross clamp removal in patients under nicorandil in the mitral valve replacement group. Interestingly, although not significantly different, in CABG patients four nicorandil patients compared to two placebo patients had creatine kinase (CK-MB) levels higher than 75 IU L−1. Within the patients treated with nicorandil in the CABG group, as far as it can be concluded from Table 3, the rise in CK-MB mainly occurred between 6 and 24 h postoperatively, which is delayed in respect to the nicorandil patients for mitral valve replacement where the peak is at 6 h postoperatively. You conclude that these patients receiving nicorandil had a lower incidence of significant elevation of postoperative myocardial damage, suggesting that ongoing myocardial injury in the postoperative period might possibly be due to reperfusion injury and that nicorandil may have attenuated this due to its anti-inflammatory property. This could be correct but is in contrast to the finding that four nicorandil CABG patients compared to two placebo CABG patients had significantly elevated CK-MB levels. Theoretically, the incidence of significantly elevated CK-MB levels should be lower in the nicorandil group. Nonetheless, this could mean that there are multifactorial causes for postoperative myocardial infarction in your patients. When did the myocardial infarction actually happen in both groups and could other factors have been responsible such as the quality of coronary vessels? In addition, could postoperative factors rather than perioperative management have influenced the occurrence of postoperative infarction in the cases you mentioned? Unfortunately, the limited number of patients in your study precludes any conclusion on this issue. Ideally, troponin I should also have been measured. Nevertheless, the general outcome in your publication is favourable for the use of nicorandil during cardioplegia. Just as a remark, because enzymes of the potassium channel openers work more efficiently during normothermia, warm cardioplegia could have improved results due to increased potassium permeability leading to rapid sinus node arrest. In addition, Cohen and colleagues [6] concluded that terminal infusion of warm blood cardioplegia repleted myocardial ATP levels and improved postoperative myocardial function. In general, hypothermic cardioplegia should be avoided because it can trigger CAS, thereby inducing ventricular fibrillation leading to depletion of myocardial ATP. Warm cardioplegia was not applied in your study. On the other hand, CK-MB levels under cold cardioplegia were improved in mitral valve replacement patients with nicorandil, which make your results even more interesting because CAS is more enhanced by hypothermia. In spite of above remarks, we want to congratulate Chinnan and co-workers for their excellent paper which brings us one step further ahead to optimize coronary blood flow during CPB and thereby decreasing events of myocardial ischaemia.

Homomers of Kir.3.4 in atrial myocytes: their relevance to atrial fibrillation

The vagus slows the heart by raising potassium permeability and the activated K+ current – IK(ACh) – is carried by heteromers of the G-protein-gated inward rectifier potassium channel subunits Kir3.1 and Kir3.4 (for review, see Stanfield et al. 2002). However, in atrial myocytes only about half of Kir3.4 can be purified in heteromers with Kir3.1 and the remainder forms Kir3.4 homomers (Corey & Clapham, 1998). Quite what these homomeric channels do physiologically has been a puzzle: mice with Kir3.1 knocked out show mild tachycardia, which fails to respond to carbachol suggesting that Kir3.4 homomers do not contribute significantly to IK(ACh) (Bettahi et al. 2002). Clinical interest in the molecular biology of these channels is heightened by the need to manage atrial fibrillation, a common complication of heart failure and valvular disease. The dysrhythmia leads to fast, ‘irregularly irregular’ pacing of the ventricles, compromising cardiac output. In addition, blood is held in atria, and thrombus formation, particularly in the left auricle, increases risk of stroke. Atrial fibrillation of congestive heart failure (CHF) has often been managed with digoxin in the UK, and the drug has been held, on the basis of ECG monitoring, to have an effect via the vagus nerve, activating IK(ACh) (Krum et al. 1995). Atrial fibrillation (AF) is followed by electrophysiological remodelling of the heart, a process that is reversed after restoration of sinus rhythm (Brundel et al. 2001; Gaborit et al. 2005). In atrial myocytes, Kir3.1 expression is reduced relative to that of Kir3.4. Kir3.0 currents become constitutively active and the response to carbachol is reduced. The shortening of atrial action potentials may be corrected by the bee venom tertiapin, to which there is increased sensitivity (Dobrev et al. 2005), and use of derivatives of this venom has been seen as a possible method of management, mirroring use of the K+ channel blocker amiodarone. In this issue of The Journal of Physiology, Mintert et al. (2007) address some of these issues, using over-expression of Kir3.4 to mimic part of what happens in persistent AF. The homomeric channels produced show weaker inward rectification than is seen with Kir3.1/3.4 heteromers (the residues binding gating polyamines come from both Kir3.1 and 3.4, e.g. Stanfield et al. 2002). Unlike Kir3.1/3.4 heteromers, which must be gated open by Gβγ, these homomers are constitutively active. Nonetheless, FRET studies show homomers do interact with Gβγ. Kir3.0 channels require the membrane lipid phosphatidyl inositol bis phosphate (PIP2) as cofactor. Channels are thought to be activated by intracellular Na+ (Sui et al. 1998), Na+ binding to an aspartate residue in the C-terminus (Ho & Murrell-Lagnado, 1999) of Kir3.4. (The residue is Asn in Kir3.1.) By masking the negative charge of aspartate, Na+ aids the binding of PIP2. Surprisingly in view of this, the paper by Mintert et al. (2007) shows that IK(ACh) of atrial myocytes, carried by Kir3.1/3.4 heteromers, is not activated by internal Na+ at concentrations up to 60 mm; but Kir3.4 homomers are highly sensitive to Na+. The sensitivity to Na+ does not require activation through Gβγ since GTP-β-S has little inhibitory effect. Na+-activated currents are little inhibited by partial depletion of PIP2: a higher affinity probably means that PIP2 can bind from a depleted pool. Finally, channels are exquisitely sensitive to tertiapin-Q, affinity being at least 10-fold greater than that of Kir3.1/Kir3.4 heteromers. Do these findings concerning activation only of homomeric channels, which come to dominate over heteromers in atrial fibrillation, explain how digoxin controls AF? The drug might raise internal Na+ and activate Kir3.4 currents. In fact Mintert et al. (2007) are careful not to draw this conclusion, which would be controversial in the light of clinical literature describing the shortening of atrial action potentials as central to AF, with derivatives of tertiapin being seen as a solution to management. Further studies of this common dysrhythmia are sure to follow.

TASK-3 Two-Pore Domain Potassium Channels Enable Sustained High-Frequency Firing in Cerebellar Granule Neurons

The ability of neurons, such as cerebellar granule neurons (CGNs), to fire action potentials (APs) at high frequencies during sustained depolarization is usually explained in relation to the functional properties of voltage-gated ion channels. Two-pore domain potassium (K(2P)) channels are considered to simply hyperpolarize the resting membrane potential (RMP) by increasing the potassium permeability of the membrane. However, we find that CGNs lacking the TASK-3 type K(2P) channel exhibit marked accommodation of action potential firing. The accommodation phenotype was not associated with any change in the functional properties of the underlying voltage-gated sodium channels, nor could it be explained by the more depolarized RMP that resulted from TASK-3 channel deletion. A functional rescue, involving the introduction of a nonlinear leak conductance with a dynamic current clamp, was able to restore wild-type firing properties to adult TASK-3 knock-out CGNs. Thus, in addition to the accepted role of TASK-3 channels in limiting neuronal excitability, by increasing the resting potassium conductance TASK-3 channels also increase excitability by supporting high-frequency firing once AP threshold is reached.

Enhanced large intestinal potassium permeability in end‐stage renal disease

The capacity of the colon for potassium (K+) secretion increases in end-stage renal disease (ESRD), to the extent that it makes a substantial contribution to K+ homeostasis. This colonic K+ adaptive response may reflect enhanced active K+ secretion, and be associated with an increase in apical membrane K+ permeability. In this study, this hypothesis was tested in patients with normal renal function or ESRD, by evaluating the effect of barium ions (a K+ channel inhibitor) on rectal K+ secretion using a rectal dialysis technique, and the expression of high conductance (BK) K+ channel protein in colonic mucosa by immunohistochemistry. Under basal conditions, rectal K+ secretion was almost threefold greater (p < 0.02) in ESRD patients (n = 8) than in patients with normal renal function (n = 10). Intraluminal barium (5 mmol/l) decreased K+ secretion in the ESRD patients by 45% (p < 0.05), but had no effect on K+ transport in patients with normal renal function. Immunostaining using a specific antibody to the BK channel alpha-subunit revealed greater (p < 0.001) levels of BK channel protein expression in surface colonocytes and crypt cells in ESRD patients (n = 9) than in patients with normal renal function (n = 9), in whom low levels of expression were mainly restricted to surface colonocytes. In conclusion, these results suggest that enhanced colonic K+ secretion in ESRD involves an increase in the apical K+ permeability of the large intestinal epithelium, which most likely reflects increased expression of apical BK channels.

Passive potassium transport by symbiosomes from broad bean root nodules

Passive potassium release from symbiosomes isolated from broad bean root nodules and placed into K +-free assay medium was investigated by following acidification of the symbiosome interior and tetraphenylphosphonium (TPP +)-induced change in turbidity of symbiosome suspension observed under the chosen experimental conditions. The kinetics of acid pH shift inside the symbiosomes was substantially accelerated by the K +-specific ionophore valinomycin, the protonophore carbonyl cyanide 4-trifluoromethoxyphenylhydrazone (FCCP), but rapidly reversed by centimolar concentrations of K + as well as by TPP + added to the assay medium. In addition, the acidification process was shown to be markedly inhibited by tetraethylammonium (TEA), the known blocker of K +-channels in cell membranes, and to be strongly suppressed by removal of free calcium ions from the assay medium with the Ca 2+ chelator EGTA. TPP + was found to rapidly trigger the increase in turbidity of symbiosome suspension in the same assay medium. The kinetics of the TPP +-triggered process appeared to be markedly inhibited by TEA as well. Taken together, these results indicate relatively high potassium permeability of the peribacteroid membrane (PBM) of broad bean root nodules and provide evidence that the responses tested are due to K + release from the symbiosomes through TEA-sensitive K +-permeable channel in the PBM down transmembrane electrochemical gradient of K + ions. The data obtained suggest that such a channel is voltage-gated, and Ca 2+ ions on the cytosolic side of the PBM are required for functioning of this transporter.

Potassium-specific effects of levosimendan on heart mitochondria

In this study, we evaluated levosimendan, a new drug developed for the treatment of acute and decompensated heart failure, as a potential activator of ATP-sensitive potassium flux to the matrix of cardiac mitochondria. We estimated the K ATP channel openers-induced increase in mitochondrial inner membrane permeability for potassium by registering changes in membrane potential of heart mitochondria, oxidizing endogenous substrates. We compared the effect of levosimendan with the effects of the known K ATP channel openers diazoxide and pinacidil. Levosimendan (1 μM) accelerated potassium-specific Δ Ψ decrease by 0.15%/s, whereas 50 μM diazoxide by 0.10%/s, and 50 μM pinacidil by 0.08%/s, respectively. These results were confirmed by swelling experiments of non-respiring mitochondria in potassium nitrate medium. We found that levosimendan with an EC 50 of 0.83 ± 0.24 μM activates potassium flux to the mitochondrial matrix. This effect is discussed as a possible explanation of the anti-ischemic action of levosimendan.

In skeletal muscle the relaxation of the resting membrane potential induced by K(+) permeability changes depends on Cl(-) transport.

In resting skeletal muscle the potassium permeability is determined by the permeability of the inwardly potassium rectifier. Continuous resting membrane potential measurements are done to follow the relaxation of the membrane potential upon changes in potassium permeability. Inhibition of the inwardly potassium rectifier, by extracellular application of 80 microM Ba(2+), causes the cell to depolarize with mean time constants as follows: in control 127+/-7 s ( n=23), in the presence of bumetanide, as an inhibitor of the Na(+)/K(+)/2Cl(-) cotransporter, 182+/-23 s ( n=7), in hypertonic media (340 mosmol/kg) 90.4+/-5 s ( n=7) and in reduced chloride medium 64+/-8 s ( n=5). The depolarizing relaxation of the membrane potential induced by reduction of extracellular potassium produces similar results. These time constants are at least three orders of magnitude slower than the time constants reported in the literature for the inhibition of the inwardly potassium rectifier. Chloride transport affects the relaxation of the membrane potential. A further characterization of chloride transport is done by following the relaxation of the membrane potential upon application of chloride transport modulators. It is argued that the electroneutral cotransporter, for which a flux was preliminarily estimated of 13.4 pmol cm(-2) s(-1), has a considerable role in the processes related to the resting membrane potential.

Simplified interpretation of the pacemaker potential as a tool for teaching membrane potentials.

Most courses of physiology start by teaching about membrane potentials in different cells. Many of our students find these ideas difficult to understand. Often they try to memorize facts rather than understand mechanisms. The most difficult task may be interpretation of the pacemaker potential

Convergent Excitation of Dorsal Raphe Serotonin Neurons by Multiple Arousal Systems (Orexin/Hypocretin, Histamine and Noradrenaline)

Dorsal raphe serotonin neurons fire tonically at a low rate during waking. In vitro, however, they are not spontaneously active, indicating that afferent inputs are necessary for tonic firing. Agonists of three arousal-related systems impinging on the dorsal raphe (orexin/hypocretin, histamine and the noradrenaline systems) caused an inward current and increase in current noise in whole-cell patch-clamp recordings from these neurons in brain slices. The inward current induced by all three agonists was significantly reduced in extracellular solution containing reduced sodium (25.6 mm). In extracellular recordings, all three agonists increased the firing rate of serotonin neurons; the excitatory effects of histamine and orexin A were occluded by previous application of phenylephrine, suggesting that all three systems act via common effector mechanisms. The dose-response curve for orexin B suggested an effect mediated by type II (OX2) receptors. Single-cell PCR demonstrated the presence of both OX1 and OX2 receptors in tryptophan hydroxylase-positive neurons. The effects of histamine and the adrenoceptor agonist, phenylephrine, were blocked by antagonists of histamine H1 and alpha1 receptors, respectively. The inward current induced by orexin A and phenylephrine was not blocked by protein kinase inhibitors or by thapsigargin. Three types of current-voltage responses were induced by all three agonists but in no case did the current reverse at the potassium equilibrium potential. Instead, in many cases the orexin A-induced current reversed in calcium-free medium at a value (-23 mV) consistent with the activation of a mixed cation channel (with relative permeabilities for sodium and potassium of 0.43 and 1, respectively).

The search is on for the voltage sensor-to-gate coupling.

[Hodgkin and Huxley (1952)][1] demonstrated the presence of voltage-activated sodium (Na+) and potassium (K+) permeabilities in the squid giant axon. Ever since then, biophysicists have tried to understand how voltage activates the ionic pathways responsible for the action potential. [Hodgkin and

Hypotonic medium increases calcium permeant channels activity in human normal and dystrophic myotubes

Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin and an elevated intracellular calcium level. Single-channel recordings were performed with the cell-attached configuration of the patch-clamp technique. The present study shows, on human co-cultured normal and dystrophic muscle cells, the evidence for an increased activity of calcium permeant cationic mechano-sensitive channels under hypotonic medium stimulation. This activity was particularly enhanced in DMD cells. The hypotonic medium induced drastic changes in the single-channel activity characteristics, which are: a large increase of the calcium over potassium permeability ratio; and a great enhancement of the quantity of current crossing through these channels. These channels could contribute to a significant calcium entry, which could participate in the abnormal calcium homeostasis observed in DMD muscle.

Identification of Inwardly Rectifying Potassium Channels in Bovine Retinal and Choroidal Endothelial Cells

Ion channels were studied using the whole-cell patch clamp technique in bovine retinal and choroidal microvascular endothelial cells (MVEC) cultured under the same conditions. The two types of MVEC expressed inward currents at hyperpolarizing voltage steps and showed small outward currents at depolarizing steps. The extrapolated reversal potentials of the inward currents were near to the potassium equilibrium potential. Cs⁺ and the K⁺ channel blocker TEA reduced the amplitudes of the currents indicating the selectivity and permeability for potassium. This was confirmed by changes of outside K⁺ concentration shifting the I-V curves to the right. RT-PCR studies revealed the presence of mRNA of Kir2.1, an inwardly rectifying K⁺ channel, in retinal and choroidal MVEC. The profile of the small outward currents is related to the Kv family but not identical with the Kv1.4 subtype.

Effect of inhibitors of cyclic nucleotide phosphodiesterases on electrical and contractile activity of smooth muscle cells.

Double sucrose gap experiments were carried out to study the effect of phosphodiesterase inhibitors and penetrating analogs of cyclic nucleotides on action potential and contraction of guinea pig ureteral smooth muscle cells. 3-Isobutyl-1-methylxanthine (10 microM) and dibutyryl-cAMP (20 microM) shortened the plateau of action potential and inhibited contraction of smooth muscle cells by increasing potassium permeability of their membrane. Vinpocetine (1 microM) and dibutyryl-cAMP (100 microM) strengthened contraction of smooth muscle cells and shortened action potentials by decreasing sodium permeability of their membrane.

Analysis of a putative voltage-gated prokaryotic potassium channel.

Most of the completely sequenced prokaryotic genomes contain genes of potassium channel homologues, but there is still not much known about the role of these proteins in prokaryotes. Here we describe the large-scale overproduction and purification of a prokaryotic voltage-gated potassium channel homologue, Kch, from Escherichia coli. After successful overproduction of the protein, a specific increase in the potassium permeability of the cells was found. Kch could be purified in large amounts using classical purification methods to prevent aggregation of the protein. The physiological state of the protein was revealed to be a homotetramer and the protein was shown to be localized to the cytoplasmic membrane of the cells. In the course of the localization studies, we found a specific increase in the density of the cytoplasmic membrane on Kch production. This was linked to the observed increase in the protein to lipid ratio in the membranes. Another observed change in the membrane composition was an increase in the cardiolipin to phosphatidylglycerol ratio, which may indicate a specific cardiolipin requirement of Kch. On the basis of some of our results, we discuss a function for Kch in the maintenance of the membrane potential in E. coli.

The Use of Permeability of Potassium in the Diagnosis of Interstitial Cystitis

The Use of Permeability of Potassium in the Diagnosis of Interstitial Cystitis

Potassium inhibition of sodium‐activated potassium (KNa) channels in guinea‐pig ventricular myocytes

N(a+)-activated potassium channels (K(Na) channels) were studied in inside-out patches from guinea-pig ventricular myocytes at potentials between -100 and +80 mV. External K(+) (K(+)(o)) was set to 140 mM. For inwardly directed currents with 105 mM internal K(+) (K(+)(1)), the unitary current-voltage relationship was fitted by the constant field equation with a potassium permeability coefficient, P(K), of 3.72 x 10(-13) cm(3) s(-1). The slope conductance (-100 to -10 mV) was 194 +/- 4.5 pS (mean +/- s.d., n = 4) with 105 mM K(+)(i) (35 mM Na(+)(i)) but it decreased to 181 +/- 5.6 pS (n = 5) in 70 mM K(+)(i) (70 mM Na(+)(i)). K(Na) channels were activated by internal Na(+) in a concentration-dependent fashion. With 4 mM K(+)(i), maximal activation was recorded with 100 mM Na(+)(i) (open probability, P(o), about 0.78); half-maximal activation required about 35 mM Na(+)(i). When K(+)(i) was increased to 70 mM, half-maximal activation shifted to about 70 mM Na(+)(i). With Na(+)(i) set to 105 mM, channel activity was markedly inhibited when K(+)(i) was increased from 35 to 105 mM. Channel openings were abolished with 210 mM K(+)(i). The inhibitory effect of internal K(+) was also observed at more physiological conditions of osmolarity, ionic strength and chloride concentration. With 35 mM Na(+)(i) and 4 mM K(+)(i), P(o) was 0.48 +/- 0.10 (n = 6); when K(+)(i)was increased to 35 mM, P(o) was reduced to 0.04 +/- 0.05 (n = 7, P < 0.001). The relationship between P(o) and Na(+)(i) concentration at different levels of K(+)(i) is well described by a modified Michaelis-Menten equation for competitive inhibition; the Hill coefficients were 4 for the P(o)-Na(+)(i) relationship and 1.2 for the P(o)-K(+)(i) relationship. It is suggested that Na(+) and K(+) compete for a superficial site on the channel's permeation pathway. K(Na) channels would be most likely to be activated in vivo when an increase in Na(+)(i) is accompanied by a decrease of K(+)(i).

Electrogenic properties of the sodium pump in a dynamic model of membrane transport.

The general purpose of this theoretical work is to contribute to understand the physiological role of the electrogenic properties of the sodium pump, by studying a dynamic model that integrates diverse processes of ionic and water transport across the plasma membrane. For this purpose, we employ a mathematical model that describes the rate of change of the intracellular concentrations of Na+, K+ and Cl-, of the cell volume, and of the plasma membrane potential (Vm). We consider the case of a nonexcitable, nonpolarized cell expressing the sodium pump; Na+, K+, Cl- and water channels, and cotransporters of KCl and NaCl in its plasma membrane. We particularly analyze here the conditions under which the physiological Vm can be generated in a predominantly electrogenic fashion, as a result of the activity of the sodium pump. A major conclusion of this study is that, for the cell model considered, a low potassium permeability is not a sufficient condition for a predominantly electrogenic generation of the Vm by the sodium pump. The presence of an electroneutral exchange of Na+ and K+ represents a necessary additional requirement.

TASK (TWIK-related acid-sensitive K+ channel) is expressed in glomerulosa cells of rat adrenal cortex and inhibited by angiotensin II.

The present study was conducted to explore the possible contribution of a recently described leak K+ channel, TASK (TWIK-related acid-sensitive K+ channel), to the high resting K+ conductance of adrenal glomerulosa cells. Northern blot analysis showed the strongest TASK message in adrenal glomerulosa (capsular) tissue among the examined tissues including heart and brain. Single-cell PCR demonstrated TASK expression in glomerulosa cells. In patch-clamp experiments performed on isolated glomerulosa cells the inward current at -100 mV in 30 mM [K+] (reflecting mainly potassium conductance) was pH sensitive (17+/-2% reduction when the pH changed from 7.4 to 6.7). In Xenopus oocytes injected with mRNA prepared from adrenal glomerulosa tissue the expressed K+ current at -100 mV was virtually insensitive to tetraethylammonium (3 mM) and 4-aminopyridine (3 mM). Ba2+ (300 microM) and Cs+ (3 mM) induced voltage-dependent block. Lidocaine (1 mM) and extracellular acidification from pH 7.5 to 6.7 inhibited the current (by 28% and 16%, respectively). This inhibitory profile is similar (although it is not identical) to that of TASK expressed by injecting its cRNA. In oocytes injected with adrenal glomerulosa mRNA, TASK antisense oligonucleotide reduced significantly the expression of K+ current at -100 mV, while the sense oligonucleotide failed to have inhibitory effect. Application of angiotensin II (10 nM) both in isolated glomerulosa cells and in oocytes injected with adrenal glomerulosa mRNA inhibited the K+ current at -100 mV. Similarly, in oocytes coexpressing TASK and ATla angiotensin II receptor, angiotensin II inhibited the TASK current. These data together indicate that TASK contributes to the generation of high resting potassium permeability of glomerulosa cells, and this background K+ channel may be a target of hormonal regulation.

Volatile anesthetics activate the human tandem pore domain baseline K+ channel KCNK5.

Previous studies have identified a volatile anesthetic-induced increase in baseline potassium permeability and concomitant neuronal inhibition. The emerging family of tandem pore domain potassium channels seems to function as baseline potassium channels in vivo. Therefore, we studied the effects of clinically used volatile anesthetics on a recently described member of this family. A cDNA clone containing the coding sequence of KCNK5 was isolated from a human brain library. Expression of KCNK5 in the central nervous system was determined by Northern blot analysis and reverse-transcription polymerase chain reaction. Functional expression of the channel was achieved by injection of cRNA into Xenopus laevis oocytes. Expression of KCNK5 was detected in cerebral cortex, medulla, and spinal cord. When heterologously expressed in Xenopus oocytes, KCNK5 currents exhibited delayed activation, outward rectification, proton sensitivity, and modulation by protein kinase C. Clinical concentrations of volatile general anesthetics potentiated KCNK5 currents by 8-30%. Human KCNK5 is a tandem pore domain potassium channel exhibiting delayed activation and sensitivity to volatile anesthetics and may therefore have a role in suppressing cellular excitability during general anesthesia.

Ionic permeability of the frog sciatic nerve perineurium: parallel studies of potassium and lanthanum penetration using electrophysiological and electron microscopic techniques.

The isolated sciatic nerve of the frog Rana temporaria was used for a parallel electrophysiological and electron microscopic examination of the ionic permeability of the perineurium, one component of the blood-nerve barrier. Nerves mounted in a grease-gap chamber for electrophysiological recording showed negligible changes in DC potential (Delta DC) or compound action potential on challenge with 100 mM K(+) Ringer, evidence that the perineurium was tight to K(+). In preparations then fixed and exposed to 5 mM lanthanum in the fixative, and examined in the electron microscope, electron-dense lanthanum deposits were seen between perineurial lamellae, but lanthanum was not detectable within the endoneurium, confirming that the perineurium was also tight to lanthanum. Absence of lanthanum penetration was confirmed by X-ray analysis of electron microscopic sections. In nerves exposed to 2 mM sodium deoxycholate (DOC) in the recording chamber, then challenged with high [K(+)], a moderate increase in perineurial K(+) permeability (P(K)) was observed, but lanthanum was still excluded. Exposure of nerves to 4 mM DOC caused a greater increase in perineurial potassium permeability, and the two nerves with the greatest permeability (P(K) > 1 x 10(-5) cm x sec(-1)) also showed detectable lanthanum within the endoneurium. The results indicate that DOC causes a dose-dependent increase in tight junctional permeability in the perineurium, and that the electrophysiological monitoring of K(+) penetration is a more sensitive measure of small ion permeability than electron microscopical analysis using lanthanum as tracer. Vesicular profiles observed in perineurial lamellae did not form open channels for ion flux across the perineurium in control nerves, or in those exposed to DOC. In preparations where lanthanum reached the endoneurium, lanthanum was observed in dense deposits in the extracellular spaces around nodes of Ranvier, and in the outer mesaxon cleft, but did not penetrate the internodal periaxonal space, the myelin intraperiod line, or the Schmidt-Lanterman incisures, in contrast to observations in mammalian nerves. The apparent differences in accessibility of the internodal periaxonal space in frog and mammalian axons are discussed in relation to axonal physiology. The study illustrates the value of parallel electrophysiological and electron microscopic examination in elucidating the properties of extracellular ionic pathways and their role in neural function.