Exogenous Potassium Research Articles

Impact of Disease-causing SUR1 Mutations on the KATP Channel Subunit Interface Probed with a Rhodamine Protection Assay

The function of the ATP-sensitive potassium (K(ATP)) channel relies on the proper coupling between its two subunits: the pore-forming Kir6.2 and the regulator SUR. The conformation of the interface between these two subunits can be monitored using a rhodamine 123 (Rho) protection assay because Rho blocks Kir6.2 with an efficiency that depends on the relative position of transmembrane domain (TMD) 0 of the associated SUR (Hosy, E., Dérand, R., Revilloud, J., and Vivaudou, M. (2007) J. Physiol. 582, 27-39). Here we find that the natural and synthetic K(ATP) channel activators MgADP, zinc, and SR47063 induced a Rho-insensitive conformation. The activating mutation F132L in SUR1, which causes neonatal diabetes, also rendered the channel resistant to Rho block, suggesting that it stabilized an activated conformation by uncoupling TMD0 from the rest of SUR1. At a nearby residue, the SUR1 mutation E128K impairs trafficking, thereby reducing surface expression and causing hyperinsulinism. To augment channel density at the plasma membrane to investigate the effect of mutating this residue on channel function, we introduced the milder mutation E126A at the matching residue of SUR2A. Mutation E126A imposed a hypersensitive Rho phenotype indicative of a functional uncoupling between TMD0 and Kir6.2. These results suggest that the TMD0-Kir6.2 interface is mobile and that the gating modes of Kir6.2 correlate with distinct positions of TMD0. They further demonstrate that the second intracellular loop of SUR, which contains the two residues studied here, is a key structural element of the TMD0-Kir6.2 interface.

Lethal injection of potassium chloride: first description of the pathological appearance of organs

Lethal injection of potassium chloride (KCl) can be used as a method of either suicide or homicide. As biological tests are still inadequate to differentiate endogenous from exogenous potassium, at the scene of death the cause can only be suspected. We wished to determine the usefulness of conventional pathological examination in this context and carried out a study in four fetuses after medical termination of pregnancy for serious disease. Pregnancy was terminated by KCl injection in two cases and by injection of lidocaine and sufentanil in the other two cases. In each of the two fetuses in which KCl injection was performed, macroscopic examination showed whitish deposits on the tissues and histological examination showed clumps of lanceolate crystals in the internal organs. In the two fetuses which received lidocaine and sufentanil injection, no deposits were visible on macroscopic examination and no crystals were seen on histological examination. These findings suggest that pathological study may have useful applications in forensic medicine when death by potassium injection is suspected.

Injury‐induced neurogenesis in Bax‐deficient mice: evidence for regulation by voltage‐gated potassium channels

Adult neural stem and progenitor cells may help remodel the brain in response to injury. The pro-apoptotic molecule Bax has recently been identified as a key player in adult neural stem cell survival. In Bax-deficient mice that have undergone traumatic brain injury, we find increased numbers of neural progenitor cells in the dentate gyrus and improved remodeling of the hippocampus. Exogenous potassium chloride mimics spreading depression (SD)-like events in vitro, and Bax-deficient neural stem cells proliferate in response to these events more robustly than wild-type neural stem cells. Selective potassium channel blockers interrupt SD-mediated stimulation of stem cells. In addition, the potassium channel Kv4.1 is expressed within neural stem and progenitor cells in the dentate gyrus and is increased in Bax-deficiency. These data suggest that the neuroprotection observed after injury in Bax-deficiency may be due to increased neurogenesis via activation of the Kv4 family of potassium channels.

A serum potassium level above 10 mmol/l in a patient predisposed to hypokalemia

A 58-year-old man, previously diagnosed with Bartter's syndrome, presented with a short history of vomiting, diarrhea and weakness. He had severe hyperkalemia (serum potassium levels >10 mmol/l), which was successfully managed. Post hoc investigation suggested that the patient had Gitelman's rather than Bartter's syndrome. Physical examination, urine and blood analyses, chest radiography, electrocardiogram, renal ultrasound, and genetic analysis focusing on the SLC12A3 gene, which encodes the thiazide-sensitive Na/Cl cotransporter. Gitelman's syndrome and hyperkalemia secondary to acute renal failure plus exogenous potassium supplementation. Intravenous calcium gluconate, insulin and dextrose administration. Temporary continuous venovenous hemodiafiltration. Genetic confirmation of the underlying molecular defect. Long-term treatment for Gitelman's syndrome with oral potassium and magnesium supplements and epithelial sodium channel-blocking drugs. Review of patient education regarding renal salt-wasting syndromes.

Regulation of axon growth in vivo by activity-based competition

The formation of functional neural networks requires precise regulation of the growth and branching of the terminal arbors of axons, processes known to be influenced by early network electrical activity. Here we show that a rule of activity-based competition between neighbouring axons appears to govern the growth and branching of retinal ganglion cell (RGC) axon arbors in the developing optic tectum of zebrafish. Mosaic expression of an exogenous potassium channel or a dominant-negative SNARE protein was used to suppress electrical or neurosecretory activity in subsets of RGC axons. Imaging in vivo showed that these forms of activity suppression strongly inhibit both net growth and the formation of new branches by individually transfected RGC axon arbors. The inhibition is relieved when the activity of nearby 'competing' RGC axons is also suppressed. These results therefore identify a new form of activity-based competition rule that might be a key regulator of axon growth and branch initiation.

Effect of insulin-induced hypokalemia on lumbar sympathetic nerve activity in anesthetized rats.

Acute euglycemic hyperinsulinemia produces sympathoexcitation and a profound fall in plasma potassium levels. Because hypokalemia may activate the renin-angiotensin system to produce the observed increases in sympathetic nerve activity (SNA), the present study was designed to determine whether acute euglycemic-hyperinsulinemia in rats causes decreases in plasma potassium accompanied by increases in plasma renin activity (PRA) as well as elevations in SNA, and whether these alterations would be prevented by maintaining normokalemia with an exogenous potassium infusion. We infused vehicle (control; n = 10) or insulin (10 mU/min) in anesthetized untreated rats (insulin; n = 11), or in rats receiving simultaneous KCl infusion (Insulin + K+; n = 10), while measuring mean arterial pressure (MAP), heart rate (HR), SNA, plasma potassium, and PRA during euglycemic clamp. As expected, insulin rats had a large fall in plasma potassium (4.6 +/- 0.1 to 3.9 +/- 0.1 mEq/l), contrasting with no change in the control (4.8 +/- 0.2 to 4.8 +/- 0.2 mEq/l) and insulin + K+ (4.4 +/- 0.1 to 4.6 +/- 0.2 mEq/l) groups. However, PRA levels at study completion were not different in the three experimental groups. In addition, insulin rats had large increases in lumbar SNA (194 +/- 11% from 100% baseline) compared with modest elevations in control rats (122 +/- 10%), and prevention of hypokalemia failed to affect sympathetic increases (213 +/- 20%) in insulin + K+ rats. MAP and HR did not change in any of the experimental groups. These findings indicate that insulin per se, rather than insulin-induced hypokalemia or hormonal and compensatory adjustments secondary to hypokalemia, is the main mechanism that triggers increases in lumbar SNA.

Post-Traumatic Hyperexcitability Is Not Caused by Impaired Buffering of Extracellular Potassium

Impaired extracellular potassium buffering has been proposed as one of the major mechanisms underlying the increased risk for temporal lobe epilepsy after brain injury (D'Ambrosio et al., 1999). The present study systematically tested this hypothesis by measuring the resting [K+]o and recovery of the stimulation-evoked [K+]o increases in the dentate gyrus after experimental head trauma, using a combination of whole-cell recordings and ion-selective microelectrode recordings in rat hippocampal slices. Despite the presence of hyperexcitability, the resting [K+]o was not increased after injury. The faster rate of increase and larger amplitude of the orthodromically evoked [K+]o elevation after head trauma occurred in association with a greater population spike with shorter response latency. Contrary to the assumption in previous studies that the evoked activity in control and injured neuronal circuits is the same during antidromic activation, stimulation of granule cell axons in glutamate receptor antagonists evoked a greater [K+]o increase and a larger population spike. Although perforant path stimulation resulted in a larger [K+]o elevation after injury, the rate of clearance of the [K+]o transients evoked either by neuronal activity or by external application of potassium was not compromised. The [K+]o increase evoked by activation of the presynaptic afferents in isolation was not increased. In addition, the postsynaptic neuronal depolarization and firing evoked by exogenous potassium application was decreased after trauma. These results show that the regulation of [K+]o is not impaired after injury and indicate that the larger [K+]o increase evoked by neuronal activity is a consequence, rather than the primary mechanism underlying post-traumatic hyperexcitability.

A developmental gradient in the mechanism of K+ uptake during light-stimulated leaf growth in Nicotiana tabacum L.

Light causes growth of dicotyledonous leaves by stimulating proton efflux, cell wall acidification and loosening, and solute accumulation for turgor maintenance. For cells still undergoing cell division at the base of expanding tobacco ( Nicotiana tabacum L. cv. Xanthi) leaves, light-stimulated growth depends on K+ uptake, and is inhibited by the potassium channel blocker tetraethylammonium (TEA). The generality of this mechanism has been tested by comparing the effect of light on the growth-associated physiology of dividing and expanding cells in the base with cells at the tip growing by cell expansion only. The magnitude of the light-induced growth response of excised leaf discs is greatest at the leaf base and declines as cells mature. Basal tissue is more sensitive to exogenous potassium, which enhances light-stimulated growth at <1 mM, whereas tip tissue requires higher levels (>10 mM). Growth is inhibited by TEA similarly in tip and base. However, light-stimulated K+ uptake and proton efflux respond differently to TEA in tip and basal tissue. In basal tissue, TEA reduces light-stimulated K+ uptake by 60% and inhibits light-stimulated proton efflux. These results agree with those presented by M. Claussen et al. (1997, Planta 201:227-234) showing that auxin-stimulated H+ pump activity and growth in coleoptiles require K+ uptake as an electrical counterbalance to H+ efflux. In contrast, in tip tissue, TEA inhibits light-stimulated K+ uptake by only 17% and does not inhibit proton efflux. Our results suggest that the basipetal gradient in the effect of TEA on light-regulated growth physiology can be explained by TEA effects on K+ uptake: TEA inhibits light-stimulated H+ pump activity, wall acidification and membrane hyperpolarization only in cells dependent on TEA-sensitive channels for light-stimulated K+ uptake. Further, our data suggest that younger, basal tissue is dependent on TEA-sensitive, sucrose-stimulatable channels for light-stimulated K+ uptake whereas older, tip tissue is able to use an additional, TEA-insensitive K+ transporter to mediate light-stimulated K+ uptake.

Effects of isoflurane anesthesia on pulmonary vascular response to K+ ATP channel activation and circulatory hypotension in chronically instrumented dogs.

The objective of this study was to evaluate the effects of isoflurane anesthesia on the pulmonary vascular responses to exogenous adenosine triphosphate-sensitive potassium (K+ ATP) channel activation and circulatory hypotension compared with responses measured in the conscious state. In addition, the extent to which K+ ATP channel inhibition modulates the pulmonary vascular response to circulatory hypotension in conscious and isoflurane-anesthetized dogs was assessed. Fifteen conditioned, male mongrel dogs were fitted with instruments for long-term monitoring to measure the left pulmonary vascular pressure-flow relation. The dose-response relation to the K+ ATP channel agonist, lemakalim, and the pulmonary vascular response to circulatory hypotension were assessed in conscious and isoflurane-anesthetized (approximately 1.2 minimum alveolar concentration) dogs. The effect of the selective K+ ATP channel antagonist, glibenclamide, on the pulmonary vascular response to hypotension was also assessed in conscious and isoflurane-anesthetized dogs. Isoflurane had no effect on the baseline pulmonary circulation, but it attenuated (P<0.05) the pulmonary vasodilator response to lemakalim. Reducing the mean systemic arterial pressure to approximately 50 mm Hg resulted in pulmonary vasoconstriction (P<0.05) in the conscious state, and this response was attenuated (P<0.05) during isoflurane. Glibenclamide had no effect on the baseline pulmonary circulation, but it potentiated (P<0.05) the pulmonary vasoconstrictor response to hypotension in conscious and isoflurane-anesthetized dogs. These results indicate that K+ ATP-mediated pulmonary vasodilation and the pulmonary vasoconstrictor response to hypotension are attenuated during isoflurane anesthesia. Endogenous K+ ATP channel activation modulates the pulmonary vasoconstrictor response to hypotension in the conscious state, and this effect is preserved during isoflurane anesthesia.

Gossypol-induced hypokalemia and role of exogenous potassium salt supplementation when used as an antispermatogenic agent in male langur monkey

In our earlier study, we have observed that hypokalemia in langur monkeys, following gossypol acetic acid (GAA) treatment (5 mg dose level) when used as an antispermatogenic agent, and potassium salt supplementation partially maintained body potassium level of the animals. The aims of the present investigation was to confirm further occurrence of hypokalemia in the monkey (comparatively at two higher dose levels) and the role of potassium salt in preventing occurrence of gossypol-induced hypokalemia. Highly purified gossypol acetic acid alone at two dose levels (7.5 and 10 mg/animal/day; oral) and in combination with potassium chloride (0.50 and 0.75 mg/animal/day; oral) was given for 180 days. Treatment with gossypol alone as well as with the supplementation of potassium salt resulted in severe oligospermia and azoospermia. Animals receiving gossypol alone showed significant potassium deficiency with signs of fatigue at both dose levels. Enhanced potassium loss through urine was found in potassium-deficient animals, whereas animals receiving gossypol acetic acid plus potassium salt showed normal serum potassium with a less significant increase in urine potassium level during treatment phases. Other parameters of the body remained within normal range except gradual and significant elevation in serum transaminases activity. The animals gradually returned to normalcy following 150 and 180 days of termination of the treatment.

Thiocyanate Levels in Human Saliva: Quantitation by Fourier Transform Infrared Spectroscopy

A new quantitative method, based on Fourier transform infrared spectroscopy, was developed to evaluate the thiocyanate concentration in human saliva. Saliva samples were collected following a typical protocol and infrared spectra obtained from very small volumes (5 μl) deposited on a barium fluoride substrate. Exogenous potassium thiocyanate was used for calibration of the endogenous thiocyanate. This methodology does not require separation or extraction procedures. Human saliva spectra contain a characteristic marker band, due to thiocyanate, at 2058 cm−1. The integrated area of this band can be used for linear regression analysis and provides a good correlation between band area and thiocyanate concentration. Recovery of thiocyanate added to saliva was 100%. Centrifugation and dialysis experiments demonstrated that thiocyanate in saliva exists as a free or loosely bound ion. Saliva collected in the afternoon from 25 different subjects had a thiocyanate concentration of 0.83 ± 0.42 (mean ± SD) mmol/liter. In 4 subjects whose circardian pattern was investigated there was evidence of a higher thiocyanate concentration in saliva samples collected in the morning hours.

Xenopus laevis Oocytes Contain Endogeneous Large Conductance Ca2+-activated K+ Channels

Xenopus laevis oocytes have become a pre-eminent tool for studying cloned ion channels, primarily because they intrinsically express low levels of most types of ion channels. However, when these cells are used for single channel studies, it is essential to determine whether or not oocytes contain even low levels of endogenous ion channels with properties similar to the channel being investigated. We show here that X. laevis oocytes express endogenous large-conductance Ca2+-activated K+ channels with properties similar to mammalian isoforms of this channel. The endogenous channels exhibit a voltage-dependence of 12–14 mV per e-fold change in open probability (po), can be activated by micromolar Ca2+ concentrations, and have a single channel conductance of approximately 200 pS in symmetrical 110 mM K+ solutions. Patch clamp experiments indicate that this endogenous channel is present at low densities (∼1 channel/3000 μm2). If endogenous channel subunits can form functional tetramers with other exogenous potassium channel subunits, then they will give rise to the expression of a heterogeneous channel population. Therefore, studies involving the heterologous expression of large-conductance Ca2+-activated K+ channels in Xenopus laevis oocytes require careful analysis and interpretation. Copyright © 1996 Elsevier Science Ltd

Potassium Homeostasis and Hyperkalemic Syndromes

Acute hyperkalemia can be a life-threatening consequence of a variety of pathologic, pharmacologic, and iatrogenic disorders. Recognition and prompt therapy often reverse the electrophysiologic complications within minutes. Therefore, physicians must have a thorough understanding of potassium homeostasis. The causes of hyperkalemia, intracellular-to-extracellular shifts of potassium, excess exogenous potassium loads, and pseudohyperkalemia are discussed.

Hexacyanoferrate (III) stimulation of elongation in coleoptile segments fromZea mays L.

The influence of exogenous potassium hexacyanoferrate (III) (HCF III) on elongation of maize (Zea mays L.) coleoptile segments was investigated. Addition of HCF III led to a strong stimulation of growth both in the presence and absence of indole-3-acetic acid (IAA). The magnitude of growth stimulation was dependent on the presence of IAA, HCF III concentration, incubation time, and phase growth. The reduced form, potassium hexacyanoferrate (II), was without effect on growth. In the presence of HCF III, elongation was suppressed when coleoptile segments were treated with N,N′-dicyclohexylcarbodiimide, cycloheximide or atebrine (quinacrine). The addition of HCF III stimulated the IAA-induced proton extrusion, and the e−/H+ ratio decreased with incubation time. HCF III also strongly stimulated elongation ofAvena saliva L. coleoptile segments andGlycine max L. hypocotyl segments. These results suggested that a plasma membrane redox system (NADH oxidase type I) may be involved in the regulation of growth through the activity of the plasma membrane-bound ATPase.

Epithelial modulation of cholinergic responses in human tracheal smooth muscle.

The aim of this study was to test the hypothesis that the increase in maximal responses to histamine, acetylcholine, and cholinergic electrical field stimulation and decreased relaxant responses to isoprenaline reported in asthmatic tracheal smooth muscle result from the epithelial damage observed in asthma. The effect of mechanical removal of the epithelium on contractile and relaxant responses was examined in normal human postmortem tracheal smooth muscle strips. The epithelium was removed from alternate tracheal strips obtained from 25 subjects within 14 h of sudden death from nonrespiratory causes. In paired samples, contractile cholinergic and inhibitory nonadrenergic, noncholinergic (i-NANC) neural responses to electrical field stimulation and responses to exogenous histamine, potassium chloride, theophylline, and isoprenaline were unaffected by removal of the epithelium. However, the maximal isometric tension (Tmax) induced by methacholine increased by 70.1 +/- 19.8% (mean +/- SE, p < 0.005, n = 9), without alteration in EC50. These data suggest that disruption of the epithelium is unlikely to be the explanation of the abnormalities observed in trachea in fatal asthma. Explanations of the increase in maximal response to methacholine following removal of the epithelium include loss of an epithelium-derived relaxant factor released via an epithelial muscarinic receptor or loss of a specific permeability or metabolic barrier imposed by the epithelium for methacholine.

Effect of Exogenous Potassium on the Reduction in Tar, Nicotine and Carbon Monoxide Deliveries in the Mainstream Smoke of Cigarettes

Abstract Deliveries of tar, nicotine and carbon monoxide in mainstream smoke decreased as the exogenous-potassium content of tobacco increased. The puff count was almost unchanged with added potassium malate, but slightly decreased with added nitrate. Potassium malate was more effective than the nitrate for the reduction of carbon monoxide, whereas the opposite was observed in the case of nicotine. Both the exogenous and the endogenous potassium contents influenced the reduction of carbon-monoxide delivery through a decrease in the peak temperature of the burning cone. The pungency and off-taste peculiar to the tobacco were eliminated to some extent by the addition of potassium malate at levels less than 2 % expressed as potassium.

Adenylate cyclase activation is not sufficient to stimulate somatostatin release from dispersed cerebral cortical and diencephalic cells in glia-free cultures

Under conditions in which vasoactive intestinal peptide (VIP) induces somatostatin release from cortical and diencephalic neuronal cultures, VIP causes large increases in intracellular cyclic AMP. Both the release of somatostatin and the increase in cyclic AMP elicited by VIP require exogenous calcium, can be blocked by cobalt ion, and can be qualitatively mimicked by depolarizing concentrations of exogenous potassium ion. Direct activation of adenylate cyclase by forskolin causes large in increases in cyclic AMP content but does not induce somatostatin release. In the absence of VIP, the calcium ionophore, ionomycin, and the phorbol ester, phorbol 12-myristate-12-acetate, also stimulate somatostatin release. These results indicate that VIP-stimulation of cyclic AMP formation and VIP-stimulation of somatostatin release are calcium-dependent and that the two phenomena are dissociatable. Cyclic AMP formation is not a necessary condition for VIP-induced somatostatin release. Nucleotide formation may be a sufficient condition for release or, possibly in association with calcium influx, it may be an event unrelated to the release process.

Hyperkalemia: Disorders of Internal and External Potassium Balance

The serum potassium level is normally preserved de spite changes in potassium intake and output (the exter nal potassium balance) and changes in its distribution in the body (the internal potassium balance). External potassium homeostasis depends primarily on renal ex cretion of the daily exogenous potassium burden. Inter nal homeostasis depends on the extrarenal regulation of potassium. Skeletal muscle and liver are the dominant sites of that regulation. The two chief regulators of inter nal balance are insulin and catecholamines, the latter acting through β-adrenergic receptors. Acid-base bal ance and the cellular potassium content are other important regulators of internal balance. The major disorders of external balance are renal failure, hypo reninemic hypoaldosteronism, interstitial nephritis, and a variety of drugs that impair renal potassium excretion. The major disorders of internal balance are diabetes mellitus, acidosis, medications, and release of endoge nous potassium during vigorous exercise, traumatic muscle injury, or tumor lysis chemotherapy. These dis orders frequently result in troublesome elevations of serum potassium in the intensive care setting. Their re view in this article includes a thorough discussion of the evaluation and proper management of the hyperkalemic patient.

Effect of insulin on renal potassium metabolism.

The effect of insulin on renal potassium excretion was examined by employing the euglycemic insulin clamp technique in combination with renal clearance measurements. While euglycemia was maintained, insulin was infused at rates of 4.8 (n = 7) and 12 (n = 5) mU X kg-1 X min-1. Steady-state plasma insulin levels of 164 +/- 8 and 370 +/- 15 microU/ml were achieved in the low- and high-dose studies, respectively. Base-line plasma potassium concentration declined progressively by a mean of 0.14 +/- 0.09 (P less than 0.05) and 0.40 +/- 0.05 meq/liter (P less than 0.01) during the low- and high-dose insulin infusion protocols. Urinary potassium excretion did not change significantly from base line with either insulin dose. Because the decline in plasma potassium concentration could have masked a stimulatory effect of insulin on UKV, six rats received a 12-mU X kg-1 X min-1 euglycemic insulin clamp in combination with an exogenous potassium infusion to maintain the plasma potassium concentration constant at the basal level (4.03 +/- 0.03 vs. 4.05 +/- 0.05 meq/l). Under these conditions of normokalemia, insulin augmented UKV 2.4-fold, from 0.20 +/- 0.05 to 0.48 +/- 0.04 meq/l (P less than 0.001).

Vascular effects of free radicals generated by electrical stimulation

Electrical field stimulation (9 V, 1.0 ms, 4 Hz) of isolated segments of rat tail arteries and dog coronary arteries inhibits contractile responses to exogenous norepinephrine and elevated potassium concentration. This inhibitory effect of electrical stimulation is blocked by various agents that alter oxygen metabolism: superoxide dismutase, catalase, glutathione, ascorbate, and dimethyl sulfoxide. The observations suggest that the inhibitory effect is due to an action of oxygen free radical metabolites that are generated by the electrical stimulation of the oxygen-rich buffer. These free radical metabolites have two actions: 1) they oxidize drugs in the experimental system, and 2) they exert a direct inhibitory action on vascular smooth muscle.