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Results for “Potassium Channel Blockers”

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PubMed2002

Plasmalemmal KATP channels shape triggered calcium transients in metabolically impaired rat atrial myocytes.

The relative role of plasmalemmal and mitochondrial ATP-sensitive K(+) (K(ATP)) channels in calcium homeostasis of the atrium is little understood. Electrically triggered (1 Hz) cytoplasmic calcium transients were measured by 340-to-380-nm wavelength fura 2 emission ratios in cultured rat atrial myocytes. CCCP, a mitochondrial protonophore (100-400 nmol/l), dose dependently reduced the transient amplitude by up to 85%, caused a slow rise in baseline calcium, and reduced the recovery time constant of the transient from 143 to 91 ms (P < 0.05). However, neither 5-hydroxydecanoate, a mitochondrial K(ATP) channel blocker, nor diazoxide (500 micromol/l) affected the amplitude, baseline, or time constant in CCCP-treated cells. HMR-1098 (30 micromol/l), a plasmalemmal K(ATP) channel blocker, and glibenclamide (1 micromol/l) increased the amplitude in CCCP-treated myocytes by 69-82%, sharply elevated the calcium baseline, and prolonged the recovery time constant to 181-193 ms (P < 0.01). Thus opening of plasmalemmal but not mitochondrial K(ATP) channels reduces the calcium overload in metabolically compromised but otherwise intact atrial myocytes. Mitochondrial K(ATP) channels probably operate through a different mechanism to afford ischemic protection.

Adenosine Triphosphate↗

PubMed1997

Inhibition of K(V) and K(Ca) channels antagonizes NO-induced relaxation in pulmonary artery.

Endogenous nitric oxide (NO) may contribute to the maintenance of normal pulmonary vasomotor tone, and inhaled NO is used to treat patients with pulmonary hypertension. Because pulmonary vascular tone is regulated by intracellular free Ca2+ concentration and membrane potential, which are controlled by the K+ channel activity in pulmonary artery (PA) smooth muscle cells, we sought to determine whether K+ channels are involved in NO-induced relaxation and, if so, which types of K+ channels are responsible. Authentic NO (approximately 0.3 microM) and sodium nitroprusside (SNP, 10 pM) both produced significant relaxation in isolated PA rings precontracted by increasing extracellular K+ concentration. Further elevation of the K+ concentration from 20 to 60 mM resulted in a significant increase in contraction but caused a marked decline in SNP- and NO-mediated PA relaxation. The dependence of SNP- and NO-induced relaxation on transmembrane K+ gradient suggests that K+ efflux through K+ channels is involved in these effects. Furthermore, 4-aminopyridine (4-AP, 5-10 mM), which blocks voltage-gated K+ channels (K(V)), and charybdotoxin (200 nM), which blocks Ca2+-activated K+ channels (K(Ca)), both significantly inhibited NO- and SNP-induced PA relaxation. The ATP-sensitive K+ channel blocker glibenclamide, however, had no effect on the relaxation response. The blocking of guanylate cyclase diminished, but did not abolish, the NO-induced relaxation, whereas 4-AP further decreased the NO-induced relaxant response in the presence of the guanylate cyclase inhibitor LY-83583. These data suggest that activation of both K(V) channels and K(Ca) channels by guanosine 3',5'-cyclic monophosphate-dependent and -independent pathways is a mechanism, at least in part, by which NO induces PA relaxation.

4-Aminopyridine↗

PubMed2003

A radiolabeled peptide ligand of the hERG channel, [125I]-BeKm-1.

The wild-type scorpion toxin BeKm-1, which selectively blocks human ether-a-go-go related (hERG) channels, was radiolabeled with iodine at tyrosine 11. Both the mono- and di-iodinated derivatives were found to be biologically active. In electrophysiological patch-clamp recordings mono-[127I]-BeKm-1 had a concentration of half-maximal inhibition (IC50 value) of 27 nM, while wild-type BeKm-1 inhibited hERG channels with an IC50 value of 7 nM. Mono-[125I]-BeKm-1 was found to bind in a concentration-dependent manner and with picomolar affinity to hERG channel protein in purified membrane vesicles from transfected human embryonic kidney cells (HEK-293). Under optimized conditions the equilibrium dissociation constant ( Kd) values from saturation and kinetic binding analysis were 13 and 14 pM, respectively. Both the association and dissociation of [(125)I]-BeKm-1 were fast (association rate constant, k(on)=3.6 x 10(7) M(-1)s(-1); dissociation rate constant, k(off)=0.005 s(-1)). Wild-type BeKm-1 displaced binding of [125I]-BeKm-1 with half-maximal inhibitory concentrations of 44 pM. In contrast, competition experiments with a BeKm-1 mutant BeKm-1-K18A, in which the toxin interaction site is disrupted, resulted in a drop in affinity by more than 300-fold as compared to the wild-type toxin. Iberiotoxin and apamin, peptide inhibitors of Ca2+-activated K+-channels, had no effect on [125I]-BeKm-1 binding. Adding the classical rapid delayed rectifier current (IKr) blocker E-4031 reduced binding of [125I]-BeKm-1 to the hERG channel to an IC50 of 7 nM. In autoradiographic studies on rat hearts, binding of [125I]-BeKm-1 was dose-dependent and could partially be displaced by the addition of excess amounts of non-radioactive BeKm-1. The density of the radioactive signal was equally distributed in the myocardium of both the ventricle and atria indicating a homogenous expression of hERG channels throughout the heart.

Animals↗

PubMed2002

Classic ischemic but not pharmacologic preconditioning is abrogated following genetic ablation of the TNFalpha gene.

OBJECTIVE: Tumor necrosis factor alpha (TNFalpha) is known to mimic ischemic preconditioning (IP). However, it is not known whether TNFalpha-preconditioning is mediated by 'established' preconditioning signaling or via novel signaling cascades. Moreover, whether TNFalpha is required to induce the ischemic preconditioning phenotype has not been determined. METHODS: To evaluate the role of TNFalpha, we determined the infarct-sparing effect of IP comparing TNFalpha null (TNFalpha-/-) and wild-type mice. The IP protocol included 4x5 min ischemia/reperfusion (I/R) prior to the index 35 min of global ischemia followed by 45 min of reperfusion in isolated perfused murine hearts. Infarct size was measured as a percentage of cardiac volume. To evoke particular signaling pathways numerous pharmacologic studies were performed. RESULTS: Following IP, infarct size was significantly reduced by 43% in wild-type mice. In contrast, infarct size was not attenuated by IP in the TNFalpha-/- group versus I/R controls (Infarct size-36+/-3%). Interestingly, pharmacologic preconditioning with adenosine (100 microM) and diazoxide (30 microM) mimicked IP in both the wild-type (infarct size-11+/-4% and 18+/-2%) and in TNFalpha-/- mice (infarct size-15+/-4% and 23+/-3%) versus respective I/R controls. Recombinant TNFalpha (0.5 ng/ml) administered for 7 min followed by a 10-min washout mimicked IP in wild-type mice but not in the TNFalpha deficient mouse hearts. The cardioprotective effects of IP, adenosine and TNFalpha were abolished by the co-administration of the putative mitochondrial K(ATP) blocker 5-hydroxydecanoate. CONCLUSIONS: We demonstrate that cardiac TNFalpha production is required for ischemic preconditioning-induced cardioprotection but not necessary in pharmacologic preconditioning with adenosine or diazoxide in TNFalpha-/- mice. Moreover, TNFalpha administration is sufficient to activate preconditioning in wild-type mice. Finally, as 5-hydroxydecanoate abrogates ischemic, adenosine and TNFalpha induced preconditioning, this suggests that diverse signaling pathways converge at the level of mitochondrial K(ATP) channel activation to mediate this cardioprotection.

Adenosine↗

PubMed1996

Novel nonpeptide agents potently block the C-type inactivated conformation of Kv1.3 and suppress T cell activation.

The nonpeptide agent CP-339,818 (1-benzyl-4-pentylimino-1,4-dihydroquinoline) and two analogs (CP-393,223 and CP-394,322) that differ only with respect to the type of substituent at the N1 position, potently blocked the Kv1.3 channel in T lymphocytes. A fourth compound (CP-393,224), which has a smaller and less-lipophilic group at N1, was 100-200-fold less potent, suggesting that a large lipophilic group at this position is necessary for drug activity. CP-339,818 blocked Kv1.3 from the outside with a IC50 value of approximately 200 nM and 1:1 stoichiometry and competitively inhibited 125I-charybdotoxin from binding to the external vestibule of Kv1.3. This drug inhibited Kv1.3 in a use-dependent manner by preferentially blocking the C-type inactivated state of the channel. CP-339,818 was a significantly less potent blocker of Kv1.1, Kv1.2, Kv1.5, Kv1.6, Kv3.1-4, and Kv4.2; the only exception was Kv1.4, a cardiac and neuronal A-type K+ channel. CP-339,818 had no effect on two other T cell channels (I(CRAC) and intermediate-conductance K(Ca)) implicated in T cell mitogenesis. This drug suppresses human T cell activation, suggesting that blockade of Kv1.3 alone is sufficient to inhibit this process.

HeLa Cells↗

PubMed2000

Do KATP channels open as a prominent and early feature during ischaemia in the Langendorff-perfused rat heart?

The objective was to investigate whether myocardial adenosine triphosphate-sensitive K+ (KATP) channels open during the first 10 min of regional ischaemia in Langendorff-perfused rat hearts. Changes in monophasic action potentials and arrhythmias were studied during myocardial ischaemia in both the presence and absence of pharmacological KATP modulation. Ligation of the left main coronary artery for 10 min did not shorten the action potential duration (APD). The APD50 and APD80 (15.5 +/- 1.0 and 38.1 +/- 2.3 ms, respectively [mean +/- S.E., n = 15 hearts], immediately prior to ligation) increased transiently during the first 4 min of ligation (by 160 and 79% respectively, P < 0.05), before returning to pre-ligation values, but without a significant below-baseline-shortening. The cardiac electrogram showed no accompanying ventricular tachyarrhythmia (VT). These results raised the possibility that the myocardial KATP channels had not opened during the ligation. The KATP opener Ro 31-6930 (0.5 and 5 microM) shortened the APD50 and APD80 during coronary ligation, to significantly below both their control and pre-occlusion values (P < 0.05), and caused a concentration-dependent increase in both the incidence and duration of VT during the ligation. Ro 31-6930 at 5 microM also shortened APD50 and APD80 even before ligation (by 50 and 62% respectively, P < 0.05), and abolished the normal APD-lengthening seen during ischaemia. The KATP blocker glibenclamide (1 microM) abolished both the APD-shortening and pro-arrhythmic effects of the KATP opener, both before and during coronary ligation, yet when delivered on its own, at the same concentration which abolished the effects of KATP activation, it had no significant effect on the APD changes seen during the coronary ligation alone. These results suggest that, in Langendorff-perfused rat hearts in the absence of drugs, KATP channels do not open during early myocardial ischaemia.

Action Potentials↗

PubMed2006

K+-independent actions of diazoxide question the role of inner membrane KATP channels in mitochondrial cytoprotective signaling.

Activation by diazoxide and inhibition by 5-hydroxydecanoate are the hallmarks of mitochondrial ATP-sensitive K+ (K(ATP)) channels. Opening of these channels is thought to trigger cytoprotection (preconditioning) through the generation of reactive oxygen species. However, we found that diazoxide-induced oxidation of the widely used reactive oxygen species indicator 2',7'-dichlorodihydrofluorescein in isolated liver and heart mitochondria was observed in the absence of ATP or K+ and therefore independent of K(ATP) channels. The response was blocked by stigmatellin, implying a role for the cytochrome bc1 complex (complex III). Diazoxide, though, did not increase hydrogen peroxide (H2O2) production (quantitatively measured with Amplex Red) in intact mitochondria, submitochondrial particles, or purified cytochrome bc1 complex. We confirmed that diazoxide inhibited succinate oxidation, but it also weakly stimulated state 4 respiration even in K+-free buffer, excluding a role for K(ATP) channels. Furthermore, we have shown previously that 5-hydroxydecanoate is partially metabolized, and we hypothesized that fatty acid metabolism may explain the ability of this putative mitochondrial K(ATP) channel blocker to inhibit diazoxide-induced flavoprotein fluorescence, commonly used as an assay of K(ATP) channel activity. Indeed, consistent with our hypothesis, we found that decanoate inhibited diazoxide-induced flavoprotein oxidation. Taken together, our data question the "mitochondrial K(ATP) channel" hypothesis of preconditioning. Diazoxide did not evoke superoxide (which dismutates to H2O2) from the respiratory chain by a direct mechanism, and the stimulatory effects of this compound on mitochondrial respiration and 2',7'-dichlorodihydrofluorescein oxidation were not due to the opening of K(ATP) channels.

Animals↗

PubMed2001

The endocannabinoid anandamide is a direct and selective blocker of the background K(+) channel TASK-1.

TASK-1 encodes an acid- and anaesthetic-sensitive background K(+) current, which sets the resting membrane potential of both cerebellar granule neurons and somatic motoneurons. We demonstrate that TASK-1, unlike the other two pore (2P) domain K(+) channels, is directly blocked by submicromolar concentrations of the endocannabinoid anandamide, independently of the CB1 and CB2 receptors. In cerebellar granule neurons, anandamide also blocks the TASK-1 standing-outward K(+) current, IKso, and induces depolarization. Anandamide-induced neurobehavioural effects are only partly reversed by antagonists of the cannabinoid receptors, suggesting the involvement of alternative pathways. TASK-1 constitutes a novel sensitive molecular target for this endocannabinoid.

Animals↗

PubMed2000

The role of the delayed rectifier component IKs in dog ventricular muscle and Purkinje fibre repolarization.

1. The relative contributions of the rapid and slow components of the delayed rectifier potassium current (IKr and IKs, respectively) to dog cardiac action potential configuration were compared in ventricular myocytes and in multicellular right ventricular papillary muscle and Purkinje fibre preparations. Whole-cell patch-clamp techniques, conventional microelectrode and in vivo ECG measurements were made at 37C. 2. Action potential duration (APD) was minimally increased (less than 7%) by chromanol 293B (10 microM) and L-735,821 (100 nM), selective blockers of IKs, over a range of pacing cycle lengths (300-5000 ms) in both dog right ventricular papillary muscles and Purkinje fibre strands. D-Sotalol (30 microM) and E-4031 (1 microM), selective blockers of IKr, in the same preparations markedly (20-80%) lengthened APD in a reverse frequency-dependent manner. 3. In vivo ECG recordings in intact anaesthetized dogs indicated no significant chromanol 293B (1 mg kg-1 i.v.) effect on the QTc interval (332.9 +/- 16.1 ms before versus 330.5 +/- 11.2 ms, n = 6, after chromanol 293B), while D-sotalol (1 mg kg-1 i.v.) significantly increased the QTc interval (323.9 +/- 7.3 ms before versus 346.5 +/- 6.4 ms, n = 5, after D-sotalol, P < 0.05). 4. The current density estimated during the normal ventricular muscle action potential (i.e. after a 200 ms square pulse to +30 mV or during a 250 ms long 'action potential-like' test pulse) indicates that substantially more current is conducted through IKr channels than through IKs channels. However, if the duration of the square test pulse or the 'action potential-like' test pulse was lengthened to 500 ms the relative contribution of IKs significantly increased. 5. When APD was pharmacologically prolonged in papillary muscle (1 microM E-4031 and 1 microg ml-1 veratrine), 100 nM L-735,821 and 10 microM chromanol 293B lengthened repolarization substantially by 14.4 +/- 3.4 and 18. 0 +/- 3.4% (n = 8), respectively. 6. We conclude that in this study IKs plays little role in normal dog ventricular muscle and Purkinje fibre action potential repolarization and that IKr is the major source of outward current responsible for initiation of final action potential repolarization. Thus, when APD is abnormally increased, the role of IKs in final repolarization increases to provide an important safety mechanism that reduces arrhythmia risk.

Action Potentials↗

PubMed2003

Modification of cardiovascular response of posterior hypothalamic adenosine A(2) receptor stimulation by adenylate cylase, guanylate cyclase and by K(ATP) channel blockade in anesthetized rats.

Cardiovascular inhibitory effects induced by posterior hypothalamic adenosine A(2) receptors and their modulation by nitric oxide were suggested by our previous report. In this experiment, we examined the modulation of cardiovascular effects of adenosine A(2) receptor stimulation by adenylate cyclase, guanylate cyclase and ATP-sensitive K(+) channel in the posterior hypothalamus. Posterior hypothalamic injection of drugs was performed in anesthetized, artificially ventilated male Sprague-Dawley rats. Injection of adenosine A(2) receptor agonist 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA; 1, 2 and 5 nmol) produced a dose-dependent decrease of blood pressure and heart rate. Pretreatment with adenosine A(2) receptor antagonist 3,7-dimethyl-1-propargylxanthine (10 nmol) blocked the depressor and bradycardiac effects of CPCA (5 nmol). Pretreatments with adenylate cyclase inhibitor MDL-12330 (10 nmol) and guanylate cyclase inhibitor LY-83583 (5 nmol) attenuated the depressor and bradycardiac effects of CPCA (5 nmol). In addition, pretreatment with ATP-sensitive K(+) channel blocker glipizide (20 nmol) attenuated the depressor and bradycardiac responses of CPCA (5 nmol). These results suggest that posterior hypothalamic adenosine A(2) receptors play an inhibitory role in the central cardiovascular regulation and that both adenylate cyclase and guanylate cyclase mediate the depressor and bradycardiac actions of adenosine A(2) receptors. Also, ATP-sensitive K(+) channel mediates the posterior hypothalamic cardiovascular regulations of adenosine A(2) receptors.

Adenosine↗

PubMed2005

Reduced molecular expression of K(+) channel proteins in vascular smooth muscle from rats made hypertensive with N{omega}-nitro-L-arginine.

Smooth muscle membrane potential (E(m)) depends on K(+) channels, and arteries from rats made hypertensive with N(omega)-nitro-l-arginine (LHR) are depolarized compared with control. We hypothesized that decreased K(+) channel function, due to decreased K(+) channel protein expression, underlies E(m) depolarization. Furthermore, K(+) channel blockers should move control E(m) (-46 +/- 1 mV) toward that in LHR (-37 +/- 2 mV) and normalize contraction. The E(m) vs. K(+) relationship was less steep in LHR (23 +/- 2 vs. 28 +/- 1 mV/log K(+) concentration), and contractile sensitivity to K(+) was increased (EC(50) = 37 +/- 1 vs. 23 +/- 1 mM). Iberiotoxin (10 nM), an inhibitor of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels, depolarized control and LHR E(m) to -35 +/- 1 and -30 +/- 2 mV, respectively; however, effects on K(+) sensitivity were more profound in LHR (EC(50) = 25 +/- 2 vs. 15 +/- 3 mM). The voltage-dependent K(+) (K(V)) channel blocker 4-aminopyridine (3 mM) depolarized control E(m) to the level of LHR (-28 +/- 1 vs. -28 +/- 1 mV); however, effects on K(+) sensitivity were greater in LHR (EC(50) = 17 +/- 4 vs. 4 +/- 4 mM). Western blots revealed reduced BK(Ca) and K(V)1.5 channel expression in LHR arteries. The findings suggest that diminished expression of K(+) channels contributes to depolarization and enhanced contractile sensitivity. These conclusions are supported by direct electrophysiological assessment of BK(Ca) and K(V) channel function in control and LHR smooth muscle cells.

Animals↗

PubMed2006

A1 adenosine receptor-mediated modulation of neuronal ATP-sensitive K channels in rat substantia nigra.

ATP-sensitive K (K(ATP)) channels, widely expressed in cytoplasmic membranes of neurons, couple cell metabolism to excitability. They are considered to play important roles in controlling seizure activity during hypoxia and in neuroprotection against cell damage during hypoxia, ischemia and excitotoxicity. It is known that adenosine augments the opening of cardiac surface K(ATP) channels by reducing the sensitivity of these channels to ATP blockade. We investigated whether a similar modulation occurs in neuronal channels. Whole cell voltage-clamp recordings were made using rat midbrain slices to record the membrane current and conductance in principal neurons of the substantia nigra pars compacta (SNc). When the pipette solution contained 1 mM ATP, the membrane current at -60 mV and cellular conductance remained stable for at least 15 min. When slices were treated with (-)-N(6)-2-phenylisopropyl adenosine (R-PIA), a selective agonist for A(1) adenosine receptors, in the same condition, the outward current developed slowly to the amplitude of 109.9+/-26.6 pA, and conductance increased to 229+/-50% of the baseline. These changes were strongly inhibited by 200 microM tolbutamide, a K(ATP) channel blocker, suggesting that opening of K(ATP) channels mediated these changes. Pretreatment with 8-cyclopentyltheophylline (CPT), a selective A(1) adenosine receptor antagonist, abolished the outward current and conductance increases. Treatment of adenosine resulted in the similar changes sensitive to tolbutamide. These changes were abolished by CPT. These results suggest that activation of A(1) adenosine receptors promotes the opening of K(ATP) channels in principal neurons of the SNc by removing the blockade by ATP.

Adenosine↗

PubMed2004

Electrophysiological and pharmacological characterization of K+-currents in muscle fibres isolated from the ventral sucker of Fasciola hepatica.

Fibres isolated from the ventral sucker of Fasciola hepatica were identified as muscle on the basis of their contractility, and their actin and myosin staining. They were voltage-clamped at a holding potential of -40 mV and depolarization-activated outward currents were characterized both electrophysiologically and pharmacologically. Activation was well fitted by a Boltzmann equation with a half-maximal potential of + 9 mV and a slope factor of -14.3 mV, and the kinetics of activation and deactivation were voltage-sensitive. Tail current analysis showed that the reversal potential was shifted by +46+/-3 mV when E(K) was increased by 52 mV, confirming that this was a K+-current with electrophysiological characteristics similar to delayed rectifier and Ca2+-activated K+-currents in other tissues. The peak current at + 60 mV was inhibited by 76+/-6% by tetrapentylammonium chloride (1 mM) and by 84+/-7% by Ba2+ (3 mM), but was completely resistant to block by tetraethylammonium (30 mM), 3,4-diaminopyridine (100 microM) and 4-aminopyridine (10 mM). Penitrem A, a blocker of high-conductance Ca2+-activated K+-channels reduced the current at +60 mV by 23+/-5%. When the effects of Ca2+-channel blocking agents were tested, the peak outward current at + 60 mV was reduced by 71+/-7% by verapamil (30 microM) and by 59+/-4% by nimodipine (30 microM). Superfusion with BAPTA-AM (50 microM), which is hydrolysed intracellularly to release the Ca2+-buffer BAPTA, also decreased the current by 44+/-16%. We conclude that voltage-and Ca2+-sensitive K+-channels are expressed in this tissue, but that their pharmacology differs considerably from equivalent channels in other phyla.

Actins↗

PubMed2002

Ion channel diversity in the feline smooth muscle esophagus.

We have characterized ion-channel identity and density differences along the feline smooth muscle esophagus using patch-clamp recording. Current clamp recording revealed that the resting membrane potential (RMP) of esophageal smooth muscle cells (SMC) from the circular layer at 4 cm above the lower esophageal sphincter (EBC4; LES) were more depolarized than at 2 cm above LES. Higher distal Na(+) permeability (but not Cl(-) permeability) contributes to this RMP difference. K(+) channels but not large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels contribute to RMP at both levels, because nonspecific K(+)-channel blockers depolarize all SMC. Depolarization of SMC under voltage clamp revealed that the density of voltage-dependent K(+) channels (K(V)) was greatest at EBC4 due to increased BK(Ca.) Delayed rectifier K(+) channels (K(DR)), compatible with subtype K(V)1.2, were present at both levels. Differences in K(Ca)-to-K(DR) channel ratios were also manifest by predictable shifts in voltage-dependent inactivation at EBC4 when BK(Ca) channels were blocked. We provide the first evidence for regional electrophysiological differences along the esophageal body resulting from SMC ion channel diversity, which could allow for differential muscular responses to innervation and varied muscular contribution to peristaltic contractions along the esophagus.

4-Aminopyridine↗

PubMed2004

KCNQ/M channels control spike afterdepolarization and burst generation in hippocampal neurons.

KCNQ channel subunits are widely expressed in peripheral and central neurons, where they give rise to a muscarinic-sensitive, subthreshold, and noninactivating K+ current (M-current). It is generally agreed that activation of KCNQ/M channels contributes to spike frequency adaptation during sustained depolarizations but is too slow to influence the repolarization of solitary spikes. This concept, however, is based mainly on experiments with muscarinic agonists, the multiple effects on membrane conductances of which may overshadow the distinctive effects of KCNQ/M channel block. Here, we have used selective modulators of KCNQ/M channels to investigate their role in spike electrogenesis in CA1 pyramidal cells. Solitary spikes were evoked by brief depolarizing current pulses injected into the neurons. The KCNQ/M channel blockers linopirdine and XE991 markedly enhanced the spike afterdepolarization (ADP) and, in most neurons, converted solitary ("simple") spikes to high-frequency bursts of three to seven spikes ("complex" spikes). Conversely, the KCNQ/M channel opener retigabine reduced the spike ADP and induced regular firing in bursting neurons. Selective block of BK or SK channels had no effect on the spike ADP or firing mode in these neurons. We conclude that KCNQ/M channels activate during the spike ADP and limit its duration, thereby precluding its escalation to a burst. Consequently, down-modulation of KCNQ/M channels converts the neuronal firing pattern from simple to complex spiking, whereas up-modulation of these channels exerts the opposite effect.

Action Potentials↗

PubMed2000

Participation of a K(+) channel modulated directly by cGMP in the speract-induced signaling cascade of strongylocentrotus purpuratus sea urchin sperm.

Speract, a decapeptide from Strongylocentrotus purpuratus sea urchin eggs, transiently stimulates a membrane guanylyl cyclase and activates a K(+)-selective channel that hyperpolarizes sperm. However, previous studies of sperm and of sperm membrane vesicles reached conflicting conclusions about the mechanisms that open these channels. We find that speract hyperpolarizes and increases the cGMP content of flagellar vesicles. We confirm previous findings that intravesicular GTPgammaS and GTP enhance this hyperpolarization, but not GDPbetaS. The G protein activators AlF(-)(4) and mastoparan also are ineffective. Thus, it is unlikely that a G protein participates in the speract response. In contrast, hyperpolarization responses to speract are increased by 3-isobutyl-1-methylxanthine, which preferentially inhibits cGMP-selective phosphodiesterases of sperm, and the 8Br-cGMP derivative hyperpolarizes vesicles in the absence of speract. The responses to speract and to 8Br-cGMP have similar ionic selectivities (K(+) > Rb(+) > > Li(+) > Na(+)) and sensitivities to the channel blockers 4-aminopiridine and 3, 4-dichlorobenzamil, indicating that they likely result from opening of the same K(+) channel. Inhibitors that preferentially inhibit cAMP-selective phosphodiesterases do not alter responses to speract, and permeant cAMP analogs do not hyperpolarize vesicles. In addition, inhibitors of protein kinases and phosphatases fail to alter vesicle hyperpolarization by speract. The increase in vesicular cGMP content produced by speract therefore may directly mediate opening of the channel that hyperpolarizes sperm membrane vesicles. Similar mechanisms presumably operate in intact sperm.

4-Aminopyridine↗

PubMed2000

Increased mitochondrial K(ATP) channel activity during chronic myocardial hypoxia: is cardioprotection mediated by improved bioenergetics?

Increased resistance to myocardial ischemia in chronically hypoxic immature rabbit hearts is associated with activation of ATP-sensitive K(+) (K(ATP)) channels. We determined whether chronic hypoxia from birth alters the function of the mitochondrial K(ATP) channel. The K(ATP) channel opener bimakalim (1 micromol/L) increased postischemic recovery of left ventricular developed pressure in isolated normoxic (FIO(2)=0.21) hearts to values (42+/-4% to 67+/-5% ) not different from those of hypoxic controls but did not alter postischemic recovery of developed pressure in isolated chronically hypoxic (FIO(2)=0.12) hearts (69+/-5% to 72+/-5%). Conversely, the K(ATP) channel blockers glibenclamide (1 micromol/L) and 5-hydroxydecanoate (5-HD, 300 micromol/L) attenuated the cardioprotective effect of hypoxia but had no effect on postischemic recovery of function in normoxic hearts. ATP synthesis rates in hypoxic heart mitochondria (3.92+/-0.23 micromol ATP. min(-1). mg mitochondrial protein(-1)) were significantly greater than rates in normoxic hearts (2.95+/-0.08 micromol ATP. min(-1). mg mitochondrial protein(-1)). Bimakalim (1 micromol/L) decreased the rate of ATP synthesis in normoxic heart mitochondria consistent with mitochondrial K(ATP) channel activation and mitochondrial depolarization. The effect of bimakalim on ATP synthesis was antagonized by the K(ATP) channel blockers glibenclamide (1 micromol/L) and 5-HD (300 micromol/L) in normoxic heart mitochondria, whereas glibenclamide and 5-HD alone had no effect. In hypoxic heart mitochondria, the rate of ATP synthesis was not affected by bimakalim but was attenuated by glibenclamide and 5-HD. We conclude that mitochondrial K(ATP) channels are activated in chronically hypoxic rabbit hearts and implicate activation of this channel in the improved mitochondrial bioenergetics and cardioprotection observed.

Adaptation, Physiological↗

PubMed1997

Role of K+ channels in EDHF-dependent relaxation induced by acetylcholine in canine coronary artery.

To identify the K+ channels responsible for endothelium-derived hyperpolarizing factor (EDHF)-dependent relaxation, we studied the effects of various K+ channel blockers on acetylcholine-induced relaxation, which persists even in the presence of both an inhibitor of nitric oxide synthase and that of cyclooxygenase, in canine coronary artery rings. A nonselective K+ channel blocker, tetrabutylammonium (TBA), a large and intermediate conductance Ca2+-activated K+ channel blocker, charybdotoxin (CTX), and a voltage-dependent K+ channel blocker, 4-aminopyridine (4-AP), significantly inhibited this residual relaxation. A combined treatment with CTX and 4-AP almost completely blocked the relaxation. Neither a large (iberiotoxin) nor a small (apamin) conductance Ca2+-activated K+ channel blocker blocked the relaxation. We also investigated effects of K+ channel blockers on basal tone to determine whether or not EDHF is involved in regulating basal tone. TBA and CTX substantially raised basal tone to a greater degree in endothelium-intact preparations than in endothelium-denuded preparations. These results indicate that EDHF may exert its relaxing action through intermediate conductance Ca2+-activated and voltage-dependent K+ channels in canine coronary arteries. In addition, EDHF may play a role in maintaining basal vascular tone.

4-Aminopyridine↗