Sodium (2-sulfonatoethyl) methanethiosulfonate prevents S-nitroso-L-cysteine activation of Ca2+-activated K+ (BKCa) channels in myocytes of the guinea-pig taenia caeca

1. The modulation of large conductance Ca(2+)-activated K(+) (BK(Ca)) channels by the nitric oxide (NO(.)) donor, S-nitroso-L-cysteine (NOCys) and three sulfhydryl-specific methanethiosulfonate (MTS) reagents, positively charged 2-aminoethyl MTS hydrobromide (MTSEA C(3)H(9)NO(2)S(2)HBr) and [2-(trimethylammonium) ethyl MTS bromide (MTSET C(6)H(16)NO(2)S(2)Br), and negatively charged sodium (2-sulfonatoethyl) MTS (MTSES C(3)H(7)O(5)S(3)Na) were compared in excised inside-out membrane patches of the guinea-pig taenia caeca. 2. In membrane patches bathed in a low Ca(2+) (15 nM) high K(+) physiological salt solution, 1-3 BK(Ca) channels opened with a low probability (N.P(o)) of 0.019+/-0.011 at 0 mV. N.P(o) readily increased with membrane depolarization, raised Ca(2+) concentration or upon the addition of NOCys (10 micro M for 2-5 min) such that 5-15 open BK(Ca) channels were evident. 3. MTSEA (2.5 mM) decreased, while MTSES (2.5 mM) increased N.P(o) (at 0 mV) and the number of open BK(Ca) channels at positive potentials. These changes in channel activity remained after a prolonged washout of these two MTS reagents. 4. MTSET (2.5 mM) increased N.P(o) (at 0 mV) and voltage-dependently decreased BK(Ca) current amplitudes in a manner readily reversed upon washout. 5. Pre-exposure of excised membrane patches to MTSES or N-ethylmaleimide (NEM 1 mM), a specific alkylating agent of cysteine sulfhydryls, but not MTSEA or MTSET, prevented the excitatory actions of NOCys (10 micro M). 6. It was concluded that NOCys-evoked increases in BK(Ca) channel activity occur via the S-nitrosylation of cysteine residues within basic regions of the channel alpha subunit that have an accessible water interface.

Effects of intermediate-conductance Ca2+-activated K+ channel modulators on human prostate cancer cell proliferation

The effects of 1-ethyl-2-benzimidazolinone (1-EBIO) and riluzole on human prostate cancer cells, LNCaP and PC-3, were evaluated using rubidium (86Rb(+)) efflux and proliferation assays. 1-EBIO and riluzole evoked concentration-dependent increases in 86Rb(+) efflux from LNCaP and PC-3 cells that were sensitive to inhibition by intermediate-conductance Ca(2+)-activated K(+) channel (IK(Ca)) blockers clotrimazole and charybdotoxin. Blockers of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel, iberiotoxin, or small-conductance Ca(2+)-activated K(+) (SK(Ca)) channel, apamin or scyllatoxin, had no effect. Concurrently, both 1-EBIO and riluzole evoked concentration-dependent increases in proliferation from human prostate cancer cell lines (LNCaP and PC-3 cells). Clotrimazole and charybdotoxin, but not iberiotoxin, apamin or scyllatoxin, inhibited 1-EBIO- and riluzole-evoked increases in proliferation from LNCaP and PC-3 cells. N-(3-(trifluoromethyl)phenyl)-N'-(2-hydroxy-5-chlorophenyl)urea (NS-1608) and 2-amino-5-(2-fluorophenyl)-4-methyl-1H-pyrrole-3-carbonitrile (NS-8), BK(Ca) channel openers had no effect on LNCaP and PC-3 proliferation. These results demonstrate that IK(Ca) channels play an important role in the regulation of human prostate cancer cell proliferation.

The contribution of resurgent sodium current to high-frequency firing in Purkinje neurons: an experimental and modeling study

Purkinje neurons generate high-frequency action potentials and express voltage-gated, tetrodotoxin-sensitive sodium channels with distinctive kinetics. Their sodium currents activate and inactivate during depolarization, as well as reactivate during repolarization from positive potentials, producing a "resurgent" current. This reopening of channels not only generates inward current after each action potential, but also permits rapid recovery from inactivation, leading to the hypothesis that resurgent current may facilitate high-frequency firing. Mutant med mice are ataxic and lack expression of the Scn8a gene, which encodes the NaV1.6 protein. In med Purkinje cells, transient sodium current inactivates more rapidly than in wild-type cells, and resurgent current is nearly abolished. To investigate how NaV1.6-specific kinetics influence firing patterns, we recorded action potentials of Purkinje neurons isolated from wild-type and med mice. We also recorded non-sodium currents from Purkinje cells of both genotypes to test whether the Scn8a mutation induced changes in other ion channels. Last, we modeled action potential firing by simulating eight currents directly recorded from Purkinje cells in both wild-type and med mice. Regular, high-frequency firing was slowed in med Purkinje neurons. In addition to disrupted sodium currents, med neurons had small but significant changes in potassium and leak currents. Simulations indicated that these modified non-sodium currents could not account for the reduced excitability of med cells but instead slightly facilitated spiking. The loss of NaV1.6-specific kinetics, however, slowed simulated spontaneous activity. Together, the data suggest that across a range of conditions, sodium currents with a resurgent component promote and accelerate firing.

AT1 receptor blockade improves vasorelaxation in experimental renal failure

It is not known whether angiotensin II type 1 receptor antagonists can influence the function and morphology of small arteries in renal failure. We investigated the effect of 8-week losartan therapy (20 mg/kg per day) on isolated mesenteric resistance arteries by wire and pressure myographs in 5/6 nephrectomized rats. Plasma urea nitrogen was elevated 1.6-fold after nephrectomy, and ventricular synthesis of atrial and B-type natriuretic peptides was increased 2.2-fold and 1.7-fold, respectively, whereas blood pressure was not affected. Losartan did not influence these variables. The endothelium-mediated relaxation to acetylcholine was impaired in nephrectomized rats in the absence and presence of nitric oxide synthase and cyclooxygenase inhibition. Blockade of calcium-activated potassium channels by charybdotoxin and apamin reduced the remaining acetylcholine response, and this effect was less marked in nephrectomized than in sham-operated rats. Relaxation to levcromakalim, a vasodilator acting through adenosine triphosphate-sensitive potassium channels, was also impaired after nephrectomy. The arteries of nephrectomized rats showed eutrophic inward remodeling: Wall-to-lumen ratio was increased without change in wall cross-sectional area. All changes in arterial relaxation and morphology were normalized by losartan therapy. Aortic ACE content, measured by autoradiography, directly correlated to the plasma level of urea nitrogen, suggesting that renal failure has an enhancing influence on the vascular renin-angiotensin system. Losartan normalized relaxation and morphology of resistance arteries in experimental renal failure, independent of its influence on blood pressure, impaired kidney function, or volume overload. The mechanism of improved vasodilation by losartan may include enhanced relaxation through potassium channels.

Contributions of nitric oxide, EDHF, and EETs to endothelium-dependent relaxation in renal afferent arterioles

BACKGROUND

Acetylcholine-induced endothelium-dependent relaxation in the renal afferent arteriole has been ascribed to nitric oxide, but the role of endothelium-derived hyperpolarizing factors (EDHFs) and 14,15-epoxyeicosatrienoic acid (14,15-EET) are unclear.

METHODS

Single afferent arterioles were dissected from kidney of normal rabbits and microperfused in vitro at 60 mm Hg. Vessels were preconstricted submaximally with norepinephrine (10(-8) mol/L). Relaxation was assessed following cumulative addition of ACh (10(-9) to 10(-4) mol/L) alone, or in the presence of indomethacin (to inhibit cyclooxygenase), Nw-nitro-L-arginine (L-NNA) (to inhibit nitric oxide synthase), methylene blue (to inhibit soluble guanylate cyclase), or a combination of L-NNA + methylene blue. To assess contributions by EDHF, studies were repeated with either apamin + charybdotoxin [to block Ca2+-activated K+ channels (KCa)] or with 40 mmol/L KCl. To asses the role of 14,15-EET, relaxations were evaluated in the presence of its competitive inhibitor 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE).

RESULTS

Relaxation by acetylcholine was abolished following endothelial denudation. It was unaffected by indomethacin but was inhibited 54%+/- 5% (P < 0.001) by L-NNA, 57%+/- 5% by methylene blue, and 60%+/- 4% by the combination of L-NNA plus methylene blue. Relaxation was inhibited further by KCl (80%+/- 6%) or by apamin + charybdotoxin (96%+/- 2%). 14,15-EEZE, alone, inhibited acetylcholine-induced relaxation by 29%+/- 3%, and by 80%+/- 5% in the presence of L-NNA.

CONCLUSION

Acetylcholine-induced afferent arteriolar relaxation depends strongly on both nitric oxide, acting via soluble guanylate cyclase, and on an EDHF, likely 14,15-EET, acting via KCa.

Voltage-gated potassium currents in rat vas deferens smooth muscle cells

Voltage-gated components of the outward current in single smooth muscle cells isolated from the epididymal part of the rat vas deferens were studied using amphotericin B perforated patch-clamp techniques. The complex kinetics of the net outward current elicited by positive voltage steps from -80 mV to +40 mV suggested the presence of several components. Bath application of 200 nM charybdotoxin, a potent blocker of large-conductance, Ca(2+)-dependent K(+) channels (BK(Ca)), reduced the current amplitude significantly. When BK(Ca) channels were suppressed, fast-inactivating (I(K,f)) and delayed rectifying (I(K,dr)) components of the outward current were identified. I(K,f) was characterized by fast kinetics of current decay, negative steady-state activation and inactivation dependencies and sensitivity to 4-aminopyridine with an apparent K(d) of 0.32 mM, properties similar to those of the A-type K(+) current. In contrast, I(K,dr) activated and inactivated at more positive potentials. The time constant of activation of I(K,dr) was voltage dependent with an e-fold decrease per 21 mV depolarization. I(K,dr) was inhibited by clofilium, a blocker of voltage-gated K(+) channels, with an IC(50) of 12 micro M and was not blocked by 5 mM 4-aminopyridine. The possible significance of the voltage-gated currents is discussed.

Calcium influx via L- and N-type calcium channels activates a transient large-conductance Ca2+-activated K+ current in mouse neocortical pyramidal neurons

Ca2+-activated K+ currents and their Ca2+ sources through high-threshold voltage-activated Ca2+ channels were studied using whole-cell patch-clamp recordings from freshly dissociated mouse neocortical pyramidal neurons. In the presence of 4-aminopyridine, depolarizing pulses evoked transient outward currents and several components of sustained currents in a subgroup of cells. The fast transient current and a component of the sustained currents were Ca2+ dependent and sensitive to charybdotoxin and iberiotoxin but not to apamin, suggesting that they were mediated by large-conductance Ca2+-activated K+ (BK) channels. Thus, mouse neocortical neurons contain both inactivating and noninactivating populations of BK channels. Blockade of either L-type Ca2+ channels by nifedipine or N-type Ca2+ channels by omega-conotoxin GVIA reduced the fast transient BK current. These data suggest that the transient BK current is activated by Ca2+ entry through both N- and L-type Ca2+ channels. The physiological role of the fast transient BK current was also examined using current-clamp techniques. Iberiotoxin broadened action potentials (APs), indicating a role of BK current in AP repolarization. Similarly, both the extracellular Ca2+ channel blocker Cd2+ and the intracellular Ca2+ chelator BAPTA blocked the transient component of the outward current and broadened APs in a subgroup of cells. Our results indicate that the outward current in pyramidal mouse neurons is composed of multiple components. A fast transient BK current is activated by Ca2+ entry through high-threshold voltage-activated Ca2+ channels (L- and N-type), and together with other voltage-gated K+ currents, this transient BK current plays a role in AP repolarization.

Loperamide mobilizes intracellular Ca2+ stores in insulin-secreting HIT-T15 cells

1 We have investigated the effects of loperamide on intracellular Ca(2+) stores and membrane K(+) channels in insulin-secreting hamster insulinoma (HIT-T15) cells. 2 In cell-attached patch-clamp mode, loperamide (3-250 micro M) activated large single-channel currents. The loperamide-activated currents were tentatively identified as Ca(2+)-activated K(+) channel (K(Ca)) currents based on their single-channel conductance (145 pS), apparent reversal potential, and insensitivity to tolbutamide. Smaller single-channel currents with a conductance (32 pS) indicative of adenosine triphosphate-sensitive K(+) channels (K(ATP) channels) were also recorded, but were insensitive to loperamide. 3 Surprisingly, the loperamide-activated currents persisted in the absence of extracellular Ca(2+). Yet under these conditions, we still measured loperamide-induced Ca(2+) increases. These effects are dose dependent. Loperamide had no effects in the inside-out patch configuration, suggesting that loperamide does not directly activate the channels with large conductance, but does so secondarily to release of Ca(2+) from intracellular stores. 4 Carbachol (100 micro M), an agonist of muscarinic receptors, which mediates IP(3)-dependent intracellular Ca(2+) release, enhanced the effects of loperamide on K(Ca) channels. 5 Both the putative K(Ca) currents and Ca(2+) signals induced by loperamide (with '0' [Ca(2+)](o)) were abolished when the intracellular Ca(2+) stores had been emptied by pretreating the cells with either carbachol or thapsigargin, an endoplasmic reticulum Ca(2+)-ATPase inhibitor that blocks reuptake of calcium. 6 These data indicate that loperamide in insulin-secreting beta-cells evokes intracellular Ca(2+) release from IP(3)-gated stores and activates membrane currents that appear to be carried by K(Ca), rather than K(ATP) channels.

Effect of phosphatidylserine on unitary conductance and Ba2+ block of the BK Ca2+-activated K+ channel: re-examination of the surface charge hypothesis

Incorporation of BK Ca2+-activated K+ channels into planar bilayers composed of negatively charged phospholipids such as phosphatidylserine (PS) or phosphatidylinositol (PI) results in a large enhancement of unitary conductance (gch) in comparison to BK channels in bilayers formed from the neutral zwitterionic lipid, phospatidylethanolamine (PE). Enhancement of gch by PS or PI is inversely dependent on KCl concentration, decreasing from 70% at 10 mM KCl to 8% at 1,000 mM KCl. This effect was explained previously by a surface charge hypothesis (Moczydlowski, E., O. Alvarez, C. Vergara, and R. Latorre. 1985. J. Membr. Biol. 83:273-282), which attributed the conductance enhancement to an increase in local K+ concentration near the entryways of the channel. To test this hypothesis, we measured the kinetics of block by external and internal Ba2+, a divalent cation that is expected to respond strongly to changes in surface electrostatics. We observed little or no effect of PS on discrete blocking kinetics by external and internal Ba2+ at 100 mM KCl and only a small enhancement of discrete and fast block by external Ba2+ in PS-containing membranes at 20 mM KCl. Model calculations of effective surface potential sensed by the K+ conduction and Ba2+-blocking reactions using the Gouy-Chapman-Stern theory of lipid surface charge do not lend support to a simple electrostatic mechanism that predicts valence-dependent increase of local cation concentration. The results imply that the conduction pore of the BK channel is electrostatically insulated from the lipid surface, presumably by a lateral distance of separation (>20 A) from the lipid head groups. The lack of effect of PS on apparent association and dissociation rates of Ba2+ suggest that lipid modulation of K+ conductance is preferentially coupled through conformational changes of the selectivity filter region that determine the high K+ flux rate of this channel relative to other cations. We discuss possible mechanisms for the effect of anionic lipids in the context of specific molecular interactions of phospholipids documented for the KcsA bacterial potassium channel and general membrane physical properties proposed to regulate membrane protein conformation via energetics of bilayer stress.

[Effects of potassium channel blockers on the proliferation of rat bronchial smooth muscle cells]

AIM

To investigate the effects of blockers of the three kinds of potassium channels: voltage-dependent K+ channel(KV), calcium-activated K+ channel(KCa) and ATP-sensitive K+ channel(KATP), on the proliferation of rat bronchial smooth muscle cells (BSMCs).

METHODS

The effects of three kinds of potassium channel blockers on the proliferation of BSMCs were detected by MTT method, immunocytochemistry staining and flow-cytometry. Their effects on the dynamic changes of Ca2+ concentration in BSMCs were investigated by the fluorophotometry of Fura-2/AM.

RESULTS

The KV blocker 4-aminopyridine (4-AP) was shown to significantly increase the expression of proliferating cell nucleus antigen in cultured rat BSMCs (P < 0.01), but the KCa blocker tetraethylammonium (TEA) and KATP blocker glibenclamide(Glib) did not show such effect (P > 0.05). 4-AP was found to significantly increase the optical density value of the cultured BSMCs (P < 0.01) by MTT method and the numbers of S + G2M BSMCs(P < 0.05) by flow-cytometry. TEA and Glib did not show such effects. 4-AP significantly increased the Ca2+ concentration in cultured BSMCs(P < 0.01). TEA and Glib did not show such effects.

CONCLUSION

This result suggests that inhibition of KV activity can increase intracellular Ca2+ and proliferation of rat BSMCs, but inhibition of KCa and KATP showed no effect.

A novel conotoxin from Conus betulinus, kappa-BtX, unique in cysteine pattern and in function as a specific BK channel modulator

A novel conotoxin, kappa-conotoxin (kappa-BtX), has been purified and characterized from the venom of a worm-hunting cone snail, Conus betulinus. The toxin, with four disulfide bonds, shares no sequence homology with any other conotoxins. Based on a partial amino acid sequence, its cDNA was cloned and sequenced. The deduced sequence consists of a 26-residue putative signal peptide, a 31-residue mature toxin, and a 13-residue extra peptide at the C terminus. The extra peptide is cleaved off by proteinase post-processing. All three Glu residues are gamma-carboxylated, one of the two Pro residues is hydroxylated at position 27, and its C-terminal residue is Pro-amidated. The monoisotopic mass of the toxin is 3569.0 Da. Electrophysiological experiments show that: 1) among voltage-gated channels, kappa-BtX is a specific modulator of K(+) channels; 2) among the K channels, kappa-BtX specifically up-modulates the Ca(2+)- and voltage-sensitive BK channels (252 +/- 47%); 3) its EC(50) is 0.7 nm with a single binding site (Hill = 0.88); 4) the time constant of wash-out is 8.3 s; and 5) kappa-BtX has no effect on single channel conductance, but increases the open probability of BK channels. It is concluded that kappa-BtX is a novel specific biotoxin against BK channels.

Different modulation by Ca2+-activated K+ channel blockers and herbimycin of acetylcholine- and flow-evoked vasodilatation in rat mesenteric small arteries

1. The present study addressed whether endothelium-dependent vasodilatation evoked by acetylcholine and flow are mediated by the same mechanisms in isolated rat mesenteric small arteries, suspended in a pressure myograph for the measurement of internal diameter. 2. In pressurized arterial segments contracted with U46619 in the presence of indomethacin, shear stress generated by the flow evoked relaxation. Thus, in endothelium-intact segments low (5.1+/-0.6 dyn cm(-2)) and high (19+/-2 dyn cm(-2)) shear stress evoked vasodilatations that were reduced by, respectively, 68+/-11 and 68+/-8% (P<0.05, n=7) by endothelial cell removal. Acetylcholine (0.01-1 microM) evoked concentration-dependent vasodilatation that was abolished by endothelial cell removal. 3. Incubation with indomethacin alone did not change acetylcholine and shear stress-evoked vasodilatation, while the combination of indomethacin with the nitric oxide (NO) synthase inhibitor, N(G),N(G)-asymmetric dimethyl-L-arginine (ADMA 1 mM), reduced low and high shear stress-evoked vasodilatation with, respectively, 52+/-15 and 58+/-10% (P<0.05, n=9), but it did not change acetylcholine-evoked vasodilatation. 4. Inhibition of Ca(2+)-activated K(+) channels with a combination of apamin (0.5 microM) and charybdotoxin (ChTX) (0.1 microM) did not change shear stress- and acetylcholine-evoked vasodilatation. In the presence of indomethacin and ADMA, the combination of apamin (0.5 microM) and ChTx (0.1 microM) increased contraction induced by U46619, but these blockers did not change the vasodilatation evoked by shear stress. In contrast, acetylcholine-evoked vasodilatation was abolished by the combination of apamin and charybdotoxin. 5. In the presence of indomethacin, the tyrosine kinase inhibitor, herbimycin A (1 microM), inhibited low and high shear stress-evoked vasodilatation with, respectively, 32+/-12 and 68+/-14% (P<0.05, n=8), but it did not change vasodilatation induced by acetylcholine. In the presence of indomethacin and ADMA, herbimycin A neither changed shear stress nor acetylcholine-evoked vasodilatation. 6. The present study suggests that Ca(2+)-activated K(+) channels sensitive for the combination of apamin and ChTx are involved in acetylcholine-evoked, mainly non-NO nonprostanoid factor-mediated, vasodilatation, while an Src tyrosine kinase plays a role for flow-evoked NO-mediated vasodilatation in rat mesenteric small arteries.

Lipid factor (bVLF) from bovine vitreous body evokes in EGFR-T17 cells a Ca2+-dependent K+ current associated with inositol 1,4,5-trisphosphate-independent Ca2+ mobilization

Bovine vitreous lipid factor (bVLF) is a complex phospholipid isolated from bovine vitreous body with strong Ca(2+)-mobilizing activity. In this study, the effects of bVLF on membrane potential were investigated in EGFR-T17 fibroblasts with the whole-cell patch clamp technique on monolayer cells, as well as with the fluorescent dye bis-oxonol as membrane potential-sensitive probe on monolayer and suspension cells. bVLF induced a transient hyperpolarization characterized by an initial peak and subsequent return to resting membrane potential levels within 1-2 min. The increase of [Ca(2+)](i) was concomitant with an outward current responsible for the hyperpolarizing response. Results with: (a) high [K(+)](o) media; (b) the monovalent cation ionophore gramicidin; and (c) substitution of K(+) with Cs(+) in the intracellular solution were consistent with the involvement of K(+) channels. The bVLF-induced hyperpolarization was blocked by the K(+) channel blockers, quinine and tetraethylamonium chloride, and partially affected by 4-aminopyridine. The calcium ionophore ionomycin caused a similar hyperpolarization as bVLF. When intracellular calcium was buffered by adding BAPTA to the pipette solution, bVLF-activated outward current was prevented. Moreover, the hyperpolarization response was strongly reduced at low doses (3 nM) of specific Ca(2+)-activated K(+) channel blockers, charybdotoxin and iberiotoxin. Based on these observations we conclude that bVLF hyperpolarizes the cells via the activation of a Ca(2+)-dependent K(+) current. In addition, it was observed that bVLF did not have a significant effect on intercellular communication measured by a single patch-electrode technique. Thus, membrane potential changes appeared to belong to the earliest cellular responses triggered by bVLF, and are closely associated with phosphatidic acid-dependent [Ca(2+)](i) mobilization.

ICA-17043, a novel Gardos channel blocker, prevents sickled red blood cell dehydration in vitro and in vivo in SAD mice

A prominent feature of sickle cell anemia is the presence of dehydrated red blood cells (RBCs) in circulation. Loss of potassium (K(+)), chloride (Cl(-)), and water from RBCs is thought to contribute to the production of these dehydrated cells. One main route of K(+) loss in the RBC is the Gardos channel, a calcium (Ca(2+))-activated K(+) channel. Clotrimazole (CLT), an inhibitor of the Gardos channel, has been shown to reduce RBC dehydration in vitro and in vivo. We have developed a chemically novel compound, ICA-17043, that has greater potency and selectivity than CLT in inhibiting the Gardos channel. ICA-17043 blocked Ca(2+)-induced rubidium flux from human RBCs with an IC(50) value of 11 +/- 2 nM (CLT IC(50) = 100 +/- 12 nM) and inhibited RBC dehydration with an IC(50) of 30 +/- 20 nM. In a transgenic mouse model of sickle cell disease (SAD), treatment with ICA-17043 (10 mg/kg orally, twice a day) for 21 days showed a marked and constant inhibition of the Gardos channel activity (with an average inhibition of 90% +/- 27%, P <.005), an increase in RBC K(+) content (from 392 +/- 19.9 to 479.2 +/- 40 mmol/kg hemoglobin [Hb], P <.005), a significant increase in hematocrit (Hct) (from 0.435 +/- 0.007 to 0.509 +/- 0.022 [43.5% +/- 0.7% to 50.9% +/- 2.2%], P <.005), a decrease in mean corpuscular hemoglobin concentration (MCHC) (from 340 +/- 9.0 to 300 +/- 15 g/L [34.0 +/- 0.9 to 30 +/- 1.5 g/dL], P <.05), and a left-shift in RBC density curves. These data indicate that ICA-17043 is a potent inhibitor of the Gardos channel and ameliorates RBC dehydration in the SAD mouse.

Roles of cyclic AMP and Ca2+-activated K+ channels in endothelium-independent relaxation by urocortin in the rat coronary artery

OBJECTIVE

Urocortin possesses cardioprotective properties against the damaging effects of ischemia/reperfusion injury. Our previous study demonstrated that urocortin can induce both endothelium-dependent and -independent coronary relaxation. However, the mechanisms thereby urocortin triggers endothelium-independent relaxation have not been investigated. The present study aimed to examine the role of cyclic AMP and Ca(2+)-activated K(+) channels in the relaxant response to urocortin in the isolated endothelium-denuded rat left anterior descending coronary arteries.

METHODS

Changes in vessel tension were measured by using a force transducer built in a Multi Myograph System.

RESULTS

In 9,11-dideoxy-11alpha,9alpha-epoxy-methanoprostaglandin F(2alpha) (U46619)-contracted rings, urocortin-induced relaxation (pD(2): 8.40+/-0.04) was significantly reduced by cyclic AMP-dependent protein kinase (PKA) inhibitors, Rp-cAMPS triethylamine (Rp-cAMPS) and KT 5720. Treatment with the large-conductance Ca(2+)-activated K(+) channel blockers, iberiotoxin or tetraethylammonium ions (TEA(+)) attenuated urocortin-induced relaxation; this effect was abolished in the presence of 200 nmol/l KT 5720. In contrast, apamin (small-conductance Ca(2+)-activated K(+) channel blocker), glibenclamide (ATP-sensitive K(+) channel blocker), or BaCl(2) (inwardly rectifier K(+) channel blocker) had no effect. Urocortin-induced relaxation was reduced in rings contracted with increasing concentrations of extracellular K(+) (35 and 50 mmol/l). Treatment with TEA(+) or Rp-cAMPS inhibited the relaxant effect of urocortin in 35 mmol/l K(+)-contracted rings. Combined treatment with TEA(+) and Rp-cAMPS had no additional effect. Similarly, forskolin produced significantly less relaxant response in 50 mmol/l K(+)-contracted than U46619-contracted rings. Forskolin-induced relaxation was attenuated by pretreatment with 3 mmol/l TEA(+).

CONCLUSION

Urocortin relaxed the rat coronary artery in substantial part via activation of the vascular Ca(2+)-activated K(+) channels and this effect appears to be primarily mediated through PKA-dependent intracellular mechanisms.

Afterhyperpolarization regulates firing rate in neurons of the suprachiasmatic nucleus

Cluster I neurons of the suprachiasmatic nucleus (SCN), which are thought to be pacemakers supporting circadian activity, fire spontaneous action potentials that are followed by a monophasic afterhyperpolarization (AHP). Using a brain slice preparation, we have found that the AHP has a shorter duration in cells firing at higher frequency, consistent with circadian modulation of the AHP. The AHP is supported by at least three subtypes of K(Ca) channels, including apamin-sensitive channels, iberiotoxin-sensitive channels, and channels that are insensitive to both of these antagonists. The latter K(Ca) channel subtype is involved in rate-dependent regulation of the AHP. Voltage-clamped, whole-cell Ca(2+) channel currents recorded from SCN neurons were dissected pharmacologically, revealing all of the major high-voltage activated subtypes: L-, N-, P/Q-, and R-type Ca(2+) channel currents. Application of Ca(2+) channel antagonists to spontaneously firing neurons indicated that predominantly L- and R-type currents trigger the AHP. Our findings suggest that apamin- and iberiotoxin-insensitive K(Ca) channels are subject to diurnal modulation by the circadian clock and that this modulation either directly or indirectly leads to the expression of a circadian rhythm in spiking frequency.

Mechanism of inhibitory actions of oxidizing agents on calcium-activated potassium current in cultured pigment epithelial cells of the human retina

PURPOSE

To identify the mechanisms by which oxidative stress with oxidizing agents alters the activity of ion channels in human retinal pigment epithelial (RPE) cells.

METHODS

The effects of oxidizing agents on ion currents were investigated in human RPE R-50 cells with the aid of the whole-cell, cell-attached, and inside-out configurations of the patch-clamp technique.

RESULTS

In the whole-cell configuration, t-butyl hydroperoxide (t-BHP; 1 mM), thimerosal (30 microM), and 4,4'-dithiodipyridine (DTDP; 30 microM) suppressed voltage-dependent K(+) current (I(K)) that was sensitive to inhibition by iberiotoxin or paxillin, yet not by apamin or 5-hydroxydecanoate sodium. Meclofenamic acid or Evans blue, but not diazoxide, reversed the decrease in I(K) caused by t-BHP. In cells dialyzed with ceramide (30 microM), neither t-BHP (1 mM) nor thimerosal (30 microM) had any effect on I(K), whereas DTDP (30 microM) slightly suppressed it. In cell-attached recordings, t-BHP (1 mM), thimerosal (30 microM), and DTDP (30 microM) suppressed the activity of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels. Dithiothreitol (10 microM) reversed DTDP-induced decrease in channel activity. Under current-clamp conditions, cell exposure to oxidizing reagents caused membrane depolarization. In cells dialyzed with ceramide (30 microM), membrane potential remained unaltered in the presence of t-BHP.

CONCLUSIONS

The results demonstrate that hydrophilic oxidants (e.g., t-BHP and thimerosal) suppress I(K) and suggest that the underlying mechanism of this inhibitory action may involve the generation of intracellular ceramide. However, the inhibition of BK(Ca) channels by DTDP, a membrane-permeable oxidant, in human RPE cells may result from the direct inhibition of BK(Ca) channels and indirectly from an increase in the intracellular production of ceramide.

Metabotropic glutamate receptor activation enhances the activities of two types of Ca2+-activated k+ channels in rat hippocampal astrocytes

The influence of activation of glutamate receptor (GluR) on outward K(+) current in cultured neonate rat hippocampal astrocytes was investigated. Patch-clamp analysis of K(+) channel currents in cultured astrocytes identified the existence of 71 +/- 6 and 161 +/- 11 pS single-channel K(+) currents that were sensitive to changes in voltage and [Ca(2+)](i) and blocked by external TEA but not by charybdotoxin, iberiotoxin, apamin, or 4-aminopyridine. Reverse transcriptase (RT)-PCR and Northern blot analysis revealed transcripts of the Ca(2+)-activated K(+) channel (K(Ca)) beta(4)-subunit (beta4) (KCNMB4) in cultured astrocytes. Expression of the metabotropic glutamate receptor (mGluR) subtypes mGluR1 and mGluR5 and the ionotropic glutamate receptor (iGluR) subtypes iGluR1 and iGluR4 were detected by RT-PCR and immunofluorescence analysis in cultured astrocytes. The mGluR agonists L-glutamate and quisqualate increased the open state probability (NP(o)) of the 71 and 161 pS K(+) channel currents that were prevented by the mGluR receptor antagonists 1-aminoindan-1,5-dicarboxylic acid or L-(+)-2-amino-3-phosphonopropionic acid and not by the iGluR antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate or CNQX. Activation of the two types of K(+) channel currents by mGluR agonists was attenuated by pertussis toxin and by inhibition of phospholipase C (PLC) or cytochrome P450 arachidonate epoxygenase. These results indicate that brain astrocytes contain the KCNMB4 transcript and express two novel types of K(Ca) channels that are gated by activation of a G-protein coupled metabotropic glutamate receptor functionally linked to PLC and cytochrome P450 arachidonate epoxygenase activity.

Endothelium-derived hyperpolarizing factor in human internal mammary artery is 11,12-epoxyeicosatrienoic acid and causes relaxation by activating smooth muscle BK(Ca) channels

BACKGROUND

Left internal mammary arteries (LIMAs) synthesize endothelium-derived hyperpolarizing factor (EDHF), a short-lived K(+) channel activator that persists after inhibition of nitric oxide (NO) and prostaglandin synthesis. EDHF hyperpolarizes and relaxes smooth muscle cells (SMCs). The identity of EDHF in humans is unknown. We hypothesized that EDHF (1) is 11,12-epoxyeicosatrienoic acid (11,12-EET); (2) is generated by cytochrome P450-2C, CYP450-2C; and (3) causes relaxation by opening SMC large-conductance Ca(2+)-activated K(+) channels (BK(Ca)).

METHODS AND RESULTS

The identity of EDHF and its mechanism of action were assessed in 120 distal human LIMAs and 20 saphenous veins (SVs) obtained during CABG. The predominant EET synthesized by LIMAs is 11,12-EET. Relaxations to exogenous 11,12-EET and endogenous EDHF are of similar magnitudes. Inhibition of EET synthesis by chemically distinct CYP450 inhibitors (17-octadecynoic acid, N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide), or a selective EET antagonist (4,15-epoxyeicosa-5(Z)-enoic acid) impairs EDHF relaxation. 11,12-EET activates a BK(Ca) current and hyperpolarizes LIMA SMCs. Inhibitors of BK(Ca) but not inward-rectifier or small-conductance K(Ca) channels abolish relaxation to endogenous EDHF and exogenous 11,12-EET. BK(Ca) and CYP450-2C mRNA and proteins are more abundant in LIMAs than in SVs, perhaps explaining the lack of EDHF activity of the SV. Laser capture microdissection and quantitative RT-PCR demonstrate that BK(Ca) channels are primarily in vascular SMCs, whereas the CYP450-2C enzyme is present in both the endothelium and SMCs.

CONCLUSIONS

In human LIMAs, EDHF is 11,12-EET produced by an EDHF synthase CYP450-2C and accounting for approximately 40% of net endothelial relaxation. 11,12-EET causes relaxation by activating SMC BK(Ca) channels.

Fast rhythmic bursting can be induced in layer 2/3 cortical neurons by enhancing persistent Na+ conductance or by blocking BK channels

Fast rhythmic bursting (or "chattering") is a firing pattern exhibited by selected neocortical neurons in cats in vivo and in slices of adult ferret and cat brain. Fast rhythmic bursting (FRB) has been recorded in certain superficial and deep principal neurons and in aspiny presumed local circuit neurons; it can be evoked by depolarizing currents or by sensory stimulation and has been proposed to depend on a persistent g(Na) that causes spike depolarizing afterpotentials. We constructed a multicompartment 11-conductance model of a layer 2/3 pyramidal neuron, containing apical dendritic calcium-mediated electrogenesis; the model can switch between rhythmic spiking (RS) and FRB modes of firing, with various parameter changes. FRB in this model is favored by enhancing persistent g(Na) and also by measures that reduce [Ca(2+)](i) or that reduce the conductance of g(K(C)) (a fast voltage- and Ca(2+)-dependent conductance). Axonal excitability plays a critical role in generating fast bursts in the model. In vitro experiments in rat layer 2/3 neurons confirmed (as shown previously by others) that RS firing could be switched to fast rhythmic bursting, either by buffering [Ca(2+)](i) or by enhancing persistent g(Na). In addition, our experiments confirmed the model prediction that reducing g(KC) (with iberiotoxin) would favor FRB. During the bursts, fast prepotentials (spikelets) could occur that did not originate in apical dendrites and that appear to derive from the axon. We suggest that modulator-induced regulation of [Ca(2+)] dynamics or of BK channel conductance, for example via protein kinase A, could play a role in determining the firing pattern of neocortical neurons; specifically, such modulation could play a role in regulating whether neurons respond to strong stimulation with fast rhythmic bursts.

Selective blockade of endothelial Ca2+-activated small- and intermediate-conductance K+-channels suppresses EDHF-mediated vasodilation

1. Activation of Ca(2+)-activated K(+)-channels (K(Ca)) has been suggested to play a key role in endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilation. However, due to the low selectivity of commonly used K(Ca)-channel blockers it is still elusive which endothelial K(Ca)-subtypes mediate hyperpolarization and thus initiate EDHF-mediated vasodilation. 2. Using the non-cytochrome P450 blocking clotrimazole-derivatives, 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (TRAM-34) and 2-(2-chlorophenyl)-2,2-diphenylacetonitrile (TRAM-39) as highly selective IK1-inhibitors, we investigated the role of the intermediate-conductance K(Ca) (rIK1) in endothelial hyperpolarization and EDHF-mediated vasodilation. 3. Expression and function of rIK1 and small-conductance K(Ca) (rSK3) were demonstrated in situ in single endothelial cells of rat carotid arteries (CA). rIK1-currents were blocked by TRAM-34 or TRAM-39, while rSK3 was blocked by apamin. In current-clamp experiments, endothelial hyperpolarization in response to acetylcholine was abolished by the combination of apamin and TRAM-34. 4. In phenylephrine-preconstricted CA, acetylcholine-induced NO and prostacyclin-independent vasodilation was almost completely blocked by ChTX, CLT, TRAM-34, or TRAM-39 in combination with the SK3-blocker apamin. Apamin, TRAM-34, and CLT alone or sulphaphenzole, a blocker of the cytochrome P450 isoform 2C9, were ineffective in blocking the EDHF-response. 5. In experiments without blocking NO and prostacyclin synthesis, the combined blockade of SK3 and IK1 reduced endothelium-dependent vasodilation. 6. In conclusion, the use of selective IK1-inhibitors together with the SK3-blocker apamin revealed that activation of both K(Ca), rIK1 and rSK3 is crucial in mediating endothelial hyperpolarization and generation of the EDHF-signal while the cytochrome P450 pathway seems to play a minor or no role in rat CA.

[Characterisation of a novel toxin active on potassium apanaine channels, purified from Buthus occitanus tunetanus scorpion venom]

A new peptidyl inhibitor of the small-conductance Ca(2+)-activated K+ channels (SKca) was purified to homogeneity from the venom of the Tunisian scorpion Buthus occitanus tunetanus. The molecular mass determined by SDS-PAGE, shows that it's a short peptide (3300 Da). The primary sequence of this toxin shows that it is a 31-residue polypeptide cross-linked by three disulfide bridges and structurally related to subfamily 5 of short scorpion toxins. This molecule shows similar pharmacological properties with this group of peptides inducing high toxicity in mice after intracerebro-ventricular injection, and competing with iodinated apamin for binding to its receptor site from rat brain synaptosomes (K0.5 = 4 nM).

Calcium activation of BK(Ca) potassium channels lacking the calcium bowl and RCK domains

In many physiological systems such as neurotransmitter release, smooth muscle relaxation and frequency tuning of auditory hair cells, large-conductance calcium-activated potassium (BK(Ca)) channels create a connection between calcium signalling pathways and membrane excitability. BK(Ca) channels are activated by voltage and by micromolar concentrations of intracellular calcium. Although it is possible to open BK(Ca) channels in the absence of calcium, calcium binding is essential for their activation under physiological conditions. In the presence of intracellular calcium, BK(Ca) channels open at more negative membrane potentials. Many experiments investigating the molecular mechanism of calcium activation of the BK(Ca) channel have focused on the large intracellular carboxy terminus, and much evidence supports the hypothesis that calcium-binding sites are located in this region of the channel. Here we show that BK(Ca) channels that lack the whole intracellular C terminus retain wild-type calcium sensitivity. These results show that the intracellular C terminus, including the 'calcium bowl' and the RCK domain, is not necessary for the calcium-activated opening of these channels.

The Ca2+-activated K+ channel of intermediate conductance:a possible target for immune suppression

The intermediate conductance Ca2+-activated K+ (IK) channel is distinguished from the functionally related Ca2+-activated K+ channels of smaller and larger unitary conductance by its molecular structure, pharmacology, tissue distribution and physiology. Like many K+ channels, IK is an assembly of four identical subunits each spanning the membrane six times and each contributing equally to the K+ selectivity pore positioned centrally in the complex. The IK channel gains its high sensitivity to intracellular Ca2+ from tightly bound calmodulin, and its activity is independent of the membrane potential. Several toxins including charybdotoxin and the more selective mutant, Glu32-charybdotoxin, maurotoxin and stichodactyla toxin potently block IK channels. Among blockers of the IK channel are also several small organic molecules including the antimycotic clotrimazole and the close analogues TRAM-34 and ICA-17043, as well as the antihypertensive, nitrendipine. The IK channel is distributed in peripheral tissues, including secretory epithelia and blood cells, but it appears absent from neuronal and muscle tissue. An important physiological role of the IK channel is to help maintain large electrical gradients for the sustained transport of ions such as Ca2+ influx that controls T lymphocyte (T cell) proliferation. In this review, special attention is given to an analysis of the use of IK blockers as potential immunosuppressants for the treatment of autoimmune disorders such as rheumatoid arthritis, inflammatory bowel disease and multiple sclerosis.

20-Hydroxyeicosatetraenoic acid potentiates stretch-induced contraction of canine basilar artery via PKC alpha-mediated inhibition of KCa channel

1. The present study was undertaken to elucidate whether PKCalpha plays a role in the mechanism of the stretch-induced contraction potentiated by 20-hydroxyeicosatetraenoic acid (20-HETE). The effects of 20-HETE on the canine basilar artery were compared with those of iberiotoxin, a blocker of large conductance Ca(2+)-activated K(+) channels (K(Ca) channels), as this blocker was shown earlier to sensitize these arteries to mechanical stretch. 2. Slow stretch at rates of 0.1 to 3 mm s(-1) did not produce any contraction in normal physiological solution. 3. In the presence of 20-HETE, the slow stretch could produce contraction, which was inhibited by nicardipine, a 1,4-dihydropyridine Ca(2+) channel blocker, and gadolinium, a blocker of stretch-activated cation channels. 4. 20-HETE inhibited whole-cell K(+) current and depolarized the membrane by approximately 10 mV. These effects of 20-HETE were similar to those of iberiotoxin. 5. Calphostin C, an inhibitor of protein kinase C (PKC), inhibited the action of 20-HETE, but not that of iberiotoxin. 6. In response to 20-HETE PKCalpha isoform was translocated from the cytosol to the membrane fraction, which translocation was inhibited by calphostin C. 7. These results suggest that 20-HETE induced sensitization of the canine basilar artery to stretch was caused by PKCalpha-mediated inhibition of K(Ca) channel activity.

Oxidant stress-induced increase in myogenic activation of skeletal muscle resistance arteries in obese Zucker rats

This study characterized myogenic activation of skeletal muscle (gracilis) resistance arteries from lean (LZR) and obese Zucker rats (OZR). Arteries from OZR exhibited increased myogenic activation versus LZR; this increase was impaired by endothelium denudation or nitric oxde synthase inhibition. Treatment of vessels with 17-octadecynoic acid impaired responses in both strains by comparable amounts. Dihydroethidine microfluorography indicated elevated vascular superoxide levels in OZR versus LZR; immunohistochemistry demonstrated elevated vascular nitrotyrosine levels in OZR, indicating increased peroxynitrite presence. Vessel treatment with oxidative radical scavengers (polythylene glycol-superoxide dismutase/catalase) or inhibition of Ca(2+)-activated K(+) (K(Ca)) channels (iberiotoxin) did not alter myogenic activation in LZR but normalized activation in OZR. Application of peroxynitrite to vessels of OZR caused a greater vasoconstriction versus LZR; the response was impaired in OZR by elevated intraluminal pressure and was abolished in both strains by iberiotoxin. These results suggest that enhanced myogenic activation of gracilis arteries of OZR versus LZR 1) is not due to alterations in cytochrome P-450 contribution, and 2) may be due to elevated peroxynitrite levels inhibiting K(Ca) channels following increased intraluminal pressure.

Muscarinic M2 receptors inhibit Ca2+-activated K+ channels in rat bladder smooth muscle

PURPOSE

To clarify the functional relationship between M2 muscarinic receptor and Ca2+-activated K+ channel, we investigated the effect of carbachol (CCh) on the membrane current of rat bladder smooth muscle cells.

METHODS

Rat bladder single smooth muscle cells were patch clamped with whole-cell configuration.

RESULTS

CCh (10 micro mol/L) transiently induced an outward current in the presence of K+ in the pipette solution. A high Ca2+ concentration in the pipette solution persistently induced an outward current, which was inhibited by CCh. In the presence of M2 inhibitor, AFDX-384, CCh induced the outward current persistently, indicating that M2 was involved in the current inhibition. In pertussis toxin pretreated cells, CCh did not apparently inhibit the outward current. The CCh-induced outward current was inhibited by iberiotoxin, a selective inhibitor of large-conductance Ca2+-activated K+ channels (BKCa).

CONCLUSION

CCh induces BKCa, which is inhibited by M2- and Gi-mediated signal transduction pathway. This M2-mediated pathway may enhance contraction which is initiated by M3-stimulation in rat bladder smooth muscle.

Fertilization and activation currents in bovine oocytes

One of the first events that occurs at fertilization is a transient modification of the electrical properties of the oocyte plasma membrane. The whole-cell voltage clamp technique was used to demonstrate an outward ion current and a hyperpolarization of the plasma membrane after fertilization in bovine oocytes. These electrical events, together with measurement of internal calcium concentrations, were also recorded after injection with sperm factor and exposure to parthenogenetic activators, such as Ca(2+) ionophore, ethanol and thapsigargin. Experiments were carried out simultaneously in immature and in vitro matured oocytes. Significant differences were recorded in the activation current and hyperpolarization among oocyte activators and between immature and matured oocytes. However, outward ion current and Ca(2+) release showed similar dynamics. The injection of the calcium chelator EGTA completely abolished both ion current and hyperpolarization, indicating that these electrical events are calcium dependent. Addition of specific calcium releasers, such as 1,4,5-inositol trisphosphate (IP(3)) and caffeine, triggered ion activation current and hyperpolarization indicating that IP(3) and ryanodine receptors are active in both immature and matured oocytes. Different ion channel inhibitors were used to characterize the channels underlying outward currents. Only addition of rIberiotoxin caused a complete inhibition of the current, indicating the involvement of high conductance Ca(2+)-activated K(+) channels in generating activation current. In conclusion, these findings provide evidence that bovine oocyte activation is associated with Ca(2+)-dependent electrical events. Oocytes have the potential to react to different activators even when immature; however, oocyte maturation seems to increase sensitivity to physiological activators, such as spermatozoa and sperm factor, and chemicals, such as ethanol.

Peroxynitrite inhibits Ca2+-activated K+ channel activity in smooth muscle of human coronary arterioles

We examined the hypothesis that ONOO-, a product of the interaction between superoxide (O2*-) and nitric oxide (NO), inhibits calcium-activated K+ (KCa) channel activity in vascular smooth muscle cells (VSMCs) of human coronary arterioles (HCAs), thereby reducing hyperpolarization-mediated vasodilation. HCAs were dissected from right atrial appendages. The interaction of ONOO- with microvessels was determined by immunohistochemistry using a nitrotyrosine antibody. Strong staining was observed in arteries exposed to authentic ONOO- or to sodium nitroprusside (SNP)+xanthine (XA)+xanthine oxidase (XO). Dilation to 10(-8) mol/L bradykinin (BK) was abolished in vessels exposed to ONOO- (-2.5+/-8%; P<0.05) but not DC-ONOO- (65+/-8%). Reduced dilation to BK was also observed after application of XO and SNP. Dilation to NS1619 (KCa channel opener) was reduced in endothelial denuded arterioles treated with ONOO-. In isolated VSMCs, whole-cell peak K+ current density was reduced by ONOO- (control 65+/-15 pA/pF; ONOO- 42+/-9 pA/pF; P<0.05). Iberiotoxin had no further effect on whole-cell K+ current. In inside-out patches, ONOO- but not DC-ONOO- decreased open state probability (NP(o)) of KCa channel by 50+/-12%. O2*- generated by XA+XO had no effect on BK-induced dilation and NP(o) of KCa channels. These results suggest that ONOO-, but not O2*-, inhibits KCa channel activity in VSMCs possibly by a direct effect. This mechanism may contribute to impaired EDHF-mediated dilation in conditions such as ischemia/reperfusion where increased activity of NO synthase occurs in the presence of excess of O2*-.

Partial apamin sensitivity of human small conductance Ca2+-activated K+ channels stably expressed in Chinese hamster ovary cells

The bee venom toxin apamin is an important drug tool for characterising small conductance Ca(2+)-activated K(+) channels (SK channels). In recombinant expression systems both rSK2 and rSK3 channels are potently blocked by apamin, whilst the sensitivity of SK1 channels is somewhat less clear. In the present study we have conducted a detailed analysis by patch clamp electrophysiology of the effects of apamin on human SK channels (SK1, SK2 and SK3) stably expressed in Chinese hamster ovary (CHO-K1) cells. CHO-K1 cell lines expressing either hSK1, 2 or 3 channels were first validated using specific antibodies and Western blotting. Specific protein bands of a size corresponding to the predicted channel tetramer (approximately 250-290 kDa) were detected. In each cell line, but not wild-type untransfected cells, large, time-independent inwardly rectifying Ca(2+)-dependent K(+) currents were observed under voltage-clamp. In CHO-hSK1, this current was markedly reduced by apamin (IC(50) value 8 nM), however, a significant fraction of the current remained unblocked (39+/-5%), even at saturating concentrations (1 microM apamin). The apamin-sensitive and -insensitive currents possess very similar biophysical and pharmacological properties. Each are Ca(2+)-dependent, inwardly rectify and have relative ionic permeabilities of K(+)>Cs(+)>Li(+)=Na(+). Both components were resistant to block by charybdotoxin and iberiotoxin, known IK and BK channel blockers, but were attenuated by the tricyclic antidepressant cyproheptadine (>95% block at 1 mM). The SK channel opener 1-EBIO could still produce channel activation in the presence of apamin. Importantly, hSK2 and hSK3 channels also exhibit partial apamin sensitivity in our experimental paradigm (IC(50) values of 0.14 nM and 1.1 nM, respectively, and maximal percentage inhibition values of 47+/-7% and 58+/-9%, respectively). Our data indicate that, at least in a recombinant expression system, all three SK channels can be partially apamin-sensitive. The explanation for this finding is presently unclear but may be due to regulatory subunits, phosphorylation or other types of post translational modification. Ascribing particular SK channels to physiological roles using apamin as a drug tool needs to be done cautiously in light of these findings.

The peripheral antinociceptive effect of resveratrol is associated with activation of potassium channels

The possible participation of K(+) channels in the antinociceptive action induced by resveratrol was assessed in the 1% formalin test. Local administration of resveratrol produced a dose-dependent antinociception in the second phase of the test. The antinociception produced by resveratrol was due to a local action as its administration in the contralateral paw was not active. Local pretreatment of the injured paw with glibenclamide, tolbutamide or glipizide (ATP-sensitive K(+) channel inhibitors) did not modify resveratrol-induced antinociception. In contrast, charybdotoxin and apamin (large and small conductance Ca(2+) activated-K(+) channel blockers, respectively), 4-aminopyridine or tetraethylammonium (voltage-dependent K(+) channel inhibitors) dose-dependently prevented resveratrol-induced antinociception. Local peripheral administration of glibenclamide, but not charybdotoxin or apamin, significantly reduced the antinociceptive effect produced by peripheral morphine (positive control). At the highest effective doses, none of the drugs used induced behavioral side effects as revealed by the evaluation of stepping, righting, corneal and pinna reflexes. In addition, when given alone, none of the inhibitors modified the nociceptive behavior induced by 1% formalin. The results suggest that resveratrol opens large and small conductance Ca(2+)-activated K(+) channels, but not ATP-sensitive K(+) channels, in order to produce its peripheral antinociceptive effect in the formalin test. The participation of voltage-dependent K(+) channels was also suggested, but since non-selective inhibitors were used the data awaits further confirmation.

Protection of potassium channel inhibitors against hypoxia/reoxygenation-induced death of cultured hippocampal neurons

OBJECTIVE

To investigate the effects of potassium channel inhibitors on hypoxia/reoxygenation-induced death of cultured hippocampal neurons.

METHODS

Cultured 8 d in vitro, hippocampal neurons were exposed to hypoxia (in mixture of 95% N2 and 5% CO2) for 6 h, and then reoxygenated till the 72nd hour. Different potassium channel inhibitors were applied to the culture solution separately after reoxygenation. Neuron death was analyzed with cell counting and MTT assay.

RESULTS

Hypoxia/reoxygenation procedure induced a delayed death of cultured hippocampal neurons. Application of tetraethylammonium (TEA) offered concentration-dependent protection of the neurons against death. Selective high-conductance calcium-activated potassium channel (BK) inhibitor iberiotoxin (IbTX) showed significant neuron protection (P<0.001). However, A-type potassium channel inhibitor 4-aminopyridine presented no protection against neuron death (P>0.05).

CONCLUSION

Following hypoxia/reoxygenation, enhanced activity of potassium channel, especially BK channel, may induce neuron death.

Ca(2+)-activated potassium (K(Ca)) channel inhibition decreases neuronal activity-blood flow coupling

A number of possible mediators have been proposed to couple neuronal activity with local cerebral metabolic activity and blood flow, but the mechanisms by which these mediators act is still unclear. In order to explore these coupling mechanisms, we used the rodent whisker-barrel cortex (WBC) model to test the hypothesis that modulation of K(Ca) channels is an important step in this coupling process. Anesthetized rats were prepared for laser-Doppler flowmetry (LDF) or evoked potential recordings utilizing a thinned cranial window over WBC. Superfusion of the K(Ca) channel blockers tetraethylammonium (TEA) or iberiotoxin directly onto WBC attenuated the magnitude of the whisker evoked LDF changes. Similar effects were seen after intravenous administration of TEA. Although attenuated, neither the temporal profile of the elicited blood flow responses nor the evoked electrical activity in WBC were affected by K(Ca) blockade. These data suggest that the process of cerebral metabolism/blood flow coupling in the rodent WBC involves K(Ca) channels.

Amelioration of ischemia- and reperfusion-induced myocardial injury by the selective estrogen receptor modulator, raloxifene, in the canine heart

OBJECTIVES

We sought to investigate whether raloxifene reduces ischemia-reperfusion injury and what mechanisms are involved in the cardioprotective effects.

BACKGROUND

Estradiol-17-beta reduces myocardial infarct size in ischemia-reperfusion injury. Raloxifene, a selective estrogen receptor modulator, demonstrates immediate coronary artery vasorelaxing effects.

METHODS

The myocardial ischemia-reperfusion model included anesthetized open-chest dogs after 90-min occlusion of the left anterior descending coronary artery (LAD) and subsequent 6-h reperfusion. Raloxifene and/or other drugs were infused into the LAD from 10 min before coronary occlusion to 1 h after reperfusion without an occlusion period.

RESULTS

Infarct size was reduced in the raloxifene (5 microg/kg per min) group compared with the control group (7.2 +/- 2.5% vs. 40.9 +/- 3.9% of the area at risk, p < 0.01). Either N(G)-nitro-L-arginine methyl ester (L-NAME), the inhibitor of nitric oxide (NO) synthase, or charybdotoxin, the blocker of Ca(2+)-activated K+ (K(Ca)) channels, partially attenuated the infarct size-limiting effect, and both of them completely abolished the effect. The incidence of ventricular fibrillation was also less in the raloxifene group than in the control group (11% vs. 44%, p < 0.05). Activity of p38 mitogen-activated protein (MAP) kinase increased with 15-min ischemia, and raloxifene pretreatment inhibited the activity. Myeloperoxidase activity of the 6-h reperfused myocardium was also attenuated by raloxifene.

CONCLUSIONS

These data demonstrate that raloxifene reduces myocardial ischemia-reperfusion injury by mechanisms dependent on NO and the opening of K(Ca) channels in canine hearts. Deactivation of p38 MAP kinase and myeloperoxidase by raloxifene may be involved in the cellular mechanisms of cardioprotection.

BK channel activity determines the extent of cell degeneration after oxygen and glucose deprivation: a study in organotypical hippocampal slice cultures

BK channels are voltage- and calcium-dependent potassium channels whose activation tends to reduce cellular excitability. In hippocampal pyramidal cells, BK channels repolarize somatic action potentials, and recent immunogold and electrophysiological analyses have revealed a presynaptic pool of BK channels that can regulate glutamate release. Agents that modulate BK channel activity would therefore be expected to affect cell excitability and neurotransmitter release also under pathological conditions. We have investigated the role of BK potassium channels in a model of ischemia-induced nerve cell degeneration. Organotypical slice cultures of rat hippocampus were exposed to oxygen and glucose deprivation (OGD), and cell death was assessed by the fluorescent dye propidium iodide. OGD induced cell death in the CA1 region and to a lesser extent in CA3. Treatment with the BK channel blockers, paxilline and iberiotoxin, during and after OGD induced increased cell death in CA1 and CA3. Both BK channel blockers also sensitized the relatively resistant granule cells in fascia dentata to OGD. The effect of paxilline and iberiotoxin was evident from 3 h after OGD, indicating a role of BK channels early in the post-ischemic phase or during OGD itself. The BK channel opener, NS1619, turned out to be gliotoxic, and this effect was not counteracted by paxilline and iberiotoxin. Our data show that blockade of BK channels aggravates OGD-induced cell damage and suggest that BK channels act as a kind of 'emergency brake' during and/or after ischemia. Accordingly, the BK channel is a potential molecular target for neuroprotective therapy in stroke.

K(Ca) channel blockers reveal hyperpolarization and relaxation to K+ in rat isolated mesenteric artery

Raising extracellular K+ concentration ([K+](o)) around mesenteric resistance arteries reverses depolarization and contraction to phenylephrine. As smooth muscle depolarizes and intracellular Ca(2+) and tension increase, this effect of K+ is suppressed, whereas efflux of cellular K+ through Ca(2+)-activated K+ (K(Ca)) channels is increased. We investigated whether K+ efflux through K(Ca) suppresses the action of exogenous K+ and whether it prestimulates smooth muscle Na(+)-K(+)-ATPase. Under isometric conditions, 10.8 mM [K+](o) had no effect on arteries contracted >10 mN, unless 100 nM iberiotoxin (IbTX), 100 nM charybdotoxin (ChTX), and/or 50 nM apamin were present. Simultaneous measurements of membrane potential and tension showed that phenylephrine depolarized and contracted arteries to -32.2 +/- 2.3 mV and 13.8 +/- 1.6 mN (n = 5) after blockade of K(Ca), but 10.8 mM K+ reversed fully the responses (107.6 +/- 8.6 and 98.8 +/- 0.6%, respectively). Under isobaric conditions and preconstriction with phenylephrine, 10.7 mM [K+](o) reversed contraction at both 50 mmHg (77.0 +/- 8.5%, n = 9) and 80 mmHg (83.7 +/- 5.5%, n = 5). However, in four additional vessels at 80 mmHg, raising K+ failed to reverse contraction unless ChTX was present. Increases in isometric and decreases in isobaric tension with phenylephrine were augmented by either ChTX or ouabain (100 microM), whereas neither inhibitor altered tension under resting conditions. Inhibition of cellular K+ efflux facilitates hyperpolarization and relaxation to exogenous K+, possibly by indirectly reducing the background activation of Na(+)-K(+)-ATPase.

Effects of imipramine on ion channels and proliferation of IGR1 melanoma cells

Human IGR1 cells are a model for malignant melanoma. Since progression through the cell cycle is accompanied by transient cell hyperpolarization, we studied the properties of potassium and chloride ion channels and their impact on cell growth. The major potassium current components were mediated by outward rectifying ether à go-go (hEAG) channels and Ca2+-activated channels (KCa) of the IK/SK type. The major chloride channel component was activated by osmotic cell swelling (Clvol). To infer about the contribution of these channels to proliferation, specific inhibitors are required. Since there is no specific blocker for hEAG available, we used the tricyclic antidepressant imipramine, which blocked all channels mentioned, in combination with blockers for KCa (charybdotoxin) and Clvol (DIDS and pamoic acid). Incubation of IGR1 cells for 48 hr in 10-15 mM imipramine reduced DNA synthesis and metabolism without significant effects on apoptosis. hEAG channels were most sensitive to imipramine (IC50: 3.4 microM at +50 mV), followed by KCa (13.8 microM at +50 mV) and Clvol (12 microM at -100 mV), indicating that hEAG expression may be of importance for proliferation of melanoma cells. The contribution of KCa channels could be excluded, as 500 nM charybdotoxin, which completely blocked KCa, had no effect on proliferation. The impact of Clvol also seems to be minor, because 500 microM pamoic acid, which completely blocked Clvol, did not affect proliferation either.

Protein tyrosine kinase and mitogen-activated protein kinase activation contribute to K(ATP) and K(ca) channel impairment after brain injury

Previous studies have observed that pial artery dilation to activators of the ATP sensitive K (K(ATP)) and calcium sensitive K (K(ca)) channel was blunted following fluid percussion brain injury (FPI) in the piglet. In recent studies in the rat, protein tyrosine kinase (PTK) activation was observed to contribute to K(ATP) channel impairment after FPI, but such a role in K(ca) channel impairment was unclear. This study investigated the role of PTK and mitogen activated protein kinase (MAPK) activation in blunted pial dilation to K(ATP) and K(ca) channel agonists following FPI in piglets equipped with a closed cranial window. Cromakalim and NS1619 (10(-8), 10(-6) M) induced pial artery dilation was blunted after FPI, but partially restored by the PTK inhibitors genistein (10(-6) M) and tyrphostin A23 (10(-5) M) (10+/-1 and 19+/-1%, sham control; 2+/-1 and 4+/-1%, FPI; and 7+/-1 and 11+/-1% FPI-genistein pretreated for NS1619 10(-8), 10(-6) M, respectively). Cromakalim- and NS1619-induced pial dilation was also partially restored after FPI by pretreatment with the MAPK inhibitors U0126 (10(-6) M) and PD98059 (10(-5) M) (12+/-1 and 21+/-1%, sham control; 2+/-1 and 4+/-1%, FPI; and 6+/-1 and 10+/-2%, FPI-U0126 pretreated for NS1619 10(-8), 10(-6) M, respectively). These data suggest that PTK and MAPK activation contribute to K(ATP) and K(ca) channel impairment following FPI.

Dual and opposing roles of presynaptic Ca2+ influx for spontaneous GABA release from rat medial preoptic nerve terminals

Calcium influx into the presynaptic nerve terminal is well established as a trigger signal for transmitter release by exocytosis. By studying dissociated preoptic neurons with functional adhering nerve terminals, we here show that presynaptic Ca2+ influx plays dual and opposing roles in the control of spontaneous transmitter release. Thus, application of various Ca2+ channel blockers paradoxically increased the frequency of spontaneous (miniature) inhibitory GABA-mediated postsynaptic currents (mIPSCs). Similar effects on mIPSC frequency were recorded upon washout of Cd2+ or EGTA from the external solution. The results are explained by a model with parallel Ca2+ influx through channels coupled to the exocytotic machinery and through channels coupled to Ca2+-activated K+ channels at a distance from the release site.

Sildenafil reverses O2 constriction of the rabbit ductus arteriosus by inhibiting type 5 phosphodiesterase and activating BK(Ca) channels

Oxygen constriction causes functional closure of the ductus arteriosus (DA) at birth. Although DA closure is crucial for postnatal adaptation, patency of the DA is critical for survival of newborns with duct-dependent cardiac malformations. In these cases, DA patency is achieved by i.v. infusion of prostaglandin E1, which, though effective, is often associated with complications. We hypothesized that sildenafil, a specific phosphodiesterase type 5 inhibitor, is an effective DA vasodilator. In isolated DA rings from term (d 30) fetal rabbits, sildenafil (10(-6)-10(-4) M) and diethylamine NONOate (10(-7)-10(-5) M) induced dose-dependent relaxation of oxygen-constricted DA (-52 +/- 4% and -51 +/- 6%, respectively) that was inhibited by the soluble guanylyl-cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (5 x 10(-5) M). Sildenafil increased cyclic GMP levels. Iberiotoxin (200 nM), an inhibitor of calcium-sensitive potassium channels, decreased the vasodilatory effect of sildenafil and diethylamine NONOate (-30 +/- 2% and -27 +/- 4%, respectively). Oxygen inhibition of whole-cell K+ current and membrane depolarization were partially restored by sildenafil, and this was inhibited by iberiotoxin. Immunohistochemistry and immunoblotting confirmed the presence of phosphodiesterase type 5 and calcium-sensitive potassium channels in the DA smooth muscle cells. This is the first study to demonstrate that sildenafil dilates the DA by increasing soluble guanylyl-cyclase-derived cGMP levels and thereby activating calcium-sensitive potassium channels, causing membrane hyperpolarization. Sildenafil, already approved for human usage, might be an alternative or a useful adjunct to prostaglandin E1 as a bridge to cardiac surgery.

Role of CA(2+)-activated K+ channels in the regulation of basilar arterial tone in spontaneously hypertensive rats

1. Ionic channels appear to play an important role in contractile responses of the cerebral arteries and, thereby, contribute to the regulation of cerebral circulation. In the present study, we investigated the role of large-conductance Ca(2+)-activated K+ (BK(Ca)) channels in the regulation of cerebral arterial tone during chronic hypertension. 2. Ring segments of the basilar artery from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were placed in bath chambers and the isometric tension of each ring was measured. 3. Application of inhibitors of BK(Ca) channels, namely tetraethylammonium (TEA; > or = 0.1 mmol/L) and charybdotoxin (CTX; > or = 0.1 nmol/L), produced spontaneous contraction with rhythmic oscillation in the basilar artery from SHR. 4. The oscillatory contraction was not induced by 5-hydroxytryptamine (0.01-10 micromol/L) or depolarization by external high K+ (20-60 mmol/L). 5. The rhythmic contraction was completely abolished by either the removal of external Ca(2+) or the application of nicardipine (10 nmol/L). 6. The oscillation was not affected by the substitution of external Cl(-) by various equimolar anions (i.e. acetate, benezenesulphonate, bromide and isethianate). 7. The amplitude of the oscillation was dose-dependently increased by the vasodilators forskolin and sodium nitroprusside, as well as by stimulation of the endothelium with histamine and acetylcholine, whereas the frequency was decreased. 8. In contrast, the oscillation was eliminated by depletion of Ca(2+) stores by caffeine. Neither TEA (10 mmol/L) nor CTX (10 nmol/L) produced any significant contraction of the basilar artery in WKY rats. 9. These results suggest that BK(Ca) channels may play an important role in regulating the resting tone of the cerebral artery in SHR.

Large-conductance, ca(2+)-activated k(+) channels: function, pharmacology and drugs

Because of the physiological role played by the hyperpolarisation process resulting from a K(+) outflow, it is not surprising that compounds able to activate outward K(+) channels are considered as promising drugs, with exciting perspectives for the treatment of several cardiovascular, respiratory, neurological and urological diseases. Among the different and numerous K(+) channel families, medicinal chemistry has focused its major interest onto two channel types: the ATP-sensitive channels (K(ATP)) and the large conductance subtype (BK), that belongs to the wide family of calcium-activated K(+) channels. BK channels are almost ubiquitous and exhibit single channel conductance of 100-300 pS, a property which justifies the potent role of these channels in the control of the membrane potential. BK channels have been investigated as potential therapeutic targets for different neuropathies, because of their profound influence on the neuronal activity. Moreover, BK channels are expected to have applications for the therapy of cardiovascular diseases. A potent feed-back control of the vascular and non-vascular smooth muscle tone is mediated by these channels, whose activation can be promoted by both a rise of the intracellular free calcium concentration as well as a membrane depolarisation. Additionally, BK channel activation can also be induced by other factors, such as cAMP-mediated phosphorylation, G-proteins, GMP and cGMP. The aim of this paper is to give a concise overview of the biological and pharmacological properties and potential therapeutic applications of activators of BK channels present at the vascular level. The "state of the art" in the pharmaceutical development of natural and synthetic BK-activators, with a description of the lead chemical structures, will be also described.

Effects of large conductance Ca(2+)-activated K(+) channels on nitroglycerin-mediated vasorelaxation in humans

Nitric oxide (NO)-induced vasorelaxation and the regulation of endothelial superoxide anion levels is partly mediated by vascular large conductance Ca(2+)-activated K(+) (BK(Ca)) channels. Nitroglycerin acts through the release of NO and its effect is modulated by changes in endothelial superoxide levels. This study examines the effect of BK(Ca) channel blockade on nitroglycerin-induced vasorelaxation in human arterial and venous vascular segments and whether responses to BK(Ca) channel blockade are influenced by the development of venous nitroglycerin tolerance. Dose-relaxation curves to nitroglycerin (10(-10)-10(-4) M) were obtained in segments of the saphenous vein and the left mammary artery. Studies were performed with and without pre-incubation with the BK(Ca) channel blocker iberiotoxin (10(-7) M) and venous tolerance to nitroglycerin were induced by a 24-h i.v. infusion (0.5 microg/kg/min). Iberiotoxin reduced the vasorelaxant effect of nitroglycerin (E(max)) by 60% in endothelium-intact arteries and 13% in endothelium-denuded arteries (P<0.05). Development of nitroglycerin tolerance did not affect the response to iberiotoxin in the venous vascular segments (P>0.05) and (compared to arterial segments) veins were less sensitive to BK(Ca) channel blockade (30% reduction in E(max)) or endothelial removal. The results suggest that primarily arterial effects of nitroglycerin are significantly inhibited by changes in the activity of the endothelial BK(Ca) channels. Although endothelial BK(Ca) are likely regulators of mechanisms underlying arterial tolerance development to nitroglycerin, they do not appear to play a role in human venous nitroglycerin tolerance development.

Beta-agkistrodotoxin inhibits large-conductance calcium-activated potassium channels in rat hippocampal CA1 pyramidal neurons

Effect of beta-agkistrodotoxin (beta-AgTx), a presynaptic neurotoxin purified from snake venom, on large-conductance calcium-activated potassium channels (BK(Ca)) was studied in rat hippocampal CA1 pyramidal neurons using inside-out configuration of patch-clamp technique. The results showed that in equimolar K+ (150 mM) and 1 microM intracellular Ca2+ conditions, internal application of beta-AgTx inhibited the activity of BK(Ca) by reducing open probability (P(o)) of the channels in a concentration-dependent manner. High concentration (74 nM) of beta-AgTx completely eliminated opening of the channels. However, 37 nM beta-AgTx (at -40 mV) decreased P(o) from 0.49+/-0.07 to 0.03+/-0.03, switched two open time constants (0.51+/-0.32 and 8.77+/-1.63 ms) to be a single time constant of 0.46+/-0.40 ms. The results indicate that inhibition of BK(Ca) by beta-AgTx may account for the facilitatory phase of the toxin on acetylcholine release from nerve terminals.

Pharmacological and molecular characterisation of SK3 channels in the TE671 human medulloblastoma cell line

The expression of the small conductance calcium-activated potassium channels SK1, SK2 and SK3 was investigated in the TE671 human medulloblastoma cell line using RT-PCR and transcripts were detected only for SK3. Immunodetection experiments confirmed this result, demonstrating the presence of the SK3 protein. This potassium channel was characterised in TE671 cells using whole-cell patch-clamp recordings. Voltage steps to -100 mV from a holding potential of 0 mV in equimolar 140 mM intra- and extracellular K(+) (K(+)(in/out)) elicited an inward current. The reversal potential of this current shifted 56.6 mV per 10-fold increase in K(+)(out) thus suggesting K(+) selectivity. This current was dependent on the concentration of Ca(2+)(in) with an EC(50) of 104.2 nM. A pharmacological characterisation of this current revealed that it was not blocked by 1 microM charybdotoxin (ChTX), 0.3 microM iberiotoxin (IbTX) or 10 microM clotrimazole (CLT) and only modestly inhibited (<50%) by 30 nM scyllatoxin (ScTX), 200 microM dequalinium chloride (Deq) or 300 microM d-tubocurarine (d-TC). The non-selective SK blocker d-TC blocked the current with an IC(50) of 43.2 microM while apamin blocked the current to a much greater extent (87.8% at 1 microM) with an IC(50) of 4.3 nM. Furthermore, the current was significantly increased (132.6+/-5.2%, n=7) by 500 microM 1-ethyl-2-benzimidazolinone (EBIO). Collectively, these data demonstrate the presence of an endogenous SK3 channel in human TE671 cells.

Potassium channels and human corporeal smooth muscle cell tone: further evidence of the physiological relevance of the Maxi-K channel subtype to the regulation of human corporeal smooth muscle tone in vitro

PURPOSE

Recent evidence indicates that the large conductance, voltage dependent, Ca2+ sensitive K channel or Maxi-K has an important role in the modulation of human corporeal smooth muscle tone and, thus, in erectile capacity. We further clarified the contribution of the Maxi-K channel subtype to the generation of contractile responses in isolated human corporeal tissue strips.

MATERIALS AND METHODS

We performed pharmacological studies of phenylephrine contracted isolated corporeal tissue strips in the presence and absence of the 2 Maxi-K channel blockers tetraethylammonium chloride (TEA) and charybdotoxin, and the Maxi-K opener NS1619. K channel treatment effects were evaluated using 2 parameters, including 1) the steady state parameter of the empirically determined peak magnitude of the steady state contractile response and 2) the kinetic parameter of time required to achieve half of the peak steady state contractile response or half-time. Electrophysiological studies in freshly isolated and cultured myocytes were performed in parallel to corroborate findings further at the tissue level.

RESULTS

Pre-incubating isolated human corporeal tissue strips with 1 mM. TEA and 1 microM. charybdotoxin was associated with an approximate 20% increase in the peak steady state contractile response and a corresponding approximate 20% decrease in the half-time of the phenylephrine induced contractile response. Conversely, pre-incubation with 10 microM. NS1619 produced a significant, approximately 20% decrease in the peak steady state contractile response and an approximate 38% increase in the half-time of the phenylephrine induced contractile response. Adding 30 to 180 microM. NS1619 to phenylephrine pre-contracted smooth muscle strips resulted in a 30% to 50% reduction in steady state contractile tension. No detectable effect of NS1619 was observed in 120 mM. KCl or 100 mM. TEA pre-contracted corporeal tissue strips. Whole cell recordings of freshly isolated and cultured corporeal myocytes confirmed that 30 microM. NS1619 induced a charybdotoxin sensitive hyperpolarizing current mediated by the Maxi-K channel.

CONCLUSIONS

These in vitro studies confirm and extend previous observations indicating the importance of the Maxi-K channel for regulating human corporeal smooth muscle tone, and by extension, erectile capacity and function.

K+ currents generated by NMDA receptor activation in rat hippocampal pyramidal neurons

Long lasting outward currents mediated by Ca2+-activated K+ channels can be induced by Ca2+ influx through N-methyl-D-aspartate (NMDA)-receptor channels in voltage-clamped hippocampal pyramidal neurons. Using specific inhibitors, we have attempted to identify the channels that underlie these outward currents. At a holding potential of -50 mV, applications of 1 mM NMDA to the soma of cultured hippocampal pyramidal neurons induced the expected inward currents. In 44% of cells tested, these were followed by outward currents (average amplitude 60 +/- 7 pA) that peaked 2.5 s after the initiation of the inward NMDA currents and decayed with a time constant of 1.4 s. In 43% of those cells exhibiting an outward current, SK channel inhibitors, UCL 1848 (100 nM) and apamin (100 nM) abolished the outward current. In the remainder of the cells, the outward currents were either insensitive or only partly inhibited (44 +/- 4%) by 100 nM UCL 1848. In these cells, the outward currents were reduced by the slow afterhyperpolarization (sAHP) inhibitors, muscarine (3 microM; 43 +/- 9%), UCL 1880 (3 microM; 34 +/- 10%), and UCL 2027 (3 microM; 57 +/- 6%). Neither the BK channel inhibitor, charybdotoxin (100 nM), nor the Na+/K+ ATPase inhibitor, ouabain (100 microM), reduced these outward currents. Irrespective of the pharmacology, the time course of the outward current did not differ. Interestingly, no correlation was observed between the presence of a slow apamin-insensitive afterhyperpolarization and an outward current insensitive to SK channel blockers following NMDA-receptor activation. It is concluded that an NMDA-mediated rise in [Ca2+]i can result in the activation of apamin-sensitive SK channels and of the channels that underlie the sAHP. The activation of these channels may, however, depend on their location relative to NMDA receptors as well as on the spatial Ca2+ buffering within individual neurons.

Role of calcium-activated potassium channels in impaired acetylcholine vasodilatory responses in diabetic rats

Muscarinic agonists produce endothelium-dependent vasodilatation in the presence of nitric oxide synthase (NOS) inhibition. The importance of this mechanism was assessed in the methoxamine-preconstricted perfused mesenteric vascular bed (MVB) of streptozotocin diabetic Sprague-Dawley rats. At 9 weeks of age, male rats were treated with streptozotocin (55 mg/kg in citrate buffer) or with citrate buffer alone. The superior mesenteric artery was cannulated and the MVB was detached from its intestinal borders. Concentration-response curves to acetylcholine were determined in the presence and in the absence of indomethacin, tetrabutylammonium (a calcium-activated potassium channel blocker), high extracellular potassium, or NOS inhibition with Nomega-nitro-l-arginine and l-NG-nitro-l-arginine. There was a rightward shift in the concentration-response curve with an increase in median inhibitory concentration (p < 0.05) and a reduction in acetylcholine IMAX (p < 0.05) values in 14-week streptozotocin rats. The ability of NOS inhibition to attenuate vasodilatation was reduced in the 14-week streptozotocin group relative to the 2-week streptozotocin treatment group (p < 0.05). However, the ability of tetrabutylammonium to block acetylcholine-mediated vasodilatation remained consistent in streptozotocin rats at both stages. The results demonstrate that an alternate pathway involving calcium-activated potassium channels may compensate for diminished nitric oxide bioactivity. This effect is contingent on the duration of diabetes. This study provides insight into the development and progression of altered diabetic vascular responses.

Decreases in Ca2+-dependent K+-currents due to cyclic guanosine monophosphate are not dependent on phosphorylation

Two-microelectrode voltage clamping experiments were performed on isolated snail neurons to measure the Ca2+-dependent. potential-dependent K+ current (I(C)), with assessment of the effects of penetrating cGMP analogs on this current - dibutyryl cGMP (dcGMP) and 8-Br-cGMP. Both of these penetrating cGMP analogs rapidly and reversibly decreased the amplitude of I(C). cGMP analogs produced no shifts in the volt-ampere characteristics of the efflux current along the voltage axis. dcGMP and 8-Br-cGMP had no effect on the influx Ca2+ current. The non-specific protein kinase inhibitor H-8 decreased or had no effect on I(C) in different cells. The effects of both dcGMP and 8-Br-cGMP persisted in the presence of H-8. Decreases in I(C) in the presence of cGMP analogs also persisted in the presence of the protein phosphatase inhibitor okadaic acid. These results lead to the conclusion that decreased conductivity of Ca2+-dependent K+ channels occurring in response to cGMP is not associated with phosphorylation.

Tritylamino aromatic heterocycles and related carbinols as blockers of ca 2+-activated potassium ion channels underlying neuronal hyperpolarization

A series of novel aromatic tritylamino heterocycles has been synthesized and the compounds have been tested in comparison with clotrimazole for their ability to inhibit the slow afterhyperpolarization current (sI (AHP)) in cultured rat hippocampal pyramidal neurones. Some analogues of the clotrimazole metabolite, 2-chlorophenyl-diphenyl methanol, having different chlorination substitution in the triphenyl group have also been examined. Two compounds in particular, 3-[(2-chlorophenyl)-diphenylmethylamino] pyridine (3a, UCL 1880) and 2-tritylaminothiazole (6, UCL 2027), are of special interest; they are effective blockers of the sI (AHP) (IC (50) = 1.1-1.2 microM) and are much more selective than clotrimazole since they have less effect on the high voltage-activated Ca2+ current.