Electromechanical and pharmacomechanical signalling pathways for conducted vasodilatation along endothelium of hamster feed arteries

Conducted vasodilatation (CVD) reflects the initiation and rapid (>mm s(-1)) spread of hyperpolarization along the endothelium and into smooth muscle. The ion channels that initiate CVD remain unclear as do signalling pathways that may complement electromechanical relaxation. Using isolated pressurized (75 mmHg; 37 degrees C) feed arteries (n=63; diameter: rest: 53 +/- 2 microm, maximal: 98 +/- 2 microm) from hamster retractor skeletal muscle, we investigated the contribution of calcium-activated potassium channels (KCa) and endothelium-derived autacoids to CVD. Local delivery (1 microm micropipette tip; 500-2000 ms pulse) of acetylcholine (ACh) at the downstream end initiated a local increase in endothelial cell [Ca2+]i (Fura-PE3; Deltaratio 340/380 nm = 0.215 +/- 0.032) that preceded CVD along the entire vessel. During local perifusion with KCa antagonists, iberiotoxin (5 microm) had no effect, but charybdotoxin (CTX, 5 microm) + apamin (APA, 10 microm) abolished CVD reversibly. Remarkably, this local inhibition of KCa unmasked a 'slow-conducted vasodilatation' (SCVD) that spread >1200 microm at approximately 21 microm s(-1) (n=27). Recorded 500 microm upstream from the ACh stimulus, a rise in endothelial cell [Ca2+]i (Deltaratio 340/380 nm) = 0.146 +/- 0.017; P<0.05) preceded SCVD (Deltadiameter = 14 +/- 3 microm) by approximately 10 s. Before KCa inhibition, antagonism of nitric oxide synthase (Nomega-nitro-L-arginine, 250 microm; l-NNA) and cyclooxygenase (indomethacin, 5 microm; INDO) had no effect on the amplitude of CVD yet response duration decreased by one-third (P<0.05). During local CTX + APA perifusion, L-NNA + INDO abolished SCVD while conducted [Ca2+]i responses remained intact. Thus, ACh triggers electromechanical relaxation of smooth muscle cells along the vessel initiated by local KCa, and the ensuing 'wave' of Ca2+ along the endothelium releases autacoids to promote pharmacomechanical relaxation.

Novel 384-Well Population Patch Clamp Electrophysiology Assays for Ca2+-Activated K+ Channels

Planar array electrophysiology techniques were applied to assays for modulators of recombinant hIK and hSK3 Ca2+-activated K+ channels. In CHO-hIK—expressing cells, under asymmetric K+ gradients, small-molecule channel activators evoked time- and voltage-independent currents characteristic of those previously described by classical patch clamp electrophysiology methods. In single-hole (cell) experiments, the large cell-to-cell heterogeneity in channel expression rendered it difficult to generate activator concentration-response curves. However, in population patch clamp mode, in which signals are averaged from up to 64 cells, well-to-well variation was substantially reduced such that concentration-response curves could be easily constructed. The absolute EC50 values and rank order of potency for a range of activators, including 1-EBIO and DC-EBIO, corresponded well with conventional patch clamp data. Activator responses of hIK and hSK3 channels could be fully and specifically blocked by the selective inhibitors TRAM-34 and apamin, with IC50 values of 0.31 μM and 3 nM, respectively. To demonstrate assay precision and robustness, a test set of 704 compounds was screened in a 384-well format of the hIK assay. All plates had Z′ values greater than 0.6, and the statistical cutoff for activity was 8%. Eleven hits (1.6%) were identified from this set, in addition to the randomly spiked wells with known activators. Overall, our findings demonstrate that population patch clamp is a powerful and enabling method for screening Ca2+-activated K+ channels and provides significant advantages over single-cell electrophysiology (IonWorksHT) and other previously published approaches. Moreover, this work demonstrates for the 1st time the utility of population patch clamp for ion channel activator assays and for non—voltage-gated ion channels.

Novel functions of small conductance Ca2+-activated K+ channel in enhanced cell proliferation by ATP in brain endothelial cells

Brain capillary endothelial cells (BCECs) form the blood-brain barrier (BBB), which is essential for maintaining homeostasis of the brain. Net cellular turnover, which results from the balance between cell death and proliferation, is important in maintaining BBB homeostasis. Here we report a novel mechanism that underlies ATP-induced cell proliferation in t-BBEC 117, a cell line derived from bovine brain endothelial cells. Application of 0.1-30 mum ATP to t-BBEC 117 concentration-dependently increased intracellular Ca(2+) concentration ([Ca(2+)](i)) in two phases: an initial transient phase and a later and smaller sustained one. These two phases of [Ca(2+)](i) rise were mainly due to Ca(2+) release and sustained Ca(2+) influx, respectively. The pretreatment with apamin, a selective blocker of small conductance Ca(2+)-activated K(+) channels (SK), significantly reduced both the [Ca(2+)](i) increase and K(+) current induced by ATP. Transcripts corresponding to P2Yx, SK2, and transient receptor potential channels were detected in t-BBEC 117. Knock down of SK2 protein, which was the predominant Ca(2+)-activated K(+) channel expressed in t-BBEC 117, by siRNA significantly reduced both the sustained phase of the [Ca(2+)](i) rise and the K(+) current induced by ATP. Cell proliferation was increased significantly by the presence of the stable ATP analogue ATPgammaS. This effect was blunted by UCL1684, a synthesized SK blocker. In conclusion, in brain endothelial cells ATP-induced [Ca(2+)](i) rise activates SK2 current, and the subsequent membrane hyperpolarization enhances Ca(2+) entry presumably through transient receptor potential channels. This positive feedback mechanism can account for the augmented cell proliferation by ATP.

Bradykinin induces microvascular preconditioning through the opening of calcium-activated potassium channels

BACKGROUND

This study was designed to investigate whether the activation of calcium-activated potassium (K(Ca)) or adenosine triphosphate sensitive potassium (K(ATP)) channels are required for bradykinin-induced microvascular preconditioning.

METHODS

Isolated rabbit hearts underwent retrograde perfusion with Krebs-Henseleit buffer (KHB) followed by 60 minutes of ischemic arrest with cold crystalloid cardioplegia (CCCP). Eight CCCP hearts received no pretreatment. Six bradykinin-preconditioned hearts received a 10-minute coronary infusion of 10(-8) mol/L bradykinin-enriched KHB followed by a 5-minute recovery period before CCCP. Six hearts received both 10(-8) mol/L charybdotoxin (a K(Ca) channel blocker) and bradykinin preconditioning. Finally, 6 other hearts received 10(-5 degrees ) mol/L glibenclamide (a K(ATP) channel blocker) to bradykinin-enriched KHB. All hearts were reperfused for 30 minutes with KHB.

RESULTS

Bradykinin preconditioning significantly improved the recovery of left ventricular and microvascular function, as compared with control. On the other hand, bradykinin preconditioning significantly reduced the contractile responses to U46619, a thromboxane A2 analogue. Charybdotoxin significantly inhibited the improved recovery of bradykinin-induced left ventricular and microvascular function. Glibenclamide tended to diminish the bradykinin preconditioning-enhanced recovery of left ventricular function, but failed to affect bradykinin preconditioning-improved recovery of microvascular function.

CONCLUSION

Both K(Ca) and K(ATP) channels were involved partially in bradykinin-induced myocardial preconditioning. However, bradykinin induces microvascular preconditioning through the opening of K(Ca) channels rather than K(ATP) channels.

Combination of Ca2+ -activated K+ channel blockers inhibits acetylcholine-evoked nitric oxide release in rat superior mesenteric artery

BACKGROUND AND PURPOSE

The present study investigated whether calcium-activated K+ channels are involved in acetylcholine-evoked nitric oxide (NO) release and relaxation.

EXPERIMENTAL APPROACH

Simultaneous measurements of NO concentration and relaxation were performed in rat superior mesenteric artery and endothelial cell membrane potential and intracellular calcium ([Ca2+]i) were measured.

KEY RESULTS

A combination of apamin plus charybotoxin, which are, respectively, blockers of small-conductance and of intermediate- and large-conductance Ca2+ -activated K channels abolished acetylcholine (10 microM)-evoked hyperpolarization of endothelial cell membrane potential. Acetylcholine-evoked NO release was reduced by 68% in high K+ (80 mM) and by 85% in the presence of apamin plus charybdotoxin. In noradrenaline-contracted arteries, asymmetric dimethylarginine (ADMA), an inhibitor of NO synthase inhibited acetylcholine-evoked NO release and relaxation. However, only further addition of oxyhaemoglobin or apamin plus charybdotoxin eliminated the residual acetylcholine-evoked NO release and relaxation. Removal of extracellular calcium or an inhibitor of calcium influx channels, SKF96365, abolished acetylcholine-evoked increase in NO concentration and [Ca2+]i. Cyclopiazonic acid (CPA, 30 microM), an inhibitor of sarcoplasmic Ca2+ -ATPase, caused a sustained NO release in the presence, but only a transient increase in the absence, of extracellular calcium. Incubation with apamin and charybdotoxin did not change acetylcholine or CPA-induced increases in [Ca2+]i, but inhibited the sustained NO release induced by CPA.

CONCLUSIONS AND IMPLICATIONS

Acetylcholine increases endothelial cell [Ca2+]i by release of stored calcium and calcium influx resulting in activation of apamin and charybdotoxin-sensitive K channels, hyperpolarization and release of NO in the rat superior mesenteric artery.

A computational analysis of central CO2 chemosensitivity in Helix aspersa

We created a single-compartment computer model of a CO(2) chemosensory neuron using differential equations adapted from the Hodgkin-Huxley model and measurements of currents in CO(2) chemosensory neurons from Helix aspersa. We incorporated into the model two inward currents, a sodium current and a calcium current, three outward potassium currents, an A-type current (I(KA)), a delayed rectifier current (I(KDR)), a calcium-activated potassium current (I(KCa)), and a proton conductance found in invertebrate cells. All of the potassium channels were inhibited by reduced pH. We also included the pH regulatory process to mimic the effect of the sodium-hydrogen exchanger (NHE) described in these cells during hypercapnic stimulation. The model displayed chemosensory behavior (increased spike frequency during acid stimulation), and all three potassium channels participated in the chemosensory response and shaped the temporal characteristics of the response to acid stimulation. pH-dependent inhibition of I(KA) initiated the response to CO(2), but hypercapnic inhibition of I(KDR) and I(KCa) affected the duration of the excitatory response to hypercapnia. The presence or absence of NHE activity altered the chemosensory response over time and demonstrated the inadvisability of effective intracellular pH (pH(i)) regulation in cells designed to act as chemostats for acid-base regulation. The results of the model indicate that multiple channels contribute to CO(2) chemosensitivity, but the primary sensor is probably I(KA). pH(i) may be a sufficient chemosensory stimulus, but it may not be a necessary stimulus: either pH(i) or extracellular pH can be an effective stimuli if chemosensory neurons express appropriate pH-sensitive channels. The lack of pH(i) regulation is a key feature determining the neuronal activity of chemosensory cells over time, and the balanced lack of pH(i) regulation during hypercapnia probably depends on intracellular activation of pH(i) regulation but extracellular inhibition of pH(i) regulation. These general principles are applicable to all CO(2) chemosensory cells in vertebrate and invertebrate neurons.

In Vivo Evidence for Nitric Oxide–Mediated Calcium-Activated Potassium-Channel Activation During Human Endotoxemia

BACKGROUND

During septic shock, the vasoconstrictor response to norepinephrine is seriously blunted. Animal experiments suggest that hyperpolarization of smooth muscle cells by opening of potassium (K) channels underlies this phenomenon. In the present study, we examined whether K-channel blockers and/or nitric oxide (NO) synthase inhibition could restore norepinephrine sensitivity during experimental human endotoxemia.

METHODS AND RESULTS

Volunteers received 2 ng/kg Escherichia coli endotoxin intravenously. Forearm blood flow (FBF) was measured with venous occlusion plethysmography. Infusion of 4 dose steps of norepinephrine into the brachial artery decreased the FBF ratio (ratio of FBF in the experimental arm to FBF in the control arm) to 84 +/- 4%, 70 +/- 4%, 55 +/- 4%, and 38 +/- 4% (mean +/- SEM) of its baseline value. After endotoxin administration, norepinephrine-induced vasoconstriction was attenuated (FBF ratio, 101 +/- 4%, 92 +/- 4%, 83 +/- 6%, and 56 +/- 7%; n = 30; P = 0.0018; pooled data). Intrabrachial infusion of the K-channel blocker tetraethylammonium (TEA) completely restored the vasoconstrictor response to norepinephrine from 104 +/- 5%, 93 +/- 7%, 93 +/- 12%, and 69 +/- 12% to 89 +/- 9%, 73 +/- 4%, 59 +/- 5%, and 46 +/- 8% (n = 6; P = 0.045). Other K-channel blockers did not affect the response to norepinephrine. The NO synthase inhibitor N(G)-monomethyl-l-arginine (L-NMMA; 0.2 mg x min(-1) x dL(-1) intra-arterially) also restored the norepinephrine sensitivity. In the presence of L-NMMA, TEA did not have an additional effect on the norepinephrine-induced vasoconstriction (n = 6; P = 0.9).

CONCLUSIONS

The K-channel blocker TEA restores the attenuated vasoconstrictor response to norepinephrine during experimental human endotoxemia. Coadministration of L-NMMA abolishes this potentiating effect of TEA, suggesting that NO mediates the endotoxin-induced effect on vascular K channels. In the absence of an effect of the selective adenosine triphosphate-dependent K-channel blocker tolbutamide, we conclude that the blunting effect of endotoxin on norepinephrine-induced vasoconstriction is caused by NO-mediated activation of calcium-activated K channels in the vascular wall.

A role for heterocellular coupling and EETs in dilation of rat cremaster arteries

OBJECTIVE

The authors probed endothelium-dependent dilation and endothelial cell Ca2+ handling in myogenically active resistance arteries.

METHODS

First-order arteries were removed from rat cremaster muscles, cannulated, and pressurized (75 mmHg). Vessel diameter and endothelial cell Ca2+ were monitored using confocal microscopy, and arterial ultrastructure was determined using electron microscopy.

RESULTS

Acetylcholine (ACh) stimulated elevations and oscillations in endothelial cell Ca2+, and concentration-dependently dilated arteries with myogenic tone. NO-independent dilation was blocked by 35 mM K+. Combined IK(Ca) (1 microM TRAM-34) and SK(Ca) (100 nM apamin) blockade partially inhibited NO-independent relaxations, with residual relaxations sensitive to BK(Ca) or cytochrome P-450 inhibition (100 nM iberiotoxin, and 20 microM 17-ODYA or 10 microM MS-PPOH). 11,12-EET stimulated iberiotoxin-sensitive dilation, but did not affect endothelial cell Ca2+. 15 mM K+ evoked dilation sensitive to inhibition of K(IR) (30 microM Ba2+) and Na+/K+-ATPase (10 microM ouabain), whereas these blockers did not affect ACh-mediated dilations. Homo- and heterocellular gap junctions were identified in radial sections through arteries.

CONCLUSION

These data suggest that rises in endothelial cell Ca2+ stimulate SK(Ca) and IK(Ca) channels, leading to hyperpolarization and dilation, likely due to electrical coupling. In addition, a component was unmasked following SK(Ca) and IK(Ca) blockade, attributable to activation of BK(Ca) channels by cytochrome P-450 metabolites.

EDHF-mediated rapid restoration of hypotensive response to acetylcholine after chronic, but not acute, nitric oxide synthase inhibition in rats

Several in vitro studies have shown that endothelium-dependent vasodilatation is maintained by endothelium-derived hyperpolarizing factor (EDHF) or prostacyclin in vessels isolated from endothelial nitric oxide synthase knockout mice. Since this has not been addressed by in vivo studies, we sought to define the magnitude and the onset time of this compensation by recording blood pressure responses to endothelium-dependent vasodilators in rats treated acutely or chronically with the NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME). Groups of male Sprague-Dawley rats were given plain water (control) or L-NAME (0.7 mg/ml) in drinking water for 1 day, 5 days, 3 wks or 6 wks. Dose-dependent hypotensive responses to acetylcholine, bradykinin and sodium nitroprusside were determined in anesthetized rats before and after acute intravenous infusion of either L-NAME or a combination of apamin plus charybdotoxin that would selectively inhibit EDHF. Acute L-NAME treatment increased the mean arterial pressure and inhibited acetylcholine- and bradykinin-induced fall in blood pressure in control but not in chronic L-NAME treated rats. The endothelium-dependent hypotensive responses to acetylcholine and bradykinin were restored in rats treated with L-NAME after a time period of 24 h along with increased sensitivity to sodium nitroprusside and reduced plasma nitrate+nitrite levels. While apamin+charybdotoxin pretreatment inhibited the responses to acetylcholine and bradykinin in both acute and chronic L-NAME treated groups, it was more pronounced in the latter group. In conclusion, chronic inhibition of nitric oxide synthase results in the development of a compensatory hypotensive response to acetylcholine within 24 h and this is mediated by EDHF.

The influence of hypotonicity on large-conductance calcium-activated potassium channels in human retinal pigment epithelial cells

The aim of this study was to characterize the effects of hypotonicity on the activity of large-conductance Ca(2+)-activated K+ (BK(Ca)) channels in human retinal pigment epithelial (RPE R-50) cells. Effects of hypotonicity on ion currents were investigated with the aid of the patch-clamp technique. A regulatory volume decrease in response to a hypotonic solution (200 mOsm/L) was observed that could be blunted by paxilline. In whole-cell current recordings, a hypotonic solution (200 mOsm/L) reversibly increased the amplitude of K+ outward currents (I(K)). The increase of I(K) could be reversed by iberiotoxin (200 nM), paxilline (1 microM), or tetrandrine (5 microM), but not by glibenclamide (10 microM), disulphonic acid (DIDS) (100 microM), or dequalinium dichloride (10 microM). In RPE R-50 cells pretreated with thapsigargin, aristolochic acid, or pertussis toxin, the increased amplitude of I(K) in response to hypotonicity was unaltered. In cell-attached patches, an increase in BK(Ca)-channel activity was observed during hypotonicity-induced cell swelling. The enhanced channel activity elicited under this condition was mainly mediated by an increase in the number of long-lived openings. These findings support the evidence for the coupling of volume swelling to the functional activity of BK(Ca) channels.

Effects of capsazepine on human small airway responsiveness unravel a novel class of bronchorelaxants

Capsazepine is known as a transient receptor potential channel vanilloid subfamily 1 (TRPV(1)) antagonist that inhibits bronchoconstriction evoked in animals by TRPV(1) agonists. In this study, effects of capsazepine and chemically related analogues, so called capsazepinoids, were examined in vitro on contractile effects in human small airway preparations. Repeated cycles with 1h of LTD(4)-free physiological saline solution followed by 30min exposure to LTD(4) (10nM) demonstrated that the contractile responsiveness of the preparations exhibited little change over time despite repeated challenges (>12h). Capsazepine (1-100microM) reversibly and concentration-dependently inhibited the contractile response to LTD(4) with EC(50) approximately 10microM and approximately 90% relaxation at 100microM. Capsazepine (10microM) was approximately equally effective to attenuate the contractions evoked by several different inflammatory contractile agonists (LTD(4), PGD(2), histamine), and it relaxed preparations with established tonic contraction due to LTD(4). Higher concentrations of capsazepine were needed to relax ACh-contractions. The effect of capsazepine on LTD(4)-induced contractions was not significantly reduced by pre-treating the preparations with either of propranolol (10microM)+atropine (1microM), L-NAME (1mM), indomethacin (1microM), iberiotoxin (0.1microM), capsaicin (10microM), and nifedipine (10microM). Although the mechanism of action of the present capsazepine-induced bronchorelaxation remains unknown it emerged here that they represent a generally effective principle exerting a functional antagonism against contractile mediators but distinct from beta receptor agonists and inhibitors of L-type calcium channels. The inhibitory effect of capsazepine is shared by chemical analogues, but not with other TRPV(1) antagonists, suggesting the possibility that capsazepine represents a novel class of bronchorelaxants effective in human small airways. These findings were not predicted by previous observations that have concerned quite limited effects of capsazepine on airway tone in different animal test systems. If potency can be further increased and the results translated to in vivo, compounds representing the capsazepinoid class of bronchorelaxants might become useful in the treatment of patients suffering from asthma and COPD.

Mitochondrial potassium channels: from pharmacology to function

Mitochondrial potassium channels, such as ATP-regulated or large conductance Ca2+ -activated and voltage gated channels were implicated in cytoprotective phenomenon in different tissues. Basic effects of these channels activity include changes in mitochondrial matrix volume, mitochondrial respiration and membrane potential, and generation of reactive oxygen species. In this paper, we describe the pharmacological properties of mitochondrial potassium channels and their modulation by channel inhibitors and potassium channel openers. We also discuss potential side effects of these substances.

Membrane potassium currents in human radial artery and their regulation by nitric oxide donor

OBJECTIVE

The human radial artery has demonstrated superior long-term results as a graft in coronary bypass surgery, but undesirable post-surgical spasm limits its clinical application. Few have examined its excitatory properties, especially the underlying ion channel mechanisms. In this study, we investigated the kinetic and pharmacological properties of the smooth muscle membrane potassium currents of this important artery.

METHODS AND RESULTS

Using whole cell patch-clamp techniques, we found the K(+) current to be voltage-dependent and outwardly rectifying. Voltage-dependent inactivation was observed, being half-maximal at +28.0 mV but incomplete even at +40 mV. The K(+) currents were predominantly sensitive to the K(Ca) blocker tetraethylammonium (TEA; 63.9+/-12.1% inhibition, p<0.05), less sensitive to the Kv blocker 4-aminopyridine (4-AP; 32.8+/-4.4% inhibition, p<0.05), and the K(ATP) blocker glibenclamide (28.7+/-8.5% inhibition), at -20 mV testing potential. Resting membrane potential was -52.0+/-6.8 mV (n=5), and suppression of K(+) currents by TEA and iberiotoxin (IbTx) caused membrane depolarization. Western blot analysis with channel-specific antibodies confirmed the presence of K(Ca) and Kv channel proteins. TEA evoked 20.7+/-9.9% of the contractile response to 60 mM KCl, whereas IbTx caused about 10% of the above response at 10(-7) M. The nitric oxide donor SNAP augmented membrane K(+) currents in a concentration-dependent fashion; the augmentation was completely suppressed by TEA, but was relatively insensitive to the guanylate cyclase inhibitor ODQ.

CONCLUSIONS

The radial artery manifests mainly Ca(2+)-dependent K(+) currents at rest; this current is augmented by nitric oxide through a cGMP- and protein kinase G-independent action. The relatively depolarized membrane potential, as well as its muscular structure, predisposes the radial artery to spasm. Agents that activate the Ca(2+)-dependent K(+) current could be of therapeutic value in preventing post-surgical vasospasm.

Paraoxon suppresses Ca2+ spike and afterhyperpolarization in snail neurons: Relevance to the hyperexcitability induction

The effects of organophosphate (OP) paraoxon, active metabolite of parathion, were studied on the Ca(2+) and Ba(2+) spikes and on the excitability of the neuronal soma membranes of land snail (Caucasotachea atrolabiata). Paraoxon (0.3 muM) reversibly decreased the duration and amplitude of Ca(2+) and Ba(2+) spikes. It also reduced the duration and the amplitude of the afterhyperpolarization (AHP) that follows spikes, leading to a significant increase in the frequency of Ca(2+) spikes. Pretreatment with atropine and hexamethonium, selective blockers of muscarinic and nicotinic receptors, respectively, did not prevent the effects of paraoxon on Ca(2+) spikes. Intracellular injection of the calcium chelator BAPTA dramatically decreased the duration and amplitude of AHP and increased the duration and frequency of Ca(2+) spikes. In the presence of BAPTA, paraoxon decreased the duration of the Ca(2+) spikes without affecting their frequency. Apamin, a neurotoxin from bee venom, known to selectively block small conductance of calcium-activated potassium channels (SK), significantly decreased the duration and amplitude of the AHP, an effect that was associated with an increase in spike frequency. In the presence of apamin, bath application of paraoxon reduced the duration of Ca(2+) spike and AHP and increased the firing frequency of nerve cells. In summary, these data suggest that exposure to submicromolar concentration of paraoxon may directly affect membrane excitability. Suppression of Ca(2+) entry during the action potential would down regulate Ca(2+)-activated K(+) channels leading to a reduction of the AHP and an increase in cell firing.

Endothelium-independent relaxation by adrenomedullin in pregnant rat mesenteric artery: role of cAMP-dependent protein kinase A and calcium-activated potassium channels

The mechanisms of relaxation of adrenomedullin were investigated in isolated mesenteric artery from pregnant rats. Adrenomedullin (1 nM-0.3 microM) produced concentration-dependent relaxation of endothelium-denuded mesenteric artery rings precontracted with norepinephrine at a concentration required to produce 70% of maximal response (ED70). The concentration-response curve of adrenomedullin was shifted to the right by adrenomedullin receptor antagonist adrenomedullin(22-52) (10 microM) or calcitonin gene-related peptide(8-37) (1 microM). Inhibition of adenylate cyclase by 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ22536) (10 microM) or protein kinase A [Rp-cyclic adenosine monophosphorothioate (Rp-cAMP); 10 microM] reduced the adrenomedullin-induced relaxation to the same magnitude. Adrenomedullin increased the intracellular cAMP level from 0.38 +/- 0.07 to 2.00 +/- 0.47 pmol/mg tissues, which was completely inhibited by adrenomedullin(22-52) (100 microM). Extracellular high potassium (80 mM), which inactivates the potassium channels, reduced the adrenomedullin-induced relaxation. Blockade of ATP-sensitive, voltage-gated, or inward rectifier potassium channels did not affect the adrenomedullin-induced relaxation. Blockade of calcium-activated K+ channels (KCa) by tetraethylammonium (1 mM) or iberiotoxin (100 nM) inhibited the adrenomedullin-induced relaxation, whereas there was no additional inhibition by SQ22536 or Rp-cAMP when KCa channels were already inhibited. In conclusion, this study provides evidence that cAMP-dependent protein kinase A and KCa channels seem to mediate as the cellular pathways in the adrenomedullin-induced endothelium-independent relaxation of mesenteric artery from pregnant rats.

Single transmembrane domain insulin-like growth factor-II/mannose-6-phosphate receptor regulates central cholinergic function by activating a G-protein-sensitive, protein kinase C-dependent pathway

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor is a single-pass transmembrane glycoprotein that plays an important role in the intracellular trafficking of lysosomal enzymes and endocytosis-mediated degradation of IGF-II. However, its role in signal transduction after IGF-II binding remains unclear. In the present study, we report that IGF-II/M6P receptor in the rat brain is coupled to a G-protein and that its activation by Leu27IGF-II, an analog that binds rather selectively to the IGF-II/M6P receptor, potentiates endogenous acetylcholine release from the rat hippocampal formation. This effect is mediated by a pertussis toxin (PTX)-sensitive GTP-binding protein and is dependent on protein kinase Calpha (PKCalpha)-induced phosphorylation of downstream substrates, myristoylated alanine-rich C kinase substrate, and growth associated protein-43. Additionally, treatment with Leu27IGF-II causes a reduction in whole-cell currents and depolarization of cholinergic basal forebrain neurons. This effect, which is blocked by an antibody against the IGF-II/M6P receptor, is also sensitive to PTX and is mediated via activation of a PKC-dependent pathway. These results together revealed for the first time that the single transmembrane domain IGF-II/M6P receptor expressed in the brain is G-protein coupled and is involved in the regulation of central cholinergic function via the activation of specific intracellular signaling cascades.

Control of spontaneous firing patterns by the selective coupling of calcium currents to calcium-activated potassium currents in striatal cholinergic interneurons

The spontaneous firing patterns of striatal cholinergic interneurons are sculpted by potassium currents that give rise to prominent afterhyperpolarizations (AHPs). Large-conductance calcium-activated potassium (BK) channel currents contribute to action potential (AP) repolarization; small-conductance calcium-activated potassium channel currents generate an apamin-sensitive medium AHP (mAHP) after each AP; and bursts of APs generate long-lasting slow AHPs (sAHPs) attributable to apamin-insensitive currents. Because all these currents are calcium dependent, we conducted voltage- and current-clamp whole-cell recordings while pharmacologically manipulating calcium channels of the plasma membrane and intracellular stores to determine what sources of calcium activate the currents underlying AP repolarization and the AHPs. The Cav2.2 (N-type) blocker omega-conotoxin GVIA (1 microM) was the only blocker that significantly reduced the mAHP, and it induced a transition to rhythmic bursting in one-third of the cells tested. Cav1 (L-type) blockers (10 microM dihydropyridines) were the only ones that significantly reduced the sAHP. When applied to cells induced to burst with apamin, dihydropyridines reduced the sAHPs and abolished bursting. Depletion of intracellular stores with 10 mM caffeine also significantly reduced the sAHP current and reversibly regularized firing. Application of 1 microM omega-conotoxin MVIIC (a Cav2.1/2.2 blocker) broadened APs but had a negligible effect on APs in cells in which BK channels were already blocked by submillimolar tetraethylammonium chloride, indicating that Cav2.1 (Q-type) channels provide the calcium to activate BK channels that repolarize the AP. Thus, calcium currents are selectively coupled to the calcium-dependent potassium currents underlying the AHPs, thereby creating mechanisms for control of the spontaneous firing patterns of these neurons.

Early pharmacological preconditioning by erythropoietin mediated by inducible NOS and mitochondrial ATP-dependent potassium channels in the rat heart

Administration of recombinant human erythropoietin (rhEPO) is known to induce protection against cardiac ischaemia injury improving functional recovery and reducing apoptosis. But the underlying mechanisms are not elucidated. We determined the role of nitric oxide synthases (NOS) as well as ATP-dependent (K(ATP)) and calcium-activated (K(Ca)) potassium channels in the early cardioprotection induced by rhEPO. Wistar male rats were divided into two experimental groups treated by rhEPO (5,000 IU/kg, i.p.) or saline (control group). One hour later, rats were anaesthetized, hearts isolated, retrogradely perfused and submitted to a 30-min no-flow global ischaemia followed by 120 min of reperfusion sequence. Cardiac functional recovery (left ventricular developed pressure, LVDP) was significantly higher in the group treated by rhEPO (LVDP at 30 min reperfusion: 71.7 +/- 2.3 mmHg) compared with the control group (57.4 +/- 5.8 mmHg). We observed the same significant effect on its derivative (dP/dt). The rhEPO-induced improvement in ventricular function was abolished by perfusion prior to ischaemia with either N-nitro-l-arginine methyl ester (l-NAME, a nonspecific NOS inhibitor) or N-(3-(aminomethyl)benzyl)acetamidine (1,400W, a specific inducible NOS inhibitor) or 5-hydroxydecanoic acid (5HD, a mitochondrial K(ATP) channel blocker) but not with paxilline (a K(Ca) channel inhibitor). Thus, in vivo rhEPO administration provides early preconditioning against ischaemic injury in the isolated perfused rat heart that is dependent on iNOS and mitochondrial K(ATP) channels.

Ca2+-activated K+ (KCa) channels are involved in the relaxations elicited by sildenafil in penile resistance arteries

The aim of the present study was to evaluate the role of K+ channels in the vasorelaxant effect of the phosphodiesterase 5 inhibitor, sildenafil, in isolated horse penile resistance arteries mounted in microvascular myographs. In phenylephrine-precontracted arteries, sildenafil elicited potent relaxations which were markedly reduced by raising extracellular K+, by the non-selective blocker of Ca2+-activated K+ channels (KCa), tetraethylammonium and by the blocker of large- and intermediate-conductance KCa channels, charybdotoxin. Sildenafil relaxant responses were also reduced by the selective inhibitor of large conductance KCa (BK(Ca)) channels iberiotoxin, but not by the blocker of small conductance KCa channels apamin. The inhibitor of the cGMP-dependent protein kinase (PKG), Rp-8-Br-PET-cGMPS, reduced the relaxations elicited by sildenafil but combined treatment with iberiotoxin and Rp-8-Br-PET-cGMPS did not further inhibit these relaxations, compared to the effect of either blocker alone. Iberiotoxin also shifted to the right the relaxations elicited by both the NO donor, S-nitrosoacetyl-D,L-penicillamine (SNAP) and the adenylate cyclase activator forskolin; treatment with both iberiotoxin and Rp-8-Br-PET-cGMPS did cause an additional inhibition. The present results demonstrate that the relaxant effect of sildenafil and NO in penile resistance arteries is due in part to activation of BK(Ca) channels through a PKG-dependent mechanism.

Effect of progesterone on calcium activated potassium currents and intracellular calcium in guinea pig colon myocytes

AIMS

To study the effects of progesterone on contractile activity of smooth muscle strips and on ion currents and intracellular Ca2+ ([Ca2+]i) intensity in single colonic myocytes in guinea pig proximal colons.

METHODS

Strips and single cells were dissected from female guinea pig proximal colon. Contraction of strips through an isotonic transducer was assessed and the responsible currents to progesterone were recorded with EPC-9 amplifier in nystatin perforated whole-cell configuration. Detection of [Ca2+]i fluorescence loading fura-2 acetoxymethylester (fura-2/AM) was measured with confocal microscope.

RESULTS

Progesterone significantly inhibited contraction of guinea pig colon strips in a dose-dependent pattern. Inhibitory concentration 50 (IC50) of progesterone in longitudinal strips and circular strips was, respectively, 9.7 microM and 1.0 nM. Iberiotoxin (IbTX) partially blocked inhibition of progesterone in both oriented smooth muscle strips. Ca2+ activated K+ (K(Ca)) channel currents recorded by depolarizing pulse protocol were enhanced by progesterone to 138% +/- 13% (n = 9, p < 0.01), and to 143% +/- 12% (n = 8, p < 0.01) when perfused with 10 mcM onapristone. Progesterone reduced L-Ca2+ currents to 67% +/- 6% (n = 7, p < 0.01) and had no effect with 5 microM nicardipine in bath solution. [Ca2+]i fluorescence was reduced by progesterone to 75% +/- 12% (n = 8, p < 0.01).

CONCLUSION

Progesterone decreases the contraction of colonic smooth muscles by enhancing K(Ca) currents and reducing Ca2+ influx.

Ca2+-dependent K+ channels are targets for bradykinin B1 receptor ligands and for lipopolysaccharide in the rat aorta

Although rat aorta smooth muscle cells in culture constitutively express bradykinin B1 receptors, the normotensive rat aorta does not respond to the bradykinin B1 receptor agonist des-Arg9-bradykinin, whereas vessels from the spontaneously hypertensive rat (SHR) respond to bradykinin B1 receptor agonists with cell membrane hyperpolarization and relaxation. Bacterial lipopolysaccharide also is inactive on the normotensive rat but hyperpolarizes the SHR aorta. To determine whether this could be due to the increased intracellular Ca2+ concentration ([Ca2+]i) in the SHR, we raised [Ca2+]i in normotensive rats by treatment with thapsigargin. In the thapsigargin-treated aorta, both lipopolysaccharide and des-Arg9-bradykinin induced hyperpolarization, which was reversed by the Ca2+-dependent K+ channel inhibitor iberiotoxin and by the bradykinin B1 receptor antagonists Lys-[Leu8]-des-Arg9-bradykinin and [Leu8]-des-Arg9-bradykinin. Thus the bradykinin B1 receptor, as well as lipopolysaccharide, needs activated Ca2+-dependent K+ channels for functional expression. The two bradykinin B1 receptor inhibitors, however, have effects on Ca2+-dependent K+ channels which are not mediated by bradykinin B1 receptors.

IKs blockade reduces dispersion of repolarization in heart failure

BACKGROUND

The slow and rapid (I(Kr)) components of I(K) are major determinants of ventricular repolarization. Unlike I(Kr), which is homogeneously expressed across the transmural wall, I(Ks) expression is reduced in midmyocardial cells and presumably contributes significantly to transmural dispersion of repolarization. Increased dispersion of repolarization during pharmacologic blockade of I(Kr) is proarrhythmic, primarily due to relatively selective prolongation of midmyocardial cell action potential duration (APD). The mechanisms underlying proarrhythmia in heart disease associated with impaired repolarization, such as heart failure, are unknown. We hypothesize that, in contrast to I(Kr) blockade, I(Ks) blockade will have little effect on midmyocardial cells and hence decrease dispersion of repolarization in heart failure.

OBJECTIVES

The purpose of this study was to determine the effect of blockade of the slow component of the delayed rectifier current (I(Ks)) on arrhythmogenic dispersion of repolarization and proarrhythmia in heart failure.

METHODS

Optical action potentials were simultaneously recorded from 256 sites spanning the transmural wall of the arterially perfused canine wedge preparation. Hearts from dogs with heart failure induced by rapid pacing (n = 6) were compared with normals (n = 6).

RESULTS

Baseline dispersion of repolarization, as measured from the range of transmural APD during stimulation at a cycle length of 2,000 ms, was significantly higher in heart failure (75 +/- 24 ms) compared with controls (39 +/- 21 ms, P < .04). I(Ks) blockade with 30 microM chromanol decreased dispersion of repolarization by 40% (P < .02) in heart failure, reducing it to values found in normals. Decreased dispersion of repolarization was due to a larger, relatively selective, drug-induced APD prolongation of epicardial (23%) compared with midmyocardial cells (9%, P < .02). VT could not be induced in failing hearts under conditions of I(Ks) blockade, and no proarrhythmia was observed.

CONCLUSION

I(Ks) blockade significantly reduced heart failure-induced dispersion of repolarization to values seen in nonfailing hearts. By prolonging repolarization without increasing dispersion of repolarization, I(Ks) blockade may have antiarrhythmic effects without creating proarrhythmia.

Quantification and distribution of big conductance Ca2+-activated K+ channels in kidney epithelia

Big conductance Ca2+ activated K+ channels (BK channels) is an abundant channel present in almost all kind of tissue. The accurate quantity and especially the precise distribution of this channel in kidney epithelia are, however, still debated. The aim of the present study has therefore been to examine the presence of BK channels in kidney epithelia and determine the actual number and distribution of these channels. For this purpose, a selective peptidyl ligand for BK channels called iberiotoxin or the radiolabeled double mutant analog 125I-IbTX-D19Y/Y36F has been employed. The presence of BK channels were determined by a isotope flux assay where up to 44% of the total K+ channel activity could be inhibited by iberiotoxin indicating that BK channels are widely present in kidney epithelia. Consistent with these functional studies, 125I-IbTX-D19Y/Y36F binds to membrane vesicles from outer cortex, outer medulla and inner medulla with Bmax values (in fmol/mg protein) of 6.8, 2.6 and 21.4, respectively. These studies were performed applying rabbit kidney epithelia tissue. The distinct distribution of BK channels in both rabbit and rat kidney epithelia was confirmed by autoradiography and immunohistochemical studies. In cortical collecting ducts, BK channels were exclusively located in principal cells while no channels could be found in intercalated cells. The abundant and distinct distribution in kidney epithelia talks in favor for BK channels being important contributors in maintaining salt and water homeostasis.

Homocysteine inhibits store-mediated calcium entry in human endothelial cells: evidence for involvement of membrane potential and actin cytoskeleton

The role of homocysteine for store-operated calcium influx was investigated in human umbilical cord endothelial cell line. Homocysteine significantly decreased thapsigargin-evoked Ca2+ entry, membrane hyperpolarization and actin polymerization. GSH and DTT prevented homocysteine-induced inhibition of thapsigargin-evoked Ca2+ entry, membrane hyperpolarization and actin polymerization; while GSSG had the opposite effect. Homocysteine blocked large conductance Ca2+-activated K+ (BK(Ca)) channels in a concentration-dependent manner and related to the redox status of the endothelial cells. BK(Ca) channels opener NS1619 reversed thapsigargin-evoked Ca2+ entry, membrane hyperpolarization and actin polymerization; BK(Ca) channels inhibitor iberiotoxin had the opposite effect. The findings suggest that homocysteine is involved in store-regulated Ca2+ entry through membrane potential-dependent and actin cytoskeleton-dependent mechanisms, redox status of homocysteine and BK(Ca) channels may play a regulatory role in it.

Pharmacological evidence for the activation of Ca2+-activated K+ channels by meloxicam in the formalin test

The possible participation of K+ channels in the antinociceptive action of meloxicam was assessed in the 1% formalin test. Local peripheral administration of meloxicam produced a dose-dependent antinociception only during the second phase of the formalin test. K+ channel blockers alone did not modify formalin-induced nociceptive behavior. However, local peripheral pretreatment of the paw with charybdotoxin and apamin (large- and small-conductance Ca2+-activated K+ channel inhibitors, respectively), 4-aminopyridine and tetraethylammonium (non-selective voltage-dependent K+ channel inhibitors), but not glibenclamide or tolbutamide (ATP-sensitive K+ channel inhibitors), dose-dependently prevented meloxicam-induced antinociception. It is concluded that meloxicam could open large- and small-conductance Ca2+-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 await further confirmation.