Differences in Gating Dynamics of BK Channels in Cellular and Mitochondrial Membranes from Human Glioblastoma Cells Unraveled by Short- and Long-Range Correlations Analysis

The large-conductance voltage- and Ca2+-activated K+ channels (BK) are encoded in humans by the Kcnma1 gene. Nevertheless, BK channel isoforms in different locations can exhibit functional heterogeneity mainly due to the alternative splicing during the Kcnma1 gene transcription. Here, we would like to examine the existence of dynamic diversity of BK channels from the inner mitochondrial and cellular membrane from human glioblastoma (U-87 MG). Not only the standard characteristics of the spontaneous switching between the functional states of the channel is discussed, but we put a special emphasis on the presence and strength of correlations within the signal describing the single-channel activity. The considered short- and long-range memory effects are here analyzed as they can be interpreted in terms of the complexity of the switching mechanism between stable conformational states of the channel. We calculate the dependencies of mean dwell-times of (conducting/non-conducting) states on the duration of the previous state, Hurst exponents by the rescaled range R/S method and detrended fluctuation analysis (DFA), and use the multifractal extension of the DFA (MFDFA) for the series describing single-channel activity. The obtained results unraveled statistically significant diversity in gating machinery between the mitochondrial and cellular BK channels.

Pharmacology of hSlo3 channels and their contribution in the capacitation-associated hyperpolarization of human sperm

Slo3 channels (mSlo3) primarily mediate mouse sperm K(+) currents and are essential for the capacitation-associated hyperpolarization (CAH). Whether Slo3 and/or Slo1, two Slo family K(+) channels are functionally expressed in human sperm is controversial. Our recent pharmacological studies of the human sperm CAH suggested the participation of both. Lack of a detailed pharmacology of heterologously expressed human Slo3 (hSlo3) prevented precisely identifying the K(+) channel(s) involved. In the present report, we compare the pharmacological profile of expressed hSlo3 in CHO cells with that of the CAH to advance this matter. Whole-cell patch-clamp recordings showed that hSlo3 currents are inhibited: significantly by progesterone, Ba(2+) and quinidine; partially by Penitrem A and Charybdotoxin; and poorly by Iberiotoxin and Slotoxin. Surprisingly, hSlo3 currents were resistant to Clofilium and 60 mM TEA(+) which inhibit mSlo3. Pharmacological comparison of the CAH and hSlo3 profiles indicates in addition to hSlo3, other K(+) channels, possibly Slo1, may participate in CAH. The pharmacological profile of heterologously expressed hSlo3 channels differs from that of mSlo3 K(+) channels, consistent with species-specific differences observed among other sperm ion channels. While the pharmacological correlation analysis of the hSlo3 currents and the CAH confirmed the participation of hSlo3 channels, it suggests that additional K(+) channels may be involved, in particular Slo1 channels.

[Impact of BKCa channel in prostate smooth muscle cells on the membrane potential in rats with chronic abacterial prostatitis]

OBJECTIVE

To investigate the impact of the BKCa channel in prostate smooth muscle cells (PSMCs) on the membrane potential in SD rats with chronic abacterial prostatitis (CAP).

METHODS

CAP models were established in 20 SD rats by castration and injection of 17 beta-estrogen, and another 20 were taken as normal controls. PSMCs were cultured and purified in vitro, and treated with DiBAC4, followed by quantitative observations on the dynamic changes of the cell membrane potential by laser confocal microscopy.

RESULTS

The extracellular calcium ion concentration ([Ca2+]o) was increased and the BKCa channel was activated, which induced the hyperpolarization of the PSMC membrane in both the CAP models and normal control rats. This effect was weakened with Iberiotoxin (IbTX), a specific blocker of the BKCa channel, but the amplitude of the hyperpolarization was obviously lower in the CAP than in the control group. The DiBAC4 fluorescence intensity induced by hyperpolarization was 18.78 +/- 2.92 in the former and 38.85 +/- 7.10 in the latter (P < 0.05), while that induced by IbTX was 1.61 +/- 0.46 and 6.12 +/- 1.32 (P < 0.05), respectively.

CONCLUSION

Significant decrease of BKCa-mediated hyperpolarization in the CAP model can reduce its abilities of regulating the membrane potential and suppressing the excessive contraction of PSMCs, which may result in pelvic pain syndrome and lower urinary tract symptoms.

Vasodilation of retinal arterioles induced by activation of BKCa channels is attenuated in diabetic rats

The large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels modulate the retinal vascular tone, but question of whether the impairment of the channel function contributes to abnormalities of retinal circulation has not yet been completely elucidated. The purpose of this study was to examine effects of diabetes on the vasodilation induced by activation of BK(Ca) channels. Male Wistar rats were treated with streptozotocin and experiments were performed 2 weeks later. The streptozotocin-treated animals were given drinking water containing 5% d-glucose to shorten the term in the development of retinal vascular dysfunction. The retinal vascular responses were assessed by measuring diameter of retinal arterioles in the fundus images that were captured with an original fundus camera system. In non-diabetic rats, vasodilator effects of acetylcholine on retinal arterioles were significantly reduced by iberiotoxin, an inhibitor of BK(Ca) channels. However, the inhibitory effect of iberiotoxin was not observed in diabetic rats, and the responses to the BK(Ca) channel opener BMS-191011 were almost completely abolished. The retinal vasodilator response to acetylcholine, possibly an endothelium-derived hyperpolarizing factor-mediated response, observed after treatment with N(G)-nitro-l-arginine methyl ester and indomethacin was markedly reduced in diabetic rats. The responses to pinacidil, an opener of ATP-sensitive K(+) channels, were unchanged. These results suggest that the retinal vasodilator response mediated through mechanisms involving activation of BK(Ca) channels is diminished at the early stage of diabetes in rats. The impairment of BK(Ca) channel function may contribute to abnormal retinal hemodynamics in diabetes and consequently play an important role in the pathogenesis of diabetic retinopathy.

Movements of individual BKCa channels in live cell membrane monitored by site-specific labeling using quantum dots

The movements of BK(Ca) channels were investigated in live cells using quantum dots (QDs). The extracellular N-terminus was metabolically tagged with biotin, labeled with streptavidin-conjugated QDs and then monitored using real-time time-lapse imaging in COS-7 cells and cultured neurons. By tracking hundreds of channels, we were able to determine the characteristics of channel movements quantitatively. Channels in COS-7 cells exhibited a confined diffusion in an area of 1.915 μm(2), with an initial diffusion coefficient of 0.033 μm(2)/s. In neurons, the channel movements were more heterogeneous and highly dependent on subcellular location. While the channels in soma diffused slowly without clear confinement, axodendritic channels showed more rapid and pseudo-one-dimensional movements. Intriguingly, the channel movement in somata was drastically increased by the neuronal β4 subunit, in contrast to the channels in the axodendritic area where the mobility were significantly decreased. Thus, our results demonstrate that the membrane mobility of BK(Ca) channels can be greatly influenced by the expression system used, subunit composition, and subcellular location. This QD-based, single-molecule tracking technique can be utilized to investigate the cellular mechanisms that determine the mobility as well as the localization of various membrane proteins in live cells.

[Changes of BKCa during aging and its correlation with blood pressure]

AIM

To investigate the changes of large-conductance calcium-activated potassium channels (BKCa, MaxiK) during aging and relations between the changes and blood pressure.

METHODS

Male spontaneously hypertensive rats (SHR) aged 9, 15, 21, 27, 33 weeks (the number of each weeks SHR was 4) were selected as hypertension group rats, corresponding gender, weeks and number Wistar-Kyoto rats (WKY) as control group rats. Blood pressure of abdominalis aorta of each weeks SHR and WKY were measured by BL-420F experimental system of biological function. The arteria mesenteric minor (AMM) were isolated in blunt dissection method. The vascular smooth muscle cells (VSMCs) of AMM were isolated with prolease. The potassium current, the current after BKCa were blockaded by Tetraethylammonium (TEA) and the capacitance of membrane (Cm) of VSMCs of AMM were recorded with using whole cell patch clamp, and calculated the BKCa current and the BKCa current density. Probe the correlation of the changes of BKCa current density with MABP during aging.

RESULTS

The potassium current density and BKCa current density of VSMCs of AMM of SHR were decreasing during aging, however, the changes of WKY had no statistically significance (P > 0.05). The BKCa current density was extremely correlative with MABP in SH R (the values of r were -0.7174), in WKY, the BKCa current density was correlative with MAB P r = -0.4832.

CONCLUSION

BKCa current and current density attenuate with aging, the level of blood pressure is response of the attenuated degree. The BKCa current density is extremely correlative with the blood pressure.

Modulation of the conductance-voltage relationship of the BK(Ca) channel by shortening the cytosolic loop connecting two RCK domains

Calcium-dependent gating of large-conductance calcium-activated potassium (BK(Ca)) channels is mediated by the intracellular carboxyl terminus, which contains two domains of regulator of K(+) conductance (RCK). In mammalian BK(Ca) channels, the two RCK domains are separated by a protein segment of 101 residues that is poorly conserved in evolution and predicted to have no regular secondary structures. We investigated the functional importance of this loop using a series of deletion mutations. We found that the length, rather than the specific sequence at the central region of the segment, is critical for the functionality of the channel. As the length of the loop is progressively shorted, the conductance-voltage relationship gradually shifts toward more positive voltages with a minimum length of 70 amino acids, in an apparent response to increased tension within the loop. Thus, the functional activity of the BK(Ca) channel can be modulated by altering the tension of this loop region.

Calcium ions regulate K⁺ uptake into brain mitochondria: the evidence for a novel potassium channel

The mitochondrial response to changes of cytosolic calcium concentration has a strong impact on neuronal cell metabolism and viability. We observed that Ca(2+) additions to isolated rat brain mitochondria induced in potassium ion containing media a mitochondrial membrane potential depolarization and an accompanying increase of mitochondrial respiration. These Ca(2+) effects can be blocked by iberiotoxin and charybdotoxin, well known inhibitors of large conductance potassium channel (BK(Ca) channel). Furthermore, NS1619 - a BK(Ca) channel opener - induced potassium ion-specific effects on brain mitochondria similar to those induced by Ca(2+). These findings suggest the presence of a calcium-activated, large conductance potassium channel (sensitive to charybdotoxin and NS1619), which was confirmed by reconstitution of the mitochondrial inner membrane into planar lipid bilayers. The conductance of the reconstituted channel was 265 pS under gradient (50/450 mM KCl) conditions. Its reversal potential was equal to 50 mV, which proved that the examined channel was cation-selective. We also observed immunoreactivity of anti-beta(4) subunit (of the BK(Ca) channel) antibodies with ~26 kDa proteins of rat brain mitochondria. Immunohistochemical analysis confirmed the predominant occurrence of beta(4) subunit in neuronal mitochondria. We hypothesize that the mitochondrial BK(Ca) channel represents a calcium sensor, which can contribute to neuronal signal transduction and survival.

trans-Arachidonic acids induce a heme oxygenase-dependent vasorelaxation of cerebral microvasculature

Nitrative stress is an important regulator of vascular tone. We have recently described that trans-arachidonic acids (TAA) are major products of NO(2)(.)-mediated isomerization of arachidonic acid in cell membranes and that nitrative stress increases TAA levels leading to neural microvascular degeneration. In the present study, we explored whether TAA exert acute effects on neuromicrovascular tone and investigated potential mechanisms thereof. TAA induced an endothelium-dependent vasorelaxation of rat brain pial microvasculature. This vasorelaxation was independent of nitric oxide, prostanoids, lipoxygenase products, and CYP(450) metabolite trans-hydroxyeicosatetraenoic acids. However, inhibition of heme oxygenase (using zinc protoporphyrin IX) and of dependent soluble guanylate cyclase (sGC; using ODQ) significantly diminished (by approximately 70%) the TAA-induced vasorelaxation. Consistent with these findings, TAA stimulated heme oxygenase (HO)-2-dependent bilirubin (using siRNA HO-2) and cGMP formation, and the HO product carbon monoxide (using CO-releasing CORM-2) reproduced the sGC-dependent cGMP formation and vasorelaxation. Further exploration revealed that TAA-induced vasorelaxation and bilirubin formation (HO activation) were nearly abrogated by large-conductance calcium-dependent potassium channels (BK(Ca)) (using TEA and iberiotoxin). Opening of BK(Ca) with the selective activator NS1619 induced a concentration-dependent vasorelaxation, which was inhibited by HO and sGC inhibitors. Coimmunoprecipitation suggested a molecular complex interaction between BK(Ca) and HO-2 (but not HO-1). Collectively, these findings identify new properties of TAA, specifically cerebral vasorelaxation through interactive activation of BK(Ca) with HO-2 and, in turn, sGC. Our findings provide new insights into the characterization of nitrative stress-derived TAA products, by showing they can act as acute mediators of nitrative stress on neurovascular tone.

Increased spontaneous tone in renal arteries of spontaneously hypertensive rats

The spontaneous tone of vascular smooth muscle is augmented in hypertension. The present study examined the role of nitric oxide (NO), cyclooxygenase (COX), thromboxane A2/prostanoid (TP) and PGE2/prostanoid (EP-1) receptors, reactive oxygen species, and large-conductance Ca2+-activated K+ (BKCa) channels in the regulation of spontaneous tone in renal arteries of young and mature Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Rings of arteries, with and without endothelium, were suspended in a myograph for isometric force recording. Spontaneous tone (increase above initial tension) was observed only in arteries of mature SHR and was greater in arteries without endothelium. Nω-nitro-l-arginine methyl ester (l-NAME, an inhibitor of NO synthases) induced larger contractions in arteries of SHR than WKY. Indomethacin (a COX inhibitor), SC-19220 (an EP-1 receptor antagonist), and terutroban (a TP receptor antagonist) reduced the l-NAME-evoked contractions. Tiron (a superoxide anion scavenger), catalase (an enzyme that degrades H2O2), and deferoxamine (a hydroxyl radical scavenger) augmented the l-NAME-induced contractions in arteries of mature SHR. Charybdotoxin (a BKCa channel blocker) caused contractions in arteries of mature SHR without endothelium and in arteries with endothelium incubated with l-NAME. A decreased protein level of endothelial NO synthase, an increased release of prostacyclin, and an increased expression of EP-1 receptors were observed in arteries of mature SHR. The present study suggests that spontaneous tone is precipitated by age and hypertension. The reduced production of NO, leading to decreased activation of BKCa channels, may leave the actions of endogenous vasoconstrictors unopposed. COX products that activate EP-1 and TP receptors are involved in the development of spontaneous tone.

Hypersensitivity of BK Ca to Ca 2+ Sparks Underlies Hyporeactivity of Arterial Smooth Muscle in Shock

Large conductance Ca 2+ -activated K + channels (BK Ca ) play a critical role in blood pressure regulation by tuning the vascular smooth muscle tone, and hyposensitivity of BK Ca to Ca 2+ sparks resulting from its altered β1 subunit stoichiometry underlies vasoconstriction in animal models of hypertension. Here we demonstrate hypersensitivity of BK Ca to Ca 2+ sparks that contributes to hypotension and blunted vasoreactivity in acute hemorrhagic shock. In arterial smooth muscle cells under voltage-clamp conditions (0 mV), the amplitude and duration, but not the frequency, of spontaneous transient outward currents of BK Ca origin were markedly enhanced in hemorrhagic shock, resulting in a 265% greater hyperpolarizing current. Concomitantly, subsurface Ca 2+ spark frequency was either unaltered (at 0 mV) or decreased in hyperpolarized resting cells. Examining the relationship between spark and spontaneous transient outward current amplitudes revealed a hypersensitive BK Ca activity to Ca 2+ spark in hemorrhagic shock, whereas the spark–spontaneous transient outward current coupling fidelity was near unity in both groups. Importantly, we found an acute upregulation of the β1 subunit of the channel, and single-channel recording substantiated BK Ca hypersensitivity at micromolar Ca 2+ , which promotes the α and β1 subunit interaction. Treatment of shock animals with the BK Ca inhibitors iberiotoxin and charybdotoxin partially restored vascular membrane potential and vasoreactivity to norepinephrine and blood reinfusion. Thus, the results underscore a dynamic regulation of the BK Ca –Ca 2+ spark coupling and its therapeutic potential in hemorrhagic shock–associated vascular disorders.

Potent stimulation of large-conductance Ca2+-activated K+ channels by rottlerin, an inhibitor of protein kinase C-delta, in pituitary tumor (GH3) cells and in cortical neuronal (HCN-1A) cells

The effects of rottlerin, a known inhibitor of protein kinase C-delta activation, on ion currents were investigated in pituitary tumor (GH3) cells. Rottlerin (0.3-100 microM) increased the amplitude of Ca2+-activated K+ current (I K(Ca)) in a concentration-dependent manner with an EC50 value of 1.7 microM. In intracellular perfusion with rottlerin (1 microM) or staurosporine (10 microM), phorbol 12-myristate 13-acetate-induced inhibition of I K(Ca) in these cells was abolished. In cell-attached mode, rottlerin applied on the extracellular side of the membrane caused activation of large-conductance Ca2+-activated K+ (BK(Ca)) channels, and a further application of BAPTA-AM (10 microM) to the bath had no effect on rottlerin-stimulated channel activity. When cells were exposed to rottlerin, the activation curve of these channels was shifted to less positive potential with no change in the slope factor. Rottlerin increased BK(Ca)-channel activity in outside-out patches. Its change in kinetic behavior of BK(Ca) channels is primarily due to an increase in mean open time. With the aid of minimal kinetic scheme, a quantitative description of rottlerin stimulation on BK(Ca) channels in GH3 cells was also provided. Under current-clamp configuration, rottlerin (1 microM) decreased the firing of action potentials. I K(Ca) elicited by simulated action potential waveforms was enhanced by this compound. In human cortical HCN-1A cells, rottlerin (1 microM) could also interact with the BK(Ca) channel to stimulate I K(Ca). Therefore, rottlerin may directly activate BK(Ca) channels in neurons or endocrine cells.

Matrix metalloproteinase 2-induced venous dilation via hyperpolarization and activation of K+ channels: relevance to varicose vein formation

BACKGROUND

Varicose veins are a common disorder of extensive venous dilation and remodeling with an as-yet unclear mechanism. Studies have shown increased plasma and tissue levels of matrix metalloproteinases (MMPs) in human varicose veins and animal models of venous hypertension. Although the effects of MMPs are generally attributed to extracellular matrix degradation, their effects on the mechanisms of venous contraction/relaxation are unclear. Our preliminary experiments have demonstrated that MMP-2 causes inhibition of phenylephrine-induced venous contraction. The purpose of this study was to determine whether MMP-induced inhibition of venous contraction involves an endothelium-dependent and/or -independent pathway.

METHODS

Circular segments of the inferior vena cava (IVC) were isolated from male Sprague-Dawley rats and suspended between two wire hooks in a tissue bath, and the effects of MMP-2 on phenylephrine- and KCl-induced contraction were measured. To study the role of endothelium-derived vasodilators, experiments were performed in the presence and absence of endothelium; N(G)-l-nitro-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthesis; indomethacin, an inhibitor of prostacyclin synthesis; cromakalim, an activator of adenosine triphosphate-sensitive K+ channels (K(ATP)); and iberiotoxin, a blocker of large-conductance Ca2+-dependent K+ channels (BK(Ca)) and smooth muscle hyperpolarization.

RESULTS

In endothelium-intact IVC segments, phenylephrine (10(-5) mol/L) caused significant contraction that slowly declined to 82.0% in 30 minutes. The addition of MMP-2 (1 microg/mL) caused a gradual decrease of phenylephrine contraction to 39.5% at 30 minutes. In endothelium-denuded IVC, MMP-2 induced a greater reduction of phenylephrine contraction, to 7.6%. In the presence of L-NAME (10(-4) mol/L), MMP-2 caused a marked decrease in phenylephrine contraction, to 4.4%. Large MMP-2-induced inhibition of phenylephrine contraction was also observed in IVC treated with L-NAME plus indomethacin. MMP-2 caused relaxation of phenylephrine contraction in IVC pretreated with cromakalim (10(-7) mol/L), an activator of K(ATP) channels. MMP-2-induced inhibition of phenylephrine contraction was abrogated in the presence of iberiotoxin (10(-8) mol/L), a blocker of BK(Ca). MMP-2 did not inhibit venous contraction during membrane depolarization by 96 mmol/L KCl, a condition that prevents outward K+ conductance and cell hyperpolarization.

CONCLUSIONS

MMP-2 causes significant IVC relaxation that is potentiated in the absence of endothelium or during blockade of endothelium-mediated nitric oxide and prostacyclin synthesis. The lack of effects of MMP-2 on KCl contraction and in iberiotoxin-treated veins suggests MMP-2-induced smooth muscle hyperpolarization and activation of BK(Ca) channels--a novel effect of MMP that may play a role in the early stages of venous dilation and varicose vein formation.

Protein phosphatase 2A and Ca2+-activated K+ channels contribute to 11,12-epoxyeicosatrienoic acid analog mediated mesenteric arterial relaxation

Epoxyeicosatrienoic acids (EETs) are considered to be endothelium-derived hyperpolarizing factors, and are potent activators of the large-conductance, Ca(2+)-activated K(+) (BK(Ca)) channel in vascular smooth muscle. Here, we investigate the signal transduction pathway involved in the activation of BK(Ca) channels by 11,12-EET and 11,12-EET stable analogs in rat mesenteric vascular smooth muscle cells. 11,12-EET and the 11,12-EET analogs, 11-nonyloxy-undec-8(Z)-enoic acid (11,12-ether-EET-8-ZE), 11-(9-hydroxy-nonyloxy)-undec-8(Z)-enoic acid (11,12-ether-EET-8-ZE-OH) and 11,12-trans-oxidoeicosa-8(Z)-enoic acid (11,12-tetra-EET-8-ZE), caused vasorelaxation of mesenteric resistance arteries. Mesenteric myocyte whole-cell (perforated-patch) currents were substantially (approximately 150%) increased by 11,12-EET and 11,12-EET analogs. Single-channel recordings were conducted to identify the target for 11,12-EET. 11,12-EET and 11,12-EET analogs also increased mesenteric myocyte BK(Ca) channel activity in cell-attached patches. Similar results were obtained in cell-free patches. Baseline mesenteric myocyte BK(Ca) channel activity (NPo) in cell-free patches averaged less than 0.001 at +50 mV and 11,12-EET (1 micromol/L) increased NPo to 0.03+/-0.02 and 11,12-EET analogs (1 micromol/L) increased NPo to 0.09+/-0.006. Inhibition of protein phosphatase 2A (PP2A) activity with okadaic acid (10 nmol/L) completely reversed 11,12-EET stimulated BK(Ca) channel activity and greatly attenuated 11,12-ether-EET-8-ZE mesenteric resistance artery vasorelaxation. 11,12-EET and 11,12-EET analogs increased mesenteric myocyte PP2A activity by 3.5-fold. Okadaic acid and the EET inhibitor, 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE) inhibited the 11,12-EET mediated increase in PP2A activity. These findings provide initial evidence that PP2A activity contributes to 11,12-EET and 11,12-EET analog activation of mesenteric resistant artery BK(Ca) channels and vasorelaxation.

[Effect of exercise stress on cigarette smoking induced downregulation of BKca and Kv1.5 expression in pulmonary arterial smooth muscle cells of rats]

OBJECTIVE

To investigate the effect of exercise stress on chronic cigarette smoking induced downregulation of large conductance calcium-activated potassium channel (BKca) and voltage-dependent delayed rectifier potassium channel (Kv1.5) expression in pulmonary arterial smooth muscle cells of rats.

METHODS

Rats were divided into three groups: the normal control group, the smoking control group and the smoking + exercise group. The plasma cortisol level, the potassium channel expression and the pathological changes in lung tissue were determined with HE staining, the immunohistochemistry and the in-situ hybridization.

RESULTS

(1) In the smoking + exercise group, the plasma cortisol level was determined immediately after exercise [(1528.7 +/- 469.7) ng/L] and was higher than that determined before exercise [(672.4 +/- 235.7) ng/L] (P < 0.01); (2) The HE staining showed that the chronic pulmonary inflammatory response in the smoking control group was severe while it was mild in the smoking + exercise group; (3) The mRNA and protein expression (OD value) of BKca in the smoking control group (mRNA: 0.2206 +/- 0.0415 for big artery and 0.3935 +/- 0.1378 for small artery; protein: 0.2634 +/- 0.1219 for big artery and 0.0995 +/- 0.0851 for small artery) were less than those in the normal control group. The mRNA expression of BKca in the smoking + exercise group (OD value) (0.5022 +/- 0.1134 for big artery and 0.6408 +/- 0.2135 for small artery) was higher than that in the smoking control group; (4) The mRNA and protein expression of Kv1.5 in the smoking control group (OD value) (mRNA: 0.9354 +/- 0.3290 for big artery and 0.5012 +/- 0.1170 for small artery; protein: 1.1112 +/- 0.3310 for big artery and 0.4736 +/- 0.1250 for small artery) were less than those in the normal control group. The protein expression of Kv1.5 in the smoking + exercise group (0.7445 +/- 0.2690) in small artery was higher than that in the smoking control group.

CONCLUSION

Proper exercise stress can decrease inhibition effect of the chronic smoking on the expression of potassium channel BKca and Kv1.5, which perhaps partly results from exercise induced increase of cortisol secretion.

KMUP-1 activates BKCa channels in basilar artery myocytes via cyclic nucleotide-dependent protein kinases

This study investigated whether KMUP-1, a synthetic xanthine-based derivative, augments the delayed-rectifier potassium (K(DR))- or large-conductance Ca2+-activated potassium (BKCa) channel activity in rat basilar arteries through protein kinase-dependent and -independent mechanisms. Cerebral smooth muscle cells were enzymatically dissociated from rat basilar arteries. Conventional whole cell, perforated and inside-out patch-clamp electrophysiology was used to monitor K+- and Ca2+ channel activities. KMUP-1 (1 microM) had no effect on the K(DR) current but dramatically enhanced BKCa channel activity. This increased BKCa current activity was abolished by charybdotoxin (100 nM) and iberiotoxin (100 nM). Like KMUP-1, the membrane-permeable analogs of cGMP (8-Br-cGMP) and cAMP (8-Br-cAMP) enhanced the BKCa current. BKCa current activation by KMUP-1 was markedly inhibited by a soluble guanylate cyclase inhibitor (ODQ 10 microM), an adenylate cyclase inhibitor (SQ 22536 10 microM), competitive antagonists of cGMP and cAMP (Rp-cGMP, 100 microM and Rp-cAMP, 100 microM), and cGMP- and cAMP-dependent protein kinase inhibitors (KT5823, 300 nM and KT5720, 300 nM). Voltage-dependent L-type Ca2+ current was significantly suppressed by KMUP-1 (1 microM), and nearly abolished by a calcium channel blocker (nifedipine, 1 microM). In conclusion, KMUP-1 stimulates BKCa currents by enhancing the activity of cGMP-dependent protein kinase, and in part this is due to increasing cAMP-dependent protein kinase. Physiologically, this activation would result in the closure of voltage-dependent calcium channels and the relaxation of cerebral arteries.

Hypoxia modulates cyclic AMP activation of BkCa channels in rat pulmonary arterial smooth muscle

The signal transduction mechanisms defining the role of cyclic nucleotides in the regulation of potassium channel activity in pulmonary vascular smooth muscle are currently an area of great interest. Normally, signaling mechanisms that elevate cyclic AMP (cAMP) open potassium channels. Modulation of the large-conductance, calcium- and voltage-activated potassium (BKCa) channel is important in the regulation of pulmonary arterial pressure, and inhibition (closing) of the BKCa channel has been implicated in the development of pulmonary vasoconstriction. Accordingly, studies were done to determine the effect of cAMP-elevating agents on BKCa channel activity under normoxic and hypoxic conditions using patch-clamp studies in pulmonary arterial smooth muscle cells (PASMC) of the fawn-hooded rat (FHR). Forskolin (10 microM; n = 4), a stimulator of adenylate cyclase and an activator of cAMP-dependent protein kinase (PKA), and CPT-cAMP (100 microM; n = 3), a membrane-permeable derivative of cAMP, opened BKCa channels in single FHR PASMC under normoxic conditions. Exposure of FHRs to 4 weeks of 10% O2 (hypoxia) significantly attenuated the effect of both forskolin (n = 7) and CPT-cAMP (n = 14) on BKCa channel activity in PASMC. These results suggest that this phenomenon may serve as a physiological mechanism to cause hypoxic vasoconstriction in the pulmonary circulation via modulation of BKCa channels.

Involvement of BK α subunit tyrosine phosphorylation in vascular hyporesponsiveness of superior mesenteric artery following hemorrhagic shock in rats

OBJECTIVE

Vascular hyporesponsiveness is a major complication following severe trauma and shock. It plays important roles in the development of shock and seriously interferes with the treatment of shock. The mechanism responsible for the occurrence of vascular hyporesponsiveness has not been fully understood. The purpose of this study was to determine whether the alpha subunit tyrosine sites of large conductance calcium-activated potassium channel (BK(Ca)) could be phosphorylated and whether the phosphorylation of BK(Ca) was closely associated with the activation of BK(Ca) and the development of vascular hyporesponsiveness following hemorrhagic shock in rats.

METHODS

A hemorrhagic shock (30 mm Hg for 0.5, 2, 4 h) model of Wistar rats was established. Phosphorylation of tyrosine residues of the BK(Ca) alpha subunit from vascular smooth muscle cells (VSMC) in superior mesenteric arteries (SMA) was detected by immunoprecipitation and Western blotting. BK(Ca) activity was evaluated by cell-attached patch clamping. The vascular responsiveness of SMA to norepinephrine was measured with an isolated organ perfusion system.

RESULTS

The level of BK(Ca) alpha subunit tyrosine phosphorylation was increased in a time-dependent manner following hemorrhagic shock, which was mediated by protein tyrosine kinases (PTK) and protein tyrosine phosphatases (PTP). The activation of VSMC BK(Ca) following hemorrhagic shock was inhibited by genistein (2x10(-5) mol/L), the permeable isoflavone PTK inhibitor, and was potentiated by the PTP inhibitor sodium orthovanadate (Na(3)VO(4), 10(-3) mol/L). The decreased vasoresponsiveness following hemorrhagic shock was partly restored by genistein (10(-5) mol/L) or by the BK(Ca)-selective inhibitor tetrabutylammonium chloride (0.1 mmol/L), while it was further decreased by Na(3)VO(4) (10(-5) mol/L).

CONCLUSION

The tyrosine residues of BK(Ca) alpha subunit of SMA were phosphorylated following hemorrhagic shock, which was regulated by PTK and PTP and appeared to be related to the activation of BK(Ca) and the development of vascular hyporesponsiveness following hemorrhagic shock.

Membrane proteins involved in potassium shifts during muscle activity and fatigue

Muscle activity is associated with potassium displacements, which may cause fatigue. It was reported previously that the density of the large-conductance Ca2+-dependent K+ (BK(Ca)) channel is higher in the T tubule membrane than in the sarcolemmal membrane and that the opposite is the case for the ATP-sensitive K+ (K(ATP)) channel. In the present experiments, we investigated the subcellular localizations of the strong inward rectifier 2.1 K+ (Kir2.1) channel and the Na+-K+-2Cl- (NKCC)1 cotransporter with Western blot analysis of different muscle fractions. Furthermore, muscle function was studied while trying to manipulate the opening probability or transport capacity of these proteins during electrical stimulation of isolated soleus muscles. All experiments were made with excised muscle from male Wistar rats. Kir2.1 channels were almost undetectable in the sarcolemmal membrane but present in the T tubule membrane, whereas NKCC1 cotransporters were present in the sarcolemmal membrane. For muscles incubated in a buffer containing pinacidil, NS1619, Ba2+, or bumetanide, there was a faster reduction in peak force (P < 0.05). Furthermore, bumetanide incubation reduced the peak force at the onset of electrical stimulation (P < 0.05). Thus the effects on muscle force indicate that these drugs can affect K+-transporting proteins and thereby influence K+ accumulation, especially in the T tubules, suggesting that K(ATP) and BK(Ca) channels are responsible for K+ release and decrease in force during repeated muscle contractions, whereas Kir2.1 and NKCC1 may have a role in K+ reuptake.

Ca2+-independent activation of BKCa channels at negative potentials in mammalian inner hair cells

The defining characteristic of large-conductance Ca(2)(+)- and voltage-activated K(+) channels (BK(Ca)) is their allosteric activation by two distinct stimuli, membrane depolarization and cytosolic Ca(2)(+) ions. In this allosteric gating, increasing cytosolic Ca(2)(+) concentration ([Ca(2)(+)](i)) shifts the depolarization required for channel opening into the physiological voltage range. In fact, according to present knowledge, elevation of [Ca(2)(+)](i) to micromolar levels is the only means to activate BK(Ca) at membrane potentials below 0 mV. We recorded BK(Ca)-mediated currents from auditory inner hair cells (IHCs) in acutely isolated organs of Corti using the patch-clamp technique in whole-cell and excised patch configuration. In inside-out and outside-out patches, activation of BK(Ca) channels from IHCs showed the prototypic sensitivity to increased [Ca(2)(+)](i). However, channel activation at 0 [Ca(2)(+)](i) occurred at unusually negative potentials (half-maximal activation (V(h)) around 0 mV), indicating that a large fraction of the channels can be activated at physiological voltages without elevated [Ca(2)(+)](i). In intact IHCs, the activation curve of BK(Ca) currents recorded in whole-cell configuration exhibited a V(h) of -42 mV together with a high voltage dependence (slope factor of 10 mV) and submillisecond onset of current. Surprisingly, this activation was independent of changes in local [Ca(2)(+)](i) as shown by experiments that interfered with Ca(2)(+) influx through voltage-gated Ca(2)(+) (Cav) channels, release of Ca(2)(+) from internal stores, or intracellular buffer capacity. This behaviour is not due to beta-subunits of BK(Ca) (BKbeta), as genetic inactivation of the beta-subunit expressed in IHCs, KCNMB1, did not affect BK(Ca) gating. We conclude that the BK(Ca) channel protein in IHCs may be modified in order to rapidly activate and deactivate at resting [Ca(2)(+)](i). Our results suggest that BK(Ca) may function as a purely voltage-gated K(+) channel with exceptionally rapid activation kinetics, challenging the view that both increased cytosolic Ca(2)(+) and depolarization are generally required for activation of BK(Ca).

Identification and functional characterization of cereblon as a binding protein for large-conductance calcium-activated potassium channel in rat brain

Large-conductance Ca2+-activated K+ (BK(Ca)) channels are activated by membrane depolarization and modulated by intracellular Ca2+. Here, we report the direct interaction of cereblon (CRBN) with the cytosolic carboxy-terminus of the BK(Ca) channel alpha subunit (Slo). Rat CRBN contained the N-terminal domain of the Lon protease, a 'regulators of G protein-signaling' (RGS)-like domain, a leucine zipper (LZ) motif, and four putative protein kinase C (PKC) phosphorylation sites. RNA messages of rat cereblon (rCRBN) were widely distributed in different tissues with especially high-levels of expression in the brain. Direct association of rCRBN with the BK(Ca) channel was confirmed by immunoprecipitation in brain lysate, and the two proteins were co-localized in cultured rat hippocampal neurons. Ionic currents evoked by the rSlo channel were dramatically suppressed upon coexpression of rCRBN. rCRBN decreased the formation of the tetrameric rSlo complex thus reducing the surface expression of functional channels. Therefore, we suggest that CRBN may play an important role in assembly and surface expression of functional BK(Ca) channels by direct interaction with the cytosolic C-terminus of its alpha-subunit.

Characterization of ionic currents in human mesenchymal stem cells from bone marrow

This study characterized functional ion channels in cultured undifferentiated human mesenchymal stem cells (hMSCs) from bone marrow with whole-cell patch clamp and reverse transcription polymerase chain reaction (RT-PCR) techniques. Three types of outward currents were found in hMSCs, including a noise-like rapidly activating outward current inhibited by the large conductance Ca(2+)-activated K(+) channel (I(KCa)) blocker iberiotoxin, a transient outward K(+) current (I(to)) suppressed by 4-aminopyridine (4-AP), and a delayed rectifier K(+) current (IK(DR))-like ether-à-go-go (eag) K(+) channel. In addition, tetrodotoxin-sensitive sodium current (I(Na.TTX)) and nifedipine-sensitive L-type Ca(2+) current (I(Ca.L)) were also detected in 29% and 15% hMSCs, respectively. Moreover, RT-PCR revealed the molecular evidence of high levels of mRNA for the functional ionic currents, including human MaxiK for I(KCa), Kv4.2 and Kv1.4 for I(to), heag1 for IK(DR), hNE-Na for I(Na.TTX), and CACNAIC for I(Ca.L). These results demonstrate that multiple functional ion channel currents--that is, I(KCa), I(to), heag1, I(Na.TTX), and I(Ca.L)--are expressed in hMSCs from bone marrow.

Cardioprotective effects of estradiol include the activation of large-conductance Ca(2+)-activated K(+) channels in cardiac mitochondria

The molecular components of the large-conductance Ca(2+)-activated K(+) channels that are functionally expressed in mitochondria (mitoK(Ca)) in cardiac myocytes have not been identified. Our experimental results show that the transcript corresponding to the large-conductance Ca(2+)-activated K(+) channel beta1-subunit (BK-beta1) is substantially expressed in mammalian heart. A yeast two-hybrid assay showed the BK-beta1 protein can interact with a mitochondrial protein, cytochrome c oxidase subunit I (Cco1). Results from immunocytochemical experiments also demonstrated that BK-beta1 interacted with Cco1 and colocalized in rat cardiac mitochondria. Furthermore, 17beta-estradiol, which enhances the activity of the BK channel alpha-subunit only in the presence of the beta1-subunit, significantly increased flavoprotein oxidation in rat ventricle myocytes and decreased the rate of cell death under simulated ischemia. Single-channel recordings from mitochondrial inner membrane indicated that the activity of mitoK(Ca), which had a conductance of approximately 270 pS, was enhanced by 17beta-estradiol and blocked by paxilline. In combination, the present study revealed a new mechanism for the cardioprotective effects of 17beta-estradiol, which include the activation of mitoK(Ca) via the interaction with BK-beta1. BK-beta1 may be an important molecular component that functionally couples with both Cco1 and mitoK(Ca) pore-forming alpha-subunit.

MaxiK channel roles in blood vessel relaxations induced by endothelium-derived relaxing factors and their molecular mechanisms

The endothelium of blood vessels plays a crucial role in the regulation of blood flow by controlling mechanical functions of underlying vascular smooth muscle. The regulation by the endothelium of vascular smooth muscle relaxation and contraction is mainly achieved via the release of vasoactive substances upon stimulation with neurohumoural substances and physical stimuli. Nitric oxide (NO) and prostaglandin I2 (prostacyclin, PGI2) are representative endothelium-derived chemicals that exhibit powerful blood vessel relaxation. NO action involves activation of soluble guanylyl cyclase and PGI2 action is initiated by the stimulation of a cell-surface receptor (IP receptor, IPR) that is coupled with Gs-protein-adenylyl cyclase cascade. Many studies on the mechanisms by which NO and PGI2 elicit blood vessel relaxation have highlighted a role of the large conductance, Ca2+-activated K+ (MaxiK, BKCa) channel in smooth muscle as their common downstream effector. Furthermore, their molecular mechanisms have been unravelled to include new routes different from the conventionally approved intracellular pathways. MaxiK channel might also serve as a target for endothelium-derived hyperpolarizing factor (EDHF), the non-NO, non-PGI2 endothelium-derived relaxing factor in some blood vessels. In this brief article, we review how MaxiK channel serves as an endothelium-vascular smooth muscle transducer to communicate the chemical signals generated in the endothelium to control blood vessel mechanical functions and discuss their molecular mechanisms.

Alternative splicing of Slo channel gene programmed by estrogen, progesterone and pregnancy

STREX alternative-exon adds to Slo channel a phosphorylation sequence that can invert protein kinase A (PKA) regulation from excitatory to inhibitory. Because pregnancy switches Slo responsiveness to PKA from inhibitory to excitatory, we hypothesized that STREX expression diminishes with pregnancy and is regulated by sex hormones. Different from total-rSlo, which is elevated around mid-pregnancy and decreases at term, STREX transcripts progressively decreased with pregnancy near 80% at term. STREX downregulation was mimicked by estrogen, and opposed by estrogen-receptor antagonist ICI 182,780 or progesterone (Pg). The regulation of STREX splicing directed by estrogen and Pg provides a mechanism for Slo's PKA-related phenotypic alteration with pregnancy.

Alcohol tolerance in large-conductance, calcium-activated potassium channels of CNS terminals is intrinsic and includes two components: decreased ethanol potentiation and decreased channel density

Tolerance is an important element of drug addiction and provides a model for understanding neuronal plasticity. The hypothalamic-neurohypophysial system (HNS) is an established preparation in which to study the actions of alcohol. Acute application of alcohol to the rat neurohypophysis potentiates large-conductance calcium-sensitive potassium channels (BK), contributing to inhibition of hormone secretion. A cultured HNS explant from adult rat was used to explore the molecular mechanisms of BK tolerance after prolonged alcohol exposure. Ethanol tolerance was intrinsic to the HNS and consisted of: (1) decreased BK potentiation by ethanol, complete within 12 min of exposure, and (2) decreased current density, which was not complete until 24 hr after exposure, indicating that the two components of tolerance represent distinct processes. Single-channel properties were not affected by chronic exposure, suggesting that decreased current density resulted from downregulation of functional channels in the membrane. Indeed, we observed decreased immunolabeling against the BK alpha-subunit on the surface of tolerant terminals. Analysis using confocal microscopy revealed a reduction of BK channel clustering, likely associated with the internalization of the channel.

Divalent cation sensitivity of BK channel activation supports the existence of three distinct binding sites

Mutational analyses have suggested that BK channels are regulated by three distinct divalent cation-dependent regulatory mechanisms arising from the cytosolic COOH terminus of the pore-forming α subunit. Two mechanisms account for physiological regulation of BK channels by μM Ca²⁺. The third may mediate physiological regulation by mM Mg²⁺. Mutation of five aspartate residues (5D5N) within the so-called Ca²⁺ bowl removes a portion of a higher affinity Ca²⁺ dependence, while mutation of D362A/D367A in the first RCK domain also removes some higher affinity Ca²⁺ dependence. Together, 5D5N and D362A/D367A remove all effects of Ca²⁺ up through 1 mM while E399A removes a portion of low affinity regulation by Ca²⁺/Mg²⁺. If each proposed regulatory effect involves a distinct divalent cation binding site, the divalent cation selectivity of the actual site that defines each mechanism might differ. By examination of the ability of various divalent cations to activate currents in constructs with mutationally altered regulatory mechanisms, here we show that each putative regulatory mechanism exhibits a unique sensitivity to divalent cations. Regulation mediated by the Ca²⁺ bowl can be activated by Ca²⁺ and Sr²⁺, while regulation defined by D362/D367 can be activated by Ca²⁺, Sr²⁺, and Cd²⁺. Mn²⁺, Co²⁺, and Ni²⁺ produce little observable effect through the high affinity regulatory mechanisms, while all six divalent cations enhance activation through the low affinity mechanism defined by residue E399. Furthermore, each type of mutation affects kinetic properties of BK channels in distinct ways. The Ca²⁺ bowl mainly accelerates activation of BK channels at low [Ca²⁺], while the D362/D367-related high affinity site influences both activation and deactivation over the range of 10–300 μM Ca²⁺. The major kinetic effect of the E399-related low affinity mechanism is to slow deactivation at mM Mg²⁺ or Ca²⁺. The results support the view that three distinct divalent-cation binding sites mediate regulation of BK channels.

Heme regulates allosteric activation of the Slo1 BK channel

Large conductance calcium-dependent (Slo1 BK) channels are allosterically activated by membrane depolarization and divalent cations, and possess a rich modulatory repertoire. Recently, intracellular heme has been identified as a potent regulator of Slo1 BK channels (Tang, X.D., R. Xu, M.F. Reynolds, M.L. Garcia, S.H. Heinemann, and T. Hoshi. 2003. Nature. 425:531-535). Here we investigated the mechanism of the regulatory action of heme on heterologously expressed Slo1 BK channels by separating the influences of voltage and divalent cations. In the absence of divalent cations, heme generally decreased ionic currents by shifting the channel's G-V curve toward more depolarized voltages and by rendering the curve less steep. In contrast, gating currents remained largely unaffected by heme. Simulations suggest that a decrease in the strength of allosteric coupling between the voltage sensor and the activation gate and a concomitant stabilization of the open state account for the essential features of the heme action in the absence of divalent ions. At saturating levels of divalent cations, heme remained similarly effective with its influence on the G-V simulated by weakening the coupling of both Ca(2+) binding and voltage sensor activation to channel opening. The results thus show that heme dampens the influence of allosteric activators on the activation gate of the Slo1 BK channel. To account for these effects, we consider the possibility that heme binding alters the structure of the RCK gating ring and thereby disrupts both Ca(2+)- and voltage-dependent gating as well as intrinsic stability of the open state.

Cysteine modification alters voltage- and Ca(2+)-dependent gating of large conductance (BK) potassium channels

The Ca²⁺-activated K⁺ (BK) channel α-subunit contains many cysteine residues within its large COOH-terminal tail domain. To probe the function of this domain, we examined effects of cysteine-modifying reagents on channel gating. Application of MTSET, MTSES, or NEM to mSlo1 or hSlo1 channels changed the voltage and Ca²⁺ dependence of steady-state activation. These reagents appear to modify the same cysteines but have different effects on function. MTSET increases I_K and shifts the G_K–V relation to more negative voltages, whereas MTSES and NEM shift the G_K–V in the opposite direction. Steady-state activation was altered in the presence or absence of Ca²⁺ and at negative potentials where voltage sensors are not activated. Combinations of [Ca²⁺] and voltage were also identified where P_o is not changed by cysteine modification. Interpretation of our results in terms of an allosteric model indicate that cysteine modification alters Ca²⁺ binding and the relative stability of closed and open conformations as well as the coupling of voltage sensor activation and Ca²⁺ binding and to channel opening. To identify modification-sensitive residues, we examined effects of MTS reagents on mutant channels lacking one or more cysteines. Surprisingly, the effects of MTSES on both voltage- and Ca²⁺-dependent gating were abolished by replacing a single cysteine (C430) with alanine. C430 lies in the RCK1 (regulator of K⁺ conductance) domain within a series of eight residues that is unique to BK channels. Deletion of these residues shifted the G_K–V relation by >−80 mV. Thus we have identified a region that appears to strongly influence RCK domain function, but is absent from RCK domains of known structure. C430A did not eliminate effects of MTSET on apparent Ca²⁺ affinity. However an additional mutation, C615S, in the Haem binding site reduced the effects of MTSET, consistent with a role for this region in Ca²⁺ binding.

Tertiapin-Q blocks recombinant and native large conductance K+ channels in a use-dependent manner

Tertiapin, a short peptide from honey bee venom, has been reported to specifically block the inwardly rectifying K(+) (Kir) channels, including G protein-coupled inwardly rectifying potassium channel (GIRK) 1+GIRK4 heteromultimers and ROMK1 homomultimers. In the present study, the effects of a stable and functionally similar derivative of tertiapin, tertiapin-Q, were examined on recombinant human voltage-dependent Ca(2+)-activated large conductance K(+) channel (BK or MaxiK; alpha-subunit or hSlo1 homomultimers) and mouse inwardly rectifying GIRK1+GIRK2 (i.e., Kir3.1 and Kir3.2) heteromultimeric K(+) channels expressed in Xenopus oocytes and in cultured newborn mouse dorsal root ganglion (DRG) neurons. In two-electrode voltage-clamped oocytes, tertiapin-Q (1-100 nM) inhibited BK-type K(+) channels in a use- and concentration-dependent manner. We also confirmed the inhibition of recombinant GIRK1+GIRK2 heteromultimers by tertiapin-Q, which had no effect on endogenous depolarization- and hyperpolarization-activated currents sensitive to extracellular divalent cations (Ca(2+), Mg(2+), Zn(2+), and Ba(2+)) in defolliculated oocytes. In voltage-clamped DRG neurons, tertiapin-Q voltage- and use-dependently inhibited outwardly rectifying K(+) currents, but Cs(+)-blocked hyperpolarization-activated inward currents including I(H) were insensitive to tertiapin-Q, baclofen, barium, and zinc, suggesting absence of functional GIRK channels in the newborn. Under current-clamp conditions, tertiapin-Q blocked the action potential after hyperpolarization (AHP) and increased action potential duration in DRG neurons. Taken together, these results demonstrate that the blocking actions of tertiapin-Q are not specific to Kir channels and that the blockade of recombinant BK channels and native neuronal AHP currents is use-dependent. Inhibition of specific types of Kir and voltage-dependent Ca(2+)-activated K(+) channels by tertiapin-Q at nanomolar range via different mechanisms may have implications in pain physiology and therapy.

Calcium-sensitive potassium channelopathy in human epilepsy and paroxysmal movement disorder

The large conductance calcium-sensitive potassium (BK) channel is widely expressed in many organs and tissues, but its in vivo physiological functions have not been fully defined. Here we report a genetic locus associated with a human syndrome of coexistent generalized epilepsy and paroxysmal dyskinesia on chromosome 10q22 and show that a mutation of the alpha subunit of the BK channel causes this syndrome. The mutant BK channel had a markedly greater macroscopic current. Single-channel recordings showed an increase in open-channel probability due to a three- to fivefold increase in Ca(2+) sensitivity. We propose that enhancement of BK channels in vivo leads to increased excitability by inducing rapid repolarization of action potentials, resulting in generalized epilepsy and paroxysmal dyskinesia by allowing neurons to fire at a faster rate. These results identify a gene that is mutated in generalized epilepsy and paroxysmal dyskinesia and have implications for the pathogenesis of human epilepsy, the neurophysiology of paroxysmal movement disorders and the role of BK channels in neurological disease.

Estrogen regulates {beta}1-subunit expression in Ca(2+)-activated K(+) channels in arteries from reproductive tissues

Daily estradiol-17beta (E(2)beta) increases basal uterine blood flow (UBF) and enhances acute E(2)beta-mediated increases in UBF in ovariectomized nonpregnant ewes. The acute E(2)beta-mediated rise in UBF involves vascular smooth muscle (VSM) large-conductance Ca(2+)-activated K(+) channels (BK(Ca)). BK(Ca) consist of pore-forming alpha-subunits and regulatory beta(1)-subunits that modulate channel function and E(2)beta responsiveness. It is unclear whether E(2)beta also alters subunit expression and thus channel density and/or function, thereby contributing to the rise in basal UBF and enhanced UBF responses that follow daily E(2)beta. Therefore, we examined BK(Ca) subunit expression by using reverse transcription-PCR and immunoblot analysis of arterial VSM from reproductive and nonreproductive tissues and myometrium from ovariectomized nonpregnant ewes after daily E(2)beta (1 microg/kg iv) or vehicle without or with acute E(2)beta (1 microg/kg). Tissue distribution was determined by immunohistochemistry. Acute E(2)beta did not alter alpha- or beta(1)-subunit expression in any tissue (P > 0.1). Daily E(2)beta also did not affect alpha-subunit mRNA or protein in any tissue (P > 0.1) or mesenteric arterial VSM beta(1)-subunit. However, daily E(2)beta increased uterine and mammary arterial VSM beta(1)-subunit mRNA by 32% and 83% (P < 0.05), uterine VSM protein by 30%, and myometrial beta(1)-subunit mRNA and protein by 74% (P < or = 0.005). Immunostaining of uterine arteries, myometrium, and intramyometrial arteries paralleled immunoblot analyses for both subunits. Although BK(Ca) density is unaffected by daily and acute E(2)beta, daily E(2)beta increases beta(1)-subunit in proximal and distal uterine arterial VSM. Thus prolonged E(2)beta exposure may alter BK(Ca) function, estrogen responsiveness, and basal vascular tone and reactivity in reproductive arteries by modifying alpha:beta(1) stoichiometry.

Dietary K+ regulates apical membrane expression of maxi-K channels in rabbit cortical collecting duct

The cortical collecting duct (CCD) is a final site for regulation of K(+) homeostasis. CCD K(+) secretion is determined by the electrochemical gradient and apical permeability to K(+). Conducting secretory K(+) (SK/ROMK) and maxi-K channels are present in the apical membrane of the CCD, the former in principal cells and the latter in both principal and intercalated cells. Whereas SK channels mediate baseline K(+) secretion, maxi-K channels appear to participate in flow-stimulated K(+) secretion. Chronic dietary K(+) loading enhances the CCD K(+) secretory capacity due, in part, to an increase in SK channel density (Palmer et al., J Gen Physiol 104: 693-710, 1994). Long-term exposure of Ambystoma tigrinum to elevated K(+) increases renal K(+) excretion due to an increase in apical maxi-K channel density in their CDs (Stoner and Viggiano, J Membr Biol 162: 107-116, 1998). The purpose of the present study was to test whether K(+) adaptation in the mammalian CCD is associated with upregulation of maxi-K channel expression. New Zealand White rabbits were fed a low (LK), control (CK), or high (HK) K(+) diet for 10-14 days. Real-time PCR quantitation of message encoding maxi-K alpha- and beta(2-4)-subunits in single CCDs from HK animals was greater than that detected in CK and LK animals (P < 0.05); beta(1)-subunit was not detected in any CCD sample but was present in whole kidney. Indirect immunofluorescence microscopy revealed a predominantly intracellular distribution of alpha-subunits in LK kidneys. In contrast, robust apical labeling was detected primarily in alpha-intercalated cells in HK kidneys. In summary, K(+) adaptation is associated with an increase in steady-state abundance of maxi-K channel subunit-specific mRNAs and immunodetectable apical alpha-subunit, the latter observation consistent with redistribution from an intracellular pool to the plasma membrane.

Interaction of BKCa channel modulators with adrenergic agonists in the rat aorta is influenced by receptor reserve

Our main objective was to study the interaction of BKCa channel modulators with adrenergic agonists UK 14304 and noradrenaline (NA), acting on alpha1-adrenoceptors, in the rat aorta and how this is affected by receptor reserve. NA and UK 14304 evoked concentration-dependent contractions of the rat aorta. UK 14304 was a partial agonist relative to NA in this preparation. The BK(Ca) channel blocker tetraethylammonium (TEA, 1 mM) and opener NS 1619 (3 x 10(-5) M) modulated NA- and UK 14304-induced contractions, and were more effective on UK 14304-induced contractions. TEA (1 mM) increased the maximum response to NA and UK 14304 by about 13% and 300%, respectively, while NS 1619 (3 x 10(-5) M) reduced the maximum response to UK 14304 by about 81% compared to 31% for noradrenaline. The effect of TEA on the noradrenaline concentration-response curve was increased after treatment of the aorta with phenoxybenzamine (PBZ), an irreversible alpha1-adrenoceptor antagonist, to reduce receptor reserve. We concluded that the interaction of BKCa channel modulators with alpha1-adrenergic agonists in the rat aorta was influenced by receptor reserve.

The beta1 subunit enhances oxidative regulation of large-conductance calcium-activated K+ channels

Oxidative stress may alter the functions of many proteins including the Slo1 large conductance calcium-activated potassium channel (BK_Ca). Previous results demonstrated that in the virtual absence of Ca²⁺, the oxidant chloramine-T (Ch-T), without the involvement of cysteine oxidation, increases the open probability and slows the deactivation of BK_Ca channels formed by human Slo1 (hSlo1) α subunits alone. Because native BK_Ca channel complexes may include the auxiliary subunit β1, we investigated whether β1 influences the oxidative regulation of hSlo1. Oxidation by Ch-T with β1 present shifted the half-activation voltage much further in the hyperpolarizing direction (−75 mV) as compared with that with α alone (−30 mV). This shift was eliminated in the presence of high [Ca²⁺]_i, but the increase in open probability in the virtual absence of Ca²⁺ remained significant at physiologically relevant voltages. Furthermore, the slowing of channel deactivation after oxidation was even more dramatic in the presence of β1. Oxidation of cysteine and methionine residues within β1 was not involved in these potentiated effects because expression of mutant β1 subunits lacking cysteine or methionine residues produced results similar to those with wild-type β1. Unlike the results with α alone, oxidation by Ch-T caused a significant acceleration of channel activation only when β1 was present. The β1 M177 mutation disrupted normal channel activation and prevented the Ch-T–induced acceleration of activation. Overall, the functional effects of oxidation of the hSlo1 pore-forming α subunit are greatly amplified by the presence of β1, which leads to the additional increase in channel open probability and the slowing of deactivation. Furthermore, M177 within β1 is a critical structural determinant of channel activation and oxidative sensitivity. Together, the oxidized BK_Ca channel complex with β1 has a considerable chance of being open within the physiological voltage range even at low [Ca²⁺]_i.

Differential effects of beta 1 and beta 2 subunits on BK channel activity

High conductance, calcium- and voltage-activated potassium (BK) channels are widely expressed in mammals. In some tissues, the biophysical properties of BK channels are highly affected by coexpression of regulatory (β) subunits. β1 and β2 subunits increase apparent channel calcium sensitivity. The β1 subunit also decreases the voltage sensitivity of the channel and the β2 subunit produces an N-type inactivation of BK currents. We further characterized the effects of the β1 and β2 subunits on the calcium and voltage sensitivity of the channel, analyzing the data in the context of an allosteric model for BK channel activation by calcium and voltage (Horrigan and Aldrich, 2002). In this study, we used a β2 subunit without its N-type inactivation domain (β2IR). The results indicate that the β2IR subunit, like the β1 subunit, has a small effect on the calcium binding affinity of the channel. Unlike the β1 subunit, the β2IR subunit also has no effect on the voltage sensitivity of the channel. The limiting voltage dependence for steady-state channel activation, unrelated to voltage sensor movements, is unaffected by any of the studied β subunits. The same is observed for the limiting voltage dependence of the deactivation time constant. Thus, the β1 subunit must affect the voltage sensitivity by altering the function of the voltage sensors of the channel. Both β subunits reduce the intrinsic equilibrium constant for channel opening (L₀). In the allosteric activation model, the reduction of the voltage dependence for the activation of the voltage sensors accounts for most of the macroscopic steady-state effects of the β1 subunit, including the increase of the apparent calcium sensitivity of the BK channel. All allosteric coupling factors need to be increased in order to explain the observed effects when the α subunit is coexpressed with the β2IR subunit.

MaxiK channel-triggered negative feedback system is preserved in the urinary bladder smooth muscle from streptozotocin-induced diabetic rats

MaxiK channel, the large-conductance Ca2+-sensitive K+ channel, facilitates a negative feedback mechanism to oppose excitation and contraction in various types of smooth muscles including urinary bladder smooth muscle (UBSM). In this study, we investigated how the contribution of MaxiK channel to the regulation of basal UBSM mechanical activity is altered in streptozotocin-induced diabetic rats. Although the urinary bladder preparations from both control and diabetic rats were almost quiescent in their basal mechanical activities, they generated spontaneous rhythmic contractions in response to a MaxiK channel blocker, iberiotoxin (IbTx). The effect of IbTx on the mechanical activity was significantly greater in diabetic rat than in control animal. Similarly, the basal mechanical activity was increased with apamin, an inhibitor for some types of small conductance Ca2+-sensitive K+ channels, and this effect was more pronounced for diabetic rat. However, in both control and diabetic animals, IbTx action was stronger than that of apamin. Diabetes also enhanced the responses to BayK 8644, an L-type Ca2+ channel agonist. The extent of this enhancement in diabetic bladder vs. control was, however, almost the same as that attained with IbTx. Expression levels for MaxiK channel as well as apamin-sensitive K+ channels and L-type Ca2+ channel were not altered by diabetes, when determined as their corresponding mRNA levels. These results indicate that diabetes can potentially increase the basal UBSM mechanical activity. However, in diabetic UBSM, the main negative-feedback system triggered by MaxiK channel is still preserved enough to counteract the possible enhancement of this smooth muscle mechanical activity.

Human podocytes possess a stretch-sensitive, Ca2+-activated K+ channel: potential implications for the control of glomerular filtration

Podocytes express many proteins characteristic of smooth muscle, such as actin and myosin. They also express receptors to several vasoactive agents, including acetylcholine and angiotensin II; these phenotypic properties suggest that podocytes are not static entities but may respond to physiologic stimuli. The electrophysiologic properties of a conditionally immortalized human podocyte cell line that expresses the specific podocyte proteins nephrin, podocin, and synaptopodin were examined by patch clamp. Channels that were highly K(+)-selective and had a conductance of 224 +/- 11.5 pS in symmetrical 150 mM K(+) solutions were identified. Channel activity was Ca(2+)- and voltage-dependent, being increased with an increase in Ca(2+) or depolarization, and inhibited by penitrem A. The conductance and voltage- and Ca(2+)-dependence suggest that this is the large-conductance calcium-activated K(+) channel, BK (KCNMA1)-this was supported by reverse transcription-PCR experiments that showed the presence of the BK encoding mRNA, along with expression of KCNMB subunit types 3 and 4. In sections of human glomeruli, immunocytochemistry revealed that BK co-localizes with the podocyte-specific protein nephrin, indicating that these channels are present in native human podocytes. In whole-cell experiments, penitrem A inhibited outward currents to the same extent as tetra-ethyl ammonium (TEA) but did not affect the membrane potential. Channel activity was also increased by applying suction to the patch pipette or by dilution of the bathing medium, indicating that these channels are stretch sensitive. Thus, these channels do not contribute to the resting membrane potential but are activated by a rise in intracellular Ca(2+), membrane depolarization, cell swelling, or membrane stretch. By implication, these results suggest that podocytes may be able to respond to changes in the glomerular capillary pressure and modulate the GFR.

Regulation of K+ flow by a ring of negative charges in the outer pore of BKCa channels. Part I: Aspartate 292 modulates K+ conduction by external surface charge effect

The pore region of the majority of K⁺ channels contains the highly conserved GYGD sequence, known as the K⁺ channel signature sequence, where the GYG is critical for K⁺ selectivity (Heginbotham, L., T. Abramson, and R. MacKinnon. 1992. Science. 258:1152–1155). Exchanging the aspartate residue with asparagine in this sequence abolishes ionic conductance of the Shaker K⁺ channel (D447N) (Hurst, R.S., L. Toro, and E. Stefani. 1996. FEBS Lett. 388:59–65). In contrast, we found that the corresponding mutation (D292N) in the pore forming α subunit (hSlo) of the voltage- and Ca²⁺-activated K⁺ channel (BK_Ca, MaxiK) did not prevent conduction but reduced single channel conductance. We have investigated the role of outer pore negative charges in ion conduction (this paper) and channel gating (Haug, T., R. Olcese, T. Ligia, and E. Stefani. 2004. J. Gen Physiol. 124:185–197). In symmetrical 120 mM [K⁺], the D292N mutation reduced the outward single channel conductance by ∼40% and nearly abolished inward K⁺ flow (outward rectification). This rectification was partially relieved by increasing the external K⁺ concentration to 700 mM. Small inward currents were resolved by introducing an additional mutation (R207Q) that greatly increases the open probability of the channel. A four-state multi-ion pore model that incorporates the effects of surface charge was used to simulate the essential properties of channel conduction. The conduction properties of the mutant channel (D292N) could be predicted by a simple ∼8.5-fold reduction of the surface charge density without altering any other parameter. These results indicate that the aspartate residue in the BK_Ca pore plays a key role in conduction and suggest that the pore structure is not affected by the mutation. We speculate that the negative charge strongly accumulates K⁺ in the outer vestibule close to the selectivity filter, thus increasing the rate of ion entry into the pore.

Regulation of K+ flow by a ring of negative charges in the outer pore of BKCa channels. Part II: Neutralization of aspartate 292 reduces long channel openings and gating current slow component

Neutralization of the aspartate near the selectivity filter in the GYGD pore sequence (D292N) of the voltage- and Ca²⁺-activated K⁺ channel (MaxiK, BKCa) does not prevent conduction like the corresponding mutation in Shaker channel, but profoundly affects major biophysical properties of the channel (Haug, T., D. Sigg, S. Ciani, L. Toro, E. Stefani, and R. Olcese. 2004. J. Gen. Physiol. 124:173–184). Upon depolarizations, the D292N mutant elicited mostly gating current, followed by small or no ionic current, at voltages where the wild-type hSlo channel displayed robust ionic current. In fact, while the voltage dependence of the gating current was not significantly affected by the mutation, the overall activation curve was shifted by ∼20 mV toward more depolarized potentials. Several lines of evidence suggest that the mutation prevents population of certain open states that in the wild type lead to high open probability. The activation curves of WT and D292N can both be fitted to the sum of two Boltzmann distributions with identical slope factors and half activation potentials, just by changing their relative amplitudes. The steeper and more negative component of the activation curve was drastically reduced by the D292N mutation (from 0.65 to 0.30), suggesting that the population of open states that occurs early in the activation pathway is reduced. Furthermore, the slow component of the gating current, which has been suggested to reflect transitions from closed to open states, was greatly reduced in D292N channels. The D292N mutation also affected the limiting open probability: at 0 mV, the limiting open probability dropped from ∼0.5 for the wild-type channel to 0.06 in D292N (in 1 mM [Ca²⁺]_i). In addition to these effects on gating charge and open probability, as already described in Part I, the D292N mutation introduces a ∼40% reduction of outward single channel conductance, as well as a strong outward rectification.

Identification and functional characterization of ankyrin-repeat family protein ANKRA as a protein interacting with BKCa channel

Ankyrin-repeat family A protein (ANKRA) was originally cloned in mouse as an interacting protein to megalin, a member of low-density lipoprotein receptor superfamily. Here, we report that the isolation of rat ANKRA as a new binding partner for the alpha-subunit of rat large-conductance Ca2+-activated K+ channel (rSlo). We mapped the binding region of each protein by using yeast two-hybrid and in vitro binding assays. ANKRA expressed together with rSlo channels were colocalized near the plasma membrane and coimmunoprecipitated in transfected cells. We also showed that BKCa channel in rat cerebral cortex coprecipitated with rANKRA and colocalized in cultured rat hippocampal neuron. Although the coexpression of ANKRA did not affect the surface expression of rSlo, the gating kinetics of rSlo channel was significantly altered and the effects were highly dependent on the intracellular calcium. These results indicate that ANKRA could modulate the excitability of neurons by binding directly to endogenous BKCa channel and altering its gating kinetics in a calcium-dependent manner.

Ligand-dependent activation of Slo family channels is defined by interchangeable cytosolic domains

Large-conductance Ca2+- and voltage-regulated K+ channels (Slo1 BK-type) are controlled by two physiological stimuli, membrane voltage and cytosolic Ca2+. Regulation by voltage is similar to that in voltage-dependent K+ channels, arising from positively charged amino acids primarily within the S4 transmembrane helices. The basis for regulation by Ca2+ remains controversial. One viewpoint suggests that the extensive cytosolic C terminus contains the Ca2+ regulatory machinery, whereas another suggests that the pore-forming module contains the Ca2+-sensing elements. To address this issue, we take advantage of another Slo family member, the pH-regulated homolog Slo3. We reason that if the ligand-sensing apparatus is uniquely associated with a particular domain (either the pore or the cytosolic domain), exchange of those domains between Slo1 and Slo3 should result in exchange of ligand dependence in association with the key domain. The results show that the Slo3 cytosolic module confers pH-dependent regulation on the Slo1 pore module, whereas the Slo1 cytosolic module confers Ca2+-dependent regulation on the Slo3 pore module. Thus, ligand-specific regulation is defined by interchangeable cytosolic regulatory modules.

Functional effects of auxiliary beta4-subunit on rat large-conductance Ca(2+)-activated K(+) channel

Large-conductance calcium-activated potassium (BK(Ca)) channels are composed of the pore-forming alpha-subunit and the auxiliary beta-subunits. The beta4-subunit is dominantly expressed in the mammalian central nervous system. To understand the physiological roles of the beta4-subunit on the BK(Ca) channel alpha-subunit (Slo), we isolated a full-length complementary DNA of rat beta4-subunit (rbeta4), expressed heterolgously in Xenopus oocytes, and investigated the detailed functional effects using electrophysiological means. When expressed together with rat Slo (rSlo), rbeta4 profoundly altered the gating characteristics of the Slo channel. At a given concentration of intracellular Ca(2+), rSlo/rbeta4 channels were more sensitive to transmembrane voltage changes. The activation and deactivation rates of macroscopic currents were decreased in a Ca(2+)-dependent manner. The channel activation by Ca(2+) became more cooperative by the coexpression of rbeta4. Single-channel recordings showed that the increased Hill coefficient for Ca(2+) was due to the changes in the open probability of the rSlo/rbeta4 channel. Single BK(Ca) channels composed of rSlo and rbeta4 also exhibited slower kinetics for steady-state gating compared with rSlo channels. Dwell times of both open and closed events were significantly increased. Because BK(Ca) channels are known to modulate neuroexcitability and the expression of the beta4-subunit is highly concentrated in certain subregions of brain, the electrophysiological properties of individual neurons should be affected profoundly by the expression of this second subunit.

Hemoxygenase-2 is an oxygen sensor for a calcium-sensitive potassium channel

Modulation of calcium-sensitive potassium (BK) channels by oxygen is important in several mammalian tissues, and in the carotid body it is crucial to respiratory control. However, the identity of the oxygen sensor remains unknown. We demonstrate that hemoxygenase-2 (HO-2) is part of the BK channel complex and enhances channel activity in normoxia. Knockdown of HO-2 expression reduced channel activity, and carbon monoxide, a product of HO-2 activity, rescued this loss of function. Inhibition of BK channels by hypoxia was dependent on HO-2 expression and was augmented by HO-2 stimulation. Furthermore, carotid body cells demonstrated HO-2-dependent hypoxic BK channel inhibition, which indicates that HO-2 is an oxygen sensor that controls channel activity during oxygen deprivation.

Function and clustered expression of MaxiK channels in cerebral myocytes remain intact with aging

The incidence of stroke increases significantly in the aging population where stroke related deaths boost at >75 years and survivors are often permanently disabled. Aging is known to decrease cerebral blood flow likely due to an increase in arterial tone. Although MaxiK channels are key regulators of cerebral arterial tone their pattern of expression and function in cerebral blood vessels during aging is unknown. Using specific antibodies against the alpha-subunit of MaxiK channels and current recordings, we now demonstrate that in aging cerebral myocytes, MaxiK channels remain healthy. Furthermore, we show for the first time that in the vasculature, MaxiK channels are expressed in clusters. Clusters have an estimated radius of approximately 200 nm in young rats (3-5 month old Fisher 344 rats) which remains normal in old (25-30 month rats) cerebral myocytes. Consistent with a healthy MaxiK channel expression in old cerebral arteries, MaxiK current density, kinetics and Ca(2+) sensitivity were practically identical in young and old myocytes. Sensitivity to nanomolar concentrations of dehydrosoyasaponin-I that activates channels formed by alpha and beta subunits is also the same in young and old myocytes. These results demonstrate that MaxiK channels maintain normal expression during cerebral aging which is in sharp contrast to our previous finding of loss of expression in aging coronary arteries. It seems therefore, that cerebral myocytes have developed a protective anti-aging mechanism leading to the continued expression of MaxiK channels.

Deletion of the Ca2+-activated potassium (BK) alpha-subunit but not the BKbeta1-subunit leads to progressive hearing loss

The large conductance voltage- and Ca2+-activated potassium (BK) channel has been suggested to play an important role in the signal transduction process of cochlear inner hair cells. BK channels have been shown to be composed of the pore-forming alpha-subunit coexpressed with the auxiliary beta1-subunit. Analyzing the hearing function and cochlear phenotype of BK channel alpha-(BKalpha-/-) and beta1-subunit (BKbeta1-/-) knockout mice, we demonstrate normal hearing function and cochlear structure of BKbeta1-/- mice. During the first 4 postnatal weeks also, BKalpha-/- mice most surprisingly did not show any obvious hearing deficits. High-frequency hearing loss developed in BKalpha-/- mice only from approximately 8 weeks postnatally onward and was accompanied by a lack of distortion product otoacoustic emissions, suggesting outer hair cell (OHC) dysfunction. Hearing loss was linked to a loss of the KCNQ4 potassium channel in membranes of OHCs in the basal and midbasal cochlear turn, preceding hair cell degeneration and leading to a similar phenotype as elicited by pharmacologic blockade of KCNQ4 channels. Although the actual link between BK gene deletion, loss of KCNQ4 in OHCs, and OHC degeneration requires further investigation, data already suggest human BK-coding slo1 gene mutation as a susceptibility factor for progressive deafness, similar to KCNQ4 potassium channel mutations.

Functional and molecular evidence of MaxiK channel beta1 subunit decrease with coronary artery ageing in the rat

Large-conductance, voltage- and Ca2+ -activated K+ channels (MaxiK, BK) are key regulators of vascular tone. Vascular MaxiK are formed by the pore-forming alpha subunit and the modulatory beta1 subunit, which imprints unique kinetics, Ca2+/voltage sensitivities and pharmacology to the channel. As age progresses, alpha subunit functional expression and protein levels diminish in coronary myocytes. However, whether ageing modifies beta1 subunit expression or the mechanism of alpha subunit reduction is unknown. Thus, we examined functional and pharmacological characteristics of MaxiK, as well as alpha and beta1 transcript levels in coronary myocytes from young and old F344 rats. The mechanism of age-dependent alpha subunit protein reduction involves its transcript downregulation. A corresponding loss of beta1 transcripts was also detected in old myocytes, suggesting a proportional age-dependent decrease of beta1 to alpha subunit protein. Indeed, MaxiK channel properties, defined by coassembly of beta1 and alpha subunits, were equivalent in young versus old, for example in terms of (i) activation kinetics, (ii) sensitivity to Ca2+ levels > 1 microm (iii) dehydrosoyasaponin-I-induced activation, and (iv) iberiotoxin blockade. Consistent with less MaxiK expression/function in older myocytes, the ability of iberiotoxin to contract coronary rings was reduced approximately 50% with ageing confirming our previous findings. 5-Hydroxytryptamine (5-HT) contractile efficacy was reduced by iberiotoxin pretreatment in young > old coronary arteries (explained by larger iberiotoxin-induced contraction and decreased dynamic range for 5-HT contraction in young versus old) with no apparent differences in nitroglycerine-induced relaxation. We propose that the age-related MaxiK reduction involves a parallel decrease of alpha and beta1 functional expression via a transcript downregulatory mechanism; a major impact on basal and possibly stimulated coronary contraction may contribute to altered coronary flow regulation and coronary morbidity in the elderly.

An endoplasmic reticulum trafficking signal prevents surface expression of a voltage- and Ca2+-activated K+ channel splice variant

Protein delivery to restricted plasma membrane domains is exquisitely regulated at different stages of the cell trafficking machinery. Traffic control involves the recognition of export/retention/retrieval signals in the endoplasmic reticulum (ER)/Golgi complex that will determine protein fate. A splice variant (SV), SV1, of the voltage- and Ca(2+)-activated K(+) channel alpha-subunit accumulates the channel in the ER, preventing its surface expression. We show that SV1 insert contains a nonbasic, hydrophobic retention/retrieval motif, CVLF, that does not interfere with proper folding and tetramerization of SV1. Localization of proteins in the ER by CVLF is independent of its position; originally, on the first internal loop, SV1 insert or CVLF perform equally well if placed at the middle or end of the alpha-subunit intracellular carboxyl terminus. Also, CVLF is able to restrict the traffic of an independently expressed transmembrane protein, beta 1-subunit. CVLF is present in proteins across species and in lower organisms. Thus, CVLF may have evolved to serve as a regulator of cellular traffic.

Direct interaction between BKCa potassium channel and microtubule-associated protein 1A

The BKCa channel, a potassium channel that is allosterically activated by voltage and calcium, is expressed in both excitable and non-excitable cells. The channel plays an important role in regulating membrane excitability. The channel activity can be modulated by post-translational modifications such as phosphorylation. Recently, hippocampal BKCa channels were shown to be directly modulated by assembly/disassembly of the submembranous actin cytoskeleton. Here, we report that the BKCa channel physically interacts with the light chain of microtubule associated protein 1A (MAP1A). The light chain was isolated in a yeast two-hybrid screen of a human brain cDNA library. The specificity of the interaction was demonstrated in biochemical experiments utilizing GST fusion protein pulldown assays and reciprocal co-immunoprecipitations from rat brain. Furthermore, utilizing immunofluorescence, the BKCa channel and MAP1A co-localize in the Purkinje cell layer of the cerebellum. These studies identify a novel interaction between the C-terminal tail of the BKCa channel and the light chain of MAP1A, which enables channel association with and modulation by the cytoskeleton.

Extracellular ATP induces oscillations of intracellular Ca2+ and membrane potential and promotes transcription of IL-6 in macrophages

The effects of low concentrations of extracellular ATP on cytosolic Ca(2+), membrane potential, and transcription of IL-6 were studied in monocyte-derived human macrophages. During inflammation or infection many cells secrete ATP. We show here that application of 10 microM ATP or 10 microM UTP induces oscillations in cytosolic Ca(2+) with a frequency of approximately 12 min(-1) and oscillations in membrane potential. RT-PCR analysis showed expression of P2Y(1), P2Y(2), P2Y(11), P2X(1), P2X(4), and P2X(7) receptors, large-conductance (KCNMA1 and KCNMB1-4), and intermediate-conductance (KCNN4) Ca(2+)-activated K(+) channels. The Ca(2+)oscillations were unchanged after removal of extracellular Ca(2+), indicating that they were mainly due to movements of Ca(2+) between intracellular compartments. Comparison of the effects of different nucleotides suggests that the Ca(2+) oscillations were elicited by activation of P2Y(2) receptors coupled to phospholipase C. Patch-clamp experiments showed that ATP induced a transient depolarization, probably mediated by activation of P2X(4) receptors, followed by membrane potential oscillations due to opening of Ca(2+)-activated K(+) channels. We also found that 10 microM ATP gamma S increased transcription of IL-6 approximately 40-fold within 2 h. This effect was abolished by blockade of P2Y receptors with 100 microM suramin. Our results suggest that ATP released from inflamed, damaged, or metabolically impaired cells represents a "danger signal" that plays a major role in activating the innate immune system.