Differential Ca2+ sensitivities of BK(Ca) isochannels in bovine mesenteric vascular smooth muscle

BK Channels in Tail Artery Vascular Smooth Muscle Cells of Normotensive (WKY) and Hypertensive (SHR) Rats Possess Similar Calcium Sensitivity But Different Responses to the Vasodilator Iloprost

It has been reported that, in the spontaneously hypertensive rat (SHR) model of hypertension, different components of the G-protein/adenylate cyclase (AC)/Calcium-activated potassium channel of high conductance (BK) channel signaling pathway are altered differently. In the upstream part of the pathway (G-protein/AC), a comparatively low efficacy has been established, whereas downstream BK currents seem to be increased. Thus, the overall performance of this signaling pathway in SHR is elusive. For a better understanding, we focused on one aspect, the direct targeting of the BK channel by the G-protein/AC pathway and tested the hypothesis that the comparatively low AC pathway efficacy in SHR results in a reduced agonist-induced stimulation of BK currents. This hypothesis was investigated using freshly isolated smooth muscle cells from WKY and SHR rat tail artery and the patch-clamp technique. It was observed that: (1) single BK channels have similar current-voltage relationships, voltage-dependence and calcium sensitivity; (2) BK currents in cells with a strong buffering of the BK channel activator calcium have similar current-voltage relationships; (3) the iloprost-induced concentration-dependent increase of the BK current is larger in WKY compared to SHR; (4) the effects of activators of the PKA pathway, the catalytic subunit of PKA and the potent and selective cAMP-analogue Sp-5,6-DCl-cBIMPS on BK currents are similar. Thus, our data suggest that the lower iloprost-induced stimulation of the BK current in freshly isolated rat tail artery smooth muscle cells from SHR compared with WKY is due to the lower efficacy of upstream elements of the G-Protein/AC/BK channel pathway.

Important marine areas for endangered African penguins before and after the crucial stage of moulting

The population of the Endangered African penguin Spheniscus demersus has decreased by > 65% in the last 20 years. A major driver of this decrease has been the reduced availability of their principal prey, sardine Sardinops sagax and anchovy Engraulis encrasicolus. To date, conservation efforts to improve prey availability have focused on spatial management strategies to reduce resource competition with purse-seine fisheries during the breeding season. However, penguins also undergo an annual catastrophic moult when they are unable to feed for several weeks. Before moulting they must accumulate sufficient energy stores to survive this critical life-history stage. Using GPS tracking data collected between 2012 and 2019, we identify important foraging areas for pre- and post-moult African penguins at three of their major colonies in South Africa: Dassen Island and Stony Point (Western Cape) and Bird Island (Eastern Cape). The foraging ranges of pre- and post-moult adult African penguins (c. 600 km from colony) was far greater than that previously observed for breeding penguins (c. 50 km from colony) and varied considerably between sites, years and pre- and post-moult stages. Despite their more extensive range during the non-breeding season, waters within 20 and 50 km of their breeding colonies were used intensively and represent important foraging areas to pre- and post-moult penguins. Furthermore, penguins in the Western Cape travelled significantly further than those in the Eastern Cape which is likely a reflection of the poor prey availability along the west coast of South Africa. Our findings identify important marine areas for pre- and post-moult African penguins and support for the expansion of fisheries-related spatio-temporal management strategies to help conserve African penguins outside the breeding season.

N1-Methylnicotinamide: Is It Time to Consider as a Dietary Supplement for Athletes?

Abstract:

Exercise is considered to be a “medicine” due to its modulatory roles in metabolic disorders such as diabetes and obesity. The intensity and duration of exercise determine the mechanism of energy production by various tissues of the body, especially by muscles, in which the requirement for adenosine triphosphate (ATP) increases by as much as 100-fold. Naturally, athletes try to improve their exercise performance by dietary supplementation with, e.g., vitamins, metabolites, and amino acids. MNAM, as a vitamin B3 metabolite, reduces serum levels and liver contents of triglycerides, and cholesterol and induces lipolysis. It stimulates gluconeogenesis and prohibits liver cholesterol and fatty acid synthesis through the expression of sirtuin1 (SIRT1). It seems that MNAM is not responsible for the actions of NNMT in the adipose tissues as MNAM inhibits the activity of NNMT in the adipose tissue and acts like inhibitors of its activity. NNMT-MNAM axis is more activated in the muscles of participants who were undergoing the high-volume-low-intensity exercise and caloric restriction. Therefore, MNAM could be an important myokine during exercise and fasting where it provides the required energy for muscles through the induction of lipolysis and gluconeogenesis in the liver and adipose tissues, respectively. Increased levels of MNAM in exercise and fasting led us to propose that the consumption of MNAM during training especially endurance training could boost exercise capacity and improves performance. Therefore, in this review, we shed light on the potential of MNAM as a dietary supplement in sports medicine.

BK potassium channel modulation by leucine-rich repeat-containing proteins

Molecular diversity of ion channel structure and function underlies variability in electrical signaling in nerve, muscle, and nonexcitable cells. Regulation by variable auxiliary subunits is a major mechanism to generate tissue- or cell-specific diversity of ion channel function. Mammalian large-conductance, voltage- and calcium-activated potassium channels (BK, K(Ca)1.1) are ubiquitously expressed with diverse functions in different tissues or cell types, consisting of the pore-forming, voltage- and Ca(2+)-sensing α-subunits (BKα), either alone or together with the tissue-specific auxiliary β-subunits (β1-β4). We recently identified a leucine-rich repeat (LRR)-containing membrane protein, LRRC26, as a BK channel auxiliary subunit, which causes an unprecedented large negative shift (∼140 mV) in voltage dependence of channel activation. Here we report a group of LRRC26 paralogous proteins, LRRC52, LRRC55, and LRRC38 that potentially function as LRRC26-type auxiliary subunits of BK channels. LRRC52, LRRC55, and LRRC38 produce a marked shift in the BK channel's voltage dependence of activation in the hyperpolarizing direction by ∼100 mV, 50 mV, and 20 mV, respectively, in the absence of calcium. They along with LRRC26 show distinct expression in different human tissues: LRRC26 and LRRC38 mainly in secretory glands, LRRC52 in testis, and LRRC55 in brain. LRRC26 and its paralogs are structurally and functionally distinct from the β-subunits and we designate them as a γ family of the BK channel auxiliary proteins, which potentially regulate the channel's gating properties over a spectrum of different tissues or cell types.

Role of vascular potassium channels in the regulation of renal hemodynamics

K+ conductance is a major determinant of membrane potential ( Vm) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by Vm through the action of voltage-operated Ca2+ channels (VOCC) in VSMC. Increased K+ conductance leads to hyperpolarization and vasodilation, while inactivation of K+ channels causes depolarization and vasoconstriction. K+ channels in EC indirectly participate in the control of vascular tone by several mechanisms, e.g., release of nitric oxide and endothelium-derived hyperpolarizing factor. In the kidney, a change in the activity of one or more classes of K+ channels will lead to a change in hemodynamic resistance and therefore of renal blood flow and glomerular filtration pressure. Through these effects, the activity of renal vascular K+ channels influences renal salt and water excretion, fluid homeostasis, and ultimately blood pressure. Four main classes of K+ channels [calcium activated (KCa), inward rectifier (Kir), voltage activated (KV), and ATP sensitive (KATP)] are found in the renal vasculature. Several in vitro experiments have suggested a role for individual classes of K+ channels in the regulation of renal vascular function. Results from in vivo experiments are sparse. We discuss the role of the different classes of renal vascular K+ channels and their possible role in the integrated function of the renal microvasculature. Since several pathological conditions, among them hypertension, are associated with alterations in K+ channel function, the role of renal vascular K+ channels in the control of salt and water excretion deserves attention.

Diet quality affects postnuptial molting and feather quality of the house sparrow (Passer domesticus): interaction with humoral immune function?

We investigated the effects of nutritional limitation, humoral immune activation, and their interaction on postnuptial molting of aviary-kept house sparrows ( Passer domesticus (L., 1758)). In a 2 × 2 experimental design, we measured the progress of molting and the quality of feathers produced during molting by house sparrows exposed to different diet qualities (high and low) and humoral immune activation with sheep red blood cells (SRBC). Food quality, but not the activation of humoral immunity, affected significantly the body mass and the process of molting. Sparrows feeding on low-quality food had decreased body mass and longer molts than the high-quality group. Low-quality food, but not the activation of humoral immunity, reduced significantly the length and mass (i.e., the quality) of primaries grown during molting. Birds responded significantly to injection with SRBC compared with the control group, but the immune response was similar between nutritional groups. The absence of a negative effect of humoral immunity on molting in house sparrows might be related to the low energy and nutritional requirements of mounting and maintaining a humoral immune response.

Renovascular BKCachannels are not activated in vivo under resting conditions and during agonist stimulation

We investigated the role of large-conductance Ca2+-activated K+(BKCa) channels for the basal renal vascular tone in vivo. Furthermore, the possible buffering by BKCaof the vasoconstriction elicited by angiotensin II (ANG II) or norepinephrine (NE) was investigated. The possible activation of renal vascular BKCachannels by cAMP was investigated by infusing forskolin. Renal blood flow (RBF) was measured in vivo using electromagnetic flowmetry or ultrasonic Doppler. Renal preinfusion of tetraethylammonium (TEA; 3.0 μmol/min) caused a small reduction of baseline RBF, but iberiotoxin (IBT; 0.3 nmol/min) did not have any effect. Renal injection of ANG II (1–4 ng) or NE (10–40 ng) produced a transient decrease in RBF. These responses were not affected by preinfusion of TEA or IBT. Renal infusion of the BKCaopener NS-1619 (90.0 nmol/min) did not affect basal RBF or the response to NE, but it attenuated the response to ANG II. Coadministration of NS-1619 with TEA or IBT abolished this effect. Forskolin caused renal vasodilation that was not inhibited by IBT. The presence of BKCachannels in the preglomerular vessels was confirmed by immunohistochemistry. Despite their presence, there is no indication for a major role for BKCachannels in the control of basal renal tone in vivo. Furthermore, BKCachannels do not have a buffering effect on the rat renal vascular responses to ANG II and NE. The fact that NS-1619 attenuates the ANG II response indicates that the renal vascular BKCachannels can be activated under certain conditions.

BKCa Channels Activating at Resting Potential without Calcium in LNCaP Prostate Cancer Cells

Large-conductance Ca2+-dependent K+ (BK(Ca)) channels are activated by intracellular Ca2+ and membrane depolarization in an allosteric manner. We investigated the pharmacological and biophysical characteristics of a BK(Ca)-type K+ channel in androgen-dependent LNCaP (lymph node carcinoma of the prostate) cells with novel functional properties, here termed BK(L). K+ selectivity, high conductance, activation by Mg2+ or NS1619, and inhibition by paxilline and penitrem A largely resembled the properties of recombinant BK(Ca) channels. However, unlike conventional BK(Ca) channels, BK(L) channels activated in the absence of free cytosolic Ca2+ at physiological membrane potentials; the half-maximal activation voltage was shifted by about -100 mV compared with BK(Ca) channels. Half-maximal Ca2+-dependent activation was observed at 0.4 microM: for BK(L) (at -20 mV) and at 4.1 microM: for BK(Ca) channels (at +50 mV). Heterologous expression of hSlo1 in LNCaP cells increased the BK(L) conductance. Expression of hSlo-beta1 in LNCaP cells shifted voltage-dependent activation to values between that of BK(L) and BK(Ca) channels and reduced the slope of the P (open) (open probability)-voltage curve. We propose that LNCaP cells harbor a so far unknown type of BK(Ca) subunit, which is responsible for the BK(L) phenotype in a dominant manner. BK(L)-like channels are also expressed in the human breast cancer cell line T47D. In addition, functional expression of BK(L) in LNCaP cells is regulated by serum-derived factors, however not by androgens.

Nitric oxide stimulates a large-conductance Ca-activated K+ channel in human skin fibroblasts through protein kinase G pathway

In order to investigate the large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel and determine the effects of nitric oxide (NO) on the channel in human skin fibroblasts, we performed electrophysiological patch clamp recordings on 5th-passage cells of human genital skin cultures. The whole-cell outward K(+) current was increased with depolarization, and proved to be sensitive to NS1619 (a selective BK(Ca) channel activator) and iberiotoxin (a specific BK(Ca )channel inhibitor). The single-channel currents showed 226 pS of mean conductance in symmetrical K(+). Sodium nitroprusside (SNP; an NO donor) significantly increased the K(+) current amplitude in the whole-cell mode, and open probability of the channel (NPo) in the cell-attached mode, but not in the inside-out mode. S-nitroso-N-acetylpenicillamine (an NO donor) and 8-Br-cGMP (a membrane-permeant cGMP analogue) also increased the BK(Ca )channel activity. The stimulatory effect of SNP on BK(Ca) channels was inhibited by pretreatment with 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (a soluble guanylyl cyclase inhibitor), or KT5823 [a specific protein kinase G (PKG) inhibitor]. Cytoplasmic PKG also increased the channel activity in inside-out patches. In conclusion, the present data indicate that BK(Ca) channels constitute a significant fraction of K(+) current in human skin fibroblasts, and that NO increases NPo of BK(Ca) channels, which are mediated via the cGMP/PKG pathway, without direct effects on the channel.

Pharmacological evidence for a key role of voltage-gated K+ channels in the function of rat aortic smooth muscle cells

The role of voltage-dependent (I(K(v))) and large conductance Ca(2+)-activated (BK(Ca)) K(+) currents in the function of the rat aorta was investigated using specific BK(Ca) and K(V) channel inhibitors in single rat aortic myocytes (RAMs) with patch-clamp technique and in endothelium-denuded aortic rings with isometric tension measurements. The whole-cell K(+) currents were recorded in RAMs dialysed with 200 and 444 nm Ca(2+) and in perforated-patch configuration. Electrophysiological analysis demonstrated that I(K(v)) appeared at >/=-40 mV, while BK(Ca) (isolated using 1 microm paxilline) were seen positive to -20 mV in all conditions. Voltage-dependent characteristics, but not maximal conductance, of I(K(v)) was significantly altered in increased [Ca(2+)](i). Correolide (1 microm) (a K(V)1 channel blocker) did not inhibit the I(K(v)), whereas millimolar concentration of TEA (IC(50)=3.1+/-0.6 mm, n=5) and 4-aminopyridine (4-AP, IC(50)=5.9+/-1.9 mm, n=7) suppressed I(K(v)). These results and immunocytochemical analysis suggest the K(V)2.1 channel to be a molecular correlate for I(K(v)). In nonstimulated aortic rings 1-5 mm TEA and 4-AP (inhibitors of I(K(v))), but not paxilline (1 microm), caused contraction. The frequency of contractile responses to TEA and 4-AP was increased in the presence of 10 mm KCl, which itself did not significantly affect the aortic basal tone. Phenylephrine (15-40 nm) induced sustained tension with superimposed slow oscillatory contractions (termed OWs). OWs were blocked by diltiazem, ryanodine and cyclopiazonic acid, suggesting the involvement of L-type Ca(2+) channels and ryanodine-sensitive Ca(2+) stores in this process. TEA and 4-AP, but not IbTX, paxilline or correolide, increased the duration and amplitude of OWs, indicating that I(K(v)) is involved in the control of oscillatory activity. In conclusion, our findings suggest that the K(V)2.1-mediated I(K(v)), and not BK(Ca), plays an important role in the regulation of the excitability and contractility of rat aorta.

Detection and implications of potassium channel alterations

Potassium channel dysfunction plays a role in the pathogenesis of a number of vascular diseases including pulmonary and systemic hypertension, diabetes, and complications of atherosclerosis. Two types of K+ channels that are known to be prevalent and contribute significantly to the repolarization of vascular smooth muscle cell (SMC) membranes are the high-conductance Ca(2+)- and voltage-activated K+ (BKCa) channels, and the voltage-gated K+ (KV) channels. Alterations in either BKCa or KV channel function can have dramatic effects on vascular tone. To date, hereditary and congenital mutations in genes encoding K+ channels, abnormalities in transcription, posttranslational modifications, and altered responses to intracellular second messengers have been described as potential mechanisms for several cardiovascular diseases. Comprehensive approaches including genetic, biochemical, molecular biological, and electrophysiological analyses are necessary to identify the levels at which K+ channel expression patterns or function are disrupted. Additionally, reproducing clinical pathologies in animal, organ, and virtual models has been important in studying the discrete mechanisms by which the structure and function of these channels are altered in pathophysiological conditions. This article will describe approaches that are currently used to identify abnormalities in BKCa and KV channels that may exist in diseases involving vascular dysfunction.

Potent inhibition of the aortic smooth muscle maxi-K channel by clinical doses of ethanol

We investigated the effects of clinically relevant ethanol concentrations (5-20 mM) on the single-channel kinetics of bovine aortic smooth muscle maxi-K channels reconstituted in lipid bilayers (1:1 palmitoyl-oleoyl-phosphatidylethanolamine: palmitoyl-oleoyl-phosphatidylcholine). Ethanol at 10 and 20 mM decreased the channel open probability (P(o)) by 75 +/- 20.3% mainly by increasing the mean closed time (+82 to +960%, n = 7). In some instances, ethanol also decreased the mean open time (-40.8 +/- 22. 5%). The P(o)-voltage relation in the presence of 20 mM ethanol exhibited a rightward shift in the midpoint of voltage activation (DeltaV(1/2) congruent with 17 mV), a slightly steeper relationship (change in slope factor, Deltak, congruent with -2.5 mV), and a decreased maximum P(o) (from approximately 0.82 to approximately 0. 47). Interestingly, channels inhibited by ethanol at low Ca(2+) concentrations (2.5 microM) were very resistant to ethanol in the presence of increased Ca(2+) (>/= 20 microM). Alcohol consumption in clinically relevant amounts may alter the contribution of maxi-K channels to the regulation of arterial tone.

The influence of phenylephrine outward potassium currents in single smooth muscle cells from the rabbit mesenteric artery

In mesenteric artery smooth muscle cells, depolarizing voltage steps activated outward K+ currents whose amplitude was decreased by about 20% with phenylephrine (1-10 microM: n = 14 cells). Attenuation of outward current was only partly dependent on [Ca2+]i, because it persisted, although reduced, with 10 mM BAPTA in the patch pipette and was abolished in the presence of 3 mM 3,4-diaminopyridine (n = 13). In outside-out patches, phenylephrine did not exert any direct effect on the unitary current amplitude or open probability of large conductance K+ channels. Outward current was significantly increased (>100% in both cases) by 10 mM caffeine, presumably owing to the release of internal Ca2+ stores. With 10 mM BAPTA in the pipette, the only response to caffeine was a small decrease (9 +/- 3.7%, n = 10) in the K+ current. These observations show that a minor effect of phenylephrine is to reduce outward K+ current (probably Kv) in mesenteric cells.

Rat supraoptic magnocellular neurones show distinct large conductance, Ca2+-activated K+ channel subtypes in cell bodies versus nerve endings

1. Large conductance, Ca2+-activated K+ (BK) channels were identified in freshly dissociated rat supraoptic neurones using patch clamp techniques. 2. The single channel conductance of cell body BK channels, recorded from inside-out patches in symmetric 145 mM K+, was 246.1 pS, compared with 213 pS in nerve ending BK channels (P<0.01). 3. At low open probability (Po), the reciprocal of the slope in the ln(NPo)-voltage relationship (N, number of available channels in the patch) for cell body and nerve ending channels were similar: 11 vs. 14 mV per e-fold change in NPo, respectively. 4. At 40 mV, the [Ca2+]i producing half-maximal activation was 273 nM, as opposed to >1.53 microM for the neurohypophysial channel, indicating the higher Ca2+ sensitivity of the cell body isochannel. 5. Cell body BK channels showed fast kinetics (open time constant, 8.5 ms; fast closed time constant, 1.6 and slow closed time constant, 12.7 ms), identifying them as 'type I' isochannels, as opposed to the slow gating (type II) of neurohypophysial BK channels. 6. Cell body BK activity was reduced by 10 nM charybdotoxin (NPo, 37% of control), or 10 nM iberiotoxin (NPo, 5% of control), whereas neurohypophysial BK channels are insensitive to charybdotoxin at concentrations as high as 360 nM. 7. Whilst blockade of nerve ending BK channels markedly slowed the repolarization of evoked single spikes, blockade of cell body channels was without effect on repolarization of evoked single spikes. 8. Ethanol reversibly increased neurohypophysial BK channel activity (EC50, 22 mM; maximal effect, 100 mM). In contrast, ethanol (up to 100 mM) failed to increase cell body BK channel activity. 9. In conclusion, we have characterized BK channels in supraoptic neuronal cell bodies, and demonstrated that they display different electrophysiological and pharmacological properties from their counterparts in the nerve endings.

In situ characterization of the Ca2+ sensitivity of large conductance Ca2+-activated K+ channels: implications for their use as near-membrane Ca2+ indicators in smooth muscle cells

The Ca2+ sensitivity of large conductance Ca2+- and voltage-activated K+ channels (BKV,Ca) has been determined in situ in freshly isolated myocytes from the guinea pig urinary bladder. In this study, in situ denotes that BKV,Ca channel activity was recorded without removing the channels from the cell. By combining patch clamp recording in the cell-attached configuration and microfluorometry of fura-2, we were able to correlate BKV,Ca channel activity with changes in cytoplasmic intracellular [Ca2+] ([Ca2+]i). The latter were induced by ionomycin, an electroneutral Ca2+ ionophore. At 0 mV, the Hill coefficient (nH) and the [Ca2+]i to attain half of the maximal BKV,Ca channel activity (Ca50) were 8 and 1 microM, respectively. The data suggest that this large Hill number was not a consequence of the difference between the near-membrane [Ca2+] ([Ca2+]s) and the bulk [Ca2+]i, indicated by fura-2. High Hill numbers in the activation by Ca2+ of BKV,Ca channels have been seen by different groups (e.g., filled squares in Fig. 4 of Silberberg, S. D., A. Lagrutta, J. P. Adelman, and K. L. Magleby. 1996. Biophys. J. 70:2640-2651). However, such high nH has always been considered a peculiarity rather than the rule. This work shows that a high Ca2+ cooperativity is the normal situation for BKV,Ca channels in myocytes from guinea pig urinary bladder. Furthermore, the Ca50 did not display any significant variation among different channels or cells. It was also evident that BKV,Ca channel activity could decrease in elevated [Ca2+]i, either partially or completely. This work implies that the complete activation of BKV,Ca channels occurs with a smaller increment in [Ca2+]s than previously expected from in vitro characterization of the Ca2+ sensitivity of these channels. Additionally, it appears that the activity of BKV,Ca channels in situ does not strictly follow changes in near-membrane [Ca2+].

Glomerular mesangial cells: electrophysiology and regulation of contraction

Mesangial cells are smooth muscle-like pericytes that abut and surround the filtration capillaries within the glomerulus. Studies of the fine ultrastructure of the glomerulus show that the mesangial cell and the capillary basement membrane form a biomechanical unit capable of regulating filtration surface area as well as intraglomerular blood volume. Structural and functional studies suggest that mesangial cells regulate filtration rate in both a static and dynamic fashion. Mesangial excitability enables a homeostatic intraglomerular stretch reflex that integrates an increase in filtration pressure with a reduction in capillary surface area. In addition, mesangial tone is regulated by diverse vasoactive hormones. Agonists, such as angiotensin II, contract mesangial cells through a signal transduction pathway that releases intracellular stores of Ca2+, which subsequently activate nonselective cation channels and Cl- channels to depolarize the plasma membrane. The change in membrane potential activates voltage-gated Ca2+ channels, allowing Ca2+ cell entry and further activation of depolarizing conductances. Contraction and entry of cell Ca2+ are inhibited only when Ca2+-activated K+ channels (BK(Ca)) are activated and the membrane is hyperpolarized toward the K+ equilibrium potential. The mesangial BK(Ca) is a weak regulator of contraction in unstimulated cells; however, the gain of the feedback is increased by atrial natriuretic peptide, nitric oxide, and the second messenger cGMP, which activates protein kinase G and decreases both the voltage and Ca2+ activation thresholds of BK(Ca) independent of sensitivity. This enables BK(Ca) to more effectively counter membrane depolarization and voltage-gated Ca2+ influx. After hyperpolarizing the membrane, BK(Ca) rapidly inactivates because of dephosphorylation by protein phosphatase 2A. Regulation of ion channels has been linked casually to hyperfiltration during early stages of diabetes mellitus. Determining the signaling pathways controlling the electrophysiology of glomerular mesangial cells is important for understanding how glomerular filtration rate is regulated in health and disease.

Nitric oxide (NO)-induced activation of large conductance Ca2+-dependent K+ channels (BK(Ca)) in smooth muscle cells isolated from the rat mesenteric artery

1. To assess the action of nitric oxide (NO) and NO-donors on K+ current evoked either by voltage ramps or steps, patch clamp recordings were made from smooth muscle cells freshly isolated from secondary and tertiary branches of the rat mesenteric artery. 2. Inside-out patches contained channels, the open probability of which increased with [Ca2+]i. The channels had a linear slope conductance of 212+/-5 pS (n = 12) in symmetrical (140 mM) K+ solutions which reversed in direction at 4.4 mV. In addition, the channels showed K+ selectivity, in that the reversal potential shifted in a manner similar to that predicted by the Nernst potential for K+. Barium (1 mM) applied to the intracellular face of the channel produced a voltage-dependent block and external tetraethylammonium (TEA; at 1 mM) caused a large reduction in the unitary current amplitude. Taken together, these observations indicate that the channel most closely resembled BK(Ca). 3. In five out of six inside-out patches, NO (45 or 67 microM) produced an increase in BK(Ca) activity. In inside-out patches, BK(Ca) activity was also enhanced in some patches with 100 or 200 microM 3-morpholino-sydnonimine (SIN-1) (4/11) and 100 microM sodium nitroprusside (SNP) (3/8). The variability in channel opening with the NO donors may reflect variability in the release of NO from these compounds. 4. In inside-out patches, 100 microM SIN-1 failed to increase BK(Ca) activity (in all 4 patches tested), while at a higher (500 microM) concentration SIN-1 had a direct blocking effect on the channels (n = 3). NO applied directly to inside-out patches increased (P < 0.05) BK(Ca) activity in two patches. 5. In the majority of cells (6 out of 7), application of NO (45 or 67 microM) evoked an increase in the amplitude of whole-cell currents in perforated patches. This action was not affected by the soluble guanylyl cyclase inhibitor, 1H-[1,2,4] oxadiazolo [4,3-a]quinoxalin-1-one (ODQ). An increase in whole-cell current was also evoked with either of the NO donors, SIN-1 or SNP (each at 100 microM). With SIN-1, the increase in current was blocked with the BK(Ca) channel blocker, iberiotoxin (50 nM). 6. With conventional whole-cell voltage clamp, the increase in the outward K+ current evoked with SIN-1 (50-300 microM) showed considerable variability. Either no effect was obtained (11 out of 18 cells), or in the remaining cells, an average increase in current amplitude of 38.7+/-10.2% was recorded at 40 mV. 7. In cell-attached patches, large conductance voltage-dependent K+ channels were stimulated by SIN-1 (100 microM) applied to the cell (n = 5 patches). 8. These data indicate that NO and its donors can directly stimulate BK(Ca) activity in cells isolated from the rat mesenteric artery. The ability of NO directly to open BK(Ca) channels could play an important functional role in NO-induced relaxation of the vascular smooth muscle cells in this small resistance artery.

Kinetic structure of large-conductance Ca2+-activated K+ channels suggests that the gating includes transitions through intermediate or secondary states. A mechanism for flickers

Mechanisms for the Ca2+-dependent gating of single large-conductance Ca2+-activated K+ channels from cultured rat skeletal muscle were developed using two-dimensional analysis of single-channel currents recorded with the patch clamp technique. To extract and display the essential kinetic information, the kinetic structure, from the single channel currents, adjacent open and closed intervals were binned as pairs and plotted as two-dimensional dwell-time distributions, and the excesses and deficits of the interval pairs over that expected for independent pairing were plotted as dependency plots. The basic features of the kinetic structure were generally the same among single large-conductance Ca2+-activated K+ channels, but channel-specific differences were readily apparent, suggesting heterogeneities in the gating. Simple gating schemes drawn from the Monod- Wyman-Changeux (MWC) model for allosteric proteins could approximate the basic features of the Ca2+ dependence of the kinetic structure. However, consistent differences between the observed and predicted dependency plots suggested that additional brief lifetime closed states not included in MWC-type models were involved in the gating. Adding these additional brief closed states to the MWC-type models, either beyond the activation pathway (secondary closed states) or within the activation pathway (intermediate closed states), improved the description of the Ca2+ dependence of the kinetic structure. Secondary closed states are consistent with the closing of secondary gates or channel block. Intermediate closed states are consistent with mechanisms in which the channel activates by passing through a series of intermediate conformations between the more stable open and closed states. It is the added secondary or intermediate closed states that give rise to the majority of the brief closings (flickers) in the gating.

Increased expression of Ca2+-sensitive K+ channels in the cerebral microcirculation of genetically hypertensive rats: evidence for their protection against cerebral vasospasm

The Ca2+-sensitive K+ channel (K(Ca) channel) plays a key role in buffering pressure-induced constriction of small cerebral arteries. An amplified current through this channel has been reported in vascular smooth muscle cells obtained from hypertensive animals, implying that the expression or properties of K(Ca) channels may be regulated by in vivo blood pressure levels. In this study, we investigated this hypothesis and its functional relevance by comparing the properties, expression levels, and physiological role of K(Ca) channels in cerebral resistance arteries from normotensive and genetically hypertensive rats. Whole-cell patch-clamp experiments revealed a 4.7-fold higher density of iberiotoxin-sensitive K(Ca) channel current at physiological membrane potentials in spontaneously hypertensive rat (SHR) compared with Wistar-Kyoto (WKY) rat cerebrovascular smooth muscle cells (n = 18 and 21, respectively). However, additional single-channel analysis in detached patches showed similar levels of unitary conductance, voltage, and Ca2+ sensitivity in K(Ca) channels from WKY and from SHR membranes. In contrast, Western analysis using an antibody directed against the K(Ca) channel alpha-subunit revealed a 4.1-fold increase in the corresponding 125-kD immunoreactive signal in cerebrovascular membranes from SHR compared with WKY rats. The functional impact of this enhanced K(Ca) channel expression was assessed in SHR and WKY rat pial arterioles, which were monitored by intravital microscopy through in situ cranial windows. Progressive pharmacological block of K(Ca) channels by iberiotoxin (0.1 to 100 nmol/L) dose-dependently constricted pial arterioles from SHR and WKY rats (n = 6 to 8). The arterioles in SHR constricted 2- to 4-fold more intensely, and vasospasm occurred in some vessels. These data provide the first direct evidence that elevated levels of in situ blood pressure induce K(Ca) channel expression in cerebrovascular smooth muscle membranes. This homeostatic mechanism may critically regulate the resting tone of cerebral arterioles during chronic hypertension. Furthermore, the overexpression of distinct K+ channel types during specific cardiovascular pathologies may provide for the upregulation of novel disease-specific membrane targets for vasodilator therapies.

Ca(2+)-dependent non-selective cation and potassium channels activated by bradykinin in pig coronary artery endothelial cells

1. Using the cell-attached and inside-out modes of the patch-clamp technique, we studied the Ca(2+)-dependent ionic channels activated by bradykinin in cultured pig coronary artery endothelial cells to further understand electrophysiological events underlying cellular activation. 2. In the cell-attached mode, bradykinin (94 nM) activated two types of Ca(2+)-dependent channels: a high conductance K+ channel (285 pS in high symmetrical K+), whose open state probability was increased by depolarization, and a lower conductance inwardly rectifying non-selective cation channel (44 pS in high symmetrical K+). 3. The 285 pS K+ channel was half-maximally activated by cytosolic Ca2+ levels of 1.6 and 4.5 microM at +10 and -30 mV, respectively. Such local concentrations should be reached in the presence of bradykinin, which induces a mean maximal cytosolic Ca2+ rise of 1.3 microM. 4. The 285 pS K+ channel was inhibited by d-tubocurarine, which acted by reducing the mean open time duration (flickering pattern), finally reducing the channel conductance. 5. Divalent cations such as Ca2+ could flow through the 44 pS non-selective cation channel, with nearly the same permeability (P) as monovalent cations (PK: PNa: PCa = 1:1:0.7). 6. The cation channel appeared to be more sensitive to Ca2+ than the K+ channel, with a half-maximal open probability induced by 0.7 microM Ca2+ on the intracellular side of the membrane. 7. In contrast to the K+ channel, the cation channel mean open time was clearly increased by bradykinin. This effect was delayed compared with the increase in the channel open state probability and was rapidly lost in the inside-out configuration. Caffeine also activated the cation channel but more transiently than bradykinin and without any effect on the open duration. 8. In the absence of extracellular Ca2+, the bradykinin-induced increase in cytosolic free Ca2+ was shortened temporally by 52% and reduced in amplitude by 88%, whereas the bradykinin-induced hyperpolarization was not significantly reduced in amplitude but was shortened by 70%, thus illustrating the major role of Ca2+ influx in endothelial cell activation by bradykinin. 9. We conclude that bradykinin activates two types of Ca(2+)-dependent channels in coronary endothelial cells: a high conductance K+ channel regulated by membrane potential, and an inwardly rectifying cation channel allowing Ca2+ entry, the cation channel being about 6 times more sensitive to Ca2+ than the K+ channel. The increase in cation channel open state probability involves an increase in open number, like the K+ channel, but also involves a rise in channel open duration. Ca2+ entry via cation channels could contribute to increase the cytoplasmic Ca2+ level, activate Ca(2+)-dependent K+ channels, thus triggering membrane hyperpolarization when the endothelial cell is stimulated by a vasoactive agonist such as bradykinin.

Synergistic action of NS-004 and internal Ca2+ concentration in modulating pulmonary artery K+ channels

Considering the singular vasomotor behavior of the pulmonary artery, we were interested to test NS-004 (1-(2'-hydroxy-5'-chlorophenyl)-5-trifluoromethyl-2(3H)-benzimidazolo ne) on pulmonary artery smooth muscle cells. Using the patch clamp technique, we identified a delayed rectifier K+ current and a Ca(2+)-activated K+ current. With a low free intracellular Ca2+ concentration ([Ca2+]i), 10-50 microM NS-004 activated a noisy outward current which was blocked by iberiotoxin. 50 microM NS-004 also inhibited a smooth inactivating outward current. Under these conditions, 10 microM NS-004 induced no change in the resting membrane potential. With a higher free [Ca2+]i, 10 microM NS-004 was 3.5 times more efficacious in increasing the noisy current and it induced a hyperpolarization. We concluded that increasing free [Ca2+]i induced potentiation of the NS-004-induced activation of high conductance Ca(2+)-sensitive K+ channels and of the NS-004-induced hyperpolarization of the cell. The delayed rectifier K+ channel was inhibited by NS-004 as well as by an increased free [Ca2+]i.

Physiological role of large, Ca2+-activated K+ channels in human glomerular mesangial cells

1. Contraction assays and patch clamp methods were used to determine the role of K+ channels in the regulation of contractile tone of human mesangial cells (MC) in culture. 2. MC contraction was induced by vasoconstrictor agents, such as angiotensin II (AngII; 100 nmol/L) and glybenclamide (Glyb), but not by iberiotoxin (IbTX), a blocker of large Ca2+-activated K+ channels (BK(Ca)). These results suggest that Glyb-sensitive K+ channels, but not BK(Ca) channels, were active at rest. 3. In the presence of 100 nmol/L IbTX, contraction by AngII was slightly, but not significantly, enhanced, indicating that BK(Ca) has a minimal role as a negative feedback regulator of contraction. Nitroprusside (NP; 100 mu mol/L) a nitric oxide (NO) donor, atrial natriuretic peptide (ANP; 1.0 mu mol/L) and db-cGMP (10 mu mol/L) attenuated AngII-induced contraction in the absence, but not in the presence, of IbTX, suggesting that BK(Ca) channels were activated by cGMP. 4. In patch clamp experiments, three distinct K+-selective channels of 9, 65 and 150 pS (outward currents) were found in excised, inside-out patches. The 150 pS channel was completely inhibited by 100 nmol/L IbTX and displayed voltage- and calcium-dependent gating qualitatively similar to BK(Ca) in other cell types. 5. In cell attached (CA) patches, the response of BK(Ca) to bath AngII (100 nmol/L) was relatively minor in control solutions, but was considerably greater in the presence of db-cGMP. 6. In excised patches, Mg-ATP (1 mmol/L) plus db-cGMP (1 mu mol/L) activated BK(Ca) in the absence, but not the presence, of the non-specific kinase inhibitor, staurosporine. 7. Separate experiments showed that BK(Ca) were also activated by arachidonic acid and high ambient glucose concentrations. 8. These results indicate that: (i) resting MC tone is sensitive to glybenclamide and apamin; and (ii) the role of BK(Ca) as a negative feedback regulator of contraction is minimal under normal conditions but is markedly enhanced by cGMP-stimulating relaxants and arachidonic acid.

Potassium currents in human freshly isolated bronchial smooth muscle cells

1. K+ currents were studied in smooth muscle cells enzymatically dissociated from human bronchi, by use of the patch-clamp technique. 2. In whole-cell recordings a depolarization-induced, 4-aminopyridine (4-AP)-sensitive current was observed in only 26 of 155 cells, and in 20 of these 26 cells its amplitude at a test potential of 0 mV was less than 100 pA. 3. In the majority of cells depolarization to -40 mV or more positive potentials induced a noisy outward current which activated within milliseconds and showed almost no inactivation even during a 5 s depolarizing voltage step. This current was insensitive to 4-AP (up to 5 mM) but was strongly inhibited in the presence of tetraethylammonium (TEA, 1 mM), charybdotoxin (ChTX, 100 nM) or iberiotoxin (IbTX, 50 nM) in the bath. The same current was also recorded by the nystatin-perforated patch technique. 4. Single channels with a conductance of about 210 pS were recorded in cell-attached patch, inside-out patch, outside-out patch and whole-cell recording configurations. Channel open state probability in inside-out patches was 0.5 at a membrane potential of 4 +/- 14 mV (mean +/- s.d., n = 13) mV even with a free Ca2+ concentration on the cytosolic side of the patch of less than 0.1 nM. Open state probability increased with depolarization and internal Ca2+ concentration. Single channels could be reversibly blocked by externally applied TEA, ChTX and IbTX. 5. In current-clamp recordings with 100 nM free Ca2+ in the intracellular solution both TEA and ChTX caused substantial concentration-dependent depolarization. 6. These results suggest that in human bronchial smooth muscle cells, in marked contrast to other species, the majority of the outward current induced by depolarization is not due to a delayed rectifier,but to the activity of a large conductance, ChTX-sensitive K+ channel. The Ca2+- and voltage-dependency of this channel may well allow a sufficiently high open state probability for it to play a partin the regulation of the resting membrane potential.