Calcium-activated potassium channels in single smooth muscle cells of rabbit jejunum and guinea-pig mesenteric artery

Effects of prostaglandin F2alpha on membrane currents in rabbit middle cerebral arterial smooth muscle cells

Although PGF(2alpha) affects contractility of vascular smooth muscles, no studies to date have addressed the electrophysiological mechanism of this effect. The purpose of our investigation was to examine the direct effects of PGF(2alpha) on membrane potentials, Ca(2+)-activated K(+) (K(Ca)) channels, delayed rectifier K(+) (K(V)) channels, and L-type Ca(2+) channels with the patch-clamp technique in single rabbit middle cerebral arterial smooth muscle cells (SMCs). PGF(2alpha) significantly hyperpolarized membrane potentials and increased the amplitudes of total K(+) currents. PGF(2alpha) increased open-state probability but had little effect on the open and closed kinetics of K(Ca) channels. PGF(2alpha) increased the amplitudes of K(V) currents with a leftward shift of the activation and inactivation curves and a decrease in the activation time constant. PGF(2alpha) decreased the amplitudes of L-type Ca(2+) currents without any significant change in threshold or apparent reversal potentials. This study provides the first finding that the direct effects of PGF(2alpha) on middle cerebral arterial SMCs, at least in part, could attenuate vasoconstriction.

Vasorelaxant effects of eugenol on rat thoracic aorta

Eugenol is a natural pungent substance and the main component of clove oil, with vasorelaxant action. To elucidate some of the possible mechanisms involved in this action isometric tension was measured in aortic rings from male Wistar rats precontracted with phenylephrine (PHE, 10(-7) M) or KCl (75 mM). Responses to increasing concentrations of eugenol (10(-6)-10(-2) M) were obtained in the presence and absence of endothelium. In the presence of eugenol, dose-response curves to PHE (10(-9) to 10(-4) M) and KCl (5-125 mM) were displaced downwards. Concentration-dependent relaxation was observed in rings precontracted with PHE (10(-7) M) and KCl (75 mM). The tension increment produced by increasing external calcium concentration (0.25-3 mM) was also reduced by eugenol (300 microM) treatment. The inhibitory effects of eugenol (300 microM) were compared to those induced by nifedipine (0.01 microM), a selective Ca(2+) channel blocker, producing similar relaxant effects. Two other protocols were performed. After precontraction with PHE (10(-7) M), increasing concentrations of eugenol (10(-6)-10(-2) M) were used before and after N(w)-nitro-L-arginine (L-NAME, 10(-4) M) and methylene blue (10(-5) M) treatment. Eugenol-induced relaxation was reduced by endothelial damage (rubbing), L-NAME and methylene blue treatments. Results suggested that eugenol produces smooth muscle relaxation resulting from the blockade of both voltage-sensitive and receptor-operated channels that are modulated by endothelial-generated nitric oxide.

Ammonium ion enhances the calcium-dependent gating of a mammalian large conductance, calcium-sensitive K+ channel

We observed that the current amplitude and activation of expressed, mouse brain large conductance, calcium-sensitive K+ channels (BKCa channels) may be reversibly enhanced following addition of low concentrations of the weakly permeant cation NH4+ to the cytoplasmic face of the channel in excised, inside-out membrane patches from HEK 293 cells. Conductance-voltage relations were left-shifted along the voltage axis by addition of NH4Cl in a concentration-dependent manner, with an EC50 of 18.5 mM. Furthermore, this effect was observed in the presence of cytosolic free calcium (~1 µM), but was absent in a cytosolic bath solution containing nominally zero free calcium (e.g., 5 mM EGTA only), a condition under which these channels undergo largely voltage-dependent gating. Recordings of single BKCa channel events indicated that NH4+ increased the channel open probability of single channel activity ~3-fold, but did not alter the amplitude of single channel currents. These findings suggest that the calcium-sensitive gating of mammalian BKCa channels may be modified by other ions present in cytosolic solution.Key words: potassium channel, calcium, modulation, electrophysiology.

Diversity of K+ channels in circular smooth muscle of opossum lower esophageal sphincter

We previously demonstrated that a balance of K+ and Ca2+-activated Cl– channel activity maintained the basal tone of circular smooth muscle of opossum lower esophageal sphincter (LES). In the current studies, the contribution of major K+ channels to the LES basal tone was investigated in circular smooth muscle of opossum LES in vitro. K+ channel activity was recorded in dispersed single cells at room temperature using patch-clamp recordings. Whole-cell patch-clamp recordings displayed an outward current beginning to activate at –60 mV by step test pulses lasting 400 ms (–120 mV to +100 mV) with increments of 20 mV from holding potential of –80 mV ([K+]I = 150 mM, [K+]o = 2.5 mM). However, no inward rectification was observed. The outward current peaked within 50 ms and showed little or no inactivation. It was significantly decreased by bath application of nifedipine, tetraethylammonium (TEA), 4-aminopyridine (4-AP), and iberiotoxin (IBTN). Further combination of TEA with 4-AP, nifedipine with 4-AP, and IBTN with TEA, or vice versa, blocked more than 90% of the outward current. Ca2+-sensitive single channels were recorded at asymetrical K+ gradients in cell-attached patch-clamp configurations (100.8 ± 3.2 pS, n = 8). Open probability of the single channels recorded in inside-out patch-clamp configurations were greatly decreased by bath application of IBTN (100 nM) (Vh = –14.4 ± 4.8 mV in control vs. 27.3 ± 0.1 mV, n = 3, P < 0.05). These data suggest that large conductance Ca2+-activated K+ and delayed rectifier K+ channels contribute to the membrane potential, and thereby regulate the basal tone of opossum LES circular smooth muscle.Key words: large conductance Ca2+-activated K+ channels, delayed rectifier K+ channels, patch-clamp recording, visceral smooth muscle.

Inhibition of Ca2+-activated K+ channels by tyrosine phosphatase inhibitors in rat mesenteric artery

To investigate the possible regulation of large-conductance Ca2+-activated K+ channels (BKCa) by tyrosine phosphatases (Tyr-PPs), single-channel currents of myocytes from rat mesenteric artery were recorded in open cell-attached patches. Two structurally different Tyr-PP inhibitors, sodium orthovanadate (Na3VO4) and dephostatin, were used. The channels (236 pS) evoked at +40 mV and pCa 6, were significantly inhibited by 1 mM Na3VO4 (-81 ± 3%, n = 10; P < 0.005). Similarly, 100 µM dephostatin strongly inhibited the BKCa channels (-80 ± 7%, n = 7 ; P < 0.05). Therefore, BKCa channels in vascular smooth muscle cells may be regulated by tyrosine phosphatase-dependent signal transduction pathways, whose inhibition could attenuate the channel activity.Key words: Ca2+-activated K+ channel, vascular smooth muscle, tyrosine phosphatase, vanadate, dephostatin.

Potassium channels in gastrointestinal smooth muscle

1. Electromechanical coupling in smooth muscle serves to coordinate the contractile activity of the syncytium. Electrical activity of smooth muscle of the gut is generated by ionic conductances that regulate and in turn are regulated by the membrane potential of smooth muscle cells. This activity determines the extent of Ca2+ entry into smooth muscle cells, and thus, the timing and intensity of contractions. 2. Potassium channels play an important role in regulating the excitability of the syncytium. The different types of K+ channel are characterized by different sensitivities to membrane potential, to intracellular Ca2+ levels and to modulation by agonists. 3. This review highlights the different types of K+ channels found in gut smooth muscle and describes their possible roles in regulating the electrical activity of the muscle.

3-Morpholinosydnonimine (SIN-1) and K(+) channels in smooth muscle cells of the rabbit and guinea pig carotid arteries

Experiments were designed to determine the subtype of K(+) channels activated by the nitrovasodilator 3-morpholinosydnonimine (SIN-1) in smooth muscle cells of the rabbit and guinea pig carotid arteries. Membrane potential was recorded in isolated segments with intracellular microelectrode and K(+) currents in freshly dissociated smooth muscle cells, with the patch-clamp technique. In the guinea pig carotid artery, SIN-1 caused a glibenclamide-sensitive hyperpolarization. The nitrovasodilator did not affect the whole-cell K(+) current, but activated a glibenclamide-sensitive K(+) current. In the rabbit carotid artery, SIN-1 induced only an iberiotoxin-sensitive repolarization in phenylephrine-depolarized tissue and in isolated cells, enhanced the activity of an iberiotoxin-sensitive K(+) current. These findings demonstrate that the population of K(+) channels activated by nitric oxide (NO) is species-dependent and support the conclusion that, in the guinea pig carotid artery, in contrast to the rabbit carotid artery, the release of NO cannot account for the responses attributed to endothelium-derived hyperpolarizing factor (EDHF).

Peroxynitrite reversibly inhibits Ca(2+)-activated K(+) channels in rat cerebral artery smooth muscle cells

Peroxynitrite (ONOO(-)) is a contractile agonist of rat middle cerebral arteries. To determine the mechanism responsible for this component of ONOO(-) bioactivity, the present study examined the effect of ONOO(-) on ionic current and channel activity in rat cerebral arteries. Whole cell recordings of voltage-clamped cells were made under conditions designed to optimize K(+) current. The effects of iberiotoxin, a selective inhibitor of large-conductance Ca(2+)-activated K(+) (BK) channels, and ONOO(-) (10-100 microM) were determined. At a pipette potential of +50 mV, ONOO(-) inhibited 39% of iberiotoxin-sensitive current. ONOO(-) was selective for iberiotoxin-sensitive current, whereas decomposed ONOO(-) had no effect. In excised, inside-out membrane patches, channel activity was recorded using symmetrical K(+) solutions. Unitary currents were sensitive to increases in internal Ca(2+) concentration, consistent with activity due to BK channels. Internal ONOO(-) dose dependently inhibited channel activity by decreasing open probability and mean open times. The inhibitory effect of ONOO(-) could be overcome by reduced glutathione. Glutathione, added after ONOO(-), restored whole cell current amplitude to control levels and reverted single-channel gating to control behavior. The inhibitory effect of ONOO(-) on membrane K(+) current is consistent with its contractile effects in isolated cerebral arteries and single myocytes. Taken together, our data suggest that ONOO(-) has the potential to alter cerebral vascular tone by inhibiting BK channel activity.

Nitric oxide and thiol reagent modulation of Ca2+-activated K+ (BKCa) channels in myocytes of the guinea-pig taenia caeci

The modulation of large conductance Ca2+-activated K+ (BKCa) channels by the nitric oxide (NO) donors S-nitroso-L-cysteine (NOCys) and sodium nitroprusside (SNP) and agents which oxidize or reduce reactive thiol groups were compared in excised inside-out membrane patches of the guinea-pig taenia caeci. When the cytosolic side of excised patches was bathed in a physiological salt solution (PSS) containing 130 mM K+ and 15 nM Ca2+, few BKCa channel openings were recorded at potentials negative to 0 mV. However, the current amplitude and open probability (NPo) of these BKCa channels increased with patch depolarization. A plot of ln(NPo) against the membrane potential (V) fitted with a straight line revealed a voltage at half-maximal activation (V0.5) of 9.4 mV and a slope (K) indicating an e-fold increase in NPo with 12.9 mV depolarization. As the cytosolic Ca2+ was raised to 150 nM, V0.5 shifted 11.5 mV in the negative direction, with little change in K (13.1 mV). NOCys (10 microM) and SNP (100 microM) transiently increased NPo 16- and 3. 7-fold, respectively, after a delay of 2-5 min. This increase in NPo was associated with an increase in the number of BKCa channel openings evoked at positive potentials by ramped depolarizations (between -60 and +60 mV). Moreover, this NOCys-induced increase in NPo was still evident in the presence of 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM), the specific blocker of soluble guanylyl cyclase. The sulfhydryl reducing agents dithiothreitol (DTT; 10 and 100 microM) and reduced glutathione (GSH; 1 mM) also significantly increased NPo (at 0 mV) 7- to 9-fold, as well as increasing the number of BKCa channel openings evoked during ramped depolarizations. Sulfhydryl oxidizing agents thimerosal (10 microM) and 4,4'-dithiodipyridine (4,4DTDP; 10 microM) and the thiol-specific alkylating agent N-ethylmaleimide (NEM; 1 mM) significantly decreased NPo (at 0 mV) to 40-50% of control values after 5-10 min. Ramped depolarizations to +100 mV evoked relatively few BKCa channel openings. The effects of thimerosal on NPo were readily reversed by DTT, while the effects of NOCys were prevented by NEM. It was concluded that both redox modulation and nitrothiosylation of cysteine groups on the cytosolic surface of the alpha subunit of the BKCa channel protein can alter channel gating.

Augmentation of SR Ca(2+) release by rapamycin and FK506 causes K(+)-channel activation and membrane hyperpolarization in bladder smooth muscle

1. The immunosuppressants rapamycin and FK506 are known to relax smooth muscle despite facilitating Ca(2+) release through ryanodine-receptors of the sarcoplasmic reticulum (SR). The apparent contradiction was studied in isolated guinea-pig urinary bladder myocytes. 2. Modulation of spontaneous SR Ca(2+) release was monitored by means of spontaneous transient outward currents (or STOCs) in isolated smooth muscle cells voltage-clamped to -20 mV. Rapamycin (10 microM, n=18) significantly increased amplitude (50+/-12%, mean+/-s.d.), life time (77+/-19%), and time integral of STOCs (113+/-22%), and it reduced the interval between STOCs (20+/-7%). FK506 (20 microM, n=24) increased amplitude (15+/-7%), life time (50+/-7%), time integral (104+/-26%). Cyclosporin A (20 microM, n=18) had no significant effects on STOCs. 3. The basal cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) measured by Indo1-fluorescence was insensitive to rapamycin or FK506. Pretreatment with rapamycin (20 microM, 2 min) did not impair the SR Ca(2+) load as can be concluded from caffeine-induced Ca(2+)-transients. 4. As it was expected from the enhanced STOC activity, the non-clamped membrane was hyperpolarized by rapamycin (15+/-2 mV) or by FK506 (15+/-3 mV). 5. The data are consistent with the idea that rapamycin and FK506 augment spontaneous SR Ca(2+) release by removal of FK-binding proteins from the RyR-complex. Smooth muscle relaxation is interpreted as negative Ca(2+) feedback: augmented Ca(2+) activation of STOCs induces membrane hyperpolarization that reduces Ca(2+) influx through voltage gated channels.

Calcium sparks in smooth muscle

Local intracellular Ca(2+) transients, termed Ca(2+) sparks, are caused by the coordinated opening of a cluster of ryanodine-sensitive Ca(2+) release channels in the sarcoplasmic reticulum of smooth muscle cells. Ca(2+) sparks are activated by Ca(2+) entry through dihydropyridine-sensitive voltage-dependent Ca(2+) channels, although the precise mechanisms of communication of Ca(2+) entry to Ca(2+) spark activation are not clear in smooth muscle. Ca(2+) sparks act as a positive-feedback element to increase smooth muscle contractility, directly by contributing to the global cytoplasmic Ca(2+) concentration ([Ca(2+)]) and indirectly by increasing Ca(2+) entry through membrane potential depolarization, caused by activation of Ca(2+) spark-activated Cl(-) channels. Ca(2+) sparks also have a profound negative-feedback effect on contractility by decreasing Ca(2+) entry through membrane potential hyperpolarization, caused by activation of large-conductance, Ca(2+)-sensitive K(+) channels. In this review, the roles of Ca(2+) sparks in positive- and negative-feedback regulation of smooth muscle function are explored. We also propose that frequency and amplitude modulation of Ca(2+) sparks by contractile and relaxant agents is an important mechanism to regulate smooth muscle function.

Desensitization to ANG II in guinea pig ileum depends on membrane repolarization: role of maxi-K(+) channel

Desensitization of ANG II tonic contractile response of the guinea pig ileum is related to membrane repolarization determined by Ca(2+)-activated K(+) (maxi-K(+)) channel opening. ANG II-stimulated depolarized myocytes presented sustained activation of maxi-K(+) channels, characterized by reduction from 415 to 12 ms of the closed time constant. ANG II desensitization was prevented by 100 nM iberiotoxin, being reversible within 30 min. Depolarization by KCl, higher than 4 mM, impaired desensitization, suggesting that the membrane potential must attain a threshold to counteract the repolarization induced by maxi-K(+) channel opening. Once this value is attained, there is no time dependency because the desensitization process was shut off by addition of KCl along the time course of the tonic response. In contrast, the sustained ACh tonic component was not altered by these maneuvers. We conclude that desensitization of the ANG II tonic component is foremost due to the opening of maxi-K(+) channels, leading to membrane repolarization, thus closing the voltage-dependent Ca(2+) channels responsible for the Ca(2+) influx that sustains the tonic component in this muscle.

Large-conductance Ca2+-activated potassium channels in secretory neurons

Large-conductance Ca2+-activated K+ channels (BK) are believed to underlie interburst intervals and contribute to the control of hormone release in several secretory cells. In crustacean neurosecretory cells, Ca2+ entry associated with electrical activity could act as a modulator of membrane K+ conductance. Therefore we studied the contribution of BK channels to the macroscopic outward current in the X-organ of crayfish, and their participation in electrophysiological activity, as well as their sensitivity toward intracellular Ca2+, ATP, and voltage, by using the patch-clamp technique. The BK channels had a conductance of 223 pS and rectified inwardly in symmetrical K+. These channels were highly selective to K+ ions; potassium permeability (PK) value was 2.3 x 10(-13) cm(3) s(-1). The BK channels were sensitive to internal Ca2+ concentration, voltage dependent, and activated by intracellular MgATP. Voltage sensitivity (k) was approximately 13 mV, and the half-activation membrane potentials depended on the internal Ca2+ concentration. Calcium ions (0.3-3 microM) applied to the internal membrane surface caused an enhancement of the channel activity. This activation of BK channels by internal calcium had a KD(0) of 0.22 microM and was probably due to the binding of only one or two Ca2+ ions to the channel. Addition of MgATP (0.01-3 mM) to the internal solution increased steady state-open probability. The dissociation constant for MgATP (KD) was 119 microM, and the Hill coefficient (h) was 0.6, according to the Hill analysis. Ca2+-activated K+ currents recorded from whole cells were suppressed by either adding Cd2+ (0.4 mM) or removing Ca2+ ions from the external solution. TEA (1 mM) or charybdotoxin (100 nM) blocked these currents. Our results showed that both BK and K(ATP) channels are present in the same cell. Even when BK and K(ATP) channels were voltage dependent and modulated by internal Ca2+ and ATP, the profile of sensitivity was quite different for each kind of channel. It is tempting to suggest that BK and KATP channels contribute independently to the regulation of spontaneous discharge patterns in crayfish neurosecretory cells.

Molecular diversity of K(V) alpha- and beta-subunit expression in canine gastrointestinal smooth muscles

Voltage-activated K(+) (K(V)) channels play an important role in regulating the membrane potential in excitable cells. In gastrointestinal (GI) smooth muscles, these channels are particularly important in modulating spontaneous electrical activities. The purpose of this study was to identify the molecular components that may be responsible for the K(V) currents found in the canine GI tract. In this report, we have examined the qualitative expression of eighteen different K(V) channel genes in canine GI smooth muscle cells at the transcriptional level using RT-PCR analysis. Our results demonstrate the expression of K(V)1.4, K(V)1.5, K(V)1.6, K(V)2.2, and K(V)4.3 transcripts in all regions of the GI tract examined. Transcripts encoding K(V)1.2, K(V)beta1.1, and K(V)beta1.2 subunits were differentially expressed. K(V)1.1, K(V)1.3, K(V)2.1, K(V)3.1, K(V)3.2, K(V)3.4, K(V)4.1, K(V)4.2, and K(V)beta2.1 transcripts were not detected in any GI smooth muscle cells. We have also determined the protein expression for a subset of these K(V) channel subunits using specific antibodies by immunoblotting and immunohistochemistry. Immunoblotting and immunohistochemistry demonstrated that K(V)1.2, K(V)1.4, K(V)1.5, and K(V)2.2 are expressed at the protein level in GI tissues and smooth muscle cells. K(V)2.1 was not detected in any regions of the GI tract examined. These results suggest that the wide array of electrical activity found in different regions of the canine GI tract may be due in part to the differential expression of K(V) channel subunits.

Calcium dependence of C-type natriuretic peptide-formed fast K(+) channel

The lipid bilayer technique was used to characterize the Ca(2+) dependence of a fast K(+) channel formed by a synthetic 17-amino acid segment [OaCNP-39-(1-17)] of a 39-amino acid C-type natriuretic peptide (OaCNP-39) found in platypus (Ornithorhynchus anatinus) venom (OaV). The OaCNP-39-(1-17)-formed K(+) channel was reversibly dependent on 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-buffered cis (cytoplasmic) Ca(2+) concentration ([Ca(2+)](cis)). The channel was fully active when [Ca(2+)](cis) was >10(-4) M and trans (luminal) Ca(2+) concentration was 1.0 mM, but not at low [Ca(2+)](cis). The open probability of single channels increased from zero at 1 x 10(-6) M cis Ca(2+) to 0.73 +/- 0.17 (n = 22) at 10(-3) M cis Ca(2+). Channel openings to the maximum conductance of 38 pS were rapidly and reversibly activated when [Ca(2+)](cis), but not trans Ca(2+) concentration (n = 5), was increased to >5 x 10(-4) M (n = 14). Channel openings to the submaximal conductance of 10.5 pS were dominant at >/=5 x 10(-4) M Ca(2+). K(+) channels did not open when cis Mg(2+) or Sr(2+) concentrations were increased from zero to 10(-3) M or when [Ca(2+)](cis) was maintained at 10(-6) M (n = 3 and 2). The Hill coefficient and the inhibition constant were 1 and 0.8 x 10(-4) M cis Ca(2+), respectively. This dependence of the channel on high [Ca(2+)](cis) suggests that it may become active under 1) physiological conditions where Ca(2+) levels are high, e.g., during cardiac and skeletal muscle contractions, and 2) pathological conditions that lead to a Ca(2+) overload, e.g., ischemic heart and muscle fatigue. The channel could modify a cascade of physiological functions that are dependent on the Ca(2+)-activated K(+) channels, e.g., vasodilation and salt secretion.

Inflammatory modulation of calcium-activated potassium channels in canine colonic circular smooth muscle cells

BACKGROUND & AIMS

The characteristics of colonic circular smooth muscle slow waves are altered during inflammation. The aim of this study was to examine whether inflammation modulates the open-state probability of Ca2+-activated K+ (KCa) channels in these cells to contribute to these alterations.

METHODS

The experiments were performed on freshly dissociated single smooth muscle cells from the canine colon using standard patch clamp methods. Inflammation was induced by mucosal exposure to ethanol and acetic acid.

RESULTS

Inflammation decreased the open-state probability of large-conductance KCa (BK) channels in the cell-attached and excised inside-out configurations. The voltage sensitivity of the channels was also reduced during inflammation. Inflammation had no significant effect on the large, medium, and small conductances or the unitary current levels of channel openings. However, it decreased the maximum number of simultaneous channel openings. The channels were Ca2+-dependent and were blocked by tetraethylammonium and charybdotoxin in normal and inflamed cells.

CONCLUSIONS

Inflammation decreases the open-state probability of BK channels. This may partially reverse the decrease in duration and amplitude of slow waves and depolarization of membrane potential seen in inflammation.

Characterization and regulation of Ca2+-dependent K+ channels in human esophageal smooth muscle

We examined the properties of K+ channels in smooth muscle cells dissociated from human esophagus using patch-clamp recording in the cell-attached configuration. The predominant channel observed had a conductance of 224 +/- 4 pS, and current reversal was dependent on K+ concentration. Channel activity was voltage dependent and increased with elevation of intracellular free Ca2+ concentration ([Ca2+]i), consistent with this being the large-conductance Ca2+-dependent K+ (KCa) channel. ACh as well as caffeine caused transient increases in KCa channel activity, and the effects of ACh persisted in Ca2+-free solution, indicating that Ca2+ release from stores contributed to channel activation. Simultaneous patch clamp and fluorescence revealed that KCa channel activity was well correlated with elevation of [Ca2+]i. The functional role of KCa channels in esophagus was studied by measuring ACh-induced contraction of strips of muscle. Tetraethylammonium and iberiotoxin, blockers of KCa channels, increased ACh-induced contraction, consistent with a role for K+ channels in limiting excitation and contraction. These studies are the first to characterize KCa channels and their regulation in human esophageal smooth muscle.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

An intermediate conductance K+ channel in the cell membrane of mouse intestinal smooth muscle

Single channel currents were recorded from cell-attached and inside-out patches in smooth muscle cells of the mouse ileum in order to identify TEA-sensitive Ca2+-dependent K+ channels. Cells were bathed in high-K+ (150 mM) solution with [Ca2+] buffered to 80-150 nM with EGTA and patch pipettes were filled with low-K+ (2.5 mM) physiological solution. Two distinct TEA-sensitive unitary outward current levels were identified at a holding potential (Vh) of 0 mV, corresponding to intermediate conductance (IK, approximately 40 pS) and large conductance (BK, >200 pS) K+ channels. The open probability (Po) of IK channels increased with depolarization, the voltage for half-maximal activation averaging +12 mV in 80 nM Cabath2+. Raising the [Ca2+] in the high-K+ solution from 80 nM to 150 nM increased the Po of IK channels at Vh=0 mV from 0.078 to 0.21. Likewise, the open probability of BK channels at 0 mV was increased from 0.003 to 0.026. Unlike BK channels, IK channels inactivated with maintained depolarization with a voltage for half-maximal inactivation of -66 mV. IK channels were blocked by 2-5 mM external TEA and were sensitive to both charybdotoxin (100 nM) and apamin (500 nM). Our results suggest that IK channels contribute significantly to the Ca2+-dependent K+ conductance in visceral smooth muscle.

Effect of nitric oxide on calcium-activated potassium channels in colonic smooth muscle of rabbits

Nitric oxide (NO) hyperpolarizes intestinal smooth muscle cells. This study was designed to determine the mechanism whereby NO activates KCa channels of circular smooth muscle of the rabbit colon. Transmural biopsies of the rabbit colon were stained for NADPH-diaphorase. Freshly dispersed circular smooth muscle cells were studied in the whole cell configuration, as well as in on-cell and excised inside-out patch recording configurations, while KCa current and the activity of KCa channels, respectively, were monitored. NADPH-diaphorase-positive nerve fibers were found in both muscle layers. NO (1%) increased whole cell net outward current by 79% and hyperpolarized resting membrane voltage from -59 to -73 mV (n = 8 cells, P < 0.01). In the on-cell patch recording configuration. NO (0.5% or 1%) in the bath increased NPo of KCa channels; charybdotoxin (125 nM) in the pipette solution blocked this effect. In the excised inside-out patch recording configuration, NO (1%) had no effect on NPo of KCa channels. In the on-cell patch recording configuration, methylene blue (1 microM) or cystamine (5 mM) in the bath solution decreased the effect of NO (1%) on NPo of KCa channels. NPo was increased by 8-bromo-cGMP (8-BrcGMP; 1 mM), a cGMP analog, and zaprinast (100 microM), an inhibitor of cGMP phosphodiesterase. These data suggest that NO increased whole cell outward K+ current by activating KCa channels through a cGMP pathway.

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.

Effect of motilin and erythromycin on calcium-activated potassium channels in rabbit colonic myocytes

BACKGROUND & AIMS

Motilin and erythromycin are prokinetic agents that act on the same receptor in gastrointestinal smooth muscle to cause contraction. Both agonists may also cause an increase in outward current. The aim of this study was to determine whether motilin and erythromycin activate calcium-activated potassium (KCa) channels.

METHODS

Freshly dispersed longitudinal smooth muscle cells of the rabbit colon were used to measure whole-cell outward current and single-channel activity using patch clamp recording methods.

RESULTS

Erythromycin and motilin increased a calcium-dependent outward potassium current and increased the open probability of KCa channels of cell- attached patches.

CONCLUSIONS

Erythromycin and motilin activate KCa channels via an intracellular second messenger system. This effect may modulate the increase in contractility caused by these agonists.

Effects of nitric oxide donors, S-nitroso-L-cysteine and sodium nitroprusside, on the whole-cell and single channel currents in single myocytes of the guinea-pig proximal colon

1. The nature of the membrane channels underlying the membrane conductance changes induced by the nitric oxide (NO) donors, S-nitroso-L-cysteine (NOCys) and sodium nitroprusside (SNP) were investigated in single myocytes isolated from the circular muscle layer of the guinea-pig proximal colon, by use of standard whole-cell and single channel recording techniques. 2. Under voltage clamp, depolarizing steps from -60 mV elicited a rapidly-developing, little-inactivating outward K+ current (IK) at potentials positive to -40 mV (at 20-25 degrees C). The steady-state level (ISS) of this K current increased in amplitude as the step potential was made to more positive potentials. If the depolarizing steps were made from a holding potential of -80 mV an additional rapidly activating and inactivating outward K+ current was also elicited, superimposed on IK. 3. At 20-25 degrees C, NOCys (2.5 microM), SNP (100 microM) and 8-bromo-cyclic GMP (500 microM) increased the amplitude of ISS of IK elicited from a holding potential of -60 mV. In contrast, NOCys (2-5 microM) had little effect on ISS at 35 degrees C. Higher concentrations (> or = 5 microM at 20-25 degrees C and > or = 10 microM at 35 degrees C) of NOCys decreased the peak amplitude (I[Peak]) and ISS of IK in a concentration-dependent manner. This blockade of IK with NOCys was always associated with an increase of the holding current (IHold), due to the activation of a membrane conductance with a reversal potential between 0 and + 30 mV and which was reduced approximately 50% upon the addition of Cd2+ (1 mM). 4. NOCys (2.5 to 10 microM) or SNP (100 microM) increased the activity of large conductance Ca2+-activated (BK) K' channels in both cell-attached and excised inside-out patches, bathed in either a symmetrical high K+ (130 mM) or an asymmetrically K+ (6 mMout: 130 mMin) physiological saline. Increases in BK channel activity in NOCys (10 microM) or SNP (100 microM) were associated with an increase in the probability of BK channel opening (N.Po), and with a negative shift of the plots of ln(N.Po) against the patch potential, with little change in the slopes of these plots. In cell-attached patches, the increase in N.Po with NOCys was often associated with a decrease in the BK single channel conductance. 5. In both cell-attached and excised patches, NOCys (2.5 to 10 microM) also activated an additional population of channels which allowed inward current flow at potentials positive to EK. In excised inside-out patches bathed in asymmetrical K+ physiological saline, these single channel currents were 2-3 pA in amplitude at -30 mV and reversed in direction near + 10 mV, even if the NaCl (126 mM) concentration in the pipette solution had been replaced with an equimolar concentration of Na gluconate. 6. Under current clamp, NOCys (2.5 microM) and SNP (100 microM) had variable effects on the membrane potential of colonic myocytes, inducing either a small membrane hyperpolarization of <5 mV, or a slowly-developing membrane depolarization of about 5 mV. In contrast, NOCys (5 microM) produced a transient membrane hyperpolarization which was followed by a large depolarization of the membrane potential to positive potentials. The electrotonic potentials elicited in response to an injection of constant hyperpolarizing current (10 pA for 400 ms) were little changed during the NOCys (5 PM)-induced membrane hyperpolarization, but significantly reduced (to 61% of control) during the periods of membrane depolarization. 7. It was concluded that NOCys and SNP, directly increased the number of active BK channels in the membrane of colonic myocytes which leads to a small rapidly oscillating membrane hyperpolarization. The following rebound depolarization in NOCys arises from both the direct opening of a population of cationic channels and the blockade of voltage- and Ca-activated K+ conductances. Finally, the apamin-sensitive K+channels underlying the initial transient hyperpolarization recorded in the intact proximal colon, in response to nerve-released or directly-applied NO, have yet to be identified at the single channel or whole-cell current level.

Characterization and function of Ca(2+)-activated K+ channels in arteriolar muscle cells

We examined the functional role of large-conductance Ca(2+)-activated K+ (KCa) channels in the hamster cremasteric microcirculation by intravital videomicroscopy and characterized the single-channel properties of these channels in inside-out patches of membrane from enzymatically isolated cremasteric arteriolar muscle cells. In second-order (39 +/- 1 microns, n = 8) and third-order (19 +/- 2 microns, n = 8) cremasteric arterioles with substantial resting tone, superfusion with the KCa channel antagonists tetraethylammonium (TEA, 1 mM) or iberiotoxin (IBTX, 100 nM) had no significant effect on resting diameters (P > 0.05). However, TEA potentiated O2-induced arteriolar constriction in vivo, and IBTX enhanced norepinephrine-induced contraction of cremasteric arteriolar muscle cells in vitro. Patch-clamp studies revealed unitary K(+)-selective and IBTX-sensitive currents with a single-channel conductance of 240 +/- 2 pS between -60 and 60 mV (n = 7 patches) in a symmetrical 140 mM K+ gradient. The free Ca2+ concentration ([Ca2+]) for half-maximal channel activation was 44 +/- 3, 20 +/- 1, 6 +/- 0.4, and 3 +/- 0.5 microM at membrane potentials of -60, -30, +30, and +60 mV, respectively (n = 5), with a Hill coefficient of 1.9 +/- 0.2. Channel activity increased e-fold for a 16 +/- 1 mV (n = 6) depolarization. The plot of log[Ca2+] vs. voltage for half-maximal activation (V1/2) was linear (r2 = 0.9843, n = 6); the change in V1/2 for a 10-fold change in [Ca2+] was 84 +/- 5 mV, and the [Ca2+] for half-maximal activation at 0 mV (Ca0; the Ca2+ set point) was 9 microM. Thus, in vivo, KCa channels are silent in cremasteric arterioles at rest but can be recruited during vasoconstriction. We propose that the high Ca0 is responsible for the apparent lack of activity of these channels in resting cremasteric arterioles, and we suggest that this may result from expression of unique KCa channels in the microcirculation.

Small-conductance Ca(2+)-dependent K+ channels activated by ATP in murine colonic smooth muscle

The patch-clamp technique was used to determine the ionic conductances activated by ATP in murine colonic smooth muscle cells. Extracellular ATP, UTP, and 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP) increased outward currents in cells with amphotericin B-perforated patches. ATP (0.5-1 mM) did not affect whole cell currents of cells dialyzed with solutions containing ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Apamin (3 x 10(-7) M) reduced the outward current activated by ATP by 32 +/- 5%. Single channel recordings from cell-attached patches showed that ATP, UTP, and 2-MeS-ATP increased the open probability of small-conductance, Ca(2+)-dependent K+ channels with a slope conductance of 5.3 +/- 0.02 pS. Caffeine (500 microM) enhanced the open probability of the small-conductance K+ channels, and ATP had no effect after caffeine. Pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS, 10(-4) M), a nonselective P2 receptor antagonist, prevented the increase in open probability caused by ATP and 2-MeS-ATP. PPADS had no effect on the response to caffeine. ATP-induced hyperpolarization in the murine colon may be mediated by P2y-induced release of Ca2+ from intracellular stores and activation of the 5.3-pS Ca(2+)-activated K+ channels.

Roles of outward potassium currents in the action potentials of guinea pig ureteral myocytes

Outward currents of freshly dissociated ureteral myocytes consist mainly of Ca(2+)-activated K+ current (IKCa) and a transient outward current (ITO). No delayed rectifier current was apparent. IKCa is small and nondecaying and fluctuates actively and irregularly. Blocking IKCa decreased resting membrane conductance and prolonged action potential plateaus, showing its roles in maintaining the resting potential and in repolarizing action potentials. It is also responsible for the membrane potential fluctuations on action potential plateaus. Neither 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride nor caffeine reduced the fluctuations in the outward current or in the action potentials, indicating that internal Ca2+ storage contributes little to the fluctuations. ITO has fast activation and inactivation kinetics with inactivation time constants of approximately 15 and 150 ms, respectively. Its highly negative voltage-availability relationship (V0.5 = -70.5 mV) suggests a low availability (< 5%) at normal resting potentials. It has only trivial effects on action potentials.

Ca(2+)-activated K+ channels regulate action potential repolarization in urinary bladder smooth muscle

The goal of this study was to examine the role of large conductance Ca(2+)-activated K+ channels in the regulation of cell excitability in urinary bladder smooth muscle from the guinea pig. Ca(2+)-activated K+ channels were studied with single-channel recording techniques and found to be intracellular Ca2+ and voltage dependent and sensitive to external tetraethylammonium and blocked by nanomolar concentrations of iberiotoxin (apparent dissociation constant of 4 nM). Spontaneous action potentials recorded from intact tissue strips depended on external Ca2+ and were inhibited by Ca2+ channel blockers. Iberiotoxin (100 nM) significantly altered the configuration of the action potential by increasing the duration and peak amplitude of the action potential and decreasing the rate of decay. Iberiotoxin also increased the action potential frequency from 0.11 to 0.29 Hz. This study suggests that Ca(2+)-activated K+ channels play a significant role in the repolarization of the action potential and in the maintenance of the resting membrane potential of the urinary bladder smooth muscle.

Mammalian retinal glial (Müller) cells express large-conductance Ca(2+)-activated K+ channels that are modulated by Mg2+ and pH and activated by protein kinase A

The cell-attached and excised patch configurations of the patch clamp technique were used to characterize Ca(2+)-activated maxi-K+ channels in freshly-isolated Müller glial cells. The cells were dissociated from postmortem adult human and porcine retinas. The maxi-K+ channels in Müller cells of both species display a single channel conductance of 175 pS in cell-attached and inside-out patches (125/110 mM K+). The channels are activated by membrane depolarization and by elevation of intracellular Ca2+. In the presence of 10(-5), 10(-4), and 10(-3) M intracellular free Ca2+, the half-activation voltages are +7.2, -26.6, and -47.5 mV, respectively. The half-activation-[Ca2+] at +10 mV is 8.1 microM, and the Hill coefficient of Ca2+ binding is 1.7, Ba2+ exerts a voltage-dependent block of the open-state probability. The maxi-K+ channels of Müller cells are activated by raising of the intracellular pH as well as by Mg2- ions at the cytosolic face of the channels. Phosphorylation of the channel after cytosolic addition of the catalytic subunit of a cAMP dependent protein kinase in the presence of Mg-ATP caused a shift of the activation curve to negative membrane potentials. Between -40 and -80 mV membrane potentials, the open-state probability rose to 190.3% of the control value (100%) after phosphorylation of the channel. Therefore, phosphorylation enhances sensitivity of the channels to Ca2+ and voltage. The maxi-K+ channels may provide a link between second messenger systems and membrane conductance of retinal Müller cells and may have an important function in repolarization of the Müller cell membrane and, therefore, in the maintainance of the retinal spatial K+ buffering mechanisms.

Regulation of the resting potential of rabbit pulmonary artery myocytes by a low threshold, O2-sensing potassium current

1. The contributions of specific K+ currents to the resting membrane potential of rabbit isolated, pulmonary artery myocytes, and their modulation by hypoxia, were investigated by use of the whole-cell, patch-clamp technique. 2. In the presence of 10 microM glibenclamide the resting potential (-50 +/- 4 mV, n = 18) was unaffected by 10 microM tetraethylammonium ions, 200 nM charybdotoxin, 200 nM iberiotoxin, 100 microM ouabain or 100 microM digitoxin. The negative potential was therefore maintained without ATP-sensitive (KATP) or large conductance Ca(2+)-sensitive (BKCa) K channels, and without the Na(+)-K+ ATPase. 3. The resting potential, the delayed rectifier current (IK(V)) and the A-like K+ current (IK(A)) were all reduced in a concentration-dependent manner by 4-aminopyridine (4-AP) and by quinine. 4. 4-AP was equally potent at reducing the resting potential and IK(V), 10 mM causing depolarization from -44 mV to -22 mV with accompanying inhibition of IK(V) by 56% and IK(A) by 79%. In marked contrast, the effects of quinine on resting potential were poorly correlated with its effects on both IK(A) and IK(V). At 10 mM, quinine reduced IK(V) and IK(A) by 47% and 38%, respectively, with no change in the resting potential. At 100 microM, both currents were almost abolished while the resting potential was reduced < 50%. Raising the concentration to 1 mM had little further effect on IK(A) or IK(V), but essentially abolished the resting potential. 5. Reduction of the resting potential by quinine was correlated with inhibition of a voltage-gated, low threshold, non-inactivating K+ current, IK(N). Thus, 100 microM quinine reduced both IK(N) and the resting potential by around 50%. 6. The resting membrane potential was the same whether measured after clamping the cell at -80 mV, or immediately after a prolonged period of depolarization at 0 mV, which inactivated IK(A) and IK(V), but not IK(N). 7. When exposed to a hypoxic solution, the O2 tension near the cell fell from 125 +/- 6 to 14 +/- 2 mmHg (n = 20), resulting in a slow depolarization of the myocyte membrane to -35 +/- 3 mV (n = 16). The depolarization occurred without a change in the amplitude of IK(V) or IK(A), but it was accompanied by 60% inhibition of IK(N) at 0 mV. 8. Our findings suggest that the resting potential of rabbit pulmonary artery myocytes depends on IK(N), and that inhibition of IK(N) may mediate the depolarization induced by hypoxia.

The chemical modification of KCa channels by carbon monoxide in vascular smooth muscle cells

The chemical modification of big conductance calcium-activated potassium (KCa) channels in rat tail artery smooth muscle cells by carbon monoxide (CO) was investigated using the cell-free single channel recording technique. Exposure of the internal surface of cell membranes to diethyl pyrocarbonate (DEPC) neither affected the characteristics of KCa channels nor modified the stimulatory effect of CO on KCa channels. However, when DEPC was applied to the external surface of cell membranes, the open probability of KCa channels was reduced. The pH and concentration dependence of the effect of DEPC indicated the specific modification of histidine residues. Kinetic analysis suggested that one externally located histidine residue was modified by DEPC. Treatment of the external surface of cell membranes with DEPC abolished the CO-induced increase in the open probability of KCa channels. Likewise, the presence of CO partially protected KCa channels from inhibition by DEPC. Moreover, photooxidation of the histidine residue located on the external membrane surface abolished the CO-induced activation of KCa channels. Our study demonstrates that the CO-induced increase in the open probability of KCa channels may rely specifically on the structure and topological locations of histidine residues.

Apamin blocks the direct relaxant effect of melatonin on rat ileal smooth muscle

The present study investigated the mechanisms of melatonin-induced inhibition of the ileal smooth muscle contraction. Rat isolated ileal smooth muscle strips were stimulated in an organ bath using carbachol (CAR) or potassium chloride (KCl) depolarization. Under these conditions, melatonin produced a concentration-dependent inhibition of muscle contraction (mean inhibitory concentration, IC50: 17.3 x 10(-6) M), which was not blocked by either tetrodotoxin (10(-6) M), hexamethonium (10(-4) M), or phentolamine (10(-6) M). The inhibitory effect of melatonin during CAR stimulation was blocked in a concentration-dependent manner by the presence of apamin (4.8 x 10(-9) M), a K(+)-channel blocker. By contrast, other K(+)-channel blockers such as 4-aminopyridine (10(-4) M to 5 x 10(-3) M), tetraethylammonium (10(-4) to 10(-1) M), and glibenclamide (10(-5) M) were ineffective. Additionally, the Ca(2+)-channel antagonists nitrendipine (IC50: 2.4 x 10(-9) M) and verapamil (IC50: 1.1 x 10(-7) M) also blocked the inhibitory action of melatonin. These results suggest that melatonin may interact with an apamin-sensitive, possibly Ca(2+)-activated, K+ channel and thus cause an inhibition of ileal smooth muscle contractions.

Properties of a novel K+ current that is active at resting potential in rabbit pulmonary artery smooth muscle cells

1. An outward current (IK(N)) was identified in rabbit pulmonary artery myocytes, which persisted after Ca(2+)-activated and ATP-sensitive K+ currents were blocked by TEA (10 mM) and glibenclamide (10 microM), respectively, and after A-like (IK(A)) and delayed rectifer (IK(V)) K+ currents were inactivated by clamping the cell at 0 mV for 10 min. It was found in smooth muscle cells at all levels of the pulmonary arterial tree. 2. The relationship between the reversal potential of IK(N) and the extracellular K+ concentration ([K+]o) was close to that expected for a K(+)-selective channel. Deviation from Nernstian behaviour at low [K+)o could be accounted for by the presence of an accompanying leakage current. 3. IK(N) is voltage gated. It has a low threshold for activation, between -80 and -65 mV, and activates slowly without delay. Activation follows an exponential time course with a time constant of 1.6 s at -60 mV. Deactivation is an order of magnitude faster than activation, with a time constant of 107 ms at -60 mV. 4. IK(N) showed a similar sensitivity to 4-aminopyridine as IK(A) and IK(V), with 49% inhibition at 10 mM. The current was not blocked by microM quinine, which did inhibit IK(A) and IK(V), by 51 and 47%, respectively. 5. Activation of IK(N) was detected at potentials close to the resting membrane potential of pulmonary artery smooth muscle cells, under physiological conditions. Thus it is likely to contribute to the resting membrane potential of these cells.

Role of Ca(2+)-activated K+ channels in the regulation of membrane potential and tone of smooth muscle in human pial arteries

Smooth muscle cells (SMCs) in 58% of human pial arteries obtained during surgery showed no spontaneous contractions and displayed a stable resting membrane potential (MP) of -54.7 +/- 1.5 mV. Those that exhibited periodic spontaneous contractions associated with periodic depolarization and generation of spontaneous action potentials (APs) had a less negative MP of -43.1 +/- 0.5 mV (42%). Inhibition of calcium-activated potassium (KCa) channels in the silent arteries by charybdotoxin (CTX) and tetraethylammonium ions (TEA) induced dose-dependent depolarization, AP generation, and contraction. TEA and CTX enhanced the spontaneous depolarization and force in arteries that exhibited spontaneous activity. They also prolonged the spontaneous APs up to several times and increased their upstroke amplitude. Both TEA and CTX failed to produce significant depolarization in arteries treated with nifedipine. It is concluded that KCa channels are important regulators of human pial artery SMC resting MP and tone. They are also involved in the control of AP amplitude and duration and the associated contractions. These data suggest that alterations in the activity of SMC KCa channels could be responsible for the appearance of spontaneous activity in human pial arteries in vitro and that impaired function of these channels might be related to vasospastic phenomena in human cerebral circulation.

ATP increases Ca(2+)-activated K+ channel activity in isolated rat arterial smooth muscle cells

Large conductance Ca(2+)-activated K+ (Kca) channels are known to be activated by phosphorylation through cAMP- and cGMP-dependent kinase activation. In pulmonary arterial smooth muscle KCa channels are directly activated by ATP (but not by non-hydrolysable analogues) independently of the presence of cyclic nucleotides or the catalytic subunits of protein kinases. This study was designed to determine whether direct activation of KCa channels by ATP is apparent in other types of arterial smooth muscle. KCa channels of similar conductance to those of rat pulmonary artery (approximately 250 pS) were found in membrane patches excised from isolated smooth muscle cells from rat aorta, mesenteric and basilar arteries. In myocytes isolated from each of these arteries, intracellular application of ATP (in the absence of exogenous cyclic nucleotides or catalytic subunits) reversibly increased the open state probability of KCa channels: a response markedly reduced by a specific inhibitor of protein kinase A. Nucleotide sequence analysis of KCa channels revealed no homology with the majority of protein kinases. It is concluded that phosphorylation of KCa channels through the activity of a membrane tethered kinase related to protein kinase A (but lacking its regulatory subunits) may play an important role in controlling K+ flux in a range of arterial smooth muscle cell types.

Endothelin-induced electrical activity and calcium dynamics in vascular smooth muscle cells: a model study

A model is proposed to describe the electrical activity and intracellular calcium dynamics of vascular smooth muscle cells (SMC) induced by endothelin (ET1). The conductance of the nonselective channels (NSCs), proportional to the ET1-receptor complex (ET . R), is intracellular calcium dependent. Inositol (1,4,5)-trisphosphate (IP3) produced by ET1 releases Ca2+ from the IP3-sensitive Ca2+ store. The transient increase of intracellular Ca2+ triggers the release of Ca2+ from the Ca(2+)-sensitive store by a Ca(2+)-induced Ca2+ (CICR) mechanism and activates the Ca(2+)-activated K+ current (IK,Ca). The inward current (Iin) via the NSC can depolarize the cell to a level at which the L-type Ca2+ current becomes activated (ICa). The level of depolarization is determined by the relative amplitude of (Iin + ICa + IK,Ca) and the voltage- and time-dependent K+ current. The model simulations show that (a) in cells without a CICR mechanism, short-lasting stimulation by ET1 elicits higher membrane potential and Ca2+ than long-lasting stimulation; (b) in cells with or without a CICR mechanism, a reduction of normal membrane capacitance (1 muf/cm2) results in either significant and sustaining or oscillatory membrane potential and intracellular calcium concentration. The applicability of the model to the study of electrical activity and calcium dynamics associated with hypercholesterolemia is discussed.

Spontaneous transient outward currents in smooth muscle cells

Spontaneous transient outward currents (STOCs) lasting about 100 ms occur in single smooth muscle cells and represent the simultaneous opening of up to a hundred calcium-activated potassium (BK) channels. The recent observation of brief focal releases of sarcoplasmic reticulum (SR) calcium ('sparks') in smooth muscle cells has provided support for the original suggestion that STOCs arise due to the spontaneous releases of calcium from the SR close to the sarcolemma. However, it is possible that such releases occur in a region of close apposition of SR membrane and sarcolemma about 0.1 microns wide ('junctional space') in which case they would be detectable by endogenous calcium-sensitive molecules such as BK channels but, using present confocal microscopy technique, not by calcium-indicator dyes introduced into the cell; should calcium escape from the junctional space then it may be visualised as 'sparks' by the fluorescent emission from calcium-indicator dyes using confocal microscopy. Some STOCs seem too large to represent the effect of a single 'spark' and some form of calcium-induced calcium release or 'macrospark' may be involved in their generation. Depletion of calcium stores by caffeine, ryanodine, or by activation of receptors linked to the phospholipase C/inositol trisphosphate system abolishes STOCs. However, low concentrations of caffeine or inositol trisphosphate accelerate STOC discharge by an unknown mechanism and often decrease STOC size presumably by depleting store calcium; similar effects are produced by agents such as cyclopiazonic acid and thapsigargin which inhibit calcium storage mechanisms (largely the SR calcium pump).

Block of BK (maxi K) channels of rat pituitary melanotrophs by Na+ and other alkali metal ions

The block of large-conductance calcium-activated potassium (BK) channels by internal and external alkali metal ions was studied in adult rat melanotrophs. Internal but not external 20 mM Na+ produced a strongly voltage-dependent, flickery block that was well-fitted to the Woodhull model by using a value of 140 mM for the dissociation rate constant at 0 mV [Kd(0)] and an equivalent valence (zdelta) of 0.9. At a concentration of 20 mM external K+, Cs+ and Rb+, but not Li+, caused a rightward shift of the voltage dependence of the intracellular Na+ (Na+i ) block. This effect of K+, Cs+ and Rb+ was modelled by an equilibrium knock-out mechanism in which the block-relieving ion binds to a site located within the voltage field and consequently increases the off-rate of Na+. Internal Li+ caused little or no block whereas internal Cs+ caused a voltage-dependent block [Kd(0) approximately 150 mM]. Flickery channel block observed in cell-attached patches was consistent with a cytoplasmic Na+ activity between 1 and 10 mM.

Expression and function of voltage-dependent potassium channel genes in human airway smooth muscle

Patch clamp and RNA-polymerase chain reaction methods were used to determine the expression of voltage-dependent potassium channel currents and mRNAs in human airway smooth muscle cells, and tension measurements were used to examine the functional role of specific potassium channel gene products in human bronchial smooth muscle. RNA from airway smooth muscle tissue revealed the presence of Kv1.2 (11 kilobases (kb)) and Kv1.5 (3.5 and 4.4 kb) transcripts, as well as Kv1.1 mRNA (9.5 kb), which has not previously been reported in smooth muscle; transcripts from other gene families were not detected. RNA-polymerase chain reaction from cultured human myocytes confirmed that the identified transcripts were expressed by smooth muscle cells. The available voltage-dependent potassium current in human airway myocytes was insensitive to charybdotoxin (200 nM) but blocked by 4-aminopyridine. Dendrotoxin (1-300 nM; inhibits Kv1.1 and Kv1.2 channels), charybdotoxin (10 nM to 1 microM; inhibits KCa and Kv1.2 channels), and glybenclamide (0.1-100 microM; inhibits KATP channels) had no effect on resting tone. Conversely, 4-aminopyridine increased resting tension with an EC50 (1.8 mM) equivalent to that observed for current inhibition (1.9 mM). Human airway myocytes express mRNA from several members of the Kv1 gene family; the channel that underlies the predominate voltage-dependent current and the regulation of basal tone appears to be Kv1.5.

Effects of potassium channel blockers on resting tone in isolated coronary arteries

The effects of several potassium channel blockers on resting vasomotor tone were studied in porcine isolated coronary arteries. Coronary artery rings were suspended in organ baths for isometric tension recording. The nonselective potassium channel blockers tetraethylammonium (TEA 10(-5)-3 x 10(-2) M) and 4-aminopyridine (4-AP 10(-5)-10(-2) M) caused concentration-dependent contractions that were similar in rings with and without endothelium. The concentration-response curves to TEA and 4-AP were unaffected by treatment with phentolamine (3 x 10(-6) M),propranolol (10(-6) M), or atropine (10(-6) M). Diltiazem (10(-6) M) almost abolished the contractions evoked by TEA and 4-AP. Charybdotoxin (10(-9)-10(-7) M) and apamin (10(-8)-10(-6) M), selective blockers of large and small calcium-activated potassium channels, respectively, and glyburide (10(-8)-10(-6) M), a selective blocker of ATP-sensitive potassium channels, caused little or no contraction in rings with or without endothelium. Therefore, in isolated coronary arteries, TEA and 4-AP caused contractions that were independent of the release of vasoactive mediators from the endothelium or perivascular nerves. These effects are not mediated by ATP-sensitive potassium channels or by large and small conductance calcium-activated potassium channels. The data are consistent with an effect of TEA and 4-AP on resting membrane potassium conductance in coronary arteries, resulting in contractions that are sensitive to inhibition by diltiazem. This pattern of responsiveness of isolated coronary arteries to potassium channel blockers differs from that observed in vessels from other vascular beds.

Opening of Ca(2+)-dependent K+ channels by nordihydroguaiaretic acid in porcine coronary arterial smooth muscle cells

Effects of nordihydroguaiaretic acid (NDGA) on the Ca(2+)-dependent K+ channel (BK channel) were examined by the patch clamp technique in single smooth muscle cells of porcine coronary artery. The open probability of BK channels in inside-out patches increased by about 30 times at the holding potential of 0 mV, when 10 microM NDGA was added to the bathing solution (pCa 7.0). The effect of NDGA was concentration-dependent in the range of 1-100 microM and partly removed by washout. The enhancement of BK channels by NDGA was not observed when the cytosolic Ca2+ concentration was very low (pCa > 8.5). These results clearly indicate that NDGA possesses a BK channel opening property in coronary arterial myocytes.

Model predictions of myoelectrical activity of the small bowel

A mathematical model for the periodic electrical activity of a functional unit of the small intestine is developed. Based on real morphological and electrophysiological data, the model assumes that: the functional unit is an electromyogenic syncytium; the kinetics of L, T-type Ca2+, mixed Ca(2+)-dependent K+, potential sensitive K+ and Cl- channels determines electrical activity of the functional unit; the basic neural circuit, represented by a single cholinergic neurone, provides an excitatory input to the functional unit via receptor-linked L-type Ca2+ channels. Numerical simulation of the model has shown that it is capable of displaying the slow waves and that slight modifications of some of the parameters result in different electrical responses. The effects of the variations of the main parameters have been analyzed for their ability to reproduce various electrical patterns. The results are in good qualitative and quantitative agreement with results of experiments conducted on the small intestine.

Calcium-activated potassium channels in adrenal chromaffin cells

Rat chromaffin cells express an interesting diversity of Ca(2+)-dependent K+ channels, including a voltage-independent, small-conductance, apamin-sensitive SK channel and two variants of voltage-dependent, large-conductance BK channels. The two BK channel variants are differentially segregated among chromaffin cells, such that BK current is completely inactivating in about 75-80% of rat chromaffin cells, while the remainder express a mix of inactivating and non-inactivating current or mostly non-inactivating BKs current. The single-channel conductance of BKi channels is identical to that of BKs channels. Although rates of current activation are similar in the two variants, the deactivation kinetics of the two channels also differ. Furthermore, BKi channels are somewhat less sensitive to scorpion toxins than BKs channels. The slow component of BKi channel deactivation may be an important determinant of the functional role of these channels. During blockade of SK current, cells with BKi current fire tonically during sustained depolarizing current injection, whereas cells with BKs current tend to fire only a few action potentials before becoming quiescent. The ability to repetitively fire requires functional BKi channels, since partial blockade of BKi channels by CTX makes a BKi cell behave much like a BKs cell. In contrast, the physiological significance of BKi inactivation may arise from the ability of secretagogue-induced [Ca2+]i elevations to regulate the availability of BKi channels during subsequent action potentials (Herrington et al., 1995). By reducing the number of BK channels available for repolarization, the time course of action potentials may be prolonged. This possibility remains to be tested directly. These results raise a number of interesting questions pertinent to the control of secretion in rat adrenal chromaffin cells. An interesting hypothesis is that cells with a particular kind of BK current may reflect particular subpopulations of chromaffin cells. These subpopulations might differ either in the nature of the material secreted from the cell (e.g., Douglass and Poisner, 1965) or in the responsiveness to particular secretagogues. The differences in electrical behavior between cells with BKi and BKs current suggest that the pattern of secretion that might be elicited by a single type of stimulus could differ. For BKi cells, secretion may occur in a tonic fashion during sustained depolarization, while secretion from cells with BKs current may be more phasic. In the absence of specific structural information about the domains responsible for inactivation of BKi channels, our understanding of the mechanism of inactivation remains indirect. BKi inactivation shares many features with N-terminal inactivation of voltage-dependent K+ channels. However, there are provocative differences between the two types of inactivation which require us to propose that the native inactivation domain of BKi channels may occlude access of permeant ions to the BK channel permeation pathway in a position at some distance from the actual mouth of the channel. Further understanding of the structural and mechanistic basis of inactivation of BKi channels promises to provide new insights into both the cytoplasmic topology of BK channels and the Ca(2+)- and voltage-dependent steps involved in channel activation.

Action of angiotensin II, 5-hydroxytryptamine and adenosine triphosphate on ionic currents in single ear artery cells of the rabbit

1. Angiotensin II, 5-hydroxytryptamine (5-HT) and adenosine triphosphate (ATP) evoked a transient inward current in isolated single car artery cells of rabbit held at -60 mV by whole cell voltage clamp in physiological saline using a KCL-containing pipette solution. Under these conditions agonist did not activate a calcium-dependent potassium current. 2. Responses to each agonist were transient and desensitized rapidly. Inward current at -60 mV holding potential was not abolished by blockade of voltage-dependent calcium channels or by buffering intracellular calcium with BAPTA, a calcium chelator, or following depletion of intracellular calcium stores with ryanodine. 3. The shape of the current-voltage relationships and the reversal potentials of the current induced by angiotensin II, 5-HT and ATP were similar under a variety of ionic conditions. Agonist-induced current was unaffected by replacing intracellular chloride with citrate ions or by replacing intracellular sodium with caesium or extracellular sodium with barium or calcium. Replacement of extracellular sodium with Tris shifted the reversal potential in all cases by around 30 mV negatively. 4. These data suggest that angiotensin II, 5-HT and ATP activate similar cationic conductances which are relatively non-selective allowing mono- and divalent cations to cross the smooth muscle cell membrane. These channels may allow the influx of calcium under physiological conditions.

Role of potassium channels in cerebral blood vessels

BACKGROUND

Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. In cerebral blood vessels, four different potassium channels have been described: ATP-sensitive potassium channels, calcium-activated potassium channels, delayed rectifier potassium channels, and inward rectifier potassium channels.

SUMMARY OF REVIEW

Activation of ATP-sensitive and calcium activated potassium channels appears to play a major role in relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, and hypoxia. Both calcium-activated and delayed rectifier potassium channels may contribute to a negative feedback system that regulates tone in large cerebral arteries. The influence of ATP-sensitive and calcium-activated potassium channels is altered in disease states such as hypertension, diabetes, and atherosclerosis.

CONCLUSIONS

Activation of potassium channels is a major mechanism of cerebral vasodilatation. Alteration of activity of potassium channels and impairment of vasodilatation may contribute to the development or maintenance of cerebral ischemia or vasospasm.

Modulation of calcium channel currents by arachidonic acid in single smooth muscle cells from vas deferens of the guinea-pig

1. Effects of arachidonic acid (AA) on voltage-dependent Ca channel currents were investigated by whole-cell-clamp methods in single smooth muscle cells freshly isolated from vas deferens of the guinea-pig. 2. Ca channel current was decreased by application of 1-30 microM AA in a concentration-dependent manner. When Ca2+ or Ba2+ was the charge carrier, Ca channel current (ICa or IBa) was reduced by AA to a similar extent (IC50 = 10 and 6 microM, respectively). Addition of 15 mM BAPTA to the pipette solution did not affect the reduction of IBa by 10 microM AA. 3. The effect of AA on IBa was not prevented by internal application of 1 mM nordihydroguaiaretic acid (NDGA) and 1 mM indomethacin (Indo). When the pipette solution contained 0.1 mM guanosine-5'-triphosphate (GTP), IBa was decreased slightly but significantly by application of 30 microM prostaglandin F2 alpha (PGF2 alpha) but not by PGE2. This effect of PGF2 alpha was irreversible or not observed when the pipette solution contained 0.3 mM guanosine-5'-(3-thiotriphosphate) (GTP gamma S) or both GTP or guanosine-5'-O-(2-thiodiphosphate) (GDP beta S), respectively. 4. External application of 100 units ml-1 superoxide dismutase slightly but significantly attenuated the inhibition of IBa by 1-30 microM AA. Intracellular application of 1 mM GDP beta S or 0.3 mM GTP gamma S did not significantly change the effect of AA. Intracellular application of 0.1 mM 1-(5-isoquinolinesulphonyl)-2-methylepiperazine (H-7) also did not change the effect of AA. 5. These results indicate that the decrease in Ca channel currents in vas deferens smooth muscle cells is mainly due to AA itself, as opposed to its metabolites. The effect of AA may be due to AA itself, as opposed to its metabolites. The effect of AA may be due to its direct action on Ca channels or membrane phospholipids, but may not be mediated by activation of GTP binding proteins or protein kinase C. The inhibition of Ca channel current by AA may be partly induced by superoxide radicals derived from AA oxidation. PGF2A also reduces Ca channel currents but probably by a separate mechanism via activation of a GTP binding protein.

Voltage-gated K+ currents regulate resting membrane potential and [Ca2+]i in pulmonary arterial myocytes

The membrane potential (Em) of pulmonary arterial smooth muscle cells (PASMCs) regulates pulmonary arterial tone by controlling voltage-gated Ca2+ channel activity, which is a major contributor to [Ca2+]i. The resting membrane is mainly permeable to K+; thus, the resting Em is controlled by K+ permeability through sarcolemmal K+ channels. At least three K+ currents, voltage-gated K+ (KV) currents, Ca(2+)-activated K+ (KCa) currents, and ATP-sensitive (KATP) currents, have been identified in PASMCs. In this study, both patch-clamp and quantitative fluorescent microscopy techniques were used to determine which kind(s) of K+ channels (KV, KCa, and/or KATP) is responsible for controlling Em and [Ca2+]i under resting conditions in rat PASMCs. When the bath solution contained 1.8 mmol/L Ca2+ and the pipette solution included 0.1 mmol/L EGTA, depolarizations (-40 to +80 mV) elicited both KCa and KV currents. Removal of extracellular Ca2+ and increase of intracellular EGTA concentration (to 10 mmol/L) eliminated the Ca2+ influx-dependent KCa current. 4-Aminopyridine (4-AP, 5 to 10 mmol/L) but not charybdotoxin (ChTX, 10 to 20 nmol/L) significantly reduced KV current under these conditions. In current-clamp experiments, 4-AP decreased Em (depolarization) and induced Ca(2+)-dependent action potentials; this depolarization increased [Ca2+]i in intact PASMCs. Neither ChTX nor the specific blocker of KATP channels, glibenclamide (2 to 10 mumol/L), caused membrane depolarization and the increase in [Ca2+]i. However, pretreatment of PASMCs with ChTX enhanced the 4-AP-induced increase in [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

[Ca2+]i inhibition of K+ channels in canine pulmonary artery. Novel mechanism for hypoxia-induced membrane depolarization

Experiments were performed on smooth muscle cells isolated from canine pulmonary artery to identify the type of K+ channel modulated by hypoxia and examine the possible role of [Ca2+]i in hypoxic K+ channel inhibition. Whole-cell patch-clamp experiments revealed that hypoxia (induced by the O2 scavenger, sodium dithionite) reduced macroscopic K+ currents, an effect that could be prevented by strong intracellular buffering of [Ca2+]i. The inhibitory effects of hypoxia were mimicked by acute exposure of cells to caffeine and could be prevented by caffeine pretreatment, suggesting an important obligatory role of [Ca2+]i in hypoxic inhibition of K+ currents. Exposure of cells to low concentrations of 4-aminopyridine (4-AP, 1 mmol/L) prevented hypoxic inhibition of macroscopic K+ currents, whereas low concentrations of tetraethylammonium were without effect, suggesting that the target K+ channel inhibited by hypoxia is a voltage-dependent delayed rectifier K+ channel, which is inhibited by [Ca2+]i. Hypoxia failed to consistently modify the activity of large-conductance (118 picosiemens [pS] in physiological K+) Ca(2+)-activated K+ channels in inside-out membrane patches but reduced open probability of smaller-conductance (25-pS) delayed rectifier K+ channels in cell-attached membrane patches. In inside-out membrane patches, 1 mumol/L Ca2+ added to the cytoplasmic surface significantly reduced open probability of small-conductance (25-pS) 4-AP-sensitive delayed rectifier K+ channels. Whole-cell current measurements using symmetrical K+ to increase driving force for small currents active near the cell's resting membrane potential revealed the presence of a 4-AP-sensitive K+ current that activated near -65 mV and was inhibited by hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)