Different types of K+ channel current are generated by different levels of a single mRNA

A cloned human voltage-sensitive K+ channel HLK3 which is present in T-lymphocytes and in the brain was expressed in Xenopus oocytes and after permanent transfection of a human B-lymphocyte cell line (IM9). Injections of low cRNA concentrations into Xenopus oocytes led to the expression of a transient K+ current, with saturating current-voltage (I-V) relationship, which was abolished by repetitive stimulations due to a slow recovery from inactivation. This transient K+ channel current was fully inhibited by 10 nM charybdotoxin. Injection of high concentrations of the same RNA led to a non-inactivating K+ current, with linear I-V curve, which did not undergo use-dependent inactivation and was hardly sensitive to 10 nM charybdotoxin. Intermediate behaviour due to changing proportions of these two types of K+ channel expression were observed at intermediate RNA concentrations. Transient and non-inactivating K+ currents were also observed by both whole-cell and single channel patch-clamp recording from HLK3 transfected IM9 cells. The main conductance of the channel in the two different modes (inactivating and charybdotoxin-sensitive or non-inactivating and charybdotoxin-resistant) is the same (12-14 pS). Destruction of the cytoskeletal elements with cytochalasin D, colchicine or botulinum C2 toxin in oocyte experiments prevented expression of the sustained mode of the K+ channel. The results suggest that the sustained mode obtained at high RNA concentrations corresponds to channel clustering involving cytoskeletal elements. This differential functional expression of K+ channels associated with different levels of mRNA appears as a new important factor to explain the biophysical and pharmacological diversity of voltage-sensitive K+ channels.

Ca(2+)-activated K+ channels from an insulin-secreting cell line incorporated into planar lipid bilayers

This study evaluates the use of the planar lipid bilayer as a functional assay of Ca(2+)-activated K+ channel activity for use in purification of the channel protein. Ca(2+)-activated K+ channels from the plasma membrane of an insulin-secreting hamster Beta-cell line (HIT T15) were incorporated into planar lipid bilayers. The single channel conductance was 233 picoSiemens (pS) in symmetrical 140 mmol/l KCl and the channel was strongly K(+)-selective (PCl/PK = 0.046; PNa/PK = 0.027). Channels incorporated into the bilayer with two orientations. In 65% of cases, the probability of the channel being open was increased by raising calcium on the cis side of the bilayer (to which the membrane vesicles were added) or by making the cis side potential more positive. At a membrane potential of + 20 mV, which is close to the peak of the Beta-cell action potential, channel activity was half-maximal at a Ca2+ concentration of about 15 mumol/l. Charybdotoxin greatly reduced the probability of the channel being open when added to the side opposite to that at which Ca2+ activated the channel. These results resemble those found for Ca(2+)-activated K+ channels in native Beta cell membranes and indicate that the channel properties are not significantly altered by incorporation in a planar lipid bilayer.

Role of potassium channels in bronchodilator responses in human airways

The plasma membrane of airway smooth muscle contains a high density of K+ channels of various types that mainly regulate membrane potential. To examine whether these K+ channels are involved in bronchodilating mechanisms in human airways, relaxation concentration-response studies to isoproterenol, theophylline, and a K(+)-channel opener, lemakalim (BRL 38227), were obtained in the presence or absence of charybdotoxin (ChTX) (10 or 100 nM), an inhibitor of large conductance Ca(2+)-activated K+ channels (KCa) in smooth muscle. The effects of other potassium channel blockers, apamin (0.1 microM, a small-conductance KCa blocker) and BRL 31660 (10 microM, an ATP-sensitive K(+)-channel blocker) on isoproterenol-induced bronchodilation were also examined. All relaxation studies were performed on spontaneous tone and in the presence of 1 microM indomethacin. ChTX produced a dose-dependent significant rightward shift in the isoproterenol relaxation response curves without changing maximum relaxation; geometric mean values of EC50 were 4.6 nM without and 19 nM with 10 nM ChTX (n = 7, p less than 0.005), and 3.4 nM without and 41 nM with 100 nM ChTX (n = 4, p less than 0.05), respectively. The theophylline relaxation responses were inhibited to a lesser extent by ChTX (10 nM) (ED50 of 32 microM without and 71 microM with ChTX, n = 7, p less than 0.05), whereas lemakalim-induced relaxation response was not affected. Other K(+)-channel blockers, apamin and BRL31660, failed to affect isoproterenol-induced bronchodilation. These results suggest that ChTX-sensitive K+ channels are involved in bronchodilation induced by beta-agonists and theophylline in human airways.

The effect of NH4Cl on Rb+ fluxes in resting and stimulated rat submandibular acinar cells

Dispersed salivary acini isolated from the rat submandibular gland were incubated in a HEPES-buffered Krebs-Ringer solution or in the same buffer containing 20 mM NH4Cl and the accumulation and efflux of K+ were measured with the radiotracer 86Rb+, in the presence and absence of acetylcholine and of transport inhibitors. Exposure to NH4Cl caused a significant (greater than 50%) reduction in tracer accumulation. This effect was blocked by 0.1 mM bumetanide, but not by 1 mM ouabain. The effect of NH4Cl was, on the other hand, nearly additive with that of 1 microM acetylcholine. In cells preincubated with tracer, acute addition of NH4Cl caused a significant net efflux of isotope, so that the tracer content fell to 45% of the control value within 10 min. Bumetanide added to preloaded cells in the same fashion had no effect on tracer content and did not modify the efflux of 86Rb+ induced by 1 microM acetylcholine. However, this inhibitor essentially abolished the NH4Cl-induced tracer efflux. Exposure to NH4Cl during tracer loading did not appear to affect subsequent agonist-stimulated tracer efflux. These results suggest that: (1) the inhibition of K+ entry by NH4Cl is due to an effective competition by the NH4+ ion with Rb+ (and K+) for uptake via a bumetanide-sensitive Na+/K+/2Cl- contransporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Charybdotoxin and apamin sensitivity of the calcium-dependent repolarization and the afterhyperpolarization in neostriatal neurons

1. Intracellular recordings from neostriatal neurons in an in vitro slice preparation of the rat brain were used to analyze the pharmacological sensitivity of the action potential (AP) repolarization and the afterhyperpolarization (AHP) that follows a single action potential. The interspike voltage trajectory and the AHP could be divided into two main parts: a fast component lasting a few milliseconds and better observed during a train of spikes, and a slow component lasting approximately 250 ms and that comprises the main portion of the AHP. In some cells, a slow (up to 1 s) component of low amplitude was also detected. 2. Single APs were elicited at two imposed membrane potentials (around -60 and around -80 mV). The AP amplitude was larger, the repolarization rate was faster, and the duration was shorter when spikes were evoked at -80 mV. When measured from the -60 mV holding potential, the afterpotential was an AHP with peak amplitude of -5 mV. The afterpotential became a delayed depolarization (DD) at -80 mV. 3. Firing frequency adaptation was voltage sensitive. The firing of APs induced by long intracellular current pulses from a holding potential of -80 mV exhibited only a slow-frequency adaptation (time constant of seconds). However, at -60 mV, an initial and faster frequency adaptation was evident (time constant of tens of milliseconds). 4. The Ca2+ channel blocker Cd2+ retarded AP repolarization rate. This effect correlated with a significant block of the fast and slow components of the AHP. In contrast, Ni2+ had no significant effects on the same parameters.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of charybdotoxin-sensitive K+ channel in the relaxation of bovine tracheal smooth muscle by glyceryl trinitrate and sodium nitroprusside

To elucidate the involvement of K+ channels in the smooth muscle relaxation by glyceryl trinitrate (GTN) and sodium nitroprusside (SNP), effects of several K+ channel antagonists on the relaxant responses to GTN, SNP and 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) were studied in bovine tracheal smooth muscle. Although an antagonist of large conductance Ca(++)-activated K+ channel, charybdotoxin, produced no definite effect on the relaxation induced by GTN, SNP and atriopeptin in the rabbit aortic ring preparation, this antagonist inhibited the relaxation by GTN, SNP, atriopeptin and 8-Br-cGMP in the bovine tracheal smooth muscle. Methylene blue, a soluble guanylate cyclase inhibitor, also had an inhibitory effect on the relaxation by GTN and SNP. Both apamin, a small conductance Ca(++)-activated K+ channel antagonist, and glibenclamide, an ATP-sensitive K+ channel antagonist, did not exhibit any inhibitory effect on the relaxant responses to GTN and SNP. GTN and SNP increased cGMP content. The increment was attenuated by methylene blue, whereas it was unaffected by charybdotoxin. These results indicate the involvement of large conductance Ca(++)-activated K+ channel in the relaxation of bovine tracheal smooth muscle by GTN, SNP and 8-Br-cGMP. The activation of K+ channel by GTN and SNP is thought to occur via increases in cGMP content.

High affinity inhibition of Ca(2+)-dependent K+ channels by cytochrome P-450 inhibitors

The Ca(2+)-dependent K+ channel of human red cells was inhibited with high affinity by several imidazole antimycotics which are potent inhibitors of cytochrome P-450. IC50 values were (in microM): clotrimazole, 0.05; tioconazole, 0.3; miconazole, 1.5; econazole, 1.8. Inhibition of the channel was also found with other drugs with known cytochrome P-450 inhibitory effect. However, no inhibition was obtained with carbon monoxide (CO). This suggests that, given the high selectivity of the above inhibitors for the heme moiety, a different but closely related to cytochrome P-450 kind of hemoprotein may be involved in the regulation of the red cell Ca(2+)-dependent K+ channel. Clotrimazole also inhibited two other charybdotoxin-sensitive Ca(2+)-dependent K+ channels, those of rat thymocytes (IC50 = 0.1-0.2 microM) and of Ehrlich ascites tumor cells (IC50 = 0.5 microM). Imidazole antimycotics inhibit also receptor-operated Ca2+ channels (Montero, M., Alvarez, J. and García-Sancho, J. (1991) Biochem. J. 277, 73-79). This suggests that both Ca2+ and Ca(2+)-dependent K+ channels might have a similar regulatory mechanism involving a cytochrome.

Autoradiographic localization of [125I]charybdotoxin binding sites in rat brain

Charybdotoxin, a 37 amino acid peptide isolated from scorpion venom, is a potent inhibitor of potassium channel function. [125I]charybdotoxin was originally believed to be a selective ligand for the Ca(2+)-sensitive channel in many tissues, but it appears to bind only to a voltage-sensitive potassium channel in brain. We found high densities of [125I]charybdotoxin binding in lateral olfactory tract, interpeduncular nucleus and a variety of mesencephalic nuclei. Moderate levels were found in the cerebral cortex, medial thalamus, hypothalamus and selected thalamic nuclei. These results indicate that [125I]charybdotoxin identifies a potassium channel or channels with a unique distribution in the brain.

Transformation of renal tubule epithelial cells by simian virus-40 is associated with emergence of Ca(2+)-insensitive K+ channels and altered mitogenic sensitivity to K+ channel blockers

We compared the pattern of K+ channels and the mitogenic sensitivity to K+ channel blocking agents in primary cultures of rabbit proximal tubule cells (PC.RC) (Ronco et al., 1990) and two derived SV-40-transformed cell lines exhibiting specific functions of proximal (RC.SV1) and more distal (RC.SV2) tubule cells (Vandewalle et al., 1989). First, K+ channel equipment surveyed by the patch-clamp technique was modified after SV-40 transformation in both cell lines; although a high conductance Ca(2+)-activated K+ channel [K+200 (Ca2+)] remained the most frequently recorded K+ channel, the transformed state was characterized by emergence of three Ca(2+)-insensitive K+ channels (150, 50, and 30 pS), virtually absent from primary culture, contrasting with reduced frequency of two Ca(2+)-sensitive K+ channels (80 and 40 pS). Second, quinine (Q), tetraethylammonium ion (TEA) and charybdotoxin (CTX), at concentrations not affecting cell viability, all decreased 3H-TdR incorporation and cell growth in PC.RC cultures, but only TEA had similar effects in transformed cells. The latter were further characterized by paradoxical effects of Q that induced a marked increase in thymidine incorporation. Q also exerted contrasting effects on channel activity: it inhibited the [K+200 (Ca2+)] when the channel was highly active, with a Ki (0.2 mM) similar to that measured for 3H-TdR incorporation in PC.RC cells (0.3 mM), but increased the mean current through poorly active channels. TEA blocked all K+ channels with conductance greater than or equal to 50 pS, including the [K+200 (Ca2+)], in a range of concentrations that substantially affected cell proliferation. The unique effect of TEA on SV-40-transformed cells might be related to broad inhibition of K+ channels.

Effects of cromakalim on the contraction and the membrane potential of the circular smooth muscle of guinea-pig stomach

1. The effects of cromakalim on mechanical and electrical activities of the circular smooth muscles of guinea-pig stomach antrum were observed. 2. Cromakalim (greater than 1 x 10(-7) M) decreased the amplitude of spontaneous rhythmic contractions and also the acetylcholine-enhanced spontaneous contractions. Cromakalim was less effective against the 25.9 mM and 35.9 mM K(+)-induced tonic contractions. 3. Glibenclamide (1 x 10(-6) M) itself caused no detectable change in the spontaneous contractions, those potentiated by acetylcholine or tonic contractions induced by high K+ solutions, but attenuated the actions of cromakalim. On the other hand, charybdotoxin (3 x 10(-8) M) increased the amplitude of spontaneous contractions but failed to affect the actions of cromakalim. 4. Cromakalim (greater than 1 x 10(-6) M) decreased the amplitude and duration of slow waves, and hyperpolarized the membrane. These actions of cromakalim were completely antagonized by 1 x 10(-6) M glibenclamide, whereas part of the effects of cromakalim on mechanical activity was resistant to glibenclamide. 5. The results suggest that the inhibition by cromakalim of the electrical activity and the hyperpolarization, which may be associated with the opening of glibenclamide-sensitive K+ channel, are responsible for its inhibitory action on circular smooth muscle of guinea-pig stomach. Further, some effects independent of glibenclamide-sensitive K+ channel may also be responsible for the mechanical effect.

Diverse K+ channels in primary human T lymphocytes

We used patch clamp techniques to identify and characterize a variety of K+ channels in primary human peripheral T lymphocytes. The most common channel observed in cell-attached configuration was voltage gated and inactivating. In ensemble averages, the kinetics of its activation and inactivation were similar to those of the whole-cell, voltage-gated K+ current described previously (Cahalan, M. D., K. G. Chandy, T. E. DeCoursey, and S. Gupta. 1985. J. Physiol. [Lond.]. 358:197-237; Deutsch, C., D. Krause, and S. C. Lee. 1986. J. Physiol. [Lond.]. 372:405-423), suggesting that this channel underlies the major portion of the outward current in lymphocytes. A small fraction of the time, this or another very similar channel was observed to inactivate significantly more slowly. Another channel type observed in cell-attached recording was seen less frequently and was transient in its appearance. This channel has a unitary conductance of approximately 10 pS, similar to the voltage-gated channel, but its voltage-independent gating, lack of inactivation, and different kinetic parameters showed it to be distinct. In whole-cell recording there is often a significant plateau current during sustained depolarization. Experiments using whole-cell and excised outside-out configurations indicate that at least part of this residual current is carried by K+ and, as opposed to the predominant voltage-gated current, is charybdotoxin insensitive. These findings are consistent with evidence that implicates charybdotoxin-sensitive and -insensitive components in T lymphocyte proliferation and volume regulation.

Potassium currents of rat basilar artery smooth muscle cells

Primary isolates of smooth muscle cells from the basilar artery of the rat were studied using whole-cell and single-channel patch-clamp techniques. Two distinct potassium currents were characterized. With low intracellular calcium, depolarization above 0 mV elicited an outward current of a few hundred pA (at +120 mV) with sigmoidal onset and little inactivation during 1.25 s steps. This current was reduced by bath application of 1 mM procaine or 1 mM strychnine, but not by 500 nM charybdotoxin. These are characteristics of the delayed rectifier potassium current in other preparations. With higher intracellular calcium, depolarization above 0 mV elicited a non-inactivating potassium current of several nA (at +120 mV). This current persisted in the presence of 1 mM procaine or strychnine but was reduced by bath application of 100 nM charybdotoxin. In whole-cell recordings in which intracellular calcium was unbuffered with EGTA, spontaneous transient outward currents were manifest and displayed voltage dependence and tail currents similar to the calcium-dependent current. The spontaneous transient current and the calcium-dependent current had similar sensitivity to charybdotoxin. Cell-free membrane patches contained one or more channels of 220 pS (in solutions symmetrical with respect to potassium) with similar voltage and calcium dependence. These are characteristics of the large conductance calcium-activated potassium current in other preparations.

Characterization of a delayed rectifier potassium current in chicken growth plate chondrocytes

With the use of the whole cell arrangement of the patch-clamp technique, an outward-directed time-dependent potassium current was identified in cultured chicken growth plate chondrocytes. This delayed rectifier potassium current (IK) activated with a sigmoidal time course during voltage steps to potentials positive to -40 mV. The half-maximal voltage required for current activation was determined to be -8 mV. The reversal potential (Erev) for IK, measured using deactivating tail currents, was -72 mV in the presence of 140 mM internal and 5 mM external [K+] solutions. Changes in external [K+] caused Erev to shift in a manner expected for a potassium-selective channel. In addition, increasing external [K+] from 5 to 50 mM caused the slope conductance of the tail currents to increase twofold. The chondrocyte IK was inhibited by the potassium-channel blocker 4-aminopyridine (4-AP) at concentrations of 0.5-4 mM and by the scorpion venom toxin charybdotoxin (CTX; 10 nM) but was unaffected by 10 mM tetraethylammonium (TEA). Addition of 20 microM ZnCl2 reduced IK in a voltage-dependent manner with the greatest inhibition found to occur at potentials near the threshold for current activation. Reduction of IK by ZnCl2 was accompanied by a slowing in the kinetics of IK activation. On the basis of the gating and pharmacological properties of this current, it is suggested that the chondrocyte channel belongs to a superfamily of K+ channels found in bone and immune system cells. The chondrocyte K+ channel may contribute to the unusually high [K+] found in the extracellular fluid of growth plate cartilage.

IP3- and cAMP-induced responses in isolated olfactory receptor neurons from the channel catfish

Olfactory receptor neurons enzymatically dissociated from channel catfish olfactory epithelium were depolarized transiently following dialysis of IP3 or cAMP (added to the patch pipette) into the cytoplasm. Voltage and current responses to IP3 were blocked by ruthenium red, a blocker of an IP3-gated Ca(2+)-release channel in sarcoplasmic reticulum. In contrast, the responses to cAMP were not blocked by extracellularly applied ruthenium red, nor by L-cis-diltiazem or amiloride and two of its derivatives. The current elicited by cytoplasmic IP3 in neurons under voltage clamp displayed a voltage dependence different from that of the cAMP response which showed marked outward rectification. A sustained depolarization was caused by increased cytoplasmic IP3 or cAMP when the buffering capacity for Ca2+ of the pipette solution was increased, when extracellular Ca2+ was removed or after addition of 20-200 nM charybdotoxin to the bathing solution, indicating that the repolarization was caused by an increase in [Cai] that opened Ca(2+)-activated K+ channels. The results suggest that different conductances modulated by either IP3 or cAMP are involved in mediating olfactory transduction in catfish olfactory receptor neurons and that Ca(2+)-activated K+ channels contribute to the termination of the IP3 and cAMP responses.

The increased K+ leak of malaria-infected erythrocytes is not via a Ca(2+)-activated K+ channel

Charybdotoxin and nitrendipine both inhibited K+(86Rb+) influx via the Ca(2+)-activated channel of uninfected erythrocytes but had no effect on K+(86Rb+) transport in malaria-infected cells. Activation of the channel in uninfected cells in which the cytoplasmic [Na+]/[K+] ratio was adjusted to be comparable with that of late-stage malaria-infected erythrocytes resulted in a large (nitrendipine-sensitive) increase in K+(86Rb+) influx. These results suggest that the endogenous Ca(2+)-activated K+ channel remains inactive in human red cells infected with late-stage parasites. The identity of the pathway which mediates the increased K(+)-leak in infected erythrocytes remains to be established.

Cloning, functional expression, and regulation of two K+ channels in human T lymphocytes

Low stringency screening of a Jurkat cDNA library with a rat brain K+ channel (RCK1) probe has resulted in the isolation of HLK3, a voltage-gated K+ channel. In Xenopus oocytes, the HLK3 clone directs the expression of a rapidly activating transient outward K+ current similar to the type n K+ current recorded in Jurkat T cells. The HLK3 gene is located on the short arm of human chromosome 1 (p13.3). Polymerase chain reaction was used to clone HIsK from Jurkat cDNA. The HIsK clone shares the same sequence with a previously described genomic clone (Murai, T., Kazikuka, A., Takumi, T., Ohkubo, H., and Nakanishi, S. (1989) Biochem. Biophys. Res. Commun. 161, 176-181). In Xenopus oocytes, it encodes a slowly activating, noninactivating K+ channel which cannot be recorded in Jurkat cells by conventional patch-clamp techniques. Transcripts of both clones are present at a similar level before and after activation of purified human T lymphocytes and Jurkat cells, reflecting a constitutive expression of K+ channel messages. This finding is in good agreement with the electrophysiological results for type n K+ current density on the same cells. HLK3 current is very sensitive to the scorpion toxin charybdotoxin (IC50 = 0.8 nM). HIsK current is totally insensitive to this toxin but is blocked by the antiarrhythmic clofilium (IC50 = 80 microM). While charybdotoxin has no effect on interleukin 2 mRNA induction, clofilium potently inhibits interleukin 2 mRNA expression upon mitogen-induced T cell activation. It is concluded that the HLK3 channel is not an important component of the T cell mitogenic response. Other targets for K+ channel blockers, such as the HIsK protein, could be involved in the activation process.

Regulation of arterial tone by activation of calcium-dependent potassium channels

Blood pressure and tissue perfusion are controlled in part by the level of intrinsic (myogenic) vascular tone. However, many of the molecular determinants of this response are unknown. Evidence is now presented that the degree of myogenic tone is regulated in part by the activation of large-conductance calcium-activated potassium channels in arterial smooth muscle. Tetraethylammonium ion (TEA+) and charybdotoxin (CTX), at concentrations that block calcium-activated potassium channels in smooth muscle cells isolated from cerebral arteries, depolarized and constricted pressurized cerebral arteries with myogenic tone. Both TEA+ and CTX had little effect on arteries when intracellular calcium was reduced by lowering intravascular pressure or by blocking calcium channels. Elevation of intravascular pressure through membrane depolarization and an increase in intracellular calcium may activate calcium-activated potassium channels. Thus, these channels may serve as a negative feedback pathway to control the degree of membrane depolarization and vasoconstriction.

Large conducting potassium channel reconstituted from airway smooth muscle

Microsomal fractions were prepared from canine and bovine airway smooth muscle (ASM) by differential and gradient centrifugations. Surface membrane vesicles were characterized by binding assays and incorporated into planar lipid bilayers. Single-channel activities were recorded in symmetric or asymmetric K+ buffer systems and studied under voltage and Ca2+ clamp conditions. A large-conductance K(+)-selective channel (greater than 220 pS in 150 mM K+) displaying a high Ca2+, low Ba2+, and charybdotoxin (CTX) sensitivity was identified. Time analysis of single-channel recordings revealed a complex kinetic behavior compatible with the previous schemes proposed for Ca(2+)-activated K+ channels in a variety of biological surface membranes. We now report that the open probability of the channel at low Ca2+ concentration is enhanced on in vitro phosphorylation, which is mediated via an adenosine 3',5'-cyclic monophosphate-dependent protein kinase. In addition to this characterization at the molecular level, a second series of pharmacological experiments were designed to assess the putative role of this channel in ASM strips. Our results show that 50 nM CTX, a specific inhibitor of the large conducting Ca(2+)-dependent K+ channel, prevents norepinephrine transient relaxation on carbamylcholine-precontracted ASM strips. It was also shown that CTX reversed the steady-state relaxation induced by vasoactive intestinal peptide and partially antagonized further relaxation induced by cumulative doses of this potent bronchodilatator. Thus it is proposed that the Ca(2+)-activated K+ channels have a physiological role because they are indirectly activated on stimulation of various membrane receptors via intracellular mechanisms.

Mapping hydrophobic residues of the interaction surface of charybdotoxin

Images

A small-conductance charybdotoxin-sensitive, apamin-resistant Ca(2+)-activated K+ channel in aortic smooth muscle cells (A7r5 line and primary culture)

A small conductance K+ channel was identified in smooth muscle cells of the rat aortic cell line A7r5 and also in rat aortic smooth muscle cells in primary culture, using conventional single-channel recording techniques. The single-channel conductance shows no rectification, either in the range -70 to +40 mV under asymmetrical conditions (9.1 pS), or in the range -100 to +50 mV in symmetrical 150 mM K+ (37 pS). Channel activity is reversibly inhibited by extracellular application of charybdotoxin, with a concentration of 8 nM producing half-maximal inhibition. It is unaffected by apamin or scyllatoxin. Channel activity depends on the presence of free Ca2+ on the cytosolic face of the membrane, with an activation zone between 0.1 and 1 microM. This small-conductance, charybdotoxin-sensitive, Ca(2+)-regulated K+ channel is activated by vasoconstrictors such as vasopressin and endothelin.

Toxin pharmacology of the ATP-induced hyperpolarization in Madin-Darby canine kidney cells

The effects of Leiurus quinquestriatus hebraeus (LQH) venom, mamba venom, Buthus tamulus (BT) venom, purified apamin and synthetic charybdotoxin on the membrane hyperpolarization induced by extracellular ATP were examined in Madin-Darby canine kidney cells. For this we used a membrane potential probe (bisoxonol) to determine the potential variations. The relation between bisoxonal fluorescence and membrane potential was established by treating Madin-Darby canine kidney cells suspended in solutions containing various external sodium concentrations with gramicidin. Extracellular ATP induced a rapid hyperpolarization that was blocked by LQH venom and synthetic charybdotoxin. BT venom also blocked the response but at a much higher concentration than that of LQH. Mamba venom (Dendroaspis polylepis) and apamin did not modify the ATP-induced hyperpolarization. We concluded that the ATP induced hyperpolarization was due to the augmentation of the potassium conductance probably through Ca(2+)-activated K+ channels sensitive to charybdotoxin but not to mamba venom. The interaction previously described between charybdotoxin and dendrotoxin (the main toxin of mamba venom) was not observed in our case.

Calcium-activated potassium channels mediate prejunctional inhibition of peripheral sensory nerves

Activation of several receptors, including mu-opioid, alpha 2-adrenergic, and neuropeptide Y receptors, inhibits excitatory nonadrenergic noncholinergic (NANC) neural responses in airways, which were mediated by the release of peptides from capsaicin-sensitive sensory nerves. This raises the possibility of a common inhibitory mechanism, which may be related to an increase in K+ conductance in sensory nerves. To examine this hypothesis, we have studied whether K(+)-channel blockers inhibit the effects of neuromodulators of sensory nerves in guinea pig bronchi by using selective K(+)-channel blockers. Charybdotoxin (ChTX; 10 nM), which blocks large conductance Ca(2+)-activated K(+)-channel function, completely blocked and reversed the inhibitory effects of a mu-opioid agonist, neuropeptide Y, and an alpha 2-adrenoceptor agonist on excitatory NANC responses. Neither inhibitors of ATP-sensitive K+ channels (BRL 31660 or glibenclamide, both at 10 microM) nor an inhibitor of small conductance Ca(2+)-activated K+ channels (apamin; 0.1 microM) were effective. This suggests that ChTX-sensitive K(+)-channel activation may be a common mechanism for the prejunctional modulation of sensory nerves in airways. This may have important implications for the control of neurogenic inflammation.

Ca(2+)-activated K+ channels in human leukemic T cells

Using the patch-clamp technique, we have identified two types of Ca(2+)- activated K+ (K(Ca)) channels in the human leukemic T cell line. Jurkat. Substances that elevate the intracellular Ca2+ concentration ([Ca2+]i), such as ionomycin or the mitogenic lectin phytohemagglutinin (PHA), as well as whole-cell dialysis with pipette solutions containing elevated [Ca2+]i, activate a voltage-independent K+ conductance. Unlike the voltage-gated (type n) K+ channels in these cells, the majority of K(Ca) channels are insensitive to block by charybdotoxin (CTX) or 4- aminopyridine (4-AP), but are highly sensitive to block by apamin (Kd less than 1 nM). Channel activity is strongly dependent on [Ca2+]i, suggesting that multiple Ca2+ binding sites may be involved in channel opening. The Ca2+ concentration at which half of the channels are activated is 400 nM. These channels show little voltage dependence over a potential range of -100 to 0 mV and have a unitary conductance of 4-7 pS in symmetrical 170 mM K+. In the presence of 10 nM apamin, a less prevalent type of K(Ca) channel with a unitary conductance of 40-60 pS can be observed. These larger-conductance channels are sensitive to block by CTX. Pharmacological blockade of K(Ca) channels and voltage- gated type n channels inhibits oscillatory Ca2+ signaling triggered by PHA. These results suggest that K(Ca) channels play a supporting role during T cell activation by sustaining dynamic patterns of Ca2+ signaling.

Mechanisms of action of cicletanine in human and guinea pig resistance arteries

Human subcutaneous (s.c.) arteries and guinea pig mesenteric vessels (internal diameter 150-570 microns) were isolated and mounted on a microvascular myograph. Cicletanine-induced relaxation curves were constructed after preconstriction with either depolarising potassium (KPSS) or norepinephrine (NE) and in the presence and absence of indomethacin, glibenclamide, or tetraethylammonium chloride (TEA). Using only guinea pig vessels, cicletanine relaxation curves were constructed with and without charybdotoxin. In human vessels, there was no significant difference between cicletanine-induced relaxation in vessels preconstricted with either NE or KPSS, and neither ATP-sensitive or Ca(2+)-activated K channels were involved. However, with indomethacin added, in human vessels the maximal response to cicletanine (30 microM) was reduced by 51% (p less than 0.05). In guinea pig mesenteric arteries, cicletanine (30 microM) produced a 95% relaxation of the NE-constricted vessel and only 30% relaxation of the KPSS-constricted vessel (p less than 0.001). There was no evidence for any involvement of the ATP-sensitive K channel or the eicosanoid system. The relaxation to cicletanine (less than 3 microM) in with TEA added was greatly reduced (p less than 0.001) and with charybdotoxin added the concentration response curve to cicletanine was shifted approximately 3 log units to the right (p less than 0.001), suggesting an involvement of Ca(2+)-activated K channels. The acute vasodilator action of cicletanine is complex, with multiple mechanisms of action, and the relative importance of these varies in different species or at least in different vascular beds.

Effect of K+ channel-modulating drugs on the vasoconstrictor responses of leukotrienes C4, D4 and angiotensin II in the guinea-pig isolated perfused heart

1. The vascular actions of leukotrienes C4 (LTC4) and LTD4 in the guinea-pig isolated perfused heart were studied in the presence of potassium (K+) channel modulatory compounds. 2. Cromakalim (0.35-10 microM), a K+ channel activator, inhibited the vasoconstrictor responses of LTC4 (30 pmol), LTD4 (30 pmol) and angiotensin II (AII) (100 pmol) in a concentration-dependent manner. 3. Glyceryl trinitrate (10 mgl-1) and vasoactive intestinal peptide (10 nM) induced a similar vasodilator action to cromakalim in the isolated heart but had no effect on responses to LTC4 and LTD4. 4. The inhibitory action by cromakalim (10 microM) on the LTC4 (30 pmol) response could be reversed in the presence of an equimolar concentration of glibenclamide. However, glibenclamide (10 microM) only partially restored the LTD4 (30 pmol) actions. 5. Galanin (10 nM) and charybdotoxin (60 nM) had no effect on the vascular responses to LTC4 and LTD4 (30 pmol). 6. Inhibition by cromakalim of coronary vasospasm induced by vascular LTC4, LTD4 and AII appears to be separate from its vasodilator action and it is postulated that a cromakalim-sensitive mechanism in the coronary vasculature is important in the vasoconstrictor responses to LTC4, LTD4 and AII.

Activation of K+ current in macrophages by platelet activating factor

Puff application of platelet activating factor (10(-8) M) onto peritoneal macrophages from thioglycollate-stimulated mice induced an outward current at a holding potential of -63 mV. The current was suppressed by an antagonist Y-24180 but not by CV-3988. Charybdotoxin (10(-6) M) suppressed the current. Reversal potentials were dependent on external K+ concentrations. The current was not suppressed in Ca(2+)-free EGTA-containing solution but was completely abolished in BAPTA-AM containing solution. These results suggest that platelet activating factor activates a Ca(2+)-dependent K+ channel.

Effect of shear stress on cytosolic Ca2+ of calf pulmonary artery endothelial cells

The purpose of the present study was to determine if hemodynamic shear stress increases free cytosolic Ca2+ concentration ([Ca2+]i) of cultured pulmonary artery endothelial cells exposed to steady laminar fluid flow in a parallel plate chamber. Average [Ca2+]i was estimated by measuring cell-associated fura-2 fluorescence using microfluorimetric analysis. To determine [Ca2+]i close to the membrane surface, 86Rb+ efflux via Ca(2+)-dependent K+ channels was measured. Upon initiation of flow or upon step increases in flow, no change in [Ca2+]i was observed using fura-2. However, increases in shear stress produced a large, transient increase in 86Rb+ efflux. The shear stress-dependent increase in 86Rb+ efflux was not blocked by either tetrabutylammonium ions (20 mM) or by charybdotoxin (10 nM), two specific inhibitors of the Ca(2+)-dependent K+ channel of vascular endothelial cells. These results demonstrate that shear stress per se has little effect on either the average cytosolic [Ca2+]i as measured by fura-2 or on [Ca2+]i close to the cytoplasmic surface of the plasmalemma as measured by the activity of Ca(2+)-dependent K+ channels.

Presynaptic calcium signals and transmitter release are modulated by calcium-activated potassium channels

The regulation of synaptic transmission by Ca(2+)-activated potassium (gKca) channels was investigated at the frog neuromuscular junction (nmj). Charybdotoxin (CTX), a blocker of certain types of gKca channels, induced a twofold increase of transmitter release. Similar results were obtained with purified natural toxin, synthetic toxin, and recombinant toxin. Apamin, a blocker of a different type of gKca channel, did not alter transmitter release. CTX was ineffective after intraterminal Ca2+ buffering was increased by application of the membrane-permeant Ca2+ buffer dimethyl-BAPTA-AM. By itself, the permeant buffer first caused a slight increase in transmitter release before release was eventually decreased. This increase of release did not occur when the buffer was applied in the presence of CTX or Ba2+, another gKca channel blocker. Stimulus-evoked entry of Ca2+ in nerve terminals, detected with the fluorescent Ca2+ indicator FLUO-3, was increased after blockade of gKca channels by CTX. CTX had no effect on the amount or the time course of synaptic depression. The results are consistent with the hypothesis that CTX-sensitive gKca channels normally narrow the presynaptic action potential and thus, by indirectly regulating Ca2+ entry, can serve as powerful modulators of evoked transmitter release. In order to affect presynaptic action potentials, the gKca channels must be located close to Ca2+ channels.

Refined structure of charybdotoxin: common motifs in scorpion toxins and insect defensins

Conflicting three-dimensional structures of charybdotoxin (Chtx), a blocker of K+ channels, have been previously reported. A high-resolution model depicting the tertiary structure of Chtx has been obtained by DIANA and X-PLOR calculations from new proton nuclear magnetic resonance (NMR) data. The protein possesses a small triple-stranded antiparallel beta sheet linked to a short helix by two disulfides and to an extended fragment by one disulfide, respectively. This motif also exists in all known structures of scorpion toxins, irrespective of their size, sequence, and function. Strikingly, antibacterial insect defensins also adopt this folding pattern.

Two outward K+ currents in bovine pigmented ciliary epithelial cells: IK(Ca) and IK(V)

Pigmented ciliary epithelial cells were studied using the whole cell voltage-clamp technique. Depolarizing steps from a holding potential of -80 mV resulted in a small initial inward current followed by a large outward current. Prolonged depolarizing voltage steps revealed inactivating and noninactivating components of outward current. Outward current was sensitive to the level of Ca2+ in the pipette and was increased by the calcium ionophore A23187; it was blocked by tetraethylammonium (TEA+), quinine, and 4-aminopyridine (4-AP). 4-AP blocked 70% of the outward current with a Ki of 7 x 10(-5) M, and part of the remaining current was abolished by Ni2+. Ni2+ caused a reduction in outward current by blocking IK(Ca) indirectly via decreasing Ca2+ entry through T-type Ca2+ channels. Separating Ni(2+)-sensitive from -insensitive outward conductance gives components that correspond notionally to IK(Ca) and IK(V), respectively. On this basis IK(Ca) represents approximately 28% of K+ outward current. Charybdotoxin blocked 26% of the outward conductance at very depolarized voltage steps as calculated from the slope of the current-voltage curve in this region. It is concluded that there are two major components to the outward current: IK(V), an inactivating voltage-sensitive K+ current, and IK(Ca), which is dependent on the entry of Ca2+ through T-type Ca2+ channels and comprises approximately a quarter of the total K+ outward current under the conditions described. Because of their relative voltage-activation properties, IK(Ca) will be the more important in terms of K+ transport and the secretion of aqueous humor by the ciliary epithelium.

Calcium-activated potassium conductance in presynaptic terminals at the crayfish neuromuscular junction

Membrane potential changes that typically evoke transmitter release were studied by recording intracellularly from the excitor axon near presynaptic terminals of the crayfish opener neuromuscular junction. Depolarization of the presynaptic terminal with intracellular current pulses activated a conductance that caused a decrease in depolarization during the constant current pulse. This conductance was identified as a calcium-activated potassium conductance, gK(Ca), by its disappearance in a zero-calcium/EGTA medium and its block by cadmium, barium, tetraethylammonium ions, and charybdotoxin. In addition to gK(Ca), a delayed rectifier potassium conductance (gK) is present in or near the presynaptic terminal. Both these potassium conductances are involved in the repolarization of the membrane during a presynaptic action potential.

Effects of K+ channel blockers and cromakalim (BRL 34915) on the mechanical activity of guinea pig detrusor smooth muscle

There is strong evidence that cromakalim (BRL 34915) relaxes smooth muscle by opening cell membrane K+ channels. The aim of this study was to use relatively selective K+ channel blockers to investigate 1) the K+ channel type(s) opened by cromakalim in guinea pig detrusor and 2) the role of different K+ channel types in the control of basal tension. Cromakalim produced a concentration-related relaxation (IC50 = 0.50 +/- 0.03 microM, n = 42) of 15 mM K(+)-evoked mechanical activity. The ATP-sensitive K+ channel blocker glyburide (0.3-3 microM) antagonized the effects of cromakalim in an apparently competitive manner (pA2 = 6.76). Charybdotoxin and iberiatoxin (3-30 nM), blockers of the large conductance, Ca(++)-activated K+ channel, appeared to functionally antagonize cromakalim. Apamin (1 microM) and leiurotoxin I (0.3 microM), blockers of the small conductance, Ca(++)-activated K+ channel, and noxiustoxin (0.3 microM), a blocker of squid axon delayed rectifer K+ channels, all failed to antagonize cromakalim. Cumulative administration of charybdotoxin and iberiatoxin produced marked, concentration-related stimulation of mechanical activity per se whereas glyburide, noxiustoxin, apamin and leiurotoxin I had no effect. Apamin and leiurotoxin I did stimulate mechanical activity to a small extent when administered noncumulatively, however. The results suggest that cromakalim opens ATP-sensitive K+ channels in detrusor and suggest that cromakalim does not open CA(++)-activated K+ channels and noxiustoxin-sensitive, delayed rectifier K+ channels. The marked stimulatory effects of charybdotoxin and iberiatoxin per se suggest an important role for large conductance, Ca(++)-activated K+ channels in the control of basal tension and, presumably, membrane potential in detrusor smooth muscle cells.

Characterization of the solubilized charybdotoxin receptor from bovine aortic smooth muscle

Monoiodotyrosine ([125I]ChTX) binds with high affinity to a single class of receptors present in bovine aortic smooth muscle sarcolemmal membranes that are functionally associated with the high-conductance Ca(2+)-activated K+ channel [maxi-K channel; Vázquez, J., et al. (1989) J. Biol. Chem. 265, 20902-20909]. Cross-linking experiments carried out with this preparation in the presence of [125I]ChTX and disuccinimidyl suberate indicate specific incorporation of radioactivity into a protein of Mr 35,000. The smooth muscle ChTX receptor can be solubilized in active form in the presence of selected detergents. Treatment of membranes with digitonin releases about 50% of the ChTX binding sites. The solubilized receptor retains the same biochemical and pharmacological properties that are characteristic of toxin interaction with membrane-bound receptors. The solubilized receptor binds specifically to wheat germ agglutinin-Sepharose resin, suggesting that it is a glycoprotein. Functional ChTX binding sites can also be solubilized in 3-[(3-cholamidopropyl)dimethylamino]-1-propanesulfonate (CHAPS). Sucrose density gradient centrifugation of either digitonin or CHAPS extracts indicates that the ChTX receptor has a high apparent sedimentation coefficient (s20,w = 23 and 18 S, respectively). Cross-linking experiments indicate that the appearance of the 35-kDa membrane protein correlates with ChTX binding activity after both wheat germ agglutinin-Sepharose and sucrose density gradient centrifugation steps. Given the high apparent sedimentation coefficient of the ChTX receptor, the 35-kDa membrane protein may be a subunit of a higher molecular weight complex which forms the maxi-K channel in smooth muscle sarcolemma.

Charybdotoxin-sensitive, Ca(2+)-dependent membrane potential changes are not involved in human T or B cell activation and proliferation

The involvement of ion channels in B and T lymphocyte activation is supported by many reports of changes in ion fluxes and membrane potential after mitogen binding. Human T and B lymphocytes demonstrate an early and transient hyperpolarization after ligand binding. Inasmuch as the change in membrane potential is dependent on elevation of free cytosolic calcium, the hyperpolarization is presumably through opening of Ca(2+)-stimulated K+ channels. We have used charybdotoxin, a known inhibitor of Ca(2+)-dependent K+ channels, to study the role of these channels in lymphocyte activation and mitogenesis. We demonstrate that charybdotoxin inhibits the ligand-induced transient membrane hyperpolarization in B and T cells in a dose-dependent fashion, without affecting changes in cytosolic Ca2+. However, blockade of the Ca(2+)-activated K+ channel is not associated with changes in cell-cycle gene activation, IL-2 production, IL-2R expression or B and T cell mitogenesis. These results imply that membrane potential changes secondary to the ligand-dependent opening of Ca(2+)-activated K+ channels are not involved in B and T lymphocyte activation and mitogenesis.

Flow activates an endothelial potassium channel to release an endogenous nitrovasodilator

Flow-mediated vasodilation is endothelium dependent. We hypothesized that flow activates a potassium channel on the endothelium, and that activation of this channel leads to the release of the endogenous nitrovasodilator, nitric oxide. To test this hypothesis, rabbit iliac arteries were perfused at varying flow rates, at a constant pressure of 60 mm Hg. Increments in flow induced proportional increases in vessel diameter, which were abolished by L,N-mono-methylarginine (the antagonist of nitric-oxide synthesis). Barium chloride, depolarizing solutions of potassium, verapamil, calcium-free medium, and antagonists of the KCa channel (charybdotoxin, iberiotoxin) also blocked flow-mediated vasodilation. Conversely, responses to other agonists of endothelium-dependent and independent vasodilation were unaffected by charybdotoxin or iberiotoxin. To confirm that flow activated a specific potassium channel to induce the release of nitric oxide, endothelial cells cultured on micro-carrier beads were added to a flow chamber containing a vascular ring without endothelium. Flow-stimulated endothelial cells released a diffusible vasodilator; the degree of vasorelaxation was dependent upon the flow rate. Relaxation was abrogated by barium, tetraethylammonium ion, or charybdotoxin, but was not affected by apamin, glybenclamide, tetrodotoxin, or ouabain. The data suggest that transmission of a hyperpolarizing current from endothelium to the vascular smooth muscle is not necessary for flow-mediated vasodilation. Flow activates a potassium channel (possibly the KCa channel) on the endothelial cell membrane that leads to the release of nitric oxide.

Inositol trisphosphate-dependent periodic activation of a Ca(2+)-activated K+ conductance in glucose-stimulated pancreatic beta-cells

Glucose-stimulated insulin secretion is associated with the appearance of electrical activity in the pancreatic beta-cell. At intermediate glucose concentrations, beta-cell electrical activity follows a characteristic pattern of slow oscillations in membrane potential on which bursts of action potentials are superimposed. The electrophysiological background of the bursting pattern remains unestablished. Activation of Ca(2+)-activated large-conductance K+ channels (KCa channel) has been implicated in this process but seems unlikely in view of recent evidence demonstrating that the beta-cell electrical activity is unaffected by the specific KCa channel blocker charybdotoxin. Another hypothesis postulates that the bursting arises as a consequence of two components of Ca(2+)-current inactivation. Here we show that activation of a novel Ca(2+)-dependent K+ current in glucose-stimulated beta-cells produces a transient membrane repolarization. This interrupts action potential firing so that action potentials appear in bursts. Spontaneous activity of this current was seen only rarely but could be induced by addition of compounds functionally related to hormones and neurotransmitters present in the intact pancreatic islet. K+ currents of the same type could be evoked by intracellular application of GTP, the effect of which was mediated by mobilization of Ca2+ from inositol 1,4,5-trisphosphate (InsP3)-sensitive intracellular Ca2+ stores. These observations suggest that oscillatory glucose-stimulated electrical activity, which is correlated with pulsatile release of insulin, results from the interaction between the beta-cell and intraislet hormones and neurotransmitters. Our data also provide evidence for a close interplay between ion channels in the plasma membrane and InsP3-induced mobilization of intracellular Ca2+ in an excitable cell.

Effects of charybdotoxin and iberiotoxin on the spontaneous motility and tonus of different guinea pig smooth muscle tissues

Charybdotoxin (ChTX) and iberiotoxin (IbTX), two potent peptidyl blockers of the high conductance Ca(2+)-activated K+ channel (PK,Ca) were used to probe the role of this channel in regulating the contractility of various smooth muscles isolated from the guinea pig. Of the spontaneously contracting tissues that have been investigated, bladder and taenia coli are affected by ChTX, whereas portal vein and uterus are relatively insensitive. In the former two tissues, ChTX (10-100 nM) produces a concentration-dependent increase in contractility, with bladder being most sensitive to action of the toxin. ChTX also causes a contracture of quiescent aortic rings, although not affecting indomethacin-treated trachea. In order to demonstrate that the effects of ChTX are due specifically to blockage of PK,Ca, rather than to inhibition of some other K+ channel, two other inhibitors of PK,Ca were monitored. IbTX (10-100 nM), a selective inhibitor of PK,Ca, and tetraethylammonium ion, which blocks PK,Ca at low (0.1-3 mM) concentrations, both increase the myogenic activity of bladder, but not portal vein. In addition, IbTX causes a sustained contracture of aorta. Taken together, these data indicate that the increased contractility of certain guinea pig smooth muscles produced by ChTX, IbTX and tetraethylammonium ion is the result of selective inhibition of PK,Ca. It is suggested that in spontaneously active bladder and taenia coli, PK,Ca provides a repolarization pathway after tissue depolarization, whereas in quiescent aorta, PK,Ca maintains cellular resting potential. In contrast, in indomethacin-treated trachealis muscle, ChTX-sensitive K+ channel pathways are not involved in controlling resting tension.(ABSTRACT TRUNCATED AT 250 WORDS)

The cystine-stabilized alpha-helix: a common structural motif of ion-channel blocking neurotoxic peptides

Neurotoxic peptides from venoms of scorpions and honey bees exhibit a consensus pattern in the two disulfide bridgings related to the sequence portions Cys-X-Cys and Cys-X-X-X-Cys. A revised three-dimensional structure of charybdotoxin, as determined by two-dimensional nmr spectroscopy, confirms that the consensus cystine dislocation generates in all these toxins a common structural element, i.e., the cystine-stabilized alpha-helical (CSH) motif, which may be correlated with their common ion channel blocking activity.

N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W7) stimulation of K+ transport in a human salivary epithelial cell line

Treatment of a human salivary epithelial cell line, HSG-PA, with the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W7; 20-70 microM) increased 86Rb (K+) influx and efflux in a manner similar to that resulting from muscarinic (carbachol; Cch) or calcium ionophore (A23187) stimulation. Unlike the Cch or A23187 responses, the W7 responses were not blocked by 0.1 mM atropine (muscarinic antagonist) or phorbol-12-myristate-13-acetate (0.1 microM). Like Cch- or A23187-stimulated 86Rb fluxes, W7-stimulated 86Rb fluxes were substantially blocked by the K+ channel inhibitors quinine (0.25 mM) and scorpion venom-containing charybdotoxin (33 micrograms/mL), while 5 mM tetraethylammonium chloride (K+ channel blocker), furosemide (0.1 mM; Na+,K+,2Cl- co-transport inhibitor) and ouabain (10 microM; Na+,K(+)-ATPase inhibitor) were ineffective. Purified charybdotoxin (10 nM) also blocked W7-stimulated 86Rb influx, as well as 86Rb influx stimulated by Cch or A23187. Although Quin 2 fluorescence measurements indicated that W7 increased free intracellular Ca2+ concentration ([Ca2+]i), the magnitude of the increase appeared to be insufficient to solely account for the W7-stimulated increases in 86Rb fluxes (i.e. K+ channel activity). Ca2+ was involved in the W7 response, however, as lack of Ca2+ in the incubation medium reduced the W7-stimulated increases in 86Rb influx and efflux. Taken together, our results suggest that W7 increased K+ fluxes in HSG-PA cells by interacting, directly or indirectly, with the K+ transport machinery (K+ channels) in a manner different from that observed during muscarinic stimulation, and also in a manner not accounted for solely by the formation of a typical muscarinic- or calcium ionophore-generated calcium signal.

Glyburide-sensitive K+ channels in cultured rat hippocampal neurons: activation by cromakalim and energy-depleting conditions

Previous studies in our laboratory have shown that cromakalim activates a tetraethylammonium-sensitive K+ current in cultured embryonic rat hippocampal neurons. This phenomenon was further characterized using whole-cell voltage-clamp and single-channel recording techniques. Glyburide (1-25 microM), an antagonist of ATP-sensitive K+ channels, produced a concentration-dependent depression of the cromakalim-activated current. In contrast, charybdotoxin (100 nM), an antagonist of some Ca(2+)-dependent and other K+ channels, not only failed to block the effect of cromakalim but actually produced a moderate enhancement of the cromakalim-activated K+ current. Neither glyburide nor charybdotoxin affected resting or voltage-activated K+ currents in the absence of cromakalim. Exposure of the cells to energy-depleting conditions (0.24 micrograms/ml oligomycin and 10 mM 2-deoxy-D-glucose) also activated an outward current. Single-channel recordings in the cell-attached configuration showed that cromakalim (100 microM) stimulated the opening of flickery single channels having a unitary conductance of approximately 26 pS and a prolonged burst duration (mean open time, approximately 131 msec); similar channel openings were observed in patches from cells exposed to energy-depleting conditions. In patches containing a single K+ channel, the open probability in the presence of cromakalim was approximately 0.6 and in the presence of energy-depleting conditions was approximately 0.8; in the absence of either of these treatments, channel openings were not observed. Glyburide produced a reversible inhibition of the channels activated by cromakalim and energy-depleting conditions. These data provide additional support for the existence of ATP-sensitive K+ channels in central neurons and indicate that the K+ channels whose opening is stimulated by cromakalim are likely to be of the ATP-sensitive type.

Potassium currents in freshly dispersed myometrial cells

K+ currents in freshly dispersed cells from rat myometrium at estrus were studied with the patch-clamp technique (whole cell). Three types of K+ currents were identified: 1) a fast-activating current (IKf), 2) a slowly activating current (IKs), and 3) a transient current (IKt). IKf had a half-activation voltage of 12 mV and a time constant of activation (tau on) of approximately 3 ms at +50 mV. IKs had a tau on of approximately 9 ms at +50 mV and a half-activation potential of 33 mV. Both IKf and IKs were sustained and became potentiated by the entrance of Ca2+ from the patch pipette. These two Ca(2+)-activated K+ currents were inhibited by 100 nM external charybdotoxin and were blocked by external tetraethylammonium (TEA, 2-20 mM). The third current (IKt) was transient, had a faster tau on (approximately 1 ms), and a decay phase with a time constant of approximately 8 ms at +50 mV. This current had a half-activation potential of 22 mV. IKt was not potentiated by intracellular Ca2+, was sensitive to 4-aminopyridine (1 mM), was insensitive to external charybdotoxin (100 nM) and TEA (2 mM), and spontaneously decreased with time.

Ca(2+)-activated K+ channels in rat thymic lymphocytes: activation by concanavalin A

1. The role of ion channels in the mitogenic response of rat thymic lymphocytes to concanavalin A (ConA) was studied using single-channel patch-clamp recordings and measurements of membrane potential with the fluorescent probe bis-oxonol. 2. ConA (20 micrograms ml-1) evoked a rapid membrane hyperpolarization; Indo-1 measurements indicated a concurrent increase in [Ca2+]i. The hyperpolarization was blocked by cytoplasmic loading with the Ca2+ buffer BAPTA (bis(O-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid), or charybdotoxin, a component of scorpion venom known to block K+ channels in lymphocytes. 3. Cell-attached patch-clamp recordings showed that both ConA and the Ca2+ ionophore ionomycin activated channels with high selectivity for K+. Two conductance levels were observed -6-7 pS and 17-18 pS-measured as inward chord conductance at 60 mV from reversal potential (Erev) with 140 mM-KCl in the pipette. The current-voltage relationship for the larger channel displayed inward rectification and channel open probability was weakly dependent upon membrane potential. 4. These experiments provide the first direct evidence for mitogen-activated Ca(2+)-gated K+ channels (IK(Ca)) in lymphocytes. This conductance is relatively inactive in unstimulated rat thymocytes but following the intracellular Ca2+ rises induced by ConA, IK(Ca) channels are activated and produce a significant hyperpolarization of the cell potential.

ATP-sensitive K+ channels from aortic smooth muscle incorporated into planar lipid bilayers

Glibenclamide binding sites were identified in a membrane preparation from canine aortic smooth muscle. The dissociation constant for [3H]glibenclamide binding was 10 +/- 2 nM, with a density of 420 +/- 108 fmol/mg protein. The properties of ATP-sensitive potassium (KATP) channels from the same membrane preparation incorporated into planar lipid bilayers were investigated. ATP was a potent inhibitor of the channels with half-maximal inhibition of channel activity by 41 microM ATP. Glibenclamide inhibited channel activity, and cromakalim activated the channel in the presence of ATP. Blockers of Ca(2+)-activated K+ (KCa) channels (charybdotoxin and tetraethylammonium ions) did not affect KATP channels in concentrations that caused significant block of KCa channels in bilayers. This membrane preparation should allow further biochemical and functional characterization of KATP channels and glibenclamide receptors in arterial smooth muscle.

Guinea-pig isolated trachealis: the effects of charybdotoxin on mechanical activity, membrane potential changes and the activity of plasmalemmal K(+)-channels

1. A study has been made, in guinea-pig isolated trachealis, of the effects of charybdotoxin in modulating (a) the activity of large conductance K(+)-channels, (b) the spontaneous electrical activity of intact cells and (c) the mechanical effects of some bronchodilator drugs. 2. Single smooth muscle cells were isolated from guinea-pig trachealis by enzymic digestion and were studied by the patch clamp recording technique. Recordings were made from outside-out plasmalemmal patches when the medium bathing the external surface of the patches contained 1.2 mM Ca2+ and 6 mM K+ while that bathing the cytosolic surface contained 0.1 microM Ca2+ and 140 mM K+. Charybdotoxin (100 nM), applied to the external surface of patches held at 0 mV, abolished the unitary currents associated with the opening of large conductance K(+)-channels. 3. Opened segments of guinea-pig trachea were used for the simultaneous recording of membrane potential and tension changes. In these experiments charybdotoxin (100 nM) caused the conversion of spontaneous electrical slow waves into spike-like action potentials. This effect was accompanied by a very small reduction in resting membrane potential. 4. Tissue bath recording showed that charybdotoxin (100 nM) increased the spontaneous mechanical tone of the tissue, antagonized (2.8 fold in each case) the relaxant actions of isoprenaline and theophylline but did not antagonize the relaxant actions of cromakalim or RP 49356. 5. It is concluded that charybdotoxin is an effective inhibitor of large conductance K(+)-channels in guinea-pig trachealis cells. The ability of charybdotoxin to convert spontaneous slow waves into spike-like action potentials suggests that the large, charybdotoxin-sensitive, K+-channels play an important role in determining the strong outward rectifying behaviour of the cells. The ability of charybdotoxin to antagonize isoprenaline and theophylline, but not to antagonize cromakalim and RP 49356, suggests that opening of the large conductance, charybdotoxin-sensitive K+-channel is implicated in the action of the former but not the latter pair of bronchodilator drugs.

A Ca-activated K channel from rabbit renal brush-border membrane vesicles in planar lipid bilayers

Rabbit renal brush-border membranes were fused to planar lipid bilayers to gain insight into the nature and properties of ion channels from the luminal membrane of the proximal tubule. Fusion was obtained using osmotic gradients. A large conductance channel was commonly observed. Measurements of reversal potentials indicated that the channel was selective for K over Rb, Na, and Cl. Channel open probability was increased by membrane depolarization and by increased Ca activity on the intracellular face of the channel. The channel was inhibited by charybdotoxin (CTX), a protein from leiurus venom, from the external side of the channel. The channel was also blocked by Ba and quinidine added to the intracellular bathing solution. Na added to the intracellular bathing solution reduced current amplitude in a voltage-dependent fashion. In addition, methylisobutyl amiloride, an analogue of the K-sparing diuretic amiloride, inhibited channel activity when added to the external solution. The possible physiological role of the channel is discussed. The usefulness to the study of renal ion channels of the technique of fusing membrane vesicles to planar lipid bilayers is evaluated.

Charybdotoxin blocks cation-channels in the vacuolar membrane of suspension cells of Chenopodium rubrum L

Using the patch-clamp technique, we studied the action of charybdotoxin which blocks Ca(2+)-activated large-conductance K+ channels in animal tissue on the slow-activating (SV), Ca(2+)-activated cation channel in the vacuolar membrane of suspension-cells of Chenopodium rubrum L. The toxin reversibly reduced the vacuolar current with EC50 approximately 20 nM suggesting structural similarities between ion channels in animal and plant membranes.

Somatostatin stimulates Ca(2+)-activated K+ channels through protein dephosphorylation

The neuropeptide somatostatin inhibits secretion from electrically excitable cells in the pituitary, pancreas, gut and brain. In mammalian pituitary tumour cells somatostatin inhibits secretion through two distinct pertussis toxin-sensitive mechanisms. One involves inhibition of adenylyl cyclase, the other an unidentified cyclic AMP-independent mechanism that reduces Ca2+ influx by increasing membrane conductance to potassium. Here we demonstrate that the predominant electrophysiological effect of somatostatin on metabolically intact pituitary tumour cells is a large, sustained increase in the activity of the large-conductance Ca(2+)- and voltage-activated K+ channels (BK). This action of somatostatin does not involve direct effects of Ca2+, cAMP or G proteins on the channels. Our results indicate instead that somatostatin stimulates BK channel activity through protein dephosphorylation.

Rapid secretagogue-induced activation of Na+H+ exchange in rat parotid acinar cells. Possible interrelationship between volume regulation and stimulus-secretion coupling

We demonstrate a rapid activation of the Na+/H+ exchanger in intact rat parotid acini in response to muscarinic (carbachol; K1/2 = 0.4 microM) and alpha-adrenergic (epinephrine; K1/2 = 0.1 microM) stimulation. This rapid activation is apparently distinct from the relatively "slow" activation of the exchanger (t1/2 greater than or equal to 5 min) reported previously (Manganel, M., and Turner, R. J. (1989) J. Membr. Biol. 111, 191-198). This rapid activation is not produced by treatment of acini with active diacylglycerol analogues nor prevented by protein kinase inhibitors, arguing against the involvement of protein kinase C-dependent processes. Stimulation of the exchanger is, however, produced by concentrations of ionomycin which yield intracellular calcium levels in the physiologic (secretagogue-induced) range. In addition, chelation of intracellular calcium with 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid blocks the effect of carbachol, but calmodulin antagonists are without effect. The possibility that the rapid activation of the Na+/H+ exchanger may be associated with cell shrinkage arising from carbachol-induced calcium mobilization is explored. In support of this suggestion we present evidence that: (i) the Na+/H+ exchanger is stimulated by shrinkage of these cells, (ii) the carbachol dose dependence of Na+/H+ exchange activation correlates well with that of shrinkage (but not with that of intracellular calcium levels), and (iii) maneuvers which blunt carbachol- or calcium-induced shrinkage also blunt activation of the exchanger. We suggest that this osmoregulatory response may play an important role in maintaining ionic homeostasis during the acinar fluid secretory process.

Calcium-activated potassium channels from coronary smooth muscle reconstituted in lipid bilayers

This work is the initial characterization of Ca(2+)-activated K+ (KCa) channels from coronary smooth muscle reconstituted into lipid bilayers. The channels were obtained from a surface membrane preparation of porcine coronary smooth muscle. KCa channels were the predominant K+ channels in this preparation. The conductance histogram (n = 137 channels) revealed two main populations of "maxi" KCa channels with conductances of 245 and 295 pS. Each population could be subdivided in two "isoforms" or "isochannels" with different functional properties (voltage and Ca2+ sensitivities and kinetics). The analysis of "burst" probability of opening showed that at pCa 4 the two isochannels of 245 pS (KCa-1 and KCa-1') had half-activation potentials (V1/2) of -80 and 6 mV, respectively. The isochannels of 295 pS (KCa-2 and KCa-2') had V1/2 of -28 and -66 mV, respectively. KCa-1 had the highest Ca2+ sensitivity; at -60 mV, the concentration of half-activation value for Ca2+ was 1.2 +/- 0.3 microM (n = 5). External tetraethylammonium reduced channel amplitude in a voltage-dependent manner; dissociation constant was 180 +/- 6 and 466 +/- 41 microM at -40 and +80 mV, respectively (n = 5). Charybdotoxin (5-50 nM) produced typical long closings. These effects were similar in all the channels. We conclude that coronary smooth muscle possesses isoforms of maxi KCa channels with Ca2+ and voltage sensors with different properties, which may confer to each channel a specific functional role.