Characteristics and modulation by thyrotropin-releasing hormone of an inwardly rectifying K+ current in patch-perforated GH3 anterior pituitary cells

Hyperpolarization of patch-perforated GH₃ rat anterior pituitary cells in high-K⁺ Ca²⁺-free medium reveals an inwardly rectifying K⁺ current. This current showed potential-dependent activation and inactivation kinetics, complete inactivation during strong hyperpolarization and rectification at depolarized potentials. The current was blocked by millimolar concentrations of external Cs⁺, Ba²⁺, Cd²⁺ and Co²⁺, but it was almost insensitive to tetraethylammonium, 4-aminopyridine and two dihydropyridines, nisoldipine and nitrendipine. Verapamil and methoxyverapamil produced a strong and reversible inhibition of the current. In the presence of 100 nM thyrotropin-releasing hormone (TRH), the current was reduced. This reduction was increased by holding the cell at more negative potentials and was accompanied by a shift in steady-state voltage dependence of inactivation towards more positive voltages. Furthermore, the current slowly returned to the initial levels upon washout. Treatment of the cell with the protein phosphatase inhibitor okadaic acid increased the magnitude of the inhibition caused by TRH. Moreover, the current did not return towards the control level during a 30-min washout period. It is concluded that protein phosphatases participate in modulation of the GH₃ cell inwardly rectifying K⁺ channels by TRH. Furthermore, these data indicate that either a protein phosphatase or a factor necessary for its activation is lost under whole-cell mode, which could account for the permanent reduction of the current in response to TRH.     

Caffeine enhancement of electrical activity through direct blockade of inward rectifying K+ currents in GH3 rat anterior pituitary cells

Treatment of rat anterior pituitary GH₃ cells with caffeine causes a reversible enhancement of electrical activity superimposed over a depolarization of the plasma membrane potential. Similar results are obtained with theophylline, but not with isobutylmethylxanthine or forskolin. The effects of caffeine are not related to Ca²⁺ liberation from intracellular stores since they are not affected by incubation of the cells with ryanodine or thapsigargin. Furthermore, caffeine-induced hyperpolarization of the membrane is not detectable even in cells in which Ca²⁺ liberation from inositol 1,4,5-trisphosphate-sensitive compartments produces a prominent transient hyperpolarization in response to thyrotropin-releasing hormone. Reductions of Ca²⁺-dependent K⁺ currents caused by partial block of L-type Ca²⁺ channels by caffeine are not sufficient to explain the effects of the xanthine, since the results obtained with caffeine are not mimicked by direct blockade of Ca²⁺ channels with nisoldipine. GH₃ cell inwardly rectifying K⁺ currents are inhibited by caffeine. Studies on the voltage dependence of the caffeine-induced effects indicate a close correlation between alterations of electrical parameters and reported values of steady-state voltage dependence of inactivation of these currents. We conclude that, as previously shown for thyrotropin-releasing hormone, modulation of inwardly rectifying K⁺ currents plays a major role determining the firing rate of GH₃ cells and its enhancement by caffeine.     

Different G proteins are involved in the biphasic response of clonal rat pituitary cells to thyrotropin-releasing hormone

In rat anterior pituitary tumour cells (GH₃/B₆) thyrotropin-releasing hormone (TRH) elicits a biphasic response. First, a release of intracellularly stored Ca²⁺ induces a hyperpolarization of the cell. Second, a depolarization thought to be induced by a reduction of the inward-rectifying K⁺ current (K_IR) causes an increase in action potential frequency and a plateau-like increase in [Ca²⁺]_i. It has been proposed that the two phases are induced by the actions of inositol 1,4,5-trisphosphate (InsP ₃) and protein kinase C (PKC), respectively, but we demonstrate here that PKC is not responsible for the second phase increase in [Ca²⁺]_i and suggest that the pathways diverge at the level of receptor and G protein coupling. Both phases of the TRH response were insensitive to pertussis toxin, but cholera toxin (CTX) selectively affected the second phase. After CTX pretreatment cells had a high spontaneous spiking frequency and smaller K_IR amplitude. In these cells TRH failed to increase the action potential frequency after the first phase hyperpolarization, elicited only a transient peak increase in [Ca²⁺]_i with no plateau phase and could only slightly reduce K_IR. These effects of CTX are not mediated by its ability to increase cAMP via activation of G_S, as increased cAMP levels neither inhibit K_IR nor prevent its reduction by TRH. In addition, inhibition of protein kinase A activation did not block the second phase increase in [Ca²⁺]_i induced by TRH, suggesting that the CTX-sensitive G protein mediating the second phase of the TRH response is not G_S.On leave from Uchtomsky-Institute, St. Petersburg University, Russia     

Intracellular Ca2+ inactivates an outwardly rectifying K+ current in human adenomatous parathyroid cells

We have used whole-cell patch-clamp techniques to study the conductances in the plasma membranes of human parathyroid cells. With a KCl-rich pipette solution containing Ca²⁺ buffered to a concentration of 0.1 mol/l, the zero current potential was –71.1±0.5 mV (n=19) and the whole-cell current/ voltage (I/V) relation had an inwardly rectifying and an outwardly rectifying component. The inwardly rectifying current activated instantaneously on hyperpolarization of the plasma membrane to potentials more negative than –80 mV, and a semi-logarithmic plot of the reversal potential of the inward current (estimated by extrapolation from the range in which it was linear) as a function of extracellular K⁺ concentration ([K⁺]_o) revealed a linear relation with a slope of 64 mV per decade change in [K⁺]_o, which is not significantly different from the Nernstian slope, demonstrating that the current was carried by K⁺ ions. The conductance exhibited a square root dependence on [K⁺]_o as has been observed for inward rectifiers in other tissues. The current was blocked by the presence of Ba²⁺ (1 mmol/l) or Cs⁺ (1.5 mmol/l) in the bath. The outwardly rectifying current was activated by depolarization of the membrane potential to potentials more positive than –20 mV. It was inhibited by replacement of pipette K⁺ with Cs⁺, indicating that it also was a K⁺ current: it was partially (42%) blocked when tetraethylammonium (TEA⁺, 10 mmol/l) was added to the bath. The outwardly rectifying, but not the inwardly rectifying K⁺ current, was regulated by intracellular free Ca²⁺ concentration ([Ca²⁺]_i) such that increasing [Ca²⁺]_i above 10 nmol/l inhibited the outwardly rectifying current, the half-maximum effect being seen at 1 mol/l. Since it is known that increases in [Ca²⁺]_o produce increases in [Ca²⁺]_i, and that they depolarize parathyroid cells by reducing the membrane K⁺ conductance, we suggest that it is the reduction of the outwardly rectifying K⁺ conductance by increases in [Ca²⁺]_i which is responsible for the reduction in K⁺ conductance seen when [Ca²⁺]_o is increased.     

Demonstration of an inwardly rectifying K+ current component modulated by thyrotropin-releasing hormone and caffeine in GH3 rat anterior pituitary cells

 Reduction of an inwardly rectifying K⁺ current by thyrotropin-releasing hormone (TRH) and caffeine has been considered to be an important determinant of electrical activity increases in GH₃ rat anterior pituitary cells. However, the existence of an inwardly rectifying K⁺ current component was recently regarded as a misidentification of an M-like outward current, proposed to be the TRH target in pituitary cells, including GH₃ cells. In this report, an inwardly rectifying component of K⁺ current is indeed demonstrated in perforated-patch voltage-clamped GH₃ cells. The degree of rectification varied from cell to cell, but both TRH and caffeine specifically blocked a fraction of current with strong rectification in the hyperpolarizing direction. Use of ramp pulses to continuously modify the membrane potential demonstrated a prominent blockade even in cells with no current reduction at voltages at which M-currents are active. Depolarization steps to positive voltages at the maximum of the inward current induced a caffeine-sensitive instantaneous outward current followed by a single exponential decay. The magnitude of this current was modified in a biphasic way according to the duration of the previous hyperpolarization step. The kinetic characteristics of the current are compatible with the possibility that removal from inactivation of a fast-inactivating delayed rectifier causes the hyperpolarization-induced current. Furthermore, the inwardly rectifying current was blocked by astemizole, a potent and selective inhibitor of human ether-á-go-go -related gene (HERG) K⁺ channels. Along with other pharmacological and kinetic evidence, this indicates that the secretagogue-regulated current is probably mediated by a HERG-like K⁺ channel. Addition of astemizole to current-clamped cells induced clear increases in the frequency of action potential production. Thus, an inwardly-rectifying K⁺ current and not an M-like outward current seems to be involved in TRH and caffeine modulation of electrical activity in GH₃ cells. Received: 15 May 1997 / Received after revision and accepted: 24 July 1997     

Capacitative Ca2+ entry contributes to the Ca2+ influx induced by thyrotropin-releasing hormone (TRH) in GH3 pituitary cells

Treatment of GH₃ cells with either hypothalamic peptide thyrotropin-releasing hormone (TRH), the endomembrane Ca²⁺-ATPase inhibitor thapsigargin or the Ca²⁺ ionophore ionomycin mobilized, with different kinetics, essentially all of the Ca²⁺ pool from the intracellular Ca²⁺ stores. Any of the above-described treatments induced a sustained increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i), which was dependent on extracellular Ca²⁺ and was prevented by Ni²⁺ but not by dihydropyridines (DHPs), suggesting that it was due to capacitative Ca²⁺ entry via activation of a plasma membrane pathway which opened upon the emptying of the intracellular Ca²⁺ stores. The increase of the plasma membrane permeability to Ca²⁺ correlated negatively with the filling degree of the intracellular Ca²⁺ stores and was reversed by refilling of the stores. The mechanism of capacitative Ca²⁺ entry into GH₃ cells differed from similar mechanisms described in several types of blood cells in that the pathway was poorly permeable to Mn²⁺ and not sensitive to cytochrome P₄₅₀ inhibitors. In GH₃ cells, TRH induced a transient [Ca²⁺]_i increase due to Ca²⁺ release from the stores (phase 1) followed by a sustained [Ca²⁺]_i increase due to Ca²⁺ entry (phase 2). At the single-cell level, phase 2 was composed of a DHP-insensitive sustained [Ca²⁺]_i increase, due to activation of capacitative Ca²⁺ entry, superimposed upon which DHP-sensitive [Ca²⁺]_i oscillations took place. The two components of the TRH-induced Ca²⁺ entry differed also in that [Ca²⁺]_i oscillations remained for several minutes after TRH removal, whereas the sustained [Ca²⁺]_i increase dropped quickly to prestimulatory levels, following the same time course as the refilling of the stores. The drop was prevented when the refilling was inhibited by thapsigargin. It is concluded that, even though the mechanisms of capacitative Ca²⁺ entry may show differences from cell to cell, it is also present and may contribute to the regulation of physiological functions in excitable cells such as GH₃. There, capacitative Ca²⁺ entry cooperates with voltage-gated Ca²⁺ channels to generate the [Ca²⁺]_i increase seen during phase 2 of TRH action. This contribution of capacitative Ca²⁺ entry may be relevant to the enhancement of prolactin secretion induced by TRH.     

Histamine modulates three types of K+ current in a human intestinal epithelial cell line

K⁺ conductance species in a human intestinal epithelial cell line (Intestine 407) were studied in connection with their sensitivities to an intestinal secretagogue, histamine, using the tight-seal whole-cell patch-clamp technique. Applications of positive command pulses rapidly induced outward K⁺ currents. The conductance became progressively larger with increasing command voltages, exhibiting an outwardly rectifying current voltage relation. Inward K⁺ currents were also rapidly activated upon applications of hyperpolarizing pulses at potentials negative to the equilibrium potential of K⁺ (E _K), and the conductance inwardly rectified. Application of a Ca²⁺ ionophore, ionomycin, brought about activation of additional K⁺ currents. An inhibitor of protein kinase C, polymyxin B, did not affect the ionomycin-induced response. Histamine (10–200 μM) also activated a similar K⁺ current which was abolished by cytosolic Ca²⁺ chelation. Under conditions where Ca²⁺ mobilization was minimized, histamine was found to significantly augment inwardly rectifying K⁺, but suppress outwardly rectifying K⁺, currents. Polymyxin B blocked these effects of histamine. An activator of protein kinase C, 1-oleoyl-2-acetylglycerol, mimicked the histamine effects. It is concluded that the intestinal epithelial cell has three distinct types of K⁺ conductance and that histamine modulates not only Ca²⁺-activated K⁺ conductance via Ca²⁺ mobilization, but also inward- and outward-rectifier K⁺ conductances via activation of protein kinase C.     

Caffeine-induced oscillations of cytosolic Ca2+ in GH3 pituitary cells are not due to Ca2+ release from intracellular stores but to enhanced Ca2+ influx through voltage-gated Ca2+ channels

Caffeine, a well known facilitator of Ca²⁺-induced Ca²⁺ release, induced oscillations of cytosolic free Ca²⁺ ([Ca²⁺]_i) in GH₃ pituitary cells. These oscillations were dependent on the presence of extracellular Ca²⁺ and blocked by dihydropyridines, suggesting that they are due to Ca²⁺ entry through L-type Ca²⁺ channels, rather than to Ca²⁺ release from the intracellular Ca²⁺ stores. Emptying the stores by treatment with ionomycin or thapsigargin did not prevent the caffeine-induced [Ca²⁺]_i oscillations. Treatment with caffeine occluded phase 2 ([Ca²⁺]_i oscillations) of the action of thyrotropin-releasing hormone (TRH) without modifying phase 1 (Ca²⁺ release from the intracellular stores). Caffeine also inhibited the [Ca²⁺]_i increase induced by depolarization with high-K⁺ solutions (56% at 20 mM), suggesting direct inhibition of the Ca²⁺ entry through voltage-gated Ca²⁺ channels. We propose that the [Ca²⁺]_i increase induced by caffeine in GH₃ cells takes place by a mechanism similar to that of TRH, i.e. membrane depolarization that increases the firing frequency of action potentials. The increase of the electrical activity overcomes the direct inhibitory effect on voltage-gated Ca²⁺ channels with the result of increased Ca²⁺ entry and a rise in [Ca²⁺]_i. Consideration of this action cautions interpretation of previous experiments in which caffeine was assumed to increase [Ca²⁺]_i only by facilitating the release of Ca²⁺ from intracellular Ca²⁺ stores.     

Stimulatory actions of di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate (di-8-ANEPPS), voltage-sensitive dye, on the BKCa channel in pituitary tumor (GH3) cells

Di-8-ANEPPS (4-{2-[6-(dibutylamino)-2-naphthalenyl]-ethenyl}-1-(3-sulfopropyl)pyridinium inner salt) has been used as a fast-response voltage-sensitive styrylpyridinium probe. However, little is known regarding the mechanism of di-8-ANEPPS actions on ion currents. In this study, the effects of this dye on ion currents were investigated in pituitary GH₃ cells. In whole-cell configuration, di-8-ANEPPS (10 μM) reversibly increased the amplitude of Ca²⁺-activated K⁺ current. In inside-out configuration, di-8-ANEPPS (10 μM) applied to the intracellular surface of the membrane caused no change in single-channel conductance; however, it did enhance the activity of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels with an EC₅₀ value of 7.5 μM. This compound caused a left shift in the activation curve of BK_Ca channels with no change in the gating charge of these channels. A decrease in mean closed time of the channels was seen in the presence of this dye. In the cell-attached mode, di-8-ANEPPS applied on the extracellular side of the membrane also activated BK_Ca channels. However, neither voltage-gated K⁺ nor ether-à-go-go-related gene (erg)-mediated K⁺ currents in GH₃ cells were affected by di-8-APPNES. Under current-clamp configuration, di-8-ANEPPS (10 μM) decreased the firing of action potentials in GH₃ cells. In pancreatic βTC-6 cells, di-8-APPNES (10 μM) also increased BK_Ca-channel activity. Taken together, this study suggests that during the exposure to di-8-ANEPPS, the stimulatory effects on BK_Ca channels could be one of potential mechanisms through which it may affect cell excitability.     

Effects of caffeine on intracellular calcium,calcium current and calcium-dependent potassium current in anterior pituitary GH3 cells

Caffeine elicits physiological responses in a variety of cell types by triggering the mobilization of Ca²⁺ from intracellular organelles. Here we investigate the effects of caffeine on intracellular Ca²⁺ concentration ([Ca²⁺]_i) and ionic currents in anterior pituitary cells (GH₃) cells. Caffeine has a biphasic effect on Ca²⁺-activated K⁺ current [I _K(Ca)]: it induces a transient increase superimposed upon a sustained inhibition. While the transient increase coincides with a rise in [Ca²⁺]_i, the sustained inhibition of I _K(Ca) is correlated with a sustained inhibition of the L-type Ca²⁺ current. The L-type Ca²⁺ current is also inhibited by other agents that mobilize intracellular Ca²⁺, including thyrotropin releasing hormone (TRH) and ryanodine, but in a matter distinct from caffeine. Unlike the caffeine effect, the TRH-induced inhibition washes-out under whole-cell patch-clamp conditions and is eliminated by intracellular Ca²⁺ chelators. Likewise, the ryanodine-induced inhibition desensitizes while the caffeine-induced inhibition does not. Simultaneous [Ca²⁺]_i and Ca²⁺ current measurements show that caffeine can inhibit Ca²⁺ current without changing [Ca²⁺]_i. Single-channel recordings show that caffeine reduces mean open time without affecting single-channel conductance of L-type channels. Hence the effects of caffeine on ion channels in GH₃ cells are attributable both to mobilization of intracellular Ca²⁺ and to a direct effect on the gating of L-type Ca²⁺ channels.     

Caffeine and histamine-induced oscillations of K(Ca) current in single smooth muscle cells of rabbit cerebral artery

In the present experiment, we characterized the intracellular Ca²⁺ oscillations induced by caffeine (1 mM) or histamine (1–3 M) in voltage-clamped single smooth muscle cells of rabbit cerebral (basilar) artery. Superfusion of caffeine or histamine induced periodic oscillations of large whole-cell K⁺ current with fairly uniform amplitudes and intervals. The oscillatory K⁺ current was abolished by inclusion of ethylenebis(oxonitrilo)tetraacetate (EGTA, 5 mM) in the pipette solution. Caffeine- and histamine-induced periodic activation of the large-conductance Ca²⁺-activated K⁺ [K_(Ca)] channel was recorded in the cell-attached patch mode. These results suggest that the oscillations of K⁺ current are carried by the K_(Ca) channel and reflect the oscillations of intracellular Ca²⁺ concentration ([Ca²⁺]_i). Ryanodine (1–10 M) abolished both caffeine- and histamine-induced oscillations. Caffeine- induced oscillations were abolished by the sarcoplasmic reticulum Ca²⁺-adenosine 5-triphosphatase (Ca²⁺-ATPase) inhibitor, cyclopiazonic acid (10 M), and a high concentration of caffeine (10 mM). Inclusion of heparin (3 mg/ml) in the pipette solution blocked histamine-induced oscillations, but did not block caffeine-induced oscillations. By the removal of extracellular Ca²⁺, but not by the addition of verapamil and Cd²⁺, the caffeine-induced oscillations were abolished. Increasing Ca²⁺ influx rate increased the frequencies of caffeine-induced oscillations. Spontaneous oscillations were also observed in cells that were not superfused with agonists, and had similar characteristics to the caffeine-induced oscillations. From the above results, it is concluded, that in smooth muscle cells of the rabbit cerebral (basilar) artery, ryanodine-sensitive Ca²⁺-induced Ca²⁺ release pools play key roles in the generation of caffeine- and histamine-induced intracellular Ca²⁺ oscillations.     

Characterisation of Ca2+-dependent inwardly rectifying K+ currents in HeLa cells

The whole-cell configuration of the patch-clamp technique was used to examine K⁺ currents in HeLa cells. Under quasi-physiological ionic gradients, using an intracellular solution containing 10^–7 mol/l free Ca²⁺, mainly outward currents were observed. Large inwardly rectifying currents were elicited in symmetrical 145 mmol/l KCl. Replacement of all extracellular K⁺ by isomolar Na⁺, greatly decreased inward currents and shifted the reversal potential as expected for K⁺ selectivity. The inwardly rectifying K⁺ currents exhibited little or no apparent voltage dependence within the range of from –120 mV to 120 mV. A square-root relationship between chord conductance and [K⁺]₀ at negative potentials could be established. The inwardly rectifying nature of the currents was unaltered after removal of intracellular Mg²⁺ and chelation with ATP and ethylenediaminetetraacetic acid (EDTA). Permeability ratios for other monovalent cations relative to K⁺ were: K⁺ (1.0)>Rb⁺ (0.86)>Cs⁺ (0.12)>Li⁺ (0.08)>Na⁺ (0.03). Slope conductance ratios measured at –100 mV were: Rb⁺ (1.66)>K⁺ (1.0)>Na⁺ (0.09)>Li⁺ (0.08)>Cs⁺ (0.06). K⁺ conductance was highly sensitive to intracellular free Ca²⁺ concentration. The relationship between conductance at 0 mV and Ca²⁺ concentration was well described by a Hill expression with a dissociation constant, K _D, of 70 nmol/l and a Hill coefficient, n, of 1.81. Extracellular Ba²⁺ blocked the currents in a concentration- and voltage-dependent manner. The dependence of the K _D for the blockade was analysed using a Woodhull-type treatment, locating the ion interaction site at 19 % of the distance across the electrical field of the membrane and a K _D (0 mV) of 7 mmol/l. Tetraethylammonium and 4-aminopyridine were without effect whilst quinine and quinidine blocked the currents with concentrations for half-maximum effects equal to 7 mol/l and 3.5 mol/l, respectively. The unfractionated venom of the scorpion Leiurus quinquestriatus (LQV) blocked the K⁺ currents of HeLa cells. The toxins apamin and scyllatoxin had no detectable effect whilst charybdotoxin, a component of LQV, blocked in a voltage-dependent manner with half-maximal concentrations of 40 nmol/l at –120 mV and 189 nmol/l at 60 mV; blockade by charybdotoxin accounts for the effect of LQV. Application of ionomycin (5–10 mol/l), histamine (1 mmol/l) or bradykinin (1–10 mol/l) to cells dialysed with low-buffered intracellular solutions induced K⁺ currents showing inward rectification and a lack of voltage dependence.     

Contribution of different Ca2+‐activated K+ channels to the first phase of the response to TRH in clonal rat anterior pituitary cells

Aims: Thyrotropin‐releasing hormone (TRH) induces biphasic changes in electrical activity, cytosolic free Ca²⁺ level ([Ca²⁺]_i), and prolactin secretion from both clonal GH cells and native lactotrophs. The first phase of the TRH response is characterized by hyperpolarization because of activation of Ca²⁺‐activated K⁺ channels (K_Ca). In the present study, the relative contribution of BK, SK, and IK channels to the first phase of the TRH response in GH₄ cells was assessed. Methods: The expression of IK channels was confirmed by PCR with specific primers for SK4 (IK). The response to TRH was studied using the perforated patch technique and Ca²⁺ microfluoromety (fura‐2). The involvement of different K_Ca channels was estimated by employing the specific channel blockers iberiotoxin (BK), apamin (SK) and clotrimazole (IK). Results: Application of 100 nm iberiotoxin, 1 μm apamin, and 10 μm clotrimazole reduced the peak value of the outward K⁺ current during the first phase of the TRH response by 33, 26, and 33%, respectively. Clotrimazole also shortened the duration of the outward current response by 60%, causing a reduction of total charge movement by 73%. All these toxin‐induced reductions were significant (P < 0.05). A combination of all three toxins abolished the current response almost completely. Conclusion: All the three main types of K_Ca channels are involved in the first phase of the TRH response, with IK as the major contributor. This is the first demonstration of a dominant role of IK compared with BK and SK channels in excitable cells.     

K+ and Cl− currents in freshly isolated rat osteoclasts

Membrane electrical properties of freshly isolated rat osteoclasts were studied using patch-clamp recording methods. Characterization of the passive membrane properties indicated that the osteoclast cell membrane behaved as an isopotential surface. The specific membrane capacitance was 1.2±0.3 F/cm² (mean ±SD), with no difference between cells plated on glass and those adhering to a permeable collagen substrate. The current/voltage (I/V) relationship of all cells showed inward rectification and I/V curves shifted 51 mV positive per tenfold increase of [K⁺]_out, indicating an inwardly rectifying K⁺ conductance. The voltage dependence of the K⁺ chord conductance (g _K) also shifted positive along the voltage axis, and the maximum conductance increased, with elevation of [K⁺]_out. g _K for cells bathed in 4.7 mM [K⁺]_out increased e-fold per 12mV hyperpolarization, and half-maximal activation was at –89 mV. Approximately 18% (50 pS/pF) of the maximum g _K was active at –70 mV. Inward single-channel currents were recorded in cell-attached patches at hyperpolarizing potentials. With symmetrical K⁺, channel conductance was 25±3 pS and reversal was close to the K⁺ equilibrium potential, consistent with this K⁺ channel underlying the whole-cell K⁺ currents. With both conventional whole-cell and perforated-patch recording, no voltage-activated Ca²⁺ current was detected. In approximately 30% of osteoclasts studied, an outwardly rectifying current was observed, which was reversibly blocked by 4,4-diisothiocyanostilbene-2,2-disulphonic acid (DIDS) and 4-acetamido-4-isothiocyanostilbene2,2-disulphonic acid (SITS). This DIDS- and SITS-sensitive current reversed direction at the chloride equilibrium potential. We conclude that an inwardly rectifying K⁺ current is present in all rat osteoclasts and that some osteoclasts also exhibit an outwardly rectifying Cl^– current. Both these membrane conductances may play an important physiological role by dissipating the potential that arises from the electrogenic transport of H⁺ across the ruffled membrane of the osteoclast.     

Calcitonin-gene-related peptide activates the muscarinic-gated K+ current in atrial cells

A high density of nerve fibers containing calcitonin-gene-related peptide (CGRP) is present in the atria. Recently CGRP was reported to open ATP-sensitive K channels in arterial smooth muscle cells. This study examines whether CGRP activates a similar K⁺ channel in cardiac cells. In voltage-clamped whole cells loaded with GTP and ATP, CGRP reversibly evoked an inwardly rectifying K⁺ current. To identify the K⁺ channel that gives rise to this current, three types of K⁺ channel (resting, ATP-sensitive and acetylcholine-activated) were examined. CGRP failed to activate or inhibit the ATP-sensitive or the resting K⁺ channel. However, CGRP (0.1–1 M) caused activation of single channels with kinetics similar to that of the muscarinic K⁺ channel (35–40 pS conductance and approx. 1 ms mean open time in symmetrical 140 mM K⁺). In excised, inside-out (CGRP in pipette) or in outside-out (GTP in pipette) patches, the K⁺ current was activated by perfusion with GTP or CGRP, respectively, suggesting that CGRP activated the muscarinic K⁺ channel via GTP-binding protein. Treatment with pertussis toxin inhibited the activation of the K⁺ channel, suggesting that CGRP receptor may be coupled to a G_i or a G_o type of GTP-binding protein. Together with previous findings, these results suggest that CGRP modulates several types of ion channels to produce its cellular effects.This work was supported in part by HL40586 and American Heart Association grant-in-aid.     

Cell type influences the molecular mechanisms involved in hormonal regulation of ERG K<Superscript>+</Superscript> channels

While the thyrotropin-releasing hormone (TRH) effect of raising intracellular Ca²⁺ levels has been shown to rely on G_q/11 and PLC activation, the molecular mechanisms involved in the regulation of ERG K⁺ channels by TRH are still partially unknown. We have analysed the effects of βγ scavengers, Akt/PKB inactivation, and TRH receptor (TRH-R) overexpression on such regulation in native and heterologous expression cell systems. In native rat pituitary GH₃ cells β-ARK/CT, Gα_t, and phosducin significantly reduced TRH inhibition of rERG currents, whereas in HEK-H36/T1 cells permanently expressing TRH-R and hERG, neither of the βγ scavengers affected the TRH-induced shift in V _1/2. Use of specific siRNAs to knock Akt/PKB expression down abolished the TRH effect on HEK-H36/T1 cell hERG, but not on rERG from GH₃ cells. Indeed, wortmannin or long insulin pretreatment also blocked TRH regulation of ERG currents in HEK-H36/T1 but not in GH₃ cells. To determine whether these differences could be related to the amount of TRH-Rs in the cell, we studied the TRH concentration dependence of the Ca²⁺ and ERG responses in GH₃ cells overexpressing the receptors. The data indicated that independent of the receptor number additional cellular factor(s) contribute differently to couple the TRH-R to hERG channel modulation in HEK-H36/T1 cells. We conclude that regulation of ERG currents by TRH and its receptor is transduced in GH₃ and HEK-H36/T1 cell systems through common and different elements, and hence that the cell type influences the signalling pathways involved in the TRH-evoked responses.     

Intracellular Na+ modulates large conductance Ca2+-activated K+ currents in human umbilical vein endothelial cells

We studied the effects of Na⁺ influx on large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in cultured human umbilical vein endothelial cells (HUVECs) by means of patch clamp and SBFI microfluorescence measurements. In current-clamped HUVECs, extracellular Na⁺ replacement by NMDG⁺ or mannitol hyperpolarized cells. In voltage-clamped HUVECs, changing membrane potential from 0 mV to negative potentials increased intracellular Na⁺ concentration ([Na⁺]_i) and vice versa. In addition, extracellular Na⁺ depletion decreased [Na⁺]_i. In voltage-clamped cells, BK_Ca currents were markedly increased by extracellular Na⁺ depletion. In inside-out patches, increasing [Na⁺]_i from 0 to 20 or 40 mM reduced single channel conductance but not open probability (NPo) of BK_Ca channels and decreasing intracellular K⁺ concentration ([K⁺]_i) gradually from 140 to 70 mM reduced both single channel conductance and NPo. Furthermore, increasing [Na⁺]_i gradually from 0 to 70 mM, by replacing K⁺, markedly reduced single channel conductance and NPo. The Na⁺–Ca²⁺ exchange blocker Ni²⁺ or KB-R7943 decreased [Na⁺]_i and increased BK_Ca currents simultaneously, and the Na⁺ ionophore monensin completely inhibited BK_Ca currents. BK_Ca currents were significantly augmented by increasing extracellular K⁺ concentration ([K⁺]_o) from 6 to 12 mM and significantly reduced by decreasing [K⁺]_o from 12 or 6 to 0 mM or applying the Na⁺–K⁺ pump inhibitor ouabain. These results suggest that intracellular Na⁺ inhibit single channel conductance of BK_Ca channels and that intracellular K⁺ increases single channel conductance and NPo. GH Liang and MY Kim contributed equally to this publication and therefore share the first authorship.     

Small-conductance Ca2+-activated K+ channels in bovine chromaffin cells

Simultaneous whole-cell patch-clamp and fura-2 fluorescence [Ca²⁺]_i measurements were used to characterize Ca²⁺-activated K⁺ currents in cultured bovine chromaffin cells. Extracellular application of histamine (10 M) induced a rise of [Ca²⁺]_i concomitantly with an outward current at holding potentials positive to –80 mV. The activation of the current reflected an increase in conductance, which did not depend on membrane potential in the range –80 mV to –40 mV. Increasing the extracellular K⁺ concentration to 20 mM at the holding potential of –78 mV was associated with inwardly directed currents during the [Ca²⁺]_i elevations induced either by histamine (10 M) or short voltage-clamp depolarizations. The current reversal potential was close to the K⁺ equilibrium potential, being a function of external K⁺ concentration. Current fluctuation analysis suggested a unit conductance of 3–5 pS for the channel that underlies this K⁺ current. The current could be blocked by apamin (1 M). Whole-cell current-clamp recordings snowed that histamine (10 M) application caused a transient hyperpolarization, which evolved in parallel with the [Ca²⁺]_i changes. It is proposed that a small-conductance Ca²⁺-activated K⁺ channel is present in the membrane of bovine chromaffin cells and may be involved in regulating catecholamine secretion by the adrenal glands of various species.     

ATP-sensitive K+ channels regulated by intracellular Ca2+ and phosphorylation in normal (T84) and cystic fibrosis (CFPAC-1) epithelial cells

The elementary K⁺ conductance activated by the cAMP or the Ca²⁺ second messenger pathways was investigated in the model salt-secreting epithelium, the human T84 cell line. Under Cl^–-free conditions, an inwardly rectifying whole-cell K⁺ current was evoked by either forskolin 10 (mol/l) or acetylcholine 1 (mol/l) and blocked by extracellular charybdotoxin 10 (nmol/l). In the cell-attached mode, both secretory agonists induced the opening of a channel showing inward rectification with a unitary chord conductance of 36.8±2.5 pS (n=26) for inward currents. In inside-out patches, a 35-pS inwardly rectifying K⁺ channel that corresponded to the channel recorded in the cell-attached configuration was recorded in the presence of 0.3 mol/l free Ca²⁺ at the inner side of the membrane. This channel was blocked by Ba²⁺ (5 mol/l) and by charybdotoxin (50 nmol/l). Its open probability was enhanced by intracellular Ca²⁺ with and EC₅₀ of 0.25 mol/l and strongly reduced by intracellular MgATP with an IC₅₀ of 600 mol/l. In the continuous presence of ATP, the channel activity was consistently increased by 125 kU/l catalytic subunit of cAMP-dependent protein kinase. In the cystic fibrosis pancreatic duct cell line CFPAC-1, a K⁺ channel was also recorded, with similar characterictics and regulation as the 35-pS channel in T84 cells. We conclude that an ATP-sensitive K⁺ channel regulated by intracellular Ca²⁺ and phosphorylation supports the main K⁺ current activated by secretory agonists in normal cystic fibrosis cell lines. This conductance possibly represents the major pathway for K⁺ recycling at the basolateral membrane during transepithelial fluid secretion.     

[Ca2+]i-dependent membrane currents in guinea-pig ventricular cells in the absence of Na/Ca exchange

Transient inward currents (I _ti) during oscillations of intracellular [Ca²⁺] ([Ca²⁺]_i) in ventricular myocytes have been ascribed to Na/Ca exchange. We have investigated whether other Ca²⁺-dependent membrane currents contribute to I _ti in single guinea-pig ventricular myocytes, by examining membrane currents during [Ca²⁺]_i oscillations and during caffeine-induced Ca²⁺ release from the sarcoplasmic reticulum in the absence of Na⁺. Membrane currents were recorded during whole-cell voltage clamp and [Ca²⁺]_i measured simultaneously with fura-2. In the absence of Na/Ca exchange, i.e., with Li⁺, Cs⁺ or N-methyl-D-glucamine (NMDG⁺) substituted for Na⁺, the cell could be loaded with Ca²⁺ by repetitive depolarizations to +10 mV, resulting in spontaneous [Ca²⁺]_i oscillations. During these oscillations, no inward currents were seen, but instead spontaneous Ca²⁺ release was accompanied by a shift of the membrane current in the outward direction at potentials between –40 mV and +60 mV. This [Ca²⁺]_i-dependent outward current shift was not abolished when NMDG⁺ was substituted for internal monovalent cations, nor was it sensitive to substitution of external Cl^–. It was however, sensitive to the blockade of I_Ca by verapamil. These results suggest that the transient outward current shift observed during spontaneous Ca²⁺ release represents [Ca²⁺]_idependent transient inhibition of I _Ca. Similarly, during the [Ca²⁺]_i transients induced by brief caffeine (10 mM) applications, we could not detect membrane currents attributable to a Ca²⁺-activated nonselective cation channel, or to a Ca²⁺-activated Cl^– channel; however, transient Ca²⁺-dependent inhibition of I _Ca was again observed. We conclude that neither the Ca²⁺-activated nonselective cation channel nor the Ca²⁺-activated Cl^– channel contribute significantly to the membrane currents during spontaneous [Ca²⁺]_i oscillations in guineapig ventricular myocytes. However, in the voltage range between –40 mV and +60 mV Ca²⁺-dependent transient inhibition of I _Ca will contribute to the oscillations of the membrane current.