Delayed rectifying and calcium-activated K+ channels and their significance for action potential repolarization in mouse pancreatic beta-cells

The contribution of Ca2(+)-activated and delayed rectifying K+ channels to the voltage-dependent outward current involved in spike repolarization in mouse pancreatic beta-cells (Rorsman, P., and G. Trube. 1986. J. Physiol. 374:531-550) was assessed using patch-clamp techniques. A Ca2(+)-dependent component could be identified by its rapid inactivation and sensitivity to the Ca2+ channel blocker Cd2+. This current showed the same voltage dependence as the voltage-activated (Cd2(+)-sensitive) Ca2+ current and contributed 10-20% to the total beta-cell delayed outward current. The single-channel events underlying the Ca2(+)-activated component were investigated in cell-attached patches. Increase of [Ca2+]i invariably induced a dramatic increase in the open state probability of a Ca2(+)-activated K+ channel. This channel had a single-channel conductance of 70 pS [( K+]o = 5.6 mM). The Ca2(+)-independent outward current (constituting greater than 80% of the total) reflected the activation of an 8 pS [( K+]o = 5.6 mM; [K+]i = 155 mM) K+ channel. This channel was the only type observed to be associated with action potentials in cell-attached patches. It is suggested that in mouse beta-cells spike repolarization results mainly from the opening of the 8-pS delayed rectifying K+ channel.     

Voltage-gated and Ca(2+)-activated K+ channels in intact human T lymphocytes. Noninvasive measurements of membrane currents, membrane potential, and intracellular calcium

Voltage-gated n-type K(V) and Ca(2+)-activated K+ [K(Ca)] channels were studied in cell-attached patches of activated human T lymphocytes. The single-channel conductance of the K(V) channel near the resting membrane potential (Vm) was 10 pS with low K+ solution in the pipette, and 33 pS with high K+ solution in the pipette. With high K+ pipette solution, the channel showed inward rectification at positive potentials. K(V) channels in cell-attached patches of T lymphocytes inactivated more slowly than K(V) channels in the whole-cell configuration. In intact cells, steady state inactivation at the resting membrane potential was incomplete, and the threshold for activation was close to Vm. This indicates that the K(V) channel is active in the physiological Vm range. An accurate, quantitative measure for Vm was obtained from the reversal potential of the K(V) current evoked by ramp stimulation in cell-attached patches, with high K+ solution in the pipette. This method yielded an average resting Vm for activated human T lymphocytes of -59 mV. Fluctuations in Vm were detected from changes in the reversal potential. Ionomycin activates K(Ca) channels and hyperpolarizes Vm to the Nernst potential for K+. Elevating intracellular Ca2+ concentration ([Ca2+]i) by ionomycin opened a 33-50-pS channel, identified kinetically as the CTX-sensitive IK-type K(Ca) channel. The Ca2+ sensitivity of the K(Ca) channel in intact cells was determined by measuring [Ca2+]i and the activity of single K(Ca) channels simultaneously. The threshold for activation was between 100 and 200 nM; half-maximal activation occurred at 450 nM. At concentrations > 1 microM, channel activity decreased. Stimulation of the T-cell receptor/CD3 complex using the mitogenic lectin, PHA, increased [Ca2+]i, and increased channel activity and current amplitude resulting from membrane hyperpolarization.     

Selectivity and gating of the type L potassium channel in mouse lymphocytes

Type l voltage-gated K+ channels in murine lymphocytes were studied under voltage clamp in cell-attached patches and in the whole-cell configuration. The kinetics of activation of whole-cell currents during depolarizing pulses could be fit by a single exponential after an initial delay. Deactivation upon repolarization of both macroscopic and microscopic currents was mono-exponential, except in Rb-Ringer or Cs-Ringer solution in which tail currents often displayed "hooks," wherein the current first increased or remained constant before decaying. In some cells type l currents were contaminated by a small component due to type n K+ channels, which deactivate approximately 10 times slower than type l channels. Both macroscopic and single channel currents could be dissected either kinetically or pharmacologically into these two K+ channel types. The ionic selectivity and conductance of type l channels were studied by varying the internal and external permeant ion. With 160 mM K+ in the cell, the relative permeability calculated from the reversal potential with the Goldman-Hodgkin-Katz equation was K+ (identical to 1.0) greater than Rb+ (0.76) greater than NH4+ = Cs+ (0.12) much greater than Na+ (less than 0.004). Measured 30 mV negative to the reversal potential, the relative conductance sequence was quite different: NH4+ (1.5) greater than K+ (identical to 1.0) greater than Rb+ (0.5) greater than Cs+ (0.06) much greater than Na+, Li+, TMA+ (unmeasurable). Single channel current rectification resembled that of the whole-cell instantaneous I-V relation. Anomalous mole-fraction dependence of the relative permeability PNH4/PK was observed in NH4(+)-K+ mixtures, indicating that the type l K+ channel is a multi-ion pore. Compared with other K+ channels, lymphocyte type l K+ channels are most similar to "g12" channels in myelinated nerve.     

Single sodium channels from the squid giant axon

Since the work of A. L. Hodgkin and A. F. Huxley (1952. J. Physiol. [Lond.].117:500-544) the squid giant axon has been considered the classical preparation for the study of voltage-dependent sodium and potassium channels. In this preparation much data have been gathered on macroscopic and gating currents but no single sodium channel data have been available. This paper reports patch clamp recording of single sodium channel events from the cut-open squid axon. It is shown that the single channel conductance in the absence of external divalent ions is approximately 14 pS, similar to sodium channels recorded from other preparations, and that their kinetic properties are consistent with previous results on gating and macroscopic currents obtained from the perfused squid axon preparation.     

The feasibility and limitation of patch-clamp recordings from neonatal rat cardiac ventricular slices

Examine the feasibility of whole-cell patch-clamp recordings from the cardiac ventricular slices of newborn (P(3)-P(7)) Sprague-Dawley rats to identify a better substitute for single cardiac myocytes prepared using enzymatic treatment. High resistance seals (>1 G?) were obtained from cardiac ventricle tissues prepared without using enzymatic treatment. Thereafter, cell-attached and whole-cell patch-clamp techniques were used on thin cardiac slices (200 μm thick) in 2009 in the Institute of Molecular Medicine of Peking University. An averaged sodium current (n=11 cells) was recorded in the cell-attached mode, and this displayed features similar to those previously reported for isolated rat ventricular myocytes. The outward potassium current, hyperpolarization-activated cation channel or I (f) channel (HCN channel), and action potential were recorded in the whole-cell mode (n=2 cells), and the identical properties were observed from the cardiac slices. The cell-attached mode is stable and reliable for recording the ion current. The resting potential for cardiac slices measured using current-clamp recording in the whole-cell mode was -50 to -70 mV. The resting potential of cardiac slices has properties similar to those of enzyme-prepared cardiomyocytes, with the exception that it is positive. We achieved whole-cell patch-clamp recordings from cardiac slices and affirmed the feasibility and values of both cell-attached and whole-cell recording modes using this technique. Nevertheless, there remain difficulties and limitations associated with the application of whole-cell patch-clamping to cardiac slices, due primarily to the existence of large amounts of connective tissue even in newborn rats.     

Temperature dependence of K(+)-channel properties in human T lymphocytes.

We have used whole-cell patch clamp to determine the temperature dependence of the conductance and gating kinetics of the voltage-gated potassium channel in quiescent, human peripheral blood T lymphocytes. Threshold for activation, steady-state inactivation, and the reversal potential are the same at 22 degrees and 37 degrees C. However, the time-constants for activation, inactivation, deactivation, and release from inactivation are quite sensitive to temperature, changing by at least a factor of five in each case over this range of temperatures. The onset of cumulative inactivation at 22 degrees and 37 degrees C reflects the time-course of deactivation. Peak outward current is approximately twofold greater at 37 degrees C than at 22 degrees C; this increase is also manifest at the single channel level. Energies of activation for conductance, activation, inactivation, deactivation, and release from inactivation are 8.2, 22.1, 25.0, 36.2, and 42.2 kcal/mol, respectively. No new channels were observed at 37 degrees C, and there was no evidence for alteration of the K+ conductance by putative modulators at 22 or 37 degrees C.     

Single calcium channel behavior in native skeletal muscle

The purpose of this study was to use whole-cell and cell-attached patches of cultured skeletal muscle myotubes to study the macroscopic and unitary behavior of voltage-dependent calcium channels under similar conditions. With 110 mM BaCl2 as the charge carrier, two types of calcium channels with markedly different single-channel and macroscopic properties were found. One class was DHP-insensitive, had a single-channel conductance of approximately 9 pS, yielded ensembles that displayed an activation threshold near -40 mV, and activated and inactivated rapidly in a voltage-dependent manner (T current). The second class could only be well resolved in the presence of the DHP agonist Bay K 8644 (5 microM) and had a single-channel conductance of approximately 14 pS (L current). The 14-pS channel produced ensembles exhibiting a threshold of approximately -10 mV that activated slowly (tau act approximately 20 ms) and displayed little inactivation. Moreover, the DHP antagonist, (+)-PN 200-110 (10 microM), greatly increased the percentage of null sweeps seen with the 14-pS channel. The open probability versus voltage relationship of the 14-pS channel was fitted by a Boltzmann distribution with a VP0.5 = 6.2 mV and kp = 5.3 mV. L current recorded from whole-cell experiments in the presence of 110 mM BaCl2 + 5 microM Bay K 8644 displayed similar time- and voltage-dependent properties as ensembles of the 14-pS channel. Thus, these data are the first comparison under similar conditions of the single-channel and macroscopic properties of T current and L current in native skeletal muscle, and identify the 9- and 14-pS channels as the single-channel correlates of T current and L current, respectively.     

Voltage- and time-dependent K+ channel currents in the basolateral membrane of villus enterocytes isolated from guinea pig small intestine

Patch-clamp studies were carried out in villus enterocytes isolated from the guinea pig proximal small intestine. In the whole-cell mode, outward K+ currents were found to be activated by depolarizing command pulses to -45 mV. The activation followed fourth order kinetics. The time constant of K+ current activation was voltage-dependent, decreasing from approximately 3 ms at -10 mV to 1 ms at +50 mV. The K+ current inactivated during maintained depolarizations by a voltage- independent, monoexponential process with a time constant of approximately 470 ms. If the interpulse interval was shorter than 30 s, cumulative inactivation was observed upon repeated stimulations. The steady state inactivation was voltage-dependent over the voltage range from -70 to -30 mV with a half inactivation voltage of -46 mV. The steady state activation was also voltage-dependent with a half- activation voltage of -22 mV. The K+ current profiles were not affected by chelation of cytosolic Ca2+. The K+ current induced by a depolarizing pulse was suppressed by extracellular application of TEA+, Ba2+, 4-aminopyridine or quinine with half-maximal inhibitory concentrations of 8.9 mM, 4.6 mM, 86 microM and 26 microM, respectively. The inactivation time course was accelerated by quinine but decelerated by TEA+, when applied to the extracellular (but not the intracellular) solution. Extracellular (but not intracellular) applications of verapamil and nifedipine also quickened the inactivation time course with 50% effective concentrations of 3 and 17 microM, respectively. Quinine, verapamil and nifedipine shifted the steady state inactivation curve towards more negative potentials. Outward single K+ channel events with a unitary conductance of approximately 8.4 pS were observed in excised inside-out patches of the basolateral membrane, when the patch was depolarized to -40 mV. The ensemble current rapidly activated and thereafter slowly inactivated with similar time constants to those of whole-cell K+ currents. It is concluded that the basolateral membrane of guinea pig villus enterocytes has a voltage-gated, time-dependent, Ca(2+)-insensitive, small-conductance K+ channel. Quinine, verapamil, and nifedipine accelerate the inactivation time course by affecting the inactivation gate from the external side of the cell membrane.     

Single inward rectifier potassium channels in guinea pig ventricular myocytes. Effects of quinidine.

The effects of quinidine on single inward rectifier K channels were investigated in cell-attached patches with 4.5 mM pipette potassium concentrations. Under these conditions, the single-channel slope conductance of the predominant conductance level of the inward rectifier channels was 3.9 +/- 0.3 pS at membrane potentials between -75 and -150 mV. Quinidine reversibly decreased the likelihood of channel opening to the main conductance level without reducing the single-channel conductance, and also reduced the probability of channel opening to subconducting levels. Quinidine had no significant effects on the channel open times, and the inhibition of channel opening was only slightly voltage dependent over the range of membrane potentials investigated. Quinidine induced a complete cessation of channel openings for brief periods (up to 2 min), suggesting that quinidine promoted occupancy of a state from which opening was less likely. Occasional long periods (up to an hour) with an absence of channel activity were also observed but quinidine did not appear to promote this behavior. The data suggest that quinidine decreases the ability of the channel to enter both main and subconducting states. By binding to a particular closed conformation of the channel, quinidine could reduce the likelihood of channel opening. The main features of these observations could be accounted for using the three-state kinetic model proposed by Sakmann, B. and G. Trube (1984b. J. Physiol. [Lond.]. 347:659-683.) with quinidine binding to the middle closed state.     

Inwardly rectifying whole-cell and single-channel K currents in the murine macrophage cell line J774.1

Summary Inward currents in the murine macrophage-like cell line J774.1 were studied using the whole-cell and cell-attached variations of the patch-clamp technique. When cells were bathed in Na Hanks' (KCl=4.5mm, NaCl=145mm), and the electrode contained Na-free K Hanks' (KCl=145mm) single-channel currents were observed at potentials below –40 mV which showed inward rectification, were K-selective, and were blocked by 2.5mm Ba in the pipette. Single-channel conductance was 29 pS, and was proportional to the square root of [K] _o . Channels manifested complex kinetics, with multiple open and closed states. The steady-state open probability of the channel was voltage dependent, and declined from 0.9 to 0.45 between –40 and –140 mV. When hyperpolarizing voltage pulses were repetitively applied in the cell-attached patch mode, averaged single-channel currents showed inactivation. Inactivation of inwardly rectifying whole-cell current was measured in Na Hanks' and in two types of Na-free Hanks': one with a normal K concentration (4.5mm) and the other containing 145mm K. Inactivation was shown to have Na-dependent and Na-independent components. Properties of single-channel current were found to be sufficient to account for the behavior of the macroscopic current, except that single-channel current showed a greater degree of Na-independent inactivation than whole-cell current.     

Two types of potassium channels in murine T lymphocytes

The properties of two types of K+ channels in murine T lymphocytes are described on the basis of whole-cell and isolated-patch recordings using the gigohm-seal technique. Type l (standing for "lpr gene locus" or "large") channels were characterized mainly in T cells from mutant MRL/MpJ-lpr/lpr mice, in which they are present in large numbers. Type n ("normal") K+ channels are abundant and therefore most readily studied in concanavalin A-activated T cells from four strains of mice, MRL-+/+, CBA/J, C57BL/6J, and BALB/c. Type l channels, compared with type n, are activated at potentials approximately 30 mV more positive, and close much more rapidly upon repolarization. Type l channels inactivate more slowly and less completely than type n during maintained depolarization, but recover from inactivation more rapidly, so that little inactivation accumulates during repetitive pulses. Type l channels have a higher unitary conductance (21 pS) than type n (12 pS) and are less sensitive to block by external Co++, but are 100-fold more sensitive to block by external tetraethylammonium (TEA), with half-block of type l channels at 50-100 microM TEA compared with 8-16 mM for type n. TEA blocks both types of channels by reducing the apparent single channel current amplitude, with a dose-response relation similar to that for blocking macroscopic currents. Murine type n K+ channels resemble K+ channels in human T cells.     

Curcumin potently blocks Kv1.4 potassium channels

Curcumin, a major constituent of the spice turmeric, is a nutriceutical compound reported to possess therapeutic properties against a variety of diseases ranging from cancer to cystic fibrosis. In whole-cell patch-clamp experiments on bovine adrenal zona fasciculata (AZF) cells, curcumin reversibly inhibited the Kv1.4K+ current with an IC50 of 4.4 microM and a Hill coefficient of 2.32. Inhibition by curcumin was significantly enhanced by repeated depolarization; however, this agent did not alter the voltage-dependence of steady-state inactivation. Kv1.4 is the first voltage-gated ion channel demonstrated to be inhibited by curcumin. Furthermore, these results identify curcumin as one of the most potent antagonists of these K+ channels identified thus far. It remains to be seen whether any of the therapeutic actions of curcumin might originate with its ability to inhibit Kv1.4 or other voltage-gated K+ channel.     

Increased voltage-gated potassium conductance during interleukin 2- stimulated proliferation of a mouse helper T lymphocyte clone

Recent work has demonstrated the presence of voltage-gated potassium channels in human peripheral blood T lymphocytes (Matteson, R., and C. Deutsch, 1984, Nature (Lond.), 307:468-471; DeCoursey T. E., T. G. Chandy, S. Gupta, and M. D. Cahalan, 1984, Nature (Lond.), 307:465-468) and a murine cytolytic T-cell clone (Fukushima, Y., S. Hagiwara, and M. Henkart, 1984, J. Physiol., 351:645-656). Using the whole cell patch clamp, we have found a potassium conductance with similar properties in a murine noncytolytic T lymphocyte clone, L2. Under voltage clamp, a step from a holding potential of -70 mV to +50 mV produces an average outward current of 100-150 pA in "quiescent" L2 cells at the end of their weekly maintenance cycle. When these cells are stimulated with human recombinant interleukin 2 (rIL2, 100 U/ml), they grow in size and initiate DNA synthesis at approximately 24 h. Potassium conductance is increased as early as 8 h after stimulation with rIL2 and rises to a level 3-4 times that of excipient controls by 24 h. The level remains elevated through 72 h, but as the cells begin to leave the cell cycle at 72-96 h, the conductance decreases quickly to a value only slightly higher than the initial one. Quinine, a blocker of this conductance, markedly reduces the rate at which L2 cells traverse the cell cycle, while also reducing the rate of stimulated protein synthesis. The regulation of potassium conductance in L2 cells during rIL2-stimulated proliferation suggests that potassium channel function may play a role in support of the proliferative response.     

Patch-clamp studies in human macrophages: Single-channel and whole-cell characterization of two K+ conductances

Summary Human peripheral blood monocytes cultured for varying periods of time were studied using whole-cell and single-channel patch-clamp recording techniques. Whole-cell recordings revealed both an outward K current activating at potentials >20 mV and an inwardly rectifying K current present at potentials negative to –60 mV. Tail currents elicited by voltage steps that activated outward current reversed nearE _K, indicating that the outward current was due to a K conductance. TheI–V curve for the macroscopic outward current was similar to the mean single-channelI–V curve for the large conductance (240 pS in symmetrical K) calcium-activated K channel present in these cells. TEA and charybdotoxin blocked the whole-cell outward current and the single-channel current. Excised and cell-attached single-channel data showed that calcium-activated K channels were absent in freshly isolated monocytes but were present in >85% of patches from macrophages cultured for >7 days. Only 35% of the human macrophages cultured for >7 days exhibited whole-cell inward currents. The inward current was blocked by external barium and increased when [K] _o increased. Inward-rectifying single-channel currents with a conductance of 28 pS were present in cells exhibiting inward whole-cell currents. These single-channel currents are similar to those described in detail in J774.1 cells (L.C. McKinney & E.K. Gallin,J. Membrane Biol. 103:41–53, 1988).     

Regulation of Cl- channels by multifunctional CaM kinase.

cAMP kinase has been shown to mediate the cAMP pathway for regulation of Cl- channels in lymphocytes, but the mediator of an alternative, Ca2+ pathway has not been identified. We show here that Ca2+ ionophore activates Cl- currents in cell-attached and whole-cell patch-clamp recordings of Jurkat T lymphocytes, but this activation is not direct. The effect of Ca2+ ionophore on whole-cell Cl- currents is inhibited by a specific peptide inhibitor of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase). Furthermore, Cl- channels are activated in excised patches by purified CaM kinase in a fashion that mimics the effect of Ca2+ ionophore in cell-attached recordings. These results suggest that CaM kinase mediates the Ca2+ pathway of Cl- channel activation.     

Recovery from C-type inactivation is modulated by extracellular potassium.

Extracellular potassium modulates recovery from C-type inactivation of Kv1.3 in human T lymphocytes. The results of whole-cell patch clamp recordings show that there is a linear increase in recovery rate with increasing [K+]o. An increase from 5 to 150 mM K+o causes a sixfold acceleration of recovery rate at a holding potential of -90 mV. Our results suggest that 1) a low-affinity K+ binding site is involved in recovery, 2) the rate of recovery increases with hyperpolarization, 3) potassium must bind to the channel before inactivation to speed its recovery, and 4) recovery rate depends on external [K+] but not on the magnitude of the driving force through open channels. We present a model in which a bound K+ ion destabilizes the inactivated state to increase the rate of recovery of C-type inactivation, thereby providing a mechanism for autoregulation of K+ channel activity. The ability of K+ to regulate its own conductance may play a role in modulating voltage-dependent immune function.     

Extracellular sodium interacts with the HERG channel at an outer pore site

Most voltage-gated K(+) currents are relatively insensitive to extracellular Na(+) (Na(+)(o)), but Na(+)(o) potently inhibits outward human ether-a-go-go-related gene (HERG)-encoded K(+) channel current (Numaguchi, H., J.P. Johnson, Jr., C.I. Petersen, and J.R. Balser. 2000. Nat. Neurosci. 3:429-30). We studied wild-type (WT) and mutant HERG currents and used two strategic probes, intracellular Na(+) (Na(+)(i)) and extracellular Ba(2+) (Ba(2+)(o)), to define a site where Na(+)(o) interacts with HERG. Currents were recorded from transfected Chinese hamster ovary (CHO-K1) cells using the whole-cell voltage clamp technique. Inhibition of WT HERG by Na(+)(o) was not strongly dependent on the voltage during activating pulses. Three point mutants in the P-loop region (S624A, S624T, S631A) with intact K(+) selectivity and impaired inactivation each had reduced sensitivity to inhibition by Na(+)(o). Quantitatively similar effects of Na(+)(i) to inhibit HERG current were seen in the WT and S624A channels. As S624A has impaired Na(+)(o) sensitivity, this result suggested that Na(+)(o) and Na(+)(i) act at different sites. Extracellular Ba(2+) (Ba(2+)(o)) blocks K(+) channel pores, and thereby serves as a useful probe of K(+) channel structure. HERG channel inactivation promotes relief of Ba(2+) block (Weerapura, M., S. Nattel, M. Courtemanche, D. Doern, N. Ethier, and T. Hebert. 2000. J. Physiol. 526:265-278). We used this feature of HERG inactivation to distinguish between simple allosteric and pore-occluding models of Na(+)(o) action. A remote allosteric model predicts that Na(+)(o) will speed relief of Ba(2+)(o) block by promoting inactivation. Instead, Na(+)(o) slowed Ba(2+) egress and Ba(2+) relieved Na(+)(o) inhibition, consistent with Na(+)(o) binding to an outer pore site. The apparent affinities of the outer pore for Na(+)(o) and K(+)(o) as measured by slowing of Ba(2+) egress were compatible with competition between the two ions for the channel pore in their physiological concentration ranges. We also examined the role of the HERG closed state in Na(+)(o) inhibition. Na(+)(o) inhibition was inversely related to pulsing frequency in the WT channel, but not in the pore mutant S624A.     

Divalent ion trapping inside potassium channels of human T lymphocytes

Using the patch-clamp whole-cell recording technique, we investigated the influence of external Ca2+, Ba2+, K+, Rb+, and internal Ca2+ on the rate of K+ channel inactivation in the human T lymphocyte-derived cell line, Jurkat E6-1. Raising external Ca2+ or Ba2+, or reducing external K+, accelerated the rate of the K+ current decay during a depolarizing voltage pulse. External Ba2+ also produced a use-dependent block of the K+ channels by entering the open channel and becoming trapped inside. Raising internal Ca2+ accelerated inactivation at lower concentrations than external Ca2+, but increasing the Ca2+ buffering with BAPTA did not affect inactivation. Raising [K+]o or adding Rb+ slowed inactivation by competing with divalent ions. External Rb+ also produced a use-dependent removal of block of K+ channels loaded with Ba2+ or Ca2+. From the removal of this block we found that under normal conditions approximately 25% of the channels were loaded with Ca2+, whereas under conditions with 10 microM internal Ca2+ the proportion of channels loaded with Ca2+ increased to approximately 50%. Removing all the divalent cations from the external and internal solution resulted in the induction of a non-selective, voltage-independent conductance. We conclude that Ca2+ ions from the outside or the inside can bind to a site at the K+ channel and thereby block the channel or accelerate inactivation.     

Apical K+ channels in Necturus taste cells. Modulation by intracellular factors and taste stimuli

The apically restricted, voltage-dependent K+ conductance of Necturus taste receptor cells was studied using cell-attached, inside-out and outside-out configurations of the patch-clamp recording technique. Patches from the apical membrane typically contained many channels with unitary conductances ranging from 30 to 175 pS in symmetrical K+ solutions. Channel density was so high that unitary currents could be resolved only at negative voltages; at positive voltages patch recordings resembled whole-cell recordings. These multi-channel patches had a small but significant resting conductance that was strongly activated by depolarization. Patch current was highly K+ selective, with a PK/PNa ratio of 28. Patches containing single K+ channels were obtained by allowing the apical membrane to redistribute into the basolateral membrane with time. Two types of K+ channels were observed in isolation. Ca(2+)-dependent channels of large conductance (135-175 pS) were activated in cell-attached patches by strong depolarization, with a half-activation voltage of approximately -10 mV. An ATP-blocked K+ channel of 100 pS was activated in cell-attached patches by weak depolarization, with a half-activation voltage of approximately -47 mV. All apical K+ channels were blocked by the sour taste stimulus citric acid directly applied to outside-out and perfused cell-attached patches. The bitter stimulus quinine also blocked all channels when applied directly by altering channel gating to reduce the open probability. When quinine was applied extracellularly only to the membrane outside the patch pipette and also to inside-out patches, it produced a flickery block. Thus, sour and bitter taste stimuli appear to block the same apical K+ channels via different mechanisms to produce depolarizing receptor potentials.     

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.