Nonselective Cation and BK Channels in Apical Membrane of Outer Sulcus Epithelial Cells

The outer sulcus epithelium was recently shown to absorb cations from the lumen of the gerbil cochlea. Patch clamp recordings of excised apical membrane were made to investigate ion channels that participate in this reabsorptive flux. Three types of channel were observed: (i) a nonselective cation (NSC) channel, (ii) a BK (large conductance, maxi K or K _Ca ) channel and (iii) a small K⁺ channel which could not be fully characterized. The NSC channel found in excised insideout patch recordings displayed a linear current-voltage (I-V) relationship (27 pS) and was equally conductive for Na⁺ and K⁺, but not permeable to Cl⁻ or N-methyl-d-glucamine. Channel activity required the presence of Ca²⁺ at the cytosolic face, but was detected at Ca²⁺ concentrations as low as 10⁻⁷ m (open probability (P _o ) = 0.11 ± 0.03, n= 8). Gadolinium decreased P _o of the NSC channel from both the external and cytosolic side (IC₅₀∼ 0.6 μm). NSC currents were decreased by amiloride (10 μm− 1 mm) and flufenamic acid (0.1 mm). The BK channel was also frequently (38%) observed in excised patches. In symmetrical 150 mm KCl conditions, the I-V relationship was linear with a conductance of 268 pS. The Goldman-Hodgkin-Katz equation for current carried solely by K⁺ could be fitted to the I-V relationship in asymmetrical K⁺ and Na⁺ solutions. The channel was impermeable to Cl⁻ and N-methyl-d-glucamine. P _o of the BK channel increased with depolarization of the membrane potential and with increasing cytosolic Ca²⁺. TEA (20 mm), charybdotoxin (100 nm) and Ba²⁺ (1 mm) but not amiloride (1 mm) reduced P _o from the extracellular side. In contrast, external flufenamic acid (100 μm) increased P _o and this effect was inhibited by charybdotoxin (100 nm). Flufenamic acid inhibited the inward short-circuit current measured by the vibrating probe and caused a transient outward current. We conclude that the NSC channel is Ca²⁺ activated, voltage-insensitive and involved in both constitutive K⁺ and Na⁺ reabsorption from endolymph while the BK channel might participate in the K⁺ pathway under stimulated conditions that produce an elevated intracellular Ca²⁺ or depolarized membrane potential. Received: 14 October 1999/Revised: 10 December 1999     

Role and Source of ATP for Activation of Nonselective Cation Channels by AlF Complex in Guinea Pig Chromaffin Cells

Intracellular dialysis with the solution containing the G protein activator, AlF complex, induced an inward nonselective cation current (I _NS) at −55 mV in chromaffin cells. Amplitudes of I _NS induced by dialysis with ATP-free AlF solutions progressively diminished as cells were pretreated with cyanide, a mitochondrial inhibitor. After a 10-min pretreatment, generation of I _NS by the AlF complex depended on exogenous ATP delivered from pipette solution. The relationship between amplitudes of I _NS and concentrations of MgATP was well expressed by a rectangular hyperbola with an EC₅₀ of 0.265 mm. This result suggests that the cyanide treatment almost depleted ATP near the plasma membrane. On the other hand, a similar cyanide treatment of adrenal medullary preparations did not induce a marked decrease in cellular ATP content. GTP, ITP, or UTP could not substitute for ATP in generation of I _NS by the AlF complex. Similarly, the substitution of ATP with non- or poorly hydrolyzable ATP analogues did not aid in generating I _NS. Bath application of the kinase inhibitor, H-7 (100 μm), suppressed AlF-induced I _NS in a manner depending on intracellular Mg²⁺. We conclude that ATP is a prerequisite for generation of I _NS as a phosphoryl donor and that mitochondria is the main source of ATP. Received: 17 April 1996/Revised: 26 July 1996     

A Novel Large Conductance, Nonselective Cation Channel in Pheochromocytoma (PC12) Cells

A new type of nonselective cation channel was identified and characterized in pheochromocytoma (PC12) cells using inside-out and cell-attached patch-clamp recordings. The channel shows a large unitary conductance (274 pS in symmetric 145 mm K⁺) and selectivity for Na⁺≈ K⁺ > Li⁺, and is practically impermeable to Cl⁻. The channel activity-voltage relationship is bell-shaped, showing maximal activation at ≈−10 mV. The overall activity of this channel is unmodified by [Na⁺] _ic , or [Ca⁺⁺] _ic . However, increases in [Ca⁺⁺] _ic lead to a decrease in the unitary current amplitude. In addition, overall activity is mildly increased when suction is applied to the back of the patch pipette. Together, these characteristics distinguish the present channel from all other large conductance nonselective cation channels reported so far in a variety of preparations. The frequency of appearance of this channel type is similar in undifferentiated and NGF-treated PC12 cells (≈8–27% of patches). The combination of large conductance, permeability to Na⁺, and existence of conducting states at negative potentials, may provide a significant pathway for inward current and depolarization in PC12 cells. Received: 14 February 1997/Revised: 28 July 1997     

A Nonselective High Conductance Channel in Bovine Pigmented Ciliary Epithelial Cells

An ion channel activated by hyperpolarization was identified in excised patches of bovine pigmented ciliary epithelial cells using the single channel patch clamp technique. In symmetrical NaGluconate, the channel had a maximum conductance of 285 pS. The channel was characterized by frequent flickery transitions between the fully open and closed levels. The channel did not discriminate very clearly between anions and cations; when the cytoplasmic face of excised patches was bathed in a dilute NaCl solution, the channel had a chloride-to-sodium permeability ratio (P _Cl/P _Na) of 1.3. However, the channel showed a small anion selectivity (P _Cl/P _Na= 3.7) when bathed in a concentrated NaCl solution. Gd³⁺ blocked the channel reversibly. Channel kinetics were characterised by slow (≈ min) voltage-dependent activation and inactivation rate constants. The channel was most active in the range −60 to −140 mV and showed a peak at −120 mV. A similar time- and voltage-dependent activation was also observed in cell-attached recordings. In conclusion, hyperpolarization of pigmented ciliary epithelial cell membrane patches activated a large conductance, nonselective ion channel. This combination of nonselectivity and hyperpolarizing activation is consistent with the involvement of this channel in ion loading from the blood into pigmented ciliary epithelial cells—the first phase in the secretion of aqueous humor. Received: 30 June 1995/Revised: 16 November 1995     

Interconverting Gating Modes of a Nonselective Cation Channel from the Tapeworm Echinococcus granulosus Reconstituted on Planar Lipid Bilayers

A 107-pS (symmetrical 150 mm KCl), nonselective cation channel was reconstituted from a microsomal membrane fraction of the larval stage of the tapeworm Echinococcus granulosus. Most of the time, it displayed a high open probability (>0.95) irrespective of either the applied voltage, Ca²⁺, Ba²⁺, or tetraethylammonium concentration. Nevertheless, in contrast with this ``leaklike' behavior, less frequently this ``all-the-time-open' channel reversibly entered two different kinetic modes. One of them was characterized by lower P _o values and some voltage sensitivity (V _?≅ 129 mV, and an equilibrium constant for channel closing changing e-fold per 63-mV change) the kinetic analysis revealing that it resulted from the appearance of voltage-sensitivity in the mean closed times and a sixfold increase in the equilibrium constant for channel closing at 0 mV. The other mode was characterized by a very fast open-close activity leading to poorly resolved current levels and a P _o around 0.6–0.7 which, occasionally and in a voltage-sensitive manner, entered a long-lived nonconducting state. However, the rare nature of these mode-shifting transitions precluded a more detailed analysis of their kinetics. The conductive properties of the channel were not affected by these switches. Model gating alone does not seem to ensure any physiological role of this channel and, instead, some other channel changes must occur if this phenomenon were to be of regulatory importance in vivo. Thus, mode-shifting might constitute an alternative target for channel activity modulation also in tapeworms. Received: 30 August 1996/Revised: 31 January 1997     

Mechanism of Shrinkage Activation of Nonselective Cation Channels in M-1 Mouse Cortical Collecting Duct Cells

It has previously been shown that osmotic cell shrinkage activates a nonselective cation (NSC) channel in M-1 mouse cortical collecting duct cells [54] and in a variety of other cell types [20]. In the present study we further characterized the shrinkage-activated NSC channel in M-1 cells and its mechanism of activation using whole-cell current recordings. Osmotic cell shrinkage induced by addition of 100 mm sucrose to the bath solution caused a 20-fold increase in whole-cell inward currents from −10.8 ± 1.5 pA to −211 ± 10.2 pA (n= 103). A similar response was observed when cell shrinkage was elicited using a hypo-osmotic pipette solution. This indicates that cell shrinkage and not extracellular osmolarity per se is the signal for current activation. Cation substitution experiments revealed that the activated channels discriminate poorly between monovalent cations with a selectivity sequence NH₄ (1.2) ≥ Na⁺ (1) ≈ K⁺ (0.9) ≈ Li⁺ (0.9). In contrast there was no measurable permeability for Ca²⁺ or Ba²⁺ and the cation-to-anion permeability ratio was about 14. The DPC-derivatives flufenamic acid, 4-methyl-DPC and DCDPC were the most effective blockers followed by LOE 908, while amiloride and bumetanide were ineffective. The putative channel activator maitotoxin had no effect. Current activation was dependent upon the presence of intracellular ATP and Mg²⁺ and was inhibited by staurosporine (1 μm) and calphostin C (1 μm). Moreover, cytochalasin D (10 μm) and taxol (2 μm) reduced the current response to cell shrinkage. These findings suggest that the activation mechanism of the shrinkage-activated NSC channel involves protein kinase mediated phosphorylation steps and cytoskeletal elements. Received: 3 May 2000/Revised: 6 July 2000     

Histamine Increases [Ca2+] in and Activates Ca-K and Nonselective Cation Currents in Cultured Human Capillary Endothelial Cells

We characterized the effects of histamine on intracellular Ca²⁺ and activation of ionic currents in human capillary endothelial cells. Histamine produced both a transient and sustained increase in intracellular Ca²⁺. The transient response was mediated largely through intracellular Ca²⁺ release and the sustained response was due to extracellular Ca²⁺ entry. The increase in intracellular Ca²⁺ by histamine was not affected by the H2 blocker cimetidine. But was entirely blocked by the H1 antagonist diphenhydramine showing that the histamine response in these cells is mediated through the H1 receptor. A transient ionic current is coactivated with the histamine-induced increase in intracellular Ca²⁺ and this current has several properties of a nonselective, Ca²⁺ permeable, cation channel (NSC). The magnitude of the NSC current does not strictly correlate with intracellular Ca²⁺ levels. A Ca²⁺-activated K⁺ current (BKCA) is activated by the increase in intracellular Ca²⁺ and this current is blocked by the selective BKCA blocker iberiotoxin. Received: 16 June 1999/Revised: 22 September 1999     

An L-type Calcium Channel in Renal Epithelial Cells

A voltage-activated Ca⁺⁺ channel has been identified in the apical membranes of cultured rabbit proximal tubule cells using the patch-clamp technique. With 105 mm CaCl₂ solution in the pipette and 180 NaAsp in the bath, the channel had a conductance of 10.4 ± 1.0 pS (n= 8) in on-cell patches, and 9.8 ± 1.1 pS (n= 8) in inside-out patches. In both on-cell and inside-out patches, the channel is active by membrane depolarization. For this channel, the permeation to Ba⁺⁺ and Ca⁺⁺ is highly selective over Na⁺ and K⁺ (P_Ca(Ba):P_Na(K) >200:1). The sensitivity to dihydropyridines is similar to that for L-type channels where the channel was blocked by nifedipine (10 μm), and activated by Bay K 8644 (5 μm). When activated by Bay K 8644, the channel showed subconductance levels. Treatment with forskolin (12.5 μm), phorbol ester (1 μm), or stretching (40 cm water) did not activate this channel. These results indicate that this Ca⁺⁺ channel is mostly regulated by membrane voltage, and appears to be an epithelial class of L-type Ca⁺⁺ channel. As such, it may participate in calcium reabsorption during periods of enhanced sodium reabsorption, or calcium signaling in volume regulation, where membrane depolarization occurs for prolonged periods. Received: 1 April 1996/Revised: 5 August 1996     

G Protein-mediated Activation of a Nonspecific Cation Current in Cultured Rat Retinal Pigment Epithelial Cells

We used whole-cell patch-clamp recording techniques to investigate G protein-activated currents in cultured rat retinal pigment epithelial (RPE) cells. Using 140 mm KCl intracellular and 130 mm NaCl extracellular solutions, rat RPE cells possessed both inward and outward K⁺ currents. Upon addition of the nonhydrolyzable guanine triphosphate analogue, guanosine-5′-O-(3-thiophosphate) (GTPγS, 0.1 mm), to the recording electrode, a nonspecific cation (NSC) current was elicited. The NSC current had a mean reversal potential of +5.7 mV in 130 mm extracellular NaCl with Cs⁺-aspartate in the pipette, and was not affected by alterations in the extracellular Ca²⁺ or Cl⁻ concentration. The GTPγS-activated current was found to be permeable to several monovalent cations (K⁺, Na⁺, choline, TRIS, and NMDG). Addition of fluoroaluminate, an activator of large molecular weight heterotrimeric GTP-binding proteins (G proteins), to the intracellular recording solution activated the NSC current. The G protein involved was pertussis toxin (PTX)-sensitive, since GTPγS failed to activate the NSC current in cells pretreated with PTX. Further investigation of second messenger molecules suggested that activation of the NSC current was not affected by alterations in intracellular Ca²⁺ or ATP. From these results, we conclude that a G protein-regulated NSC current is present in rat RPE cells. Activation of the NSC current may sufficiently depolarize RPE cells to activate outward K⁺ currents. This would provide a mechanism by which these cells could rid themselves of accumulated K⁺. Received: 25 January 1996/Revised: 24 April 1996     

Expression of Cyclic Nucleotide-Gated Cation Channels in Airway Epithelial Cells

Using the whole-cell patch-clamp technique, the selectivity and pharmacology of 8-Br-cGMP-stimulated currents in the human alveolar cell line A549 was compared to 8-Br-cGMP-stimulated currents in HK293 cells transfected with hαCNC1. Whole cell currents stimulated by 8-Br-cGMP in HK293 cells transfected with hαCNC1 or A549 cells are carried by inward sodium and outward potassium with nearly the same selectivity. The whole-cell inward currents that are stimulated by 8-Br-cGMP in HK293 cells transfected with hαCNC1 are inhibited by l-cis-diltiazem with an IC₅₀ of 154 μm, by 2′,4′-dichlorobenzamil with an IC₅₀ of 50 μm and by amiloride with an IC₅₀ of 133 μm. The whole-cell inward currents in A549 cells that are stimulated by 8-Br-cGMP, are inhibited by l-cis-diltiazem with an IC₅₀ of 87 μm, by 2′4′-dichlorobenzamil with an IC₅₀ of 38 μm and by amiloride with an IC₅₀ of 32 μm suggesting that these airway cells contain cyclic nucleotide-gated cation channels. RT-PCR data suggest that mRNA of both αCNC1 and βCNC subunits are present in A549 cells and the presence of the βCNC subunit, may as previously reported, increase the affinity of these channel blockers compared to the hαCNC1 subunit alone. The mRNA of two other isoforms of this channel, CNC2 and CNC3, are also expressed in the A549 cell line. This study documents the IC₅₀ of externally applied channel blockers that can be used for in vitro or in vivo experiments to document sodium absorption via cyclic nucleotide-gated cation channels in airway cells. Received: 24 February/Revised: 28 May 1999     

Gating and Conduction Properties of a Sodium-activated Cation Channel from Lobster Olfactory Receptor Neurons

The gating and conduction properties of a channel activated by intracellular Na⁺ were studied by recording unitary currents in inside-out patches excised from lobster olfactory receptor neurons. Channel openings to a single conductance level of 104 pS occurred in bursts. The open probability of the channel increased with increasing concentrations of Na⁺. At 210 mm Na⁺, membrane depolarization increased the open probability e-fold per 36.6 mV. The distribution of channel open times could be fit by a single exponential with a time constant of 4.09 msec at −60 mV and 90 mm Na⁺. The open time constant was not affected by the concentration of Na⁺, but was increased by membrane depolarization. At 180 mm Na⁺ and −60 mV, the distribution of channel closed times could be fit by the sum of four exponentials with time constants of 0.20, 1.46, 8.92 and 69.9 msec, respectively. The three longer time constants decreased, while the shortest time constant did not vary with the concentration of Na⁺. Membrane depolarization decreased all four closed time constants. Burst duration was unaffected by the concentration of Na⁺, but was increased by membrane depolarization. Permeability for monovalent cations relative to that of Na⁺ (P _X /P _Na ), calculated from the reversal potential, was: Li⁺ (1.11) > Na⁺ (1.0) > K⁺ (0.54) > Rb⁺ (0.36) > Cs⁺ (0.20). Extracellular divalent cations (10 mm) blocked the inward Na⁺ current at −60 mV according to the following sequence: Mn²⁺ > Ca²⁺ > Sr²⁺ > Mg²⁺ > Ba²⁺. Relative permeabilities for divalent cations (P _Y /P _Na ) were Ca²⁺ (39.0) > Mg²⁺ (34.1) > Mn²⁺ (15.5) > Ba²⁺ (13.8) > Na⁺ (1.0). Both the reversal potential and the conductance determined in divalent cation-free mixtures of Na⁺ and Cs⁺ or Li⁺ were monotonic functions of the mole fraction, suggesting that the channel is a single-ion pore that behaves as a multi-ion pore when the current is carried exclusively by divalent cations. The properties of the channel are consistent with the channel playing a role in odor activation of these primary receptor neurons. Received: 17 September 1996/Revised: 15 November 1996     

Characterization of the Stretch-activated Chloride Channel in Isolated Human Atrial Myocytes

Macroscopic and unitary currents through stretch-activated Cl⁻ channels were examined in isolated human atrial myocytes using whole-cell, excised outside-out and inside-out configurations of the patch-clamp technique. When K⁺ and Ca²⁺ conductances were blocked and the intracellular Ca²⁺ concentration ([Ca²⁺] _i ) was reduced, application of positive pressure via the pipette activated membrane currents under whole-cell voltage-clamp conditions. The reversal potential of the current shifted by 60 mV per 10-fold change in the external Cl⁻ concentration, indicating that the current was Cl⁻ selective. The current was inhibited by bath application of 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) and 9-anthracenecarboxylic acid (9-AC). β-Adrenergic stimulation failed to activate a Cl⁻ current. In single channel recordings from outside-out patches, positive pressure in the pipette activated the unitary current with half-maximal activation of 14.7 mm Hg at +40 mV. The current-voltage relationship of single channel activity obtained in inside-out patches was linear in symmetrical Cl⁻ solution with the averaged slope conductance of 8.6 ± 0.7 pS (mean ±sd, n= 10). The reversal potential shift of the channel by changing Cl⁻ concentration was consistent with a Cl⁻ selective channel. The open time distribution was best described by a single exponential function with mean open lifetime of 80.4 ± 9.6 msec (n= 9), while at least two exponentials were required to fit the closed time distributions with a time constant for the fast component of 11.5 ± 2.2 msec (n= 9) and that for the slow component of 170.2 ± 21.8 msec (n= 9). Major changes in the single channel activity in response to pressure were caused by changes in the interburst interval. Single channel activity was inhibited by DIDS and 9-AC in a manner similar to whole-cell configuration. These results suggest that membrane stretch induced by applying pressure via the pipette activated a Cl⁻ current in human atrial myocytes. The current was sensitive to Cl⁻ channel blockers and exhibited membrane voltage-independent bursting opening without sensitive to β-adrenergic stimulation. Received: 21 October 1996/Revised: 17 December 1997     

The Kinetics of Quinine Blockade of the Maxi Cation Channel in the Plasma Membrane of Rye Roots

The maxi cation channel from the plasma membrane of rye (Secale cereale L.) roots was studied following its incorporation into planar phosphatidylethanolamine bilayers. Current recordings were made in the presence of 100-mm KCl containing quinine on both sides of the bilayer. Quinine produced voltage- and concentration-dependent blockade of the channel, reducing its apparent unitary current and open probability. The voltage-dependence suggested that blockade was effected from the cytoplasmic side by cationic quinine. Blockade was modelled using a kinetic scheme with two independent blocked states termed B1 and B2 (B1⇆O⇄B2). Rate constants promoting fast kinetics (k ₁ and k ₋₁ ) were found to be several orders of magnitude greater than those promoting slow kinetics (k ₂ and k ₋₂ ). Analysis of the fast kinetics indicated that the rate constants for blockade of the open channel at the first site (k ₁ ) and its clearance (k ₋₁ ) had voltage-dependencies (zδ _p ) of 0.41 and −0.71, respectively, and that the equilibrium dissociation constant for the binding site (K _d (0)) was about 1 mm. Analysis of the slow kinetics indicated that the rate constants for blockade of the open channel at the second site (k ₂ ) and its clearance (k ₋₂ ) had zδ _p values of 0.12 and −1.27, respectively. The K _d (0) value for the second binding site was about 10 mm. Received: 20 January 1998/Revised: 1 May 1998     

Single-Channel Characterization of a Nonselective Cation Channel from Human Placental Microvillous Membranes. Large Conductance, Multiplicity of Conductance States, and Inhibition by Lanthanides

The rate-limiting step for the maternofetal exchange of low molecular-weight solutes in humans is constituted by transport across a single epithelial layer (syncytiotrophoblast) of the placenta. Other than the well-established presence of a large-conductance, multisubstate Cl⁻ channel, the ionic channels occurring in this syncytial tissue are, for the most part, unknown. We have found that fusion of apical plasma membrane-enriched vesicle fractions with planar lipid bilayers leads, mainly (96% of 353 reconstitutions), to the reconstitution of nonselective cation channels. Here we describe the properties of this novel placental conductance at the single-channel level. The channel has a large (>200 pS) and variable conductance, is cation selective (P _Cl /P _K ≅ 0.024), is reversibly inhibited (presumably blocked) by submillimolar La³⁺, has very unstable kinetics, and displays a large number (>10) of current sublevels with a ``promiscuous' connectivity pattern. The occurrence of both ``staircaselike' and ``all-or-nothing' transitions between the minimum and maximum current levels was intriguing, particularly considering the large number of conductance levels spanned at a time during the concerted current steps. Single-channel data simulated according to a multistate linear reaction scheme, with rate constants that can vary spontaneously in time, reproduce many aspects of the recorded subconductance behavior. The channel's sensitivity to lanthanides is reminiscent of stretch-sensitive channels which, in turn, suggests a physiological role for this ion channel as a mechanotransducer during syncytiotrophoblast-volume regulation. Received: 30 August 1999/Revised: 12 November 1999     

Regulation of Cation-selective Channels in Liver Cells

In liver cells, cation-selective channels are permeable to Ca²⁺ and have been postulated to represent a pathway for receptor-mediated Ca²⁺ influx. This study examines the mechanisms involved in the regulation of these channels in a model liver cell line. Using patch-clamp recording techniques, it is shown that channel open probability is a saturable function of cytosolic [Ca²⁺], with half-maximal opening at 660 nm. By contrast, channel opening is not affected by membrane voltage or cytosolic pH. In intact cells, reduction of cytosolic [Cl⁻], a physiological response to Ca²⁺-mobilizing hormones and cell swelling, is also associated with an increase in channel opening. Finally, channel opening is inhibited by intracellular ATP through a mechanism that does not involve ATP hydrolysis. These findings suggest that opening of cation-selective channels is coupled to the metabolic state of the cell and provides a positive feedback mechanism for regulation of receptor-mediated Na⁺ and Ca²⁺ influx. Received: 8 October 1996     

cAMP Increases Apical IsK Channel Current and K+ Secretion in Vestibular Dark Cells

Adenosine 3′,5′-cyclic monophosphate (cAMP) is known to stimulate exogenous I_sK channel current in the Xenopus oocyte expression system. The present study was performed to determine whether elevation of cytosolic cAMP in a native mammalian epithelium known to secrete K⁺ through endogenously expressed I_sK channels would stimulate K⁺ secretion through these channels. The equivalent short circuit current (I _sc ) across vestibular dark cell epithelium in gerbil was measured in a micro-Ussing chamber and the apical membrane current (I _IsK ) and conductance (g _IsK ) of I_sK channels was recorded with both the on-cell macro-patch and nystatin-perforated whole-cell patch-clamp techniques. It has previously been shown that I _sc can be accounted for by transepithelial K⁺ secretion and that the apical I_sK channels constitute a significant pathway for K⁺ secretion. The identification of the voltage-dependent whole-cell currents in vestibular dark cells was strengthened by the finding that a potent blocker of I_sK channels, chromanol 293B, strongly reduced I _IsK from 646 ± 200 to 154 ± 22 pA (71%) and g _IsK from 7.5 ± 2.6 to 2.8 ± 0.4 nS (53%). Cytoplasmic cAMP was elevated by applying dibutyryl cyclic AMP (dbcAMP), or the phosphodiesterase inhibitors 3-isobutyl-1-methylxanthine (IBMX) and Ro-20-1724. dbcAMP (1 mm) increased I _sc and I _IsK from 410 ± 38 to 534 ± 40 μA/cm² and from 4.3 ± 0.8 to 11.4 ± 2.2 pA, respectively. IBMX (1 mm) caused transient increases of I _sc from 415 ± 30 to 469 ± 38 μA/cm² and Ro-20-1724 (0.1 mm) from 565 ± 43 to 773 ± 58 μA/cm². IBMX increased I _IsK from 5.5 ± 1.5 to 16.9 ± 5.8 pA in on-cell experiments and from 191 ± 31 to 426 ± 53 pA in whole-cell experiments. The leak conductance due to all non-I_sK channel sources did not change during dbcAMP and IBMX while 293B in the presence of dbcAMP reduced I _IsK by 84% and g _IsK by 62%, similar to unstimulated conditions. These results demonstrate that the cAMP pathway is constitutively active in vestibular dark cells and that the cAMP pathway stimulates transepithelial K⁺ secretion by increasing I_sK channel current rather than by altering another transport pathway. Received: 9 June 1995/Revised: 17 October 1996     

Ion Permeation and Block of P2X2 Purinoceptors: Single Channel Recordings

P2X₂ purinoceptors are cation-selective channels activated by ATP and its analogues. Using single channel measurements we studied the channel's selectivity for the alkali metal ions and organic monovalent cations NMDG⁺, Tris⁺, TMA⁺, and TEA⁺. The selectivity sequence for currents carried by alkali metal ions is: K⁺ > Rb⁺ > Cs⁺ > Na⁺ > Li⁺, which is Eisenman sequence IV. This is different from the mobility sequence of the ions in free solution suggesting there is weak interaction between the ions and the channel interior. The relative conductance for alkali ions increases linearly in relation to the Stokes radius. The organic ions NMDG⁺, Tris⁺, TMA⁺ and TEA⁺ were virtually impermeant. The divalent ions (Mn²⁺, Mg²⁺, Ca²⁺ and Ba²⁺) induced a fast block visible as a reduction in amplitude of the unitary currents. Using a single-site binding model, the divalent ions exhibited an equilibrium affinity sequence of Mn²⁺ > Mg²⁺ > Ca²⁺ > Ba²⁺. Received: 3 May 1999/Revised: 23 August 1999     

Nitric Oxide Inhibition of the Rat Olfactory Cyclic Nucleotide-Gated Cation Channel

The effects of nitric oxide (NO) and other cysteine modifying agents were examined on cyclic nucleotide-gated (CNG) cation channels from rat olfactory receptor neurons. The NO compounds, S-nitroso-cysteine (SNC) and 3-morpholino-sydnonomine (SIN-1), did not activate the channels when applied for up to 10 min. The cysteine alkylating agent, N-ethylmaleimide (NEM), and the oxidising agent, dithionitrobensoate (DTNB), were also without agonist efficacy. Neither SNC nor DTNB altered the cAMP sensitivity of the channels. However, 2-min applications of SIN-1, SNC and DTNB inhibited the cAMP-gated current to approximately 50% of the control current level. This inhibition showed no spontaneous reversal for 5 min but was completely reversed by a 2-min exposure to DTT. The presence of cAMP protected the channels against NO-induced inhibition. These results indicate that inhibition is caused by S-nitrosylation of neighboring sulfhydryl groups leading to sulfhydryl bond formation. This reaction is favored in the closed channel state. Since recombinantly expressed rat olfactory α and β CNG channel homomers and α/β heteromers are activated and not inhibited by cysteine modification, the results of this study imply the existence of a novel subunit or tightly bound factor which dominates the effect of cysteine modification in the native channels. As CNG channels provide a pathway for calcum influx, the results may also have important implications for the physiological role of NO in mammalian olfactory receptor neurons. Received: 30 March 1998/Revised: 17 June 1998     

An Energy-Barrier Model for the Permeation of Monovalent and Divalent Cations Through the Maxi Cation Channel in the Plasma Membrane of Rye Roots

The depolarization-activated, high-conductance ``maxi' cation channel in the plasma membrane of rye (Secale cereale L.) roots is permeable to a wide variety of monovalent and divalent cations. The permeation of K⁺, Na⁺, Ca²⁺ and Ba²⁺ through the pore could be simulated using a model composed of three energy barriers and two ion binding sites (a 3B2S model), which assumed single-file permeation and the possibility of double cation occupancy. The model had an asymmetrical free energy profile. Differences in permeation between cations were attributed primarily to differences in their free energy profiles in the regions of the pore adjacent to the extracellular solution. In particular, the height of the central free energy peak differed between cations, and cations differed in their affinities for ion binding sites. Significant ion repulsion occurred within the pore, and the mouths of the pore had considerable surface charge. The model adequately described the diverse current vs. voltage (I/V) relationships obtained over a wide variety of experimental conditions. It described the phenomena of non-Michaelian unitary conductance vs. activity relationships for K⁺, Na⁺ and Ca²⁺, differences in selectivity sequences obtained from measurements of conductance and permeability ratios, changes in relative cation permeabilities with solution composition, and the complex effects of Ba²⁺ and Ca²⁺ on K⁺ currents through the channel. The model enabled the prediction of unitary currents and ion fluxes through the maxi cation channel under physiological conditions. It could be used, in combination with data on the kinetics of the channel, as input to electrocoupling models allowing the relationships between membrane voltage, Ca²⁺ influx and Ca²⁺ signaling to be studied theoretically. Received: 29 April 1998/Revised: 20 November 1998