Ionic channel rundown in excised membrane patches

Functional stability of CFTR depends on tight binding of ATP at its degenerate ATP-binding site

Opening of the cystic fibrosis transmembrane conductance regulator (CFTR) channel is coupled to the motion of its two nucleotide-binding domains: they form a heterodimer sandwiching two functionally distinct ATP-binding sites (sites 1 and 2). While active ATP hydrolysis in site 2 triggers rapid channel closure, the functional role of stable ATP binding in the catalysis-incompetent (or degenerate) site 1, a feature conserved in many other ATP-binding cassette (ABC) transporter proteins, remains elusive. Here, we found that CFTR loses its prompt responsiveness to ATP after the channel is devoid of ATP for tens to hundreds of seconds. Mutants with weakened ATP binding in site 1 and the most prevalent disease-causing mutation, F508del, are more vulnerable to ATP depletion. In contrast, strengthening ligand binding in site 1 with N⁶ -(2-phenylethyl)-ATP, a high-affinity ATP analogue, or abolishing ATP hydrolysis in site 2 by the mutation D1370N, helps sustain a durable function of the otherwise unstable mutant channels. Thus, tight binding of ATP in the degenerate ATP-binding site is crucial to the functional stability of CFTR. Small molecules targeting site 1 may bear therapeutic potential to overcome the membrane instability of F508del-CFTR. KEY POINTS: During evolution, many ATP-binding cassette transporters - including the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, whose dysfunction causes cystic fibrosis (CF) - lose the ability to hydrolyse ATP in one of the two ATP-binding sites. Here we show that tight ATP binding at this degenerate site in CFTR is central for maintaining the stable, robust function of normal CFTR. We also demonstrate that membrane instability of the most common CF-causing mutant, F508del-CFTR, can be rescued by strengthening ATP binding at CFTR's degenerate site. Our data thus explain an evolutionary puzzle and offer a potential therapeutic strategy for CF.

Antispasmodic effects of the essential oil of Croton zehnteneri, anethole, and estragole, on tracheal smooth muscle

Croton zehntneri is a plant well adapted to the semi-arid climate of northeastern region of Brazil. The essential oil of C. zehntneri (EOCz) has been described to have several pharmacologic properties, including effect on airflow resistance of in vivo respiratory system. For this reason, we investigated the hypothesis that EOCz and its major constituents, anethole and estragole, have antispasmodic activity on tracheal muscle. In tracheal rings of Wistar rats, maintained in Krebs-Henseleit's solution, EOCz, anethole and estragole inhibited contractions induced by 60mM [K⁺], ACh (10μM), Ba²⁺ and Phorbol dibutirate (1 μM). For EOCz, anethole and estragole, the IC₅₀ for inhibition of KCl-induced contractions were 145.8 ± 14.8, 89.9 ± 7.4 and 181.0 ± 23.3 μg/mL, respectively, and for ACh-induced contraction, they were 606.1 ± 122.0, 160.5 ± 33.0 and 358.6 ± 49.2 μg/mL. Pharmacodynamic efficacy was maximal in all cases. These data in Ba²⁺-induced contraction and the differential IC₅₀ suggested that blockade of Voltage Dependent Calcium Channels (VDCC) is a component of the mechanism of action of the three agents. Evaluation of the direct effect of anethole, on VDCC, showed inhibition of the Ca²⁺ current through this type of channel. These results show that EOCz and the constituents have antispasmodic activity and the mechanism includes blockade of VDCC channels.

Isoform-specific regulation of HCN4 channels by a family of endoplasmic reticulum proteins

Ion channels in excitable cells function in macromolecular complexes in which auxiliary proteins modulate the biophysical properties of the pore-forming subunits. Hyperpolarization-activated, cyclic nucleotide-sensitive HCN4 channels are critical determinants of membrane excitability in cells throughout the body, including thalamocortical neurons and cardiac pacemaker cells. We previously showed that the properties of HCN4 channels differ dramatically in different cell types, possibly due to the endogenous expression of auxiliary proteins. Here, we report the discovery of a family of endoplasmic reticulum (ER) transmembrane proteins that associate with and modulate HCN4. Lymphoid-restricted membrane protein (LRMP, Jaw1) and inositol trisphosphate receptor-associated guanylate kinase substrate (IRAG, Mrvi1, and Jaw1L) are homologous proteins with small ER luminal domains and large cytoplasmic domains. Despite their homology, LRMP and IRAG have distinct effects on HCN4. LRMP is a loss-of-function modulator that inhibits the canonical depolarizing shift in the voltage dependence of HCN4 in response to the binding of cAMP. In contrast, IRAG causes a gain of HCN4 function by depolarizing the basal voltage dependence in the absence of cAMP. The mechanisms of action of LRMP and IRAG are independent of trafficking and cAMP binding, and they are specific to the HCN4 isoform. We also found that IRAG is highly expressed in the mouse sinoatrial node where computer modeling predicts that its presence increases HCN4 current. Our results suggest important roles for LRMP and IRAG in the regulation of cellular excitability, as tools for advancing mechanistic understanding of HCN4 channel function, and as possible scaffolds for coordination of signaling pathways.

K+ Accumulation and Clearance in the Calyx Synaptic Cleft of Type I Mouse Vestibular Hair Cells

Vestibular organs of Amniotes contain two types of sensory cells, named Type I and Type II hair cells. While Type II hair cells are contacted by several small bouton nerve terminals, Type I hair cells receive a giant terminal, called a calyx, which encloses their basolateral membrane almost completely. Both hair cell types release glutamate, which depolarizes the afferent terminal by binding to AMPA post-synaptic receptors. However, there is evidence that non-vesicular signal transmission also occurs at the Type I hair cell-calyx synapse, possibly involving direct depolarization of the calyx by K⁺ exiting the hair cell. To better investigate this aspect, we performed whole-cell patch-clamp recordings from mouse Type I hair cells or their associated calyx. We found that [K⁺] in the calyceal synaptic cleft is elevated at rest relative to the interstitial (extracellular) solution and can increase or decrease during hair cell depolarization or repolarization, respectively. The change in [K⁺] was primarily driven by G_K,L, the low-voltage-activated, non-inactivating K⁺ conductance specifically expressed by Type I hair cells. Simple diffusion of K⁺ between the cleft and the extracellular compartment appeared substantially restricted by the calyx inner membrane, with the ion channels and active transporters playing a crucial role in regulating intercellular [K⁺]. Calyx recordings were consistent with K⁺ leaving the synaptic cleft through postsynaptic voltage-gated K⁺ channels involving K_V1 and K_V7 subunits. The above scenario is consistent with direct depolarization and hyperpolarization of the calyx membrane potential by intercellular K⁺.

Molecular and electrophysiological characterization of anion transport in Arabidopsis thaliana pollen reveals regulatory roles for pH, Ca2+ and GABA

We investigated the molecular basis and physiological implications of anion transport during pollen tube (PT) growth in Arabidopsis thaliana (Col-0). Patch-clamp whole-cell configuration analysis of pollen grain protoplasts revealed three subpopulations of anionic currents differentially regulated by cytoplasmic calcium ([Ca²⁺ ]_cyt ). We investigated the pollen-expressed proteins AtSLAH3, AtALMT12, AtTMEM16 and AtCCC as the putative anion transporters responsible for these currents. AtCCC-GFP was observed at the shank and AtSLAH3-GFP at the tip and shank of the PT plasma membrane. Both are likely to carry the majority of anion current at negative potentials, as extracellular anionic fluxes measured at the tip of PTs with an anion vibrating probe were significantly lower in slah3^-/- and ccc^-/- mutants, but unaffected in almt12^-/- and tmem16^-/- . We further characterised the effect of pH and GABA by patch clamp. Strong regulation by extracellular pH was observed in the wild-type, but not in tmem16^-/- . Our results are compatible with AtTMEM16 functioning as an anion/H⁺ cotransporter and therefore, as a putative pH sensor. GABA presence: (1) inhibited the overall currents, an effect that is abrogated in the almt12^-/- and (2) reduced the current in AtALMT12 transfected COS-7 cells, strongly suggesting the direct interaction of GABA with AtALMT12. Our data show that AtSLAH3 and AtCCC activity is sufficient to explain the major component of extracellular anion fluxes, and unveils a possible regulatory system linking PT growth modulation by pH, GABA, and [Ca²⁺ ]_cyt through anionic transporters.

An allosteric mechanism of inactivation in the calcium-dependent chloride channel BEST1

Bestrophin proteins are calcium (Ca²⁺)-activated chloride channels. Mutations in bestrophin 1 (BEST1) cause macular degenerative disorders. Whole-cell recordings show that ionic currents through BEST1 run down over time, but it is unclear whether this behavior is intrinsic to the channel or the result of cellular factors. Here, using planar lipid bilayer recordings of purified BEST1, we show that current rundown is an inherent property of the channel that can now be characterized as inactivation. Inactivation depends on the cytosolic concentration of Ca²⁺, such that higher concentrations stimulate inactivation. We identify a C-terminal inactivation peptide that is necessary for inactivation and dynamically interacts with a receptor site on the channel. Alterations of the peptide or its receptor dramatically reduce inactivation. Unlike inactivation peptides of voltage-gated channels that bind within the ion pore, the receptor for the inactivation peptide is on the cytosolic surface of the channel and separated from the pore. Biochemical, structural, and electrophysiological analyses indicate that binding of the peptide to its receptor promotes inactivation, whereas dissociation prevents it. Using additional mutational studies we find that the "neck" constriction of the pore, which we have previously shown to act as the Ca²⁺-dependent activation gate, also functions as the inactivation gate. Our results indicate that unlike a ball-and-chain inactivation mechanism involving physical occlusion of the pore, inactivation in BEST1 occurs through an allosteric mechanism wherein binding of a peptide to a surface-exposed receptor controls a structurally distant gate.

Biphasic voltage-dependent inactivation of human NaV 1.3, 1.6 and 1.7 Na+ channels expressed in rodent insulin-secreting cells

KEY POINTS

Na+ current inactivation is biphasic in insulin-secreting cells, proceeding with two voltage dependences that are half-maximal at ∼-100 mV and -60 mV. Inactivation of voltage-gated Na+ (NaV ) channels occurs at ∼30 mV more negative voltages in insulin-secreting Ins1 and primary β-cells than in HEK, CHO or glucagon-secreting αTC1-6 cells. The difference in inactivation between Ins1 and non-β-cells persists in the inside-out patch configuration, discounting an involvement of a diffusible factor. In Ins1 cells and primary β-cells, but not in HEK cells, inactivation of a single NaV subtype is biphasic and follows two voltage dependences separated by 30-40 mV. We propose that NaV channels adopt different inactivation behaviours depending on the local membrane environment.

ABSTRACT

Pancreatic β-cells are equipped with voltage-gated Na+ channels that undergo biphasic voltage-dependent steady-state inactivation. A small Na+ current component (10-15%) inactivates over physiological membrane potentials and contributes to action potential firing. However, the major Na+ channel component is completely inactivated at -90 to -80 mV and is therefore inactive in the β-cell. It has been proposed that the biphasic inactivation reflects the contribution of different NaV α-subunits. We tested this possibility by expression of TTX-resistant variants of the NaV subunits found in β-cells (NaV 1.3, NaV 1.6 and NaV 1.7) in insulin-secreting Ins1 cells and in non-β-cells (including HEK and CHO cells). We found that all NaV subunits inactivated at 20-30 mV more negative membrane potentials in Ins1 cells than in HEK or CHO cells. The more negative inactivation in Ins1 cells does not involve a diffusible intracellular factor because the difference between Ins1 and CHO persisted after excision of the membrane. NaV 1.7 inactivated at 15--20 mV more negative membrane potentials than NaV 1.3 and NaV 1.6 in Ins1 cells but this small difference is insufficient to solely explain the biphasic inactivation in Ins1 cells. In Ins1 cells, but never in the other cell types, widely different components of NaV inactivation (separated by 30 mV) were also observed following expression of a single type of NaV α-subunit. The more positive component exhibited a voltage dependence of inactivation similar to that found in HEK and CHO cells. We propose that biphasic NaV inactivation in insulin-secreting cells reflects insertion of channels in membrane domains that differ with regard to lipid and/or membrane protein composition.

Protein phosphorylation maintains the normal function of cloned human Cav2.3 channels

Ca_v2.3 Ca²⁺ channels are subject to cytosolic regulation, which has been difficult to characterize in native cells. Neumaier et al. demonstrate the role of phosphorylation in the function of these channels and suggest a close relationship between voltage dependence and the phosphorylation state.

R-type currents mediated by native and recombinant Ca_v2.3 voltage-gated Ca²⁺ channels (VGCCs) exhibit facilitation (run-up) and subsequent decline (run-down) in whole-cell patch-clamp recordings. A better understanding of the two processes could provide insight into constitutive modulation of the channels in intact cells, but low expression levels and the need for pharmacological isolation have prevented investigations in native systems. Here, to circumvent these limitations, we use conventional and perforated-patch-clamp recordings in a recombinant expression system, which allows us to study the effects of cell dialysis in a reproducible manner. We show that the decline of currents carried by human Ca_v2.3+β₃ channel subunits during run-down is related to adenosine triphosphate (ATP) depletion, which reduces the number of functional channels and leads to a progressive shift of voltage-dependent gating to more negative potentials. Both effects can be counteracted by hydrolysable ATP, whose protective action is almost completely prevented by inhibition of serine/threonine but not tyrosine or lipid kinases. Protein kinase inhibition also mimics the effects of run-down in intact cells, reduces the peak current density, and hyperpolarizes the voltage dependence of gating. Together, our findings indicate that ATP promotes phosphorylation of either the channel or an associated protein, whereas dephosphorylation during cell dialysis results in run-down. These data also distinguish the effects of ATP on Ca_v2.3 channels from those on other VGCCs because neither direct nucleotide binding nor PIP₂ synthesis is required for protection from run-down. We conclude that protein phosphorylation is required for Ca_v2.3 channel function and could directly influence the normal features of current carried by these channels. Curiously, some of our findings also point to a role for leupeptin-sensitive proteases in run-up and possibly ATP protection from run-down. As such, the present study provides a reliable baseline for further studies on Ca_v2.3 channel regulation by protein kinases, phosphatases, and possibly proteases.

A comparison of serotonin neuromodulation of mouse spinal V2a interneurons using perforated patch and whole cell recording techniques

Whole cell recordings (WCRs) are frequently used to study neuronal properties, but may be problematic when studying neuromodulatory responses, due to dialysis of the cell's cytoplasm. Perforated patch recordings (PPR) avoid cellular dialysis and might reveal additional modulatory effects that are lost during WCR. We have previously used WCR to characterize the responses of the V2a class of Chx10-expressing neurons to serotonin (5-HT) in the neonatal mouse spinal cord (Zhong et al., 2010). Here we directly compare multiple aspects of the responses to 5-HT using WCR and PPR in Chx10-eCFP neurons in spinal cord slices from 2 to 4 day old mice. Cellular properties recorded in PPR and WCR were similar, but high-quality PP recordings could be maintained for significantly longer. Both WCR and PPR cells could respond to 5-HT, and although neurons recorded by PPR showed a significantly greater response to 5-HT in some parameters, the absolute differences between PPR and WCR were small. We conclude that WCR is an acceptable recording method for short-term recordings of neuromodulatory effects, but the less invasive PPR is preferable for detailed analyses and is necessary for stable recordings lasting an hour or more.

Premotor synaptic plasticity limited to the critical period for song learning

Synaptic plasticity has been hypothesized to underlie learning and memory. Understanding of how such plasticity might produce motor learning is limited, in part because of the paucity of model systems with a tractable learned behavior under control of a discrete neural circuit. Songbirds possess both of these traits, thereby providing an excellent model for studying vertebrate motor learning. We report unique evidence of long-term depression (LTD) in the juvenile songbird premotor robust nucleus of the arcopallium (RA). LTD induction at RA recurrent collateral synapses requires NMDA receptors, postsynaptic depolarization, and postsynaptic calcium, and can be reversed by high-frequency stimulation. In adult birds, which have exited the critical period for sensorimotor learning and cannot modify their song, we were no longer able to induce LTD at RA collateral synapses. Furthermore, testosterone-induced premature maturation of song in juveniles abolishes LTD. LTD in nucleus RA therefore makes an excellent candidate mechanism to mediate song learning during development and is well-suited to provide insight into other forms of vertebrate motor learning.

Calcium-regulated anion channels in the plasma membrane of Lilium longiflorum pollen protoplasts

• Currents through anion channels in the plasma membrane of Lilium longiflorum pollen grain protoplasts were studied under conditions of symmetrical anionic concentrations by means of patch-clamp whole-cell configuration. • With Cl(-) -based intra- and extracellular solutions, three outward-rectifying anion conductances, I(Cl1) , I(Cl2) and I(Cl3) , were identified. These three activities were discriminated by differential rundown behaviour and sensitivity to 5-nitro-2-(phenylpropylamino)-benzoate (NPPB), which could not be attributed to one or more channel types. All shared strong outward rectification, activated instantaneously and displayed a slow time-dependent activation for positive potentials. All showed modulation by intracellular calcium ([Ca(2+) ](in) ), increasing intensity from 6.04 nM up to 0.5 mM (I(Cl1) ), or reaching a maximum value with 8.50 μM (I(Cl2) and I(Cl3) ). • After rundown, the anionic currents measured using NO(3) (-) -based solutions were indistinguishable, indicating that the permeabilities of the channels for Cl(-) and NO(3) (-) are similar. Additionally, unitary anionic currents were measured from outside-out excised patches, confirming the presence of individual anionic channels. • This study shows for the first time the presence of a large anionic conductance across the membrane of pollen protoplasts, resulting from the presence of Ca(2+) -regulated channels. A similar conductance was also found in germinated pollen. We hypothesize that these putative channels may be responsible for the large anionic fluxes previously detected by means of self-referencing vibrating probes.

Stimulation of Wild-Type, F508del- and G551D-CFTR Chloride Channels by Non-Toxic Modified pyrrolo[2,3-b]pyrazine Derivatives

Cystic fibrosis (CF) is a major inherited disorder involving abnormalities of fluid and electrolyte transport in a number of different organs due to abnormal function of cystic fibrosis transmembrane conductance regulator (CFTR) protein. We recently identified a family of CFTR activators, which contains the hit: RP107 [7-n-butyl-6-(4-hydroxyphenyl)[5H]-pyrrolo[2,3-b]pyrazine]. Here, we further evaluated the effect of the chemical modifications of the RP107-OH radical on CFTR activation. The replacement of the OH radical by a fluorine atom at position 2 (RP193) or 4 (RP185) significantly decreased the toxicity of the compounds without altering the ability to activate CFTR, especially for RP193. The non-toxic compound RP193 has no effect on cAMP production but stimulates the channel activity of wild-type CFTR in stably transfected CHO cells, in human bronchial epithelial NuLi-1 cells, and in primary culture of human bronchial epithelial cells (HBEC). Whole-cell and single patch-clamp recordings showed that RP193 induced a linear, time- and voltage-independent current, which was fully inhibited by two different and selective CFTR inhibitors (CFTRinh-172 and GP_inh5a). Moreover, RP193 stimulates CFTR in temperature-rescued CuFi-1 (F508del/F508del) HBEC and in CHO cells stably expressing G551D-CFTR. This study shows that it is feasible to reduce cytotoxicity of chemical compounds without affecting their potency to activate CFTR and to rescue the class 2 F508del-CFTR and class 3 G551D-CFTR CF mutant activities.

A single amino acid mutation attenuates rundown of voltage-gated calcium channels

The activity of voltage-gated calcium channels (VGCCs) decreases with time in whole-cell and inside-out patch-clamp recordings. In this study we found that substituting a single amino acid (I1520) at the intracellular end of IIIS6 in the alpha(1) subunit of P/Q-type Ca(2+) channels with histidine or aspartate greatly attenuated channel rundown in inside-out patch-clamp recordings. The homologous mutations also slowed rundown of N- and L-type Ca(2+) channels, albeit to a lesser degree. In P/Q-type channels, the attenuation of rundown is accompanied by an increased apparent affinity for phosphatidylinositol-4,5-bisphosphate, which has been shown to be critical for maintaining Ca(2+) channel activity [L. Wu, C.S. Bauer, X.-G. Zhen, C. Xie, J. Yang, Dual regulation of voltage-gated calcium channels by PtdIns(4,5)P2. Nature 419 (2002) 947-952]. Furthermore, the histidine mutation significantly stabilized the open state, making the channels easier to open, slower to close, harder to inactivate and faster to recover from inactivation. Our finding that mutation of a single amino acid can greatly attenuate rundown provides an easy and efficient way to slow the rundown of VGCCs, facilitating functional studies that require direct access to the cytoplasmic side of the channel.

Phosphatidylinositol-4,5-bisphosphate, PIP2, controls KCNQ1/KCNE1 voltage-gated potassium channels: a functional homology between voltage-gated and inward rectifier K+ channels

Phosphatidylinositol-4,5-bisphosphate (PIP(2)) is a major signaling molecule implicated in the regulation of various ion transporters and channels. Here we show that PIP(2) and intracellular MgATP control the activity of the KCNQ1/KCNE1 potassium channel complex. In excised patch-clamp recordings, the KCNQ1/KCNE1 current decreased spontaneously with time. This rundown was markedly slowed by cytosolic application of PIP(2) and fully prevented by application of PIP(2) plus MgATP. PIP(2)-dependent rundown was accompanied by acceleration in the current deactivation kinetics, whereas the MgATP-dependent rundown was not. Cytosolic application of PIP(2) slowed deactivation kinetics and also shifted the voltage dependency of the channel activation toward negative potentials. Complex changes in the current characteristics induced by membrane PIP(2) was fully restituted by a model originally elaborated for ATP-regulated two transmembrane-domain potassium channels. The model is consistent with stabilization by PIP(2) of KCNQ1/KCNE1 channels in the open state. Our data suggest a striking functional homology between a six transmembrane-domain voltage-gated channel and a two transmembrane-domain ATP-gated channel.

The Ca-activated Cl channel and its control in rat olfactory receptor neurons

Odorants activate sensory transduction in olfactory receptor neurons (ORNs) via a cAMP-signaling cascade, which results in the opening of nonselective, cyclic nucleotide-gated (CNG) channels. The consequent Ca2+ influx through CNG channels activates Cl channels, which serve to amplify the transduction signal. We investigate here some general properties of this Ca-activated Cl channel in rat, as well as its functional interplay with the CNG channel, by using inside-out membrane patches excised from ORN dendritic knobs/cilia. At physiological concentrations of external divalent cations, the maximally activated Cl current was approximately 30 times as large as the CNG current. The Cl channels on an excised patch could be activated by Ca2+ flux through the CNG channels opened by cAMP. The magnitude of the Cl current depended on the strength of Ca buffering in the bath solution, suggesting that the CNG and Cl channels were probably not organized as constituents of a local transducisome complex. Likewise, Cl channels and the Na/Ca exchanger, which extrudes Ca2+, appear to be spatially segregated. Based on the theory of buffered Ca2+ diffusion, we determined the Ca2+ diffusion coefficient and calculated that the CNG and Cl channel densities on the membrane were approximately 8 and 62 micro m-2, respectively. These densities, together with the Ca2+ diffusion coefficient, demonstrate that a given Cl channel is activated by Ca2+ originating from multiple CNG channels, thus allowing low-noise amplification of the olfactory receptor current.

PF4, a FMRFamide-related peptide, gates low-conductance Cl(-) channels in Ascaris suum

Here we describe the actions of the peptide Lys-Pro-Asn-Phe-Ile-Arg-Phe-NH(2), or PF4, on inside-out membrane patches (n=164), recorded from vesicles derived from Ascaris suum somatic muscle cells. We observed numerous, small-amplitude Cl(-) channels in the membrane patches. The conductance of the Cl(-) channels ranged from 1.09 to 7.07 pS, the open probability (P(open)) ranged from 0.047+/-0.015 (mean+/-S.E.M.) at 0 microM PF4 to 0.156+/-0.026 at 0.1 microM PF4. The channel mean open time was more variable and prolonged at negative potentials than when the membrane patch was clamped at positive potentials: at 0.03 microM PF4, the mean open time (+/-S.E.M) at -80 mV was 522+/-333 ms; at+80 mV, it was 25+/-7 ms. When patches were isolated from the parent vesicle, there were no changes in channel characteristics, suggesting that the channels function without the involvement of cytoplasmic components. Similarly, the channel characteristics were not affected by the G-protein inhibitor, guanosine-5'-O-(2-thiodiphosphate), indicating that the ion channels do not require a G-protein to function. These data indicate that the PF4-activated Cl(-) channels function independently of intracellular signal transducers and are, therefore, directly gated by PF4.

Role of intracellular and extracellular pH in the chemosensitive response of rat locus coeruleus neurones

The chemosensitive response of locus coeruleus (LC) neurones to changes in intracellular pH (pH(i)), extracellular pH (pH(o)) and molecular CO(2) were investigated using neonatal rat brainstem slices. A new technique was developed that involves the use of perforated patch recordings in combination with fluorescence imaging microscopy to simultaneously measure pH(i) and membrane potential (V(m)). Hypercapnic acidosis (15 % CO(2), pH(o) 6.8) resulted in a maintained fall in pH(i) of 0.31 pH units and a 93 % increase in the firing rate of LC neurones. On the other hand, isohydric hypercapnia (15 % CO(2), 77 mM HCO(3)(-), pH(o) 7.45) resulted in a smaller and transient fall in pH(i) of about 0.17 pH units and an increase in firing rate of 76 %. Acidified Hepes (N-2-hydroxyethylpiperazine-N'-2- ethanesulfonic acid)-buffered medium (pH(o) 6.8) resulted in a progressive fall in pH(i) of over 0.43 pH units and an increase in firing rate of 126 %. Isosmotic addition of 50 mM propionate to the standard HCO(3)(-)-buffered medium (5 % CO(2), 26 mM HCO(3)(-), pH(o) 7.45) resulted in a transient fall in pH(i) of 0.18 pH units but little increase in firing rate. Isocapnic acidosis (5 % CO(2), 7 mM HCO(3)(-), pH(o) 6.8) resulted in a slow intracellular acidification to a maximum fall of about 0.26 pH units and a 72 % increase in firing rate. For all treatments, the changes in pH(i) preceded or occurred simultaneously with the changes in firing rate and were considerably slower than the changes in pH(o). In conclusion, an increased firing rate of LC neurones in response to acid challenges was best correlated with the magnitude and the rate of fall in pH(i), indicating that a decrease in pH(i) is a major part of the intracellular signalling pathway that transduces an acid challenge into an increased firing rate in LC neurones.

Secretory modulation of basolateral membrane inwardly rectified K(+) channel in guinea pig distal colonic crypts

Cell-attached recordings revealed K(+) channel activity in basolateral membranes of guinea pig distal colonic crypts. Inwardly rectified currents were apparent with a pipette solution containing 140 mM K(+). Single-channel conductance (gamma) was 9 pS at the resting membrane potential. Another inward rectifier with gamma of 19 pS was observed occasionally. At a holding potential of -80 mV, gamma was 21 and 41 pS, respectively. Identity as K(+) channels was confirmed after patch excision by changing the bath ion composition. From reversal potentials, relative permeability of Na(+) over K(+) (P(Na)/P(K)) was 0.02 +/- 0.02, with P(Rb)/P(K) = 1.1 and P(Cl)/P(K) < 0.03. Spontaneous open probability (P(o)) of the 9-pS inward rectifier ((gp)K(ir)) was voltage independent in cell-attached patches. Both a low (P(o) = 0.09 +/- 0.01) and a moderate (P(o) = 0.41 +/- 0.01) activity mode were observed. Excision moved (gp)K(ir) to the medium activity mode; P(o) of (gp)K(ir) was independent of bath Ca(2+) activity and bath acidification. Addition of Cl(-) and K(+) secretagogues altered P(o) of (gp)K(ir). Forskolin or carbachol (10 microM) activated the small-conductance (gp)K(ir) in quiescent patches and increased P(o) in low-activity patches. K(+) secretagogues, either epinephrine (5 microM) or prostaglandin E(2) (100 nM), decreased P(o) of (gp)K(ir) in active patches. This (gp)K(ir) may be involved in electrogenic secretion of Cl(minus sign) and K(+) across the colonic epithelium, which requires a large basolateral membrane K(+) conductance during maximal Cl(-) secretion and, presumably, a lower K(+) conductance during primary electrogenic K(+) secretion.

Oxidative regulation of large conductance calcium-activated potassium channels

Reactive oxygen/nitrogen species are readily generated in vivo, playing roles in many physiological and pathological conditions, such as Alzheimer's disease and Parkinson's disease, by oxidatively modifying various proteins. Previous studies indicate that large conductance Ca(2+)-activated K(+) channels (BK(Ca) or Slo) are subject to redox regulation. However, conflicting results exist whether oxidation increases or decreases the channel activity. We used chloramine-T, which preferentially oxidizes methionine, to examine the functional consequences of methionine oxidation in the cloned human Slo (hSlo) channel expressed in mammalian cells. In the virtual absence of Ca(2+), the oxidant shifted the steady-state macroscopic conductance to a more negative direction and slowed deactivation. The results obtained suggest that oxidation enhances specific voltage-dependent opening transitions and slows the rate-limiting closing transition. Enhancement of the hSlo activity was partially reversed by the enzyme peptide methionine sulfoxide reductase, suggesting that the upregulation is mediated by methionine oxidation. In contrast, hydrogen peroxide and cysteine-specific reagents, DTNB, MTSEA, and PCMB, decreased the channel activity. Chloramine-T was much less effective when concurrently applied with the K(+) channel blocker TEA, which is consistent with the possibility that the target methionine lies within the channel pore. Regulation of the Slo channel by methionine oxidation may represent an important link between cellular electrical excitability and metabolism.

Association of cystic fibrosis transmembrane conductance regulator and protein phosphatase 2C

Cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels are rapidly deactivated by a membrane-bound phosphatase activity. The efficiency of this regulation suggests CFTR and protein phosphatases may be associated within a regulatory complex. In this paper we test that possibility using co-immunoprecipitation and cross-linking experiments. A monoclonal anti-CFTR antibody co-precipitated type 2C protein phosphatase (PP2C) from baby hamster kidney cells stably expressing CFTR but did not co-precipitate PP1, PP2A, or PP2B. Conversely, a polyclonal anti-PP2C antibody co-precipitated CFTR from baby hamster kidney membrane extracts. Exposing baby hamster kidney cell lysates to dithiobis (sulfosuccinimidyl propionate) caused the cross-linking of histidine-tagged CFTR (CFTR(His10)) and PP2C into high molecular weight complexes that were isolated by chromatography on Ni(2+)-nitrilotriacetic acid-agarose. Chemical cross-linking was specific for PP2C, because PP1, PP2A, and PP2B did not co-purify with CFTR(His10) after dithiobis (sulfosuccinimidyl propionate) exposure. These results suggest CFTR and PP2C exist in a stable complex that facilitates regulation of the channel.

Rundown of the hyperpolarization-activated KAT1 channel involves slowing of the opening transitions regulated by phosphorylation

Disappearance of the functional activity or rundown of ion channels upon patch excision in many cells involves a decrease in the number of channels available to open. A variety of cellular and biophysical mechanisms have been shown to be involved in the rundown of different ion channels. We examined the rundown process of the plant hyperpolarization-activated KAT1 K+ channel expressed in Xenopus oocytes. The decrease in the KAT1 channel activity on patch excision was accompanied by progressive slowing of the activation time course, and it was caused by a shift in the voltage dependence of the channel without any change in the single-channel amplitude. The single-channel analysis showed that patch excision alters only the transitions leading up to the burst states of the channel. Patch cramming or concurrent application of protein kinase A (PKA) and ATP restored the channel activity. In contrast, nonspecific alkaline phosphatase (ALP) accelerated the rundown time course. Low internal pH, which inhibits ALP activity, slowed the KAT1 rundown time course. The results show that the opening transitions of the KAT1 channel are enhanced not only by hyperpolarization but also by PKA-mediated phosphorylation.

ATP counteracts the rundown of gap junctional channels of rat ventricular myocytes by promoting protein phosphorylation

1. The degree of cell-to-cell coupling between ventricular myocytes of neonatal rats appeared well preserved when studied in the perforated version of the patch clamp technique or, in double whole-cell conditions, when ATP was present in the patch pipette solution. In contrast, when ATP was omitted, the amplitude of junctional current rapidly declined (rundown). 2. To examine the mechanism(s) of ATP action, an 'internal perfusion technique' was adapted to dual patch clamp conditions, and reintroduction of ATP partially reversed the rundown of junctional channels. 3. Cell-to-cell communication was not preserved by a non-hydrolysable ATP analogue (5'-adenylimidodiphosphate, AMP-PNP), indicating that the effect most probably did not involve direct interaction of ATP with the channel-forming proteins. 4. An ATP analogue supporting protein phosphorylation but not active transport processes (adenosine 5'-O-(3-thiotriphosphate), ATPgammaS) maintained normal intercellular communication, suggesting that the effect was due to kinase activity rather than to altered intracellular Ca2+. 5. A broad spectrum inhibitor of endogenous serine/threonine protein kinases (H7) reversibly reduced the intercellular coupling. A non-specific exogenous protein phosphatase (alkaline phosphatase) mimicked the effects of ATP deprivation. The non-specific inhibition of endogenous protein phosphatases resulted in the preservation of substantial cell-to-cell communication in ATP-free conditions. 6. The activity of gap junctional channels appears to require both the presence of ATP and protein kinase activity to counteract the tonic activity of endogenous phosphatase(s).

Divalent cations inhibit IsK/KvLQT1 channels in excised membrane patches of strial marginal cells

The IsK/KvLQT1 K+ channel in the apical membrane of strial marginal cells and vestibular dark cells is an essential ion transport pathway for the secretion of K+ into the endolymph of the inner ear. Study of this control point has been impeded by rundown of channel activity upon excision into commonly used cytosolic solutions. This paper describes conditions under which patches of apical membrane of strial marginal cells and vestibular dark cells from gerbil containing this channel can be excised, retaining its characteristic voltage dependence, kinetic properties, ion permeability sequence and pharmacological sensitivity, similar to those found during on-cell and perforated-patch whole cell recordings (Shen et al., Audit. Neurosci. 3 (1997) 215-230). Those excised-patch conditions include removal of Mg2+ from the cytosolic solution and use of a K+-rich pipette electrolyte. The inhibition of channel activity by Mg2+ was found to be a general feature of divalent cations; the channel was also inhibited by Ca2+, Ba2+ and Sr2+. The concentrations causing 50% inhibition of IsK/KvLQT1 channel current were 7 x 10(-5) M, 6 x 10(-6) M, 3 x 10(-4) M and 7 x 10(-5) M, respectively. It was also found that a chemical cross-linking agent, 3,3'-dithio-bis(sulfosuccinimidyl propionate) (DTSSP), which was previously shown to persistently activate IsK/KvLQTI channels expressed in Xenopus oocytes, maintained in excised patches channel activity which retained voltage dependence and pharmacological sensitivity. These data demonstrate that (1) the channel complex is inhibited by Ca2+, Mg2+ and other divalent cations, (2) the activation by Ca2+ observed previously in whole-cell preparations was due to action via other cellular pathways. These findings must be taken into account when considering the action of receptors which alter the cytosolic Ca2+ activity.

Partially active channels produced by PKA site mutation of the cloned renal K+ channel, ROMK2 (kir1.2)

The activity of the cloned renal K+ channel (ROMK2) is dependent on a balance between phosphorylation and dephosphorylation. There are only three protein kinase A (PKA) sites on ROMK2, with the phosphorylated residues being serine-25 (S25), serine-200 (S200), and serine-294 (S294) (Z.-C. Xu, Y. Yang, and S. C. Hebert. J. Biol. Chem. 271: 9313-9319, 1996). We previously mutated these sites from serine to alanine to study the contribution of each site to overall channel function. Here we have studied each of these single PKA site mutants using the single-channel configuration of the patch-clamp technique. Both COOH-terminal mutations at sites S200A and S294A showed a decreased open channel probability (Po), whereas the NH2-terminal mutation at site S25A showed no change in Po compared with wild-type ROMK2. The decrease in Po for the S200A and S294A mutants was caused by the additional presence of a long closed state. In contrast, the occurrence of the S25A channel was approximately 66% less, suggesting fewer active channels at the membrane. The S200A and S294A channels had different kinetics compared with wild-type ROMK2 channels, showing an increased occurrence of sublevels. Similar kinetics were observed when wild-type ROMK2 was excised and exposed to dephosphorylating conditions, indicating that these effects are specifically a property of the partially phosphorylated channel and not due to an unrelated effect of the mutation.