Regulation of neuronal ion channels via P2Y receptors

Within the last 15 years, at least 8 different G protein-coupled P2Y receptors have been characterized. These mediate slow metabotropic effects of nucleotides in neurons as well as non-neural cells, as opposed to the fast ionotropic effects which are mediated by P2X receptors. One class of effector systems regulated by various G protein-coupled receptors are voltage-gated and ligand-gated ion channels. This review summarizes the current knowledge about the modulation of such neuronal ion channels via P2Y receptors. The regulated proteins include voltage-gated Ca²⁺ and K⁺ channels, as well as N-methyl-d-aspartate, vanilloid, and P2X receptors, and the regulating entities include most of the known P2Y receptor subtypes. The functional consequences of the modulation of ion channels by nucleotides acting at pre- or postsynaptic P2Y receptors are changes in the strength of synaptic transmission. Accordingly, ATP and related nucleotides may act not only as fast transmitters (via P2X receptors) in the nervous system, but also as neuromodulators (via P2Y receptors). Hence, nucleotides are as universal transmitters as, for instance, acetylcholine, glutamate, or -aminobutyric acid.     

Membrane channels as integrators of G-protein-mediated signaling

A variety of extracellular stimuli regulate cellular responses via membrane receptors. A well-known group of seven-transmembrane domain-containing proteins referred to as G protein-coupled receptors, directly couple with the intracellular GTP-binding proteins (G proteins) across cell membranes and trigger various cellular responses by regulating the activity of several enzymes as well as ion channels. Many specific populations of ion channels are directly controlled by G proteins; however, indirect modulation of some channels by G protein-dependent phosphorylation events and lipid metabolism is also observed. G protein-mediated diverse modifications affect the ion channel activities and spatio-temporally regulate membrane potentials as well as of intracellular Ca^2 + concentrations in both excitatory and non-excitatory cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.     

Regulation of K+ channels in the basolateral membrane ofNecturus oxyntic cells

Summary Patch-clamp methods were used to study single-channel events in isolated oxyntic cells and gastric glands fromNecturus maculosa. Cell-attached, excised inside-out and outside-out patches from the basolateral membrane frequently contained channels which had conductances of 67±21 pS in 24% of the patches and channels of smaller conductance, 33±6 pS in 56% of the patches. Channels in both classes were highly selective for K⁺ over Na⁺ and Cl^–, and shared linear current-voltage relations. The 67-pS channel was activated by membrane depolarization, whereas the activity of the 33-pS channel was relatively voltage independent. The larger conductance channels were activated by intracellular Ca²⁺ in the range between 5 and 500nm, but unaffected by cAMP. The smaller conductance channels were activated by cAMP, but not Ca²⁺. The presence of K⁺ channels in the basolateral membrane which are regulated by these known second messengers can account for the increase in conductance and the hyperpolarization of the membrane observed upon secretagogue stimulation.     

Roles of G Proteins in Coupling of Receptors to Ionic Channels and Other Effector System

Abstract

Guanine nucleotide binding (G) proteins are heterotrimers that couple a wide range of receptors to ionic channels. The coupling may be indirect, via cytoplasmic agents, or direct, as has been shown for two K⁺ channels and two Ca²⁺ channels. One example of direct G protein gating is the atrial muscarinic K⁺ channel K⁺ [ACh], an inwardly rectifying K¹ channel with a slope conductance of 40 pS in symmetrical isotonic K⁺ solutions and a mean open lifetime of 1.4 ms at potentials between -40 and - 100 mV. Another is the clonal GH, muscarinic or somatostatin K⁺ channel, also inwardly rectifying but with a slope conductance of 55 pS. AG protein, G., purified from human red blood cells WC) activates K⁺[ACh] channels at subpicomolar concentrations; its a subunit is equi-potent. Except for being irreversible, their effects on gating precisely mimic physiological gating produced by muscarinic agonists. The α_k effects are general and are similar in atria from adult guinea pig, neonatal rat, and chick embryo. The hydrophilic βγ from transducin has no effect while hydropho-bic βγ from brain, hRBCs, or retina has effects at nanomolar concentrations which in our hands cannot be disSociated from detergent effects. An anti-α_k monoclonal antibody blocks muscarinic activation, supporting the concept that the physiological mediator is the a subunit not the βγ dimer. The techniques of molecular biology are now being used to specify G protein gating. A “bacterial” α_i-3 expressed in Escherichia coli using a pT7 expression system mimics the gating produced by hRBC α_k.     

G Proteins Modulate D2 Receptor-Coupled K(ATP) Channels in Rat Dopaminergic Terminals

The presynaptic dopamine (DA) D2 receptor-mediated regulation of ATP-sensitive potassium (K⁺ _ATP) channels was examined in slices of the rat caudate-putamen. When slices were incubated with the specific D2 receptor antagonist (–)-sulpiride (SLP), a concentration-dependent increase of extracellular DA release was observed. SLP-induced enhancement was completely antagonized by coincubation with the K⁺ _ATP channel opener diazoxide (DIA). Treatment of slices with the D2 receptor agonist quinpirole (QUI) almost completely inhibited DA outflow induced by the K⁺ _ATP channel blocker butanedione-monoxime (BDM). Coincubation of SLP and guanosine triphosphate (GTP) or its non-hydrolizable analogue guanylyl-5-imidodiphosphate [Gpp(NH)p], significantly reduced the SLP-induced effect on DA levels. Furthermore, we observed that BDM-induced DA outflow was markedly inhibited by G protein activators suggesting an additional receptor-independent regulation of K⁺ _ATP channel gating. Our results suggest that PTX-sensitive G proteins are involved in the signal transduction between D2 receptors and K⁺ _ATP channels. Furthermore, K⁺ _ATP channels can be modulated in a receptor-independent mechanism by G protein activators.     

Presynaptic modulation by eicosanoids in cortical synaptosomes

In continuing experiments to determine the ionic basis of inhibitory presynaptic modulation, rat cortical synaptosomes were employed and receptor-activated K⁺ efflux was determined with a K⁺ sensitive electrode. When synaptosomes were sub-optimally depolarized by veratridine, the addition of agents that activated purinergic, ₂, muscarinic and opioid receptors all promoted K⁺ efflux. With 2-chloroadenosine as a model inhibitory presynaptic modulator, the increased K⁺ efflux evoked by this agent was blocked by the cyclooxygenase inhibitor indomethacin suggesting that arachidonic acid or its metabolites was an intermediary in opening the channel. When arachidonic acid and PGE₂ were tested, both promoted K⁺ efflux that was inhibited by dendrotoxin and mast cell degranulating peptide, two agents that are known to inhibit a delayed rectifier K⁺ current. Our results suggest that via eicosanoid second messengers, inhibitory presynaptic modulators open a sub-class of K channels that hyperpolarize nerve terminals, therefore less Ca²⁺ would enter per nerve impulse and thus the evoked release of neurotransmitters would be decreased.Abbreviations DTX dendrotoxin - MCDP mast cell degranulating peptide - NHGA norhydroguairetic acid - PGE₂ prostaglandin E₂     

Satellite glial cells In Situ within mammalian prevertebral ganglia express K+ channels active at rest potential

Patch-clamp experiments were performed on satellite glial cells wrapped around sympathetic neurons in the rabbit coeliac ganglion. With the cleaning method used, the glial cells could be kept in place and were directly accessible to the patch-clamp pipettes. Whole-cell recordings showed that glial cells had almost ohmic properties. Their resting potential (–79.1±1.2 mV) was found to be very nearly the same as the K⁺ reversal potential and 20 mV more negative than that of the neurons they encapsulated. Unitary currents from ionic channels present in the glial membrane were recorded in the cell-attached configuration with pipettes filled with various amounts of K⁺, Na⁺ and gluconate. Only K⁺-selective channels with slight inwardly rectifying properties (in the presence of 150 mM [K⁺]₀) were detected. These channels were active (P ₀=0.7–0.8) at the cell resting potential. The channel conductance, but not its opening probability, was dependent on the [K⁺] in the pipette. Cl^–-selective channels (outwardly rectifying and large conductance channels) were detected in excised patches.The properties of the K⁺ channels (increased inward current with [K⁺] and detectable outward current at low [K⁺]) are well suited for siphoning the K⁺ released by active neurons.     

Channel-mediated K+ flux in barley aleurone protoplasts

Gibberellic acid (GA₃) stimulates K⁺ efflux from the barley (Hordeum vulgare L. cv. Himalaya) aleurone. We investigated the mechanism of K⁺ flux across the plasma membrane of aleurone protoplasts using patch-clamp techniques. Potassium-ion currents, measured over the entire surface of the protoplast plasma membrane, were induced when the electrochemical gradient for K⁺ was inward (into the cytoplasm). The magnitude and voltage-dependence of this inward current were the same in protoplasts treated with GA₃ and in control protoplasts (no GA₃). Inward currents activated by negative shifts in the membrane potential (E_M) from the Nernst potential for K⁺ (E_K) showed membrane conductance to be a function of the electrochemical gradient (i.e. E_M-E_K). Single-channel influx currents of K⁺ were recorded in small patches of the plasma membrane. These channels had a single-channel conductance of 5–10 pS with 100 mM K⁺ on the inside and 10 mM K⁺ on the outside of the plasma membrane. Single-channel currents, like whole-cell currents, were the same in protoplasts treated with GA₃ and control protoplasts. Voltage-gated efflux currents were found only in protoplasts tha thad been incubated without GA₃. We conclude that K⁺ influx in the aleurone is mediated by channels and these membrane proteins are not greatly effected by GA₃.Abbreviations and symbols F_K Nernst potential for K⁺ - E_M membrane potential - E_rev reversal potential - GA₃ gibberellic acid - K_i concentration of K⁺ inside the cell - K_o concentration of K⁺ outside the cell - R gas constant - S conductance (siemens) - T temperature (^oK) - _i ionic activity coefficient for internal (cytoplasmic) solution - _o ionic activity coefficient for external medium     

Patch clamp analysis of chemically activated and modulated ionic channels in isolated mammalian cardiomyocytes

Techniques routinely utilized in this laboratory for recording currents through single ionic channels of isolated atrial and ventricular rat cardiomyocytes are described. Emphasis is placed in two main areas: first, on methods for obtaining a sufficient yield of Ca⁺⁺-tolerant myocytes suitable for patch clamp experiments, and secondly, on methods for analyzing the temporal characteristics of patched ionic channels. These methods were used on acetylcholine activated K⁺ channels in isolated atrial myocytes and on an inwardly-rectifying K⁺ channel in ventricular myocytes. The latter is an example of a hormonally modulated K⁺ channel, since its activity could be substantially increased by norepinephrine. Analysis of the closed and open time distributions suggested that one of the closed states of this channel is markedly abbreviated by norepinephrine, whereas the open state is nearly unaffected. Norepinephrine was effective when channel activity was recorded from on-cell patches and the hormone was added to the solution bathing the cell membrane outside of the patched area. This indicates that a second messenger substance is probably mediating the action of norepinephrine.     

The gating of nucleotide-sensitive K+ channels in insulin-secreting cells can be modulated by changes in the ratio ATP4−/ADP3− and by nonhydrolyzable derivatives of both ATP and ADP

Summary The³¹P-NMR technique has been used to assess the intracellular ratios and concentrations of mobile ATP and ADP and the intracellular pH in an insulin-secreting cell line, RINm5F. The single-channel current-recording technique has been used to investigate the effects of changes in the concentrations of ATP and ADP on the gating of nucleotide-dependent K⁺ channels. Adding ATP to the membrane inside closes these channels. However, in the continued presence of ATP adding ADP invariably leads to the reactivation of ATP-inhibited K⁺ channels, even at ATP^4–/ADP^3– concentration ratios greater than 71. Interactions between ATP^4– and ADP^3– seem competitive. An increase in the concentration ratio ATP^4–/ADP^3– consistently evoked a decrease in the open-state probability of K⁺ channels; conversely a decrease in ATP^4–/ADP^3– increased the frequency of K⁺ channel opening events. Channel gating was also influenced by changes in the absolute concentrations of ATP^4– and ADP^3–, at constant free concentration ratios. ADP-evoked stimulation of ATP-inhibited channels did not result from phosphorylation of the channel, as ADP--S, a nonhydrolyzable analog of ADP, not only stimulated but enhanced ADP-induced activation of K⁺ channels, in the presence of ATP. Similarly, ADP was able to activate K⁺ channels in the presence of two nonhydrolyzable derivatives of ATP, AMP-PNP and methylene ATP.     

ATP-sensitive inward rectifier and voltage- and calcium-activated K+ channels in cultured pancreatic islet cells

Summary K⁺ channels in cultured rat pancreatic islet cells have been studied using patch-clamp single-channel recording techniques in cell-attached and excised inside-out and outside-out membrane patches. Three different K⁺-selective channels have been found. Two inward rectifier K⁺ channels with slope conductances of about 4 and 17 pS recorded under quasi-physiological cation gradients (Na⁺ outside, K⁺ inside) and maximal conductances recorded in symmetrical K⁺-rich solutions of about 30 and 75 pS, respectively. A voltage- and calcium-activated K^– channel was recorded with a slope conductance of about 90 pS under the same conditions and a maximal conductance recorded in symmetrical K⁺-rich solutions of about 250 pS. Single-channel current recording in the cell-attached conformation revealed a continuous low level of activity in an apparently small number of both the inward rectifier K⁺ channels. But when membrane patches were excised from the intact cell a much larger number of inward rectifier K⁺ channels became transiently activated before showing an irreversible decline. In excised patches opening and closing of both the inward rectifier K⁺ channels were unaffected by voltage, internal Ca²⁺ or externally applied tetraethyl-ammonium (TEA) but the probability of opening of both inward rectifier K⁺ channels was reduced by internally applied 1–5mm adenosine-5-triphosphate (ATP). The large K⁺ channel was not operational in cell-attached membrane patches, but in excised patches it could be activated at negative membrane potentials by 10^–7 to 10^–6 m internal Ca²⁺ and blocked by 5–10mm external TEA.     

Pharmacology of K+ channels in the plasmalemma of the green algaChara corallina

Summary The outer membranes of plant cells contain channels which are highly selective for K⁺. However, many of their properties and their similarities to K⁺ channels found in animal cells had not previously been established. The channels open when the cells are depolarized in solutions with a high K⁺/Ca²⁺ ratio. In this work, the pharmacology of a previously identified plant K⁺ channel was examined. This survey showed that the channels have many properties which are similar to those of high-conductance Ca²⁺-activated K⁺ channels (highG K⁺(Ca²⁺)). K⁺ currents inChara were reduced by TEA⁺, Na⁺, Cs⁺, Ba²⁺, decamethonium and quinine, all inhibitors of, among other things, highG K⁺(Ca²⁺) channels. Tetracaine also inhibited K⁺ currentsChara, but its effect on most types of K⁺ channels in animal tissues is unknown. The currents were not inhibited by 4-aminopyridine (4AP), caffeine, tolbutamide, dendrotoxin, apamin or tubocurarine, which do not inhibit highG K⁺(Ca²⁺) channels, but affect other classes of K⁺ channels. The channels were locked open by 4AP, in a remarkably similar manner to that reported for K⁺(Ca²⁺) channels of a molluscan neuron. No evidence for the role of the inositol cycle in channel behavior was found, but its role in K⁺ channel control in animal cells is obscure. Potassium conductance was slightly decreased upon reduction of cytoplasmic ATP levels by cyanide + salicylhydroxamic acid (SHAM), consistent with channel control by phosphorylation. The anomalously strong voltage dependence of blockade by some ions (e.g. Cs⁺) is consistent with the channels being multiion pores. However, the channels also demonstrate some differences from the highG K⁺(Ca²⁺) channels found in animal tissues. The venom of the scorption,Leiurus quinquestriatus (LQV), and a protein component, charybdotoxin (CTX), an apparently specific inhibitor of highG K⁺(Ca²⁺) channels in various animal tissues, had no effect on the K⁺ channels in theChara plasmalemma. Als,, pinacidil, an antihypertensive drug which may increase highG K⁺(Ca²⁺) channel activity had no effect on the channels inChara. Although the described properties of theChara K⁺ channels are most similar to those of high conductance K⁺(Ca²⁺) in animal cells, the effects of CTX and pinacidil are notably different; the channels are clearly of a different structure to those found in animal cells, but are possibly related.     

Oscillating activity of a calcium-activated K+ channel in normal and cancerous mammary cells in culture

Summary Calcium-activated potassium channels were the channels most frequently observed in primary cultured normal mammary cell and in the established mammary tumor cell, MMT060562. In both cells, single-channel and whole-cell clamp recordings sometimes showed slow oscillations of the Ca²⁺-gated K⁺ current. The characteristics of the Ca²⁺-activated K⁺ channels in normal and cancerous mammary cells were quite similar. The slope conductances changed from 8 to 70 pS depending on the mode of recording and the ionic composition in the patch electrode. The open probability of this channel increased between 0.1 to 1 m of the intracellular Ca²⁺, but it was independent of the membrane potential.Charybdotoxin reduced the activity of the Ca²⁺-activated K⁺ channel and the oscillation of the membrane current, but apamin had no apparent effect. The application of tetraethylammonium (TEA) from outside and BaCl₂ from inside of the cell diminished the activity of the channel. The properties of this channel were different from those of both the large conductance (BK or MAXI K) and small conductance (SK) type Ca²⁺-activated K⁺ channels.     

Ion channels in the membrane ofChara inflata

Summary Voltage-clamped steps in the electric potential difference (PD) across the membrane in cells of the green alga,Chara inflata, cause voltage- and time-dependent current flows, interpreted to arise from opening and closing of various types of ion channel in the membrane. With cells in the light, these channels are normally closed, and the resting PD is probably determined by the operation of an H⁺ efflux pump. Positive steps in PD from the resting level often caused the opening of K⁺ channels with sigmoid kinetics. The channels began to show opening when the PD–120 mV for an external concentration of K⁺ of 1.0mm. Return of the PD to the resting level caused closing of the channels with complex kinetics. Various treatments of the cell could cause these K⁺ channels to open, and remain open continuously, with the PD then lying closer to the Nernst PD for K⁺. The K⁺ channels have been identified by the blocking effects of TEA⁺. Another group of channels, probably Cl^– and Ca²⁺ associated with the action potential open when the PD is stepped to values less negative than –50 mV. Negative steps from the resting PD cause the slow opening, with a time course of seconds, of yet another type of channel, probably Cl^–.     

Metabolic control of the K+ channel of human red cells

Summary The effects of cAMP, ATP and GTP on the Ca²⁺-dependent K⁺ channel of fresh (1–2 days) or cold-stored (28–36 days) human red cells were studied using atomic absorption flame photometry of Ca²⁺-EGTA loaded ghosts which had been resealed to monovalent cations in dextran solutions. When high-K⁺ ghosts were incubated in an isotonic Na⁺ medium, the rate constant of Ca²⁺-dependent K⁺ efflux was reduced by a half on increasing the theophylline concentration to 40mm. This effect was observed in ghosts from both fresh and stored cells, but only if they were previously loaded with ATP. The inhibition was more marked when Mg²⁺ was added together with ATP, and it was abolished by raising free Ca²⁺ to the micromolar level. Like theophylline, isobutyl methylxanthine (10mm) also affected K⁺ efflux. cAMP (0.2–0.5mm), added both internally and externally (as free salt, dibutyryl or bromide derivatives), had no significant effect on K⁺ loss when the ghost free-Ca²⁺ level was below 1 m, but it was slightly inhibitory at higher concentrations. The combined presence of cAMP (0.2mm) plus either theophylline (10mm), or isobutyl methylxanthine (0.5mm), was more effective than cAMP alone. This inhibition showed a strict requirement for ATP plus Mg²⁺ and it, was not overcome by raising internal Ca²⁺. Ghosts from stored cells seemed more sensitive than those from fresh cells, to the combined action of cAMP and methylxanthines. Loading ATP into ghosts from fresh or stored cells markedly decreased K⁺ loss. Although this effect was observed in the absence of added Mg²⁺ (0.5mm EDTA present), it was potentiated upon adding 2mm Mg²⁺. The K⁺ efflux from ATP-loaded ghosts was not altered by dithio-bis-nitrobenzoic acid (10mm) or acridine orange (100 m), while it was increased two-to fourfold by incubating with MgF₂ (10mm), or MgF₂ (10mm)+theophylline (40mm), respectively. By contrast, a marked efflux reduction was obtained by incorporating 0.5mm GTP into ATP-containing ghosts. The degree of phosphorylation obtained by incubating membranes with (-³²P)ATP under various conditions affecting K⁺ channel activity, was in direct correspondence to their effect on K⁺ efflux. The results suggest that the K⁺ channel of red cells is under complex metabolic control, via cAMP-mediated and nonmediated mechanisms, some which require ATP and presumably, involve phosphorylation of the channel proteins.     

Beta-Adrenergic modulation of K+ current in human T lymphocytes

Summary The whole-cell voltage-clamp technique was employed to study the -adrenergic modulation of voltage-gated K⁺ currents in CD8⁺ human peripheral blood lymphocytes. The -receptor agonist, isoproterenol, decreased the peak current amplitude and increased the rate of inactivation of the delayed rectifier K⁺ current. In addition, isoproterenol decreased the voltage dependence of steady-state inactivation and shifted the steady-state inactivation curve to the left. Isoproterenol, on the other hand, had no significant effect on the steady-state parameters of current activation. The isoproterenol-induced decrease in peak current amplitude was inhibited by the -blocker propranolol. Bath application of dibutyryl cAMP (1mm) mimicked the effects of isoproterenol on both K⁺ current amplitude and time course of inactivation. Furthermore, the reduction in the peak current amplitude in response to isoproterenol was attenuated when PKI_5–24 (2–5 m), a synthetic peptide inhibitor of cAMP-dependent protein kinase, was present in the pipette solution. The increase in the rate of inactivation of the K⁺ currents in response to isoproterenol was mimicked by the internal application of GTP--S (300 m) and by exposure of the cell to cholera toxin (1 g/ml), suggesting the involvement of a G protein. These results demonstrate that the voltage-dependent K⁺ conductance in T lymphocytes can be modulated by -adrenergic stimulation. The effects of -agonists, i.e., isoproterenol, appear to be receptor mediated and could involve cAMP-dependent protein kinase as well as G proteins. Since inhibition of the delayed rectifier K⁺ current has been found to decrease the proliferative response in T lymphocytes, the -adrenergic modulation of K⁺ current may well serve as a feedback control mechanism limiting the extent of cellular proliferation.     

Glutathione-gated K+ channels of Escherichia coli carry out K+ efflux controlled by the redox state of the cell

The kinetics of K⁺ efflux across the membranes of i) wild-type Escherichia coli poisoned by the thiol reagent N-ethylmaleimide, ii) K⁺ retention mutants and iii) glutathione-deficient mutants, have revealed a common K⁺ leaky phenotype; it is characterized by a very high rate of K⁺ efflux. The results suggest that the products of kefB and kefC genes could encode two K⁺ channels, both gated by glutathione. The possible function of these K⁺ channels seems to be a K⁺ exit controlled by the redox state of the cell; indeed, it can be inferred from the effects of several oxidants and reductants that turning on and off of the K⁺ efflux mediated by the channels can be correlated with the redox state of glutathione.     

K+ Channel Expression in Human Breast Cancer Cells: Involvement in Cell Cycle Regulation and Carcinogenesis

K⁺ channels are a most diverse class of ion channels in the plasma membrane and are distributed widely throughout a variety of cells including cancer cells. Evidence has been accumulating from fundamental studies indicating that tumour cells possess various types of K⁺ channels and that these K⁺ channels play important roles in regulating tumor cell proliferation, cell cycle progression and apoptosis. Moreover, a significant increase in K⁺ channel expression has been correlated with tumorigenesis, suggesting the possibility of using these proteins as transformation markers and perhaps reducing the tumor growth rate by selectively inhibiting their functional activity. Significant progress has been made in defining the properties of breast K⁺ channels, including their biophysical and pharmacological properties and distribution throughout different phases of the cell cycle in breast cell line MCF-7. This review aims to provide a comprehensive overview of the current state of research into K⁺ channels/currents in breast cancer cells. The possible mechanisms by which K⁺ channels affect tumor cell proliferation and cell cycle progression are discussed.     

Ca-dependent K channels in exocrine salivary glands

In the last 15 years, remarkable progress has been realized in identifying the genes that encode the ion-transporting proteins involved in exocrine gland function, including salivary glands. Among these proteins, Ca²⁺-dependent K⁺ channels take part in key functions including membrane potential regulation, fluid movement and K⁺ secretion in exocrine glands. Two K⁺ channels have been identified in exocrine salivary glands: (1) a Ca²⁺-activated K⁺ channel of intermediate single channel conductance encoded by the KCNN4 gene, and (2) a voltage- and Ca²⁺-dependent K⁺ channel of large single channel conductance encoded by the KCNMA1 gene. This review focuses on the physiological roles of Ca²⁺-dependent K⁺ channels in exocrine salivary glands. We also discuss interesting recent findings on the regulation of Ca²⁺-dependent K⁺ channels by protein–protein interactions that may significantly impact exocrine gland physiology.     

Effects of ADP upon the ATP-sensitive K+ channel in rat ventricular myocytes

Summary The effects of ADP upon the gating of ATP-sensitive K⁺ channels from rat ventricular myocytes have been investigated by patch-clamp single-channel current recording experiments. ADP was applied to the internal surface of excised insideout membrane patches and depending upon the experimental protocol and the concentration it was found that ADP could either inhibit or stimulate openings of ATP-sensitive K⁺ channels. In the absence of inactivation, ATP-sensitive K⁺ channels were inhibited by ADP in a dose-dependent manner. Partially inactivated channels, on the other hand, were stimulated by low (10 to 250 M) and inhibited by high (>250 M) concentrations of ADP. ATP-sensitive K⁺ channels which were being inhibited by ATP (<1 mM) could be opened by the simultaneous application of ADP (50 M to 1 mM). ADP had no effect upon channels inhibited by mM concentrations of ATP. The situation was further complicated when it was found that inhibition evoked by ADP was strongly attenuated by the presence of Mg²⁺ ions whilst channel stimulation, whether of partially inactivated channels or channels inhibited by ATP, required the presence of Mg²⁺ ions. The analog of ADP, ADPS, always evoked inhibition of ATP-sensitive K⁺ channels which was not affected by the presence or absence of Mg²⁺ ions.