PIP2 and PIP as determinants for ATP inhibition of KATP channels

A region of the sulfonylurea receptor critical for a modulation of ATP-sensitive K(+) channels by G-protein betagamma-subunits

To determine the interaction site(s) of ATP-sensitive K(+) (K(ATP)) channels for G-proteins, sulfonylurea receptor (SUR2A or SUR1) and pore-forming (Kir6.2) subunits were reconstituted in the mammalian cell line, COS-7. Intracellular application of the G-protein betagamma2-subunits (G(betagamma)(2)) caused a reduction of ATP-induced inhibition of Kir6.2/SUR channel activities by lessening the ATP sensitivity of the channels. G(betagamma)(2) bound in vitro to both intracellular (loop-NBD) and C-terminal segments of SUR2A, each containing a nucleotide-binding domain (NBD). Furthermore, a single amino acid substitution in the loop-NBD of SUR (Arg656Ala in SUR2A or Arg665Ala in SUR1) abolished the G(betagamma)(2)-dependent alteration of the channel activities. These findings provide evidence that G(betagamma) modulates K(ATP) channels through a direct interaction with the loop-NBD of SUR.

K(ATP) channels: linker between phospholipid metabolism and excitability

ATP-sensitive potassium (K(ATP)) channels couple electrical activity to cellular metabolism via their inhibition by intracellular ATP. When examined in excised patches, ATP concentrations required for half-maximal inhibition (IC(50)) varied among tissues and were reported to be as low as 10 microM. This set up a puzzling question on how activation of K(ATP) channels can occur under physiological conditions, where the cytoplasmic concentration of ATP is much higher than that required for channel inhibition. A new twist was added to this puzzle when two recent reports showed that phospholipids such as phosphatidylinositol-4,5-bisphosphate (PIP(2)) and phosphatidyl-4-phosphate (PIP) are able to shift ATP-sensitivity of K(ATP) channels from the micro- into the millimolar range and thus provide a mechanism for physiological activation of the channels. This commentary describes how phospholipids control ATP inhibition of K(ATP) channels and how this mechanism is regulated effectively by receptor-mediated stimulation of phospholipase C.

Regulation of cloned ATP-sensitive K channels by phosphorylation, MgADP, and phosphatidylinositol bisphosphate (PIP(2)): a study of channel rundown and reactivation

Kir6.2 channels linked to the green fluorescent protein (GFP) (Kir6. 2-GFP) have been expressed alone or with the sulfonylurea receptor SUR1 in HEK293 cells to study the regulation of K(ATP) channels by adenine nucleotides, phosphatidylinositol bisphosphate (PIP(2)), and phosphorylation. Upon excision of inside-out patches into a Ca(2+)- and MgATP-free solution, the activity of Kir6.2-GFP+SUR1 channels spontaneously ran down, first quickly within a minute, and then more slowly over tens of minutes. In contrast, under the same conditions, the activity of Kir6.2-GFP alone exhibited only slow rundown. Thus, fast rundown is specific to Kir6.2-GFP+SUR1 and involves SUR1, while slow rundown is a property of both Kir6.2-GFP and Kir6.2-GFP+SUR1 channels and is due, at least in part, to Kir6.2 alone. Kir6. 2-GFP+SUR1 fast phase of rundown was of variable amplitude and led to increased ATP sensitivity. Excising patches into a solution containing MgADP prevented this phenomenon, suggesting that fast rundown involves loss of MgADP-dependent stimulation conferred by SUR1. With both Kir6.2-GFP and Kir6.2-GFP+SUR1, the slow phase of rundown led to further increase in ATP sensitivity. Ca(2+) accelerated this process, suggesting a role for PIP(2) hydrolysis mediated by a Ca(2+)-dependent phospholipase C. PIP(2) could reactivate channel activity after a brief exposure to Ca(2+), but not after prolonged exposure. However, in both cases, PIP(2) reversed the increase in ATP sensitivity, indicating that PIP(2) lowers the ATP sensitivity by increasing P(o) as well as by decreasing the channel affinity for ATP. With Kir6.2-GFP+SUR1, slow rundown also caused loss of MgADP stimulation and sulfonylurea inhibition, suggesting functional uncoupling of SUR1 from Kir6.2-GFP. Ca(2+) facilitated the loss of sensitivity to MgADP, and thus uncoupling of the two subunits. The nonselective protein kinase inhibitor H-7 and the selective PKC inhibitor peptide 19-36 evoked, within 5-15 min, increased ATP sensitivity and loss of reactivation by PIP(2) and MgADP. Phosphorylation of Kir6.2 may thus be required for the channel to remain PIP(2) responsive, while phosphorylation of Kir6.2 and/or SUR1 is required for functional coupling. In summary, short-term regulation of Kir6.2+SUR1 channels involves MgADP, while long-term regulation requires PIP(2) and phosphorylation.

Lipid metabolism as a target for potassium channel effectors

K(+) channel effectors are widely used in the treatment of various diseases, including diabetes mellitus type II, hypertension, and cardiac arrhythmia. In addition, a constantly growing body of literature reveals that some of these substances, despite their direct effect on K(+) channels, may influence cellular lipid metabolism. As a result, membrane lipid content and cellular concentrations of lipid messengers are changed. Due to the dependence of K(+) channel activity on membrane lipids, these observations seem to be of particular importance not only to characterize secondary effects of K(+) channel effectors but also to understand the long-term effects of these agents on K(+) channel activity.

Leptin activation of ATP-sensitive K+ (KATP) channels in rat CRI-G1 insulinoma cells involves disruption of the actin cytoskeleton

1. The role of the cytoskeleton in leptin-induced activation of ATP-sensitive K+ (KATP) channels was examined in rat CRI-G1 insulin-secreting cells using patch clamp and fluorescence imaging techniques. 2. In whole cell recordings, dialysis with the actin filament stabiliser phalloidin (10 microM) prevented KATP channel activation by leptin. 3. Application of the actin filament destabilising agents deoxyribonuclease type 1 (DNase 1; 50 microg ml-1) or cytochalasin B (10 microM) to intact cells or inside-out membrane patches also increased KATP channel activity in a phalloidin-dependent manner. 4. The anti-microtubule agents nocodazole (10 microM) and colchicine (100 microM) had no effect on KATP channel activity. 5. Fluorescence staining of the cells with rhodamine-conjugated phalloidin revealed rapid disassembly of actin filaments by cytochalasin B and leptin, the latter action being prevented by the phosphoinositide 3 (PI 3)-kinase inhibitor LY 294002. 6. Activation of KATP channels by the PI 3-kinase product phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) was also prevented by phalloidin. This is consistent with the notion that leptin activates KATP channels in these cells by an increase in PtdIns(3,4,5)P3 or a similar 3-phosphorylated phosphoinositol lipid, resulting in actin filament disruption.

Molecular basis of protein kinase C-induced activation of ATP-sensitive potassium channels

Potassium channels that are inhibited by internal ATP (K(ATP) channels) provide a critical link between metabolism and cellular excitability. Protein kinase C (PKC) acts on K(ATP) channels to regulate diverse cellular processes, including cardioprotection by ischemic preconditioning and pancreatic insulin secretion. PKC action decreases the Hill coefficient of ATP binding to cardiac K(ATP) channels, thereby increasing their open probability at physiological ATP concentrations. We show that PKC similarly regulates recombinant channels from both the pancreas and heart. Surprisingly, PKC acts via phosphorylation of a specific, conserved threonine residue (T180) in the pore-forming subunit (Kir6.2). Additional PKC consensus sites exist on both Kir and the larger sulfonylurea receptor (SUR) subunits. Nonetheless, T180 controls changes in open probability induced by direct PKC action either in the absence of, or in complex with, the accessory SUR1 (pancreatic) or SUR2A (cardiac) subunits. The high degree of conservation of this site among different K(ATP) channel isoforms suggests that this pathway may have wide significance for the physiological regulation of K(ATP) channels in various tissues and organelles.

The I182 region of k(ir)6.2 is closely associated with ligand binding in K(ATP) channel inhibition by ATP

The ATP-inhibited potassium (K(ATP)) channel is assembled from four inward rectifier potassium (K(ir)6.x) subunits and four sulfonylurea receptor (SURx) subunits. The inhibitory action of ATP is mediated by at least two distinct functional domains within the C-terminal cytoplasmic tail of K(ir)6.2. The G334D mutation of K(ir)6.2 virtually eliminates ATP-dependent gating with no effect on ligand-independent gating, suggesting a role in linkage of the site to the gate or in the ATP binding site, itself. The T171A mutation of K(ir)6.2 strongly disrupts both ATP-dependent and ligand-independent gating, suggesting a role for T171 in the gating step. A neighboring mutation, I182Q, virtually eliminates ATP inhibition, but its effect on ligand-independent gating remained unknown. We have now characterized both the K(i) values for inhibition by ATP and the ligand-independent gating kinetics of 15 substitutions at position 182. All substitutions decreased ATP-dependent inhibition gating as measured by the K(i), many profoundly so, yet had little or no effect on ligand-independent gating kinetics. Thus, substitutions at position 182 are unlikely to act by disrupting inhibition gate movement. Our results indicate an indispensable role for I182 in a step of the ATP binding mechanism, the linkage mechanism coupling the ATP binding site to the inhibition gate, or both.

Receptor-mediated hydrolysis of plasma membrane messenger PIP2 leads to K+-current desensitization

Phosphatidylinositol bisphosphate (PIP2) directly regulates functions as diverse as the organization of the cytoskeleton, vesicular transport and ion channel activity. It is not known, however, whether dynamic changes in PIP2 levels have a regulatory role of physiological importance in such functions. Here, we show in both native cardiac cells and heterologous expression systems that receptor-regulated PIP2 hydrolysis results in desensitization of a GTP-binding protein-stimulated potassium current. Two receptor-regulated pathways in the plasma membrane cross-talk at the level of these channels to modulate potassium currents. One pathway signals through the betagamma subunits of G proteins, which bind directly to the channel. Gbetagamma subunits stabilize interactions with PIP2 and lead to persistent channel activation. The second pathway activates phospholipase C (PLC) which hydrolyses PIP2 and limits Gbetagamma-stimulated activity. Our results provide evidence that PIP2 itself is a receptor-regulated second messenger, downregulation of which accounts for a new form of desensitization.

Extracellular ATP inhibits the small-conductance K channel on the apical membrane of the cortical collecting duct from mouse kidney

We have used the patch-clamp technique to study the effects of changing extracellular ATP concentration on the activity of the small-conductance potassium channel (SK) on the apical membrane of the mouse cortical collecting duct. In cell-attached patches, the channel conductance and kinetics were similar to its rat homologue. Addition of ATP to the bathing solution of split-open single cortical collecting ducts inhibited SK activity. The inhibition of the channel by ATP was reversible, concentration dependent (K(i) = 64 microM), and could be completely prevented by pretreatment with suramin, a specific purinergic receptor (P(2)) blocker. Ranking of the inhibitory potency of several nucleotides showed strong inhibition by ATP, UTP, and ATP-gamma-S, whereas alpha, beta-Me ATP, and 2-Mes ATP failed to affect channel activity. This nucleotide sensitivity is consistent with P(2)Y(2) purinergic receptors mediating the inhibition of SK by ATP. Single channel analysis further demonstrated that the inhibitory effects of ATP could be elicited through activation of apical receptors. Moreover, the observation that fluoride mimicked the inhibitory action of ATP suggests the activation of G proteins during purinergic receptor stimulation. Channel inhibition by ATP was not affected by blocking phospholipase C and protein kinase C. However, whereas cAMP prevented channel blocking by ATP, blocking protein kinase A failed to abolish the inhibitory effects of ATP. The reduction of K channel activity by ATP could be prevented by okadaic acid, an inhibitor of protein phosphatases, and KT5823, an agent that blocks protein kinase G. Moreover, the effect of ATP was mimicked by cGMP and blocked by L-NAME (N(G)-nitro-l-arginine methyl ester). We conclude that the inhibitory effect of ATP on the apical K channel is mediated by stimulation of P(2)Y(2) receptors and results from increasing dephosphorylation by enhancing PKG-sensitive phosphatase activity.

Intracellular pH regulation by Na(+)/H(+) exchange requires phosphatidylinositol 4,5-bisphosphate

The carrier-mediated, electroneutral exchange of Na(+) for H(+) across the plasma membrane does not directly consume metabolic energy. Nevertheless, acute depletion of cellular ATP markedly decreases transport. We analyzed the possible involvement of polyphosphoinositides in the metabolic regulation of NHE1, the ubiquitous isoform of the Na(+)/H(+) exchanger. Depletion of ATP was accompanied by a marked reduction of plasmalemmal phosphatidylinositol 4,5-bisphosphate (PIP(2)) content. Moreover, sequestration or hydrolysis of plasmalemmal PIP(2), in the absence of ATP depletion, was associated with profound inhibition of NHE1 activity. Examination of the primary structure of the COOH-terminal domain of NHE1 revealed two potential PIP(2)-binding motifs. Fusion proteins encoding these motifs bound PIP(2) in vitro. When transfected into antiport-deficient cells, mutant forms of NHE1 lacking the putative PIP(2)-binding domains had greatly reduced transport capability, implying that association with PIP(2) is required for optimal activity. These findings suggest that NHE1 activity is modulated by phosphoinositides and that the inhibitory effect of ATP depletion may be attributable, at least in part, to the accompanying net dephosphorylation of PIP(2).

On the mechanism of ADP-induced alteration of sulphonylurea sensitivity in cardiac ATP-sensitive K(+) channels

1. To study the mechanism of regulation of sulphonylurea sensitivity in ATP-sensitive K(+) (K(ATP)) channels, we used the inside-out patch clamp technique in guinea-pig ventricular myocytes. 2. In the absence of nucleotides, the half maximal concentration of tolbutamide inhibition of K(ATP) channels (IC(50)) was 0.4 mM, and it decreased to 0.1 mM when 0.1 mM ATP was added. 3. Increasing the ADP concentration from 0 to 0.1 and 0.3 mM in the absence of ATP shifted the IC(50) from 0.4 to 5.3 and 11.4 mM, respectively. Increasing the ADP concentration further to 1 and 3 mM conversely reduced the IC(50) to 9.5 and 4.4 mM, respectively. 4. In the absence of Mg(2+) and ADP, the IC(50) was calculated to 16.6 mM which was found to be less, 12.3, 5.1 and 2.5 mM, respectively, when the ADP concentration was increased to 0.1, 0.3 and 1 mM. 5. The IC(50)s for tolbutamide obtained at various concentrations of ADP in the presence of Mg(2+) were best fitted by equations reflecting a model that assumed two binding sites for ADP; one is a high affinity site that reduces the sensitivity to the sulphonylurea, while the other is a low affinity site that increases such sensitivity. Dissociation constants calculated for ADP to sites 1 and 2 were 2.6 microM and 46.7 mM, respectively. In the absence of Mg(2+), data were fitted by equations corresponding to a single site model (site 2); the dissociation constant for ADP was 25.0 mM. 6. It is concluded that ADP modifies tolbutamide sensitivity by binding to two sites. The high affinity site is strongly Mg(2+)-dependent, whereas the low affinity site is Mg(2+)-independent.

Biophysical and molecular mechanisms underlying the modulation of heteromeric Kir4.1-Kir5.1 channels by CO2 and pH

CO2 chemoreception may be related to modulation of inward rectifier K+ channels (Kir channels) in brainstem neurons. Kir4.1 is expressed predominantly in the brainstem and inhibited during hypercapnia. Although the homomeric Kir4.1 only responds to severe intracellular acidification, coexpression of Kir4.1 with Kir5.1 greatly enhances channel sensitivities to CO2 and pH. To understand the biophysical and molecular mechanisms underlying the modulation of these currents by CO2 and pH, heteromeric Kir4. 1-Kir5.1 were studied in inside-out patches. These Kir4.1-Kir5.1 currents showed a single channel conductance of 59 pS with open-state probability (P(open)) approximately 0.4 at pH 7.4. Channel activity reached the maximum at pH 8.5 and was completely suppressed at pH 6.5 with pKa 7.45. The effect of low pH on these currents was due to selective suppression of P(open) without evident effects on single channel conductance, leading to a decrease in the channel mean open time and an increase in the mean closed time. At pH 8.5, single-channel currents showed two sublevels of conductance at approximately 1/4 and 3/4 of the maximal openings. None of them was affected by lowering pH. The Kir4.1-Kir5.1 currents were modulated by phosphatidylinositol-4,5-bisphosphate (PIP2) that enhanced baseline P(open) and reduced channel sensitivity to intracellular protons. In the presence of 10 microM PIP2, the Kir4.1-Kir5.1 showed a pKa value of 7.22. The effect of PIP2, however, was not seen in homomeric Kir4.1 currents. The CO2/pH sensitivities were related to a lysine residue in the NH2 terminus of Kir4.1. Mutation of this residue (K67M, K67Q) completely eliminated the CO2 sensitivity of both homomeric Kir4.1 and heteromeric Kir4.1-Kir5.1. In excised patches, interestingly, the Kir4.1-Kir5.1 carrying K67M mutation remained sensitive to low pHi. Such pH sensitivity, however, disappeared in the presence of PIP2. The effect of PIP2 on shifting the titration curve of wild-type and mutant channels was totally abolished when Arg178 in Kir5.1 was mutated. Thus, these studies demonstrate a heteromeric Kir channel that can be modulated by both acidic and alkaline pH, show the modulation of pH sensitivity of Kir channels by PIP2, and provide information of the biophysical and molecular mechanisms underlying the Kir modulation by intracellular protons.

Coronary microcirculation: physiology and pharmacology

Coronary microvessels play a pivotal role in determining the supply of oxygen and nutrients to the myocardium by regulating the coronary flow conductance and substance transport. Direct approaches analyzing the coronary microvessels have provided a large body of knowledge concerning the physiological and pharmacological characteristics of the coronary circulation, as has the rapid accumulation of biochemical findings about the substances that mediate vascular functions. Myogenic and flow-induced intrinsic vascular controls that determine basal tone have been observed in coronary microvessels in vitro. Coronary microvascular responses during metabolic stimulation, autoregulation, and reactive hyperemia have been analyzed in vivo, and are known to be largely mediated by metabolic factors, although the involvement of other factors should also be taken into account. The importance of ATP-sensitive K(+) channels in the metabolic control has been increasingly recognized. Furthermore, many neurohumoral mediators significantly affect coronary microvascular control in endothelium-dependent and -independent manners. The striking size-dependent heterogeneity of microvascular responses to all of these intrinsic, metabolic, and neurohumoral factors is orchestrated for optimal perfusion of the myocardium by synergistic and competitive interactions. The regulation of coronary microvascular permeability is another important factor for the nutrient supply and for edema formation. Analyses of collateral microvessels and subendocardial microvessels are important for understanding the pathophysiology of ischemic hearts and hypertrophied hearts. Studies of the microvascular responses to drugs and of the impairment of coronary microvessels in diseased conditions provide useful information for treating microvascular dysfunctions. In this article, the endogenous regulatory system and pharmacological responses of the coronary circulation are reviewed from the microvascular point of view.

The kinetic and physical basis of K(ATP) channel gating: toward a unified molecular understanding

K(ATP) channels can be formed from Kir6.2 subunits with or without SUR1. The open-state stability of K(ATP) channels can be increased or reduced by mutations throughout the Kir6.2 subunit, and is increased by application of PIP(2) to the cytoplasmic membrane. Increase of open-state stability is manifested as an increase in the channel open probability in the absence of ATP (Po(zero)) and a correlated decrease in sensitivity to inhibition by ATP. Single channel lifetime analyses were performed on wild-type and I154C mutant channels expressed with, and without, SUR1. Channel kinetics include a single, invariant, open duration; an invariant, brief, closed duration; and longer closed events consisting of a "mixture of exponentials," which are prolonged in ATP and shortened after PIP(2) treatment. The steady-state and kinetic data cannot be accounted for by assuming that ATP binds to the channel and causes a gate to close. Rather, we show that they can be explained by models that assume the following regarding the gating behavior: 1) the channel undergoes ATP-insensitive transitions from the open state to a short closed state (C(f)) and to a longer-lived closed state (C(0)); 2) the C(0) state is destabilized in the presence of SUR1; and 3) ATP can access this C(0) state, stabilizing it and thereby inhibiting macroscopic currents. The effect of PIP(2) and mutations that stabilize the open state is then to shift the equilibrium of the "critical transition" from the open state to the ATP-accessible C(0) state toward the O state, reducing accessibility of the C(0) state, and hence reducing ATP sensitivity.

Phosphatidylinositol 4,5-bisphosphate and intracellular pH regulate the ROMK1 potassium channel via separate but interrelated mechanisms

ROMK channels are responsible for K(+) secretion in kidney. The activity of ROMK is regulated by intracellular pH (pH(i)) with acidification causing channel closure (effective pK(a) approximately 6.9). Recently, we and others reported that a direct interaction of the channels with phosphatidyl-4,5-bisphosphate (PIP(2)) is critical for opening of the inwardly rectifying K(+) channels. Here, we investigate the relationship between the mechanisms for regulation of ROMK by PIP(2) and by pH(i). We find that disruption of PIP(2)-ROMK1 interaction not only decreases single-channel open probability (P(o)) but gives rise to a ROMK1 subconductance state. This state has an increased sensitivity to intracellular protons (effective pK(a) shifted to pH approximately 7.8), such that the subconductance channels are relatively quiescent at physiological pH(i). Open probability for the subconductance channels can then be increased by intracellular alkalinization to supra-physiological pH. This increase in P(o) for the subconductance channels by alkalinization is not associated with an increase in PIP(2)-channel interaction. Thus, direct interaction with PIP(2) is critical for ROMK1 to open at full conductance. Disruption of this interaction increases pH(i) sensitivity for the channels via emergence of the subconductance state. The control of open probability of ROMK1 by pH(i) occurs via a mechanism distinct from the regulation by PIP(2).

Targeted overactivity of beta cell K(ATP) channels induces profound neonatal diabetes

A paradigm for control of insulin secretion is that glucose metabolism elevates cytoplasmic [ATP]/[ADP] in beta cells, closing K(ATP) channels and causing depolarization, Ca2+ entry, and insulin release. Decreased responsiveness of K(ATP) channels to elevated [ATP]/[ADP] should therefore lead to decreased insulin secretion and diabetes. To test this critical prediction, we generated transgenic mice expressing beta cell K(ATP) channels with reduced ATP sensitivity. Animals develop severe hyperglycemia, hypoinsulinemia, and ketoacidosis within 2 days and typically die within 5. Nevertheless, islet morphology, insulin localization, and alpha and beta cell distributions were normal (before day 3), pointing to reduced insulin secretion as causal. The data indicate that normal K(ATP) channel activity is critical for maintenance of euglycemia and that overactivity can cause diabetes by inhibiting insulin secretion.

Fluorescently labeled neomycin as a probe of phosphatidylinositol-4, 5-bisphosphate in membranes

Phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)), a minor component of the plasma membrane, is important in signal transduction, exocytosis, and ion channel activation. Thus fluorescent probes suitable for monitoring the PI(4,5)P(2) distribution in living cells are valuable tools for cell biologists. We report here three experiments that show neomycin labeled with either fluorescein or coumarin can be used to detect PI(4,5)P(2) in model phospholipid membranes. First, addition of physiological concentrations of PI(4,5)P(2) (2%) to lipid vesicles formed from mixtures of phosphatidylcholine (PC) and phosphatidylserine (PS) enhances the binding of labeled neomycin significantly (40-fold for 5:1 PC/PS vesicles). Second, physiological concentrations of inositol-1,4,5-trisphosphate (10 microM I(1,4,5)P(3)) cause little translocation of neomycin from PC/PS/PI(4,5)P(2) membranes to the aqueous phase, whereas the same concentrations of I(1,4,5)P(3) cause significant translocation of the green fluorescent protein/phospholipase C-delta pleckstrin homology (GFP-PH) constructs from membranes (Hirose et al., Science, 284 (1999) 1527). Third, fluorescence microscopy observations confirm that one can distinguish between PC/PS vesicles containing either 0 or 2% PI(4, 5)P(2) by exposing a mixture of the vesicles to labeled neomycin. Thus fluorescently labeled neomycin could complement GFP-PH constructs to investigate the location of PI(4,5)P(2) in cell membranes.

Regulation of ATP-sensitive potassium channel function by protein kinase A-mediated phosphorylation in transfected HEK293 cells

ATP-sensitive potassium (K(ATP)) channels regulate insulin secretion, vascular tone, heart rate and neuronal excitability by responding to transmitters as well as the internal metabolic state. K(ATP) channels are composed of four pore-forming alpha-subunits (Kir6.2) and four regulatory beta-subunits, the sulfonylurea receptor (SUR1, SUR2A or SUR2B). Whereas protein kinase A (PKA) phosphorylation of serine 372 of Kir6.2 has been shown biochemically by others, we found that the phosphorylation of T224 rather than S372 of Kir6.2 underlies the catalytic subunits of PKA (c-PKA)- and the D1 dopamine receptor-mediated stimulation of K(ATP) channels expressed in HEK293 cells. Specific changes in the kinetic properties of channels treated with c-PKA, as revealed by single-channel analysis, were mimicked by aspartate substitution of T224. The T224D mutation also reduced the sensitivity to ATP inhibition. Alteration of channel gating and a decrease in the apparent affinity for ATP inhibition thus underlie the positive regulation of K(ATP) channels by PKA phosphorylation of T224 in Kir6.2, which may represent a general mechanism for K(ATP) channel regulation in different tissues.

ATP-sensitive potassium channels may participate in the coupling of neuronal activity and cerebrovascular tone

K(+) dilate and constrict cerebral vessels in a dose-dependent fashion. Modest elevations of abluminal K(+) cause vasodilatation, whereas larger extracellular K(+) concentration ([K(+)](out)) changes decrease cerebral blood flow. These dilations are believed to be mediated by opening of inward-rectifier potassium channels sensitive to Ba(2+). Because BaCl(2) also blocks ATP-sensitive K(+) channels (K(ATP)), we challenged K(+) dilations in penetrating, resistance-size (<60 mmu) rat neocortical vessels with the K(ATP) channel blocker glibenclamide (1 microM). Glibenclamide reduced K(+) responses from 138 +/- 8 to 110 +/- 0.8%. K(+) constrictions were not affected by glibenclamide. The Na(+)-K(+)-pump inhibitor ouabain (200 microM) did not significantly change resting vessel diameter but decreased K(+) dilations (from 153 +/- 9 to 99 +/- 2%). BaCl(2) blocked K(+) dilations with a half-maximal dissociation constant of 2.9 microM and reduced dilations to the specific K(ATP) agonist pinacidil with equal potency. We conclude that, in resistance vessels, K(+) dilations are mediated by K(ATP); we hypothesize that [K(+)](out) causes activation of Na(+)-K(+) pumps, depletion of intracellular ATP concentration, and subsequent opening of K(ATP). This latter hypothesis is supported by the blocking effect of ouabain.

Cytoplasmic unsaturated free fatty acids inhibit ATP-dependent gating of the G protein-gated K(+) channel

This study reports the identification of an endogenous inhibitor of the G protein-gated (K(ACh)) channel and its effect on the K(ACh) channel kinetics. In the presence of acetylcholine in the pipette, K(ACh) channels in inside-out atrial patches were activated by applying GTP to the cytoplasmic side of the membrane. In these patches, addition of physiological concentration of intracellular ATP (4 mM) upregulated K(ACh) channel activity approximately fivefold and induced long-lived openings. However, such ATP-dependent gating is normally not observed in cell-attached patches, indicating that an endogenous substance that inhibits the ATP effect is present in the cell. We searched for such an inhibitor in the cell. ATP-dependent gating of the K(ACh) channel was inhibited by the addition of the cytosolic fraction of rat atrial or brain tissues. The lipid component of the cytosolic fraction was found to contain the inhibitory activity. To identify the lipid inhibitor, we tested the effect of approximately 40 different lipid molecules. Among the lipids tested, only unsaturated free fatty acids such as oleic, linoleic, and arachidonic acids (0.2-2 microM) reversibly inhibited the ATP-dependent gating of native K(ACh) channels in atrial cells and hippocampal neurons, and of recombinant K(ACh) channels (GIRK1/4 and GIRK1/2) expressed in oocytes. Unsaturated free fatty acids also inhibited phosphatidylinositol-4, 5-bisphosphate (PIP(2))-induced changes in K(ACh) channel kinetics but were ineffective against ATP-activated background K(1) channels and PIP(2)-activated K(ATP) channels. These results show that during agonist-induced activation, unsaturated free fatty acids in the cytoplasm help to keep the cardiac and neuronal K(ACh) channels downregulated by antagonizing their ATP-dependent gating. The opposing effects of ATP and free fatty acids represent a novel regulatory mechanism for the G protein-gated K(+) channel.

Essential role of phosphoinositide 3-kinase in leptin-induced K(ATP) channel activation in the rat CRI-G1 insulinoma cell line

The mechanism by which leptin increases ATP-sensitive K(+) (K(ATP)) channel activity was investigated using the insulin-secreting cell line, CRI-G1. Wortmannin and LY 294002, inhibitors of phosphoinositide 3-kinase (PI3-kinase), prevented activation of K(ATP) channels by leptin. The inositol phospholipids phosphatidylinositol bisphosphate and phosphatidylinositol trisphosphate (PtdIns(3,4,5)P(3)) mimicked the effect of leptin by increasing K(ATP) channel activity in whole-cell and inside-out current recordings. LY 294002 prevented phosphatidylinositol bisphosphate, but not PtdIns(3,4,5)P(3), from increasing K(ATP) channel activity, consistent with the latter lipid acting as a membrane-associated messenger linking leptin receptor activation and K(ATP) channels. Signaling cascades, activated downstream from PI 3-kinase, utilizing PtdIns(3,4,5)P(3) as a second messenger and commonly associated with insulin and cytokine action (MAPK, p70 ribosomal protein-S6 kinase, stress-activated protein kinase 2, p38 MAPK, and protein kinase B), do not appear to be involved in leptin-mediated activation of K(ATP) channels in this cell line. Although PtdIns(3,4,5)P(3) appears a plausible and attractive candidate for the messenger that couples K(ATP) channels to leptin receptor activation, direct measurement of PtdIns(3,4,5)P(3) demonstrated that insulin, but not leptin, increased global cellular levels of PtdIns(3,4,5)P(3). Possible mechanisms to explain the involvement of PI 3-kinases in K(ATP) channel regulation are discussed.

Modulation of nucleotide sensitivity of ATP-sensitive potassium channels by phosphatidylinositol-4-phosphate 5-kinase

ATP-sensitive potassium channels (K(ATP) channels) regulate cell excitability in response to metabolic changes. K(ATP) channels are formed as a complex of a sulfonylurea receptor (SURx), a member of the ATP-binding cassette protein family, and an inward rectifier K(+) channel subunit (Kir6.x). Membrane phospholipids, in particular phosphatidylinositol (PI) 4,5-bisphosphate (PIP(2)), activate K(ATP) channels and antagonize ATP inhibition of K(ATP) channels when applied to inside-out membrane patches. To examine the physiological relevance of this regulatory mechanism, we manipulated membrane PIP(2) levels by expressing either the wild-type or an inactive form of PI-4-phosphate 5-kinase (PIP5K) in COSm6 cells and examined the ATP sensitivity of coexpressed K(ATP) channels. Channels from cells expressing the wild-type PIP5K have a 6-fold lower ATP sensitivity (K(1/2), the half maximal inhibitory concentration, approximately 60 microM) than the sensitivities from control cells (K(1/2) approximately 10 microM). An inactive form of the PIP5K had little effect on the K(1/2) of wild-type channels but increased the ATP-sensitivity of a mutant K(ATP) channel that has an intrinsically lower ATP sensitivity (from K(1/2) approximately 450 microM to K(1/2) approximately 100 microM), suggesting a decrease in membrane PIP(2) levels as a consequence of a dominant-negative effect of the inactive PIP5K. These results show that PIP5K activity, which regulates PIP(2) and PI-3,4,5-P(3) levels, is a significant determinant of the physiological nucleotide sensitivity of K(ATP) channels.

Inactivation of P2X2 purinoceptors by divalent cations

1. P2X2 channels are activated by extracellular ATP. Despite being commonly described as non-desensitizing, P2X2 receptors do desensitize or inactivate. In the unspliced, 472 amino acid isoform of the P2X2 receptor, inactivation required membrane disruption and the presence of extracellular Ca2+. 2. The ability to inactivate whole-cell currents developed slowly after breaking in. In contrast, currents from excised patches exhibited rapid (approximately 100 ms) inactivation with a dependence on extracellular Ca2+, ATP and voltage. 3. The inactivation rate increased with the fourth power of [Ca2+] suggesting that the functional channel may be a tetramer. Ca2+ had both a higher affinity and a larger Hill coefficient for inactivation than Mg2+, Ba2+ or Mn2+. Trivalent cations at concentrations up to the solubility product of ATP had no effect. The change in apparent co-operativity with ionic species suggests the presence of experimentally unresolved ligand-insensitive kinetic steps. 4. Based on the weak voltage dependence of inactivation and the lack of effect of intracellular Ca2+ buffers, the Ca2+-binding sites are probably located near the extracellular surface of the membrane. 5. The recovery from inactivation was slow, with a time constant of approximately 7 min. 6. Ca2+-sensitive inactivation only appeared when the membrane was disrupted in some manner. Treatment with actin and microtubule reagents did not induce inactivation, suggesting that an intact cytoskeleton is not necessary. 7. Inactivation rates observed in different patch configurations suggest that the induction of Ca2+-dependent inactivation was due to the loss of a diffusible cofactor located in the membrane or the cytoplasm.

Structure and dynamics of the pore of inwardly rectifying K(ATP) channels

Inwardly rectifying K(+) currents are generated by a complex of four Kir (Kir1-6) subunits. Pore properties are conferred by the second transmembrane domain (M2) of each subunit. Using cadmium ions as a cysteine-interacting probe, we examined the accessibility of substituted cysteines in M2 of the Kir6.2 subunit of inwardly rectifying K(ATP) channels. The ability of Cd(2+) ions to inhibit channels was used as the estimate of accessibility. The distribution of Cd(2+) accessibility is consistent with an alpha-helical structure of M2. The apparent surface of reactivity is broad, and the most reactive residues correspond to the solvent-accessible residues in the bacterial KcsA channel crystal structure. In several mutants, single channel measurements indicated that inhibition occurred by a single transition from the open state to a zero-conductance state. Analysis of currents expressed from mixtures of control and L164C mutant subunits indicated that at least three cysteines are required for coordination of the Cd(2+) ion. Application of phosphatidylinositol 4,5-diphosphate to inside-out membrane patches stabilized the open state of all mutants and also reduced cadmium sensitivity. Moreover, the Cd(2+) sensitivity of several mutants was greatly reduced in the presence of inhibitory ATP concentrations. Taken together, these results are consistent with state-dependent accessibility of single Cd(2+) ions to coordination sites within a relatively narrow inner vestibule.

Molecular aspects of ATP-sensitive K+ channels in the cardiovascular system and K+ channel openers

ATP-sensitive K+ (K(ATP)) channels are inhibited by intracellular ATP (ATPi) and activated by intracellular nucleoside diphosphates and thus, provide a link between cellular metabolism and excitability. K(ATP) channels are widely distributed in various tissues and may be associated with diverse cellular functions. In the heart, the K(ATP) channel appears to be activated during ischemic or hypoxic conditions, and may be responsible for the increase of K+ efflux and shortening of the action potential duration. Therefore, opening of this channel may result in cardioprotective, as well as proarrhythmic, effects. These channels are clearly heterogeneous. The cardiac K(ATP) channel is the prototype of K(ATP) channels possessing approximately 80 pS of single-channel conductance in the presence of approximately 150 mM extracellular K+ and opens spontaneously in the absence of ATPi. A vascular K(ATP) channel called a nucleoside diphosphate-dependent K+ (K(NDP)) channel exhibits properties significantly different from those of the cardiac K(ATP) channel. The K(NDP) channel has the single-channel conductance of approximately 30-40 pS in the presence of approximately 150 mM extracellular K+, is closed in the absence of ATPi, and requires intracellular nucleoside di- or triphosphates, including ATPi to open. Nevertheless, K(ATP) and K(NDP) channels are both activated by K+ channel openers, including pinacidil and nicorandil, and inhibited by sulfonylurea derivatives such as glibenclamide. It recently was found that the cardiac K(ATP) channel is composed of a sulfonylurea receptor (SUR)2A and a two-transmembrane-type K+ channel subunit Kir6.2, while the vascular K(NDP) channel may be the complex of SUR2B and Kir6.1. By precisely comparing the functional properties of the SUR2A/Kir6.2 and the SUR2B/Kir6.1 channels, we shall show that the single-channel characteristics and pharmacological properties of SUR/Kir6.0 channels are determined by Kir and SUR subunits, respectively, while responses to intracellular nucleotides are determined by both SUR and Kir subunits.

Glucose regulation of free Ca(2+) in the endoplasmic reticulum of mouse pancreatic beta cells

Free Ca(2+) was measured in organelles of individual mouse pancreatic beta cells loaded with the low affinity indicator furaptra. After removal of cytoplasmic indicator by controlled digitonin permeabilization the organelle Ca(2+) was located essentially in the endoplasmic reticulum (ER), >90% being sensitive to inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPases. The Ca(2+) accumulation in the ER of intact beta cells depended in a hyperbolic fashion on the glucose concentration with half-maximal and maximal filling at 5.5 and >20 mM, respectively. Also elevation of cytoplasmic Ca(2+) by K(+) depolarization significantly enhanced the Ca(2+) accumulation. In permeabilized beta cells 1-3 mM ATP caused rapid Ca(2+) filling of the ER reaching almost 500 microM. At 50 nM, Ca(2+) ER became half-maximally filled at 45 microM ATP, whereas only 3.5 microM ATP was required at 200 nM Ca(2+). Inositol 1,4,5-trisphosphate induced a rapid release of about 65% of the ER Ca(2+), and its precursor phosphatidylinositol 4,5-bisphosphate was found to slowly mobilize 75% by another mechanism. It is concluded that glucose is an efficient stimulator of Ca(2+) uptake in the ER of pancreatic beta cells both by increasing ATP and cytoplasmic Ca(2+). Because physiological concentrations of cytoplasmic ATP are in the mM range, Ca(2+) sequestration can be anticipated to be modulated by factors reducing its ATP sensitivity.

pH gating of ROMK (K(ir)1.1) channels: control by an Arg-Lys-Arg triad disrupted in antenatal Bartter syndrome

Inward-rectifier K(+) channels of the ROMK (K(ir)1.1) subtype are responsible for K(+) secretion and control of NaCl absorption in the kidney. A hallmark of these channels is their gating by intracellular pH in the neutral range. Here we show that a lysine residue close to TM1, identified previously as a structural element required for pH-induced gating, is protonated at neutral pH and that this protonation drives pH gating in ROMK and other K(ir) channels. Such anomalous titration of this lysine residue (Lys-80 in K(ir)1.1) is accomplished by the tertiary structure of the K(ir) protein: two arginines in the distant N and C termini of the same subunit (Arg-41 and Arg-311 in K(ir)1.1) are located in close spatial proximity to the lysine allowing for electrostatic interactions that shift its pK(a) into the neutral pH range. Structural disturbance of this triad as a result from a number of point mutations found in patients with antenatal Bartter syndrome shifts the pK(a) of the lysine residue off the neutral pH range and results in channels permanently inactivated under physiological conditions. Thus, the results provide molecular understanding for normal pH gating of K(ir) channels as well as for the channel defects found in patients with antenatal Bartter syndrome.

Phospholipase C-linked receptors regulate the ATP-sensitive potassium channel by means of phosphatidylinositol 4,5-bisphosphate metabolism

In the COS7 cells transfected with cDNAs of the Kir6.2, SUR2A, and M(1) muscarinic receptors, we activated the ATP-sensitive potassium (K(ATP)) channel with a K(+) channel opener and recorded the whole-cell K(ATP) current. The K(ATP) current was reversibly inhibited by the stimulation of the M(1) receptor, which is linked to phospholipase C (PLC) by the G(q) protein. The receptor-mediated inhibition was observed even when protein kinase C (PKC) was inhibited by H-7 or by chelating intracellular Ca(2+) with 10 mM 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) included in the pipette solution. However, the receptor-mediated inhibition was blocked by U-73122, a PLC inhibitor. M(1)-receptor stimulation failed to inhibit the K(ATP) current activated by the injection of exogenous phosphatidylinositol 4,5-bisphosphate (PIP(2)) through the whole-cell patch pipette. The receptor-mediated inhibition became irreversible when the replenishment of PIP(2) was blocked by wortmannin (an inhibitor of phosphatidylinositol kinases), or by including adenosine 5'-[beta,gamma-imido]triphosphate (AMPPNP, a nonhydrolyzable ATP analogue) in the pipette solution. In inside-out patch experiments, the ATP sensitivity of the K(ATP) channel was significantly higher when the M(1) receptor in the patch membrane was stimulated by acetylcholine. The stimulatory effect of pinacidil was also attenuated under this condition. We postulate that stimulation of PLC-linked receptors inhibited the K(ATP) channel by increasing the ATP sensitivity, not through PKC activation, but most probably through changing PIP(2) levels.

Novel inositol polyphosphate 5-phosphatase localizes at membrane ruffles

We have cloned a novel inositol polyphosphate 5-phosphatase from the rat brain cDNA library. It contains two highly conserved 5-phosphatase motifs, both of which are essential for its enzymatic activity. Interestingly, the proline content of this protein is high and concentrated in its N- and C-terminal regions. One putative SH3-binding motif and six 14-3-3 zeta-binding motifs were found in the amino acid sequence. This enzyme hydrolyzed phosphate at the D-5 position of inositol 1,4,5-trisphosphate, inositol 1,3,4, 5-tetrakisphosphate, and phosphatidylinositol 4,5-bisphosphate, consistent with the substrate specificity of type II 5-phosphatase, OCRL, synaptojanin and synaptojanin 2, already characterized 5-phosphatases. When the Myc-epitope-tagged enzyme was expressed in COS-7 cells and stained with anti-Myc polyclonal antibody, a signal was observed at ruffling membranes and in the cytoplasm. We prepared several deletion mutants and demonstrated that the 123 N-terminal amino acids (311-433) and a C-terminal proline-rich region containing 277 amino acids (725-1001) were essential for its localization to ruffling membranes. This enzyme might regulate the level of inositol and phosphatidylinositol polyphosphates at membrane ruffles.

Distinct specificities of inwardly rectifying K(+) channels for phosphoinositides

Activation of several inwardly rectifying K(+) channels (Kir) requires the presence of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)). The constitutively active Kir2.1 (IRK1) channels interact with PtdIns(4,5)P(2) strongly, whereas the G-protein activated Kir3.1/3.4 channels (GIRK1/GIRK4), show only weak interactions with PtdIns(4,5)P(2). We investigated whether these inwardly rectifying K(+) channels displayed distinct specificities for different phosphoinositides. IRK1, but not GIRK1/GIRK4 channels, showed a marked specificity toward phosphates in the 4,5 head group positions. GIRK1/GIRK4 channels were activated with a similar efficacy by PtdIns(3,4)P(2), PtdIns(3,5)P(2), PtdIns(4,5)P(2), and PtdIns(3,4,5)P(3). In contrast, IRK1 channels were not activated by PtdIns(3,4)P(2) and only marginally by high concentrations of PtdIns(3,5)P(2). Similarly, high concentrations of PtdIns(3,4,5)P(3) were required to activate IRK1 channels. For either channel, PtdIns(4)P was much less effective than PtdIns(4,5)P(2), whereas PtdIns was inactive. In contrast to the dependence on the position of phosphates of the phospholipid head group, GIRK1/GIRK4, but not IRK1 channel activation, showed a remarkable dependence on the phospholipid acyl chains. GIRK1/GIRK4 channels were activated most effectively by the natural arachidonyl stearyl PtdIns(4,5)P(2) and much less by the synthetic dipalmitoyl analog, whereas IRK1 channels were activated equally by dipalmitoyl and arachidonyl stearyl PtdIns(4,5)P(2). Incorporation of PtdInsP(2) into the membrane is necessary for activation, as the short chain water soluble diC(4) PtdIns(4,5)P(2) did not activate either channel, whereas activation by diC(8) PtdIns(4, 5)P(2) required high concentrations.

Regulation of the ROMK channel: interaction of the ROMK with associate proteins

The ROMK channel plays an important role in K recycling in the thick ascending limb (TAL) and K secretion in the cortical collecting duct (CCD). A large body of evidence indicates that the ROMK channel is a key component of the native K secretory channel identified in the apical membrane of the TAL and the CCD. Although the ROMK channel shares several key regulatory mechanisms with the native K secretory channel in a variety of respects, differences in the channel modulatory mechanism are clearly present between the ROMK channel and the native K secretory channel. Therefore, it is possible that additional associate proteins are required to interact with the ROMK channel to assemble the native K secretory channel. This notion is supported by recent reports showing that cystic fibrosis transmembrane conductance regulator (CFTR) and A kinase anchoring proteins (AKAP) interact with the ROMK channels to restore the response to ATP sensitivity and protein kinase A stimulation. This review is an attempt to summarize the up-to-date progress regarding the interaction between the ROMK channel and the associate proteins in forming the native K secretory channel.

Synergistic activation of G protein-gated inwardly rectifying potassium channels by the betagamma subunits of G proteins and Na(+) and Mg(2+) ions

Native and recombinant G protein-gated inwardly rectifying potassium (GIRK) channels are directly activated by the betagamma subunits of GTP-binding (G) proteins. The presence of phosphatidylinositol-bis-phosphate (PIP(2)) is required for G protein activation. Formation (via hydrolysis of ATP) of endogenous PIP(2) or application of exogenous PIP(2) increases the mean open time of GIRK channels and sensitizes them to gating by internal Na(+) ions. In the present study, we show that the activity of ATP- or PIP(2)-modified channels could also be stimulated by intracellular Mg(2+) ions. In addition, Mg(2+) ions reduced the single-channel conductance of GIRK channels, independently of their gating ability. Both Na(+) and Mg(2+) ions exert their gating effects independently of each other or of the activation by the G(betagamma) subunits. At high levels of PIP(2), synergistic interactions among Na(+), Mg(2+), and G(betagamma) subunits resulted in severalfold stimulated levels of channel activity. Changes in ionic concentrations and/or G protein subunits in the local environment of these K(+) channels could provide a rapid amplification mechanism for generation of graded activity, thereby adjusting the level of excitability of the cells.

Molecular mechanism for sodium-dependent activation of G protein-gated K+ channels

1. G protein-gated inwardly rectifying K+ (GIRK) channels are activated independently by Gbetagamma and internal Na+ via mechanisms requiring phosphatidylinositol phosphates. An aspartate (Asp) at position 226 in GIRK2 is crucial for Na+-dependent activation of GIRK1-GIRK2 heteromeric channels. We expressed wild-type and mutant GIRK1-GIRK2 channels in Xenopus oocytes and tested the effects of Na+ and neutralizing Asp226 on the functional interactions of the channels with phosphatidylinositol 4, 5-bisphosphate (PIP2). 2. The rate of inhibition of GIRK1-GIRK2 currents by application of anti-PIP2 antibody to inside-out membrane patches was slowed > 2-fold by the D226N mutation in GIRK2 and by increasing internal [Na+]. The reverse mutation in GIRK1 (N217D) increased the rate of inhibition. 3. The dose-response relationship for activation by purified PIP2 was shifted to lower concentrations in the presence of 20 mM Na+. 4. Three synthetic isoforms of PIP2, PI(4,5)P2, PI(3,4)P2 and PI(3,5)P2, activated GIRK channels with similar potencies. 5. We conclude that Na+ directly interacts with Asp226 of GIRK2 to reduce the negative electrostatic potential and promote the functional interaction of the channels with PIP2.

Expression density and functional characteristics of the outer hair cell motor protein are regulated during postnatal development in rat

1. The non-linear capacitance (Cnon-lin) of postnatal outer hair cells (OHCs) of the rat was measured by a patch-clamp lock-in technique. Cnon-lin is thought to result from a membrane protein that provides the molecular basis for the unique electromotility of OHCs by undergoing conformational changes in response to changes in membrane potential (Vm). Protein conformation is coupled to Vm by a charged voltage sensor, which imposes Cnon-lin on the OHC. Cnon-lin was investigated in order to characterize the surface expression and voltage dependence of this motor protein during postnatal development. 2. On the day of birth (P0), Cnon-lin was not detected in OHCs of the basal turn of the cochlea, whilst it was 89 fF in apical OHCs. Cnon-lin increased gradually during postnatal development and reached 2.3 pF (basal turn, P9) and 7.5 pF (apical turn, P14) at the oldest developmental stages covered by our measurements. The density of the protein in the plasma membrane, deduced from non-linear charge movement per membrane area, increased steeply between P6 and P11 and reached steady state (4200 e- microm-2) at about P12. 3. Voltage at peak capacitance (V) shifted with development from hyperpolarized potentials shortly after birth (-88.3 mV, P2) to the depolarized potential characteristic of mature OHCs (-40.8 mV, P14). This developmental difference in V was also observed in outside-out patches immediately after patch excision. During subsequent wash-out V shifted towards the depolarized value found in the adult state, suggesting a direct modulation of the molecular motor. 4. Thus, the density of the motor protein in the plasma membrane and also its voltage dependence change concomitantly in the postnatal period and reach adult characteristics right at the onset of hearing.

ATP-sensitive potassium channel blockade enhances spontaneous alternation performance in the rat: a potential mechanism for glucose-mediated memory enhancement

Peripheral and central injections of D-glucose enhance learning and memory in rats, and block memory impairments produced by morphine. The mechanism(s) for these effects is (are) as yet unknown. One mechanism by which glucose might act on memory and other brain functions is by regulating the ATP-sensitive potassium channel. This channel may couple glucose metabolism and neuronal excitability, with channel blockade increasing the likelihood of stimulus-evoked neurotransmitter release. The present experiments explored the effects of intra-septal injections of glucose and the ATP-sensitive potassium channel blocker glibenclamide on spontaneous alternation behavior in the rat. Intra-septal injections of glucose (20 nmol) or glibenclamide (10 nmol), 30 min prior to plus-maze spontaneous alternation performance, significantly enhanced alternation scores compared to rats receiving vehicle injections. Glibenclamide enhanced spontaneous alternation performance in an inverted-U dose-response manner. Individually sub-effective doses of glucose (5 nmol) and glibenclamide (5 nmol) significantly enhanced plus-maze alternation scores when co-injected into the septal area. Glibenclamide (10 nmol), when co-administered with morphine (4 nmol) 30 min prior to Y-maze spontaneous alternation performance, attenuated the performance-impairing effects of morphine alone. The present findings show that intra-septal injections of the direct ATP-sensitive potassium channel blocker glibenclamide, both alone and in conjunction with a sub-effective dose of glucose, enhance spontaneous alternation performance and attenuate the performance-impairing effects of morphine. The similarity of the results obtained with glibenclamide and glucose, together with their similar actions on ATP-sensitive potassium channel function, suggests that glucose may modulate memory-dependent behavior in the rat by regulating the ATP-sensitive potassium channel.

Phosphoinositides decrease ATP sensitivity of the cardiac ATP-sensitive K(+) channel. A molecular probe for the mechanism of ATP-sensitive inhibition

Anionic phospholipids modulate the activity of inwardly rectifying potassium channels (Fan, Z., and J.C. Makielski. 1997. J. Biol. Chem. 272:5388-5395). The effect of phosphoinositides on adenosine triphosphate (ATP) inhibition of ATP-sensitive potassium channel (K(ATP)) currents was investigated using the inside-out patch clamp technique in cardiac myocytes and in COS-1 cells in which the cardiac isoform of the sulfonylurea receptor, SUR2, was coexpressed with the inwardly rectifying channel Kir6.2. Phosphoinositides (1 mg/ml) increased the open probability of K(ATP) in low [ATP] (1 microM) within 30 s. Phosphoinositides desensitized ATP inhibition with a longer onset period (>3 min), activating channels inhibited by ATP (1 mM). Phosphoinositides treatment for 10 min shifted the half-inhibitory [ATP] (K(i)) from 35 microM to 16 mM. At the single-channel level, increased [ATP] caused a shorter mean open time and a longer mean closed time. Phosphoinositides prolonged the mean open time, shortened the mean closed time, and weakened the [ATP] dependence of these parameters resulting in a higher open probability at any given [ATP]. The apparent rate constants for ATP binding were estimated to be 0.8 and 0.02 mM(-1) ms(-1) before and after 5-min treatment with phosphoinositides, which corresponds to a K(i) of 35 microM and 5.8 mM, respectively. Phosphoinositides failed to desensitize adenosine inhibition of K(ATP). In the presence of SUR2, phosphoinositides attenuated MgATP antagonism of ATP inhibition. Kir6.2DeltaC35, a truncated Kir6.2 that functions without SUR2, also exhibited phosphoinositide desensitization of ATP inhibition. These data suggest that (a) phosphoinositides strongly compete with ATP at a binding site residing on Kir6.2; (b) electrostatic interaction is a characteristic property of this competition; and (c) in conjunction with SUR2, phosphoinositides render additional, complex effects on ATP inhibition. We propose a model of the ATP binding site involving positively charged residues on the COOH-terminus of Kir6.2, with which phosphoinositides interact to desensitize ATP inhibition.

Inwardly rectifying potassium channels

Inwardly rectifying potassium (Kir) channels regulate the resting membrane potential of the cell and thereby modulate the electrical activity of cardiac and neuronal cells, insulin secretion and epithelial K(+) transport. Considerable progress in understanding the molecular structure of Kir channels and the way in which they are regulated by extracellular and intracellular modulators has been made during the past year.

Sulfonylurea and K(+)-channel opener sensitivity of K(ATP) channels. Functional coupling of Kir6.2 and SUR1 subunits

The sensitivity of K(ATP) channels to high-affinity block by sulfonylureas and to stimulation by K(+) channel openers and MgADP (PCOs) is conferred by the regulatory sulfonylurea receptor (SUR) subunit, whereas ATP inhibits the channel through interaction with the inward rectifier (Kir6.2) subunit. Phosphatidylinositol 4, 5-bisphosphate (PIP(2)) profoundly antagonized ATP inhibition of K(ATP) channels expressed from cloned Kir6.2+SUR1 subunits, but also abolished high affinity tolbutamide sensitivity. By stabilizing the open state of the channel, PIP(2) drives the channel away from closed state(s) that are preferentially affected by high affinity tolbutamide binding, thereby producing an apparent loss of high affinity tolbutamide inhibition. Mutant K(ATP) channels (Kir6. 2[DeltaN30] or Kir6.2[L164A], coexpressed with SUR1) also displayed an "uncoupled" phenotype with no high affinity tolbutamide block and with intrinsically higher open state stability. Conversely, Kir6. 2[R176A]+SUR1 channels, which have an intrinsically lower open state stability, displayed a greater high affinity fraction of tolbutamide block. In addition to antagonizing high-affinity block by tolbutamide, PIP(2) also altered the stimulatory action of the PCOs, diazoxide and MgADP. With time after PIP(2) application, PCO stimulation first increased, and then subsequently decreased, probably reflecting a common pathway for activation of the channel by stimulatory PCOs and PIP(2). The net effect of increasing open state stability, either by PIP(2) or mutagenesis, is an apparent "uncoupling" of the Kir6.2 subunit from the regulatory input of SUR1, an action that can be partially reversed by screening negative charges on the membrane with poly-L-lysine.

Activation of inwardly rectifying K+ channels by distinct PtdIns(4,5)P2 interactions

Direct interactions of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) with inwardly rectifying potassium channels are stronger with channels rendered constitutively active by binding to PtdIns(4,5)P2, such as IRK1, than with G-protein-gated channels (GIRKs). As a result, PtdIns(4,5)P2 alone can activate IRK1 but not GIRKs, which require extra gating molecules such as the beta gamma subunits of G proteins or sodium ions. Here we identify two conserved residues near the inner-membrane interface of these channels that are critical in interactions with PtdIns(4,5)P2. Between these two arginines, a conservative change of isoleucine residue 229 in GIRK4 to the corresponding leucine found in IRK1 strengthens GIRK4-PtdIns(4,5)P2 interactions, eliminating the need for extra gating molecules. A negatively charged GIRK4 residue, two positions away from the most strongly interacting arginine, mediates stimulation of channel activity by sodium by strengthening channel-PtdIns(4,5)P2 interactions. Our results provide a mechanistic framework for understanding how distinct gating mechanisms of inwardly rectifying potassium channels allow these channels to subserve their physiological roles.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

The bacterial K+ channel structure and its implications for neuronal channels

Voltage-gated K+ (Kv) channels play a central role in generating action potentials and rhythmic patterns, as well as in dendritic signal processing in neurons. Recently, the first structure of a member of the K+ channel family was solved. Although this channel is from bacteria and has a streamlined body plan with no voltage gating, it establishes the architecture of the functional core of the voltage-gated (K+) channels and their relatives. This architecture explains the crucial features of ion permeation and blockade, and gives some strong hints about gating. The bacterial K+ channel structure is the central piece in a puzzle; it remains to be seen how it will fit together with other domains of the Kv channels, with auxiliary subunits, and with other signal transduction molecules.

Modulation of rat atrial G protein-coupled K+ channel function by phospholipids

1. G protein-gated K+ channels (KACh channels) in the heart and brain are activated by the betagamma subunit of inhibitory G protein. Phosphatidylinositol-4,5-bisphosphate (PIP2) has recently been reported to directly activate KACh channels (GIRK) expressed in oocytes, as well as to support activation by the betagamma subunit in the presence of Na+. We examined the effect of Na+, PIP2 and other phospholipids on the KACh channel to understand better their role in KACh channel activation and modulation. 2. In atrial membrane patches, none of the phospholipids tested including PIP2 caused activation of the KACh channel in either the presence or the absence of 30 mM Na+. PIP2 (3 microM) and other phospholipids (30 microM) blocked acetylcholine-induced activation of the KACh channel. 3. When KACh channels were first activated with GTPgammaS, however, all phospholipids (100 microM) tested augmented the KACh channel activity 1.5- to 2-fold. Phosphatidylinositol-4-phosphate (PIP) and PIP2 were an order of magnitude more potent than other phospholipids. The increase in KACh channel activity was the result of a shift in the gating mode of the channel from a short-lived to a longer-lived open state. Such a modulatory effect was qualitatively similar to that produced by intracellular ATP. Trypsin blocked the ATP effect but not the phospholipid effect on the KACh channel kinetics. 4. The phosphate group linked to the glycerol backbone was important for KACh channel modulation by phospholipids. The higher potency of PIP and PIP2 was due to the presence of inositol phosphates. 5. Intracellular Na+ (30 mM) increased the frequency of KACh channel opening approximately 2-fold if the channels were already active, but did not affect modulation by phospholipids. The effects of Na+ and phospholipids on KACh channel activity were additive. 6. A low concentration of ATP (20 microM), which had no effect on the KACh channel by itself, potentiated the stimulatory action of phospholipids, indicating that ATP and phospholipids interacted to modulate KACh channel function. 7. We conclude that exogenously applied PIP2 and other phospholipids block agonist-mediated KACh channel activation. However, if the KACh channel is already activated with GTPgammaS, phospholipids augment the existing activity by increasing the number of longer-lived channel openings. The evidence for and against the role of PIP and PIP2 in the stimulatory effect of ATP on the KACh channel is presented and discussed.

Phosphoinositide 3-kinase lipid products regulate ATP-dependent transport by sister of P-glycoprotein and multidrug resistance associated protein 2 in bile canalicular membrane vesicles

Bile acid transport and secretion in hepatocytes require phosphatidylinositol (PI) 3-kinase-dependent recruitment of ATP-dependent transporters to the bile canalicular membrane and are accompanied by increased canalicular PI 3-kinase activity. We report here that the lipid products of PI 3-kinase also regulate ATP-dependent transport of taurocholate and dinitrophenyl-glutathione directly in canalicular membranes. ATP-dependent transport of taurocholate and dinitrophenyl-glutathione in isolated canalicular vesicles from rat liver was reduced 50-70% by PI 3-kinase inhibitors, wortmannin, and LY294002, at concentrations that are specific for Type I PI 3-kinase. Inhibition was reversed by addition of lipid products of PI 3-kinase (PI 3,4-bisphosphate and, to a lesser extent, PI 3-phosphate and PI 3,4,5-trisphosphate) but not by PI 4, 5-bisphosphate. A membrane-permeant synthetic 10-mer peptide that binds polyphosphoinositides and leads to activation of PI 3-kinase in macrophages doubled PI 3-kinase activity in canalicular membrane vesicles and enhanced taurocholate and dinitrophenyl-glutathione transport in canalicular membrane vesicles above maximal ATP-dependent transport. The effect of the peptide was blocked by wortmannin and LY294002. PI 3-kinase activity was also necessary for function of the transporters in vivo. ATP-dependent transport of taurocholate and PI 3-kinase activity were reduced in canalicular membrane vesicles isolated from rat liver that had been perfused with taurocholate and wortmannin. PI 3,4-bisphosphate enhanced ATP-dependent transport of taurocholate in these vesicles above control levels. Our results indicate that PI 3-kinase lipid products are necessary in vivo and in vitro for maximal ATP-dependent transport of bile acid and nonbile acid organic anions across the canalicular membrane. Our results demonstrate regulation of membrane ATP binding cassette transporters by PI 3-kinase lipid products.

Glucose generates sub-plasma membrane ATP microdomains in single islet beta-cells. Potential role for strategically located mitochondria

Increases in the concentration of free ATP within the islet beta-cell may couple elevations in blood glucose to insulin release by closing ATP-sensitive K+ (KATP) channels and activating Ca2+ influx. Here, we use recombinant targeted luciferases and photon counting imaging to monitor changes in free [ATP] in subdomains of single living MIN6 and primary beta-cells. Resting [ATP] in the cytosol ([ATP]c), in the mitochondrial matrix ([ATP]m), and beneath the plasma membrane ([ATP]pm) were similar ( approximately 1 mM). Elevations in extracellular glucose concentration (3-30 mM) increased free [ATP] in each domain with distinct kinetics. Thus, sustained increases in [ATP]m and [ATP]pm were observed, but only a transient increase in [ATP]c. However, detectable increases in [ATP]c and [ATP]pm, but not [ATP]m, required extracellular Ca2+. Enhancement of glucose-induced Ca2+ influx with high [K+] had little effect on the apparent [ATP]c and [ATP]m increases but augmented the [ATP]pm increase. Underlying these changes, glucose increased the mitochondrial proton motive force, an effect mimicked by high [K+]. These data support a model in which glucose increases [ATP]m both through enhanced substrate supply and by progressive Ca2+-dependent activation of mitochondrial enzymes. This may then lead to a privileged elevation of [ATP]pm, which may be essential for the sustained closure of KATP channels. Luciferase imaging would appear to be a useful new tool for dynamic in vivo imaging of free ATP concentration.

Alternative splicing of sur2 Exon 17 regulates nucleotide sensitivity of the ATP-sensitive potassium channel

ATP-sensitive potassium channels (KATP) are implicated in a diverse array of physiological functions. Previous work has shown that alternative usage of exons 14, 39, and 40 of the muscle-specific KATP channel regulatory subunit, sur2, occurs in tissue-specific patterns. Here, we show that exon 17 of the first nucleotide binding fold of sur2 is also alternatively spliced. RNase protection demonstrates that SUR2(Delta17) predominates in skeletal muscle and gut and is also expressed in bladder, fat, heart, lung, liver, and kidney. Polymerase chain reaction and restriction digest analysis of sur2 cDNA demonstrate the existence of at least five sur2 splice variants as follows: SUR2(39), SUR2(40), SUR2(Delta17/39), SUR2(Delta17/40), and SUR2(Delta14/39). Electrophysiological recordings of excised, inside-out patches from COS cells cotransfected with Kir6.2 and the sur2 variants demonstrated that exon 17 splicing alters KATP sensitivity to ATP block by 2-fold from approximately 40 to approximately 90 microM for exon 17 and Delta17, respectively. Single channel kinetic analysis of SUR2(39) and SUR2(Delta17/39) demonstrated that both exhibited characteristic KATP kinetics but that SUR2(Delta17/39) exhibited longer mean burst durations and shorter mean interburst dwell times. In sum, alternative splicing of sur2 enhances the observed diversity of KATP and may contribute to tissue-specific modulation of ATP sensitivity.

Effects of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate on a Na+-gated nonselective cation channel

Olfactory receptor neurons in the lobster express a nonselective cation channel that is activated by intracellular Na+ and carries a substantial part of the depolarizing receptor current. Here, we show that phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and phosphatidylinositol 4-phosphate [PI(4)P] applied to the intracellular face of cell-free patches activate the channel in the absence of Na+ and that antibodies against the respective phospholipids irreversibly inhibit the evoked activity. Further, we show that applying PI(4,5)P2 or PI(4)P in the presence of Na+ decreases the concentration of Na+ required to activate the channel from an EC50 of 74 to 22 mM for PI(4,5)P2 and to 29 mM for PI(4)P, respectively. Na+-gated channel activity was irreversibly inhibited by monoclonal antibodies against PI(4,5)P2 and PI(4)P in patches never exposed to exogenous phosphatidylinositols, suggesting that endogenous inositol phospholipids are required for the activation of the channel by intracellular Na+. Our findings suggest that PI(4,5)P2 and/or PI(4)P may serve as intracellular signaling molecules in these primary sensory neurons and provide a general mechanism to explain how the sensitivity of Na+-gated channels to Na+ could be much greater in intact cells than in excised membrane patches.

Transmembrane structure of an inwardly rectifying potassium channel

Inwardly rectifying potassium channels (K(ir)), comprising four subunits each with two transmembrane domains, M1 and M2, regulate many important physiological processes. We employed a yeast genetic screen to identify functional channels from libraries of K(ir) 2.1 containing mutagenized M1 or M2 domains. Patterns in the allowed sequences indicate that M1 and M2 are helices. Protein-lipid and protein-water interaction surfaces identified by the patterns were verified by sequence minimization experiments. Second-site suppressor analyses of helix packing indicate that the M2 pore-lining inner helices are surrounded by the M1 lipid-facing outer helices, arranged such that the M1 helices participate in subunit-subunit interactions. This arrangement is distinctly different from the structure of a bacterial potassium channel with the same topology and identifies helix-packing residues as hallmark sequences common to all K(ir) superfamily members.