Regulation of cardiac Na+,Ca2+ exchange and KATP potassium channels by PIP2

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.

Functional analysis of a disulfide bond in the cardiac Na(+)-Ca(2+) exchanger

The electrophoretic mobility of the cardiac Na(+)-Ca(2+) exchange protein is different under reducing and nonreducing conditions. This mobility shift is eliminated in a cysteine-less exchanger, suggesting that the presence or absence of an intramolecular disulfide bond alters the conformation and mobility of the exchanger. Using cysteine mutagenesis and biochemical analysis, we have identified the cysteine residues involved in the disulfide bond. Cysteine 792 in loop h of the exchanger forms a disulfide bond with either cysteine 14 or 20 near the NH(2) terminus. Because the NH(2) terminus is extracellular, the data establish that loop h must also be extracellular. A rearrangement of disulfide bonds has previously been implicated in the stimulation of exchange activity by combinations of reducing and oxidizing agents. We have investigated the role of cysteines in the stimulation of the exchanger by the combination of FeSO(4) and dithiothreitol (Fe-DTT). Using the giant excised patch technique, we find that stimulation of the wild type exchanger by Fe-DTT is primarily due to the removal of a Na(+)-dependent inactivation process. Analysis of mutated exchangers, however, indicates that cysteines are not responsible for stimulation of the exchange activity by Fe-DTT. Ca(2+) blocks modification of the exchanger by Fe-DTT. Disulfide bonds are not involved in redox stimulation of the exchanger, and the modification reaction is unknown. Modulation of Na(+)-dependent inactivation may be a general mechanism for regulation of Na(+)-Ca(2+) exchange activity and may have physiological significance.

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.

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.

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.

Mode-specific inhibition of sodium-calcium exchange during protein phosphatase blockade

The effects of the protein phosphatase inhibitors calyculin A and okadaic acid on Na(+)/Ca(2+) exchange activity were examined in transfected Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger. Incubating the cells for 5-10 min with 100 nM calyculin A reduced exchange-mediated (45)Ca(2+) uptake or Ba(2+) influx by 50-75%. Half-maximal inhibition of (45)Ca(2+) uptake was observed at 15 nM calyculin A. The nonselective protein kinase inhibitors K252a and staurosporine provided partial protection against the effects of calyculin A. Okadaic acid, another protein phosphatase inhibitor, nearly completely blocked exchange-mediated Ba(2+) influx. Chinese hamster ovary cells expressing a mutant exchanger in which 420 out of 520 amino acid residues were deleted from the central hydrophilic domain of the exchanger remained sensitive to the inhibitory effects of calyculin A and okadaic acid. Surprisingly, Na(o)(+)-dependent Ca(2+) efflux appeared to be only modestly inhibited, if at all, by calyculin A or okadaic acid. We conclude that protein hyperphosphorylation during protein phosphatase blockade selectively inhibits the Ca(2+) influx mode of Na(+)/Ca(2+) exchange, probably by an indirect mechanism that does not involve phosphorylation of the exchanger itself.

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.

Alpha1-adrenoceptor-mediated formation of glycerophosphoinositol 4-phosphate in rat heart: possible role in the positive inotropic response

In the present study, we investigated whether phospholipase A2 (PLA2)/lysophospholipase activity producing glycerophosphoinositols from phosphoinositides was operating in rat heart and could be stimulated by alpha1-adrenergic agonists. PLA2/lysophospholipase activity was found in homogenates from rat right ventricles. The stimulation of PLA2/lysophospholipase activity by noradrenaline (NA) was prevented either by the alpha1-adrenergic antagonist prazosin or arachidonyl trifluoromethyl ketone, a selective inhibitor of the 85-110 kDa, sn-2-arachidonyl-specific cytosolic PLA2. The selective alpha1-adrenergic agonist phenylephrine induced a concentration- and time-dependent increase in glycerophosphoinositol (GroPIns) and glycerophosphoinositol 4-phosphate (GroPIns4P) in rat right ventricle slices prelabelled with D-myo-[3H]inositol. In electrically driven strips of rat right ventricles, prelabelled with D-myo-[3H]inositol, the positive inotropic effect induced by 20 microM NA in the presence of propranolol was accompanied by the formation of GroPIns and GroPIns4P. The concentration of the formed GroPIns4P (1.33+/-0.12 microM, N = 6) was similar to that previously reported to inhibit the Na+/Ca2+ exchanger in cardiac sarcolemmal vesicles (Luciani S, Antolini M, Bova S, Cargnelli G, Cusinato F, Debetto P, Trevisi L and Varotto R, Biochem Biophys Res Commun 206: 674-680, 1995). These findings show that the stimulation of alpha1-adrenoceptors in rat heart is followed by an increase in the formation of GroPIns4P, which may contribute to the positive inotropic effect of alpha1-adrenergic agonists by inhibition of the Na+/Ca2+ exchanger.

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.

A molecular mechanism of aluminium-induced Alzheimer's disease?

An abundance of research has continued to link aluminium (Al) with Alzheimer's disease (AD) (Strong et al., J. Toxicol. Environ. Health 48 (1996) 599; Savory et al., J. Toxicol. Environ. Health 48 (1996) 615). Animals loaded with Al develop both symptoms and brain lesions that are similar to those found in AD. However, these animal models of Al intoxication are not representative of human exposure to Al. They have not addressed the significance of a truly chronic exposure to Al. If Al is a cause of AD it is effective at the level of our everyday exposure to the metal and AD will be one possible outcome of the life-long presence of a low, though burgeoning, brain Al burden. Individual susceptibility to AD will be as much to do with differences in brain physiology as with changes in our everyday exposure to the metal. There will be a chemical response and indeed biochemical/physiological response in the brain to Al. The question is whether brain Al homeostasis could impact upon brain function. In reviewing the recent literature covering the neurotoxicity of Al and, in particular, of the known and probable mechanisms involved in brain Al homeostasis I have identified a mechanism through which a truly chronic exposure to Al would bring about subtle and persistent changes in neurotransmission which, in time, could instigate the cascade of events known collectively as AD. This mechanism involves the potentiation of the activities of neurotransmitters by the action of Al-ATP at adenosine 5'-triphosphate (ATP) receptors in the brain.

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.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Changes in sarcolemmal PLC isoenzymes in postinfarct congestive heart failure: partial correction by imidapril

We have examined the changes in quantity and activity of cardiac sarcolemmal (SL) phosphoinositide-phospholipase C (PLC)-beta(1), -gamma(1), and -delta(1) in a model of congestive heart failure (CHF) secondary to large transmural myocardial infarction (MI). We also instituted a late in vivo monotherapy with imidapril, an ANG-converting enzyme (ACE) inhibitor, to test the hypothesis that its therapeutic action is associated with the functional correction of PLC isoenzymes. SL membranes were purified from the surviving left ventricle of rats in a moderate stage of CHF at 8 wk after occlusion of the left anterior descending coronary artery. SL PLC isoenzymes were examined in terms of protein mass and hydrolytic activity. CHF resulted in a striking reduction (to 6-17% of controls) of the mass and activity of gamma(1)- and delta(1)-isoforms in combination with a significant increase of both PLC beta(1) parameters. In vivo treatment with imidapril (1 mg/kg body wt, daily, initiated 4 wk after coronary occlusion) improved the contractile function and induced a partial correction of PLCs. The mass of SL phosphatidylinositol 4,5-bisphosphate and the activities of the enzymes responsible for its synthesis were significantly reduced in post-MI CHF and partially corrected by imidapril. The results indicate that profound changes in the profile of heart SL PLC-beta(1), -gamma(1), and -delta(1) occur in CHF, which could alter the complex second messenger responses of these isoforms, whereas their partial correction by imidapril may be related to the mechanism of action of this ACE inhibitor.

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.

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

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

ATP dependence is not an intrinsic property of Na+/H+ exchanger NHE1: requirement for an ancillary factor

Na+/H+ exchange is a passive process not requiring expenditure of metabolic energy. Nevertheless, depletion of cellular ATP produces a marked inhibition of the antiport. No evidence has been found for direct binding of nucleotide to exchangers or alteration in their state of phosphorylation, suggesting ancillary factors may be involved. This possibility was tested by comparing the activity of dog red blood cells (RBC) and their resealed ghosts. Immunoblotting experiments using isoform-specific polyclonal and monoclonal antibodies indicated RBC membranes express Na+/H+ exchanger isoform 1 (NHE1). In intact RBC, uptake of Na+ was greatly stimulated when the cytosol was acidified. The stimulated uptake was largely eliminated by amiloride and by submicromolar concentrations of the benzoyl guanidinium compound HOE-694, consistent with mediation by NHE1. Although exchange activity could also be elicited by acidification in resealed ghosts containing ATP, the absolute rate of transport was markedly diminished at comparable pH. Dissipation of the pH gradient was ruled out as the cause of diminished transport rate in ghosts. This was accomplished by a "pH clamping" procedure based on continued export of base equivalents by the endogenous anion exchanger. These observations suggest a critical factor required to maintain optimal Na+/H+ exchange activity is lost or inactivated during preparation of ghosts. Depletion of ATP, achieved by incubation with 2-deoxy-D-glucose, inhibited Na+/H+ exchange in intact RBC, as reported for nucleated cells. In contrast, the rate of exchange was similar in control and ATP-depleted resealed ghosts. Interestingly, the residual rate of Na+/H+ exchange in ATP-depleted but otherwise intact cells was similar to the transport rate of ghosts. Therefore, we tentatively conclude that full activation of NHE1 requires both ATP and an additional regulatory factor, which may mediate the action of the nucleotide. Ancillary phosphoproteins or phospholipids or the kinases that mediate their phosphorylation are likely candidates for the regulatory factor(s) that is inactivated or missing in ghosts.

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.

Regulation of ROMK1 channel by protein kinase A via a phosphatidylinositol 4,5-bisphosphate-dependent mechanism

ROMK inward-rectifier K+ channels control renal K+ secretion. The activity of ROMK is regulated by protein kinase A (PKA), but the molecular mechanism for regulation is unknown. Having found that direct interaction with membrane phosphatidylinositol 4, 5-bisphosphate (PIP2) is essential for channel activation, we investigate here the role of PIP2 in regulation of ROMK1 by PKA. By using adenosine-5'-[gamma-thio]triphosphate) (ATP[gammaS]) as the substrate, we found that PKA does not directly activate ROMK1 channels in membranes that are devoid of PIP2. Rather, phosphorylation by PKA + ATP[gammaS] lowers the concentration of PIP2 necessary for activation of the channels. In solution-binding assays, anti-PIP2 antibodies bind PIP2 and prevent PIP2-channel interaction. In inside-out membrane patches, antibodies inhibit the activity of the channels. PKA treatment then decreases the sensitivity of ROMK1 for inhibition by the antibodies, indicating an enhanced interaction between PIP2 and the phosphorylated channels. Conversely, mutation of the PKA phosphorylation sites in ROMK1 decreases PIP2 interaction with the channels. Thus, PKA activates ROMK1 channels by enhancing PIP2-channel interaction.

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.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

Changes in phosphoinositide metabolism with days in culture affect signal transduction pathways in galdieria sulphuraria

The metabolism of phosphatidylinositol-4,5-bisphosphate (PIP2) changed during the culture period of the thermoacidophilic red alga Galdieria sulphuraria. Seven days after inoculation, the amount of PIP2 in the cells was 910 +/- 100 pmol g-1 fresh weight; by 12 d, PIP2 levels increased to 1200 +/- 150 pmol g-1 fresh weight. In vitro assays indicated that phosphatidylinositol monophosphate (PIP) kinase specific activity increased from 75 to 230 pmol min-1 mg-1 protein between d 7 and 12. When G. sulphuraria cells were osmostimulated, transient increases of up to 4-fold could be observed in inositol-1,4,5-trisphosphate (IP3) levels within 90 s, regardless of the age of the cells. In d-12 cells, the increase in IP3 was preceded by a transient increase of up to 5-fold in specific PIP kinase activity, whereas no such increase was detected after osmostimulation of d-7 cells. The increase in PIP kinase activity before IP3 signaling in d-12 cells indicates that there is an additional pathway for regulation of phosphoinositide metabolism after stimulation other than an initial activation of phospholipase C. Also, the rapid activation of PIP2 biosynthesis in cells with already-high PIP2 levels suggests that the PIP2 present was not available for signal transduction. By comparing the response of the cells at d 7 and 12, we have identified two potentially distinct pools of PIP2.

Unique topology of the internal repeats in the cardiac Na+/Ca2+ exchanger

Hydropathy analysis predicts 11 transmembrane helices in the cardiac Na+/Ca2+ exchanger. Using cysteine susceptibility analysis and epitope tagging, we here studied the membrane topology of the exchanger, in particular of the highly conserved internal alpha-1 and alpha-2 repeats. Unexpectedly, we found that the connecting loop in the alpha-1 repeat forms a re-entrant membrane loop with both ends facing the extracellular side and one residue (Asn-125) being accessible from the inside and that the region containing the alpha-2 repeat is mostly accessible from the cytoplasm. Together with other data, we propose that the exchanger may consist of nine transmembrane helices.

Wortmannin, an inhibitor of phosphatidylinositol kinases, blocks the MgATP-dependent recovery of Kir6.2/SUR2A channels

1. In order to investigate the mechanism underlying MgATP-dependent recovery of ATP-sensitive potassium (KATP) channels, we expressed Kir6.2/SUR2A (inwardly rectifying K+ channel subunit/sulfonylurea receptor) or C-terminal-truncated Kir6.2 (Kir6.2DeltaC26) in COS7 cells (Green monkey kidney cells), and carried out inside-out patch clamp experiments. 2. After patch excision in ATP-free internal solution, the activity of Kir6.2/SUR2A channels could be maximally recovered by the application of 5 mM MgATP. Subsequent application of 100 microM Ca2+ induced a rapid decay of Kir6.2/SUR2A activity to 11.6 +/- 1.1 % (mean +/- s.e.m.) of the control level (Ca2+-induced run-down; n = 64). 3. MgATP (5 mM) recovered 99.4 +/- 4.2 % (n = 13) of the Ca2+-induced run-down. Protein kinase inhibitors such as W-7, H-7, H-8 and genistein did not inhibit this reaction. However, wortmannin, an inhibitor of phosphatidylinositol 3- and 4-kinases, blocked the MgATP-dependent recovery in a concentration-dependent manner; the magnitudes of recovery were 35.7 +/- 7.2 % (10 microM) and 4.3 +/- 2.5 % (100 microM) of the Ca2+-induced run-down. 4. MgUDP (10 mM) reversed the Ca2+-induced run-down of Kir6.2/SUR2A channels by 60.4 +/- 7.6 % (n = 5). Wortmannin failed to modify this reaction. 5. Kir6.2DeltaC26 channels, which opened in the absence of SUR2A, were less sensitive to Ca2+; Kir6.2DeltaC26 channels were inactivated to 44.8 +/- 4.4 % (n = 14) by 100 microM Ca2+. MgATP recovered the Ca2+-induced run-down of Kir6.2DeltaC26 by 89.8 +/- 7. 7 % (n = 9), and 100 microM wortmannin inhibited this reaction (1.8 +/- 2 %, n = 7). 6. Application of 10 microM phosphatidylinositol-4, 5-bisphosphate (PI-4,5-P2) recovered the activity of Kir6.2/SUR2A channels after Ca2+-induced run-down (104.3 +/- 6.4 %, n = 10). Even after the MgATP-dependent recovery was blocked by 100 microM wortmannin, PI-4,5-P2 reactivated the channels (102.3 +/- 8.6 %, n = 5). Similar results were obtained with Kir6.2DeltaC26. 7. These results suggest that the entity of MgATP-dependent recovery may be membrane lipid phosphorylation rather than protein phosphorylation, and that synthesis of PI-4,5-P2 or phosphatidylinositol-3,4, 5-trisphosphate may upregulate Kir6.2 channels.

Interaction of the Na(+)-Ca2+ exchanger with small molecules on cell Ca2+ signaling

Interactions of the Na(+)-Ca2+ exchanger with small molecules on cell Ca2+ signaling were elucidated in Chinese hamster ovary (CHO) C1 cells, which transfected a control vector without any expression of the Na(+)-Ca2+ exchanger's gene while CHO CK1.4 cells transfected an expression vector encoding the bovine cardiac Na(+)-Ca2+ exchanger's cDNA, treated with lithium- or sodium-buffer medium respectively, by using L16(2)15 multifactorial orthogonal statistics and fura-2 fluorescence real-time imaging. In contrast to controls of Li(+)-treated C1 cells, the store-dependent Ca(2+)-influx (SDCI) was enhanced by either the Na(+)-Ca2+ exchanger, Na(+), 1-¿(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl¿-1H-imidazole HCl (SK&F96365) or ouabain, and by interactions of the Na(+)-Ca2+ exchanger with either Na+, SK&F96365 or both SK&F96365 and ouabain; and ATP-induced Ca2+ release (AICR) was activated by SK&F96365 or Na+ alone, interactions of the Na(+)-Ca2+ exchanger with SK&F96365 or Na+, and an interaction between SK&F96365 and ouabain. The dramatic interaction of the Na(+)-Ca2+ exchanger with small molecules indicates that cell Ca2+ signaling is generated by inositol triphosphate (InsP3)-dependent pathways, allosteric effects of the G-protein coupled P2y&2u purinoceptor and multi-site recognition. Our findings provide meaningful clues for designing new strategies of cardiocerebral vascular oxidative diseases.

pH-dependent gating of ROMK (Kir1.1) channels involves conformational changes in both N and C termini

ROMK channels (Kir1.1) are members of the superfamily of inward rectifier potassium channels (Kir) and represent the channels underlying K+ secretion in the kidney. As their native counterparts, Kir1.1 channels are gated by intracellular pH, with acidification leading to channel closure. Although a lysine residue (Lys80) close to the first hydrophobic segment M1 has been identified as the pH sensor, little is known about how opening and closing of the channel is accomplished. Here we investigate the gating process of Kir1.1 channels exploiting their state-dependent modification by water-soluble oxidants and sulfhydryl reagents. Mutagenesis of all intracellular cysteines either alone or in combination revealed two residues targeted by these reagents, one in the N terminus (Cys49) and one in the C terminus (Cys308) of the channel protein. Both sites reacted with the thiol reagents only in the closed state and not in the open state. These results indicate that pH-dependent gating of Kir1.1 channels involves movement of protein domains in both N and C termini of the Kir1.1 protein.

PIPkins1, their substrates and their products: new functions for old enzymes

The phosphatidylinositolphosphate kinases (PIPkins) are a unique family of enzymes that catalyse the production of phosphorylated inositol lipids. Recent advances have revealed that, due to their ability to utilise a number of different lipid substrates (at least in vitro), this family is potentially able to generate several distinct, physiologically important inositol lipids. Despite their importance, however, our understanding of the regulation of the PIPkins and of their physiological role in cellular signalling and regulation is still poor. Here we describe in turn the diverse physiological functions of the known substrates and major products of the PIPkins. We then examine what is known about the members of the PIPkin family themselves, and their characteristics and regulation.

Activation of KATP channels by Na/K pump in isolated cardiac myocytes and giant membrane patches

Strophanthidin inhibits KATP channels in 2,4-dinitrophenol-poisoned heart cells (). The current study shows that the Na/K pump interacts with KATP current (IK-ATP) via submembrane ATP depletion in isolated giant membrane patches and in nonpoisoned guinea pig cardiac cells in whole-cell configuration. IK-ATP was inhibited by ATP, glibenclamide, or intracellular Cs+. Na/K pump inactivation by substitution of cytoplasmic Na+ for Li+ or N-methylglucamine decreased both IK-ATP by 1/3 (1 mM ATP, zero calcium), and IC50 of ATP for IK-ATP (0.3 +/- 0.1 mM) by 2/5. The Na+/Li+ replacement had no effect on IK-ATP at low pump activity ([ATP] </= 0.1 mM or 100 microM ouabain) or when IK-ATP was completely inhibited by 10 mM ATP. In whole-cell configuration, ouabain inhibited up to 60% of inwardly rectifying IK-ATP at 1 mM ATP in the pipette but not at 10 mM ATP and 10 mM phosphocreatine when IK-ATP was always blocked. However, mathematical simulation of giant-patch experiments revealed that only 20% of ATP depletion may be attributed to the ATP concentration gradient in the bulk solution, and the remaining 80% probably occurs in the submembrane space.

PIP2 and PIP as determinants for ATP inhibition of KATP channels

Adenosine triphosphate (ATP)-sensitive potassium (KATP) channels couple electrical activity to cellular metabolism through their inhibition by intracellular ATP. ATP inhibition of KATP channels varies among tissues and is affected by the metabolic and regulatory state of individual cells, suggesting involvement of endogenous factors. It is reported here that phosphatidylinositol-4, 5-bisphosphate (PIP2) and phosphatidylinositol-4-phosphate (PIP) controlled ATP inhibition of cloned KATP channels (Kir6.2 and SUR1). These phospholipids acted on the Kir6.2 subunit and shifted ATP sensitivity by several orders of magnitude. Receptor-mediated activation of phospholipase C resulted in inhibition of KATP-mediated currents. These results represent a mechanism for control of excitability through phospholipids.

Membrane phospholipid control of nucleotide sensitivity of KATP channels

Adenosine triphosphate (ATP)-sensitive potassium (KATP) channels couple cell metabolism to electrical activity. Phosphatidylinositol phosphates (PIPs) profoundly antagonized ATP inhibition of KATP channels when applied to inside-out membrane patches. It is proposed that membrane-incorporated PIPs can bind to positive charges in the cytoplasmic region of the channel's Kir6.2 subunit, stabilizing the open state of the channel and antagonizing the inhibitory effect of ATP. The tremendous effect of PIPs on ATP sensitivity suggests that in vivo alterations of membrane PIP levels will have substantial effects on KATP channel activity and hence on the gain of metabolism-excitation coupling.

Cytoplasmic terminus domains of Kir6.x confer different nucleotide-dependent gating on the ATP-sensitive K+ channel

1. In order to investigate the structural basis for the nucleotide-dependent gating of ATP-sensitive K+ channels (KATP), Kir6.1 (uKATP-1), Kir6.2 (BIR1) and chimeric channels were co-expressed with a common subtype of sulphonylurea receptor, SUR1, in COS7 cells. Representing the amino terminal domain-transmembrane domain-carboxyl-terminal domain of Kir6.1 as 1-1-1 and of Kir6.2 as 2-2-2, chimeric Kir6.x channels were constructed by swapping the amino and/or carboxyl terminal domains between Kir6.1 and Kir6.2 to give the chimeric x-1-x channels 1-1-2, 2-1-1 and 2-1-2, and the chimeric x-2-x channels 2-2-1, 1-2-2 and 1-2-1. 2. Inside-out patch clamp experiments revealed that both wild-type Kir6.1 and Kir6.2 formed inwardly rectifying K+ channels. Single-channel conductances were 36.3 and 66.1 pS, respectively. Chimeric x-1-x channels, whose transmembrane domain was that of Kir6.1, showed similar ion-pore properties to wild-type Kir6.1. Likewise, chimeric x-2-x channels had similar ion-pore properties to wild-type Kir6.2. 3. Wild-type Kir6.1 and Kir6.2 possessed distinct gating properties towards intracellular nucleotides. The activity of Kir6.1 was entirely dependent on Mg2+ and nucleotide diphosphates (NDPs) such as UDP. In contrast, Kir6.2 was activated upon excision of patch membrane. When Kir6.2 underwent rundown, UDP reactivated the channel. 4. In order to eliminate UDP dependence from Kir6.1, it was necessary to replace both N- and C-termini; chimera 2-1-2 opened in UDP-free conditions. With Kir6.2, substitution of the N-terminus with that of Kir6.1 conferred UDP dependence on chimeras 1-2-2 and 1-2-1. Chimera 2-2-1 opened in UDP-free conditions, but UDP potentiated the channel activity by > 20-fold. 5. The kinetics of UDP-dependent activation were significantly different between Kir6.1 and Kir6.2. Kir6.1 maximally activated by UDP was sensitive to intracellular ATP, although its ATP sensitivity was significantly lower than that of Kir6.2 measured in identical conditions. The kinetics of UDP-dependent activation and ATP sensitivity could be transferred between Kir6.1 and Kir6.2 only when both N- and C-termini were replaced. We therefore concluded that nucleotide-dependent gating was regulated by the N- and C-terminal domains irrespective of the transmembrane domains.

Mechanism of cloned ATP-sensitive potassium channel activation by oleoyl-CoA

Insulin secretion from pancreatic beta cells is coupled to cell metabolism through closure of ATP-sensitive potassium (KATP) channels, which comprise Kir6.2 and sulfonylurea receptor (SUR1) subunits. Although metabolic regulation of KATP channel activity is believed to be mediated principally by the adenine nucleotides, other metabolic intermediates, including long chain acyl-CoA esters, may also be involved. We recorded macroscopic and single-channel currents from Xenopus oocytes expressing either Kir6.2/SUR1 or Kir6. 2DeltaC36 (which forms channels in the absence of SUR1). Oleoyl-CoA (1 microM) activated both wild-type Kir6.2/SUR1 and Kir6.2DeltaC36 macroscopic currents, approximately 2-fold, by increasing the number and open probability of Kir6.2/SUR1 and Kir6.2DeltaC36 channels. It was ineffective on the related Kir subunit Kir1.1a. Oleoyl-CoA also impaired channel inhibition by ATP, increasing the Ki values for both Kir6.2/SUR1 and Kir6.2DeltaC36 currents by approximately 3-fold. Our results indicate that activation of KATP channels by oleoyl-CoA results from an interaction with the Kir6.2 subunit, unlike the stimulatory effects of MgADP and diazoxide which are mediated through SUR1. The increased activity and reduced ATP sensitivity of KATP channels by oleoyl-CoA might contribute to the impaired insulin secretion observed in non-insulin-dependent diabetes mellitus.

Cardiac cell volume: crystal clear or murky waters? A comparison with other cell types

The osmolarity of bodily fluids is strictly controlled so that most cells do not experience changes in osmotic pressure under normal conditions, but osmotic changes can occur in pathological states such as ischemia, septic shock, and diabetic coma. The primary effect of a change in osmolarity is to acutely alter cell volume. If the osmolarity around a cell is decreased, the cell swells, and if increased, it shrinks. In order to tolerate changes in osmolarity, cells have evolved volume regulatory mechanisms activated by osmotic challenge to normalise cell volume and maintain normal function. In the heart, osmotic stress is encountered during a period of myocardial ischemia when metabolites such as lactate accumulate intracellularly and to a certain degree extracellularly, and cause cell swelling. This swelling may be exacerbated further on reperfusion when the hyperosmotic extracellular milieu is replaced by normosmotic blood. In this review, we describe the theory and mechanisms of volume regulation, and draw on findings in extracardiac tissues, such as kidney, whose responses to osmotic change are well characterised. We then describe cell volume regulation in the heart, with particular emphasis on the effect of myocardial ischemia. Finally, we describe the consequences of osmotic cell swelling for the cell and for the heart, and discuss the implications for antiarrhythmic drug efficacy. Using computer modelling, we have summated the changes induced by cell swelling, and predict that swelling will shorten the action potential. This finding indicates that cell swelling is an important component of the response to ischemia, a component modulating the excitability of the heart.

Recent advances on renal inward rectifier K+ channels

Inwar J rectifier K+ channels control resting membrane potential, cell excitability, insulin secretion and renal K+ transport. Much progress has been made in the structure and regulation, as well as the molecular identity and physiological role played by the channels. This review discusses recent advances in inward rectifier K+ channels with a special emphasis on the renal channels.

Regulation of Na+/Ca2+ exchange activity by cytosolic Ca2+ in transfected Chinese hamster ovary cells

Transfected Chinese hamster ovary cells stably expressing the bovine cardiac Na+/Ca2+ exchanger (CK1.4 cells) were used to determine the range of cytosolic Ca2+ concentrations ([Ca2+]i) that activate Na+/Ca2+ exchange activity. Ba2+ influx was measured in fura 2-loaded, ionomycin-treated cells under conditions in which the intracellular Na+ concentration was clamped with gramicidin at approximately 20 mM. [Ca2+]i was varied by preincubating ionomycin-treated cells with either the acetoxymethyl ester of EGTA or medium containing 0-1 mM added CaCl2. The rate of Ba2+ influx increased in a saturable manner with [Ca2+]i, with the half-maximal activation value of 44 nM and a Hill coefficient of 1.6. When identical experiments were carried out with cells expressing a Ca2+-insensitive mutant of the exchanger, Ba2+ influx did not vary with [Ca2+]i. The concentration for activation of exchange activity was similar to that reported for whole cardiac myocytes but approximately an order of magnitude lower than that reported for excised, giant patches. The reason for the difference in Ca2+ regulation between whole cells and membrane patches is unknown.

Functional coupling of phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-trisphosphate receptor

The inositol 1,4,5-trisphosphate receptor (InsP3R) plays a key role in intracellular Ca2+ signaling. InsP3R is activated by InsP3 produced from phosphatidylinositol 4,5-bisphosphate (PIP2) by phospholipase C cleavage. Using planar lipid bilayer reconstitution technique, we demonstrate here that rat cerebellar InsP3R forms a stable inhibitory complex with endogenous PIP2. Disruption of InsP3R-PIP2 interaction by specific anti-PIP2 monoclonal antibody resulted in 3-4-fold increase in InsP3R activity and 10-fold shift in apparent affinity for InsP3. Exogenously added PIP2 blocks InsP3 binding to InsP3R and inhibits InsP3R activity. Similar results were obtained with a newly synthesized water soluble analog of PIP2, dioctanoyl-(4,5)PIP2, indicating that insertion of PIP2 into membrane is not required to exert its inhibitory effects on the InsP3R. We hypothesize that the functional link between InsP3R and PIP2 described in the present report provides a basis for a local, rapid, and efficient coupling between phospholipase C activation, PIP2 hydrolysis, and intracellular Ca2+ wave initiation in neuronal and non-neuronal cells.

Functional Modulation of Cardiac ATP-Sensitive K(+) Channels

ATP-sensitive K(+) (K(ATP)) channels are inhibited by intracellular ATP, but MgATP is necessary to maintain the channel activity. Numerous cofactors modulate channel function. K(+) channel openers activate and sulfonylureas inhibit K(ATP) channels. The structure of cardiac K(ATP) channel is a complex of mainly K(IR)6.2 and SUR2a. Activation of cardiac K(ATP) channels contributes to action potential shortening during ischemia and plays a role in cardioprotection.

Cloning, expression, and characterization of the squid Na+-Ca2+ exchanger (NCX-SQ1)

We have cloned the squid neuronal Na+-Ca2+ exchanger, NCX-SQ1, expressed it in Xenopus oocytes, and characterized its regulatory and ion transport properties in giant excised membrane patches. The squid exchanger shows 58% identity with the canine Na+-Ca2+ exchanger (NCX1.1). Regions determined to be of functional importance in NCX1 are well conserved. Unique among exchanger sequences to date, NCX-SQ1 has a potential protein kinase C phosphorylation site (threonine 184) between transmembrane segments 3 and 4 and a tyrosine kinase site in the Ca2+ binding region (tyrosine 462). There is a deletion of 47 amino acids in the large intracellular loop of NCX-SQ1 in comparison with NCX1. Similar to NCX1, expression of NCX-SQ1 in Xenopus oocytes induced cytoplasmic Na+-dependent 45Ca2+ uptake; the uptake was inhibited by injection of Ca2+ chelators. In giant excised membrane patches, the NCX-SQ1 outward exchange current showed Na+-dependent inactivation, secondary activation by cytoplasmic Ca2+, and activation by chymotrypsin. The NCX-SQ1 exchange current was strongly stimulated by both ATP and the ATP-thioester, ATP gamma S, in the presence of F- (0.2 mM) and vanadate (50 microM), and both effects reversed on application of a phosphatidylinositol-4',5'-bisphosphate antibody. NCX1 current was stimulated by ATP, but not by ATP gamma S. Like NCX1 current, NCX-SQ1 current was strongly stimulated by phosphatidylinositol-4',5'-bisphosphate liposomes. In contrast to results in squid axon, NCX-SQ1 was not stimulated by phosphoarginine (5-10 mM). After chymotrypsin treatment, both the outward and inward NCX-SQ1 exchange currents were more strongly voltage dependent than NCX1 currents. Ion concentration jump experiments were performed to estimate the relative electrogenicity of Na+ and Ca2+ transport reactions. Outward current transients associated with Na+ extrusion were much smaller for NCX-SQ1 than NCX1, and inward current transients associated with Ca2+ extrusion were much larger. For NCX-SQ1, charge movements of Ca2+ transport could be defined in voltage jump experiments with a low cytoplasmic Ca2+ (2 microM) in the presence of high extracellular Ca2+ (4 mM). The rates of charge movements showed "U"-shaped dependence on voltage, and the slopes of both charge-voltage and rate-voltage relations (1,600 s-1 at 0 mV) indicated an apparent valency of -0.6 charges for the underlying reaction. Evidently, more negative charge moves into the membrane field in NCX-SQ1 than in NCX1 when ions are occluded into binding sites.

A view of sur/KIR6.X, KATP channels

ATP-sensitive potassium channels, termed KATP channels, link the electrical activity of cell membranes to cellular metabolism. These channels are heteromultimers of sulfonylurea receptor (SUR) and KIR6.X subunits associated with a 1:1 stoichiometry as a tetramer (SUR/KIR6.X forms the pores, whereas SUR regulates their activity. Changes in [ATP]i and [ADP]i gate the channel. The diversity of KATP channels results from the assembly of SUR and KIR6.X subtypes KIR6.1-based channels differ from KIR6.2 channels mainly by their smaller unitary conductance. SUR1- and SUR2-based channels are distinguished by their differential sensitivity to sulfonylureas, whereas SUR2A-based channels are distinguished from SUR2B channels by their differential sensitivity to diazoxide. Mutations that result in the loss of KATP channels in pancreatic beta-cells have been identified in SUR1 and KIR6.2. These mutations lead to familial hyperinsulinism. Understanding the mutations in SUR and KIR6.X is allowing insight into how these channels respond to nucleotides, sulfonylureas, and potassium channel openers, KCOs.

Na+/Ca2+ exchange and cellular Ca2+ homeostasis

The Na+/Ca2+ exchange system is the primary Ca2+ efflux mechanism in cardiac myocytes, and plays an important role in controlling the force of cardiac contraction. The exchanger protein contains 11 transmembrane segments plus a large hydrophilic domain between the 5th and 6th transmembrane segments; the transmembrane regions are responsible for mediating ion translocation while the hydrophilic domain is responsible for regulation of activity. Exchange activity is regulated in vitro by interconversions between an active state and either of two inactive states. High concentrations of cytosolic Na+ or the absence of cytosolic Ca2+ promote the formation of the inactive states; phosphatidylinositol-(4,5)bisphosphate (or other negatively charged phospholipids) and cytosolic Ca2+ counteract the inactivation process. The importance of these mechanisms in regulating exchange activity under normal physiological conditions is uncertain. Exchanger function is also dependent upon cytoskeletal interactions, and the exchanger's location with respect to intracellular Ca2+-sequestering organelles. An understanding of the exchanger's function in normal cell physiology will require more detailed information on the proximity of the exchanger and other Ca2+-transporting proteins, their interactions with the cytoskeleton, and local concentrations of anionic phospholipids and transported ions.

Differential up-regulation of Na+-Ca2+ exchange by phosphoarginine and ATP in dialysed squid axons

1. The aim of this study was to characterize further the two main metabolic pathways of regulation of the Na+-Ca2+ exchanger in squid axons induced by its two naturally ocurring high-energy compounds: ATP and phosphoarginine (Pa). [Na+]o-dependent Ca2+ efflux (forward Na+o-Ca2+i exchange) and [Ca2+]o-dependent Ca2+ efflux (Ca2+o-Ca2+i exchange) were measured in internally dialysed squid axons at 16-17 C. 2. Measurements of changes in the apparent affinity of the Na+-Ca2+ exchanger for transporting (Na+o, Na+i, Ca2+o, Ca2+i) and regulatory (Ca2+i) ions induced by ATP and Pa show marked differences for the two substrates: (i) ATP strongly alters the affinity for Na+o and Na+i, while Pa does not, and (ii) in the absence of Na+i, ATP has no stimulatiory effect; on the other hand, Pa causes a dramatic increase in Na+o-Ca2+i exchange with little activation of Ca2+o-Ca2+i exchange. 3. The MgATP analogue chromium-ATP (CrATP) completely inhibits MgATP stimulation of the Na+-Ca2+ exchanger. Nevertheless, even with the effects of the nucleotide blocked, Pa exhibits its usual activation of the [Na+]o-dependent Ca2+ efflux. 4. None of the classical serine-threonine-tyrosine kinase inhibitors, nor the PP1 and PP2 phosphatase inhibitors, affects either the ATP or the Pa effect. However, intracellular microinjections of an exogenous phosphatase (alkaline phosphatase) completely reverses the stimulation of the Na+-Ca2+ exchange induced by ATP and Pa. 5. Prolonged intracellular dialysis with highly permeable porous capillaries (18 kDa molecular weight cut-off), which normally induces a complete run-down of the MgATP effect, does not alter the Pa stimulation of the exchanger, even after 6 h of continuous dialysis. 6. We conclude that the ATP and Pa modulation of Na+-Ca2+ exchange in an invertebrate nerve fibre are two genuinely different mechanisms, which affect the carrier properties in very different ways. An interesting similarity between ATP and Pa is that a phosphorylation-dephosphorylation process seems to be a common feature of these two regulation modes.