The effects of magnesium upon adenosine triphosphate-sensitive potassium channels in a rat insulin-secreting cell line

The Pharmacology of ATP-Sensitive K+ Channels (KATP)

ATP-sensitive K⁺ channels (K_ATP) are inwardly-rectifying potassium channels, broadly expressed throughout the body. K_ATP is regulated by adenine nucleotides, characteristically being activated by falling ATP and rising ADP levels thus playing an important physiological role by coupling cellular metabolism with membrane excitability. The hetero-octameric channel complex is formed of 4 pore-forming inward rectifier Kir6.x subunits (Kir6.1 or Kir6.2) and 4 regulatory sulfonylurea receptor subunits (SUR1, SUR2A, or SUR2B). These subunits can associate in various tissue-specific combinations to form functional K_ATP channels with distinct electrophysiological and pharmacological properties. K_ATP channels play many important physiological roles and mutations in channel subunits can result in diseases such as disorders of insulin handling, cardiac arrhythmia, cardiomyopathy, and neurological abnormalities. The tissue-specific expression of K_ATP channel subunits coupled with their rich and diverse pharmacology makes K_ATP channels attractive therapeutic targets in the treatment of endocrine and cardiovascular diseases.

The Ion Channel and GPCR Toolkit of Brain Capillary Pericytes

Brain pericytes reside on the abluminal surface of capillaries, and their processes cover ~90% of the length of the capillary bed. These cells were first described almost 150 years ago (Eberth, 1871; Rouget, 1873) and have been the subject of intense experimental scrutiny in recent years, but their physiological roles remain uncertain and little is known of the complement of signaling elements that they employ to carry out their functions. In this review, we synthesize functional data with single-cell RNAseq screens to explore the ion channel and G protein-coupled receptor (GPCR) toolkit of mesh and thin-strand pericytes of the brain, with the aim of providing a framework for deeper explorations of the molecular mechanisms that govern pericyte physiology. We argue that their complement of channels and receptors ideally positions capillary pericytes to play a central role in adapting blood flow to meet the challenge of satisfying neuronal energy requirements from deep within the capillary bed, by enabling dynamic regulation of their membrane potential to influence the electrical output of the cell. In particular, we outline how genetic and functional evidence suggest an important role for G_s-coupled GPCRs and ATP-sensitive potassium (K_ATP) channels in this context. We put forth a predictive model for long-range hyperpolarizing electrical signaling from pericytes to upstream arterioles, and detail the TRP and Ca²⁺ channels and G_q, G_i/o, and G_12/13 signaling processes that counterbalance this. We underscore critical questions that need to be addressed to further advance our understanding of the signaling topology of capillary pericytes, and how this contributes to their physiological roles and their dysfunction in disease.

Mechanism of KATP hyperactivity and sulfonylurea tolerance due to a diabetogenic mutation in L0 helix of sulfonylurea receptor 1 (ABCC8)

Activating mutations in different domains of the ABCC8 gene-coded sulfonylurea receptor 1 (SUR1) cause neonatal diabetes. Here we show that a diabetogenic mutation in an unexplored helix preceding the ABC core of SUR1 dramatically increases open probability of (SUR1/Kir6.2)(4) channel (KATP) by reciprocally changing rates of its transitions to and from the long-lived, inhibitory ligand-stabilized closed state. This kinetic mechanism attenuates ATP and sulfonylurea inhibition, but not Mg-nucleotide stimulation, of SUR1/Kir6.2. The results suggest a key role for L0 helix in KATP gating and together with previous findings from mutant KATP clarify why many patients with neonatal diabetes require high doses of sulfonylureas.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Both G i and G o heterotrimeric G proteins are required to exert the full effect of norepinephrine on the beta-cell K ATP channel

The effects of norepinephrine (NE), an inhibitor of insulin secretion, were examined on membrane potential and the ATP-sensitive K+ channel (K ATP) in INS 832/13 cells. Membrane potential was monitored under the whole cell current clamp mode. NE hyperpolarized the cell membrane, an effect that was abolished by tolbutamide. The effect of NE on K ATP channels was investigated in parallel using outside-out single channel recording. This revealed that NE enhanced the open activities of the K ATP channels approximately 2-fold without changing the single channel conductance, demonstrating that NE-induced hyperpolarization was mediated by activation of the K ATP channels. The NE effect was abolished in cells preincubated with pertussis toxin, indicating coupling to heterotrimeric G i/G o proteins. To identify the G proteins involved, antisera raised against alpha and beta subunits (anti-G alpha common, anti-G beta, anti-G alpha i1/2/3, and anti-G alpha o) were used. Anti-G alpha common totally blocked the effects of NE on membrane potential and K ATP channels. Individually, anti-G alpha i1/2/3 and anti-G alpha o only partially inhibited the action of NE on K ATP channels. However, the combination of both completely eliminated the action. Antibodies against G beta had no effects. To confirm these results and to further identify the G protein subunits involved, the blocking effects of peptides containing the sequence of 11 amino acids at the C termini of the alpha subunits were used. The data obtained were similar to those derived from the antibody work with the additional information that G alpha i3 and G alpha o1 were not involved. In conclusion, both G i and G o proteins are required for the full effect of norepinephrine to activate the K ATP channel.

Novel mechanism of chronic exposure of oleic acid-induced insulin release impairment in rat pancreatic beta-cells

A sustained, high circulating level of free fatty acids (FFAs) is an important risk factor for the development of insulin resistance, islet beta-cell dysfunction, and pathogenesis of type 2 diabetes. Here, we report a novel mechanism of chronic exposure of oleic acid (OA)-induced rat insulin release impairment. Following a 4-day exposure to 0.1 mM OA, there was no significant difference in basal insulin release when comparing OA-treated and untreated islets in the presence of 2.8 mM glucose, whereas 16.7 mM glucose-stimulated insulin release increased 2-fold in control, but not in OA-treated, islets. Perforated patch-clamp recordings showed that untreated beta-cells exhibited a resting potential of -62.1 +/- 0.9 mV and were electrically silent, whereas OA-treated beta-cells showed more positive resting potentials and spontaneous action potential firing. Cell-attached single-channel recordings revealed spontaneous opening of ATP-sensitive potassium (K(ATP)) channels in control, but not in OA-treated, beta-cells. Inside-out excised patch recordings showed similar activity in both OA-treated and untreated beta-cells in the absence of ATP on the inside of the cellular membrane, whereas in the presence of ATP, K(ATP) channel activity was significantly reduced in OA-treated beta-cells. Electron microscopy demonstrated that chronic exposure to OA resulted in the accumulation of triglycerides in beta-cell cytoplasm and reduced both the number of insulin-containing granules and insulin content. Collectively, chronic exposure to OA closed K(ATP) channels by increasing the sensitivity of K(ATP) channels to ATP, which in turn led to the continuous excitation of beta-cells, depletion of insulin storage, and impairment of glucose-stimulated insulin release.

Properties of an inwardly rectifying K(+) channel in the basolateral membrane of mouse TAL

We investigated the properties of K(+) channels in the basolateral membrane of the cortical thick ascending limb (CTAL) using the patch-clamp technique. Approximately 34% of cell-attached patches contained an inwardly rectifying K(+) channel (K(+)-to-Na(+) permeability ratio approximately 22), having an inward conductance (G(in)) of 44 pS and an outward conductance (G(out)) of approximately 10 pS (G(in)/G(out) approximately 4). Channel activity (NP(o)) increased with depolarization. When the cytosolic sides of inside-out patches were exposed to an Mg(2+)-free medium, the channel had a G(in) of 50 pS and was weakly inwardly rectifying (G(in)/G(out) approximately 1). Cytosolic Mg(2+) reduced G(out), yielding a G(in)/G(out) of 3.8 at 1.3 mM Mg(2+). Internal Na(+) also yielded a G(in)/G(out) of 1.6 at 20 mM Na(+). Spermine reduced NP(o) on inside-out membrane patches. Sensitivity to spermine at depolarizing voltages [half-maximal inhibitory concentration (K(i)) = 0.2 microM] was much greater than at hyperpolarizing voltages (K(i) = 26 microM). Half-inactivation by 0.5 microM spermine occurred at a clamp potential of 43 mV, with an effective valence of 1.25. A sigmoid relationship between bath pH and NP(o) of inside-out membrane patches was observed, with a pK of 7.6 and a Hill coefficient of 1.8. Intracellular acidification also reduced the NP(o) of cell-attached patches. This channel is probably a major component of K(+) conductance in the CTAL basolateral membrane.

Feedback control of the ATP-sensitive K(+) current by cytosolic Ca(2+) contributes to oscillations of the membrane potential in pancreatic beta-cells

During glucose stimulation, pancreatic beta-cells display membrane potential oscillations that correspond to intermittent Ca(2+) influx, leading to oscillations of the cytosolic free calcium concentration ([Ca(2+)](c)) and insulin secretion. The role of ATP-sensitive K(+) (K(+)-ATP) channels in the control of these oscillations was investigated by measuring the K(+)-ATP current (I(KATP)) with the perforated mode of the patch-clamp technique. No oscillations of I(KATP) were observed when glucose-stimulated beta-cells were kept hyperpolarized, thus with low and stable [Ca(2+)](c). However, increasing [Ca(2+)](c) by Ca(2+) influx (depolarizing pulses) or Ca(2+) mobilization (acetylcholine) transiently augmented I(KATP). This effect was abolished by tolbutamide, attenuated by increasing the glucose concentration in the medium, and prevented by abrogation of the [Ca(2+)](c) rise, which demonstrates that the current is really I(KATP) and that its increase is Ca(2+)-dependent. Injection of a current of a similar amplitude to that of the Ca(2+)-induced increase in I(KATP) was sufficient to repolarize glucose-stimulated beta-cells. These results suggest that, in the absence of [Ca(2+)](c) oscillations, no metabolic oscillations affect I(KATP) in pancreatic beta-cells. In contrast, [Ca(2+)](c) oscillations evoke I(KATP) oscillations. This mechanism may constitute the feedback loop controlling the glucose-induced oscillating electrical activity in beta-cells.

Intracellular Ca(2+) modulation of ATP-sensitive K(+) channel activity in acetylcholine-induced activation of rat pancreatic beta-cells

We investigated the mechanism by which acetylcholine (ACh) regulates insulin secretion from rat pancreatic beta-cells. In an extracellular solution with 5.5 mM glucose, ACh increased the rate of insulin secretion from rat islets. In islets treated with bisindolylmaleimide (BIM), a PKC inhibitor, ACh still increased insulin secretion, but the increment was lower than that without BIM. In the presence of nifedipine, an L-type Ca(2+) channel blocker, on the other hand, ACh did not increase insulin secretion. In isolated rat pancreatic beta-cells, ACh caused depolarization followed by action potentials. This ACh effect was observed even in cells treated with BIM. In the presence of nifedipine, ACh caused only depolarization. These ACh effects were prevented by atropine. In the perforated whole-cell configuration, ramp pulses from -90 to -50 mV induced membrane currents mostly through ATP-sensitive K(+) channels (K(ATP)). These currents were reduced in size by ACh in cells either treated or untreated with BIM; whereas the loading of cells with U-73122 (a phospholipase C inhibitor) or BAPTA/AM (a Ca(2+) chelator) abolished the ACh effect. In the standard whole-cell configuration, ACh reduced the currents through K(ATP) with 0.5 mM EGTA, but not with 10 mM EGTA, in the pipette solution. Intracellular application of GDPbetaS or heparin also inhibited the ACh effect. In the inside-out single-channel recordings, elevation of the Ca(2+) concentration inside the membrane from 10 nM-10 microM decreased K(ATP) activity only in the presence of ATP. The affinity of ATP to K(ATP) became 4.5 times higher with the higher concentration of Ca(2+). These results suggest that Ca(2+) from ACh receptor signaling modulates the sensitivity of K(ATP) to ATP. A positive-feedback mechanism of intracellular Ca(2+)-dependent Ca(2+) influx was also demonstrated.

A conserved inhibitory and differential stimulatory action of nucleotides on K(IR)6.0/SUR complexes is essential for excitation-metabolism coupling by K(ATP) channels

The mechanism by which ubiquitous adenine nucleotide-gated K(IR)6.0(4)/SUR(4) channels link membrane excitability with cellular metabolism is controversial. Is a decreased sensitivity to inhibitory ATP required, or is the Mg-ADP/ATP-dependent stimulatory action of the ATPase, sulfonylurea receptor (SUR), on K(IR) sufficient to elicit a physiologically significant open channel probability? To evaluate the roles of nucleotide inhibition versus stimulation, we compared K(IR)6.1-based K(NDP) channels with K(IR)6.2-based K(ATP) channels and all possible K(IR)6.1/6.2 hybrids. Although K(NDP) channels are thought to be poorly sensitive to inhibitory ATP and to require Mg-nucleotide diphosphates for activity, we demonstrate that, like K(ATP), and hybrid channels, they are inhibited with an IC(50(ATP)) 100-fold lower than [ATP](i). K(IR)6.1 is, however, more efficiently stimulated by SUR than K(IR)6.2, thus providing a mechanism for differential nucleotide regulation, in addition to the known differential interactions of Mg-nucleotides with SUR isoforms. The on-cell and spontaneous activities of K(NDP), K(ATP), and hybrid channels identified in native cells, are different; thus, their similar IC(50(ATP)) values argue the regulatory "beta" SUR subunits play a preeminent role in coupling excitation to metabolism and pose questions about the physiologic significance of models, which assume the ATP insensitivity of open K(IR)s.

Zinc-induced changes in ionic currents of clonal rat pancreatic -cells: activation of ATP-sensitive K+ channels

The effects of zinc (Zn2+) on excitability and ionic conductances were analysed on RINm5F insulinoma cells under whole-cell and outside-out patch-clamp recording conditions. We found that extracellular application of 10-20 microM Zn2+ induced a reversible abolition of Ca2+ action potential firing, which was accompanied by an hyperpolarisation of the resting membrane potential. Higher concentrations of Zn2+, in the tens to hundreds micromolar range, induced a reversible reduction of voltage-gated Ca2+ and, to a lesser extent, K+ currents. Low-voltage-activated Ca2+ currents were more sensitive to Zn2+ block than high voltage-activated Ca2+ currents. The Zn2+-induced hyperpolarisation arose from a dose-dependent increase in a voltage-independent K+ conductance that was pharmacologically identified as an ATP-sensitive K+ (KATP) conductance. The effect was rapid in onset, readily reversible, voltage independent, and related to intracellular ATP concentration. In the presence of 1 mM intracellular ATP, half-maximal activation of KATP channels was obtained with extracellular application of 1.7 microM Zn2+. Single channel analysis revealed that extracellular Zn2+ increased the KATP channel open-state probability with no change in the single channel conductance. Our data support the hypothesis that Zn2+ binding to KATP protein subunits results in an activation of the channels, therefore regulating the resting membrane potential and decreasing the excitability of RINm5F cells. Taken together, our results suggest that Zn2+ can influence insulin secretion in pancreatic beta-cells through a negative feedback loop, involving both KATP and voltage-gated conductances.

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.

Effects of inhibitors of guanine nucleotide synthesis on membrane potential and cytosolic free Ca2+ levels in insulin-secreting cells

Adenine nucleotides play an important role in the control of membrane potential by acting on ATP-sensitive K+ (K(ATP)) channels and, in turn, modulating the open probability of voltage-gated Ca2+ channels in pancreatic islet beta-cells. Here, we provide evidence that guanine nucleotides (GNs) also may be involved in the modulation of these events in vivo. GNs were depleted by treatment of HIT-T15 cells with mycophenolic acid (MPA). Resting membrane potential was more depolarized in cells treated for 3 and 6 hr with MPA than in control cells, and this effect was inhibited by diazoxide. After 6 hr of exposure to MPA, basal cytosolic free Ca2+ concentrations ([Ca2+]i) were elevated by 20%. Increments in [Ca2+]i induced by submaximal concentrations of K+ (10-15 mM) or bombesin were enhanced by > 50%. Opening K(ATP) channels with diazoxide lowered basal [Ca2+]i in MPA-treated cells to normal and abrogated the enhanced [Ca2+]i responses. However, an L-type Ca2+ channel blocker only abolished the enhanced [Ca2+]i response to stimuli and had no effect on the elevated basal [Ca2+]i, in contrast to EGTA, which obliterated both, implying that the latter was due to Ca2+ influx via non-L-type Ca2+ channels. These effects on ion fluxes were attributable specifically to GN depletion, since guanosine, which restores GTP content and the GTP/GDP ratio, but not adenosine, prevented all MPA-induced ion changes; furthermore, the latter were mimicked by mizoribine (a structurally dissimilar GTP synthesis inhibitor). It is concluded that, in addition to adenine nucleotides, GNs might contribute to the modulation of K(ATP) channels in intact beta-cells. In addition, GN depletion appeared to be able to reduce stimulated insulin secretion by a mechanism largely independent of the changes of ion fluxes observed above.

ATP-sensitive potassium channels: structures, functions, and pathophysiology

ATP-sensitive potassium channels (KATP channels) play important roles in various tissues by coupling cell metabolic status to electrical activity. Recently, molecular biological and electrophysiological techniques have revealed the molecular basis of the KATP channels to be a complex of the Kir6.0 subunit, a member of the inwardly rectifying K+ channel subfamily Kir6.0, and the sulfonylurea receptor (SUR) subunit, a member of ATP-binding cassette (ABC) superfamily; the functional diversity of the various KATP channels is being determined by a combination of the Kir6.0 subunit (Kir6.1 or Kir6.2) and the SUR subunit (SUR1 or SUR2) comprising it. Recent studies of the KATP channels have suggested mechanisms of KATP channel regulation and pathophysiology and also a new model in which ABC proteins regulate the functional expression of ion channels.

The sulphonylurea receptor SUR1 regulates ATP-sensitive mouse Kir6.2 K+ channels linked to the green fluorescent protein in human embryonic kidney cells (HEK 293)

1. Using a chimeric protein comprising the green fluorescent protein (GFP) linked to the C-terminus of the K+ channel protein mouse Kir6.2 (Kir6.2-C-GFP), the interactions between the sulphonylurea receptor SUR1 and Kir6.2 were investigated in transfected human embryonic kidney cells (HEK 293) by combined imaging and patch clamp techniques. 2. HEK 293 cells transfected with mouse Kir6.2-C-GFP and wild-type Kir6.2 exhibited functional K+ channels independently of SUR1. These channels were inhibited by ATP (IC50 = 150 microM), but were not responsive to stimulation by ADP or inhibition by sulphonylureas. Typically 15 +/- 7 active channels were found in an excised patch. 3. The distribution of Kir6.2-C-GFP protein was investigated by imaging of GFP fluorescence. There was a lamellar pattern of fluorescence labelling inside the cytoplasm (presumably associated with the endoplasmic reticulum and the Golgi apparatus) and intense punctate labelling near the cell membrane, but little fluorescence was associated with the plasma membrane. 4. In contrast, cells co-transfected with Kir6.2-C-GFP and SUR1 exhibited intense uniform plasma membrane labelling, and the lamellar and punctate labelling seen without SUR1 was no longer prominent. 5. In cells co-transfected with Kir6.2-C-GFP and SUR1, strong membrane labelling was associated with very high channel activity, with 484 +/- 311 active channels per excised patch. These K+ channels were sensitive to inhibition by ATP (IC50 = 17 microM), stimulated by ADP and inhibited by sulphonylureas. 6. We conclude that co-expression of SUR1 and Kir6.2 generates channels with the properties of native KATP channels. In addition, SUR1 promotes uniform insertion of Kir6.2-C-GFP into the plasma membrane and a 35-fold increase in channel activity, suggesting that SUR1 facilitates protein trafficking of Kir6.2 into the plasma membrane.

Effects of divalent cations on single-channel conduction properties of Xenopus IP3 receptor

The effects of Mg2+ and Ba2+ on single-channel properties of the inositol 1,4,5-trisphosphate receptor (IP3R) were studied by patch clamp of isolated nuclei from Xenopus oocytes. In 140 mM K+ the IP3R channel kinetics and presence of conductance substates were similar over a range (0-9.5 mM) of free Mg2+. In 0 mM Mg2+ the channel current-voltage (I-V) relation was linear with conductance of approximately 320 pS. Conductance varied slowly and continuously over a wide range (SD approximately 60 pS) and sometimes fluctuated during single openings. The presence of Mg2+ on either or both sides of the channel reduced the current (blocking constant approximately 0.6 mM in symmetrical Mg2+), as well as the range of conductances observed, and made the I-V relation nonlinear (slope conductance approximately 120 pS near 0 mV and approximately 360 pS at +/-70 mV in symmetrical 2.5 mM Mg2+). Ba2+ exhibited similar effects on channel conductance. Mg2+ and Ba2+ permeated the channel with a ratio of permeability of Ba2+ to Mg2+ to K+ of 3.5:2.6:1. These results indicate that divalent cations induce nonlinearity in the I-V relation and reduce current by a mechanism involving permeation block of the IP3R due to strong binding to site(s) in the conduction pathway. Furthermore, stabilization of conductance by divalent cations reveals a novel interaction between the cations and the IP3R.

MgATP activates the beta cell KATP channel by interaction with its SUR1 subunit

ATP-sensitive potassium (KATP) channels in the pancreatic beta cell membrane mediate insulin release in response to elevation of plasma glucose levels. They are open at rest but close in response to glucose metabolism, producing a depolarization that stimulates Ca2+ influx and exocytosis. Metabolic regulation of KATP channel activity currently is believed to be mediated by changes in the intracellular concentrations of ATP and MgADP, which inhibit and activate the channel, respectively. The beta cell KATP channel is a complex of four Kir6.2 pore-forming subunits and four SUR1 regulatory subunits: Kir6.2 mediates channel inhibition by ATP, whereas the potentiatory action of MgADP involves the nucleotide-binding domains (NBDs) of SUR1. We show here that MgATP (like MgADP) is able to stimulate KATP channel activity, but that this effect normally is masked by the potent inhibitory effect of the nucleotide. Mg2+ caused an apparent reduction in the inhibitory action of ATP on wild-type KATP channels, and MgATP actually activated KATP channels containing a mutation in the Kir6.2 subunit that impairs nucleotide inhibition (R50G). Both of these effects were abolished when mutations were made in the NBDs of SUR1 that are predicted to abolish MgATP binding and/or hydrolysis (D853N, D1505N, K719A, or K1384M). These results suggest that, like MgADP, MgATP stimulates KATP channel activity by interaction with the NBDs of SUR1. Further support for this idea is that the ATP sensitivity of a truncated form of Kir6.2, which shows functional expression in the absence of SUR1, is unaffected by Mg2+.

Modulation of reconstituted ATP-sensitive K(+)-channels by GTP-binding proteins in a mammalian cell line

1. The action of GTP-binding proteins on ATP-sensitive potassium (KATP) channels was investigated. KATP channels were expressed in a mammalian cell line (COS-1 cells) by cotransfecting vectors carrying the sulphonylurea receptor (SUR1) and BIR (Kir6.2), a member of the inward rectifier K+ channel family. G proteins were also tested on KATP channels composed of an isoform of SUR1, SUR2A, in combination with Kir6.2. 2. The alpha and beta gamma subunits of the GTP binding protein G1 were tested separately in inside-out patches under continuous recording. G alpha-11 increases the activity of SUR1-Kir6.2 and SUR2A-Kir6.2 channels by 200 and by 30%, respectively. 3. G alpha-12 does not increase the activity of SUR1-Kir6.2 channels, but increase the activity of SUR2A-Kir6.2 channels by 30%. 4. Control experiments showed that GTP gamma S, a specific activator of G proteins, and heat-inactivated G alpha-11 do not increase the single channel activity. 5. No effects of the other subunits (beta gamma) from either G11 or G12 on the single channel activity were observed. 6. The protein kinase C inhibitors H7 and an inhibitory peptide (FARKGALRQKNV) had no effect on the modulatory action of G alpha-11 on SUR1-Kir6.2 channels. 7. We conclude that both types of reconstituted KATP channels are modulated by G proteins.

Operative condition-dependent response of cardiac ATP-sensitive K+ channels toward sulfonylureas

A defining property of ATP-sensitive K+ (K[ATP]) channels is inhibition by sulfonylurea drugs, yet the response of cardiac K[ATP] channels toward sulfonylureas during myocardial ischemia is not consistent. Altered channel sensitivity toward sulfonylureas has, in part, been ascribed to antagonism by cytosolic nucleotide diphosphates, although the mechanism of interaction remains unclear. Herein, in inside-out patches excised from cardiomyocytes, we observed a dual response of K[ATP] channels toward the sulfonylurea drug, glyburide, in the presence of cytosolic UDP. Specifically, glyburide failed to inhibit spontaneous K[ATP] channel activity in the presence of UDP but inhibited UDP-induced channel activity after rundown of spontaneous channel openings. Such behavior of K[ATP] channels cannot be explained by differences in the level of channel activity or by UDP-induced displacement of glyburide. Rather, the dual response toward the sulfonylurea could be attributed to a property of K[ATP] channels to switch between operative conditions (spontaneous versus UDP-induced) each associated with a distinct responsiveness toward ligands. Conversion of post-rundown K[ATP] channels to the spontaneously operative channel condition, by Mg-ATP, restored the ability of UDP to antagonize the inhibitory action of glyburide lost after rundown, suggesting that the response of the channel to glyburide is phosphorylation dependent. The existence of distinct operative conditions of cardiac K[ATP] channels could be the basis for the inconsistent response of the channel toward sulfonylurea drugs and should be considered when sulfonylureas are used to implicate the opening of K[ATP] channels in the myocardium.

Ligand-insensitive state of cardiac ATP-sensitive K+ channels. Basis for channel opening

The mechanism by which ATP-sensitive K+ (KATP) channels open in the presence of inhibitory concentrations of ATP remains unknown. Herein, using a four-state kinetic model, we found that the nucleotide diphosphate UDP directed cardiac KATP channels to operate within intraburst transitions. These transitions are not targeted by ATP, nor the structurally unrelated sulfonylurea glyburide, which inhibit channel opening by acting on interburst transitions. Therefore, the channel remained insensitive to ATP and glyburide in the presence of UDP. "Rundown" of channel activity decreased the efficacy with which UDP could direct and maintain the channel to operate within intraburst transitions. Under this condition, the channel was sensitive to inhibition by ATP and glyburide despite the presence of UDP. This behavior of the KATP channel could be accounted for by an allosteric model of ligand-channel interaction. Thus, the response of cardiac KATP channels towards inhibitory ligands is determined by the relative lifetime the channel spends in a ligand-sensitive versus -insensitive state. Interconversion between these two conformational states represents a novel basis for KATP channel opening in the presence of inhibitory concentrations of ATP in a cardiac cell.

ATP-sensitive K+ channels in pancreatic, cardiac, and vascular smooth muscle cells

ATP-sensitive K+ (KATP) channels are therapeutic targets for several diseases, including angina, hypertension, and diabetes. This is because stimulation of KATP channels is thought to produce vasorelaxation and myocardial protection against ischemia, whereas inhibition facilitates insulin secretion. It is well known that native KATP channels are inhibited by ATP and sulfonylurea (SU) compounds and stimulated by nucleotide diphosphates and K+ channel-opening drugs (KCOs). Although these characteristics can be shared with KATP channels in different tissues, differences in properties among pancreatic, cardiac, and vascular smooth muscle (VSM) cells do exist in terms of the actions produced by such regulators. Recent molecular biology and electrophysiological studies have provided useful information toward the better understanding of KATP channels. For example, native KATP channels appear to be a complex of a regulatory protein containing the SU-binding site [sulfonylurea receptor (SUR)] and an inward-rectifying K+ channel (Kir) serving as a pore-forming subunit. Three isoforms of SUR (SUR1, SUR2A, and SUR2B) have been cloned and found to have two nucleotide-binding folds (NBFs). It seems that these NBFs play an essential role in conferring the MgADP and KCO sensitivity to the channel, whereas the Kir channel subunit itself possesses the ATP-sensing mechanism as an intrinsic property. The molecular structure of KATP channels is thought to be a heteromultimeric (tetrameric) assembly of these complexes: Kir6.2 with SUR1 (SUR1/Kir6.2, pancreatic type), Kir6.2 with SUR2A (SUR2A/ Kir6.2, cardiac type), and Kir6.1 with SUR2B (SUR2B/Kir6.1, VSM type) [i.e., (SUR/Kir6.x)4]. It remains to be determined what are the molecular connections between the SUR and Kir subunits that enable this unique complex to work as a functional KATP channel.

Function, regulation, pharmacology, and molecular structure of ATP-sensitive K+ channels in the cardiovascular system

ATP-sensitive K+ (K[ATP]) channels are inhibited by intracellular ATP 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. In the vascular smooth muscle, the K(ATP) channel is believed to mediate the relaxation of vascular tone. Thus, K(ATP) channels play important regulatory roles in the cardiovascular system. Furthermore, K(ATP) channels are the targets of two important classes of drugs, i.e., the antidiabetic sulfonylureas, which block the channels, and a series of vasorelaxants called "K+ channel openers," which tend to maintain the channels in an open conformation. Recently, the molecular structure of K(ATP) channels has been clarified. The K(ATP) channel in pancreatic beta-cells is a complex composed of at least two subunits, a member of inwardly rectifying K+ channels and a sulfonylurea receptor. Subsequently, two additional homologs of the sulfonylurea receptor, which form cardiac and smooth muscle type K(ATP) channels, respectively, have been reported. Further works are now in progress to understand the molecular mechanisms of K(ATP) channel function.

Activation and inhibition of K-ATP currents by guanine nucleotides is mediated by different channel subunits

The ATP-sensitive potassium channel (K-ATP channel) plays a key role in insulin secretion from pancreatic beta-cells. It is closed by glucose metabolism, which stimulates secretion, and opened by the drug diazoxide, which inhibits insulin release. Metabolic regulation is mediated by changes in ATP and MgADP concentration, which inhibit and potentiate channel activity, respectively. The beta-cell K-ATP channel consists of a pore-forming subunit, Kir6.2, and a regulatory subunit, SUR1. The site at which ATP mediates channel inhibition lies on Kir6.2, while the potentiatory action of MgADP involves the nucleotide-binding domains of SUR1. K-ATP channels are also activated by MgGTP and MgGDP. Furthermore, both nucleotides support the stimulatory actions of diazoxide. It is not known, however, whether guanine nucleotides mediate their effects by direct interaction with one or more of the K-ATP channel subunits or indirectly via a GTP-binding protein. We used a truncated form of Kir6.2, which expresses independently of SUR1, to show that GTP blocks K-ATP currents by interaction with Kir6.2 and that the potentiatory effects of GTP are endowed by SUR1. We also showed that mutation of the lysine residue in the Walker A motif of either the first (K719A) or second (K1384M) nucleotide-binding domain of SUR1 abolished both the potentiatory effects of GTP and GDP on K-ATP currents and their ability to support stimulation by diazoxide. This argues that the stimulatory effects of guanine nucleotides require the presence of both Walker A lysines.

RIN14B: a pancreatic delta-cell line that maintains functional ATP-dependent K+ channels and capability to secrete insulin under conditions where it no longer secretes somatostatin

The delta-cell line RIN14B was characterized with regard to ATP-regulated K+ (K(ATP)) channel activity and hormone release. By applying the patch-clamp technique, dose-response curves for ATP and the sulfonylurea tolbutamide were obtained in inside-out patches. The concentration causing half-maximal K(ATP) channel inhibition was found to be 23.7 and 27.6 microM for ATP and tolbutamide, respectively. ADP and diazoxide stimulated K(ATP) channel activity, an effect dependent on the presence of intracellular Mg2+. The stimulatory effect of diazoxide also required the presence of ATP. The kinetic properties of the K(ATP) channel were analysed in the presence of ATP, a combination of ADP and ATP and in nucleotide-free solutions. The distribution of K(ATP) channel open time could be described by a single exponential function with a time constant of approximately 30 ms in nucleotide-free and in ATP-containing solutions. The presence of both ATP and ADP resulted in the appearance of an additional time constant of > 150 ms. Single-channel unitary current-voltage (i-V) relation was characterised for the K((ATP) channel present in RIN14B cells. The slope conductance, measured at the reversal potential was found to be 19.1 +/- 2.4 pS. The permeability for K+ ions was calculated to be 0.31 x 10(-13) cm3 x s(-1). We have not been able to confirm the somatostatin releasing profile of the RIN14B cells using radioimmunoassays, nor could we find positive somatostatin stain with immunocytochemical techniques. We conclude that the RIN14B cell line, previously characterized as a somatostatin-secreting cell line, contains K(ATP) channels with properties closely resembling the K(ATP) channel described in the pancreatic beta-cell. However, the cell line appears to have dedifferentiated with regard to the ability to secrete somatostatin, maintaining the highly differentiated function of both insulin biosynthesis and exocytosis.

Effects of levcromakalim and nucleoside diphosphates on glibenclamide-sensitive K+ channels in pig urethral myocytes

1. Effects of levcromakalim and nucleoside diphosphates (NDPs) on both membrane currents and unitary currents in pig proximal urethra were investigated by use of patch clamp techniques (conventional whole-cell configuration, nystatin perforated patch, cell-attached configuration and inside-out patches). 2. Levcromakalim produced a concentration-dependent outward current at a holding potential of -50 mV. The peak current amplitude showed little variation when measured by either conventional whole-cell or nystatin perforated patch configurations. 3. In conventional whole-cell configuration, the levcromakalim (100 microM)-induced outward current decayed by about 90% in 18 min. In contrast, with the nystatin perforated patch, approximately 86% of the levcromakalim-induced outward current still remained after 18 min. 4. The peak amplitude of the levcromakalim (100 microM)-induced outward membrane current recorded by the conventional whole-cell configuration was greatly reduced by inclusion of 5 mM EDTA in the pipette. The much smaller but significant outward membrane current remaining was abolished by glibenclamide. 5. In conventional whole-cell recordings, inclusion of an NDP in the pipette solution induced a small outward current which slowly reached a maximal amplitude (in 2 to 10 min) and was suppressed by glibenclamide. Addition of 100 microM levcromakalim after the NDP-induced current had peaked activated a further outward current which was larger than that recorded in the absence of NDPs. Approximately 50% of this current still remained at 18 min, even when conventional whole-cell configuration was used. 6. In the cell-attached mode in symmetrical 140 mM K+ conditions, glibenclamide inhibited the 100 microM levcromakalim-activated 43 pS K+ channel in a concentration-dependent manner, showing an inhibitory dissociation constant (Ki) of approximately 520 nM. 7. In inside-out patches in which the glibenclamide-sensitive K+ channel had run down after exposure to levcromakalim, both uridine 5'-diphosphate (UDP) and MgATP were capable of reactivating the channel. Further application of Mg2+ to the UDP-reactivated K+ channels enhanced the channel activity reversibly. 8. In inside-out patches UDP was capable of activating the glibenclamide-sensitive K+ channel without levcromakalim, providing that there was free Mg2+ present (either UDP in 5 mM EGTA or UDP in 5 mM EDTA with Mg2+). Additional application of levcromakalim caused a further reversible activation of channel opening. 9. In the presence of levcromakalim, application of adenosine 5'-triphosphate (ATP) to the inner surface of the membrane patch inhibited UDP-reactivated channel opening in a concentration-dependent manner. 10. Addition of an untreated cytosolic extract of pig proximal urethra reactivated the glibenclamide-sensitive K+ channel in the presence of 100 microM levcromakalim in inside-out patches. 11. These results demonstrate the presence in the pig proximal urethra of a glibenclamide-sensitive K+ channel that is blocked by intracellular ATP and can be activated by levcromakalim. Intracellular UDP can reactivate the channel after rundown. Additionally, intracellular Mg2+ may play an important role in regulating the channel activity.

Similarity of ATP-dependent K+ channels in skeletal muscle fibres from normal and mutant mdx mice

1. ATP-dependent K+ (KATP) channels were studied in fibres isolated from flexor digitorum brevis and interosseal skeletal muscles of normal and mutant mdx mice using the patch clamp technique in the presence of asymmetrical K+ concentrations (5 mM K+ in the pipette and in vivo intracellular [K+] or 145 mM K+ at the cytoplasmic face). 2. In cell-attached patches from mdx muscle fibres bathed in K(+)-rich solution, cell poisoning with fluorodinitrobenzene induced partially reversible opening of channels carrying an outward current of an amplitude of 1.2 pA at 0 mV. Exposure of fibres to the K+ channel opener cromakalim led to opening of the same type of channel. These channels were assumed to be KATP channels. 3. On excision of inside-out patches from mdx muscle fibres, in the absence of intracellular ATP, KATP channels were active: they carried a unitary outward current of 1.6 pA at 0 mV and were inhibited by intracellular ATP and glibenclamide. The number of KATP channels per patch was not significantly different in muscles from normal and mdx mice. 4. In inside-out patches, in the presence of 1 mM intracellular Mg2+, slope conductances of 21 and 20.3 pS were found for KATP channels in normal and mdx muscle, respectively. In the absence of Mg2+, slope conductances of KATP channels were 31.3 and 32 pS in normal and mdx muscle, respectively and KATP channel activity was augmented in mdx muscle in the same way as in normal muscle. Activity of the same KATP channel was observed in extensor digitorum longus muscle from normal and mdx mice. 5. In inside-out patches held at 0 mV, the relationship between KATP channel activity and intracellular ATP was described by a Hill equation: Ki values were 23 and 21 microM and Hill coefficients were 1.8 and 1.9 in normal and mdx muscle, respectively. 6. These results indicate that the distribution, the conductance properties and ATP sensitivity of KATP channels do not differ in normal and in mdx mouse skeletal muscle.

Regulation of an outwardly rectifying Cl- conductance in single proximal tubule cells isolated from frog kidney

1. A previous study has identified a Cl- conductance (GCl) in single proximal tubule cells isolated from frog kidney, which was activated by a protein kinase C (PKC)-dependent mechanism. 2. The whole-cell patch clamp technique was employed to examine further the properties and regulation of GCl. 3. GCl showed outward rectification, outward conductance was significantly greater than the inward conductance (56.1 +/- 15.6 vs. 16.8 +/- 6.4 microS cm-2, respectively, n = 8). DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) blocked GCl in a dose- and voltage-dependent manner. 4. Other anions permeated the conductance. The anion selectivity sequence, I- > Br- > Cl- > gluconate, followed Eisenman's sequence I. 5. GCl could be activated by ATP. This process was dependent on ATP hydrolysis and channel phosphorylation. 6. G-protein activation inhibited the ATP-dependent activation of GCl. 7. These data support the hypothesis that activation of GCl by an increase in cell volume is dependent on ATP hydrolysis and channel phosphorylation via PKC.

Concentration dependence and time course of the effects of glucose on adenine and guanine nucleotides in mouse pancreatic islets

Changes in the ATP:ADP ratio in pancreatic B cells may participate in the regulation of insulin secretion by glucose. Here, we have investigated the possible role of guanine nucleotides. Mouse islets were incubated in a control medium (when K+-ATP channels are the major site of regulation) or in a high K+ medium (when glucose modulates the effectiveness of cytosolic Ca2+ on exocytosis). Glucose induced a concentration-dependent (0-20 m) increase in GTP and a decrease in GDP in both types of medium, thus causing a progressive rise of the GTP:GDP ratio. ATP and ADP levels were 4-5-fold higher but varied in a similar way as those of guanine nucleotides. Insulin secretion was inversely correlated with ADP and GDP levels and positively correlated with the ATP:ADP and GTP:GDP ratios between 6 and 20 m glucose in control medium and between 0 and 20 m glucose in high K+ medium. The increases in the GTP:GDP and ATP:ADP ratios induced by a rise of glucose were faster than the decreases induced by a fall in glucose, but the changes of both ratios were again parallel. In conclusion, glucose causes large, concentration-dependent changes in guanine as well as in adenine nucleotides in islet cells. This raises the possibility that both participate in the regulation of nutrient-induced insulin secretion.

Role of intracellular Ca2+ in the K channel opener action of CGRP in the guinea-pig ureter

1. The aim of this study was to assess the role of sarcoplasmic reticulum (SR) calcium (Ca2+) in the smooth muscle relaxant and hyperpolarizing actions of calcitonin gene-related peptide (CGRP) in the guinea-pig ureter. 2. CGRP (0.1 microM) rapidly and transiently reduced myogenic phasic contractions (twitches) produced by electrical field stimulation (EFS). Approximately 70% of the response to CGRP was antagonized by glibenclamide (1 microM). 3. Cyclopiazonic acid (CPA, 10 microM), ryanodine (100 microM) and thapsigargin (1 microM) reduced only the glibenclamide-sensitive component of the response to CGRP (0.1 microM) but did not modify the mechano-inhibitory effect of cromakalim (3 microM). A low concentration of CPA (1 microM), assumed to produce a limited impairment of Ca2+ uptake from the stores, prolonged the duration of the inhibitory response to CGRP. Pre-exposure to caffeine (5 mM) inhibited the suppression of twitches by CGRP or cromakalim. 4. When the frequency of EFS was increased, the suppression of twitches by CGRP was reduced. Under these conditions, CPA (1 microM) again prolonged the duration of the inhibitory response to CGRP. 5. CGRP (0.1 microM) and cromakalim (3 microM) markedly depressed the phasic component of contractions to 80 mM KCl. CPA (10 microM) antagonized the inhibitory effect of CGRP but not that of cromakalim. Inhibition of the tonic contraction to 80 mM KCl by CGRP was insensitive to CPA. 6. In sucrose gap experiments, a 5 min exposure to CGRP (0.1 microM) or cromakalim (3 microM) produced a sustained membrane hyperpolarization. Caffeine (5 mM) produced a glibenclamide-sensitive transient hyperpolarization followed by a sustained depolarization. When tested in a Ca(2+)-free medium the hyperpolarization produced by CGRP, cromakalim or caffeine was reduced. In normal Krebs, pre-exposure to CPA (10 microM, 60 min) only abolished the hyperpolarization induced by CGRP. In contrast, 5 min after a caffeine challenge (5 mM) the hyperpolarizations induced by CGRP or cromakalim were reduced. The CGRP-induced hyperpolarization was insensitive to apamin (0.1 microM) or charybdotoxin (0.1 microM). 7. We conclude that the K channel-opening action of CGRP in the guinea-pig ureter requires the mobilization of intracellular Ca2+ from a caffeine- and CPA-sensitive store, leading to transient activation of glibenclamide-sensitive K channels. The K channel-opening action of caffeine appears to involve Ca2+ mobilization from a store which is insensitive to depletion by CPA.

Block of pancreatic ATP-sensitive K+ channels and insulinotrophic action by the antiarrhythmic agent, cibenzoline

1. We investigated the effect of cibenzoline (a class Ia antiarrhythmic drug) on basal insulin secretory activity of rat pancreatic islets and ATP-sensitive K+ channels (KATP) in single pancreatic beta cells of the same species, using radioimmunoassay and patch clamp techniques. 2. Micromolar cibenzoline had a dose-dependent insulinotrophic action with an EC50 of 94.2 +/- 46.4 microM. The compound inhibited the activity of the KATP channel recorded from a single beta-cell in a concentration-dependent manner. The IC50 was 0.4 microM in the inside-out mode and 5.2 microM in the cell-attached mode, at pH 7.4. 3. In the cell-attached mode, alkalinization of extracellular solution increased the inhibitory action of cibenzoline and the IC50 was reduced from 26.8 microM at pH 6.2 to 0.9 microM at pH 8.4. On the other hand, the action of cibenzoline in the excised inside-out mode was acute in onset with a small IC50, indicating that the drug attains its binding site from the cytoplasmic side of the cell membrane. 4. In the inside-out mode, micromolar ADP reactivated the cibenzoline-blocked KATP channels in a manner similar to that by which ADP restored ATP-dependent block of the channel. 5. The binding of [3H]-glibenclamide to pancreatic islets was inhibited by glibenclamide but not by cibenzoline. In contrast, the [3H]-cibenzoline binding was displaced by unlabelled cibenzoline but not by glibenclamide. It is concluded that cibenzoline blocks pancreatic KATP channels via a binding site distinct from the sulphonylurea receptor.

Cardiac ATP-sensitive K+ channels: regulation by intracellular nucleotides and K+ channel-opening drugs

ATP-sensitive K+ (KATP) channels are present at high density in membranes of cardiac cells where they regulate cardiac function during cellular metabolic impairment. KATP channels have been implicated in the shortening of the action potential duration and the cellular loss of K+ that occurs during metabolic inhibition. KATP channels have been associated with the cardioprotective mechanism of ischemia-related preconditioning. Intracellular ATP (ATPi) is the main regulator of KATP channels. ATPi has two functions: 1) to close the channel (ligand function) and 2) in the presence of Mg2+, to maintain the activity of KATP channels (presumably through an enzymatic reaction). KATP channel activity is modulated by intracellular nucleoside diphosphates that antagonize the ATPi-induced inhibition of channel opening or induce KATP channels to open. How nucleotides will affect KATP channels depends on the state of the channel. K+ channel-opening drugs are pharmacological agents that enhance KATP channel activity through different mechanisms and have great potential in the management of cardiovascular conditions. KATP channel activity is also modulated by neurohormones. Adenosine, through the activation of a GTP-binding protein, antagonizes the ATPi-induced channel closure. Understanding the molecular mechanisms that underlie KATP channel regulation should prove essential to further define the function of KATP channels and to elucidate the pharmacological regulation of this channel protein. Since the molecular structure of the KATP channel has now become available, it is anticipated that major progress in the KATP channel field will be achieved.

Modification by protons of frog skeletal muscle KATP channels: effects on ion conduction and nucleotide inhibition

1. The molecular mechanisms underlying pH regulation of skeletal muscle ATP-sensitive K+ (KATP) channels were studied using the patch clamp technique in the inside-out configuration. Two effects of intracellular protons were studied in detail: the decrease in magnitude of single-channel currents and the increase in open probability (Po) of nucleotide-inhibited channels. 2. The pH dependence of inward unit currents under different ionic conditions was in poor agreement with either a direct block of the pore by protons or an indirect proton-induced conformational change, but was compatible with the protonation of surface charges located near the cytoplasmic entrance of the pore. This latter electrostatic mechanism was modelled using Gouy-Chapman-Stern theory, which predicted the data accurately with a surface charge density of about 0.1 negative elementary charges per square nanometre and a pK (pH value for 50% effect) value for protonation of these charges of 6.25. The same mechanism, i.e. neutralization of negative surface charges by cation binding, could also account for the previously reported reduction of inward unit currents by Mg2+. 3. Intracellular alkalization did not affect Po of the KATP channels. Acidification increased Po. In the presence of 0.1 mM ATP (no Mg2+), the channel activation vs. pH relationship could be fitted with a sigmoid curve with a Hill coefficient slightly above 2 and a pK value of 6. This latter value was dependent on the ATP concentration, decreasing from 6.3 in 30 microM ATP to 5.3 in 1 microM ATP. 4. Conversely, the channel inhibition vs. ATP concentration curve was shifted to the right when the pH was lowered. At pH 7.1, the ATP concentration causing half-maximal inhibition was about 10 microM. At pH 5.4, it was about 400 microM. The Hill coefficient values remained slightly below 2. Similar effects were observed when ADP was used as the inhibitory nucleotide. 5. These results confirm that a reciprocal competitive link exists between proton and nucleotide binding sites. Quantitatively, they are in full agreement with a steady-state model of a KATP channel possessing four identical protonation sites (microscopic pK, 6) allosterically connected to the channel open state and two identical nucleotide sites (microscopic ATP dissociation constant, approximately 30 microM) connected to the closed state.

Characterisation of Ca(2+)-dependent inwardly rectifying K+ currents in HeLa cells

The whole-cell configuration of the patch-clamp technique was used to examine K+ currents in HeLa cells. Under quasi-physiological ionic gradients, using an intracellular solution containing 10(-7) mol/l free Ca2+, mainly outward currents were observed. Large inwardly rectifying currents were elicited in symmetrical 145 mmol/l KCl. Replacement of all extracellular K+ by isomolar Na+, greatly decreased inward currents and shifted the reversal potential as expected for K+ selectivity. The inwardly rectifying K+ currents exhibited little or no apparent voltage dependence within the range of from -120 mV to 120 mV. A square-root relationship between chord conductance and [K+] at negative potentials could be established. The inwardly rectifying nature of the currents was unaltered after removal of intracellular Mg2+ and chelation with ATP and ethylenediaminetetraacetic acid (EDTA). Permeability ratios for other monovalent cations relative to K+ were: K+ (1.0) > Rb+ (0.86) > Cs+ (0.12) > Li (0.08) > Na+ (0.03). Slope conductance ratios measured at -100 mV were: Rb+ (1.66) > K+ (1.0) > Na+ (0.09) > Li (0.08) > Cs+ (0.06). K+ conductance was highly sensitive to intracellular free Ca2+ concentration. The relationship between conductance at 0 mV and Ca2+ concentration was well described by a Hill expression with a dissociation constant, KD, of 70 nmol/l and a Hill coefficient, n, of 1.81. Extracellular Ba2+ blocked the currents in a concentration- and voltage-dependent manner. The dependence of the KD for the blockade was analysed using a Woodhull-type treatment, locating the ion interaction site at 19% of the distance across the electrical field of the membrane and a KD (0 mV) of 7 mmol/l. Tetraethylammonium and 4-aminopyridine were without effect whilst quinine and quinidine blocked the currents with concentrations for half-maximum effects equal to 7 mumol/l and 3.5 mumol/l, respectively. The unfractionated venom of the scorpion Leiurus quinquestriatus (LQV) blocked the K+ currents of HeLa cells. The toxins apamin and scyllatoxin had no detectable effect whilst charybdotoxin, a component of LQV, blocked in a voltage-dependent manner with half-maximal concentrations of 40 nmol/l at -120 mV and 189 nmol/l at 60 mV; blockade by charybdotoxin accounts for the effect of LQV. Application of ionomycin (5-10 mumol/l), histamine (1 mmol/l) or bradykinin (1-10 mumol/l) to cells dialysed with low-buffered intracellular solutions induced K+ currents showing inward rectification and a lack of voltage dependence.

Activation of ATP-dependent K+ channels by metabolic poisoning in adult mouse skeletal muscle: role of intracellular Mg(2+) and pH

1. The effects of metabolic poisoning, intracellular Mg(2+) and pH on ATP-dependent K+ (K+ATP) channels were examined in adult mouse isolated skeletal muscle fibres using the patch clamp technique. 2. In cell-attached membrane patches, while openings of one kind of channel could only rarely be detected under control conditions, cell poisoning with fluorodinitrobenzene (FDNB), dinitrophenol (DNP) and cyanide (CN) induced a strong and partially reversible increase in channel activity. 3. Slope conductance and glibenclamide sensitivity of this outward current indicated that the channel activated during poisoning was the K+ATP channel. 4. Single channel current amplitude was reduced during poisoning, but remained unchanged when activation of the K+ATP channel was induced by cromakalim. 5. In inside-out membrane patches, in the absence of intracellular ATP, intracellular application of Mg2+ decreased channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 6. In the absence of intracellular ATP, a decrease in intracellular pH induced a reduction in channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 7. The reduction of single channel current amplitude during poisoning was attributed to an increase in intracellular Mg2+ concentration caused by a fall in intracellular ATP concentration. These results also show that metabolic poisoning causes direct activation of K+ATP channels in skeletal muscle, and that is activation is at least partially mediated through an increase in intracellular Mg(2+) concentration and a decrease in intracellular pH.

Spermine gates inward-rectifying muscarinic but not ATP-sensitive K+ channels in rabbit atrial myocytes. Intracellular substance-mediated mechanism of inward rectification

The effect of spermine, a low molecular mass aliphatic amine with positive charges, on the strongly inwardly rectifying muscarinic K+ (KACh) channel was examined in rabbit atrial myocytes. In inside-out patch membranes, the single channel current-voltage relationship of KACh channels activated by guanosine 5'-3-O-(thio)triphosphate became linear in the absence of intracellular Mg2+. The open probability (Po) of the channels did not show significant voltage dependence under these conditions. Spermine specifically reduced Po of outwardly flowing KACh channel currents without affecting the unitary current amplitude at depolarized potentials, but had no effect on inward KACh currents under hyperpolarization. This voltage dependence of Po of KACh channels in the presence of spermine resembled that normally observed in the whole cell or open cell-attached configurations. Spermine (300 nM to 3 microM) also restored the relaxation of KACh currents which had been lost in the inside-out configuration. The effect of spermine was concentration-dependent with IC50 of approximately 10 nM at +40 mV. The order of potency of polyamines in reducing Po at +40 mV was spermine > or = spermidine > putrescine > ornithine; arginine had no significant effect. Intracellular Mg2+ antagonized the effect of spermine. Neither the single channel conductance nor Po of the ATP-sensitive K+ channel, a weak inward rectifier, was affected by spermine. Because submillimolar concentrations of spermine and spermidine are available in the cytosol of most cells, these substances may be the unidentified intracellular gating factors for strong inward rectifiers such as KACh and IK1 channels.

Biophysical, pharmacological and developmental properties of ATP-sensitive K+ channels in cultured myotomal muscle cells from Xenopus embryos

Unlike mammalian muscle cells in culture, cultured myotomal muscle cells of Xenopus embryos express ATP-sensitive K+ (KATP) channels. The KATP channels are blocked by internal ATP (half-maximal inhibition K0.5 = 16 microM) and to a lesser extent by internal ADP, are voltage independent, have an inward rectification at positive potentials and are inhibited by glibenclamide (K0.5 = 2 microM). Surprisingly, these KATP channels are not sensitive to K+ channel openers such as cromakalim. Opening of these KATP channels does not occur under normal physiological conditions. It is elicited by metabolic exhaustion of the muscle cell and it precedes the development of an irreversible rigor state. Neither intracellular acidosis nor an increase of intracellular Ca2+ are involved in KATP channel opening. Different types of K+ channels are successively expressed after plating of myotomal muscle cells: (1) sustained delayed-rectifier K+ channels; (2) KATP channels; (3) inward-rectifier K+ channels; (4) transient delayed-rectifier K+ channels. The current density associated with KATP channels far exceeds that of voltage-dependent K+ channels. Innervation controls the expression of these KATP channels. Co-culture of muscle cells with neurons from the neural tube decreases the number of active KATP channels per patch. Similarly, in situ innervated submaxillaris muscle of tadpoles at stage 50-55 has a very low density of KATP channels.

Characterization of the G protein coupling of a glucagon receptor to the KATP channel in insulin-secreting cells

The G-protein-mediated coupling of a glucagon receptor to ATP-dependent K channels--KATP--has been studied in insulin-secreting cells using the patch clamp technique. In excised outside-out patches, KATP channel activity was inhibited by low concentrations of glucagon (IC50 = 2.4 nM); the inhibitory effect vanished at concentrations greater than 50 nM. In cell-attached patches, inhibition by bath-applied glucagon was seen most often, although stimulation was observed in a few cases. A dual action of the hormone is proposed to resolve these apparently divergent results. In excised inside-out patches, KATP channel activity was inhibited by addition of beta gamma subunits purified from either erythrocyte or retina (IC50 = 50 pM and 1 nM, respectively). Subsequent exposure of the patch to alpha i or alpha o reversed this effect. In excised inside-out patches, increasing Mg2+ in the bath stimulated the channel activity between 0 and 0.5 nM, but blocked it at higher concentrations (IC50 = 2.55 mM). In most cases (70%), GTP had a stimulatory effect at concentrations up to 100 microns. However, in three cases, similar GTP levels had clear inhibitory effects. In excised inside-out patches, cholera toxin (CTX) caused channel inhibition. Although the effect could not be reversed by removal of the toxin, the activity was restored by subsequent addition of purified alpha i or alpha o. These results are compatible with a model whereby channel inhibition by activated Gs-coupled receptors occurs, at least in part, via association of the beta gamma subunits of Gs with alpha i/alpha o subunits and deactivation of the alpha i/alpha o-dependent stimulatory pathway. On the basis of this hypothesis, a model is developed to describe the effects of G proteins on the KATP channel, as well as to account for the concentration-dependent stimulation and inhibition of KATP channel by Mg2+. An interpretation of the ability of glucagon to potentiate, but not initiate, insulin release is also given in terms of this model and the effects of ATP on KATP channels.

Regulation of ROMK1 K+ channel activity involves phosphorylation processes

An inwardly rectifying, ATP-regulated K+ channel with a distinctive molecular architecture, ROMK1, was recently cloned from rat kidney. Using patch clamp techniques, we have investigated the regulation of ROMK1 with particular emphasis on phosphorylation/dephosphorylation processes. Spontaneous channel rundown occurred after excision of membrane patches into ATP-free bath solutions in the presence of Mg2+. Channel rundown was almost completely abolished after excision of patches into either Mg(2+)-free bathing solutions or after preincubation with the broad-spectrum phosphatase inhibitor, orthovanadate, in the presence of Mg2+. MgATP preincubation also inhibited channel rundown in a dose-dependent manner. In addition, the effect of the specific phosphatase inhibitors okadaic acid (1 microM) and calyculin A (1 microM) was also investigated. The presence of either okadaic acid or calyculin A failed to inhibit channel rundown. Taken together, these data suggest that rundown of ROMK1 involves a Mg(2+)-dependent dephosphorylation process. Channel activity was also partially restored after the addition of MgATP to the bath solution. Addition of exogenous cAMP-dependent protein kinase A (PKA) catalytic subunit led to a further increase in channel open probability. Addition of Na2ATP, in the absence of Mg2+, was ineffective, suggesting that restoration of channel activity is a Mg(2+)-dependent process. Addition of the specific PKA inhibitor, PKI, to the bath solution led to a partial, reversible inhibition in channel activity. Thus, PKA-dependent phosphorylation processes are involved in the modulation of channel activity. This observation is consistent with the presence of potential PKA phosphorylation sites on ROMK1.

Dualistic behavior of ATP-sensitive K+ channels toward intracellular nucleoside diphosphates

ATP-sensitive K+ (KATP) channels are intracellular ligand-gated channels which regulate diverse cellular functions. Intracellular nucleoside diphosphates (NDPs) are essential for the physiological opening of KATP channels which would otherwise be permanently closed by their overt sensitivity to intracellular ATP. We find that KATP channels exhibit dualistic behavior toward NDPs depending on their operative condition. When channels are in the spontaneous operative condition, NDPs antagonize channel inhibition by intracellular ATP. When channels have "run down", NDPs induce channel opening but no longer antagonize intracellular ATP. The switch of the KATP channel response to the same ligand, i.e., NDPs, is controlled by a Mg-ATP-dependent reaction. The condition of the target protein therefore determines the effect of the ligand. This property provides a novel basis to evaluate the dynamic regulation of ion channels by their ligands.

Effects of chemical modification of amino and sulfhydryl groups on KATP channel function and sulfonylurea binding in CRI-G1 insulin-secreting cells

The effects of several group-specific chemical reagents were examined upon the activity of the ATP-sensitive potassium (KATP) channel in the CRI-G1 insulin-secreting cell line. Agents which interact with the sulfhydryl moiety (including 1 mM N-ethyl-maleimide (NEM), 1 mM 5,5'-dithio-bis-(2-nitrobenzoic acid) (DNTB) and 1 mM o-iodobenzoate) produced an irreversible inhibition of KATP channel activity when applied to the intracellular surface of excised inside-out patches. This inhibition was substantially reduced when attempts were made to eliminate Mg2+ from the intracellular compartment. ATP 50 microM and 100 microM tolbutamide were each shown to protect against the effects of these reagents. The membrane impermeable DNTB was significantly less effective when applied to the external surface of outside-out patches. Agents which interact with peptide terminal amine groups and epsilon amino groups of lysine [1 mM methyl acetimidate and 1 mM trinitrobenzene sulfonic acid (TNBS)] and also the guanido group of arginine (1 mM methyl glyoxal) produced a Mg(2+)-dependent irreversible inhibition of KATP channel activity which could be prevented by ATP but not tolbutamide. The irreversible activation of the KATP channel produced by the proteolytic enzyme trypsin was prevented only when methyl glyoxal and methyl acetimidate were used in combination to inhibit channel activity. Radioligand binding studies showed that the binding of 3H glibenclamide was unaffected by any of the above agents with the exception of TNBS which completely inhibited binding with a EC50 of 307 +/- 6 microM. These results provide evidence for the presence of essential sulfhydryl (possibly cysteine), and basic amino acid (possibly lysine and arginine) residues associated with the normal functioning of the KATP channel. Furthermore, we believe that the sulfhydryl group in question is situated at the internal surface of the membrane, possibly near to the channel pore.

G proteins activate ATP-sensitive K+ channels by antagonizing ATP-dependent gating

To determine whether G proteins activate cardiac ATP-sensitive K+ (KATP) channels by regulating intracellular ATP (ATPi)-dependent gating, currents were measured in inside-out patches. When ATPi closed KATP channels, activators of endogenous G proteins, GTP (plus adenosine or acetylcholine), GTP gamma S, or AlF-4 stimulated channels, an effect prevented by GDP beta S. In the absence of ATPi, G protein activators were ineffective. Intracellular nucleoside diphosphates restored KATP channel openings after the "rundown" of spontaneous activity. Only when ATPi suppressed nucleoside diphosphate-induced openings, GTP gamma S or AlF-4 enhanced KATP channel activity. Active forms of exogenous G protein subunits (G alpha i-1, G alpha i-2, or G alpha o) activated only KATP channels closed by ATPi. G proteins stimulate cardiac KATP channels apparently by antagonizing ATPi-dependent inhibitory gating. Regulation of ligand-dependent gating represents a distinct type of G protein modulation of ion channels.

Internal Na+ and Mg2+ blockade of DRK1 (Kv2.1) potassium channels expressed in Xenopus oocytes. Inward rectification of a delayed rectifier

Delayed rectifier potassium channels were expressed in the membrane of Xenopus oocytes by injection of rat brain DRK1 (Kv2.1) cRNA, and currents were measured in cell-attached and inside-out patch configurations. In intact cells the current-voltage relationship displayed inward going rectification at potentials > +100 mV. Rectification was abolished by excision of membrane patches into solutions containing no Mg2+ or Na+ ions, but was restored by introducing Mg2+ or Na+ ions into the bath solution. At +50 mV, half-maximum blocking concentrations for Mg2+ and Na+ were 4.8 +/- 2.5 mM (n = 6) and 26 +/- 4 mM (n = 3) respectively. Increasing extracellular potassium concentration reduced the degree of rectification of intact cells. It is concluded that inward going rectification resulting from voltage-dependent block by internal cations can be observed with normally outwardly rectifying DRK1 channels.

Mg(2+)-dependent inhibition of KATP by sulphonylureas in CRI-G1 insulin-secreting cells

1 Patch-clamp recording techniques were used to examine the effects of tolbutamide, glibenclamide, meglitinide and thiopentone on KATP in CRI-GI insulin-secreting cells in the presence and absence of Mg2+. 2 In the absence of Mg2+ in the intracellular bathing solution, tolbutamide was significantly less effective when applied either to the intracellular or to the extracellular surfaces of cell-free patches. Removal of extracellular Mg2+ did not alter the effectiveness of tolbutamide provided that Mg2+ was present at the intracellular surface of the patch. 3 Tolbutamide was also significantly less effective when applied to the intracellular surface of cell-free patches when Mn2+ was used as a replacement for Mg2+. 4 Both the sulphonylurea, glibenclamide and the non-sulphonylurea derivative, meglitinide also showed Mg2+ dependent inhibitory effects in cell-free patches. In contrast, the barbiturate thiopentone inhibited KATP in a Mg(2+)-independent manner. 5 Whole-cell IK(ATP) were used to quantify the effects of tolbutamide and glibenclamide in the presence and absence of intracellular Mg2+. Concentration-inhibition curves, in the presence of intracellular Mg2+, resulted in IC50 values of 12.1 microM and 2.1 nM for tolbutamide and glibenclamide, respectively. In the absence of intracellular Mg2+, the corresponding IC50 values were 25.3 mM and 3.6 microM, respectively. The values of IC50 for thiopentone in the presence and absence of intracellular Mg2+ were 69.4 microM and 69.2 microM, respectively. 6 With respect to the high affinity binding sites for [3H]-glibenclamide in CRI-G1 membranes, no significant differences were found between the dissociation constants for, or the maximal binding capacities of, [3H]-glibenclamide in the presence or absence of Mg2+. 7. In the CRI-G1 insulin-secreting cell line, it is concluded that intracellular Mg2+ does not influence the affinity of the sulphonylureas for the sulphonylurea receptor but that this ion is critically important for the interaction between the sulphonylurea receptor and KATP.

Polymyxin B has multiple blocking actions on the ATP-sensitive potassium channel in insulin-secreting cells

The action of polymyxin B (0.1 microM) on ATP-sensitive K+ (K+ATP) channels in RINm5F insulin-secreting cells was investigated by patch-clamp techniques. Using inside-out patches, open-cells and outside-out patches, polymyxin B was found to block K+ATP channels by, on average, approximately 90-95% of the initial control level of channel activity. The effects were rapid in onset, sustained and readily reversible. Similar effects were found in patches excised from cells pretreated overnight with 1 microM of the phorbol ester phorbol myristate acetate (PMA). External block of channels was associated with a marked decrease in single-channel current amplitude, whereas these effects were not seen when polymyxin B was added to the inside face of the membrane. In patches bathed with internally applied ATP (0.5 mM) and ADP (0.5 mM), polymyxin B inhibited channels but its actions were not reversible upon removal of the compound. However, when the same protocol was undertaken upon cells pre-treated with PMA, the effects of polymyxin B were readily reversed. Our data suggests that polymyxin B is a novel modulator of K+ATP channels, exhibiting multiple blocking actions that may possibly involve a direct effect upon the channel and indirect effects mediated through the inhibition of endogenous protein kinase(s).

Calcium-mediated agonists activate an inwardly rectified K+ channel in colonic secretory cells

Single-channel recording techniques were used to identify and characterize the K+ channel activated by Ca(2+)-mediated secretory agonists in T84 cells. Carbachol (CCh; 100 microM) and taurodeoxycholate (TDC; 0.75 mM) stimulated oscillatory outward K+ currents. With K gluconate in bath and pipette, cell-attached single-channel K+ currents stimulated by CCh and ionomycin (2 microM) were inwardly rectified and reversed at 0 mV. The single-channel chord conductance was 32 pS at -90 mV and 14 pS at +90 mV. Similar properties were observed in excised inside-out patches in symmetric K+, permitting further characterization of channel properties. Partial substitution of bath or pipette K+ with Na+ gave a K(+)-to-Na+ selectivity ratio of 5.5:1. Channel activity increased with increasing bath Ca2+ concentration in the physiological range of 50-800 nM. Maximal channel activity occurred at intracellular pH 7.2 and decreased at more acidic or alkaline pH values. Extracellular charybdotoxin (CTX; 50 nM) blocked inward but not outward currents. Extracellular tetraethylammonium (TEA; 10 mM) reduced single-channel amplitude at all voltages. No apparent block of the channel was observed with extracellular Ba2+ (1 mM), apamin (1 microM), 4-aminopyridine (4-AP; 4 mM), quinine (500 microM), or glyburide (10 microM). Cytosolic quinine and 4-AP blocked both inward and outward currents, whereas Ba2+ blocked only outward currents. Apamin, CTX, TEA, and glyburide did not affect channel activity. The agonist activation and pharmacological profile of this inwardly rectified K+ channel indicate that it is responsible for the increase in basolateral K+ conductance stimulated by Ca(2+)-mediated agonists in T84 cells.

The ATP-sensitive K+ channel

A small conductance K+ channel, that is inactivated by ATP, was recently found in the inner membrane of rat liver mitochondria (Inoue et al., 1991). This finding clearly indicates that a variety of K+ channels, showing ATP-sensitivity, are widely distributed. ATP is an important compound in view of its participation in oxidative phosphorylation and as the source of high-energy phosphate for nearly every energy-requiring reaction in the cell. Therefore, it is easy to speculate that transducing the ATP concentration within a cell into an electrical signal is vital for most living cells. The opening of the ATP-sensitive K+ channel by a decrease in the ATP level shifts the membrane potential in a negative direction and in general depresses cell function. The closing of the channel by an increase in ATP depolarizes the membrane and enhances membrane excitability. It might be speculated that a sequence of amino acids common for the binding site of ATP is preserved and combined with different types of K+ channels, so that the gating with ATP is quite similar between different K+ channels, but the conductance properties are different. The large variability in the value of K1/2ATP in the same cells or between different tissues might be due to modulation of the reaction of ATP and the binding site. These ideas will be substantiated by clarifying the molecular structure of the ATP-sensitive K+ channel in the near future. The molecular mechanisms for the selective channel blockers, sulfonylureas, and for the K+ channel openers should also be clarified.

Modification of K-ATP channels in pancreatic beta-cells by trypsin

The inside-out configuration of the patch-clamp method was used to study the effects of trypsin on the activity of ATP-sensitive potassium (K-ATP) channels from isolated mouse pancreatic beta-cells. Trypsin (20 micrograms/ml) irreversibly enhanced channel activity around twofold by reducing the interburst intervals without altering the burst kinetics. No effect on the single channel conductance or the inward rectification produced by internal Mg2+ was observed: however, the protease did reduce the inhibitory effect of Mg2+ on channel activity. Trypsin both prevented rundown of K-ATP channel activity and reactivated the channels after complete rundown. These effects of trypsin were absent in the presence of trypsin inhibitor. The protease also reduced the inhibitory effect of ATP on channel activity, increasing the dissociation constant from 7 to 49 microM. Trypsin removed the activating effect of ADP (0.1 mmol/l) on channel activity and reduced the inhibitory effect of tolbutamide (0.5 mmol/l). Carboxypeptidase A did not activate K-ATP channels in excised patches, although it was able to slightly reactivate channels after complete rundown, whereas chymotrypsin increased K-ATP channel activity but it did not produce reactivation. The effects of papain were similar to those of trypsin.