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

Mechanisms of Post-Pancreatitis Diabetes Mellitus and Cystic Fibrosis-Related Diabetes: A Review of Preclinical Studies

Anatomical proximity and functional correlations between the exocrine and endocrine pancreas warrant reciprocal effects between the two parts. Inflammatory diseases of the exocrine pancreas, such as acute or chronic pancreatitis, or the presence of cystic fibrosis disrupt endocrine function, resulting in diabetes of the exocrine pancreas. Although novel mechanisms are being increasingly identified, the intra- and intercellular pathways regulating exocrine-endocrine interactions are still not fully understood, making the development of new and more effective therapies difficult. Therefore, this review sought to accumulate current knowledge regarding the pathogenesis of diabetes in acute and chronic pancreatitis, as well as cystic fibrosis.

Running out of time: the decline of channel activity and nucleotide activation in adenosine triphosphate-sensitive K-channels

K_ATP channels act as key regulators of electrical excitability by coupling metabolic cues—mainly intracellular adenine nucleotide concentrations—to cellular potassium ion efflux. However, their study has been hindered by their rapid loss of activity in excised membrane patches (rundown), and by a second phenomenon, the decline of activation by Mg-nucleotides (DAMN). Degradation of PI(4,5)P₂ and other phosphoinositides is the strongest candidate for the molecular cause of rundown. Broad evidence indicates that most other determinants of rundown (e.g. phosphorylation, intracellular calcium, channel mutations that affect rundown) also act by influencing K_ATP channel regulation by phosphoinositides. Unfortunately, experimental conditions that reproducibly prevent rundown have remained elusive, necessitating post hoc data compensation. Rundown is clearly distinct from DAMN. While the former is associated with pore-forming Kir6.2 subunits, DAMN is generally a slower process involving the regulatory sulfonylurea receptor (SUR) subunits. We speculate that it arises when SUR subunits enter non-physiological conformational states associated with the loss of SUR nucleotide-binding domain dimerization following prolonged exposure to nucleotide-free conditions. This review presents new information on both rundown and DAMN, summarizes our current understanding of these processes and considers their physiological roles.

This article is part of the themed issue ‘Evolution brings Ca²⁺ and ATP together to control life and death’.

Chronic exposure to KATP channel openers results in attenuated glucose sensing in hypothalamic GT1-7 neurons

Individuals with Type 1 diabetes (T1D) are often exposed to recurrent episodes of hypoglycaemia. This reduces hormonal and behavioural responses that normally counteract low glucose in order to maintain glucose homeostasis, with altered responsiveness of glucose sensing hypothalamic neurons implicated. Although the molecular mechanisms are unknown, pharmacological studies implicate hypothalamic ATP-sensitive potassium channel (K_ATP) activity, with K_ATP openers (KCOs) amplifying, through cell hyperpolarization, the response to hypoglycaemia. Although initial findings, using acute hypothalamic KCO delivery, in rats were promising, chronic exposure to the KCO NN414 worsened the responses to subsequent hypoglycaemic challenge. To investigate this further we used GT1-7 cells to explore how NN414 affected glucose-sensing behaviour, the metabolic response of cells to hypoglycaemia and K_ATP activity. GT1-7 cells exposed to 3 or 24 h NN414 exhibited an attenuated hyperpolarization to subsequent hypoglycaemic challenge or NN414, which correlated with diminished K_ATP activity. The reduced sensitivity to hypoglycaemia was apparent 24 h after NN414 removal, even though intrinsic K_ATP activity recovered. The NN414-modified glucose responsiveness was not associated with adaptations in glucose uptake, metabolism or oxidation. K_ATP inactivation by NN414 was prevented by the concurrent presence of tolbutamide, which maintains K_ATP closure. Single channel recordings indicate that NN414 alters K_ATP intrinsic gating inducing a stable closed or inactivated state. These data indicate that exposure of hypothalamic glucose sensing cells to chronic NN414 drives a sustained conformational change to K_ATP, probably by binding to SUR1, that results in loss of channel sensitivity to intrinsic metabolic factors such as MgADP and small molecule agonists.

Domain organization of the ATP-sensitive potassium channel complex examined by fluorescence resonance energy transfer

K(ATP) channels link cell metabolism to excitability in many cells. They are formed as tetramers of Kir6.2 subunits, each associated with a SUR1 subunit. We used mutant GFP-based FRET to assess domain organization in channel complexes. Full-length Kir6.2 subunits were linked to YFP or cyan fluorescent protein (CFP) at N or C termini, and all such constructs, including double-tagged YFP-Kir6.2-CFP (Y6.2C), formed functional K(ATP) channels. In intact COSm6 cells, background emission of YFP excited by 430-nm light was ∼6%, but the Y6.2C construct expressed alone exhibited an apparent FRET efficiency of ∼25%, confirmed by trypsin digestion, with or without SUR1 co-expression. Similar FRET efficiency was detected in mixtures of CFP- and YFP-tagged full-length Kir6.2 subunits and transmembrane domain only constructs, when tagged at the C termini but not at the N termini. The FRET-reported Kir6.2 tetramer domain organization was qualitatively consistent with Kir channel crystal structures: C termini and M2 domains are centrally located relative to N termini and M1 domains, respectively. Additional FRET analyses were performed on cells in which tagged full-length Kir6.2 and tagged SUR1 constructs were co-expressed. These analyses further revealed that 1) NBD1 of SUR1 is closer to the C terminus of Kir6.2 than to the N terminus; 2) the Kir6.2 cytoplasmic domain is not essential for complexation with SUR1; and 3) the N-terminal half of SUR1 can complex with itself in the absence of either the C-terminal half or Kir6.2.

Activation of the K(ATP) channel by Mg-nucleotide interaction with SUR1

The mechanism of adenosine triphosphate (ATP)-sensitive potassium (K_ATP) channel activation by Mg-nucleotides was studied using a mutation (G334D) in the Kir6.2 subunit of the channel that renders K_ATP channels insensitive to nucleotide inhibition and has no apparent effect on their gating. K_ATP channels carrying this mutation (Kir6.2-G334D/SUR1 channels) were activated by MgATP and MgADP with an EC₅₀ of 112 and 8 µM, respectively. This activation was largely suppressed by mutation of the Walker A lysines in the nucleotide-binding domains of SUR1: the remaining small (∼10%), slowly developing component of MgATP activation was fully inhibited by the lipid kinase inhibitor LY294002. The EC₅₀ for activation of Kir6.2-G334D/SUR1 currents by MgADP was lower than that for MgATP, and the time course of activation was faster. The poorly hydrolyzable analogue MgATPγS also activated Kir6.2-G334D/SUR1. AMPPCP both failed to activate Kir6.2-G334D/SUR1 and to prevent its activation by MgATP. Maximal stimulatory concentrations of MgATP (10 mM) and MgADP (1 mM) exerted identical effects on the single-channel kinetics: they dramatically elevated the open probability (P_O > 0.8), increased the mean open time and the mean burst duration, reduced the frequency and number of interburst closed states, and eliminated the short burst states. By comparing our results with those obtained for wild-type K_ATP channels, we conclude that the MgADP sensitivity of the wild-type K_ATP channel can be described quantitatively by a combination of inhibition at Kir6.2 (measured for wild-type channels in the absence of Mg²⁺) and activation via SUR1 (determined for Kir6.2-G334D/SUR1 channels). However, this is not the case for the effects of MgATP.

A new benzoxazine compound blocks KATP channels in pancreatic beta cells: molecular basis for tissue selectivity in vitro and hypoglycaemic action in vivo

BACKGROUND AND PURPOSE

The 2-propyl-1,4 benzoxazine (AM10) shows a peculiar behaviour in skeletal muscle, inhibiting or opening the ATP-sensitive K(+) (KATP) channel in the absence and presence of ATP, respectively. We focused on tissue selectivity and mechanism of action of AM10 by testing its effects on pancreatic KATP channels by means of both in vitro and in vivo investigations.

EXPERIMENTAL APPROACH

In vitro, patch-clamp recordings were performed in native pancreatic beta cells and in tsA201 cells expressing the Kir6.2 Delta C36 channel. In vivo, an intraperitoneal glucose tolerance test was performed in normal mice.

KEY RESULTS

In contrast with what observed in the skeletal muscle, AM10, in whole cell perforated mode, did not augment KATP current (I(KATP)) of native beta cells but it inhibited it in a concentration-dependent manner (IC(50): 11.5 nM; maximal block: 60%). Accordingly, in current clamp recordings, a concentration-dependent membrane depolarization was observed. On excised patches, AM10 reduced the open-time probability of KATP channels without altering their single channel conductance; the same effect was observed in the presence of trypsin in the bath solution. Moreover, AM10 inhibited, in an ATP-independent manner, the K(+) current resulting from expressed Kir6.2 Delta C36 (maximal block: 60% at 100 microM; IC(50): 12.7 nM) corroborating an interaction with Kir. In vivo, AM10 attenuated the glycemia increase following a glucose bolus in a dose-dependent manner, without, at the dose tested, inducing fasting hypoglycaemia.

CONCLUSION AND IMPLICATIONS

Altogether, these results help to gain insight into a new class of tissue specific KATP channel modulators.

Functional coupling between sulfonylurea receptor type 1 and a nonselective cation channel in reactive astrocytes from adult rat brain

We previously identified a novel, nonselective cation channel in native reactive (type R1) astrocytes (NR1As) from injured rat brain that is regulated by cytoplasmic Ca2+ and ATP (NC(Ca-ATP)) and exhibits sensitivity to block by adenine nucleotides similar to that of sulfonylurea receptor type 1 (SUR1). Here we show that SUR1 is involved in regulation of this channel. NR1As within the site of injury and after isolation exhibited specific binding of FITC-tagged glibenclamide and were immunolabeled with anti-SUR1 antibody, but not with anti-SUR2, anti-Kir6.1 or anti-Kir6.2 antibodies, indicating absence of ATP-sensitive K+ (KATP) channels. RT-PCR confirmed transcription of mRNA for SUR1 but not SUR2. Several properties previously associated exclusively with SUR1-regulated KATP channels were observed in patch-clamp experiments using Cs+ as the charge carrier: (1) the sulfonylureas, glibenclamide and tolbutamide, inhibited NCCa-ATP channels with EC50 values of 48 nm and 16.1 microm, respectively; (2) inhibition by sulfonylureas was lost after exposure of the intracellular face to trypsin or anti-SUR1 antibody; (3) channel inhibition was caused by a change in kinetics of channel closing, with no change in channel amplitude or open-channel dwell times; and (4) the SUR activator ("KATP channel opener"), diazoxide, activated the NCCa-ATP channel, whereas pinacidil and cromakalin did not. Also, glibenclamide prevented cell blebbing after ATP depletion, whereas blebbing was produced by exposure to diazoxide. Our data indicate that SUR1 is functionally coupled to the pore-forming portion of the NC(Ca-ATP) channel, providing the first demonstration of promiscuity of SUR1 outside of the K+ inward rectifier family of channels.

Modification of ATP-sensitive K+ channels by proteolysis in smooth muscle cells from pig urethra

Patch-clamp experiments have been performed to investigate the effects of endoproteases (such as trypsin, carboxypeptidase B) on both membrane currents and unitary currents in isolated smooth muscle cells from pig proximal urethra (conventional whole-cell configuration, cell-attached configuration, and inside-out patches). Application of either trypsin (1 mg/mL) or carboxypeptidase B (0.1 mg/mL) to the intracellular surface of the excised membrane patches stimulated the activity of a 2.1 pA K+ channel (in symmetrical 140 mM K+ conditions) at a holding potential of -50 mV. The trypsin-induced K+ channels in inside-out configuration exhibited the same amplitude and similar channel opening kinetics to the levcromakalim-induced ATP-sensitive K+ channel (i.e. K ATP channel) in cell-attached patches of the same membrane; however, the sensitivity of the channels to glibenclamide was greatly reduced after the trypsin-treatment. The activity of the trypsin-induced K+ channel was reversibly inhibited by cibenzoline in an inside-out configuration (Ki = 5 microM). It is concluded that trypsin and carboxypeptidase B reactivate the channel with an intact pore activity but the different pharmacological properties of the channels may reflect some change in the conformation in channel proteins after proteolysis.

KATP channel inhibition by ATP requires distinct functional domains of the cytoplasmic C terminus of the pore-forming subunit

ATP-sensitive potassium ("KATP") channels are rapidly inhibited by intracellular ATP. This inhibition plays a crucial role in the coupling of electrical activity to energy metabolism in a variety of cells. The KATP channel is formed from four each of a sulfonylurea receptor (SUR) regulatory subunit and an inwardly rectifying potassium (Kir6.2) pore-forming subunit. We used systematic chimeric and point mutagenesis, combined with patch-clamp recording, to investigate the molecular basis of ATP-dependent inhibition gating of mouse pancreatic beta cell KATP channels expressed in Xenopus oocytes. We identified distinct functional domains of the presumed cytoplasmic C-terminal segment of the Kir6.2 subunit that play an important role in this inhibition. Our results suggest that one domain is associated with inhibitory ATP binding and another with gate closure.

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.

Direct inhibition of the pancreatic beta-cell ATP-regulated potassium channel by alpha-ketoisocaproate

The ATP-regulated potassium (KATP) channel plays an essential role in the control of insulin release from the pancreatic beta-cell. In the present study we have used the patch-clamp technique to study the direct effects of alpha-ketoisocaproate on the KATP channel in isolated patches and intact pancreatic beta-cells. In excised inside-out patches, the activity of the KATP channel was dose-dependently inhibited by alpha-ketoisocaproate, half-maximal concentration being approximately 8 mM. The blocking effect of alpha-ketoisocaproate was fully reversible. Stimulation of channel activity by the addition of ATP/ADP (ratio 1) did not counteract the inhibitory effect of alpha-ketoisocaproate. In the presence of the metabolic inhibitor sodium azide, alpha-ketoisocaproate was still able to inhibit single channel activity in excised patches and to block whole cell KATP currents in intact cells. No effect of alpha-ketoisocaproate could be obtained on either the large or the small conductance Ca2+-regulated K+ channel. Enzymatic treatment of the patches with trypsin prevented the inhibitory effect of alpha-ketoisocaproate. Based on these observations, it is unlikely that the blocking effect of alpha-ketoisocaproate is due to an unspecific effect on K+ channel pores. Leucine, the precursor of alpha-ketoisocaproate, did not affect KATP channel activity in excised patches. Our findings are compatible with the view that alpha-ketoisocaproate not only affects the beta-cell stimulus secretion coupling by generation of ATP but also by direct inhibition of the KATP channel.

Inhibition of the ATP-sensitive potassium channel from mouse pancreatic beta-cells by surfactants

1. We have used patch-clamp methods to study the effects of the detergents, Cremophor, Tween 80 and Triton X100 on the K(ATP) channel in the pancreatic beta-cell from mouse. 2. All three detergents blocked K(ATP) channel activity with the following order of potency: Tween 80 (Ki< approximately 83 nM)>Triton X100 (Ki=350 nM)>Cremophor. In all cases the block was poorly reversible. 3. Single-channel studies suggested that at low doses, the detergents act as slow blockers of the K(ATP) channel. 4. Unlike the block produced by tolbutamide, that produced by detergent was not affected by intracellular Mg2+-nucleotide, diazoxide or trypsin treatment, nor did it involve an acceleration of rundown or increase in ATP sensitivity of the chanel. 5. The detergents could block the pore-forming subunit, Kir6.2deltaC26, which can be expressed independently of SUR1 (the regulatory subunit of the K(ATP) channel). These data suggest that the detergents act on Kir6.2 and not SUR1. 6. The detergents had no effect on another member of the inward rectifier family: Kir1.1a (ROMK1). 7. Voltage-dependent K-currents in the beta-cell were reversibly blocked by the detergents with a far lower potency than that found for the K(ATP) channel. 8. Like other insulin secretagogues that act by blocking the K(ATP) channel, Cremophor elevated intracellular Ca2+ in single beta-cells to levels that would be expected to elicit insulin secretion. 9. Given the role of the K(ATP) channel in many physiological processes, we conclude that plasma borne detergent may have pharmacological actions mediated through blockage of the K(ATP) channel.

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.

Regulation of KATP channel activity by diazoxide and MgADP. Distinct functions of the two nucleotide binding folds of the sulfonylurea receptor

KATP channels were reconstituted in COSm6 cells by coexpression of the sulfonylurea receptor SUR1 and the inward rectifier potassium channel Kir6.2. The role of the two nucleotide binding folds of SUR1 in regulation of KATP channel activity by nucleotides and diazoxide was investigated. Mutations in the linker region and the Walker B motif (Walker, J.E., M.J. Saraste, M.J. Runswick, and N.J. Gay. 1982. EMBO [Eur. Mol. Biol. Organ.] J. 1:945-951) of the second nucleotide binding fold, including G1479D, G1479R, G1485D, G1485R, Q1486H, and D1506A, all abolished stimulation by MgADP and diazoxide, with the exception of G1479R, which showed a small stimulatory response to diazoxide. Analogous mutations in the first nucleotide binding fold, including G827D, G827R, and Q834H, were still stimulated by diazoxide and MgADP, but with altered kinetics compared with the wild-type channel. None of the mutations altered the sensitivity of the channel to inhibition by ATP4-. We propose a model in which SUR1 sensitizes the KATP channel to ATP inhibition, and nucleotide hydrolysis at the nucleotide binding folds blocks this effect. MgADP and diazoxide are proposed to stabilize this desensitized state of the channel, and mutations at the nucleotide binding folds alter the response of channels to MgADP and diazoxide by altering nucleotide hydrolysis rates or the coupling of hydrolysis to channel activation.

Evidence for an Na(+)-K(+)-Cl- cotransporter in mammalian type I vestibular hair cells

In amniotes, there are two types of hair cells, designated I and II, that differ in their morphology, innervation pattern, and ionic membrane properties. Type I cells are unique among hair cells in that their basolateral surfaces are almost completely enclosed by an afferent calyceal nerve terminal. Recently, several lines of evidence have ascribed a motile function to type I hair cells. To investigate this, elevated external K+, which had been used previously to induce hair cell shortening, was used to induce shape changes in dissociated mammalian type I vestibular hair cells. Morphologically identified type I cells shortened and widened when the external K+ concentration was raised isotonically from 2 to 125 mM. The shortening did not require external Ca2+ but was abolished when external Cl- was replaced with gluconate or sulfate and when external Na+ was replaced with N-methyl-D-glucamine. Bumetanide (10-100 microM), a specific blocker of the Na(+)-K(+)-Cl- cotransporter, significantly reduced K(+)-induced shortening. Hyposmotic solution resulted in type I cell shape changes similar to those seen with high K+, i.e., shortening and widening. Type I cells became more spherical in hyposmotic solution, presumably as a result of a volume increase due to water influx. In hypertonic solution, cells became narrower and increased in length. These results suggest that shape changes in type I hair cells induced by high K+ are due, at least in part, to ion and solute entry via an Na(+)-K(+)-Cl- cotransporter, which results in cell swelling. A scheme is proposed whereby the type I hair cell depolarizes and K+ leaves the cell via voltage-dependent K+ channels and accumulates in the synaptic space between the type I hair cell and calyx. Excess K+ could then be removed from the intercellular space by uptake via the cotransporter.

Characterization of a nicotinamide-adenine dinucleotide-dependent cation channel in the CRI-G1 rat insulinoma cell line

1. Cell-free excised membrane patches were used to examine the properties of a novel nicotinamide-adenine dinucleotide (beta-NAD+)-activated ion channel in the rat insulin-secreting cell line, CRI-G1. 2. In inside-out recordings, beta-NAD+ (0.05-1.0 mM) induced the appearance of a channel characterized by extremely slow kinetics, with mean open times in the range of seconds. The estimated EC50 for activation was 114 microM. Channel activity declined with time (run-down) following activation by beta-NAD+ in excised patches and this was not prevented by intracellular application of trypsin. 3. The single channel current-voltage relationship was linear with a conductance of 74 pS in symmetrical NaCl. The channel appears equally permeable to Na+, K+ and Cs+, exhibits an appreciable permeability to Ca2+, Mg2+ and Ba2+, but excludes anions. 4. The channel displays an unusual voltage sensitivity, with an abrupt increase in open-state probability at depolarized voltages. 5. Channel opening, in the presence of beta-NAD+, required both Ca2+ and Mg2+ to be present at the internal side of the membrane. Activation by Ca2+ required a concentration of at least 10 microM and was maximal at 0.1 mM. Ba2+ did not substitute for Ca2+ in inducing channel activity nor did it inhibit activation by Ca2+. Increasing the concentration of intracellular Mg2+ stabilized the open state of NAD(+)-activated channels. 6. The non-selective cation channel reported here differs in its gating and modulatory characteristics from non-selective cation channels described in other tissues. This channel may play a role in the pathophysiological responses of beta-cells to oxidative stress.

Phentolamine block of KATP channels is mediated by Kir6.2

The ATP-sensitive K+-channel (KATP channel) plays a key role in insulin secretion from pancreatic beta cells. It is closed both by glucose metabolism and the sulfonylurea drugs that are used in the treatment of noninsulin-dependent diabetes mellitus, thereby initiating a membrane depolarization that activates voltage-dependent Ca2+ entry and insulin release. The beta cell KATP channel is a complex of two proteins: Kir6.2 and SUR1. The former is an ATP-sensitive K+-selective pore, whereas SUR1 is a channel regulator that endows Kir6.2 with sensitivity to sulfonylureas. A number of drugs containing an imidazoline moiety, such as phentolamine, also act as potent stimulators of insulin secretion, but their mechanism of action is unknown. We have used a truncated form of Kir6.2, which expresses independently of SUR1, to show that phentolamine does not inhibit KATP channels by interacting with SUR1. Instead, our results argue that phentolamine may interact directly with Kir6.2 to produce a voltage-independent reduction in channel activity. The single-channel conductance is unaffected. Although the ATP molecule also contains an imidazoline group, the site at which phentolamine blocks is not identical to the ATP-inhibitory site, because phentolamine block of an ATP-insensitive mutant (K185Q) is normal. KATP channels also are found in the heart where they are involved in the response to cardiac ischemia: they also are blocked by phentolamine. Our results suggest that this may be because Kir6.2, which is expressed in the heart, forms the pore of the cardiac KATP channel.

Evidence for a unique long chain acyl-CoA ester binding site on the ATP-regulated potassium channel in mouse pancreatic beta cells

The mechanism by which long chain acyl-CoA (LC-CoA) esters affect the ATP-regulated potassium channel (KATP channel) was studied in inside-out patches isolated from mouse pancreatic beta cells. Addition of LC-CoA esters dramatically increased KATP channel activity. The stimulatory effect of the esters could be explained by the induction of a prolonged open state of the channel and did not involve alterations in single channel unitary conductance. Under control conditions, absence of adenine nucleotides, the distribution of KATP channel open time could be described by a single exponential, with a time constant of about 25 ms. Exposing the same patch to LC-CoA esters resulted in the appearance of an additional component with a time constant of >>150 ms, indicating a conformational change of the channel protein. LC-CoA esters were also able to potently activate channel activity at different ratios of ATP/ADP. Simultaneous additions of MgADP and LC-CoA esters resulted in a supra-additive effect on channel mean open time, characterized by openings of very long duration. Following modification of the KATP channel by a short exposure of the patch to the protease trypsin, the stimulatory effect of ADP on channel activity was lost while activation by LC-CoA esters still persisted. This indicates that LC-CoA esters and MgADP do not bind to the same site. We conclude that LC-CoA esters may play an important role in the physiological regulation of the KATP channel in the pancreatic beta cell by binding to a unique site and thereby inducing repolarization of the beta cell-membrane potential.

Differential effects of diet and obesity on high and low affinity sulfonylurea binding sites in the rat brain

The brain contains neurons which alter their firing rates when ambient glucose concentrations change. An ATP-sensitive K+ (Katp) channel on these neurons closes and increases cell firing when ATP is produced by intracellular glucose metabolism. Binding of the antidiabetic sulfonylurea drugs to a site linked to this channel has a similar effect. Here rats with a propensity to develop diet-induced obesity (DIO) or to be diet-resistant (DR) when fed a diet moderately high in fat, energy and sucrose (HE diet) had low and high affinity sulfonylurea binding assessed autoradiographically with [3H]glyburide in the presence or absence of Gpp(NH)p. Before HE diet exposure, chow-fed DIO- and DR-prone rats were separated by their high vs. low 24 h urine NE levels. In DR-prone rats, low affinity [3H]glyburide binding sites comprised up to 45% of total binding with highest concentrations in the hypothalamus and amygdala. But DIO-prone rats had few or no low affinity binding sites throughout the forebrain. High affinity [3H]glyburide binding was similar between phenotypes. When rats developed DIO after 3 months on HE diet, their low affinity binding increased slightly. DR rats fed the HE diet gained the same amount of weight as chow-fed controls but their low affinity binding sites were reduced to DIO levels and both were significantly lower than chow-fed controls. By contrast, high affinity [3H]glyburide binding was increased in DR rats throughout the forebrain so that it significantly exceeded that in both DIO and chow-fed control rats. These studies demonstrate a significant population of low affinity sulfonylurea binding sites throughout the forebrain which, along with high affinity sites, are regulated as a function of both weight gain phenotype and diet composition.

Trypsin alters ATP sensitivity of KATP channels in control and hypertrophied myocytes

Increased ATP-sensitive potassium (KATP) channel activity in cardiac muscle during hypertension and myocardial hypertrophy may be induced by the release of endogenous proteases, altering inhibitory binding sites for intracellular ATP. To test this hypothesis, we studied the effects of trypsin (1.5 mg/ml) on channel sensitivity to ATP in myocytes from control (WKY) and spontaneously hypertensive rats (SHR). Trypsin increased channel activity in 63% of membrane patches from WKY rats, but in only 29% from SHR. Pre-treatment with trypsin decreased sensitivity to the inhibitory effects of ATP in both groups. These results support the possibility that KATP channel modification during chronic metabolic stress is caused by intracellular proteolysis, which decreases sensitivity to [ATP]i.

New mechanisms for sulfonylurea control of insulin secretion

Oral antidiabetic sulfonylureas like tolbutamide and glyburide have been used to treat patients with noninsulin dependent diabetes mellitus. These agents lower blood glucose by stimulating insulin secretion from the pancreatic islets of Langerhans. A major component of this stimulation is sulfonylurea-mediated closure of the ATP-inhibited potassium channels (K(ATP) channels) of islet β-cells. Closure of these channels leads to cell depolarization, calcium uptake, and insulin exocytosis. Progress leading up to the recent cloning of the high-affinity sulfonylurea receptor and reconstitution of the K(ATP) channel is reviewed in this article together with new data showing that sulfonylureas may control secretion by activating a novel chloride ion channel, inhibiting an islet Na/K/ATPase or via distal stimulation of granule exocytosis by a kinase C dependent mechanism.

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.

Direct demonstration of sulphonylurea-sensitive KATP channels on nerve terminals of the rat motor cortex

We examined whether ATP-sensitive potassium (KATP) channels are present on presynaptic terminals of the rat motor cortex, an area of the CNS exhibiting a high density of sulphonylurea binding. A novel fused nerve terminal preparation was developed which produced structures amenable to patch clamp methods. In inside-out recordings a K+ channel was observed which possessed all the major features of the Type 1 KATP channel, including sensitivity to ATP and the antidiabetic sulphonylureas.

Effects of internal chloride on ATP-sensitive K-channels in mouse pancreatic beta-cells

The effect of internal Cl- on the K-ATP channel of pancreatic beta-cells was examined. Reducing Cl- from 140mM to 14mM potentiated channel activity (NPO) approximately 4.5 fold in 60% of patches but was without effect in 40% of patches. The Ki for channel inhibition by ATP was not changed. The inhibitory effect of Cl- interacted with the stimulatory action of MgGDP. In 140mM Cl-, 1mM MgGDP increased NPO in Cl(-)-sensitive patches only: when NPO was potentiated by 14mM Cl-, no further increase was produced by MgGDP. These observations suggest that MgGDP and low Cl- solution may increase channel activity via a similar mechanism.

Localization of high affinity [3H]glibenclamide binding sites within the substantia nigra zona reticulata of the rat brain

The rat substantia nigra zona reticulata contains a high density of binding sites for glibenclamide, an adenosine triphosphate-sensitive potassium channel inhibitor, but the precise location of glibenclamide binding sites within this area has not previously been examined. By combining neurochemical lesion and autoradiographical studies we have shown that high affinity [3H]glibenclamide binding sites are located on striatonigral terminals. Unilateral injections of 6-hydroxydopamine into the medial forebrain bundle or of quinolinic acid into the striatum were performed in anaesthetized adult rats to lesion the nigrostriatal and striatonigral pathways respectively. Autoradiography was performed on coronal sections of midbrain with [3H]glibenclamide, [3H]YM-09151-2 (dopamine D2 receptor antagonist) and [3H]SCH 23390 (dopamine D1 receptor antagonist) at three rostrocaudal levels of the substantia nigra. Under the conditions of the incubation [3H]glibenclamide binds primarily to the high affinity site. Following the 6-hydroxydopamine nigrostriatal lesion, D2 receptor binding was reduced (by up to 67%) on the lesioned side at all three levels of the substantia nigra whereas D1 receptor and glibenclamide binding were not significantly affected. In contrast, following striatonigral pathway lesion with quinolinic acid D2 receptor binding was unchanged on the lesioned side, but both D1 receptor and glibenclamide binding were reduced at all three levels (by up to 85% and 63% in the area of maximum lesion, respectively). In adjacent sections, the pattern of D1 binding loss was closely paralleled by the loss of glibenclamide binding. These results demonstrate that the high affinity glibenclamide binding sites of the substantia nigra zona reticulata are, at least in part, located on the terminals of striatonigral projection neurons.

Effects of intracellular pH on ATP-sensitive K+ channels in mouse pancreatic beta-cells

1. The effects of intracellular pH (pHi) on the ATP-sensitive K+ channel (K+ATP channel) from mouse pancreatic beta-cells were examined in inside-out patches exposed to symmetrical 140 mM K+ solutions. 2. The relationship between channel activity and pHi was described by the Hill equation with half-maximal inhibition (Ki) at pHi 6.25 and a Hill coefficient of 3.7. 3. Following exposure to pHi < 6.8, channel activity did not recover to its original level. Subsequent application of trypsin to the intracellular membrane surface restored channel activity to its initial level or above. 4. At -60 mV the relationship between pHi and the single-channel current amplitude was described by a modified Hill equation with a Hill coefficient of 2.1, half-maximal inhibition at pHi 6.48 and a maximum inhibition of 18.5%. 5. A decrease in pHi reduced the extent of channel inhibition by ATP: Ki was 18 microM at pH 7.2 and 33 microM at pH 6.4. The Hill coefficient was also reduced, being 1.65 at pH 7.2 and 1.17 at pH 6.4. 6. When channel activity was plotted as a function of ATP4- (rather than total ATP) there was no effect of pHi on the relationship. This suggests that ATP4- is the inhibitory ion species and that the effects of reducing pHi are due to the lowered concentration of ATP4-. 7. Changes in external pH had little effect on either single-channel or whole-cell K+ATP currents. 8. The effects of pHi do not support a role for H+ in linking glucose metabolism to K+ATP channel inhibition in pancreatic beta-cells.