Diazoxide-sensitivity of the adenosine 5'-triphosphate-dependent K+ channel in mouse pancreatic beta-cells

Water intake disorder in a DEND syndrome afflicted patient with R50P mutation

In this study, we present a case of developmental delay, epilepsy and neonatal diabetes (DEND) syndrome in a young male patient with the R50P mutation located in the Kir6.2 subunit of the ATP-sensitive K(+) (KATP) channel. Whereas most patients with DEND syndrome are resistant to sulfonylurea therapy, our patient was responsive to sulfonylurea, lacked the most common neurological symptoms, such as epilepsy, but refused to drink water. His serum electrolytes and plasma osmolarity were normal but the serum vasopressin level was increased. To investigate the underlying mechanism of his water intake disorder, a 5 μL aliquot of 340 μM KATP channel opener diazoxide or 100 μM KATP channel inhibitor glibenclamide was injected into the third ventricle of the rat brain, and water intake was monitored. Although the injection of glibenclamide had no effect, injection of diazoxide significantly increased water intake by about 1.5 fold without affecting food intake. This result indicates that the KATP channel activity in the brain may have an influence on water intake. Here, we present the first case of a DEND syndrome-afflicted patient with water intake disorder and increased serum vasopressin level, possibly related to altered KATP channel activity.

The physiology of rodent beta-cells in pancreas slices

Beta-cells in pancreatic islets form complex syncytia. Sufficient cell-to-cell electrical coupling seems to ensure coordinated depolarization pattern and insulin release that can be further modulated by rich innervation. The complex structure and coordinated action develop after birth during fast proliferation of the endocrine tissue. These emergent properties can be lost due to various reasons later in life and can lead to glucose intolerance and diabetes mellitus. Pancreas slice is a novel method of choice to study the physiology of beta-cells still embedded in their normal cellulo-social context. I present major advantages, list drawbacks and provide an overview on recent advances in our understanding of the physiology of beta-cells using the pancreas slice approach.

KATP-channels in beta-cells in tissue slices are directly modulated by millimolar ATP

In pancreatic beta-cells, inhibition of K(ATP)-channels plays a pivotal role in signal transduction of glucose-induced insulin release. However, the extreme sensitivity of K(ATP)-channels to its ligand ATP as found in inside-out patches is not directly compatible with modulation of these channels at physiological [ATP](i). We studied K(ATP)-channel sensitivity to ATP in beta-cells in dispersed culture and in fresh pancreatic tissue slices. Physiological [ATP](i) blocks more than 99% of K(ATP)-channels in cultured beta-cells, while only 90% in beta-cells in slices, indicating reduced sensitivity to ATP in the fresh slices. Applying cytosolic factors like ADP, phosphatidylinositol-4,5-bisphosphate (PIP(2)) or oleoyl-CoA did not restore the K(ATP)-channel sensitivity in cultured beta-cells. Our data suggest that interaction between SUR1 and Kir6.2 subunit of the K(ATP)-channel could be a factor in sensitivity modulation. Tissue slices are the first beta-cell preparation to study direct K(ATP)-channel modulation by physiological [ATP](i).

Nucleotide sensitivity of pancreatic ATP-sensitive potassium channels and type 2 diabetes

Type 2 diabetes is generally perceived as a polygenic disorder, with disease development being influenced by both hereditary and environmental factors. However, despite intensive investigations, little progress has been made in identifying the genes that impart susceptibility to the common late-onset forms of the disease. E23K, a common single nucleotide polymorphism in K(IR)6.2, the pore-forming subunit of pancreatic beta-cell ATP-sensitive K(+) (K(ATP)) channels, significantly enhances the spontaneous open probability of these channels, and thus modulates sensitivities toward inhibitory and activatory adenine nucleotides. Based on previous association studies, we present evidence that with an estimated attributable proportion of 15% in Caucasians, E23K in K(IR)6.2 appears to be the most important genetic risk factor for type 2 diabetes yet identified.

K(IR)6.2 polymorphism predisposes to type 2 diabetes by inducing overactivity of pancreatic beta-cell ATP-sensitive K(+) channels

E23K, a common single nucleotide polymorphism in K(IR)6.2, the pore-forming subunit of pancreatic beta-cell ATP-sensitive K(+) channels, significantly enhanced open probability of these channels, thus reducing their sensitivity toward inhibitory ATP(4-) and increasing the threshold concentration for insulin release. Previous association studies and high allelic frequency suggest this effect to critically inhibit secretion and play a major role in pathogenesis of common type 2 diabetes. Based on evidence for functional relevance of E23K in both the heterozygous (E/K; with E in position 23 of K(IR)6.2 in one allele and K in the other) and homozygous (K/K; with K in position 23 of K(IR)6.2 in both alleles) genotype, we propose a model in which enhanced susceptibility to type 2 diabetes is associated with evolutionary advantage of the E/K state.

Regulation of glucagon release in mouse -cells by KATP channels and inactivation of TTX-sensitive Na+ channels

The perforated patch whole-cell configuration of the patch-clamp technique was applied to superficial glucagon-secreting alpha-cells in intact mouse pancreatic islets. alpha-cells were distinguished from the beta- and delta-cells by the presence of a large TTX-blockable Na+ current, a TEA-resistant transient K+ current sensitive to 4-AP (A-current) and the presence of two kinetically separable Ca2+ current components corresponding to low- (T-type) and high-threshold (L-type) Ca2+ channels. The T-type Ca2+, Na+ and A-currents were subject to steady-state voltage-dependent inactivation, which was half-maximal at -45, -47 and -68 mV, respectively. Pancreatic alpha-cells were equipped with tolbutamide-sensitive, ATP-regulated K+ (KATP) channels. Addition of tolbutamide (0.1 mM) evoked a brief period of electrical activity followed by a depolarisation to a plateau of -30 mV with no regenerative electrical activity. Glucagon secretion in the absence of glucose was strongly inhibited by TTX, nifedipine and tolbutamide. When diazoxide was added in the presence of 10 mM glucose, concentrations up to 2 microM stimulated glucagon secretion to the same extent as removal of glucose. We conclude that electrical activity and secretion in the alpha-cells is dependent on the generation of Na+-dependent action potentials. Glucagon secretion depends on low activity of KATP channels to keep the membrane potential sufficiently negative to prevent voltage-dependent inactivation of voltage-gated membrane currents. Glucose may inhibit glucagon release by depolarising the alpha-cell with resultant inactivation of the ion channels participating in action potential generation.

Regulation of glucagon release in mouse -cells by KATP channels and inactivation of TTX-sensitive Na+ channels

The perforated patch whole-cell configuration of the patch-clamp technique was applied to superficial glucagon-secreting alpha-cells in intact mouse pancreatic islets. alpha-cells were distinguished from the beta- and delta-cells by the presence of a large TTX-blockable Na+ current, a TEA-resistant transient K+ current sensitive to 4-AP (A-current) and the presence of two kinetically separable Ca2+ current components corresponding to low- (T-type) and high-threshold (L-type) Ca2+ channels. The T-type Ca2+, Na+ and A-currents were subject to steady-state voltage-dependent inactivation, which was half-maximal at -45, -47 and -68 mV, respectively. Pancreatic alpha-cells were equipped with tolbutamide-sensitive, ATP-regulated K+ (KATP) channels. Addition of tolbutamide (0.1 mM) evoked a brief period of electrical activity followed by a depolarisation to a plateau of -30 mV with no regenerative electrical activity. Glucagon secretion in the absence of glucose was strongly inhibited by TTX, nifedipine and tolbutamide. When diazoxide was added in the presence of 10 mM glucose, concentrations up to 2 microM stimulated glucagon secretion to the same extent as removal of glucose. We conclude that electrical activity and secretion in the alpha-cells is dependent on the generation of Na+-dependent action potentials. Glucagon secretion depends on low activity of KATP channels to keep the membrane potential sufficiently negative to prevent voltage-dependent inactivation of voltage-gated membrane currents. Glucose may inhibit glucagon release by depolarising the alpha-cell with resultant inactivation of the ion channels participating in action potential generation.

Molecular biology of adenosine triphosphate-sensitive potassium channels

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

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.

Interaction of N-benzoyl-D-phenylalanine and related compounds with the sulphonylurea receptor of beta-cells

1. The structure activity relationships for the insulin secretagogues N-benzoyl-D-phenylalanine (NBDP) and related compounds were examined at the sulphonylurea receptor level by use of cultured HIT-T15 and mouse pancreatic beta-cells. The affinities of these compounds for the sulphonylurea receptor were compared with their potencies for K(ATP)-channel inhibition. In addition, the effects of cytosolic nucleotides on K(ATP)-channel inhibition by NBDP were investigated. 2. NBDP displayed a dissociation constant for binding to the sulphonylurea receptor (K(D) value) of 11 microM and half-maximally effective concentrations of K(ATP)-channel inhibition (EC50 values) between 2 and 4 microM (in the absence of cytosolic nucleotides or presence of 0.1 mM GDP or 1 mM ADP). 3. In the absence of cytosolic nucleotides or presence of GDP (0.1 mM) maximally effective concentrations of NBDP (0.1-1 mM) reduced K(ATP)-channel activity to 47% and 44% of control, respectively. In the presence of ADP (1 mM), K(ATP)-channel activity was completely suppressed by 0.1 mM NBDP. 4. The L-isomer of N-benzoyl-phenylalanine displayed a 20 fold lower affinity and an 80 fold lower potency than the D-isomer. 5. Introduction of a p-nitro substituent in the D-phenylalanine moiety of NBDP did not decrease lipophilicity but lowered affinity and potency by more than 30 fold. 6. Introduction of a p-amino substituent in the D-phenylalanine moiety of NBDP (N-benzoyl-p-amino-D-phenylalanine, NBADP) reduced lipophilicity and lowered affinity and potency by about 10 fold. This loss of affinity and potency was compensated for by formation of the phenylpropionic acid derivative of NBADP. A similar difference in affinity was observed for the sulphonylurea carbutamide and its phenylpropionic acid derivative. 7. Replacing the benzene ring in the D-phenylalanine moiety of NBDP by a cyclohexyl ring increased lipophilicity, and the K(D) and EC50 values were slightly lower than for NBDP. Exchange of both benzene rings in NBDP by cyclohexyl rings further increased lipophilicity without altering affinity and potency. 8. This study shows that N-acylphenylalanines interact with the sulphonylurea receptor of pancreatic beta-cells in a stereospecific manner. Their potency depends on lipophilic but not aromatic properties of their benzene rings. As observed for sulphonylureas, interaction of N-acylphenylalanines with the sulphonylurea receptor does not induce complete inhibition of K(ATP)-channel activity in the absence of inhibitory cytosolic nucleotides.

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.

KATP-channel on the somata of spiny neurones in rat caudate nucleus: regulation by drugs and nucleotides

1. The aim of the present study was to characterize the pharmacological properties of the adenosine 5'-triphosphate(ATP)-sensitive K+ channel (KATP-channel) on the somata of spiny neurones in rat caudate nucleus and to compare them with those of beta-cells. For that purpose we tested the effects of several KATP-channel-inhibiting and -activating drugs on the opening activity of the KATP-channel in caudate nucleus by use of the patch-clamp technique. In addition, the modulation of drug responses by cytosolic nucleotides was examined. 2. When KATP-channels in caudate nucleus were activated in cell-attached patches by inhibition of mitochondrial energy production, meglitinide (a benzoic acid derivative), Hoe36320 (a sulphonylurea of low lipophilicity) and glipizide reduced KATP-channel activity half-maximally at 0.4 microM, 0.4 microM and about 0.5 nM, respectively. 3. In inside-out patches (presence of 0.7 mM free Mg2+ at the cytoplasmic membrane side), tolbutamide (0.1 mM) caused only partial inhibition of KATP-channels in the absence of cytosolic nucleotides but complete inhibition in the simultaneous presence of the channel-activating nucleotide guanosine 5'-diphosphate (GDP; 1 mM) and the channel-inhibiting nucleotide adenylyl-imidodiphosphate (AMP-PNP; 0.2 mM). 4. Diazoxide (0.3 mM) strongly increased channel activity in the presence of ATP (0.1 mM) or GDP (0.03 mM), but was ineffective in the presence of AMP-PNP (0.1 mM). In the absence of cytosolic nucleotides diazoxide even decreased channel activity. 5. In the presence of 0.1 mM ATP, diazoxide activated KATP-channels half-maximally at 38 microM. 6. When KATP-channel activity was inhibited by 0.1 mM ATP, (-)-pinacidil (0.5 mM) elicited a slight activation of KATP-channels in caudate nucleus, whereas (+)-pinacidil (0.5 mM) and lemakalim (0.3 mM) were ineffective. 7. Since our data indicate similar control by drugs and nucleotides of KATP-channels in the somata of spiny neurones and pancreatic beta-cells, we conclude that the high affinity sulphonylurea receptors of these tissues are probably closely related.

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.

Sulfonylurea-sensitive potassium current evoked by sodium-loading in rat midbrain dopamine neurons

In Parkinson's disease, there is evidence of impaired mitochondrial function which reduces the capacity to synthesize ATP in dopamine neurons. This would be expected to reduce the activity of the sodium pump (Na+/K+ ATPase), causing increased intracellular levels of Na+. Patch pipettes were used to introduce Na+ (40 mM in pipette solutions) into dopamine neurons in the rat midbrain slice in order to study the electrophysiological effects of increased intracellular Na+. We found that intracellular Na+ loading evoked 100-300 pA of outward current (at -60 mV) and increased whole-cell conductance; these effects developed gradually during the first 10 min after rupture of the membrane patch. Extracellular Ba2+ reduced most of the outward current evoked by Na+ loading; this Ba(2+)-sensitive current reversed direction at the expected reversal potential for K+ (EK), and was also blocked by extracellular tetraethylammonium (30 mM) and intracellular Cs+ (which replaced K+ in pipette solutions). The sulfonylurea drugs glipizide (IC50 = 4.9 nM), tolbutamide (IC50 = 23 microM) and glibenclamide (1 microM) were as effective as 300 microM Ba2+ in reducing the K+ current evoked by Na+ loading. When recording with "control" pipettes containing 15 mM Na+, diazoxide (300 microM) increased chord conductance and evoked outward current at -60 mV, which also reversed direction near EK. Effects of diazoxide were blocked by glibenclamide (1 microM) or glipizide (300 nM). Diazoxide (300 microM) and baclofen (3 microM), which also evoked K(+)-mediated outward currents recorded with control pipettes, caused little additional increases in outward currents during Na+ loading. Raising ATP concentrations to 10 mM in pipette solutions failed to significantly reduce currents evoked by diazoxide or Na+ loading, suggesting that these currents may not be mediated by ATP-sensitive K+ channels. Finally, Na+ loading using pipettes containing Cs+ in place of K+ evoked a relatively small outward current (50-150 pA at -60 mV), which developed gradually over the first 10 min after rupturing the membrane patch. This current was reduced by dihydro-ouabain (3 microM) and a low extracellular concentration of K+ (0.5 mM instead of 2.5 mM), but was not affected by Ba2+. We conclude that intracellular Na+ loading evokes a current generated by Na+/K+ ATPase in addition to sulfonylurea-sensitive K+ current. This Na(+)-dependent K+ current is unusual in its sensitivity to sulfonylureas, and could protect dopamine neurons against toxic effects of intracellular Na+ accumulation.

Influence of ion channel modulation on in vitro interferon-gamma induced MHC class I and II expression on macrophages

The in vitro effect of K+ channel blockers quinidine and verapamil, anion channel blocker SITS and K+ channel openers diazoxide, pinacidil, and BRL 38227 on interferon-gamma (IFN-gamma) induced MHC class I and II expression of Lewis rat peritoneal macrophages was investigated by cell ELISA assay. MHC class I expression was significantly enhanced by diazoxide at concentrations of 10(-5)M to 10(-6)M and by pinacidil and BRL 38227 at the concentration of 10(-6)M. MHC class II expression was enhanced by pinacidil and BRL 38227 at concentrations of 10(-5)M to 10(-6)M. The enhancing effect of pinacidil could be blocked by inhibitors of the protein kinases PKA and PKC suggesting that activation of both is required for optimum induction of MHC molecule expression. K+ and anion channel blockers were less active in modulation of MHC molecule expression. Verapamil had no influence, quinidine suppressed MHC class I expression at concentrations of 10(-4)M to 10(-5)M, and SITS suppressed MHC class I expression at the concentration of 10(-3)M. Since MHC class II expression is essential for efficient antigen presentation to T helper cells and MHC class I expression is required for target cell lysis by cytotoxic T cells, ion channel modulating drugs may be potential candidates for immunopharmacological intervention in inflammatory diseases.

Location of the sulphonylurea receptor at the cytoplasmic face of the beta-cell membrane

1. In insulin-secreting cells the location of the sulphonylurea receptor was examined by use of a sulphonylurea derivative representing the glibenclamide molecule devoid of its cyclohexy moiety (compound III) and a benzenesulphonic acid derivative representing the glibenclamide molecule devoid of its cyclohexylurea moiety (compound IV). At pH 7.4 compound IV is only present in charged form. 2. Lipid solubility declined in the order tolbutamide > compound III > compound IV. 3. The dissociation constant (KD) for binding of compound IV to the sulphonylurea receptor in HIT-cells (pancreatic beta-cell line) was similar to the KD value for tolbutamide and fourfold higher than the KD value for compound III. 4. In mouse pancreatic beta-cells, drug concentrations inhibiting adenosine 5'-triphosphate-sensitive K+ channels (KATP-channels) half-maximally (EC50) were determined by use of the patch-clamp technique. When the drugs were applied to the extracellular side of outside-out or the intracellular side of inside-out membrane patches, the ratio of extracellular to intracellular EC50 values was 281 for compound IV, 25.5 for compound III and 1.2 for tolbutamide. 5. In mouse pancreatic beta-cells, measurement of KATP-channel activity in cell-attached patches and recording of insulin release displayed much higher EC50 values for compound IV than inside-out patch experiments. A corresponding, but less pronounced difference in EC50 values was observed for compound III, whereas the EC50 values for tolbutamide did not differ significantly. 6. It is concluded that the sulphonylurea receptor is located at the cytoplasmic face of the beta-cell plasma membrane. Receptor activation is induced by the anionic forms of sulphonylureas and their analogues.

Embryopathic effects of diazoxide and the reduction of sulfonylurea-induced dysmorphogenesis in vitro

Diazoxide is a benzothiadiazine used in oral and intravenous preparations to treat hypertension and hypoglycaemia, but its effect on embryonic development has not been well studied. Previous in vivo work has suggested placental transfer and teratogenicity of diazoxide in humans and laboratory animals, but this study represents the first in vitro investigation of the effect of diazoxide on the embryo. The in vitro method of whole-embryo culture was used to expose mouse embryos to specific levels of diazoxide (0-200 mug/ml) during organogenesis at well-defined (4-6 and 20-25 somite) stages of development. In addition, the combined effect of diazoxide [ATP-sensitive K(+) (K(ATP)) channel opener] and the sulfonylurea oral hypoglycaemic agent chlorpropamide (K(ATP) channel blocker), was evaluated in vitro in embryos with 4-6 somites. Diazoxide produced malformations and growth retardation in mouse embryos exposed to 100 mug/ml or more for 24 hr in vitro at both stages of organogenesis. In addition, a subteratogenic concentration of diazoxide (25 mug/ml) reduced the embryopathic effects of chlorpropamide (130 mug/ml) in embryos with 4-6 somites. A potential mechanism for these effects involves K(ATP) channels.

Imidazolines stimulate release of insulin from RIN-5AH cells independently from imidazoline I1 and I2 receptors

The effect on insulin release of efaroxan, an alpha 2-adrenoceptor antagonist and a highly potent drug at imidazoline I1 receptors, and the effects of seven other imidazoline compounds selective for the imidazoline I1 or I2 receptors, were studied in the rat insulinoma cell line RIN-5AH. The cells released insulin in response to glucose (0.3-10 mM), and efaroxan (100 microM) potentiated glucose-induced insulin release. (-)-Adrenaline completely displaced the binding of [125I]p-iodoclonidine to membranes of RIN-5AH cells, indicating that these cells do not express imidazoline I1 receptors. Cirazoline and idazoxan (100 microM), both highly potent drugs at imidazoline I2 receptors, and the guanidines guanoxan and amiloride (200 microM), also promoted insulin release from RIN-5AH cells. Irreversible blockade of imidazoline I2 receptors with 10 microM clorgyline did not prevent the stimulatory effects of cirazoline or idazoxan; however, these compounds completely reversed the inhibition by diazoxide (250 microM), an opener of ATP-dependent K+ channels (K+ATP channels), of glucose-induced insulin release. These data indicate that the imidazoline/guanidine compounds promote insulin release from RIN-5AH cells, by interacting with a novel binding site related to K+ATP channels that does not represent any of the known imidazoline I1 or I2 receptors.

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.

Influence of K+ channel openers on interferon-gamma dependent immune response in experimental allergic neuritis (EAN)

We examined the influence of the K+ channel opening drugs BRL 38227, pinacidil and diazoxide on cellular immune response and clinical course of experimental allergic neuritis (EAN) actively induced in Lewis rats by bovine peripheral myelin (BPM). T cell functions of EAN lymph node cells were assessed by measurement of proliferation and by counting of interferon-gamma secreting cells (IFN-gamma sc) in response to the specific antigen BPM and the T cell mitogen phytohemagglutinin (PHA). BRL 38227 and diazoxide at concentrations of 10(-5)M-10(-6)M and pinacidil at concentrations of 10(-5)M-10(-7)M enhanced the proliferative response to both BPM and PHA. The number of IFN-gamma sc was suppressed by the K+ channel openers in the same concentration range. There was a tendency of stronger suppression of cultures with high numbers of BPM-reactive IFN-gamma sc than of cultures with low numbers of BPM-reactive IFN-gamma sc. The applied K+ channel openers are primarily acting on ATP-sensitive K+ channels, which have not been found in T cells so far. The drugs may, therefore, exert non-selective effects on conventional voltage- and/or Ca(++)-dependent channels of T cells. A first trial with in vivo administration of 2.5 mg/kg x day of the drugs resulted in more severe neurological deficits in the early phase of EAN with BRL 38227, whereas pinacidil and diazoxide had no significant effects.

Interaction of tolbutamide and cytosolic nucleotides in controlling the ATP-sensitive K+ channel in mouse beta-cells

1. In mouse pancreatic beta-cells the role of cytosolic nucleotides in the regulation of the sulphonylurea sensitivity of the adenosine 5'-triphosphate-sensitive K+ channel (KATP-channel) was examined. Patch-clamp experiments with excised inside-out membrane patches were carried out using an experimental protocol favouring phosphorylation of membrane proteins. 2. In the absence of Mg2+, the KATP-channel-inhibiting potency of cytosolic nucleotides decreased in the order ATP = adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) > adenosine 5'-diphosphate (ADP) > adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) = adenylyl-imidodiphosphate (AMP-PNP) > 2'-deoxyadenosine 5'-triphosphate (dATP) > uridine 5'-triphosphate (UTP) > 2'-deoxyadenosine 5'-diphosphate (dADP) > guanosine 5'-triphosphate (GTP) > guanosine 5'-diphosphate (GDP) > uridine 5'-diphosphate (UDP). 3. In the presence of Mg2+, the inhibitory potency of cytosolic nucleotides decreased in the order ATP gamma S > ATP > AMP-PNP > ADP beta S > dATP > UTP. In the presence of Mg2+, the KATP-channels were activated by dADP, GTP, GDP and UDP. 4. Tolbutamide inhibited the KATP-channels not only in the presence but also in the prolonged absence of Mg2+. In nucleotide-free solutions, the potency of tolbutamide was very low. When about half of the KATP-channel activity was inhibited by ATP, AMP-PNP, ADP beta S or ADP (absence of Mg2+), the potency of tolbutamide was increased. 5. Tolbutamide (100 microM) slightly enhanced the channel-inhibiting potency of AMP-PNP and inhibited the channel-activating effect of MgGDP in a non-competitive manner. 6. Channel activation by MgGDP (0.5 mM) competitively antagonized the inhibitory responses to AMP-PNP (1 MicroM- 1 mM). This effect of GDP was neutralized by tolbutamide (100 MicroM).7. The stimulatory effect of 0.5 mM MgGDP was neutralized by 200 MicroM AMP-PNP. Under these conditions the potency of tolbutamide was much higher than in the presence of 0.5 mM MgGDP alone or in the absence of any nucleotides.8. dADP (0.3-1 mM) increased the potency of tolbutamide. Additional application of 200 MicroM AMPPNP caused a further increase in the potency of tolbutamide.9. In conclusion, in the simultaneous presence of inhibitory and stimulatory nucleotides, binding of sulphonylureas to their receptor causes direct inhibition of channel activity, non-competitive inhibition of the action of stimulatory nucleotides and interruption of the competitive interaction between stimulatory and inhibitory nucleotides. The latter effect increases the proportion of KATP- channels staying in the nucleotide-blocked state. In addition, this state potentiates the direct effect of sulphonylureas.

Tolbutamide- and diazoxide-sensitive K+ channel in neurons of substantia nigra pars reticulata

Single-channel K+ currents were recorded in cell-attached patches from slices of rat substantia nigra. On the somata of neurons in the caudal half of the substantia nigra pars reticulata a K+ selective channel with a unitary conductance of 71 pS (154 mmol/l K+ in pipette filling solution) was identified. The channel was activated both by application of diazoxide (300 mumol/l) and by energy-depleting conditions (200 mumol/l cyanide) and was reversibly blocked by tolbutamide (0.1-1 mmol/l). It is concluded that neurons in the substantia nigra pars reticulata of the rat contain a typical ATP-sensitive K+ channel the activity of which can be modulated by diazoxide and sulfonylureas.