The binding properties of the particulate and solubilized sulfonylurea receptor from cerebral cortex are modulated by the Mg2+ complex of ATP

Localization of the ATP-sensitive K(+) channel regulatory subunits SUR2A and SUR2B in the rat brain

ATP-sensitive K(+) (K(ATP)) channel subunits SUR2A and SUR2B in the rat brain were investigated by RT-PCR assay, western blot analysis, in situ hybridization histochemistry, and immunohistochemical staining. The results show that the mRNA and protein of SUR2A and SUR2B are expressed in whole rat brain extracts and selected regions. SUR2 mRNA is widely expressed in many neurons and glial cells as revealed by in situ hybridization histochemistry. Immunohistochemical staining shows SUR2A to be widely expressed in neurons of the brain, especially in the large pyramidal neurons and their main dendrites in the neocortex and in the Purkinje cells of the cerebellar cortex. In contrast to SUR2A, SUR2B is potently expressed in small cells in the corpus callosum and cerebellar white matter, but is also weakly expressed in some neurons. Double immunostaining shows SUR2B to be localized in astrocytes and oligodendrocytes, while SUR2A is only localized in oligodendrocytes. These results suggest that SUR2A might be mainly a regulatory subunit of the K(ATP) channel in most neurons and part of oligodendrocytes, while SUR2B might be mainly a regulatory subunit of the K(ATP) channel in astrocytes, oligodendrocytes, and some neurons.

Incomplete dissociation of glibenclamide from wild-type and mutant pancreatic K ATP channels limits their recovery from inhibition

BACKGROUND AND PURPOSE

The antidiabetic sulphonylurea, glibenclamide, acts by inhibiting the pancreatic ATP-sensitive K(+) (K(ATP)) channel, a tetradimeric complex of K(IR)6.2 and sulphonylurea receptor 1 (K(IR)6.2/SUR1)(4). At room temperature, recovery of channel activity following washout of glibenclamide is very slow and cannot be measured. This study investigates the relation between the recovery of channel activity from glibenclamide inhibition and the dissociation rate of [(3)H]-glibenclamide from the channel at 37 degrees C.

EXPERIMENTAL APPROACH

K(IR)6.2, K(IR)6.2DeltaN5 or K(IR)6.2DeltaN10 (the latter lacking amino-terminal residues 2-5 or 2-10 respectively) were coexpressed with SUR1 in HEK cells. Dissociation of [(3)H]-glibenclamide from the channel and recovery of channel activity from glibenclamide inhibition were determined at 37 degrees C.

KEY RESULTS

The dissociation kinetics of [(3)H]-glibenclamide from the wild-type channel followed an exponential decay with a dissociation half-time, t(1/2)(D) = 14 min; however, only limited and slow recovery of channel activity was observed. t(1/2)(D) for K(IR)6.2DeltaN5/SUR1 channels was 5.3 min and recovery of channel activity exhibited a sluggish sigmoidal time course with a half-time, t(1/2)(R) = 12 min. t(1/2)(D) for the DeltaN10 channel was 2.3 min; recovery kinetics were again sigmoidal with t(1/2)(R) approximately 4 min.

CONCLUSIONS AND IMPLICATIONS

The dissociation of glibenclamide from the truncated channels is the rate-limiting step of channel recovery. The sigmoidal recovery kinetics are in quantitative agreement with a model where glibenclamide must dissociate from all four (or at least three) sites before the channel reopens. It is argued that these conclusions hold also for the wild-type (pancreatic) K(ATP) channel.

Affinities of dihydrocodeine and its metabolites to opioid receptors

Dihydrocodeine is metabolized to dihydromorphine, dihydrocodeine-6-O-, dihydromorphine-3-O- and dihydromorphine-6-O-glucuronide, and nordihydrocodeine. The current study was conducted to evaluate the affinities of dihydrocodeine and its metabolites to mu-, delta- and kappa-opioid receptors. Codeine, morphine, d,1-methadone and levomethadone were used as controls. Displacement binding experiments were carried out at the respective opioid receptor types using preparations of guinea pig cerebral cortex and the specific opioid agonists [5H]DAMGO (mu-opioid receptor), [3H]DPDPE (delta-opioid receptor) and [3H]U69,593 (K-opioid receptor) as radioactive ligands at concentrations of 0.5, 1.0 and 1.0 nmol/l, respectively. All substances had their greatest affinity to the mu-opioid receptor. The affinities of dihydromorphine and dihydromorphine-6-O-glucuronide were at least 70 times greater compared with dihydrocodeine (Ki 0.3 micromol/l), whereas the other metabolites yielded lower affinities. For the delta-opioid receptor, the order of affinities was similar with the exception that dihydrocodeine-6-O-glucuronide revealed a doubled affinity in relation to dihydrocodeine (Ki 5.9 micromol/l). In contrast, for the K-opioid receptor, dihydrocodeine-6-O- and dihydromorphine-6-O-glucuronide had clearly lower affinities compared to the respective parent compounds. The affinity of nordihydrocodeine was almost identical to that of dihydrocodeine (Ki 14 micromol/l), whereas dihydromorphine had a 60 times higher affinity. These results suggest that dihydromorphine and its 6-O-glucuronide may provide a relevant contribution to the pharmacological effects of dihydrocodeine. The O-demethylation of dihydrocodeine to dihydromorphine is mediated by the polymorphic cytochrome P-450 enzyme CYP2D6, resulting in different metabolic profiles in extensive and poor metabolizers. About 7% of the caucasian population which are CYP2D6 poor metabolizers thus may experience therapeutic failure after standard doses.

Interaction of K(ATP) channel modulators with sulfonylurea receptor SUR2B: implication for tetramer formation and allosteric coupling of subunits

Sulfonylurea receptors (SURs) are subunits of ATP-sensitive K(+) channels (K(ATP) channels); they mediate the channel-closing effect of sulfonylureas such as glibenclamide and the channel-activating effect of K(ATP) channel openers such as the pinacidil analog P1075. We investigated the inhibition by MgATP and P1075 of glibenclamide binding to SUR2B, the SUR subtype in smooth muscle. To increase specific binding, experiments were also performed using SUR2B(Y1206S), a mutant with higher affinity for glibenclamide than for the wild-type (K(D )= 4 versus 22 nM, respectively) but otherwise exhibiting similar pharmacological properties. In the absence of MgATP, [(3)H]glibenclamide binding to both SURs was homogenous. MgATP inhibited [(3)H]glibenclamide binding to both SURs to 25% by reducing the apparent number of glibenclamide binding sites, leaving the affinity unchanged. In the absence of MgATP, P1075 inhibited [(3)H]glibenclamide binding in a monophasic manner with K(i) approximately 1 microM. In the presence of MgATP (1 mM), inhibition was biphasic with one K(i) value resembling the true affinity of P1075 for SUR2B (2-6 nM) and the other resembling K(i) in the absence of MgATP (approximately 1 microM). The data show that (1) MgATP induces heterogeneity in the glibenclamide sites; (2) the high-affinity glibenclamide sites remaining with MgATP are linked to two classes of P1075 sites; and (3) P1075 interacts specifically with SUR2B also in the absence of MgATP. The data are discussed with the assumption that SUR2B, expressed alone, forms tetramers; that MgATP induces allosteric interactions between the subunits; and that mixed SUR2B-glibenclamide-P1075 complexes can exist at equilibrium.

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.

A view of sur/KIR6.X, KATP channels

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

Pharmacological characterization of the sulphonylurea receptor in rat isolated aorta

1. The binding of the sulphonylurea [3H]-glibenclamide, a blocker of adenosine 5'-triphosphate (ATP)-sensitive K+ channels (KATP channels), was studied in endothelium-denuded rings from rat aorta. 2. [3H]-glibenclamide labelled two classes of binding sites with KD values of 20 +/- 5 nM and 32 +/- 1 microM. The high affinity component, which comprised 17% of total binding at 1 nM [3H]-glibenclamide, had an estimated binding capacity of 150 fmol mg-1 wet weight. 3. Other sulphonylureas such as glipizide and glibornuride and the sulphonylurea-related carboxylate, AZ-DF 265, inhibited high affinity [3H]-glibenclamide binding with the potencies expected from their K+ channel activity. At very high concentrations, AZ-DF 265 and glipizide started to interact also with the low affinity component of [3H]-glibenclamide binding. 4. Openers of the ATP-sensitive K+ channel belonging to different structural groups inhibited only the high affinity [3H]-glibenclamide binding; the potencies in this assay were similar to those obtained in functional (i.e. vasorelaxation) studies. 5. High affinity [3H]-glibenclamide binding was abolished by prolonged hypoxia combined with metabolic inhibition. 6. The data indicate that the high affinity component of [3H]-glibenclamide binding mediates the block of the KATP channel by the sulphonylureas in rat aorta; hence, it represents the sulphonylurea receptor in this vessel. The pharmacological properties of this binding site resemble those of the binding site for the openers of the KATP channel; present evidence suggests that these two classes of sites are negatively allosterically coupled.

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.

Morphine-6-O-beta-D-glucuronide but not morphine-3-O-beta-D-glucuronide binds to mu-, delta- and kappa- specific opioid binding sites in cerebral membranes

We investigated the nature of interaction of morphine-3-O-beta-D-glucuronide (M3G) and morphine-6-O-beta-D-glucuronide (M6G) with opioid binding sites at the mu-, delta- and kappa-opioid receptors (mu-OR, delta-OR and kappa-OR) in cerebral membranes. Saturation binding experiments revealed a competitive interaction of M6G with all three opioid receptors. Inhibition binding experiments at the mu-OR employing combinations of morphine and M6G resulted in a rightward shift of the IC50 for morphine proportional to the M6G concentration, thus strengthening the finding of competitive interaction of M6G at the mu-opioid binding site. Data in absence and presence of M6G were included in a three-dimensional model. Compared to a model with one binding site a model with two binding sites significantly improved the fits. This might indicate that different mu-OR subtypes are involved. Hydrolysis of M6G to morphine was investigated and did not occur. Therefore the effects of M6G on binding to the mu-OR were due to M6G and not due to morphine. In contrast, M3G at the three opioid receptors was found to inhibit binding being about 300 times weaker than morphine. This effect was well explained by the amount of contaminating morphine (about 0.3%) identified by HPLC. We conclude that M6G binds to mu-, delta- and kappa-OR in a competitive manner. Some of our results on the mu-OR suggest two binding sites for agonists at the mu-OR and that M6G binds to both sites. Our results suggest that the high potency of M6G as an analgesic is mediated through opioid receptors. In contrast, M3G does not interact with the mu-, delta- or kappa-OR. We therefore doubt that any effect of M3G is mediated via opioid receptors.

Characterization and solubilization of the sulphonylurea receptor in rat brain

The binding characteristics of the sulphonylurea receptor were investigated using rat brain microsomes. Scatchard plots for binding of [3H]glibenclamide, a potent sulphonylurea which inhibits the ATP-sensitive K-channel, suggested the presence of both high and low affinity binding sites with Kd of 0.58 and 17 nM, and beta max of 123 and 392 fmol/mg protein, respectively. When brain microsomes were solubilized with CHAPS, high affinity sites were retained with Kd and beta max of 1.2 nM and 42.1 fmol/mg protein, respectively, whereas the low affinity sites disappeared. The specific binding was displaced by non-labelled glibenclamide, meglitinide, and tolbutamide with IC50 at 5 nM, 25 microM and 130 microM, respectively. ATP and GTP inhibited [3H]glibenclamide binding in a Mg-dependent manner whereas the inhibition by ADP and GDP was Mg-independent [3H]Glibenclamide binding to the solubilized receptor was similarly inhibited by those nucleotides. Diazoxide inhibited [3H]glibenclamide binding in the presence of MgATP, but after CHAPS-solubilization diazoxide failed to inhibit [3H]glibenclamide binding even with MgATP. These findings suggest the brain sulphonylurea receptor has similar features to the beta-cell receptor. However, inhibition of the binding by nucleotides is not identical, possibly reflecting differences in the nucleotide-binding subunit.