Pharmacological characterization of the sulphonylurea receptor in rat isolated aorta

Endoplasmic Reticulum-Targeted Aggregation-Induced Emission Luminogen for Synergetic Tumor Ablation with Glibenclamide

Photodynamic therapy based on fluorescence illumination of subcellular organelles and in situ bursts of reactive oxygen species (ROS) has been recognized as a promising strategy for cancer theranostics. However, the short life of ROS and unclarified anticancer mechanism seriously restrict the application. Herein, we rationally designed and facilely synthesized a 2,6-dimethylpyridine-based triphenylamine (TPA) derivative TPA-DMPy with aggregation-induced emission (AIE) features and production of type-I ROS. Except for its selective binding to the endoplasmic reticulum (ER), TPA-DMPy, in synergy with glibenclamide, a medicinal agent used against diabetes, induced significant apoptosis of cancer cells in vitro and in vivo. Additionally, TPA-DMPy greatly incited the release of calcium from ER upon light irradiation to further aggravate the depolarization of ER membrane potential caused by glibenclamide, thus inducing fatal ER stress and crosstalk between ER and mitochondria. Our study extends the biological design and application of AIE luminogens and provides new insights into discovering novel anticancer targets and agents.

Antidiabetic activity and toxicity of Zizyphus spina-christi leaves

The effect of the butanol extract of Zizyphus spina-christi (L.), Willd (Rhamnaceae) leaves and its major saponin glycoside, christinin-A, on the serum glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment either with 100 mg/kg butanol extract or christinin-A potentiated glucose-induced insulin release in non-diabetic control rats. In type-II but not in type-I diabetic rats pretreatment with the butanol extract or christinin-A improved the oral glucose tolerance and potentiated glucose-induced insulin release. Treatment either with 100 mg/kg butanol extract or christinin-A reduced the serum glucose level and increased the serum insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of the butanol extract and christinin-A were similar. Pretreatment of non-diabetic control and type-II diabetic rats either with 100 mg/kg butanol extract or christinin-A enhanced the glucose lowering and insulinotropic effects of 5 g/kg glibenclamide. The hyperglycemic and hypoinsulinemic effects of 30 mg/kg diazoxide in non-diabetic control and type-II diabetic rats were inhibited and antagonized, respectively by pretreatment with the butanol extract or christinin-A. The relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips were inhibited by 100 micromol/l christinin-A or 10 micromol/l glibenclamide. The combination of glibenclamide and christinin-A led to complete inhibition of the relaxant effects of different concentrations of diazoxide. At a dose level much higher than that required to produce satisfactory insulinotropic and hypoglycemic effects, the butanol extract of Zizyphus spina-christi leaves produced a depressant effect on the central nervous system in rats. Treatment of rats with 100mg/kg butanol extract for 3 months produced no functional or structural disturbances in liver and kidney and no haematological changes. In addition, the oral LD50 of the butanol extract in mice was 3820 mg/kg, while that of glibenclamide was 3160 mg/kg. Thus, Zizyphusspina-christi leaves appears to be a safe alternative to lower blood glucose. The safe insulinotropic and subsequent hypoglycemic effects of Zizyphus spina-christi leaves may be due to a sulfonylurea-like activity.

The pore-forming subunit of the K(ATP) channel is an important molecular target for LPS-induced vascular hyporeactivity in vitro

ATP-sensitive K(+) (K(ATP)) channel activation is implicated in the vascular hyporeactivity occurring in septic shock. However, channel inhibition with the sulphonylurea receptor (SUR) antagonist, glibenclamide (Glib) fails to reverse lipopolysaccharide (LPS)-induced vascular hyporeactivity in vitro. We investigated whether inhibitors that act by binding to the K(ATP) channel pore could be effective. Ring segments of endothelium-intact rat mesenteric artery were incubated with LPS in culture media for either 6 or 20 h before contractile responses to phenylephrine were assessed in the absence or presence of K(ATP) channel inhibitors. The pore-forming subunit inhibitors barium chloride (BaCl(2); 300 microM) and PNU-37883A (1 microM) significantly reversed hyporeactivity at both time points, although less so at 20 h. In contrast, the SUR inhibitors, Glib (10 microM), tolbutamide (Tolb) (1 mM) and PNU-99963 (1 microM) were ineffective. In LPS-incubated tissues, Glib and Tolb antagonised contractions to the thromboxane A2 mimetic, U46619 (9,11-dideoxy-9alpha, 11alpha-methanoepoxy prostaglandin F(2alpha)) (10(-7) M), whereas the pinacidil-derived inhibitor, PNU-99963, did not. Contractions to 60 mM KCl were unaffected by LPS at 6 h, but were significantly depressed by LPS at 20 h, suggesting that K(+)-channel-independent pathways contribute to hyporeactivity at the later time point. The inducible nitric oxide synthase (iNOS) inhibitor, 1400 W (10 microM) and Tolb inhibited the production of nitrite induced by LPS, whereas BaCl(2) and PNU-37883A had no effect. In conclusion, K(ATP) channels contribute to LPS-induced vascular hyporeactivity via the iNOS pathway in rat mesenteric artery. The effectiveness of pore inhibitors over SUR inhibitors of the K(ATP) channel suggests altered SUR function following LPS administration, which cannot be explained by thromboxane receptor inhibition.

Alteration of binding sites for [3H]P1075 and [3H]glibenclamide in renovascular hypertensive rat aorta

AIM

The alterations of the binding sites for ATP-sensitive K+ channel (K(ATP)) openers and blockers in aortic strips were investigated in hypertensive rats.

METHODS

Radioligand binding techniques were used to compare the specific binding properties of [3H]P1075 and [3H]glibenclamide (Gli) in normotensive (NWR) and reno-vascular hypertensive rat (RVHR) aortic strips.

RESULTS

The KD values of [3H]P1075 binding were increased by 1.5-fold, while the Bmax values were unchanged in RVHR. The IC50 values of P1075 and pinacidil (Pin) for displacing the [3H]P1075 binding in RVHR were increased by 1.8- and 1.7-fold, respectively. The kinetic processes of association and dissociation of [3H]P1075 binding were slower in RVHR. Glibenclamide pretreatment slowed down the kinetic processes of the association and dissociation of [3H]P1075 binding in NWR, but failed to alter the kinetic processes of [3H]P1075 binding in RVHR. The IC50 values of Gli for displacing the [3H]Gli binding at high-affinity sites were increased by 3-fold, while those at low-affinity sites remained to be unchanged in RVHR. The kinetic processes of association of [3H]Gli binding were decreased and those of the dissociation were accelerated in RVHR. The treatment with Pin slowed down the association kinetic processes but accelerated the process of the dissociation of [3H]Gli binding in NWR, but did not alter the kinetics of [3H]Gli binding in RVHR.

CONCLUSION

The affinity of binding sites for [3H]P1075 and of high-affinity binding sites for [3H]Gli are decreased, and the negative allosteric interactions between the two binding sites are impaired in RVHR aorta.

Vasorelaxing effects of flavonoids: investigation on the possible involvement of potassium channels

A flavonoid-rich diet has been associated with a lower incidence of cardiovascular diseases, probably because of the antioxidant and vasoactive properties of flavonoids. Indeed, many flavonoids show vasorelaxing properties, due to different and often not yet completely clarified mechanisms of action. Among them, the activation of vascular potassium channels has been indicated as a possible pathway, accounting, at least in part, for the vasodilatory action of some flavonoid derivatives, such as apigenin and dioclein. Therefore, this work aims at evaluating, on in vitro isolated rat aortic rings, the endothelium-independent vasorelaxing effects of a number of flavonoid derivatives, to identify a possible activation of calcium-activated and/or ATP-sensitive potassium channels and to indicate some possible structure-activity relationships. Among the several flavonoids submitted to the pharmacological assay, only baicalein and quercetagetin were almost completely ineffective, while quercetin, hesperidin, quercitrin and rhoifolin exhibited only a partial vasorelaxing effect. On the contrary, acacetin, apigenin, chrysin, hesperetin, luteolin, pinocembrin, 4'-hydroxyflavanone, 5-hydroxyflavone, 5-methoxyflavone, 6-hydroxyflavanone and 7-hydroxyflavone, belonging to the chemical classes of flavones and flavanones, showed full vasorelaxing effects. The vasodilatory activity of hesperetin, luteolin, 5-hydroxyflavone and 7-hydroxyflavone were antagonised by tetraethylammonium chloride, indicating the possible involvement of calcium-activated potassium channels. Moreover, iberiotoxin clearly antagonised the effects of 5-hydroxyflavone, indicating the probable importance of a structural requirement (the hydroxy group in position 5) for a possible interaction with large-conductance, calcium-activated potassium channels. Finally, glibenclamide inhibited the vasorelaxing action of luteolin and 5-hydroxyflavone, suggesting that ATP-sensitive potassium channels may also be involved in their mechanism of action.

Analysis of adenosine vascular effect in isolated rat aorta: possible role of Na+/K+-ATPase

The present experiments were undertaken in order to examine the effect of adenosine in isolated rat aorta, to investigate the possible role of intact endothelium and endothelial relaxing factors in this action and to determine which population of adenosine receptors is involved in rat aorta response to adenosine. Adenosine (0.1-300 microM) produced concentration-dependent (intact rings: pD2=4.39+/-0.09) and endothelium-independent (denuded rings: pD2=4.52+/-0.12) relaxation of isolated rat aorta. In the presence of high concentration of K+ (100 mM) adenosine-evoked relaxation was significantly reduced (maximal relaxation in denuded rings: control - 92.1+/-9.8 versus K+- 54.4+/-5.0). Similar results were obtained after incubation of ouabain (100 microM) or glibenclamide (1 microM). In K+-free solution, K+ (1-10 mM)-induced rat aorta relaxant response was significantly inhibited by ouabain (100 microM). Application of indomethacin (10 microM), NG-nitro-L-arginine (10 microM) or tetraethylammonium (500 microM) did not alter the adenosine-elicited effect in rat aorta. 8-(3-Chlorostyril)-caffeine (0.3-3 microM), a selective A2A-receptor antagonist, significantly reduced adenosine-induced relaxation of rat aorta in a concentration-dependent manner (pKB=6.57). Conversely, 1,3-dipropyl-8-cyclopentylxanthine (10 nM), an A1-receptor antagonist, did not affect adenosine-evoked dilatation. These results indicate that in isolated rat aorta, adenosine produces endothelium-independent relaxation, which is most probably dependent upon activation of smooth muscle Na+/K+-ATPase, and opening of ATP-sensitive K+ channels, to a smaller extent. According to receptor analysis, vasorelaxant action of adenosine in rat aorta is partly induced by activation of smooth muscle adenosine A2A receptors.

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.

The potential antidiabetic activity of some alpha-2 adrenoceptor antagonists

The effect of alpha-2 adrenoceptor antagonists, yohimbine and efaroxan, on the plasma glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment with either yohimbine or efaroxan potentiated glucose-induced insulin release in non-diabetic control rats and produced an improvement of the oral glucose tolerance and potentiated glucose-induced insulin release in type-II but not in type-I diabetic rats. Treatment with either yohimbine or efaroxan reduced the plasma glucose level and increased the plasma insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of efaroxan were more marked. Pretreatment of non-diabetic control and type-II diabetic rats with either yohimbine or efaroxan inhibited clonidine-induced hyperglycaemia and suppressed or reversed clonidine-induced hypoinsulinaemia. Also, pretreatment of these animals with either yohimbine or efaroxan enhanced the hypoglycaemic and insulinotropic effects of glibenclamide. The combination of glibenclamide and efaroxan led to a synergistic increase in insulin secretion, while that of glibenclamide and yohimbine led to an additive increase. The hyperglycaemic effect of diazoxide in non-diabetic control and type-II diabetic rats was inhibited by pretreatment with either yohimbine or efaroxan. The hypoinsulinaemic effect of diazoxide in these animals was antagonized and reversed by pretreatment with yohimbine and efaroxan, respectively. In type-I diabetic rats, there was no change in the plasma glucose and insulin levels induced by the treatment of animals with each of clonidine or diazoxide alone or in combination with either yohimbine or efaroxan. Glibenclamide produced a slight decrease in the plasma glucose level of type-I diabetic rats, at the end of the 120 min period of investigation but there was no change in the plasma insulin level. Pretreatment of these animals with either yohimbine or efaroxan produced no change in glibenclamide effects. Additionally, bath application of efaroxan or glibenclamide inhibited the relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips, while the application of yohimbine produced insignificant changes. The combination of glibenclamide and efaroxan led to complete inhibition of the relaxant effects of different concentrations of diazoxide, while that of glibenclamide and yohimbine did not produce such an effect. It is concluded that yohimbine, via blockade of postsynaptic alpha-2 adrenoceptors, and efaroxan, via blockade of postsynaptic alpha-2 adrenoceptors and adenosine triphosphate-sensitive potassium channels in the pancreatic beta-cell membrane, produce insulinotropic and subsequent hypoglycaemic effects.

Modulation by nucleotides of binding sites for [3H]glibenclamide in rat aorta and cardiac ventricular membranes

Radioligand binding techniques were employed to determine the modulation by nucleotides of the specific [3H]glibenclamide (Gli) binding to rat aortic and cardiac ventricular preparations. Saturation analysis revealed a single binding site with K(D) value of 31.3 nM and Bmax of 180 fmol/mg wet weight in aortic preparations. We also observed that [3H]Gli bound reversibly and specifically to cardiac membranes. Unlabeled glibenclamide displaced [3H]Gli-specific binding of cardiac membranes completely with K(I) of 54.4 nM. In cardiac membranes, adenosine triphosphate (ATP), adenosine diphosphate (ADP), and uridine diphosphate (UDP) (from 0.01-5 mM) concentration dependently inhibited [3H]Gli binding independent of Mg2+. The values of K(I) were 0.47, 0.22, and 0.58 mM, respectively. However, in aortic preparations, [3H]Gli-specific binding was increased by ATP of 5 and 10 mM and showed a biphasic response to ADP. At concentrations to 1 mM, ADP inhibited binding; above 5 mM, the specific [3H]Gli binding was increased. UDP did not alter the binding up to 5 mM. In the presence of Mg2+ (20 mM), the inhibitory effects of ATP (0.01-1 mM) or ADP (0.01-5 mM) on the binding in cardiac membranes were abolished, whereas the facilitatory effects of ATP or ADP in aortic preparations were strengthened. Analysis of kinetics showed that the time of [3H]Gli association and dissociation in cardiac and aortic preparations was monophasic. The association was delayed with dissociation unchanged by ATP, ADP, and UDP of 1 mM, respectively, in cardiac membranes. In aorta, however, at the same concentration ATP accelerated association and retarded dissociation and vice versa for ADP. Association and dissociation were not changed by UDP of 5 mM. We conclude that ATP, ADP, and UDP are all major allosteric modulators of K(ATP) channels and they affect the antagonist binding to heart (sulfonylurea receptor 2A) and aorta (sulfonylurea receptor 2B) differently.

Involvement of K(ATP) channels in diethylstilbestrol-induced relaxation in rat aorta

The estrogens prevent cardiovascular diseases that among other effects could be related to the modulation of the vascular tone via modifying ionic channel permeability. ATP-sensitive K(+) (K(ATP)) channels seem to be involved in diethylstilbestrol-induced relaxation in isolated rat aorta precontracted by noradrenaline (30 nM), since the effect is inhibited by glibenclamide (1--10 microM), and 1 mM tetraethylammonium, but not by 30 mM tetraethylammonium or paxilline. The antiestrogen tamoxifen, the inhibitor of protein kinase A, Rp-cAMPS, and the inhibitor of ornithine decarboxylase, difluoromethylornithine, antagonized diethylstilbestrol-induced relaxation. The association of glibenclamide with these compounds separately did not modify the effect of glibenclamide alone on diethylstilbestrol-induced relaxation. Functional K(ATP) channels are present in rat aorta, since diazoxide induced relaxation sensitive to glibenclamide. Papaverine, dibutyryl cyclic AMP and spermine relaxed isolated rat aorta although this was not sensitive to glibenclamide. The relaxation to forskolin was antagonized by glibenclamide. We conclude that diethylstilbestrol-induced relaxation in rat aorta is related to the modulation of K(ATP) channels. Cyclic AMP-dependent mechanisms and polyamine synthesis may mediate this modulation.

Identification of the Potassium Channel Opener Site on Sulfonylurea Receptors

Diversity of sulfonylurea receptor (SUR) subunits underlies tissue specific pharmacology of K(ATP) channels, which represent critical regulators of electrical activity in numerous cells. Notably, the neuronal/pancreatic beta-cell receptor, SUR1, imparts high sensitivity to hypoglycemic sulfonylureas (SUs; e.g. glibenclamide) and low to potassium channel openers (KCOs; e.g. P1075), whereas the opposite drug sensitivities are conferred by cardiovascular receptors, SUR2A and SUR2B. By exchanging domains between SUR1 and SUR2B, we identify two regions (KCO I: Thr(1059)-Leu(1087) and KCO II: Arg(1218)-Asn(1320); rat SUR2 numbering) within the second set of transmembrane domains (TMDII) as critical for KCO binding. Swapping both regions reconstitutes KCO affinities and sensitivities of the donor SUR isoform. High glibenclamide affinity of SUR1 is not reduced by transfer of KCO I plus II from SUR2B, demonstrating that high SU and KCO affinity can coexist in the same SUR molecule. Consistently, high SU affinity was imparted on SUR2B by substituting the region separating KCO I and II (Ile(1088)-Val(1217)) with the corresponding domain of SUR1. We infer the receptor sites for KCOs and SUs to be closely associated within a regulatory domain (Thr(1059)-Asn(1320)) in TMDII of SURs.

(+)-[3H]isradipine and [3H]glyburide bindings to heart and lung membranes from rats with monocrotaline-induced pulmonary hypertension

We examined the binding of a 1,4-dihydropyridine-sensitive Ca2+ channel ligand, (+)-[3H]isradipine (PN200-110), and that of an ATP-sensitive K+ (K(ATP)) channel ligand, [3H]glyburide, to heart, lung and brain membranes isolated from Sprague-Dawley rats made pulmonary hypertensive by monocrotaline, a pyrrolizidine alkaloid. A single subcutaneous injection of monocrotaline increased right ventricular systolic pressure, a measure of pulmonary arterial pressure, and the thickness of the right ventricular free wall in 3 to 4 weeks. The (+)-[3H]PN200-110 and [3H]glyburide binding site densities (Bmax) were reduced in hypertrophied right ventricles when normalized per unit protein in comparison with those of age-matched control (sham) rats, whereas the values of the dissociation constant (Kd) of both ligands bound to the hypertrophied right ventricle were not significantly changed. The [3H]PN200-110 binding to the lung membranes of the monocrotaline-induced pulmonary hypertensive rats was increased. The results indicate that the change in the binding of 1,4-dihydropyridine Ca2+ and K(ATP) channel ligands to heart membranes may contribute to the pathological alteration of cardiopulmonary structure and functions in rats with pulmonary hypertension induced by monocrotaline.

Pharmacological evidence for a KATP channel in renin-secreting cells from rat kidney

1. Openers of the ATP-sensitive potassium channel (KATP channel) increase and blockers decrease renin secretion. Here we report the effects of levcromakalim (LCRK, a channel opener) and glibenclamide (GBC, a blocker) on membrane potential, whole-cell current and the cytoplasmic Ca2+ concentration of renin-secreting cells (RSC). Studies were performed on afferent arterioles from the kidney of Na+-depleted rats. 2. As monitored with the fluorescent oxonol dye DiBAC4(3), LCRK (0.3 and 1 microM) induced a hyperpolarization of approximately 15 mV which was abolished by GBC (1 microM). 3. Whole-cell current-clamp experiments showed that RSC had a membrane potential of -61 +/- 1 mV (n = 16). LCRK (1 microM) induced a hyperpolarization of 9.9 +/- 0.2 mV (n = 16) which, in the majority of cells, decreased slowly with time. 4. Capacitance measurements showed a strong electrical coupling of the cells in the preparation. 5. At -60 mV, LCRK induced a hyperpolarizing current in a concentration-dependent manner with an EC50 of 152 +/- 31 nM and a maximum current of about 200 pA. 6. Application of GBC (1 microM) produced no effect; however, when applied after LCRK (300 nM), GBC inhibited the opener-induced hyperpolarizing current with an IC50 of 103 +/- 36 nM. 7. LCRK (0.3 and 1 microM) did not significantly affect the cytoplasmic Ca2+ concentration either at rest or after stimulation by angiotensin II. 8. The data show that LCRK induces a GBC-sensitive hyperpolarizing current in rat RSC. This current presumably originates from the activation of KATP channels which pharmacologically resemble those in vascular smooth muscle cells. The stimulatory effect of KATP channel opening on renin secretion is not mediated by a decrease in intracellular Ca2+ concentration.

Impairment of skeletal muscle adenosine triphosphate-sensitive K+ channels in patients with hypokalemic periodic paralysis

The adenosine triphosphate (ATP)-sensitive K+ (KATP) channel is the most abundant K+ channel active in the skeletal muscle fibers of humans and animals. In the present work, we demonstrate the involvement of the muscular KATP channel in a skeletal muscle disorder known as hypokalemic periodic paralysis (HOPP), which is caused by mutations of the dihydropyridine receptor of the Ca2+ channel. Muscle biopsies excised from three patients with HOPP carrying the R528H mutation of the dihydropyridine receptor showed a reduced sarcolemma KATP current that was not stimulated by magnesium adenosine diphosphate (MgADP; 50-100 microM) and was partially restored by cromakalim. In contrast, large KATP currents stimulated by MgADP were recorded in the healthy subjects. At channel level, an abnormal KATP channel showing several subconductance states was detected in the patients with HOPP. None of these were surveyed in the healthy subjects. Transitions of the KATP channel between subconductance states were also observed after in vitro incubation of the rat muscle with low-K+ solution. The lack of the sarcolemma KATP current observed in these patients explains the symptoms of the disease, i.e., hypokalemia, depolarization of the fibers, and possibly the paralysis following insulin administration.

Mg2+ and ATP dependence of K(ATP) channel modulator binding to the recombinant sulphonylurea receptor, SUR2B

1. The binding of modulators of the ATP-sensitive K+ channel (KATP channel) to the murine sulphonylurea receptor, SUR2B, was investigated. SUR2B, a proposed subunit of the vascular KATP channel, was expressed in HEK 293 cells and binding assays were performed in membranes at 37 degrees C using the tritiated KATP channel opener, [3H]-P1075. 2. Binding of [3H]-P1075 required the presence of Mg2+ and ATP. MgATP activated binding with EC50 values of 10 and 3 microM at free Mg2+ concentrations of 3 microM and 1 mM, respectively. At 1 mM Mg2+, binding was lower than at 3 microM Mg2+. 3. [3H]-P1075 saturation binding experiments, performed at 3 mM ATP and free Mg2+ concentrations of 3 microM and 1 mM, gave KD values of 1.8 and 3.4 nM and BMAX values of 876 and 698 fmol mg(-1), respectively. 4. In competition experiments, openers inhibited [3H]-P1075 binding with potencies similar to those determined in rings of rat aorta. 5. Glibenclamide inhibited [3H]-P1075 binding with Ki values of 0.35 and 2.4 microM at 3 Mm and 1 mM free Mg2+, respectively. Glibenclamide enhanced the dissociation of the [3H]-P1075-SUR2B complex suggesting a negative allosteric coupling between the binding sites for P1075 and the sulphonylureas. 6. It is concluded that an MgATP site on SUR2B with microM affinity must be occupied to allow opener binding whereas Mg2+ concentrations > or = 10 microM decrease the affinities for openers and glibenclamide. The properties of the [3H]-P1075 site strongly suggest that SUR2B represents the drug receptor of the openers in vascular smooth muscle.

Binding of K(ATP) channel modulators in rat cardiac membranes

1. The binding of [3H]-P1075, a potent opener of adenosine-5'-triphosphate-(ATP)-sensitive K+ channels, was studied in a crude heart membrane preparation of the rat, at 37 degrees C. 2. Binding required MgATP. In the presence of an ATP-regenerating system, MgATP supported [3H]-P1075 binding with an EC50 value of 100 microM and a Hill coefficient of 1.4. 3. In saturation experiments [3H]-P1075 binding was homogeneous with a KD value of 6+/-1 nM and a binding capacity (Bmax) of 33+/-3 fmol mg(-1) protein. 4. Upon addition of an excess of unlabelled P1075, the [3H]-P1075-receptor complex dissociated in a mono-exponential manner with a dissociation rate constant of 0.13+/-0.01 min(-1). If a bi-molecular association mechanism was assumed, the dependence of the association kinetics on label concentration gave an association rate constant of 0.030+/-0.003 nM(-1) min(-1). From the kinetic experiments the KD value was calculated as 4.7+/-0.6 nM. 5. Openers of the ATP-sensitive K+ channel belonging to different structural classes inhibited specific [3H]-P1075 binding in a monophasic manner to completion; an exception was minoxidil sulphate where maximum inhibition was 68%. The potencies of the openers in this assay agree with published values obtained in rat cardiocytes and are on average 3.5 times lower than those determined in rat aorta. 6. Sulphonylureas, such as glibenclamide and glibornuride and the sulphonylurea-related carboxylate, AZ-DF 265, inhibited [3H]-P1075 binding with biphasic inhibition curves. The high affinity component comprised about 60% of the curves with the IC50 value of glibenclamide being approximately 90 nM; affinities for the low affinity component were in the microM concentration range. The fluorescein derivative, phloxine B, showed a monophasic inhibition curve with an IC50 value of 6 microM, a maximum inhibition of 94% and a Hill coefficient of 1.5. 7. It is concluded that binding studies with [3H]-P1075 are feasible in rat heart membranes in the presence of MgATP and of an ATP-regenerating system. The pharmacological profile of the [3H]-P1075 binding sites in the cardiac preparation, which probably contains sulphonylurea receptors (SURs) from cardiac myocytes (SUR2A) and vascular smooth muscle cells (SUR2B), differs from that expected for SUR2A and SUR2B.

Characterization of K(ATP) channels in intact mammalian skeletal muscle fibres

1. The aim of this study was to characterize the K(ATP) channel of intact rat skeletal muscle (rat flexor digitorum brevis muscle). Changes in membrane currents were recorded with two-electrode voltage-clamp of whole fibres. 2. The K(ATP) channel openers, levcromakalim and pinacidil (10-400 microM), caused a concentration-dependent increase in whole-cell chord conductance (up to approximately 1.5 mScm(-2)). The activated current had a weak inwardly rectifying current-voltage relation, a reversal potential near E(K) and nanomolar sensitivity to glibenclamide--characteristic of a K(ATP) channel current. Concentration-effect analysis revealed that levcromakalim and pinacidil were not particularly potent (EC50 approximately 186 microM, approximately 30 microM, respectively), but diazoxide was completely inactive. 3. The ability of both classical K(ATP) channel inhibitors (glibenclamide, tolbutamide, glipizide and 5-hydroxydecanoic acid) and a number of structurally related glibenclamide analogues to antagonize the levcromakalim-induced current was determined. Glibenclamide was the most potent compound with an IC50 of approximately 5 nM. However, the non-sulphonylurea (but cardioactive) compound 5-hydroxydecanoic acid was inactive in this preparation. 4. Regression analysis showed that the glibenclamide analogues used have a similar rank order of potency to that observed previously in vascular smooth muscle and cerebral tissue. However, two compounds (glipizide and DK13) were found to have unexpectedly low potency in skeletal muscle. 5. These experiments revealed K(ATP) channels of skeletal muscle to be at least 10x more sensitive to glibenclamide than previously found; this may be because of the requirement for an intact intracellular environment for the full effect of sulphonylureas to be realised. Pharmacologically, K(ATP) channels of mammalian skeletal muscle appear to resemble most closely K(ATP) channels of cardiac myocytes.

The effects of levcromakalim and pinacidil on the human internal mammary artery

The present study was undertaken to examine the effects of pinacidil and levcromakalim, two potassium, channel openers, on human internal mammary artery (HIMA) obtained from patients undergoing coronary artery bypass surgery, and to clarify the contribution of different K+ channel subtypes in pinacidil and levcromakalim action in this blood vessel. Pinacidil and levcromakalim induced a concentration-dependent relaxation of the precontracted arterial segments (pEC50 = 5.77 +/- 0.05 and 6.89 +/- 0.03, respectively), 4-Aminopyridine (3 mM), a non-selective blocker of K+ channels, induced significant shifts to the right of the concentration-response curves for pinacidil and levcromakalim. Tetraethylammonium (6 mM), charybdotoxin (0.4 microM) and apamin (0.1 microM), blockers of Ca(2+)-sensitive K+ channels, had no effect on the pinacidil- and levcromakalim-evoked relaxation. Glibenclamide (0.1-10 microM), a selective blocker of adenosine triphosphate (ATP)-sensitive K+ channels, competitively antagonized the response to levcromakalim (pKB = 7.92 +/- 0.07). In contrast, glibenclamide, in significantly higher concentrations (3-30 microM), non-competitively antagonized the response to pinacidil. High concentrations of pinacidil (> 10 microM) relaxed arterial rings bathed by a medium containing 100 mM K+ with maximum response 83 +/- 6%. Under the same conditions, the maximum levcromakalim-induced relaxation on HIMA was almost abolished (15 +/- 2%). It is concluded that pinacidil and levcromakalim do not relax the HIMA through the same subtype of K+ channel. ATP-sensitive K+ channels are probably involved in levcromakalim- but not in a pinacidil-induced relaxation in the HIMA. In addition, in pinacidil-induced relaxation of the HIMA, K+ channel-independent mechanisms seem to be involved.

Binding and effects of KATP channel openers in the vascular smooth muscle cell line, A10

1. The ATP-sensitive K+ channel (KATP channel) in A10 cells, a cell line derived from rat thoracic aorta, was characterized by binding studies with the tritiated KATP channel opener, [3H]-P1075, and by electrophysiological techniques. 2. Saturation binding experiments gave a KD value of 9.2 +/- 5.2 nM and a binding capacity (BMax) of 140 +/- 40 fmol mg-1 protein for [3H]-P1075 binding to A10 cells; from the BMax value a density of binding sites of 5-10 per microns2 plasmalemma was estimated. 3. KATP channel modulators such as the openers P1075, pinacidil, levcromakalim and minoxidil sulphate and the blocker glibenclamide inhibited [3H]-P1075 binding. The extent of inhibition at saturation depended on the compound, levcromakalim inhibiting specific [3H]-P1075 binding by 85%, minoxidil sulphate and glibenclamide by 70%. The inhibition constants were similar to those determined in strips of rat aorta. 4. Resting membrane potential, recorded with microelectrodes, was -51 +/- 1 mV. P1075 and levcromakalim produced a concentration-dependent hyperpolarization by up to -25 mV with EC50 values of 170 +/- 40 nM and 870 +/- 190 nM, respectively. The hyperpolarization induced by levcromakalim (3 microM) was completely reversed by glibenclamide with an IC50 value of 86 +/- 17 nM. 5. Voltage clamp experiments were performed in the whole cell configuration under a physiological K+ gradient. Levcromakalim (10 microM) induced a current which reversed around -80 mV; the current-voltage relationship showed considerable outward rectification. Glibenclamide (3 microM) abolished the effect of levcromakalim. 6. Analysis of the noise of the levcromakalim (10 microM)-induced current at -40 and -20 mV yielded estimates of the channel density, the single channel conductance and the probability of the channel to be open of 0.14 micron-2, 8.8 pS and 0.39, respectively. 7. The experiments showed that A10 cells are endowed with functional KATP channels which resemble those in vascular tissue; hence, these cells provide an easily accessible source of channels for biochemical and pharmacological studies. The density of binding sites for [3H]-P1075 was estimated to be one order of magnitude higher than the density of functional KATP channels; assuming a plasmalemmal localization of the binding sites this suggests a large receptor reserve for the openers in A10 cells.