Coexpression with the inward rectifier K(+) channel Kir6.1 increases the affinity of the vascular sulfonylurea receptor SUR2B for glibenclamide

ATP-sensitive K(+) channels are closed by the hypoglycemic sulfonylureas like glibenclamide (GBC) and activated by a class of vasorelaxant compounds, the K(+) channel openers. These channels are octamers of Kir6.x and sulfonylurea receptor (SUR) subunits with 4:4 stoichiometry. The properties of the opener-sensitive K(+) channel in the vasculature are well matched by the SUR2B/Kir6.1 channel; however, the GBC sensitivity of the recombinant channel is unknown. In binding experiments we have determined the affinity of GBC for SUR2B and the SUR2B/Kir6.1 channel and compared the results with the channel blocking potency of GBC. All experiments were performed in whole transfected human embryonic kidney cells at 37 degrees C. The equilibrium dissociation constants (K(D)) of GBC binding to SUR2B and to the SUR2B/Kir6.1 complex were determined to be 32 and 6 nM, respectively; the K(D) value of the opener P1075 (N-cyano-N'-(1, 1-dimethylpropyl)-N"-3-pyridylguanidine) ( approximately 5 nM) was, however, not affected by cotransfection. In whole cell voltage-clamp experiments, GBC inhibited the SUR2B/Kir6.1 channel with IC(50) approximately 43 nM. The data show that, in the intact cell: 1) SUR2B, previously considered to be a low-affinity SUR, has a rather high affinity for GBC; 2) coexpression with the inward rectifier Kir6.1 increases the affinity of SUR2B for GBC; 3) the recombinant channel exhibits the same GBC affinity as the opener-sensitive K(+) channel in vascular tissue; and 4) the K(D) value of GBC binding to the octameric channel is 7 times lower than the IC(50) value for channel inhibition. The latter finding suggests that occupation of all four GBC sites per channel is required for channel closure.

Potent inhibition of the CFTR chloride channel by suramin

After phosphorylation by protein kinase A and in the presence of ATP, the cystic fibrosis transmembrane conductance regulator (CFTR) functions as a Cl- channel. In this study we have examined the effects of suramin on the CFTR Cl- current (I(CFTR)) in excised inside-out macropatches from Xenopus oocytes expressing human CFTR; glibenclamide, the standard inhibitor of I(CFTR), and some congeners were tested in comparison. I(CFTR) was activated by addition of the catalytic subunit of protein kinase A and MgATP to the bath. Suramin inhibited I(CFTR) with an IC50 value of 1 microM and a Hill coefficient close to 1; the inhibition showed little voltage dependence and was easily reversed upon washout of the drug. In comparison, glibenclamide inhibited I(CFTR) with an IC50 value of approximately 20 microM. When tested against I(CFTR) in whole oocytes, bath application of suramin was ineffective whereas glibenclamide was about four times weaker than in the inside-out patch configuration. The data show that suramin is the most potent inhibitor of CFTR yet described and suggest that the compound approaches its site of action from the cytosol.

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.

ATP-Sensitive K+ channel modulator binding to sulfonylurea receptors SUR2A and SUR2B: opposite effects of MgADP

KATP channels are heteromeric complexes of inwardly rectifying K+ channel subunits and sulfonylurea receptors (SURs). SUR2A and SUR2B, which differ within the carboxyl terminal exon 38, are characteristic for the cardiac and smooth muscle type channels, respectively. Here we compare binding of the tritiated KATP channel opener, [3H]P1075, to membranes from human embryonic kidney (HEK) cells transfected with murine SUR2A and 2B at 37 degrees C. Binding to both SURs required addition of Mg2+ and ATP in the low micromolar range. In the presence of MgATP, micromolar concentrations of MgADP, formed by the ATPase activity of the membrane preparation, increased binding to SUR2A but inhibited binding to SUR2B. Decreasing temperatures strongly reduced [3H]P1075 binding to SUR2A, whereas binding to SUR2B was increased in a bell-shaped manner. Kinetic experiments revealed a faster dissociation of the [3H]P1075-SUR2A complex, whereas the association rate constants for [3H]P1075 binding to SUR2A and 2B were similar. Openers inhibited [3H]P1075 binding to SUR2A with potencies approximately 4 times lower than to SUR2B; in contrast, glibenclamide inhibited [3H]P1075 binding to SUR2A approximately 8 times more potently than to SUR2B. The data suggest that SUR2A and 2B represent the opener receptors of cardiac and vascular smooth muscle KATP channels, respectively, and show that MgADP is an important modulator of opener binding to SUR. The different carboxyl termini of SUR2A and 2B lead to differences in the MgADP dependence and the thermodynamics of [3H]P1075 binding, as well as in the affinities for openers and glibenclamide, underlining the importance of this part of the molecule for KATP channel modulator binding.

A fast and sensitive TLD method for measurement of energy and homogeneity of electron beams using transmitted radiation through lead

A simple and fast, but sensitive TLD method for the measurement of energy and homogeneity of therapeutically used electron beams has been developed and tested. This method is based on the fact that when small thicknesses of high-Z absorbers such as lead are interposed in the high-energy electron beams, the transmitted radiation increases with the energy of the electron beams. Consequently, the ratio of readouts of TLDS held on the two sides of a lead plate varied sharply (by factor of 70) with a change in energy of the electron beam from 5 MeV to 18 MeV, offering a very sensitive method for the measurement of the energy of electron beams. By using the ratio of TL readouts of two types of TLD ribbon with widely different sensitivities, LiF TLD-700 ribbons on the upstream side and highly sensitive CaF2:Dy TLD-200 ribbons on the downstream side, an electron energy discrimination of better than +/- 0.1 MeV could be achieved. The homogeneity of the electron beam energy and the absorbed dose was measured by using a jig in which the TLDS were held in the desired array on both sides of a 4 mm thick lead plate. The method takes minimal beam time and makes it possible to carry out measurements for the audit of the quality of electron beams as well as for intercomparison of beams by mail.

Effects of ions on adenosine binding and enzyme activity of purified S-adenosylhomocysteine hydrolase from bovine kidney

The present investigation was undertaken to determine the effect of various ions on the characteristics of S-adenosylhomocysteine (SAH) hydrolase from bovine kidney. The binding sites of [3H]-adenosine to purified SAH hydrolase were not influenced by phosphate, magnesium, potassium, sodium, chloride or calcium ions at physiological cytosolic concentrations. To test whether NAD+ in the SAH hydrolase is essential for adenosine binding, we prepared the apoenzyme by removing NAD+ with ammonium sulfate. The resulting apoenzyme did not exhibit any [3H]-adenosine binding. Since the apoenzyme was enzymatically inactive, it is suggested that adenosine binds to the active site and not to an allosteric site of the intact enzyme. The kinetics of the hydrolysis and the synthesis of SAH catalyzed by the enzyme SAH hydrolase were measured in the presence and absence of phosphate and magnesium. Phosphate increased the Vmax for both synthesis and hydrolysis. However, only the affinity of adenosine for SAH synthesis was significantly enhanced from 10.1+/-1.3 microM to 5.4+/-0.5 microM by phosphate. This effect was already maximal at a phosphate concentration of 1 mM. All other tested ions were without effect on the enzyme activity. Our results show that phosphate at physiological concentrations shifts the thermodynamic equilibrium of SAH hydrolase in the direction of SAH synthesis. These findings imply that SAH-sensitive transmethylation reactions are inhibited during renal hypoxia when intracellular levels of phosphate, adenosine, and SAH are elevated.

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.

Endovascular brachytherapy--treatment planning and radiation protection

The risk of restenosis, main late effect limiting the success of percutaneous transluminal coronary artery angioplasty, can be reduced significantly by vascular radiotherapy, subsequent to PTCA. This discovery lead to the development of new irradiation techniques. Endovascular brachytherapy is the choice in treatment of coronary artery stenosis. Successful irradiation, however, requires precise treatment planning. This review addresses the physical possibilities and problems of intravascular brachytherapy planning, and the radiobiologically based definition of the target volume and of structures at risk. Recommendations for dose specification, recording and reporting are given. The criteria for selecting a vascular radiotherapy technique are discussed as well as the possibilities of dosimetric treatment planning and quality assurance based on precise plastic scintillator dosimetry and intravascular ultrasound. Radiation protection and safety must be reconsidered prior to the usage of therapeutic radiation sources in the catheter laboratory and for the decision about emergency plans. Finally, the design of clinical trials, the role of medical physicists, and the future of irradiation treatment of stenosis is discussed.

Interaction of the diuretics torasemide and U-37883A with the K(ATP) channel in rat isolated aorta

In this study we have investigated the interaction of the loop diuretics torasemide and furosemide and of the eukalemic diuretic U-37883A (4-morpholinocarboximidine-N-1-adamantyl-N'-cyclohexylhyd rochloride) with the ATP-sensitive K+ channel (K(ATP) channel) in rat aortic rings. Torasemide contains a sulphonylurea group which might enable the compound to interfere with K(ATP) channels; this group is lacking in furosemide. U-37883A blocks several types of K(ATP) channels. The interaction with the vascular K(ATP) channel was probed in binding studies, 86Rb+ efflux experiments and vasorelaxation assays. Torasemide inhibited the binding of the K(ATP) channel inhibitor [3H]glibenclamide and of the opener [3H]P1075 with IC50 values of 19 and 45 microM, respectively; furosemide and U-37883A were inactive or interfered with binding in a nonspecific way. In 86Rb+ efflux experiments, the loop diuretics, at microM concentrations, inhibited basal tracer efflux to 50% whereas U-37883A had no effect. P1075-stimulated 86Rb+ efflux, a qualitative measure of K(ATP) channel opening, was inhibited by U-37883A and torasemide with IC50 values of 0.06 and 130 microM, respectively; furosemide induced only a small (23%) inhibition. In experiments measuring isometric force, torasemide and furosemide partially relaxed endothelium-denuded aortic rings precontracted with noradrenaline or KCl with EC50 values between 6 and 10 microM. The vasorelaxant effect of P1075 was inhibited in a noncompetitive manner by torasemide (300 microM) but unaffected by furosemide. U-37883A increased noradrenaline-induced force and inhibited the vasorelaxant effect of P1075 in an apparently competitive manner with an inhibition constant of 0.4 microM. The data show that torasemide interferes specifically with the binding of the K(ATP) channel modulators [3H]glibenclamide and [3H]P1075 and with the K(ATP) channel opening and vasorelaxant effects of P1075 whereas furosemide is inactive. This suggests that the interaction of torasemide with the vascular K(ATP) channel is due to the sulphonylurea group present in torasemide. U-37883A, which does not inhibit P1075 binding, is one of the most potent blockers of P1075-induced 86Rb+ efflux yet described but is relatively weak as an inhibitor of P1075-mediated vasorelaxation. The opposite vascular actions of torasemide and U-37883A are expected to contribute to the renal effects of these drugs.

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.

[Oral vaccines against poliomyelitis and vaccination-related paralytic poliomyelitis in Germany. Do we need a new immunization strategy?]

Oral vaccination against poliomyelitis, which was carried out worldwide, lead to eradication of poliomyelitis in the United States, in South America and parts of Europe; in other parts of the world, paralytic poliomyelitis is still a severe risk of health. In those countries where poliomyelitis has been eradicated, it is presently discussed whether the vaccination schedules should be changed to an inactivated polio vaccine (IPV), as in polio-free countries only cases of paralytic poliomyelitis after vaccinations have been reported. Behringwerke's data from a 30-year period of analysing adverse drug reaction reveal the following: using the trivalent oral polio vaccine (OPV), based on WHO case definition, the risk for vaccine-associated paralytic poliomyelitis with permanent damage is approximately 1 case for 4.5 million vaccinations (0.22 per million) in vaccinees, and approximately 1 case for 11 million (0.09 per million) in contact persons. This low risk is in line with the data ascertained worldwide.

Disruption of the actin cytoskeleton abolishes high affinity 3H-glibenclamide binding in rat aortic rings

The interaction between the cytoskeleton and the ATP-sensitive K+ channel (KATP channel) was studied in rat aortic rings by examining the binding of the sulphonylurea blocker, 3H-glibenclamide, and of the opener, 3H-P1075. The actin cytoskeleton disrupting agents, cytochalasin D (1 microM) and latrunculin B (1 microM), abolished the high affinity component of 3H-glibenclamide binding. Preincubation with the actin cytoskeleton stabilizing agent, phalloidin (10 microM) prevented the effect of cytochalasin D. In contrast, binding of the opener, 3H-P1075, and inhibition of this binding by glibenclamide, were unaffected by cytochalasin D (3 microM). Colchicine (100 microM), which disassembles microtubules, had no effect on the binding of 3H-glibenclamide and 3H-P1075. The data show that high affinity binding of glibenclamide, which mediates the effects of the sulphonylurea in this preparation, requires the presence of an intact actin cytoskeleton. Binding of the opener is unaffected by the state of the cytoskeleton and preserves a conformational state in which high affinity binding of glibenclamide to the sulphonylurea receptor can occur.

[Intravascular irradiation in the combined therapy and prevention of restenosis. Overview]

Despite numerous efforts in catheter technology and procedural approaches the problem of restenosis in interventional cardiology persists. Although the implantation of coronary stents has significantly reduced restenosis rates based on the inhibition of elastic recoil, intimal proliferation as the second major mechanism for postinterventional restenosis could not effectively be suppressed. Intimal proliferation is the response to vessel injury following interventional procedure, e.g. balloon angioplasty. It results in the adhesion of mono- and lymphocytes which themselves trigger the colonisation of myofibroblasts. Intracoronary irradiation seeks to prevent this proliferative process as it destroys or irreversibly alters DNA structures of cells at the site of balloon injury. The antiproliferative effect depends on the irradiation dosis, the timing and the cell cycle phase. Mainly beta- and gamma-radiation is used for intracoronary irradiation. Beta-emitters are characterized by a sharp decline of dose rate within millimeters from the actual source. The exposure to surrounding tissue as well the catheter staff can be kept to a minimum. The high intensity of beta-emitters allow a short treatment period of minutes to gain an effective radiation dose to the target. In contrast, gamma-emitters have a low radial dose distribution resulting in high dosage even centimeters away from the source. These emitters require additional shielding in the catheter laboratory and lead to excessive whole body doses. To achieve a sufficient dose in the target tissue, irradiation times of more than 20 minutes are necessary which prolongs the interventional procedure substantially. At present, catheter based systems or radioactive implantable stents are available to deliver the required dose. Catheter based systems seem more flexible in a number of considerations. On the other hand they require a substantial amount of hardware. Beta-emitting stents are implanted via a conventional stent delivery system with small shielding modifications. However, stents emit an inhomogeneous radiation profile due to the mesh-like structure. In addition, not every lesion can be reached by a stent nor does every lesion require a stent solely to deliver radiation. External irradiation is presently not recommended due to its ineffectiveness and the high rate of side effects. In the experimental setting the porcine model comes closest to the clinical situation in man. Animal experiments have demonstrated the effective reduction of intimal proliferation using beta- and gamma-sources in a wide dose range of 3 to 56 Gy. Although the initial and early results are convincing little is know about the long term results. Only few studies have been and are currently performed in patients. Some of these investigations demonstrate a significant reduction of restenosis rate after 6 months. Again, information on long-term results are lacking. It has to be considered that perivascular fibrosis, which may occur with a delay of 5 to 10 years depending on the dosage, could curtail the initial success. Intracoronary irradiation is a promising method for the prevention of restenosis. The dose finding with respect to the dose effect relation, the determination of the therapeutic window and the timing of irradiation have to be further defined in the clinical setting. Nevertheless, intracoronary irradiation remains high on the priority list in fighting restenosis.

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.

Interaction between thiol-modifying agents and P1075, a K(ATP) channel opener, in rat isolated aorta

In vascular smooth muscle, openers of ATP-dependent potassium channels (K(ATP) channels), such as P1075 (N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine), produce relaxation. In this study we have investigated the effects of thiol-modifying agents on the binding of P1075 and on the 86Rb+ efflux stimulating and vasorelaxant effects of the opener in rat aortic rings. The increase in 86Rb+ efflux induced by P1075 was taken as a qualitative measure of K+ channel opening. The hydrophilic SH-group-oxidizing substance, thimerosal (1 to 100 microM), abolished specific binding of [3H]-P1075 with an IC50 value of 7.6+/-1.2 microM; at 30 microM, the half time for inhibition was 38 min. Two other thioloxidizing agents, PMB (4-hydroxy-mercuribenzoic acid) and DTBNP (2,2'-dithio-bis(5-nitropyridine)), inhibited binding up to 86% and 44%, respectively. The disulphide bond reducing substance, DTT (1,4-dithiothreitol, 0.1 to 1 mM), reduced [3H]-P1075 binding by up to 20% and partially reversed the inhibitory effect of thimerosal. In 86Rb+ efflux experiments, thimerosal (3 to 100 microM) concentration-dependently increased basal efflux but inhibited P1075-stimulated tracer efflux with an IC50 value of 7+/-1 microM. The inhibitory effect occurred with a half-time of approximately 8 min and was essentially reversed by DTT. In rings precontracted with noradrenaline, thimerosal inhibited the vasorelaxant effect in a noncompetitive manner, shifting the concentration-relaxation curves to the right and reducing maximum relaxation. The data show that oxidation of thiol groups interferes with the binding of the K(ATP) channel opener, P1075; concomitantly, the 86Rb+ efflux stimulating and the vasorelaxant effects are inhibited. Reduction of disulphide bonds by DTT has only minor effects on the action of P1075. Collectively, the results suggest that intact thiol groups are essential for the functioning of the K(ATP) channel in rat aorta. The different kinetics governing the inhibition of opener binding and of opener-stimulated 86Rb+ efflux suggest that the SH-groups involved in the two processes differ in their accessibility to thimerosal and/or in their reactivity.

Inhibition by protein kinase C of the 86Rb+ efflux and vasorelaxation induced by P1075, a K(ATP) channel opener, in rat isolated aorta

In rat aortic rings, P1075, an opener of ATP-dependent potassium channels (K(ATP) channels), produces relaxation and 86Rb+ efflux from preloaded tissues; the increase in 86Rb+ efflux qualitatively reflects K(ATP) channel opening. In this study we have investigated the effects of protein kinase C modulation on the 86Rb+ efflux stimulating, the vasorelaxant and the binding properties of P1075. Phorbol 12,13-dibutyrate (PDBu), a direct activator of protein kinase C, inhibited the 86Rb+ efflux produced by P1075 with an IC50 value of 20+/-2 nM. Phorbol 12-myristate 13-acetate (PMA), another stimulator of protein kinase C, was 150 times weaker in this respect whereas 4alpha-PDBu, the inactive stereoisomer of PDBu, was ineffective. Staurosporine (300 nM), an inhibitor of protein kinase C, induced a small but significant increase of P1075-induced tracer efflux and partially reversed the inhibitory effect of PDBu on P1075-stimulated tracer efflux. The vasorelaxant effect of P1075 was inhibited only to a moderate degree by PDBu at concentrations which inhibited P1075-induced 86Rb+ efflux to >90%; however, in the presence of PDBu, the relaxation kinetics of P1075 were increasingly slowed. The vasorelaxant effect of P1075 in the presence of PDBu was still sensitive to inhibition by glibenclamide (100 nM), the standard inhibitor of the K(ATP) channel openers. Specific binding of [3H]-P1075 to rat aortic rings was unaffected by PDBu and PMA even in the micromolar concentration range. The data show that stimulation of protein kinase C inhibits the K+ channel opening effect of P1075 in rat aorta and suggest that protein kinase C may exert a weak tonic inhibition on the K(ATP) channels in this vessel under quasiphysiological conditions. At concentrations of PDBu which essentially abolished P1075-induced tracer efflux, the glibenclamide-sensitive vasorelaxant effect of P1075 was slowed down but not prevented; this supports earlier suggestions that K+ channel openers are also able to relax smooth muscle cells by a mechanism independent of K(ATP) channel opening.

Binding and effects of P1075, an opener of ATP-sensitive K+ channels, in the aorta from streptozotocin-treated diabetic rats

Diabetes mellitus is associated with major vascular complications. It was the aim of this study to examine the function of the ATP-sensitive K+ channel (K(ATP) channel) in aortic rings prepared from diabetic rats and from age-matched controls. Diabetes was induced by injection of streptozotocin (60 mg/kg i.p.) and the animals were sacrificed 10 weeks after treatment. The binding of the K(ATP) channel opener, P1075 (N-cyano-N'-(1,1dimethylpropyl)-N"-3-pyridylguanidine), as well as the vasorelaxant and 86Rb+ efflux stimulating effects of the drug were measured. In endothelium-denuded rings from diabetic rats, the maximum contraction and sensitivity to noradrenaline were increased; in rings with intact endothelium, the acetylcholine-induced (endothelium-dependent) relaxation was similar in the two groups. In rings from diabetic rats the relaxation-concentration curve of the K(ATP) channel opener P1075 against noradrenaline was shifted rightwards by a factor of 1.3 and the maximum relaxation was reduced from 81 to 71% of initial tension (P <0.01). However, specific binding of 3H-P1075 was increased by 20% without a change in affinity, indicating that the number of binding sites for the opener was increased as a consequence of diabetes. In addition, P1075-induced 86Rb+ efflux, a qualitative measure of K(ATP) channel opening, was augmented by 50%. The data show that in the aorta from diabetic rats the K+ channel opening response to P1075 is markedly increased; however, the vasorelaxant effect to the K(ATP) channel opener is slightly impaired. A possible explanation of these findings is that the vasorelaxant mechanisms (which are in part independent of plasmalemmal K(ATP) channel opening) may be altered; alternatively, the link between membrane potential and smooth muscle tone may be changed in this model of insulin-dependent diabetes mellitus.

Activators of protein kinase A induce a glibenclamide-sensitive 86Rb+ efflux in rat isolated aorta

The effect of activators of protein kinase A on membrane K+ permeability and the interaction of these compounds with cromakalim, an opener of ATP-sensitive K+ channels (K(ATP) channels), were investigated. Membrane K+ permeability was assessed by measuring 86Rb+ efflux from rings of rat aorta. Forskolin, an activator of adenylate cyclase, and isobutylmethylxanthine (IBMX), a nonselective phosphodiesterase inhibitor, induced small, concentration-dependent increases in tracer efflux up to 20-40% over the basal level. The effect of forskolin was abolished by the K+ channel blocker tedisamil (1 microM) and partially inhibited by glibenclamide (1 microM), a relatively selective blocker of K(ATP) channels. Further studies were conducted in the presence of 35 mM KCI in the bath in order to increase the size of the 86Rb+ efflux stimulated by forskolin and IBMX. At high concentrations, these compounds produced a biphasic effect with a peak increase being followed by a lower plateau value. Glibenclamide inhibited the 86Rb+ efflux response to forskolin and IBMX by 50-80%. The K+ channel blockers tedisamil (1 microM), Ba2+ (1 mM) and tetraethylammonium (10 mM) also reduced the peak response to forskolin by about 50% and abolished or greatly inhibited the plateau response. In addition to the small effect on basal 86Rb+ efflux, forskolin (0.3 microM) increased cromakalim-induced 86Rb+ efflux 3.4 times. At higher concentrations, however, a concentration-dependent inhibition was observed with an IC50 value of 7.6 +/- 0.4 microM. 1,9-dideoxyforskolin, which does not increase cAMP, increased neither basal nor cromakalim-induced 86Rb+ efflux; however, it inhibited cromakalim-stimulated tracer efflux with an IC50 value of 22 +/- 2 microM. It is concluded that forskolin and IBMX, probably by increasing intracellular cAMP levels, induce a 86Rb+ efflux from rat aorta, the major part of which is glibenclamide-sensitive and may pass through K(ATP) channels. In addition, low concentrations of forskolin greatly facilitate the K(ATP) channel opening effect of cromakalim whereas high concentrations block the channel; this blocking effect of forskolin is unrelated to the cAMP elevating action.

Sulphonylurea binding in rat isolated glomeruli: pharmacological characterization and dependence on cell metabolism and cytoskeleton

The kidney is endowed with ATP-sensitive K+ channels (KATP channels) both at the vascular and at the epithelial level. In this study we have characterized the binding of the sulphonylurea glibenclamide, the most widely used blocker of KATP channels, in rat isolated glomeruli. In metabolically intact glomeruli, 3H-glibenclamide labelled two different binding components with affinities of 47 +/- 12 nM and 10 +/- 1 microM and estimated binding capacities of 1.2 +/- 0.1 and 501 +/- 11 pmol/mg protein, respectively. 3H-glibenclamide binding was inhibited differentially by other sulphonylureas (tolbutamide, glibornuride, gliquidone and glipizide) and benzoic acid analogues such as meglitinide, AZ-DF 265 and UL-DF 9. Sulphonylureas interacted with the high affinity component and, in some cases, also with the low affinity component whereas the benzoic acid derivatives inhibited exclusively low affinity glibenclamide binding. Severe metabolic stress affected both components of glibenclamide binding by shifting high affinity binding to the right and reducing the capacity of the low affinity component. Disruption of the cytoskeletal actin filaments by cytochalasin B and D mimicked the effect of metabolic stress on the high affinity component but left the low affinity component unchanged. In crude membranes, the affinity of the first component was again reduced and a major loss of the low affinity sites was observed. The data show that the two binding components of glibenclamide binding in rat isolated glomeruli have very different properties. The high affinity component is not recognized by the benzoic acid derivatives; its affinity is modulated by cell metabolism and the actin component of the cytoskeleton. The low affinity sites are, in their majority, cytosolic. The function and cellular localization of the high affinity sites are under further study.

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.

Binding of [3H]-P1075, an opener of ATP-sensitive K+ channels, to rat glomerular preparations

ATP-sensitive K+ channels (KATP channels) in the kidney have been found in the tubular system and in the afferent arteriole. In this study we have examined the binding of [3H]-P1075 ([3H]-N-cyano-N'-(1, 1-dimethylpropyl)-N"-3-pyridylguanidine), a selective opener of KATP channels, in rat glomerular preparations. Equilibrium (saturation, competition) and kinetic experiments indicated that [3H]-P1075 binds to a single class of sites with a dissociation constant of about 3 nM and a maximum binding capacity of 10 fmol mg-1 glomerular protein. The association rate constant of the complex was 6,5 x 10(7) M-1 min-1; dissociation occurred with a half-time of 6.2 min. Specific [3H]-P1075 binding was strongly reduced when the metabolic state of the glomerular preparation was impaired during the preparation procedure or the binding assay or when the preparation was subjected to mild collagenase treatment. In different metabolically competent preparations, the amount of specific [3H]-P1075 binding correlated well with the number of vascular endings adherent to the glomeruli; no specific binding was found in mesangial cells in culture. Specific [3H]-P1075 binding was inhibited by representatives of the different classes of KATP channel openers and by sulphonylurea-type blockers with inhibition constants similar to those obtained in rat aortic rings. It is concluded that rat glomerular preparations possess specific binding sites for KATP channel openers with vascular characteristics. The sensitivity of binding to mild collagenase treatment suggests that these sites are located on a membrane protein; in addition, the data suggest that these sites are localized on smooth muscle and/or renin secreting cells of the afferent vascular endings attached to some of the glomeruli. Their estimated density (1,500 microns-2) is much higher than that of KATP channels in smooth muscle.