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

Summary— 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 μM) and apamin (0.1 μM), blockers of Ca²⁺-sensitive K⁺ channels, had no effect on the pinacidil- and levcromakalim-evoked relaxation. Glibenclamide (0.1–10 μM), a selective blocker of adenosine triphosphate (ATP)-sensitive K⁺ channels, competitively antagonized the response to levcromakalim (pK_B = 7.92 ± 0.07). In contrast, glibenclamide, in significantly higher concentrations (3–30 μM), non-competitively antagonized the response to pinacidil. High concentrations of pinacidil (> 10 μM) 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.     

Role of K ATP channels in cephalic vasodilatation induced by calcitonin gene-related peptide, nitric oxide, and transcranial electrical stimulation in the rat

Objective.— The objective of this study was to explore the role of K_ATP channels in vasodilatation induced by calcitonin gene‐related peptide (CGRP), nitric oxide (NO), and transcranial electrical stimulation (TES) in intracranial arteries of rat. Background.— Dilatation of cerebral and dural arteries causes a throbbing, migraine‐like pain. Both CGRP and NO are potent vasodilators that can induce migraine. Their antagonists are effective in the treatment of migraine attacks. K_ATP channel openers cause headache in the majority of healthy subjects suggesting a role for K_ATP channels in migraine pathogenesis. We hypothesized that vasodilatation induced by CGRP and the NO donor glyceryltrinitrate (GTN) is mediated via K_ATP channels. Methods.— We examined the effects of the K_ATP channel inhibitor glibenclamide on dural and pial vasodilatation induced by CGRP, NO, and endogenously released CGRP by TES. A rat genuine closed cranial window model was used for in vivo studies and myograph baths for studying the effect in vitro. In the closed cranial window model the diameter of dural vessels was measured directly in anesthetized animals to investigate the vascular effects of infused CGRP, NO, and endogenous CGRP after electrical stimulation. Also diameter changes of pial arteries, mean arterial blood pressure and local cerebral blood flow by Laser Doppler flowmetry (LCBF_Flux) were measured. Results.— CGRP, NO, and TES caused dilatation of the 2 arteries in vivo and in vitro. In anesthetized rats glibenclamide significantly attenuated CGRP induced dural and TES induced dural/pial artery dilatation (P = .001; P = .001; P = .005), but had no effect on dural/pial vasodilatation induced by GTN. In vitro glibenclamide failed to significantly inhibit CGRP‐ and GTN‐induced vasodilatation. Conclusions.— These results show that a K_ATP channel blocker in vivo but not in vitro inhibits CGRP, but not GTN‐induced dilatation of dural and pial arteries, a mechanism thought to be important in migraine.     

Differential effects of pinacidil and levcromakalim on the contractions elicited electrically or by noradrenaline in the portal vein of the rabbit

The present study was undertaken to examine the antivasoconstrictor effects of pinacidil and levcromakalim, two potassium channel openers (PCOs), on the isolated rabbit portal vein and to define the role for different subtypes of pre- and/or post-synaptic K+ channels in the antivasoconstrictor action of the PCOs. The vein strips were contracted by electrical field stimulation (EFS) or by exogenous noradrenaline (NA). The results of this study showed that pinacidil produced a more potent inhibition of the neurogenic contractions (pD2 = 6.04 +/- 0.05) than of contractions induced by exogenous NA (pD2 = 4.90 +/- 0.10). Glibenclamide (1 microM), a selective blocker of adenosine triphosphate (ATP)-sensitive K+ channels (K(ATP)), did not affect the pinacidil-induced inhibition of contractions evoked by exogenous NA. In contrast, glibenclamide (0.1-10 microM) significantly antagonized the effect of pinacidil on EFS evoked contractions in a noncompetitive manner. There was no difference between the inhibitory effects of levcromakalim on neurogenic contractions (pD2 = 7.58 +/- 0.05) and contractions evoked by exogenous NA (pD2 = 7.64 +/- 0.08). Glibenclamide (1 microM) antagonized in the same manner the levcromakalim-induced inhibition of neurogenic contractions and contractions evoked by exogenous NA. Moreover, glibenclamide competitively antagonized the effect of levcromakalim on EFS induced contractions of the rabbit portal vein (pA2 = 6.40 +/- 0.10). Charybdotoxin (0.4 microM) and apamin (0.1 microM) did not influence the inhibitory effects of pinacidil and levcromakalim, both on contractions evoked by EFS and contractions evoked by exogenous NA. These results suggest that the antivasoconstrictor effect of levcromakalim might be postsynaptic and associated with opening of the smooth muscle K(ATP) channels. In contrast, it is hypothesized that the effect of pinacidil on neurogenic contractions is due to an interference with K(ATP) channels in the neuromuscular synapse. It seems that the action of pinacidil on the NA contractions is mediated by another still undefined mechanisms of pinacidil.     

Comparison of effects of cromakalim and pinacidil on mechanical activity and 86Rb efflux in dog coronary arteries

Effects of two K+ channel openers, cromakalim and pinacidil, on mechanical activity and on 86Rb efflux were compared in strips of dog coronary arteries. Cromakalim and pinacidil produced the relaxation in 20.9 mM K(+)-contracted strips with a pD2 of 6.53 and 5.95, respectively. In 65.9 mM K(+)-contracted strips, high concentrations of pinacidil, but not cromakalim, produced relaxation. Ca+(+)-induced contractions in 80 mM K(+)-depolarized strips were also inhibited by pinacidil but not by cromakalim. Glibenclamide, a blocker of ATP-regulated K+ (KATP) channels, competitively antagonized the relaxant responses to cromakalim with a pA2 value of 7.62. However, the antagonism by glibenclamide of the relaxant responses to pinacidil was not a typical competitive type, suggesting the contribution of other effects than the KATP channel opening activity to the relaxant effects of pinacidil. In resting strips preloaded with 86Rb, cromakalim and pinacidil increased the basal 86Rb efflux in a dose-dependent manner. The increase in the 86Rb efflux induced by cromakalim was greater than that by pinacidil. When the effects of cromakalim and pinacidil on the 86Rb efflux were determined in the 20.9 or 65.9 mM K(+)-contracted strips, both drugs increased the 86Rb efflux. Under the same conditions nifedipine, a Ca(+)+ channel blocker, produced the relaxation that is accompanied by the decrease in 86Rb efflux. The increase in the 86Rb efflux induced by cromakalim was much greater than that by pinacidil.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relaxation of human placental arteries and veins by ATP-sensitive potassium channel openers

BACKGROUND: Adenosine triphosphate (ATP)-sensitive potassium channels (K(ATP)) are important modulators of vascular tone. Preliminary data from our laboratory suggests that K(ATP) channels are expressed in the fetoplacental vasculature where addition of pinacidil, a specific K(ATP) opener, promotes relaxation. We aimed to assess the effects of KRN2391 and KRN4884 on the fetoplacental vasculature, which are putative K(ATP) channel openers. MATERIALS AND METHODS: Functional activity of K(ATP) channels was assessed in chorionic plate arteries and veins using wire myography. Cromakalim-, KRN2391- and KRN4884-induced relaxations were assessed in the presence and absence of agonist-induced pretone. Cromakalim, an established K(ATP) channel opener, acted as control. RESULTS: KRN2391 evoked significantly greater relaxation of chorionic plate arteries and veins than either KRN4884 or cromakalim. KRN2391-induced relaxation of precontracted arteries and veins was reduced in the presence of inhibitors of the nitric oxide pathway (L-NNA or LY83583). With KRN4884, there was no contribution of nitric oxide to the induced relaxation. CONCLUSIONS: We conclude that K(ATP) channels play an important role in controlling placental vascular tone. KRN2391 induces relaxation of human placental blood vessels by activation of K(ATP) channels and via activation of nitric oxide-dependent pathways.     

Protein kinase A-dependent and -independent effects of isoproterenol in rat isolated mesenteric artery: interactions with levcromakalim

The effect of beta-adrenoceptor activation on levcromakalim-induced relaxation was investigated in myograph-mounted rat mesenteric arteries. The nonselective beta-adrenoceptor agonist isoproterenol (at a concentration causing approximately 30% relaxation of methoxamine-induced tone) potentiated relaxation to levcromakalim; higher concentrations exerted no additional effect. The modulatory and relaxant effects of isoproterenol were inhibited by the beta(1)-adrenoceptor antagonist atenolol, but the ATP-sensitive K(+) (K(ATP)) channel inhibitor glibenclamide did not inhibit relaxations to isoproterenol. The protein kinase A inhibitor Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPS) inhibited the ability of isoproterenol to modulate levcromakalim relaxation. However, neither Rp-cAMPS nor N-[2-(p-bromocinnamylamino)ethyl]-6-isoquinolinesulfonamide (H-89) (another protein kinase A inhibitor) markedly reduced isoproterenol-induced relaxation, although Rp-cAMPS inhibited relaxations induced by forskolin (an adenylyl cyclase activator). Iberiotoxin (50 nM), an inhibitor of large conductance Ca(2+)-activated K(+) channels (BK(Ca)), attenuated isoproterenol relaxation. Moreover, both Rp-cAMPS and H-89 caused inhibition of the effects of isoproterenol in the presence of iberiotoxin, whereas glibenclamide did not. We conclude that isoproterenol modulates the actions of levcromakalim through beta(1)-adrenoceptors and protein kinase A, even though K(ATP) channels do not contribute to its relaxant effects. However, the major relaxant mechanism for isoproterenol appears to be protein kinase A-independent activation of BK(Ca), with cyclic AMP-dependent mechanisms only being unmasked when the BK(Ca) mechanism is inhibited. Although direct G protein-mediated activation of BK(Ca) has been demonstrated previously in electrophysiological studies of single smooth muscle cells, this is the first time that such a mechanism has been shown to be functionally important in an intact blood vessel preparation.     

Differential effects of sulphonylureas on the vasodilatory response evoked by K(ATP) channel openers

The potency of three sulphonylureas, glibenclamide, glimepiride and gliclazide in antagonizing the vasorelaxant action of openers of adenosine triphosphate (ATP)-regulated K+ channel (KATP) was studied in vivo and in vitro in micro- and macrovessels, respectively. In the hamster cheek pouch, the vasodilatation and the increase in vascular diameter and blood flow induced by diazoxide were markedly reduced by the addition of either glibenclamide or glimepiride (0.8 microm) while they were not affected by gliclazide up to 12 microm. Similarly, in rat and guinea-pig isolated aortic rings, glibenclamide, glimepiride and gliclazide reduced the vasodilator activity of cromakalim. However, the inhibitory effect of gliclazide was considerably less when compared with either glimepiride or glibenclamide. These results suggest that, in contrast to glibenclamide and glimepiride, therapeutically relevant concentrations of gliclazide do not block the vascular effects produced by KATP channel openers in various in vitro and in vivo animal models.     

Role of BK(Ca) channels in cephalic vasodilation induced by CGRP, NO and transcranial electrical stimulation in the rat

Both calcitonin gene-related peptide (CGRP) and nitric oxide (NO) are potent vasodilators that have been shown to induce headache in migraine patients. Their antagonists are effective in the treatment of migraine attacks. In the present study, we hypothesize that vasodilation induced by the NO donor glyceryltrinitrate (GTN) or by CGRP is partially mediated via large conductance calcium-activated potassium (BK(Ca)) channels. The effects of the BK(Ca) channel selective inhibitor iberiotoxin on dural and pial vasodilation induced by CGRP, GTN and endogenously released CGRP by transcranial electrical stimulation (TES) were examined. Iberiotoxin significantly attenuated GTN-induced dural and pial artery dilation in vivo and in vitro, but had no effect on vasodilation induced by CGRP and TES. Our results show that GTN- but not CGRP-induced dural and pial vasodilation involves opening of BK(Ca) channels in rat.     

猪冠状动脉内皮细胞舒张过程中高钾溶液的抑制作用

背景:目前高钾心脏停搏液已在心脏手术中广泛应用。但有关高钾溶液对冠状动脉内皮细胞舒张血管功能影响的研究尚少。 目的:探讨高钾溶液对猪冠状动脉内皮细胞功能的影响及其机制。 设计:随机对照的实验研究。 地点、材料和干预:2000-06/09从北京大红门肉联厂共取17只刚宰杀的生猪猪心(雌雄不限)。均置入4℃的KH中,30min内送回北京阜外医院体外循环研究室,10min内选其心外膜下冠状动脉前降支中下三分之一,切成3段3mm的血管环。按单纯随机抽样法将49条猪冠状动脉血管环分为7组,每组7条;分别为对照组、20mmol/L高钾组、50mmol/L高钾组、四乙胺组、格列苯脲(商品名:优降糖)组、50mmol/L高钾 四乙胺组、50mmol/L高钾 格列苯脲组。采用器官槽法,分别用Krebs-Henseleit重碳酸盐缓冲液(Krebs solution, KH)、20,50mmol/L的高钾溶液浸泡血管环1h。 主要观察指标:7μmol/L环加氧酶阻断剂吲哚美辛(商品名:消炎痛)和300μmol/L一氧化氮合成酶阻断剂N-硝基-L-精氨酸、1mmol/L钙激动性钾通道阻断剂四乙胺或3μmol/L ATP敏感性钾通道阻断剂优降糖的作用下,30nmol/L前列腺素F_(2α)。引发的预收缩强度和非受体介导钙离子载体(1×10~(-10)～1×10~(-6)mol/L)引发的内皮源性舒张反应。 结果:格列苯脲组、50mmol/L高钾 四乙     

Molecular aspects of ATP-sensitive K+ channels in the cardiovascular system and K+ channel openers

ATP-sensitive K+ (K(ATP)) channels are inhibited by intracellular ATP (ATPi) and activated by intracellular nucleoside diphosphates and thus, provide a link between cellular metabolism and excitability. K(ATP) channels are widely distributed in various tissues and may be associated with diverse cellular functions. In the heart, the K(ATP) channel appears to be activated during ischemic or hypoxic conditions, and may be responsible for the increase of K+ efflux and shortening of the action potential duration. Therefore, opening of this channel may result in cardioprotective, as well as proarrhythmic, effects. These channels are clearly heterogeneous. The cardiac K(ATP) channel is the prototype of K(ATP) channels possessing approximately 80 pS of single-channel conductance in the presence of approximately 150 mM extracellular K+ and opens spontaneously in the absence of ATPi. A vascular K(ATP) channel called a nucleoside diphosphate-dependent K+ (K(NDP)) channel exhibits properties significantly different from those of the cardiac K(ATP) channel. The K(NDP) channel has the single-channel conductance of approximately 30-40 pS in the presence of approximately 150 mM extracellular K+, is closed in the absence of ATPi, and requires intracellular nucleoside di- or triphosphates, including ATPi to open. Nevertheless, K(ATP) and K(NDP) channels are both activated by K+ channel openers, including pinacidil and nicorandil, and inhibited by sulfonylurea derivatives such as glibenclamide. It recently was found that the cardiac K(ATP) channel is composed of a sulfonylurea receptor (SUR)2A and a two-transmembrane-type K+ channel subunit Kir6.2, while the vascular K(NDP) channel may be the complex of SUR2B and Kir6.1. By precisely comparing the functional properties of the SUR2A/Kir6.2 and the SUR2B/Kir6.1 channels, we shall show that the single-channel characteristics and pharmacological properties of SUR/Kir6.0 channels are determined by Kir and SUR subunits, respectively, while responses to intracellular nucleotides are determined by both SUR and Kir subunits.     

Potassium channel openers dilate large epicardial coronary arteries in conscious dogs by an indirect, endothelium-dependent mechanism.

Cromakalim and pinacidil, two potassium channel openers, dilate both large and small coronary arteries in conscious dogs. Because flow-mediated dilation of large arteries is endothelium-dependent, the consequences of in vivo endothelium removal (balloon denudation) on the response of large epicardial coronary arteries to cromakalim (10 micrograms/kg) and pinacidil (30 micrograms/kg) were investigated in six dogs chronically instrumented for the measurement of arterial pressure, left circumflex coronary artery diameter and coronary blood flow. Endothelium removal abolished the dilation of large coronary arteries induced by acetylcholine (endothelium-dependent dilation) and reactive hyperemia (flow-mediated dilation), but only slightly reduced (-18%) that induced by nitroglycerin. Before endothelium removal, both cromakalim and pinacidil induced a significant decrease in coronary resistance (-71 +/- 2 and -63 +/- 2%, respectively) and a significant increase in coronary diameter (8.5 +/- 1.3 and 6.7 +/- 0.9%). After endothelium removal, the decreases in coronary resistance were unaffected, but the increases in coronary diameter were reduced by 93 and 98% as compared to predenudation responses with cromakalim and pinacidil, respectively (both P < .01). In contrast, in vitro studies performed in isolated large epicardial coronary arteries obtained from five additional dogs showed that cromakalim evoked relaxations that were not affected by prior in vivo endothelium removal. Thus, despite the presence of potassium channels in isolated denuded large coronary arteries, our data demonstrate that cromakalim- and pinacidil-induced dilation of large arteries in vivo is an indirect, flow-mediated effect which is entirely endothelium-dependent.     

The relaxation induced by S-nitroso-glutathione and S-nitroso-N-acetylcysteine in rat aorta is not related to nitric oxide production.

S-nitroso-glutathione (GSNO) and S-nitroso-N-acetylcysteine (NACysNO) are nitrosothiols that release nitric oxide (NO) and mimic the effects of endogenous NO. This study investigated the relaxation induced by GSNO and NACysNO in rat aorta and the relation between relaxation and NO formation. Both compounds at concentrations from 10(-9) M to 10(-4) M relaxed the rat aorta in a concentration-dependent manner. However, NO production depended on the concentration of nitrosothiols present and was detected only above 100 microM GSNO or NACysNO. To determine whether K+ channels are involved in the relaxation induced by nitrosothiols, the contractions were induced with KCl at concentrations of 30, 60, or 90 mM. The concentration-effect curves for the relaxation induced by nitrosothiols were shifted to the right for all the K+ concentrations compared with aortas precontracted with phenylephrine. These results indicate the participation of K+ channels in the relaxation induced by GSNO and NACysNO. A selective inhibitor of soluble guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, significantly inhibited the relaxation induced by the nitrosothiols. The relaxation induced by GSNO and NACysNO was inhibited by the K+ channel blockers glibenclamide, selective K(ATP) channels, and apamin, selective for low-conductance Ca2+-activated K+ channels in rat aorta, but was not inhibited by charybdotoxin, a potent and selective Ca2+-activated K+ channel blocker, or by 4-aminopyridine, a voltage-gated K+ channel blocker. These results indicate that relaxation induced by GSNO and NACysNO is partially due to activation of K(ATP) channels and partially due to activation of low-conductance Ca2+-activated K+ channels. However, the ability of the nitrosothiol compounds to overcome the inhibitory effect of high extracellular K+ concentrations suggests another mechanism of relaxation contributing to the nitrosothiol response. The most intriguing finding is that relaxation is not related to the NO produced in rat aorta.     

Evaluation of bosentan,pinacidil and nitroprusside on human pulmonary arteries: comparison with rat pulmonary arteries*

Summary— Bosentan (endothelin ET_A/ET_B antagonist), pinacidil (potassium channel opener) and nitroprusside (nitric oxide donor) were examined on isolated ring preparations of human intralobar pulmonary artery (3rd-5th generation; internal radius > 1 mm), rat main pulmonary artery (1st generation; internal radius > 1 mm) and rat intralobar pulmonary artery (3rd generation; internal radius ≥ 0.1–0.3 mm). The potency of endothelin-1 was the same in all three artery types. In human intralobar artery and rat main pulmonary artery, bosentan (3 and 10 μM) shifted the endothelin-1 concentration response curve to a higher concentration range (endothelin-1 concentration ratios, in human intralobar and rat main pulmonary artery, respectively: 3 liM bosentan, 4.5 and 8.1; 10 /xM bosentan, 13.5 and 19.5), but caused no significant block of endothelin-1 in rat intralobar artery. The latter finding may be due to the reported presence of ET_B receptors in rat intralobar arteries and the higher potency of bosentan on ET_A than on ET_B receptors. In contrast, the potencies of nitroprusside and pinacidil (relaxation of submaximal contractions to the thromboxane-mimetic, U46619) agreed on human and rat intralobar arteries but were 6 to 16-fold lower than on rat main pulmonary artery. We conclude that data obtained on pulmonary arteries from rats can be useful in predicting the effects of vasoactive drugs in human pulmonary arteries but selection of the most appropriate rat artery for study will depend on the drug group under investigation. For potassium channel openers and nitric oxide donors, good agreement between human and rat data will be found when using pulmonary arteries from the same anatomical location even though they differ markedly in size. In contrast, for endothelin antagonists, agreement is more likely to be found in arteries of comparable size, despite their different anatomical locations.     

Intracellular ATP can regulate afferent arteriolar tone via ATP-sensitive K+ channels in the rabbit.

Studies were performed to assess whether ATP-sensitive K+ (KATP) channels on rabbit preglomerular vessels can influence afferent arteriolar (AA) tone. K+ channels with a slope conductance of 258 +/- 13 (n = 7) pS and pronounced voltage dependence were demonstrated in excised patches from vascular smooth muscle cells of microdissected preglomerular segments. Channel activity was markedly reduced by 1 mM ATP and in a dose-dependent fashion by glibenclamide (10(-9) M to 10(-6) M), a specific antagonist of KATP channels. 10(-5) M diazoxide, a K+ channel opener, activated these channels in the presence of ATP, and this effect was also blocked by glibenclamide. To determine the role of these KATP channels in the control of vascular tone, diazoxide was tested on isolated perfused AA. After preconstriction from a control diameter of 13.1 +/- 1.1 to 3.5 +/- 2.1 microns with phenylephrine (PE), addition of 10(-5) M diazoxide dilated vessels to 11.2 +/- 0.7 microns, which was not different from control. Further addition of 10(-5) M glibenclamide reconstricted the vessels to 5.8 +/- 1.5 microns (n = 5; P less than 0.03). In support of its specificity for KATP channels, glibenclamide did not reverse verapamil induced dilation in a separate series of experiments. To determine whether intracellular ATP levels can effect AA tone, studies were conducted to test the effect of the glycolytic inhibitor 2-deoxy-D-glucose. After preconstriction from 13.4 +/- 3.2 to 7.7 +/- 1.3 microns with PE, bath glucose was replaced with 6 mM 2-deoxy-D-glucose. Within 10 min, the arteriole dilated to a mean value of 11.8 +/- 1.4 microns (n = 6; NS compared to control). Subsequent addition of 10(-5) M glibenclamide significantly reconstricted the vessels to a diameter of 8.6 +/- 0.5 micron (P less than 0.04). These data demonstrate that KATP channels are present on the preglomerular vasculature and that changes in intracellular ATP can directly influence afferent arteriolar tone via these channels.     

Sirolimus causes relaxation of human vascular smooth muscle: a novel action of sirolimus mediated via ATP-sensitive potassium channels

Little is known about the vasomotor effects of sirolimus, and preliminary studies using animal models have provided conflicting results. The present study was designed to determine the effects of sirolimus on vasomotor tone in human blood vessels. Human radial artery segments were cut into rings, denuded of endothelium, and placed into organ chambers for isometric tension recording. Sirolimus (10(-10) to 10(-6) M) caused concentration-dependent relaxation of human arteries contracted with U46619 (9,11-dideoxy-11alpha,9alpha-epoxymethano-prostaglandin F(2alpha); 10(-8) M) [-log (M) EC(50) (pD(2)) = 7.28 +/- 0.1; E(max) = 57 +/- 6%] or phenylephrine (10(-6) M) (pD(2) = 7.16 +/- 0.4; E(max) = 45 +/- 9%). Sirolimus-induced relaxation was unaffected by treatment with indomethacin (10(-5) M) but was nearly abolished in tissues contracted by depolarization with elevated K(+) (60 mM). In U46619-contracted rings, the response to sirolimus was markedly inhibited in the presence of the specific ATP-sensitive potassium (K(ATP)) channel blocker, glyburide (10(-6) M), but was unaffected by treatment with blockers of large conductance, calcium-activated potassium channel (iberiotoxin, 10(-7) M), small conductance, calcium-activated potassium channel (apamin, 10(-6) M), or voltage-gated potassium channel (4-aminopyridine, 10(-3) M). The K(ATP) channel opener, aprikalim (10(-7) to 10(-5) M), caused concentration-dependent relaxations that were inhibited by glyburide (10(-6) M) and abolished in tissues contracted with elevated K(+) (60 mM), thus confirming that K(ATP) channel opening causes relaxation of these arteries. These data suggest that sirolimus, at concentrations attained in vivo, causes relaxation of human arteries, and this effect is mediated by opening of K(ATP) channels in vascular smooth muscle. Reduced vasomotor tone is a heretofore unrecognized action of sirolimus that could potentially contribute to its efficacy in drug-eluting stents.     

Vascular K(ATP) channel blockade by glibenclamide, but not by acarbose, in patients with Type II diabetes

Glibenclamide inhibits the opening of vascular ATP-sensitive potassium (K(ATP)) channels, which represents a protective mechanism during ischaemia. This effect may imply harmful cardiovascular effects of glibenclamide when used under conditions of ischaemia in patients with Type II diabetes. Acarbose is not associated with effects on the cardiovascular system, because the drug is not absorbed from the bowel. Therefore we hypothesized that treatment of Type II diabetes patients with glibenclamide will impair the vasodilator function of K(ATP) opening, unlike treatment with acarbose. A double-blind randomized cross-over study in 12 patients with Type II diabetes was performed to compare the effects of glibenclamide with those of acarbose on the vasodilator responses to K(ATP) channel opening in the forearm vascular bed. The study consisted of two periods: 8 weeks of treatment with orally administered glibenclamide (10 mg x day(-1)) followed by 8 weeks of treatment with acarbose (300 mg x day(-1)), or vice versa. At the end of each treatment period, forearm blood flow (venous occlusion plethysmography) in response to intra-arterially administered diazoxide, acetylcholine and dipyridamole and to forearm ischaemia was measured. The diazoxide-mediated increase in the forearm blood flow ratio (infused/control arm) was significantly less pronounced after glibenclamide than after acarbose (290 +/- 58% and 561 +/- 101% respectively; P<0.0005). Forearm blood flow responses to acetylcholine, dipyridamole and forearm ischaemia were similar during glibenclamide and acarbose treatment. Thus, in patients with Type II diabetes mellitus, treatment with glibenclamide is associated with an attenuated response to K(ATP) opening as compared with treatment with acarbose. This implies that glibenclamide may affect defensive mechanisms under conditions of K(ATP) channel activation.     

Alterations by glyburide of effects of BRL 34915 and P 1060 on contraction, 86Rb efflux and the maxi-K+ channel in rat portal vein

Effects of the K+ channel blocking agent, glyburide, on the actions of two K+ channel openers, BRL 34915 (cromakalim) and P 1060 (Leo), a potent pinacidil derivative (N-(t-butyl)-N"-cyano-N'-3-pyridyl-guanidine), were ascertained. Tension responses and 86Rb fluxes in rat portal vein strips and single channel electrophysiological recordings in enzymatically dissociated rat portal vein cells were obtained. Glyburide (0.3 microM) increased spontaneous contractile activity and caused concentration-dependent shifts in the relaxation responses to BRL 34915 and P 1060. Increases in 86Rb efflux were obtained only at much higher concentrations of BRL 34915 or P 1060, and these increases were blocked only at higher concentrations of glyburide (5.0 microM). BRL 34915 and P 1060 specifically increase the open-state probability of the Ca+(+)-activated K+ (maxi-K+) channel, and these actions are blocked by glyburide and also by charybdotoxin. Changes in single channel activity and contractile responsiveness occur at similar concentrations of agonists and antagonists. Thus, the membrane channel in rat portal vein affected by glyburide, BRL 34915 and P 1060 appears to be the Ca+(+)-activated maxi-K+ channel (that does not show ATP dependence under the conditions of these experiments). Concentrations of agonists and antagonists effective on maxi-K+ channel activity correspond to those affecting contractile responsiveness and are lower than those eliciting changes in 86Rb flux.