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H Karaki

Publications and source records attributed to H Karaki.

At least 19 recordsLinked to original sources
PubMed1998

Regulatory mechanisms of calcium sensitization of contractile elements in smooth muscle.

It is evident that smooth muscle contraction is regulated not only by the Ca2+/calmodulin/myosin light chain kinase system but also by modulation of Ca2+ sensitivity. Changes in free calmodulin concentrations, myosin light chain phosphorylation elicited by rho/rho-kinase, regulation of myosin phosphatase activity and thin filament-linked mechanisms are the possible mechanisms for regulation of Ca2+ sensitivity.

Animals↗
PubMed1998

Effects of adrenomedullin and calcitonin gene-related peptide on contractions of the rat aorta and porcine coronary artery.

1. Effects of adrenomedullin and alpha-calcitonin gene-related peptide (CGRP) on the contractions and cytosolic Ca2+ concentrations ([Ca2+]i) of the rat aorta and porcine coronary artery were investigated. Characteristics of the receptors mediating the effects of adrenomedullin and alpha-CGRP were also investigated. 2. Adrenomedullin and alpha-CGRP caused a concentration-dependent relaxation in the rat aorta contracted with noradrenaline. The IC50 values for adrenomedullin and alpha-CGRP were 2.4 nM and 4.0 nM, respectively. The relaxant effects of these peptides were abolished by removal of the endothelium and significantly attenuated by an inhibitor of nitric oxide synthase, NG-monomethyl-L-arginine (L-NMMA, 100 microM), but not by a cyclo-oxygenase inhibitor, indomethacin (10 microM). 3. Adrenomedullin and alpha-CGRP increased the endothelial [Ca2+]i in the rat aorta with endothelium, whereas they did not change [Ca2+]i in the smooth muscle. 4. An antagonist of the CGRP1 receptor, CGRP (8-37), antagonized the relaxant effects of alpha-CGRP and the beta-isoform of CGRP (beta-CGRP) but not those of adrenomedullin in the rat aorta. 5. In the porcine coronary artery contracted with U46619, adrenomedullin and alpha-CGRP caused a concentration-dependent relaxation with an IC50 of 27.6 and 4.1 nM, respectively. Removal of the endothelium altered neither the IC50 values nor the maximal relaxations induced by adrenomedullin or alpha-CGRP. When the artery was contracted with high K+ solution (72.7 mM), these peptides caused a small relaxation. 6. Adrenomedullin and alpha-CGRP increased cyclic AMP content and decreased the smooth muscle [Ca2+]i in the porcine coronary artery. 7. CGRP (8-37) significantly antagonized the relaxant effects of adrenomedullin and alpha-CGRP in the porcine coronary artery. However, it had little effect on the relaxations induced by the beta-isoform of CGRP (beta-CGRP). 8. These results suggest that in the rat aorta, adrenomedullin and alpha-CGRP increase the endothelial [Ca2+]i, activate nitric oxide synthase and release nitric oxide, without a direct inhibitory action on smooth muscle. In the porcine coronary artery, in contrast, adrenomedullin and alpha-CGRP directly act on smooth muscle, increase cyclic AMP content, decrease the smooth muscle [Ca2+]i and inhibit contraction. The rat aortic endothelium seems to express the CGRP receptor which is sensitive to alpha-CGRP, beta-CGRP and CGRP (8-37) and the adrenomedullin specific receptor. The porcine coronary smooth muscle, in contrast, seems to express two types of CGRP receptor; one of which is sensitive to alpha-CGRP, CGRP (8-37) and adrenomedullin and the other is sensitive only to beta-CGRP.

Adrenomedullin↗
PubMed1998

Actin-depolymerizing effect of dimeric macrolides, bistheonellide A and swinholide A.

We compared the effects of dimeric marine toxins, bistheonellide A, and swinholide A, on actin polymerization. Bistheonellide A and swinholide A possess two identical side chains with similar structures to those of other marine toxins, mycalolide B, and aplyronine A. By monitoring changes in fluorescent intensity of pyrenyl-actin, bistheonellide A was found to inhibit polymerization of G-actin and to depolymerize F-actin in a concentration-dependent manner. The relationship between the concentration of bistheonellide A and its inhibitory activity on actin polymerization suggested that one molecule of bistheonellide A binds two molecules of G-actin. We demonstrated by SDS-PAGE that the complex of G-actin with bistheonellide A, swinholide A, or mycalolide B could not interact with myosin. No evidence was found that bistheonellide A severs F-actin at the concentrations examined (molar ratio to actin; 0. 025-2.5), while swinholide A showed severing activity, although it was weaker than that of mycalolide B. We also demonstrated that the depolymerizing effect of bistheonellide A or mycalolide B is irreversible. Bistheonellide A increased, while swinholide A decreased, the rate of nucleotide exchange in G-actin, suggesting that binding of these toxins induces different conformational changes in the actin molecule. These results suggest that bistheonellide A intervenes between two actin molecules, forms a tertiary complex with each of its side chains bound to G-actin, and inhibits polymerization by sequestering G-actin from incorporation into F-actin. A difference in structure at the end of the side chain between dimeric macrolides and mycalolide B may account for the weak severing activity of the former.

Actins↗
PubMed1998

Distinct endothelin-B receptors mediate the effects of sarafotoxin S6c and IRL1620 in the ileum.

In the guinea pig ileum, both sarafotoxin S6c (S6c) and IRL1620 induced a biphasic effect (relaxation and contraction). S6c induced strong tachyphylaxis of both components of the response, but IRL1620 induced tachyphylaxis mainly of the contractile component. Whereas the tissues rendered tachyphylactic to S6c did not respond to IRL1620, a normal biphasic response to S6c was observed in the tissues rendered tachyphylactic to IRL1620. In the presence of IRL1620, S6c could induce its biphasic effect, whereas in the presence of S6c, IRL1620 was ineffective. BQ-123, a specific ETA antagonist, did not affect the biphasic response induced by either agonist. PD145065, a potent ETA/ETB antagonist, was a competitive and a noncompetitive antagonist, respectively, of the contractile components of IRL1620 and S6c. RES-701-1, a specific ETB1 antagonist, inhibited both components of the response induced by IRL1620. However, it inhibited mainly the relaxant component induced by low doses of S6c. Apamin had different effects on endothelin-1 (ET-1), S6c, and IRL1620. Our results suggest that there are at least two distinct populations of ETB receptors mediating the biphasic response: the ETB1 receptor, sensitive to RES-701-1 and PD145065, and the ETB2 receptor, less sensitive to RES-701-1 and PD145065.

Animals↗
PubMed1998

Cicletanine-induced decreases in cytosolic Ca2+ level and contraction in vascular smooth muscle.

The mechanism by which cicletanine (3-(4-chlorophenyl)-1,3-dihydro-7-hydroxy-6-methylfuro-[3,4-c]pyri dine) induces vasodilatation was examined in isolated vascular smooth muscle. Cicletanine inhibited the contraction induced by high K+, norepinephrine (NE) and prostaglandin F2alpha in a concentration-dependent manner in rat aorta. High K+ (15.8-72.7 mM) elicited elevation of cytosolic Ca2+ level ([Ca2+]i) and contraction in a concentration-dependent manner. Cicletanine (300 microM) inhibited the high K+-induced contractions without changing the [Ca2+]i/tension relationship. NE (3-300 nM) elicited greater contractions than high K+ at a given [Ca2+]i, suggesting that NE increased Ca2+ sensitivity of the contractile elements. Cicletanine inhibited the NE-induced contractions without changing the slope of the [Ca2+]i/tension relationship. Cicletanine inhibited the transient increases in both [Ca2+]i and muscle tension elicited by NE but not the transient increase in [Ca2+]i elicited by caffeine in Ca2+-free solution. Cicletanine did not inhibit contraction induced by Ca2+ in the permeabilized rabbit mesenteric artery with alpha-toxin. These results suggest that cicletanine inhibits vascular smooth muscle contraction by multiple mechanisms: 1) inhibition of Ca2+ influx via voltage-dependent Ca2+ channel and 2) inhibition of Ca2+ release mediated by the alpha-adrenoceptors, but not by caffeine.

Animals↗
PubMed1998

Inhibition of rat platelet aggregation by mycalolide-B, a novel inhibitor of actin polymerization with a different mechanism of action from cytochalasin-D.

In vitro effects of mycalolide-B (MB), isolated from marine sponge, were investigated with regard to the activation of rat platelets. Collagen-induced platelet aggregation in platelet-rich plasma (PRP) was slightly but significantly potentiated by lower concentrations of MB (0.3 and 1 microM) but was inhibited by higher concentrations (3 and 10 microM). ADP-induced platelet aggregation in PRP was also significantly prevented by MB (1-10 microM). Potentiation of ADP-induced aggregation by MB (0.3 microM) was hardly observed. G-actin contents, determined by DNase I inhibition assay, were increased in resting washed platelets incubated with MB (3 microM). In contrast, cytochalasin-D (CD) at 3 microM slightly reduced G-actin contents in resting platelets. After platelet aggregation with collagen (3 microg/ml) or ADP (10 microM), G-actin contents in platelets were reduced, indicating de novo actin polymerization. MB (3 microM) and CD (3 microM) abolished both ADP (10 microM)- and collagen (3 microg/ml)-induced platelet aggregation and actin polymerization in washed platelets. MB (1-10 microM) had no effects on intracellular Ca2+ concentrations in ADP (10 microM)-stimulated platelets. [125I]-fibrinogen binding to activated platelets with ADP (10 microM)(was inhibited by MB (0.3-3 microM) in a concentration-dependent manner. Thrombin-induced platelet-fibrin clot retraction was inhibited by MB (1 and 10 microM). These results suggest that MB inhibits platelet activation by interfering with actin polymerization through a different mechanism of action from CD. MB may be a useful tool for studying the role of actin polymerization in various cells.

Actins↗
PubMed1998

Mechanism of relaxant effect of clonidine in isolated bovine tracheal smooth muscle.

The relaxant effect of clonidine and the possible involvement of imidazoline I1 receptors in bovine tracheal smooth muscle (BTSM) were examined. Clonidine caused concentration-dependent significant relaxation in BTSM precontracted with 0.1 or 1 microM carbachol (CCh) but not in 72.7 mM KCl-induced contraction. The relaxation in CCh-contracted BTSM was inhibited by yohimbine (1 microM) and idazoxan (10 and 30 microM) but not by tetrodotoxin, indomethacin and other adrenoceptor antagonists. Oxymetazoline (0.1-100 microM) and phentolamine (0.1-100 microM) caused concentration-dependent relaxation, which was attenuated by idazoxan (10 microM). Norepinephrine (0.1-100 microM) produced concentration-dependent relaxation, which was completely abolished by propranolol (10 microM) but not by yohimbine (1 microM). In fura-PE3/AM-loaded BTSM, CCh and 72.7 mM KCl increased intracellular calcium concentration ([Ca++]i) followed by contraction. The high K+-induced increase in [Ca++]i was not affected by clonidine. In CCh-stimulated BTSM, clonidine decreased [Ca++]i and muscle force in parallel, whereas verapamil decreased [Ca++]i more strongly than muscle force. Clonidine (100 microM) inhibited the transient increase in [Ca++]i induced by CCh but not by caffeine (20 mM) in Ca++-free solution. Clonidine did not change the cAMP content in the presence of either 72.7 mM KCl or CCh. These results indicate that clonidine relaxes CCh-stimulated BTSM through the inhibition of CCh-induced increases in Ca++-influx, Ca++-release and intracellular signal transduction probably via imidazoline I1 receptors.

Adrenergic alpha-Agonists↗
PubMed1997

Interleukin-1beta-induced, nitric oxide-dependent and -independent inhibition of vascular smooth muscle contraction.

Stimulation of vascular smooth muscle by bacterial lipopolysaccharide has been shown to produce interleukin-1beta and to induce vasodilation in septic shock. To understand the mechanisms of interleukin-1beta-induced relaxation, we examined the effects of interleukin-1beta on contractility and cyclic GMP contents of vascular smooth muscle. After treatment of the rat aorta with interleukin-1beta (20 ng/ml) for 6 h, the cyclic GMP content increased and the contraction induced by phenylephrine (1 microM) was partially inhibited. An inhibitor of nitric oxide (NO) synthase, N(G)-monomethyl-L-arginine (L-NMMA, 100 microM), prevented the inhibitory effect of interleukin-1beta. After treatment with interleukin-1beta for 24 h, the phenylephrine-induced contraction was inhibited more strongly. Neither L-NMMA (100 microM) nor aminoguanidine (100 microM) reversed the inhibition, whereas methylene blue (10 microM) partially reversed the inhibition. After treatment with interleukin-1beta for 12 or 24 h, the cyclic GMP content increased but to a level lower than that obtained with a 6-h treatment. The effects of sodium nitroprusside (1 microM) to inhibit the phenylephrine-induced contraction and to increase the cyclic GMP content were markedly suppressed by the 24-h interleukin-1beta treatment. In contrast, the 24-h interleukin-1beta treatment did not change the ability of 8-bromo-cGMP to relax the phenylephrine-stimulated aorta. Addition of L-NMMA (1 mM) during the 24 h treatment prevented NO production and preserved the sodium nitroprusside-induced cGMP generation by interleukin-1beta. The 24 h interleukin-1beta treatment increased the threshold concentration of KCl needed to induce contraction without changing the maximum contraction. In the presence of 25.4 mM KCl or the non-selective K+ channel inhibitor, tetraethylammonium, the inhibitory effect of the 24-h interleukin-1beta treatment on phenylephrine-induced contraction was restored. These results suggest that interleukin-1beta inhibits vascular smooth muscle contraction by a time-dependent, dual mechanism. After a 6-h treatment with interleukin-1beta, the NO/cyclic GMP system is activated. After a 24-h interleukin-1beta treatment, in contrast, the NO/cyclic GMP system may be desensitized and the contraction of vascular smooth muscle is inhibited by another mechanism, possibly membrane hyperpolarization.

Animals↗
PubMed1997

Palytoxin-induced increase in endothelial Ca2+ concentration in the rabbit aortic valve.

Palytoxin (PTX) is one of the most potent toxins isolated from marine coelenterates of the genus Palythoa. It induces depolarization in various types of cells by increasing the permeability for monovalent cations. It has been reported that PTX induces endothelium-dependent relaxation of vascular smooth muscle. In this study, we examined the effect of PTX on the cytosolic Ca2+ concentration ([Ca2+]i) in the endothelium of rabbit aortic valves loaded with fluorescent Ca2+ indicators, fura-PE3 or fluo-3. PTX (10 pM-300 nM) irreversibly increased endothelial [Ca2+]i in a concentration-dependent manner. ATP and thapsigargin also increased [Ca2+]i. Imaging of [Ca2+]i with a confocal microscope revealed that PTX increased [Ca2+]i in all endothelial cells studied (n = 13). An inorganic Ca2+ entry blocker, La3+ (30 microM), had no effect on the increase in [Ca2+]i induced by PTX whereas it inhibited the sustained phase of the increase in [Ca2+]i induced by ATP or thapsigargin. The PTX-induced increase in [Ca2+]i was partially inhibited by ouabain and was abolished by removal of external Ca2+ although decrease of Na+ concentration in the incubation medium was ineffective. Activation of protein kinase C by 1 microM 12-deoxyphorbol 13-isobutyrate or inhibition of phosphatase by 10 nM calyculin-A had no effect on the increase in [Ca2+]i induced by PTX, whereas both agents inhibited the sustained phase of the increase in [Ca2+]i induced by ATP or thapsigargin. Mn2+ influx, measured by the quenching of fura-PE3 fluorescence, was accelerated by ATP or thapsigargin, but not by PTX. These results suggest that PTX increases [Ca2+]i in the endothelium of the rabbit aortic valve by increasing Ca2+ influx through a pathway which is different from that activated by ATP or thapsigargin.

Acrylamides↗
PubMed1997

Stellettamide-A, a novel inhibitor of calmodulin, isolated from a marine sponge.

1. Stellettamide A (ST-A), a novel marine toxin isolated from a marine sponge, inhibited high K+(72.7 mM)-induced contraction in the smooth muscle of guinea-pig taenia coli with an IC50 of 88 microM. 2. In the taenia permeabilized with Triton X-100, ST-A inhibited Ca2+ (3 and 10 microM)-induced contractions with an IC50 of 46 microM for 3 microM Ca2+ and 105 microM for 10 microM Ca2+. In the permeabilized taenia, calyculin-A (300 nM), a potent inhibitor of type-1 and type-2A phosphatases, induced sustained contraction in the absence of Ca2+. ST-A had no effect on this contraction. 3. ST-A inhibited Mg2+-ATPase activity in native actomyosin prepared from chicken gizzard with an IC50 of 25 microM. 4. In a reconstituted smooth muscle contractile system containing calmodulin, myosin light chain (MLC) and MLC kinase, ST-A inhibited MLC phosphorylation with an IC50 of 152 microM. The inhibitory effect of ST-A was antagonized by increasing the concentration of calmodulin. 5. ST-A inhibited calmodulin activity, assessed by Ca2+/calmodulin-dependent enzymes, (Ca2+-Mg2+)-ATPase of erythrocyte membrane, with an IC50 of 100 microM and phosphodiesterase prepared from bovine cardiac muscle with an IC50 of 52 microM. The inhibitory effect on phosphodiesterase activity was antagonized by increasing the calmodulin concentration. 6. Interaction between ST-A and calmodulin was demonstrated by instantaneous quenching of the intrinsic tyrosine fluorescence of calmodulin by ST-A (3-300 microM). Similar results were obtained in the presence or absence of Ca2+ suggesting that ST-A binds to calmodulin and that Ca2+ is not essential for the binding of ST-A to calmodulin. 7. These results suggest that ST-A, isolated from marine metabolites, is a novel inhibitor of calmodulin.

Animals↗
PubMed1997

[Effect of discodermin A, an antimicrobial peptide, on the cytoplasm membrane].

The effects of discodermin A, an antimicrobial peptide extracted from sea sponge Discodermia kiiensis, on cell membranes were investigated using vascular smooth muscle cells and erythrocytes. At lower concentrations (0.1-3 microM), discodermin A increased muscle tension with an increase in intracellular free Ca2+ concentration ([Ca2+]i) indicated by the fluorescence of Ca2+ indicator, fura-PE3, in rat aortic smooth muscle. On the other hand, the higher concentration of discodermin A (10-30 microM) accelerated the leakage of loaded fura-PE3 from cells. In rabbit mesenteric artery treaded with discodermin A, addition of micromolar concentration of Ca2+ evoked contraction in the presence of ATP, suggesting that permeability of the membrane to Ca2+ and ATP is increased by discodermin A. Confocal fluorescence microscopy showed that discodermin A permeabilized the plasma membrane of A10 cells to fluorescent agents EthD-1 and the intracellular esterase coupled with another fluorescent agent calcein. Discodermin A also showed a hemolytic effect on rabbit erythrocytes, suggesting that discodermin A permitted transmembrane passage of hemoglobin. These results suggest that discodermin A form pores of different sizes on the cytoplasm membrane in concentration- and time-dependent manners. Discodermin A may be a saponin-like bioactive peptide.

Amphibian Proteins↗
PubMed1997

Effect of phorbol esters on cytosolic Ca2+ level, myosin phosphorylation and muscle tension in high K(+)-stimulated bovine tracheal smooth muscle.

To determine the role of protein kinase C (PKC) in bovine tracheal smooth muscle contractility, we examined the effects of phorbol esters on cytosolic Ca2+ level ([Ca2+]i), myosin light chain (MLC) phosphorylation and contractile force in intact muscle and contraction in a permeabilized preparation. In intact muscle, 12-deoxyphorbol 13-isobutyrate (DPB, 1 microM) increased the force without changing [Ca2+]i. High K+ (72.7 mM) induced sustained contraction with sustained increase in [Ca2+]i. In the muscle stimulated by high K+, 50 nM DPB increased the contractile force without changing [Ca2+]i, and 1 microM DPB increased the contractile force with decreasing [Ca2+]i. Thus DPB shifted the [Ca2+]i/force relationship for high K+ to the lower [Ca2+]i in a concentration-dependent manner. In permeabilized muscle, DPB did not induce contraction in the absence of Ca2+ (< < 0 nM), but shifted the Ca2+/force relationship to the lower Ca2+ levels. In the muscle stimulated with high K+, DPB (50 nM and 1 microM) increased MLC phosphorylation and force without changing the MLC phosphorylation/force relationship. DPB (1 microM) increased PKC activity estimated by the translocation from the cytoplasm to the membrane. These results suggest that DPB increases the Ca2+ sensitivity of MLC phosphorylation via the activation of PKC. Furthermore, DPB at higher concentration has an inhibitory effect on stimulated [Ca2+]i.

Animals↗
PubMed1996

Effects of chronic oral administration of a high dose of nicorandil on in vitro contractility of rat arterial smooth muscle.

Nicorandil, which is structurally a nitrate and also a nicotinamide, has a vasodilator action by stimulating cyclase and ATP-sensitive K+ channel. The aim of present study was to examine the effects of chronic oral administration of a high dose of nicorandil on in vitro vascular reactivity. Nicorandil (30 mg/kg), at a dose 6-10-times higher than to decrease blood pressure in rat, was orally administered 2-times daily for a 2-4 weeks to the rats. At the end of the administration period, thoracic aorta was isolated for in vitro study. Treatment with nicorandil for 4 weeks markedly reduced the relaxant effect of nicorandil itself and other vasodilators including sodium nitroprusside, nitric oxide, endothelium-derived relaxing factor released by carbachol, 8-Br-cyclic guanosine 3',5'-monophosphate (cGMP), a K+ channel opener, levcromakalim, and forskolin. Increase in cGMP content induced by nicorandil and sodium nitroprusside was less in the aorta from nicorandil-treated rat than in the vehicle-control rat. Chronic administration of nicorandil altered neither the contractile responses to norepinephrine nor the vasodilator effect of verapamil. On the other hand, a 4-week treatment with a dose of nicorandil (2 mg/kg) sufficient to decrease blood pressure in rat showed no change in aortic response. These results suggest that in vivo chronic treatment with a high dose of nicorandil inactivates not only the guanylate cyclase activity but also the mechanism mediated by cGMP; it also attenuates the sensitivity of K+ channels to levcromakalim. Prolonged activation of the specific site may desensitize its site of action.

Administration, Oral↗
PubMed1996

Antihypertensive effect of angiotensin I-converting enzyme inhibitory peptides derived from hemoglobin.

From proteolytic digest of swine hemoglobin, we isolated four peptide, E-1 (Phe-Gln-Lys-Val-Val-Ala), E-2 (Phe-Gln-Lys-Val-Val-Ala-Gly), peptide 30-3 (Phe-Gln-Lys-Val-Val-Ala-Lys) and H-1 (Gly-Lys-Lys-Val-Leu-Gln). These peptides inhibited angiotensin I-converting enzyme activity with an IC50 of 5.8, 7.4, 2.1 and 1.9 microM, respectively. Oral administration of 50 mg/kg E-1 and 50 mg/kg H-1 decreased blood pressure in spontaneously hypertensive rats. In normotensive rats, oral administration of 500 mg/kg E-1 and 500 mg/kg H-1 inhibited the pressor effect of i.v. administrated 300 ng/kg angiotensin I, possibly by inhibiting its conversion to angiotensin II. These results suggest that these peptides are orally effective inhibitors of angiotensin I-converting enzyme that have a hypotensive effect.

Angiotensin-Converting Enzyme Inhibitors↗
PubMed1996

Lack of endothelin ETB receptor binding and function in the rat with a mutant ETB receptor gene.

Congenital aganglionosis rat is a mutant with an autosomal recessive gene (sl). Recent studies have revealed that the endothelin ETB receptor gene of sl/sl rat has a deletion of 301-bp region spanning exon 1 and intron 1 corresponding to the first and the second transmembrane domains of the receptor. In the present experiments, we examined the functions of ETB receptors in the sl/sl rats. In the membranes of cerebellum, heart, and lung of control (+/+ and sl/+) rats, ET-1 induced a monophasic, competitive displacement of [125I]ET-1 binding, whereas ET-3, IRL 1620, and BQ-123 showed biphasic displacement. In the membranes of sl/sl rats, in contrast, ET-1, BQ-123, ET-3, and IRL 1620 showed only monophasic displacement. Scatchard analysis revealed a single [125I]ET-3 binding site in the membrane of control heart but not in the sl/sl rat heart, and the specific binding sites for [125I]ET-1 in both control and sl/sl rat hearts. In the control rat aorta but not in the sl/sl rat aorta, ET-3 induced endothelium-dependent relaxation. These results suggest that sl/sl rats do not have functional ETB receptors.

Animals↗
PubMed1996

Effects of chronic oral administration of levcromakalim on in vitro contractile responses of arterial smooth muscle.

It has been shown that oral administration of 0.038-0.15 mg/kg levcromakalim elicits a dose-related antihypertensive response in spontaneously hypertensive rats (Clapham et al., Arzneim. Forsch. 41 (1991) 385). In the present study, we examined the effects of long term administration of a high dose of levcromakalim on in vitro vascular contractility. Levcromakalim (2.25 mg/kg/day) was administered to the rats for 2 weeks and the thoracic aorta was then isolated. The levcromakalim treatment markedly reduced the relaxant effect of levcromakalim itself on norepinephrine-induced contraction. Relaxant effects of sodium nitroprusside and 8-bromo-cGMP were also attenuated by the levcromakalim treatment, although the relaxant effects of verapamil and forskolin were unchanged. The levcromakalim treatment decreased the threshold concentration for KCl and norepinephrine to induce contraction. The chronic levcromakalim treatment did not affect the cGMP production due to 3-isobutyl-1-methylxanthine and/or sodium nitroprusside. The aorta isolated from spontaneous hypertensive rats did not exhibit spontaneous activity in normal solution. After treatment with levcromakalim, however, the aorta showed spontaneous rhythmic contractions. Verapamil (10 microM) completely suppressed the spontaneous activity and decreased the basal tension below the original level. Similar to the effects of chronic treatment with levcromakalim, high-K+ solution (15.4 mM) augmented the contractile response to norepinephrine in the aorta of normotensive rats and induced rhythmic contractions in the aorta of spontaneously hypertensive rats. These results suggest that chronic treatment with a high dose of levcromakalim attenuates not only the effects of levcromakalim itself but also the cGMP-mediated relaxation, possibly by desensitizing the K+ channel.

Animals↗