BRL 38227 (levcromakalim)-induced hyperpolarization reduces the sensitivity to Ca2+ of contractile elements in canine coronary artery

Endothelium-Independent Relaxation of Vascular Smooth Muscle Induced by Persimmon-Derived Polyphenol Phytocomplex in Rats

The vasorelaxant effect of polyphenols is well known, and the mortality rate due to coronary artery disease is low in people who consume polyphenol-containing foods. We aimed to elucidate the mechanism by which polyphenols derived from persimmon juice (PJ) and persimmon leaves (PLs) induce vasorelaxation and suppress vasocontraction in the superior mesenteric arteries isolated from male Sprague Dawley rats. Vasocontraction was induced with 1 µM phenylephrine, and polyphenol-induced vasorelaxation was expressed as a percentage of the previous tone induced by phenylephrine. PJ powder (100 mg/L) induced higher levels of vasorelaxation (mean ± standard error of the mean, 88.6% ± 4.4%) than PLs powder (1 g/L; 72.0% ± 10.8%). Nitric oxide pathway inhibitors (NG-nitro-L-arginine methyl ester + carboxy-PTIO) did not affect persimmon-derived polyphenol-induced vasorelaxation, whereas potassium chloride, tetraethylammonium, and potassium-channel inhibitors did. Vasorelaxation was endothelium independent with both extracts. Phenylephrine-induced vasocontraction was suppressed by pretreatment with PJ and PLs powder, even when inositol triphosphate-mediated Ca²⁺ release and extracellular Ca²⁺ influx were inhibited. These results suggest that persimmon-derived polyphenol phytocomplex cause vasorelaxation and inhibit vasocontraction through hyperpolarization of smooth muscle cells. Persimmon-derived polyphenols may be able to prevent cardiovascular diseases caused by abnormal contraction of blood vessels.

Introduction to ion channels and calcium signaling in the microcirculation

The microcirculation is the network of feed arteries, arterioles, capillaries and venules that supply and drain blood from every tissue and organ in the body. It is here that exchange of heat, oxygen, carbon dioxide, nutrients, hormones, water, cytokines, and immune cells takes place; essential functions necessary to maintenance of homeostasis throughout the life span. This chapter will outline the structure and function of each microvascular segment highlighting the critical roles played by ion channels in the microcirculation. Feed arteries upstream from the true microcirculation and arterioles within the microcirculation contribute to systemic vascular resistance and blood pressure control. They also control total blood flow to the downstream microcirculation with arterioles being responsible for distribution of blood flow within a tissue or organ dependent on the metabolic needs of the tissue. Terminal arterioles control blood flow and blood pressure to capillary units, the primary site of diffusional exchange between blood and tissues due to their large surface area. Venules collect blood from capillaries and are important sites for fluid exchange and immune cell trafficking. Ion channels in microvascular smooth muscle cells, endothelial cells and pericytes importantly contribute to all of these functions through generation of intracellular Ca²⁺ and membrane potential signals in these cells.

Extracranial activation of ATP-sensitive potassium channels induces vasodilation without nociceptive effects

Introduction

Levcromakalim opens ATP-sensitive potassium channels (KATP channel) and induces head pain in healthy volunteers and migraine headache in migraine patients, but no pain in other parts of the body. KATP channels are expressed in C- and Aδ-fibers, and these channels might directly activate nociceptors and thereby evoke pain in humans.

Methods

To assess the local effect of KATP channel opening in trigeminal and extra-trigeminal regions, we performed a crossover, double-blind, placebo-controlled study in healthy volunteers. Participants received intradermal and intramuscular injections of levcromakalim and placebo in the forehead and the forearms.

Results

Intradermal and intramuscular injections of levcromakalim did not evoke more pain compared to placebo in the forehead ( p > 0.05) and the forearms ( p > 0.05). Intradermal injection of levcromakalim caused more flare ( p < 0.001 ), skin temperature increase ( p < 0.001), and skin blood flow increase ( p < 0.001) compared to placebo in the forehead and the forearms.

Conclusion

These findings suggest that it is unlikely that levcromakalim induces head pain by direct activation of peripheral neurons.

KV channels and the regulation of vascular smooth muscle tone

VSMCs in resistance arteries and arterioles express a diverse array of K_V channels with members of the K_V 1, K_V 2 and K_V 7 families being particularly important. Members of the K_V channel family: (i) are highly expressed in VSMCs; (ii) are active at the resting membrane potential of VSMCs in vivo (-45 to -30 mV); (iii) contribute to the negative feedback regulation of VSMC membrane potential and myogenic tone; (iv) are activated by cAMP-related vasodilators, hydrogen sulfide and hydrogen peroxide; (v) are inhibited by increases in intracellular Ca²⁺ and vasoconstrictors that signal through G_q -coupled receptors; (vi) are involved in the proliferative phenotype of VSMCs; and (vii) are modulated by diseases such as hypertension, obesity, the metabolic syndrome and diabetes. Thus, K_V channels participate in every aspect of the regulation of VSMC function in both health and disease.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

Gynura procumbens Merr. decreases blood pressure in rats by vasodilatation via inhibition of calcium channels

INTRODUCTION

Gynura procumbens has been shown to decrease blood pressure via inhibition of the angiotensinconverting enzyme. However, other mechanisms that may contribute to the hypotensive effect have not been studied.

OBJECTIVES

To investigate the cardiovascular effects of a butanolic fraction of Gynura procumbens in rats.

METHODS

Anaesthetized rats were given intravenous bolus injections of butanolic fraction at doses of 2.5-20 mg/kg in vivo. The effect of butanolic fraction on vascular reactivity was recorded in isolated rat aortic rings in vitro.

RESULTS

Intravenous administrations of butanolic fraction elicited significant (p < 0.001) and dose-dependent decreases in the mean arterial pressure. However, a significant (p < 0.05) decrease in the heart rate was observed only at the higher doses (10 and 20 mg/kg). In isolated preparations of rat aortic rings, phenylephrine (1 × 10⁻⁶ M)- or potassium chloride (8 × 10⁻² M)-precontracted endothelium-intact and -denuded tissue; butanolic fraction (1 × 10⁻⁶ - 1 × 10⁻¹ g/ml) induced similar concentration-dependent relaxation of the vessels. In the presence of 2.5 × 10⁻³ and 5.0 × 10⁻³ g/ml butanolic fraction, the contractions induced by phenylephrine (1 × 10⁻⁹-3 × 10⁻⁵ M) and potassium chloride (1 × 10⁻² - 8 × 10⁻² M) were significantly antagonized. The calcium-induced vasocontractions (1 × 10⁻⁴-1 × 10⁻²M) were antagonized by butanolic fraction concentration-dependently in calcium-free and high potassium (6×10⁻² M) medium, as well as in calcium- and potassium-free medium containing 1×10⁻⁶ M phenylephrine. However, the contractions induced by noradrenaline (1 × 10⁻⁶ M) and caffeine (4.5 × 10⁻² M) were not affected by butanolic fraction.

CONCLUSION

Butanolic fraction contains putative hypotensive compounds that appear to inhibit calcium influx via receptor-operated and/or voltage-dependent calcium channels to cause vasodilation and a consequent fall in blood pressure.

Endothelin-I and angiotensin II inhibit arterial voltage-gated K+ channels through different protein kinase C isoenzymes

AIMS

Voltage-gated K+ (Kv) channels of arterial smooth muscle (ASM) modulate arterial tone and are inhibited by vasoconstrictors through protein kinase C (PKC). We aimed to determine whether endothelin-1 (ET-1) and angiotensin II (AngII), which cause similar inhibition of Kv, use the same signalling pathway and PKC isoenzyme to exert their effects on Kv and to compare the involvement of PKC isoenzymes in contractile responses to these agents.

METHODS AND RESULTS

Kv currents recorded using the patch clamp technique with freshly isolated rat mesenteric ASM cells were inhibited by ET-1 or AngII. Inclusion of a PKCepsilon inhibitor peptide in the intracellular solution substantially reduced inhibition by AngII, but did not affect that by ET-1. Kv inhibition by ET-1 was reduced by the conventional PKC inhibitor Gö 6976 but not by the PKCbeta inhibitor LY333531. Selective peptide inhibitors of PKCalpha and PKCepsilon were linked to a Tat carrier peptide to make them membrane permeable and used to show that inhibition of PKCalpha prevented ET-1 inhibition of Kv current, but did not affect that by AngII. In contrast, inhibition of PKCepsilon prevented Kv inhibition by AngII but not by ET-1. The Tat-linked inhibitor peptides were also used to investigate the involvement of PKCalpha and PKCepsilon in the contractile responses of mesenteric arterial rings, showing that ET-1 contractions were substantially reduced by inhibition of PKCalpha, but unaffected by inhibition of PKCepsilon. AngII contractions were unaffected by inhibition of PKCalpha but substantially reduced by inhibition of PKCepsilon.

CONCLUSION

ET-1 inhibits Kv channels of mesenteric ASM through activation of PKCalpha, while AngII does so through PKCepsilon. This implies that ET-1 and AngII target Kv channels of ASM through different pathways of PKC-interacting proteins, so each vasoconstrictor enables its distinct PKC isoenzyme to interact functionally with the Kv channel.

Cellular and molecular mechanisms regulating vascular tone. Part 2: regulatory mechanisms modulating Ca2+ mobilization and/or myofilament Ca2+ sensitivity in vascular smooth muscle cells

Understanding the physiological mechanisms regulating vascular tone would lead to better circulatory management during general anesthesia. This two-part review provides an overview of current knowledge about the cellular and molecular mechanisms regulating the contractile state of vascular smooth muscle cells (i.e., vascular tone). The first part reviews basic mechanisms controlling the cytosolic Ca2+ concentration in vascular smooth muscle cells, and the Ca2+-dependent regulation of vascular tone. This second part reviews the regulatory mechanisms modulating Ca2+ mobilization and/or myofilament Ca2+ sensitivity in vascular smooth muscle cells-including Rho/Rho kinase, protein kinase C, arachidonic acid, Ca2+/calmodulin-dependent protein kinase II, caldesmon, calponin, mitogen-activated protein kinases, tyrosine kinases, cyclic nucleotides, Cl- channels, and K+ channels.

K(ATP) channel therapeutics at the bedside

The family of potassium channel openers regroups drugs that share the property of activating adenosine triphosphate-sensitive potassium (K(ATP)) channels, metabolic sensors responsible for adjusting membrane potential-dependent functions to match cellular energetic demands. K(ATP) channels, widely represented in metabolically-active tissue, are heteromultimers composed of an inwardly rectifying potassium channel pore and a regulatory sulfonylurea receptor subunit, the site of action of potassium channel opening drugs that promote channel activity by antagonizing ATP-induced pore inhibition. The activity of K(ATP) channels is critical in the cardiovascular adaptive response to stress, maintenance of neuronal electrical stability, and hormonal homeostasis. Thereby, K(ATP) channel openers have a unique therapeutic spectrum, ranging from applications in myopreservation and vasodilatation in patients with heart or vascular disease to potential clinical use as bronchodilators, bladder relaxants, islet cell protector, antiepileptics and promoters of hair growth. While the current experience in practice with potassium channel openers remains limited, multitude of ongoing investigations aims at defining the benefit of this emerging family of therapeutics in diverse disease conditions associated with metabolic distress.

Potassium channels in the peripheral microcirculation

Vascular smooth muscle (VSM) cells, endothelial cells (EC), and pericytes that form the walls of vessels in the microcirculation express a diverse array of ion channels that play an important role in the function of these cells and the microcirculation in both health and disease. This brief review focuses on the K+ channels expressed in smooth muscle and endothelial cells in arterioles. Microvascular VSM cells express at least four different classes of K+ channels, including inward-rectifier K+ channels (Kin), ATP-sensitive K+ channels (KATP), voltage-gated K+ channels (Kv), and large conductance Ca2+-activated K+ channels (BKCa). VSM KIR participate in dilation induced by elevated extracellular K+ and may also be activated by C-type natriuretic peptide, a putative endothelium-derived hyperpolarizing factor (EDHF). Vasodilators acting through cAMP or cGMP signaling pathways in VSM may open KATP, Kv, and BKCa, causing membrane hyperpolarization and vasodilation. VSMBKc. may also be activated by epoxides of arachidonic acid (EETs) identified as EDHF in some systems. Conversely, vasoconstrictors may close KATP, Kv, and BKCa through protein kinase C, Rho-kinase, or c-Src pathways and contribute to VSM depolarization and vasoconstriction. At the same time Kv and BKCa act in a negative feedback manner to limit depolarization and prevent vasospasm. Microvascular EC express at least 5 classes of K+ channels, including small (sKCa) and intermediate(IKCa) conductance Ca2+-activated K+ channels, Kin, KATP, and Kv. Both sK and IK are opened by endothelium-dependent vasodilators that increase EC intracellular Ca2+ to cause membrane hyper-polarization that may be conducted through myoendothelial gap junctions to hyperpolarize and relax arteriolar VSM. KIR may serve to amplify sKCa- and IKCa-induced hyperpolarization and allow active transmission of hyperpolarization along EC through gap junctions. EC KIR channels may also be opened by elevated extracellular K+ and participate in K+-induced vasodilation. EC KATP channels may be activated by vasodilators as in VSM. Kv channels may provide a negative feedback mechanism to limit depolarization in some endothelial cells.

Potassium channel openers: therapeutic potential in cardiology and medicine

Potassium (K(+)) channel openers (KCOs) define a class of chemically diverse agents that share a common molecular target, the metabolism-regulated ATP-sensitive K(+) (K(ATP)) channel. In view of the unique function that K(ATP) channels play in the maintenance of cellular homeostasis, this novel class of ion channel modulators adds to existent pharmacotherapy with potential in promoting cellular protection under conditions of metabolic stress. Indeed, experimental studies have demonstrated broad therapeutic potential for KCOs, including roles as cardioprotective agents, vasodilators, bronchodilators, bladder relaxants, anti-epileptics, insulin secretagogues and promoters of hair growth. However, clinical experience with these drugs is limited and their place in patient management needs to be fully established.

Effects of KRN2391 on ionic currents in rabbit femoral arterial myocytes

The effects of KRN2391, an ATP-sensitive K+ channel opener (KCO) which also acts as a nitrate, on ionic membrane currents in rabbit femoral arterial myocytes were examined. Under whole-cell clamp conditions where cells were superfused with physiological salts solution containing 5.9 mM K+, KRN2391 elicited an outward current at a holding potential of -30 mV. KRN2391-induced current had a reversal potential of -78 mV and was abolished by glibenclamide (glib). KRN2391 was approximately 25 times more potent than nicorandil to activate an ATP-sensitive K+ current (I:(KATP)). On the other hand, 10 microM KRN2391 did not affect either voltage-dependent Ca(2+) or delayed rectifier K+ channel currents. In the inside-out patch configuration, KRN2391 activated 47 pS K+ channels in the presence of nucleotide diphosphates (NDPs) under the symmetrical 140 mM K+ conditions. Glib and intracellular ATP reversibly inhibited the activity of the 47 pS K+ channels. The 47 pS K+ channels activated by KRN2391 are similar in their conductance and other properties to NDP-sensitive K+ channels (K(NDP) channels) described in other smooth muscles and the cloned channels. KRN2391 is a potent activator of the 47 pS K+ channels and the activation can contribute to the KRN2391-induced vasodilation in arterial muscles.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Mechanisms of vasorelaxation induced by eicosapentaenoic acid (20:5n-3) in WKY rat aorta

The vasorelaxant activity of eicosapentaenoic acid (EPA, 20:5n-3), the omega-3 polyunsaturated fatty acid, was investigated in isolated Wistar Kyoto (WKY) rat aortae by measuring isometric tension. Eicosapentaenoic acid (1 - 100 microM) relaxed rat aortae contracted with high K(+) (80 mM) or noradrenaline (NA, 1 microM) in a concentration-dependent manner. Contractions induced by Bay K 8644 or increasing concentrations of calcium were unaffected by EPA. The relaxant effect of EPA (3 - 100 microM) was significantly inhibited by indomethacin (10 microM), the cyclo-oxygenase inhibitor, but not by the nitric oxide (NO) synthesis inhibitor, N(omega)-nitro-L-arginine methyl ester hydrochloride (L-NAME, 100 microM). Removal of the endothelium did not alter EPA-induced relaxations. In Ca(2+)-free, EGTA 2 mM solution, EPA (10 - 30 microM significantly inhibited NA-sustained contractions. Incubation with EPA (5, 10 microM) diminished both NA-induced (1 microM) phasic and sustained contractions. The vasorelaxant effects of EPA (> or =30 microM) on NA-induced (1 microM) contractions were significantly inhibited by the K(+) channel blocker, glibenclamide (10 microM), but not tetraethylammonium (1 mM). Moreover, indomethacin and glibenclamide combined significantly inhibited EPA-induced (1 - 100 microM) responses. These results indicate EPA exerts its endothelium-independent vasorelaxant effects in WKY rat aortae through production of prostanoids which activate K(+)(ATP) channels. Inhibition of Ca(2+) mobilization from intracellular pools and influx through the non-L-type, but not the L-type, Ca(2+) channel are also possible mechanisms action of EPA's.

Hyperpolarization-induced dilatation of submucosal arterioles in the guinea-pig ileum

1. The effects of inhibition of acetylcholine (ACh)-induced hyperpolarization on dilatation of submucosal arterioles were investigated in the guinea-pig ileum. 2. In smooth muscles of the arterioles depolarized by Ba(2+) (0.5 mM) to about -40 mV, ACh (3 microM) repolarized the membrane to about -65 mV (hyperpolarization), irrespective of the absence or presence of L-N(omega)-nitroarginine (L-NOARG, 0.1 mM) and diclofenac (1 microM), and increased the diameter (dilatation). 3. Combined application of charybdotoxin (CTX, 50 nM) and apamin (0.1 microM), inhibitors of some types of K(+)-channels, abolished the ACh-induced hyperpolarization and dilatation. 4. 18 beta-Glycerrhetinic acid (18 beta-GA, 30 microM), a known inhibitor of gap junctions, depolarized the membrane to about -36 mV, either in the absence or in the presence of Ba(2+), with no associated contraction of the arterioles. In the presence of 18 beta-GA, ACh-induced hyperpolarization was abolished, however the dilatation was inhibited only partially, with associated inhibition of constriction produced by Ba(2+) and NA. 5. 18 beta-GA inhibited the dilatation produced by sodium nitroprusside, an NO donor. 6. The ACh-induced hyperpolarization and dilatation were abolished in the presence of 2-aminoethoxydiphenyl borate (30 microM), an inhibitory modulator of inositol trisphosphate receptor-mediated Ca(2+) release from intracellular stores. 7. It is concluded that in submucosal arterioles, hyperpolarizations produced by ACh have causal relationship to the arteriolar dilatation. 18 beta-GA did not induce parallel relationship between hyperpolarization and dilatation produced by ACh. 18 beta-GA may have unidentified inhibitory effects on agonist-mediated actions, in addition to the inhibition of gap junctions.

Altered beta-adrenoceptor-mediated responses in the gastric smooth muscle of hypertensive rats

Effects of isoproterenol on contraction and membrane potential of gastric smooth muscle were studied in stroke prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar Kyoto rats (WKY). Circular muscle preparation from the gastric fundus developed tonic contraction by re administration of Ca2+ to a nominally Ca2+-free solution. The contraction was inhibited by nifedipine or nicardipine. Isoproterenol induced relaxation when it was applied to the Ca2+-induced contraction. The amplitude of isoproterenol induced relaxation was concentration-dependent. Propranolol 10(-6) M abolished the relaxation induced by isoproterenol 10(-7) M. In the preparation from SHRSP, the amplitude of isoproterenol induced relaxation was smaller than that from WKY between 3 x 10(-9) and 10(-7) M. Forskolin, an adenylate cyclase activator, induced concentration-dependent relaxation. There was no difference in the relaxation induced by forskolin between preparations from WKY and SHRSP. Dibutilyl cyclic AMP, a membrane permeable analogue of cyclic AMP, also induced similar relaxation in preparations from WKY and SHRSP. Resting membrane potential of smooth muscle cell was not different between preparations from WKY and SHRSP. Isoproterenol hyperpolarized the membrane concentration-dependently. Isoproterenol-induced hyperpolarization in the preparation from SHRSP was smaller than that from WKY between 10(-8) and 10(-6) M. When the membrane was depolarized by Tyrode's solution containing 40 mM K+, isoproterenol-induced hyperpolarization was almost abolished. In this condition, the isoproterenol-induced relaxation was inhibited partly, however, there was no difference in the amplitude of relaxation between preparations from WKY and SHRSP. Therefore, isoproterenol-induced hyperpolarization contributed at least partly to the relaxation. Forskolin hyperpolarized the membrane by the same amplitude in the preparations from WKY and SHRSP. These results indicate that a decrease in hyperpolarization may contribute to the decreased relaxation by isoproterenol in the preparation from SHRSP.

Ion channels and vascular tone

Ion channels in the plasma membrane of vascular muscle cells that form the walls of resistance arteries and arterioles play a central role in the regulation of vascular tone. Current evidence indicates that vascular smooth muscle cells express at least 4 different types of K(+) channels, 1 to 2 types of voltage-gated Ca(2+) channels, >/=2 types of Cl(-) channels, store-operated Ca(+) (SOC) channels, and stretch-activated cation (SAC) channels in their plasma membranes, all of which may be involved in the regulation of vascular tone. Calcium influx through voltage-gated Ca(2+), SOC, and SAC channels provides a major source of activator Ca(2+) used by resistance arteries and arterioles. In addition, K(+) and Cl(-) channels and the Ca(2+) channels mentioned previously all are involved in the determination of the membrane potential of these cells. Membrane potential is a key variable that not only regulates Ca(+2) influx through voltage-gated Ca(2+) channels, but also influences release of Ca(2+) from internal stores and Ca(2+)- sensitivity of the contractile apparatus. By controlling Ca(2+) delivery and membrane potential, ion channels are involved in all aspects of the generation and regulation of vascular tone.

Levcromakalim decreases cytoplasmic Ca2+ and vascular tone in basilar artery of SAH model dogs

We investigated the effects of levcromakalim, a K+ channel opener, on [Ca2+]i and the contractile force of basilar arteries obtained from normal dogs and subarachnoid hemorrhage (SAH) dogs. The responsiveness to serotonin was increased more in the SAH group than in the control group. Levcromakalim decreased the resting [Ca2+]i and force more profoundly than did a Ca2+ channel blocker, nicardipine, and these effects were more prominent in the SAH group than in the control group. Levcromakalim diminished the increases in [Ca2+]i and contractile force induced by serotonin more profoundly than nicardipine did, and these effects were equal in both groups. The effects of levcromakalim were not inhibited by iberiotoxin but were antagonized completely by glibenclamide. These results suggest that levcromakalim-induced opening of adenosine triphosphate (ATP)-sensitive K+ (K(ATP)) channels reduces [Ca2+]i more effectively than does nicardipine and that levcromakalim exerts the vasodilator effects under the condition in which large conductance Ca2+-activated K+ (BK) channels are blocked with iberiotoxin. Consequently, K+ channel openers like levcromakalim may be useful drug candidates to treat delayed cerebral vasospasm after SAH.

Levcromakalim decreases vascular tone, cytoplasmic Ca2+ and Ca2+ sensitivity in canine basilar artery

The involvement of large conductance Ca(2+)-activated K+ channels (BK) and ATP-sensitive K+ (KATP) channels in the regulation of canine basilar arterial tone was estimated in the presence of the agonist and blockers of these channels, by simultaneously measuring the changes in intracellular Ca2+ concentration ([Ca2+]i) with the fura-2 microfluorimetric method. In the resting condition, levcromakalim reduced [Ca2+]i and vascular tone. Levcromakalim suppressed the serotonin-induced increases in [Ca2+]i and force of contraction, the maximum effects of which were much greater than those of nicardipine. The inhibitory effects of levcromakalim were blocked by glibenclamide but not by tetraethylammonium (TEA) or iberiotoxin (IbTX). In the presence of levcromakalim, the curve relating [Ca2+]i with force in the presence of serotonin at different extracellular Ca2+ concentration ([Ca2+]o) was shifted down- and right-ward compared with that in the absence of levcromakalim, suggesting that levcromakalim may reduce the Ca(2+)-sensitivity of the contractile proteins. Thus, levcromakalim may be a good candidate to suppress delayed cerebral vasospasm after subarachnoid hemorrhage.

Differential sensitivity to ATP-sensitive potassium channel openers of norepinephrine-induced contraction of guinea pig and rat aorta

Vasorelaxant effects of ATP-sensitive potassium (K(ATP)) channel openers were examined on the tonic phase of vascular contraction induced by norepinephrine (NE) in guinea pig and rat aorta. K(ATP) channel openers, NIP-121 and cromakalim, produced glibenclamide-sensitive and concentration-dependent relaxations in guinea pig and rat aorta preconstricted with NE. However, the vascular relaxations induced by both K(ATP) channel openers were less pronounced in guinea pig aorta than in rat aorta. D-cis-Diltiazem, at the concentration up to 10(-5) M, did not appreciably inhibit the NE-induced contraction of guinea pig aorta, whereas the compound almost completely inhibited the NE-induced contraction of rat aorta at the same concentration. By contrast, sodium nitroprusside relaxed the NE-induced contractions in both guinea pig and rat aorta with similar potencies. These findings suggest that vasorelaxant effects of K(ATP) channel openers on the NE-induced sustained contraction in guinea pig aorta is not attributable to the subsequent inhibition of Ca2+ influx through L-type voltage-gated Ca2+ channels. Lower sensitivity of guinea pig aortic smooth muscle to K(ATP) channel openers is most likely due to the low dependence of NE-induced contraction on the Ca2+ influx in this vascular smooth muscle.

Effects of levcromakalim and nucleoside diphosphates on glibenclamide-sensitive K+ channels in pig urethral myocytes

1. Effects of levcromakalim and nucleoside diphosphates (NDPs) on both membrane currents and unitary currents in pig proximal urethra were investigated by use of patch clamp techniques (conventional whole-cell configuration, nystatin perforated patch, cell-attached configuration and inside-out patches). 2. Levcromakalim produced a concentration-dependent outward current at a holding potential of -50 mV. The peak current amplitude showed little variation when measured by either conventional whole-cell or nystatin perforated patch configurations. 3. In conventional whole-cell configuration, the levcromakalim (100 microM)-induced outward current decayed by about 90% in 18 min. In contrast, with the nystatin perforated patch, approximately 86% of the levcromakalim-induced outward current still remained after 18 min. 4. The peak amplitude of the levcromakalim (100 microM)-induced outward membrane current recorded by the conventional whole-cell configuration was greatly reduced by inclusion of 5 mM EDTA in the pipette. The much smaller but significant outward membrane current remaining was abolished by glibenclamide. 5. In conventional whole-cell recordings, inclusion of an NDP in the pipette solution induced a small outward current which slowly reached a maximal amplitude (in 2 to 10 min) and was suppressed by glibenclamide. Addition of 100 microM levcromakalim after the NDP-induced current had peaked activated a further outward current which was larger than that recorded in the absence of NDPs. Approximately 50% of this current still remained at 18 min, even when conventional whole-cell configuration was used. 6. In the cell-attached mode in symmetrical 140 mM K+ conditions, glibenclamide inhibited the 100 microM levcromakalim-activated 43 pS K+ channel in a concentration-dependent manner, showing an inhibitory dissociation constant (Ki) of approximately 520 nM. 7. In inside-out patches in which the glibenclamide-sensitive K+ channel had run down after exposure to levcromakalim, both uridine 5'-diphosphate (UDP) and MgATP were capable of reactivating the channel. Further application of Mg2+ to the UDP-reactivated K+ channels enhanced the channel activity reversibly. 8. In inside-out patches UDP was capable of activating the glibenclamide-sensitive K+ channel without levcromakalim, providing that there was free Mg2+ present (either UDP in 5 mM EGTA or UDP in 5 mM EDTA with Mg2+). Additional application of levcromakalim caused a further reversible activation of channel opening. 9. In the presence of levcromakalim, application of adenosine 5'-triphosphate (ATP) to the inner surface of the membrane patch inhibited UDP-reactivated channel opening in a concentration-dependent manner. 10. Addition of an untreated cytosolic extract of pig proximal urethra reactivated the glibenclamide-sensitive K+ channel in the presence of 100 microM levcromakalim in inside-out patches. 11. These results demonstrate the presence in the pig proximal urethra of a glibenclamide-sensitive K+ channel that is blocked by intracellular ATP and can be activated by levcromakalim. Intracellular UDP can reactivate the channel after rundown. Additionally, intracellular Mg2+ may play an important role in regulating the channel activity.

Vascular endothelium: vasoactive mediators

In most blood vessels, the endothelium generates both vasodilator and growth-stabilizing mediators under normal physiological circumstances. The vasodilator influence of the endothelium modulates the vasoconstriction induced by adrenergic nerves, bloodborne substances, and local autacoids. Nitric oxide (NO) is a major endothelium-derived vasodilator, along with prostacyclin. A third substance called endothelium-derived hyperpolarizing factors (EDHF) mediates vasodilatation in certain conduit arteries and in most resistance vessels. EDHF may be a cytochrome P-450 metabolite of arachidonic acid. NO acts mostly through an elevation of cyclic guanosine monophosphate in vascular smooth muscle, whereas prostacyclin stimulates adenylate cyclase. The mode of action of EDHF involves the activation of K+ channels. The multiplicity of the factors released by the endothelium, as well as the complexity of the interactions among these factors and those with other nonendothelial mediators, determine the extent of vasomotor control exerted locally by the endothelium.

Effects of the potassium channel openers KRN4884 and levcromakalim on the contraction of rat aorta induced by A23187, compared with nifedipine

We examined the different vasodilatory effects of the K+ channel openers levcromakalim and 5-amino-N- [2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine (KRN4884), and the Ca2+ channel blocker nifedipine in the rat aorta. KRN 4884 (10(-10)-10(-5) M) and nifedipine (10(-10)-10(-5) M) produced concentration-dependent relaxation in the rat aorta precontracted by 25 mM KCl. The K+ channel blocker glibenclamide (1 microM) inhibited the relaxation induced by KRN4884 but did not influence nifedipine-induced relaxation. KRN4884 had almost no effect on contraction induced by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl. These results indicate that KRN4884 is a K+ channel opener. We investigated the relaxant effects of KRN4884 (10(-10)-10(-5) M), levcromakalim (10(-9)-10(-5) M) and nifedipine (10(-9)-10(-5) M) on A23187 (1 microM)-induced contraction. KRN4884 and levcromakalim had a potent relaxant effect but nifedipine only a weak effect on the smooth muscle contracted by A23187. Glibenclamide (1 microM) inhibited the relaxation induced by KRN4884 and levcromakalim, but did not influence the nifedipine-induced relaxation. KRN4884 (1 microM) produced a larger relaxation of A23187-induced contraction but had little effect on the increase in intracellular [Ca2+] induced by A23187. These results suggest that KRN4884 is a specific K+ channel opener and its vasodilating mechanisms involve not only deactivation of Ca2+ channels but also a decrease in the Ca2+ sensitivity of contractile elements.

Comparative relaxant effects of cromakalim and pinacidil on the tonic contraction of canine coronary artery induced by phorbol 12,13-dibutylate

1. Phorbol esters, activators of protein kinase C (PKC), have been widely used to investigate the role of PKC in the regulation of smooth muscle contraction. However, limited studies have so far been made of the sensitivity of the contraction induced by phorbol esters to relaxant drugs. We therefore examined the relaxant effects of the K+ channel openers, cromakalim and pinacidil, on the tonic contraction of canine isolated coronary artery rings induced by phorbol 12,13-dibutylate (PDBu). 2. In rings contracted with 10(-7) mol/L PDBu, cromakalim and pinacidil, as well as nifedipine, produced a concentration-dependent relaxation. At their maximum effects, both cromakalim and nifedipine caused a partial relaxation, whereas pinacidil produced nearly a full relaxation. 3. The relaxant effects of cromakalim and pinacidil were effectively antagonized by the ATP-sensitive K+ channel blocker, glibenclamide (10(-6) mol/L). 4. In the presence of nifedipine, high K+ was ineffective while PDBu (10(-7) mol/L) still induced a tonic contraction. This PDBu-induced contraction was inhibited by concentrations of cromakalim and pinacidil higher than those needed in the absence of nifedipine. 5. In rings partially depolarized with 35 mmol/L KCl, the ability of cromakalim to inhibit PDBu-induced contractions in the presence of nifedipine was completely abolished. Under the same conditions, the relaxant response to pinacidil, unlike cromakalim, was inhibited only partially. 6. These results suggest that cromakalim inhibits PDBu-induced contractions through an opening of K+ channels. Pinacidil does so by a mechanism shared with cromakalim as well as by another that is independent of this K+ channel opening action. These multiple modes of action may confer on pinacidil a vasorelaxant activity greater than that of cromakalim.

Endothelium-derived hyperpolarizing factor: a key mediator of the vasodilator action of bradykinin

Bradykinin causes vasodilatation by stimulating the production of vasodilator prostanoids and nitric oxide (NO). However, there is an additional component that is mediated by a diffusible endothelium-derived hyperpolarizing factor (EDHF). The non-selective inhibitor of arachidonic acid metabolism eicosatetraynoic acid inhibits the EDHF-mediated component of the relaxation to bradykinin. Therefore, EDHF may be an archidonic acid metabolite. The diffusible nature of EDHF has been disputed because of the inability to consistently detect the factor using perfusion bioassay techniques. However, administration of the acyltransferase inhibitor thimerosal facilitates the release of EDHF by endothelial cells in culture. Further studies are warranted to identify EDHF and explore further its functions in vasomotion.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

Divergent mechanisms of ATP-sensitive K+ channel-induced vasodilation in renal afferent and efferent arterioles. Evidence of L-type Ca2+ channel-dependent and -independent actions of pinacidil

K+ channel openers (PCOs), such as pinacidil, elicit vasodilation primarily by hyperpolarization-induced inhibition of L-type Ca2+ channel activation. The physiological role of other mechanisms suggested to contribute to PCO-induced vasodilation is not well established. In the renal microcirculation, L-type Ca2+ channels play a prominent role in vasoconstriction of the afferent arteriole (AA) but are absent or physiologically silent in the efferent arteriole (EA). Thus, L-type Ca2+ channel-dependent and -independent mechanisms can readily be distinguished in this model. In the present study, we found that pinacidil potently inhibited Bay K 8644-induced AA vasoconstriction. Pinacidil also preferentially inhibited angiotensin II-induced AA vasoconstriction (approximately ninefold greater potency than EA). These results are consistent with an AA effect of pinacidil on L-type Ca2+ channel activation. Unexpectedly, 10 mumol/L pinacidil inhibited AA and EA responses to similar extents (84 +/- 10% and 71 +/- 9%, respectively). In both AAs and EAs, glibenclamide restored normal reactivity, indicating an involvement of the ATP-sensitive K+ channels. In the EA, however, pretreatment with diltiazem did not alter the effects of pinacidil. Nevertheless, 45 mmol/L KCl reversed the EA actions of pinacidil, indicating an essential requirement for a normal K+ gradient. These findings suggest that the EA actions of pinacidil involve alterations in membrane potential but not changes in L-type Ca2+ channel activity. Overall, our findings do support the premise that L-type Ca2+ channel modulation is involved in PCO-induced vasodilation in the renal microcirculation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of KC399, a newly synthesized K+ channel opener, on acetylcholine-induced electrical and mechanical activities in rabbit tracheal smooth muscle

1. Effects of KC399, an opener of ATP-sensitive K+ channels were investigated on membrane potential, isometric force and intracellular Ca2+ ([Ca2+]i) mobilization induced by acetylcholine (ACh) in smooth muscle from the rabbit trachea. 2. In these smooth muscle cells, ACh (0.1 and 1 microM) depolarized the membrane in a concentration-dependent manner, KC399 (1-100 nM) hyperpolarized the membrane whether in the presence or absence of ACh. When the concentration of ACh was increased, the absolute values of the membrane potential induced by the maximum concentration of KC399 were less negative. 3. ACh (0.1 to 10 microM) concentration-dependently produced a phasic, followed by a tonic increase in both [Ca2+]i and force. KC399 (above 3 nM) lowered the resting [Ca2+]i and attenuated the ACh-induced phasic and tonic increases in [Ca2+]i and force, in a concentration-dependent manner. The magnitude of the inhibition was greater for the ACh-induced tonic responses than for the phasic ones. Nicardipine (0.3 microM), a blocker of the L-type Ca2+ channel, attenuated the ACh-induced tonic, but not phasic, increases in [Ca2+]i and force. KC399 further attenuated the ACh-induced tonic responses in the presence of nicardipine. 4. In beta-escin-skinned strips, Ca2+ (0.3-10 microM) produced a contraction in a concentration-dependent manner. KC399 (0.1 microM) had no effect on the Ca(2+)-force relationship in the presence or absence of ATP with GTP. However, at a very high concentration (1 microM), this agent slightly shifted the relationship to the right and attenuated the maximum Ca(2+)-induced contraction. 5. We conclude that, in rabbit tracheal smooth muscle, the membrane hyperpolarization induced byKC399 attenuates the ACh-induced tonic increase in [Ca2+], through an inhibition of nicardipinesensitive and -insensitive Ca2+-influxes, thus causing an inhibition of the ACh-induced tonic contraction. The ACh-induced phasic increase in [Ca2+]i and force are also inhibited, but less effectively than the tonic ones, suggesting that the action of such K+ channel openers on agonist-induced responses may be slightly different in tracheal from vascular smooth muscle.

The effect of K+ channel openers on submucosal gland function and epithelial transport of the ferret trachea, in vitro

The effects of three K+ channel openers on lysozyme output from submucosal gland serous cells and epithelial albumin transport following maintained submaximal stimulation by the secretagogues methacholine and phenylephrine were examined in the ferret trachea in vitro preparation. The K+ channel openers Ro 31-6930, 2-(6-cyano-2,2-dimethyl-2H-1-benzopyran-4-yl)-pyridine 1-oxide (10 nM-10 microM), levcromakalim, BRL38227 (10 nM-10 microM) and pinacidil (100 nM-10 microM) produced a concentration dependent inhibition of (20 microM) methacholine-induced lysozyme output, with pD2 values of 7.64, 7.72 and 7.28 respectively. Ro 31-6930 (10 nM-10 microM), levcromakalim (10 nM-10 microM) and pinacidil (1 nM-10 microM) also produced a concentration dependent inhibition of (100 microM) phenylephrine-induced lysozyme output, with pD2 values of 7.64, 6.55 and 9.16 respectively. Furthermore, glibenclamide (1 microM) produced a modest attenuation of the K+ channel opener effects on secretagogue-induced lysozyme output. All three K+ channel openers failed to produce any significant change in either methacholine or phenylephrine-induced albumin outputs. The K+ channel openers exerted marked effects on airway secretion processes, suggesting that these compounds may have an antisecretory effect. The relevance of the use of the K+ channel openers in airway disease remains to be determined.

Effects of semotiadil fumarate, a novel Ca2+ antagonist, on cytosolic Ca2+ level and force of contraction in porcine coronary arteries

1. The mechanisms of action of semotiadil fumarate, a novel Ca2+ antagonist, were examined by measuring the cytosolic Ca2+ level ([Ca2+]i) and force of contraction in porcine coronary arteries, and by determining [3H]-pyrilamine binding to bovine cerebellar membranes. 2. Semotiadil or verapamil (0.1 and 1 microM) inhibited both the high KCl-induced increases in [Ca2+]i and force in a concentration-dependent manner. 3. Histamine (30 microM) produced transient increases followed by sustained increases in [Ca2+]i and force, which were inhibited by semotiadil and verapamil (1 and 10 microM). The agents were different in that semotiadil reduced the maximum [Ca2+]i and force responses to histamine, but not pD2 values, whereas verapamil did reduce the pD2 values for histamine, but not the maximum responses. 4. Verapamil (10 microM), but not semotiadil, inhibited histamine-induced increases in [Ca2+]i and force in Ca(2+)-free solution. Neither semotiadil nor verapamil affected the increases in [Ca2+]i and force induced by caffeine. Semotiadil even at the higher concentration (10 microM) did not displace specific binding of [3H]-pyrilamine to bovine cerebellar membranes. 5. These results suggest that semotiadil inhibits both KCl- and histamine-induced contractions mainly by blocking voltage-dependent L-type Ca2+ channels.

Effect of Kil769, a novel K(+)-channel opener, on sensitivity to Ca2+ of contractile elements and inositol phosphate formation in porcine coronary artery

To determine whether Kil769, a novel K(+)-channel opener, acts intracellularly in vasorelaxation, we compared the effects of Kil769 on force of contraction, intracellular Ca2+ concentration ([Ca2+]i) and inositol phosphate (IP1) formation with those of Ca(2+)-channel blockers in isolated porcine coronary artery. Kil769 (10 microM) and verapamil (1 microM), which produced submaximal relaxation, reduced the increase in [Ca2+]i and force of contraction induced by 25 mM KCl. Verapamil reduced [Ca2+]i and the force of contraction to a similar extent but Kil769 reduced force of contraction more strongly than it did [Ca2+]i. Kil769 also inhibited U46619 (9,11-dideoxy-9 alpha,11 alpha-methano-epoxy-PGF2 alpha)-induced IP1 formation and glibenclamide blocked its inhibitory effect. These results suggest that the opening of K+ channels induced by Kil769 reduces the Ca2+ sensitivity of contractile elements and inositol phospholipid hydrolysis which is related to the Ca2+ release from intracellular storage.

The effects of potassium channel openers and blockers on the specific binding sites for [3H]glibenclamide in rat tissues

The effects of K+ channel openers (PCOs), NIP-121, levcromakalim and nicorandil, and the blockers of the specific binding sites for [3H]glibenclamide, ATP-sensitive K+ channel blocker, were investigated in rat brain and cardiac ventricle membrane preparations. When the microsomes were incubated with [3H]glibenclamide, the specific glibenclamide binding was fully inhibited by unlabeled glibenclamide (1 microM) and apamin (100 microM). However, the specific glibenclamide binding was not influenced by excess NIP-121, levcromakalim and nicorandil, although glibenclamide antagonized the increase in the 86Rb+ efflux by PCOs. On the other hand, the binding of [3H]glibenclamide after a long pre-incubation (60 min) at 37 degrees C with NIP-121 and levcromakalim at pharmacological effective concentrations (10 nM to 1 microM) was significantly influenced. Both PCOs partially reduced both Kd and Bmax values of the specific [3H]glibenclamide binding in a concentration-dependent manner that was not regulated by GTP gamma S. The dose-effect relationships for the Bmax's of NIP-121 and levcromakalim seemed similar to those for vasorelaxation. These findings indicate that the pharmacological effect of PCO may be caused by the binding to its own specific sites but not to the specific sulfonylurea sites. The binding of PCOs may inhibit, in a negative allosteric manner the binding of sulfonylureas.

The effects of a novel vasodilator, LP-805, on cytosolic Ca2+ concentrations and on tension in rabbit isolated femoral arteries

1. LP-805, 8-tert-butyl-6,7-dihydropyrrolo-[3,2-e]-5-methylpyrazolo- [1,5a]-pyrimidine-3-carbonitrile, is a newly synthesized potent vasodilator. To investigate the cellular mechanisms of vasorelaxation induced by LP-805, we simultaneously determined the effects of LP-805 on cytosolic Ca2+ concentrations ([Ca2+]i) and on tension of smooth muscle of rabbit femoral arterial strips, with or without the endothelium, using front-surface fluorometry and fura-2. 2. In the absence of the endothelium, LP-805, in a concentration-dependent manner, decreased [Ca2+]i and tension during the contraction induced by K(+)-depolarization, at relatively low concentrations ([K+]o < or = 30 mM). The decreases in [Ca2+]i and tension were fully antagonized by treatment with 2 x 10(-6) M glibenclamide. The [Ca2+]i-tension relationship in the LP-805-induced relaxation was similar to that of K(+)-depolarization-induced contractions. 3. LP-805, in a concentration-dependent manner (IC50 for inhibition of tension; 1.7 x 10(-6) M), decreased both [Ca2+]i and tension during the steady-state of contractions induced by 1 x 10(-7) M noradrenaline (NA) in the strips without the endothelium. Glibenclamide completely inhibited these reductions of [Ca2+]i and tension. At the steady-state of relaxation induced by LP-805 during NA-induced contraction, [Ca2+]i-tension relation was shifted to the left of that obtained with high K(+)-induced contraction. 4. NA induced transient increases in [Ca2+]i and tension in the absence of extracellular Ca2+. LP-805 (up to 3 x 10(-6) M) had no effect on these intracellular Ca2+ mobilisation and tension development induced by NA. 5. In strips with an intact endothelium, LP-805 decreased both [Ca2+]i and tension during contraction induced by 1 x 10(-7) M NA. The concentration-response curve for inhibition of [Ca2+]i and tension obtained in the presence of the endothelium was shifted to the left from that obtained in the absence of endothelium. IC50 for the inhibition of tension obtained in the strips with the endothelium was 4.0 x 10(-7) M. Treatment with 1 x 10(-4) M NG-nitro-L-arginine (L-NOARG) attenuated reductions of both [Ca2+]i and tension induced by LP-805 and the concentration-response curve shifted to the right and overlapped that obtained in the absence of the endothelium. Treatment with glibenclamide almost fully overcame the reduction of [Ca2+]i induced by LP-805, while the reversion of tension was 50% at most. 6. In the presence of the endothelium with L-NOARG, LP-805 reduced the tension to the extent of that expected from the reduction of [Ca2'ji, as based on the [Ca2+]i-tension relationship obtained with LP-805 in the absence of endothelium. On the contrary, in the presence of the endothelium without L-NOARG, LP-805 induced a greater reduction of tension than expected from the reduction of [Ca2+J1.This effect became more apparent after treatment with glibenclamide.7. These results suggest that: (1) LP-805 relaxes smooth muscle mainly by activating ATP-sensitive K+channels of smooth muscle and by releasing endothelium-derived relaxing factor (EDRF). (2) Activation of ATP-sensitive K+ channels decrease [Ca2+]i and thereby relax smooth muscle with no effect on Ca2"-sensitivity of the contractile apparatus of smooth muscle or on the agonist-induced Ca2"-release process. (3) EDRF induced by LP-805 relaxes smooth muscle not only by decreasing [Ca2+]i but also decreasing Ca2+-sensitivity of the contractile apparatus of smooth muscle. In the presence of an intact endothelium, a decrease in Ca2+-sensitivity of the contractile apparatus may play an important role in LP-805-induced relaxation.

Potassium channel openers and other regulators of KATP channels

1. Interest in ATP-sensitive K (KATP) channels first arose when it was shown that hypoglycaemic sulphonylureas, such as glibenclamide, closed these channels in pancreatic beta-cells to cause insulin release. The demonstration that certain smooth muscle relaxants (K channel openers) may exert their actions through opening a similar channel in vascular smooth muscle fueled further investigation of these channels and their physiological role in a variety of tissue types, including various types of smooth muscle, cardiac and skeletal muscle and neural and endocrine organ function. 2. The K channel openers have a variety of potential therapeutic applications, including disorders of smooth muscle hyperreactivity, such as hypertension, and a great deal of research has focused on this field. More recently, attention has turned to the cardiac actions of these compounds and this area is discussed in detail. One of the current problems is the lack of selectivity of KATP channel regulators. However, there have been a number of recent encouraging reports suggesting that, under certain pathophysiological conditions, the action of the K channel openers may be enhanced, conferring upon them some degree of selectivity. 3. A number of endogenous regulators of these channels have been identified, particularly in the category of endogenous openers of these channels. At present though, the physiological role of these channels and the endogenous regulators identified, is unclear. 4. It is evident that, although advances have been made, much work is still required to increase our understanding and ultimately to allow selective pharmacological manipulation of these channels to become a therapeutic reality.

Levcromakalim-induced modulation of membrane potassium currents, intracellular calcium and mechanical activity in rat mesenteric artery

In freshly-dispersed cells from rat mesenteric artery, levcromakalim (1 and 10 microM) induced a non-inactivating potassium current (IKCO), an event which was associated with increased current noise. IKCO was fully inhibited in the presence of 10 microM glibenclamide. Stationary fluctuation analysis of the current noise associated with IKCO induced by levcromakalim at a holding potential of -10 mV indicated that the unitary conductance of the underlying K-channels was 10.2 pS at 0 mV under the quasi-physiological conditions of the experiment. In isolated arterioles both levcromakalim (10 nM-10 microM) and nifedipine (10 nM-10 microM) each elicited full, concentration-dependent, parallel reductions of the increases in [Ca2+]i (assessed using fura-2) and tension induced by 10 microM noradrenaline. However, the effects of both drugs on KCl-induced increases in tension and in [Ca2+]i, did not follow a simple relationship. Levcromakalim relaxed KCl- and noradrenaline-induced sustained contractions with a similar potency. This was in contrast to nifedipine which was approximately 20 times more potent against KCl-induced contractions. It is concluded that levcromakalim relaxes rat mesenteric arterioles primarily by the opening of a small conductance, glibenclamide-sensitive K-channel. An additional action of levcromakalim is suggested by its relative inability to suppress the increase in [Ca2+]i produced by 30 mM K(+)-PSS.

A possibility that the ATP-sensitive potassium channel in coronary artery has a high-affinity internal binding site for tetraalkylammonium

The functionally responsible sites for the blocking action of tetraalkylammonium ions (TAAs) in ATP-sensitive K+ (KATP) channels opened by levcromakalim were estimated in canine coronary artery. Tetraethylammonium (TEA) and tetrabutylammonium (TBA) inhibited the levcromakalim-induced relaxation in a noncompetitive manner. Analyses of the noncompetitive antagonism revealed that the binding constant of TBA was about 900 times lower than that of TEA, although the reported affinity of TBA for the internal binding site in various K+ channels was only 10 times higher than that of TEA. TBA is much more lipid-soluble and permeable through membranes than TEA. Thus, TBA blocks KATP channels by binding to a possible high-affinity internal site for TAAs, whereas TEA seems to bind to the external site.

Do the K+ channel openers relax smooth muscle by opening K+ channels?

During the past decade, a group of chemically heterogeneous compounds known as the K+ channel openers has emerged. These compounds open a certain class of K+ channels (ATP-sensitive K+ channels) in the sarcolemma of vascular smooth muscle cells, which leads to hyperpolarization of the cell membrane and relaxation of the tissue. The mechanisms by which hyperpolarization affects smooth muscle contraction and contractility can thus be examined. Hyperpolarization induced by these K+ channel openers prevents Ca2+ entry through voltage-operated Ca2+ channels. Surprisingly, and by mechanisms not yet defined, hyperpolarization of the cell also reduces agonist-induced accumulation of inositol 1,4,5-trisphosphate (and consequently, Ca2+ mobilization from intracellular stores), and the Ca2+ sensitivity of the contractile apparatus. In addition, recent evidence reviewed here by Ulrich Quast suggests that the K+ channel openers possess further mechanisms of vasorelaxation not linked to the opening of plasmalemmal K+ channels.