Antagonism by lipophilic quaternary ions of the K+ channel opener, levcromakalim, in vascular smooth muscle

Vasorelaxation and hyperpolarisation of rat small mesenteric artery by the quaternary anion tetraphenylboron

This study characterises the vasorelaxation and hyperpolarisation effects of the negatively charged quaternary compound tetraphenylboron (TPB) in the rat small mesenteric artery. Segments of rat small mesenteric artery were mounted in a myograph and vessel tone and membrane potential were measured simultaneously. In vessels pre-contracted with vasopressin (0.3-0.6 nM), U46619 (30-90 nM) or methoxamine (0.3-3 microM), TPB (0.1-100 microM) produced a marked endothelium-independent relaxation. However, vasorelaxation responses to TPB were abolished in tissues pre-contracted with K(+) (50 mM), and significantly inhibited by glibenclamide (glib, 10 microM). In the absence of tone, TPB (1-30 microM) caused a concentration-dependent membrane hyperpolarisation of rat mesenteric artery smooth muscle cells, which was not dependent on the endothelium, but sensitive to glibenclamide (10 microM). In methoxamine (0.3-3 microM) pre-contracted vessels, the relaxation response was associated with a marked hyperpolarisation, which was also sensitive to glibenclamide (10 microM), further inhibited by a combination of K(+) channel blockers (glib [10 microM], charybdotoxin [100 nM], apamin [100 nM], 4-aminopyridine [1 mM] and Ba(2+) [30 microM]) and abolished by 50 mM K(+). The results of this study show that TPB causes a vasorelaxation and hyperpolarisation response in the rat small mesenteric artery through a direct action on the vascular smooth muscle. TPB exerts its effects partially via the activation of K(ATP) channels, but also by another mechanism involving K(+)-dependent hyperpolarisation.

Mitochondrial potassium transport: the K(+) cycle

Potassium transport plays three distinct roles in mitochondria. Volume homeostasis to prevent excess matrix swelling is a housekeeping function that is essential for maintaining the structural integrity of the organelle. This function is mediated by the K(+)/H(+) antiporter and was first proposed by Peter Mitchell. Volume homeostasis to prevent excess matrix contraction is a recently discovered function that maintains a fully expanded matrix when diffusive K(+) influx declines due to membrane depolarization caused by high rates of electron transport. Maintaining matrix volume under these conditions is important because matrix contraction inhibits electron transport and also perturbs the structure-function of the intermembrane space (IMS). This volume regulation is mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). Cell signaling functions to protect the cell from ischemia-reperfusion injury and also to trigger transcription of genes required for cell growth. This function depends on the ability of mitoK(ATP) opening to trigger increased mitochondrial production of reactive oxygen species (ROS). This review discusses the properties of the mitochondrial K(+) cycle that help to understand the basis of these diverse effects.

Functional and electrophysiological effects of a novel imidazoline-based K(ATP) channel blocker, IMID-4F

1. The functional and electrophysiological effects of IMID-4F (2-[N-(2, 6-dichlorophenyl)-N-(4-flurorobenzyl)amino]imidazoline), a fluoro-benzyl derivative of clonidine, on vascular K(ATP) channels were investigated. In pig coronary artery, IMID-4F inhibited the vasorelaxation response to the K(ATP) channel opener levcromakalim with a pK(B) value of approximately 7.1. IMID-4F (30 microM) did not affect the vasorelaxation response to sodium nitroprusside (SNP). 2. In rat mesenteric artery smooth muscle cells IMID-4F (1 - 10 microM) caused a concentration-dependent depolarization of membrane potential. IMID-4F (10 microM) abolished the hyperpolarizing effects of levcromakalim (10 microM). 3. In patch clamp experiments using rat mesenteric artery smooth muscle cells, K(ATP) channel currents induced by levcromakalim (10 microM) were inhibited by IMID-4F (0.3 - 3 microM) in a concentration-dependent manner. The calculated IC(50) for IMID-4F inhibiting K(ATP) channel current was approximately 0.8 microM. 4. Radioligand binding studies using bovine aortic smooth muscle cell membranes showed that IMID-4F (30 microM) did not displace binding to the K(ATP) channel opener [(3)H]-P1075. However, both levcromakalim (10 microM) and glibenclamide (10 microM) caused significant displacement of [(3)H]-P1075. 5. These studies show that the imidazoline compound IMID-4F is one of the most potent antagonists of arterial K(ATP) channels identified. Vasorelaxation, hyperpolarization and K(+) currents through K(ATP) channels were all inhibited by IMID-4F at micromolar concentrations. Radioligand binding studies indicate that IMID-4F does not bind to the same site as levcromakalim or as glibenclamide. Considering other evidence, it is likely that IMID-4F acts by interacting directly with the pore of the K(IR) channel, rather than through the sulphonylurea subunit of the K(ATP) channel complex.

KATP channel blocking actions of quaternary ions play no role in their antiproliferative action on mouse leukaemia and rat vascular smooth muscle cells in vitro

1. The aim of the present study was to investigate the possibility that, in the two cell lines examined, alterations in cell growth caused by lipophilic quaternary ions may involve KATP channels. We examined the effect of tetraphenylphosphonium (TPP), tetraphenylboron (TPB), rhodamine 123, dequalinium chloride (DECA) and the non-quaternary ion cisplatin on the proliferation of L1210 mouse leukaemia cells and rat smooth muscle cells in vitro. The KATP channel opener levcromakalim (LKM) and the KATP channel antagonist glibenclamide were also tested. 2. From growth-inhibition studies, the rank order of potency (based on pIC50 values) using L1210 leukaemia cells was: DECA (6.61) > cisplatin (6.09) = rhodamine 123 (6.01) > TPP (5.61) > TPB (4.25). Levcromakalim and glibenclamide were found to be inactive at the maximum concentrations used (100 mumol/L). A different rank order of potency was obtained in rat aortic smooth muscle cells: cisplatin (6.33) > DECA (5.67) > TPP (4.96) > rhodamine 123 (4.1). Tetraphenylboron (30 mumol/L), LKM (100 mumol/L) and glibenclamide (100 mumol/L) were found to be inactive. 3. When the negatively charged TPB (30 mumol/L) was combined with some of the active agents, the potency of the active agents was increased. Thus, in L1210 cells, rhodamine 123, DECA and TPP were all more potent at inhibiting cell growth in the presence of TPB. Tetraphenylboron had no effect on cisplatin in this cell line. In rat smooth muscle cells, TPB (30 mumol/L) potentiated the effect of rhodamine 123 but had no effect on the actions of cisplatin, DECA or TPP. 4. In functional studies, rhodamine 123 was a weak antagonist of the vasorelaxant responses to the KATP channel opener LKM in the porcine right circumflex artery in vitro. The pKB value obtained for rhodamine 123 at 100 mumol/L was 4.95. Dequalinium chloride was inactive. 5. We found no correlation between the actions of the compounds tested to antagonise KATP channels and their ability to inhibit cell proliferation. In addition, compounds known to regulate KATP channel activity failed to influence proliferative rates. These results suggest that KATP channels are not involved in the antiproliferative action of TPP and other quaternary ions in the two cell lines studied.

Interspecies differences in thromboxane A2 receptors are distinguished by glibenclamide

The ability of the thromboxane A2 receptor antagonist, GR32191 ([1R-[1alpha(Z),2beta3beta,5alpha]]-7-[5-[[(1,1'-biphe nyl)-4-yl]methoxy]-3-hydroxy-2-(1-piperidinyl)cyclopentyl]-4-heptenoic acid), and the sulphonylurea, glibenclamide, to antagonise contractions to the thromboxane A2 mimetic, U46619 ((15S)-hydroxy-11alpha,9alpha(epoxymethano)prosta-5Z, 13E-dienoic acid), were assessed in rat and guinea-pig isolated large (aorta) and small (mesentery and coronary) arteries. U46619 concentration-response curves were constructed in the absence and presence of GR32191 and glibenclamide and pKB values calculated. GR32191 caused significant rightward shifts in U46619 concentration-response curves and was a more potent antagonist in guinea-pig vessels (pKB approximately 9.4) than rat arteries (pKB approximately 7.9). Conversely, glibenclamide failed to inhibit contractions to U46619 in guinea-pig vessels but antagonised responses to U46619 in rat aorta (pKB = 6.1) and mesenteric artery (pKB = 6.3). In combination, GR32191 and glibenclamide caused a shift in the concentration-effect curve to U46619 in rat aorta that was additive. These results suggest that glibenclamide can discriminate between species differences in thromboxane A2 receptors and may exert its inhibitory effect upon U46619-mediated contractions at the level of the thromboxane A2 receptor.

Unique effect of tetrapentylammonium ions on sympathetic transmission in rat vas deferens

1. Effects of tetrapentylammonium ions (TPA+) on contractile responses induced by electric field stimulation and by exogenous agonist such as noradrenaline, ATP and high K+ were examined in the isolated epididymal half of the rat vas deferens. 2. The electrically evoked monophasic contractions (0.5 Hz, 1 ms pulse duration, 40 V) were potentiated by TPA+ at low concentrations (1-3 microM) and inhibited by TPA+ at a concentration greater than 10 microM. 3. TPA+ (0.3-3 microM) potentiated the contractile responses induced by noradrenaline and ATP, whilst, TPA+ (10 microM) concentration-dependently reduced the agonist-induced contractions. TEA+ enhanced both electrically evoked and agonist-induced contractions. 4. TPA+ (0.3 microM) potentiated 30 mM K+-induced phasic contraction which was inhibited by pretreatment with alpha,beta-methylene ATP (3 microM) and prazosin (3 microM). 5. TPA+ (10-300 microM) reduced the contractions induced by 100 mM extracellular K+ while TBA+ and TEA+ had no effect. 6. The present results show that TPA+ at low concentrations may act at prejunctional nerve membranes to enhance release of contractile transmitters and act as a putative K+ channel blocker at postjunctional membranes to increase muscle contractility in rat vas deferens; whilst at high concentrations TPA+ mainly acts on smooth muscle probably as a non-selective relaxant against the agonist-induced contraction probably through inhibition of Ca2+ influx; this inhibitory effect appears unique for TPA+ since TEA+ and TBA+ did not induce muscle relaxation.

The electrophysiological effects of tetraphenylphosphonium on vascular smooth muscle

The effect of the lipophilic quaternary ion, tetraphenylphosphonium, on membrane potential of segments of rat small mesenteric artery and on the current in single voltage-clamped smooth muscle cells from rabbit portal vein was studied. In rat small mesenteric artery, tetraphenylphosphonium (1-30 microM) caused membrane depolarization of approximately 23 mV and decreased or abolished the hyperpolarization induced by the KATP channel opener, levcromakalim (0.1-3 microM). In rabbit portal vein K+ currents induced by levcromakalim (10 microM) or pinacidil (10 microM) were completely inhibited by tetraphenylphosphonium (IC50 0.5 microM). The results show that tetraphenylphosphonium antagonizes the KATP current induced by K+ channel openers in vascular smooth muscle possibly by acting on the KATP channel itself.

The thromboxane A2 and K(ATP) channel antagonist actions of a series of sulphonylurea derivatives in the pig coronary artery

The ability of a series of sulphonylurea derivatives to antagonise the vasorelaxant actions of the ATP-dependent K+ channel (K(ATP)) opener, levcromakalim, and the vasoconstrictor responses of the thromboxane A2 mimetic, U46619, were assessed in the pig coronary artery. The sulphonylurea derivatives of glibenclamide caused a rightward shift in the concentration-vasorelaxant response curve obtained to levcromakalim in arterial segments pre-constricted with acetylcholine (0.5 microM). From these shifts pK(B) were calculated to estimate the potency of these compounds as levcromakalim antagonists. Similarly U46619 concentration-vasoconstrictor responses curves were constructed in the absence and in the presence of a sulphonylurea derivative and pK(B) values calculated. Regression analysis of pK(B) values showed that there was a significant correlation between the potency of these compounds in the two systems studied indicating similar structure-activity relationships apply in both cases. That sulphonylureas regulate K(ATP) channel opening is well known and they do so through a specific receptor associated with the channel. The results obtained in this study may indicate that a sulphonylurea receptor may also be associated with thromboxane A2 excitation-contraction coupling.

Structure-activity relationship of quaternary ion antagonism of levcromakalim-induced relaxation in pig coronary artery

The aim of this study was to investigate the interaction between the K+ channel opener levcromakalim and several quaternary ions. Cumulative vasorelaxant-response curves to levcromakalim were constructed in the absence and in the presence of the quaternary ions, in the pig coronary artery. The most potent compounds (based on 'apparent pKB' values) were: propyltriphenylphosphonium (7.33), butyltriphenylphosphonium (7.04), tetraphenylarsonium (6.86), tetraphenylphosphonium (6.81), ethyltriphenylphosphonium (6.70), and hexyltriphenylphosphonium (6.63). Tetrabutylphosphonium (6.06), tetrabutylammonium (5.12), methyltriphenylphosphonium (5.25), clofilium (5.66) and guanethidine (5.61) were significantly less potent. Tetrapropylammonium, tetrapentyltin and tetraphenylboron were inactive at the maximum concentrations used (30 microM). Tetraphenylboron (10-100 microM) fully reversed tetraphenylphosphonium, tetraphenylarsonium (both at 3 microM), tetrabutylammonium (30 microM) and clofilium (10 microM) and partially reversed guanethidine (10 microM) antagonism of levcromakalim responses indicating a similarity in the mechanism of action of these chemically distinct compounds. The results show that quaternary ions similar in structure to tetraphenylphosphonium, i.e., containing phosphonium ion centre and phenyl side chains, are the most potent antagonists of levcromakalim, in pig coronary artery. It is also apparent that marked changes can be made in the substitution on the phosphonium ion (ethyl to hexyl) with little or no effect on their potency.