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

Abstract

1 The aim of this study was to characterize the interaction between the K+ channel opener levcromakalim (LKM) and several quaternary ions, in vascular smooth muscle, in vitro. Segments of isolated, thoracic aorta of the rat were suspended in organ baths filled with Krebs solution at 37C. Cumulative concentration-response curves to LKM were obtained in the absence and in the presence of increasing concentrations of quaternary ions using a number of agents to pre-constrict the vessel. The ions tested were tetraphenylphosphonium (chloride TPP-C1 and bromide TPP-Br salts), tetrapentylammonium (TPeA), tetraethylammonium (TEA), tetraphenylarsonium (TPAs) and tetraphenylboron (TPB). 2 For the compounds which antagonized the vasorelaxation responses of LKM, 'apparent pKB' values were estimated on the basis of a single concentration of antagonist. These were then used to obtain the following order of potency: TPP-Br (7.22 ± 0.25) = TPAs (7.12 ± 0.04) = TPP-Cl (7.11 ± 0.15) > TPeA (6.23 ± 0.20). TEA and TPB were both found to be inactive at the maximum concentrations used. 3 The interaction between the cationic TPP and anionic TPB was also investigated. The shift in the LKM concentration-response curve constructed in the presence of both of these compounds was compared to that when each agent was present separately. We found that TPB, at concentrations greater than 1 gLM, reversed the blockade of the LKM-mediated relaxation induced by TPP (3 IsM). 4 Similar experiments were undertaken combining TPB with either alinidine or glibenclamide (both functional antagonists of K+ channel openers). It was found that TPB (10 gM) partially reversed the antagonism induced by alinidine (30 and 100 gM) but had no effect on the action of glibenclamide (3 gM). 5These studies show that lipophilic cations such as TPP and TPAs are potent antagonists of levcromakalim-mediated vasorelaxation responses in the rat thoracic aorta. The mechanism by which these compounds cause their antagonism is not known. However, given the lipophilicity of these compounds, it is possible they may act at a number of sites including the KATP channel itself or possibly via some other intracellular mechanism.

The identification of a potent imidazoline-based vascular K ATP channel antagonist

“…At this time it is uncertain where lipophilic quaternary ions interact with the K ATP channel. However, previous studies with other quaternary ions have suggested that they interact with the pore of the K + channel to inhibit K + efflux from the cell (see McPherson and Piekarska 1994). The potency and nature of the antagonism displayed by the imidazolines characterised in this study are similar to that displayed by the lipophilic quaternary ions.…”

“…Another potent group of levcromakalim antagonists that have been identified are the lipophilic quaternary ions such as tetraphenylphosphonium (pK B =7.2; McPherson and Piekarska 1994;Piekarska and McPherson 1997). While displaying the same potency as glibenclamide, tetraphenylphosphonium (and related compounds) show a markedly different type of antagonism as that displayed by the sulphonylureas.…”

“…Thus the imidazolines cause a non-competitive antagonism of the K ATP channel-opening effects of levcromakalim. Since imidazolines can be positively charged at physiological pH (see McPherson and Piekarska 1994), it is possible that these compounds may affect K ATP channel opening through a mechanism similar to that utilized by the lipophilic quaternary ions.…”

Functional and electrophysiological effects of a novel imidazoline‐based KATP channel blocker, IMID‐4F

“…Thus when using sulphonylurea based compounds the levcromakalim concentration‐effect curve is shifted to the right in a parallel manner with no obvious effect on the maximum response. Another potent group of levcromakalim antagonists that we have identified are the lipophilic quaternary ions such as tetraphenylphosphonium (pK B =7.2; McPherson & Piekarska, 1994; Piekarska & McPherson, 1997). While displaying the same potency as glibenclamide, tetraphenylphosphonium (and related compounds) show a markedly different type of antagonism as that displayed by the sulphonylureas.…”

Abolition of endothelium-dependent relaxation in the rat aorta by tetraoctylammonium ions

“…TPA + constricts the rat mesenteric artery at low concentrations (5-10 µM), but acts as a non-selective relaxant in rat arteries at concentrations above 10 µM, whereby this effect is not associated with its blocking effect on vascular K + channels (Huang 1997). Two other lipophilic quaternary compounds, tetraphenylphosphonium and tetraphenylarsonium, are among the most potent antagonists of levcromakalim-mediated vasorelaxation (McPherson and Piekarska 1994). Some quaternary ions are highly lipophilic, which allows them to cross the hydrophobic core of the plasma membrane to interact with K + channels or other intracellular molecules.…”