Mechanism of uptake of the fluorescent dye 2-(4-dimethylaminostyryl)-1-ethylpyridinium cation (DMP+) by phospholipid vesicles

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.

Use of fluorescence probes to monitor function of the subunit proteins of the MexA-MexB-oprM drug extrusion machinery in Pseudomonas aeruginosa

The MexA-MexB-OprM efflux pump of Pseudomonas aeruginosa consists of two inner membrane proteins, MexA and MexB, and one outer membrane protein, OprM. We investigated the role of the components of this drug extrusion system by evaluating the repercussions of deleting these subunit components on the accumulation of several fluorescent probes. Fluorescence intensities of positively charged 2-(4-dimethylaminostyryl)-1ethylpyridinium and uncharged N-phenyl-1-naphtylamine were 7 and 4 times higher, respectively, in the mutant lacking OprM and 4 and 1.7 times higher, respectively, in the mutants lacking MexA or MexB than in the wild type strain. This order of fluorescence intensity was fully consistent with a previously reported minimum inhibitory concentration of antibiotics such as tetracycline, chloramphenicol, and fluoroquinolones. Ethidium bromide accumulation in all the Mex mutants proceeded at about 5 times faster than the rate in the wild type cells. This result is in accord with the minimum inhibitory concentration of beta-lactam antibiotics. These results suggest that the fluorescence probes could be successfully used in real time monitoring of the function of the drug extrusion machinery in Gram-negative bacteria. The downhill extrusion kinetics of 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene, which orients perpendicular to the inner leaflet of the cytoplasmic membrane, from preloaded cells lacking the extrusion pump was preceded by a slow increase in fluorescence intensity, whereas the wild type cell immediately released the dye. This observation was explained by a slow trans-cytoplasmic membrane crossing of intracellular dye in the mutants. These results reflected higher accumulation of the probe in the cytoplasmic membrane in the mutants and strengthened the hypothesis that extrusion of hydrophobic substrate mediated by MexA-MexB-OprM mainly takes place from the interior of the cytoplasmic membrane.

The role of efflux systems and the cell envelope in fluorescence changes of the lipophilic cation 2-(4-dimethylaminostyryl)-1-ethylpyridinium in Escherichia coli

The interaction of the fluorescent dye 2-(4-dimethylaminostyryl)-1-ethlypyridinium cation (DMP+) with cells of Escherichia coli AN120 (uncA) and AS-1 (acrA) was studied to elucidate the role of the envelope and of efflux systems in the uptake of lipophilic cations. DMP+ bound to the two strains in a different manner. With AS-1 the bound dye was displaced only to a small extent by addition of Mg2+ or other divalent cations. By contract, 50% of the DMP+ was displaced by micromolar concentrations of Mg2+ from resting cells of AN120. Energization of the cells by substrate oxidation resulted in the loss in AN120 of 50% of the bound dye and a decrease of the fluorescence in the cell suspension. With AS-1, energization caused more DMP+ to be taken up from the medium. This was associated with an increase in fluorescence in the cell suspension. The extent of the quenching by addition of Mg2+ was not increased. Right-side out vesicles from AN120, like those of AS-1, showed DMP+ fluorescence behaviour which resembled that of intact cells of AS-1. Transformation of AS-1 with plasmids encoding the E. coli Mvr and EmrAB efflux systems resulted in the DMP+ fluorescence response of this strain becoming like that of AN120. It is suggested that with strain AN120 the changes in binding of DMP+ and fluorescence intensity were associated with activation of efflux systems on cell energization. With AS-1, it is suggested that the observed fluorescence and binding changes are due to inactivation of the AcrAB efflux system by the acrA mutation. Thus, the net entry of lipophilic cations is facilitated. Energization of dye update and release is driven by an electrochemical gradient of protons. ATP is not directly involved in energizing the movement of the dye.