monensin and ethylisopropylamiloride

The amount of neurotransmitter stored in a single synaptic vesicle can determine the size of the postsynaptic response, but the factors that regulate vesicle filling are poorly understood. A proton electrochemical gradient (Δμ(H+)) generated by the vacuolar H(+)-ATPase drives the accumulation of classical transmitters into synaptic vesicles. The chemical component of Δμ(H+) (ΔpH) has received particular attention for its role in the vesicular transport of cationic transmitters as well as in protein sorting and degradation. Thus, considerable work has addressed the factors that promote ΔpH. However, synaptic vesicle uptake of the principal excitatory transmitter glutamate depends on the electrical component of Δμ(H+) (Δψ). We found that rat brain synaptic vesicles express monovalent cation/H(+) exchange activity that converts ΔpH into Δψ, and that this promotes synaptic vesicle filling with glutamate. Manipulating presynaptic K(+) at a glutamatergic synapse influenced quantal size, indicating that synaptic vesicle K(+)/H(+) exchange regulates glutamate release and synaptic transmission.

Topics: Amiloride; Analysis of Variance; Animals; Animals, Newborn; Arthropod Proteins; Aspartic Acid; Biological Transport; Biophysical Phenomena; Brain; Carbonyl Cyanide m-Chlorophenyl Hydrazone; Cations; Dose-Response Relationship, Drug; Electric Stimulation; Excitatory Postsynaptic Potentials; Gluconates; Glutamic Acid; Hydrogen-Ion Concentration; In Vitro Techniques; Ionophores; Membrane Potential, Mitochondrial; Monensin; Oligopeptides; Potassium; Presynaptic Terminals; Radionuclide Imaging; Rats; Rats, Wistar; Sodium Isotopes; Synapses; Synaptic Vesicles; Synaptosomes; Tritium; Vesicular Glutamate Transport Proteins

Multidrug resistant (MDR) tumor cells exhibit an altered pH gradient across different cell compartments, which favors a reduced intracellular accumulation of antineoplastic drugs and a decreased therapeutic effect. In our study, we have observed that the activity and expression of Na+/H+ exchanger (NHE), which is involved in the homeostasis of intracellular pH (pHi), are increased in doxorubicin-resistant (HT29-dx) human colon carcinoma cells in comparison with doxorubicin-sensitive HT29 cells. The pH(i) was significantly higher in HT29-dx cells, which accumulated less doxorubicin than HT29 cells. The NHE inhibitor 5-(N-ethyl-N-isopropyl)amiloride (EIPA) significantly reduced the pHi value and increased the intracellular accumulation of doxorubicin in both cell populations: in the presence of EIPA HT29-dx cells accumulated as much drug as control HT29 cells. On the other hand, monensin, a Na+/H+ ionophore mimicking NHE activation, and phorbol 12-myristate 13-acetate (PMA), which stimulates NHE, significantly increased the pHi and decreased the drug accumulation in HT29 cells to values similar to those observed in control HT29-dx cells. EIPA potentiated the cytotoxic effect of doxorubicin in HT29 cells, and made HT29-dx cells as sensitive to the cytotoxic effect of the drug as control HT29 cells. Instead, PMA and monensin made HT29 cells as insensitive to doxorubicin as HT29-dx cells. These results suggest that in MDR cells the higher cytosolic pH is likely to decrease drug accumulation, and that such resistance can be reverted by inhibiting the NHE activity. This result opens the possibility to revert MDR with the clinical use of NHE inhibitors.

Topics: Amiloride; Colonic Neoplasms; Doxorubicin; Drug Resistance, Neoplasm; HT29 Cells; Humans; Hydrogen-Ion Concentration; Monensin; Sodium-Hydrogen Exchangers; Tetradecanoylphorbol Acetate

Desmopressin (DDAVP) action on platelets is associated with the development of procoagulant response but the underlying mechanism of this phenomenon is not known. We investigated whether this effect of DDAVP might be due to activation of plasma membrane Na+/H+ exchanger. The DDAVP-induced platelet procoagulant response, measured as phospholipid-dependent thrombin generation, was dose dependent and significantly weaker than that produced by collagen or monensin (mimics Na+/H+ antiport). Both the DDAVP- and collagen-produced procoagulant responses were less pronounced in the presence of EIPA, an Na+/H+ exchanger inhibitor. Flow cytometry studies revealed that in vitro treatment of platelets with DDAVP or collagen was associated with the appearance of both degranulated (and fragmented) and swollen cells. The DDAVP-evoked rise in size and granularity heterogeneity was similar to that produced by collagen or monensin and was not observed in the presence of EIPA. Using flow cytometry and annexin V-FITC as a probe for phosphatidylserine (PS) we demonstrated increased and uniform binding of this marker to all subsets of DDAVP-treated platelet population. The DDAVP-evoked PS expression was dose dependent, strongly reduced by EIPA and weaker than that caused by monensin or collagen. As judged by optical swelling assay, DDAVP in a dose dependent manner produced a rise in platelet volume. The swelling was inhibited by EIPA and its kinetics was similar to that observed in the presence of monensin. Electronic cell-sizing measurements showed an increase in mean platelet volume and a decrease in platelet count and platelet crit upon treatment with DDAVP. DDAVP elicited a slow (much slower than collagen) alkalinization of platelet cytosol. Altogether the data indicate an involvement of Na+/H+ exchanger in the generation of procoagulant activity in DDAVP-treated platelets.

Topics: Amiloride; Blood Coagulation; Blood Platelets; Cell Membrane; Cell Size; Collagen; Deamino Arginine Vasopressin; Humans; In Vitro Techniques; Monensin; Phosphatidylserines; Platelet Activation; Platelet Count; Sodium-Hydrogen Exchangers; Thrombin

SH-SY5Y neural cells expressing mu- and delta-opioid receptors were maintained viable in isotonic, sodium-free buffer in vitro. Intracellular sodium levels were manipulated by various methods, and ligand binding to intact cells was studied. In physiological buffer containing 118 mM sodium, [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol ([3H]-DAMGO) and [3H]naltrexone bound to mu receptor with KD values of 3.1 and 0.32 nM and Bmax values of 94 and 264 fmol/mg of protein, respectively. Replacement of sodium by choline decreased the affinity of the antagonist and increased Bmax for [3H]DAMGO, without significantly affecting the other corresponding binding parameters. Depolarizing concentrations of KCl (34 mM) in physiological buffer decreased the intracellular sodium levels by 67%, but this did not decrease the [3H]DAMGO binding to the cells. Incubation of cells with monensin and ouabain increased the intracellular sodium levels dramatically (from 78 to 250 and 300 nmol/mg, respectively), with no changes in agonist binding parameters. Ethylisopropylamiloride inhibited [3H]DAMGO and [3H]naloxone binding to intact cells with EC50 values of 24 and 3,600 nM, respectively. Adenylyl cyclase activities measured in intact cells, at different concentrations of sodium, showed the physiological significance of this ion in signal transduction. Potency of DAMGO in inhibiting the forskolin-stimulated adenylyl cyclase activity was significantly higher at lower concentrations of sodium. However, inhibition reached the maximal level only at 50 mM sodium, and typical sigmoidal dose-response curves were obtained only in the presence of 118 mM sodium. Furthermore, even at low or high intracellular sodium levels, DAMGO inhibition of cyclic AMP levels was normal. These results support a role for extracellular sodium in regulating not only the ligand interactions with the receptor, but also the signal transduction through the mu receptor.

Topics: Adenylyl Cyclase Inhibitors; Amiloride; Cyclic AMP; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Extracellular Space; Humans; Intracellular Membranes; Monensin; Naltrexone; Neurons; Osmolar Concentration; Ouabain; Receptors, Opioid, mu; Sodium; Tumor Cells, Cultured

The purpose of this study was to characterize the role of ions other than Ca2+ in hepatic responses to alpha 1-adrenergic stimulation. We report that the alpha 1-adrenoreceptor activation of hepatic functions is accompanied by extracellular acidification and an increase in intracellular pH. These effects are dependent on extracellular Na+ concentration and are inhibited by the Na+/H+ antiporter blocker 5-(N-ethyl-N-isopropyl) amiloride under conditions that preclude antagonistic effects on agonist binding. Thus, the activation of plasma membrane Na+/H+ exchange is an essential feature of the hepatic alpha-adrenoreceptor-coupled signaling pathway. The following observations indicate that the sustained hepatic alpha 1-adrenergic actions rely on a functional coupling between the plasma membrane Na+/H+ and Na+/Ca2+ exchangers, resulting in the stimulation of Ca2+ influx. 1) Inhibition of the Na+/K(+)-ATPase does not prevent the alpha 1-adrenergic effects. However, alpha 1-adrenoreceptor stimulation fails to induce intracellular alkalinization and to acidify the extracellular medium in the absence of extracellular Ca2+. 2) A non-receptor-induced increase in intracellular Na+ concentration, caused by the ionophore monensin, stimulates Ca2+ influx and increases vascular resistance. 3) Inhibition of Na+/Ca2+ exchange prevents, in a concentration-dependent manner, most of the alpha 1-agonist-induced responses. 4) The actions of Ca(2+)-mobilizing vasoactive peptide receptors or alpha 2-adrenoreceptors, which produce neither sustained extracellular acidification nor release of Ca2+, are insensitive to Na+/H+ exchange blockers.

Topics: Amiloride; Animals; Calcium; Carrier Proteins; Hydrogen-Ion Concentration; Liver; Membrane Potentials; Monensin; Ouabain; Rats; Rats, Wistar; Receptors, Adrenergic, alpha; Sodium; Sodium-Calcium Exchanger; Sodium-Hydrogen Exchangers; Vasopressins

Following the observation that interleukin 1 beta (IL-1 beta) production in lipopolysaccharide (LPS)activated monocytes increases in concert with a rise in intracellular pH (pHi), the role of ion transport in IL-1 beta production was investigated. The amiloride analogue 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an inhibitor of the Na(+)-H+ antiporter, inhibited extracellular IL-1 beta. The replacement of Na+ in the culture medium with sucrose or choline chloride also prevented monocyte activation. The sodium ionophore monensin, in doses from 100 pM to 1 microM, potentiated LPS-stimulated extracellular IL-1 beta when compared with LPS alone. In the absence of LPS activation, monensin by itself at 10 nM stimulated IL-1 beta production to 63%. EIPA at 10 microM inhibited the Na+ influx, the rise in pHi, and intra- and extracellular IL-1 beta production in activated monocytes; this inhibition was reversed by 10 nM monensin. In the absence of bicarbonate, or in the presence of 10 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, the pHi of activated monocytes and the total protein synthesis did not change, but the production of IL-1 beta was inhibited. The data suggest that the stimulated influx of Na+ via the Na(+)-H+ antiporter regulates both pHi and IL-1 beta production in LPS-activated monocytes. The requirement for bicarbonate indicates an additional mechanism(s), separate from the modulation of pHi and intracellular Na+.

Topics: 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; 4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; Amiloride; Bicarbonates; Chlorides; Culture Media; Extracellular Space; Humans; Hydrogen-Ion Concentration; Infant, Newborn; Interleukin-1; Intracellular Membranes; Lipopolysaccharides; Monensin; Monocytes; Protein Biosynthesis; Sodium

Opossum kidney (OK) cells (an epithelial cell line) were examined by flame photometry of cellular Na+ and K+ and by microfluorometric measurements of the intracellular pH (pHi) of single cells loaded with 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF). The work concentrates on defining resting pHi values under different experimental conditions and examines factors that contribute to the maintenance of resting pHi. To use nigericin to calibrate the intracellular response of BCECF, cellular K+ levels were measured by a null point analysis, and the stability and magnitude of cellular Na+ and K+ levels were determined vs. time. Resting pHi in medium without added CO2 was high when measured by null point analysis of the population (pHi 7.6) and from measurements of single cells that have recovered from an acid load caused by NH4 prepulse (pHi 7.76 +/- 0.03, n = 20 cells). In single-cell measurements, addition of CO2-HCO3- to the medium results in cellular acidification of the steady-state pHi by 0.35 +/- 0.04 pH units. In medium equilibrated with room air, the resting pHi is shown to be a dynamic steady state composed of net flux due to apical Na(+)-dependent transport (Na(+)H+ exchange) plus acidifying processes. It is concluded that although 5-[N-ethyl-N-isopropyl]amiloride (EIPA) inhibits the forward reaction of Na(+)-H+ exchange, EIPA is either ineffective as an inhibitor of the reverse reaction of Na(+)-H+ exchange or Na(+)-H+ exchange does not reverse measurably in the OK cells.

Topics: Amiloride; Animals; Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; Cell Line; Cells, Cultured; Chlorides; Culture Media; Homeostasis; Hydrogen-Ion Concentration; Kidney; Kinetics; Monensin; Opossums; Ouabain; Potassium; Quaternary Ammonium Compounds

Thrombin-stimulated endothelial cells produce platelet-activating factor (PAF) in a dose-dependent manner: the activation of a Ca2+-dependent lyso-PAF acetyltransferase is the rate-limiting step in this process. The present study shows that acetyltransferase activation and consequent PAF production induced by thrombin in human endothelial cells are markedly inhibited in Na+-free media or after addition of the amiloride analog 5-(N-ethyl-N-isopropyl)amiloride, suggesting that a Na+/H+ antiport system is present in endothelial cells and plays a prominent role in thrombin-induced PAF synthesis. Accordingly, thrombin elicits a sustained alkalinization in 6-carboxyfluorescein-loaded endothelial cells, that is abolished in either Na+-free or 5-(N-ethyl-N-isopropyl)amiloride-containing medium. Extracellular Ca2+ influx induced by thrombin (as measured by quin2 and 45Ca methods) is completely blocked in the same experimental conditions, and monensin, a Na+/H+ ionophore mimicking the effects of the antiporter activation, evokes a dose-dependent PAF synthesis and a marked Ca2+ influx, which are abolished in Ca2+-free medium. An amiloride-inhibitable Na+/H+ exchanger is present in the membrane of human endothelial cells, its apparent Km for extracellular Na+ is 25 mM, and its activity is greatly enhanced when the cytoplasm is acidified. These results suggest that Na+/H+ exchange activation by thrombin and the resulting intracellular alkalinization play a direct role in the induction of Ca2+ influx and PAF synthesis in human endothelial cells.

Topics: Acetyltransferases; Amiloride; Calcium; Carrier Proteins; Cells, Cultured; Endothelium, Vascular; Humans; Hydrogen-Ion Concentration; Kinetics; Monensin; Platelet Activating Factor; Sodium-Hydrogen Exchangers; Thrombin; Umbilical Veins