Enhancement of the electrical excitability of neuroblastoma cells by valinomycin

Assay of Bacillus cereus Emetic toxin produced in orange squash

The contamination of squash by B. cereus, an enterotoxin producer, was found to range between 7.5×104  and 1.8×104 CFU/g in orange squash (during storage), that is hazardous. Orange squash is widely produced and consumed in India, but has a low rating of 3 on the scale of 10 (on feedback), mostly due to high sugars, not preferred these days. It can be preserved for >9 months due to added sugars and preservatives. During processing squash, if juice is not quickly cooled and/or squash is kept for long at temperatures <48 °C after processing, it can be a source of food poisoning. Reason, a large number of toxins can be produced by B. cereus. B. cereus strains, isolated from squash, produce heat stable toxin. Vacuolar assay confirmed them as emetic toxins, produced in squash. The toxin behaved like an ionophore in assay using mitochondria, extracted from liver cells of chicken with potassium ions in buffer. The toxicity of toxin by assay was 3200 IU/ng (BC IV strain) and 800 IU/ng (BC X strain). By the vacuolar expansions of mitochondria in assay, toxins of B. cereus demonstrated a toxic effect, in the range of 20.93 to 60.94 % by BC IV toxin and 43.28 to 45.02 % by BC X toxin, on the 3rd day growth of B. cereus in squash and toxin extraction for assay. It was also possible to produce antibodies against the B. cereus whole cell and toxin of BC IV, as an attempt to detect B. cereus contaminations in foods, by Ouchterlony’s immune-diffusion test

Assessment of murine neuroblastoma (N1E-115) resting membrane potential by confocal microscopy

Digital imaging (confocal microscopy) and a slow potentiometric dye (tetramethylrhodamine methyl ester) were used to assess the resting membrane potential (V m) of murine neuroblastoma cells (N1E-115). The averageV m was found to be -64.0±2.0 mV. The difference between this and the previously reported higher values was attributed to the use of glass microelectrode techniques that probably caused mechanical injury to the cell membranes: Digital imaging of N1E-115V m was found to be sensitive, reproducible, fast, and simple.

Density-dependent hyperpolarization in cultured aortic smooth muscle cells

The membrane potential (Em) of cultured aortic smooth muscle cells from Sprague-Dawley (SD), Wistar-Kyoto (WKY), and spontaneously hypertensive (SHR) rats was measured in proliferating primary cultures. Em of SD cells in high-density cultures was -51 to -58 mV, whereas that of low-density cultures (1-2 days) was -30 mV. This difference was due to a continuous process of hyperpolarization during proliferation in culture. Em of WKY and SHR hyperpolarized similarly, from -12 to -42 and -38 mV, respectively. Hyperpolarization of Em of SD, WKY, and SHR cells was related to cell density rather than time in culture. Em may be a sensitive and significant indicator of the changes in the differentiated state expressed by proliferating smooth muscle in vitro.

Plasma membrane potential of murine erythroleukemia cells: approach to measurement and evidence for cell-density dependence

The plasmamembrane potential (delta psi p) of murine erythroleukemia (MEL) cells has been determined by measuring the distribution of the lipophilic cation tetraphenylphosphonium (TPP+) across the plasmamembrane. TPP+ accumulation within the cells (experimental accumulation ratio, AR exp) was measured by adding 2 microM TPP+ directly to the culture flasks, avoiding any other perturbation of the experimental system. The mitochondrial contribution to AR exp, evaluated by adding carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) or 2,4-dinitrophenol (DNP), was apparently negligible in standard cultures, AR exp being substantially the same in either the absence or presence of these uncouplers. However, the addition of oligomycin produced a strong AR exp enhancement, which was abolished by FCCP, suggesting that MEL cell mitochondria are in state 3. The aspecific TPP+ binding was estimated by a new mathematical approach worked out to fit AR exp values measured in the presence of valinomycin at various extracellular K+ concentrations and plotted against the ratio of intracellular to extracellular K+ concentration ([K+]i/[K+]e). This binding was found to be close to zero in MEL cells. By applying the Nernst equation directly to AR exp values, delta psi p of these cells was then measured; this potential varying from -65 mV to -16 mV (inside negative) is inversely related to the cell density on the culture surface on which the cells sediment (cells/cm2; CD). The dependence of delta psi p on CD is practically eliminated by valinomycin and appears to be related to a cell interaction with the culture surface of the flasks, suggesting that in the immediate environment of MEL cells one or more factors are produced that modulate the K+ plasma membrane permeability (PK).

Ultrastructural localization of voltage-sensitive sodium channels using [125I]alpha scorpion toxin

The distribution of alpha scorpion toxin (alpha-ScTx) receptors was examined in differentiated mouse neuroblastoma cell cultures (N IE 115 clone) by electron microscope autoradiography using [125I]alpha-ScTx. This neurotoxin binds specifically to voltage-sensitive sodium channels, slowing down the inactivation of the sodium permeability. Quantitative analysis demonstrated that only plasma membranes were labelled. The alpha-ScTx receptors seemed to be randomly dispersed on both cell bodies and cell processes. Microvilli protruding from the cell bodies carried more sodium channels than other parts of the membrane. The specific binding site density for alpha-ScTx varied from 4 (cell body membrane) to 13 (cell process membrane) per square micrometer.

Effects of valinomycin on lymphocytes independent of potassium permeability

10(-7) M valinomycin affects human lymphocytes in the following manner: (1) it is non-toxic; (2) it inhibits mitogenesis; (3) it causes a reduction in cell ATP; and (4) it causes a marked increase in steady-state Na+ exchange. However, it has a minimal effect on cell ion (K+, Na+, Ca2+, Mg2+) contents and no effect whatever on K+ exchange. Neither the fast nor the slow fraction of steady-state K+ exchange is affected by 10(-7) M valinomycin. The various reported effects of valinomycin on lymphocyte functions cannot be assumed to be due to changes in plasma membrane K+ permeability. The mechanism of the increase in steady-state Na+ exchange, and whether or not it is related to inhibition of mitogenesis, are unsettled issues.

Neurite formation and membrane changes of mouse neurobalstoma cells induced by valinomycin

A clonal cell line of mouse neuroblastoma cells was found to undergo morphological differentiation in the presence of a K+ ionophore, valinomycin, in the assay medium. This effect was blocked by increasing the concentration of KCl of the medium, suggesting that the changes in resting membrane potential and ion fluxes may be involved in the mechanism of the formation of neurites. No enhancement of the neurite formation was observed in salines containing high concentrations of KCl in the absence of valinomycin. Depolarizing agents including veratridine, gramicidin and ouabain did not stimulate the outgrowth of neurites. Neither electrophoretic mobility of the cells nor molecular anisotropy of fluorescence probes in the membranes was modified by the treatment of valinomycin. Instead, it modified the slow binding phase in kinetics of the interaction of 1-anilinonaphthalene-8-sulfonate (ANS) with the cells, which is related to the penetration process of the probe into membranes. Valinomycin also enhanced the fluorescence intensity of ANS by increasing the binding sites in neuroblastoma cells.

Membrane currents examined under voltage clamp in cultured neuroblastoma cells

Examination of ionic membrane currents in a voltage-clamped neuronal cell line derived from the mouse C1300 neuroblastoma disclosed four kinetically different components: sodium, potassium, calcium, and leakage current. The kinetics, voltage dependence, and pharmacological properties of the sodium and potassium currents qualitatively resemble those of the corresponding currents in squid giant axon and frog myelinated nerve fiber, suggesting that the molecular structures of the sodium and potassium channels in neuroblastoma are similar to those of the non-mammalian preparations.

A potassium ionophore (valinomycin) inhibits lymphocyte proliferation by its effects on the cell membrane

Valinomycin is a depsipeptide antibiotic which selectively translocates potassium across biologic membranes. This potassium ionophore was observed to inhibit phytohemagglutinin-stimulated blastogenesis and proliferation in human lymphocytes. The effect was not due to toxicity to the cells, nor appeared to be due to the effects of valinomycin as an uncoupler of oxidative phosphorylation. Furthermore, the inhibitory effect on phytohemagglutinin stimulated lymphocytes was prevented by increasing the potassium concentration of the external media. These results suggest that the interaction of mitogens with specific receptors at the cell membrane may involve mechanisms affecting cation fluxes and membrane potential. These ionic events may play a role in the transduction of membrane signals for lymphocyte stimulation.