Increase in lipid microviscosity of unilamellar vesicles upon the creation of transmembrane potential

Summary Diffusion potential of potassium ions was formed in unilamellar vesicles of phosphatidyl choline. The vesicles, which included potassium sulfate buffered with potassium phosphate, were diluted into an analogous salt solution made of sodium sulfate and sodium phosphate. The diffusion potential was created by the addition of the potassium-ionophore, valinomycin. The change in lipid microviscosity, ensuing the formation of membrane potential, was measured by the conventional method of fluorescence depolarization with 1,6-diphenyl-1,3,5-hexatriene as a probe. Lipid microviscosity was found to increase with membrane potential in a nonlinear manner, irrespective of the potential direction. Two tentative interpretations are proposed for this observation. The first assumes that the membrane potential imposes an energy barrier on the lipid flow which can be treated in terms of Boltzmann-distribution. The other interpretation assumes a decrease in lipid-free volume due to the pressure induced by the electrical potential. Since increase in lipid viscosity can reduce lateral and rotational motions, as well as increase exposure of functional membrane proteins, physiological effects induced by transmembrane potential could be associated with such dynamic changes.     

Cell damage by cytolysin. Spontaneous recovery and reversible inhibition by divalent cations

Cytolysin-induced membrane damage (which requires low Ca2+) has been studied 1) in E by assay of hemolysis, 2) in Lettre cells by measurement of transmembrane potential, intracellular content of K+ and Na+, leakage of phosphoryl[3H]choline or 51Cr from [3H]choline-labeled or 51CrO4(2-)-labeled cells and leakage of lactate dehydrogenase, and 3) in phospholipid bilayers by measurement of electrical conductivity changes. In Lettre cells, damage is restricted and reversible: little lactate dehydrogenase leaks from cells that leak substantial amounts of Na+, K+, and phosphoryl[3H]choline; at low amounts of cytolysin, membrane potential and intracellular content of Na+ and K+ recover within minutes. In E and Lettre cells, membrane damage is inhibited by Zn2+, by high Ca2+, or by low pH. Inhibition is reversible: addition of EGTA to Zn2+-protected E or Lettre cells (incubated in the presence of cytolysin, low Ca2+ and Zn2+) initiates leakage; removal of Zn2+ (and cytolysin and Ca2+) by washing also initiates leakage; such leakage is again sensitive to Zn2+, high Ca2+, or H+. In phospholipid bilayers, channels induced by cytolysin (at low Ca2+) are partially closed by negative voltage; Ca2+, Zn2+, or H+ promote channel closure. Channels are re-opened (only partially in the case of Zn2+) by positive voltage. From all these results it is concluded that the action of cytolysin on membranes is similar to that of other pore-forming agents: damage does not necessarily lead to lysis of nucleated cells, and can be prevented by Ca2+, Zn2+, or H+.     

Eimeria spp. of the domestic fowl: resolution of chromosomes by field inversion gel electrophoresis

The molecular karyotypes of five species of chicken coccidia, viz., Eimeria acervulina, E. brunetti, E. maxima, E. necatrix, and E. tenella, were determined using field inversion gel electrophoresis (FIGE). Each species has a distinctive set of resolvable chromosomes which range from about 1 to greater than 5.7 megabases. We were able to resolve at least 8 chromosomes for E. acervulina, 5 for E. brunetti, 10 for E. maxima, 6 for E. necatrix, and 9 for E. tenella. If the value of 67 megabases for the genomic DNA of E. tenella is accurate, then under the conditions used here only about 60% of its chromosomal complement has been resolved.     

Gut contents and diel feeding rhythm in dominant copepods in the ice-covered Weddell Sea,March 1992

Zooplankton was sampled at a diurnal station during the drift of the Ice Station Weddell-1 in the western Weddell Sea in March 1992. Gut contents of Metridia gerlachei, Calanoides acutus, Calanus propinquus and Rhincalanus gigas were studied. Calanoides acutus was trophically inactive, but 18%, 33% and 45% of Calanus propinquus, R. gigas and M. gerlachei, respectively, contained food in their guts. Diel changes in gut levels and percentage of individuals with full guts with maximum during the dark period were observed in M. gerlachei copepodite stage V and females. The possible reasons for observed differences in foraging patterns of the four copepod species are discussed.