Ref ID : 996
J.L. Browning and D.L. Nelson; Biochemical studies of the excitable membrane of Paramecium aurelia I. [45]Ca2+ fluxes across resting and excited membrane. Biochim.Biophys.Acta 448:338-351, 1976
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The characteristics of Ca2+ transport across the excitable membrane of Paramecium aurelia were studied by measuring [45]Ca2+ influx and efflux. The intracellular concentration of free Ca2+ in resting P. aurelia was at least ten times less than the extracellular concentration. Ca2+ influx was easily measurable at 0 degrees C, but not at 23 degrees C. The influx of [45]Ca2+ was stimulated by the same conditions which cause membrane depolarization and ciliary reversal. Addition of Na+ and K+ (which stimulate ciliary reversal) resulted in a 10-fold increase in the rate of Ca2+ influx. An externally applied, pulsed, electric field (1-2 mA/cm2 of electrode surface), caused the rate of Ca2+ influx to increase 3-5 times, with the extent of stimulation dependent on the current density and the pulse width. Ca2+ influx had the characteristics of a passive transport system and was associated with the chemically or electrically triggered Ca2+ "gating" mechanism, which has been studied electro-physiologically. In contrast, Ca2+ efflux appeared to be catalyzed by an active transport system. With cells previously loaded at 0 degrees C with [45]Ca2+, Ca2+ efflux was rapid at 23 degrees C, but did not occur at 0 degrees C. This active Ca2+ efflux mechanism is probably responsible for maintaining the low internal Ca2+ levels in unstimulated cells.