Biochemical studies of the excitable membrane of Paramecium aurelia. I. 45Ca2+ fluxes across resting and excited membrane.
The characteristics of Ca2+ transport across the excitable membrane of Paramecium aurelia were studied by measuring 45Ca2+ 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 45Ca2+ 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 electrophysiologically. In contrast, Ca2+ efflux appeared to be catalyzed by an active transport system. With cells previously loaded at 0 degrees C with 45Ca2+, 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.
D001386:Azides / Q000494:pharmacology; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent / Q000494:pharmacology; D002414:Cations, Monovalent / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism*; D003486:Cyanides / Q000494:pharmacology; D004558:Electric Stimulation; D006863:Hydrogen-Ion Concentration; D007444:Inulin / Q000378:metabolism; D010247:Paramecium / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology; D013696:Temperature
D001386:Azides; D002413:Cations, Divalent; D002414:Cations, Monovalent; D003486:Cyanides; D007444:Inulin; D012964:Sodium; D011188:Potassium; D002118:Calcium