Biochemical studies of the excitable membrane of Paramecium aurelia. I. 45Ca 2+ fluxes across resting and excited membrane

Abstract

The characteristics of Ca 2+ transport across the excitable membrane of Paramecium aurelia were studied by measuring 45Ca 2+ influx and efflux. The intracellular concentration of free Ca 2+ in resting P. aurelia was at least ten times less than the extracellular concentration. Ca 2+ influx was easily measurable at 0°C, but not at 23°C. The influx of 45Ca 2+ 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 Ca 2+ influx. An externally applied, pulsed, electric field (1–2 mA/cm 2 of electrode surface), caused the rate of Ca 2+ influx to increase 3–5 times, with the extent of stimulation dependent on the current density and the pulse width Ca 2+ influx had the characteristics of a passive transport system and was associated with the chemically or electrically triggered Ca 2+ “gating” mechanism, which has been studied electrophysiologically. In contrast, Ca 2+ efflux appeared to be catalyzed by an active transport system. With cells previously loaded at 0°C with 45Ca 2+, Ca 2+ efflux was rapid at 23°C, but did not occur at 0°C. This active Ca 2+ efflux mechanism is probably responsible for maintaining the low internal Ca 2+ levels in unstimulated cells.