Phagosome formation in Paramecium: effects of solid particles.

Digestive vacuole (DV) formation in Paramecium can be separated into four steps: sequestration and recycling of the spent DV membrane, sweeping and concentrating of particles into the nascent DV, vacuole growth, and the release of the nascent DV. How the size, load and surface charge of solid particles affected formation and subsequently the digestive processes in log-phase cells were investigated. Pulsing cells continuously with beads resulted in an initial linear increase, followed by a steady state, of labelled DVs. Above a certain threshold concentration, the rate of formation and the size of the DVs formed (as well as the steady state) all increased with increasing bead load, so that for a 16-fold increase in bead load, a corresponding fourfold increase in the incorporation of the recycled membrane into the DVs was observed. The threshold values, which depended on the sensitivity of the technique used to score these DVs, were lowered as bead size increased. The steady state of labelled DVs was shown to correspond to a strict balance between the formation and defecation rates as shown by efflux measurements and pulse-chase protocols using two consecutive labels. The duration of the pulse required to reach these steady states was inversely proportional to the logarithm of the bead number at low bead loads but remained constant at higher bead loads. The formation rates and the DV size were similar when cells were pulsed with beads from 0.5 to 3 microns, but DV size increased using beads of 4.5 and 5.7 microns. The maximal bead size that could be ingested was approximately 10 microns.(ABSTRACT TRUNCATED AT 250 WORDS)