SPATIAL-ORGANIZATION OF CALCIUM SIGNALING INVOLVED IN CELL-VOLUME CONTROL IN THE FUCUS RHIZOID

Subprotoplasts prepared from different regions of rhizoid and thallus cells of Fucus zygotes displayed mechanosensitive plasma membrane chan nels in cell-attached patch-clamp experiments by using laser microsurg ery. In excised patches, this channel was found to be voltage gated, c arrying K+ outward and Ca2+ inward, with a relative permeability of Ca 2+/K+ of 0.35 to 0.5, and an increased open probability at membrane po tentials more positive than -80 mv. No significant difference was foun d in the density of this channel type from different regions of rhizoi d or thallus cells. Hypoosmotic treatment of intact zygotes induced dr amatic transient elevations of cytoplasmic Ca2+, initiating at the rhi zoid apex and propagating in a wavelike manner to subapical regions. L ocalized initiation of the Ca2+ transient correlated with greater osmo tic swelling at the rhizoid apex compared with other regions of the zy gote. Ca2+ transients exhibited a refractory period between successive hypoosmotic shocks, during which additional transients could not be e licited and the ability to osmoregulate was impaired. Buffering the Ca 2+ transients with microinjected Br(2)BAPTA similarly reduced the abil ity of rhizoid cells to osmoregulate. Ca2+ influx was associated with the initiation of the Ca2+ transient in apical regions, whereas intrac ellular sources contributed to its propagation. Thus, localized signal transduction is patterned by interactions of the cell wall, plasma me mbrane, and intracellular Ca2+ stores.