The effect of an extracellular mucilage on the response to osmotic shock in the charophyte alga Lamprothamnium papulosum

We have used current/voltage (IN) analysis to investigate the role played b y extracellular mucilage in the cellular response to osmotic shock in Lampr othamnium papulosum. Cells lacking extracellular mucilage originated in a b rackish environment (1/3 seawater). These were compared, first with cells c oated with thick (similar to 50 mu m) extracellular mucilage, collected fro m a marine lake, and second, with equivalent mucilaginous marine cells, tre ated with heparinase enzyme to disrupt the mucilage layer. Histochemical st ains Toluidine Blue and Alcian Blue at low pH identified the major componen t of the extracellular mucilage as sulfated polysaccharides. Treating mucil age with heparinase removed the capacity for staining with cationic dyes at low pH, although the mucilage was not removed, and remained as a substanti al unstirred layer. Cells lacking mucilage responded to hypotonic shock wit h depolarization (by similar to 95 mV), cessation of cyclosis, due to trans ient opening of Ca2+ channels, and opening of Ca2+-activated Cl- channels a nd K+ channels. Cell conductance transiently increased tenfold, but after 6 0 min was restored to the conductance prior to hypotonic shock. Mucilaginou s cells depolarized by a small amount (similar to 18 mV), but Ca2+ channels failed to open in large enough numbers for cyclosis to cease. Likewise mos t Ca2+-activated Cl- channels failed to open and conductance increased only similar to 1.2-fold above the prehypotonic level. After 60 min conductance was less than the conductance prior to hypotonic shock. Heparinased mucila ginous cells recovered several aspects of the hypotonic response in cells l acking mucilage. These cells depolarized (by similar to 103 mV); cyclosis c eased, indicating that Ca2+ channels had opened, and conductance increased to similar to 4 times the value prior to hypotonic shock, indicating that C a2+-activated Cl- channels opened. However, after 60 min, these cells had n either restored membrane potential land remained at positive values), nor d ecreased their conductance. It was not possible to determine whether K+ cha nnels had opened. The heparinased cells recovered the normal hypotonic resp onse of mucilaginous cells when heparinase was washed out. Apical seawater cells, which lacked mucilage, were unaffected by heparinase treatment. The results demonstrate that the presence of extracellular sulfated polysacchar ide mucilage impacts upon the electrophysiology of the response to osmotic shock in Lamprothamnium cells. The role of such sulfated mucilages in marin e algae and animal cells is compared and discussed.