DISCRIMINATION BETWEEN PLASMA-MEMBRANE AND INTRACELLULAR TARGET SITESOF SPHINGOSYLPHOSPHORYLCHOLINE

On the background of the emerging concept of G protein-coupled sphingo lipid receptors, Ca2+ mobilization by sphingosylphosphoryl-choline (SP PC) in intact cells and SPPC-induced Ca2+ release in permeabilized cel ls, both occurring at similar, micromolar concentrations, were charact erized and compared. In intact human embryonic kidney (HEK-293) cells, SPPC rapidly increased [Ca2+](i) by mobilization of Ca2+ from thapsig argin-sensitive stores. In saponin-permeabilized HEK-293 cells, SPPC r eleased stored Ca2+, in a manner similar to but independent of inosito l 1,4,5-trisphosphate. Only the action of SPPC on intact cells, but no t that in permeabilized cells, was, at least in part, sensitive to per tussis toxin. In addition and most important, Ca2+ release by SPPC in permeabilized cells was not stereoselective, whereas in intact cells o nly the naturally occurring D-erythro-SPPC, but not L-threo-SPPC, incr eased [Ca2+](i). Stereoselectivity of SPPC-induced [Ca2+](i) increase was also demonstrated in bovine aortic endothelial cells. In conclusio n, Ca2+ mobilization by SPPC in intact cells is independent of the pre viously described SPPC-gated Ca2+ channel on endoplasmic reticulum but probably mediated by a membrane sphingolipid receptor. Thus, SPPC can regulate Ca2+ homeostasis by acting apparently at two cellular target s, which exhibit clearly distinct recognition patterns. (C) 1998 Elsev ier Science B.V. All rights reserved.