DIFFERENTIAL EXPRESSION OF PHOSPHOLIPASE-C SPECIFIC FOR INOSITOL PHOSPHOLIPIDS AT THE CELL-SURFACE OF RAT GLIAL-CELLS AND REF52 RAT EMBRYO FIBROBLASTS

Phosphatidylinositol(PI)-specific phospholipase C activity was detected on the surface of rat astrocytes, rat C6 glioma cells, and rat embryo (REF52) fibroblasts. The cell surface phospholipase C (ecto-PLC) activity was calcium-dependent, did not result from secreted phopholipase C, and was not released from the cell surface by bacterial PI-specific phospholipase C. Agents known to stimulate intracellular PI turnover, including carbachol, L-glutamic acid, acetylcholine, and orthovanadate, did not induce measurable alterations in the activity of the ecto-PLC. The expression of ecto-PLC activity by REF52 fibroblasts was density-dependent: subconfluent cultures of REF52 exhibited low levels of activity (less than 80 pmol of inositol phosphate formed/min/10(6) cells), whereas in confluent cultures ecto-PLC activity increased to approximately 300 pmol/min/10(6) cells. In contrast to this behavior and that exhibited by previously reported ecto-PLC-positive cell types, the ecto-PLC activity exhibited by astrocytes (approximately 1,000 pmol/min/10(6)cells)and by C6 glioma cells (approximately 100 pmol/min/10(6) Cells) was independent of cell culture density up to confluence. The constitutive expression of ecto-PLC activity of astroglial cells may be related to their function as accessory cells in close association with neurons.