The function of the astroglial membrane assemblies, the orthogonal arrays of intramembranous particles revealed by freeze-fracture electron microscopy, is unknown. Previously, we have shown that agents which raise cAMP levels increased the astroglial assembly concentrations. We now show that protein kinase C (PKC) is not involved in regulating assembly number with the use of both PKC activators and inhibitors. The chemical nature of the assemblies was assessed. In view of previously work indicating resistance to proteolytic enzymes, we studied the response of assemblies to phospholipases. Assemblies were unchanged by choline dependent phospholipase C (PLC), and phospholipases, A2 and D. However, background, non-assembly particles in the astroglial membrane were affected by PLC. After 6-7 hrs of exposure (0.05-0.1u/ml), individual intramembranous particles redistributed into patches of aggregates interspersed with particle-free areas. The assemblies remained unchanged, but were flanked by other aggregated particles. By 26 hrs, and without changing the media, the astroglial membrane recovered its pattern of evenly distributed particles and assemblies. In contrast, the membrane structure of brain endothelial cells, fibroblasts, cerebellar granule cells and PC-12 cells was not affected by PLC even at concentrations which were 10 times higher (0.5-lu/ml) than those affecting astrocytes. The amount of choline released from these cells after PLC treatment was equivalent. Thus the enzyme interacts with these different cell types in a similar fashion. It therefore appears that the structure of astroglial plasma membrane is much more sensitive to changes in phosphotidylcholine content than the plasma membrane of other cells types.
