THE ROLE OF INTRACELLULAR CALCIUM AND PROTEIN-KINASE-C IN ENDOTHELIN-STIMULATED PROLIFERATION OF RAT TYPE-I ASTROCYTES

The increased expression of immunoreactive endothelin-1 (ET-1) in reac tive astrocytes and its mitogenic effects on astrocytes and glioma cel l lines, have implicated endothelins in the development of reactive gl iosis. In this study, an increase in DNA synthesis in fat type I astro cytes was observed after cultures were transiently exposed to ET-1 for 15 min, suggesting that early signal transduction events are essentia l and sufficient for the propagation of the ET-1-induced mitogenic sig nal. Prompt increases in inositol triphosphate (IP3) formation and [Ca 2+](i) were observed upon the addition of ET-1 to these cells. The ET- 1-evoked increase in [Ca2+](i) consisted of an initial peak which was preserved in Ca2+-free medium, and a sustained phase which was abolish ed in Ca2+-free medium and partly attenuated by nifedipine. ET-1 also increased the activity of membrane-associated protein kinase C (PKC) a nd induced the in vivo phosphorylation of the 85 kD MARCKS protein, an endogenous PKC-specific substrate. The ET-1-evoked increases in DNA s ynthesis, IP3, [Ca2+](i) membrane PKC, and 85 kD MARCKS protein phosph orylation in rat cortical astrocytes were prevented by either the sele ctive endothelin ETA receptor antagonist, BQ-123, or the phospholipase C (PLC)specific inhibitor, U-73122. However, the inhibition of PKC ac tivity did not affect ET-1-induced DNA synthesis in rat cortical astro cytes. These results suggest that ET-1-induced IP3 and/or [Ca2+](i) re sponses, but not the activation of PKC, are essential for the growth-f actor like actions of ET-1 in rat cortical astrocytes. (C) 1995 Wiley- Liss, Inc.