The role of intracellular calcium and protein kinase C in endothelin‐stimulated proliferation of rat type I astrocytes

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Abstract
The increased expression of immunoreactive endothelin‐1 (ET‐1) in reactive astrocytes and its mitogenic effects on astrocytes and glioma cell lines, have implicated endothelins in the development of reactive gliosis. In this study, an increase in DNA synthesis in rat type I astrocytes was observed after cultures were transiently exposed to ET‐1 for 15 min, suggesting that early signal transduction events are essential and sufficient for the propagation of the ET‐1‐induced mitogenic signal. Prompt increases in inositol triphosphate (IP3) formation and [Ca2+]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 abolished in Ca2+‐free medium and partly attenuated by nifedipine. ET‐1 also increased the activity of membrane‐associated protein kinase C (PKC) and induced the in vivo phosphorylation of the 85 kD MARCKS protein, an endogenous PKC‐specific substrate. The ET‐1‐evoked increases in DNA synthesis, IP3, [Ca2+]i, membrane PKC, and 85 kD MARCKS protein phosphorylation in rat cortical astrocytes were prevented by either the selective endothelin ETA receptor antagonist, BQ‐123, or the phospholipase C (PLC)‐specific inhibitor, U‐73122. However, the inhibition of PKC activity did not affect ET‐1‐induced DNA synthesis in rat cortical astrocytes. These results suggest that ET‐1‐induced IP3 and/or [CA2+]i responses, but not the activation of PKC, are essential for the growth‐factor like actions of ET‐1 in rat cortical astrocytes.