Norepinephrine-mediated protein phosphorylation in astrocytes

Studies were conducted to determine if norepinephrine activates both protein kinase C and the cyclic AMP-dependent protein kinase in cultured rat astrocytes using phosphoproteins as markers. Norepinephrine was found to decrease 32P incorporation into an acidic 80,000 M(R) protein. A similar response was observed with isoproterenol and cyclic AMP analogs. In contrast, phorbol myristate acetate (PMA) increased 32P incorporation into this protein. Further studies looked at phosphorylation sites on glial fibrillary acidic protein and vimentin using two-dimensional tryptic phosphopeptide maps. The pattern of phosphorylation of these two proteins by norepinephrine resembles that of 8-bromo cyclic AMP and isoproterenol, and not that of PMA. Additionally, the effect of norepinephrine on the phosphorylation of GFAP and vimentin was blocked by alprenolol. One difference noted between norepinephrine and isoproterenol was the phosphorylation of an 18,000 M(R) protein. Norepinephrine increased, and isoproterenol decreased, 32P incorporation into this protein; however, the mechanism which mediates the norepinephrine effect remains to be determined. Overall, these studies indicate that the most prominent phosphorylation events mediated by norepinephrine are the consequence of the activation of cyclic AMP-dependent protein kinase.