Rapid stimulation of mitogen-activated protein kinase of rat liver by prolactin.

Intraperitoneal prolactin administration to female rats caused a rapid and transient stimulation of hepatic mitogen-activated kinase (MAP kinase) activity measured in vitro as cytosolic phosphotransferase capacity towards two specific substrates. Myelin basic protein kinase activity of MAP kinase immunoprecipitates confirmed the specificity and magnified the prolactin effect. Immunoblot experiments with anti-(MAP kinase) and anti-phosphotyrosine antibodies showed changes in both electrophoretic mobility and phosphotyrosine content of 40 and 44 kDa isoenzymes suggesting that prolactin affects these isoforms. Concomitant with the increase in MAP kinase activity, prolactin induced tyrosine phosphorylation in a number of liver proteins, suggesting a rapid involvement of tyrosine kinases which might be correlated in some way with MAP kinase activation. Protein kinase C activity, which has been implicated in the regulation of MAP kinase and in mediating the prolactin effect, does not seem to participate in MAP kinase activation.