Activin A, an autocrine factor produced by hepatocytes, inhibits mitogen-stimulated DNA synthesis and induces apoptotic death of cultured rat hepatocytes. Several lines of evidence indicate that norepinephrine (NE), as a comitogenic growth factor, alters the balance between growth stimulation and inhibition and acts as a trigger for the initiation of hepatocyte proliferation. In the present study, we examined whether NE modulated the effects of activin A on rat hepatocytes in primary culture. Activin A, at a concentration of 10(-9) mol/L, blocked the effect of epidermal growth factor (EGF) on DNA synthesis, that was assessed by measuring [3H] thymidine incorporation and nuclear labeling, almost completely, and NE reversed the inhibitory effect of activin A on DNA synthesis. This effect of NE was dose-dependent, being significant at concentrations of 10(-6) mol/L and above, but was overcome by higher concentrations of activin A, and was attenuated by prazosin, but not by yohimbine or propranolol. NE exerted its effect during the first 24 hours of culture, but was ineffective when added after 24 hours. EGF augmented the release of follistatin, an activin-binding protein known to block the action of activin A, by hepatocytes and NE did not affect the amount of follistatin they released. In addition to inhibiting DNA synthesis by hepatocytes cultured with EGF, activin A induced death of hepatocytes cultured in the absence of EGF. The nuclear morphology of cells cultured with activin A alone was strikingly changed compared with untreated control cells and marked identation of the nuclear membranes and moderate chromatin condensation were observed. Fragmentation of DNA was also observed, suggesting that activin A induced apoptosis, and activin-mediated cell death was prevented significantly by NE. These results indicate that NE, acting on alpha 1-adrenergic receptors, attenuates the effects of activin A on DNA synthesis by and apoptosis of cultured rat hepatocytes.