Both alpha 1-adrenergic agonists (e.g. norepinephrine, NE*) and tumor-promoting phorbol esters (e.g. phorbol myristate acetate, PMA) are known to activate protein kinase C (PKC) (Abdel-Latif, 1986, Niedel and Blackshear, 1986). However, alpha 1 agonists and PMA produce very different effects on cardiac function (see Simpson, 1985; Benfey, 1987; Meidell et al., 1986; Leatherman et al., 1987; Yuan et al., 1987; for examples). PKC activation in heart cells has been studied only for PMA treated perfused heart (Yuan et al., 1987). Therefore, acute activation and chronic regulation of PKC by NE and PMA were compared in cultured neonatal rat heart myocytes. NE acutely and transiently activated PKC, as measured by translocation of PKC activity to the cell particulate fraction (Niedel and Blackshear, 1986). Particulate PKC activity peaked at 23% of total after NE for 30 s, as compared with 8% for control (P less than 0.001). By contrast, acute PKC activation by PMA was more pronounced and persistent, with particulate PKC activity 62% of total at 5 min (P less than 0.001). Calcium/lipid-independent kinase activity increased acutely with PMA, but not with NE. Chronic treatment with NE (24 to 48 h) increased total per cell PKC activity and 3H-phorbol dibutyrate (PDB) binding sites, an index of the number of PKC molecules (Niedel and Blackshear, 1986), by 30 to 60% over control (all P less than 0.05 to 0.01). In contrast with NE, chronic treatment with PMA down-regulated PKC, reducing total per cell PKC activity and 3H-PDB binding sites to 3% and 12% of control, respectively (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)