The sympathetic nervous system has been indicated to influence the severity of inflammatory disease including rheumatoid arthritis. In this study, we elucidated the effects of catecholamine on the synovial cell populations. Stimulation with epinephrine or norepinephrine for 1 - 2 weeks dose- and time-dependently increased the number of synovial A (macrophage-like) cells but decreased that of B (fibroblast-like) cells. These responses in A and B cells were inhibited by the α2-antagonist yohimbine, the G-protein inactivator pertussis toxin and the phospholipase C (PLC) inhibitor U-73122. Furthermore, the protein kinase C (PKC) inhibitor calphostin C and mitogen-activated protein (MAP) kinase inhibitors PD98059 and wortmannin also abolished the norepinephrine effects on A and B cell numbers. In A cells cloned from an A and B cell mixture, norepinephrine also increased the cell number. In immunoblotting and immunocytostaining analyses, among the PKC isozymes, only PKC βII immunoreactivity was observed in the cytoplasm of unstimulated A and B cells. After α2-adrenoceptor stimulation, PKC βII immunoreactivity increased in the plasma membranes of both A and B cells with decreases in the cytoplasm. These findings indicated that α2-adrenoceptor stimulation of type A and B synoviocytes produced an increase and a decrease in the respective cell number, probably through Gi-coupled PLC activation and the resulting stimulation of the PKC βII/MAP kinase.