Vascular endothelial growth factor (VEGF) promotes angiogenesis and endothelial cell (EC) migration and proliferation by affecting intracellular mediators, only some of which are known, distal to its receptors. Protein kinase C (PKC) participates in the function of VEGF, but the role of individual PKC isoenzymes is unknown. In this study, we tested the importance of the activity of specific PKC isoenzymes in human EC migration and proliferation in response to VEGF. PKCdelta specific activity was depressed by the addition of VEGF (by 41+/-8% [P<0.05] at 24 hours) in human umbilical vein ECs (HUVECs) and in a HUVEC-derived EC line, ECV, without changing the total amount of either protein or mRNA encoding PKCdelta. Neither basic fibroblast growth factor (FGF-2) nor serum altered PKCdelta specific activity. The VEGF-induced decrease of PKCdelta activity, which began at 8 hours after stimulation, was strongly blocked by pretreatment with the nitric oxide (NO) synthase inhibitor N(G)-monomethyl-L-arginine in HUVECs; NO release peaked within 2 hours after stimulation. An exogenous NO donor, sodium nitroprusside, also decreased PKCdelta activity. The inhibition by N(G)-monomethyl-L-arginine of VEGF-induced HUVEC migration and proliferation, but not that induced by FGF-2 or serum, suggested that the decrease in PKCdelta via NO pathway is required for VEGF-induced EC migration and proliferation. Overexpression of PKCdelta in ECV cells specifically prevented EC response to VEGF but not to FGF-2 or serum. Thus, we conclude that suppression of PKCdelta activity via a NO synthase mechanism is required for VEGF-induced EC migration and proliferation, but not for that induced by FGF-2 or serum.