The goal of the present studies was to determine whether anion fluxes are involved in thrombin-and histamine-activated signal transduction pathways in human umbilical vein endothelial cells (HUVECs). 125Iodine (125I) efflux techniques were used to test the sensitivity of anion fluxes to increases in [Ca2+]i and activation of protein kinase C. HUVECs exhibited constant 125I efflux rates under basal conditions. Administration of thrombin or histamine stimulated an increase in 125I efflux rates which returned to control values after approximately 1-2 min. Since both agonists stimulate increases in [Ca2+]i, we tested the hypothesis that 125I efflux was sensitive to changes in [Ca2+]i. When HUVECs were exposed to ionomycin or thapsigargin, the 125I efflux rate increased and remained elevated for several minutes. In subsequent experiments, HUVECs were incubated with the cell permanent Ca2+ chelator, 1,2-bis-(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid-AM, to buffer changes in [Ca2+]i. This treatment reduced both basal and thrombin-stimulated 125I efflux. However, when Ca2+ was removed from the efflux buffer and replaced with EGTA, peak thrombin-stimulated 125I efflux remained unchanged. This anion efflux was also sensitive to activation of protein kinase C since phorbol 12-myristate 13-acetate and phorbol, 12,13-dibutyrate blunted thrombin-mediated increases in 125I efflux. Preincubation of HUVECs with protein kinase C inhibitor peptide [19-36] antagonized the phorbol ester-mediated decrease in thrombin-stimulated 125I efflux. We suggest that 125I efflux in HUVECs represents a Ca2+-sensitive anion conductance and that intracellular Ca2+ release, but not extracellular Ca2+ influx, is sufficient to initiate channel activity. © 1994 Springer-Verlag New York Inc.