Insulin increases nitric oxide (NO) synthesis via the endothelial NO synthase (eNOS), and L-arginine transport via the cationic amino acid transporters (hCATs) in human umbilical vein endothelial cells (HUVECs) (González et al. 2004). Ex vivo experiments demonstrated that insulin causes relaxation of human umbilical vein by an endothelium-and hCATs-dependent mechanism, under acute (30 min) or chronic (8 h) incubation (González et al., 2011). We here investigated the involvement of large conductance calcium activated potassium channels (BKCa channels) in the acute effects of insulin in the vascular reactivity of human chorionic veins, and on effects of insulin on L-arginine transport in HUVECs. Chorionic vein rings (368±18 µm diameter) and HUVECs were isolated from normal pregnancies (ethics committee approval and informed patient consent were obtained). Vessel rings were mounted on a wire myograph using standar condition (Wareing et al., 2006). Contractions were standardized using 90 mM KCl. Vessels were washed, stabilized and pre-treated (30 min) with insulin (10 nM) and/or tetraethylammonium (TEA, 5 µM, K+ channels blocker) or iberiotoxin (IbTx, 100 nM, BKCa channels blocker). Then vessels were constricted by adding hydrogen peroxide (H2O2) (0.01-1 mM). HUVECs were isolated by collagenase digestion (37 celsius degree) and cultured in medium 199 (M199) supplemented with 20% newborn and fetal calf sera. After incubation (1-30 min) with insulin (1 nM), TEA and/or IbTx, L-arginine transport (100 µM L-arginine, 2 µCi/mL L[3H]-arginine, 1 min, 37 celsius degree) was measured. Significant (ANOVA unpaired Student’s t test, P<0.05, n=5-10) contraction (94±11% KCl response) was obtained with 100 µM H2O2; contraction decreased with insulin pre-treatment (48±12% KCl response). The insulin effect on H2O2-induced contraction was reversed with TEA and IbTx co-incubation. In HUVECs, insulin increased L-arginine transport in a time-dependent manner, with a maximal effect (2.4-fold) after 30 min of incubation. This effect on L-arginine transport was blocked by TEA and IbTx co-incubation. In conclusion, insulin pre-treatment attenuated H2O2-induced contraction of human chorionic plate veins. Insulin pre-treatment also significantly increased L-arginine transport in umbilical vein endothelial cells. Selective potassium channel blockade partially reversed these effects suggesting a role for BKCa channels in insulin’s actions on human placental vascular tissues.