A, effect of isoproterenol 1 μm, forskolin 1 μm or dibutyryl cAMP 1 mm on l-arginine uptake as compared with basal values (vehicle treatment), by HUVEC. *, **: P < 0.05 and < 0.01, respectively, versus basal. B, effect of isoproterenol on percentage increase in l-arginine uptake above basal, in HUVEC, in the presence of the selective β1AR antagonist CGP 20712A 300 nm, the selective β2AR antagonist ICI 118551 100 nm, or neither antagonist. ***P < 0.001 as compared with no antagonist. C, effect of isoproterenol 1 μm or forskolin 1 μm, as compared with basal values (vehicle treatment), on l-arginine uptake by HUVEC, in the absence (filled bars) or presence (open bars) of 0.2 mmN-ethylmaleimide. **, ***: P < 0.01 and < 0.001 as compared with absence of N-ethylmaleimide. Data are mean ±s.e.m. of 8–10 experiments in different cell cultures.