OBJECTIVE The purpose of this study was to evaluate the role of endothelin-1 in serotonin-induced vasoconstriction in human umbilical artery. METHODS Umbilical arteries collected from 41 normal subjects at term were cut helically and suspended in an organ bath for recording isometric mechanical activity. In study 1 we measured the concentration-contraction response to serotonin or endothelin-1 according to a cumulative concentration schedule. In study 2 vessels were preincubated with a subthreshold concentration of endothelin-1 (100, 250, or 500 pg/ml), and then serotonin was added cumulatively. In study 3 vessels were preincubated with H-7 (3 x 10(-6) mol/L), an inhibitor of protein kinase C, or with 12-tetradecanoylphorbol-13 acetate (10(-9) to 10(-7) mol/L), an activator of protein kinase C; then serotonin was added cumulatively. In study 4 vessels were suspended in a calcium-free solution containing 2 mmol/L ethylene glycol-bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid and 60 mmol/L potassium chloride, and then a cumulative concentration-response curve to calcium chloride (10(-7) to 10(-3) mol/L) was constructed for vessels pretreated with endothelin-1, 500 pg/ml or 5 ng/ml. RESULTS The threshold concentrations of serotonin and endothelin-1 were 5 and 1 ng/ml, respectively. A subthreshold concentration of endothelin-1 (250 or 500 pg/ml) potentiated significantly the concentration response to serotonin (p < 0.084, p < 0.009, by analysis of variance). Pretreatment with H-7 significantly suppressed the amplifying effect of endothelin-1 on serotonin-induced contraction. Treatment with 12-tetradecanoylphorbol-13 acetate (10(-7) or 10(-8) mol/L) significantly potentiated the contractile response to serotonin (p < 0.0003, p < 0.015). The sensitivity of the arterial segments to calcium chloride in the presence of a subthreshold concentration of endothelin-1 did not differ from that in the control group. CONCLUSIONS Endothelin-1 at a subthreshold concentration amplified the smooth muscle contraction induced by serotonin. The mechanism of action may be mediated by activation of intracellular protein kinase C.