A role for the small G protein rho and rho-kinase has been shown in smooth muscle contraction regarding Ca++ sensitivity. However, there are no data in the literature assessing how this system operates in human umbilical arteries (HUA). Therefore, we evaluated the effects of HA-1077 and Y-27632, two rho-kinase inhibitors, on agonist-(5-hydroxytryptamine [5-HT]) and depolarization-induced (KCI) contractions of HUA. HA-1077 and Y-27632 inhibited 5-HT-induced contractile responses at 10(-4)M concentration but not at 10(-5)M. HA-1077 at 10(-4)M also significantly attenuated contractions induced by 20 mM KCI. In addition, HUA precontracted with 5-HT relaxed concentration dependently in response to HA-1077 and Y-27632. When precontracted with KCI, HUA also relaxed dose-dependently in response to HA-1077, but the maximal relaxation was significantly smaller than the response obtained when precontracted with 5-HT. To determine possible involvement of rho-kinase on agonist-induced intracellular calcium-mediated contractions, tissues were precontracted with 5-HT in Ca++-free Krebs solution before cumulative addition of HA-1077 or Y27632 (10(-7) to 10(-4)M). Both rho-kinase inhibitors relaxed HUA completely. Maximum relaxations of HIJA to HA-1077 and Y-27632 were significantly larger than the responses seen in normal Krebs solution and were obtained with lower concentrations of the drugs considered to be more specific for rho-kinase inhibition. However, preincubation of HIJA with HA-1077 or Y-27632 (10(-5)M for both) did not affect the 5-HT-induced contractions in this medium. Finally, immunoblot experiments revealed the expression of rho-kinase isoform rockII protein in HUA. These results indicate that rhoA/rho-kinase pathway can contribute to agonist-induced contractions of HUA. However, this effect appears to be limited to intracellular calcium-induced contractions and may be more important in sustaining contractions rather than the initial phase of force development.