Objective: This study investigated signaling pathways that may contribute to the potent positive inotropic effect of human urotensin-II (hU-II) in human isolated right atrial trabeculae obtained from patients with coronary artery disease. Methods: Trabeculae were set up in tissue baths and stimulated to contract at 1 Hz. Tissues were incubated with 20 nM hU-II with or without phorbol 12-myristate 13-acetate (PMA, 10 μM) to desensitize PKC, the PKC inhibitor chelerythrine (10 μM), 10 μM 4α-phorbol that does not desensitize PKC, the myosin light chain kinase inhibitor wortmannin (50 nM, 10 μM), or the Rho kinase inhibitor Y-27632 (0.1–10 μM). Activated RhoA was determined by affinity immunoprecipitation, and phosphorylation of signaling proteins was determined by SDS-PAGE. Results: hU-II caused a potent positive inotropic response in atrial trabeculae, and this was concomitant with increased phosphorylation of regulatory myosin light chain (MLC-2, 1.8±0.4-fold, P<0.05, n=6) and PKCα/βII (1.4±0.2-fold compared to non-stimulated controls, P<0.05, n=7). Pretreatment of tissues with PMA caused a marked reduction in the inotropic effect of hU-II, but did not affect hU-II-mediated phosphorylation of MLC-2. The inotropic response was inhibited by chelerythrine, but not 4α-phorbol or wortmannin. Although Y-27632 also reduced the positive inotropic response to hU-II, this was associated with a marked reduction in basal force of contraction. RhoA.GTP was immunoprecipitated in tissues pretreated with or without hU-II, with findings showing no detectable activation of RhoA in the agonist stimulated tissues. Conclusions: The findings indicated that hU-II increased force of contraction in human heart via a PKC-dependent mechanism and increased phosphorylation of MLC-2, although this was independent of PKC. The positive inotropic effect was independent of myosin light chain kinase and RhoA-Rho kinase signaling pathways.