The objectives of this study were to investigate the effects of 6-[4-(4'-pyridylamino)phenyl]-4,5-dihydro-3(2H)-pyridazinone hydrochloride trihydrate (MCI-154), a newly developed cardiotonic agent, on vascular reactivity and contractile responses to extracellular Ca2+ ([Ca2+]o) after hemorrhagic shock and primarily explore its mechanism. In vivo, the effects of MCI-154 (0.1, 0.5, 1.0, and 2.0 mg/kg) on the pressor effect of norepinephrine (NE) in rats subjected to hemorrhagic shock (30 mm Hg for 2 h) were observed and in vitro, the effects of MCI-154 (10(-7), 10(-6), 10(-5), 10(-4) mol/L) on vascular reactivity and contractile responses to [Ca2+]o of superior mesenteric artery (SMA) from hemorrhagic shock rats and its relationship to Rho-kinase, protein kinase C (PKC), and protein kinase G (PKG) were observed. The results showed that the NE-induced pressor response after hemorrhagic shock was significantly decreased (P<0.01), and MCI-154 made it decrease further. In vitro, MCI-154 further decreased the contractile responses of SMA to NE and Ca2+ after hemorrhagic shock as compared with untreated hemorrhagic shock group (P<0.01). Angiotensin II (Ang II), with Rho-kinase stimulating action, and PMA, a PKC agonist increased the contractile responses to [Ca2+]o of SMA after hemorrhagic shock. MCI-154 (10(-5) mol/L) partly inhibited Ang II and PMA-induced increase of the contractile responses to [Ca2+]o of SMA (P<0.01). KT-5823, the PKG antagonist, antagonized MCI-154-induced decrease of the contractile responses to [Ca2+]o. Taken together, these results suggested that the vascular reactivity and contractile responses to [Ca2+]o of vascular smooth muscle after hemorrhagic shock were significantly decreased. MCI-154 worsened hemorrhagic shock-induced vascular hyporeactivity and the decrease of contractile responses to [Ca2+]o. These effects were possibly regulated by Rho-kinase, PKC, and PKG, but this needs further confirmation.