Background Human urotensin Ⅱ (UⅡ) is the most potent mammalian vasoconstrictor identified so far. Our previous study showed that UⅡ is a potent mitogen of airway smooth muscle cells (ASMC) inducing ASMC proliferation in a dose-dependent manner. The signal transduction pathway of UⅡ mitogenic effect remains to be clarified. This study was conducted to investigate the signal transduction pathway in the proliferation of ASMC induced by UⅡ. Methods In primary cultures of rat ASMCs,activities of protein kinase C (PKC),mitogen-activated protein kinase (MAPK) and calcineurin (CaN) induced by UⅡ were measured. The effect of CaN on PKC and MAPK was studied by adding cyclosporin A (CsA),a specific inhibitor of CaN. Using H_7 and PD_ 98059 , inhibitors of PKC and MAPK,respectively,to study the effect of PKC and MAPK on CaN. The cytosolic free calcium concentration induced by UⅡ was measured using Fura-2/AM. Results UⅡ 10 -7 mol/L stimulated ASMC PKC and MAPK activities by 44% and 24% ( P <0.01), respectively,after incubating for 20 minutes. It increased CaN activity in a time-dependent manner,being 1.68 times as that of control for 24 hours ( P <0.01). It promoted the cytosolic free calcium concentration increase of 18% ( P <0.01). CsA 10 -6 mol/L and H_7 50 μmol/L inhibited UⅡ-stimulated CaN activity by 45% ( P <0.01) and 21% ( P <0.05),respectively,while PD_ 98059 50 μmol/L had no effect on CaN activity ( P >0.05). CsA 10 -6 mol/L inhibited UⅡ-stimulated PKC activity by 14% ( P <0.05),while having no effect on MAPK activity ( P >0.05). Conclusions UⅡ increases cytosolic free calcium concentration and activates PKC,MAPK and CaN. The signal transduction pathway between PKC and CaN has cross-talk. Background Human urotensin II (UII) is the most potent mammalian vasoconstrictor identified so far. Our previous study showed that UII is a potent mitogen of airway smooth muscle cells (ASMC) inducing ASMC proliferation in a dose- dependent manner. The signal transduction pathway of UII mitogenic effect remains to be clarified. This study was conducted to investigate the signal transduction pathway in the proliferation of ASMC induced by UII. Methods In primary cultures of rat ASMCs, activities of protein kinase C (PKC), mitogen-activated protein kinase ( MAPK) and calcineurin (CaN) induced by UII were measured. The effect of CaN on PKC and MAPK was studied by adding cyclosporin A (CsA), a specific inhibitor of CaN. Using H_7 and PD_ 98059, inhibitors of PKC and MAPK, respectively , to study the effect of PKC and MAPK on CaN. The cytosolic free calcium concentration induced by UII was measured using Fura-2 / AM. Results UII 10 -7 mol / L stimulated ASMC PKC and MAPK activities It increased by 44% and 24% (P <0.01), respectively, after incubating for 20 minutes. It increased the CaN activity in a time-dependent manner, being 1.68 times as that of control for 24 hours (P <0.01) (P <0.01) and 21% (P <0.05), respectively, and CsA 10 -6 mol / L and H_7 50 μmol / L inhibited UⅡ-stimulated CaN activity by 45% while PD_ (98059) 50 μmol / L had no effect on CaN activity (P> 0.05). CsA 10 -6 mol / L inhibited UⅡ-stimulated PKC activity by 14% (P> 0.05). Conclusions UII increases cytosolic free calcium concentration and activates PKC, MAPK and CaN. The signal transduction pathway between PKC and CaN has cross-talk.