Aim:To investigate the mechanism of silibinin-protected isoproterenol-inducedapoptosis in rat cardiac myocytes.Methods:The viability of rat cardiac myocyteswas measured by MTT method.The apoptotic ratio was measured by terminaldeoxynucleotidyl transferase-mediated dUTP nick end-labeling.Protein kinase C(PKC) activity assay was carried out according to the instructions of the PepTagnon-radioactive protein kinase C assay kit.Western blot analysis was used toevaluate the level of Ras,Raf-1 and mitogen-activated protein kinase (MAPK)expression.Results:The protective effects of silibinin were significantly sup-pressed by inhibitors,including genistein,manumycin A and GW5074 [inhibitorsfor protein tyrosine kinases (PTK),Ras and Raf-1,respectively].The exposure ofrat cardiac myocytes to isoproterenol alone caused decreased PKC activity,whichwas prevented by pretreatment with silibinin dose-dependently.Simultaneously,the increased expression of Ras and Raf-1 activated by silibinin were blocked bythe PKC inhibitor,stauroporine.In addition,the extracellularly responsive kinase(ERK) inhibitor,PD98059,suppressed silibinin-protected apoptosis,whereas thep38 MAPK inhibitor,SB203580,protected cardiac myocytes from isoproterenol-induced injury,and the c-Jun N-terminal kinase (JNK) inhibitor,SP600125 had noprotective effects.Furthermore,Western blot analysis showed that the expres-sion of phosphorylated ERK was increased by silibinin,the expression of phos-phorylated p38 MAPK was decreased and total ERK,p38,JNK and phosphory-lated JNK MAPK did not change after treatment with both isoproterenol andsilibinin.Furthermore,pretreatment of cardiac myocyte with PKC,Ras and Rafinhibitors significantly blocked ERK phosphorylation.Conclusion:Silibinin issuggested to protect isoproterenol-induced rat cardiac myocyte apoptosis byactivating the tyrosine kinase pathway,PKC and MAPK pathways. Aim:To investigate the mechanism of silibinin-protected isoproterenol-inducedapoptosis in rat myocytes.Methods:The viability of rat myocyteswas measured by MTT method.The apoptotic ratio was measured by terminaldeoxynucleotidyl transferase-mediated dUTP nick end-labeling.Protein kinase C (PKC) activity assay was carried out according to the instructions of the PepTagnon-radioactive protein kinase C assay kit. Western blot analysis was used toevaluate the level of Ras,Raf-1 and mitogen-activated protein kinase (MAPK)expression.Results: The protective effects of silibinin were significantly inhibited by inhibitors, including genistein, manumycin A and GW5074 [inhibitors for protein tyrosine kinases (PTK), Ras and Raf-1, respectively]. The exposure ofrat cardiac myocytes to isoproterenol alone caused decreased PKC activity. ,which was prevented by pretreatment with silibinin dose-dependently.Simultaneously,the increased expression of Ras and Raf-1 activated by silibinin were blocked Bythe PKC inhibitor,stauroporine.In addition,the extracellularly responsive kinase(ERK) inhibitor,PD98059,suppressed silibinin-protected apoptosis,whereas the p38 MAPK inhibitor,SB203580,protected cardiac myocytes from isoproterenol-induced injury,and the c-Jun N-terminal Kinase (JNK) inhibitor, SP600125 had noprotective effects.Furthermore, Western blot analysis showed that the expres-sion of phosphorylated ERK was increased by silibinin, the expression of phos-phorylated p38 MAPK was decreased and total ERK,p38,JNK and phosphory- Lated JNK MAPK did not change after treatment with both isoproterenol andsilibinin.Furthermore,pretreatment of cardiac myocyte with PKC, Ras and Rafinhibitors significantly blocked ERK phosphorylation.Conclusion:Silibinin issuggested to protect isoproterenol-induced rat myocyte apoptosis by activating the tyrosine kinase pathway,PKC And MAPK pathways.