Effects of topdressing nitrogen on apparent quality, dry matter accumulation and contents of active components in the root of Isatis indigotica Fort. Pharmacokinetic-pharmacodynamic binding model of febrile rats in vivo for study on epigoitrin from total alkaloids of Radix lsatidis Content determination of epigoitrin in Radix Isatidis and its preparation by RP-HPLC Correlation between chemical composition in process of Isatidis Radix preparation and its pharmacological activity