Purpose:To investigate the mechanism of incompatibility pairs between liquorice root and Peking spurge in the clinical practice of Traditional Chinese Medicine as far as affecting the P-glycoprotein (P-gp) function and changing Rhodamine 123 (R123),one p-gp substrate transportation via gastrointestinal membranes were concerned.Methods:Three different decoctions of liquorice root and Peking spurge served as the experimental group which were prepared based on the requirement of clinical use of Traditional Chinese Medicine and were taken their quality control by high performance liquid Chromatography.As the known P-gp inhibitors,solution of verapamil was set as the positive control in experiment.Negative control group:saline only.First of all,R123 intestinal epithelial transport via rat intestinal membranes was investigated by ussing chamber method in both absorptive and secretory directions,after oral administration of saline,verapamil solution and three different decoctions of liquorice root and Peking spurge.Furthermore,the effects of verapamil solution and decoctions of liquorice root and Peking spurge on the jejunum absorption of R123 by an in situ closed loop method were examined.Finally,the change of the function and expression of P-gp for the administration of the above solutions was carried out by quantitative RT-PCR analyses in jejunum.As the model drug of non-p-gp substrate transported by a passive diffusion,fluorescein sodium (CF)was as same as R123 investigated after oral administration in vitro and in vivo experiment.Results:Quality control of extracts of liquorice root and Peking spurge is according to Chinese pharmacopoeia requirements.The determination of the amount of ammonium glycyrrhizinate and glycyrrhizin in liquorice root extract was 0.6% and 2.2% (n=5),respectively,and the amount of ammonium euphol and salvianolic acids in Peking spurge extracts was 0.1% and 2.0% (n=5),respectively.In our study,the permeability of R123 via jejunum membrane was higher than other membranes in both directions by ussing chamber method (ER was 5.69±4.13,3.51±2.20,2.02±0.84 on the three membranes,respectively).However,after oral administration of verapamil solution(the positive group,15mg/kg),and combine decoction of the liquorice root and Peking spurge (the experimental group,2g/kg of each dried herb),the permeability of R123 was reduced.(ER was 0.42±0.33,and 0.61±0.40,respectively,p<0.05) Similar effects obtained following the in situ closed loop method showed that R123 was significantly enhanced in absorption directions.After oral administration of combine decoction of the two herbs,P-gp expression levels decreased,whereas decoction of liquorice root group or Peking spurge had no significant effect compared with negative control group (p<0.05).Conclusion:Combined decoction of the liquorice root and Peking spurge may exert strong effect to inhibit the P-gp-mediated efflux system,thereby increasing the intestinal absorption of p-gp substrates such as R123.Therefore,one of reasons about the incompatibility pairs between liquorice root and peking spurge in Traditional Chinese Medicine may contribute to its increasing absorption of some toxic p-gp substrates from ether liquorice root or peking spurge when co-administrated.In other hand,it hints that the use of Peking spurge and liquorice root in oral drug formulations may improve the bio-availability of co-prescribed drugs,if they are P-gp substrates.