OBJECTIVE To identify the effective fraction of Platycodon grandiflorum polysaccharides(PGPs) that promotes the intestinal absorption of platycodin D(PD). It also seeks to compare the microstructure differences among different sub-fractions.

METHODS An in vivo single-pass intestinal perfusion model in rats was employed. The apparent permeability coefficient(P_app), absorption rate constant(K_a), and cumulative absorption per unit area per hour(Q) of PD were used as indicators. These indicators helped compare the effects of PGPs on PD’s intestinal absorption in the duodenum, jejunum, ileum, and colon. PGPs were fractionated into three sub-fractions(PGPs-1, PGPs-2, and PGPs-3) via DEAE-cellulose column chromatography. Elution was carried out with NaCl solutions of different concentrations. High-performance liquid gel permeation chromatography was used to determine the molecular weights of the three sub-fractions. Their effects on PD absorption in the ileum were further compared. P-glycoprotein(P-gp) inhibitors were utilized to investigate P-gp’s role in the promotion of PD intestinal absorption by PGPs-2. A laser particle size analyzer and scanning electron microscopy were employed to characterize the microstructure(including particle size and morphology) of five perfusates. These perfusates included the Platycodon grandiflorum aqueous extract perfusate, platycosides extract(PE) perfusate, PE+PGPs mixed perfusate, PGPs-2 perfusate, and PE+PGPs-2 perfusate. The spatial structure and morphology of the PGPs-2+PE perfusate were observed using an atomic force microscope.

RESULTS In the jejunum and ileum, PGPs significantly increased PD’s P_app, K_a, and Q(P<0.05 or P<0.01). This effect was comparable to that observed in the Platycodon grandiflorum aqueous extract group. In the colon, PGPs also promoted PD absorption. However, the promoting effect was less significant than that in the jejunum and ileum. In the duodenum, PGPs exerted no obvious promoting effect on PD absorption. The molecular weights of PGPs-1, PGPs-2, and PGPs-3 were 3638.72 Da, 15826.11 Da, and 22475.36 Da, respectively. Among the three polysaccharides, only PGPs-2 could significantly increase the P_app, K_a, and Q of PD in ileum(P<0.01), but PGPs-1 and PGPs-3 had no significant effect. No significant difference was found in the promoting effect on PD intestinal absorption between PGPs-2 and the original PGPs. However, the promotion of PD absorption by PGPs-2 was independent of P-gp. PGPs-2 could self-assemble with PD to form nanoparticles. These nanoparticles had an average particle size of approximately (305.24±0.57)nm and uniform dispersion(polydispersity index, PDI=0.2723±0.0108). This indicated that the nano-system formed by PGPs-2 and PD is more stable.

CONCLUSION PGPs can enhance the intestinal absorption of PD, with PGPs-2 identified as the active fraction responsible for its absorption-promoting effect. The absorption-facilitating mechanism may be associated with the formation of stable nanoparticles via self-assembly of PGPs-2 and PD. This study provides a reference for research on the interactions among multiple components in traditional Chinese medicine.