title: Effect of HEPES buffer on the uptake and transport of P-glycoprotein substrates and large neutral amino acids.
abstract: HEPES has been widely employed as an organic buffer agent in cell culture medium as well as uptake and transport experiments in vitro. However, concentrations of HEPES used in such studies vary from one laboratory to another. In this study, we investigated the effect of HEPES on the uptake and bidirectional transport of P-gp substrates employing both Caco-2 and MDCK-MDR1 cells. ATP-dependent uptake of glutamic acid was also examined. ATP production was further quantified applying ATP Determination Kit. An addition of HEPES to the growth and incubation media significantly altered the uptake and transport of P-gp substrates in both Caco-2 and MDCK-MDR1 cells. Uptake of P-gp substrates substantially diminished as the HEPES concentration was raised to 25 mM. Bidirectional (A-B and B-A) transport studies revealed that permeability ratio of P(appB-A) to P(appA-B) in the presence of 25 mM HEPES was significantly higher than control. The uptake of phenylalanine is an ATP-independent process, whereas the accumulation of glutamic acid is ATP-dependent. While phenylalanine uptake remained unchanged, glutamic acid uptake was elevated with the addition of HEPES. Verapamil is an inhibitor of P-gp mediated uptake; elevation of cyclosporine uptake in the presence of 5 muM verapamil was compromised by the presence of 25 mM HEPES. The results of ATP assay indicated that HEPES stimulated the production of ATP. This study suggests that the addition of HEPES in the medium modulated the energy dependent efflux and uptake processes. The effect of HEPES on P-gp mediated drug efflux and transport may provide some mechanistic insight into possible reasons for inconsistencies in the results reported from various laboratories.
authors.affiliation: Division of Pharmaceutical Sciences, School of Pharmacy, University of Missouri-Kansas City, Kansas City, Missouri 64108-2718, USA.
authors.forename: Shuanghui
authors.initials: S
authors.lastname: Luo
authors.forename: Dhananjay
authors.initials: D
authors.lastname: Pal
authors.forename: Sujay J
authors.initials: SJ
authors.lastname: Shah
authors.forename: Deep
authors.initials: D
authors.lastname: Kwatra
authors.forename: Kalyani D
authors.initials: KD
authors.lastname: Paturi
authors.forename: Ashim K
authors.initials: AK
authors.lastname: Mitra
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D021542:Amino Acids, Neutral / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects*; D018938:Caco-2 Cells; D002460:Cell Line; D002463:Cell Membrane Permeability / Q000187:drug effects; D016572:Cyclosporine / Q000378:metabolism; D004285:Dogs; D018698:Glutamic Acid / Q000378:metabolism; D006531:HEPES / Q000494:pharmacology*; D006801:Humans; D061466:Lopinavir; D010649:Phenylalanine / Q000378:metabolism; D011744:Pyrimidinones / Q000378:metabolism; D019438:Ritonavir / Q000378:metabolism; D014700:Verapamil / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D021542:Amino Acids, Neutral; D011744:Pyrimidinones; D061466:Lopinavir; D018698:Glutamic Acid; D010649:Phenylalanine; D016572:Cyclosporine; D000255:Adenosine Triphosphate; D014700:Verapamil; D019438:Ritonavir; D006531:HEPES