title: Transport mechanisms of carnosine in SKPT cells: contribution of apical and basolateral membrane transporters.
abstract: OBJECTIVE
  The aim of this study was to investigate the transport properties of carnosine in kidney using SKPT cell cultures as a model of proximal tubular transport, and to isolate the functional activities of renal apical and basolateral transporters in this process.
  
  
  METHODS
  The membrane transport kinetics of 10 microM [3H]carnosine was studied in SKPT cells as a function of time, pH, potential inhibitors and substrate concentration. A cellular compartment model was constructed in which the influx, efflux and transepithelial clearances of carnosine were determined. Peptide transporter expression was probed by RT-PCR.
  
  
  RESULTS
  Carnosine uptake was 15-fold greater from the apical than basolateral surface of SKPT cells. However, the apical-to-basolateral transepithelial transport of carnosine was severely rate-limited by its cellular efflux across the basolateral membrane. The high-affinity, proton-dependence, concentration-dependence and inhibitor specificity of carnosine supports the contention that PEPT2 is responsible for its apical uptake. In contrast, the basolateral transporter is saturable, inhibited by PEPT2 substrates but non-concentrative, thereby, suggesting a facilitative carrier.
  
  
  CONCLUSIONS
  Carnosine is expected to have a substantial cellular accumulation in kidney but minimal tubular reabsorption in blood because of its high influx clearance across apical membranes by PEPT2 and very low efflux clearance across basolateral membranes.
authors.affiliation: Department of Pharmaceutical Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA.
authors.forename: Dilara
authors.initials: D
authors.lastname: Jappar
authors.forename: Yongjun
authors.initials: Y
authors.lastname: Hu
authors.forename: Richard F
authors.initials: RF
authors.lastname: Keep
authors.forename: David E
authors.initials: DE
authors.lastname: Smith
mesh_terms: D000465:Algorithms; D000818:Animals; D002336:Carnosine / Q000378:metabolism*; D048429:Cell Size; D002478:Cells, Cultured; D004848:Epithelium / Q000378:metabolism; D007202:Indicators and Reagents; D007687:Kidney Tubules, Proximal / Q000378:metabolism; D007700:Kinetics; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D011918:Rats, Inbred SHR; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013379:Substrate Specificity; D027981:Symporters / Q000235:genetics / Q000378:metabolism
chemical_list: D007202:Indicators and Reagents; D026901:Membrane Transport Proteins; D027981:Symporters; C093694:hydrogen-coupled oligopeptide transporter PepT2; D002336:Carnosine