H/dipeptide absorption across the human intestinal epithelium is controlled indirectly via a functional Na/H exchanger.
OBJECTIVE
For optimal nutrient absorption to occur, the enterocyte must express a range of specialist ion-driven carrier proteins that function cooperatively in a linked and mutually dependent fashion. Thus, absorption via the human intestinal H(+)-coupled di/tripeptide transporter (hPepT1) is dependent on maintenance of the trans-apical driving force (the H(+)-electrochemical gradient) established, in part, by brush-border Na(+)/H(+) exchanger (NHE3) activity. This study aimed to examine whether physiologic regulation of NHE3 activity can limit hPepT1 capacity and, therefore, protein absorption after a meal.


METHODS
hPepT1 and NHE3 activities were determined in intact human intestinal epithelial Caco-2 cell monolayers by measurements of [(14)C]glycylsarcosine transport and uptake, (22)Na(+)-influx, H(+)-influx, and H(+)-efflux. Expression of NHE regulatory factors was determined by reverse-transcriptase polymerase chain reaction.


RESULTS
Optimal dipeptide transport was observed in the presence of a transapical pH gradient and extracellular Na(+). At apical pH 6.5, and only in Na(+)-containing media, protein kinase A activation (by forskolin or vasoactive intestinal peptide) or selective NHE3 inhibition (by S1611) reduced transepithelial dipeptide transport and cellular accumulation by a reduction in the capacity (without effect on affinity) of dipeptide uptake.


CONCLUSIONS
Protein kinase A-mediated modulation of intestinal dipeptide absorption is indirect via effects on the apical Na(+)/H(+) exchanger.
Thwaites
David T
DT
Department of Physiological Sciences, Medical School, University of Newcastle upon Tyne, Newcastle upon Tyne, United Kingdom. d.t.thwaites@ncl.ac.uk
Kennedy
David J
DJ
Raldua
Demetrio
D
Anderson
Catriona M H
CM
Mendoza
Maria E
ME
Bladen
Catherine L
CL
Simmons
Nicholas L
NL
D001692:Biological Transport; D018938:Caco-2 Cells; D002352:Carrier Proteins / Q000502:physiology*; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology; D004151:Dipeptides / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007408:Intestinal Absorption; D007413:Intestinal Mucosa / Q000378:metabolism; D000074011:Peptide Transporter 1; D010750:Phosphoproteins / Q000235:genetics; D012333:RNA, Messenger / Q000032:analysis; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000502:physiology*; D027981:Symporters; D014660:Vasoactive Intestinal Peptide / Q000378:metabolism
D002352:Carrier Proteins; D004151:Dipeptides; D000074011:Peptide Transporter 1; D010750:Phosphoproteins; D012333:RNA, Messenger; C082186:SLC15A1 protein, human; C000613385:SLC9A3 protein, human; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D027981:Symporters; C069139:growth factor-activatable Na-H exchanger NHE-1; C093376:sodium-hydrogen exchanger regulatory factor; D005576:Colforsin; D014660:Vasoactive Intestinal Peptide; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases