The activity of the proton-coupled apical peptide transporter of human intestinal cells (hPepT1) is regulated indirectly through a functional couple with the apical Na⁺/H⁺ exchanger NHE3 (Thwaites et al. 2002). In contrast, the peptide transporter present at the basolateral membrane shows a different dependence upon pH and is directly dependent upon medium Na⁺ but independent of basolateral NHE1 activity (Henderson et al. 2002). We now report that the basolateral peptide transporter is negatively regulated by vasoactive intestinal peptide (VIP) acting via intracellular cAMP.

[¹⁴C]Gly-Sar (10 µM, 0.5 µCi ml^-1, 30 min) uptake (37 °C ) across the basolateral membrane was measured as described previously (Henderson et al. 2002) in Na⁺-containing (137 mM NaCl) or Na⁺-free (choline-Cl replacement) Krebs-Ringer solution (pH 5.5). In Na⁺-containing media, basolateral [¹⁴C]Gly-Sar uptake was significantly inhibited by 10 nM VIP and 10 mM forskolin from 15.73 ± 1.26 to respectively 8.44 ± 0.68 and 7.76 ± 0.57 pmol cm^-2. (30 min)^-1 (means ± S.E.M., n = 6, P < 0.001 vs. control, ANOVA). Ten nanomolar pituitary adenylate-cyclase activating peptide (PACAP) and 10 mM 8-Br-cAMP also inhibited basolateral [¹⁴C]Gly-Sar uptake, but 10 mM dideoxyforskolin was without inhibitory effect. Additions of VIP, PACAP, forskolin and 8-Br-cAMP in combination did not inhibit basolateral [¹⁴C]Gly-Sar uptake further. In Na⁺-free media, basolateral [¹⁴C]Gly-Sar uptake was inhibited from 19.54 ± 1.98 to 9.06 ± 0.72 pmol cm^-2 (30 min)^-1 (n = 6, P < 0.001 vs. control), and VIP, PACAP, forskolin and 8-Br-cAMP were without effect. The kinetics of basolateral Gly-Sar uptake in Na⁺-containing media are best fitted by a saturable component (K_m 5.78 ± 1.31 mM, V_max 5686 ± 1343 pmol cm^-2 (30 min)^-1) and linear component of 81.8 ± 56.9 X 10³ cm^-1 (30 min)^-1. In the presence of forskolin (10 mM) inhibition is associated with a marked reduction of the V_max of the saturable component to 168 ± 27 pmol cm^-2 (30 min)^-1. Finally, in Na⁺-containing medium the inhibitory action of VIP was of high affinity (half-maximal inhibition at 27 ± 17 pM (S.E.M.)).

In conclusion VIP acting via cAMP downregulates the saturable Na⁺-dependent component of dipeptide uptake at the basolateral membrane of human intestinal Caco-2 epithelial cells.

F.D.H.’s studentship was sponsored by GlaxoSmithKline.