Glucagon-like peptide-1-(7-36)-amide (GLP-1) is involved in satiety control and glucose homeostasis. Animal studies suggest a physiological role for GLP-1 in water and salt homeostasis. This study's aim was to define the effects of GLP-1 on water and sodium excretion in both healthy and obese men. Fifteen healthy subjects and 16 obese men (mean body mass index, 36 kg/m2) were examined in a double-blind, placebo-controlled, crossover study to demonstrate the effects of a 3-h infusion of GLP-1 on urinary sodium excretion, urinary output, and the glomerular filtration rate after an i.v. 9.9-g salt load. Infusion of GLP-1 evoked a dose-dependent increase in urinary sodium excretion in healthy subjects (from 74 +/- 8 to 143 +/- 18 mmol/180 min, P = 0.0013). In obese men, there was a significant increase in urinary sodium excretion (from 59 to 96 mmol/180 min, P = 0.015), a decrease in urinary H+ secretion (from 1.1 to 0.3 pmol/180 min, P = 0.013), and a 6% decrease in the glomerular filtration rate (from 151 +/- 8 to 142 +/- 8 ml/min, P = 0.022). Intravenous infusions of GLP-1 enhance sodium excretion, reduce H+ secretion, and reduce glomerular hyperfiltration in obese men. These findings suggest an action at the proximal renal tubule and a potential renoprotective effect.

To identify specific transporters that drive xenobiotics from central nervous system to blood, the accumulation of fluorescent drugs was studied in isolated capillaries from rat and pig brain using confocal microscopy and quantitative image analysis. Luminal accumulation of daunomycin and of fluorescent derivatives of cyclosporine A (CSA) and ivermectin was concentrative, specific, and energy-dependent (inhibition by NaCN). Transport was reduced by PSC 833, ivermectin, verapamil, CSA, and vanadate, but not by leukotriene C(4) (LTC(4)), indicating the involvement of P-glycoprotein. Luminal accumulation of the fluorescent organic anions sulforhodamine 101 and fluorescein methotrexate was also concentrative, specific, and energy-dependent. LTC(4), chlorodinitrobenzene, and vanadate reduced transport of these compounds, but PSC 833 and verapamil did not, indicating the involvement of a multidrug resistance-associated protein (Mrp). Immunostaining localized P-glycoprotein and Mrp2 to the luminal surface of the capillary endothelium and quantitative polymerase chain reaction showed Mrp1 and Mrp2 expression. Finally, the HIV protease inhibitors saquinavir and ritonavir were potent inhibitors of transport mediated by both P-glycoprotein and Mrp. These results validate a new method for studying drug transport in isolated brain capillaries and implicate both P-glycoprotein and one or more members of the Mrp family in drug transport from central nervous system to blood.

The human brain endothelial capillary cell line hCMEC/D3 has been developed recently as a model for the human blood‐brain barrier. In this study a further characterization of this model was performed with special emphasis on permeability properties and active drug transport. Para‐ or transcellular permeabilities (Pe) of inulin (0.74 × 10−3 cm/min), sucrose (1.60 × 10−3 cm/min), lucifer yellow (1.33 × 10−3 cm/min), morphine (5.36 × 10−3 cm/min), propranolol (4.49 × 10−3 cm/min) and midazolam (5.13 × 10−3 cm/min) were measured. By addition of human serum the passive permeability of sucrose could be reduced significantly by up to 39%. Furthermore, the expression of a variety of drug transporters (ABCB1, ABCG2, ABCC1–5) as well as the human transferrin receptor was demonstrated on the mRNA level. ABCB1, ABCG2 and transferrin receptor proteins were detected and functional activity of ABCB1, ABCG2 and the ABCC family was quantified in efflux experiments. Furthermore, ABCB1‐mediated bidirectional transport of rhodamine 123 was studied. The transport rate from the apical to the basolateral compartment was significantly lower than that in the inverse direction, indicating directed p‐glycoprotein transport. The results of this study demonstrate the usefulness of the hCMEC/D3 cell line as an in vitro model to study drug transport at the level of the human blood‐brain barrier.

. The interaction of cyclosporin A (CyA) with p‐glycoprotein during intestinal uptake was investigated by a combination of in vitro experiments with human Caco‐2 cells and an intubation study in healthy volunteers. . CyA uptake into the cells was not saturable and exhibited only a low temperature sensitivity, suggesting passive diffusion. When the permeation of CyA across Caco‐2 monolayers from the apical to the basolateral side was determined, overall transport had an apparently saturable component up to a concentration of 1 μm. At higher concentrations permeation increased over‐proportionally. Calculation of the kinetic parameters of apical to basolateral permeation suggested a diffusional process with a KD of 0.5 μl min−1 per filter, which was overlayed by an active system in basolateral to apical direction with a KM of 3.8 μm and a Jmax of 6.5 picomol min−1 per filter. . CyA permeation was significantly higher when the drug was given from the basolateral side as compared to the permeation from the apical side. Apical to basolateral transport of CyA was increased in the presence of vinblastine, daunomcyin and a non‐immunosuppressive CyA‐derivative. All compounds inhibit p‐glycoprotein‐mediated transport processes. Basolateral to apical permeation of CyA showed a dose‐dependent decrease in the presence of vinblastine. Permeation of daunomycin across Caco‐2 cell monolayers was also higher from the basolateral to the apical side than vice versa. Basolateral to apical permeation was decreased in the presence of SDZ PSC 833 and cyclosporin A. . Western blot analysis of Caco‐2 cells with the monoclonal antibody C219 confirmed the presence of p‐glycoprotein in the used cell system. . When the absorption of CyA in the gastrointestinal (G***I)‐tract of healthy volunteers was determined, a remarkable decrease of the plasma AUC could be observed dependent on the location of absorption in the rank order stomach > jejunum/ileum > colon. The decrease in absorption exhibited a marked correlation (r = 0.994) to the expression of mRNA for p‐glycoprotein over the G**I‐tract (stomach < jejunum < colon). . All data provide evidence that CyA is a substrate of p‐glycoprotein in the G***I‐tract, which might explain the local differences and the high variability in cyclosporin absorption found in vivo.

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