Effects of cyclosporin a on paracellular and transcellular transport of horseradish peroxidase in perfused rat livers

The immunosuppressive agent cyclosporin A (CsA) is known to cause cholestasis. Transcellular and paracellular transport of macromolecules contribute, albeit to a minor extent, to bile formation, but little is known about the effects of CsA on these pathways. The aims of this study were to investigate the influence of CsA on tight junction (paracellular) permeability and on transcytotic vesicular pathways labeled with horseradish peroxidase (HRP) in perfused rat liver. Livers from male Sprague-Dawley rats were perfused with Krebs-Henseleit buffer (albumin 1%, RBC 20%, and amino acid mixture). Taurodehydrocholate (1 mu M/min) was coinfused into the portal vein; 1 mu g/ml of CsA, dissolved in Cremophor-EL (CsA livers), or the vehicle alone (GEL livers), was added to the medium. Tight junction permeability was assessed by administering HRP (25 mg) as a short pulse to perfused rat livers, operating under single-pass conditions. Under such conditions, HRP output into the bile shows two components: an initial peak at approximately 3-5 min, corresponding to paracellular transfer across tight junctions, and a second peak at approximately 15 min, corresponding to vesicular transport. Furthermore, we assessed the vesicular transport pathway by examining HRP-labeled vesicles in the perisinusoidal (PS) and pericanalicular (PC) areas using ultrastructural morphometric analysis. To analyze HRP in hepatocytes and to study rapid and late transcytotic vesicular pathways, a 1-min pulse of a high dose of HRP (500 and 200 mg, respectively) was given. Two and 18 min after single-pass perfusion the livers were fixed with 2.5% glutaraldehyde-0.8% paraformaldehyde in 0.1 mM cacodylate buffer, pH 7.8. The total pericanalicular area, the HRP-containing structures, were quantified morphometrically in liver samples. At concentrations of 1.2 mu g/ml, CsA produced a twofold increase in the paracellular transfer of HRP to bile. The areas under the second peak (transcellular vesicular pathway) of the biliary HRP secretion curve were similar in CEL- and CsA-treated livers. Morphometric analysis confirmed that CsA treatment did not affect the percentage area of HRP-labeled vesicles in either the pericanalicular or in the perisinusoidal area at 2 min (rapid pathway) and 18 min (late pathway). These results indicate that CsA increases tight junctional permeability whereas it does not inhibit rapid or late transcytotic vesicle pathways.