Tamai I, Saheki A, Saitoh R, Sai Y, Yamada I, Tsuji A
Faculty of Pharmaceutical Sciences, Kanazawa University, Japan.
J Pharmacol Exp Ther. 1997 Oct;283(1):108-15.
The mechanism of the nonlinear concentration dependence of intestinal absorption of the 5-hydroxytryptamine receptor antagonist azasetron was studied by use of rat in situ intestinal perfusion, as well as an in vitro Ussing-type chamber method mounted with rat intestinal tissue and cultured monolayers of human adenocarcinoma Caco-2 cells. The intestinal absorption rate constant of azasetron evaluated by the Doluisio method increased significantly with increasing concentration of azasetron up to 10 mM in a nonlinear fashion and tended to decrease at higher concentrations. Mucosal-to-serosal directed permeation of [14C]azasetron across rat ileal sheets evaluated by the in vitro Ussing-type chamber method also increased in a nonlinear fashion in a low concentration range, followed by a decrease as the concentration was further increased, whereas serosal-to-mucosal directed permeation decreased in a concentration-dependent manner. Vectorial transport of [14C]azasetron across a Caco-2 cell monolayer was observed, with higher transport in the basolateral-to-apical direction at a trace concentration of azasetron. When the initial uptake rate of azasetron by Caco-2 cells was measured, it was saturable with an apparent half-saturation concentration of 15 mM and was reduced in the presence of several cationic compounds. These observations suggest that azasetron is taken up by a carrier-mediated transport mechanism across the intestinal epithelial cells. When the steady-state uptake of [14C]azasetron was measured, it was increased in the presence of unlabeled azasetron and ondansetron. In addition, the steady-state uptake was enhanced in the presence of a P-glycoprotein inhibitor, cyclosporin A, and by ATP-depletion of the cells, although these treatments had no effect on the initial uptake of [14C]azasetron. Furthermore, the multidrug-resistant cancer cell line K562/ADM that overexpresses P-glycoprotein accumulated azasetron less extensively than did the parental drug-sensitive K562 cells. These results strongly suggest that azasetron is secreted into the intestinal lumen predominantly by P-glycoprotein. We conclude that intestinal transport of azasetron involves specialized transporters in both the absorptive and secretory directions, and the complex nonlinear intestinal absorption characteristics can be ascribed to the participation of multiple transport mechanisms.
采用大鼠原位肠灌注以及安装有大鼠肠组织和人腺癌Caco - 2细胞培养单层的体外Ussing型小室法，研究了5 - 羟色胺受体拮抗剂阿扎司琼肠道吸收的非线性浓度依赖性机制。用Doluisio法评估的阿扎司琼肠道吸收速率常数，在阿扎司琼浓度增加至10 mM时呈非线性显著增加，而在更高浓度时趋于下降。通过体外Ussing型小室法评估的[14C]阿扎司琼跨大鼠回肠片的黏膜到浆膜方向的渗透，在低浓度范围内也呈非线性增加，随后随着浓度进一步增加而下降，而浆膜到黏膜方向的渗透则呈浓度依赖性下降。观察到[14C]阿扎司琼跨Caco - 2细胞单层的矢量转运，在阿扎司琼痕量浓度下，从基底外侧到顶端方向的转运更高。当测量Caco - 2细胞对阿扎司琼的初始摄取速率时，其具有饱和性，表观半饱和浓度为15 mM，并且在几种阳离子化合物存在下降低。这些观察结果表明，阿扎司琼通过载体介导的转运机制被肠道上皮细胞摄取。当测量[14C]阿扎司琼的稳态摄取时，在未标记的阿扎司琼和昂丹司琼存在下其增加。此外，在P - 糖蛋白抑制剂环孢素A存在下以及细胞ATP耗竭时，稳态摄取增强，尽管这些处理对[14C]阿扎司琼的初始摄取没有影响。此外，过表达P - 糖蛋白的多药耐药癌细胞系K562/ADM比亲代药物敏感的K562细胞积累阿扎司琼的程度更低。这些结果强烈表明，阿扎司琼主要通过P - 糖蛋白分泌到肠腔中。我们得出结论，阿扎司琼在肠道的转运在吸收和分泌方向均涉及特殊转运体，并且复杂的非线性肠道吸收特征可归因于多种转运机制的参与。
