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P-glycoprotein (Pgp) is expressed in various normal tissues and plays an important role in drug absorption and disposition. In addition, it is supposed that alterations in the expression levels of Pgp are involved in the inter- and intraindividual variability of pharmacokinetics of many drugs. Since pharmacokinetic properties of various drugs are altered in patients with thyroid disorders, we examined the expression of Pgp and mdr1a/1b mRNA in the kidney, liver, jejunum, and ileum from euthyroid and hyperthyroid rats. Western blot analysis revealed that Pgp expression was markedly increased in the kidney and liver of hyperthyroid rats. In contrast, it was slightly increased in the jejunum and ileum. mdr1a/1b mRNA levels were significantly increased in the kidney of hyperthyroid rats. However, they were not increased in the liver as well as in the jejunum and ileum of hyperthyroid rats. Expression levels of bile salt export pump and mdr2 mRNA were also unchanged in hyperthyroid rat liver. Taken together, these findings suggest that thyroid hormone induces Pgp expression in a tissue-selective manner, and that the modulation of mdr1a/1b mRNA expression in the hyperthyroid state varies among tissues.

Purpose.To examine the effect of thyroid hormone status on PEPT1 in vivo, the activity and expression of PEPT1 in the small intestine were examined in euthyroid and hyperthyroid rats. Methods.Hyperthyroidism was induced by treating rats with l-thyroxine (12 mg/L) in the drinking water for 21 days. Transport activity was measured by everted small intestinal preparations and in situintestinal loop technique. Expressions of PEPT1 mRNA and protein were evaluated by competitive polymerase chain reaction and Western blotting, respectively. Results.The uptake of [14C]glycylsarcosine by everted small intestinal preparations was significantly decreased in hyperthyroid rats, whereas that of methyl-α-d-[14C(U)]-glucopyranoside was not altered. Kinetic analysis showed that the Vmaxvalue for [14C]glycylsarcosine uptake was significantly decreased in hyperthyroid rats, whereas the Kmvalue was not affected. The mean portal vein concentrations after intrajejunal administration of [14C]glycylsarcosine were also decreased in hyperthyroid rats. Moreover, hyperthyroidism caused a significant decrease in the expression of PEPT1 mRNA in the small intestine, whereas the expression of Na+/glucose cotransporter (SGLT1) mRNA was not changed. The level of PEPT1 protein was also decreased in the small intestine of hyperthyroid rats. Conclusions.These results indicate that in hyperthyroid rats, the activity and expression of PEPT1 were decreased in the small intestine.