Substrate specificity and pH dependence of the transport system for diphenhydramine were investigated in Caco-2 cell monolayers. Diphenhydramine uptake was not affected by any typical substrate for the renal organic cation transport system except procainamide. Along with procainamide, tertiary amine compounds with N-dimethyl or N-diethyl moieties in their structures inhibited the diphenhydramine uptake. Moreover, accumulation of diphenhydramine was stimulated by preloading the Caco-2 cells with these tertiary amines (trans-stimulation effect), indicating the existence of the specific transport system for tertiary amines with N-dimethyl or N-diethyl moieties. Efflux of diphenhydramine from monolayers was enhanced by medium acidification. In addition, intracellular acidification resulted in marked stimulation of diphenhydramine accumulation. ATP depletion of the cells caused an enhancement of diphenhydramine accumulation, suggesting the involvement of an active secretory pathway. However, P-glycoprotein did not mediate the diphenhydramine transport. These findings indicate that a novel pH-dependent tertiary amine transport system that recognizes N-dimethyl or N-diethyl moieties is involved in diphenhydramine transport in Caco-2 cells.

An oligopeptide transporter (PEPT1) in the small intestine plays an important role in the absorption of small peptides and peptide-like drugs. We examined the effect of thyroid hormone 3,5,3′-l-triiodothyronine (T3) on the activity and expression of PEPT1 in human intestinal Caco-2 cells. Treatment of Caco-2 cells with T3 inhibited [14C]glycylsarcosine uptake in a time- and dose-dependent manner. [14C]glycylsarcosine uptake was reduced by pretreatment of the cells with 100 nM T3 for 4 days (67% of control value), whereas methyl-α-d-[U-14C]glucopyranoside and [3H]threonine uptake were not decreased. Kinetic analysis showed that T3 treatment significantly decreased the maximum uptake (Vmax) value for [14C]glycylsarcosine uptake but had no effect on the K m value. Moreover, T3 treatment caused a significant decrease in the amount of PEPT1 mRNA (25% of the control). Western blotting indicated that the amount of PEPT1 protein in the apical membrane was decreased (70% of the control). These findings indicate that T3 treatment inhibits the uptake of [14C]glycylsarcosine by decreasing the transcription and/or stability of PEPT1 mRNA.

ABSTRACT: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.P-glycoprotein (Pgp) is expressed in various tissues such as brain, liver, kidney, and intestine (Cordon-Cardo et al., 1990;Brady et al., 2002) and plays an important role in defining the pharmacokinetics of many drugs. Pgp functions as a drug efflux pump and exports hydrophobic, bulky drugs such as anticancer agents, cardiac glycosides, ␤-blockers, calcium channel blockers, and immunosuppressants. Since Pgp has a broad substrate recognition, the concomitant administration of drugs often causes drug interactions by inhibiting Pgpmediated transport (Yu, 1999). For example, inhibition of digoxin transport in cultured epithelial cell lines expressing Pgp by various drugs such as quinidine (Tanigawara et al., 1992;Fromm et al., 1999), verapamil (Tanigawara et al., 1992, and cyclosporin A (Okamura et al., 1993) has been reported. We have also demonstrated that the renal clearance of digoxin was decreased in patients receiving a concomitant administration of clarithromycin and, accordingly, the plasma concentration of digoxin was increased (Wakasugi et al., 1998). The mechanism of this interaction was explained by the inhibition of Pgp-mediated tubular secretion of digoxin. On the other hand, recent studies have demonstrated that changes in the expression levels of Pgp affect the pharmacokinetic properties of Pgp substrates. Greiner et al. (1999) reported that rifampin administration induced Pgp expression in the small intestine and reduced the plasma concentration of orally administered digoxin, suggesting that alterations in the expression levels of Pgp are closely involved in the inter-and intraindividual variability of pharmacokinetics of Pgp substrates.Thyroid hormone is secreted from the thyroid gland to maintain normal growth and development, normal body temperature, and normal energy levels. Most of its effect...

These results indicate that in hyperthyroid rats, the activity and expression of PEPT1 were decreased in the small intestine.