H+ coupled uphill transport of aminocephalosporins via the dipeptide transport system in rabbit intestinal brush-border membranes. postadministration in either the intestine or the liver. Meanwhile, in the kidney, AMPC persisted until 12 h postadministration to a high degree in the proximal tubules, especially in the S3 segment cells in the tubular lumen, in which numerous small bodies that strongly reacted with the antibody were observed. All these sites of AMPC accumulation correspond well to specific sites where certain transporter systems for penicillins occur, suggesting that AMPC is actually and actively assimilated, eliminated, or excreted at these CycLuc1 sites, possibly through such certain penicillin transporters. Amoxicillin (AMPC) is usually a moderate-spectrum, bacteriolytic, -lactam antibiotic used to treat bacterial infections caused by susceptible microorganisms, acting by inhibiting the synthesis of the bacterial cell wall. In chemotherapy, the ability of a drug to reach its site of action for a desired duration is dependent on absorption, distribution, metabolism, and excretion, all of which are intimately related to the transport mechanisms in the barrier epithelia (56). Actually, pharmacotherapeutic efficacy and toxicity are governed by a multitude of pharmacodynamic and pharmacokinetic factors. Recently, a variety of transporters for penicillins have been demonstrated at the molecular level, especially in the kidney and liver, in which numerous potentially toxic xenobiotics and drugs are eliminated (23). It has been postulated that the following may be involved in the transport of penicillins: in the small intestine, the proton-coupled oligopeptide transporter PEPT1 (1); in the liver, the organic anion transporter (OAT) (5, 22, 44, 54), multidrug resistance-associated protein (Mrp2) (6, 26, 39, 42), and sodium-dependent phosphate transport protein (NPT1) (8, 21, 57); and in the kidney, the rat multispecific organic anion transporter 1 (rOAT1), rOAT2, and rOAT3 (2, 22, 23, 51, 55), Mrp2 (43), and H+/peptide cotransporters (PEPT1 and PEPT2) (23-25, 38, 45, 49, 53), etc. The conversation of such transporters with -lactam antibiotics has been studied extensively (10, 23, 25). However, the role of transporters in drug disposition, in the context of other transporters, glomerular filtration, and metabolism, has not been established. CycLuc1 Thus, knowledge of the time sequence of the distribution of penicillins in cells and tissues of animals may help develop a better understanding of the actual overall pharmacokinetics of the drugs. Also, it should be useful for developing drug therapy for infections caused by intracellular parasites, including oocytes, and renal cell lines expressing carrier-mediated transporters (10, 23, 50). However, no studies of the distribution of drugs in animal tissues are available. In the study described in this paper, we have now prepared anti-AMPC serum, characterized for its specificity, and developed an ICC procedure for determining the sites of AMPC accumulation in the rat small intestine, liver, and kidney, which represent the main organs responsible for drug absorption and elimination. Serum against a GA-conjugated AMPC was CXCR6 prepared and was demonstrated to be specific for AMPC and ABPC but very slightly specific for cephalexin. Inhibition ELISA showed that antibody binding to the solid-phase antigen was inhibited strongly with AMPC-GA-BSA, weakly with AMPC-GA, and not at all with AMPC itself or ABPC. This obtaining suggests that the antibody recognizes not only the AMPC molecule but also, in part, the carrier protein conjugation site(s) of GA. In accordance with the findings of our recent studies (18), the tissues of rats orally administered AMPC were fixed with 2% GA by perfusion in order to immobilize the antigen in the tissue as rapidly as you possibly can. Also, following systematic testing of several pretreatments aimed at demasking the immunoreactivity of AMPC in fixed tissues and reducing background staining, we succeeded in specifically localizing AMPC in the livers, kidneys, and intestine of rats injected with the drug. The H+/peptide cotransporter PEPT1, which occurs exclusively in the small intestine, is especially enriched in the microvilli of the absorptive epithelial cells and is responsible for mucosal cell transport of many peptide-like drugs, such as -lactam antibiotics (9, 29, 45, 58). The ICC method used in the present study exhibited that at 3 h postadministration, large amounts of AMPC distributed in the microvilli, as well as in the whole cell of the absorptive epithelial cells in the jejunum. This is the first study to show that AMPC is usually extensively and actively absorbed by the villus cells through their microvilli, thus strongly suggesting that this PEPT1 transporter actually mediates absorption CycLuc1 of AMPC at those.