ABSTRACT:Intestinal cytochrome P450 (P450) proteins play an important role in the biotransformation of drugs and may significantly limit their oral absorption and bioavailability. Therefore, we have investigated the amount of P450 proteins via Western blot analysis along the entire intestine of male and female rats. Despite of the use of an inbred rat strain, controlled housing conditions for the animals, and a timed sample preparation, high interindividual differences in the expression of all P450 proteins was observed. CYP3A (135-243 fmol/mg of protein) and CYP2B1 (107-645 fmol/mg of protein) were the most abundant P450 isoforms in the duodenum and jejunum of rat intestine but were present in neither the ileum nor the colon. Compared with CYP2B1 and CYP3A, CYP2D1 (25-71 fmol/mg of protein) and CYP2C6 (3-10 fmol/mg of protein) were only expressed in minor amounts. CYP2C11 could not be identified in the entire rat intestine. In conclusion, this is the first systematic evaluation and quantification of the expression of P450 proteins along the entire length of the intestine in both male and female rats. These data will provide a basis for a better understanding of the extent of intestinal metabolism along the gastrointestinal tract.Absorption through the gut is a key step in the oral delivery of drugs. The main interface between gut lumen and the bloodstream is an epithelial cell layer consisting of polarized enterocytes controlling the passage of exogenous substances into the portal circulation. Enterocytes have a variety of structural features, including tight junctions reducing paracellular permeability, numerous drug transporters, and a set of metabolic enzymes that may all affect the entry of drugs into the body. The extent to which a drug is absorbed also depends on the intrinsic properties of the compound such as solubility, permeability, efflux or uptake transport properties, and susceptibility to metabolic degradation (Martinez and Amidon, 2002;Benet et al., 2004).Although the liver is known as the major site of first-pass extraction, recent studies have indicated that the small intestine also contributes significantly to the first-pass metabolism of many drugs, e.g., cyclosporine (Wu et al., 1995), nifedipine (Iwao et al., 2002), midazolam (Paine et al., 1996, and diltiazem (Iwao et al., 2004). It is known that several uptake and efflux transporter and P450 isoforms are expressed in the human and rat intestine (van de Kerkhof et al., 2007). If a drug is a substrate of efflux transporters, it may enter and exit the enterocytes several times, and, with each cycle, small quantities may be metabolized by cytochrome P450 (P450) enzymes localized in the endoplasmic reticulum. The P450 enzymes belong to a superfamily of heme proteins that show a broad substrate specificity, with substrates ranging in size from ethylene (M r 28) to cyclosporine (M r 1201) (Isin and Guengerich, 2007). Although the liver is regarded as the main organ of drug metabolism, P450 proteins are also expressed in other tissues, e.g., kidn...

August 20, 2008; doi:10.1152/ajprenal.00465.2007.-Polyuria, hypernatremia, and hypovolemia are the major clinical signs of inherited nephrogenic diabetes insipidus (NDI). Hypernatremia is commonly considered a secondary sign caused by the net loss of water due to insufficient insertion of aquaporin-2 water channels into the apical membrane of the collecting duct cells. In the present study, we employed transcriptome-wide expression analysis to study gene expression in V2 vasopressin receptor (Avpr2)-deficient mice, an animal model for X-linked NDI. Gene expression changes in NDI mice indicate increased proximal tubular sodium reabsorption. Expression of several key genes including Na ϩ -K ϩ -ATPase and carbonic anhydrases was increased at the mRNA levels and accompanied by enhanced enzyme activities. In addition, altered expression was also observed for components of the eicosanoid and thyroid hormone pathways, including cyclooxygenases and deiodinases, in both kidney and hypothalamus. These effects are likely to contribute to the clinical NDI phenotype. Finally, our data highlight the involvement of the renin-angiotensin-aldosterone system in NDI pathophysiology and provide clues to explain the effectiveness of diuretics and indomethacin in the treatment of NDI. vasopressin receptor; diabetes insipidus; G protein-coupled receptor; signal transduction; hypernatremia THE NEUROHYPOPHYSEAL PEPTIDE arginine vasopressin (AVP) is an essential hormone in mammals that regulates the body's water and electrolyte homeostasis. AVP mediates its renal actions through activation of the tubular V2 vasopressin receptor (Avpr2). Avpr2 stimulation activates the G s protein/adenylyl cyclase signaling cascade and eventually results in the insertion of the water channel protein aquaporin-2 (AQP2) into the apical plasma membrane of the collecting duct cells (42). The major physiological importance of the AVP regulatory system becomes apparent when key components are defective. AVP deficiency results in central diabetes insipidus (CDI), whereas inactivating mutations in Avpr2 or AQP2 genes lead to inherited forms of nephrogenic diabetes insipidus (NDI). In NDI, the renal concentrating capability is impaired and the kidney produces large volumes of hypotonic urine, a situation that may lead to severe dehydration and electrolyte imbalance, in particular hypernatremia and hyperchloremia (29). Mechanistically, hypernatremia is believed to be the result of net water depletion. This assumption is supported by effective treatment of NDI patients with abundant water intake. Current therapy of NDI patients further includes a low-sodium diet and thiazide diuretics, often in combination with the potassium-sparing diuretic amiloride.Thiazide diuretics [e.g., hydrochlorothiazide (HCTZ)] paradoxically decrease urine volume and increase urine osmolality in NDI patients (21). A recent study (40) showed upregulation of AQP2 and a number of distal renal Na ϩ transporters, such as the thiazide-sensitive NaCl cotransporter (NCC) and the epithelial sodiu...

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