The balancing act of foraging: mammalian herbivores trade-off multiple risks when selecting food patches.

Animals face multiple risks while foraging such as the risk of acquiring inadequate energy from food and the risk of predation. We evaluated how two sympatric rabbits (pygmy rabbits, Brachylagus idahoensis, and mountain cottontail rabbits, Sylvilagus nuttallii) that differ in size, use of burrows, and habitat specialization in the sagebrush-steppe of western North America respond to different types and levels of perceived risks (i.e., fitness cost × probability of occurrence), including fiber and toxins in food, exposure to predation, and distance from a refuge. We measured food intake by the rabbits at paired food patches that varied in these risks and used the method of paired comparisons to create a relative ranking of habitat cues, which revealed an animal's perceived risk on a single scale representing an integrated response to a variety of risks. Pygmy rabbits perceived exposure to predation risk and distance from a burrow as riskier than did cottontails, whereas cottontails perceived dietary toxin as riskier. Pygmy rabbits consumed lower quality food, containing higher fiber or toxins, thereby avoided feeding in exposed patches or traveling far from their burrow to forage. In contrast, cottontails fed in exposed patches and traveled farther from the burrow to obtain higher quality food. We have shown how risks can be integrated into a single model that allows animals to reveal their perceptions of risks on a single scale that can be used to create a spatially explicit landscape of risk.

Effects of starch and fibre in pelleted diets on nutritional status of mule deer (Odocoileus hemionus) fawns.

To compare the effects of a low-starch, high-fibre diet [LSHF; 51.6% neutral detergent fibre (NDF), 3.0% starch, 14.8% crude protein (CP)] and a high-starch, low-fibre diet (HSLF; 33.3% NDF, 20.0% starch, 19.6% CP) on the nutritional status of captive exotic ruminants, 16 mule deer (Odocoileus hemionus) were fed one of these two diets ad libitum with ≤25% alfalfa hay cubes from 10 days to 68 weeks of age. During five sampling periods beginning in November and spaced 6-12 weeks apart thereafter, feed intake, rumen and blood chemistry, faecal scores, growth and body condition were measured. Dry matter intake, digestible energy intake, time spent ruminating and feeding and blood acetate concentration were greater for deer fed LSHF (p < 0.05 for all). Lower dietary CP led to reduced blood urea nitrogen for deer consuming LSHF (p = 0.004). Deer had the same faecal scores, growth and body fat among treatments (all p > 0.05). These findings show pelleted diets with less starch, more fibre, and reduced protein met the energy and protein requirements of growing mule deer, a medium-sized browsing ruminant, as well as traditional grain-based diets while more closely mimicking natural forages and stimulating a more natural feeding behaviour.

Digestibility, nitrogen excretion, and mean retention time by North American porcupines (Erethizon dorsatum) consuming natural forages.

North American porcupines (Erethizon dorsatum) subsist predominantly on low-protein, high-fiber, high-tannin diets. Therefore, we measured the porcupine's ability to digest dry matter, fiber, and protein by conducting digestion trials on eight natural forages and one pelleted ration varying in concentration of fiber, nitrogen, and tannins. On these diets, dry matter intake ranged from 5 to 234 g/kg(0.75)/d and dry matter digestibility ranged from 62% to 96%. Porcupines digested highly lignified fiber better than many large hindgut fermenters and ruminants. The porcupine's ability to digest fiber may be explained, in part, by their lengthy mean retention time of particles (38.43+/-0.56 h). True nitrogen digestibility was 92% for nontannin forages and pellets. Endogenous urinary nitrogen was 205 mg N/kg(0.75)/d, and metabolic fecal nitrogen was 2.8 g N/kg dry matter intake. Porcupines achieved nitrogen balance at relatively low levels of nitrogen intake (346 mg N/kg(0.75)/d). Tannins reduced the porcupines' ability to digest protein. However, the reduction in protein digestion was not predictable from the amount of bovine serum albumin precipitated. Like many herbivores, porcupines may ameliorate the effects of certain tannins in natural forages on protein digestibility through physiological and behavioral adaptations.

Metabolism and disposition of moxonidine in Fischer 344 rats.

The metabolism and disposition of moxonidine (4-chloro-5-(imidazolidin-2-ylidenimino)-6-methoxy-2-methylp yrimidine ), a potent central-acting antihypertensive agent, were investigated in F344 rats. After an i.v. or oral administration of 0.3 mg/kg of [(14)C]moxonidine, the maximum plasma concentrations of moxonidine were determined to be 146.0 and 4.0 ng/ml, respectively, and the elimination half-lives were 0.9 and 1.1 h, respectively. The oral bioavailability of moxonidine was determined to be 5.1%. The metabolic and elimination profiles of moxonidine were determined after an oral administration of 5 mg/kg of [(14)C]moxonidine. More than fifteen phase I and phase II metabolites of moxonidine were identified in the different biological matrices (urine, plasma, and bile). Oxidative metabolism of moxonidine leads to the formation of hydroxymethyl moxonidine and a carboxylic acid metabolite as the major metabolites. Several GSH conjugates, cysteinylglycine conjugates, cysteine conjugates, and a glucuronide conjugate were also identified in rat bile samples. The radiocarbon was eliminated primarily by urinary excretion in rats, with 59.5% of total radioactivity recovered in the urine and 38.4% recovered in the feces within 120 h. In bile duct-cannulated rats, about 39.7% of the radiolabeled dose was excreted in the urine, 32.6% excreted in the bile, and approximately 2% remained in the feces. The results from a quantitative whole body autoradiography study indicate that radiocarbon associated with [(14)C]moxonidine and/or its metabolites was widely distributed to tissues, with the highest levels of radioactivity observed in the kidney and liver. In summary, moxonidine is well absorbed, extensively metabolized, widely distributed into tissues, and rapidly eliminated in rats after oral administration.

Harvesting, rumination, digestion, and passage of fruit and leaf diets by a small ruminant, the blue duiker.

Because small ruminants (<15 kg) have a high ratio of metabolic rate to fermentation capacity, they are expected to select and require low-fiber, nutrient-dense concentrate diets. However, recent studies suggest that small ruminants may not be as limited in their digestive capacity as previously thought. In this study, we examined harvesting, rumination, digestion, and passage of three diets (domestic figs Ficus carica, fresh alfalfa Medicago sativa, and Pacific willow leaves Salix lasiandra) ranging from 10 to 50% neutral detergent fiber content (NDF) in captive blue duikers (Cephalophus monticola, 4 kg). Harvesting and rumination rates were obtained by observing and videotaping animals on each diet, and digestibility and intake were determined by conducting total collection digestion trials. We estimated mean retention time of liquid and particulate digesta by administering Co-EDTA and forages labelled with YbNO3 in a pulse dose and monitoring fecal output over 4 days. Duikers harvested and ruminated the fig diet faster than the alfalfa and willow diets. Likewise, they achieved higher dry matter, energy, NDF, and protein digestibility when eating figs, yet achieved a higher daily digestible energy intake on the fresh willow and alfalfa than on the figs by eating proportionately more of these forages. Duikers maintained a positive nitrogen balance on all diets, including figs, which contained only 6.3% crude protein. Mean retention time of cell wall in the duikers' digestive tract declined with increasing NDF and cellulose content of the diet. Digestibility coefficients and mean retention times of these small ruminants were virtually equivalent to those measured for ruminants two orders of magnitude larger, suggesting that they are well adapted for a mixed diet.

Flavin-containing monooxygenase-mediated N-oxidation of the M(1)-muscarinic agonist xanomeline.

The involvement of flavin-containing monooxygenases (FMOs) in the formation of xanomeline N-oxide was examined in various human and rat tissues. Expressed FMOs formed xanomeline N-oxide at a significantly greater rate than did expressed cytochromes P-450. Consistent with the involvement of FMO in the formation of xanomeline N-oxide in human liver, human kidney, rat liver, and rat kidney microsomes, this biotransformation was sensitive to heat treatment, increased at pH 8.3, and inhibited by methimazole. The latter two characteristics were effected to a lesser extent in human kidney, rat liver, and rat kidney microsomes than were observed in human liver microsomes, suggesting the involvement of a different FMO family member in this reaction in these tissues. As additional proof of the involvement of FMO in the formation of xanomeline N-oxide, the formation of this metabolite by a characterized human liver microsomal bank correlated with FMO activity. The FMO forming xanomeline N-oxide by human kidney microsomes exhibited a 20-fold lower K(M) (average K(M) = 5.5 microM) than that observed by the FMO present in human liver microsomes (average K(M) of 107 microM). The involvement of an FMO in the formation of xanomeline N-oxide in rat lung could not be unequivocally demonstrated. These data and those in the literature suggest that the increased prevalence of N-oxidized metabolites of xanomeline after s.c. dosing as compared with oral dosing may be due to differences in the affinity of various FMO family members for xanomeline or to differences in exposure to xanomeline that these enzymes receive under different dosing regimens.

Synthesis, antiviral activity, and biological properties of vinylacetylene analogs of enviroxime.

A series of vinylacetylene analogs of Enviroxime (1) was synthesized. The new compounds are potent inhibitors of poliovirus in tissue culture. Cross-sensitivity with Enviroxime-derived mutants shows that the new compounds have the same mechanism of action as Enviroxime, which involves the viral 3A protein. In studies with Rhesus monkeys, the p-fluoro derivative 12 was found to be unique in providing oral bioavailability. Metabolism studies using hepatic microsomes suggest that this procedure would be a useful in vitro method for selecting the appropriate animal model for testing oral absorption. Compound 12 was found to be efficacious by oral administration in treating a Coxsackie A21 infection in CD-1 mice.

Muscarinic analgesics with potent and selective effects on the gastrointestinal tract: potential application for the treatment of irritable bowel syndrome.

Irritable bowel syndrome (IBS) is a pathopysiolocal condition characterized by abnormal bowel habits that are frequently accompanied by abdominal pain. Current therapy based on reducing high-amplitude GI contractions with nonselective muscarinic antagonists is limited in efficacy due to typical muscarinic side effects and provides no pain relief. We have previously found potent antinociceptive agents acting through muscarinic receptors. In the present work, new 1,2,5-thiadiazole-based structures with muscarinic activity have been evaluated both for activity as analgesics in the mouse withing assay and for activity in normalizing spontaneous cluster contractions in ferret jejunum as a model of IBS in humans. (5R,6R)-exo-6-[4-[(4,4,4-Trifluorobutyl)thio]-1,2,5-thiadiazol+ ++-3-yl] -1-azabicyclo[3.2.1]octane (35, LY316108/NNC11-2192) was found to offer an exceptional profile combining analgesic potency in mouse writhing (ED50 = 0.1 mg/kg) along with potency for normalization of GI motility (ED50 = 0.17 mg/kg). This combination of GI and analgesic potency suggests 35 as an excellent candidate for evaluation as a potential treatment of IBS.

Intratumor pharmacokinetics, flow resistance, and metabolism during gemcitabine infusion in ex vivo perfused human small cell lung cancer.

The relationship between tumor physiology and the pharmacokinetics of 2',2' difluorodeoxycytidine [gemcitabine (dFdC)] in ex vivo perfused human small cell lung cancer was examined. Two small cell lung cancer sublines, 54A and 54B, with known in vivo sensitivity to dFdC, were grown as tissue-isolated tumors in athymic mice and perfused ex vivo with or without 20-40 micrometer dFdC. Arteriovenous differences in gases, pH, and metabolites were determined before and during drug infusion. The geometric flow resistance (FR) of individual tumors was calculated, and dFdC and its inactive metabolite 2',2' difluorodeoxyuridine were determined by high-performance liquid chromatography of consecutive samples from the output line. Both tumors had prominent lactate production concurrent with a significant O2 consumption. The arteriovenous pH drop was approximately 0.3 in both tumor lines. Significant metabolic differences between 54A and 54B tumors were found that elucidated previously described differences further. Pharmacokinetic analysis showed that the initial tumor uptake of dFdC was flow limited, and a significant inverse correlation between the geometric FR and initial drug uptake was found. The rate constant for recovery of the drug in the tumor outflow was greater in 54B tumors (P < 0.05), and the geometric FR was greater in 54A tumors (P < 0.01). The drug conversion rate was independent of physiological parameters. Attempts to modify the delivery of dFdC should be directed at the tumor blood flow distribution. More generally, our experimental model provides useful new insight into metabolism and intratumor pharmacokinetics of chemotherapeutic agents in solid tumors.

Comparisons of phase I and phase II in vitro hepatic enzyme activities of human, dog, rhesus monkey, and cynomolgus monkey.

The metabolism of probe substrates of phase I and phase II enzymes in vitro were compared in hepatic subcellular fractions from humans, cynomolgus monkeys, rhesus monkeys, and beagle dogs. These studies were undertaken to compare the suitability of these species as models of metabolism in drug development. Eight cytochrome P450-dependent activities were measured in microsomal incubations: ethoxyresorufin O-deethylase, coumarin 7-hydroxylase, tolbutamide 4-hydroxylase, S-mephenytoin 4'-hydroxylase, bufuralol 1'-hydroxylase, N-nitrosodimethylamine N-demethylase, midazolam 1'-hydroxylase, and erythromycin N-demethylase. Seven phase II activities were determined in the appropriate subcellular fractions:acetaminophen UDP-glucurono-syltransferase, acetaminophen sulfotransferase, 17 alpha-ethinylestradiol UDP-glucuronosyltransferase, 17 alpha-ethinylestradiol sulfotransferase, 6-mercaptopurine methylase, dichloronitrobenzene (DCNB) glutathione S-transferase, and isoniazid N-acetylase. Hepatic subcellular fractions from cynomolgus and rhesus monkeys showed significantly higher activities than those from humans for ethoxyresorufin O-deethylase, bufuralol 1'-hydroxylase, midazolam 1'-hydroxylase, erythromycin N-demethylase, acetaminophen UDP-glucuronosyltransferase, acetaminophen sulfotransferase, and tolbutamide 4-hydroxylase. Cynomolgus monkey had higher activity than humans and rhesus monkeys for S-mephenytoin 4'-hydroxylase erythromycin N-demethylase. Rhesus monkey and human cytosol displayed an apparent genetic polymorphism in the N-acetylation of isoniazid, whereas cynomolgus monkey cytosol did not. All other monkey activities were not significantly different than human. Dog subcellular fractions showed higher activity than humans for midazolam 1'-hydroxylase, erythromycin N-demethylase, acetaminophen UDP-glucuronosyltransferase, acetaminophen sulfotransferase, 17 alpha-ethinylestradiol sulfotransferase, and DCNB glutathione S-transferase. Furthermore, dog samples had significantly lower activity for coumarin 7-hydroxylase and 6-mercaptopurine methylase, and no detectable activity for tolbutamide 4-hydroxylase or isoniazid N-acetylase. All other activities were not significantly different from human. These results reveal minor differences between the cynomolgus and rhesus monkey in drug metabolism capacities in vitro, but both species are generally more metabolically active than humans in both phase I and phase II metabolism, whereas dogs had more diverse deviations from humans.

Determination of the muscarinic agent [(3-(3-1-butylthio)-1,2,5-thiadiazol-4-yl)-1-azabicyclo-2.2.2-oct ane], in rat, rabbit, and monkey plasma, using high-performance liquid chromatography in conjunction with tandem mass spectrometry.

A method for determining a selective muscarinic agent, LY297802 (compound I), [(3-(3-1-butylthio)-1,2,5-thiadiazol-4-yl)-1-azabicyclo-2.2.2-octa ne], indicated in the treatment of pain, in rat, rabbit, and monkey plasma is described. The analytes, including an internal standard, were extracted from plasma at basic pH with hexane. The organic fraction was evaporated to dryness and the residue reconstituted with mobile phase. The analytes were detected utilizing HPLC in conjunction with electrospray (ES) tandem mass spectrometry (MS-MS). The limit of quantitation was 0.25 ng/ml, and the response was linear to at least 100 ng/ml.

Determination of xanomeline (LY246708 tartrate), an investigational agent for the treatment of Alzheimer's disease, in rat and monkey plasma by capillary gas chromatography with nitrogen-phosphorus detection.

A GC method is described for the determination of xanomeline (LY246708 tartrate) and selected metabolites in rat and monkey plasma. The analytes, including an internal standard, were extracted from plasma at basic pH with hexane. The organic extract was evaporated to dryness and the residue was reconstituted in hexane. The analytes were separated from metabolites and endogenous substances using a DB1701 capillary column. The analytes were detected using nitrogen-phosphorus detection (NPD). The limit of quantitation was determined to be 8 ng/ml, and the response was linear from 8 to 800 ng/ml. The method has been successfully applied to rat and monkey samples pursuant to the development of xanomeline as an agent for the symptomatic treatment of Alzheimer's disease.

Influence of size and density of browse patches on intake rates and foraging decisions of young moose and white-tailed deer.

We examined the functional response and foraging behavior of young moose (Alces alces) and white-tailed deer (Odocoileus virginianus) relative to animal size and the size and distribution of browse patches. The animals were offered one, three, or nine stems of dormant red maple (Acer rubrum) in hand-assembled patches spaced 2.33, 7, 14, or 21 m apart along a runway. Moose took larger twig diameters and bites and had greater dry matter and digestible energy intake rates than did deer, but had lower cropping rates. Moose and deer travelled at similar velocities between patches and took similar numbers of bites per stem. We found that a model of intake rate, based on the mechanics of cropping, chewing, and encountering bites, effectively described the intake rate of moose and deer feeding in heterogeneous distributions of browses. As patch size and density declined, the animals walked faster between patches, cropped larger bites, and cropped more bites per stem, and hence, dry matter intake rates remained relatively constant. As is characteristic of many hardwood browse stems, however, potential digestible energy concentration of the red maple stems declined as the size and number of bites removed (i.e., stem diameter at point of clipping) by the animals increased. Therefore, the digestible energy content of the diet declined with decreasing patch size and density. Time spent foraging within a patch increased as patch size increased and as distance between patches increased, which qualitatively supported the marginal-value theorem. However, actual patch residence times for deer and moose exceeded those predicted by the marginal-value theorem (MVT) by approximately 250%. The difference between actual and predicted residence time may have been a result of (1) an unknown or complex gain function, (2) the artificial conditions of the experiments, or (3) assumptions of MVT that do not apply to herbivores.

Comparison of human and rhesus monkey in vitro phase I and phase II hepatic drug metabolism activities.

Twelve human and six rhesus monkey liver samples were analyzed in vitro for phase I metabolism and phase II conjugation activity. Of the eight P-450-dependent activities measured, only N-nitrosodimethylamine N-demethylase activity was not significantly different between the two species. Coumarin 7-hydroxylase activity was greater in the human as compared with the rhesus monkey samples, whereas erythromycin N-demethylase, benzphetamine N-demethylase, pentoxyresorufin O-dealkylase, ethoxycoumarin O-deethylase, and ethoxyresorufin O-deethylase activities were significantly greater in rhesus monkey microsome samples (p < or = 0.01). Cimetidine S-oxygenation and chlorpromazine N-oxygenation were 2.1- and 2.6-fold higher in rhesus monkey samples. Of the seven microsomal and cytosolic phase II activities measured, only 17 alpha-ethynylestradiol glucuronidation was significantly higher in the human samples. The genetic polymorphism for isoniazid acetylation was evident only in the human samples, with activities varying 200-fold. This study shows that, although the rhesus monkey is often used by the pharmaceutical industry as a representative mammalian species for drug testing, the in vitro metabolic capabilities of the human and rhesus monkey drug metabolizing enzymes are different.

Metabolism and disposition of gemcitabine, and oncolytic deoxycytidine analog, in mice, rats, and dogs.

Gemcitabine, 2'-deoxy-2',2'-difluorocytidine, is a broad spectrum oncolytic compound with antitumor activity in solid tumor models. The pharmacokinetics, metabolism, and disposition of gemcitabine was examined in mice, rats, and dogs. All three species metabolize gemcitabine by deamination to the uracil metabolite. However, deamination in the mouse and dog was more extensive than in the rat. The mouse deaminated gemcitabine rapidly with the plasma concentration maximum of the uracil metabolite of gemcitabine being attained at 15 min postdosing compared with approximately 3 and 6 hr in the dog and rat, respectively. The rapid deamination in the mouse was also reflected in the plasma half-life of the parent compound. The mouse exhibited the shortest plasma half-life, approximately 0.28 hr, contrasted with 2.14 and 1.38 hr half-lives in rat and dog, respectively. Plasma AUC for the uracil metabolite of gemcitabine was 73%, 10.5%, and 315% of that for gemcitabine in the mouse, rat, and dog, respectively. Tissue concentrations of gemcitabine-derived radioactivity in the rat and mouse indicated that gemcitabine was rapidly distributed throughout the body. Half-lives of radioactivity in tissues of both the rat and mouse were relatively short, with the longest tissue half-lives of 5.7 and 3.0 hr, respectively. Plasma protein binding is negligible in all three species. The major route of elimination is via the urine in all three species with 76-86% of the dose excreted in the first 24 hr. The predominant radiolabeled component isolated from urine was gemcitabine in the rat and its uracil metabolite in the mouse and dog.

Modulation of rat hepatic cytochromes P450 by chronic methapyrilene treatment.

The antihistaminic compound methapyrilene (MP) when chronically administered has been shown to be a rat-specific hepatocarcinogen. To examine the effects of chronic MP treatment on the hepatic microsomal cytochromes P450. Fischer 344 rats were gavaged for 10 weeks (5 days on, 2 days off) with either vehicle or 50, 100, or 150 mg MP/kg body weight. Chronic MP treatment was found to have a significant effect on several microsomal enzymatic activities. Small (17-28%) but significant (P less than 0.05) decreases were observed for total P450 levels and the activities of erythromycin N-demethylase (catalyzed by P450IIIA), N-nitrosodimethylamine demethylase (catalyzed by P450IIE1) and pentoxyresorufin O-dealkylase (catalyzed by P450IIB1). In addition, a relatively large decrease (approximately 80%) was observed for the activity of benzphetamine N-demethylase (representative of P450IIC11) and an induction of about 40% was observed for ethoxyresorufin O-dealkylase (catalyzed by P450IA). The metabolism of testosterone by microsomes isolated from the rats chronically treated with MP indicated that several reactions were compromized. Specifically, testosterone 2 alpha-hydroxylase, indicative of P450IIC11, was reduced greatly (86%), whereas testosterone 6 beta-hydroxylase, reflecting P450IIIA, and testosterone 7 alpha-hydroxylase, indicative of P450IIIA1, were affected only slightly by MP treatment (approximately 25%). Immunoblot analyses of the various microsomal samples were performed to determine if chronic MP treatment had direct effects on the level of expression of the cytochromes P450. Decreases in the levels of P450IIIA, IIE1, and IIC11, determined by immunoblot analyses, closely paralleled those observed for their marker catalytic activities. Further studies will be required to determine the mechanism by which MP affects the levels of the cytochromes P450 (i.e. increased degradation or decreased synthesis).

The disposition of an antileishmanial 8-aminoquinoline drug in the isolated perfused rat liver: thermospray liquid chromatography-mass spectrometry identification of metabolites.

1. The disposition of the candidate antileishmanial drug 8-(diethylaminohexylamino-6-methoxy-4-methyl quinoline dihydrochloride (I) has been investigated in the isolated perfused rat liver preparation after the administration of 5 mg/kg (25 microCi) of 14C-I. 2. The perfusate concentration of unchanged I declined biexponentially over the 4 h study period, with a distribution t1/2 of 3.3 +/- 0.3 min and a terminal t1/2 of 35.4 +/- 13.6 min. The area under the perfusate plasma concentration/time curve (AUC0-last time point) was 53.3 +/- 15.7 micrograms min/ml, representing 96% of the area under the curve extrapolated to infinity. the perfusate contained predominantly the carboxylic acid metabolite of I, as well as trace quantities of metabolites detected and identified in bile. 3. Biliary excretion of total 14C accounted for 18.2 +/- 5.0% of the dose, only 2.8 +/- 0.7% was identified by h.p.l.c. analysis as unchanged I. The remainder of the bile contained the desethyl metabolite of I as well as a minimum of 12 more polar metabolites. After 4 h, a total of 39.0 +/- 8.3% of dosed 14C was recovered from the liver tissue. Subcellular fractionation of the livers revealed 24.6 +/- 2.2% of 14C to be located in the 10,000 g pellet. 4. Thermospray liquid chromatography-mass spectrometry analysis of untreated bile and bile treated with beta-glucuronidase or aryl sulphatase permitted identification of some of these metabolites, revealing the presence of the parent drug, desethyl metabolite, 6-desmethyl glucuronide, the 6-desmethyl desethyl glucuronide and the side-chain cleaved 8-amino N-glucuronide metabolites of I, as well as the 6-desmethyl sulphate and the 6-desmethyl desethyl sulphate. Two dihydroxylated metabolites were also detected; however, further structure elucidation is required for unambiguous identification.

Disposition of the antimalarial, mefloquine, in the isolated perfused rat liver.

The disposition of mefloquine has been investigated in the isolated perfused rat liver (IPRL) preparation after the administration of [14C]mefloquine HCl (3.8 mg, 4 microCi, quinoline ring labeled). Mefloquine underwent avid hepatic uptake within 10 min of dosing. Also at this point, hepatic oxygen consumption was reduced markedly in four of the six IPRL preparations, but was restored completely by approximately 30 min post-dose. The drug concentration profile underwent a biexponential decline over the 4-hr study period, with a terminal T1/2 of 1.0 +/- 0.3 hr. The area under the perfusate plasma concentration/time curve (AUC0-infinity) was 4.0 +/- 1.8 micrograms.hr.ml-1. Mefloquine was a high clearance compound (956.0 +/- 390 ml/hr) with a large apparent volume of distribution (1416 +/- 819 ml) in the IPRL. Biliary excretion accounted for 7.5 +/- 6.5% of the dose. Mefloquine was quantitated by HPLC analysis as approximately half (3.3 +/- 1.8%) of biliary label, the remainder consisting of highly polar metabolites of mefloquine. By 4 hr, a total of 64.8 +/- 4.4% of the [14C] dose was recovered from the livers. Subsequent HPLC analysis revealed this to be mostly unchanged mefloquine. Subcellular fractionation of the homogenized livers revealed that 50.6 +/- 6.8% of the dose of mefloquine was located in the 10,000 g pellet. In summary, mefloquine was cleared rapidly from the IPRL and underwent avid hepatic uptake into the lipid-rich fractions of rat liver.

Effects of adenosine on glucuronidation and uridine diphosphate glucuronic acid (UDPGA) synthesis in isolated rat hepatocytes.

Dibutyryl cyclic adenosine 3':5'-monophosphate (DBcAMP) has been shown to inhibit glucuronidation of p-nitrophenol in a concentration-dependent manner in isolated rat hepatocytes. Adenosine (ADO) also decreased glucuronidation in a similar fashion. The effects of adenosine were examined on the variables controlling glucuronidation in intact cells. The addition of adenosine was without effect on either glucuronyltransferase or beta-glucuronidase. Adenosine decreased uridine diphosphate glucuronic acid (UDPGA) levels by 62% and, subsequently, inhibited glucuronidation by 41% in isolated rat hepatocytes. Since the synthesis of UDPGA requires NAD+ for the dehydrogenation of UDP-glucose, alterations in the redox state could account for the decrease in intracellular UDPGA levels. The effects of ADO (500 microM) on lactate and pyruvate content and redox state were examined in rat hepatocytes. ADO caused a 2.1-fold increase in lactate levels and a 2.65-fold increase in the [lactate]/[pyruvate] ratio. The NAD+/NADP ratio, therefore, was decreased by 63% in the presence of ADO. Carbohydrate reserve also affects UDPGA levels; thus, graded concentrations of glucose (5.5, 25, and 50 mM) were added to cells incubated with ADO. At 5.5 mM glucose, ADO caused a 61% decrease in glucuronide formation, while at concentrations of 25 and 50 mM glucose, the inhibition was diminished by 53 and 47% respectively. ADO appears to have decreased the synthesis of UDPGA by decreasing the NAD+/NADH ratio, thus inhibiting UDP-glucose dehydrogenase. Carbohydrate reserve also appears to be involved in the inhibition of glucuronidation mediated by ADO.