Perfusion Experiments

Gum-Saline. Gum is a galactoso—gluconic acid having molecular weight of approximately 1500. First used (16) in kidney perfusion experiments, gum—saline enjoyed great popularity as a plasma expander starting from the end of World War I. The aggregation state of gum depends on concentration, pH, salts, and temperature, and its coUoid oncotic pressure and viscosity are quite variable. Conditions were identified (17) under which the viscosity would be the same as that of whole blood. 

In practice, estimation of Laq requires information on the rate of solute removal at the membrane since aqueous resistance is calculated from experimental data defining the solute concentration profile across this barrier [7], Mean /.aq values calculated from the product of aqueous diffusivity (at body temperature) and aqueous resistance obtained from human and animal intestinal perfusion experiments in situ are in the range of 100-900 pm, compared to lumenal radii of 0.2 cm (rat) and 1 cm (human). These estimates will necessarily be a function of perfusion flow rate and choice of solute. The lower Laq estimated in vivo is rationalized by better mixing within the lumen in the vicinity of the mucosal membrane [6],... 

Most of the perfusion studies have been carried out on the isolated liver since it is in this organ that the change of fat to carbohydrate presumably takes place. On the basis of some of these studies, carbohydrate formation from fat has been postulated.161-166 In such cases more carbohydrate has been present at the end of the perfusion experiment than could be accounted for on the basis of known carbohydrate precursors. In several cases, a decrease in the fat content of the liver was accompanied by a concomitant increase in carbohydrate. However, Gregg166 has been... 

Perfusion experiments before oxygenation problems were solved often gave incomplete or misleading information. Embden s work on fatty acid oxidation in liver (see Chapter 7) suggested acetoacetate was a normal metabolite, although in the intact animal free acetoacetate is only detected when glucose oxidation is impaired. 

Figure 2.5 Mass balance model for intestinal perfusion experiments at steady state (Adapted from Johnson and Amidon [129]).

The choice of flow rates in perfusion experiments is an important consideration as it may affect hydrodynamics [35], ABL thickness [30], intestinal radius [34], intestinal surface area [45], and time to reach steady-state conditions [32], all of which can impact on Peff estimates. The intestinal radius has implications for the estimation of the permeability coefficient. The most widely used estimate for the rat intestinal radius is 0.18 cm [34], These authors found that there was a small change in intra-luminal pressure with an increase in flow... 

To facilitate a standardisation of inter-laboratory results of permeability, it is now common practice to include a range of model drugs as internal standards in initial validation (i.e. method suitability) of intestinal perfusion techniques [116]. A list of 20 model drugs has been reported by the FDA for the standardisation of the in situ intestinal perfusion experiment, whereas six drugs are recommended for human studies. Once the method has been... 

Note Data represent the mean S.E. (n = 3). MW, molecular weight P0/w, octanol-to-water partition coefficient CLapp, apparent membrane permeability clearance SI, midgut area of the small intestine NA, not available or applicable. Absorption was evaluated in our laboratory using the closed loop of the rat intestine in situ (urethane anesthesia, 1.125 g/4.5 ml/kg, i.p.) in 60 min for riboflavin and L-camitine and 30 min for the others. For those that are transported by carriers in part (riboflavin and glycerol in both colon and SI, and L-carnitine, 5-fluorouracil, and cephradine in SI), absorption was evaluated at higher concentrations where the contribution of carrier-mediated transport is negligible. Values of P0/w were obtained from a report by Leo et al. [30] except for that of D-xylose, which was determined in our laboratory. a Data by single-pass perfusion experiments. b Unpublished data from our laboratory. 

The basic principles to consider in establishing an experimental system for lung perfusion experiments are considered with regard to the apparatus and the mode of ventilation and perfusion. 

J.C. Challier. Criteria for evaluating perfusion experiments and presentation of results. Contributions to gynecology and obstetrics. 13 32-39 (1985). 

The presence of folds and villi structures on the surface area is not taken into account for the in vivo effective intestinal membrane permeability (Pefr when extrapolated from a perfusion experiment, a smooth tube is usually assumed). In humans, the fold expansion (FE) of the surface area is about threefold, and villi expansion (VE) is about 10-fold [7]. In the case of high epithelial membrane permeability (Pep) absorption occurs at the top of the villi before diffusing down the villi channels, whereas low Pep compound may diffuse down the villi channels to the crypts (Figure 6.1). Therefore, accessibility (Acc) to the surface depends on Pep and diffusion coefficient [7, 8]. The effective membrane permeability can be expressed as ... 

Simulated perfusion experiments recycle of filtrate without fresh medium addition. ETFE ethylene-tetrafluoroethylene. 

Although perfusion cultures using RDF devices have not yet been reported, the results obtained in harvesting [113] and simulated perfusion experiments [104] indicate that it could be a promising cell retention device for suspendedcell perfusion processes. 

Vasoconstriction. + indicates constriction in perfusion experiments or an action on smooth muscles of arterioles.. .. ... 

Fig. 4.9 (a) Logarithm of the apparent permeability coefficient, for perfusion experiments in rats (filled symbols) and Caco-2 (empty symbols) versus logarithm of the intrinsic permeability coefficients based on PAMPA. (b) The unstirred water layer effect in the Caco-2 data. 

Evidence from rats suggests that apoE is synthesized almost exclusively in the liver (M31, W19). Perfusion experiments show that the liver produces discoid nascent HDL particles which are rich in apoE, and which also contain apoA-I (Dl, D2, F8, H3, H5, Kll, M31). ApoE is not found in chylomicrons of intestinal lymph, and its presence in chylomicrons in blood suggests that a transfer from nascent HDL to chylomicrons occurs (G28, 14). 

The exclusive role of the liver in the biosynthesis of fibrinogen, questioned by some, has been reaffirmed in isolated liver perfusion experiments in which maximal net biosynthesis of fibrinogen of 0.6 mg. per hour per gram wet weight of liver occurs. This approximates three to four times the normal rate of fibrinogen biosynthesis in turnover in the intact normal rat and occurs only in liver perfusions in which the stimulus of extreme hypofibrinogenemia associated with the use of completely defibrinated blood is present. 

Prior to the perfusion experiment, incubate blood with either a platelet antagonist or vehicle (control) at 37 °C for 10 min. 

The concentrations of the compound are determined using appropriate analytical methods for the respective candidate compound in the perfusate at the end of the perfusion experiment, plasma, bile and hepatic tissue. From the data the site of intestinal absorption using different perfused intestinal segments in separate experiments as well as the degree of hepatobiliary elimination can be estimated. 

Appropriate analytical methods with sufficient sensitivity are used for detection of the candidate compound in the perfusate, plasma, bile and liver tissue. From the total amount of the candidate compound in the perfusate of the respective intestinal segment at the end of the perfusion experiment in relation to the total added amount of compound at time 0, the total enteral absorption rate can be estimated. The decline in the concentration of the candidate compound in the perfusate is a measure of the enteral absorption, the uptake, metabolism and elimination of the compound by the liver. The concentration in the plasma is a measure of the bioavailability. The appearance of the candidate compound in the bile is a measure of the hepatobiliary elimination of the compound. The tissue level of the candidate compound is a measure of the uptake and... 

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