Non-absorbable marker

An additional consideration when using in situ techniques is the volume of the luminal drug solution as water absorption and secretion during the perfusion may introduce errors in the lumenal concentrations and therefore in the calculated absorption. Various water flux correction methods have been published [42] including the co-perfusion of a non-absorbed marker such... 

Lennernas s group at Uppsala has performed extensive studies to confirm the validity of this in vivo experimental set-up at assessing the rate and the extent of drug absorption. Recovery of PEG 4000 (a non-absorbable marker) is more than 95%, which indicates that the absorption barrier is intact. In addition, maintenance of functional viability of the mucosa during perfusion has been demonstrated by the rapid transmucosal transport of D-glucose and L-leucine. Estimation of absorption half-lives from the measured Pefr agree well with half-lives derived from oral dose studies in humans (i.e. physiologically realistic half-lives). Human Peff estimates are well correlated with the fraction absorbed in humans, and served as the basis for BCS development, and hence the technique is ultimately the benchmark by which other in situ intestinal perfusion techniques are compared. The model has been extensively used to... 

C PEG4000 Non-absorbable marker or zero permeability marker Extent of recovery >95-99% indicates that the intestinal mucosa is intact, also used as markers for transmucosal water flux... 

Dry the appropriate amounts of each radiolabel (usually supplied in ethanol or toluene) under a stream of N2 in a glass tube or glass scintillation vial. Typically, the non-absorbed marker is used at 0.2-0.5 pCi per mouse and the cholesterol at 0.5-1.0 pCi per mouse. 

Yegen et al. (1996) studied the inhibitory effects of gastrin releasing peptide on gastric emptying in rats using methyl cellulose and phenol red as non-absorbable marker. 

It is possible that excretion of isotopes in more than one pulse is due to peristaltic reflux or variations in fecal flow for different components of feces Turnlund (34) has observed excretion of polyethyleneglycol (PEG), a non-absorbable marker, in a multiphasic pattern However, it seems unlikely that this would explain excretion of isotope as long as six weeks after ingestion ... 

In addition, a non-absorbable marker, polyethylene glycol (PEG 4000, 10 g/L) was added to each meal in order to estimate the percent recovery of the meal and to adjust the volumes at the end of the test meal digestion (Borgstrom et al., 1957). This marker also allowed to estimate the rate of gastric emptying, the volume of gastric contents and the pyloric outputs of HGL and hpolysis products (Carriere et al., 1993 a). 

Collection of duodenal contents through a duodenal tube after feeding a test meal containing a non-absorbable marker is a commonly used technique for studies of lipid digestion and absorption in man [55]. Using a multilumen tube, Simmonds et al. [56] were able to study in detail in man the absorption kinetics of cholesterol from a micellar solution. 

International Commission on Radiological Protection (1987 a) Technetium-labelled colloids (1987) In Annals of the ICRP, radiation dose to patients from radiopharmaceuticals, biokinetic models and data. ICRP publication 53, vol 18, no 1-4. Pergamon, Oxford, pp 179-183 International Commission on Radiological Protection (1987b) Technetium-labelled non-absorbable markers (1987) In Annals of the ICRP, radiation dose to patients from radiopharmaceuticals, biokinetic models and data. ICRP publication 53, vol. 18, no. 1-4. Pergamon, Oxford, pp 225-226... 

One of the many difficulties in studying nutrient absorption is that it is sometimes necessary to achieve total recovery of 24 h fecal output. This can be avoided, however, if a non-absorbable analog of the nutrient in question is available. The method also requires that the marker and nutrient can be obtained with different radiolabels, typically 3H on one and 14C on the other. 

For cholesterol absorption, one can take advantage of the fact that plant sterols (phytosterols) are poorly absorbed by mice and humans despite close structural similarity to cholesterol. Beta sitosterol, for example, differs from cholesterol only by the addition of an ethyl group to carbon 24 of the sterol side chain (see Fig. 10.1). This reduces absorption to 6% (34, 35) as compared to 60-80% for cholesterol. Sitostanol, which has a saturated C5-C6 bond in addition to the ethyl group, is only absorbed at 3% (34, 35) and is widely used as a non-absorbed sterol marker (Fig. 10.1). 

Because copper has only two stable isotopes, only one tracer isotope is available, which limits the choice of techniques for metabolic studies substantially. In most human studies, Cu has been used to determine apparent absorption, as the difference between the amount of isotope ingested and the amount recovered in feces. To check for completeness of fecal collection, holmium has been suggested and successfully used as a non-absorbable, quantitative elemental marker in copper absorption studies [259]. Studies comparing copper absorption of foods intrinsically or extrinsicaUy labeled with Cu are limited. Johnson and co-workers [260, 261] found no statistically significant difference in copper absorption from intrinsically and extrinsicaUy labeled goose fiver, goose breast, peanut butter, and wheat. Harvey et al. [262] observed significant differences in copper absorption between intrinsically and extrinsicaUy labeled sunflower seeds and soy beans. 

Figure 8. Concentration of markers of adhesion (A) and differentiation (B) in rat aortic smooth muscle cells in cultures on polyethylene (PE) modified by irradiation with ions (energy 30 keV, doses from lO to lO ions/cm ). Measured by enzymatic immunosorbent assay (ELISA) per mg of protein, absorbances expressed in % of the values obtained on pristine non-modified PE. Mean + SEM from 4 experiments. Student t-test for unpaired data, p<0.05 p<0.01 compared to the values on pristine PE.

The volume of the luminal solution, which may change as a result of absorption or secretion of water, is an important parameter when applying in situ techniques. A non- or low-absorbable volume marker, such as radiolabeled polyethyleneglycol 4000 or a fluorescent marker (lucifer yellow), and a marker of paracellular absorption (mannitol) need to be added and monitored as internal standards. 

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