Transport studies across intestinal tissues from animals are also a widely used in vitro method to study drug absorption. This method involves the isolation of the intestinal tissues, cutting it into strips of appropriate size, clamping it on a suitable device and then the rate of drug transport across this tissue is measured. The utility of this in vitro device (Ussing Chamber) was first demonstrated by Ussing and Zerahn (1951). In this method, the permeability is measured based on the appearance of drug in the serosal side rather than the disappearance of drug in the mucosal side. The unique feature of this approach is that electric resistance of the membrane can be measured during the course of the experiment. The short circuit current across the membrane, as well as the resistance across the membrane, can be monitored. These parameters are routinely used as an indicator of the viability of the intestinal tissue during the transport studies with Ussing chamber. The apparent permeability coefficient (P) is estimated using the equation (Grass and Sweetana, 1989)
where V is the volume of the receiver chamber, A is the exposed tissue surface area, C0 is the initial concentration drug in the donor chamber, and dC/dT is the change in drug concentration in the receiver with time.
The Ussing chamber technique is an ideal method to study regional differences on the absorption of drugs by mounting intestinal tissues from various intestinal regions (Ungell et al., 1998). It is also possible to perform studies with human intestinal tissues thus providing a methodology to compare the permeability values across species. The amounts of drug required for the study are relatively small (mg quantity) and the collected samples are analytically clean which facilitates quantitative analysis. Apart from the disadvantages commonly associated with any in vitro studies, the drawbacks of this technique include: lack of blood and nerve supply, rapid loss of viability of the tissues during the experiment and changes in morphology and functionality of transporter proteins during the process of surgery and mounting of tissues.
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