Strategies for Uncovering the Kinetics of Nucleoside Transport and Metabolism in Capillary Endothelial Cells

Abstract

For the analysis of signals obtained by external detection techniques such as positron tomographic imaging (PET), magnetic resonance imaging (MRI), and X-ray computed tomography (X-ray CT), investigators and diagnosticians usually obtain a sequence of images. For physiological interpretation in terms of the underlying physical and chemical events, it is essential to use models when one wants to learn more than the simplest measures. Among the simplest measures one can often include volume and flow estimates, but not always, for it commonly occurs that the distinctive estimation of these two parameters simultaneously requires using knowledge of the anatomy or of other properties of the tissue. The two most accessible measures of indicator transport are the areas under dilution curves and their mean transit times following a pulse injection. Mass conservation for substances that are not destroyed, such as radioactive tracers, relies on the general expression:

$$q(t){\text{ = }}F\int_0^t {{C_{in}}} \;dt{\text{ - }}F\int_0^t {{C_{out}}} \;dt,$$

, where q(t) represents the mass of indicator in the tissue at time t, F is the flow of indicator-containing fluid, and C_in(t) and C_out(t) represent the concentration-time curves for the indicator at the inflow and outflow. Such expressions represent whole organ behavior exactly when the organ is supplied by a single artery and drained by a single vein. When there are multiple inlets and multiple outlets, then the first term on the right is replaced by a sum of similar terms for each inlet, and likewise the second term on the right is replaced by a similar sum of the outputs (Lassen and Perl, 1979).