Membrane transport tracer studies

Blaedel and Haupert (28) demonstrated the feasibility of using this phenomenon as a preconcentration technique using isotope tracer studies on the ions Na4", Cs4", Zn24", and later Blaedel and Christensen (29) extended the work to include the anions 1 and HP01 . They found anion transport to be much slower than the previously reported cation transport. Coion transport in the anion exchange membranes was much higher and apparently dependent on the anionic charge of the bulk electrolyte. Further studies with more recently available membranes (1) seem to be needed. 

Of course, there are more bulk properties of interest than the above parameters related to transport of the fast ions and electrons. Metal cation transport is minor, but still a most crucial parameter, because it eventually leads to membrane walkout, demixing, or decomposition in chemical gradients. Methods used for investigating metal cation diffusion comprise reactivity studies, interdiffusion couples, and tracer studies, using analytical SEM, EPMA, SIMS or radioactivity for the diffusion profile analyses. 

Probably the most systematic and complete study on the influence of temperature on water transfer has been performed on mammalian red cells [10,20,28]. The dependence on temperature of both the tracer diffusional permeability coefficient (cotho) 3 nd the hydraulic conductivity (Lp) of water in human and dog red-cell membranes have been studied. The apparent activation energies calculated from these results for both processes are given in Table 2. The values for the apparent activation energies for water self-diffusion and for water transport in a lipid bilayer are also included in the table. For dog red cells, the value of 4.9 kcal/mol is not significantly different from that of 4.6-4.8 kcal/mol for the apparent activation energy of the water diffusion coefficient ( >,) in free solution. Furthermore, it can be shown that the product L — THOV )rt, where is the partial molar volume of water and the viscosity of water remains virtually independent of temperature for dog, hut not for the human red-cell membrane [20]. The similarity of the transmembrane diffusion with bulk water diffusion and the invariance of the... 

The results of the tracer studies including the elucidation of the stereochemistry involved, provided a firm basis for a biochemical approach to PA biosynthesis, i. e., characterization of the enzymes that catalyze biosynthetic key steps and the specific mechanisms involved in translocation,subcellular accumulation, and metabolism of PAs. Early tracer work was carried out with intact plants to which tracers were applied for days or weeks. Meanwhile, in vitro plant systems, such as cell cultures and root-organ cultures of PA-producing plants are available. Root cultures were found to be excellent systems for biochemical and enzymatic studies of PA biosynthesis [20-22]. Dedifferentiated cell cultures do not synthesize PAs, but retain the ability to accumulate PAs. They are excellent systems to study the membrane transport of PAs and to identify the subcellular storage sites. 

Reporter genes can also encode for extracellular receptors such as dopamine D2 (200) and SST type-2 receptors (201), or membrane transporters such as the sodium/iodide symporter (202). These human genes have been suggested as candidate reporter genes because they exhibit limited expression in the body. At the same time, radiolabeled tracers with high affinity for these extracellular gene-products have been extensively studied and are approved for human use [e.g., [ F]fluoroethylspiperone for D2 receptors (203), " in-pentetreotide for SST receptors (121), and 123/124/I3ij... 

However, although cellular uptake of [ " TcjMIBI is driven by the plasma and mitochondrial membrane potentials, the overall accumulation of the tracer is inversely proportional to the expression levels of Pgp in tumor cells. The earlier observations that intracellular accumulation of f Tc]MIBI varied widely among tumor cell lines [60] prompted several groups to explore the role of Pgp in transport kinetics of [ Tc]MIBI in tumor cells. Several studies performed in tumor cell lines exhibiting the MDR phenotype, either by selection with cytotoxic agents or by transfection with the MDR recombinant transporters, generally showed a reduced net uptake of [ Tc]MIBI, a feature that is closely related with the outward transport activity of Pgp. 

To overcome the problem of low selectivity, use of facilitated transport membranes has been proposed (2,5) and many works on the CO2 facilitated transport were well summarized in review articles (4,5). As the amine type carriers of CO2, monoethanolamine (MEA) (6-8), diethanolamine (DEA) (7,9,10) and triethanolamine (7) have been investigated. Most of the data were obtained by the "tracer transport experiments" where the tracer flux of 02 through the membrane of aqueous amine solution which had been equilibrated with a known partial pressure of CO2 was measured (6,7). However, very few quantitative studies have been performed for the "net transport experiments" where the partial pressure of CO2 in the downstream side was kept lower than that in the upstream side (9,10). 

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