Translocation of Xenobiotics

To arrive at the receptor site in the target cell, the blood must transport the absorbed xenobiotic. The time to the onset of toxicity depends on how quickly plasma levels of the toxic compound may be achieved. In contrast, the time to reach peak plasma concentration of the toxin is significantly longer after oral administration. 

Lipophilic compounds diffuse easily through capillary walls. Their diffusion velocity is related to their Upid-water partition coefficient (Goldstein et al., 1974). 

The entry of a compound into the bloodstream does not necessarily ensure that it wiU arrive unchanged at its specific receptor. As mentioned before, xenobiotics absorbed from the gastrointestinal tract are carried by the portal vein to the liver. The liver has a very active xenobiotic-metabohzing system in which chemicals may or may not be altered before being released through hepatic veins into the general circulation. Alternatively, they may be excreted into the bile and returned to the gastrointestinal tract. From there they may be excreted, all or in part, or reabsorbed and carried back to the liver. 

At the capillary subdivision, solutes are freely exchangeable between plasma and the interstitial fluid thus the concentration of a xenobiotic in tissne is proportional to that of the free xenobiotic in plasma. 

The entry of toxins into the brain and central nervous system (CNS) is frequently more difficnlt than into other tissues. The function of this blood-brain barrier is related to impaired permeability of the blood capillaries in brain tissne, the necessity for toxins to penetrate glial cells, and the low protein content of the CNS interstitial flnid (Klaassen, 1986). Lipid solnbihty of a toxin is an important factor in the penetration of the blood-brain barrier. 

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Biochemistry and Physiology. The principle clmllenge here is the multiplicity of crop plants and pests of interest. Two broadly applicable research areas are penetration and translocation of xenobiotics in plants, and their metabolism in crops and pests. The... 

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