Veterinary Taxicology

TISSUE DISTRIBUTION

• Distribution or translocation of the toxicant follows via the bloodstream to reactive sites, including storage depots.
• The selective deposition of foreign chemicals in various tissues depends on receptor sites.
• The ease of chemical distribution depends largely on its water solubility.
• Polar- or aqueous-soluble agents tend to be excreted by kidneys; lipid-soluble chemicals are more likely to be excreted via bile and get accumulated in fat depots.
• Knowledge of the translocation characteristics of poisons is necessary for proper selection of organs for analysis.
• Highest concentration of a poison is not necessarily found in the organ in which it exerts its greatest effect.
• Substances absorbed from the gut may be transported either in association with plasma proteins or circulatory erythrocytes.
• Following their absorption, toxic substances will first reach the liver via hepatic portal system.
• Hence, hepatic damage is frequently observed. However, substances that are entering after pulmonary and subcutaneous exposure may also cause hepatic damage.
• Some toxic agents are selectively deposited in certain tissues and organs like
  • iodine in thyroid
  • lead, fluorine and strontium in skeletal tissue
  • arsenic in hair and nails
  • halogenated hydrocarbons in adipose tissue
  • primaquine in liver
  • mercurials, aminoglycosides, certain antifungal drugs in kidneys
  • paraquat in lungs
• This affinity for the toxic agent does not necessarily result in the development of pathological lesions in these tissues.
• However, they are of importance while conducting toxicological analysis.
• The concentration of a xenobiotic in the tissues/organs is directly proportional to the free xenobiotic concentration in plasma, which further depends on its binding to plasma proteins.
• Protein bound xenobiotic will serve as a depot since bound toxicant cannot cross the capillary wall. This also serves to lower the intensity of toxic effects.