Mammalian Toxicology

Session 2, Feb. 7, 2002

Note the time change for class on T 2/12/02 & 2/19/02 to 1:00-2:30 PM. Note also the invitation to join me (KLC) at the Instructional Technology Center presentation on the Prometheus on-line course framework on T 2/19/02 from 10:00-12:00.

Beginning to address some questions posed in Session 1.

1) Why might a lethality (dose/response) curve plateau at a level less than 100%?

This is probably best explained by the existence of genetically determined individual variation of genes, gene expression, and protein production (expressed phenotype). Genetic variation may give rise to organisms that are not susceptible to a given toxicant either because they fail to express a protein target of the toxicant or because they possess degradative mechanisms that inactivate the toxicant. Paths to inactivation may be entirely new or they may simply be more efficient than those found in other individuals. Note that these same considerations will apply to cross-species susceptibilities, so some species will be sensitive to some toxicants or toxins while others will be much less susceptible or not susceptible at all. Examples here would be drug resistant strains of bacteria, or malarial parasites, pesticide resistant strains of mosquitos, or flies, or herbicide resistant strains of grasses.

2) What forms of dosage and exposure are there?

Total dosage may be less important than concentration but this depends on how acutely or chronically the dose is delivered, just how toxic is the toxicant, whether the toxicant is cleared or accumulated, and, whether, when delivered, the toxicant is in a form that is bioavailable.

The old saying that "The solution to pollution is dilution" has some truth to it in that many toxicant problems arise when man concentrates naturally occurring toxicants and exposes other humans, or other biotic or nonbiotic elements of his immediate environment to supranatural concentrations of these materials. Excellent examples here are the naturally occurring radionuclides used in research, medicine, and industry or the plant concentrates like cube resins used as pesticides in agriculture and home lawn care. If exposure occurs at a low level over a long enough period of time for the toxicants to be deactivated either by metabolism or adsorption to surfaces (making them unavailable), the risk of a toxic response may be low. If the same total dose (amount) is given over a short term, however, toxic response will be likely as the exposure overwhems inactivation or adsorption capacities.

Form of delivery of the toxicant is also an important factor in determining biological response. For lipophilic materials, adsorption on hydrophobic surfaces such as charcoals or clays will often make the toxicants unavailable to biological systems unless extremes of pH or mechanical agitation liberate the toxicant materials as they pass through a particular digestive system. Dilution of lipophilic materials in oils prior to introduction into a mammalian system will tend to slow release of the material to the biological system because of the physico-chemical interactions of the lipophilic solute and solvent. Charged molecules like cations may also adsorb strongly to certain zeolite clays or to humic acids in light soils. On the other hand, such materials should be readily released from aqueous suspensions.

Related issues associated with bioavailability arise when a compound is bioaccumulated following administration. Persistent hydrocarbon pesticides and PCBs are excellent examples of how lipophilic toxicants can accumulate during chronic exposure largely based on the physicochemical characteristics of the molecule in question. By partitioning into lipid droplets or biological membrane lipid layers these compounds become sequestered against biological breakdown and against biological response until they reach such levels that the fats within an animal cannot accumulate any more and/or the degradative systems become saturated so levels of the toxicant in aqueous compartments rise and evoke toxic responses. Note that fluctuations of animal adipose levels may release bioaccumulated toxicants during lean periods and reduce toxicant risk during periods of abundance. Declines in body fat levels often coincide with the breeding and gestation periods of many birds and mammals. Since fat is the most concentrated form of biological energy available, the embryo in all species is provided a yolk source for early development and in mammals is provided milk which also contains high proportions of fat. Because of fat mobilization during gestation and lactation many animals including birds, reptiles, and mammals naturally expose developing young to higher levels of bioaccumulating lipophilic toxicants than the adults are exposed to. As a great deal of the lipid delivered to the young is used in energy or macromolecule production, toxicants often become available in concentrations sufficient to elicit biological responses.

Likewise materials that can accumulate in bone, such as some ions, will also tend to be sequestered during chronic low level exposures. Since gestation and lactation mobilize the elements of bone, toxicants that are hydrophilic may also be transferred in biologically effective concentrations to developing embryos and to nursing offspring.

Evolutionarily, how might the downloading of toxicants to offspring be useful or unfavorable?

Routes of toxicant/toxin delivery include: sc, subcutaneous; im, intramuscular; iv, intravenous; ip, intraperitoneal; po, per os (via the mouth); inhalation; topical/percutaneous; and transmucosal.

Biological responses arising from a single dose at a single concentration delivered in these several ways may well differ because of the differences in rate of absorption of the dose from the site of deposition, the availability of sites for adsorption of the administered dose, the presence of degradative enzymes in circulation or target tissues, the presence of unusual or extreme conditions (e.g., stomach pH), and the rate of transit from the site of administration through the biological system and back to the environment (e.g., rate of respiration for a volatile toxicant, or rate of gut transit).

A review of blood circulation and lymphatic circulation is suggested to understand how these various routes of exposure may differ so greatly.

IV is the most direct route, but response may differ if injection is done above the heart or below it as much of the lower circulation encounters the hepatic portal circulation which carries it through the very active hepatic degradative enzyme systems. Some compounds, e.g., estradiol are readily degraded in the liver and do not survive passage except by adsorption to carrier proteins like albumin. On the other hand, volatile compounds may survive poorly if injected in the upper extremities due to passage through the pulmonary circulation with losses occurring in the lungs.

IP passes via the lymphatics or the mesenteric venous drainage to the circulatory system. The lymph systems deposit injected materials into the venous drainage above the hepatic portal system while the mesenteric vein empties into the hepatic portal vessels.

SC involves uptake by the lymphatics or by diffusion into the venous drainage of the introduction site.

IM tends to be slow delivery as lymphatic drainage tends to be less important than diffusion through the muscle tissues to blood vessels.

Transdermal/topical/percutaneous exposure will rely on much the same delivery mechanisms as SC, however, the barrier of the skin must be breached either directly or via diffusion, often in the presence of a carrier solvent.

Transmucosal exposure will be similar to transdermal or sc, except that the vascularity of mucosal membranes will tend to favor vascular drainage of the exposure sites.

Inhalation exposure will involve diffusion into the pulmonary venous drainage of the lung. Although some degradation may occur in the lung, much clearance will occur either in the liver or the kidney.

PO exposure may involve transmucosal adsorption from the mouth or GI tract. But it may also involve chemical modification in the stomach acid. Depending on the chemical involved, this may degrade the compound, hydrolyze it and make it more easily degraded or more easily absorbed, or protonate or hydrolyze it so as to make it less readily adsorbed.

Questions arising from toxicological historical background.

1.	The idea of causality arose during the middle ages and Renaissance. This builds on careful observation and testing. How might this give rise to the notions of occupational hazards or the nonmystical explanation for oncogenesis?

2.	What information already discussed is subsummed in the notion of biochemical kinetics?

3.	Modern toxicology includes the concepts of causality, risk assessment, and site specificity of action. Why are all these concepts relevant to the following agencies: USPHS, NSC, EPA, FDA, OSHA, NRC? How many of these agencies derive directly from concerns about toxicant exposures?

4.	What is risk assessment with respect to humans, animals, ecology?

RESPONSES TO STUDENT COMMENTS ON SESSION 1.

Joanne:

Allison:

Neetu: