Absorption - Pesticide Biotransformation and Disposition - page 41

in receptor fluid with acetone and soil vehicle, respectively, at 24 h. Skin concentrations
were only 2.6-3.7 and 0.11-0.14% for acetone and soil vehicles, respectively, which is
still not comparable to the in vivo data. The approximately 25% PCP absorption from
nonocclusive soil in monkeys ( Wester et al., 1993b ) compares favorably with the 29%
PCP absorption from nonocclusive soil in Yorkshire pigs in vivo ( Qiao et al., 1997 ).
Note that inhibition of soil and/or skin microorganisms can inhibit absorption
of PCP, alter local and systemic distribution, and increase plasma/blood concentra-
tion ratios in pig skin in vivo ( Qiao et al., 1997 ). It is plausible to assume that skin or
soil microorganisms and/or products of PCP microbial degradation may play a role in
PCP absorption and disposition.
Absorption from the Gastrointestinal Tract
People are potentially exposed to pesticides orally from pesticide residues in foods such
as meat, milk, fruits, and vegetables. Children may also be orally exposed to pesticides
when they place contaminated objects in their mouths. The rate and extent of absorp-
tion after oral exposure depend on the ability of the chemical to cross the plasma mem-
branes of the gastrointestinal tract. As discussed under Factors That Influence the Transfer
and Availability of Chemicals in the Body, diffusion across a plasma membrane depends
to a large extent on the lipid solubility and degree of ionization of the chemical. The
degree of ionization of weak acids and weak bases, and hence absorption, depends on
pH. The pH range in the gastrointestinal tract varies from approximately 1-3 in the
stomach to 6-8 in the intestines. Thus, the rate and extent of absorption of weak organic
acids and bases vary with location in the gastrointestinal tract; weak acids are nonionized
and are absorbed in the stomach, whereas weak bases are nonionized and are absorbed
in the intestine ( Figure 3.4 ). Removal from the site of absorption by blood flow main-
tains a concentration gradient, thus enhancing the absorption of chemicals.
Residence time in the region of the gastrointestinal tract where the chemical is
absorbed also affects absorption. The presence of food in the gut can alter the pH of
the gut contents and the intestinal motility, which in turn can affect the rate of absorp-
tion from the gastrointestinal tract. Stomach acid, gastric enzymes, and intestinal flora
may decompose the chemical before absorption can occur, which may also decrease
the potential for toxicity. Pekas (1972) demonstrated the hydrolysis of naphthyl
N -methyl carbamate in the intestine (pH 6.4) and Baynes and Bowen (1995) demon-
strated the effect of rumen microflora on methyl parathion. The large surface area of
the intestinal tract aids in absorption from this site; even chemicals that do not readily
cross the plasma membrane (e.g., weak acids) can be absorbed to a high degree in the
intestine because of the increased surface area. Particles may be absorbed in the intes-
tines by endocytosis.
Chemicals that are absorbed into the bloodstream from the gastrointestinal tract
enter the portal circulation and are delivered directly to the liver, where they may