- There has been no toxicologic proof that the long-term, low-level
exposure of man to chlorophenothane (DDT) is linked with an
increased susceptibility to toxicity or disease. Acute pretreatment
of rats with 75-100 mg/kg DDT has been demonstrated to potentiate
CCI₄-induced hepatotoxicity in that species (McLean and McLean,
1966). Since the hepatotoxic response to CC1₄ is similar among
mammalian species, it was believed man's response could also be
altered by prior exposure to DDT. Chronic pretreatment of rats with
DDT more closely duplicates man's exposure situation, which is
mainly dietary. Therefore, this present investigation was undertaken
to quantify the DDT-CCI₄ hepatotoxic interaction in rats from the
standpoint of a chronic dietary exposure to DDT.
Chronic and acute pretreatments with DDT were compared for
their relative effects on acute CC1₄ hepatotoxicity. Subgroups of rats
fed DDT at concentrations of 6-65 ppm for three weeks and 65 ppm
for 24 weeks attained body burdens of DDT and its metabolites ranging
from 6.1 to 30.6 ppm, compared to 0 ppm (no detectable) for controls.
In the acute study, subgroups of rats were dosed with 35-150 mg/kg
DDT by gavage 24 hours prior to CC1₄
exposure. Doses of CC1₄
ranging from 0.125 to 1.0 mi./kg were administered by gavage. All
pretreatments with DDT, chronic or acute, resulted in a potentiated
elevation of serum transaminase (SGPT and SGOT) activities following
treatment with CC1₄. The degree of potentiation was dose-related to
and to the total DDT body burden. In a temporal study, the
onset of the potentiated response was noted six hours after CC1₄. All
of the above-mentioned results were confirmed by histopathology.
Plasma BSP disappearance, dose-response and temporal studies
were conducted in control and DDT-fed rat groups, utilizing the BSP
test as an index of hepatic functional impairment, CC1₄-induced BSP
retention was greatly potentiated by prior dietary exposure to DDT.
This potentiation effect was obscured at a dose of 2.0 ml/kg CC1₄, due
to the severe hepatic damage produced by this dose of CCI₄
alone. The onset and development of the potentiated hepatic dysfunction
paralleled that of the parenchymal cell destruction.
The potentiated CC1₄-induced increases in SGOT activity and
centrilobularly-oriented coagulative necrosis in the DDT-fed animals
were prevented by spinal cord transection at the level of the seventh cervical vertebra. Chronic DDT pretreatment did not enhance CC1₄
uptake into the blood or livers of intact rats, and this was ruled out
as a possible mechanism for the potentiation effect. Cord-sectioning
did enhance tissue CC1₄
uptake, yet these animals were protected
against CC1₄-induced central necrosis. Neither cord-sectioning nor
the resultant hypothermia reduced the microsomal cytochrome P-450
content in control or DDT-fed rats. Hypothermia depressed the in
vitro N-demethylation of ethylmorphine, so that oxidative drug metabolism
was probably also decreased in vivo in the cordotomized
Maximal blood and liver concentrations of CC1₄
between 2-4 hours in the intact control and DDT-fed groups. These
concentrations decreased more rapidly between 4-12
hours in the DDT-fed rats, thus suggesting an increased rate of CC1₄
metabolism could have occurred at this time in vivo in these animals.
Unlike the other indices of hepatic damage, the cytochrome P-450
response preceded rather than followed the hepatocellular destruction.
About 75% of the induced cytochrome P-450 concentration in the DDTfed
rats was destroyed within six hours after CC1₄, at which time the
onset of potentiated hepatic damage was detected. In the controls,
the cytochrome P-450 concentration regenerated from a low value at
18 hours to normal by 48 hours after CC1₄. Hepatic regeneration was
impaired in the DDT-fed rats, since the cytochrome P-450
concentration continued to decline between 24-48 hours. The amount
of cytochrome P-450 destruction appeared to correlate with the
degree of hepatic damage observed. Lower absolute concentrations
of cytochrome P-450 were measured in controls 24 hours after dosing
with 0.125-2.0 ml/kg CC1₄. More cytochrome P-450 was destroyed
in the DDT-fed group, however, as a result of the induced microsomal
concentrations of this cytochrome present initially before CC1₄
given. Thus, the CCI₄- cytochrome P-450 interaction could play an
integral role in the potentiation phenomenon, as McLean and McLean
(1969) have suggested.
Liver weight increased in a dose-related manner to CC1₄ in both
control and DDT-fed groups, but the response was greater in controls.
In the control animals, the liver weight increased rapidly to a maximal
value at 18 hours, but returned to normal by 48 hours after CC1₄.
Although the onset of the response was slower in the DDT-fed rats, the
liver weight continued to increase at 48 hours. Rectal temperatures
declined between 6-24 hours only in the DDT-fed animals.
The pharmacologic agents SKF 525-A (drug metabolism
inhibitor) and lead (porphyrin biosynthesis inhibitor) were utilized to
assess the role of cytochrome P-450 in potentiation. SKF 525-A
provided protection against the potentiated CC1₄-induced centrilobular
necrosis at 11 hours after CC1₄. This protection could not be
positively ascribed to SKF 525-A binding with cytochrome P-450 or inhibiting CC1₄
metabolism, since SKF 525-A has been reported to
alter the gastrointestinal absorption of CC1₄
(Marchand, McLean and
Plaa, 1970). Lead significantly reduced the effect of prior DDT
feeding in potentiating CC1₄ hepatotoxicity, but this protection did not
result from a lowered microsomal cytochrome P-450 content. Thus,
the role of cytochrome P-450 in the potentiation effect remains to be
Mecamylamine pretreatment was used to determine the
importance of the sympathomimetic properties of DDT in potentiation.
This ganglionic blocking agent produced an insignificant tendency to
decrease the potentiated CC1₄- induced BSP retention. However, the
histologic evidence of protection against CC1₄
-induced hepatic necrosis
was not apparent in mecamylamine-pretreated animals. Thus, the
central sympathetic nervous system stimulatory effects of DDT are
probably not directly involved in potentiation.
The basis for extrapolation of these results from rat to man
could be the total DDT body burden, which was similar in these rats
(6.1 -30.6 ppm) to that estimated for man, e.g., 10 ppm by Durham
(1965). Since rats and man respond similarly to CC1₄, these results
indicate that man's chronic exposure to DDT could render him highly
susceptible to a secondary exposure to CC1₄.