Agriculture Reference
In-Depth Information
Table 5.4
continued
Highly toxic*:
Moderately toxic*:
cloethocarb (Lance)
isoprocarb (Etrofolan, MIPC)
bendiocarb (Ficam, Dycarb, Multamat,
niomil, Tattoo, Turcam).
*
Compounds are listed approximately in order of descending toxicity. "Highly toxic" N-methyl
carbamates have listed oral LD
50
values (rat) less than 50 mg/kg body weight; "moderately
toxic" agents have LD
50
values in excess of 50 mg/kg.
+
This pesticide is taken up by some plants into the foliage and sometimes into the fruit.
Toxicology and Mode of Action
The N-methyl carbamate esters cause reversible carbamylation of acetyl-
cholinesterase enzyme, allowing accumulation of acetylcholine, the neuro-
transmitter substance, at parasympathetic neuroeffector junctions (muscarinic
ef fec ts), at sk ele tal mu scl e my one ural ju nct ions an d au ton omic ga ngl ia
(nicotinic effects), and in the brain (CNS effects). The carbamyl-
acetylcholinesterase combination dissociates more readily than the phosphoryl-
acetylcholinesterase complex produced by organophosphate compounds. This
lability has several important consequences: 1) it tends to limit the duration of
N-methyl carbamate poisonings; 2) it accounts for the greater span between
symptom-producing and lethal doses than exists in the case of most organo-
phosphate compounds; and 3) it frequently invalidates the measurement of
blood cholinesterase activity as a diagnostic index of poisoning.
N-methyl carbamates are absorbed by inhalation and ingestion and some
by skin penetration. Dermal absorption of particular compounds (notably
carbofuran) is very slight. N-methyl carbamates are hydrolyzed enzymatically
by the liver and the degradation products are excreted by the kidneys and the
liver.
At cholinergic nerve junctions with smooth muscle and gland cells, high
acetylcholine concentration causes muscle contraction and secretion, respec-
tively. At skeletal muscle junctions, excess acetylcholine may be excitatory
(cause muscle twitching), but may also weaken or paralyze the cell by depolar-
izing the end-plate. In the brain, elevated acetylcholine concentrations may
cause sensory and behavioral disturbances, incoordination, and depressed motor
function (rarely seizures), even though the N-methyl carbamates do not pene-
trate the central nervous system very efficiently. Depression of respiration
combined with pulmonary edema is the usual cause of death from poisoning
by N-methyl carbamate compounds.
Figure 5.7
i
llustrates the neuralmuscular
system showing its parts and chemical transmission of nerve impulse and
point of action, the synapase, where the accumulation of acetylcholine occurs.
c. Solid Organochlorine Insecticides (chlorinated hydrocarbons, chlorinated
organics, chlorinated insecticides, and chlorinated synthetics)
The following table lists current organochlorine products.
