Parathion dermal dose-response studies

Dermal Dose-Response Studies. The dermal dose red cell ChE-response curves for parathion and thiodicarb are given in Figure 2. Para-thion, the most toxic pesticide used, inhibited 50Z of the red cell ChE activity at a dose level ot 31 t%/cm (3.2 mg/kg) of treated skin in 72 hr, tdiile 322 ug/cm of thiodicarb (33.3 mg/kg) gave 50 ChE inhibition in 24 hr. Carbaryl at dose levels up to 4,000 ng/cn of treated skin (417 sig/kg) did not produce detectable ChE inhibition 24 hr after application of the dose. Thiodicarb was dermally more toxic than carbaryl, but substantially less toxic (1/10) than parathion. 2... 

The rapid inhibition of red cell cholinesterase in workers der-mally exposed to parathion suggested that parathion was rapidly absorbed from skin. In the rat dermal dose response study (J a period of 72 hr was required for a single topical dose of parathion (32 mg/kg) to inhibit 50% of the red cell cholinesterase activity. Studies by Fredriksson (18) in he c t showed that parathion was absorbed slowly (0.35 ug hr cm ) and that parathion was unsuitable as a model substance for studying percutaneous absorption. The rate of absorption was determined by serial skin stipping and by measuring the disappearance of parathion over a period of 5 hr using a GM tube. 

Dermal dose-ChE response and percutaneous absorption studies were conducted with parathion, carbaryl, and thiodicarb in the rat. Parathion and thiodicarb inhibited 50% of the red cell cholinesterase activity at dose levels of 3.2 and 33 mg/kg of bw. Carbaryl at the highest dose level tested (417 mg/kg of bw) produced no detectable red cell cholinesterase Inhibition. 

Thlodicarb is a new insecticide being marketed by Union Carbide as Larvin for the control of insects on cotton. Thiodicarb is metabolized in the rat to acetamide, acetonitrile, CO, methomyl, and methomyl metabolites. No illnesses have been reported among applicators or field workers coming in contact with this carbamate insecticide. Thiodicarb is somewhat more persistent on crops than methomyl and is less toxic dermally in rabbit studies. The dermal dose-CliE response study showed that a dose of 33 mg/kg produced 50% ChE inhibition 24 hr. after the application of the dose, while a dose of 87 mg/kg of thiodicarb did not produce more inhibition than a dose of 44 mg/kg. The poor solubility of this carbamate in water/lipid most likely prevented the absorption of additional quantities of thiodicarb, thereby reducing its toxicity. Thiodicarb was found in lower concentrations in skin than either parathion or carbaryl during the percutaneous absorption studies. The concentrations in plasma. 

---