Parathion elimination

In a study of pregnant rats that were exposed to radiolabeled methyl parathion by single dermal application, half-life elimination rate constants for various tissues ranged from 0.04 to 0.07 hour, highest values noted in plasma, kidneys, and fetus. Of the applied radioactivity, 14% was recovered in the urine in the first hour postapplication. By the end of the 96-hour study, 91% of the applied dose had been recovered in the urine. Fecal excretion accounted for only 3% of the administered dose (Abu-Qare et al. 2000). 

Effluent containing methyl parathion may not be discharged into lakes, streams, ponds, estuaries, oceans, or public waters unless the compound is specifically identified in a National Pollutant Discharge Elimination System (NPDES) permit. Moreover, discharge of effluent that contains methyl parathion is forbidden without prior notice to the sewage treatment plant authority (NPIRS 1986). 

Of the three organic phosphorus insecticides—hexaethyl tetraphosphate, tetraethyl pyrophosphate, and parathion—the first two have been shown to be mixtures (36) that contain tetraethyl pyrophosphate as the principal active ingredient. Several methods have been proposed for the determination of this compound in the commercial products (25, 35). All are based on the separation of the tetraethyl pyrophosphate from the related ethyl phosphates, followed by its hydrolysis to diethyl orthophosphoric acid and titration with standard alkali. Both hexaethyl tetraphosphate and tetraethyl pyrophosphate are soluble in water and are rapidly hydrolyzed to monoethyl and diethyl orthophosphoric acid. This rapid hydrolysis to nontoxic products greatly limits the duration of the in- secticidal effectiveness of tetraethyl pyrophosphate, but it also eliminates the danger of toxic residues on the crops treated. 

Accessibility of the deposit to the insects by contact is the chief feature of the small vial method, but fumigant action cannot be eliminated entirely. In the case of DDT this has been found to be unimportant, for flies kept in the vials out of contact with the surface are not affected. But with 7-hexachlorocyclohexane or parathion there is a noticeable toxic effect. If the vials are stood upright, laid on the side, or hung upside down, there is a decrease in the mortality produced in the order given. The position on the side has been adopted because it avoids extremes and because practical use of an insecticide often involves limited but not accentuated ventilation. 

Our first objective was to determine whether microencapsulated methyl parathion Is unique In Its property to be carried back to the hive by bees. To that end a mixture of three commonly used insecticides along with MMP was applied to a plot of blooming rape. The agents were azlnphos-methyl (Guthlon), parathion, and carbaryl (Sevin). By using a mixture on a single plot the effects of variation In bee visitation were eliminated and the tendencies to be carried to the hive could be measured by the relative residue levels in the pollen samples. Five applications were made over a period of seventeen days. Pollen samples were collected from hives placed near the field after two, three, four, and five successive applications approximately two days after each application was made. The application rates were doubled for the last two applications. The data are shown In Table I. 

Dichlorvos is of moderate acute toxicity, with an oral LD50 value in rodents from 50 to 150mgkg. While the LC50 for inhibiting mammalian brain acetylcholinesterase is similar between dichlorvos and paraoxon (i.e., the active metabolite of parathion), the acute LD50 values for these agents are considerably different, due in part to the more rapid metabolism and elimination of dichlorvos. 

Metabolites of parathion are exclusively eliminated through urine. However, some of the unmetabolized parent compound may also be excreted through sebum. The elimination half-life of parathion was found to be 2.1 days. It was reported that, following oral administration of parathion (1 or 2 mg day in humans, 60% of parathion was excreted within... 

Simultaneous Absorption and Elimination in Plasma. The ty e-concen-tration curves for the absorption and elimination of [ C]labeled parathion, carbaryl, and thiodicarb equivalents in plasma of adult rats are giv in Figure 8 and the pharmacokinetic constants in Table III. The [ C]equivalents were found in blood shortly after the application of the dose and reached maximum concentrations in plasma in 2.5 to 12 hr. Carbaryl and paraty on [ C]equivalents were eliminated during the study, while [ C]thiodicarb equivalents... 

TABLE III. Pharmacokinetic C stants For the Simultaneous Absorption and Elimination of [ C]Labeled Parathion, Carbaryl... 

Absorption Rates. The skin absorption rates for [ C]labeled para-thion, carbaryl, thiodicarb in adult rats are given in Table IV. Female rats absorbed parathion faster than males as indicated in this Table when either the t 1/2 for plasma elimination or for skin loss was used. In males and females, the t 1/2 for skin loss gave higher absorption rates than the t 1/2 for elimination from plasma. This difference is believed to be produced by evaporative losses of parathion from skin. Carbaryl was absorbed at a slower rate than parathion in males when the t 1/2 for plasma elimination or the t 1/2 for skin loss was used. Dijfer ces in the absorption rate for carbaryl (0.18 vs 0.44 ug hr cm maybe caused by evaporation. In the case of thiodicarb, t 1/2 for skin loss (a and g phase) was used to determine the rate of absorption because steady state plasma concentrations prevented the calculation of the t 1/2 for elimination of thiodicarb from plasma. 

Knaak et al. ( ) detected parathion equivalents in low concentrations in blood soon after topical application. The parathion equivalents reached a maximum concentration within 12 hr as shown by the blood plasma absorption-elimination curve. These blood levels most likely occur in workers exposed to foliar residues of parathion and are responsible for illnesses reported in workers several hr... 

Maibach et al., (19) conducted percutaneous absorption studies in man and Shah et al. (22) in the mouse. Carbaryl was absorbed more readily than parathion in these studies. The relationship between the dose and the adverse health effect (OiE inhibition) was not examined. According to the skin loss and plasma elimination data of Knaak et al. (iL) reviewed in this paper, carbaryl was not absorbed more rapidly than parathion in the adult male rat. More carbaryl residues were found in skin (penetrated) than were found in the case of parathion. The difference (2X) may be related to the ability of the rat skin to metabolize these pesticides prior to their absorption into blood. Fredriksson (22) showed that parathion was not metabolized to any extent in rat skin. Studies by Chin et al. (24) suggest that carbaryl may be partially metabolized in skin to water soluble products prior to their absorption into blood. 

Thiodicarb was initially lost from skin at a rate similar to that of parathion and carbaryl. After 24 hr, the rate of loss decreased by a factor of 1/6. The initial loss of thiodicarb appears to be due to a combination of events which may include evaporative or other losses, the initial penetration of the dose into skin and rapid distribution to blood and other tissues. Absorption was slow after 24 hr as indicated by the t 1/2 for skin loss of 254 hr. Parathion and carbaryl, on the other hand, penetrated the skin and were absorbed at a more uniform rate after 24 hr according to the skin loss data. The plasma absorption-elimination curve for thiodicarb plateaued after 24 hr. The dose remaining on the surface of the skin acted as an infinite dose supplying the rat with a low but uniform amount of thiodicarb. 

The effect of a topically applied dose of parathion, carbaryl, and thiodicarb on red blood cell cholinesterase activity in the rat was reviewed along with pharmacokinectic data developed on their percutaneous absorption. Parathion and thiodicarb inhibited 50% of the red cell cholinesterase activity at dose levels of 3.2 and 33 mg/kg of bw, while no inhibition was detected with carbaryl at dose levels as high as 417 mg/kg of w. Parathion and carbaryl were absorbed at 0.33 and 0.18 ug/hr/cm, while thiodicarb was absorbed at rates varying from 0.27 to 0.042 ug/hr/cm of skin. Skin loss and plasma elimination data were used to calculate the values. The topically applied pesticides slowly penetrated skin and were available for absorption into blood and redistribution to other tissues. Recovery data suggested that evaporative losses occurred during the course of the 5-day study. The pesticides may be removed from skin by washing, thus reducing the amount available for absorption. 

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