Insecticide mosquitofish

The Disposition and Biotransformation of Organochlorine Insecticides in Insecticide-Resistant and -Susceptible Mosquitofish... 

Figure 1. Cytochrome P-450 contents in liver microsomes from insecticide-resistant and -susceptible mosquitofish...

Figure 2. Specific activity of NADPH-Cytochrome c reductase in liver microsomes from insecticide-resistant and -susceptible mosquitofish. A unit (U) of enzyme activity is defined as 1 pmol product formed/min.

The uptake and distribution of organochlorine insecticides has been studied under a variety of conditions. Although the results indicate that further study is needed on a characterization of extraneous factors that affect disposition, the studies clearly demonstrate the presence of a membrane barrier to insecticide penetration in the R population. This membrane barrier would aid in the protection of target sites in the R fish from the insecticide. This barrier is felt to be an important factor in resistance to organochlorine insecticides in mosquitofish. 

Further, by virtue of their larger livers, the R fish have a greater xenobiotic biotransformation potential. However, the in vivo studies show few consistent differences in metabolism between the two populations. Biotransformation may be a major contributory factor in mosquitofish resistance to other pesticides, for example, organophosphorus and botanical insecticides, since the level of resistance to these chemicals is very low (4 fold or less) 08,20,21). However, biotransformation does not appear to play a major role in organochlorine insecticide resistance. 

The levels of resistance which the mosquitofish demonstrate toward the chlorinated alicyclic insecticides (40 - 500 fold) are the most intriguing of those studied, yet they are impossible to explain in terms of disposition and biotransformation alone. Although insecticide metabolism cannot be completely discounted, it contributes little to chlorinated alicyclic resistance. 

We have, therefore, been able to indirectly assess the importance of three factors involved in chlorinated alicyclic insecticide resistance in mosquitofish disposition, metabolism and target site sensitivity. In a highly polluted environment in which mosquitofish have been placed under severe selective pressures by chronic exposure to insecticides, the system of metabolism appears to be of little significance in resistance the... 

Hepatic mixed-function oxidase activities demonstrated seasonal trends, with higher specific activities in the cold weather months in both populations with few differences in enzyme activities or cytochrome levels between the two populations. Metabolism of aldrin, dieldrin and DDT was similar between the two populations. R fish have larger relative liver size and, therefore, a greater potential for xenobiotic metabolism. However, biotransformation appears to be of minor importance in chlorinated alicyclic insecticide resistance in mosquitofish barriers to penetration appear to be of greater importance and an implied target site insensitivity appears to be the most important factor in resistance. 

Decreased sensitivity to insecticides in resistant strains of mosquitofish from Belzoni, Mississippi is partly based on their ability to metabolize the toxicant rapidly. Resistant strains converted aldrin to dieldrin and water-soluble components to a greater extent than sensitive strains89. Resistant strains also had higher activities of methyl-parathion dearylation which appear to be catalyzed by the higher levels of CYP in this strain15. 

Chambers, J.E. and J.D. Yarbrough. Organophosphate degradation by insecticide-resistant and susceptible populations of mosquitofish (Gambusia affinis). Pesti. Biochem. Physiol. 3 312-322, 1973. 

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