OBJECTIVE: This work exposed rats to low levels of the organophosphate insecticide chlorpyrifos and monitored for toxic effects, including weight gain. METHODS: Rats received either a subcutaneous injection of chlorpyrifos, 5 mg/kg/day, or an equal volume of vehicle daily for 4 months. Subjects were observed for 30 minutes after injection for signs of acute toxicity. Body weights were recorded at baseline, 2 months, 3 months, and 4 months. At the end of the experiment, the weights of hearts, medial lobe of the livers, peri-nephric fat pads, and gastrocnemius muscles were recorded. Effects of chlorpyrifos on adipocyte differentiation in culture were studied. Results were compared using RMANOVA. RESULTS: No signs of acute cholinergic toxicity were observed after injections in any subject. Rats in the 5 mg/kg group were significantly heavier than those in the control group by 2 months (335.7 +/- 16.7 g vs. 318.6 +/- 15.8 g; p = 0.034). This difference increased at 3 months (350.1 +/- 16.4 g vs. 322.3 +/- 21.3 g p = 0.006) and 4 months (374.4 +/- 22.2 g vs. 340.2 +/- 25.2 g p = 0.006). At 4 months, the weights of the perinephric fat pads were significantly increased in the chlorpyrifos group relative to controls (2.867 + 0.516 vs. 1.130 + 0.171, p = 0.0039). The two groups showed no weight differences between hearts, livers, and gastrocnemius muscles. Chlorpyrifos did not affect adipocyte differentiation in tissue culture. CONCLUSIONS: Chronic exposure to chlorpyrifos at 5 mg/kg/day caused an increase in rat body weight when compared to controls. This increase was in adipose tissue. Chlorpyrifos did not induce differentiation of adipocytes in culture.

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