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Two main functions of rat liver mitochondria namely the respiration rate and generation of membrane potential, were analysed simultaneously to evaluate the toxicity of the dinitroaniline herbicide pendimethalin. The chromatography grade pendimethalin enhanced the mitochondrial respiration in a concentration-dependent manner and markedly decreased the membrane potential starting from 8.2x10-5 M up to 5.47x10-4 M (23-154 mg/ml). A higher toxicity of technical grade pendimethalin (Stomp - a mixture of pure ingredient and adjuvant(s)) was observed at lower concentrations of the pure ingredient (pendimethalin) than those of the single chromatography grade chemical. These data indicate that: 1) pure and technical grade pendimethalin preparations act as uncouplers of oxidative phosphorylation in mitochondria (enhance respiration and diminish membrane potential), 2) these effects are weaker but comparable (of the same concentration order) to those of 2,4-dinitrophenol, and 3) technical grade pendimethalin is more toxic to oxidative phosphorylation in mitochondria than pure chemical pendimethalin (active ingredient) itself. These data clearly disclaim the statement presented in the manual "Recognition and Management of Pesticides Poisonings" (US EPA 1999) that pendimethalin does not act as an uncoupler of oxidative phosphorylation. [ABSTRACT FROM AUTHOR]

The characterization of bioavailability and toxicity of pesticides is necessary for the assessment of environmental risk caused by these chemicals. Pendimethalin is a dinitroaniline herbicide used for selective control of most annual grasses and many annual broad-leaved weeds in several crops. The technical formulation of this herbicide, Stomp 330, has been approved in Lithuania and is applied to winter rye, barley, maize, winter wheat, as well as to some vegetable crops. The objective of this study was 1) to evaluate the bioavailability and toxicity of pendimethalin to three species of aquatic microorganisms: green microalgae Selenastrum capricornutum, ciliate protozoa Tetrahymena thermophila and luminescent bacteria Vibrio fischeri; 2) to compare the toxicity of pendimethalin-standard (pure compound) and technical formulation of herbicide Stomp 330 to the microorganisms tested. The growth of algae S. capricornutum was most sensitive to pendimethalin (the value of EC50 for pendimethalinstandard--52 µg/l) In the tests with T. thermophila and V. fischeri, the values of EC50 for pendimethalin were above the water solubility level of this compound (0.3 mg/l). The effect of Stomp 330 on bacterial bioluminescence appeared in a shorter time as compared to the effect of pure pendimethalin. The difficulty in evaluating the bioavailability and toxicity of poorly watersoluble substances to microbial cells is discussed. [ABSTRACT FROM AUTHOR]