Characterization of cholinesterases in Chironomus riparius and the effects of three herbicides on chlorpyrifos toxicity.

In this study, the toxicities of four pesticides (the herbicides atrazine, terbuthylazine, metolachlor and the insecticide chlorpyrifos) previously detected in the Alqueva reservoir/dam (south of Portugal) were evaluated individually and in binary combinations of the herbicides and the insecticide using fourth-instar larvae of the aquatic midge Chironomus riparius. Chlorpyrifos induced toxicity to midges in all the 48 h toxicity bioassays performed. The swimming behaviour of the larvae was impaired, with EC50 values ranging from 0.15 to 0.17 µg/L. However, neither s-triazine (atrazine and terbuthylazine) herbicides nor metolachlor alone at concentrations up to 200 µg/L caused significant toxicity to C. riparius. When combined with both s-triazine herbicides, chlorpyrifos toxicity was enhanced by approximately 2-fold when tested in a binary mixture experimental setup, at the 50% effective concentration levels. To evaluate how chlorpyrifos toxicity was being increased, the cholinesterases (ChE) were characterized biochemically using different substrates and selective inhibitors. The results obtained suggested that the main enzyme present in this species is acetylcholinesterase (AChE) and therefore it was assayed upon C. riparius exposures to all pesticides individually and as binary mixtures. Although atrazine and terbuthylazine are not effective inhibitors of AChE, the potentiation of chlorpyrifos toxicity by the two s-triazine herbicides was associated with a potentiation in the inhibition of AChE in midges; both s-triazine herbicides at 200 µg/L increased the inhibition of the AChE activity by 7 and 8-fold, respectively. A strong correlation was observed between swimming behaviour disturbances of larvae and the inhibition of the AChE activity. In contrast, metolachlor did not affect chlorpyrifos toxicity at any of the concentrations tested. Therefore, the herbicides atrazine and terbuthylazine can act as synergists in the presence of chlorpyrifos, increasing the toxicity and consequently underestimating risk based on single chemical levels.

Synergistic effects caused by atrazine and terbuthylazine on chlorpyrifos toxicity to early-life stages of the zebrafish Danio rerio.

This study examined the effects of three widely used pesticides that have been previously detected in aquatic systems neighbouring agricultural fields on the early-life stages of the zebrafish Danio rerio. Tests involving single exposures and binary combinations of the s-triazine herbicides (atrazine and terbuthylazine) and the organophosphate insecticide chlorpyrifos were performed. Several endpoints, such as swimming behaviour, morphological abnormalities and mortality, were studied. In addition, the inhibition of acetylcholinesterase (AChE) activity was investigated in order to evaluate the mode of action and toxicity of chlorpyrifos in the presence of these herbicides. Results indicate that both binary mixtures elicited synergistic responses on the swimming behaviour of zebrafish larvae. Moreover, although the herbicides were not effective inhibitors of the AChE on their own, a synergistic inhibition of the enzyme activity was obtained by exposure to mixtures with chlorpyrifos. We observed a correlation between impairment of swimming behaviour of the larvae and inhibition of AChE activity. This study supports previous studies concerning the risk assessment of mixtures since the toxicity may be underestimated when looking only at the single toxicants and not their mixtures.

Growth rate of Pseudokirchneriella subcapitata exposed to herbicides found in surface waters in the Alqueva reservoir (Portugal): a bottom-up approach using binary mixtures.

Previous work showed the existence of ecotoxicity of water samples from the Alqueva reservoir due to the presence of the herbicides atrazine, simazine, terbuthylazine and metolachlor. In the present study we examine the effects of these herbicides singly and as binary mixtures on the growth rate of the microalgae Pseudokirchneriella subcapitata. Usually, the toxicity of mixtures is evaluated in relation to the reference models CA (concentration addiction) and IA (independent action). In this study CA model was selected to evaluate the joint effects of s-triazine herbicides on the growth of algae due to their similar mode of action. Moreover, IA reference model was chosen to evaluate the joint toxicity of the chloroacetanilide metolachlor and the s-triazine herbicides due to their different mode of action. In this study dose ratio was the common deviation obtained on both reference models. In the binary mixtures between atrazine/simazine and atrazine/terbuthylazine the increase of the mixtures toxicity (synergism) was mainly due to atrazine. Also, in the binary mixture between atrazine and metolachlor, atrazine was responsible for the increase (synergism) of the mixture toxicity. In the cases of the binary mixtures between simazine/metolachlor and terbuthylazine/metolachlor, the increase of the mixtures toxicity (synergism) was detected when metolachlor was the herbicide dominant, and antagonism was detected when simazine and terbuthylazine were dominant in both mixtures. This study represents an important step to understand the interactions among herbicides detected previously in the waters of the Alqueva reservoir.

Assessment of water quality in the Alqueva Reservoir (Portugal) using bioassays.

BACKGROUND, AIM, AND SCOPE: Alqueva Reservoir is the biggest artificial freshwater reservoir in Europe and is an important water supply for human and agricultural consumption in the Alentejo region (Portugal). Pollution can impair environmental and human health status, and to assure water quality and ecological balance, it is crucial to frequently monitor water supplies. In this study, we used an ecotoxicological test battery to identify the potential toxicity of water from this reservoir. MATERIALS AND METHODS: Water samples from the Alqueva aquatic system were collected bimonthly in 2006 from 11 different water points within the reservoir. Several bioassays were carried out: a 72-h growth test with Pseudokirchneriella subcapitata, a 6-day growth test with Chironomus riparius larvae, and the luminescence inhibition test with Vibrio fischeri (Microtox(R)). RESULTS AND DISCUSSION: Algae growth was significantly inhibited in several sampling points and periods throughout the year, mainly due to the presence of pesticides. Although in some sampling points pesticide concentrations (single and sum) were still below the maximum permissible concentrations, water samples showed high toxicities to algae, especially during the summer months. In addition, several sampling points showed pesticide concentrations above the permissible level which can pose a significant risk to humans and the environment. Chironomids showed less sensitivity to the water samples, possibly due to the low concentrations of insecticides present. V. fischeri showed no sensitivity when exposed to all the water samples collected throughout the year of 2006. CONCLUSIONS: Standardized laboratory bioassays can be useful tools to assess water quality from aquatic systems and can valuably complement chemical analysis evaluation. The results obtained in this study demonstrated that the most sensitive species used in this test battery was the microalgae P. subcapitata. The growth of C. riparius was less affected, which is probably due to the fact that low insecticide concentrations were measured and, furthermore, since this species lives in the sediment and not in the water column and is, therefore, usually more resistant to pollutants. RECOMMENDATIONS AND PERSPECTIVES: On its own, chemical analysis is not enough to derive conclusions on the water quality and/or status, which can be valuably complemented by laboratory bioassays. Single chemical, maximum permissible values, and the sum of pesticide concentrations do not take into account possible patterns of synergism, antagonism, dose level dependencies, or even the dominance of several chemicals within a mixture. In addition, several species from different levels in trophic chains are recommended due to differences in species' sensitivities to chemical compounds that are present.

Digoxin: current use and approach to toxicity.

Heralded as the oldest known cardiovascular drug, digoxin remains widely used today in the face of increasing rates in heart failure and atrial fibrillation despite the emergence of newer medications. Its hemodynamic, neurohormonal and electrophysiologic actions make it a suitable adjunctive, evidence-based therapy for the above conditions. Its narrow therapeutic index and its toxicity, however, have become more relevant as aging, comorbid diseases, and polypharmacy make more patients vulnerable. Because signs and symptoms of digoxin toxicity are mostly nonspecific, a high index of suspicion is crucial for early recognition and appropriate management.