The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and the insecticide fipronil have been used widely in agriculture and detected in aquatic ecosystems, where they threaten wildlife. This study evaluated the whole-body bioconcentration and the biochemical and morphological changes in the gills of the neotropical fish Prochilodus lineatus exposed for 96 h to 2,4-D or fipronil as single compounds or as a mixture (2,4-D + fipronil). Fish exposed to either compound alone bioconcentrated 2,4-D (77 ± 23 ng g ^− 1 fish dry mass) and fipronil (789 ± 178 ng g ^− 1 fish dry mass). Fish exposed to 2,4-D + fipronil bioconcentrated fipronil (683 ± 73 ng g ^− 1 fish dry mass) but not 2,4-D. In the gills, catalase (CAT) and glutathione-S-transferase (GST) activities and the lipid peroxidation (LPO) level increased after exposure to 2,4-D. GST activity increased after exposure to fipronil. Conversely, no changes occurred in CAT and GST activities and LPO upon exposure to 2,4-D + fipronil. Histopathological changes such as hyperplasia, cellular hypertrophy, epithelial lifting, and vascular congestion were frequent in the gills of fish exposed to 2,4-D or fipronil individually or 2,4-D + fipronil. The mitochondria-rich cell (MRC) density increased on gill surface in fish exposed to fipronil or 2,4-D + fipronil. Only exposure to 2,4-D alone induced oxidative stress in the gills. Most morphological changes showed defense responses against the pesticides; however, hypertrophy and the change in MRC indicated compensatory responses to maintain the gill osmoregulatory function. The 2,4-D + fipronil mixture showed antagonistic interaction, except for the MRC fractional area at gill surface, which showed synergistic interaction. This is the first report showing antagonistic interaction of 2,4-D and fipronil in the gills after exposing fish to the mixture of both pesticides. The biochemical and morphological changes in gills endanger the gill functions, a phenomenon that implies an energy cost for fish.

除草剂 2,4-二氯苯氧乙酸 (2,4-D) 和杀虫剂氟虫腈已广泛用于农业并在水生生态系统中检测到，它们威胁着野生动物。本研究评估了新热带鱼Prochilodus lineatus暴露于 2,4-D 或氟虫腈作为单一化合物或混合物 (2,4- D  + 氟虫腈）。暴露于单独生物浓缩 2,4-D（77 ± 23 ng g  ^{−  1}鱼干质量）和氟虫腈（789 ± 178 ng g  ^{−  1}鱼干质量）的鱼。暴露于 2,4- D  + 氟虫腈生物浓缩氟虫腈 (683 ± 73 ng g ^{−  1}条鱼干重）但不是 2,4-D。在鳃中，接触 2,4-D 后，过氧化氢酶 (CAT) 和谷胱甘肽-S-转移酶 (GST) 活性和脂质过氧化 (LPO) 水平增加。接触氟虫腈后 GST 活性增加。相反，接触 2,4- D  + 氟虫腈后 CAT 和 GST 活性以及 LPO 没有发生变化。单独暴露于 2,4-D 或氟虫腈或 2,4- D  + 氟虫腈的鱼的鳃中经常出现组织病理学变化，例如增生、细胞肥大、上皮提升和血管充血。暴露于氟虫腈或 2,4- D 的鱼鳃表面富含线粒体的细胞 (MRC) 密度增加 + 氟虫腈。仅暴露于 2,4-D 会在鳃中引起氧化应激。大多数形态变化显示对杀虫剂的防御反应；然而，肥大和 MRC 的变化表明维持鳃渗透调节功能的代偿反应。2,4- D  + 氟虫腈混合物表现出拮抗作用，除了鳃表面的 MRC 分数区域显示出协同作用。这是第一份显示将鱼暴露于两种杀虫剂的混合物后鳃中 2,4-D 和氟虫腈的拮抗相互作用的报告。鳃的生化和形态变化危及鳃功能，这种现象意味着鱼类的能量成本。