Hypoxia and mercury contamination often co-occur in tropical freshwater ecosystems, but the interactive effects of these two stressors on fish populations are poorly known. The effects of mercury (Hg) on recorded changes in the detailed form of the electrocardiogram (ECG) during exposure to progressive hypoxia were investigated in two Neotropical freshwater fish species, matrinxã, Brycon amazonicus and traíra, Hoplias malabaricus. Matrinxã were exposed to a sublethal concentration of 0.1 mg L-1 of HgCl2 in water for 96 h. Traíra were exposed to dietary doses of Hg by being fed over a period of 30 days with juvenile matrinxãs previously exposed to HgCl2, resulting in a dose of 0.45 mg of total Hg per fish, each 96 h. Both species showed a bradycardia in progressive hypoxia. Hg exposure impaired cardiac electrical excitability, leading to first-degree atrioventricular block, plus profound extension of the ventricular action potential (AP) plateau. Moreover, there was the development of cardiac arrhythmias and anomalies such as occasional absence of QRS complexes, extra systoles, negative Q-, R- and S-waves (QRS complex), and T wave inversion, especially in hypoxia below O2 partial pressures (PO2) of 5.3 kPa. Sub-chronic dietary Hg exposure induced intense bradycardia in normoxia in traira, plus lengthening of ventricular AP duration coupled with prolonged QRS intervals. This indicates slower ventricular AP conduction during ventricular depolarization. Overall, the data indicate that both acute waterborne and sub-chronic dietary exposure (trophic level transfer), at sublethal concentrations of mercury, cause damage in electrical stability and rhythm of the heartbeat, leading to myocardial dysfunction, which is further intensified during hypoxia. These changes could lead to impaired cardiac output, with consequences for swimming ability, foraging capacity, and hence growth and/or reproductive performance.

中文翻译：

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汞暴露和缺氧对两种新热带淡水鱼物种：Matrinxã，Brycon amazonicus和traíra，Hoplias malabaricus的心电图模式的相互作用。

缺氧和汞污染经常在热带淡水生态系统中同时发生，但是这两种应激因素对鱼类种群的相互作用却鲜为人知。在两种新热带淡水鱼中，Matrinxã，Brycon amazonicus和traíra，Hoplias malabaricus，研究了汞（Hg）对进行性缺氧期间心电图（ECG）详细形式记录变化的影响。Matrinxã暴露在水中的亚致命浓度为0.1 mg L-1的HgCl2达96 h。特拉伊拉（Traíra）通过在30天的时间里喂食以前服用过HgCl2的幼年母草暴露于膳食中的汞，每条鱼的总Hg剂量为0.45 mg，每96小时一次。两种物种在进行性缺氧时均表现为心动过缓。汞暴露会损害心脏电兴奋性，导致一级房室传导阻滞，以及心室动作电位（AP）平台的深度扩展。此外，还有心律不齐的发展和异常，例如偶尔缺乏QRS复合波，额外的收缩期，Q波，R波和S波阴性（QRS复合波）以及T波反转，尤其是在氧分压低于O2的低氧环境中（ PO2）为5.3 kPa。亚慢性饮食中的汞暴露会导致traira的正常性缺氧导致严重的心动过缓，加上心室AP持续时间延长和QRS间隔延长。这表明在心室去极化过程中心室AP传导较慢。总体而言，数据表明，在亚致死汞浓度下，急性水源性饮食和亚慢性饮食（营养水平转移）都会导致电稳定性和心律失常，导致心肌功能障碍，在缺氧时进一步加剧。这些变化可能导致心输出量受损，进而影响游泳能力，觅食能力，进而影响生长和/或生殖能力。