Glover Chris N, Wood Chris M
Department of Biology, McMaster University, Hamilton, Ontario L8S 4K1, Canada.
Physiol Biochem Zool. 2005 Nov-Dec;78(6):1005-16. doi: 10.1086/432858. Epub 2005 Sep 8.
Humic substances have important functions in aquatic systems. While these roles are primarily indirect, influencing the physicochemical environment, recent evidence suggests these materials may also have direct biological actions. This study investigated the mechanism by which humic substances perturb sodium metabolism in a freshwater invertebrate, the water flea Daphnia magna. Aldrich humic acid (AHA) stimulated the maximal rate of whole-body sodium influx (Jmax) when experimental pH was 6 and water calcium content was 0.5 mM. This effect persisted at pH 8 and 1 mM calcium but not at pH 8 in the absence of calcium. An indirect action of AHA on apical transporter activity was proposed to explain this effect. At pH 4 AHA promoted a linear sodium uptake kinetic relationship, attributed to altered membrane permeability due to enhanced membrane binding of humic substances at low pH. In contrast, a real-world natural organic matter sample had no consistent action on sodium influx, suggesting that impacts on sodium metabolism may be limited to commercially available humic materials. These findings question the applicability of commercially available humic substances for laboratory investigations and have significant implications for the study of environmental metal toxicity.
腐殖质在水生系统中具有重要作用。虽然这些作用主要是间接的，影响着物理化学环境，但最近的证据表明这些物质可能也具有直接的生物学作用。本研究调查了腐殖质扰乱淡水无脊椎动物大型溞钠代谢的机制。当实验pH值为6且水体钙含量为0.5 mM时，奥尔德里奇腐殖酸（AHA）刺激了全身钠流入的最大速率（Jmax）。在pH值为8且钙含量为1 mM时这种效应依然存在，但在无钙条件下pH值为8时则不存在。有人提出AHA对顶端转运体活性的间接作用来解释这种效应。在pH值为4时，AHA促进了线性钠摄取动力学关系，这归因于在低pH值下腐殖质与膜结合增强导致膜通透性改变。相比之下，一个实际的天然有机物样品对钠流入没有一致的作用，这表明对钠代谢的影响可能仅限于市售腐殖质材料。这些发现质疑了市售腐殖质在实验室研究中的适用性，并对环境金属毒性研究具有重要意义。
