Gonzalez R J, Wilson R W, Wood C M, Patrick M L, Val A L
Laboratory of Ecology and Molecular Evolution, National Institute for Amazon Research, Alameda Cosme Ferreira, 1756. 69.083-000 Manaus, Amazonas, Brazil.
Physiol Biochem Zool. 2002 Jan-Feb;75(1):37-47. doi: 10.1086/339216.
We measured unidirectional ion fluxes of fish collected directly from the Rio Negro, an extremely dilute, acidic blackwater tributary of the Amazon. Kinetic analysis of Na(+) uptake revealed that most species had fairly similar J(max) values, ranging from 1,150 to 1,750 nmol g(-1) h(-1), while K(m) values varied to a greater extent. Three species had K(m) values <33 micromol L(-1), while the rest had K(m) values >or=110 micromol L(-1). Because of the extremely low Na(+) concentration of Rio Negro water, the differences in K(m) values yield very different rates of Na(+) uptake. However, regardless of the rate of Na(+) uptake, measurements of Na(+) efflux show that Na(+) balance was maintained at very low Na(+) levels (<50 micromol L(-1)) by most species. Unlike other species with high K(m) values, the catfish Corydoras julii maintained high rates of Na(+) uptake in dilute waters by having a J(max) value at least 100% higher than the other species. Corydoras julii also demonstrated the ability to modulate kinetic parameters in response to changes in water chemistry. After 2 wk in 2 mmol L(-1) NaCl, J(max) fell >50%, and K(m) dropped about 70%. The unusual acclimatory drop in K(m) may represent a mechanism to ensure high rates of Na(+) uptake on return to dilute water. As well as being tolerant of extremely dilute waters, Rio Negro fish generally were fairly tolerant of low pH. Still, there were significant differences in sensitivity to pH among the species on the basis of degree of stimulation of Na(+) efflux at low pH. There were also differences in sensitivity to low pH of Na(+) uptake, and two species maintained significant rates of uptake even at pH 3.5. When fish were exposed to low pH in Rio Negro water instead of deionized water (with the same concentrations of major ions), the effects of low pH were reduced. This suggests that high concentrations of dissolved organic molecules in the water, which give it its dark tea color, may interact with the branchial epithelium in some protective manner.
我们测量了直接从内格罗河采集的鱼类的单向离子通量，内格罗河是亚马逊河一条极其稀释、呈酸性的黑水支流。对钠（Na⁺）摄取的动力学分析表明，大多数物种的最大摄取率（J(max)）值相当相似，范围在1150至1750纳摩尔克⁻¹小时⁻¹之间，而米氏常数（K(m)）值变化幅度更大。三个物种的K(m)值<33微摩尔升⁻¹，其余物种的K(m)值≥110微摩尔升⁻¹。由于内格罗河水的钠（Na⁺）浓度极低，K(m)值的差异导致钠（Na⁺）摄取速率差异很大。然而，无论钠（Na⁺）摄取速率如何，钠（Na⁺）外流的测量结果表明，大多数物种在极低的钠（Na⁺）水平（<50微摩尔升⁻¹）下维持钠（Na⁺）平衡。与其他具有高K(m)值的物种不同，鲶鱼朱莉科氏兵鲶（Corydoras julii）通过具有至少比其他物种高100%的J(max)值，在稀释水中维持高钠（Na⁺）摄取率。朱莉科氏兵鲶还表现出能够根据水化学变化调节动力学参数的能力。在2毫摩尔升⁻¹氯化钠中放置2周后，J(max)下降>50%，K(m)下降约70%。K(m)这种不寻常的适应性下降可能代表一种机制，以确保回到稀释水后能有高钠（Na⁺）摄取率。除了耐受极其稀释的水之外，内格罗河鱼类总体上对低pH也相当耐受。不过，根据低pH下钠（Na⁺）外流的刺激程度，物种之间对pH的敏感性存在显著差异。钠（Na⁺）摄取对低pH的敏感性也存在差异，甚至在pH 3.5时，两个物种仍维持显著的摄取率。当鱼类在含有相同主要离子浓度的内格罗河水而非去离子水中暴露于低pH时，低pH的影响会降低。这表明水中高浓度的溶解有机分子（赋予水深茶颜色）可能以某种保护方式与鳃上皮相互作用。
