Department of Biology, University of Ottawa, Ottawa, ON K1N 6N5, Canada.
J Exp Biol. 2010 Nov 1;213(Pt 21):3656-65. doi: 10.1242/jeb.045955.
Goldfish acclimated to cold water (e.g. 7°C) experience a marked reduction in functional lamellar surface area owing to the proliferation of an interlamellar cell mass (ILCM), a phenomenon termed gill remodelling. The goal of the present study was to assess the consequences of the reduced functional surface area on the capacity of goldfish to excrete ammonia. Despite the expected impact of ambient temperature on functional surface area, fish acclimated to 7°C and 25°C exhibited similar rates of ammonia excretion (J(net,amm)); the Q₁₀ values for fed and starved fish were 1.07 and 1.20, respectively. To control for possible temperature-related differences in rates of endogenous ammonia production, J(net,amm) was determined at the two acclimation temperatures after loading fish with 1.12 μmol g₋₁ of NH₄Cl. In the 3 h post-injection period, J(net,amm) was elevated to a greater extent in the 25°C fish. To estimate the potential contribution of increased ventilation and cardiac output to ammonia clearance in the warmer fish, the ammonia loading experiment was repeated on the 7°C fish immediately after they were exercised to exhaustion. The rate of excretion of ammonia was significantly increased in the exercised 7°C fish (presumably experiencing increased ventilation and cardiac output for at least some of the measurement period) suggesting that differences in external and internal convection may at least partially explain the enhanced capacity of the 25°C fish to clear the ammonia load. To more specifically assess the contribution of the different functional surface areas on the differing rates of ammonia clearance at the two acclimation temperatures, the 7°C fish were exposed for 7 days to hypoxia (P(O₂)=10 mmHg=1.33 kPa), a treatment known to cause the disappearance of the ILCM. The results demonstrated that the hypoxia-associated loss of the ILCM was accompanied by a significant increase in the rate of ammonia clearance in the 7°C fish when returned to normoxic conditions. To determine whether compensatory changes in the ammonia transporting proteins might be contributing to sustaining J(net,amm) under conditions of reduced functional lamellar surface area, the relative expression and branchial distribution of four Rh proteins were assessed by western blotting and immunocytochemistry. Although the relative expression of the Rh proteins was unaffected by acclimation temperature, there did appear to be a change in the spatial distribution of Rhag, Rhbg and Rhcg1. Specifically, these three Rh proteins (and to a lesser extent Rhcg2) appeared to localize in cells on the outer edge of the ILCM that were enriched with Na(+)/K(+)-ATPase. Thus, we suggest that despite the impediment to ammonia excretion imposed by the ILCM, goldfish acclimated to 7°C are able to sustain normal rates of excretion owing to the redistribution of ammonia transporting cells.
金鱼适应冷水（例如 7°C）时，由于板层间细胞块（ILCM）的增殖，功能板层表面积明显减少，这种现象称为鳃重塑。本研究的目的是评估功能表面积减少对金鱼排泄氨能力的影响。尽管环境温度对功能表面积有预期的影响，但适应 7°C 和 25°C 的鱼表现出相似的氨排泄率（J（net，amm））；饱食和饥饿鱼的 Q₁₀值分别为 1.07 和 1.20。为了控制可能与温度相关的内源性氨产生率的差异，在两种适应温度下，在用 1.12 μmol g₋₁的 NH₄Cl 加载鱼后，确定 J（net，amm）。在注射后 3 小时内，25°C 鱼中的 J（net，amm）升高到更大的程度。为了估计增加的通气和心输出量对温暖鱼类中氨清除的潜在贡献，在 7°C 鱼锻炼到筋疲力尽后，立即重复氨负荷实验。在锻炼后的 7°C 鱼中，氨的排泄率显著增加（推测至少在测量期的一部分时间内经历了增加的通气和心输出量），这表明外部和内部对流的差异至少部分解释了 25°C 鱼清除氨负荷的增强能力。为了更具体地评估在两种适应温度下不同功能表面积对不同氨清除率的贡献，将 7°C 鱼暴露于缺氧（P（O₂）= 10 mmHg=1.33 kPa）7 天，已知这种处理会导致 ILCM 消失。结果表明，缺氧相关的 ILCM 丧失伴随着 7°C 鱼在返回正常氧条件下氨清除率的显著增加。为了确定在功能板层表面积减少的情况下，氨转运蛋白的代偿性变化是否有助于维持 J（net，amm），通过 Western 印迹和免疫细胞化学评估了四种 Rh 蛋白的相对表达和鳃分布。尽管 Rh 蛋白的相对表达不受适应温度的影响，但 Rhag、Rhbg 和 Rhcg1 的空间分布似乎确实发生了变化。具体来说，这三种 Rh 蛋白（以及程度较小的 Rhcg2）似乎定位于 ILCM 外边缘富含 Na（+）/K（+）-ATPase 的细胞中。因此，我们认为，尽管 ILCM 对氨排泄造成了障碍，但适应 7°C 的金鱼能够维持正常的排泄率，这是由于氨转运细胞的重新分布。
