鹽度和pH值對青鱂魚仔魚排氨機制的影響

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2011

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近年文獻發現氨主要以非離子態NH3經由Rh蛋白排除。目前海水魚的排氨機制尚不清楚,本實驗以青鱂魚仔魚為模式動物,利用掃描式離子選擇電極技術(SIET)進行非侵入性測量,驗證假說(一)海水魚執行與淡水魚類似的排氨機制,(二)魚體誘導酸中毒會增加氨排放促進排酸。結果顯示,海水組排NH4+和排H+功能同時存在MRC上,且排氨速率顯著高於淡水組,並導致體表的鹼化,意味著在MRC上是以非離子態的NH3進行運輸後,在體表進行酸捕捉(acid trapping)。使用NHE抑制劑(500 μM EIPA)後,顯著降低魚體排酸、排氨,暗指NHE在排氨機制上扮演關鍵性的角色。馴養淡水酸性(pH5)環境的仔魚,顯示降低體表H+的累積,提升NH4+的排放,並增強細胞Na+的吸收,意味著酸中毒會增強排氨機制,利用排氨帶動大量酸排放,使NH3與H+結合成NH4+,促使更多的酸排出。本實驗證明(一)海水魚執行與淡水魚類似的排氨機制,(二)當魚體酸中毒會增強排氨機制,以利排酸。
Ammonia excretion in seawater (SW) teleost was rarely studied. In this study, an ion-selective microelectrode technique (SIET) was applied to analyze the ionic (H+ or NH4+) activity at the skin cells of SW-acclimated medaka (Oryzias latipes) larvae. Results showed that H+ and NH4+ were mainly excreted by mitochondrion-rich cells (MRCs). The excretion rate of NH4+ was significantly higher in SW than in FW. Elevated ammonia excretion in SW-larvae alkalized the skin surface, suggesting that non-ionic NH3 is excreted by MRCs. Both NH4+ and H+ excretion were inhibited by EIPA, suggesting that Na+/H+ exchanger (NHE) plays a critical role in the ammonia excretion. On the other hand, raising the acidity of water (FW, pH5) enhanced NH4+ excretion and Na+ uptake by MRCs whereas it reduced the H+ accumulation on the skin surface. This study demonstrated that mechanism of ammonia excretion in SW is similar to that in FW. In addition acidic water can enhance the NH4+ excretion and Na+ uptake in larvae.

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排氨機制, 青鱂魚仔魚, 海水魚, 酸中毒, ammonia excretion, medaka larvae, SW-acclimated, acidity of water

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