鹽度和pH值對青鱂魚仔魚排氨機制的影響

Abstract

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.