廣鹽性青鱂魚仔魚體表離子細胞排放鉀離子之機制

Abstract

魚類鰓與皮膚上的離子細胞是維持離子與滲透壓恆定的主要細胞。離子細胞參與鈉、氯、鈣離子的調節機制已經有許多研究，但是脊椎動物身體內重要的陽離子¬鉀離子的調節機制卻不甚明確。近期研究發現，斑馬魚離子細胞中有一群細胞的頂膜會表現鉀離子通道(ROMK)，而底側膜有鈉鉀幫浦(NKA)。在吳郭魚(Mozambique tilapia)的研究也發現類似的細胞，並且推論這種細胞是參與鉀離子分泌的離子細胞。然而，這些研究仍然缺少直接的證據去證實鉀離子分泌細胞的功能。本研究使用廣鹽性青鱂魚(Oryzias latipes)作為模式物種，利用掃描式離子選擇電極技術(SIET)測量仔魚皮膚上離子細胞鉀離子流，證實有一群離子細胞會分泌鉀離子。高鉀淡水與海水環境的馴養後會促進鉀離子的分泌。而且鉀離子的分泌會受到ROMK與NKCC抑制劑的抑制。利用原位雜交與細胞免疫組織染色也發現ROMKa確實會表現在離子細胞中。定量PCR分析魚鰓上mRNA的變化，發現高鉀環境馴養能夠增加ROMKa與NKCC1a的表現量。本研究提供直接的證據支持離子細胞分泌鉀離子的生理功能，並提出ROMKa與NKCC1參與鉀分泌的機制。

The ionocyte in gills and skin of fish plays a critical role in ionic and osmotic regulation. Molecular mechanisms of sodium, chloride and calcium regulation in the ionocytes have been well investigated. However, the regulatory mechanism of potassium, which is a major monovalent cation in fish, is still unclear. In recent studies, a sub-group of ionocyte, which expresses a renal outer medullar potassium channel (ROMK) in the apical membrane and Na+/K+-ATPase (NKA) in the basolateral membrane was identified in zebrafish and Mozambique tilapia, suggesting that those ionocytes are involved in potassium secretion. However, the potassium secreting function of ionocyte is not fully understood. In this study, we investigated the mechanism of potassium secretion by using medaka (Oryzias latipes) as an animal model. With a scanning ion-selective electrode technique (SIET), potassium secretion was detected at the apical surface of a group of ionocytes in the skin of medaka larvae. The potassium secretion was enhanced after high potassium water (HK) or seawater acclimation, and was inhibited by ROMK and Na+-K+-Cl- cotransporter (NKCC) inhibitors. In situ hybridization and immunohistochemistry showed that kcnj1a and NKCC1a were expressed in the skin ionocytes. Quantitative PCR showed that mRNA levels of kcnj1a and nkcc1a were induced by HK acclimation. Taken together, this study provides physiological and molecular evidence to show that ROMKa and NKCC1 are involved in the potassium secretion of ionocytes.