Ip Yuen K, Randall David J, Kok Timothy K T, Barzaghi Cristiana, Wright Patricia A, Ballantyne James S, Wilson Jonathan M, Chew Shit F
Department of Biological Sciences, National University of Singapore, 10 Kent Ridge Road, Singapore 117543, Republic of Singapore.
J Exp Biol. 2004 Feb;207(Pt 5):787-801. doi: 10.1242/jeb.00788.
Periophthalmodon schlosseri is an amphibious and obligatory air-breathing teleost, which is extremely tolerant to environmental ammonia. It actively excretes NH(4)(+) in ammonia loading conditions. For such a mechanism to operate efficaciously the fish must be able to prevent back flux of NH(3). P. schlosseri could lower the pH of 50 volumes (w/v) of 50% seawater in an artificial burrow from pH 8.2 to pH 7.4 in 1 day, and established an ambient ammonia concentration of 10 mmol l(-1) in 8 days. It could alter the rate of titratable acid efflux in response to ambient pH. The rate of net acid efflux (H(+) excretion) in P. schlosseri was pH-dependent, increasing in the order pH 6.0<7.0<8.0<8.5. Net acid flux in neutral or alkaline pH conditions was partially inhibited by bafilomycin, indicating the possible involvement of a V-type H(+)-ATPase. P. schlosseri could also increase the rate of H(+) excretion in response to the presence of ammonia in a neutral (pH 7.0) external medium. Increased H(+) excretion in P. schlosseri occurred in the head region where active excretion of NH(4)(+) took place. This would result in high concentrations of H(+) in the boundary water layer and prevent the dissociation of NH(4)(+), thus preventing a back flux of NH(3) through the branchial epithelia. P. schlosseri probably developed such an 'environmental ammonia detoxification' capability because of its unique behavior of burrow building in the mudflats and living therein in a limited volume of water. In addition, the skin of P. schlosseri had low permeability to NH(3). Using an Ussing-type apparatus with 10 mmol l(-1) NH(4)Cl and a 1 unit pH gradient (pH 8.0 to 7.0), the skin supported only a very small flux of NH(3) (0.0095 micromol cm(-2) min(-1)). Cholesterol content (4.5 micromol g(-1)) in the skin was high, which suggests low membrane fluidity. Phosphatidylcholine, which has a stabilizing effect on membranes, constituted almost 50% of the skin phospholipids, with phosphatidyleserine and phsophatidylethanolamine contributing only 13% and 15%, respectively. More importantly, P. schlosseri increased the cholesterol level (to 5.5 micromol g(-1)) and altered the fatty acid composition (increased total saturated fatty acid content) in its skin lipid after exposure to ammonia (30 mmol l(-1) at pH 7.0) for 6 days. These changes might lead to an even lower permeability to NH(3) in the skin, and reduced back diffusion of the actively excreted NH(4)(+) as NH(3) or the net influx of exogenous NH(3), under such conditions.
施氏鲻虾虎鱼是一种两栖且必须呼吸空气的硬骨鱼，对环境氨具有极强的耐受性。在氨负荷条件下，它会主动排泄NH₄⁺。为使这种机制有效运行，该鱼必须能够防止NH₃的逆向流动。施氏鲻虾虎鱼能在1天内将人工洞穴中50体积（w/v）的50%海水的pH值从8.2降至7.4，并在8天内使环境氨浓度达到10 mmol·l⁻¹。它能根据环境pH值改变可滴定酸的排泄速率。施氏鲻虾虎鱼的净酸排泄速率（H⁺排泄）依赖于pH值，按pH 6.0＜7.0＜8.0＜8.5的顺序增加。在中性或碱性pH条件下，净酸通量部分受到巴弗洛霉素的抑制，这表明可能涉及V型H⁺ - ATP酶。施氏鲻虾虎鱼在中性（pH 7.0）外部介质中存在氨的情况下，也能提高H⁺排泄速率。施氏鲻虾虎鱼H⁺排泄增加发生在NH₄⁺主动排泄的头部区域。这会导致边界水层中H⁺浓度升高，防止NH₄⁺解离，从而防止NH₃通过鳃上皮逆向流动。施氏鲻虾虎鱼可能由于其在泥滩中独特的打洞行为以及在有限体积的水中生活而发展出这种“环境氨解毒”能力。此外，施氏鲻虾虎鱼的皮肤对NH₃的渗透性较低。使用具有10 mmol·l⁻¹ NH₄Cl和1个单位pH梯度（pH 8.0至7.0）的尤斯灌流装置，皮肤仅支持非常小的NH₃通量（0.0095 μmol·cm⁻²·min⁻¹）。皮肤中的胆固醇含量（4.5 μmol·g⁻¹）很高，这表明膜流动性较低。对膜具有稳定作用的磷脂酰胆碱几乎占皮肤磷脂的50%，磷脂酰丝氨酸和磷脂酰乙醇胺分别仅占13%和15%。更重要的是，施氏鲻虾虎鱼在暴露于氨（pH 7.0条件下30 mmol·l⁻¹）6天后，其皮肤脂质中的胆固醇水平增加（至5.5 μmol·g⁻¹）并改变了脂肪酸组成（总饱和脂肪酸含量增加）。在这种条件下，这些变化可能导致皮肤对NH₃的渗透性更低，并减少主动排泄的NH₄⁺作为NH₃的逆向扩散或外源NH₃的净内流。
