Gonzalez R J, Brauner C J, Wang Y X, Richards J G, Patrick M L, Xi W, Matey V, Val A L
Department of Biology, University of San Diego, 5998 Alcalá Park, San Diego, California 92110, USA.
Physiol Biochem Zool. 2010 Mar-Apr;83(2):322-32. doi: 10.1086/648568.
Soon after hatching, the osteoglossid fish Arapaima gigas undergoes a rapid transition from a water breather to an obligate air breather. This is followed by a gradual disappearance of gill lamellae, which leaves smooth filaments with a reduced branchial diffusion capacity due to loss of surface area, and a fourfold increase in diffusion distance. This study evaluated the effects these changes have on gill function by examining two size classes of fish that differ in gill morphology. In comparison to smaller fish (approximately 67.5 g), which still have lamellae, larger fish (approximately 724.2 g) without lamellae took up a slightly greater percentage of O2 across the gills (30.1% vs. 23.9%), which indicates that the morphological changes do not place limitations on O2 uptake in larger fish. Both size groups excreted similar percentages of CO2 across the gills (85%-90%). However, larger fish had higher blood PCO2 (26.51.9 vs. 16.51.5 mmHg) and HCO3(-) (40.2 +/- 2.9 vs. 33.6 +/- 4.5 mmol L(-1)) concentrations and lower blood pH (7.58 +/- 0.01 vs. 7.70 +/- 0.04) than did smaller fish, despite having lower mass-specific metabolisms, suggesting a possible diffusion limitation for CO2 excretion in larger fish. With regard to ion regulation, rates of diffusive Na+ loss were about 3.5 times higher in larger fish than they were in smaller fish, despite the lowered branchial diffusion capacity, and rates of Na+ uptake were higher by about the same amount despite 40% lower activity of branchial Na+/K+-ATPase. Kinetic analysis of Na uptake revealed an extremely low-affinity (K(m) = 587.9 +/- 169.5 micromol L(-1)), low-capacity (J(max) = 265.7 +/- 56.8 nmol g(-1) h(-1)) transport system. These data may reflect a general reduction in the role of the gills in ion balance. Renal Na+/K+-ATPase activity was 5-10 times higher than Na+/K+-ATPase activity in the gills, and urine: plasma ratios for Na+ and Cl(-) were very low (0.001-0.005) relative to that of other fish, which suggested an increased role for dietary salt intake and renal salt retention and which was representative of a more "terrestrial" mode of ion regulation. Such de-emphasis of branchial ion regulation confers greatly reduced sensitivity of diffusive ion loss to low water pH. Ammonia excretion also appeared to be impacted by gill changes. Rates of ammonia excretion in larger fish were one third less than that in smaller fish, despite larger fish having blood ammonia concentrations that were twice as high.
骨舌鱼巨骨舌鱼孵化后不久，便会迅速从靠鳃呼吸转变为专靠空气呼吸。随后鳃小片逐渐消失，留下光滑的鳃丝，由于表面积减少，鳃的扩散能力降低，扩散距离增加了四倍。本研究通过检查鳃形态不同的两个大小类别的鱼，评估了这些变化对鳃功能的影响。与仍有鳃小片的较小鱼（约67.5克）相比，没有鳃小片的较大鱼（约724.2克）通过鳃摄取的氧气比例略高（30.1%对23.9%），这表明形态变化并未限制较大鱼对氧气的摄取。两个大小组通过鳃排出的二氧化碳比例相似（85%-90%）。然而，尽管较大鱼的质量特异性代谢较低，但它们的血液PCO2（26.5±1.9对16.5±1.5 mmHg）和HCO3(-)（40.2±2.9对33.6±4.5 mmol L(-1)）浓度较高，血液pH较低（7.58±0.01对7.70±0.04），这表明较大鱼在二氧化碳排泄方面可能存在扩散限制。在离子调节方面，尽管鳃的扩散能力降低，但较大鱼的扩散性Na+损失率比较小鱼高约3.5倍，尽管鳃Na+/K+-ATP酶活性降低了40%，但Na+摄取率仍高出约相同的量。对Na+摄取的动力学分析显示，其转运系统亲和力极低（K(m)=587.9±169.5 micromol L(-1)）、容量低（J(max)=265.7±56.8 nmol g(-1) h(-1)）。这些数据可能反映了鳃在离子平衡中的作用普遍降低。肾脏Na+/K+-ATP酶活性比鳃中的Na+/K+-ATP酶活性高5-10倍，相对于其他鱼类，Na+和Cl(-)的尿:血浆比率非常低（0.001-0.005），这表明饮食中盐摄入和肾脏盐保留的作用增加，这代表了一种更“陆地化”的离子调节模式。这种对鳃离子调节的弱化使得扩散性离子损失对低水环境pH的敏感性大大降低。氨排泄似乎也受到鳃变化的影响。尽管较大鱼的血氨浓度是小鱼的两倍，但较大鱼的氨排泄率比较小鱼低三分之一。
