Rawat A, Chaube R, Joy K P
Department of Zoology, Institute of Science, Banaras Hindu University, Varanasi, 221005, India.
Department of Biotechnology, Cochin University of Science and Technology, Kochi, 682022, India.
Fish Physiol Biochem. 2022 Apr;48(2):381-395. doi: 10.1007/s10695-022-01058-4. Epub 2022 Feb 15.
Heteropneustes fossilis is a facultative air-breathing freshwater catfish and inhabits ponds, ditches, swamps, marshes and rivers that dry up in summers. It possesses a pair of unique tubular accessory respiratory organ (air sac), which is a modification of the gill chamber and enables it to live in water-air transition zones. In the catfish, three vasotocin (Vt) receptor gene paralogs viz., v1a1, v1a2 and v2a were identified for Vt actions. In the present study, the receptor gene transcripts were localized in the gill and air sac by in situ hybridization, and their expression levels in relation to water and air deprivation conditions were investigated by quantitative RT-PCR. The catfish were exposed to 1 h and 2 h in gonad inactive (resting) and gonad active (prespawning) phases. The gene paralogs showed overlapping distribution in the respiratory epithelium of primary and secondary lamellae of gills and reduced lamellae of the air sacs. In water deprivation (forced aerial mode of respiration) experiment, v2a expression showed a high fold increase in the air sac, which was unchanged or inhibited in the gill. Both v1a1 and v1a2 expression was significantly upregulated in the air sac but showed varied responses in the gill. The gill v1a1 expression was unchanged in the resting phase and modestly upregulated in the prespawning phase. The gill v1a2 expression was modestly upregulated at 1 h in both phases but unchanged at 2 h. In the air deprivation experiment (forced aquatic respiration), the v2a expression in the air sac was inhibited except for a mild stimulation at 1 h in the prespawning phase. In the gill, the v2a expression was stimulated with a steep upregulation at 2 h in the prespawning phase. Both v1a1 and v1a2 expression was significantly high in the gill but only modestly increased or unchanged in the air sac. The expression patterns point to a functional distinction; the V2 type receptor expression was higher in the air sac during forced aerial respiration, and the V1 type receptor expression was highly prominent in the gill during forced aquatic respiration. Water and air deprivation treatments caused a significant increase in plasma cortisol level, and the stimulation was higher in the water deprivation fish in the resting phase but equally prominent in the water and air deprivation groups in the prespawning phase. The results indicate that the changes in the expression patterns of Vt receptor genes may be a sequel to stress (hypoxic, metabolic and osmotic), and both Vt and cortisol may interact to counter the stress responses. This study shows that Vt has a new role in the control of air sac functions.
印度胡鲶是一种兼性空气呼吸的淡水鲶鱼，栖息于池塘、沟渠、沼泽、湿地和夏季干涸的河流中。它拥有一对独特的管状辅助呼吸器官（气囊），这是鳃腔的一种变体，使其能够生活在水 - 空气过渡区。在鲶鱼中，已鉴定出三种血管紧张素（Vt）受体基因旁系同源物，即v1a1、v1a2和v2a，用于Vt的作用。在本研究中，通过原位杂交将受体基因转录本定位在鳃和气囊中，并通过定量RT - PCR研究它们在水和空气剥夺条件下的表达水平。鲶鱼在性腺静止（休息）期和性腺活跃（产卵前）期分别暴露1小时和2小时。这些基因旁系同源物在鳃的初级和次级鳃小片以及气囊的减少的鳃小片的呼吸上皮中显示出重叠分布。在水剥夺（强制空气呼吸模式）实验中，v2a在气囊中的表达显示出高倍数增加，而在鳃中不变或受到抑制。v1a1和v1a2在气囊中的表达均显著上调，但在鳃中表现出不同的反应。鳃中的v1a1在静止期表达不变，在产卵前期适度上调。鳃中的v2a在两个阶段的1小时时均适度上调，但在2小时时不变。在空气剥夺实验（强制水生呼吸）中，气囊中的v2a表达受到抑制，除了在产卵前期1小时有轻微刺激。在鳃中，v2a在产卵前期2小时时表达受到刺激并急剧上调。v1a1和v1a2在鳃中的表达均显著升高，但在气囊中仅适度增加或不变。这些表达模式表明存在功能差异；在强制空气呼吸期间，V2型受体在气囊中的表达较高，而在强制水生呼吸期间，V1型受体在鳃中的表达非常突出。水和空气剥夺处理导致血浆皮质醇水平显著升高，在静止期水剥夺的鱼中刺激更高，但在产卵前期水和空气剥夺组中同样显著。结果表明，Vt受体基因表达模式的变化可能是应激（缺氧、代谢和渗透）的结果，并且Vt和皮质醇可能相互作用以对抗应激反应。这项研究表明，Vt在气囊功能控制中具有新的作用。
