Respiratory Burst Activity in Head Kidney and Spleen Leukocytes of Tilapia (Oreochromis mossambicus) under Acute Osmotic Stress Bharath Kumar Velmurugan 1 , Ing-Fong Jiang 1 , Hung-Yuan Shih 1 , Der-Nan Lee 2 , and Ching-Feng Weng 1, * 1 Institute of Biotechnology, National Dong Hwa Univ., Hualien 974, Taiwan 2 Department of Animal Science, National Ilan Univ., 1 Shen-Lung Rd., Sec. 1, Ilan 260, Taiwan (Accepted November 2, 2012) Bharath Kumar-Velmurugan, Ing-Fong Jiang, Hung-Yuan Shih, Der-Nan Lee, and Ching-Feng Weng (2012) Respiratory burst activity in head kidney and spleen leukocytes of tilapia (Oreochromis mossambicus) under acute osmotic stress. Zoological Studies 51(8): 1290-1297. Tilapia Oreochromis mossambicus is capable of surviving in a wide range of salinities and temperatures. In the present study, we determined how salinity infuences the respiratory burst (RB) activity of tilapia. At frst, tilapia were exposed to freshwater (FW) or 25 ppt seawater (SW), and serum was isolated from these groups. Further, leukocytes of the head kidneys (HKs) and spleen were isolated from tilapia exposed to 300 or 500 mOsm osmolality. Following osmolality exposure, cells were incubated (1) without serum (as the control), (2) with serum of tilapia exposed to fresh water (FW), and (3) with serum of tilapia exposed 25 ppt seawater (SW). A cell viability assay indicated that exposure of HK and spleen leukocytes to 300 and 500 mOsm up to 24 h failed to inhibit cell viability. When comparing RB activities at the 2 osmolalities without adding serum, HK and spleen leukocytes cultured at 500 mOsm osmolality for 4, 8, and 24 h were found to have higher activities than those cultured at 300 mOsm osmolality. RB activities of HK and spleen leukocytes showed signifcant decreases in both 300 and 500 mOsm incubated cells after adding serum, and the decrease was more distinct at an osmolality of 500 mOsm. These results imply that the addition of serum might cause a suppressive effect on cellular parameters rather than improving the effects of cytokines on cellular parameters. http://zoolstud.sinica.edu.tw/Journals/51.8/1290.pdf Key words: Respiratory burst activity, Stress-activated serum, Freshwater (FW)-exposed tilapia, Hyperosmolality. *To whom correspondence and reprint requests should be addressed. Bharath Kumar Velmurugan and Ing-Fong Jiang contributed equally to this work. Tel: 886-3-8633637. Fax: 886-3-8630255. E-mail:cfweng@mail.ndhu.edu.tw A nimals have very complicated and delicate responses to stress that protect against environmental perturbations, which may be disadvantageous to their physiology, psychology, growth, and breeding. Fish change their feeding activity from diurnal to nocturnal when exposed to this type of stress combined with the relatively bright illumination during daytime (Ferter and Meyer-Rochow 2010). Gills and kidneys are the most important organs for osmoregulation in teleosts, and fundamental transporters responsible for ion movements across gill and kidney epithelia have been widely studied (Marshall and Grosell 2006, Marshall 2002). Short-term starvation (stress) dampens trypsin activity and the trypsin/ chymotrypsin ratio in relation to the growth of tilapia (Chan et al. 2008). Adverse environmental situations may acutely or chronically disrupt the health of fsh, by altering some of their biochemical parameters and suppressing their innate and adaptive immune responses (Miller et al. 2002). Changes in salinity can naturally occur as a consequence of rainwater diluting seawater (SW), mixing of estuarine waters, or the ingress of SW into normally freshwater (FW) areas. Many fish species are capable of withstanding large osmotic changes (Bowden 2008). Maintenance of the water balance in fish (osmoregulation) can Zoological Studies 51(8): 1290-1297 (2012) 1290