Viability and changes of physiological functions in the tiger frog (Hoplobatrachus rugulosus) exposed to cold stress

Abstract

Global climate change has been considered to be one of the key factors leading to declines and losses of amphibian populations. Though global warming is one of the acknowledged causes of fluctuations of amphibian population worldwide, unseasonal sharp fall in air temperature as result of global climate change may also affect the survival and stability of amphibian population severely. As amphibians are typical poikilothermic animals, environmental temperature can influence their physiological functions and enhance their susceptibility to infectious diseases. The immunological competence of amphibians may, therefore, make them vulnerable to parasites and disease particularly after discontinuous and extreme events (e. g. winter rainfall, late spring coldness). In this article, we have investigated how cold stress affected the physiology and viability of tiger frogs (Hoplobatrachus rugulosus). The winter temperatures experienced by wild tiger frogs in natural winter in the Southern Mountain of Jinhua in Zhejiang Province was monitored, and then replicated in subsequent experiments. The aim of the experiments was to determine how gradient cooling (2 °C/24 h) and cold shock (2°C) affected the mortality rate of tiger frogs, and affected various aspects of their immune function and oxidation resistance. The results showed that the survival temperature range of wild overwintering was between 0-14 °Cin 12/1/2009-3/31/2010, and minimum and maximum temperature ranged between 0-4 °C and 10-14 °C respectively. During the process of gradient cooling, we found that the accumulated mortality rate was 28. 1% at 12 °C, 87. 5% at 10 °C and 100% at 8 °C. A significant negative correction was found between environmental temperature and frog mortality within a certain temperature range (Pearson test, r =-0. 952, P<0. 05). And lethal temperature of 50% occurred at 11. 2 °C based on a fitted curve regression equation. Under cold shock,45% mortality was observed after being directly stimulated at 2 °C for 6 h,80% mortality appeared at 2 °C for 12 h, and increased to 100% after 24 h. There appeared to be positive correction between cold shock time and mortality (Pearson test, r = 0. 91, P <0. 05), and lethal time of 50% is 7. 6 h at 2 °C according to fitted curve regression equation. Moreover, cold shock significantly inhibited the respiratory burst of spleen macrophages (t =3. 827, df =6, P<0. 05), peripheral blood phagocytic activity (t =5. 388, df = 3. 037, P<0. 05) and gastric lysozyme activity (t = 6. 37, df = 6, P <0. 05) at 2 °C for (6 h). Although there was no significant change in malonaldehyde (MDA) content, the glutathione (GSH) content of the liver (t = -2. 817, df = 6, P<0. 05) and kidney (t =-11. 302, df =6, P<0. 05) increased. The activity of superoxide dismutase (SOD) also increased significantly in the liver (t =-3. 3, df =6, P<0. 05), while there was no apparent change within the kidney. The results indicate that tiger frogs are sensitive to low temperatures and that cold shock might lead to greater oxidative stress, more requirements for antioxidants and produce immunosuppression in tiger frogs. Consequently, low temperature stress will stimulate organism endocrine disorder and subject the frogs to a more intensive disturbance of physiological function, increase the survival pressure and susceptibility to infectious disease, which may affect development and dynamics of amphibian populations.