| Summary: | Antarctic fish, such as the Trematomus bemacchii, maintain a serum osmolality of around 600 mOsm, nearly twice that of temperate fish. This high serum osmolality seawater surrounding Antarctica. Upon warm acclimation Antarctic fish lower their serum osmolality, increase their drinking rate, and increase gill Na+, K+-ATPase activity. The renin-angiotensin system (RAS) and cortisol have been shown to increase drinking rate and gill Na+, K+-ATPase activity in fish, therefore regulating water and ion balance. Moreover, the RAS has been shown to increase cortisol levels in fish. Therefore, experiments were designed to examine the hypothesis that: the RAS mediates an increase in cortisol levels which decreases serum osmolality and increases Na+, K+- ATPase activity in warm acclimated T. bemacchii. Manipulations known to increase RAS activity in fish, including hemorrhage and papaverine injection did not alter cortisol levels or serum osmolality (with the exception of serum osmolality increase in papaverine at 180 min and 6 hr) in T. bemacchii. Hypervolemia significantly altered cortisol levels from the control, showing a significance (p<0.05) due to treatment upon analysis by a two-way ANOVA. Upon warm acclimation to 1??C and 4??C for four weeks, T. bemacchii lowered their serum osmolality (from 547 ?? 4 mOsm/kg to 494 ?? 6 mOsm/kg and 489 ??4 mOsm/kg, respectively) and did not alter their cortisol levels with the exception of a decrease in cortisol at 48 hr (from 28.9 ?? 6.3 ng/ml to 1.7 ?? 0.3 ng/ml and 1.7 ?? 0.2 ng/ml, respectively). Additionally, a 40 kDa protein was recognized by western blot with an angiotensin II type 1 receptor antibody. Preliminary development of a renin assay is described for future experiments. These results suggest that the RAS does not directly effectively lowers the freezing point of Antarctic fish, allowing survival in the -1.87??C alter cortisol levels in T. bernacchii and that the primary role of cortisol in Antarctic fish may be to regulate the stress response, rather than serve an osmoregulatory function as reported in other teleosts.
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