2.3. Analysis of plasma osmolality and cortisol level
The plasma osmolality was measured using a vapor pressure
osmometer (Wescor 5520, Logan, UT). The plasma osmolality values
of the FW and SW fish during the salinity stress are expressed
as mOsm/kg.
The plasma cortisol levels were analyzed by an enzyme immunoassay.
Blood was collected into 2-ml eppendorf tubes from the
caudal vessel of the FW and SW groups (n = 10) using a 1-ml plastic
syringe. Plasma was obtained by centrifugation at 10,000g for
5 min at 4 C, and the supernatant was frozen at 20 C until further
use. Cortisol was measured using the Cortisol EIA kit (Cayman
Chemical, Ann Arbor, MI). Prior to the application, a preliminary
test was performed to purify the plasma. The plasma samples
(200 ll) were extracted with 3 ml of diethyl ether. After the extraction,
the supernatant was kept at 40 C in a water bath for evaporation
and then stored at 20 C until further analysis. The extracts
of cortisol were re-suspended with 500 ll of EIA buffer (kit provided)
for the assay. Standards were prepared according to the
manufacturer’s instructions. To validate the cortisol assay, a parallelism
between the standard curve and a serial dilution of the extracted
solution was established. The plasma cortisol levels were
expressed as ng/ml.