Physiological stress responses including free and protein-bound cortisol in two strains of rainbow trout

The stress induced changes in plasma cortisol, glucose, and chloride were more extreme in wild rainbow trout (Salmo gairdneri) than in hatchery-reared rainbow trout subjected to severe short-term confinement in a net and electroshock. By 12 h of net confinement, plasma cortisol in wild fish increased from resting levels of 10 ng/mL to A80 ng/mL compared to hatchery-reared fish which increased from resting levels of 2 ng/mL to 155 ng/mL. Plasma glucose was higher in wild fish, increasing from resting levels of 55 mg/dL to 284 mg/dL compared to the increase in hatchery-reared fish from resting levels of 58 mg/dL to 196 mg/dL. Plasma concentrations of cortisol, glucose, and chloride were less altered in response to electroshock than they were in response to net confinement. One hour after galvano-narcosis, plasma cortisol concentrations in the wild fish peaked at 234 ng/mL and remained moderately elevated for 4 d; cortisol in hatchery-reared fish peaked at 70 ng/mL within 0.5 h of the electrical stimulus and then returned to resting levels within 1 h. No substantial changes in plasma glucose or plasma chloride occurred in either strain after galvanonarcosis.

Except for the initial disturbances, stress-induced changes in plasma cortisol, glucose, and chloride were only slightly varied throughout six weeks of long-term, moderate confinement for both strains. Distribution of plasma cortisol bound to a heat-labile carrier protein (corticosteroid binding-protein (CBP)) in both strains ranged iv from 5 to 10% at prestress conditions. Cortisol bound to CBP increased to 32% in the wild fish and to 28% in the hatchery-reared fish by the end of confinement; however, CBP in control hatchery-reared fish increased throughout the study. The distribution of plasma cortisol bound to a non heat-labile protein (albumin protein (Alb-P)) in both strains ranged from 35 to A5 % at prestress conditions, and fluctuated from 26 to 60 % throughout the confinement. The ratio of free to total cortisol concentration for both strains indicates that free cortisol is consistently and slightly less than one-half of total cortisol throughout the normal physiological range.