Environmental and endocrine control of gill corticosteroid receptor number and affinity in Atlantic salmon (Salmo salar) during smolting

Abstract

During smolting, cortisol acts on the gill through intracellular corticosteroid receptors (CR). Regulation of CR concentration ( B max ) and dissociation constant ( k d ) by environmental factors, however, has not been investigated. We subjected juvenile Atlantic salmon ( Salmo salar ) to changes in photoperiod and temperature to determine the effect on gill CR B max and k d . Cortisol, growth hormone (GH), insulin-like growth factor 1 (IGF-1), thyroxine (T 4 ), and triiodothyronine (T 3 ) were measured to determine endocrine factors that correlated with changes in CR B max and k d . Gill Na + ,K + -ATPase activity was measured as an indicator of smolting. Control fish were maintained under ambient Connecticut River water temperatures and natural photoperiod (LDN). In the first experiment, fish were also reared at elevated temperature (constant 10 °C), or long day photoperiod (LD 16:8; 16 h light), or a combination of these two treatments. In the second experiment, fish were subjected to an advanced river temperature regime, or short day photoperiod (LD 9:15; 9 h light), or a combination of these two treatments. Seasonal changes in CR B max were found to be significantly affected by temperature, but not photoperiod. A decline in CR B max occurred when mean daily temperature increase exceeded 1.5 °C per week, preceding the increase in gill Na + ,K + -ATPase activity. CR B max was found to be correlated positively with T 4 and negatively with IGF-1. Gill CR k d changed significantly over the spring, but manipulation of temperature and photoperiod had little effect. CR k d was found to be positively correlated with gill Na + ,K + -ATPase activity, plasma GH, cortisol, IGF-1 and T 4 . Temperature appears to influence seasonal changes in CR B max observed, whereas endocrine factors appear to be more closely related to seasonal changes in CR k d .