Seawater acclimation affects cardiac output and adrenergic control of blood pressure in rainbow trout (Oncorhynchus mykiss)—implications for salinity variations now and in the future

Greater salinity variations resulting from ongoing climate change requires consideration in conservation management as this may impact on the performance of aquatic organisms. Euryhaline fish exhibit osmoregulatory flexibility and can exploit a wide range of salinities. In seawater (SW), they drink and absorb water in the intestine, which is associated with increased gastrointestinal blood flow. Yet, detailed information on other cardiovascular changes and their control across salinities is scant. Such knowledge is fundamental to understand how fish are affected during migrations between environments with different salinities, as well as by increased future salinity variability. We used rainbow trout (Oncorhynchus mykiss) as a euryhaline model species and determined dorsal aortic blood pressure, cardiac output and systemic vascular resistance in vivo after chronic freshwater—or SW-acclimation. We also assessed α-adrenergic control of blood pressure using pharmacological tools. Dorsal aortic blood pressure and systemic vascular resistance were reduced, whereas cardiac output increased in SW. α-Adrenergic stimulation with phenylephrine caused similar dose-dependent increases in resistance and pressure across salinities, indicating unaltered α-adrenoceptor sensitivity. α-Adrenergic blockade with prazosin decreased resistance and pressure across salinities, but the absolute reduction in resistance was smaller in SW. Yet, both pressure and resistance after prazosin remained consistently lower in SW. This shows that SW-acclimation lowers systemic resistance through reduced vascular α-adrenergic tone, along with other unknown vasodilating factors. The marked changes in adrenergic regulation of the vasculature across salinities discovered here may have implications for cardiovascular and aerobic performance of fishes, with possible impacts on fitness-related traits like digestion and exercise capacity. Moreover, the evolution of more complex circulatory control systems in teleost fishes compared with elasmobranchs and cyclostomes may have been an important factor in the evolution of euryhalinity, and may provide euryhaline teleosts with competitive advantages in more variable salinity environments of the future.