Because it is much saltier than their body fluids, the ocean is a very dehydrating environment for fishes. Species that can survive in both freshwater and saltwater, such as trout and salmon, must therefore undergo significant physiological changes when transitioning to seawater. To counteract water loss, they drink more and they use the salts in the water to help drive water absorption across the gut; the salts are then sent to the kidney or gills for excretion. This process requires extra blood flow to the gastrointestinal tract. Trout achieve this extra flow solely by increasing the stroke volume (the amount of blood the heart ventricle pumps with each beat), which in turn increases the cardiac output (the flow rate of blood from the heart). However, the exact mechanisms utilized to increase the stroke volume have long been uncertain. Jeroen Brijs and his colleagues at the University of Gothenburg in Sweden, as well as collaborators at the University of Applied Sciences in The Netherlands and the University of South Bohemia in the Czech Republic, sought to clarify these mechanisms. The group also wanted to determine what might happen if the fish were challenged with warm temperatures, which typically also elicit increased cardiac output so fish can meet greater metabolic demands.
To address their questions, the researchers compared cardiovascular variables in rainbow trout acclimated to freshwater or seawater. Briefly, they measured the pressure of blood entering the heart as well as the cardiac output for 3 h at 10°C, then raised the temperature to 16.5°C and monitored these same variables for another 3 h. Finally, the scientists weighed the heart ventricles and determined the proportions of stronger and weaker muscle types in the hearts of the fish in each acclimation group.
Brijs and his colleagues discovered that seawater-acclimated trout increased their stroke volume, and thus cardiac output, by increasing the pressure of blood filling the heart. They also developed a higher proportion of strong muscle tissue in their ventricles than the freshwater-acclimated trout, likely to give their hearts the power to pump the extra blood. When challenged with warmer temperatures, however, the two groups converged on similar cardiac outputs and stroke volumes – even though the seawater-acclimated fish maintained higher pressures filling the heart – suggesting that osmoregulatory strategies in seawater environments may reduce the scope of heat-induced increases in cardiac output.
This study is the first to suggest that temperature and salinity may interact to influence changes in blood flow in fish that are tolerant of variable salinities. It is also the first study to attribute salinity-induced increases in stroke volume to higher heart filling pressures in rainbow trout, inviting further investigation into exactly which mechanisms are responsible for increasing pressure. Ultimately, it looks like these salinity champions feel the pressure when they're faced with a new environment – but they forge on with full hearts.