A trip to the doctor is enough to set most hearts racing; it's hard staying calm when you're having your blood pressure taken. And maybe fish aren't so different. When they're having their blood pressure taken, their heart rates rocket. But when Michael Axelsson and Jordi Altimiras monitored trout heart rates as they rested peacefully in a tank, their pulse rates were quite low. Could fish's racing hearts affect their cardiac performance and blood pressure? All pressure measurements taken on fish's hearts in the past had been done in perfused hearts where the heart rates were much higher than if they were resting peacefully. These measurements from the fast rate hearts suggested that the pressure in the first chamber of the fish's heart, the sinus venosus, was negative; their hearts appeared to suck blood around the body as the ventricle contracted. But no one had actually measured the pressure in the beating heart's sinus venosus directly. Altimiras wondered whether the pressure in the fish heart's first chamber would be negative if he could drop their heart rate to a more realistic rate(p. 195)? But this was easier said than done.
Altimiras realised that if the fish wouldn't do it on their own, he'd have to help them. Remembering that rat heart rates could be reduced with a dose of zatebradine, Altimiras tested the drug on trout; their heart rate fell to the resting rates he'd seen in the untroubled fish. After checking that the drug didn't alter any other aspects of their cardiovascular function, he decided to measure the pressure in the sinus venosus.
But no one had ever directly measured the pressure in the chamber, and Axelsson and Altimiras knew this was a technically challenging problem. Working together they learned how to gently cannulate the sinus venosus, ready to measure the pressure in the chamber. Finally, they were ready to test how the fish's cardiac pressure varied as they gently slowed the trout's beating heart.
Not surprisingly, as they began the experiment when the fish's heart rate was high, the pressure in the sinus was low, just as Altimiras expected. But as the team increased the dose of zatebradine and the fish's pulse rate fell,the pressure in the chamber began increasing until it became positive as the fish's heart rate approached the resting fish's pace. At the low heart rates that resting fish experience, the heart was behaving almost like a mammal's,producing a positive pressure in the sinus venosus and filling the heart.
Altimiras explains that although this increase in cardiac pressure had never been measured before, it wasn't unexpected. As the heart slows and the length of time between each heart beat increases, the ventricle has longer to fill and stretch the muscle, ready to pump the blood out with an increased pressure. If the heart rate fell low enough, it would eventually be able to generate a high enough flow to maintain a positive pressure in the sinus venosus throughout a heart beat cycle. So it seems that the trout heart's function depends on the fish's level of activity, switching from low to high pressure in the sinus venosus as it settles down for a rest.