FROM HAGFISH TO TUNA - A PERSPECTIVE ON CARDIAC-FUNCTION IN FISH

This perspective takes a comparative view of cardiac physiology in fish to provide insights into cardiac control mechanisms and the degree of cardiac plasticity between species. Between the extremes represented by tuna and hagfish, there is a 15-fold difference in cardiac output that is largely accounted for by species differences in heart rate rather than in stroke volume. The relative importance of heart rate and stroke volume is reversed in many fish in terms of explaining the up to threefold increase in cardiac output observed during swimming. A conspicuous and unexplained observation is that all fish studied so far, with the exception of tuna, have heart rates lower than 120 bpm. In many fish, luminal O2 in venous blood is the sole myocardial O2 supply, and a venous Po2 threshold may ultimately limit cardiac performance. More active fish have bigger ventricles that generally beat faster and generate higher blood pressures and flows. The bigger ventricle in active fish, while improving convection to working skeletal muscle, requires the development of a coronary circulation to supplement to varying degrees the luminal myocardial O2 supply. It is hoped that these generalities will act as a framework for future research.