Influence of swim training on cardiac activity, tissue capillarization, and mitochondrial density in muscle tissue of zebrafish larvae.

Larval zebrafish (Danio rerio) of two different age classes ("swim-up" larvae, 9 days old; "free-swimming" larvae, 21 days old) were exposed to either an endurance/continuous training or interval training. Control animals were kept in stagnant water. A comparison of cardiac activity of trained (either endurance or interval) and untrained animals at the end of the training regime revealed no differences in heart rate, end-diastolic and end-systolic ventricular volume, and cardiac output. Training also had no influence on the concentration of erythrocytes in the blood. Thus, at the level of total oxygen transport in the blood, training did not provoke any improvement during the first 32 days of development. Significant changes, however, were observed at the tissue level. In free-swimming larvae [i.e., between 21 and 32 days postfertilization (dpf)] endurance training increased the capillarization of both axial muscle caudal to the anus and the tail fin. In addition, mitochondrial density of red and intermediate muscle fibers increased significantly. In contrast to capillarization, even swim-up larvae, trained between 9 and 15 dpf, were affected. The observed increase in mitochondrial content indicates a high demand for oxygen and energy-rich metabolites for oxidative phosphorylation. In older larvae, this is met by the increase in capillarization that improves the blood supply and with it the required oxygen and metabolite supply of muscle tissue. Both of these adaptational changes result in a reduction of diffusion distances (between capillary and muscle fiber as well as mitochondria) and may contribute to a higher resistance toward oxygen deficiency. Furthermore, this study indicates that plasticity of muscle tissue is already established in early stages of development at both the tissue and cellular levels.