Role of the cardiovascular system in ammonia excretion in early life stages of zebrafish (Danio rerio)

The purpose of this study was to investigate if the cardiovascular system is important for ammonia excretion in the early life stages of zebrafish. Morpholino knockdowns of cardiac troponin T (TNNT2) or vascular endothelial growth factor (VEGFA) provided morphants with non-functional circulation. At the embryonic stage (30-36 hpf), ammonia excretion was not constrained by lack of cardiovascular function. At 2 days post fertilization (dpf) and 4 dpf, morpholino knockdowns of TNNT2 or VEGFA significantly reduced ammonia excretion in all morphants. Expression of rhag, rhbg and rhcgb showed no significant changes but the mRNA levels of the urea transporter (ut) were upregulated in the 4 dpf morphants. Taken together, rhag, rhbg, rhcgb and ut gene expression and an unchanged tissue ammonia concentration but an increased tissue urea concentration, suggest that impaired ammonia excretion led to increased urea synthesis. However, in larvae anesthetized with tricaine or clove oil, ammonia excretion was not reduced in the 4 dpf morphants compared to controls. Further, oxygen consumption was reduced in morphants regardless of anesthesia. These results suggest that cardiovascular function is not directly involved in ammonia excretion, but rather reduced activity and external convection may explain reduced ammonia excretion and compensatory urea accumulation in morphants with reduced cardiovascular function.