Neuroendocrine control of ionic balance in zebrafish.
Drinking rates in both marine and freshwater stenohaline fish were studied by measuring the ingestion of polyethylene glycol that had been added to the aquarium water. Two marine species, the long-horned sculpin and the flounder, and three freshwater species, the common goldfish, the mottled sculpin, and the common shiner, were used. Control drinking rates in freshwater fish averaged 0.03-0.1% body wt/h and in marine fish varied between 0.06 and 0.24% body wt/h. Intramuscular injections of angiotensin II (ANG II) stimulated drinking two- to threefold in the two marine species but had no effect on the drinking rate of the freshwater species. Hemorrhage (1-2% of body wt) also stimulated drinking in the two marine species (6- to 10-fold) but did not affect the drinking rate of two freshwater species. Thus exogenous ANG II and hemorrhage stimulate drinking in two marine stenohaline fish as they do in mammals. These responses were absent in the three freshwater fishes studied. However, injection of converting enzyme inhibitor (SQ 20881) or saralasin in order to block endogenous ANG II did not attenuate either basal or hemorrhage-stimulated drinking in the marine fish.