ELISA measurements of vasotocin and isotocin in plasma and pituitary of the rainbow trout: effect of salinity.

The fish neurohypophysial hormones arginine vasotocin (AVT) and isotocin (IT) were measured for the first time by ELISAs (in comparison with other techniques) in plasma and hypophysis of rainbow trout adapted stepwise from freshwater (FW) to seawater (SW). AVT concentrations were higher than IT in plasma and, conversely, lower in hypophysis. No difference appeared between FW and SW conditions, but plasma hormone concentrations fell when FW fish were moved to 1/3 SW and increased progressively when fish were moved from 1/3 SW to SW. Peptide values obtained in 1/3 SW may correspond to the lowest osmoregulatory constraints occurring in an isosmotic medium in comparison to FW or full SW. The data suggest that storage and/or release of AVT and IT differ, but vary in a similar way with external salinity, and that these peptides should play a role in teleost fish osmoregulation.