Diurnal changes in plasma arginine vasotocin and isotocin in rainbow trout adapted to fresh water and brackish Baltic water.

The plasma levels of arginine vasotocin (AVT) and isotocin (IT) in rainbow trout (Oncorhynchus mykiss) were studied to assess possible diurnal variations in neurohypophysial nonapeptides. Fish were kept under natural photoperiod and adapted to fresh and brackish Baltic water. Blood was sampled at 5:00, 11:00, 16:00, 22:30, and again at 5:00. Hormones were determined by gradient high-performance liquid chromatography preceded by solid-phase extraction. The marked diurnal changes of AVT were detected in plasma of fish adapted to both fresh and brackish waters. AVT levels (fmol/ml) were maximal at 16:00 and minimal at 5:00 (253.4 +/- 35.7 and 45.5 +/- 17.3, respectively). Unlike AVT, isotocin levels displayed no diurnal changes. AVT concentrations at 11:00, 16:00, and 22:30 were significantly higher than IT values measured throughout the day. Plasma AVT concentrations determined in brackish water-adapted fish at 16:00 were significantly lower than those of freshwater-adapted fish at the same time. These data suggest that synthesis and/or release of AVT and IT are controlled independently, so that these nonapeptides have different physiological roles in teleost fish. AVT might participate in circadian time-keeping system in fish.