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 rai nbow trout (Oncorhynchus mykiss) were studied to assess possible diurn al variations in neurohypophysial nonapeptides. Fish were kept under n atural photoperiod and adapted to fresh and brackish Baltic water. Blo od was sampled at 5:00, 11:00, 16:00, 22:30, and again at 5:00. Hormon es 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 wat ers. AVT levels (fmol/ml) were maximal at 16:00 and minimal at 5:00 (2 53.4 +/- 35.7 and 45.5 +/- 17.3, respectively). Unlike AVT, isotocin l evels displayed no diurnal changes. AVT concentrations at 11:00, 16:00 , and 22:30 were significantly higher than IT values measured througho ut the day. Plasma AVT concentrations determined in brackish water-ada pted fish at 16:00 were significantly lower than those of freshwater-a dapted fish at the same time. These data suggest that synthesis and/or release of AVT and IT are controlled independently, so that these non apeptides have different physiological roles in teleost fish. AVT migh t participate in circadian time-keeping system in fish. (C) 1996 Acade mic Press, Inc.