The levels of cortisol in blood serum and the Na + /K + activated ATP-ase activity in the homogenates of the gills and the kidneys were measured in different species of acipenserids in freshwater (control) and after different periods of acclimation to brackish water of 12.5-12.7‰ salinity. Immature sturgeons (2+ years old) were used for the study: the sterlet Acipenser ruthenus L.-a freshwater species from the middle of the Volga River; the Siberian sturgeon from the Lena River A. baerii Brandt -a freshwater species that performs short migrations to brackish waters (5-10‰) of estuaries for feeding; the Russian sturgeon A. gueldenstaedtii Brandt and the starred sturgeon A. stellatus Pallas -both are diadromous species of the Volga-Caspian basin that have regular migrations to brackish water (up to 14‰). The starred sturgeon was additionally held in 18‰ after 12.7‰. The investigated species from freshwater have similar levels of serum cortisol, but different dynamics of this hormone during exposure in brackish water. After seven days of the experiment, the level of serum cortisol increased (p<0.01) 4.1 times in the sterlet and 3 times in the Siberian sturgeon. In the Russian sturgeon, after 7 days of exposure, cortisol level was similar to levels of control fish, although after 3 and 12 hours of acclimation (in the transition period to hypo-osmotic regulation) the hormone levels decreased (p<0.05). In the starred sturgeon, cortisol levels remained without changes during all periods of its life in brackish water. There was the increase of gill Na + /K + -activated ATP-ase activity only in the starred sturgeon exposed in water of 12.7‰ and 18‰ salinity. During acclimation to brackish water of 12.5-12.7‰ salinity, the enzyme activity did not change in the Siberian and the Russian sturgeons and decreased in the starlet. Levels of kidney Na + /K + -activated ATP-ase activity at this period decreased in studied species excluding the Siberian sturgeon that kept the enzyme activity without the changes. The conclusion derived from these studies was that the changes in the serum cortisol levels and Na + /K + ATP-ase activities in gills and kidneys (after fish being transferred to brackish water) explain to a great extent the osmotic and ionic regulatory capacities in different acipenserid species.

The changes in osmolarity and ion concentrations of blood serum, urine and intestinal liquid in different acipenserid species were studied during their acclimation to sea water, identifying the osmotic and ionic regulation involved in these species. The freshwater species (Acipenser ruthenus, A. baerii from Baikal Lake) can adapt to brackish water of 12.5‰ salinity as a conformer. The freshwater species Pseudoscaphyrhinchus kaufmanni is also an osmo-conformer, but tolerates brackish water up to 10‰ salinity only. Freshwater species having short migrations to the estuary for feeding (A. baerii from Lena River) have limited capacity in osmotic and ionic regulation. The diadromous brackishwater species (A. gueldenstaedtii, A. stellatus, Huso huso) held in brackish water of 12.5-12.7‰ salinity and diadromous seawater species (A. brevirostrum and A. oxyrhynchus) held in sea water of up to 30.5‰ are capable to support blood serum osmolarity at relative constant levels during the transition phase from hyper-to hypo-osmotic regulation after the transition from fresh water. However functional level of the mechanism of osmotic and ionic homeostasis is not equal in different species. The higher the medium salinity (where species live in the nature) the higher the functional level of these mechanisms. This level rises in a number of acipenserids which rank as follow "Pseudoscaphyrhinchus kaufmanni ? A. ruthenus and A. baerii from Baikal Lake ? A. baerii from Lena River? A. gueldenstaedtii? Huso huso? A. stellatus? A. brevirostrum? A. oxyrhynchus". The development of osmotic and ionic homeostasis is connected with the evolution of kidney function (the increase of sodium re-absorption and magnesium secretory activities), with the strengthening sodium absorption function of the intestines, with the strengthening of ionic secretory function of the gills and the changes regulatory function of endocrine system.

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Morphological changes in the complex of functionally linked organs responsible for maintaining the osmotic homeostasis in immature euryhaline acipenserid species-starred sturgeon Acipenser stellatusin the process of adaptation to hyperosmotic environmental salinity of 12.5-14.6‰ (403-472 mOsm/L) have been studied with the following characteristics: dynamics of blood serum osmolarity, morphofunctional changes in the neuroendocrine structures (preoptic and tuberal nuclei in the hypothalamus, adrenocortico tropic cells of hypophysis), peripheral endocrine glands (thyroid and interrenal), effector organs (chloride cells of gills and kidney), dynamics of concentrations of cortisol and thyroid hormones (T 3 and T 4 ) in the blood serum, activity of Na + /K + ATPase in homogenates of gills, kidney, and directly in chloride cells. Tran sition from hyperosmotic (in fresh water) to hypoosmotic (in hyperosmotic environment) regulation is deter mined by the essential morphofunctional remodelling of the osmoregulatory organs under study. During this period (72 h) serum osmolarity increases. The neuroendocrine and endocrine systems respond to changes in the environmental salinity in a short time interval (1-24 h) thus stimulating morphofunctional changes in the effector organs and their transition to hypoosmotic regulation, which restores serum osmolarity close to the initial level.