Reaction of osmoregulatory system of sterlet Acipenser ruthenus Linnaeus, 1758 (Acipenseridae) on influence of hyperosmotic medium

Abstract

Some features of response to hyperosmotic environment (artificial sea water with a salinity of 12.5–12.7 ‰ (403–409 mosm/l) were studied in the osmoregulatory system of freshwater sturgeon species from the Volga river, starlet Acipenser ruthenus Linnaeus, 1758. Morphofunctional changes were traced in certain elements of the organ complex responsible for the osmoregulatory process. 72 hours after the immature sterlet individuals (age 2+) were transferred from fresh water to the hyperosmotic medium, blood serum osmolarity becomes iso-osmotic respective to the external environment. The interrenal gland responds to saline exposure by increasing cortisol concentration in the blood serum. A high cortisol level (75.13 ± 12.96 vs. 19.29 ± 6.36 ng/ml in the control group) persists throughout the entire experimental period (7 days), which indicates that fishes are under stress. The cortisol excretion into the bloodstream is not followed by an increased activity of the transport enzyme, Na+/K+ ATPase, in homogenate of the gills and an increased Na+ concentration in chloride cells, was identified by A-gold technique. The ultrastructure of chloride cells, being the main site for the removal and sorption of monovalent ions (Na+, K+, Cl-) in fishes, does not display the characteristic of an active excretion in sterlet. Thus, cortisol does not provide a stimulating effect on increasing Na+/K+ ATPase activity, an enzyme needed to transport excess ions from the body. The thyroid gland responds by increasing the thyroxin (T4) concentration during 3–6 hours of saline exposure. In the next 114 hours, the concentration of this hormone decreases to its initial level (in the control group). The thyroxin dynamics in serum does not correlate with the dynamics of serum osmolarity. So it seems possible to conclude that no functional relationship exists between the effector “organ” (the set of numerous chloride cells of the gills epithelium) and the endocrine glands (interrenal and thyroid) controlled by the hormones of tropic pituitary cells. In sterlet dwelling in hyperosmotic medium the kidney keeps a higher Na+/K+ ATPase activity, as compared with the gills. High Na+ concentration in the urine (163.2 ± 5.3 meq/l), close to its concentration in the hyperosmotic environment, high Na+ proportion (87.1 ± 0.1%) in the total concentration of major urinary cations, low level of water sorption (50.8 ± 4.0%) in the renal tubules, high diuresis (0.58 ± 0.09, ml/hr/100 g body weight) determine high level of Na+ excretion in the urine (100.95 ± 13.21 meq/ml/hr /100 g body weight). The important role of the kidney in removing Na+ excess under hyperosmotic environment underlies the osmoconformity strategy in sterlet.