High-density lipoprotein remodeling affects the osmotic properties of plasma in goldfish under critical salinity.

To investigate the stress response and physiological adaptations of goldfish (Carassius auratus) to critical salinity (CS) waters, we analyzed high-density lipoprotein (HDL) stoichiometry, stress markers (cortisol, glucose), and plasma osmotic properties (Na+ , osmolality, water content) using ichthyology, biochemistry, and proteomics approaches. After 21 days of exposure to CS, plasma concentrations of cortisol, glucose, and Na+ increased, indicating stress. Total plasma osmolality (Osmtotal ) and osmolality generated by inorganic (Osminorg ) and organic osmolytes (Osmorg ) also increased, the latter by ~2%. We associated the increase of Osmorg with (1) increased metabolite concentration (glucose), (2) dissociation of HDL particles resulting in increased HDL number per unit plasma volume (~1.5-2-fold) and (3) increased HDL osmotic activity. HDL remodeling may be the reason for the redistribution of bound and free water in plasma, which may contribute to water retention in plasma and, at the same time, to hemodynamic disturbances under CS conditions. The study's findings suggest that HDL remodeling is an important mechanism for maintaining osmotic homeostasis in fish, which is consistent with current capillary exchange models in vertebrates.

Goldfish as a model for studying the effect of hypernatremia on blood plasma lipoproteins.

AIM: To evaluate the effect of hypernatremia on the organization of blood plasma high density lipoproteins (HDL) in goldfish; to compare the state of hypernatremia in fish and humans; to assess the possible risks and consequences of the effect of hypernatremia on human plasma lipoproteins. METHODS: The fish were acclimated for 20 days at a critical salinity of 11.5 g/L; after that the salt water was gradually "desalinated". The concentration of Na+ and the content of total water were determined in tissues, cells, and body fluids. The HDL organization was assessed by the number of apolipoprotein molecules per particle. The methods of flame spectrophotometry, electrophoresis and MALDI were used. RESULTS: In fresh water, the state of normonatremia was maintained in the fish body; at critical water salinity, the state of hypernatremia. Against the background of hypernatremia, the initial signs of muscle and erythrocyte dehydration appeared in fish, the total water content in the plasma did not change, and HDL disintegrated into small particles, which, upon restoration of normonatremia, were combined into the original large forms. CONCLUSION: In goldfish at the state of normonatremia, large forms of HDL are stable while at the state of hypernatremia, the small forms of HDL are stable. Under conditions of a hypertonic environment and plasma hypernatremia, the breakdown of HDL prevents the loss of water from the fish organism and reduces the threat of their dehydration. Human hypernatremia is characterized by plasma sodium levels comparable to that in goldfish, however accompanied by life-threatening metabolic changes. The results of this study may be useful for assessing the risks of HDL breakdown at hypernatremia and for the development of protocols for the treatment of pathological conditions in humans (Fig. 4, Ref. 45).

Involvement of apolipoprotein A in maintaining tissue fluid balance in goldfish Carassius auratus.

Mammalian plasma proteins play a key role in maintaining tissue fluid balance because they are retained within capillaries and thus create colloid osmotic pressure. Likewise, fish plasma contain a considerable concentration oligomeric proteins which likely serve a similar role. To elucidate the functions of these oligomeric proteins, we analyzed blood serum (BS) and interstitial fluid (IF) complexes in goldfish from the wild and under experimental conditions using 2D electrophoresis and matrix-assisted laser desorption/ionization (MALDI). We detected protein compounds with MWs ranging from 50 to 155 kDa, organized as oligomeric complexes. The protein compounds consisted of apolipoproteins АроА-I and Аро-14 which are homological to mammalian АроА-I and АроА-II, respectively. The 155-kDa and 50-125-kDa oligomer complexes were located very low-density lipoproteins (LDL) and high-density lipoproteins (HDL) areas on the BS/IF proteomic maps, respectively. The latter resembled mammalian HDL plasma particles by size and contained lipids, so we considered them as HDL particle populations. Investigation of the uniform dissociation/association mechanism for HDL and LDL oligomers in goldfish, from the wild and under critical salinity conditions, showed the "125/110 → 85/60 kDa" reorganization. This was associated with overcoming physiological stress during spawning and under critical salinity conditions. Opposite reorganization "85/60 → 125/110 kDa" was associated with restoration of metabolic processes after stress. The association/dissociation reorganizations promoted equilibration of BS and IF osmolarities in all fish groups. We discuss the connection of these reorganizations with total protein distribution across the capillary wall and salinity, as well as the role of oligomeric apolipoproteins as universal metabolic regulators.

Oligomeric protein complexes of apolipoproteins stabilize the internal fluid environment of organism in redfins of the Tribolodon genus [Pisces; Cypriniformes, Cyprinidae].

One of the most important functions of plasma proteins in vertebrates is their participation in osmotic homeostasis in the organism. Modern concepts about plasma proteins and their capillary filtration are based on a model of large monomeric proteins that are able to penetrate the interstitial space. At the same time, it was revealed that a considerable amount of oligomeric complexes are present in the low-molecular-weight (LM) protein fraction in the extracellular fluids of fishes. The functions of these complexes are unknown. In the present study, we investigated the LM-fraction proteins in the plasma and interstitial fluid (IF) of redfins of the genus Tribolodon. This fish alternatively spends parts of its life cycle in saline and fresh waters. We identified the protein Wap65, serpins and apolipoproteins in this fraction. By combining the methods of 2D-E under native and denaturing conditions with MALDI, we demonstrated that only apolipoproteins formed complexes. We showed that serum apolipoproteins (АроА-I, Аро-14) were present in the form of homooligomeric complexes that were dissociated with the release of monomeric forms of proteins in the course of capillary filtration to IF. Dissociation of homooligomers is not directly correlated with the change in salinity but is correlated with seasonal dynamics. We found that there was a significant decrease in the total protein concentration in IF relative to plasma. Therefore, we suggested that dissociation of homooligomeric complexes from various apolipoproteins supports the isoosmoticity of extracellular fluids relative to capillary wall stabilization through a fluid medium in fish.