Chaperone roles for TMAO and HSP70 during hyposmotic stress in the spiny dogfish shark (Squalus acanthias)

Salinity decreases are experienced by many marine elasmobranchs. To understand how these fishes cope with hyposmotic stress on a cellular level, we used the spiny dogfish shark (Squalus acanthias) as a model to test whether a reciprocal relationship exists between the cell's two primary protein protection mechanisms, the chemical (e.g., trimethylamine oxide, TMAO) and molecular (e.g., heat shock protein 70, HSP70) chaperone systems. This relationship is interesting given that many elasmobranchs are expected to gain water and lose osmolytes, chemical chaperones, and ions as they osmoconform to new, lowered salinity. Dogfish were cannulated for repeated blood sampling and exposed to 70% seawater (SW) for 48 h. These hyposmotic conditions had no effect on red blood cell (RBC) and white muscle TMAO concentrations, and did not result in HSP70 induction or signs of protein damage (i.e., increased ubiquitin), suggesting that TMAO levels were sufficiently protective in these tissues. However, in the gill, we observed a significant decrease in TMAO concentration and a significant induction of HSP70 as well as signs of protein damage. In the face of this cellular stress response, gill Na(+)/K(+)-ATPase (NKA) activity significantly increased during hyposmotic conditions, as expected. We suggest that this functional preservation in the gill is partly the result of HSP70 induction with lowered salinity. We conclude a reciprocal relationship between TMAO and HSP70 in the gills of dogfish as a result of in vivo hyposmotic stress. When osmotically induced protein damage surpasses the protective capacity of remaining TMAO, HSP70 is induced to preserve tissue and organismal function.

Freshwater to seawater acclimation of juvenile bull sharks (Carcharhinus leucas): plasma osmolytes and Na+/K+-ATPase activity in gill, rectal gland, kidney and intestine

This study examined the osmoregulatory status of the euryhaline elasmobranch Carcharhinus leucas acclimated to freshwater (FW) and seawater (SW). Juvenile C. leucas captured in FW (3 mOsm l(-1) kg(-1)) were acclimated to SW (980-1,000 mOsm l(-1) kg(-1)) over 16 days. A FW group was maintained in captivity over a similar time period. In FW, bull sharks were hyper-osmotic regulators, having a plasma osmolarity of 595 mOsm l(-1) kg(-1). In SW, bull sharks had significantly higher plasma osmolarities (940 mOsm l(-1) kg(-1)) than FW-acclimated animals and were slightly hypo-osmotic to the environment. Plasma Na(+), Cl(-), K(+), Mg(2+), Ca(2+), urea and trimethylamine oxide (TMAO) concentrations were all significantly higher in bull sharks acclimated to SW, with urea and TMAO showing the greatest increase. Gill, rectal gland, kidney and intestinal tissue were taken from animals acclimated to FW and SW and analysed for maximal Na(+)/K(+)-ATPase activity. Na(+)/K(+)-ATPase activity in the gills and intestine was less than 1 mmol Pi mg(-1) protein h(-1) and there was no difference in activity between FW- and SW-acclimated animals. In contrast Na(+)/K(+)-ATPase activity in the rectal gland and kidney were significantly higher than gill and intestine and showed significant differences between the FW- and SW-acclimated groups. In FW and SW, rectal gland Na(+)/K(+)-ATPase activity was 5.6+/-0.8 and 9.2+/-0.6 mmol Pi mg(-1) protein h(-1), respectively. Na(+)/K(+)-ATPase activity in the kidney of FW and SW acclimated animals was 8.4+/-1.1 and 3.3+/-1.1 Pi mg(-1) protein h(-1), respectively. Thus juvenile bull sharks have the osmoregulatory plasticity to acclimate to SW; their preference for the upper reaches of rivers where salinity is low is therefore likely to be for predator avoidance and/or increased food abundance rather than because of a physiological constraint.

Copper homeostasis and toxicity in the elasmobranch Raja erinacea and the teleost Myoxocephalus octodecemspinosus during exposure to elevated water-borne copper

Clear nosed skate, Raja erinacea were exposed to 0.10 (control), 0.52 or 1.73 microM copper and sculpin, Myoxocephalus octodecemspinosus were exposed to 0.10 or 1.73 microM copper (as CuSO4) in Salisbury Cove seawater for up to seven days. Skate gill copper concentrations increased 40-50 fold over background in response to copper exposure at both concentrations. In comparison, sculpin gill levels only increased 3-fold. While there was no evidence for internalized copper in the skate arising from the water-borne exposure, sculpin kidneys, but not livers, exhibited elevated copper concentrations after the seven days of exposure. The marked difference in branchial copper accumulation between the skate and the sculpin likely explains why elasmobranchs appear to be more sensitive to metal exposure than most marine teleost fish. Brain tissue from both species and the skate rectal gland contained relatively high background copper concentrations. Copper exposure caused an initial transient reduction in skate plasma total ammonia (Tamm), but eventually led to elevated plasma Tamm. Despite the marked branchial copper accumulation in the skate, there was no reduction in gill Na/K-ATPase activity. Similarly, Na/K-ATPase activity in skate rectal gland and intestine, as well as in sculpin gill and intestine were not affected by copper exposure. Plasma sodium, magnesium and chloride were not affected by copper exposure in either the skate or the sculpin.

Branchial mitochondria-rich cells in the dogfish Squalus acanthias

In marine teleost fishes, the gill mitochondria-rich cells (MRCs) are responsible for NaCl elimination; however, in elasmobranch fishes, the specialized rectal gland is considered to be the most important site for salt secretion. The role of the gills in elasmobranch ion regulation, although clearly shown to be secondary, is not well characterized. In the present study, we investigated some morphological properties of the branchial MRCs and the localization, and activity of the important ionoregulatory enzyme Na(+)/K(+)-ATPase, under control conditions and following rectal gland removal (1 month) in the spiny dogfish, Squalus acanthias. A clear correlation can be made between MRC numbers and the levels of Na(+)/K(+)-ATPase activity in crude gill homogenates (r(2)=-0.69). Strong Na(+)/K(+)-ATPase immunoreactivity is also clearly associated with the basolateral membrane of these MRCs. In addition, the dogfish were able to maintain ionic balance after rectal gland removal. These results all suggest a possible role of the dogfish gill in salt secretion. MRCs were, however, unresponsive to rectal gland removal in terms of changes in number, fine structure and Na(+)/K(+)-ATPase activity, as might be expected if they were compensating for the loss of salt secretion by the rectal gland. Thus, the specific role that these MRCs play in ion regulation in the dogfish remains to be determined

Effects of environmental salinity on Na(+)/K(+)-ATPase in the gills and rectal gland of a euryhaline elasmobranch (Dasyatis sabina)

Changes in Na(+)/K(+)-ATPase activity and abundance associated with environmental salinity were investigated in the gills and rectal gland of the Atlantic stingray Dasyatis sabina. Using a ouabain-specific ATPase assay and western blotting, we found that stingrays from fresh water had the highest activity and highest relative abundance of Na(+)/K(+)-ATPase in the gills. Using immunohistochemistry, we also found that gills from freshwater stingrays had the greatest number of Na(+)/K(+)-ATPase-rich cells. When freshwater stingrays were acclimated to sea water for 1 week, the activity and abundance of Na(+)/K(+)-ATPase and the number of Na(+)/K(+)-ATPase-rich cells decreased in the gills. In seawater stingrays, the branchial activity and abundance of Na(+)/K(+)-ATPase and the number of Na(+)/K(+)-ATPase-rich cells were further reduced. In rectal glands, the activity and abundance of Na(+)/K(+)-ATPase were lower in freshwater animals than in seawater-acclimated and seawater stingrays, both of which had equivalent levels. These findings suggest that salinity-associated changes in gill and rectal gland Na(+)/K(+)-ATPase activity are due to changes in the abundance of Na(+)/K(+)-ATPase. We conclude that the gills may be important for active ion uptake in fresh water, while the rectal gland is important for active NaCl excretion in sea water. The results from this study are the first to demonstrate an effect of environmental salinity on Na(+)/K(+)-ATPase expression in the gills and rectal gland of an elasmobranch.

Resting metabolic rate, critical swimming speed, and routine activity of the euryhaline cyprinodontid, Aphanius dispar, acclimated to a wide range of salinities

Specimens of the euryhaline cyprinodontid fish, Aphanius dispar, collected in salt ponds, were acclimated to salinities of <1 (freshwater), 35 (seawater), 70, 105, and 140 ppt for 4 wk before measurement of oxygen consumption, critical swimming speed, and routine activity level. Oxygen consumption was similar in <1, 35, and 70 ppt (0.18+/-0.07, 0.17+/-0.06, and 0.16+/-0.04 mL h(-1) g(-1), respectively [mean+/-SD]) but decreased in 105 and 140 ppt (0. 12+/-0.02 and 0.09+/-0.2 mL h(-1) g(-1), respectively). Critical swimming speed and routine activity levels showed the same trend. These results suggest a general decrease in physiological function of A. dispar at extreme salinities.

Energy cost of NaCl transport in isolated gills of cutthroat trout

Few studies have made direct estimates of the energy required for ion transport in gills of freshwater (FW) and seawater (SW) fish. Oxygen consumption was measured in excised gill tissue of FW-adapted cutthroat trout (Oncorhynchus clarki clarki) to estimate the energy cost of NaCl transport in that osmoregulatory organ. Ouabain (0.5 mM) and bafilomycin A1 (1 microM) were used to inhibit the Na+-K+ and H+ pumps, respectively. Both inhibitors significantly decreased gill tissue oxygen consumption, accounting for 37% of total tissue respiration. On a whole mass basis, the cost of NaCl uptake in the FW trout gill was estimated to be 1.8% of whole animal oxygen uptake. An isolated, saline-perfused gill arch preparation was also used to compare gill energetics in FW- and SW-adapted trout. The oxygen consumption of FW gills was significantly (33%) higher than SW gills. On a whole animal basis, total gill oxygen consumption in FW and SW trout accounted for 3.9 and 2.4% of resting metabolic rate, respectively. The results of both experiments suggest that the energy cost of NaCl transport in FW and SW trout gills represents a relatively small (<4%) portion of the animal's total energy budget.