Effects of environmental water salinity on blood acid-base status in juvenile turbot (Scophthalmus maximus L.)

Physiological changes in the red drum after long-term freshwater acclimation

The effect of long-term freshwater acclimation on the blood and plasma ion composition of Red Drum Sciaenops ocellatus was investigated with the goal of elucidating the necessity of ion remediation. Four replicates (n = 50) of freshwater-acclimated (FW) fish (1.6 ± 0.2 g) were raised in 25-m(3) tanks supported by 140,000 L of recirculating water. Four replicates (n = 50) of seawater (SW) fish groups were placed in 40-m(3) offshore cages at 32-35 psu. Blood was collected from 100 fish (FW = 578 ± 50 g; SW = 686 ± 45 g) of each group (FW, SW) after 8 months of rearing. During the grow-out phase, the survival of FW and SW fish was 57.5% and 92.2%, respectively. The water ion composition (mainly the Ca(2+)/K(+) [43%] and Ca(2+)/Mg(2+) ratios [1%]) explained 56.6% of the plasmatic ion variability in the fish groups. Freshwater exposure produced significant reductions in osmolality and in several plasma indicators (Na(+), Cl(-), and Mg(2+)); the K(+) levels from FW fish were the most compromised parameter. The water Ca(2+)/Na(+) ratio had a greater influence (44%) on the plasma chemistry parameters, mainly glucose and creatinine. Freshwater-acclimated fish had a higher percentage of hematocrit, hemoglobin, and red blood cells than SW fish, but the water quality explained only 12.5% of the blood parameter variability between the FW and SW groups. The results support the conclusion that Red Drum tolerates salinity variations and can adopt a relatively stable condition for short periods; however, the data suggest that Red Drum have only a limited ability to withstand a hyposmotic environment for long periods due to their limited ability in maintaining K(+) concentrations without external supplementation. Freshwater environments with high Ca(2+)/Na(+), Ca(2+)/K(+), and Ca(2+)/Mg(2+) ratios appear to be a chronic stress factor that should be considered in future experiments.

Compensatory regulation of acid-base balance during salinity transfer in rainbow trout (Oncorhynchus mykiss)

In seawater-acclimated rainbow trout (Oncorhynchus mykiss), base secretion into the intestine is a key component of the intestinal water absorption that offsets osmotic water loss to the marine environment. Acid-base balance is maintained by the matched excretion of acid equivalents via other routes, presumably the gill and/or kidney. The goal of the present study was to examine acid-base balance in rainbow trout upon transfer to more dilute environments, conditions under which base excretion into the intestine is predicted to fall, requiring compensatory adjustments of acid excretion at the gill and/or kidney if acid-base balance is to be maintained. Net acid excretion via the gill/kidney and rectal fluid, and blood acid-base status were monitored in seawater-acclimated rainbow trout maintained in seawater or transferred to iso-osmotic conditions. As predicted, transfer to iso-osmotic conditions significantly reduced base excretion into the rectal fluid (by ~48%). Transfer to iso-osmotic conditions also significantly reduced the excretion of titratable acidity via extra-intestinal routes from 183.4 ± 71.3 to -217.5 ± 42.7 μmol kg(-1) h(-1) (N = 7). At the same time, however, ammonia excretion increased significantly during iso-osmotic transfer (by ~72%) so that the apparent overall reduction in net acid excretion (from 419.7 ± 92.9 to 189.2 ± 76.5 μmol kg(-1 )h(-1); N = 7) was not significant. Trout maintained blood acid-base status during iso-osmotic transfer, although arterial pH was significantly higher in transferred fish than in those maintained in seawater. To explore the mechanisms underlying these adjustments of acid-base regulation, the relative mRNA expression and where possible, activity of a suite of proteins involved in acid-base balance were examined in intestine, gill and kidney. At the kidney, reduced mRNA expression of carbonic anhydrase (CA; cytosolic and membrane-associated CA IV), V-type H(+)-ATPase, and Na(+)/HCO(3) (-) co-transporter were consistent with a reduced role in net acid excretion following iso-osmotic transfer. Changes in relative mRNA expression and/or activity at the intestine and gill were consistent with the roles of these organs in osmotic rather than acid-base regulation. Overall, the data emphasize the coordination of acid-base, osmoregulatory and ionoregulatory processes that occur with salinity transfer in a euryhaline fish.

Involvement of respiratory chain in the regulatory volume decrease process in turbot hepatocytes

Regulatory volume decrease (RVD) constitutes a fundamental process that turbot (Scophthalmus maximus) hepatocytes are able to perform when exposed to hypo-osmotic stress. RVD is an integrative mechanism that involves various elements of the cellular machinery. Among others, ATP is an essential protagonist: released following hypo-osmotic shock, it acts as an auto/paracrine factor to trigger other signalling events. The origin of this ATP remains unclear and, to the best of our knowledge, no information exists about the role of mitochondrial respiration in RVD. Therefore, we propose to analyse the potential link between RVD and the respiratory chain, with a focus on ATP release and exocytosis. Using inhibitors of mitochondrial respiration, RVD was shown to be dependent on respiratory chain activity. However, we demonstrated an indirect role of mitochondrial respiration: ATP could be synthesized and then stored in intracellular vesicles until the moment cells release it to face hypo-osmotic swelling. However, the involvement of exocytosis in this process needs to be further investigated.

Effects of hypo-osmotic stress on ATP release in isolated turbot (Scophthalmus maximus) hepatocytes

BACKGROUND INFORMATION

ATP is released from many cell types exposed to hypo-osmotic shock and is involved in RVD (regulatory volume decrease). Purinergic signalling events have been extensively investigated in mammals, but not in marine teleosteans.

RESULTS

The effect of hypo-osmotic shock on ATP release was examined in isolated hepatocytes from turbot (Scophthalmus maximus), a marine flatfish. Hypo-osmotic stress (240 mOsm x kg(-1)) induced a significant increase in ATP efflux, and was inhibited by a potential CFTR (cystic fibrosis transmembrane conductance regulator) inhibitor, glibenclamide, but not by the MDR1 (multidrug resistance 1) P-glycoprotein inhibitor, verapamil. ATP efflux could be a cAMP-dependent process, as IBMX (isobutylmethylxanthine) and forskolin triggered the process under iso-osmotic conditions. Protein kinases, including protein kinase C, could also be involved, as staurosporine and chelerythrine inhibited the mechanism. Calcium could contribute to ATP efflux as ionomycin, a calcium ionophore, elicited a rapid release under iso-osmotic conditions, and chelation using EGTA abolished ATP release under hypo-osmotic conditions. RVD was partially abolished by apyrase, an ATP scavenger, and suramin, a purinoceptor antagonist. Moreover, hypo-osmotic shock induced a rise in intracellular calcium which could be involved in RVD. Since extracellular ATP triggered an increase in cellular free-calcium content under iso-osmotic conditions, our results could indicate that hypo-osmotic-induced ATP efflux contributes to RVD in turbot hepatocytes by stimulating purinergic receptors, which may lead to activation of a calcium signalling pathway.

CONCLUSIONS

These data provide the first evidence of volume-sensitive ATP signalling for volume maintenance in a marine teleost fish cell type.

Effects of hyposmotic stress on exocytosis in isolated turbot, Scophthalmus maximus, hepatocytes

The effect of hyposmotic shock on exocytosis was examined in isolated hepatocytes of turbot, a marine flatfish, using the molecular probe FM1-43. Sudden exposure to a reduced osmolality caused an increase in cell exocytic activity related to the osmotic gradient between intra- and extracellular fluids. Cytoskeletal microtubules could contribute to this hyposmotic-induced exocytosis since colchicine inhibited the process. Protein kinase C, phosphatidylinositol-3 kinase, phospholipases A2, C and D could constitute key enzymes in the mechanism since their inhibition by specific agents altered the hyposmotic-induced exocytic activity. Moreover, arachidonic acid and derivates from the 5-lipoxygenase pathway as well as calcium could participate in the process. As regulatory volume decrease (RVD) exhibited by turbot hepatocytes following hyposmotic stimulation involves similar features, a potential role of exocytosis in volume regulation is suggested. In particular, exocytosis could serve RVD by contributing to ATP release since this latter process similarly appeared to be phospholipase D-dependent and related to the osmotic gradient. This study provides the first evidence of a volume-sensitive exocytosis that could aim at volume constancy in a marine teleost fish cell type.

Volume regulation following hyposmotic shock in isolated turbot (Scophthalmus maximus) hepatocytes

Regulatory volume decrease (RVD) following hyposmotic stimulation was studied in isolated turbot, Scophthalmus maximus, hepatocytes. Exposed to a reduced osmolality (from 320 to 240 mosm kg(-1)), cells first swelled and then exhibited a RVD. Volume regulation was significantly inhibited in presence of NPPB, 9-AC, acetazolamide, DIDS and barium. Taken together, these results could suggest that RVD operated via separate K+ and Cl- channels and probably Cl-/HCO3(-) exchanger in turbot hepatocytes. The K+/Cl- cotransporter could also be involved as furosemide and DIOA strongly inhibited the process whereas NEM, a K+/Cl- cotransporter activator, added under isosmotic conditions, led to cell shrinkage. RVD in turbot hepatocytes appeared also to depend on proteins p38 MAP kinase and tyrosine kinase but not on proteins ERK 1/2. Arachidonic acid and leukotrienes could also be involved since inhibition of synthesis of both these compounds by quinacrine and NDGA, respectively, inhibited the volume regulation. Likewise, Ca2+ has been proved to be an essential messenger as RVD was prevented in absence of Ca2+. Finally, this work provides bases for novel studies on cell volume regulation in marine teleosteans.

Compensation for hypercapnia by a euryhaline elasmobranch: effect of salinity and roles of gills and kidneys in fresh water

Specimens of the euryhaline elasmobranch, Dasyatis sabina were acclimated to seawater and fresh water, and exposed to normocapnic (air) and hypercapnic (1% CO2 in air) environmental water. Blood pH, PCO2, and [HCO3-], as well as whole-animal net-acid excretion, were measured for up to 24 h of hypercapnia. In a separate experimental series, urine was collected from freshwater acclimated stingrays during 8 h of normocapnia and hypercapnia. Stingrays in both salinities at least partially compensated for the respiratory acidosis by accumulating HCO3- in their extracellular spaces. The degree of compensation for blood pH was 88.5% in seawater, but only 31.0% in fresh water after 24 h of hypercapnia. Whole-animal net-acid excretion was also greater in seawater than in fresh water, as was the increase in extracellular fluid [HCO3-]. Mean urinary net-acid excretion rates were slightly negative, and never increased above normocapnic control rates during hypercapnia. Since whole-animal net-acid excretion rates increased with blood [HCO3-], and urinary excretion was always negative, the gills were probably the primary organ responsible for compensation from environmental hypercapnia. The faster, and more complete, compensation for hypercapnia in seawater than in fresh water for this euryhaline elasmobranch is consistent with data for euryhaline teleosts, and probably reflects Na+-dependent mechanisms of branchial acid excretion.

Evolutionary determinants of normal arterial plasma pH in ectothermic vertebrates

Mean values of normal arterial pH in different species of fish, amphibians and reptiles at 15 and 25°C, taken from the literature, are negatively correlated with arterial PCO2 and plasma [Na+]. At either temperature, the data accord with the hypothesis that extracellular acid–base homeostasis evolved to maintain an optimal pH at particular cell-surface sites that are similar in all species. These hypothetical sites bear fixed negative charges that attract H+, but which are partially screened by Na+; for the surface pH to be constant, the bulk interstitial pH should then vary inversely with [Na+], as is the case. At the same time, the bulk interstitial fluid must be more acid than arterial plasma by an amount that increases with decreasing arterial PCO2. With allowance made for additional screening by Ca2+ and Mg2+, the relevant cell-surface pH is probably approximately 6.2.