Introduction: cellular volume regulation--mechanisms and control

Osmoionic homeostasis in bivalve mollusks from different osmotic niches: Physiological patterns and evolutionary perspectives

Physiological knowledge gained from questions focused on the challenges faced and strategies recruited by organisms in their habitats assumes fundamental importance about understanding the ability to survive when subjected to unfavorable situations. In the aquatic environment, salinity is particularly recognized as one of the main abiotic factors that affects the physiology of organisms. Although the physiological patterns and challenges imposed by each occupied environment are distinct, they tend to converge to osmotic oscillations. From a comparative perspective, we aimed to characterize the osmoregulatory patterns of the bivalve mollusks Corbicula largillierti (purple Asian cockle), Erodona mactroides (lagoon cockle), and Amarilladesma mactroides (white clam) - inhabitants of different osmotic niches - when submitted to hypo- and/or hyperosmotic salinity variations. We determined the hemolymph osmotic and ionic concentrations, tissue hydration, and the intracellular isosmotic regulation (IIR) from the use of osmolytes (organic and inorganic) after exposure to species-specific salinity intervals. Additionally, we incorporated phylogenetic perspectives to infer and even broaden the understanding about the patterns that comprise the osmoionic physiology of Bivalvia representatives. According to the variables analyzed in the hemolymph, the three species presented a pattern of osmoconformation. Furthermore, both ionic regulation and conformation patterns were observed in freshwater, estuarine, and marine species. The patterns verified experimentally show greater use of inorganic osmolytes compared to the participation of organic molecules, which varied according to the osmotic niche occupied in the IIR for the mantle, adductor muscle, and gills. This finding widens the classic vision about the preferential use of certain osmolytes by animals from distinct niches. Our phylogenetic perspective also indicates that environmental salinity drives physiological trait variations, including hemolymph osmolality and the ion composition of the extracellular fluid (sodium, chloride, magnesium, and calcium). We also highlight the important role played by the shared ancestry, which influences the interspecific variability of the hemolymph K⁺ in selected representatives of Bivalvia.

Ionic and Amino Acid Regulation in Hard Clam (Meretrix lusoria) in Response to Salinity Challenges

Most marine mollusks are osmoconformers, in that, their body fluid osmolality changes in the direction of the change in environmental salinity. Marine mollusks exhibit a number of osmoregulatory mechanisms to cope with either hypo- or hyperosmotic stress. The effects of changes in salinity on the osmoregulatory mechanisms of the hard clam (Meretrix lusoria, an economically important species of marine bivalve for Taiwan) have not been determined. In this study, we examined the effect of exposure to hypo (10‰)- and hyper (35‰)-osmotic salinity on hard clams raised at their natural salinity (20‰). The osmolality, [Na(+)], and [Cl(-)] of the hard clam hemolymph were changed in the same direction as the surrounding salinity. Further, the contents of total free amino acids including taurine in the gills and mantles were significantly upregulated in hard clam with increasing salinity. The gill Na(+), K(+)-ATPase (NKA) activity, the important enzyme regulating cellular inorganic ions, was not affected by the changed salinity. Mantle NKA activity, however, was stimulated in the 35‰ SW treatment. The taurine transporter (TAUT) is related to the regulation of intracellular contents of taurine, the dominant osmolyte. Herein, a TAUT gene of hard clam was cloned and a TAUT antibody was derived for the immunoblotting. The TAUT mRNA expression of the mantle in hard clam was significantly stimulated in 35‰ SW, but protein expression was not modulated by the changed salinity. In gills of the hard clam with 10‰ SW, both TAUT mRNA and protein expressions were significantly stimulated, and it may reflect a feedback regulation from the decreased gills taurine content under long-term hypoosmotic acclimation. These findings suggest that TAUT expression is regulated differently in gills and mantles following exposure to alterations in environmental salinity. Taken together, this study used the physiological, biochemical and molecular approaches to simultaneously explore the osmoregulation in tissues of hard clam and may further help to understand the osmoregulation in bivalves.

The Ca2+-independent PKC (p105) mediates the PMA-activation of marine mussel hemocytes and the Ca2+-dependent PKC (p60) does not intervene

Previous works revealed the presence of a Ca(2+)-dependent protein kinase (p60) and a Ca(2+)-independent protein kinase (p105) in the mantle tissue from the sea mussel Mytilus galloprovincialis Lmk. The expression of both isoforms shows a balance between cytosolic and membrane fractions in mantle, gills, and hepatopancreas, whereas, in hemocytes, their expression is mainly cytosolic, as happens in muscle tissues with p60 alone. Both enzymatic forms contain phosphorylated serines, and no phosphorylation was detected in tyrosines. Only the form p105 mediates the PMA-induced activation of the hemocytes of M. galloprovincialis, and it does so by a process of down-regulation. The form p60 does not respond to the presence of the phorbol ester, suggesting structural differences related to the binding sites of the diacylglycerol.

Branchial carbonic anhydrase activity and ninhydrin positive substances in the Pacific white shrimp, Litopenaeus vannamei, acclimated to low and high salinities

The Pacific white shrimp, Litopenaeus vannamei, acclimated to 30 ppt salinity, was transferred to either low (15 and 5 ppt), or high (45 ppt) salinity for 7 days. Hemolymph osmolality, branchial carbonic anhydrase activity, and total ninhydrin-positive substances (TNPS) in abdominal muscle were then measured for each condition. Hemolymph osmotic concentration was regulated slightly below ambient water osmolality in shrimp acclimated to 30 ppt. At 15 and 5 ppt, shrimp were strong hyper-osmotic regulators, maintaining hemolymph osmolality between 200 and 400 mOsm above ambient. Shrimp acclimated to 30 ppt and transferred to 45 ppt salinity were strong hypo-osmotic and hypo-ionic regulators, maintaining hemolymph osmolality over 400 mOsm below ambient. Branchial carbonic anhydrase (CA) activity was low (approximately 100 micromol CO(2) mg protein(-1) min(-1)) and uniform across all 8 gills in shrimp acclimated to 30 ppt, but CA activity increased in all gills after exposure to both low and high salinities. Anterior gills had the largest increases in CA activity, and levels of increase were approximately the same for low and high salinity exposure. Branchial CA induction appears to be functionally important in both hyper- and hypo-osmotic regulations of hemolymph osmotic concentrations. Abdominal muscle TNPS made up between 19 and 38% of the total intracellular osmotic concentration in shrimp acclimated to 5, 15, and 30 ppt. TNPS levels did not change across this salinity range, over which hemolymph osmotic concentrations were tightly regulated. At 45 ppt, hemolymph osmolality increased, and muscle TNPS also increased, presumably to counteract intracellular water loss and restore cell volume. L. vannamei appears to employ mechanisms of both extracellular osmoregulation and intracellular volume regulation as the basis of its euryhalinity.

Cell volume changes during rapid temperature shifts

The effect of a rapid temperature increase on the volume of different types of cells was investigated. Experiments were carried out using continuous microscopic image analysis. Volume variation of yeast cells, yeast spheroplasts and human leukaemia cells was measured during the transient phase after a thermal shift. The thermal shift was found to induce rapid increase in cell volume for cells lacking a cell wall (yeast spheroplasts and human leukaemia cells). This increase in cell volume is assumed to be a main cause of the heat shock-induced cell death. A theoretical mechanistic model that explains the behaviour of these cells is finally proposed.

Characterization of a chloride current in the larval epidermis of the beetle Tenebrio molitor

Voltage-clamp analysis of single cuticle-attached epidermal cells dissected from the newly-ecdysed mealworm revealed the presence of a large inwardly-rectifying anion (i.e. outwardly-going) current. In many cells this current formed spontaneously on breaking into the cell with the patch pipette when the bath solution was isoosmotic with the pipette solution (415 mosmol/l). The current was evoked rapidly by electrical stimulation or by bathing the cells in hyposmotic saline (335 mosmol/l). The reversal potential of the activated current shifted in agreement with the Nernst prediction for Cl(-) when the transmembrane chloride gradient was altered by partially substituting bath or patch pipette Cl(-) with gluconate(-). Substitution of Na(+) with choline(+) or K(+) with TEA and Ba(+) in the bath or pipette solutions did not alter the reversal potential. Addition of 200 &mgr;mol/l cyclic AMP or 1 mmol/l cyclic GMP to the pipette solution increased the initial current strength and reduced the time taken to reach half peak amplitude from 117 sec to 49 sec and 41 sec, respectively. Cyclic AMP also raised the threshold at which the current developed under hyperosmotic conditions by about 20 mosmol/l. Addition of the Cl(-) channel blockers diphenylamine-2-carboxylic acid (200 &mgr;mmol/l) and diisothiocyanostilbene-2,2'-disulphonic acid (250 &mgr;mol/l) to the bath solution reduced the inwardly-rectifying anion current by 50%. This current was barely detectable in cells prepared from the mid-instar integument. This non-constitutive pattern of expression suggests that cellular Cl(-) efflux (and that of other anions) may be required during moult-cycle specific processes such as moulting fluid formation and cell volume regulation. As the strength of the epidermal anion current could be raised by the exogenous application of cytosolic cyclic nucleotides, the activity of the anion channels responsible for this current may normally be regulated by yet-to-be-identified hormone(s) or neuropeptide(s) acting on this tissue.