Volume regulation in the red coelomocytes ofGlycera dibranchiata: An interaction of amino acid and K+ effluxes

Euryhalinity of subtropical marine and estuarine polychaetes evaluated through carbonic anhydrase activity and cell volume regulation

Polychaete worms are widespread and diverse in marine and estuarine habitats subject to varying salinity, in areas influenced by tides, demanding physiological adjustment for internal homeostasis. They are typically considered and reported to be osmoconformers, but they are not often studied for their osmoregulation. Here, three species of polychaete worms from distinct coastal habitats have been investigated: the spionid Scolelepis goodbody (intertidal in saline, exposed sandy beaches), the nereidid Laeonereis culveri (estuarine polyhaline), and the nephtyid Nephtys fluviatilis (estuarine oligohaline). The general objective here was to relate ecological aspects and physiology of the studied species. Constitutive whole body osmolality and carbonic anhydrase activity (CAA, relevant for osmoregulation, acid-base balance and respiration) have been assayed. In addition, cell volume regulatory capacity (from whole body cell dissociation) was challenged under hypoosmotic and hyperosmotic shocks (50% intensity), with respect to isosmotic control. S. googdbody and L. culveri, the two species from most saline environments (marine/estuarine), showed higher CAA than N. fluviatilis, which, in turn, displayed a hyperosmotic gradient to water of salinity 15. Cells from S. goodbody and L. culveri showed regulatory volume decrease upon swelling, with S. goodbody showing the largest volume increase. As in other more studied marine invertebrate groups, polychaetes also show variability in their osmoregulatory physiology, related to distinct saline challenges faced in their coastal habitats.

Tolerance, Growth, and Physiological Responses of the Juvenile Razor Clam (Sinonovacula constricta) to Environmental Ca2+ and Mg2+ Concentrations

To facilitate transplanting razor clam (Sinonovacula constricta) populations to inland saline-alkaline waters (ISWs), we evaluated the tolerance of juvenile S. constricta (JSC) to Ca²⁺ and Mg²⁺ concentrations, and determined the effects of these ions on JSC growth and physiological parameters. After 30 days stress, the tolerable ranges of JSC to Ca²⁺ and Mg²⁺ were determined to be 0.19 mmol⋅L^-1-19.46 mmol⋅L^-1 and 0 mmol⋅L^-1-29.54 mmol⋅L^-1, respectively. The concentrations of Ca²⁺ (less than 0.65 mmol⋅L^-1 or more than 3.24 mmol⋅L^-1) and Mg²⁺ (less than 0.37 mmol⋅L^-1 or more than 14.17 mmol⋅L^-1) significantly inhibit JSC growth. Physiological enzyme activity no significant response when the concentrations range of Ca²⁺ and Mg²⁺ are 0.93 mmol⋅L^-1-6.49 mmol⋅L^-1 and 0.37 mmol⋅L^-1-14.77 mmol⋅L^-1, respectively. For transplantation practice, these data indicate that only high concentrations of Ca²⁺ (3.24-6.825 mmol⋅L^-1) and Mg²⁺ (14.77-33.69 mmol⋅L^-1) in target inland saline-alkaline water had significantly impact on growth and physiological response. In addition, present study suggests that the increase in Ca²⁺ and Mg²⁺ ion concentrations caused by ocean acidification will not affect the survival, growth and physiology of S. constricta. Current research suggests that S. constricta can adapt to extreme changes in the marine environment (Ca²⁺ and Mg²⁺) and may be an excellent candidate for inland saline-alkaline water transplantation practice.

Aminotransferase and the production of alanine during hyperosmotic stress in Paramecium calkinsi

When Paramecium calkinsi encounter hyperosmotic stress, intracellular free alanine increases. In vivo assays indicate that the reaction catalyzed by alanine aminotransferase contributes to the build up of alanine in response to hyperosmotic shock. 14C-pyruvate is converted to 14C-alanine in cells grown axenically at 200 mosm. When shifted to 600 mosm, the rate of conversion of pyruvate to alanine increases, and conversion at either 200 or 600 mosm is blocked by 1 mM aminooxyacetic acid (AOA), an inhibitor of aminotransferase. Intracellular free alanine increase is partially inhibited by AOA, and AOA prevents cells living in fresh water from acclimating to higher salinities, an indication that the increase in intracellular alanine is physiologically significant.

Ionomycin produces an improved volume recovery by an increased efflux of taurine from hypoosmotically stressed molluscan red blood cells

Nucleated erythrocytes of the blood clam, Noetia ponderosa, recover cell volume after a hypoosmotic stress by an efflux of K+, Cl- and taurine. When the cells are exposed to ionomycin followed by hypoosmotic stress, swelling is less and volume recovery is both faster and more complete than in control cells without the ionophore. The improved volume recovery is caused by a large increase in the efflux of taurine. The taurine efflux is altered by changing Ca2+ concentrations in the presence of the ionophore. Potassium regulation by the osmotically stressed erythrocytes is also increased in the presence of ionomycin, but only by a small amount, perhaps accounting for the initial decrease in swelling. Variation of Ca2+ in the presence of ionomycin without osmotic stress produces no change in the regulation of either osmolyte. These results indicate that both the osmotic stress and an increase in [Ca2+]i are required for the permeability change that produces taurine efflux.

Specific protein phosphorylation occurs in molluscan red blood cell ghosts in response to hypoosmotic stress

The regulation of cellular volume upon exposure to hypoosmotic stress is accomplished by specific plasma membrane permeability changes that allow the efflux of certain intracellular solutes (osmolytes). The mechanism of this membrane permeability regulation is not understood; however, previous data implicate Ca2+ as an important component in the response. The regulation of protein phosphorylation is a pervasive aspect of cellular physiology that is often Ca2+ dependent. Therefore, we tested for osmotically induced protein phosphorylation as a possible mechanism by which Ca2+ may mediate osmotically dependent osmolyte efflux. We have found a rapid increase in 32Pi incorporation into two proteins in clam blood cell ghosts after exposure of the intact cells to a hypoosmotic medium. The osmotic component of the stress, not the ionic dilution, was the stimulus for the phosphorylations. The osmotically induced phosphorylation of both proteins was significantly inhibited when Ca2+ was omitted from the medium, or by the calmodulin antagonist, chlorpromazine. These results correlate temporally with cell volume recovery and osmolyte (specifically free amino acid) efflux. The two proteins that become phosphorylated in response to hypoosmotic stress may be involved in the regulation of plasma membrane permeability to organic solutes, and thus, contribute to hypoosmotic cell volume regulation.

Anisosmotic cell volume regulation: a comparative view

A variety of organisms and cell types spanning the five taxonomic kingdoms are exposed, either naturally or through experimental means, to osmotic stresses. A common physiological response to these challenges is maintenance of cell volume through changes in the concentration of intracellular inorganic and organic solutes, collectively termed osmolytes. Research on the mechanisms by which the concentration of these solutes is regulated has proceeded along several experimental lines. Extensive studies on osmotically activated ion transport pathways have been carried out in vertebrate cells and tissues. Much of our knowledge on organic osmolytes has come from investigations on invertebrates, bacteria, and protists. The relative simplicity of bacterial genetics has provided a powerful and elegant tool to explore the modifications of gene expression during volume regulation. An implication of this diverse experimental approach is that phylogenetically divergent organisms employ uniquely adapted mechanisms of cell volume regulation. Given the probability that changes in extracellular osmolality were physiological stresses faced by the earliest organisms, it is more likely that cell volume regulation proceeds by highly conserved physiological processes. We review volume regulation from a comparative perspective, drawing examples from all five taxonomic kingdoms. Specifically, we discuss the role of inorganic and organic solutes in volume maintenance and the mechanisms by which the concentrations of these osmolytes are regulated. In addition, the processes that may transduce volume perturbations into regulatory responses, such as stretch activation of ion channels, intracellular signaling, and genomic regulation, are discussed. Throughout this review we emphasize areas we feel are important for future research.

Evidence of calmodulin involvement in cell volume recovery following hypo-osmotic stress

An influx of Ca2+ into red blood cells of the bivalve mollusc Noetia ponderosa occurs immediately following a hypo-osmotic stress. The volume recovery response to the stress is dependent upon [Ca2+]o and is inhibited by phenothiazines. The action of these drugs is on the amino acid regulation portion of the recovery rather than on the ionic portion. Since the phenothiazines are non-specific in action, we have conducted several experiments to decide the site of phenothiazine action on the volume recovery response. The sulfoxide derivatives of both chlorpromazine and trifluoperazine have no effect on volume regulation at the same dose where the parent compound inhibits. At 50-100 times the concentration of the parent compound, the derivatives block both volume regulation and taurine efflux. The phorbol ester, TPA, an activator of protein kinase C, alters the volume recovery, but does so by affecting K+ rather than amino acid regulation. The only phenothiazine target that we can not rule out is calmodulin, which we also demonstrate to be present in the clam red cells. Thus, the data presented suggest that calmodulin is involved in the amino acid regulatory portions of the volume recovery in response to hypo-osmotic swelling.

A Ca2+ influx in response to hypo-osmotic stress may alter osmolyte permeability by a phenothiazine-sensitive mechanism

The phenomenon of cell volume recovery following a hypo-osmotic stress mediated by intracellular osmolyte regulation is well known. In many, perhaps all, cell types, the osmolytes involved are usually inorganic ions and amino acids. The details of the regulatory mechanisms for the organic-type osmolytes are not well known. We have found that an immediate influx of external Ca2+ occurs coincident with the application of a hypo-osmotic stress into red cells of two invertebrate species. In both, the influx is initiated by the osmotic stress, not the concomitant ionic decrease. Volume recovery in clam red blood cells is blocked by phenothiazines. In addition, the effect of the phenothiazines is to reduce the amino acid efflux; the ionic portion of the volume response is unaffected. In contrast, the phenothiazines potentiate the volume recovery in worm red coelomocytes. A23187 also potentiates the volume recovery of the worm red cells. The results suggest that the Ca2+ influx is involved in the mechanism that alters cell membrane permeability permitting the amino acid efflux by a mechanism that may involve calmodulin.

Comparative electrical properties of identified neurons in Elysia chlorotica before and after low salinity acclimation

Identified neurons in the abdominal ganglion of Elysia chlorotica adapted to 50% seawater (SW) had significantly different electrical properties from the same cells in animals adapted to 100% SW. Resting potential, action potential (AP) overshoot, (AP) duration, threshold and after potential were all different following salinity acclimation. The resting potential of these cells behaves as an ideal potassium electrode above 10 mM [K+]. The action potential has both sodium and calcium components to the rising phase.