Osmotic Factors Research Articles

The Role of Tubulin and Thaumatin Genes and Osmotic Factors in Salinity Tolerance of Tomato Plants

Soil salinity is a drastic abiotic factor that affects many physiological processes and whole plants’ activities, as well as up- and down-regulating gene expression. Studying the effect of salinity on tubulin and thaumatin relative gene expression as DNA markers for salinity stress in tomato plants is a scarcely studied topic. Tubulin regulates and plays an important role in the immunolocalization of xylem and phloem fibers in stems and additionally maintains the concept of heavy microtubule contribution during cellulose microfibril confession in secondary cell walls under abiotic and biotic stresses. Like tubulin, thaumatin-like proteins are concomitant with plant defense responses against both biotic and abiotic stresses. The expression of the thaumatin gene can be meaningfully induced after plants' exposure to either drought, freezing, or salinity stresses. Thus, the present investigation was conducted to study the impact of different salinity levels (0, 75, 100, and 120 mM NaCl) on the tomato plants’ growth, osmotic adjustment, and relative gene expression of both antioxidant and salt tolerance genes. As the salt concentration intensified, the fresh and dry weight of the shoots and the roots reduced significantly, accompanied by a reduction in chlorophyll a and carotenoids. On the other hand, salinity stress significantly decreased the level of osmotica (e.g., soluble sugars and soluble proteins) in tomato tissues compared to non-saline-grown plants, while a significant accumulation of free amino acids was recorded. At the molecular level, it was observed that the relative expression of the polyphenol oxidase, peroxidase, and thaumatin genes was high at the level of 100 mM NaCl, but it was suppressed at 120 mM NaCl. In contrast, salinity down-regulated tubulin gene expression in stressed tomato plants relative to controls, revealing various mechanisms that instigated salinity tolerance, which is concentration-dependent. The study recommended the importance of amino acids as osmotica as well as the relative expressions of PPO, peroxidase, and thaumatin genes in conferring salt tolerance at low to moderate salt levels.

Laterality of blood perfusion in the lower extremities after drinking saline at different temperatures

Skin blood flux (SkBF) changes caused by drinking cold water are generally associated with vagal tone and osmotic factors in the digestive system. However, there is still a lack of relevant research on whether there are left and right differences in these SkBF change. In the current study, a total of 60 subjects were recruited. Skin blood perfusion of the bilateral lower extremities was recorded simultaneously before and after drinking saline of different temperatures saline by using Laser Doppler flowmetry (LDF). The electrogastrogram (EGG) was also monitored, and the dominant frequency of the EGG and heart rate variability were analyzed. The results indicated that after drinking saline, the laterality index of SkBF at the lower extremities was different and the laterality index changes of SkBF were mainly reflected in the frequency interval V (0.4–1.6 Hz). There was a weak negative correlation between the laterality index of endothelial NO-dependent component and change rate of root mean square of successive differences (RMSSD) after drinking 4 °C saline. However, after drinking 30 °C saline, there was a weak positive correlation between neurogenic component and RMSSD The distribution and regulation of bilateral blood flow are not symmetrical but exhibit a certain laterality.

Effect of salinity stress on quinoa germination. Influence of ionic and osmotic components

Quinoa is a salinity tolerant species that can be used as a complementary crop in saline soils in many parts of the world. Previous studies, mostly referred to the influence of sodium chloride, have shown that according to the genetic material there are different salinity degrees of tolerance. In general, germination decreases with increasing salinity. However, there are few quinoa studies related to the components of saline stress (ionic and osmotic factors). In this study, the influence of both factors, in the presence of NaCl and KCl salts on CICA Var cultivated in the Argentina Northwest was evaluated. The seeds were germinated under different salt concentrations (0, 100, 200, 300, 400 mM NaCl and KCl) and in PEG8000 in solutions osmotically equivalent to the concentrations of sodium and potassium salts. The results showed that up to 200 mM of NaCl and KCl, germination reached values higher than 90% and after 300 mM of salts the rate and final germination decreased. The ionic factor of both salts has the greatest influence on germination. This result can be explained by the physiological adjustment that this variety, originally from the mountainous area of Peru, displayed against the edaphic and microclimatic conditions of the cultivation site in the Argentine Northwest (1,995 m asl).

Genome-wide identification of a novel elovl4 gene and its transcription in response to nutritional and osmotic regulations in rabbitfish (Siganus canaliculatus)

Elongation of very long-chain fatty acids 4 (Elovl4) proteins, belong to the fatty acyl elongase families that are involved in long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis in animals. Thus far, very little is known about the roles of Elovl4 in fish. Previously, we reported an elovl4 gene named as elovl4b in rabbitfish (Siganus canaliculatus). In this study, a novel elovl4-like gene with a 957-bp open reading frame encoding a 318 amino-acid protein, which shared 67.3% similarity with rabbitfish Elovl4b and more than 80% similarity with Elovl4a from other teleost, was identified in S. canaliculatus, and thus was termed as elovl4a. Multiple protein sequences alignment, gene synteny and phylogenetic analysis indicated that two obviously different elovl4 genes are widespread in various teleost. Different with elovl4b (mainly expressed in eye and slightly expressed in brain), rabbitfish elovl4a was primarily distributed in brain, slightly examined in eye and gonad tissues while hardly detectable in other tissues. The transcription of cerebral elovl4a was significantly higher in fishes cultured in seawater (32‰) than that of those maintained at a brackish water environment (15‰), when fishes were fed with a vegetable oil (VO) enriched diet. However, the transcriptional pattern was totally inverse when fishes were fed with a fish oil (FO) enriched diet. In the brackish water, diet types had no effect on the transcription of cerebral elovl4a. Whereas in seawater, the transcription of cerebral elovl4a was significantly higher in fishes fed with the VO diet in comparison with those fed with the FO diet. Our findings suggest that elovl4a can be regulated by both nutritional and osmotic factors, and it may play an important role in endogenous biosynthesis of LC-PUFA in the neural system of rabbitfish. This study may help us to understand the effects of nutritional and osmotic factors on the LC-PUFA biosynthesis, as well as contribute to the optimization or enhancement of LC-PUFA biosynthesis in teleost.

Infectious complications of peritoneal dialysis

Peritoneal dialysis is one of three basic methods of renal replacement therapy. Others include hemodialysis and kidney transplantation. In peritoneal dialysis, the peritoneal cavity is used to clean the blood of excess unnecessary substances and water. The peritoneum is a thin membrane permeable to water and many substances, lining the organs inside the abdominal cavity. Briefly, the peritoneum can be considered a kind of filter. When performing peritoneal dialysis, a dialysis fluid is administered to the abdominal cavity, which discharges after some time, and this activity is repeated at planned intervals. The chemical composition of the fluid is appropriately selected for each patient so as to obtain optimally favorable results of dialysis. Water diffusion depends on the concentration in the fluid of glucose and other osmotic factors. Among the infectious complications of peritoneal dialysis, including drainage site infections, tunnel inflammation and peritonitis associated with peritoneal dialysis, the latter remains the most common and serious.

Steviol glycosides profile in Stevia rebaudiana Bertoni hairy roots cultured under oxidative stress-inducing conditions.

The ability to synthesize particular steviol glycosides (SvGls) was studied in Stevia rebaudiana Bertoni hairy roots (HR) grown in the light or in the dark under the influence of different osmotic active compounds. Manipulation of culture conditions led to changes in the morphology and growth rate of HR, as well as to an increase in oxidative stress manifested as an enhancement in endogenous hydrogen peroxide concentration in the cultured samples. The highest level of H2O2 was noted in HR cultured under light or in the medium with the highest osmotic potential. This correlated with the highest increase in the expression level of ent-kaurenoic acid hydroxylase, responsible for the redirection of metabolic route to SvGls biosynthesis pathway. An analysis of transcriptional activity of some UDPglucosyltransferase (UGT85c2, UGT74g1, UGT76g1) revealed that all of them were upregulated due to the manipulation of culture conditions. However, the level of their upregulation depended on the type of stress factor used in our experiment. Analysis of SvGls content revealed that HR grown under all applied conditions were able to synthesize and accumulate several SvGls but their concentration differed between the samples across the different conditions. The level of rebaudioside A concentration exceeded the content of stevioside in HR in all tested conditions. Concomitantly, the presence of some minor SvGls, such as steviolbioside and rebaudioside F, was confirmed only in HR cultured in the lowest osmotic potential of the medium while rebaudioside D was also detected in the samples cultured in the media supplemented with NaCl or PEG.Key Points● Several steviol glycosides are synthesized in hairy roots of S. rebaudiana.● Light or osmotic factors cause enhancement in oxidative stress level in hairy roots.● It correlates with a significant increase in the level of KAH expression.● UGTs expression and steviol glycosides content depends on culture conditions.

Cold signaling in plants: Insights into mechanisms and regulation.

To survive under cold temperatures plants must be able to perceive a cold signal and transduce it into downstream components that induce appropriate defense mechanisms. In addition to inducing adaptive defenses, such as the production of osmotic factors to prevent freezing and the reprogramming of transcriptional pathways, cold temperatures induce changes in plant growth and development which can affect the plant life cycle. In this review, we summarize recent progress in characterizing cold-related genes and the pathways that allow transduction of the cold signal in plants, focusing primarily on studies in Arabidopsis thaliana and rice (Oryza sativa). We summarize cold perception and signal transduction from the plasma membrane to the nucleus, which involves cold sensors, calcium signals, calcium-binding proteins, mitogen-activated protein kinase cascades, and the C-repeat binding factor/dehydration-responsive element binding pathways, as well as trehalose metabolism. Finally, we describe the balance between plant organogenesis and cold tolerance mechanisms in rice. This review encapsulates the known cold signaling factors in plants and provides perspectives for ongoing cold signaling research.

Abnormal Osmolality Gap Exists in Distal Symmetric Polyneuropathy.

Distal symmetric polyneuropathy, represented by chronic inflammatory demyelinating polyneuropathy, is a popular neurological condition. Some cases are known to be associated with genetic mutations or serum auto-antibodies, but the exact mechanisms in most cases are unknown. Recently, osmotic factors have been suggested to trigger some neurological disorders, such as neuromyelitis optica. The aim of the present study was to assess the possible association of osmotic factors in the pathogenesis of distal polyneuropathy. We prospectively measured the serum levels of osmolality, electrolytes, total protein, albumin, blood urea nitrogen, glucose, and osmolality gap in the patients with acute distal polyneuropathy before treatments (n = 12) and those with other comprehensive neurological disorders such as multiple sclerosis and neurodegenerative diseases (n = 176). Then, we compared each osmotic fraction between the two groups. As a result, all of the 12 patients with acute distal polyneuropathy, including 4 patients with chronic inflammatory demyelinating polyneuropathy, showed abnormally high or low values of osmolality gap, compared to the others (p < 0.0001, F-test). In the patients with other diseases, there were 2 patients with abnormally high osmolality gap values, which were attributable to their hyperlipidemia or high titer of serum autoantibody unrelated to polyneuropathy. In conclusion, serum osmolality gap would be elevated or decreased in the acute phase of distal symmetric polyneuropathy. Osmotic imbalance between the serum and nerve cells, based on abnormal excess or deficit of some unidentified serum osmolytes, may be one of the mechanisms in symmetric polyneuropathy with unknown causes.

Response of Blood Perfusion at ST 36 Acupoint after Drinking Cold Glucose or Saline Injection.

Skin blood flux (SkBF) changes caused by drinking cold water are generally associated with vagal tone and osmotic factors in digestive system. According to acupuncture theory, change of SkBF at ST 36 might reflect the functional changes of digestive system. The aim of this study is to analyze the changes of SkBF after drinking 3°C 0.9% saline or 5% glucose injection by monitor blood flux at bilateral ST 36. The results indicated that, after drinking different cold water, the change ratio of SkBF at right side ST 36 has been different. Because all solutions have the same temperature (3°C) and both saline and glucose solution have the same osmolality, suggesting that the SkBF changes resulting from drinking cold water are not regulated just by the vagal tone and osmolality, there must have been other factors. These results have not been consistent with the frequency domain results of heart rate variability (HRV) analysis. Coherence analysis of blood flux signals at bilateral ST 36 indicated that there have been different coherence-frequency curves among different groups in special frequency bands, which suggested that coherence analysis might provide a potential tool to evaluate different status.

Different responses of two ecotypes of C3–C4 xero-halophyte Bassia sedoides to osmotic and ionic factors of salt stress

The effects of low and moderate salinity (100 and 200 mM NaCl, respectively) and iso-osmotic stress generated by polyethylene glycol PEG (1) (–0.3 MPa) and PEG (2) (–0.6 MPa) on maximum quantum yield of photosystem II (PSII), growth, photosynthesis, transpiration, dark respiration, water use efficiency (WUE), water content, chlorophyll, proline, Na+ and K+ concentrations were investigated in shoots of two ecotypes С3–С4 xero-halophyte Bassia sedoides (Pall.) Aschers. Plants were grown from seeds of two Southern Urals populations (Makan and Podolsk) differing in their bioproductivity. Aboveground biomass of the Makan plants was approximately 10-fold higher than that of the Podolsk ecotype. The plants of both ecotypes were sensitive to water deficit. They showed similar decrease in biomass, water content, net photosynthesis and transpiration intensity under both low and moderate osmotic stress (PEG). However, the content of сhlorophyll and free proline in shoots of the Podolsk plants increased under moderate osmotic stress (PEG(2)). Under salinity the differences between transpiration, F v/F m, WUE, water content, chlorophyll and proline concentrations in shoots of two ecotypes were no found. But, the Podolsk plants showed decrease in the growth parameters (1.5-fold), increase in the dark respiration intensity (2-fold) and the Na+/K+ ratio (1.2-fold) under moderate salinity (200 mM NaCl). Thus, the reduction of bioproductivity of the Podolsk ecotype under salinity was the result of ionic rather than osmotic factor of salinity. In the Podolsk plants the additional transpiration costs and consumption of assimilates (correspondingly) increased with the toxic sodium ion accumulation under salinity. This led to decrease in the growth parameters. Thus, two B. sedoides ecotypes have different adaptive strategies of tolerance to the ionic factor of salt stress at the level of the physiological processes associated with the dark CO2 gas exchange. Moreover, in less tolerant and productive Podolsk ecotype the increase in proline content in shoots characterized comparatively low adaptation to osmotic factor, and the increase in dark respiration and the Na+/K+ ratio pointed to relatively low resistance to ion factor of salinity as compared with the Makan ecotype.

Association between the concentration of n-alkanes and tolerance to cracking in commercial varieties of sweet cherry fruits

Fruit cracking is one of the major economic losses in cherry production worldwide. Factors involved in differential tolerance among varieties are unknown. This phenomenon is mainly caused by rainfall during the harvest period and is related to osmotic factors and fruit water permeability. The cuticle is a hydrophobic extracellular membrane and soluble waxes represent a mixture of aliphatic very long chain fatty acids, its derivatives and cyclic compounds like triterpenes. Analysis of waxes from enzymatic isolated sweet cherry fruit cuticles showed that the main components are triterpenes (76%), alkanes (19%) and alcohols (1%).Cracking in vitro was investigated in five sweet cherry varieties. After removal of cuticular wax fruit cracking was significantly increased. Nuclear magnetic resonance analysis (1H and 13C NMR), revealed that fruits of different sweet cherry varieties contain primarily n-alkane with 29 carbons and no iso-alkane. Gas chromatography–mass spectrometry (GC–MS) enabled identification and quantification of n-alkanes. Varieties with significantly higher concentrations of nonacosane (Kordia, Regina and Lapins) were more tolerant to cracking compared to varieties with lower amounts (Bing and Rainier).We hypothesized that there is an association between the amount of C 29 n-alkane and cracking tolerance in sweet cherry fruits. This difference could be an important factor to explain the different tolerances to cracking in fruits of sweet cherry varieties.

Influence of osmotic stress, physicochemical factors and nitrogen supplements on embryogenesis and plantlet formation in Guizotia abyssinica Cass. (Niger)

An efficient protocol for somatic embryogenesis was developed for the edible oilseed crop Guizotia abyssinica Cass. (cv. IGP 76). Induction and proliferation of callus was achieved from cotyledonary explants on MS medium amended with 1.5 mg l−1 6-benzyladenine (BA) and 0.5 mg l−1 naphthalene acetic acid (NAA). Several auxin/cytokinin combinations, physical conditions, osmotic stresses and supplementary components, viz., amino acids and casein hydrolysate were tested for induction of embryogenic callus. Higher frequency of embryogenic calli (73.7 %) and somatic embryos per culture (22) were observed on MS + 1.5 mg l−1 BA + 0.5 mg l−1 NAA + 0.5 % NaCl. Histological analyses showed typical globular and other stages of embryo development. The MS medium lacking growth regulators was best for the maximum germination of embryos (53.4 %) and formation of plantlets (65.4 %). Regenerated plantlets were acclimatized and transferred to the field with 62 % survival rate.

Cardiovascular and metabolic responses to tap water ingestion in young humans: does the water temperature matter?

Drinking water induces short-term cardiovascular and metabolic changes. These effects are considered to be triggered by gastric distension and osmotic factors, but little is known about the influence of water temperature. We determined, in a randomized crossover study, the acute cardiovascular and metabolic responses to 500 mL of tap water at 3 °C (cold), 22 °C (room) and 37 °C (body) in 12 young humans to ascertain an effect of water temperature. We measured continuous beat-to-beat haemodynamics, skin blood flux with laser-Doppler flowmetry and resting energy expenditure by indirect calorimetry starting with a 30-min baseline followed by a 4-min drink period and a subsequent 90-min post-drink observation. Ingestion of cold- and room-tempered water led to decreased heart rate (P < 0.01) and double product (P < 0.01), and increased stroke volume (P < 0.05); these effects were not observed with body-tempered water. Drinking cold- and room-, but not body-tempered water, led to increased high frequency power of heart rate variability (P < 0.05) and baroreflex sensitivity (P < 0.05). Cold- and room-tempered water increased energy expenditure over 90 min by 2.9% (P < 0.05) and 2.3% (ns), respectively, accompanied by a diminished skin blood flux (P < 0.01), thereby suggesting that both small increases in heat production together with decreased heat loss contribute to warming up the ingested water to intra-abdominal temperature levels. Overall, ingestion of cold- and room-, but not body-tempered water reduced the workload to the heart through a reduction in heart rate and double product which could be mediated by an augmented cardiac vagal tone.

A model for tear film thinning with osmolarity and fluorescein.

We developed a mathematical model predicting dynamic changes in fluorescent intensity during tear film thinning in either dilute or quenching regimes and we model concomitant changes in tear film osmolarity. We solved a mathematical model for the thickness, osmolarity, fluorescein concentration, and fluorescent intensity as a function of time, assuming a flat and spatially uniform tear film. The tear film thins to a steady-state value that depends on the relative importance of the rates of evaporation and osmotic supply, and the resulting increase of osmolarity and fluorescein concentrations are calculated. Depending on the initial thickness, the rate of osmotic supply and the tear film thinning rate, the osmolarity increase may be modest or it may increase by as much as a factor of eight or more from isosmotic levels. Regarding fluorescent intensity, the quenching regime occurs for initial concentrations at or above the critical fluorescein concentration where efficiency dominates, while lower concentrations show little change in fluorescence with tear film thinning. Our model underscores the importance of using fluorescein concentrations at or near the critical concentration clinically so that quenching reflects tear film thinning and breakup. In addition, the model predicts that, depending on tear film and osmotic factors, the osmolarity within the corneal compartment of the tear film may increase markedly during tear film thinning, well above levels that cause marked discomfort.

Cardiovascular responses to water drinking: does osmolality play a role?

Water drinking activates the autonomic nervous system and induces acute hemodynamic changes. The actual stimulus for these effects is undetermined but might be related to either gastric distension or to osmotic factors. In the present study, we tested whether the cardiovascular responses to water drinking are related to water's relative hypoosmolality. Therefore, we compared the cardiovascular effects of a water drink (7.5 ml/kg body wt) with an identical volume of a physiological (0.9%) saline solution in nine healthy subjects (6 male, 3 female, aged 26 +/- 2 years), while continuously monitoring beat-to-beat blood pressure (finger plethysmography), cardiac intervals (electrocardiography), and cardiac output (thoracic impedance). Total peripheral resistance was calculated as mean blood pressure/cardiac output. Cardiac interval variability (high-frequency power) was assessed by spectral analysis as an index of cardiac vagal tone. Baroreceptor sensitivity was evaluated using the sequence technique. Drinking water, but not saline, decreased heart rate (P = 0.01) and increased total peripheral resistance (P < 0.01), high-frequency cardiac interval variability (P = 0.03), and baroreceptor sensitivity (P = 0.01). Neither water nor saline substantially increased blood pressure. These responses suggest that water drinking simultaneously increases sympathetic vasoconstrictor activity and cardiac vagal tone. That these effects were absent after drinking physiological saline indicate that the cardiovascular responses to water drinking are influenced by its hypoosmotic properties.

Osmotic and Autotoxic Effects of Leaf Extracts on Germination and Seedling Growth of Alfalfa

Alfalfa (Medicago sativa L.) is autotoxic to seed germination and root growth of alfalfa seedlings. The objective was to determine if effects of leaf extracts were due to toxic factors, osmotic factors, or both. Rates of imbibition were slowed slightly by the leaf extract, mainly by osmotic factors. Germination at 22°C occurred in 20 h when imbibed with distilled water but was progressively delayed at higher extract concentrations. Initiation of germination events occurred in less than 10 h, and seed had completed germination when water content was 65% of total weight. An exudate with toxic activity to root growth was released during imbibition. Seed imbibed in extract solution for 10 h and then transferred to water‐agar medium had a slight delay in germination but no effect on root elongation rate (RER). Conversely, seed imbibed in water for 10 h before transfer to the extract showed a delay in germination and strong inhibition of RER as extract concentrations increased. Transfer to polyethylene glycol (PEG 8000) at 10 h showed some of the germination delay was due to osmotic properties. Root elongation rate was increased slightly by PEG solutions greater than −0.20 MPa, which was lower than those of the strongest extracts, −0.10 MPa in agar and −0.05 MPa on filter paper. These extracts reduced RER by up to 90% due to the toxicity factors. Inhibition was slowed mainly by osmotic factors whereas delayed seed germination and, especially, reduced root elongation were due mainly to toxic factors of the leaf extract.

The dynamic placenta: II. Hypothetical model of a fetus driven transplacental water balance mechanism producing low apparent permeability in a highly permeable placenta

In vitro and isotopic studies in vivo have reported the paradox that the human placenta is highly permeable, water exchanging at 3.6 litres per hour at 35 weeks of gestation, but clinical measurements in vivo show net transfer is minimal, around 2 ml/day. Current theories are based on osmotic pressure balances, but changes in maternofetal hydrostatic pressure change much faster than osmotic factors could respond. An alternative explanation might be that net transfer is not in fact the result of passive mechanisms, but is actively controlled by the fetus itself. The fetus is well equipped to monitor changes in blood volume, such as via sensors in venous and atrial stretch receptors to control ANF and hence urine production. Transplacental water regulation requires modification of transvillus pressures. Placental sub-chorial arteries and veins (of extra-embryonic origin) have different sensitivities from fetal body tissues to some vasoactive substances, and stem villous veins have unusually well developed vascular smooth muscle. It is thus theoretically possible for the fetus to modify subchorial venous resistances, and hence villous capillary pressure with a suitable circulating placental venous constrictive agent. A computer modelling study was undertaken using a fictitious placental venous constricting agent “fictensin”, considered to be released by the fetus in proportion to disturbance of vascular volume. The effective placental permeability fell in proportion to the tightness of this fetal control mechanism, suggesting that the apparent placental permeability measured in vivo is a measure of fetal control, not true permeability. However, the range of compensatory pressures that the fetus can produce by this means is limited and failure of such a mechanism could allow flooding or dehydration of the feto-placental unit. This may shed new light on disorders such as polyhydramnios and fetal hydrops.

GERMINATION, SODIUM, AND POTASSIUM CONCENTRATIONS OF BARLEY SEEDS AS INFLUENCED BY SALINITY

Seed germination is a major factor limiting the establishment of plants under saline conditions. Six barley (Hordeum vulgare L.) cultivars were screened for salt tolerance (using NaCl) during seed germination. The uptake of water, and levels of Na+ and K+ of salt sensitive and salt tolerant cultivars, before and after imbibition, were measured to determine the contributions of ionic and osmotic factors to seed different response to salt stress. Imbibition of seeds incubated with NaCl solutions was reduced by approximately 5% for each 100mM increase in NaCl concentration once the level of 100mM NaCl was reached. The Na+ concentration of seeds after imbibition did not differ significantly among cultivars and selective uptake or exclusion of Na+ were not associated with the differences in salinity tolerance. No differences between the two tested cultivars were noted for initial K+ concentration, but the salt tolerant cultivar lost less K+ relative to Na+ uptake than did the salt sensitive cultivar. The adverse effects of salt stress on seed germination might result from internal osmotic or restricted imbibition rather than from ion toxicity effects.

Mathematical investigation of some physiological factors involved in hemodialysis hypotension.

A previously developed mathematical model is used to investigate the role of some hemodynamic, regulatory, and osmotic factors in the development of symptomatic hypotension during hemodialysis. Sensitivity analysis of the model parameters suggests that a decrease in atrial pressure, with a consequent fall in cardiac output (Frank-Starling mechanism), is the primary hemodynamic perturbation induced by ultrafiltration. Also, during the first hours of a hemodialysis session, the sympathetic mechanism working on systemic resistance is the main factor responsible for arterial pressure maintenance, and the physiological response is probably characterized by a prevalence of the cardiopulmonary over the arterial baroreflex control. During this period, a decrease in plasma osmolarity, caused mainly by urea removal, may contribute to the reduction of vascular refilling. During the last hours of the session, the arterial pressure level is also significantly affected by other factors that influence vascular refilling and mean circulatory filling pressure (systemic compliance; action of feedback mechanisms working on venous unstressed volume; plasma oncotic pressure; and, especially, capillary wall permeability and interstitial space elastance). Simulation of hemodialysis with different modalities emphasizes the importance of avoiding high ultrafiltration rates and of maintaining the sodium concentration in the dialysate close to the sodium concentration of the extracellular fluid to limit the risk of symptomatic hypotension. Higher values of sodium in the dialysate are, however, associated with poor sodium removal from the extracellular pool with risks of interdialytic morbidity. In the future, the model may be used to optimize the ultrafiltration rate and sodium profile in the dialysate according to individual patient prescriptions.

Madreporite Function and Fluid Volume Relationships in Sea Urchins.

An effort was made to demonstrate an influx of seawater through the madreporites of sea urchins and to evaluate how such an influx, along with osmotic differences and other factors, could contribute to fluid homeostasis. Fluorescent microbeads placed in the medium of Strongylocentrotus droebachiensis were taken up into the pore canals and stone canal and distributed (in small numbers) to the distal tube feet, confirming a slow bulk inflow of seawater through the madreporite, where it is partially purified. Probably none of this fluid stream is diverted to the perivisceral coelom (as it is in asteroids), since experiments with Strongylocentrotus pallidus showed no significant movement of a soluble fluorescent tracer into that compartment. The osmotic concentrations of the perivisceral coelomic fluids in these two species, and in Strongylocentrotus franciscanus, were higher than that of ambient seawater by 2.66 +/- 0.39 mosmol kg-1 (mean +/- SE). That small hyperosmoticity, along with the net hydrostatic pressure differences induced by the flexing peristome, probably stabilizes body fluid volume. Likewise, fluid in the tube feet of S. franciscanus is elevated by 7.94 +/- 1.04 mosmol kg-1 above seawater, which should contribute to their inflation. Blockage of the madreporite does not lead to an obviously reduced activity of the tube feet, but over the long term, an influx through the madreporite is necessary. Specimens of S. droebachiensis with an obstructed madreporite, fed ad libitum, had significantly (P < 0.006) reduced gut contents vs controls after 28 days, indicating impaired movement or feeding; and the body weights (i.e., volumes) of unfed specimens were significantly (P < 0.013) more reduced after 21 days. Compared to starfish, the rigid test of sea urchins reduces the need for an influx of seawater through the madreporite, but some small admission is still essential.