Cell Osmolality Research Articles

Osmotic adjustment in marine yeast

The effect of environmental salinity on cell growth, and on the composition and accumulation of compatible solutes, or osmotica, of five yeast strains (Aureobasidium pullulans, Candida sp., Cryptococcus albidus var. albidus, Debaryomyces hansenii and Rhodotorula rubra) was compared. All these yeast were isolated from manne environments, but were able to grow in the absence of salt and should therefore be considered as halotolerant strains. According to their specific cell growth rates at different salt concentrations, these strains vary in their capacity to osmotically adjust to modifications in external salinity. Candida sp. appears to be the most sensitive since the maximum salt concentration at which it can grow is 1.54 mol 1-1 NaCl; however, it showed the highest specific cell growth in the range of 0 to 1.54 mol 1-1 NaCl. Aureobasidium pullulans, on the other hand, showed the lowest specific growth rate, but the highest halotolerance range from 0 to 5.13 mol 1-1 NaCl. Debaryomyces hansenii, in contrast, showed higher specific growth at this salinity range Cryptococcus albidus var. albidus and Rhodotorula rubra showed similar specific cell growth rate values and halotolerance between 0 and 2.45 mol 1-1 NaCl. The protein and carbohydrate content of the biomass of the different yeast cells, as a result of external salinity vanation, remained practically constant. The most important effects of the increase in salt concentration in the culture medium were the reduction of cell volume and the accumulation of low-molecular-weight metabolites (LMWM). which appear to act as osmoregulators. Glycerol was found as the major compatible solute in the different marine yeasts studied herein with a total contribution of 64–96% of the internal cell osmolarity. Other LMWM, like carbohydrates and amino acids, contributed to a lesser extent to compensate for the rise in osmotic pressure promoted by the salinity of the external environment.

Cholesterol and mevalonic acid modulation in cell metabolism and multiplication

Cholesterol in animals is a major structural component of cell membranes. It may therefore play a functional role in the modulation of cell osmolarity, the process of pinocytosis and the activities of membrane-associated proteins such as ionic pumps, immune responses, etc. A major relationship exists between the cell-growth processes and the cholesterol biosynthetic pathway. The cholesterol needed for new membranes may be derived either from endogenous synthesis or from exogenous sources, principally plasma low-density-lipoproteins (LDL) which enter the cells by receptor-mediated endocytosis. Both these pathways are enhanced in rapidly growing cells. Conversely, if synthesis is inhibited and no exogenous cholesterol is available, cell growth is blocked. The 3-hydroxy-3-methylglutaryl CoA(HMGCoA) reductase (the rate-limiting reaction in cholesterol biosynthesis) is the enzyme which catalyzes the conversion of HMGCoA to mevalonic acid. It has been suggested that mevalonate may play an important role in cell proliferation. All cells need at least two products synthesized from mevalonate in order to proliferate, and the only one yet identified is cholesterol. Other melavonate-derived potential candidates as cell-cycle and cell-survival products include the dolichols ubiquinone side chains, isopentenyladenosine derivatives, etc. Furthermore, it has recently been shown that membrane association appears to be an important function in mevalonate-derive modifications of several important proteins such as cellular membrane G proteins, those coded for by oncogenes ( ras proteins) and lamins (nuclear proteins). In recent years the development of cholesterol-synthesis-inhibiting drugs, for lowering plasma cholesterol levels has mainly been centred on the control of HMGCoA reductase activity (vastatins). However, because mevalonic acid is the precursor of numerous metabolites, any reduction of such activity may potentiate pleiotropic effects. Vastatins are now, therefore, receiving increased attention as potential pharmacological tools for the control of abnormal cell growth in pathological situations i.e. tumours and vascular smooth muscle cell proliferation under atherogenic conditions. In our laboratories, we have demonstrated that simvastatin can prevent arterial myocyte proliferation both in vivo and in vitro. Simvastatin can also inhibit in vitro the rate of human glioma cell growth, since it shows a strong synergistic inhibitory effect on cell proliferation when used in association with anticancer agents such as Carmustine or β-interferon. Both simvastatin-induced cell growth inhibition and the synergy observed with these drugs can be completely reversed by incubating cells with mevalonate. This shows that the effect of simvastatin of cell proliferation is due to its specific inhibitory activity on intracellular mevalonate synthesis.

Regulation of intracellular potassium in mesangial cells: a fluorescence analysis using the dye, PBFI.

We investigated the regulatory transport processes that maintain intracellular K+ homeostasis in cultured rat glomerular mesangial cells (MCs). Intracellular K+ concentration ([K+]i) of quiescent MCs, passages 3-8, grown to subconfluence on glass cover slips, was assessed by spectrofluorometry using the K(+)-sensitive dye, K(+)-binding benzofuran isophthalate (PBFI). Serum-starved MCs were incubated at 37 degrees C in 5 microM PBFI for 90 min. Excitation ratios of luminescences at 340 and 380 nm, measured at a constant emission at 500 nm, were used to determine [K+]i. Ionophores valinomycin and nigericin were used to clamp [K+]i to known [K+]o and thereby obtain an intracellular calibration of dye. Dependence of fluorescence ratio on [K+]i conformed to Michaelis-Menten behavior, with a Km of 113 mM (n = 40). PBFI retains its sensitivity to alterations in [K+]i with pH change (pHi from 6.5 to 7.5) but is relatively insensitive when intracellular Na+ is greater than 75 mM and cell osmolarity exceeds 500 mM. Normal resting [K+]i for all experiments was determined in MCs to be 102 +/- 7 mM (n = 81) in a HCO3(-)-free HEPES-buffered solution. When MCs were exposed to ouabain, [K+]i fell to 48 +/- 6 mM and did not recover, suggesting presence of Na(+)-K(+)-ATPase. When MCs were exposed to furosemide, [K+]i transiently declined to 58 +/- 11 mM, which was followed by a rapid recovery to near steady state, indicating additional presence of Na(+)-K(+)-Cl- cotransporter. Recovery was completely abolished when MCs were exposed to ouabain.(ABSTRACT TRUNCATED AT 250 WORDS)

Salinity effects on St. Augustinegrass: A novel system to quantify stress response1

Abstract An objective and quantitative batch nutrient culture system was developed to assess responses of St. Augustinegrass (Stenotaphrum secundatum [Walt.] Kuntze) cultivars ‘Seville’ (saline tolerant) and ‘Floratam’ (saline sensitive) to salt stress. Vessels containing individual grass plugs growing in solution culture were randomized in a walk‐in growth chamber. Nutrient solutions were supplemented weekly with NaCl to gradually increase the conductivity over a three week period to treatment levels (2.4 [control], 12.4, 22.4, and 32.4 dS m‐1). Plants were sampled at the onset, and at four and eight weeks during the experiment. Cell sap was extracted from grass blades and measured with a vapor pressure osmometer to determine osmolarity and osmotic adjustment. Length and area of sample shoot and root systems and foliage color changes were measured using an adaptation of video image analysis, and dry weight (DW) was measured to determine relative growth rate (RGR). ‘Seville’ responded to increasing salt concentration with uniform stepwise increases in cell osmolarity, whereas ‘Floratam’ showed strong depression of cell osmotic potential only in the highest conductivity treatment. At high salt levels, overall shoot development was initially more inhibited for ‘Floratam’, although RGR analysis indicated recovery in the susceptible cultivar after long salt stress duration. ‘Seville’ reacted to saline stress with increased root length, whereas roots were actually stunted in saline treatments for ‘Floratam’. Stress induced an increase in the optical density of grass blades for both cultivars although foliage color was not visibly affected. A positive linear correlation (r = 0.81–0.97) was obtained between shoot area as determined by image analysis versus DW measurements. Use of this novel system maximizes assessment of turfgrass stress responses, and quantification of resistance characteristics.

A central role for cell osmolarity in isotonic fluid transport across epithelia.

Previous theoretical models for solute-solvent coupling in epithelia that dealt only with the intercellular channel did not predict isotonic transport except when very high cell membrane permeabilities were assumed. To study this issue, we have developed the formalisms for osmotic equilibration at an alternative location, the apical cell membrane (including its adjacent unstirred layer), which are somewhat simpler than those for the channel. Much as in other models, we confirm that only rather unrealistically high values of the cell membrane permeability lead to isotonic transport. We have also found, however, that isotonic transport can occur at much lower values of the cell membrane permeability if the concentration within the cell differs slightly from that in the ambient medium. This emphasizes the importance of incorporating the intracellular concentration as an integral part to any transport model, such as in the present apical membrane version of local osmosis.

The effect of scorpion venom tityustoxin on the uptake of calcium in synaptosomes

The effect of tityustoxin on calcium uptake in synaptosomes incubated in hypotonie medium low sodium and physiological medium high sodium was studied. In the hypotonie medium, calcium uptake was ATP and temperature dependent and TsTX inhibits this uptake. The inhibition of calcium uptake was not due to an osmotic effect since it was also observed in conditions of physiological cell osmolarity. In the physiological medium high sodium, tityustoxin stimulated calcium uptake in synaptosomes. The effect was inhibited by tetrodotoxin and cocaine. The stimulating effect of tityustoxin was specific for sodium concentration in the medium and neither choline nor sucrose could substitute sodium in its activation of the stimulation of calcium uptake in synaptosomes by tityustoxin.