Analysis of structural changes during hypotonic swelling in Ehrlich ascites tumor cells

Control of Insulin Secretion and Glucose Homeostasis in Exercising Diabetic Rats with Intrasplenic or Kidney Subcapsular Islet Grafts

This study was designed 1) to investigate mechanisms of insulin secretion during exercise after transplantation of islets in the spleen and under the kidney capsule, and 2) to compare these organs as transplantation site regarding an adequate portal or systemic delivery of insulin and glucose homeostasis during exercise. Diabetic rats were provided with 5 μL isogenic islet tissue in the spleen or under the kidney capsule, which results in normoglycemia, and were submitted to a swimming test Portal plasma insulin levels were higher than simultaneously sampled systemic insulin levels in the control and in the intrasplenic islet grafted group, but not in the kidney subcapsular islet-grafted group. Plasma portal and systemic insulin levels decreased, and glucose levels increased during exercise in all groups. The exercise-induced increase in levels of catecholamines was larger in systemic than in portal plasma, suggesting catecholamine extraction by the lungs or intestines. The experiments were repeated after removing of adrenal medulla, resulting in nondetectable or very low plasma adrenaline levels. Despite these low adrenaline levels, insulin levels decreased during exercise. The results indicate that 1) the exercise-induced reduction of insulin secretion is not mediated by circulating adrenaline, but is probably under control of the sympathetic nervous system, which could be the result of reinnervation of the transplanted islets. 2) Although a portal-systemic insulin gradient was absent in rats with kidney subcapsular islet grafts, the absence of a difference in glucose homeostasis during exercise between the sites revealed that all investigated sites are preferential to transplant islets.

The Ste20 kinases SPAK and OSR1 travel between cells through exosomes

Proteomics studies have identified Ste20-related proline/alanine-rich kinase (SPAK) and oxidative stress response 1 (OSR1) in exosomes isolated from body fluids such as blood, saliva, and urine. Because proteomics studies likely overestimate the number of exosome proteins, we sought to confirm and extend this observation using traditional biochemical and cell biology methods. We utilized HEK293 cells in culture to verify the packaging of these Ste20 kinases in exosomes. Using a series of centrifugation and filtration steps of conditioned culture medium isolated from HEK293 cells, we isolated nanovesicles in the range of 40-100 nm. We show that these small vesicles express the tetraspanin protein CD63 and lack endoplasmic reticulum and Golgi markers, consistent with these being exosomes. We show by Western blot and immunogold analyses that these exosomes express SPAK, OSR1, and Na-K-Cl cotransporter 1 (NKCC1). We show that exosomes are not only secreted by cells, but also accumulated by adjacent cells. Indeed, exposing cultured cells to exosomes produced by other cells expressing a fluorescently labeled kinase resulted in the kinase finding its way into the cytoplasm of these cells, consistent with the idea of exosomes serving as cell-to-cell communication vessels. Similarly, coculturing cells expressing different fluorescently tagged proteins resulted in the exchange of proteins between cells. In addition, we show that both SPAK and OSR1 kinases entering cells through exosomes are preferentially expressed at the plasma membrane and that the kinases in exosomes are functional and maintain NKCC1 in a phosphorylated state.

Myosin light chain kinase and Src control membrane dynamics in volume recovery from cell swelling

MLCK resolves membrane blebbing induced by osmotic cell swelling. Cell swelling also stimulates the formation of a Src–MLCK complex, which with cortactin and dynamin forms actin-based structures at the base of the cell to facilitate membrane retrieval for volume recovery.

 The expansion of the plasma membrane, which occurs during osmotic swelling of epithelia, must be retrieved for volume recovery, but the mechanisms are unknown. Here we have identified myosin light chain kinase (MLCK) as a regulator of membrane internalization in response to osmotic swelling in a model liver cell line. On hypotonic exposure, we found that there was time-dependent phosphorylation of the MLCK substrate myosin II regulatory light chain. At the sides of the cell, MLCK and myosin II localized to swelling-induced membrane blebs with actin just before retraction, and MLCK inhibition led to persistent blebbing and attenuated cell volume recovery. At the base of the cell, MLCK also localized to dynamic actin-coated rings and patches upon swelling, which were associated with uptake of the membrane marker FM4-64X, consistent with sites of membrane internalization. Hypotonic exposure evoked increased biochemical association of the cell volume regulator Src with MLCK and with the endocytosis regulators cortactin and dynamin, which colocalized within these structures. Inhibition of either Src or MLCK led to altered patch and ring lifetimes, consistent with the concept that Src and MLCK form a swelling-induced protein complex that regulates volume recovery through membrane turnover and compensatory endocytosis under osmotic stress.

Mechanism of efficient transfection of the nasal airway epithelium by hypotonic shock

The main barrier to gene transfer in the airway epithelium is the low rate of apical endocytosis limiting naked DNA uptake. Deionized water is known to stimulate the exocytosis of numerous intracellular vesicles during hypotonic cell swelling, in order to expand plasma membrane and prevent cell lysis. This is followed by the phase of regulatory volume decrease (RVD), during which the excess plasma membrane is retrieved by intensive endocytosis. Here we show that the more hypotonic the DNA solution, the higher the transfection of the nasal tissue. P2 receptors are known to be involved in RVD and we demonstrate that some P2 agonists and a P2 antagonist impair transfection in a time-dependent manner. Our study strongly suggests that the nasal airway epithelial cells take up plasmid DNA in deionized water during RVD, within approximately half an hour. Our simple gene delivery system may constitute a promising method for respiratory tract gene therapy.

Fat metabolism in exercise - with special reference to training and growth hormone administration

Despite abundance of fat, exclusive dependency on fat oxidation can only sustain a metabolic rate corresponding to 50-60% of VO(2max) in humans. This puzzling finding has been subject to intense research for many years. Lately, it has gained renewed interest as a consequence of increased obesity and physical inactivity imposed by Western lifestyle. Why are humans so poor at metabolizing fat? Can fat metabolism be manipulated by exercise, training, diet and hormones? And why is fat stored in specialized adipose tissue and not just as lipid droplets inside muscle cells? In the present review, human fat metabolism is discussed in relation to how human fat metabolism is designed. Limitations in this design are explored and examples of different designs for fat metabolism from animal physiology are included to illustrate these limitations. Various means of manipulating fat metabolism are discussed with special emphasis on exercise, training, growth hormone (GH) physiology and GH administration. It is concluded that fat stores, non-esterified fatty acids (NEFAs) availability and enzymes for fat oxidation can be increased substantially. However, it is almost impossible to increase fat oxidation during endurance exercise at higher intensities. It seems that, for some reason, the human being is far from optimally designed for fat oxidation during exercise. Acute GH administration has several unexpected effects on fat and carbohydrate metabolism during aerobic exercise, and future research in this area is likely to provide valuable information with respect to GH physiology and the regulation of fat and carbohydrate metabolism during aerobic exercise.

Physiological significance of volume-regulatory transporters

Research over the past 25 years has identified specific ion transporters and channels that are activated by acute changes in cell volume and that serve to restore steady-state volume. The mechanism by which cells sense changes in cell volume and activate the appropriate transporters remains a mystery, but recent studies are providing important clues. A curious aspect of volume regulation in mammalian cells is that it is often absent or incomplete in anisosmotic media, whereas complete volume regulation is observed with isosmotic shrinkage and swelling. The basis for this may lie in an important role of intracellular Cl- in controlling volume-regulatory transporters. This is physiologically relevant, since the principal threat to cell volume in vivo is not changes in extracellular osmolarity but rather changes in the cellular content of osmotically active molecules. Volume-regulatory transporters are also closely linked to cell growth and metabolism, producing requisite changes in cell volume that may also signal subsequent growth and metabolic events. Thus, despite the relatively constant osmolarity in mammals, volume-regulatory transporters have important roles in mammalian physiology.

Neuronal swelling and surface area regulation: elevated intracellular calcium is not a requirement

Neurons are mechanically robust. During prolonged swelling, molluscan neurons can triple their apparent membrane area. They gain surface area and capacitance independent of extracellular Ca concentration ([Ca]e), but it is unknown if an increase in intracellular Ca concentration ([Ca]i) is necessary. If Ca for stimulating exocytosis is unnecessary, it is possible that swelling-induced membrane tension changes directly trigger surface area readjustments. If, however, Ca-mediated but not tension-mediated membrane recruitment is responsible for surface area increases, swelling neurons should sustain elevated levels of [Ca]i. The purpose of this investigation is to determine if the [Ca]i in swelling neurons attains levels high enough to promote exocytosis and if any such increase is required. Lymnaea neurons were loaded with the Ca concentration indicator fura 2. Calibration was performed in situ using 4-bromo-A-23187 and Ca-ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), with free Ca concentration ranging from 0 to 5 microM. Swelling perturbations (medium osmolarity reduced to 25% for 5 min) were done at either a standard [Ca]e or very low [Ca]e level (0.9 mM or 0.13 microM, respectively). In neither case did the [Ca]i increase to levels that drive exocytosis. We also monitored osmomechanically driven membrane dynamics [swelling, then formation and reversal of vacuole-like dilations (VLDs)] with the [Ca]i clamped below 40 nM via 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). [Ca]i did not change with swelling, and VLD behavior was unaffected, consistent with tension-driven, [Ca]i-independent surface area adjustments. In addition, neurons with [Ca]i clamped at 0.1 microM via an ionophore could produce VLDs. We conclude that, under mechanical stress, neuronal membranes are compliant by virtue of surface area regulatory adjustments that operate independent of [Ca]i. The findings support the hypothesis that plasma membrane area is regulated in part by membrane tension.

Hypotonic stimulation induced Ca2+ release from IP3-sensitive internal stores in a green monkey kidney cell line

1. Hypotonic stimulation (180 +/- 5 mosmol l-1) increased [Ca2+]i in fura-2-loaded Green monkey kidney cells (COS-7 cells) and depolarized the membrane. 2. COS-7 cells were depolarized up to -3.5 +/- 4.4 mV from a resting membrane potential of -35.2 +/- 2.3 mV in response to hypotonic stimulation, when the patch electrode was filled with a 160 mM KCl-0.5 mM EGTA-based intracellular medium. 3. The increase in [Ca2+]i induced by hypotonic stimulation was divided into two phases. One was transient and oscillatory, and observed in Ca(2+)-free medium; the other was persistent, blocked by 100 microM La3+, and observed only in Ca(2+)-containing medium. 4. The increase in [Ca2+]i in Ca(2+)-free medium was blocked by pretreatment with 10 microM thapsigargin. The increase in [Ca2+]i induced by 10 microM thapsigargin was reduced after hypotonic stimulation which induced an increase in [Ca2+]i in Ca(2+)-free medium. 5. The increase in [Ca2+]i in Ca(2+)-free medium was not affected by treatment with 5 mM caffeine or 1-10 microM ryanodine. Neither caffeine nor ryanodine induced an increase in [Ca2+]i. 6. Adenosine 5'-O-2-thiodiphosphate (ADP-beta-S; a P2Y receptor agonist) induced an increase in [Ca2+]i in Ca(2+)-free medium and caused phosphoinositide breakdown in COS-7 cells. Exposure to 10 microM ADP-beta-S blocked the increase in [Ca2+]i induced in the Ca(2+)-free medium by hypotonic stimulation. The results of summary points 4, 5, and 6 suggest that the increase in [Ca2+]i induced by hypotonic stimulation is due to Ca2+ release from inositol 1,4,5-trisphosphate (IP3)-sensitive internal stores. 7. The hypotonic stimulation-activated hydrolysis of phosphoinositides was decreased by pertussis toxin (PTX) in a dose-dependent manner. 8. These observations strongly suggest that hypotonic stimulation induced an increase in [Ca2+]i in Ca(2+)-free medium through activation of cascades using PTX-sensitive guanine nucleotide binding protein (G protein) and IP3.

A voltage-gated chloride channel in ascidian embryos modulated by both the cell cycle clock and cell volume

1. Eggs of the ascidian Boltenia villosa have an inwardly rectifying Cl- current whose amplitude varies by more than 10-fold during each cell cycle, the largest amplitude being at exit from M-phase. We examined whether this current was also sensitive to changes in cell volume. 2. Cell swelling, produced by direct inflation through a whole-cell recording pipette, greatly increased the amplitude of the Cl- current at all stages of the cell cycle in activated eggs. Swelling was much less effective in unfertilized eggs. 3. The increase in Cl- current amplitude continued for 10-20 min after an increase in diameter that was complete in 10 s, suggesting the involvement of a second messenger system in the response. 4. Treatment of unfertilized eggs with 6-dimethylaminopurine (DMAP), an inhibitor of cell cycle-dependent protein kinases, increased the amplitude of the Cl- current and its sensitivity to swelling to levels characteristic of fertilized eggs. 5. Osmotically produced swelling also increased Cl- current amplitude in unfertilized eggs. 6. We propose that dephosphorylation renders the Cl- channel functional, and that swelling or activation of the egg increases the sensitivity of the channel to dephosphorylation, perhaps by disrupting its links to the cytoskeleton.

Responses of neurons to extreme osmomechanical stress

Neurons are often regarded as fragile cells, easily destroyed by mechanical and osmotic insult. The results presented here demonstrate that this perception needs revision. Using extreme osmotic swelling, we show that molluscan neurons are astonishingly robust. In distilled water, a heterogeneous population of Lymnaea stagnalis CNS neurons swelled to several times their initial volume, yet had a ST50 (survival time for 50% of cells) > 60 min. Cells that were initially bigger survived longer. On return to normal medium, survivors were able, over the next 24 hr, to rearborize. Reversible membrane capacitance changes corresponding to about 0.7 muF/cm2 of apparent surface area accompanied neuronal swelling and shrinking in hypo- and hyperosmotic solutions; reversible changes in cell surface area evidently contributed to the neurons' ability to accommodate hydrostatic pressures then recover. The reversible membrane area/capacitance changes were not dependent on extracellular Ca2+. Neurons were monitored for potassium currents during direct mechanical inflation and during osmotically driven inflation. The latter but not the former stimulus routinely elicited small potassium currents, suggesting that tension increases activate the currents only if additional disruption of the cortex has occurred. Under stress in distilled water, a third of the neurons displayed a quite unexpected behavior: prolonged writhing of peripheral regions of the soma. This suggested that a plasma membrane-linked contractile machinery (presumably actomyosin) might contribute to the neurons' mechano-osmotic robustness by restricting water influx. Consistent with this possibility, 1 mM N-ethyl-maleimide, which inhibits myosin ATPase, decreased the ST50 to 18 min, rendered the survival time independent of initial size, and abolished writhing activity. For neurons, active mechanical resistance of the submembranous cortex, along with the mechanical compliance supplied by insertion or eversion of membrane stores may account for the ability to withstand diverse mechanical stresses. Mechanical robustness such as that displayed here could be an asset during neuronal outgrowth or regeneration.

Volume-sensitive chloride conductance in bovine chromaffin cell membrane

1. Bovine chromaffin cells were inflated by pressure applied through a pipette or swollen during intracellular perfusion with hypertonic solutions. Effects of such procedures on electrical properties of the membrane were studied by a combination of the tight-seal whole-cell patch-clamp technique and Fura-2 fluorescence measurements of free intracellular calcium concentration ([Ca2+]i). 2. Application of air pressure (about +5 cmH2O or 490 Pa) through the patch pipette caused an increase in the cell volume and concomitant development of an inwardly directed transient current at the holding potential of -60 mV. The current gradually increased to a peak value and subsequently decayed almost to its initial level within 5-10 min. A short pulse of pressure (5-10 s) was sufficient to elicit the whole sequence of events. 3. Intracellular free Ca2+ ion concentration, [Ca2+]i, steeply increased at the beginning of the pressure pulse to about 0.2 microM and either stayed at this level or decayed back to the more usual value of approximately 0.1 microM. 4. Similar changes in the transmembrane current and [Ca2+]i were observed during intracellular perfusion of cells with hypertonic solutions (30-50 mosM difference relative to the bath solution) or during extracellular application of hypotonic solution. 5. Swelling of non-perfused cells by extracellular application of hyposmotic solution caused the appearance of inward currents in cell-attached membrane patches held at a fixed potential -30 mV relative to the cell's resting potential. The kinetics of the current resembled those of the whole-cell current. 6. Intracellular introduction of guanosine triphosphate (GTP, 300 microM) significantly prolonged the duration (from 62 +/- 10 s, n = 5, to 98 +/- 8 s, n = 4, when measured at the level of half-amplitude), while introduction of the non-hydrolysable analogue of guanosine diphosphate (GDP), guanosine 5'-O-(2-thiodiphosphate) (GDP beta S, 300 microM), decreased the maximal rate of increase (from 11.4 +/- 2.6 pA/s, n = 6, to 3.2 +/- 2.1 pA/s, n = 10) of the current activated by pressure. 7. Lowering of the intracellular free Ca2+ ion concentration by introduction of 10 mM-EGTA did not significantly affect the current amplitude or time course. However, a rapid increase in the [Ca2+]i to micromolar levels (by activation of the voltage-operated calcium channels during membrane depolarization) could terminate development of the current activated by pressure and cause its fast decay to zero-current level.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural changes induced by osmotic water flow in rabbit proximal tubule

When a transepithelial osmotic difference was imposed in perfused proximal straight tubules (270 mOsm/kg H2O in the lumen and 290 in the bath) in the absence of bath colloid, a severe vacuolation (appearance of lucent spaces) developed within the epithelium such that view of the lumen border was obscured within 5 +/- 1 min (N = 13 tubules at 23 degrees C). This vacuolation was less severe if the bath was hypotonic to the lumen or if the magnitude of the osmotic difference was reduced. If colloid (6% wt/vol of either bovine serum albumin or 70,000 molecular wt dextran) was included in the bathing medium, vacuolation was either not observed or was minimal, but became severe upon removal of the colloid and obscured the lumen within 6 +/- 1 min (N = 8 for albumin and N = 4 for dextran at 23 degrees C). At 38 degrees C, vacuolation obscured the lumen within 4 +/- 1 min following the removal of albumin (N = 5). ANOVA suggests that none of the times for vacuolation to occur differed. The rate of passive volume flow due to the osmotic difference was unaffected by vacuolation (0.9 +/- 0.1 nl.min-1.mm-1 with albumin to 0.8 +/- 0.1 without albumin and vacuolated, N = 8 at 23 degrees C, P greater than 0.2 using a paired t-test). Electron microscopic examination of tubules fixed after vacuolation showed lucent spaces within the cytoplasm. These results suggest that the presence of serosal colloid protected the epithelial cells from injury during rapid transepithelial water flow. The mechanism for this protective effect is not apparent, but may be related to effects of colloid in maintaining normal volume absorption in the proximal nephron.