Morphological and physiological studies of rat kidney cortex slices undergoing isosmotic swelling and its reversal: a possible mechanism for ouabain-resistant control of cell volume

Ultrastructural Changes of Neuroendocrine Pheochromocytoma Cell Line PC-12 Exposed In Vitro to Rotenone

Rotenone is a pesticide used in research for its ability to induce changes similar, in vivo and in vitro, to those observed in Parkinson's disease (PD). This includes a selective death of dopaminergic neurons in the substantia nigra. Nonetheless, the precise mechanism through which rotenone modifies structure and function of neurons remains unclear. The PC12 cells closely resemble dopamine terminal neurons. This makes it a preferred model for studying the morphology of central dopamine neurons and predicting neurotoxicity. In this paper, we investigated the effects of 0.5 µM rotenone for 24-48 h on PC12 cell viability and ultrastructure (TEM), trying to identify primary and more evident alterations that can be related to neuronal damages similar to that seen in animal PD models. Cell viability decreased after 24 h rotenone treatment, with a further decrease after 48 h. Ultrastructural changes included vacuolar degeneration, mitochondrial mild swelling, decrease in the number of neuropeptide granules, and the loss of cell-to-cell adhesion. These findings are in agreement with previous research suggesting that rotenone, by inhibiting energy production and increasing ROS generation, is responsible for significant alterations of the ultrastructure and cell death of PC12 cells. Our data confirm the link between rotenone exposure, neuronal damage, and changes in dopamine metabolism, suggesting its role in the pathogenesis of PD.

Myocardial Aldosterone Receptor and Aquaporin 1 Up-Regulation Is Associated with Cardiomyocyte Remodeling in Human Heart Failure

Background: Abnormal aldosterone signaling is a recognized source of cardiovascular damage. Its influence on cardiomyocyte structure, function, and hormonal receptors when associated with heart failure is still unreported. Methods: Twenty-six consecutive patients with heart failure (LVEF < 40%) and normal coronaries and valves underwent left ventricular endomyocardial biopsy (EMB) for evaluation of myocardial substrate. Biopsy samples were processed for histology, electron microscopy, immunohistochemistry, and Western blot analysis of myocardial aldosterone receptor and aquaporin-1 correlated with plasma aldosterone (AD) and renin activity (PRA). Eight patients with virus-negative inflammatory cardiomyopathy (ICM) had a control EMB after 6 months of immunosuppressive therapy and recovery of cardiac function with re-evaluation of cardiomyocyte structure and receptor expression. Results: EMB in addition to the diagnosis of myocarditis (15 cases), dilated cardiomyopathy CM (6), alcohol CM (2), and diabetic CM (3) showed vacuolar degeneration and cloudy swelling of cardiomyocytes corresponding at electron microscopy to ions and water accumulation into cytosol, membrane-bound vesicles, nucleus, and other organelles, and was associated with an increased AD, PRA, and myocardial expression of aldosterone receptor (2.6 fold) and aquaporin 1 (2.7 fold). In the 8 patients recovered from ICM, cardiomyocyte diameter reduced with disappearance of intracellular vacuoles and normalization of cytosol, nucleus, and cell organelles’ electron-density, along with down-regulation of aldosterone receptor and aquaporin-1. Conclusion: Human heart failure is associated with overexpression of myocardial aldosterone receptor and aquaporin-1. These molecular changes are paralleled by intracellular water overloading and cardiomyocyte swelling and dysfunction. Cardiac recovery is accompanied by down-regulation of hormonal receptors and normalization of cell structure and composition.

Ouabain-induced cytoplasmic vesicles and their role in cell volume maintenance

Cellular swelling is controlled by an active mechanism of cell volume regulation driven by a Na(+)/K(+)-dependent ATPase and by aquaporins which translocate water along the osmotic gradient. Na(+)/K(+)-pump may be blocked by ouabain, a digitalic derivative, by inhibition of ATP, or by drastic ion alterations of extracellular fluid. However, it has been observed that some tissues are still able to control their volume despite the presence of ouabain, suggesting the existence of other mechanisms of cell volume control. In 1977, by correlating electron microscopy observation with ion and water composition of liver slices incubated in different metabolic conditions in the presence or absence of ouabain, we observed that hepatocytes were able to control their volume extruding water and recovering ion composition in the presence of ouabain. In particular, hepatocytes were able to sequester ions and water in intracellular vesicles and then secrete them at the bile canaliculus pole. We named this "vesicular mechanism of cell volume control." Afterward, this mechanism has been confirmed by us and other laboratories in several mammalian tissues. This review summarizes evidences regarding this mechanism, problems that are still pending, and questions that need to be answered. Finally, we shortly review the importance of cell volume control in some human pathological conditions.

Effects of medium calcium, and agents affecting cytoskeletal function, on cellular volume and morphology in liver tissue in vitro

The possible role of an exocytotic, vesicular mechanism in cellular volume regulation under iso-osmotic conditions has been studied in slices of rat liver. The effects of incubation conditions and agents affecting the actin cytoskeleton were examined for changes of water, ionic composition, and ultrastructure. Slices were pre-incubated at 1°C in an iso-osmotic buffered medium to induce swelling. Upon restoration to 37°C in the same medium, tissue lost water. The Na+-K+ adenosine triphosphatase (ATPase) inhibitor ouabain inhibited water extrusion of about 50%, an effect that was accompanied by the formation of characteristic vesicles in the cytoplasmic region between the Golgi apparatus and the bile canaliculi. Water extrusion in the presence of ouabain was partially inhibited by trifluoroperazine and completely inhibited when the medium was free of Ca2+. In the presence of ouabain, brefeldin A caused a small reduction of water extrusion, whereas phalloidin and cytochalasins A, D, or E caused a marked inhibition. In these conditions there was a marked increase in size and number of cytoplasmic vesicles and a more widespread distribution of them within the cells, lacking the more specific orientation to the Golgi and canalicular regions that was seen in the presence of ouabain alone. Water extrusion was inhibited by phalloidin and cytochalasins in the absence of ouabain. In conclusion, our results are consistent with the hypothesis that iso-osmotic expulsion of water from hepatocytes can proceed partly through an accumulation of water in cytoplasmic vesicles, followed by exocytosis. This mechanism does not depend on Na+-K+ ATPase activity.

Effect of arachidonic acid metabolic inhibitors on hypoxia/reoxygenation-induced renal cell injury

The present study was undertaken to examine the role of arachidonic acid (AA) metabolites in hypoxia/reoxygenation (H/R)-induced renal cell injury in rabbit renal cortical slices using AA metabolic inhibitors. Inhibitors of cyclooxygenase (indomethacin and diclofenac sodium) and lipoxygenase pathways (nordihydroguaiaretic acid, caffeic acid, and eicosapentaenoic acid) reduced H/R-induced LDH release in a dose-dependent manner, whereas an inhibitor of cytochrome P-450 monooxygenase pathway ethoxyresorufin was not effective. AA increased LDH release in control slices, and the effect was not altered by indomethacin and nordihydroguaiaretic acid. The protective effect of indomethacin was not affected by addition of PGE2, a main product of cyclooxygenase pathway in the kidney. H2O2-induced LDH release was prevented by inhibitors of lipoxygenase but not by inhibitors of cyclooxygenase and cytochrome P-450 monooxygenase H/R-induced LDH release was not altered by iron chelators, phenanthroline and deferoxamine, and a potent antioxidant, N,N'-diphenyl-p-phenylenediamine, suggesting that the H/R-induced cell injury is not attributed to a generation of reactive oxygen species. Morphological studies showed that H/R-induced structural changes including cell necrosis were significantly prevented by indomethacin. These results suggest that inhibitors of cyclooxygenase and lipoxygenase pathways exert a direct protective effect against the H/R-induced cell injury in renal tubules. Whether these effects are mediated by alterations of AA metabolic pathways is not certain.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

The response of contractile and non-contractile vacuoles of Paramecium calkinsi to widely varying salinities

Paramecium calkinsi from tidal marshes survive a wide salinity range. Fluid output of contractile vacuoles of these cells decreased as salinity of the medium to which they were acclimated increased, and both pulse rate and vacuole volume were used to regulate output. When cells were first exposed to more dilute medium, contractile vacuoles greatly increased volume so that fluid output increased even though pulse rate decreased. In cells shifted to a more concentrated medium, contractile vacuole output decreased by decreasing pulse rate. The contractile vacuole is surrounded by a set of collecting structures which change form as the salinity changes. Distensible ampullae are found in media of low salinity and collecting canals are found in media of high salinity. When cells are shifted from high salinity to low, the number of ampullae increases and the number of canals decreases. When cells are shifted from low salinity to high, the number of ampullae decreases and the number of canals decreases. Other non-contracting vacuoles also appear in response to a hypoosmotic shock. These include vacuoles within the cell as well as "blisters" on the surface. The number and frequency of blisters increases with the size of the hypoosmotic shock. They detach from cells without resulting in any visible loss of cytoplasm. Non-contractile vacuoles may play a role in sequestering and removing excess water that the contractile vacuoles cannot handle.

Selected aspects of cell volume control in renal cortical and medullary tissue

Under normal physiological conditions, demands placed on mammalian renal cortical cells are quite different from those in the medulla. Cortical proximal tubule cells exist in an isotonic environment, but must resorb vast amounts of filtered fluid and solute, and also adjust to solute generated from cellular metabolism. In addition, cortical cells must also adjust to occasional pathological derangements in blood osmolality. By contrast, human medullary cells have a smaller solute resorptive load, but exist in a milieu where osmolality varies from 40 to more than 1200 mosmol/kg H2O, depending on water intake. Remarkably, the cells maintain a near normal size despite these stresses. Under isosmotic conditions, the primary regulator of cell volume is Na-K ATPase. In its absence, factors such as external protein, extracellular matrix and basement membrane, cytoskeleton, and perhaps formation of cytoplasmic vesicular-like structures help prevent cells from swelling massively. Under anisosmotic conditions, a variety of transport processes operating across basolateral and apical membranes either remove solute from or add solute (and water) to cells to minimize changes in their size. Medullary cells have the additional ability to accumulate organic, non-toxic, osmolytes that offset external hypertonicity and allow cells to maintain normal size without increasing cellular inorganic ion concentrations.

In vitro measurements of heat production rate of rat kidney. A microcalorimetric study

In the present study heat production rate (P) of rat kidney tissue in vitro has been measured with a microcalorimetric technique. Cortex slices (0.3-0.7 mm) were incubated in a balanced electrolyte solution or in medium E-199. The metabolic activities in these different media were found to be 0.87 microW/mg and 0.98 microW/mg, respectively, determined by using 5-mg slices after 1 h of incubation (P1h). Medulla slices with a thickness of 0.4 mm demonstrated a lower P, i.e., 0.65 microW/mg. The heat output was not influenced by the thickness of those slices, ranging from 0.3 to 0.7 mm. However, an increase in the wet weight of the tissue sample was accompanied by a reduced P, expressed as power per milligram wet weight. The magnitude of this crowding phenomenon decreased as the time of incubation increased. Storage of tissue for 3.5 h prior to measurement caused a 6-7% reduction of P. This study indicates that the microcalorimetric technique is a sensitive method for measuring metabolism in small kidney biopsies.

Effect of a high Na+ diet on cell volume and Na(+)-stimulated ATPase activities of rat kidney membranes

Proximal tubular cells from kidneys of male rats chronically fed with an isotonic NaCl solution, show a volume increase which is dependent on the length of the treatment with NaCl, when compared with control rats. Parallel to the cell volume increase, there is an increase of the ouabain-insensitive Na-ATPase activity, whereas the ouabain-sensitive Na,K-ATPase activity remains unchanged. These results establish a clear relationship between a chronic Na-diet, kidney cell volume and Na-ATPase activity.

Ouabain-insensitive, Na-ATPase activity in pure suspensions of rat kidney proximal tubules

The present work was undertaken to evaluate the distribution of the Na-ATPase activity in the different components of the rat kidney cortex. Suspensions of glomeruli, proximal and distal tubules were prepared following a collagenase digestion of outermost kidney cortex slices and a separation on a Percoll gradient. It was found that the Na-ATPase activity is higher in the fraction enriched in proximal tubules. The fraction enriched in glomeruli and in distal tubules show also a Na-ATPase activity, but it is lower.

Effects of ouabain on potassium transport and cell volume regulation in rat and rabbit liver

1. We have examined whether the apparently ouabain-resistant fraction of cellular volume regulation in liver slices under isosmotic conditions is due to a failure of ouabain to cause complete inhibition of the coupled transport of Na+ and K+. The ion and water contents of rat and rabbit liver slices were altered by pre-incubation at 1 degree C and then allowed to recover at 38 degrees C, with or without ouabain or other inhibitors. The net movements of ions and water were determined during the recovery. The influx of 86Rb under steady-state conditions was taken as a measure of unidirectional influx of K+. 2. Concentrations of ouabain for half-maximal inhibition of 86Rb influx were 0.15 mM for rat and 0.15 microM for rabbit liver slices, with maximal inhibition at 2 mM and 10 microM respectively. Inhibition of net K+ reaccumulation closely followed inhibition of 86Rb influx. 3. The 86Rb influx persisting in the presence of maximally inhibiting concentrations of ouabain was not reduced by inhibitors of cellular respiration or glycolysis. 4. In rat liver slices, about 50% of net water extrusion was resistant to 2 mM-ouabain; rabbit liver showed a much smaller, but statistically significant, extrusion of water in the presence of 10 microM-ouabain. 5. In rat liver slices, a small, net uptake of K+ continued in the presence of amytal alone, when water extrusion was completely inhibited; by contrast, ouabain gave complete inhibition of K+ uptake while permitting 50% of the control water extrusion. 6. Isolated rat hepatocytes in primary culture were pre-incubated at 4 degrees C for 20 h. They recovered their original K+ content within 60 min of restoration to 37 degrees C. Ouabain, 1-2 mM, completely prevented this recovery. 7. The results imply that ouabain completely inhibits the coupled transport of Na+ and K+ in both rat and rabbit liver slices. Thus, the fraction of total water extrusion continuing in the presence of maximally inhibiting concentrations of ouabain is the consequence of a truly ouabain-resistant mechanism.

Na-pump activity in rat kidney cortex cells and its relationship with the cell volume

The present work was undertaken to evaluate whether changes in cell water content of rat kidney cortex cells can modulate the transport activity of the ouabain-insensitive Na pump as they modulate the ouabain-insensitive Na+-ATPase. It was found that there is a close relationship between the cell volume and activity of the Na pump, whereas Na,K-pump activity is not affected by variations in cell volume. When the cell water content is low, Na-pump activity (Na+ transport and Na+-ATPase activity) is minimal. Increases in cell water content produce a concomitant increase in Na-pump activity.

Cell volume-sensitive Na+-ATPase activity in rat kidney cortex cell membranes

A ouabain-insensitive, K+-independent, sodium pump, has been demonstrated in guinea-pig and rat kidney proximal tubular cells. This pump is thought to be distinct from the ouabain-sensitive Na+/K+ pump. We present evidence here indicating the modulation of the biochemical expression of the Na+ pump, i.e. the ouabain-insensitive Na+-ATPase, by the cell volume in rat kidney proximal tubular cells. Thus, basolateral plasma membranes from swollen cells show a ouabain-insensitive Na+-ATPase activity 10-times higher than that in membranes from control cells. If the swollen cells recover their volume, the activity decreases ten times to control values. The ouabain-sensitive Na+/K+-ATPase is not affected by changes in the cell volume.

High sodium diet and Na+-stimulated ATPase activities in basolateral plasma membranes from rat kidney proximal tubular cells

The ouabain-insensitive, Na+-stimulated ATPase activity of kidney proximal tubular cells from rats fed a high Na+ diet for 4 months was increased approximately 70% when compared with control (normal diet) rats. The higher ATPase activity was not due to a change in the affinity of the system toward ATP, Mg2+ or Na+. This increase in Na+-ATPase activity may be due to either a higher number of pumps or to a higher turnover rate of the enzyme or both. The ouabain-sensitive, Na+, K+-stimulated ATPase activity, on the other hand, did not change with the high sodium diet. These results can be taken as evidence that the Na+,K+-ATPase and the Na+-ATPase of basolateral plasma membranes of proximal tubular cells from rat kidney are two different entities.

Transcytosis in cultured proximal tubular cells

Studies were designed to examine fluid-phase pinocytosis in proximal tubular cells. Canine proximal tubules were obtained from the band IV of Percoll gradient centrifugation of the dispersed renal cortex, and were seeded on collagen-coated polycarbonate membranes. Integrity of monolayers was confirmed by electrophysiologic measurements, and by scanning electron microscopy. At confluence cell monolayers were studied in Ussing chambers. The rate of transfer of a marker of fluid-phase pinocytosis, Lucifer Yellow CH, from the luminal to the basolateral bath was three times higher than that occurring in the opposite direction. Fluorescence microscopy demonstrated that Lucifer Yellow was trapped exclusively in the vesicular compartment. Electron microscopy of the monolayers incubated with cationized ferritin added to the luminal or to the basolateral both revealed that endocytic vesicles were formed only at the luminal surface. Luminal-to-basolateral transfer of Lucifer Yellow was almost completely blocked at 0 degrees C, and was significantly diminished by K+ depletion. Transcytosis of Lucifer Yellow was stimulated twofold by 1-oleoyl-2-acetyl-glycerol. Transfer of quin-2 acetoxymethylester across the monolayer was used as a marker of the paracellular pathway, demonstrating the lack of directional selectivity of this transport route. In summary, vectorial fluid-phase pinocytosis in proximal tubular cells represents an additional mechanism contributing to fluid transport in this segment of the nephron.