Tonicity is the capability of a solution to modify the volume of cells by altering their water content. Cells swell by gaining water (hypotonicity) or shrink by losing water (hypertonicity) through osmotic pressure differences between the intracellular compartment (IC) and the solution tested. Solutions are isotonic when the volume of cells suspended in them does not change by osmotic fluid transfers.
Osmosis refers to water movement, which usually moves in favor of an osmotic gradient, so that water flows where there is higher osmotic concentration across a selectively permeable membrane.
Osmolarity is the term used for describing the concentration of solutes within a fluid. Meanwhile, the words isotonic, hypertonic and hypotonic compare the osmolarity of a cell to the osmolarity of the extracellular fluid around it. Hyperosmolarity doesn't always mean hypertonicity because this depends on the solutes. For example, solutes such as Na+ and glucose need transporters, they contribute to serum tonicity and are termed effective osmoles (contributing to osmolarity), while urea and ethanol easily pass through cell membranes, thus contributing to serum osmolality but not tonicity.
Disturbances in tonicity are the major clinical disorders affecting volume, proper function and survival of body cell. It causes water movement into or out of cells, thereby diluting or concentrating intracellular ions. These alterations in the homeostasis of the cells lead to a set of mechanisms to maintain their normal size as well as their functioning. Hyperosmolality itself alters several intracellular processes including cell volume regulation, cell cycle, intracellular ion homeostasis, macromolecular and nucleic acid stability and can induce apoptosis.
Hyperglycemic and hypernatremic states are the main responsible for the alteration in tonicity. Cell shrinking secondary to hypertonicity can cause severe clinical manifestations and even death. Assessment of the clinical circumstances along with serum and urine studies help to determine the etiology and to aim for the right management.
In a hypertonic environment, cells lose water through osmotic pressure differences using membrane proteins called aquaporin channels, to go to the higher concentration medium. Because cells are permeable to water, they shrink and elevate the concentration of intracellular solutes. The survival mechanisms used by cells include accumulation of organic osmolytes and increased expression of proteins through numerous pathways, resulting in osmotolerance. The shrinkage of cells generates stress that is adjusted by some mechanisms due to the work of the Na+,K+-ATPase (in steady conditions). "Fast" volume regulation due to rapid activation of membrane ion transporters and "slow" adaptation to chronic changes in extracellular osmolarity involving modifications in gene expression and intracellular organic osmolyte content.
In steady conditions, a fundamental property of cells is that they contain a significant amount of large-molecular-weight anionic colloids (mostly proteins and organic phosphates) to which the plasma membrane is impermeable. So the existence of these proteins in only one of the compartments that are impermeable to them produces a higher concentration, thus generating osmotic forces between the extracellular and intracellular compartment (this is known as Donnan effect).
Hypertonicity denotes relative excess of the solute with extracellular distribution over body water regardless of whether body water is normal, reduced or excessive. The gain of extracellular solutes leads to the osmotic exit of water from the intracellular compartment to dilute the extracellular solutes. Sodium salts, which includes sodium chloride and sodium bicarbonate, are the major extracellular solutes and routinely indicates hypertonicity.
Under hypertonic conditions, ions such as Na, Cl, and K accumulate in the cytosol and get exchanged for compatible organic osmolytes that do not perturb intracellular protein structure or function. A fast response gets driven by the fast activation of Na+,K+,2Cl- co-transporter and the Na+/H+ exchanger which couples to Cl-/H2CO3- anion exchanger.
On the other hand, TonEBP (tonicity-responsive enhancer binding protein) which is also known as NFAT5 or OREBP, is a transcription factor that can promote cellular accumulation of organic osmolytes in the hypertonic renal medulla by stimulating expression of its target genes, but it is also abundantly expressed in brain, heart, liver and activated T-cells.
Additionally, there is evidence that osmotic stress elicits a morphological disruption of the transverse tubular system (a continuation of the surface membrane that forms a junction with the sarcoplasmic reticulum which is known for its calcium storage) in skeletal muscle fibers. It is the primary interface between the myoplasm and the extracellular environment and these arrangements are essential to produce muscle contraction.
Regulation of osmolarity and volume play an essential role in maintaining body water balance and tonicity.
The acute adaptation to hypertonicity consists in ''regulatory volume increase'' (RVI). It requires the activation of the Na+,K+,2Cl- co-transporter and the Na+/H+ exchanger which couples to the Cl-/H2CO3- anion exchanger. These last two brings NaCl and KCl into the cell and H2CO3 out of it. This H2CO3 is then converted to CO2 to go back to the pool of H+ and HCO3- inside the cells, followed by the thermodynamically obliged movement of water. Sodium ion entering the cells is extruded through Na/K ATPase in exchange for potassium, being potassium chloride which is the final salt gained intracellularly in hypertonicity.
Meanwhile, in a chronic adaptation, a general response to hypertonicity is the activation of the transcription factor tonicity (TonEBP), leading to cells to increase the expression of organic osmolyte transporters and enzymes. Some of the transcriptions of genes that TonEBP produces are aldose reductase (AR), betaine/GABA transporter(BGT1), sodium myoinositol transporter (SMIT) and taurine transporter (TauT). The transcription of Hsp70, urea transporters (UTA1 and -2) and water channel aquaporin-2 (AQP2) which is a water channel protein when activated, increases cell membrane water permeability, is also activated by TonEBP.
Cells that are shrunken by hypertonicity responds initially with RVI. It increases the uptake of inorganic salts and the osmotic influx of water, but this results in high intracellular inorganic salt concentration that perturbs cells function and structure. So to counteract this, cells activate TonEBP for the transcription of genes to code for aldose reductase (for the synthesis of sorbitol) and transporters of betaine, inositol, and taurine, thus accumulating large amounts of organic osmolytes which are important because they have an osmoprotective effect.
Most cells in mammals are generally not stressed by hypertonicity because of the close control of the concentration of NaCl in virtually all extracellular body fluids. The renal inner medulla is a striking exception. Because of its urinary concentrating mechanism, it has routine exposure to extremely high concentrations of sodium chloride (NaCl) and urea. The adaptation of medullary cells to hyperosmotic stress involves acute cellular efflux of water, cell shrinkage by NaCl, chronic accumulation of compatible organic osmolytes and acute activation of immediate-early and heat shock genes. Renal medullary cells do not restrict this mechanism, which also happens to cells in other tissues when they get exposed to pathologic conditions that produce hypertonic states.
The predominant clinical syndromes of hypertonicity are hypernatremia and hyperglycemia. Rises in tonicity from changes in body water, body solute or both can be assessed testing osmolarity in serum and urine and correlating it with the level of electrolytes in these two compartments to establish the cause of the impairment. The serum osmolarity normal range is 280 to 295mOsm/ kgH2O and normal urine osmolality is from 50mOsm/kgH2O to 1400mOsm/kgH2O. Normal serum sodium is 135 to 145mmol/L, and urinary sodium reference range varies with the diet.
Tonicity is under tightly regulation by the equilibrium between water intake and water excretion. There are normal conditions where water loss occurs like in respiration, gastrointestinal fluids, urine, and skin. The problem occurs when patients are unable to replete those losses. That's why the osmoreceptors in the hypothalamus sense the increase in serum tonicity and stimulate thirst and in consequence individuals increase their intake of water. Just 1% of the change in tonicity is enough to produce ADH release, but it needs a greater than 10% fall in extracellular volume to be released. ADH acts on V2 receptors in the principal cells of the collecting tubules in kidneys and inserts aquaporins for water movement from the tubules to the hypertonic interstitium. Kidneys concentrating urine is the primary reaction to water loss, to retain water for the dilution that excess of solutes.
One of the cardinal manifestations of hyperglycemic crisis is hypertonicity. The excess of glucose in the extracellular fluid has a hypertonic effect; hyperglycemia also produces an osmotic diuresis that makes water loss exceed the losses of sodium and potassium. Because of that, there is an elevation in sodium concentration. Hyperglycemia also produces thirst as a defense mechanism to reestablish its water content and tonicity, but this isn't as effective as in hypernatremia.
Studies have shown that the TonEBP upregulates the expression of AR under high-glucose conditions in diabetic microvascular complication, particularly diabetic nephropathy. Hyperglycemia stimulates the aldose reductase production in cells that express that enzyme. This enzyme catalyzes the reaction of glucose to sorbitol. Sorbitol cannot cross cell membranes, and its accumulation generates osmotic stress on cells by drawing water into tissues, being this one of the mechanisms that glucose produces disturbance to them. AR is present in tissues such as nerve, retina, lens, glomerulus and vascular cells.
Other cells like astrocytes play a major role in defense of brain volume in acute states of tonicity variations. Immediate adaptation to brain shrinking includes movement of fluid from the cerebrospinal fluid into the astrocytes, but this is a limited adaptive mechanism because the main adaptation occurs by RVI.
Acute hypertonicity most often affects the extremes of life (children-elderly), and patients may develop fever, nausea, and vomiting. In children, symptoms can range from irritability, restlessness, muscular twitching to hyperreflexia and seizures; and in the elderly lethargy, delirium and coma, but rarely develop seizures. On the other hand, chronic hypertonicity may manifest with only subtle neurological changes because it has more time to adapt to the medium, even when hypertonicity is severe.
Conditions causing hypernatremia are due to inadequate water intake such as:
It can also result from excessive water loss like:
Glucose as an osmotically active substance results in water movement out of the cells and subsequently in a reduction of serum sodium levels by dilution. Therefore it is crucial to correct serum sodium for hyperglycemia, which is calculated by adding to measured [Na] 1.6 mmol/L for every 100 mg/dL (5.55 mmol/L) increment of serum glucose above normal, but sometimes laboratories already report the corrected serum sodium. Under other conditions, uncontrolled hyperglycemic patients produce osmotic diuresis losing water and then causing a hypovolemic state presenting with signs such as orthostatic hypotension and increased pulse rate, decreased skin turgor, flat neck veins, dry mucous membranes, etc. So, it also can lead to hypernatremia, if there is not sufficient replacement of this water loss. For that reason, in patients like for example with diabetes mellitus, sodium concentration can be variable, demonstrating the hyperglycemia-induced water movement out of the cells that lower Na and the glucosuria induced osmotic diuresis, which can raise it.
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