Sodium in biology
Sodium ions (Na+) are necessary in small amounts for some types of
Additionally, sodium ions are essential to several cellular processes. They are responsible for the co-transport of glucose in the sodium glucose symport, are used to help maintain membrane polarity with the help of the sodium potassium pump, and are paired with water to thin the mucus of the airway lumen when the active Cystic Fibrosis Transport Receptor moves chloride ions into the airway. [4]
Sodium distribution in species
Humans
The minimum physiological requirement for sodium is between 115 and 500 mg per day depending on sweating due to physical activity, and whether the person is adapted to the climate.
Normal serum sodium levels are between approximately 135 and 145 mEq/L (135 to 145 mmol/L). A serum sodium level of less than 135 mEq/L qualifies as hyponatremia, which is considered severe when the serum sodium level is below 125 mEq/L.[14][15]
The
Some potent neurotoxins, such as batrachotoxin, increase the sodium ion permeability of the cell membranes in nerves and muscles, causing a massive and irreversible depolarization of the membranes with potentially fatal consequences. However, drugs with smaller effects on sodium ion motion in nerves may have diverse pharmacological effects that range from anti-depressant to anti-seizure actions.
Other animals
Since only some plants need sodium and those in small quantities, a completely plant-based diet will generally be very low in sodium.[citation needed] This requires some herbivores to obtain their sodium from salt licks and other mineral sources. The animal need for sodium is probably the reason for the highly conserved ability to taste the sodium ion as "salty." Receptors for the pure salty taste respond best to sodium; otherwise, the receptors respond only to a few other small monovalent cations (Li+, NH+4 and somewhat to K+). The calcium ion (Ca2+) also tastes salty and sometimes bitter to some people but, like potassium, can trigger other tastes.
Sodium ions play a diverse and important role in many physiological processes, acting to regulate blood volume, blood pressure, osmotic equilibrium and pH.[8]
Plants
In
Sodium and Water Balance
Sodium is the primary
Sodium and water balance in humans
Although the system for maintaining optimal salt and water balance in the body is a complex one,[22] one of the primary ways in which the human body keeps track of loss of body water is that osmoreceptors in the hypothalamus sense a balance of sodium and water concentration in extracellular fluids. Relative loss of body water will cause sodium concentration to rise higher than normal, a condition known as hypernatremia. This ordinarily results in thirst. Conversely, an excess of body water caused by drinking will result in too little sodium in the blood (hyponatremia), a condition which is again sensed by the hypothalamus, causing a decrease in vasopressin hormone secretion from the posterior pituitary, and a consequent loss of water in the urine, which acts to restore blood sodium concentrations to normal.
Severely dehydrated persons, such as people rescued from ocean or desert survival situations, usually have very high blood sodium concentrations. These must be very carefully and slowly returned to normal, since too-rapid correction of hypernatremia may result in brain damage from cellular swelling, as water moves suddenly into cells with high
In humans, a high-salt intake was demonstrated to attenuate nitric oxide production. Nitric oxide (NO) contributes to vessel homeostasis by inhibiting vascular smooth muscle contraction and growth, platelet aggregation, and leukocyte adhesion to the endothelium.[23]
Urinary sodium
Because the hypothalamus/osmoreceptor system ordinarily works well to cause drinking or urination to restore the body's sodium concentrations to normal, this system can be used in medical treatment to regulate the body's total fluid content, by first controlling the body's sodium content. Thus, when a powerful diuretic drug is given which causes the kidneys to excrete sodium, the effect is accompanied by an excretion of body water (water loss accompanies sodium loss). This happens because the kidney is unable to efficiently retain water while excreting large amounts of sodium. In addition, after sodium excretion, the osmoreceptor system may sense lowered sodium concentration in the blood and then direct compensatory urinary water loss in order to correct the hyponatremic (low blood sodium) state.
Sodium at a cellular level
Sodium-potassium pump
The
The sodium-potassium pump plays a large role in neural signaling due to the maintenance of cell membrane potential. This creates an action potential that causes the neurons to polarize and depolarize their membranes by opening and closing the voltage gated channels: this alters voltage potential and leads to neurotransmitter secretion and ultimately signal transmission.[24]
When the pump fails to function, patients are susceptible to illnesses like heart failure and chronic obstructive lung disease (COLD). Those who experienced an event of heart failure had on average, a 40% lower concentration of the sodium-potassium ATPase. This lack of polarization of the membrane leads to an inability of action potentials to propagate at their usual rate, leading to a lowered hear rate and potentially heart failure.[25] In COLD diagnoses, a majority of patients found to have a lowered amount of magnesium and potassium also had a decreased concentration of the sodium-potassium pump in skeletal and smooth muscle during respiratory failure. COLD is treatable in the short term by glucocorticoid which up-regulates the sodium-potassium pump, helping to support muscle endurance and increase muscle activity during these episodes of respiratory failure.[26]
Sodium-glucose symporter
In the sodium-glucose symporter, sodium moves down its concentration gradient to move glucose up its concentration gradient. Sodium has a greater concentration outside of the cell, and binds to the symporter, which is in its outward facing conformation. Once sodium is bound, glucose can bind from the extracellular space, causing the symporter to switch into the occluded formation (closed) before opening to the inside of the cell and releasing the two sodium ions and the one glucose molecule. Once both are released, the symporter re-orients itself to the outward facing conformation and the process starts all over again.[4] A major example of up-regulation of the sodium-glucose symporter is seen in patients with type 2 diabetes, where there is roughly a 3-4 fold up-regulation of the sodium-glucose symporter (SGLT1). This leads to an influx of glucose into the cell and results in hyperglycemia.[27]
Sodium's role in the Cystic Fibrosis Transport Regulator (CFTR)
The Cystic Fibrosis Transport Regulator (CFTR) works by binding two ATP to the A1 and A2, ATP-binding domain. This opens the CFTR channel and allows chloride ions to flow into the lungs and airway lumen. This influx of negatively charged chloride ions into the airway lumen causes sodium to move into the airway lumen to balance the negative charge. Water then moves in with the sodium to balance the osmotic pressure and ultimately leads to the thinning of mucus. In cases of Cystic Fibrosis, the CFTR is defective and only binds a single ATP, leading to the channel failing to open and preventing chloride ions from diffusing into the airway lumen. Since chloride ions cannot diffuse in, there is no movement of sodium into the airway lumen, and no need for water to move into the lumen, leading to thick mucus that clogs and infects the airway lumen.[4]
See also
- Biology and pharmacology of chemical elements
- Action potential – Neuron communication by electric impulses
- Calcium in biology – Use of calcium by organisms
- Iodine in biology – Use of Iodine by organisms
- Magnesium in biology – Use of Magnesium by organisms
- Membrane potential – Type of physical quantity
- Potassium in biology – Use of Potassium by organisms
- Selenium in biology – Use of Selenium by organisms
References
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- ^ Milo, Ron; Philips, Rob. "Cell Biology by the Numbers: What are the concentrations of different ions in cells?". book.bionumbers.org. Retrieved 8 March 2017.
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Thus, a minimum average requirement for adults can be estimated under conditions of maximal adaptation and without active sweating as no more than 5 mEq/day, which corresponds to 115 mg of sodium or approximately 300 mg of sodium chloride per day. In consideration of the wide variation of patterns of physical activity and climatic exposure, a safe minimum intake might be set at 500 mg/day. [Note: Table 11-1 seems to clarify that 500 mg refers to sodium, not sodium chloride]
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