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Composition of Saliva

The major constituents of saliva are water, electrolytes, and a few enzymes. The uniqueproperties of this GI juice are (1) its large volume relative to the mass of glands that secrete saliva, (2) its low osmolality, (3) its high K+ concentration, and (4) the specific organic materials it contains.
Inorganic Composition
Compared with other secretory organs of the GI tract, the salivary glands elaborate a remarkably large volume of j uice per gram (g) of tissue. Thus, for example, an entire pancreas may reach a maximal rate of secretion of 1 milliliter (mL)/minute, whereas at the highest rates of secretion in some animals, a tiny submaxillary gland can secrete 1 mL/g/minute, a 50-fold higher rate. I n humans, the salivary glands secrete at rates severalfold higher than other GI organs per unit weight of tissue.
The osmolality of saliva is significantly lower than that of plasma at all but the highest rates of secretion, when the saliva becomes isotonic with plasma. As the secretory rate of the salivon increases, the osmolality of its saliva also increases. The concentrations of electrolytes in saliva vary with the rate of secretion.
The K+ concentration of saliva is 2 to 30 ti

mes that of the plasma, depending on the rate of secretion, the nature of the stimulus, the plasma K+ concentration, and the level of mineralocorticoids in the circulation. Saliva has the highest K+ concentration of any digestive j uice; maximal concentration values approach those within cells. These remarkable levels of salivary K+ imply the existence of an energy-dependent transport mechanism within the salivon. I n most species the concentration of Na in saliva is always less than that in plasma, and, as the secretory rate increases, the Na concentration also increases. I n general, Cl− concentrations parallel those of Na. These findings suggest that Na+ and Cl− are secreted and then reabsorbed as the saliva passes through the ducts. The concentration of in saliva is higher than that in plasma, except at low flow rates. This also accounts for the changes in the pH of saliva. At basal rates of flow the pH is slightly acidic but rapidly rises to approximately 8 as flow is
stimulated. The relationships between ion concentrations and flow rates, vary somewhat, depending on
the stimulus. The relationships shown in Figure 7-3 are explained by two basic types of studies that
indicate how the final saliva is produced. First, fluid collected by micropuncture of the
intercalated ducts contains Na+, K+, Cl−, and in concentrations approximately equal to their plasma concentrations. This fluid also is isotonic to plasma. Second, if one perfuses a salivary gland duct with fluid containing ions in concentrations similar to those of plasma, Na+ and Cl− concentrations are decreased and K and concentrations are increased when the fluid is collected at the duct opening. The fluid also becomes hypotonic, and the longer the fluid remains in the duct (i.e., the slower the rate of perfusion), the greater are the changes. These data indicate, first, that the acini secrete a fluid similar to plasma in its concentration of ions and, second, that as the fluid moves down the duct, Na+ and Cl−
are reabsorbed and K+ and are secreted into the saliva. The higher the flow of saliva, the less time is available for modification, and the final saliva more closely resembles plasma in its ionic makeup. At low flow rates K+ increases considerably, and Na+ and Cl− decrease. Because most salivary agonists
stimulate secretion, the concentration remains relatively high, even at high rates of secretion. Some K+ and are reabsorbed in exchange for Na, but much more Na+ and Cl− leave the duct, thus causing the saliva to become hypotonic. Because the duct epithelium is relatively impermeable to water, the final product remains hypotonic.
Current evidence indicates that Cl− is the primary ion that is actively secreted by the acinar cells. No evidence exists for direct active secretion of Na. The secretory mechanism for Cl− is inhibited by ouabain, a finding indicating that it depends on th
e Na+/K+ pump in the basolateral membrane. The active pumping of Na out of the cell creates a diffusion gradient for Na+ to enter across the basolateral membrane. Two main ion transport pathways exploit this Na+ gradient to accumulate Cl− above its equilibrium potential. I n the first, 2Cl− are cotransported with Na into the cell to preserve electrical neutrality. This process increases the electrochemical potential of Cl− within the cell, and Cl− diffuses down this gradient into the lumen via an electrogenic ion channel that may also allow to enter the lumen. I nhibition of the Na+/K+/2Cl− cotransporter decreases salivary secretion by 65%. I n the second, Na+ enters in exchange for hydrogen (H+), which alkalinizes the cell promoting the intracellular accumulation of , which then is exchanged for Cl. Removal of from the perfusate or inhibition of the Na +/H+− exchanger by amiloride reduces secretion by 30%. I n both cases Na+ moves paracellularly through the tight j unctions and into the lumen, thus preserving electroneutrality; water follows down its osmotic gradient. Evidence indicates that water also moves into the saliva transcellularly through the
aquaporin 5 apical water channel. There may also be a Ca2+ -activated K channel in the basolateral membrane. Exodus of K+ increases the electronegativity of the cytosol and thereby increases the driving force for the entry of Cl − and into the lumen. Agents that stimulate salivary secretion increase the activity of all these channels and transport processes.
Within the ducts, Na+ and Cl− are actively absorbed and K+ and are actively secreted. These processes are also inhibited by ouabain and depend on the Na gradient created by the Na+ , K+ -adenosine triphosphatase (ATPase) in the basolateral membrane. The apical membrane contains a Na+ channel, and its movement into the cell supports the electrogenic movement of Cl− into the cell through Cl−
channels. The Na/KATPase pumps Na+ out while a Cl− channel in the basolateral membrane transports it out of the cell. Cl− reabsorption also occurs via the paracellular pathway. K is secreted through apical channels into the saliva. To secrete into the lumen, must be concentrated within the cell. This occurs via an Na/ transporter in the basolateral membrane, which is driven by the Na+ gradient. leaves the cell either through the apical cyclic adenosine monophosphate (cAMP)-activated CFTR (cystic fibrosis transmembrane regulator) Cl− channel or via the Cl−/ exchanger at the apical membrane. The tight junctions of the ductule epithelium are relatively impermeable to water when compared with those of the acini. The net results are a decrease in Na+−+ and +Cl− concentrations and an increase in K+ and concentrations, as well as pH, as the saliva moves down the duct. More ions leave than water (HO), and the saliva becomes hypotonic. Aldosterone acts at the luminal membrane to increase the absorption of Na and the secretion of K+ by increasing the numbers of their channels.

Organic Composition
Some organic materials produced and secreted by the salivary glands are mentioned earlier in the section on the functions of saliva. These materials include the enzymes αamylase (ptyalin) and lingual lipase, mucus, glycoproteins, lysozymes, and lactoferrin. Another enzyme produced by salivary glands is kallikrein, which converts a plasma protein into the potent vasodilator bradykinin. Kallikrein is released when the metabolism of the salivary glands increases; it is responsible in part for increased blood flow to the secreting glands. Saliva also contains the blood group substances A, B, AB, and O. The synthesis of salivary gland enzymes, their storage, and their release are similar to the same processes in the pancreas. The protein concentration of saliva is approximately one tenth the concentration of proteins in the plasma.

About dr.Dukagjin Zeqiraj

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