Functions of Saliva
The dilution by the secreted volume and the lubricating effect of mucin (a glycoprotein) are convenient for swallowing. At maximum rates of secretion, the salivary glands can secrete up to 1 ml / min.g tissue, i.e. the actual weight per minute. The dissolution of solid foods is important for gustatory perception.
b- Keeping the buccal and pharyngeal mucosa moist, it is important for hygiene, prevention of infections and caries. Important for speech. Xerostomia is a syndrome characterized by deficient salivary secretion.
c- Digestive function by secretion of 1,4-amylase (ptyalin). Other enzymes, such as RNAase, DNAase, lipase, lysozyme, peroxidase and kallikrein, are also secreted.
- α 1,4 amylase: similar to pancreatic amylase, it hydrolyses α (1,4) bonds internally in the polysaccharide chain. It does not hydrolyse the end bonds nor α (1, 6 ). Maltose, maltotriose and isomaltose ( α limit dextrins ) therefore result from its action .
- Immunoglobulin A (IgA) is secreted into the saliva.
Structure of the salivary glands a- Classification as to the nature of the secretion:
Serous: the parotids
Mixed : sublingual, submaxillary and small glands scattered throughout the mucosa. The mucin in the secretion of these glands makes the solution more viscous.
b- Structure of the glands: acini and intercalate ducts, which produce primary saliva, striated duct and excretory ducts that modify the electrolytic composition of the salivary solution. The zymogenic cells (secreting ptyalin) and the mucin producing cells are located in the acini of the mixed glands.
c- The circulation of blood through the glands is through a portal system: arterioles are capillaries that irrigate the ducts and coalesce into venules; these form the capillary network that perfuses the acini.
d- Control of secretion is under strict control of the neurovegetative system. Both sympathetic and parasympathetic are secretory stimulants, but there are differences in effects. The sympathetic postganglionic fibers come from the superior cervical ganglion. Parasympathetic preganglionic fibers flow into the glossopharyngeal and facial nerves.
Electrolyte composition and cellular mechanisms of saliva secretion
a- Saliva is an always hypotonic solution to plasma. The main electrolytes are Na +, K +, Cl - and bicarbonate. Other ions, such as iodide, are secreted.
b- Electrolyte concentrations are dependent on the rate of salivary secretion. At very low rates of secretion, the solution is acidic, with concentrations of K + above 20 mM, much higher than plasma concentrations. Increasing the secretion rates, the concentrations of Na +, K + and bicarbonate increase. The concentration of bicarbonate exceeds the plasma concentration, which makes alkaline the pH of saliva.
c- The experimental evidence led to the following model for salivary secretion: the acini produce a primary solution, of electrolytic composition and osmolarity very similar to that of plasma. When passing through the ducts the composition and modified, with Na + and Cl - reabsorption and secretion of K + and bicarbonate. There is resorption resulting from osmols, with dilution of saliva, because the ducts, impermeable to water, do not reabsorb.
In d- basolateral membranes of acinar cells there pump Na + -K +, K + channels, where they recirculate, the Na + -H + and Na + -2Cl cotransport - K +. This co - transport raises the electrochemical potential of the Cl -above the equilibrium values and provides the force for its transfer to light through a channel in the apical membrane. The Na + -H + exchanger shifts the CO 2 hydration reaction towards the formation of HCO 3 -and H +, raising the intracellular concentration of bicarbonate to values above the equilibrium. The bicarbonate also passes through the canal for anions of the apical membrane. The secretion of anions makes the light from the acini electrically negative and the electric field moves cations, Na + mainly, by paracellular route. As the epithelium is permeable to water there is secretion of this, driven by the osmotic gradient, created by the transport of ions.
The cells of the ducts are of a different phenotype. The basolateral membrane pumps there Na + -K +, the Na + -H + and Cl channels - and K +. In the apical membrane there are three types of exchanger: Na + -H +, K + -H + and Cl - -HCO 3 - . The operation of these mechanisms results in secretion of K + and bicarbonate and Na + and Cl - reabsorption . The magnitude of reabsorption, in moles, exceeds that of secretion, with dilution of the luminal fluid. The epithelium is impermeable to water.
f- Modification of the concentrations by the ducts will depend on the time of contact. For high flows, with lower contact time, the concentrations will tend to those produced by the acini.
The secreted proteins are encapsulated in vesicles of the Golgi Apparatus. The secretion is by exocytosis of the vesicles.
The control of salivary secretion
a- The salivary glands are under the exclusive control of the neurovegetative system. Both sympathetic and parasympathetic stimulate secretion. However, the stimulatory effect of the sympathetic is transient, whereas that of the parasympathetic is persistent. The sympathetic acting on the vessels causes vasoconstriction and contraction of myoepithelial cells. Vasodilation causes the parasympathetic. This has trophic action on the glands: parasympathetic denervation causes atrophy of the glands.
b- The sympathetic mediator is epinephrine (adrenaline). Post-ganglionic post-ganglionic endings use acetylcholine and VIP (vasoactive intestinal peptide).
c- Acinar cells have adrenergic receptors, a and b, receptors for VIP, ACh and substance P. The b-adrenergic and VIP receptors activate the cAMP cascade, activating G protein, which activates adenylate cyclase. On the other hand, α-adrenergic receptors and receptors for ACh and substance P activate the cascade of IP 3 and diacylglycerol (DAG).
d-Activated epithelial cells produce a protease, kallikrein, which hydrolyzes to 2- globulin, producing bradykinin, a nonapeptide (-Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) with potent vasodilatory action .
a- Reduction in saliva production:
- Congenital Xerostomia
- Sjorgen's syndrome: acquired atrophy of the glands b- Modification of the composition
- Cystic Fibrosis: Elevation of the concentration of Na +, Ca 2 + , and protein
- Addison's disease: Elevation in Na concentration
- Cushing's syndrome and primary hyperaldosteronism: reduction in Na + concentration.
- Digitalis: cause increased concentration of Ca 2 + and K + saliva.
- Diuretics of loop (Lasix): reduction of saliva production.
1. Review the molecular structure of the starch and indicate the cleavage points by salivary amylase. Does complete digestion result in what chemical species? Explain. Discuss the physiological relevance of amide hydrolysis by this enzyme.
2. Reproduce the graphs of Na +, K +, Cl - and bicarbonate concentration of saliva as a function of the secretion rhythms, comparing with the plasma concentrations of these electrolytes. Why is acid the pH of secreted saliva in low and alkaline rhythms pH in abundant secretions?
3. In a scheme with acini and ducts, represent the processes of secretion and resorption of electrolytes resulting in the formation of saliva.
4. Draw a diagram of the acinar cells, indicating the transport mechanisms in the membranes, essential for the production of primary saliva. Make a second scheme, for the tubular cells, with the transport mechanisms that promote the modification of the saliva concentration. Based on the model and the cellular mechanisms, explain why the concentrations are flux dependent.
5. Outline the sequence of events of the cholinergic parasympathetic to the resulting increase in the secretion of electrolytes and proteins in the saliva (receptors, intracellular signaling, possible effectors). Repeat the discussion for b-adrenergic sympathetic stimulation.
6 . Discuss the neural and humoral control of blood flow to the salivary glands.
7. Investigate the major pathologies of salivary secretion.
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salivary secretion, mucin, swallowing, salivary glands, taste, xerostomia, amylase, enzymes