Protective and Maintenance Functions of Human Saliva H

General Dentistry

Protective and maintenance functions of human saliva H. Douglas Hall*

The rote of human saliva has traditionally been considered to be to aid in digestion. However, saliva's major role is not digestion but the protection and maintenance af the alimentary canal, especially the oral cavity. Loss of salivary ßaw may be devastating to a patient. Simple tasks such as .speaking, ehewing, and swallowing may become ar- duous and uncomfortable. Oral infections and mucosal irritations may drastically affect the patient's .systemic and oral well-being. Plaque-mediated diseases and fungal infections may prevail in the absence of saliva. Understanding the functions of saliva and recognizing signs and symptoms of salivary dysfunction will allow increased evaluation and awareness oj the saliveny-distressedpatient. (Quintessence Int 1993:24:813-816.)

Introduction lubricates and cleanses epitheUnm of the oral, pharyngeal, and esophageal mucosa^''; fiushes food par- The loss of salivary flow can be orally and systemi- ticles from the mouth; possesses antibacterial, anti- cally devastating to a patient. Xerostomia (dry viral, and antifungal properties: buffers pH; aids in mouth), although not considered a disease in itself, chewing, swallowing, and digestion; facilitates oral may signal the onset or presence of a nuniber of se- soft tissue repair and electrolyte balance; facihtates rious systemic diseases and conditions.' and promotes taste; and aids in maintenance and re- Mandel- proposed that a good way to appreciate mineralization of teeth.-••'•* the importance of the many roles played by saliva in The traditional view of sahva is that it is primarily humans is to sample the complaints of people with a digestive tiuid, initiating the digestion of starch salivary dysfunction: through the secretion of amylase. Breakdown of starch to maltose indeed occurs in the mouth, but My mouth and throat are dry, rough, and sticky, I'm hoarse; it's so hard to talk, I can't wear my dentures, food is rapidly cleared through the oral cavity. Most my mouth is always sore. I have to sip fluids frequently of the digestion of starch results from the action of so my tongue won't stick to the sides or roof of my pancreatic, not salivary, amylase.'' Saliva's major role mouth. Eating is difficult and sometimes impossible. or function is the protection and tnaintenance of the Food sticks to my mouth aud teeth, I can't tell the position of food in my mouth. My mouth often feels upper gastrointestinal tract and, indirectly, the well- numb, t have difTiculty lasting and have to add more being of the alimentary canal. Therefore, many facets salt and sugar to my food. My fillings are falling cut of homeostasis are influenced by sahvary flow and and my teeth are crumbling away. function. Saliva has many functions and benefits that hnmans are deprived of following sahvary dysfunction. Saliva Antiviral functions Saliva can have a modulating influence on vtrnses, via secretory immunoglobulin A (IgA)," Acinar cells of both the parotid and siibmandibular glands produce a glycoprotein known as secretory component. This glycoprotein, together with igA, forms the secretory ' Assistant Professor, Department of Periodontics, University of Oklahoma, College of Dentistry, PO Bo>: 26901, Oklahoma City. IgA, which is active on mucosal surfaces. More than Oklahoma 73190. 90% of the IgA in saliva is of the secretory nature.

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Secretory igA neutralizes viruses and can act as an mucin components of saliva help modulate oral mi- atitibody to bacterial and food antigens. Secretory croflora and interfere with pathogen receptors." Mu- IgA is relatively resistant to proteolytic enzymes and cins also aid in concentrating salivary protein defense therefore can survive in the oral cavity and gastroin- mechanisms and thereby increase the antibacterial ac- testinal tract.*' Mucins also do effective battle with tivity of these mechanisms.- viruses, and have been shown to block attachment to Although mucins have received the most attention, host cells of the infltienza virus. Saliva, with its mucin other small proteins can aggregate bacteria, and sim- content, also offers a specific nonimmune protection ple mechanisms such as calcium bridging of the neg- against the herpes simplex virus.- ative charges on bacterial cell walls can contribute to Within the last decade the most notorious viral con- aggregation as well. Certain salivary components, cern has been the acquired immunodeficiency syn- such as lactoferrin and lysozyme, have the ability to drome epidemic. Many individuals express fear about interfere with bacterial multiplication or can kill cells contracting human immunodeficiency virus (HIV) or- directly."' The antibacterial enzyme lysozyme is found ally. Fox' reported that saliva has inhibited the infec- in both parotid and submandibular secretions. Ly- tivity of HIV-1 in vitro. This inhibitory activity is soszyme is a muramidasc; that is, it sphts bacterial present in whole saliva and in submandibular and cell walls in the glycopeptide region that contains sublingual glandular secretions. When sahva is mixed muramic acid. Lysozyme may act together with other with the virus, the ability of the virus to infect lym- antibacterial systems in saliva (eg, IgA) as a general phocytes is reduced or cornpletely eliminated. This scavenger of susceptible bacterial cell walls. Lysozyme inhibitory activity has been shown in saliva from also can aggregate bacteria. Another important an- healthy men, women, and children, as well as in men tibacterial enzyme is sialoperoxidase. Together with who are HIV-1 seropositive. The specific inhibitory hydrogen peroxide and thiocyanate, sialoperoxidase factor(s) in saliva have not been identified yet, but can affect lactobacilli and cariogcnic streptococci. A this activity may reduce the infectivity of oral secre- similar system exists in human mother's milk and is tions in vivo. In general, the recovery of HIV-1 from considered to be part of the early defense mechanism oral secretions is sporadic, and the concentration of of the newborn. Lactoferrin, originally described in virus, when present, is low. Most importantly, al- bovine milk, is widely distributed in body ñuids and though HIV-1 can be found in the mouth, saliva and granules of polymorphonuclear leucocytes. This red, the mouth have not been imphcated in transmission iron-binding glycoprotein should be an effective an- of the virus. Research techniques have detected HIV- tibacterial agent in saliva by withholding iron from I virus in macrophages, monocytes, and lymphocytes, facultative and aerobic organisms.'' infiltrating both major and minor salivary glands. Saliva may aid the body in dealing with certain However, HIV-1 has not been found within the sali- forms of inflammatory periodontal diseases. Various vary acinar and ductal elements. It follows then that proteolytic enzymes are generated in bacterial plaque the low frequency of HIV-] virus recovery from glan- around the teeth and in the gingival crevicular area. dular secretions may be explained by the implication Proteases are also produced by polymorphonuclear that infected cells travel into the salivary glands but leucocytes. The sahva from the submandibular glands do not reside in the secretory elements.*' possesses phosphoproteins that have high antipro- tease activity. The antibacterial components (eg, mucins, IgA, and buffers) of saliva are augmented by constituents of gingival crevicular fluid. Crevicular Antibacterial functions fluid contains serum antibodies, such as immuno- Saliva has many cornponents that interfere with the globulin G, that act against oral bacteria, as well as growth or inhibit the attachment of bacteria in the antibacterial products from phagocytic cells.' oral cavity.'-'"" The ability to inhibit bacterial attachment {especially to mucous membranes) is a major characteristic of secretory IgA, and is the rationale Antifungal functions for the interest in an oral vaccine against caries. Other Salivary mucins and many histidinc-rich peptides macromolecules may clump or aggregate bacteria so work against fungal overgrowth in the oral cavity. that they can no longer effectively adhere to hard or Candidiasis is frequently observed in the salivary-dys- soft tissues and are expectorated or swallowed. The functional patient.--^

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Buffer capacity has led to a greater incidence of oral soft tissue tumors in the oral mucosa of experimental animals subjeeted Bicarbonate, phosphate, and certain histidinc-rich to cbemical carcinogens, compared to the same con- peptides can act as both pH regulators and antibae- ditions and tumor rate of animals witb normal sali- terial agents.-•*•'" These salivary components can dif- vary funetion.--^ fuse into bacterial plaque and act directly by neu- Wolf et al'"* reported on a study of 298 patients tralizing the acid produced. Also, urea from saliva is referred for oral dryness. Those patients demonstrat- acted on by bacterial urease to form ammonia, which ing little or no salivary gland flow rarely had nonnal- can also neutralize acid.' appearing mucosa. This finding may be explained by Maintaining adequate salivary fiow, and therefore the faet that an adequate salivary fiow (at least 0.2 a neutral pH, is essential to guard against tooth mL/min)' will allow a film to form, lining the mueosa demineralization from eithc acid, foods and liquids, and limiting penetration of irritants and toxins gastric reflux, and acidic soft drinks, as well as to in foods, beverages, tobacco smoke, and other protect oral and esophageal linings. sourees.-'" The glyeoproteins and mucoids produced Inereasing the fiow of saliva increases the pH, be- by the major and minor salivary glands form this cause there is a corresponding increase in buffers, such protective coating for the mucous membranes. This as bicarbonate and phosphate.^ Since chewing is an coating is an effective barrier against proteolytic and effeetive stimulus to increase salivary now, gum man- hydrolytic enzymes produeed in plaque, potential ear- ufacturers have increased sales by advertising tbis cinogens (chemicals, smoke, etc), and desiccation,* finding. Mucins, in addition to lubricating the mueosa and increasing rhéologie properties to saliva, are also very resistant to proteolysis and diffusion of harmful com- Remineralization and tooth integrity pounds through the mueosa.-' Salivary mucins are Salivary calcium and phosphate aid in remineraliza- good mueosal lubricants because they bind water ef- tion of tooth snrfaces.^ Sahvary glycoproteins selec- fectively. Mucins on the surface of mucous mem- tively adsorb to enamel surfaces to form an acquired branes serve as natural waterproofing and help to or salivary pellicle.'- This pellicle is semipermeable maintain tissues in a hydrated state.'"'* Mucins also and allows selective influx and exit of ions benefieial aid in forming a proteetive barrier to retard tooth to remineralization of tooth surfaces. The salivary desiccation and decay. mucins also provide a protective barrier and lubricating film against excessive tooth wear and aeid penetration and limit outflow of mineral-forming ions.-" Soft tissue repair The flushing action and antibacterial and mueosal Lavage proteetion afforded by saliva are instrumental in initiating and maintaining adequate wound bealing. Sa- The physical flow of saliva removes harmful bacteria liva speeds blood coagulation, both by affecting an- and food debris from teeth and mueosal (oral, pha- ticoagulant directly in blood and by diluting anti- ryngeal, and esophageal) surfaces. This clearance thrombin. This is especially helpful when rough food mechanism is similar to tearing and blinking in the or traumatic injury can induce bleeding and where eye, blowing the nose, coughing, and expectorating to tissues ean bleed readily beeause of infiammatory dis- clear the throat, airway, and lungs.'-' Salivary glands are more prone to infection during xerostomia, because the diminished salivary fiow cannot prevent bacteria from ascending into the gland.' Ingestion, taste, and digestion The immediate salivary contribution to digestion is Maintenance of mueosal integrity preparatory. When an individual eats, the parotid Protection and maintenance of a viable lining of the gland is active and produces a watery saliva to help oral, pharyngeal, and esophageal lining of the ali- moisten the food. The submandibular, sublingual, mentary tract is a primary function of saliva. The and minor salivary glands produce mucin to help coat pharmacologie induction of xerostomia (dry mouth) the food.'' Without these salivary functions, simple

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chewing and swallowing are uncomfortable, if not im- References possible.'' 1, Sreebny, LM. Valdini A. Xerostomia. Part I. Relalionship to Food cannot be tasted unless it is in a dissolved oilier oral symptoms and salivary gland hypofunction. Oral state. Saliva makes foods soluble and acts then as a Surg Oral Med Orul Palhol I989;66:451^5i(, medium for receptor activation of tasle buds.''' In 2, Mandei ID. The luneLions of saliva. J Dent Res 1987;66;623- addition, salivary loss could be threatening to a pa- 627. 3, Vüidcz I. Radialion-induced salivary dysfunction; Clinical tieni who relies on the dissolving of sublingual nitro- course and significance. Spec Care Dent 1991;ll:252-255. glycerin tablets during an attack of angina.^ 4, Slome BA. Rampanl caries: A side-effect of Iricyclic anlide- Eating becomes very unpleasant without the pres- pressanl Iherapy, Gen Denl 1984;32:495-496. ence of saliva to lubricate and protect the mucosa. 5, Massler M. Influence of diet on denture-bearing tissues. Dent Denture wearers who suffer from xerostomia are es- Clin North Am I9K4;28:211-22O. 6, Grand DA, Stern IB, Listgarden MA. Periodontics, ed 6. Si pecially prone to mucosal ulcérations during eating Louis. Mosby. 1988:1.15-144. and will complain of poor-tltting dentures and sore 7, Glass BJ, Van Dis ML, Langlais RP, Miles DH, Xerostomia: mucosa.'* Studies have shown that edentulous pa- Diagnosis and trealmenl planning considerations. Oral Surg tients have a lower salivary flow rate than do dentate Oral Med Oral Palhol 1984; 58:248-252. patients.'^ 8, FoxPC. Saliva and salivary gland alterations in HIV infection, J Am Denl Assoc 1991;122:46-48. Once taste or chewing become difficult, eating hab- 9, Herrera JL, Lyons MF. Johnson LF. Saliva: Its role in health its are changed and patients may suffer nutritional and disease, J Clin GaslroeDterol 1988;10(5):69-78. deficiencies with resultant systemic as well as oral 10. Slrahl RC, Welsh S, Streckfus CF. Salivary flow rates: A di- complications." agnostic aid in treatment planning geriatric patients. Clin Prev Denl 1990:12:10-12, 11. Niessen LC, Jones JA. Oral health changes in the elderly. Summary Postgrad Med Oral Health l984;75:231-237. 12. Hall HD, Gngsby WR. Analysis of acquired pellicle fonnalion Saliva is the principal protector of the soft and hard using dansylaled salivary proteins [abstracl 655], J Dent Res oral tissues. When the process of secretion is dimin- 1984;63:243. 13. DiiJLbury AJ. Thakker NS, Waslell DG. A double-blind cross- ished, the oral tissues become susceptible to infection over Irial of a mudn-containing artificial sahva. Br Denl J and the ability to masticate, swallow, speak, and taste 1989;166:i]5-i20. may be altered, thereby affecting the quality of life." 14. Wolff AC, Cox PC, Ship JA, ei al. Oral mucosal status and major salivary gland function. Oral Surg Orai Med Oral Pathol Patients who complain of dry, burning, or sore 1990;70:45-54. mouth, or difficulty in speaking, chewing, swallowing, 15. Vissink A, Panders AK, "s-Gravenmade EJ. Vermey A. The and tasting should be evaluated for salivary dysfunc- causes and consequences of hyposalivation. Ear Nose 'Ihroal tion. Oral signs such as recurrent caries, cervical and J I988;67:165-176. root caries, tooth décalcification, lost restorations, 16. Vissink A. DeJong HP, Busschen HJ, Arends J, 's-Graven- made EJ. Wetting properties of human saliva and saliva sub- fractured cusps, and chronic mucosal irritation may stitutes. J Dent Res 1986;65:1121-1I24. signify loss of salivary flow and function. 17. Sreebny LM. Recognition and treatment of salivary induced Loss of sahvary flow may complicate the ability to conditions. Int Dent J 1989;39:197-204. ingest drugs, maintain adequate nutrition, and sustain 18. Thorselius I, Emilson CG, Osterberg T. Sahvary conditions normal homeostasis. Thus, dentists and physicians and drug consumption in older age grotips of elderly Swedish individuals. Gerodonlology 1988;4:66-70, G should be acutely aware of signs and symptoms of sahvary dysfunction in their patients.

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