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Saliva

The functions of saliva are reflected by its constituents. Mucins serve to lubricate the food, making it easier to swallow, and to keep the mouth moist to facilitate masticatory and speech-related movement. Saliva dissolves compounds in food, which is a prerequisite for taste buds stimulation p. 338) and for dental and oral hygiene. Saliva has a low NaCI concentration and is hypotonic, making it suitable for rinsing of the taste receptors (NaCl) while eating. Infants need saliva to seal the lips when suckling. Saliva also contains a-amylase, which starts the digestion of starches in the mouth, while immunoglobulin A and lysozyme are part of the immune defense system (^ p.94ff.). The high HCO3- concentration in saliva results in a pH of around 7, which is optimal for a-amylase-catalyzed digestion. Swallowed saliva is also important for buffering the acidic gastric juices refluxed into the esophagus (^ p. 238). The secretion of profuse amounts of saliva before vomiting also prevents gastric acid from damaging the enamel on the teeth. Saliva secretion is very dependent on the body water content. A low content results in decreased saliva secretion—the mouth and throat become dry, thereby evoking the sensation of thirst. This is an important mechanism for maintaining the fluid balance (^ pp.168 and 184).

Secretion rate. The rate of saliva secretion varies from 0.1 to 4mL/min (10-250 ^L/min per gram gland tissue), depending on the degree of stimulation (^ B). This adds up to about 0.5 to 1.5 L per day. At 0.5 mL/min, 95% of this rate is secreted by the parotid gland (serous saliva) and submandibular gland (mucin-rich saliva). The rest comes from the sublingual glands and glands in the buccal mucosa.

Saliva secretion occurs in two steps: The acini (end pieces) produce primary saliva (^ A, C) which has an electrolyte composition similar to that of plasma (^ B). Primary saliva secretion in the acinar cells is the result of transcellular Cl- transport: Cl- is actively taken up into the cells (secondary active transport) from the blood by means of a Na+-K+-2Cl-cotransport carrier and is released into the lumen (together with HCO3-) via anion chan nels, resulting in a lumen-negative trans-epithelial potential (LNTP) that drives Na+ pa-racellularly into the lumen. Water also follows passively (osmotic effect). Primary saliva is modified in excretory ducts, yielding secondary saliva. As the saliva passes through the excretory ducts, Na+ and Cl- are reabsorbed and K+ and (carbonic anhydrase-dependent) HCO3-is secreted into the lumen. The saliva becomes hypotonic (far below 100 mOsm/kg H2O; ^ B) because Na+ and Cl- reabsorption is greater than K+ and HCO3- secretion and the ducts are relatively impermeable to water (^ B). If the secretion rate rises to values much higher than 100^L/(min-g), these processes lag behind and the composition of secondary saliva becomes similar to that of primary saliva (^ B).

Salivant stimuli. Reflex stimulation of saliva secretion occurs in the larger salivary glands (^ D). Salivant stimuli include the smell and taste of food, tactile stimulation of the buccal mucosa, mastication and nausea. Conditioned reflexes also play a role. For instance, the routine clattering of dishes when preparing a meal can later elicit a salivant response. Sleep and dehydration inhibit saliva secretion. Saliva secretion is stimulated via the sympathetic and parasympathetic nervous systems (^C2): ! Norepinephrine triggers the secretion of highly viscous saliva with a high concentration of mucin via p2 adrenoreceptors and cAMP. VIP also increases the cAMP concentration of acinar cells. ! Acetylcholine: (a) With the aid of M1 cholinocep-tors and IP3 (^ pp. 82 and 274), acetylcholine mediates an increase in the cytosolic Ca2+ concentration of acinar cells. This, in turn, increases the conductivity of luminal anion channels, resulting in the production of watery saliva and increased exocytosis of salivary enzymes. (b) With the aid of M3 cholinergic receptors, ACh mediates the contraction of myoepithelial cells around the acini, leading to emptying of the acini. (c) ACh enhances the production of kal-likreins, which cleave bradykinin from plasma kinino-gen. Bradykinin and VIP (^ p. 234) dilate the vessels of the salivary glands. This is necessary because maximum saliva secretion far exceeds resting blood flow.

A. Saliva secretion

Secondary saliva

i— B. Electrolytes in saliva

(2OO

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