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FIGURE 2 Concentrations of major ions in saliva as a function of the secretory rate. Note that, at all rates, Na+ and Cl- concentrations are lower and that K+ and HCO- are higher than their respective concentrations in plasma.

FIGURE 2 Concentrations of major ions in saliva as a function of the secretory rate. Note that, at all rates, Na+ and Cl- concentrations are lower and that K+ and HCO- are higher than their respective concentrations in plasma.

most species, the concentration of Na+ in saliva is always less than that of plasma, and as the rate of secretion increases, the concentration of Na+ also increases. In general, Cl- concentrations parallel those of Na + . These findings indicate that Na+ and Cl- are secreted and then reabsorbed as the saliva passes through the ducts. The concentration of HCO- in saliva is higher than that found in plasma except at low flow rates, which accounts for changes in the pH of saliva as well. At unstimulated rates of flow, the pH is slightly acidic, but with stimulation it rapidly rises to around pH 8. The relationships between flow rates and ion concentrations shown in Fig. 2 vary somewhat depending on the nature of the stimulus.

Two basic types of studies indicate how the final saliva is produced and explain the relationship between ion concentrations and flow rates shown in Fig. 2. First, analysis of fluid collected by micropuncture of the intercalated ducts shows that Na + , Cl-, K + , and HCO3- are present in concentrations approximately equal to their concentrations in plasma. This fluid is also isotonic to plasma. Second, if one perfuses a salivary gland duct with a solution containing Na + , Cl-, K + , and HCO3- at concentrations similar to those present in plasma, the fluid collected at the duct opening has lower concentrations of Na+ and Cl- and higher concentrations of K + and HCO-. In addition, the fluid becomes hypotonic. The longer one allows the fluid to remain in the duct, the greater the changes. These data indicate (1) that the acini produce a fluid having ion concentrations similar to those of plasma and (2) that as the fluid moves down the ducts, Na+ and Cl- are reabsorbed and K+ and HCO- are secreted into the saliva. The higher the flow of saliva, the less time available for these changes to take place. Thus, at high secretory rates the ionic composition of saliva more closely resembles that

FIGURE 3 Fluxes of the primary ions and water across the salivon. The fluid leaving the acinus is isotonic to plasma. Na+ and Cl— leave the duct, and K+ and HCO— enter. The thickness of the arrows indicates that more Na+ and Cl— leave the duct than K+ and HCO—

enter. Because the membrane is relatively impermeable to water, the saliva becomes hypotonic.

of plasma (Fig. 2). At low rates of flow, there is considerably more K+ and considerably less Na+ and Cl—. The HCO— concentration remains fairly high even at high rates of secretion, because HCO— secretion is stimulated by most salivary gland agonists. Some K+ and HCO— enter in exchange for Na+ and Cl—, but much more Na + and Cl— leave the ducts, making the saliva hypotonic. Because the epithelia of the salivary gland ducts are relatively impermeable to water, the saliva remains hypotonic. These processes are summarized in Fig. 3.

Current evidence indicates that Cl— is the major ion species actively secreted by the acinar cells (Fig. 4A). There is no evidence for direct active secretion of Na+. The secretory mechanism for Cl— is inhibited by ouabain, indicating that it depends on the Na + -K+ pump in the basolateral membrane. Na+ moves across the apical membrane of the acinar cell into the lumen to preserve electroneutrality, and water follows down its osmotic gradient. K+ and HCCV enter saliva passively as well, but there is evidence for an active component for each. Within the ducts, Na+ is actively absorbed and K+ actively secreted (Fig. 4B). Some K+ is secreted in exchange for H + . In addition, HCO— is actively secreted in exchange for Cl—. The net result is a decrease in Na + and Cl— concentrations and an increase in K+ and HCO— concentrations and pH. The active absorption of Na+ and secretion of K+ is dependent on the (Na + ,K + )-ATPase in the basolateral membrane. The ductule epithelium is relatively impermeable to water compared with that of the acini. Thus, there is a decrease in Na+ and Cl— concentrations and an increase in K + and HCO— concentrations and pH as saliva moves through the ducts. Because more ions leave than enter, the saliva becomes hypotonic. Agents that stimulate salivary secretion increase the activity of these channels and transport processes. Aldosterone acts at the apical membrane to increase the absorption of Na+ and secretion of K+.

Organic Composition

Several of the organic materials synthesized and secreted by the salivary glands are discussed in the section describing the functions of saliva. These include the a-amylase ptyalin, lingual lipase, mucus, lysozymes, glycoproteins, and lactoferrin. Another enzyme produced by salivary glands is kallikrein, which does not have a digestive function, but converts a plasma protein into the potent vasodilator bradykinin. Kallikrein is released during increased metabolic activity of the salivary gland cells and results in 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 and are discussed later in this chapter. The total protein concentration of saliva is approximately one-tenth the concentration of proteins in the plasma.

FIGURE 4 Transport mechanisms proposed to explain salivary secretion. (A) In the acinus, Cl— enters the cell as Na+ enters down the concentration gradient created by the (Na+,K + )-ATPase. C1— then diffuses into the lumen and Na+ enters to maintain electrical neutrality. K+ and HCO— are present in amounts equal to their concentrations in plasma. (B) In the ducts Na+ leaves the lumen as it moves down the gradient created by the (Na + ,K+)-ATPase. Some Na+ leaves in exchange for K+. Cl— leaves with Na+ to preserve neutrality. Some Cl— leaves in exchange for HCO—. In both cell types, the driving force is the basolateral (Na + ,K+)-ATPase.

FIGURE 4 Transport mechanisms proposed to explain salivary secretion. (A) In the acinus, Cl— enters the cell as Na+ enters down the concentration gradient created by the (Na+,K + )-ATPase. C1— then diffuses into the lumen and Na+ enters to maintain electrical neutrality. K+ and HCO— are present in amounts equal to their concentrations in plasma. (B) In the ducts Na+ leaves the lumen as it moves down the gradient created by the (Na + ,K+)-ATPase. Some Na+ leaves in exchange for K+. Cl— leaves with Na+ to preserve neutrality. Some Cl— leaves in exchange for HCO—. In both cell types, the driving force is the basolateral (Na + ,K+)-ATPase.

Regulation of Salivary Gland Secretion

For all practical purposes, salivation is under total control of the autonomic nervous system. Aldosterone and antidiuretic hormone (ADH) modify the ionic content of saliva by decreasing the Na+ and increasing the K+ concentrations, but they do not regulate the flow of saliva. The regulation of salivary gland secretion differs in this respect from the control of the secretions of the other digestive glands. The gastrointestinal hormones have major roles in regulating the secretions of the stomach, pancreas, and liver. The control of salivation is unusual in one other respect, because although the para-sympathetic system exerts far greater influence than the sympathetic, activation of either of these systems stimulates secretion.

Stimulation of the parasympathetic nerves to the salivary glands initiates and maintains salivary secretion. Increased secretion is due to the activation of transport processes in both acinar and duct cells. Secretion is facilitated by the contraction of the myoepithelial cells, which are directly innervated by the parasympathetic nerves. Parasympathetic fibers also innervate the surrounding blood vessels, stimulating vasodilation and increasing the supply of blood to the secreting cells. Increased cellular activity in response to parasympathetic stimulation is accompanied by an increased consumption of glucose and oxygen and the production of metabolites, which also increase blood flow through vasodilation. In addition, kallikrein is released and the vasodilator bradykinin produced. Eventually, increased cellular activity results in growth of the salivary glands. Section of the parasympathetic nerves to the salivary glands causes the glands to atrophy, whereas sympathetic section has little effect. These processes are outlined in Fig. 5.

FIGURE 5 Regulation of the salivary glands by the central nervous system. (Modified from Johnson LR, ed., Gastrointestinal physiology, 6th ed. St. Louis: CV Mosby, 2001.)

Sympathetic stimulation also increases secretion, metabolism, and growth of the salivary glands, although these responses are less pronounced and of shorter duration than those produced by the parasympathetics. The myoepithelial cells also contract in response to sympathetic stimulation. Stimulation via the sympathetic nerves produces a biphasic change in blood flow to the salivary glands. The earliest response is a decrease in blood flow caused by activation of «-adrenergic receptors and vasoconstriction. However, as vasodilator metabolites are produced, blood flow increases over resting levels. The effects of sympathetic stimulation are also summarized in Fig. 5.

Salivary glands contain receptors to many different mediators, but the most important physiologically are the muscarinic cholinergic and ^-adrenergic receptors. The parasympathetic mediator is acetylcholine, which acts on muscarinic receptors. This results in the formation of inositol triphosphate and the subsequent release of Ca2+ from intracellular stores. Ca2+ may also enter the cell from the plasma. VIP and substance P are also released from neurons within the salivary glands, and they stimulate Ca2+ release. The primary sympathetic mediator is norepinephrine, which binds to adrenergic receptors, resulting in the formation of adenosine 30,50-cyclic monophosphate (cAMP). Formation of these second messengers leads to protein phosphorylation and enzyme activation, resulting eventually in increased salivary gland function. Agonists that increase cAMP usually increase the enzyme and mucus content of saliva, whereas those that increase Ca2+ have a greater effect on the volume of secretion from the acinar cells.

The common stimuli and inhibitors of salivary gland activity are shown in Fig. 5. Salivation is stimulated by the presence of food in the mouth. These stimuli include taste, smell, and the physical sensations produced by chewing and the pressure of the food. Salivation can also be initiated through cortical centers by simply thinking of appetizing food and by conditioned reflexes. Sour-tasting foods and certain chemicals present in spicy foods are also potent stimulators of saliva flow. As mentioned in Chapter 33, nausea leads to an intense production of mucus-rich saliva. Inhibition of salivary gland activity occurs during sleep and periods of dehydration. Other external events inhibiting salivary flow are fatigue and fear.

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