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Saliva Composition And Exercise
JosŽ L. Chicharro, M.D. Ph.D.
Unidad de Investigaci—n
Escuela de Medicina Deportiva
Facultad de Medicina
Universidad Complutense de Madrid
Salivary glands are nonexcitable effector organs in which a large amount of fluid and electrolytes is transferred from the interior of the body to the outside. The amount of fluid translocated each day through salivary glands approaches 750 ml, which represents approximately 20% of total plasma volume.
Saliva is secreted to the mouth by three major paired salivary glands (submaxillary, parotid, and sublingual glands) and by numerous minor mucous glands, at a rate of approximately 0.025 ml.min-1. The relative contributions of each of these glands to the total amount of saliva secreted average 65 per cent from the submandibular, 23 per cent from the parotid, 8 per cent from the minor mucous, and 4 per cent from the sublingual.
Both of the two major parenchymal sites of the salivary glands, the acini and the striated ducti, participate in salivary secretion. Transport of water and electrolytes, and synthesis of enzymes, proteins, mucin and other organic components, occur in the acini, which secrete a fluid isotonic with plasma. This fluid is then modified in the ductus system, by both reabsorption and secretion of electrolytes.
Saliva Composition
Saliva is characteristically a colorless dilute fluid, with a density ranging from 18 to 35. Its pH is usually around 6.64, and varies depending on the concentration of CO2 in the blood. When blood CO2 concentration is increased, a higher fraction of CO2 is transferred from the blood to the saliva, and salivary pH decreases. If CO2 is low in blood, on the other hand, salivary pH increases as a result of a low transfer of blood CO2 to salivary glands.
Although a variety of components is always present in saliva, the total concentration of inorganic and organic constituents is generally low when compared to serum. The fraction of saliva represented by water usually exceeds 0.99. Of the inorganic constituents, sodium and potassium (and perhaps calcium) are the cations of major osmotic importance in saliva; the major osmotically active anions are chloride and bicarbonate. Although the percentage of total proteins in saliva is low in comparison to serum, specific proteins, such as the enzyme amylase, are synthesized in the salivary glands and may be present in saliva in concentrations exceeding those of serum. Other organic components existing in saliva include: maltase, serum albumin, urea, uric acid, creatinine, mucine, vitamin C, several amino acids, lysozime, lactate, and some hormones such as testosterone and cortisol. Some gases (CO2, O2, and N2) are also present in saliva. Saliva contains immunoglobins such as Ig A and Ig G, at an average concentration of 9.4 and 0.32 mg%, respectively. The concentration of potassium, calcium, urea, uric acid, and aldosterone are highly correlated to those existing in plasma. This high degree of correlation has not been shown, however, between salivary and plasma concentrations of phosphate. The physiological significance of other constituents of saliva, such as trace minerals, epithelial growth factor, neural growth factor, several enzymes and some proteins (kallikreins, calmodulin) remains unknown.
Regulation of saliva secretion
Secretion of saliva is usually elicited in response to stimulation of the autonomic innervation to the glands. Although no direct evidence for modification of salivary flow by hormones has been demonstrated in humans, catecholamines might also be involved in the control of saliva electrolytes and protein concentrations. Both salivary output and composition depend on the activity of the autonomic nervous system, and any modification of this activity can be observed indirectly by alterations in the salivary excretion. Although normal salivary secretion is dependent on the cooperation of sympathetic and parasympathetic nerves, the nervous control of saliva secretion is not identical in all salivary glands: secretion of saliva from sublingual and minor mucous glands is mainly elicited in response to cholinergic stimulation, whereas secretion from the other glands is evoked mainly by adrenergic innervation. In any case, it is generally acknowledged that parasympathetic nerve impulses create the main stimulus for salivary control in general. Parasympathetic stimulation results in a copious flow of saliva low in organic and inorganic compounds concentrations. Sympathetic stimulation, on the other hand, produces a saliva low in volume. In addition, saliva evoked by action of adrenergic mediators is generally higher in organic content and its concentration of certain inorganic salts is also higher than saliva evoked by cholinergic stimulation. The higher organic content of saliva evoked by adrenergic stimulation trough the activity of adenyl-cyclase, includes elevated levels of total protein, especially the digestive enzyme alpha-amilase. High concentrations of alpha-amilase in saliva are indeed considered to be the best indicator of adrenergic evoked secretion of saliva. The levels of inorganic compounds, i.e., Ca++, K+ and HCO3-, are usually higher with sympathetic stimulation.
Besides the type of autonomic receptor being activated, the two other parameters that can affect salivary composition are the intensity and the duration of stimulation to the glands. The differences in composition between saliva collected after a change in the intensity or the duration of stimulation appear to be due to alterations in membrane permeability of secretory cells leading to changes in the rate at which electrolytes are lost from these cells.
The secretory cells are not the only glandular elements that respond to stimulation of the sympathetic innervation. Myoepithelial cells and blood vessels of the glands also respond to such innervation, and these responses can in turn modify the quantity and composition of the elaborated saliva. It has been shown, for example, that sympathetic stimulation to salivary glands can produce a markedly increased degree of vasoconstriction. Finally, other factors such as circannual rhythms and reflexly induced secretomotor responses might also influence salivary secretion.
Effects Of Exercise On Saliva Secretion And Its Composition
Several studies have shown decreases in salivary levels of immunoglobin A (s-Ig A) in response to high-intensity exercise. Lower resting levels of s-IgA have indeed been reported in cross-country skiers and in elite swimmers, when compared to matched controls of sedentary individuals. The levels of s-IgA decrease following intense exercise, and return to normal levels after 60 minutes from cessation of activity. Since Ig A represents the first line of defense against potentially pathogenic viruses, the exercise-induced decrease in s-IgA could contribute to the higher incidence of upper respiratory infections associated to strenuous athletic training. However, endurance exercise performed at lower intensities (i.e., training protocols within the guidelines recommended by the American College of Sports Medicine), does not seem to alter normal s-IgA levels.
Salivary flow rate appears to be modified during physical activity, according to most studies. Nevertheless, interpretation of the results obtained in these studies is sometimes difficult due to some methodological limitations, concerning mainly exercise protocols and saliva collection procedures. During exercise, salivary levels of total protein can be increased, since saliva secretion is then mainly evoked by action of adrenergic mediators. Exercise is indeed known to increase sympathetic activity and the high protein concentration following exercise may be due to increased §-sympathetic activity in salivary glands. This elevated levels of protein could also be caused by the increase in blood catecholamines associated to exercise. During prolonged exercise at low to moderate intensities (lower than 60% of O2max), salivary secretion does not seem to be significantly modified. At higher intensities, however, salivary secretion decreases. Factors associated to high-intensity exercise such as an increased §-adrenergic activity, dehydration, or evaporation of saliva through hyperventilation (although less probable) have been proposed to explain this lower secretion of saliva at high workloads.
Salivary levels of cortisol are considered to be a good indicator of the adrenocortical response to exercise by some authors, since salivary cortisol closely reflects plasma free cortisol levels, presenting advantage over total cortisol measurements. During exercise, salivary and serum concentrations of cortisol are indeed very similar. In addition, both salivary and blood levels of cortisol increase with exercise intensity until a certain exercise level, at which such increase loses it linearity. This inflection point in the increase of salivary and blood levels of cortisol coincides in most of the cases with the onset of blood lactate accumulation. It has been suggested that this lactate accumulation might activate chemoreceptors within the working muscles, which in turn could stimulate the hypothalamic-pituitary axis. However, a true cause-to-effect-relationship between these variables remains to be proven. Both increases of cortisol and lactate levels could occur as a result of a marked sympathetic activity or an increase in blood catecholamines which take place at exercise intensities above anaerobic threshold.
The effects of exercise on the salivary and serum levels of Na+ and K+ have also been studied. Prolonged exercise does not appear to have a significant effect on the serum Na+ and K+. On the other hand, the salivary Na+ concentration markedly increases whereas no noteworthy changes seem to occur in salivary K+, in response to prolonged exercise. In addition, this increase in the salivary Na+/K+ ratio is positively correlated to the exercise-induced increase in salivary protein concentration.
In our laboratory, we have studied the relationship between anaerobic threshold and variations in salivary electrolytes (Na+, K+,
Cl-) in response to incremental exercise. Our results evidenced that salivary Na+ and Cl- showed a dual response to exercise: their levels decreased or remained stable during early phases of exercise, until a certain exercise level, at which they began to show a systematic increase. In contrast, K+ levels did not significantly vary during physical activity. The inflection point in the salivary Na+ and Cl- was highly correlated (r= 0.82; p<0.01) with lactate threshold, suggesting the possibility of determining anaerobic threshold with a noninvasive method involving saliva analysis.
These changes in the concentration of salivary electrolytes which occur at a certain exercise intensity might be elicited in response to sympathetic stimulation. This sympathetic stimulation might induce changes in salivary flow and in both reabsorption and secretion of electrolytes in secretory cells. The decreased in saliva secretion associated to exercise could also be the result of a reduction of blood flow to salivary glands caused by elevated adrenal-sympathetic activity. The results of our investigations demonstrate the existence of a catecholamine threshold highly correlated with blood lactate increases (r= 0.84, p<0.01) during incremental exercise. This catecholamine response which occurred at or close to lactate threshold was in turn well correlated (r=0.75, p<0.05) to the point ("saliva threshold") at which salivary electrolytes (especially Na+) showed an inflection point. Although further research in this field is necessary, our experiments suggest that saliva composition analysis might be a good estimate of the adrenal-sympathetic response during exercise. We therefore propose this new noninvasive method for anaerobic threshold determination. We believe that its potential applications in both clinical and exercise physiology areas are numerous.
References
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