ESSENTIAL FACTORS GOVERNING THE ACID-BASE BALANCE OF SALIVA

STUDIES ON THE ACID-BASE BALANCE OF SALIVA, I. REPORT

Abstract

Examining chemical and physiological factors governing the acid-base balance of human saliva (mixed and parotid), the following results were obtained.
1) A correlation between salivary pH and content of alkali reserve in bloodcan be found in some cases, but not always.
2) It was demonstrated by ingestion of abundant alkali that changes of bicarbonate content in blood cannot directly influence the salivary pH, probably because bicarbonate ion do not readily diffuse out across the glandular membrane.
3) In respiratory acidosis, the partial pressure of CO₂, P_CO2 in blood serum can influence that in saliva, probably because the free CO2 diffuses easily across the membrane. But, as P_CO2, in blood is maintained approximately constant in physiological conditions, the salivary P_CO2, shows also an almost constant value near that of blood P_CO2. Thus it cannot be an usual factor which causes the physiological change of salivary pH.
4) Determining the main inorganic and inorganic constituents in saliva and comparing their concentrations with one another, it was found that the total mol concentration in saliva, calculated from its freezing point depression is mainly composed of Na⁺, K⁺, Cl^- and HCO₃^-. As the bicarbonate ion is the only one of these four which acts as a strong buffer, and as the salivary pH (5.8-7.8) is near its dissociation exponent (6.2 at 37°C), it is concluded that the bicarbonate is the main constituent which determines the salivary pH. It was ascertained that the salivary _pH always changed in parallel to changes of total CO₂, content i.e. bicarbonate concentration, in physiological conditions.
5) The concentration of bicarbonate in saliva and therefore its pH were found to increase with the rise of the rate of salivary flow, and vice versa. Similar relations could be found generally in the extra-cellular salts in saliva, such asNa⁺, Cl^-, Ca⁺⁺, etc. Their concentrations were all lower than those of blood serum, and they approached the latter when the rate of salivary flow becamecopious. From the facts, it is presumed that these extra-cellular salts are secretedby permeation through the glandular membrane, of which the permeability may be increased by excitation of gland cells. The concentrations of intra-cellular salts, such as K, phosphate, etc. in saliva were always higher than those in blood serum. They remained nearly constant or decreased when the rate of salivary flow became high.
6) Since the salivary bicarbonate concentration sometimes surpassed the serum concentration during increased flow, it was presumed that a part of bicarbonate was produced in the excited gland and discharged into saliva in addition to the permeated bicarbonate.
7) The mechanism of bicarbonate production was discussed, and it was concluded that it consisted of two reactions, namely the production of K-bicarbonate by the neutralization of metabolic carbonic acid with cell buffer, and that of Na-bicarbonate by cation exchange between NaCl and H₂CO₃ across the glandular membrane. Possible processes concerning the latter mechanism were discussedin some details.
8) It is concluded that the excretion of bicarbonate in saliva refers closely to the excitatory state of gland cells, so that the salivary pH can reflect the glandular activity.