ROLE OF CARBONIC ANHYDRASE IN THE BIARBONATE EXCRETION FROM SALIVARY GLANDS AND MECHANISM OF IONIC EXCRETION

Abstract

Experimental studies are made to clarify the role of carbonic anhydrase upon the bicarbonate excretion from salivary glands with special reference to excretory mechanism of ions. Animals are mainly dogs and partly rabbits which are administrated with pilocarpine to promote salivary excretion. Outline of results are as follows;
1) The bicarbonate excretion from parotid gland is depressed by administration of various carbonic anhydrase inhibitors, and the salivary pH is decreased while CO₂ partial pressure in saliva is increased. Among the inhibitors examined, Diamox is the most powerful, then comes thiophen-2-sulfonamide, and next are Na₂S, NaN₃, sulfanilamide and p-sulfonamide benzoic acid, while NaSCN exerts the minimum action. This order of inhibitory actions coincides with that of inhibitory effects upon blood carbonic anhydrase which was determined by the in vivo experiment.
2) The bicarbonate excretion in saliva differs among various salivary glands. Its excretion rates relative to rate of salivary flow from the gland examined are in the following order: parotid gland of dog>submaxillary gland of dog>parotid gland of rabbit>sublingual gland of dog. This order coincides with that of carbonic anhydrase contents in theses glands.
From these, it is concluded that the bicarbonate in saliva is produced in gland cells from metabolic CO₂ under the action of carbonic anhydrase.
By comparing the relative amount of carbonic anhydrase in these glands with their histological structures, the site of its production is suggested to be the striated part of gland duct.
3) The concentrations of Na⁺, Cl^- and HCO₃^- in parotid saliva as well as in submaxillary saliva increase as the rate of salivary flow is accelerated, and approach certain maximum levels respectively. The K⁺ concentration is maintained almost constant regardless of salivary flow except when the flowing rate is very low. The ionic concentrations in sublingual saliva of dog are all maintained approximately constant.
The total ionic concentrations in parotid and submaxillary saliva of dog and also in rabbit parotid saliva are hypotonic even at the maximum levels of concentrations, while that in sublingual saliva is approximately isotonic.
Among the ions in saliva, Na⁺ and Cl^- are the most dominating, HCO₃^- is the next and K⁺ and other ions are the least ones. Excretions of Cl^- and HCO₃^- vary considerably among various glands, while that of K⁺ does not differ so far. The concentration of Na⁺ depends upon the tonicity of saliva.
4) The regression lines of excretory rates of main ions referring to the rate of salivary flow are estimated from these data, and, by comparing the constants of regression lines (the intercept constants and the regression coefficients) of various glands with their histological structures, the excretory mechanism of ions from salivary glands were discussed.