Electrolyte concentrations in primary and final saliva of the rat sublingual gland studied by micropuncture and catheterization techniques.

Abstract

The salivary electrolyte concentrations at various secretory rates have been studied in the rat sublingual gland and compared with the concentrations found in acinar-intercalated duct (primary) fluid obtained by micropuncture. The primary fluid had the following composition: [Na]=126±1.6 (S.E.M.,n=39) mEq/l; [K]=11.9±1.2 mEq/l (n=39); [Cl]=99.3±2.3 mEq/l (n=33). The Na and K concentrations differed significantly from those of plasma and from those observed previously in the primary fluid of rat parotid and submaxillary glands. The composition of primary fluid was not altered by carbachol stimulation. After carbachol stimulation, the final saliva, like that of the human but unlike that of the cat and dog sublingual gland, was hypotonic and the electrolyte concentrations showed flow-rate dependence. At the lowest flow rate the Na concentrations averaged 24.6±3.9 mEq/l (n=25) which fell first to a minimum and then rose to a plateau of about 60 mEq/l as flow rate increased to a maximum. The excretory curve showed no tendency to approach the concentrations seen in primary saliva. The K excretory curve was typical of that seen in most other salivary glands, having high concentrations at low flow rates which fell towards a plateau above primary fluid levels as flow rate increased. The salivary bicarbonate concentration was always above that in plasma; it tended to rise to a plateau of about 48 mEq/l at the highest flow rates. This concentration is probably greater than that present in primary fluid although the primary levels have not been measured directly. The results are best interpreted in terms of a modified two-stage hypothesis. First a primary, Na-rich, K-poor secretion is formed in the acinar-intercalated duct region whose composition, although decidedly not plasma-like, is constant irrespective of the degree of gland stimulation. Then, secondarily, this fluid is modified during passage along the striated and excretory ducts by processes of Na reabsorption and K and HCO3 secretion. These secondary transport processes are not transport-maximum-limited but appear to increase with increasing degree of stimulation resulting in a phenomenon which could be termed acinar-tubular balance. Although it has not proven possible to micropuncture striated ducts in the rat submaxillary or parotid glands, in the sublingual gland we were able to do so in a few cases. For the first time we are able to offer direct proof that these ducts do contain hypotonic fluid.