Having demonstrated that the rat parotid gland is able to excrete a hypotonic saliva, we studied the formation of saliva in the different segments of the duct system, through micropuncture and microanalytical techniques. The duct system of the gland is composed of three functionally different segments, aciniintercalated duets, striated duets and excretory duets, a) Acini-intercalated ducts secrete a primary fluid with plasma-like osmolality and sodium and potassium concentration, b) Striated ducts modify the primary fluid by active reabsorption of sodium in excess of water, thus producing hypotonicity, c) Excretory ducts serve as conduits of the saliva to the mouth. A process of reequilibration of the duet fluid to isotonicity occurs m the excretory ducts of the resting gland. Secretion of small amounts of potassium also appears to take place in these ducts.
Recently, the micropuncture and microanalytical methods used by the renal physiologists were introduced in the studies of the function of exocrine glands. MA~T~-EZ et al. [11]; SC~6c~L and Young [14,19], and Sc~u~z et al. [15] applied these techmques in the study of the rat submaxillary and human sweat glands. It has been shown that these exocrine glands produce a plasma like primary fluid which is subsequently modified by processes of reabsorption and secretion of electrolytes as it flows along a duct system having low water permeability.
The rat parotid gland had long been considered to produce a more or less isotonic saliva with high sodium and low potassium concentrations [2]. Recently, we showed that this gland, like the parotids of man [l 8] and dog [1 ], produced a hypotonic saliva [9]. In the resting gland (with s~livary flow rates less than 0.1 ~l/min. gm wet gland tissue), the final saliva was isotonic with high sodium and low potassium concentrations. At moderate flow (5--10 ~l/min.gm wet gland tissue), the osmolality and sodium concentration precipitously decreased to 80 mOsm/l and 8 mEq/1 respectively. Upon further increase in flow rate (up to 130 ~l/min β’gm wet gland tissue) the osmolality and sodium concentrations gradually β’ Supported by ~ItI Grant Nr. AM 06806-03 and by Deutsche Forsehungsgemeinschaft.