Constitutive secretion of kallikreins in vivo from rat submandibular glands.

In parasympathetic saliva from rat submandibular glands the relative proportions of the various tissue kallikreins differ from those in sympathetic saliva. Kallikreins in sympathetic saliva arise from exocytosis of prepackaged granules in granular tubules, so the kallikreins in parasympathetic saliva must come from a non-granular pool, and are likely to be secreted through a constitutive vesicular route. During periods devoid of stimulation in anaesthetised rats, the kallikreins have been found to accumulate progressively in glandular lumina in parasympathetic-like proportions. As this transport of kallikrein into lumina occurs continuously in vivo, independently of any stimulation or any secretion of fluid, it must arise by constitutive vesicular secretion. During parasympathetic stimulation, the kallikreins are secreted into the saliva at a greater rate than in the resting state but their proportions remain the same and the means by which this increase occurs is open to debate. Constitutively secreted true tissue kallikrein (rK1) has been found to have a different molecular form from that in secretory granules. The submandibular glands also contribute to the kallikreins normally circulating in the blood. Serum levels of kallikrein increased equally during either parasympathetic or sympathetic stimulation and were independent of the amounts secreted into the saliva, so are likely to have arisen from constitutive secretion via the basal sides of the cells, morphological evidence for which has been found in the mouse (Penschow & Coghlan, 1993).