CONSTITUTIVE SECRETION OF KALLIKREINS IN-VIVO FROM RAT SUBMANDIBULAR GLANDS

In parasympathetic saliva from rat submandibular glands the relative p roportions of the various tissue kallikreins differ from those in symp athetic saliva. Kallikreins in sympathetic saliva arise from exocytosi s of prepackaged granules in granular tubules, so the kallikreins in p arasympathetic saliva must come from a non-granular pool, and are like ly to be secreted through a constitutive vesicular route. During perio ds devoid of stimulation in anaesthetised rats, the kallikreins have b een found to accumulate progressively in glandular lumina. in parasymp athetic-like proportions. As this transport of kallikrein into lumina occurs continuously in vivo, independently of any stimulation or any s ecretion 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 propo rtions remain the same and the means by which this increase occurs is open to debate. Constitutively secreted true tissue kallikrein (rK1) h as been found to have a different molecular form from that in secretor y granules. The submandibular glands also contribute to the kallikrein s normally circulating in the blood. Serum levels of kallikrein increa sed equally during tither parasympathetic or sympathetic stimulation a nd were independent of the amounts secreted into the saliva, so are li kely to have arisen from constitutive secretion via the basal sides of the cells, morphological evidence for which has been found in the mou se (Penschow & Coghlan, 1993).