Lysosomal enzyme secretion into pancreatic juice in rats injected with pancreatic secretagogues and augmented secretion after short-term pancreatic duct obstruction.

Abstract

To examine the possible secretion of lysosomal enzymes into the pancreatic juice in rats stimulated with pancreatic secretagogues under both physiological and pathological conditions, we investigated the changes in the secretion of cathepsin B, as a lysosomal enzyme, into pancreatic juice with stimulation of 5 different doses (0.1, 0.2, 0.5, 1.0, and 1.5 micrograms/kg.hr) of caerulein. Control rats had only pancreatic duct cannulation. In other rats, the pancreatic duct was obstructed for 3 hours and secretin was infused (0.2 CU/kg.hr). Caerulein stimulated the secretion of cathepsin B into the pancreatic juice in a dose-dependent manner, as in amylase secretion, and caerulein in higher doses (1.0 and 1.5 microgram/kg.hr) inhibited cathepsin B output as it did amylase output. There was a significantly high positive correlation between cathepsin B output and amylase output after stimulation with caerulein. The secretion of several other lysosomal enzymes was also stimulated by caerulein. Blockage of the pancreatic duct for 3 hours caused a significant but moderate rise in serum amylase levels. Redistribution of cathepsin B activity in the pancreatic subcellular fractions was noted with an increase in the amount of cathepsin B recovered from zymogen-rich pellets after 15 min of centrifugation at 1300 x g. These changes after temporary pancreatic duct obstruction are very similar to those previously noted during the early stage of diet-and caerulein-induced experimental pancreatitis and suggest colocalization of lysosomal enzymes and digestive enzymes. In addition, in duct obstructed rats, the secretion of cathepsin B and other lysosomal enzymes stimulated by caerulein was significantly greater than in animals with free-flowing pancreatic juice. These results indicate that lysosomal enzymes are secreted into pancreatic juice after stimulation by gut hormones in the same manner as classical pancreatic digestive enzymes such as amylase. Moreover, lysosomal enzymes which colocalize with zymogen granules in rats with short-term pancreatic duct obstruction are also secreted into pancreatic juice together with digestive enzymes after stimulation with gut hormones. These findings suggest that lysosomal enzymes are present in zymogen granules under normal conditions and that they may play pathophysiological roles in pancreatic juice. They also contribute to an understanding of the pathogenesis of pancreatitis, since cathepsin B can activate trypsinogen.