Structural and biochemical characterization of maximal and supramaximal hormonal stimulation of rat exocrine pancreas

To study the regulation of the successive steps along the secretory pathway in the rat exocrine pancreas the model of in vivo infusion of synthetic caerulein in conscious rats for periods up to 72 h was combined with electron microscopy and in vitro analysis of protein synthesis, intracellular protein transport and enzyme discharge using isolated pancreatic lobules. Prolonged and maximal hormonal stimulation was obtained with 0.25 microgram kg-1 h-1 caerulein and resulted in a 80-90% depletion of enzyme stores within 1 to 3 h, followed by coordinate and anticoordinate changes in individual rates of (pro-)enzyme synthesis after a lag period of 3 h. One group (two amylases) revealed a decrease in synthesis to levels about 10-fold lower than controls. A third group of proteins (one trypsinogen, lipase, proelastase) did not show changes in synthesis with hormone stimulation. The sum of such alterations led to an increase in total rate of synthesis after 6 h, which was combined with acceleration of intracellular transport, packaging, and granule discharge, thus enabling a sustained rate of secretion over the period of stimulation. In contrast, infusion of a supramaximal dose of caerulein (5.0 micrograms kg-1 h-1) induced acute edematous pancreatitis and led to an almost complete reduction of volume and protein output from the cannulated main pancreatic duct. Using freeze-fracture techniques and thin-section electron microscopy, earliest structural alterations were observed at membranes of zymogen granules and the plasma membrane. Fusion of zymogen granules among each other led to formation of large membrane-bound vacuoles within the cytoplasm. These and individual zymogen granules fused with the lateral instead of the apical plasma membrane, discharging their content into the interstitial space. Vacuole formation was associated with activation of lysosomes and with cytoplasmic destruction of acinar cells. The findings indicated severe changes in the specificity of the intracellular membrane fusion process induced by supramaximal doses of caerulein, which finally resulted in autodigestion of the pancreas.