Zymogen granules of pancreas decrease in size in response to feeding.

In the first descriptions of pancreatic enzyme secretion about 100 years ago, it was noticed that zymogen granules became smaller and disappeared from the apical region of acinar cells after feeding. We have repeated these experiments and characterized changes in granule size by quantitative electron microscopy 90 min after feeding previously fasted rats. In fasted animals, granules occupied the apical portion of the cell, had an average number of 45 +/- 3 granules per cell section (+/- SE), and measured 0.85 +/- 0.15 micrometers in diameter (+/- SD). After feeding, the number and size of granules decreased. Individual samples showed either a decrease in size alone or a decrease in both size and number, but in no case did they show a reduction in granule number alone. The mean diameter of granules decreased to 0.65+/- 0.15 microns (+/- SD) or about a 55% reduction in average granules volume as compared to controls (0.32 vs. 0.14 microns 3). The size distributions were unimodal and normal in both fasted and fed rats; however, in fed animals, the distribution was shifted to lower values (diameter range 0.40-1.40 microns for fasted rats vs. 0.10-1.30 microns for fed rats). The number of granules decreased to an average of 29 +/- 2 granules per cell section (+/- SE) after feeding, and, on the average, samples with the most granules had larger ones than samples with the fewest granules. The present results support the original observations on live rabbit pancreas that individual granules decrease in size in response to feeding. We suggest that these size changes reflect the loss of proteins across the granule membrane as proposed by the equilibrium hypothesis for digestive enzyme secretion.