GASTRIN AND THE ULTRASTRUCTURE OF G-CELLS IN THE FASTING RAT

The effect of fasting on serum and antral gastrin [G] concentrations and G cell ultrastructure in the rat was examined using a radioimmunoassay and quantitative EM. Serum G levels in fasting animals were markedly reduced and there was also a significant decrease in antral G concentrations after 48 and 72 h fasting. This was associated with a significant fall in the granule content and cytoplasmic volume of individual G cells, at its greated by 48 h. A relative absence of electron dense granules in the Golgi zones of cells from animals fasted for 72 h suggested a paucity of newly formed granules, but fasting produced no detectable change in the electron density of the granule population taken as a whole. During fasting, release and then synthesis of G is inhibited so that granule stores and cell size diminish. The correlation between the granule content of G cells and the antral content of G suggests that hormone release occurs by exocytosis, rather than by any change in the content of individual granules.