Gastrin and the ultrastructure of G cells after stimulation with acetylcholine

The effect of intragastric administration of acetylcholine on serum and antral gastrin concentrations of rats has been examined using a radioimmunoassay and quantitative electron microscopy. Exposure of the stomach of rats, previously fasted for 24h, to 2% acetylcholine for either 0.5 or 2h resulted in a significant 4--5 fold increase in serum gastrin concentrations to levels similar to those found in fed animals. Such treatment produced no detectable change in antral gastrin concentration or in the number or electron density of secretory granules in the G cells. This lack of detectable change in the G cells was not unexpected since our calculations suggest that less than 10% of the total gastrin stored in the antrum is released over 2h as a result of the stimulation with acetylcholine. The proportion of electron-lucent secretory granules was, however, markedly increased by prolonged fixation in aldehydes. The increase was similar in both ACh stimulated and control animals. These results indicate that the ultrastructural appearance of G cell secretory granules in influenced far more by the conditions of fixation than by the release of gastrin. They therefore cast considerable doubt on the hypothesis that gastrin is released by molecular dispersion from the granules.