Circadian rhythms in gastrin receptors in rat fundic stomach

Circadian rhythmicity in the number of gastrin receptors in rat fundic mucosa was characterized and was related to the concentrations of gastrin in serum and in antrum. Male Sprague-Dawley rats were acclimated to 12 hr light alternating with 12 hr darkness. Subgroups of six rats each were killed at 4-hr intervals. Fundic mucosa was collected for measurement of gastrin receptors; serum and antral tissues were collected for measurement of gastrin levels by radioimmunoassay. Circadian periodicity in the data was determined by cosinor analyses. In both freely fed and fasted rats, gastrin receptors showed circadian variation (range 2.5-10 fmol/mg protein), as did serum gastrin concentrations (range in fed rats 195-407 pg/ml). The phasing of the intrinsic circadian variation in gastrin receptor level that was observed in the fasted rats was advanced by a few hours in fed rats. This shift is probably due to food-induced gastrin release, resulting in gastrin-mediated down-regulation of gastrin receptors, followed by up-regulation of gastrin receptors. Food-related effects were thus superimposed upon the intrinsic circadian rhythms in gastrin receptor levels, causing the circadian variation in gastrin receptor levels in the fed rats to be shifted forward compared to that in the fasted rats. No significant circadian rhythms, on the other hand, were found in concentrations of gastrin in the antrum. These results suggest that changes in sensitivity of target tissues to hormones are related to both intrinsic circadian rhythms in levels of hormone receptors and also to food-related changes in hormone-receptor levels mediated by changing serum hormone levels.