Feeding Regimen, Dietary Calcium, and the Diurnal Rhythms of Serum Calcium and Calcitonin in the Rat*

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Abstract
Experiments were designed initially to investigate the possibility of a gastrointestinal involvement in the postprandial increase in calcitonin (CT) secretion. In this study, adult male rats were fed for a minimum of 5 weeks Purina rodent laboratory chow, the Harvard low calcium diet (HLCD), or the HLCD with added CaCO3. The rats had access to food only between 1730‐0830 h (dark period, 1800–0600 h). Serum levels of CT increased between 1 h before and 2 h after the beginning of the regular feeding time in all groups, even in rats deprived of food. The results showed that neither calcium nor food was essential for the rise in CT levels at the usual feeding time. Other experiments also showed that feeding did not increase CT secretion. For example, when fasted male rats were fed between 1030–1130 h (during the light period) commercial diets or the HLCD with or without added CaCCO3, no increase in serum CT occurred even in rats showing increases in serum gastrin. However the administration of CaCl2 by gavage increased serum levels of both CT and calcium, but did not affect gastrin levels. In contrast, similar amounts of calcium given as a CaCO3 gavage resulted in elevated gastrin levels, but there was no increase in serum CT or calcium. Thyroparathyroidectomy did not alter the serum gastrin response after gavage of either calcium salt. These results suggest that there is no effect of gastrin on CT secretion or of CT on gastrin secretion. Because the feeding schedule seemed to be important to the CT response, we compared the diurnal patterns of serum CT and calcium expressed by rats fed on a regulated feeding schedule (1800–0600 h, corresponding to the dark period) to those by rats fed ad libitum. Surprisingly, there were no significant differences in circadian rhythms for either CT or calcium despite marked differences in feeding patterns. These and other findings in this study, showing that food per se was not essential for increased serum CT, raise doubts about a direct food-stimulated gastrointestinal involvement in CT secretion. We conclude that circadian rhythms for CT and calcium are established in adult male rats regardless of whether the rats were fed ad libitum or on a regulated schedule, and that these rhythms, once established, are not easily altered by an abrupt change in feeding schedule.