Daily fluctuations in plasma calcium, phosphate, and their radionuclide concentrations in the rat.

Daily fluctuations in plasma calcium, phosphate and their radionuclides (injected 5 or more days previously) were determined in rats maintained on a closely controlled feeding and light schedule. Male rats (150-300 g) were trained to a 7a.m.-7p.m. "light" schedule with food available 9 p.m. to 9 a.m. All prarmeters dropped rapidly at the start of each feeding period and then rose during the day. The daily changes in radionuclide concentrations were several orders of magnitude greater than for the stable ions. Continuous access to food produced an earlier fall (5 p.m.) in all plasma values. In thyroidectomized rats (with parathyroid transplants) plasma calcium and phosphate remained relatively constant during the 24 h period. If the time of availability of food was moved 6 h earlier (no change in light cycle), the drop in these plasma values also occurred 6 h earlier. Closer examination of these daily changes indicated that all values fell at least 1 h prior to feeding. In fasted rats, plasma calcium and phosphate concentrations fell as usual; however, 45Ca and 32P rose instead of falling. It was concluded that, in normal rats, changes in endogenous calcitonin secretion may actually increase rather than decrease daily pertubations of plasma calcium and phosphate. Daily changes in these values are influenced by a condition reflex developed due to the feeding habits of the rats. Finally, it is suggested that intestinal hormones rather than calcium may be the primary control of calcitonin secretion, and that the gastrointestinal tract plays a major role, in addition to absorptive activity, in both calcium and phosphate homeostasis.