Ultrastructural and physiological evidence for calcitonin-induced postprandial calcium storage in bones of rats.

Abstract

The postulate tested by these experiments is that calcitonin directs a portion of the calcium absorbed from food into temporary storage at surfaces of bone. This study utilized young adult rats in which the parathyroid glands had been autotransplanted. Many rats were also thyroidectomized (TX). All were trained to a 0900 h feeding schedule with predetermined calcium content of food. Calcitonin was injected only during the first 4 h postprandially to some of the TX animals. Urine calcium was monitored and, after sacrifice, changes in the tibia were recorded. The following results were obtained: (a) In thyroid-intact (TI) rats, renal calcium excretion was reduced if the daily intake of calcium was less than 90 mg. In TX rats calcium excretion rose each day during the time of intestinal absorption of calcium. Calcitonin injection to TX rats reduced urinary calcium content on the day it was injected. However, on the following 2 days, renal calcium excretion rose to twice that of TX controls. (b) Electron micrographs of bone tissue from TI rats and TX rats injected with calcitonin showed greater pyroantimonate reaction within bone fluid than did tissue from TX controls. Similarly, when prepared by an anhydrous procedure, bone surfaces of tibia taken from rats with endogenous or exogenous calcitonin contained dense material accumulation (following a calcium-containing meal) not present in bones from TX rats. (c) When tibia shaft fragments of rats sacrificed 4 h after consuming a calcium-containing meal were washed in acidic saline, more calcium accumulated during the first 15 min in the media containing bones from TI than from TX rats. The final equilibration level between media and bone fragments was not affected by the calcitonin state of the rat. These experiments demonstrate both physiological and bone morphological differences between TI and TX rats which can be negated by postprandial calcitonin injection to TX animals. They support the postulate that the secretion of calcitonin postprandially aids in the conservation and storage of ingested calcium.