CALCIUM AND PHOSPHORUS METABOLISM IN THE ia RAT

¹ Biological Research Laboratories, Harvard School of Dental Medicine; Department of Pharmacology, Harvard Medical School, Boston, Mass., and Department of Histology, College of Dentistry, University of Illinois, Chicago, Ill.

1. The calcium and phosphorus metabolism of ia and normal rats were studied by determining the serum calcium and phosphorus levels in rats which had been maintained on normal diet and in those which had been calcium depleted and parathyroidectomized with and without the simultaneous administration of parathyroid extract.

2. No differences between the serum calcium levels of the ia and normal rats maintained on normal commercial chow were observed from 10 to 59 days of age with the exception of the 40- to 49-day age group in which those of the ia rat were higher.

3. The serum phosphorus levels of the ia rats were found to be consistently lower than those of the normal rats below the age of 40 days. This difference was shown by a pair-feeding experiment, in which the direction of the difference was reversed, to be largely dependent upon feeding habits.

4. Bone analyses revealed the ia rat femur to be hypercalcified relative to the normal rat femur up to the age of 49 days. A rapid rise in the bone ash content of the ia rat femur from 10 days of age to a maximum in the 30- to 39-day age period was followed by a steady decline to normal values by the 50- to 59-day age period. Such findings indicated a sudden increase in net resorption of the ia bone following the 30- to 39-day age period and the possible disappearance of the fundamental defect by this time.

5. Determination of the calcium and phosphorus contents of the bone ash suggested no significant differences between the 2 phenotypes with the exception of the bone ash calcium contents in the 10- to 19-day and 30- to 39-day age groups in which those of the ia rats were higher than normal.

6. Calcium depletion for either 4 or 13 to 14 days uncovered no strain on the serum calcium levels of the ia rat relative to that of the normal rat.

7. The administration of bovine parathyroid extract following calcium depletion and parathyroidectomy gave serum calcium levels in the ia rat as high as, if not higher than, those in the normal rat. This observation suggested that the ia rat bone responds to resorption by parathyroid hormone.

8. It is concluded that the generalized defect in bone resorption in the ia rat is not due primarily either to the existence of a chronic hypoparathyroidism or to a resistance of the ia rat bone to resorption by parathyroid hormone.