Diurnal variation of plasma calcium and calcitonin function in the rat.

1. These experiments were designed to investigate the normal physiological role of calcitonin in the control of blood calcium.2. The rat can be adapted to an artificial cycle of alternating 12-hr periods of light and darkness. Since rats eat only in the dark, calcium absorption is confined to the dark period.3. In rats thus adapted, the plasma calcium and phosphate levels show cyclic variation related, among other factors, to the calcium intake.4. The variation observed is modified by the removal of the thyroid gland. This cannot be attributed to deprivation of thyroxine and triiodothyronine because these experiments were carried out within 2 days of thyroidectomy.5. Thyroidectomy raises the plasma calcium and phosphate levels, but only if performed during the dark-fed period. This suggests the existence of a diurnal rhythm in endogenous calcitonin function.6. The magnitude of the falls in plasma calcium and phosphate, produced by exogenous calcitonin, also varies with the time of day. This shows that the rat's responsiveness to calcitonin also varies diurnally.