A New Concept of Plasma Calcium Homeostasis in the Rat

Abstract

Precise measurement of plasma calcium and phosphate concentrations were made in young rats adapted to 12 hr of darkness and 12 hr of light (not necessarily in phase with sidereal time). 1) Plasma calcium normally fluctuated between 10.9 and 11.5 mg⁄100 ml with a circadian rhythm, the minimum and maximum values occurring respectively at 10 PM and noon (sidereal time). Since rats were fed from 6 PM, the minimum value occurred 4 hr after the start of the fed period; 2) Thyroidectomy was followed by a rise in calcium and phosphate within 1 hr, but only if carried out during or just before the fed period; 3) The effects of starvation differed markedly from the effects of feeding a low-calcium, low-phosphate diet; only the latter led to significant hypophosphatemia. These results lead us to postulate the existence of “anticipatory regulation,“ a rhythmic activity of controlling systems adjusted in amplitude and frequency to the rhythms of the environment. (Endocrinology95: 480, 1974)