Endogenous nature of circadian rhythms in calcium metabolism

Rats studied when the lights are on from 0600 to 1800 daily and fed only in the dark period displayed circadian rhythms in plasma calcium (ionized and total) and 45Ca concentrations, 6 and 8 days after 45Ca administration. In rats fed a calcium-deficient diet, the amplitude of daily variation of plasma ionized and total calcium increased markedly whereas plasma 45Ca daily fluctuation remained essentially unchanged. In the calcium-deficient rats, significant correlations between plasma calcium and 45Ca and between plasma calcium and magnesium were observed throughout the 24 h; circadian periodicity of calcium metabolism persisted in rats fasted overnight, regardless of the illumination schedule. Normal daily fluctuations in plasma 45Ca, lost after thyroparathyroidectomy (TPTX), were restored by feeding the TPTX rats a high-calcium diet. These results demonstrate clearly that circadian rhythms of calcium metabolism occurred irrespective of the light-dark schedule, the calcium supply through intestines and the thyroparathyroid system. An attractive suggestion is that circadian rhythmicity originates as a result of dynamic properties involving nonlinear processes of calcium metabolism.