[Regulation of calcium metabolism ; its rhythmic variations].

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. Normal daily fluctuations in plasma 45Ca, lost after thyroparathyroidectomy, were restored by feeding rats a high-calcium diet. A suggestion is that circadian rhythmicity originates as a result of dynamic properties involving nonlinear processes of calcium metabolism.