Ionized calcium determinations in the serum of chick embryos.

The addition of a concentrated, calcium-equilibrated TRIS buffer to serum samples, facilitated the determination of the level of ionized calcium (iCa) without interference of pH changes due to CO2 loss. Determinations were carried on sera from chick embryos of various ages and compared with values obtained in the sera from young chicks. It was found that iCa levels in the embryos were significantly higher than those obtained in young chicks, the decrease beginning during the first day after hatching. These results help in the clarification of the mechanism(s) inducing the high calcitonin levels observed in the embryonic serum by previous authors.