Changes in protein‐bound, complex‐bound, and ionized calcium related to parathyroid hormone levels in healthy donors during plateletapheresis

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Abstract
In order to relate changes in protein‐bound and complex‐bound calcium to levels of citrate and secretion of parathyroid hormone during mild hypocalcemia, total, ultrafiltrable, and ionized calcium were measured in 13 apparently healthy donors during plateletapheresis. Carboxyl‐terminal parathyroid hormone was measured in seven of the donors, and both amino‐terminal and intact parathyroid hormone assays were done on one donor. Mean concentrations of total and ionized calcium decreased during most of the procedure, as did total protein, albumin, and phosphorus. Complexed calcium increased, which was attributed to the large increases in citrate levels. Protein‐bound calcium decreased more than other forms of calcium, even allowing for the decrease expected from hemodilution. The carboxyl‐terminal parathyroid hormone response varied among different donors, but on the average, decreased slightly before gradually rising during the citrate‐induced hypocalcemia. These findings suggest that calcium citrate does not affect the secretion of parathyroid hormone, and that at least a portion of protein‐bound calcium is readily exchangeable and possibly influenced by the hormone.