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.