Influence of continuous infusion of citrate on responses of immunoreactive parathyroid hormone, calcium and magnesium components, and other electrolytes in normal adults during plateletapheresis.

To study the relationships between changes in concentrations of different forms of calcium and the responses of immunoreactive PTH in humans during citrate-induced hypocalcemia, we studied 12 healthy donors undergoing continuous flow plateletapheresis. Concentrations of intact, amino-terminal, and midregion PTH; ionized, ultrafiltrable, and total calcium; total and ultrafiltrable magnesium; protein; albumin; pH; phosphate; and citrate were measured in sera collected during the first 95 min of apheresis. Although ionized calcium decreased steadily in every donor, both intact and amino-terminal PTH rose quickly in most donors to a peak level 5-15 min after starting the infusion of citrate and then declined during the remainder of apheresis. Midregion PTH also rose quickly by 5-remainder of apheresis. Midregion PTH also rose quickly by 5-15 min, but levelled off at 30-90 min. Total calcium inversely correlated with both intact PTH (r = -0.76) and amino-terminal PTH (r = -0.81) better than did ionized calcium (r = -0.47 and -0.46, respectively). The rapid rise and then gradual fall of PTH may be due to glandular depletion of stored PTH, increased peripheral metabolism of PTH, or PTH initiation of other hormonal actions that compensate for hypocalcemia. Protein-bound calcium measured, and this change probably reflected dissociation of calcium from its albumin-binding sites to minimize the changes in ionized calcium concentrations.