[Kinetic parameters of calcium binding to tissue structures in human acute hypercalcaemia]

Calcium binding kinetic by tissue structures was analysed in 19 health volunteers (13 men and 6 women) of the age group of 33 +/- 6.5 under conditions of acute hypercalcaemia followed by a drip i.v. infusion of calcium gluconate over 2.5 hours. At the end of each 30-minute period the calcium amount retained by tissue structures was recorded and the kinetic parameters of calcium binding were determined according to Langmuir and Scatchard. In all volunteers there was a segment of binding isotherm with positive cooperativity (direct regression in Scatchard) with analogous buffer capacity (beta) for calcium in Langmuir (0.58 +/- 0.24 L kg). One half of volunteers demonstrated cooperativity at [Ca++] 1.3-1.5, while another--at [Ca++] 1.0-1.3 mmol/L which corresponded to the differences in the association constant (Ka) and the number of interactive sites (n) with [Ca++] 1 mmol/L. Additionally, two segments of binding isotherm were detected with the successive binding of calcium to one set of noninteractive sites with similar kinetic parameters of calcium binding (beta, K(a), n). Four different curves of calcium binding in healthy volunteers were established. This study may serve as the basis for a functional diagnostic test of disorders of the tissue calcium-binding properties in different pathological conditions.