Kinetic parameters of calcium binding to tissue structures in human acute hypercalcemia

The kinetics of calcium binding by tissue structures was analyzed in 19 healthy volunteers (13 men and 6 women) from the age group of 33 ± 6.5 years under the conditions of acute hypercalcemia followed by drip intravenous infusion of calcium gluconate for 2.5 h. At the end of each 30-min period, the calcium amount retained by the tissue structures was recorded and the kinetic parameters of calcium binding were determined according to Langmuir and Scatchard. In all volunteers, there was a region of binding isotherm with positive cooperativity (linear regression in Scatchard’s coordinates) with the same buffer capacity (β_tis) for calcium in Langmuir’s coordinates (0.58 ± 0.24 L kg). Half of the volunteers exhibited cooperativity at [Ca²⁺] = 1.3–1.5 mmol/L, while others, at [Ca²⁺] = 1.0–1.3 mmol/L, which corresponded to the differences in the association constant (K_a) and the number of interactive sites (n) with [Ca²⁺] = 1 mmol/L. Additionally, two segments of the binding isotherm were detected with the successive binding of calcium to one set of non-interactive centers with similar kinetic parameters of calcium binding (β_tis, K_a, n). Four different types of calcium binding in healthy volunteers were found. The results of this study may serve as the basis for a functional diagnostic test of disorders of the tissue calcium-binding properties in different pathological conditions.