Deliberate quin2 overload as a method forIn Situ characterization of active calcium extrusion systems and cytoplasmic calcium binding: application to the human platelet

Summary The objectives of the title were accomplished by a four-step experimental procedure followed by a simple graphical and mathematical analysis. Platelets are (i) overloaded with the indicator quin2 to cytoplasmic concentrations of 2.9mm and (ii) are exposed to 2mm external Ca²⁺ and 1.0 m ionomycin to rapidly achieve cytoplasmic Ca²⁺ (Ca²⁺]_cyt) of ca. 1.5 m. (iii) The external Ca²⁺ is removed by EGTA addition, and (iv) the active Ca²⁺ extrusion process is then monitored as a function of time. Control experiments show that the ionophore shunts dense tubular uptake and does not contribute to the Ca²⁺ efflux process during phases iii–iv and that the extrusion process is sensitive to metabolic inhibitors.The progress curves for the decline of quin2 fluorescence (resulting from active Ca²⁺ extrusion) were analyzed as a function of Ca²⁺]_cyt using a mathematical model involving the probability that an exported Ca²⁺ was removed from a quin2 complex (vs. a cytoplasmic binding element). The observed rates of decline of quin2 fluorescence at a particular Ca²⁺]_cyt are dependent upon (i) the absolute rate of the extrusion system (a function of itsK _m, V_m and Hill coefficient (n)), (ii) the intrinsic Ca²⁺ buffer capacity of the cytoplasm (a function of the total site concentration (B] _T ) and itsK _d) and (iii) the buffer capacity of the intracytoplasmic quin2 (a function of its concentration andK _d). The contribution of (iii) was known and varied and was used to determine (ii) and (i) as a function of Ca²⁺]_cyt.The Ca²⁺ binding data were verified by⁴⁵Ca²⁺ experimentation. The data fit a single binding site (B] _T =730±200 m) with an averageK _d of 140±10n m. This can be accounted for by platelet-associated calmodulin. The rate of the Ca²⁺ extrusionvs. Ca²⁺]_cyt curve can be described by two components: A saturable one withV _m=2.3±0.3 nmol min^–1 mg-membrane^–1,K _m=80±10 andn=1.7±0.3 (probably identified with a Ca²⁺-ATPase pump) and a linear one (probably identified with a Na⁺/Ca²⁺ exchanger).