Estimation and analysis of platelet Ca²⁺ regulation. Unknown concentrations and compartment sizes in the platelet Ca²⁺ module were estimated by randomly sampling the unknown quantities in panel A and selecting only those molecular “configurations” that produced a resting [Ca²⁺]_i of 100 plus or minus 10 nM. (B) This reduced set of configurations was divided into 3 groups based on the estimated [Ca²⁺]_dts (top), which corresponded to the [Ca²⁺]_i release behavior after a 10-fold increase in [IP₃] (bottom). (C) Probability distributions for estimates of SERCA, IP₃R, IP₃, and the relative DTS volume in low-, mild-, and high-response configurations (n = 100 000). The measured IP₃ level from resting platelets³¹  is boxed. (D) Image obtained from glucose-6-phosphatase stain of the DTS from a human platelet¹⁶  was used to quantify the cytosolic (gray) and DTS (black) area from a 2D section of the platelet.