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TABLE I

Parameters for Model of Ca2+ Clearance in PC12 Cells

Mechanism
Parameter
Undifferentiated
Differentiated
Units
SERCA Mmax 61 58 μmol⋅Ca2+ l−1⋅s−1 
 Km 1.3 1.3 μM 
 nHill 2.9 3.4  
MtU Slope −8 μmol⋅Ca2+ l−1⋅s−1⋅μM−1 
PMCA Mmax 77 141 μmol⋅Ca2+ l−1⋅s−1 
 Km 0.71 0.97 μM 
 nHill 2.0 3.4  
NCX Mrate 560 470 μmol⋅Ca2+ l−1⋅s−1 
 KCaAct 0.5 0.85 μM 
Mechanism
Parameter
Undifferentiated
Differentiated
Units
SERCA Mmax 61 58 μmol⋅Ca2+ l−1⋅s−1 
 Km 1.3 1.3 μM 
 nHill 2.9 3.4  
MtU Slope −8 μmol⋅Ca2+ l−1⋅s−1⋅μM−1 
PMCA Mmax 77 141 μmol⋅Ca2+ l−1⋅s−1 
 Km 0.71 0.97 μM 
 nHill 2.0 3.4  
NCX Mrate 560 470 μmol⋅Ca2+ l−1⋅s−1 
 KCaAct 0.5 0.85 μM 

Parameter values were obtained using the Virtual Cell model in Appendix to fit the empirical data collected in this study. Values are given for the MtU, but we regard these small numbers as being in the noise level and ill determined. These adjusted parameters were used to generate the smooth traces shown in Fig. 1 A and Figs. 2, 4, and 9. Units of Mmax and Mrate are micromoles of Ca2+ pumped from the cytoplasm of a liter of cells per second. Because the cytoplasm contains Ca2+ buffers, these numbers are about 300 times higher than the observed rates of fall of free Ca2+ seen in transport curves and Table II. Referred to a unit area of the plasma membrane, the Mmax of 77 μmol⋅Ca2+ l−1⋅s−1 for PMCA for example is about equivalent to 27,000⋅Ca2+ ions pumped⋅μm−2⋅s−1 and might require 100-200 PMCA molecules μm−2 of membrane and consumption of 77 μmol⋅ATP l−1⋅s−1.

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