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	−8	μmol⋅Ca2+ l−1⋅s−1⋅μM−1
PMCA	M ma x	77	141	μmol⋅Ca2+ l−1⋅s−1
	K m	0.71	0.97	μM
	n Hill	2.0	3.4
NCX	M rate	560	470	μmol⋅Ca2+ l−1⋅s−1
	K CaAct	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 M _max and M _rate 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 M _max 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.