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Table 2.

Ca bound to BAPTA relative to other possible sites

Species Concentration of Ca binding sites referred to   Fraction relative to CaBAPTA 
 Myoplasm Final solution Kd [CaX]  
 µM µM µM µM  
BAPTA  120 0.22 16 1.0 
Free Ca2+    0.034 0.0021 
Troponin 240 0.93 0.2 0.135 0.0084 
SERCA 240 0.93 0.43 0.068 0.0043 
CSQ 5,500 21.3 780 (n = 3) 1.7 × 10−12 10−13 
Ca/Mg-binding sites with [Mg2+] = 0 
Parvalbumin 1,500 7.0 0.012 5.2 0.33 
Troponin 240 0.93 0.002 0.88 0.055 
Myosin 240 0.93 0.033 0.46 0.029 
Ca/Mg-binding sites with [Mg2+] = 2 mM 
Parvalbumin 1,500 7.0 0.276 0.77 0.05 
Troponin 240 0.93 0.202 0.134 0.0084 
Myosin 240 0.93 18.4 0.002 0.0001 
Species Concentration of Ca binding sites referred to   Fraction relative to CaBAPTA 
 Myoplasm Final solution Kd [CaX]  
 µM µM µM µM  
BAPTA  120 0.22 16 1.0 
Free Ca2+    0.034 0.0021 
Troponin 240 0.93 0.2 0.135 0.0084 
SERCA 240 0.93 0.43 0.068 0.0043 
CSQ 5,500 21.3 780 (n = 3) 1.7 × 10−12 10−13 
Ca/Mg-binding sites with [Mg2+] = 0 
Parvalbumin 1,500 7.0 0.012 5.2 0.33 
Troponin 240 0.93 0.002 0.88 0.055 
Myosin 240 0.93 0.033 0.46 0.029 
Ca/Mg-binding sites with [Mg2+] = 2 mM 
Parvalbumin 1,500 7.0 0.276 0.77 0.05 
Troponin 240 0.93 0.202 0.134 0.0084 
Myosin 240 0.93 18.4 0.002 0.0001 

Ability of BAPTA to capture Ca from muscle components. Columns 2 and 3 give the concentrations of the Ca-binding sites associated with the substance in column 1. The concentrations in columns 2 and 3 refer to different volumes, the myoplasmic water volume of the muscle sample for column 2, and the volume of the final solution for the measurement (with the homogenized muscle) for column 3. Details for the conversion from column 2 to column 3 given in the text for troponin also apply to the other muscle components. Column 4 gives the Kd for the substance in column 1. Values in columns 2 and 4 were obtained from the literature as detailed in the text. Column 5 gives the concentration of Ca in the final solution (CaX) bound to BAPTA, present as the free form, or bound to muscle components. As indicated in the text, the assumed value of 16 µM for Ca bound to BAPTA is typical of the experimentally measured values. The concentration of free Ca2+ was determined from the relative concentrations of Ca-bound and Ca-free BAPTA and its Kd in column 4. Values for Ca bound to the muscle components were determined with this value for free Ca2+ using the values in columns 3 and 4. To assess how well BAPTA is expected to capture Ca from the different sites, column 6 gives the ratio of the values of [CaX] for the different components in column 5 to that for CaBAPTA. With the exception of the last three rows, it was assumed that no Mg was present. The last three rows were calculated with a free Mg2+ concentration of 2 mM. See text for additional details.

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