Making and Using Calcium-Selective Mini- and Microelectrodes - Methods in Cell Biology - page 69

100

3

75

Indo-1

2

50

1

25

0

0

Minielectrode

Microelectrode

− 25

− 50

− 75

8 7 6 5 4 3

pCa

Fig. 1 Dynamic range of Ca 2 þ -selective electrodes and indo-1. The electrode potential of Ca 2 þ -
selective mini and MEs is shown together with the fluorescence ratio (400/470) for indo-1. Measure-
ments were performed in a KCl bu

V

er containing 140 mM KCl, 10 mM HEPES, 10 mM NaCl, and
1 mM EGTA. Notice that indo-1 is suitable for measurements between pCa 7.5 and 5, while the
dynamic range for the Ca 2 þ electrode is wider, ranging from pCa 9 to 1 for minielectrodes and 7.5 to
1 for MEs.

for the Ca 2 þ -selective ligand), the relationship becomes, for 30 C and changing
to log 10:

þ K

2

pot

30 log Ca 2 þ

NaCa Na þ

E Ca ¼ E o þ ½ ð 2 Þ

Thus for the case of no interfering ion and when extracellular Ca 2 þ ¼ [Ca 2 þ ] ref ,

erent [Ca 2 þ ] reduces

then the potential di V erence ( D E) between two solutions of di V

to the Nernst equation:

Ca 2 þ ref Þ ð 3 Þ
The response of an ion-selective electrode to changes in free [Ca 2 þ ] is much slower
than the fluorescent, bioluminescent, and metallochromic Ca 2 þ indicators. Thus,
Ca 2 þ electrodes are ideal for measurements of slow changes over wide ranges of
[Ca 2 þ ], but not appropriate for very rapid changes of free [Ca 2 þ ] although the Ca 2 þ
electrodes can respond in the millisecond range at higher [Ca 2 þ ]( Bers, 1983 ). The
response time can also be improved by using Ca 2 þ selective electrodes with a
concentric inner micropipette that reduces the longitudinal resistance of the
Ca 2 þ -selective resin and thereby decreases the electrical time constant ( Fedirko
et al., 2006; Ujec et al., 1979 ). These limitations, together with the physical size of
Ca 2 þ -selective electrodes put them at disadvantage with fluorescent and lumines-
cent calcium indicators for measurements of dynamic changes in calcium levels in

Ca 2 þ =½

DE ¼ ð½

30 log