Biology Reference
In-Depth Information
will be much larger/smaller for the same Ca 2 þ e
ux/influx on lowered background
[Ca 2 þ ]. This has led to the lowering of the background [Ca 2 þ ] in order to generate
D E with a greater signal-to-noise ratio, see Table IV . Care must be taken to ensure
that changing the background concentration does not interfere with normal cellu-
lar activity.
Rearrangement of Eq. (2) relates the [Ca 2 þ ] in the near pole of excursion to the
[Ca 2 þ ] at the far pole of excursion.
Z
S 2
S 1
i
10 D S 1
C i ð 1 Þ ¼ C
Þ
ð 3 Þ
ð
2
For an ideal electrode, the voltage output is close to the Nernstian slope over a
wide range of [Ca 2 þ ]soS 1 ¼ S 2 ¼ S ¼
2 : 3 RT
z i F
. Therefore, Eq. (3) simplifies to
10 D S
C i ðÞ ¼ C i ðÞ
ð 4 Þ
For minute fluxes that are typically measured with self-referencing, the average
concentration of Ca 2 þ at the far pole, position 2, is not too di
erent from the
average concentration of Ca 2 þ in the bulk solution. Therefore the di
V
V
erence in
[Ca 2 þ ] between the two points of excursion can be described as follows:
10 D S C bath
DC ¼ C i ðÞ C i ðÞ ¼ C bath
ð 5 Þ
A primary assumption here is that the excursion distance is small compared to the
extent that the gradient extends out into the bulk solution so that the concentration
di
erence between the two excursion points is linear. For minute gradients
measured from small cells (
V
10 m m diameter), an excursion of 10 m m will most
likely sample over a distance in which the concentration di
erence is not linear and
therefore will lead the investigator to underestimate the true flux. Incorrect estima-
tion of the true flux could also occur during a two-point measurement in a more
intense, extended gradient, where the concentration of the ion in the far pole is
substantially di
V
erent from the background concentration of the ion. In both of
these cases, a three-point measurement should be performed in order to (1) more
carefully map the concentration gradient with a third point to ensure a linear
relationship or determine a more accurate nonlinear relationship and (2) to deter-
mine the concentrations in the gradient relative to the background concentration
of the ion in the bath.
The selectivity of Ca 2 þ liquid membranes is relatively good compared to liquid
membranes for other ions. Therefore, measurement of Ca 2 þ gradients in the
presence of a constant concentration of an interfering ion or in the presence of
a gradient of an interfering ion is not a major concern. However, specific
circumstances may require the use of higher concentration of an interfering
ion and the two cases need to be addressed. Details necessary to account for
these situations have been addressed previously ( Messerli
V
et al., 2006; Smith
et al., 2007 ).
Search WWH ::




Custom Search