Biology Reference
In-Depth Information
Table III
List of Ca
2
þ
binding compounds that can act as shuttle bu
V
ers for extra-
cellular Ca
2
þ
in the 0.1-1.0 mM range
Ligand
log(K
d
)
Pyrophosphate
5.0
4.0
1
N-(2-acetamido)iminodiacetic acid (ADA)
ATP
3.8
Citric acid
3.5
Oxalic acid
3.0
Polyphosphate
3.0
N,N-bis(hydroxyethyl)-glycine (Bicine)
2.8
Values were obtained from
Dawson et al. (1986)
except as noted
1
(
Lance et al., 1983
).
have been mapped near many di
erent systems (
Borgens et al., 1989; Nuccitelli,
1986
) Extracellular electric fields generated by cells are generally very small espe-
cially in high conductivity media such as animal saline. However, in lower conduc-
tivity saline ion transport can give rise to relatively large electric fields
V
1
m
V/
10
m
m which can be detected with self-referencing microelectrodes. Plants for
example, drive transcellular currents through them, as a result of ion transport
across single cells or tissues. In low conductivity medium, these currents generate
substantial voltage gradients next to the cells, coexisting with the concentration
gradients of the transported ions. The di
>
V
erential voltage measured by the CaSM
erences due to the [Ca
2
þ
]di
will be the sum of the voltage di
V
V
erence and the
V
V
voltage di
erence during oscillating cur-
rent influx of about 9
m
V would occur over a 10-
m
m distance immediately in front
of a lily pollen tube. Peak current density around 0.4
m
A/cm
2
was measured at a
distance of about 20
m
m from the cell surface with a medium resistivity of about
5000
O
cm (
Messerli and Robinson, 1998
). This voltage di
erence. For example, a peak voltage di
V
erence is just above the
5
m
V for Ca
2
þ
,(
Messerli et al.,
1999
). The voltage signals detected by the self-referencing CaSM peaked about six
times larger than the di
background noise of the system used at that time,
erential voltage due to current flux indicating that the
extracellular electric field could have contributed to the calculated Ca
2
þ
flux by up
to 15%.
V
F. Positional Artifacts
Self-referencing of CaSMs near solid objects can generate position dependent
artifacts. Movement of Ca
2
þ
across the external interface between the liquid
membrane and bathing medium may occur through current driven and zero net
current mechanisms (
Bakker and Meyerho
, 2000
). Release of Ca
2
þ
by the CaSM
restricts its sensitivity in bulk medium by leading to a modification of the local ion
V
