Biology Reference
In-Depth Information
The next application of the nitr chelators was in an analysis of Ca
2
þ
-activated
currents in Aplysia neurons (
Land
`
and Zucker, 1989
). We found that Ca
2
þ
-
activated K
þ
and nonspecific cation currents in bursting neurons were linearly
dependent on [Ca
2
þ
]
i
jumps in the micromolar range, as measured by arsenazo
spectrophotometry and modeling studies. Both currents relaxed at similar rates
after photolysis of nitr-5 or nitr-7, reflecting di
usional equilibration of [Ca
2
þ
]
i
near the front membrane surface facing the light source. Potassium current relaxed
more quickly than nonspecific cation current, after activation by Ca
2
þ
entry during
a depolarizing pulse, because of the additional voltage sensitivity of the K
þ
channels. This di
V
erence was responsible for the more rapid decay of hyperpolar-
izing afterpotentials than of depolarizing afterpotentials.
The role of Ca
2
þ
-activated K
þ
current in shaping plateau potentials in gastric
smooth muscle was explored by
Carl et al. (1990)
. In fibers loaded with nitr-5/AM,
Ca
2
þ
photorelease accelerated repolarization during plateau potentials and
delayed the time to subsequent plateau potentials, suggesting a role for changes
in [Ca
2
þ
]
i
and Ca
2
þ
-activated K
þ
current in slow wave generation.
Another current modulated by [Ca
2
þ
]
i
is the so-called M current, a muscarine-
blocked K
þ
current in frog sympathetic neurons. Although inhibition is mediated
by G-protein coupling of the receptor to phospholipase C, resting M current is
enhanced by modest elevation of [Ca
2
þ
]
i
(some tens of nanomolar) and reduced by
greater elevation of [Ca
2
þ
]
i
, which also suppresses the response to muscarine
(
Marrion et al., 1991
). As for ventricular I
Ca
(see below), several sites of modula-
tion of M current by [Ca
2
þ
]
i
apparently exist. In these experiments, [Ca
2
þ
]
i
was
elevated by photorelease from nitr-5 and simultaneously measured with fura-2.
Step changes in [Ca
2
þ
]
i
imposed by diazo-2 photolysis and monitored with bis-
fura-2 fluorescence changes have also been used to characterize the modulation of
cGMP-gated ion channels by [Ca
2
þ
]
i
(
Rebrik et al., 2000
).
The after-hyperpolarization that follows spikes in rat hippocampal pyramidal
neurons is caused by a class of Ca
2
þ
-dependent K
þ
channels called I
AHP
channels.
This after-hyperpolarization and the current underlying it rise slowly to a peak 0.5 s
after the end of a brief burst of spikes. Ca
2
þ
photorelease fromnitr-5 orDM-nitrophen
activates this current without delay (
Lancaster andZucker, 1994
), and the currentmay
be terminated rapidly by photolysis of diazo-4 (but see conflicting results of
Sah and
Clements, 1999
), suggesting that the delay in its activation following action potentials
iscausedbyadi
V
usion delay between points of Ca
2
þ
entry and the I
AHP
channels.
The Ca
2
þ
sensitivity of the mechanoelectrical transduction current in chick
cochlear hair cells was studied using nitr-5 introduced by hydrolysis of the AM
form (
Kimitsuki and Ohmori, 1992
). Elevation of [Ca
2
þ
]
i
to 0.5
m
M (measured
with fluo-3) diminished responses to displacement of the hair bundle, and acceler-
ated adaptation during displacement when Ca
2
þ
entry occurred. Preventing Ca
2
þ
influx blocked adaptation. Evidently, adaptation of this current was the result of
an action of Ca
2
þ
ions entering through the transduction channels.
In guinea pig hepatocytes, noradrenaline evokes a rise in K
þ
conductance after a
seconds-long delay. Photorelease of Ca
2
þ
from nitr-5 and use of caged inositol
V
