Articles |
From the Department of Physiology (L.A.B.), Loyola University Chicago, Maywood, Ill; the Department of Chemistry and Institute of Biotechnology (Z.T., S.M., T.M.), Oakland University, Rochester, Mich; and the Department of Physiology (P.S.S., W.G.W.), University of Maryland, Baltimore, Md.
Correspondence to Tadeusz Malinski, PhD, Department of Chemistry, Institute of Biotechnology, Oakland University, Rochester, MI 48309-4401.
| Abstract |
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0.5 µmol/L. Simultaneous measurements of
[Ca2+]i and [NO] in BK-stimulated
endothelial cells revealed that a transient increase in
[Ca2+]i was rapidly followed by an
increase in [NO] that outlasted the
[Ca2+]i transient.
Key Words: endothelium-derived relaxation factor/nitric oxide bradykinin cytoplasmic calcium concentration porphyrinic NO microsensor fura 2
| Introduction |
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Endothelium-derived relaxation factor (EDRF) was discovered more than a decade ago6 and recently has been confirmed to be identical to nitric oxide (NO). NO has been determined to be a unique, ubiquitous messenger of cellular signals. NO is not only involved in the regulation of blood pressure but also has been characterized as a neurotransmitter and plays an important role in the immune system.7 Its chemical nature makes NO an excellent candidate for short-term and short-range signaling: NO is very lipophilic (and therefore diffuses readily through cellular membranes), it is synthesized rapidly on demand, and its short life ensures a localized response.8
NO is synthesized in vascular endothelial cells from L-arginine by the Ca2+/calmodulindependent enzyme NO synthase (NOS). NOS is fully activated by [Ca2+]i that is also encountered as a result of exogenous stimulation with various agonists such as the vasoactive peptide bradykinin (BK).9 Although there is substantial indirect evidence that synthesis and release of NO by endothelial cells is [Ca2+]i dependent, experimental limitations have heretofore precluded the direct simultaneous in situ measurements of [NO] and [Ca2+]i. Recently, a porphyrinic microsensor has been developed10 with characteristics, namely dimensions, response time, sensitivity, and selectivity, for NO, and it has been applied to the in situ monitoring of NO release from single cells.
In this study we report for the first time the simultaneous measurements of [NO] and [Ca2+]i obtained from individual vascular endothelial cells. [NO] was measured either intracellularly or directly from the cell surface by placing a flexible NO microsensor onto the cell membrane. BK caused transient increases of [Ca2+]i that were followed by longer-lasting transient increases of [NO]. A 20 µmol/L [BK] caused [NO] transients that peaked at approximately 0.5 µmol/L.
| Materials and Methods |
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3.9 mmol/L). In the nominally
Ca2+-free solution, the addition of
CaCl2 was omitted. BK and L-arginine were
obtained from Sigma.
[Ca2+]i was measured with the
fluorescent calcium indicator fura 2, using the membrane-permeant fura
2 acetoxymethyl ester (fura 2-AM). The cells were loaded with the
indicator by exposure to a physiological salt solution containing 0.5
µmol/L fura 2-AM for
30 minutes. Dye loading and experiments were
carried out at room temperature (20°C). For the
[Ca2+]i measurements, fura 2
fluorescence was excited at 360 and 380 nm. Although no intracellular
calibration was obtained, the fura 2 ratio signal was translated into a
calcium concentration term, using calibration solutions with a typical
intracellular ionic background and containing fura 2 pentapotassium
salt, in order to provide an approximate quantification of the observed
changes in [Ca2+]i. The experimental
setup for fluorescence microscopy and digital imaging of
[Ca2+]i has been described in detail
previously.11 12 Its main components are a
charge- coupled device (CCD) camera (TM-745E, PULNiX America Inc)
fiber-optically coupled to a microchannel plate intensifier (Philips
Electronic Instruments Inc) and a real-time image processor (series
151, Imaging Technology, Inc) under the control of a microcomputer.
Typically [Ca2+]i images were obtained
every 10 s, calculated from an average of eight successive
background-subtracted video frames recorded at both excitation
wavelengths.
Several microsensors were prepared as described previously.10 Briefly, a single sharpened carbon fiber (0.5 to 7.0 µm diameter) encapsulated in a glass capillary with 1 mm protruding (for the extracellular measurements) was modified by coating with nickel(II) tetrakis(3-methoxy-4-hydroxyphenyl)porphyrin by use of cyclic scanning at a potential between -0.2 and 1.0 V. The polymeric porphyrin was subsequently coated by dipping for 10 s in 1% Nafion solution and left to dry in air. Prior to the electrochemical measurements of NO, the electrode was placed on the surface of a cell and a constant potential of 0.65 V (versus saturated calomel electrode) was applied using an EC 225 IBM voltametric analyzer and current-sensitive preamplifier. The amperograms were recorded on an Omniscribe strip chart recorder. Standard solutions of NO were prepared as decribed previously.13 Data are mean±SEM of n separate experiments and were analyzed by ANOVA. Post hoc tests of the significance of difference among individual means were performed using Tukey's procedure. Significance was established at P<.05.
| Results and Discussion |
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Simultaneous Measurements of [NO] and
[Ca2+]i
Fig 2
shows a group of confluent endothelial cells
loaded with fura 2. Fig 2A
shows the fura 2 fluorescence excited at 380
nm. Fig 2B
shows the same group of cells in bright-field illumination
and the position of the NO sensor on a cell's surface. The
simultaneous measurements of [NO] and
[Ca2+]i were done with a porphyrinic
sensor and fluorescence imaging microscopy, respectively. BK-stimulated
endothelial cells revealed a transient increase of [NO] that
outlasted the [Ca2+]i transient. A
typical concentration-time profile obtained during simultaneous
measurements of [NO] and [Ca2+]i is
shown in Fig 3
. Addition of BK (20 µmol/L) initiated
an increase in [Ca2+]i within 3.0±0.4
s (n=5) (Fig 3A
). A maximum concentration of 1.0±0.1 µmol/L was
reached over 10±1 s. The peak [Ca2+]i
increase was followed by a small decrease, with formation of a
semiplateau at a level of 0.9 µmol/L. The semiplateau was observed
for about 50±5 s, after which [Ca2+]i
declined to near basal levels after about 350±20 s.
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NO was detected by the porphyrinic sensor after 8.0±0.5 s (n=5)
following the addition of BK (Fig 3B
). This was a delay of 5 s from the
release of detectable amounts of Ca2+. The initial
rate of NO release was 8.0±0.7 nmol/L per second. After 15 s, the rate
of NO release decreased to 0.5±0.1 nmol/L per second, and a plateau
was established after 175±20 s that lasted for about 200±50 s. After
that time, [NO] decayed slowly with a rate of 0.8±0.2 nmol/L per
second and reached an undetectable level after 14 minutes. The time of
appearance of a detectable signal of Ca2+ or NO
depends on several factors. The major factor is mass transport of BK
(diffusion/convection) from the point of its injection to the cell
membrane. Other factors are interaction with receptors, signal
transduction, release and diffusion of Ca2+,
and synthesis and diffusion of NO. The mass transport of BK depends on
its gradient of concentration between the point of injection and the
cell surface. Therefore, the mass transport of BK results in a lag time
between the time of injection and the time at which the analytical
signal is recorded at the cell surface. There was an observed decrease
in time between injection and measurement of the analytical signal as
the applied concentration of BK increased (15±2 s and 8.0±0.7 s for
BK concentrations of 0.1 µmol/L and 20 µmol/L, respectively).
However, the maximum [NO] remains approximately the same in the
concentration range of 0.1 to 20 µmol/L of BK. The contribution of
the response time of the sensor (10 ms for [NO] of 10 nmol/L and 1 ms
for [NO] of 1 µmol/L) is negligible in comparison with the observed
time of the appearance of detectable NO signal. The concentration-time
profile of [Ca2+]i suggests that a
rapid and transient increase is sufficient to activate NOS. However,
there is apparently no need for the continuous presence of increased
[Ca2+]i for the surge in NO
production.
[NO], when monitored on the cell membrane after stimulation with BK, can be observed for a time interval of several minutes, which coincides with previous observations for porcine aorta endothelial cells.10 NO synthesis and release from cultured aortic endothelial cells was measured with the porphyrinic microsensor either from the surface of the cells or intracellularly after impalement of the microelectrode into a single cell. A difference between surface [NO] and intracellular [NO], with a lower concentration in the cytoplasm, was observed only in the first few seconds of the NO release process. After that, the surface and intracellular concentrations reached similar levels of 0.50±0.05 µmol/L. As previously reported,8 the cell membrane does not present a barrier to the diffusion of NO, and NO tends to be preconcentrated in the hydrophobic environment of the membrane. The concentration in the membrane creates a high concentration gradient on both sides of the membrane. This gradient facilitates an efficient diffusion-controlled supply of NO to adjacent cells. Detection of NO on the surface of the cell membrane, the location with the highest level of [NO], has been found to be an efficient, accurate, and reproducible method of measuring NO release. Therefore, this method was used in the studies being presented. The profile of the NO concentration-time curve depends on the kinetics of NO release (supply) and the dynamics of NO mass transport, which is controlled by depletion or dilution of NO and its consumption due to chemical reactions. The time of duration of the plateau depends on these same three factors, and it can also be affected by NO production by neighboring cells. Generally, a maximum concentration measured on the membrane of a single isolated cell or a single cell within a group of cells will be identical within an experimental error. However, the time of duration of the plateau is shorter when the measurements are done on a single isolated cell. The depletion process (diffusion/convection), which is due to a higher gradient of [NO], will be faster for the isolated cell than for the same cell located within a group of cells.
| Acknowledgments |
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Received September 9, 1994; accepted December 29, 1994.
| References |
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