Integrative Physiology |
From the Department of Medicine, Cardiology Division, Johns Hopkins Medical Institutions, Baltimore, MD 21287-6568.
Correspondence to Joshua M. Hare, MD, Johns Hopkins Hospital, Cardiology Division, 600 N Wolfe St, Carnegie 568, Baltimore, MD 21287-6568. E-mail jhare{at}mail.jhmi.edu
| Abstract |
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Key Words: heart failure caveolae signal transduction compartmentation
| Introduction |
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These seemingly conflicting findings may be reconciled if abnormalities in NOS regulation exist in failing myocardium. NOS3, which is constitutively found in cardiac myocytes9 in addition to endothelial cells, is regulated by caveolar scaffolding proteins, the caveolins.10 11 12 Caveolin-3 (in myocytes) and caveolin-1 (in endothelial cells) bind NOS3, preventing its subsequent activation by calmodulin. Caveolin interactions have also been reported with NOS110 13 and NOS2.14 Stimuli that activate NOS via Ca2+ and calmodulin displace NOS from caveolin, thereby relieving its inhibitory effect (reviewed by Michel and Feron15 ). Given the fact that caveolins bring NOS into proximity with sarcolemmal agonists, relative increases in their protein abundance have the potential to augment NO pathway activity in response to agonist (ie, ß-adrenergic) stimulation via a compartmentation effect and at the same time to inhibit basal unstimulated activity.
The purpose of the present study was to test the hypothesis that failing myocardium exhibits increased abundance of caveolin-3 and/or caveolae. Experiments were conducted in the pacing-induced canine HF model, which exhibits NO-related inhibition of ß-adrenergic inotropic responses that is similar to the inhibition found in humans with cardiomyopathy in the absence of increased levels of ventricular NOS.16
| Materials and Methods |
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Measurement of Caveolin and NOS Protein Abundance/Enzyme
Activity
Western blotting was performed by using protein purified from
total LV free wall myocardium as described,18
with use of the following antibodies: monoclonal mouse anticaveolin-1
and caveolin-3, rabbit polyclonal anti-mouse NOS3, and polyclonal
anti-NOS2. Caveolin-3 Western blots were also performed with the use of
protein from canine myocytes, isolated by collagenase
digestion, as described.19 NOS activity was measured by
the conversion of L-[14C]arginine
to L-[14C]citrulline by using a
modification of the method of Bredt and Snyder20 as
described.18
Transmission Electron Microscopy
LV endomyocardial samples obtained from HF
and control (CTL) animals (n=2 each) immediately after euthanasia were
fixed in 2.5% glutaraldehyde, washed in 0.1 mol/L
cacodylate buffer (3 times for 5 minutes), and subjected to
postfixation with 1% osmium tetroxide in 0.1 mol/L cacodylate for 1
hour. Formvar-coated copper grids were stained with 2% uranyl acetate
for 30 minutes, which was followed by subsequent staining with 0.02%
lead citrate for 3 minutes, and imaged by use of a Philips CM 120
transmission electron microscope. Individual myocyte membranes (n=15 or
16 each for CTL and HF groups) were imaged at x27 500 (27.5 K) and
x74 000 (74 K) magnifications, and caveolae were counted by blinded
investigators.
Two-Photon Microscopy
Isolated myocytes from CTL and HF dogs (n=2 each) were attached
to coverslips with laminin, fixed in 50% methanol/50% acetone, and
incubated first with monoclonal antibodies to caveolin-3 and NOS3 and
then with anti-mouse rhodamine (Jackson Immunoresearch) and anti-rabbit
Alexa 488 (Molecular Probes). Imaging and colocalization
analysis was performed on a Nikon E600FN upright
physiological fluorescence microscope with
a Bio-Rad MRC-1024/2-P multiphoton imaging system attachment, as
described in detail in the online-only Materials and Methods section
(see http://www. circresaha.org).
Response to ß-Adrenergic Stimulation and NOS Inhibition
The influence of NO was assessed by central venous infusion of
the NOS inhibitor L-NMMA (kindly provided as
L-NG-methylarginine
hydrochloride by Glaxo-Wellcome) in 1 of 2 concentrations, 10 or 20
mg · kg-1 ·
h-1 for 1 hour. These infusion rates were
selected from clinical trials of L-NMMA for sepsis,
representing maximal and half-maximal
infusions.21 The concentration-effect relation to the
ß-adrenergic agonist dobutamine was obtained before and
after L-NMMA infusion. Dobutamine was infused via right
atrial catheter at 2.5 and 5 µg ·
kg-1 · min-1 for 5
to 7 minutes until steady-state increases in peak +dP/dt were obtained.
In addition to these infusion rates, after the induction of HF,
dobutamine was also increased to 10 and 15 µg ·
kg-1 · min-1.
After the dobutamine infusions, normal saline was infused
for 15 minutes, and baseline conditions were reestablished. The NOS
inhibitor L-NMMA was then infused at either 10 or 20 mg/kg
over 1 hour, and hemodynamic measurements were obtained
every 10 minutes. After 60 minutes, the L-NMMA was continued, and the
dobutamine infusions were repeated.
Data Analysis
Data are presented as mean±SEM and were
analyzed by t test, multiple linear regression, or
ANOVA, as appropriate.
An expanded Materials and Methods section is available online at http://www.circresaha.org.
| Results |
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Caveolin-1 and -3 Abundance
Concentrations of the muscle-specific isoform, caveolin-3,
were increased in HF versus CTL myocardium
(0.59±0.08 versus 0.29±0.08 arbitrary units, P=0.01; n=10
or 11 each; data from 2 separate blots; Figure 1
). These findings were confirmed in an
additional blot using protein from isolated canine myocytes (0.63±0.33
versus 0.27±0.25 arbitrary units for HF versus CTL, respectively; n=3
each). In contrast, the endothelial cellspecific
isoform, caveolin-1, was unchanged in HF compared with CTL
myocardium (Figure 1
).
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Caveolar Abundance
Transmission electron microscopy was performed to quantify
caveolar abundance. Caveolae, defined as 50- to 100-nm membrane
invaginations, were 2-fold more abundant (2.7±0.4 versus 1.3±0.3 per
micrometer plasmalemmal membrane,
P<0.005) in HF compared with CTL myocytes (Figure 2
).
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Two-Photon Imaging
To determine whether caveolin-3 and NOS3 had similar
colocalization in myocytes, 2-photon imaging was performed with the use
of myocytes costained with caveolin-3 and NOS3 antibodies
(Figure 3
). Caveolin-3 staining
localized to the plasmalemmal membrane as well as to
T-tubular structures in both CTL and HF myocytes. NOS3, on the hand,
exhibited a diffuse staining pattern throughout the cell. The staining
patterns colocalized only at the sarcolemma and T tubules.
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Response to NOS Inhibition in Normal Dogs
Dobutamine produced a positive inotropic effect
reflected by increases in all indices of myocardial
contractility (Table 2
).
In the CTL group, the peak infusion of dobutamine (5
µg · kg-1 ·
min-1) increased peak +dP/dt by 869±148
mm Hg/s (36±7%). This was suppressed in the HF group, with +dP/dt
increasing by 162±64 mm Hg/s or 9±2% at 5 µg ·
kg-1 · min-1
(P<0.001 versus CTL group). To achieve increases in +dP/dt
similar to those in the CTL group, dobutamine infusions
were increased to rates of 10 to 15 µg ·
kg-1 · min-1, so
as to increase peak +dP/dt by 828±26 mm Hg/s (48±7%).
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NOS inhibition was tested at 2 concentrations of L-NMMA. In the CTL
group, the lower infusion (10 mg ·
kg-1 · h-1) did
not affect the inotropic response to dobutamine, whereas
twice the concentration (20 mg ·
kg-1 · h-1)
augmented the +dP/dt response to a 1735±244 mm Hg/s increase
(66±24%, P<0.001; Table 2
). This effect was also
evident in the load-insensitive indices of
contractility, Ees and dP/dt-EDD (Table 2
). In
contrast to the CTL condition, after the induction of HF, L-NMMA
significantly augmented ß-adrenergic contractility at
both infusion rates (Table 2
). Moreover, the effect was the same
for the 10 and 20 mg · kg-1 ·
h-1 infusion rates, assessed by 3-way ANOVA.
To assess whether caveolin-3 protein abundance was correlated with the
effect of L-NMMA to augment dobutamine-stimulated +dP/dt,
we quantified caveolin-3 abundance in 4 dogs with HF. Caveolin-3
abundance positively correlated with the L-NMMA effect
(r=0.9, P=0.03; Figure 4
).
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In contrast to ß-adrenergic effects, inhibition of NOS affected resting myocardial contractility in CTL but not HF dogs, producing concentration-dependent reductions in myocardial contractility. No effect was observed at 10 mg · kg-1 · h-1, but a progressive decline in +dP/dt occurred with the 20 mg · kg-1 · h-1 infusion, reaching a plateau of -19±4% (P<0.0001) after 30 minutes. After induction of HF, neither infusion of L-NMMA affected resting contractility. These data suggest that although NO exerts an increased contribution to ß-adrenergic inotropic responses in heart failure, its effect on resting contractility is reduced.
Effect of NOS Inhibition on Loading Conditions
As anticipated, L-NMMA produced potent dose-dependent increases in
arterial afterload (Ea) in the CTL group,
consistent with increased vascular tone from inhibition of
vascular endothelial NOS. Ea increased by 40±11% and
70±15% in response to the 10 and 20 mg/kg doses, respectively. In the
HF group, pressor responses were markedly attenuated, and Ea rose by
22±5% and 15±7% for the 10 and 20 mg ·
kg-1 · h-1
infusion rates, respectively (P=0.05 for each versus
baseline, P=NS for comparison between the 2 dose responses).
Neither infusion rate of L-NMMA changed EDD (preload) in the CTL or HF
groups.
NOS Enzyme Activity and Protein Abundance
LV myocardium from pacing-induced
cardiomyopathic dogs has previously been shown to have
unchanged levels of NOS activity.16 In the present
study, we confirmed this observation: After the induction of HF,
Ca2+-dependent NOS activity was unchanged from
the CTL value (4.1±0.6 versus 3.9±0.6 pmol/mg protein in CTL versus
HF groups, respectively; n=11 or 12 each), and
Ca2+-independent (NOS2) activity was not detected
in either case. Western blotting using a NOS3 antibody revealed similar
protein abundance in HF and CTL myocardium, and NOS2 was
not detected in either CTL or HF tissue by Western blot (data not
shown).
| Discussion |
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Effect of NO on Myocardial Contractility
The results of the present study regarding myocardial
contractile responses are in agreement with several earlier studies
performed in vitro22 and in vivo23 showing
that inhibition of NOS leads to an enhancement of ß-adrenergic
inotropic responses. This observation has particular relevance to
situations such as sepsis,24 aging,25 and
HF,1 which are associated with ß-adrenergic chronotropic
and inotropic downregulation and in which NO has been implicated as
contributing to these changes. With regard to HF, we have previously
shown that L-NMMA augments ß-adrenergic contractility
to a greater degree in patients with idiopathic dilated
cardiomyopathy than in control subjects with normal
LV function.1
The mechanism(s) contributing to enhanced inotropic responses with NOS inhibition in HF remains controversial. On the one hand, it has been suggested that induction of high-output NOS25 6 26 in failing myocardium leads to excess NO production. This view has received support from studies reporting the appearance of circulating cytokines capable of stimulating NOS2 induction in HF.27 On the other hand, NOS2 induction is not observed in genetic28 and pacing-induced16 HF models, and coronary sinus NO metabolites are decreased with HF in experimental models8 and in humans.7 One possible explanation that reconciles these seemingly conflicting findings may be differential NO signaling under resting versus stimulated conditions. The NO metabolite studies were performed in the absence ß-adrenergic29 activation or heart-rate increases,30 which stimulate NO production.
Our observations regarding the effect of L-NMMA on resting contractility suggest differential NO responses at baseline and during ß-adrenergic stimulation. L-NMMA suppressed resting myocardial contractility in CTL but not HF animals. The loss of NO pathway activity in HF animals is consistent with an increased inhibitory factor, such as caveolin. Divergent NO influences on contractility (inhibiting resting contractility but augmenting stimulated contractile responses) have been previously observed.18 31 NO signaling exerts its effects on ß-adrenergic inotropy most likely via cGMP production, which antagonizes the effects of cAMP.32 33 34 Different cGMP-independent NO-signaling pathways may also contribute to NO influences on resting myocardial performance. Notably, NO activates the L-type calcium channel35 and the ryanodine receptor36 via thiol nitrosylation reactions. Further support for the role of NO in Ca2+ cycling is provided by the observation that NOS1 localizes to cardiac sarcoplasmic reticulum.37
Role of Caveolins in NO Signaling
Caveolins are scaffolding proteins found in caveolae, which are
plasmalemmal microdomains that participate in signal
transduction by means of colocalizing membrane receptors with signal
transduction effectors.38 39 40 Because caveolin inhibits
NOS activity by preventing calmodulin activation, it may
exert dual regulation of NOS: inhibition of basal activity yet
augmentation of agonist-stimulated actions. In this regard, Feron et
al41 have shown that agonist-stimulated NOS signaling is
absent in isolated cardiac myocytes transfected with
myristoylation-deficient NOS3 that is unable to interact with caveolin,
but reconstituted by transfection of NOS3 able to bind caveolin-3. As
discussed above, our physiological observations
with L-NMMA are consistent with the paradigm that NO pathway
activity in HF is reduced basally but augmented during agonist
stimulation.
Given findings previously reported16 and confirmed in the present study that NOS isoforms or activity are unchanged in failing canine ventricular myocardium, we explored the hypothesis that an endogenous regulator of NOS action might be altered in HF. Western analysis revealed increased caveolin-3 protein abundance, and electron microscopy showed increased numbers of myocyte sarcolemmal caveolae. Confocal imaging identified caveolin-3 localized to the sarcolemma and T tubules and colocalized with NOS3 at these sites in HF myocytes. Moreover, caveolin-3 abundance correlated with the augmentation of dobutamine contractility due to NOS inhibition in HF dogs.
Alterations in caveolin and/or caveolae are only recently being implicated in pathophysiology. For example, caveolin-3 gene mutations42 and increased protein abundance43 are observed in patients with muscular dystrophy. Myocardial caveolin abundance is reduced by the infusion of isoproterenol in mice,12 possibly via cAMP production, as demonstrated in rat myocytes.44 Thus, the HF-associated reduction in myocardial cAMP production may contribute to increased caveolin abundance in this condition.
These observations are limited because we could not assess functional consequences of acute perturbation of caveolin-3 abundance or activity. There are no described pharmacological means to do so, and current evaluations of caveolin function have required in vitro manipulation, such as the application of antisense RNA45 or caveolin peptide fragments.41 Accordingly, it is important to consider that increased NO pathway activity in HF may result from changes in pathways unrelated to caveolin.46 For example, factors such as oxidative stress or altered phosphodiesterase activity may modulate NO signaling.33 46 47 Moreover, increased caveolin-3 abundance may play roles in HF that are unrelated to NOS and that may influence both myocardial structure and function.48 49 50 In this regard, there are reports of both receptor translocation to38 50 and away from49 caveolae during agonist stimulation. The implications of these pathways for cardiac function in HF await future study.
The present observations clarify an earlier controversy regarding the physiological impact of NOS inhibition on myocardial contractility in normal LV function. At the lower concentration tested (10 mg · kg-1 · h-1), L-NMMA augmented ß-adrenergic inotropy in HF but not CTL animals, in a manner similar to that found in our earlier human observations.1 Only at higher L-NMMA concentrations (20 mg · kg-1 · h-1) was the inotropic response augmented in CTL animals. Thus, it is likely that earlier observations not showing augmentation of ß-adrenergic inotropy in subjects with normal LV function, including our own studies,1 were limited by either inadequate concentration or time of administration of a NOS inhibitor.
In the present study, we have demonstrated that dogs with pacing-induced HF exhibit increased sensitivity to the augmentation of ß-adrenergic inotropic responses by NOS inhibition. These functional changes occurred in the absence of changes in NOS isoform activity or abundance. Failing myocardium and myocytes exhibited increased abundance of caveolin-3 and caveolae. Thus, increases in the concentration of these caveolar scaffolding proteins that compartmentalize NOS with stimulatory agonist signals suggest a novel mechanism by which the NO pathway activity may be increased in HF.
| Acknowledgments |
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| Footnotes |
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Received September 14, 1999; accepted March 28, 2000.
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