(Circulation. 1999;99:271-275.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Departments of Surgery (J.A.M.A., H.A.B.) and Internal Medicine (F.B., A.v.d.M., F.H.d.J.), University Hospital Rotterdam, Netherlands.
| Abstract |
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Methods and ResultsCompared with healthy volunteers, patients with septic shock had increased plasma levels of nitrite/nitrate (37±5 [SEM] versus 12±5 mmol/L, P<0.01), the stable end products of NO metabolism, and ET-1 (45±7 versus 3±2 pg/mL, P<0.001). Plasma ET-1 concentration was not related to plasma nitrite/nitrate concentration or blood pressure. Continuous infusion of L-NAME (1 mg · kg-1 · h-1 IV) for 12 hours increased mean arterial pressure by 43±5% and systemic vascular resistance by 64±10% (both P<0.01). The increase in blood pressure and systemic vascular resistance correlated positively with the level of ET-1 (both P<0.005) but not with plasma nitrite/nitrate level. L-NAME infusion did not result in significant changes in the plasma concentrations of ET-1 or nitrite/nitrate.
ConclusionsNO and ET-1 may both play a role in the cardiovascular derangements of human sepsis. Although L-NAME does not increase ET-1 concentration in patients with septic shock, the vasopressor response induced by L-NAME depends on the plasma level of ET-1. These findings may indicate that inhibitors of NO synthesis unmask a tonic pressor response of ET-1 in human septic shock.
Key Words: endothelium-derived factors shock endothelin
| Introduction |
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Sepsis in humans is characterized by massive vasodilatation with low systemic vascular resistance, high cardiac output, and severe hypotension.13 Increased production of NO by an inducible isotype of NO synthase has been held responsible for the cardiovascular derangements during sepsis.5 14 High levels of nitrite and nitrate, the stable end products of NO metabolism, are found in patients with severe sepsis, and these levels may correlate with vasodilation.15 Analogues of L-arginine competitively inhibit the production of NO from L-arginine16 and can reverse hypotension in endotoxin- and cytokine-induced shock in animals.5 17 More recently, these inhibitors of NO synthesis have been used to increase blood pressure in sepsis in humans.18 19 20 At present, it is unknown whether inhibition of NO synthesis modulates endothelin release in septic shock. Because high levels of endothelin-1 have been found in patients with septic shock,21 22 we hypothesized that part of the vasoconstrictive response after inhibition of NO synthesis may result from increased production of ET-1.
The purpose of the present study was to examine whether ET-1 plays a role in the increase in blood pressure after inhibition of NO synthesis in patients with severe sepsis. Plasma concentrations of ET-1 and nitrite/nitrate, as an indirect measure of NO production, were measured during continuous infusion of the L-arginine analogue NG-nitro-L-arginine methyl ester (L-NAME) in patients with severe septic shock.
| Methods |
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Study Protocol
All patients underwent continuous ECG monitoring and had
indwelling radial artery and pulmonary artery catheters
(Criticath, Ohmeda). Measurements of systemic arterial
blood pressure, central venous pressure, pulmonary artery
pressure, and cardiac output (thermodilution method) were made at
baseline and at 0.5-, 1-, 3-, and 6-hour intervals, for a total period
of 24 hours. Systemic and pulmonary vascular resistance were
calculated according to standard formulas. After baseline measurements,
infusion of L-NAME 1 mg · kg-1 ·
h-1 was started and continued for 12 hours. To
minimize the potential of developing toxicity from high serum levels of
L-NAME, infusion was not continued for longer periods. Maximum changes
in blood pressure and systemic vascular resistance were noted after 30
minutes of L-NAME administration,20 and
hemodynamic variables at this time were used to
compare with the baseline value. Concomitant therapy was at the
discretion of the clinician managing the patient.
Blood Samples
For later determination of plasma ET-1, nitrite/nitrate, and
cortisol, arterial blood samples (10 mL) were obtained in
heparin-coated blood collection tubes (Vacutainer) at baseline (t=0),
during L-NAME infusion (t=0.5, 1, 3, 6, and 12 hours), and 3, 6, and 12
hours after L-NAME infusion had stopped (t=15, 18, and 24 hours).
Control values were established in 10 healthy volunteers. Blood samples
were immediately centrifuged (1500g, 10 minutes,
4°C), and plasma was collected and stored at -80°C until tested.
Creatinine concentration was determined by a standard
method.
Determination of ET-1
ET-1 was determined, after Sep-Pak extraction, with a
commercially available radioimmunoassay kit (Nichols Institute) as
previously described.24 Normal values in our laboratory
were 1 to 5 pg/mL. The limit of detection was 1 pg/mL.
Determination of Nitrite/Nitrate Concentration
Total nitrite plus nitrate concentration was assayed as
described by Phizackerley and Al-Dabbagh.25 To reduce
nitrate to nitrite, supernatant or standards were incubated at room
temperature in the presence of Klebsiella pneumoniae under
anaerobic conditions. Total nitrite in the supernatant was
subjected to the Griess reaction and assayed spectrophotometrically.
Data are reported as the sum of nitrite plus nitrate. Normal values in
our laboratory were 5 to 15 mmol/L. The limit of detection was
0.1 mmol/L.
Determination of Cortisol
Cortisol concentrations were determined with a commercially
available radioimmunoassay kit (DPC) as described
previously.26 Normal values were 400 to 800 nmol/L (15 to
30 mg/dL). The limit of detection was 28 nmol/L.
Drugs
L-NAME was obtained from Sigma Chemical Co. The hospital
pharmacy prepared a sterile and pyrogen-free solution of L-NAME 10
mg/mL, ready for intravenous infusion.
Data Analysis
All results are expressed as mean±SEM. Changes over time were
compared with baseline values by repeated-measures ANOVA. Differences
between groups were compared by Student's t test. Pearson
correlation and scatterplots were used for analysis of
correlation between variables. A value of P<0.05 was
considered statistically significant.
| Results |
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No correlation was found between plasma ET-1 concentration and plasma
nitrite/nitrate concentration (r=0.07, P>0.80),
blood pressure (r=-0.34, P=0.30), systemic
vascular resistance (r=-0.41, P=0.22), or
creatinine concentration (r=-0.13,
P=0.69). ET-1 concentration was 46±5 pg/mL in survivors and
43±12 pg/mL in nonsurvivors (P>0.05). Plasma
nitrite/nitrate concentration was negatively correlated with systemic
vascular resistance (r=-0.78, P=0.012) (Figure 1
) but not with creatinine
concentration (r=-0.03, P=0.93).
|
Continuous infusion of L-NAME 1 mg ·
kg-1 · h-1 IV
resulted in a maximum increase of mean arterial pressure by
43±5% and systemic vascular resistance by 64±10% (Figure 2
). There was a concomitant reduction in
cardiac output by 20±4%. The increase in blood pressure
(r=0.84, P=0.001) and systemic vascular
resistance (r=0.81, P=0.002) during L-NAME
infusion correlated positively with the baseline plasma concentration
of ET-1 (Figure 3
). The increase in blood
pressure with L-NAME did not correlate with plasma nitrite/nitrate
level (r=0.12, P=0.89). L-NAME infusion for a
period of 12 hours did not result in changes in the plasma
concentration levels of ET-1 (from 45±7 pg/mL at baseline to 43±6
pg/mL after 12 hours of L-NAME, P>0.05) and/or cortisol
(from 753±86 nmol/L at baseline to 682±73 nmol/L after 12 hours of
L-NAME, P>0.05; Figure 4
).
Plasma nitrite/nitrate did not change during L-NAME (from 37±5
mmol/L at baseline to 36±5 mmol/L after 12 hours of L-NAME,
P>0.05).
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| Discussion |
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In the present study, elevated plasma levels of ET-1 were found in patients with severe sepsis, as has been reported by others.21 22 The mechanism involved in the increase of ET-1 concentration during sepsis is largely unknown. High ET-1 levels could have resulted from increased synthesis and/or diminished clearance of ET-1. Levels of circulating cytokines, such as tumor necrosis factor and interleukins, are increased in septic shock and may stimulate ET-1 production in endothelial cells and macrophages.27 28 Furthermore, intravascular leakage of ET-1 from damaged endothelial cells may have occurred.13 Because most patients with septic shock had impaired renal function, diminished renal clearance could have played a role.29 30 However, no correlation was found between plasma ET-1 and creatinine in the present study. This supports the idea that increased generation of endothelin occurs in sepsis in humans, although other vascular beds, including liver and skeletal muscles, may also contribute to the clearance of ET-1.
The finding that high ET-1 levels coincided with high nitrite/nitrate
levels as an indirect measure of increased NO production is not
in line with previous data that indicate an inhibitory
action of NO on ET-1 release.1 6 This suggests that the
normal interaction between NO and ET-1 is disturbed during sepsis.
Abnormalities in the balance between vasoconstricting and vasodilating
factors have been reported during sepsis.13 31 At
present, however, the exact interaction of ET-1 and NO in septic
shock remains to be determined. We hypothesize that the high
circulating ET-1 concentration helps to maintain vascular tone during
sepsis and opposes the vascular smooth muscle relaxation induced by
increased NO production. Indeed, the high circulating ET-1
levels in patients with septic shock, in the present study 45 pg/mL
(
18 pmol/L), are sufficient to induce relevant
vasoconstriction.32 33 The fact that the increase in blood
pressure with L-NAME correlated with the level of ET-1 further supports
the idea of a continuous vasopressor tone by ET-1 during sepsis in
humans and that this vasopressor tone is unmasked by an
inhibitor of NO synthesis. Our findings are
consistent with previous reports in normal rats in which the
increase in blood pressure with L-NAME was reduced with the specific
endothelin receptor antagonist bosentan.9 10
In healthy humans34 and humans with chronic heart
failure,35 endothelin receptor antagonists can
reduce blood pressure. To demonstrate a direct relation between ET-1
and blood pressure, however, the use of a receptor blocker specific for
ET-1 would be necessary. In the present study, we did not test the
effects of an endothelin receptor antagonist. We
hypothesize that endothelin receptor antagonism during septic shock in
humans, in which plasma levels of ET-1 are high, would be detrimental,
because it may result in severe hypotension. Whether endothelin
receptor antagonists can prevent the increase in blood
pressure of NO synthase inhibitors in humans remains to be
determined.
In the present study, no increase was found in the plasma levels of ET-1 during 12 hours of inhibition of NO synthesis with L-NAME. These findings may suggest that ET-1 release itself is not modulated by L-NAME in patients with septic shock. In normal rats, acute inhibition of NO synthesis results in modest increases in ET-1 levels, but these ET-1 levels were measured only 15 and 35 minutes after L-NAME infusion.9 10 In contrast to these findings, chronic inhibition of NO synthesis with L-NAME (3 weeks) in normal rats does not increase ET-1 levels or gene expression.36 Only a few reports are available on the NO-endothelin interaction in humans. Although nitrovasodilators can reduce ET-1 production by human endothelial cells in culture,37 infusion of nitroglycerin, an NO-donating drug, does not change plasma ET-1 levels in healthy subjects.11 In a recent study in healthy men, the NO synthase inhibitor NG-monomethyl-L-arginine temporarily (only after 20 minutes) increased ET-1 levels from 7.6 to 9.6 pmol/L.12 In the present study, however, we did not find a temporary increase in already elevated ET-1 concentration. One possible explanation is that the continuous high release of ET-1 during sepsis resulted in a depletion of storage vesicles with ET-1 (or its precursor).38 The results from the present study and previous reports suggest that increased production of ET-1 does not play a major role in the rise in blood pressure during prolonged inhibition of NO synthesis.
However, some reservations must be made regarding interpretation of plasma ET-1 levels. Plasma levels of ET-1 may not correctly reflect production rate, and the local concentration of the peptide at the vascular smooth muscle binding sites is probably more essential. For instance, big endothelin, which needs conversion to endothelin by tissue endothelin-converting enzyme to gain activity, is able to cause vasoconstriction without notable increases in plasma endothelin.39 Because the main ET-1 release seems to occur abluminally, local concentrations of ET-1 are probably higher than in the plasma.40 In the present study, therefore, increased local production of ET-1 may have occurred with L-NAME without detectable changes in plasma concentration. Similar reservations must be made regarding nitrite/nitrate levels. NO is but one of the ways that nitrite and nitrate are formed, and L-NAME infusion did not result in reduced serum levels of nitrite and nitrate, despite increased vasoconstriction. Possibly, plasma levels of nitrite and nitrate do not directly reflect the local amount of NO released, and active excretion of nitrite/nitrate through the kidneys and gastrointestinal tract may have influenced the plasma levels currently measured.41
Cortisol has a vital role in the maintenance of vascular tone and endothelial function.42 Furthermore, cortisol potentiates the vasoconstrictor actions of catecholamines.43 Thus, increases in plasma cortisol may indirectly increase vasopressor tone. Animal studies have shown that inhibition of NO synthesis with L-NAME stimulates adrenal steroidogenesis.44 45 We hypothesized that increased cortisol production might be an additional mechanism by which L-NAME increases blood pressure. However, we found no evidence of changes in cortisol levels during inhibition of NO synthesis in patients with septic shock. We conclude that changes in cortisol levels do not contribute to the increase in blood pressure with L-NAME.
In conclusion, inhibitors of NO synthesis may unmask a tonic pressor response of ET-1 in humans with septic shock. Prolonged inhibition of NO synthesis does not influence the plasma levels of ET-1. These results suggest that ET-1 maintains blood pressure in human sepsis and that plasma ET-1 concentration may determine the vasopressor response of NO synthase inhibitors in septic shock. These findings may provide further evidence of interaction between NO and ET-1 in pathological conditions of the cardiovascular system in which an imbalance between endothelium-derived vasodilator and vasoconstrictor substances disturbs the normal regulation of vascular tone.
| Footnotes |
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Guest Editor for this article was Oscar A. Carretero, MD, Henry Ford Hospital, Detroit, Mich.
Received January 28, 1998; revision received September 25, 1998; accepted October 5, 1998.
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