Download A paradigm shift for local blood flow regulation

J Appl Physiol 116: 703–705, 2014;
doi:10.1152/japplphysiol.00964.2013.
Perspectives
VIEWPOINT
A paradigm shift for local blood flow regulation
Aleksander S. Golub and Roland N. Pittman
Department of Physiology and Biophysics, Medical College of Virginia Campus, Virginia Commonwealth University,
Richmond, Virginia
Address for reprint requests and other correspondence: R. N. Pittman, Dept.
of Physiology and Biophysics, Medical College of Virginia Campus, Virginia
Commonwealth Univ., 1101 E. Marshall St., P. O. Box 980551, Richmond,
VA 23298-0551 (e-mail: [email protected]).
http://www.jappl.org
the numerous studies leading to the formulation of the NO/O2⫺
signaling mechanism of local blood flow regulation was recently published (12).
For a new perspective on the regulation of local blood flow,
the key assumptions of the metabolic hypothesis need to be
critically re-examined: the maintenance of basal tone in the
arteriolar wall and the action of metabolic vasodilators produced by parenchymal cells in response to hypoxia. Quite the
contrary, the proposed mechanism involving NO/O2⫺ coupling
is based on the concept that functional activity is the normal
physiological state for a tissue; the corresponding normal state
for its vasculature is dilation (12). This eliminates the problem
of basal tone, because the basal dilation state is supported by
continuous production of NO by the constitutive enzyme eNOS
in microvascular endothelial cells. Active regulation of local
blood flow comes about when the rate of oxygen and glucose
supply reaches or exceeds the tissue demand, thus leading to an
abundance of cytosolic reducing agents (NADH and NADPH)
and extracellular oxygen that are the substrates for membrane
NAD(P)H oxidase in parenchymal cells and the vascular wall
(Fig. 1, bottom). That circumstance causes the production of
O2⫺ into the interstitial space and subsequent neutralization of
some of the interstitial NO, which leads to constriction of the
arterioles and decreased local blood flow.
An increase of the functional activity of an organ activates
the mitochondria, which causes the import of reducing equivalents from the cytosol into mitochondria and at the same time
reduces the oxygen tension on the surface of parenchymal
cells. This reduces the production of O2⫺ into the interstitial
space by NAD(P)H oxidase. Extracellular SOD, having a
lower rate of O2⫺ removal compared with NO (2– 4 ⫻ 109
M/s), degrades the remaining interstitial O2⫺. A low level of
O2⫺ opens the interstitial space for the diffusive flux of NO to
the smooth muscle cells in arterioles, causing them to dilate
and increase local blood flow. Thus the interstitial concentration of NO acts as an error signal whose magnitude is related
to the mismatch between oxygen and glucose delivery and their
consumption (Fig. 1, bottom). To start active hyperemia, the
tissue cells need not produce a metabolic vasodilator(s), but
just stop the emission of the NO inhibitor (O2⫺) into the
interstitial space. Reactive hyperemia in the NO/O2⫺-based
model is mechanistically different from active hyperemia,
because the blood flow interruption also affects the positive
feedback loop, stopping the basal production of NO (Fig. 1,
bottom). After reperfusion, the microvascular endothelium immediately receives nutrients and oxygen to produce NO,
whereas diffusion and metabolic limitations lead to a delay in
the release of O2⫺ from the tissue cells. Therefore, the response
of the system during reperfusion is determined by the duration
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703
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regulation is the
coordination between metabolic rate and local blood flow in
microscopic volumes of an organ. In skeletal muscle an increase in oxygen consumption (V̇O2) over a wide range evokes
a proportional (⬃5 to 6 ⫻ V̇O2) increase in blood flow (29). In
the brain, neurovascular coupling is the basis for noninvasive
blood flow methods (e.g., fMRI) for imaging functionally
active brain regions with high spatial and temporal resolution.
However, the phenomenon of local functional hyperemia in
skeletal muscle and in brain still has no reliable mechanistic
rationale.
The historically established metabolic theory of local blood
flow regulation assumes the existence of basal tone in arterioles
(Fig. 1, top). An increase in cell respiration leads to a drop in
tissue/cellular PO2 and then to the production of vasodilator
metabolites by parenchymal cells, which increase local blood
flow via a negative feedback control (30, 31, 33). However,
this mechanism implies that the restoration of an adequate
oxygen supply stops the production of the metabolic signal,
which should then reduce local blood flow. The drawback of
this model is the inevitable maintenance of tissue hypoxia. A
key issue for the traditional metabolic theory is a failure to
confirm the necessary components for this mechanism of
regulation: an active closure of capillaries and significant
change in capillary density (14, 18, 28), a functional role for
precapillary sphincters (10), and certain metabolic vasodilator(s) produced by parenchymal cells during hypoxia (31, 32).
Because of its reliance on tissue-produced vasodilators, the old
metabolic theory cannot incorporate new knowledge about the
action of the signaling radical superoxide (O2⫺) and nitric
oxide (NO), a strong vasodilator produced by the vascular
endothelium and not by tissue cells.
An alternative approach to the regulation of local blood flow
began developing a quarter of a century ago, when the mechanism of vasodilation by NO produced by the endothelium was
established (11, 15, 27). At the same time, the direct inhibitory
effect of O2⫺ on NO was demonstrated (13, 15, 23, 27, 35, 36).
It was found that O2⫺ is a specific antagonist of NO, which
reacts with NO at a rate limited by diffusion: from 6.7 ⫻ 109
M/s (4) to 1.9 ⫻ 1010 M/s (17). This knowledge formed the
basis for understanding the NO/O2⫺ system and the contributions of the constitutive enzymes eNOS and NAD(P)H oxidase
and extracellular superoxide dismutase (ecSOD) to the control
of microvascular tone by a radical signaling mechanism (1–3,
5, 7, 8, 19, 20, 22, 24, 25, 34, 37). A detailed review describing
A REMARKABLE EXAMPLE OF PHYSIOLOGICAL
Perspectives
704
METABOLIC MODEL OF REGULATION
Basal tone
+
Arteriolar
diameter
SMC
-
Metabolic
vasodilators
Blood
flow
Tissue hypoxia
O2 diffusion
Tissue
Endothelial NOS
O2 &
Glucose
NO flux
+
IS
[NO]
Blood
flow
SMC
sGC
Arteriolar
diameter
O2 & Glucose
transport
O2- flux
Sarcolemmal
NAD(P)H oxidase
Fig. 1. Top: metabolic model of regulation. When the O2 supply to a tissue
whose activity increases is insufficient to meet the increased O2 demand, tissue
hypoxia ensues and metabolic vasodilators are released from the active tissue into the interstitium, where they diffuse to the nearby arterioles, producing
a negative feedback signal, causing them to dilate. The increased blood flow
brings an elevated oxygen supply that relieves the tissue hypoxia (and the
stimulus for producing the vasodilators). Bottom: NO/O2⫺ model of regulation.
When O2 and glucose content in the blood is sufficient, eNOS produces a
constant flux of NO into the perivascular space. This provides a positive
feedback signal for the smooth muscle cells (SMC) of the arteriolar wall. This
NO flux serves as a reference signal (“set point”) and is constant if PO2 in the
blood is high enough. Transport of O2 and glucose to parenchymal cells is
restricted by diffusion and transmembrane glucose transporters. This is presented as a resistor in the diagram. The parenchymal cells and vascular wall are
consumers of glucose and O2 and are sensors of their adequate supply. With
sufficient or excessive delivery of glucose, the membrane NAD(P)H oxidase is
supplied with its substrates NADH and NADPH from the cytosol to generate
extracellular O2⫺. Thus the negative feedback signal reporting the achievement
of a high level of supply is encoded as the rate of O2⫺ emission into the
interstitial space (IS). The periarteriolar NO concentration ([NO]) in the IS is
determined by the interaction of NO and O2⫺ fluxes. The superoxide flux
depends on a sufficient supply of O2 and glucose to parenchymal cells, and the
NO flux is determined by the high content of oxygen and glucose in blood. The
interaction of these fluxes in the interstitial space is similar to the subtraction
of part of the NO flux due to its neutralization by superoxide. The result of this
subtraction is the error signal, represented as [NO] at the SMCs of the arteriolar
wall. These cells serve as a controller of vascular wall tone via the soluble
guanylyl cyclase (sGC) pathway, changing the inner diameter and hydraulic
resistance of arterioles to adjust the local blood flow. At rest, the perivascular
[NO] is low and arterioles are constricted. Activation of oxidative metabolism
reduces the production of O2⫺, thus opening the arterioles with high [NO]. The
presence of ecSOD in the interstitium accelerates removal of the residual O2⫺
and accelerates the development of hyperemia.
ACKNOWLEDGMENTS
The authors are grateful to the reviewers of this manuscript for making
insightful and thoughtful comments and suggestions which improved the
overall presentation of this material.
GRANTS
The work of the authors included in this article was supported in part by
Grant HL18292 from the National Heart, Lung, and Blood Institute.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
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NO / O2- MODEL OF REGULATION
of the ischemia (6, 16, 21) and the presence of oxygen in the
blood (9).
It should be emphasized that the NO/O2⫺-based model of
regulation is aimed to control the upper level of local blood
flow so that the constriction of arterioles during low metabolic
activity prevents the cells in a tissue from being exposed to an
excessive supply of oxygen and substrates. Thus the goal of
regulation is maintaining a sufficient, but not excessive, supply
of oxygen and substrates to the cells. Increasing blood flow
simply requires a termination of the inhibitory signal (O2⫺)
instead of the production of metabolic vasodilators. NO signaling, currently considered a redundant and stand-alone vasodilation pathway, becomes the key mechanism for regulation
of local blood flow. This hypothesis explains why the vasodilation signal is generated by microvessels and how the parenchymal cells block this signal when local blood flow is sufficient. The proposed mechanism consists of well studied components: the enzymes eNOS, NAD(P)H oxidase, ecSOD and
the two rapidly diffusing and interacting signaling radicals.
Because O2⫺ is compartmentalized in a small extracellular
space, its interstitial concentration can change quickly. Production of O2⫺ by membrane NAD(P)H oxidase depends on the
balance of glycolysis and oxidative phosphorylation via the
availability of NADH and NAD(P)H in the cytosol and the interstitial PO2. Thus the coupling of tissue metabolic activity
with local blood flow is achieved.
The NO/O2⫺ regulation hypothesis provides conceptual support for the successful practical application of NO donors and
other pharmacologic agents used to manipulate the NO signaling pathway. This model underpins the application of pharmacologic modulators of angiotensin II signaling for the correction of hypertension. It is known that most of the actions of
angiotensin II are mediated by the AT1 receptor, which can
activate membrane NAD(P)H oxidase and increase the production of superoxide into the interstitium (26). The use of blockers of angiotensin converting enzyme and AT1 receptors thus
inhibits O2⫺ production and increases NO bioavailability for
the SMCs in the arteriolar wall. The proposed mechanism of
regulation defines physiologic roles for the signaling radicals
NO and O2⫺ as key elements to control an adequate supply of
oxygen and glucose in critical organs. By the explanation of
local blood flow regulation in healthy organisms, this model
provides a solid conceptual tool to support medical research
into tissue hypoxia, development of vascular pathologies, tolerance and cross-tolerance to nitrovasodilators, and neurovascular coupling. A key role of glycolysis in maintaining the
turnover of cytosolic NADH and O2⫺ production by the
sarcolemmal NAD(P)H oxidase also allows glucose metabolism to be considered as an important factor in local blood flow
regulation during metabolic syndrome.
Perspectives
705
AUTHOR CONTRIBUTIONS
Author contributions: A.S.G. and R.N.P. conception and design of research;
A.S.G. prepared figures; A.S.G. drafted manuscript; A.S.G. and R.N.P. edited
and revised manuscript; A.S.G. and R.N.P. approved final version of manuscript.
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