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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3996_Библиотеки_им_академика_М_И_Перельмана
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to prevail over the greater outflow resistance in their particular loca-
tion, and the conditions for their optimal pumping are shifted
toward higher values of intraluminal/transmural pressures
accordingly.
Later, new evidence was obtained to demonstrate the regional
variations in pressure-induced changes in lymphatic contractility;
studies were performed on lymphatic vessels taken from four differ-
ent regions of one species—rat [44]. The local differences in pressure
sensitivities and pumping ability were determined for thoracic duct,
cervical, mesenteric, and femoral lymphatic vessels. All these lym-
phatics were able to increase their pumping during moderate
increases in transmural pressure up to some pumping maximum.
The largest pump productivity was observed at 3 cm H
2
O trans-
mural pressure for all lymphatics except mesenteric lymphatics,
where maximum pumping occurred at a pressure of 5-cm
H
2
O. Moreover, detailed analysis demonstrated that all these lym-
phatics had a range of transmural pressures over which there were
no significant large differences in pumping. Experimental data dem-
onstrate that these ranges of pressure were as follows: 2–4-cm H
2
O
for the thoracic duct, 2–8-cm H
2
O for cervical lymphatics, 2–7-cm
H
2
O for mesenteric vessels, and 2–9-cm H
2
O for femoral lymphatics.
These data reveal that all selected lymphatic vessels have their opti-
mal pumping conditions at comparatively low levels of transmural
pressure that were comparable with typical in situ lymph pressures
[39] and that these pressure levels have a tendency to be higher in
more peripheral lymphatic vessels. The highest pumping at the opti-
mal pressure levels (6–8 volumes/min) was demonstrated for mes-
enteric lymphatics, and the lowest pumping (2 volumes/min) was
seen in the thoracic duct in the rat.
Recently, similar studies of regional lymphatic contractile hetero-
geneity were performed in mice [45]. These authors identified
an important regional dichotomy in the mouse lymphatic pump
function dependent on the location of the lymphatic collectors.
They observed that lymphatic vessels isolated from five different
subcutaneous/peripheral regions; cervical, popliteal, inguinal, axil-
lary, and internodal inguinal axillary exhibited strong lymph
pumping (with maximal ejection fractions of 50%–80%) similar
to those described earlier in rat and other larger mammals, while
lymphatic vessels isolated from the deep/visceral lymphatic
64 4. Hydrodynamic regulation of lymphatic vessel transport function
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network; mesenteric, thoracic duct, and iliac were not effective
lymph pumps (with maximal EFs < 15%). This lack of an effective
lymph pump agrees with the data we have also seen in mouse vis-
ceral lymphatics [46]. To evaluate what the difference between
the active pumping subcutaneous and nonpumping visceral
mouse lymphatic function was due to, the authors assessed the
membrane potential and L-type calcium channels in the muscle cells
of those vessels. While they observed some differences in the elec-
trical and L-type calcium channel activities, they concluded that
“it is unlikely that the lack of robust spontaneous contractions in vis-
ceral vessels is explained entirely by L-type calcium channels.” The
question of what these important differences in lymphatic contrac-
tile pumping activity of the mouse mean in terms of its physiological
and pathological consequences remains to be determined. However,
the authors conclude that “care must also be taken in understanding
the limitations of using lymphatic vessels from within the visceral
cavity of the mouse” [45] in studies of lymphatic contractile function
in physiological and pathophysiological states.
Due to the importance of pressure stimuli for lymphatic contrac-
tility, the idea that distension stimuli are mandatory to generate
lymphatic contractions has dominated the literature for many
decades. But in several studies, it was reported that lymphatic ves-
sels could contract in a coordinated fashion without significant dis-
tension stimuli [5,21,34,47]. Moreover, experiments performed on
lymphatics from different tissues and species showed a high per-
centage of cases in which the contractile wave propagates along
the vessel in a direction retrograde to flow [3,6,39,48,49]. At low
or normal levels of lymph formation, in many tissues, lymphan-
gions at the end of contractions are nearly empty [9]. Due to the
presence of highly competent valves in lymphatics, the stretch-
dependent activation of several upstream lymphangions in such
situations is very unlikely. Particularly, Mislin and Rathenow noted
[3] that the contractile wave could propagate in the retrograde direc-
tion through several lymphangions unconnected with the increase
of the local transmural pressure. More recent studies [12,13,50] dem-
onstrated for 80% of lymphangions poor or no correlation between
experimentally generated fluctuations of their intraluminal pressure
and lymphatic contractions. Moreover, it was also shown [12,13,50]
that isolated bovine and rat mesenteric lymphatics can have stable
65Modulation of the lymphatic vessel contractility by pressure/stretch
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long-lasting spontaneous contractility at 0-cm H
2
O intraluminal
pressure and in the absence of significant radial or axial distensions.
These data lead to the reasonable conclusion that the distension of
the lymphatic wall by intraluminal/transmural pressure is an
important factor to modulate the contractile activity in lymphatic
vessels but is not a mandatory factor for pacemaking.
Modulation of the lymphatic vessel contractility by intrinsic
and extrinsic flows
As discussed earlier, lymph flow is generated as a resulting com-
plicated combination of influences of active and passive lymph driv-
ing forces/pumps. As the peaks of the actions of passive lymph
pumps are often not synchronized with intrinsic contractile activity
of lymphangions, flow profiles in lymphatic nets are extremely var-
iable and complex. Only the presence of valves in lymphatic vessels
prevents the existence of extended periods of backflow and sup-
ports net unidirectional lymph flow. On other hand, the presence
of lymphatic valves additionally complicates the lymph flow profile.
During the lymphangion systole, the pressure difference between
contracting and relaxing lymphangions causes the temporal lymph
backflow and leads to closure of downstream valve(s) [5,10,36].
The bulblike shape of valve sinuses (the portion of lymphangions
immediately downstream to the valve) that exist in a majority of
lymphatic vessels is a structural factor that could promote the for-
mation of local transitional/nonlaminar flows during the lymphatic
valve closure [10]. The unique shape of lymphatic valves is another
structural factor that by itself could complicate lymph flow profile
locally during the lymphangion diastolic filling. The space between
two valve leaflets is much narrower than lymphatic lumen in non-
valve areas of lymphangions. During lymphatic filling, the narrow
valve-containing section between lymphangions could be a factor
that causes local temporal accelerations and nonaxial lymph flow.
Moreover, although lymphatic valves more commonly contain
two leaflets, observations of up to five-leaflet valves are also
described in literature [51]. Our own, yet unpublished, observations
demonstrated that in rat mesenteric lymphatic vessels, the adjacent
lymphatic valves could be oriented by 90 degrees to each other,
66 4. Hydrodynamic regulation of lymphatic vessel transport function
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which may be an additional factor to form nonlaminar flow profiles
near the valves as well extended along the lymphatic vessel. Such
variability in lymphatic valve structure and position predetermines
even much more unpredictability in lymph flow profiles in lym-
phatic vessels.
Comparatively recently data were obtained on lymph flow veloc-
ities and values of shear stress in lymphatic vessels [52–54]. A high-
speed video system was used to capture multiple contraction
cycles in rat mesenteric lymphatic preparations in situ. The images
were analyzed to determine fluid velocity, volume flow rate, wall
shear stress, and retrograde flow. It was determined that immune
cell density and flux varied from 326 to 35,500 cells/μL and 206 to
2030 cells/min, respectively. Lymphatics contracted phasically,
with a mean diameter of 91 9.0 μm and amplitudes of 39%,
and lymph velocity in them varied with the phasic contractions in
both direction and magnitude with an average of 0.87 0.18 and
peaks of 2.2–9.0 mm/s. The velocity was 180 degrees out of
phase with the lymphatic contractile cycle. The average lymph
flow was 13.95 5.27 μL/h with transient periods of reversed flow.
This resulted in an average shear of 0.64 0.14 with peaks of
4–12 dynes/cm
2
. These studies confirmed that shear rate in mesen-
teric lymphatics is low but had large variations in magnitude com-
pared with that seen in blood vessels and common reversal of flow to
close the valves. Importantly, in this work, the authors actually mea-
suring the velocity fluctuations in contracting mesenteric lymphatics
and more importantly approximating the wall shear stresses that
occur in situ validated that the velocities experimentally induced
in the isolated vessels experiments were physiologically relevant.
Historically, researchers started to investigate the influences of
different lymph flows/velocities on lymphatic pumping, creating
the different levels of shear in experiments with steady increases
in lymph flow rates. Particularly, Benoit et al. [39] increased lymph
flow in rat mesenteric lymphatics by elevating lymph formation and
accordingly lymph flow rates as a result of plasma dilution. They
found the increased parameters of active lymph contractility in mes-
enteric lymphatics during the periods of increased lymph flow. But
they also mentioned that pressure in the lymphatic network became
less pulsatile in high lymph flow states. Of course, such kinds of
experiments with increased lymph formation in situ give important
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information on lymphatic contractile behavior in situations similar
to those that happen during different phases of tissue and organ
activities.
The studies on influences of imposed flow on contractile activity
of rat isolated lymphatic vessel were presented by Gashev et al
[44,55] These authors performed studies on isolated and perfused
lymphatic vessels from four different regions of the body from
the same species—rat that allowed comparisons of the imposed
flow-induced modulations of lymphatic contractility in different
regional lymphatic beds. In all of these studies, the inflow pressures
were changed simultaneously with changes of outflow pressures
made on the same level but in opposite direction to maintain the
mean transmural pressure equal during the periods of increased
imposed flow. Parameters of lymphatic contractions were evaluated
immediately after imposed flow was set or changed and have been
monitored during the next 5 min with an analysis of time-
dependent changes in 1-min intervals. In some series of experi-
ments, authors changed the direction of imposed flow in isolated
mesenteric lymphatic segments to retrograde.
Using these experimental approaches, the potent imposed flow-
dependent inhibition of the active lymph pump has been found in
mesenteric lymphatics and in the thoracic duct [55] and later in fem-
oral and cervical lymphatic vessels [44]. Imposed flow gradient
caused reductions in the frequency and amplitude of lymphatic con-
tractions. As a result of these negative chronotropic and inotropic
effects, the active pumping of lymphatics was greatly diminished.
However, it is difficult to conclude that such imposed flow-
dependent inhibition of the active lymph pump decreases the total
lymph flow in vivo. Because total lymph flow is the sum of passive
and active flows, it is likely that the increase in imposed (or passive)
flow would overwhelm any decreases in active pump lymph flow.
A potentially important overriding factor would be an enhanced
rate of lymph formation. At high levels of lymph formation, passive
lymph flow could become a greater driving force to move lymph
than the active lymph pump. Imposed flow-dependent inhibition
of the active lymph pump in such situations could be a reasonable
physiological mechanism to save metabolic energy by temporarily
decreasing or stopping contractions during the time when the lym-
phatic vessel does not need it. An additional outcome of the
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inhibition of the lymph pump under these circumstances would be a
reduction in lymph outflow resistance. This reduction in outflow
resistance is the result of the net increase in average lymphatic diam-
eter that occurs when contractions are inhibited. For example, com-
plete cessation of the mesenteric active lymph pump (at zero
imposed flow and 5-cm H
2
O transmural pressure gradient) would
result in a net increase in the time averaged diameter by about 23%,
thus theoretically reducing resistance by approximately 56% [55].
This reduction in the outflow resistance could ease the removal of
fluid from the affected compartment that is producing the high
lymph flows and facilitate the resolution of edema. Therefore, dur-
ing these periods, when “passive” lymph flow [56,57] is elevated,
such mechanisms maintain lymphatic vessel contractile function
in an energy-saving mode [55] by diminishing phasic pumping con-
tractions and tone, thereby converting lymphatic functional behav-
ior from acting primarily like a pump to acting more like a
conduit [58].
Imposed flow-induced inhibition of the lymph pump followed
two temporal patterns [55]. The first is the rapidly developing inhibi-
tion of contraction frequency. Upon imposition of flow, the contraction
frequency immediately fell and then partially recovered over time during
continued flow. This effect was dependent on the magnitude of
imposed flow but was not dependent on the direction of flow.
The effect also depended upon the rate of change in the direction
of flow. The second pattern was a slowly developing reduction of the
amplitude of the lymphatic contractions, which increased over time during
continued flow. The inhibition of contraction amplitude was depen-
dent on the direction of the imposed flow but independent of the
magnitude of flow.
Therefore, the chronotropic and inotropic imposed flow-induced
inhibitory responses appear somewhat different. In the first minute
of the initiation of imposed flow, the lymphatic response to imposed
flow occurs primarily through a rapid inhibition of the contraction
frequency. In vivo, short periods of increased flow occur very often
due to the contractions of upstream lymphangions. It is possible that
a fast chronotropic response of lymphatics is an important short-
term regulatory reaction to rapid but short-lasting periods of
increased flow. At high rates of lymph formation, which can be pre-
sent in the mesenteric lymphatic bed in vivo, long periods of
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increased flow may occur. The slow inotropic effect, which develops
in lymphatics in minutes, could be an important long-term regula-
tory reaction to slow but long-lasting periods of increased flow. This
slowly developing flow-induced inhibition of lymphatic contractil-
ity could conserve energy in lymphatics when there are sufficient
passive forces to move lymph without the active lymph pump
and decrease local outflow resistance.
Additionally, the imposed flow-induced inhibition of contraction
frequency does not depend on direction of flow, whereas imposed
flow-induced inhibition of amplitude of lymphatic contractions
does. Long-term inhibition of lymphatic contractility is stronger
during orthograde flow, which occurs in lymphatics more often in
comparison with long periods of retrograde flow, which rarely
occur because of the competency of the lymphatic valves. This evi-
dence allows proposing that the mechanisms of the rapidly develop-
ing chronotropic and the slowly developing inotropic lymphatic
responses to flow could be different.
As mentioned earlier, lymphangions work in a short-distance
pump fashion, and the force of their phasic contractions generates
flow and shear in the lymphatics by themselves. Recently, it was
demonstrated that the flow generated during phasic contractions
in the rat thoracic duct itself plays an important self-regulatory
role in the lymphatic contractile cycle in a shear-dependent man-
ner [59]. In this study, the thoracic duct was chosen as a vessel sen-
sitive to an imposed flow and for its variable contractile behavior
[28,44,60–64]. In many cases contractions may occur in one part of
the thoracic duct but do not propagate between different segments
[28,44,60–64]. Contractile waves often do not propagate along this
vessel, and many times, the phasic contractions develop locally,
while adjacent parts of the duct are not contracting [65]. This feature
of the thoracic duct was used to design the experiments to evaluate
the importance of flow and shear generated by lymphatic phasic
contractions in the regulation of the lymphatic contractile cycle.
Two types of segments of thoracic duct were taken into this study:
phasically active segments and phasically inactive segments. Close
attention was paid to maintain the input and output pressures at
the same level therefore excluding any imposed flow. Thus, in the
phasically active lymphatic segments, flow and shear occurred only
as a result of their inherent contractions. In phasically inactive
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segments, flow and shear did not take place. As a result, experimental
conditions that allowed investigating the influences of flow and shear, gen-
erated solely by the phasic lymphatic pump, on the contractile function
without any extra imposed flow were used.
The authors found [59] that lymphatic resting tone in phasically
active segments of the thoracic duct was 2–2.6 times lower than in
phasically inactive segments. The only difference between the
experimental conditions of the active and inactive segments was
the existence of actively generated flow in the contracting lymphatic
segments. Hence, in this study, a relaxation in the thoracic duct, which
was connected exclusively with its spontaneous phasic pumping activity,
was discovered. Investigating the possible mechanisms of this relax-
ation, it was demonstrated that blockade of nitric oxide synthase
by
L-NAME completely abolished this difference in lymphatic tone
between the phasically active and nonactive segments. Therefore,
this evidence indicates that the contraction-generated reduction of
lymphatic tone in the thoracic duct is mediated only by nitric oxide
(NO). Based on this fact, authors determined what happened with
active lymphatic pumping if the mechanism of the contraction-
generated reduction of tone will be completely blocked. Authors
found that the reduction of tone in lymphatic segments generated
by the phasic contractions improves their diastolic filling (enhanced
lusitropy—lowering half relaxation time—indicates the speed of
diastolic filling), makes lymphatic contractions stronger (enhanced
inotropy—higher contraction amplitudes), and propels more fluid
forward during each contraction (elevated ejection fraction) while
decreasing contraction frequency (reduced chronotropy). After
NO-synthase blockade, the lymphatic segment must contract more
often (higher contraction frequency) to maintain the minute produc-
tivity (fractional pump flow) appropriate to the existing level of pre-
load (transmural pressure). Thus, the reduction in lymphatic tone due to
the flow/shear generated by phasic contractions is a regulatory mechanism
that maintains lymphatic pumping in an energy-saving efficient mode
(stronger but fewer contractions per minute).
Importantly, the flow-mediated relaxation can exist in any phasi-
cally contracting lymphatic. But the lymph flow profile is a compli-
cated and variable sum of different forces, not only the result of
phasic lymphatic contractions. When discussing the flow conditions
in a single lymphangion, it is reasonable to divide the flow pattern
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into two components: “intrinsic flow” (meaning the flow that is a
result of the contractions of that lymphangion) and “extrinsic
flow” (meaning the flow that is a result of all influences from out-
side that single lymphangion, predominantly influences from
upstream of that single lymphangion but some influences from
downstream as well). From the experiments with imposed flow,
it was known that as the imposed flow was increased, the degree
of inhibition of lymphatic pumping increased [13,44,55].Inthetho-
racic duct, it was observed [44] that during periods of high
imposed flow (transaxial pressure gradient of 5-cm H
2
O), the dia-
stolic diameter increased, resulting in a 57% reduction in resting
tone(incomparisonwiththeabsenceofimposedflowatthesame
transmural pressure level). On the other hand, at this level of
imposed flow, the spontaneous contractions of the thoracic duct
were almost completely abolished [44].Thisleadstotheconclusion
that in situ, where the extrinsic flow varies dramatically and is
dependent on many factors, the lymphangions are constantly
operating under a combination of intrinsic and extrinsic flows.
When extrinsic flow is not enough to move lymph downstream,
the maintenance of low lymphatic tone by the extrinsic flow (dem-
onstrated in [13,44,55]) is supported or completely replaced by the
reduction in lymphatic tone mediated by intri nsic flow during the
pumping-effective phasic contractions (demonstrated in [59]).
When extrinsic flow is high enough to propel lymph by itself,
spontaneous contractions may be inhibited to save energy, and
only the lowering of tone by the extrinsic flow will exist.
Currently available data support the idea that contractile activity
of the lymphatic vessels in situ constantly adjusts to the local “need”
to propel variable volumes of lymph by a continuous interplay
between the influences of extrinsic and intrinsic flows. At low levels
of inflow in the transporting lymphatics, the influences of intrinsic
flow will dominate, and NO release due to the phasic contractions
will maintain the effective energy-saving lymphatic pumping pat-
terns. As soon as the levels of lymph formation and accordingly
inflow are increased in the transporting lymphatics, the influences
of extrinsic flow will dominate, leading to a large NO release that
will temporarily inhibit the intrinsic contractility of transporting
lymphatics. Later studies with direct measurements of NO in
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mesenteric lymphatic vessels confirmed these conclusions about the
functional role on the NO for lymphatic contractility [66,67].
In conclusion, it is important to recognize that knowledge of basic
principles of physiological adaptive reactions to the changes in main
physical factors of their environment (pressure and flow) is far from
complete. It was thought for many decades that the adaptive reac-
tions of lymphatic vessels to the increases in their volume loads
are simple and could be explained by simple “bell-shaped”
pressure-dependent regulation. Recent finding described earlier
demonstrated that the contractions of lymphangions are constantly
influenced by complicated sum of different factors of lymph dynam-
ics: pressure/stretch, generated by active and passive lymph
pumps, and subsequent combination of intrinsic and extrinsic
flows/shears. Moreover, lymphangions react differently to changes
in these factors depending on species and also on their location in
the whole body and inside the regional lymphatic nets. However,
due to the natural limitations and difficulties in possibilities of
scientists to investigate lymphatic contractility, the picture of all
possible adaptive contractile events in lymphatic vessels is far from
complete. To ease the first steps in the investigation of the basic reg-
ulation of lymphatic contractility, the researchers artificially sepa-
rated the studies on pressure and flow effects on it, but in vivo
lymphangions exist in conditions of simultaneously changes in both
pressure and flow. The next step of the research efforts on basic prin-
ciples of lymph flow must be focused on a combined evaluation of
interaction between pressure/stretch- and flow/shear-dependent
adaptive reactions in lymphangions. As an example, someone
may raise here a reasonable question: do the flow-dependent regu-
latory mechanisms act similarly or differently at different levels of
lymphatic wall distension by intraluminal diastolic lymphatic pres-
sure, because more likely the increases of extrinsic flow will follow
by the increases in the lymphangions filling pressure? Obtaining the
answers on such principle question on functioning of lymphatic ves-
sels not only will promote our abilities to create the commonly
accepted theory of physiology of lymph flow but also will help to
precisely target the clinical efforts toward the correction of the main
“driving engine” of lymph flow altered during lymphatic diseases—
partially or completely incompetent lymphatic muscle cells unable
to generate sufficient flow in lymphatic nets.
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