Active lymphatic pumping and sheep lung lymph flow

Active (intrinsic) lymphatic pumping may be an important factor determining lymph flow from the lungs. Unfortunately, in most experiments, it is very difficult to determine the influence of active pumping vs. passive factors on lymph flow. However, 1) the pumping activity (stroke volume and frequency) of isolated lymphatic segments varies nonlinearly with transmural pressure, and 2) the lung lymph flow from awake sheep varies nonlinearly with lymphatic outflow pressure. Accordingly, if lymphatic pumping significantly influences lung lymph flow, then it should be possible to describe the sheep lung lymph flow vs. outflow pressure data with the pumping activity data. To test this, we used published lymphatic pumping activity data to develop a mathematical model of the lymphatic pump for a segment of lymphatic vessel. Flow vs. outflow pressure relationships obtained from simulations with this model were very similar to the data from sheep. Our results indicate that both passive factors and active lymphatic pumping contribute to lymph flow, and our model may allow investigators to distinguish the effects of active pumping vs. passive factors in the regulation of lymph flow.     

Modulation of fluid pumping in isolated bovine mesenteric lymphatics by a thromboxane/endoperoxide analogue

A thromboxane/endoperoxide analogue (compound U46619) is known to stimulate phasic and tonic contractions in quiescent bovine lymphatic vessels and enhance contractile activity in spontaneously active vessels. In order to determine how these effects relate to changes in fluid propulsion by the lymphatics, we have assessed the effects of U46619 on the ability of isolated bovine mesenteric lymphatics to pump fluid in vitro. Bovine lymphatic segments (up to 8 cm in length with a minimum of 4 valves) were cannulated at both ends and fluid input provided from a reservoir. Flow through the vessels was regulated by intraluminal pressures. On average, changes in transmural pressures up to 8 cm H2O resulted in enhanced pumping; pressures above this level depressed flow. The dominant effect of U46619 (added to the reservoir) was to depress pumping; 10(-7) and 10(-9)M decreased flow at all transmural pressures tested; 10(-8)M had a dual effect, slightly inhibiting flow at low transmural pressures and enhancing flow at higher pressures. These results suggest that thromboxane may stimulate or inhibit lymphatic pumping depending on the concentration of the agent and the transmural pressure applied to the vessel. These effects may relate to its ability to induce variable changes in luminal diameter and frequency and force of contractions.     

Network Scale Modeling of Lymph Transport and Its Effective Pumping Parameters

The lymphatic system is an open-ended network of vessels that run in parallel to the blood circulation system. These vessels are present in almost all of the tissues of the body to remove excess fluid. Similar to blood vessels, lymphatic vessels are found in branched arrangements. Due to the complexity of experiments on lymphatic networks and the difficulty to control the important functional parameters in these setups, computational modeling becomes an effective and essential means of understanding lymphatic network pumping dynamics. Here we aimed to determine the effect of pumping coordination in branched network structures on the regulation of lymph flow. Lymphatic vessel networks were created by building upon our previous lumped-parameter model of lymphangions in series. In our network model, each vessel is itself divided into multiple lymphangions by lymphatic valves that help maintain forward flow. Vessel junctions are modeled by equating the pressures and balancing mass flows. Our results demonstrated that a 1.5 s rest-period between contractions optimizes the flow rate. A time delay between contractions of lymphangions at the junction of branches provided an advantage over synchronous pumping, but additional time delays within individual vessels only increased the flow rate for adverse pressure differences greater than 10.5 cmH₂O. Additionally, we quantified the pumping capability of the system under increasing levels of steady transmural pressure and outflow pressure for different network sizes. We observed that peak flow rates normally occurred under transmural pressures between 2 to 4 cmH₂O (for multiple pressure differences and network sizes). Networks with 10 lymphangions per vessel had the highest pumping capability under a wide range of adverse pressure differences. For favorable pressure differences, pumping was more efficient with fewer lymphangions. These findings are valuable for translating experimental measurements from the single lymphangion level to tissue and organ scales.     

Impairments in the intrinsic contractility of mesenteric collecting lymphatics in a rat model of metabolic syndrome

Numerous studies on metabolic syndrome (MetSyn), a cluster of metabolic abnormalities, have demonstrated its profound impact on cardiovascular and blood microvascular health; however, the effects of MetSyn on lymphatic function are not well understood. We hypothesized that MetSyn would modulate lymphatic muscle activity and alter muscularized lymphatic function similar to the impairment of blood vessel function associated with MetSyn, particularly given the direct proximity of the lymphatics to the chronically inflamed adipose depots. To test this hypothesis, rats were placed on a high-fructose diet (60%) for 7 wk, and their progression to MetSyn was assessed through serum insulin and triglyceride levels in addition to the expression of metabolic and inflammatory genes in the liver. Mesenteric lymphatic vessels were isolated and subjected to different transmural pressures while lymphatic pumping and contractile parameters were evaluated. Lymphatics from MetSyn rats had significant negative chronotropic effects at all pressures that effectively reduced the intrinsic flow-generating capacity of these vessels by ～50%. Furthermore, lymphatics were remodeled to a significantly smaller diameter in the animals with MetSyn. Wire myograph experiments demonstrated that permeabilized lymphatics from the MetSyn group exhibited a significant decrease in force generation and were less sensitive to Ca(2+), although there were no significant changes in lymphatic muscle cell coverage or morphology. Thus, our data provide the first evidence that MetSyn induces a remodeling of collecting lymphatics, thereby effectively reducing their potential load capabilities and impairing the intrinsic contractility required for proper lymph flow.     

In vitro effects of endotoxin on bovine and sheep lung microvascular and pulmonary artery endothelial cells

A single infusion of Escherichia coli endotoxin into sheep results in structural evidence of pulmonary endothelial injury, increases in both prostacyclin and prostaglandin E2 (PGE2) in lung lymph, and an increase in pulmonary microvascular permeability. Endotoxin-induced lung endothelial damage can also be induced in vitro, but to date these studies have utilized endothelium from large pulmonary vessels. In the present study, we have grown endothelial cells from peripheral lung vessels of cows and sheep and exposed these microvascular endothelial cells to endotoxin. Controls included lung microvascular endothelium without endotoxin and endothelial cells from bovine and sheep main pulmonary artery with and without addition of endotoxin. We found that endotoxin caused significant increases in release of prostacyclin and PGE2 from both bovine and sheep lung microvascular and pulmonary artery endothelium. Normal bovine and sheep pulmonary artery and bovine lung microvascular endothelium released greater levels of prostacyclin than PGE2 (ng/ng); release of PGE2 from the microvascular cells was greater than from the pulmonary artery endothelium in both species. Exposure of endothelial cells from cow and sheep main pulmonary artery to endotoxin results in endothelial cell retraction and pyknosis, a loss of barrier function, increased release of prostacyclin and PGE2 and eventual cell lysis. In lung microvascular cells, the increases in prostanoids were accompanied by changes in cell shape but occurred in the absence of either detectable alterations in barrier function or cytolysis. Thus, while endotoxin causes alterations to endothelial cells from both large and small pulmonary vessels, the effects are not identical suggesting site specific phenotypic expression of endothelial cells even within a single vessel. To determine whether the response of either the large or small pulmonary vessel endothelial cells in culture mimics most closely the in vivo response of the lung to endotoxin requires further study.     

Mesenteric lymph flow in adult and aged rats

The objective of study was to evaluate the aging-associated changes, contractile characteristics of mesenteric lymphatic vessels (MLV), and lymph flow in vivo in male 9- and 24-mo-old Fischer-344 rats. Lymphatic diameter, contraction amplitude, contraction frequency, and fractional pump flow, lymph flow velocity, wall shear stress, and minute active wall shear stress load were determined in MLV in vivo before and after N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME) application at 100 μM. The active pumping of the aged rat MLV in vivo was found to be severely depleted, predominantly through the aging-associated decrease in lymphatic contractile frequency. Such changes correlate with enlargement of aged MLV, which experienced much lower minute active shear stress load than adult vessels. At the same time, pumping in aged MLV in vivo may be rapidly increased back to levels of adult vessels predominantly through the increase in contraction frequency induced by nitric oxide (NO) elimination. Findings support the idea that in aged tissues surrounding the aged MLV, the additional source of some yet unlinked lymphatic contraction-stimulatory metabolites is counterbalanced or blocked by NO release. The comparative analysis of the control data obtained from experiments with both adult and aged MLV in vivo and from isolated vessel-based studies clearly demonstrated that ex vivo isolated lymphatic vessels exhibit identical contractile characteristics to lymphatic vessels in vivo.     

Characteristics of the active lymph pump in bovine prenodal mesenteric lymphatics

BACKGROUND: The functional characteristics of the bovine mesenteric postnodal lymphatics are well-described. However there are no reports of pumping characteristics of the bovine mesenteric prenodal lymphatics. We propose that the prenodal lymphatics have adapted to the local conditions of lymph flow and are functioning differently than the postnodal vessels. METHODS AND RESULTS: To evaluate pumping in bovine prenodal mesenteric lymphatics, we observed their contractility in response to the changes in transmural pressure and imposed flow. Lymphatics (diameter approximately 460 microm) were isolated, cannulated, and pressurized. Lymphatic diameters were traced from video records; the lymphatic tone index, contraction amplitude and frequency, lymphatic pump indices were calculated. Increasing transmural pressure from 3 to 6 cm H2O produced a strong inotropic response, but did not induce a significant chronotropic response. Pumping reached its maximum at transmural pressures 6-9 cm H2O and was not significantly depressed up to 15 cm H2O, whereas pumping in postnodal lymphatics is typically depressed at transmural pressures higher than 10 cm H2O. Bovine prenodal mesenteric lymphatics also demonstrated very low sensitivity to the increases in imposed flow. CONCLUSIONS: We concluded that the functional heterogeneity exists on the intraregional levels in lymphatic nets.     

Effect of endotoxin on diaphragm lymph contamination in unanesthetized sheep

The preparation for collecting lung lymph from sheep caudal mediastinal lymph node (CMN) efferent vessels is widely used to study the effects of endotoxin on lung microvascular permeability. However, there are nonpulmonary lymph vessels that drain into the CMN along with the afferent lymph vessels from the lung. Thus CMN lymph is a mixture of lymph from the lung and diaphragm lymph vessels as well as from other nonpulmonary lymph vessels. We studied the effect of 0.5-1.0 microgram/kg Escherichia coli endotoxin on the flow rates in diaphragm and CMN efferent lymph vessels (Qdi and QCMN, respectively) in unanesthetized sheep. For the time period between 2 and 5.5 h after endotoxin QCMN was increased from its base line of 7.2 +/- 4.4 (SD) to 17.3 +/- 10.6 ml/h and the lymph-to-plasma protein concentration ratio (L/PCMN) had increased from 0.68 +/- 0.11 to 0.81 +/- 0.06. During the same time period, Qdi was 4.5 +/- 3.1 ml/h compared with 1.0 +/- 0.8 ml/h at base line and the diaphragm lymph-to-plasma protein concentration ratio (L/Pdi) was 0.92 +/- 0.07 (base line = 0.74 +/- 0.15). The increases in flow rate and protein concentration were significant for each type of vessel (P less than 0.05). We conclude that the period of increased QCMN and L/PCMN after endotoxin is associated with an increase in Qdi and L/Pdi. Thus, it is difficult to determine how much of the CMN lymph response comes from the lungs and how much comes from diaphragm lymph vessels.     

Smooth muscle-endothelial cell communication activates Reelin signaling and regulates lymphatic vessel formation

Active lymph transport relies on smooth muscle cell (SMC) contractions around collecting lymphatic vessels, yet regulation of lymphatic vessel wall assembly and lymphatic pumping are poorly understood. Here, we identify Reelin, an extracellular matrix glycoprotein previously implicated in central nervous system development, as an important regulator of lymphatic vascular development. Reelin-deficient mice showed abnormal collecting lymphatic vessels, characterized by a reduced number of SMCs, abnormal expression of lymphatic capillary marker lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1), and impaired function. Furthermore, we show that SMC recruitment to lymphatic vessels stimulated release and proteolytic processing of endothelium-derived Reelin. Lymphatic endothelial cells in turn responded to Reelin by up-regulating monocyte chemotactic protein 1 (MCP1) expression, which suggests an autocrine mechanism for Reelin-mediated control of endothelial factor expression upstream of SMC recruitment. These results uncover a mechanism by which Reelin signaling is activated by communication between the two cell types of the collecting lymphatic vessels--smooth muscle and endothelial cells--and highlight a hitherto unrecognized and important function for SMCs in lymphatic vessel morphogenesis and function.     

Lymphangion coordination minimally affects mean flow in lymphatic vessels

The lymphatic system returns interstitial fluid to the central venous circulation, in part, by the cyclical contraction of a series of "lymphangion pumps" in a lymphatic vessel. The dynamics of individual lymphangions have been well characterized in vitro; their frequencies and strengths of contraction are sensitive to both preload and afterload. However, lymphangion interaction within a lymphatic vessel has been poorly characterized because it is difficult to experimentally alter properties of individual lymphangions and because the afterload of one lymphangion is coupled to the preload of another. To determine the effects of lymphangion interaction on lymph flow, we adapted an existing mathematical model of a lymphangion (characterizing lymphangion contractility, lymph viscosity, and inertia) to create a new lymphatic vessel model consisting of several lymphangions in series. The lymphatic vessel model was validated with focused experiments on bovine mesenteric lymphatic vessels in vitro. The model was then used to predict changes in lymph flow with different time delays between onset of contraction of adjacent lymphangions (coordinated case) and with different relative lymphangion contraction frequencies (noncoordinated case). Coordination of contraction had little impact on mean flow. Furthermore, orthograde and retrograde propagations of contractile waves had similar effects on flow. Model results explain why neither retrograde propagation of contractile waves nor the lack of electrical continuity between lymphangions adversely impacts flow. Because lymphangion coordination minimally affects mean flow in lymphatic vessels, lymphangions have flexibility to independently adapt to local conditions.     

Determinants of valve gating in collecting lymphatic vessels from rat mesentery

Secondary lymphatic valves are essential for minimizing backflow of lymph and are presumed to gate passively according to the instantaneous trans-valve pressure gradient. We hypothesized that valve gating is also modulated by vessel distention, which could alter leaflet stiffness and coaptation. To test this hypothesis, we devised protocols to measure the small pressure gradients required to open or close lymphatic valves and determine if the gradients varied as a function of vessel diameter. Lymphatic vessels were isolated from rat mesentery, cannulated, and pressurized using a servo-control system. Detection of valve leaflet position simultaneously with diameter and intraluminal pressure changes in two-valve segments revealed the detailed temporal relationships between these parameters during the lymphatic contraction cycle. The timing of valve movements was similar to that of cardiac valves, but only when lymphatic vessel afterload was elevated. The pressure gradients required to open or close a valve were determined in one-valve segments during slow, ramp-wise pressure elevation, either from the input or output side of the valve. Tests were conducted over a wide range of baseline pressures (and thus diameters) in passive vessels as well as in vessels with two levels of imposed tone. Surprisingly, the pressure gradient required for valve closure varied >20-fold (0.1-2.2 cmH(2)O) as a passive vessel progressively distended. Similarly, the pressure gradient required for valve opening varied sixfold with vessel distention. Finally, our functional evidence supports the concept that lymphatic muscle tone exerts an indirect effect on valve gating.     

Lymphatic vessels transition to state of summation above a critical contraction frequency

Although behavior of lymphatic vessels is analogous to that of ventricles, which completely relax between contractions, and blood vessels, which maintain a tonic constriction, the mixture of contractile properties can yield behavior unique to lymphatic vessels. In particular, because of their limited refractory period and slow rate of relaxation, lymphatic vessels lack the contractile properties that minimize summation in ventricles. We, therefore, hypothesized that lymphatic vessels transition to a state of summation when lymphatic vessel contraction frequency exceeds a critical value. We used an isovolumic, controlled-flow preparation to compare the time required for full relaxation with the time available to relax during diastole. We measured transmural pressure and diameter on segments of spontaneously contracting bovine mesenteric lymphatic vessels during 10 isovolumic volume steps. We found that beat-to-beat period (frequency(-1)) decreased with increases in diameter and that total contraction time was constant or slightly increased with diameter. We further found that the convergence of beat-to-beat period and contraction cycle duration predicted a critical transition value, beyond which the vessel does not have time to fully relax. This incomplete relaxation and resulting mechanical summation significantly increase active tension in diastole. Because this transition occurs within a physiological range, contraction summation may represent a fundamental feature of lymphatic vessel function.     

First-order approximation for the pressure-flow relationship of spontaneously contracting lymphangions

To return lymph to the great veins of the neck, it must be actively pumped against a pressure gradient. Mean lymph flow in a portion of a lymphatic network has been characterized by an empirical relationship (P(in) - P(out) = -P(p) + R(L)Q(L)), where P(in) - P(out) is the axial pressure gradient and Q(L) is mean lymph flow. R(L) and P(p) are empirical parameters characterizing the effective lymphatic resistance and pump pressure, respectively. The relation of these global empirical parameters to the properties of lymphangions, the segments of a lymphatic vessel bounded by valves, has been problematic. Lymphangions have a structure like blood vessels but cyclically contract like cardiac ventricles; they are characterized by a contraction frequency (f) and the slopes of the end-diastolic pressure-volume relationship [minimum value of resulting elastance (E(min))] and end-systolic pressure-volume relationship [maximum value of resulting elastance (E(max))]. Poiseuille's law provides a first-order approximation relating the pressure-flow relationship to the fundamental properties of a blood vessel. No analogous formula exists for a pumping lymphangion. We therefore derived an algebraic formula predicting lymphangion flow from fundamental physical principles and known lymphangion properties. Quantitative analysis revealed that lymph inertia and resistance to lymph flow are negligible and that lymphangions act like a series of interconnected ventricles. For a single lymphangion, P(p) = P(in) (E(max) - E(min))/E(min) and R(L) = E(max)/f. The formula was tested against a validated, realistic mathematical model of a lymphangion and found to be accurate. Predicted flows were within the range of flows measured in vitro. The present work therefore provides a general solution that makes it possible to relate fundamental lymphangion properties to lymphatic system function.     

A genetic Xenopus laevis tadpole model to study lymphangiogenesis

Lymph vessels control fluid homeostasis, immunity and metastasis. Unraveling the molecular basis of lymphangiogenesis has been hampered by the lack of a small animal model that can be genetically manipulated. Here, we show that Xenopus tadpoles develop lymph vessels from lymphangioblasts or, through transdifferentiation, from venous endothelial cells. Lymphangiography showed that these lymph vessels drain lymph, through the lymph heart, to the venous circulation. Morpholino-mediated knockdown of the lymphangiogenic factor Prox1 caused lymph vessel defects and lymphedema by impairing lymphatic commitment. Knockdown of vascular endothelial growth factor C (VEGF-C) also induced lymph vessel defects and lymphedema, but primarily by affecting migration of lymphatic endothelial cells. Knockdown of VEGF-C also resulted in aberrant blood vessel formation in tadpoles. This tadpole model offers opportunities for the discovery of new regulators of lymphangiogenesis.     

Valve-related modes of pump failure in collecting lymphatics: numerical and experimental investigation

Lymph is transported along collecting lymphatic vessels by intrinsic and extrinsic pumping. The walls have muscle of a type intermediate between blood-vascular smooth muscle and myocardium; a contracting segment between two valves (a lymphangion) constitutes a pump. This intrinsic mechanism is investigated ex vivo in isolated, spontaneously contracting, perfused segments subjected to controlled external pressures. The reaction to varying afterload is probed by slowly ramping up the outlet pressure until pumping fails. Often the failure occurs when the contraction raises intra-lymphangion pressure insufficiently to overcome the outlet pressure, open the outlet valve and cause ejection, but many segments fail by other means, the mechanisms of which are not clear. We here elucidate those mechanisms by resort to a numerical model. Experimental observations are paired with comparable findings from computer simulations, using a lumped-parameter model that incorporates previously measured valve properties, plus new measurements of active contractile and passive elastic properties, and the dependence of contraction frequency on transmural pressure, all taken from isobaric twitch contraction experiments in the same vessel. Surprisingly, the model predicts seven different possible modes of pump failure, each defined by a different sequence of valve events, with their occurrence depending on the parameter values and boundary conditions. Some, but not all, modes were found experimentally. Further model investigation reveals routes by which a vessel exhibiting one mode of failure might under altered circumstances exhibit another.     

Therapeutic differentiation and maturation of lymphatic vessels after lymph node dissection and transplantation

Surgery or radiation therapy of metastatic cancer often damages lymph nodes, leading to secondary lymphedema. Here we show, using a newly established mouse model, that collecting lymphatic vessels can be regenerated and fused to lymph node transplants after lymph node removal. Treatment of lymph node-excised mice with adenovirally delivered vascular endothelial growth factor-C (VEGF-C) or VEGF-D induced robust growth of the lymphatic capillaries, which gradually underwent intrinsic remodeling, differentiation and maturation into functional collecting lymphatic vessels, including the formation of uniform endothelial cell-cell junctions and intraluminal valves. The vessels also reacquired pericyte contacts, which downregulated lymphatic capillary markers during vessel maturation. Growth factor therapy improved the outcome of lymph node transplantation, including functional reconstitution of the immunological barrier against tumor metastasis. These results show that growth factor-induced maturation of lymphatic vessels is possible in adult mice and provide a basis for future therapy of lymphedema.     

Intravital two-photon microscopy of lymphatic vessel development and function using a transgenic Prox1 promoter-directed mOrange2 reporter mouse

Lymphatic vessels, the second vascular system of higher vertebrates, are indispensable for fluid tissue homoeostasis, dietary fat resorption and immune surveillance. Not only are lymphatic vessels formed during fetal development, when the lymphatic endothelium differentiates and separates from blood endothelial cells, but also lymphangiogenesis occurs during adult life under conditions of inflammation, wound healing and tumour formation. Under all of these conditions, haemopoietic cells can exert instructive influences on lymph vessel growth and are essential for the vital separation of blood and lymphatic vessels. LECs (lymphatic endothelial cells) are characterized by expression of a number of unique genes that distinguish them from blood endothelium and can be utilized to drive reporter genes in a lymph endothelial-specific fashion. In the present paper, we describe the Prox1 (prospero homeobox protein 1) promoter-driven expression of the fluorescent protein mOrange2, which allows the specific intravital visualization of lymph vessel growth and behaviour during mouse fetal development and in adult mice.     

Endothelin-mediated constriction of prenodal lymphatic vessels in the canine forelimb

Endothelin is a 21 amino acid peptide which is produced by the vascular endothelium and is believed to be the mediator of endothelium-dependent vasoconstriction. In the current study we assessed the ability of synthetic human endothelin-1 to affect prenodal lymphatic vessel contractility in the canine forelimb. Intralymphatic infusion of endothelin at 1.09 x 10(-9), 1.09 x 10(-8) and 1.09 x 10(-7) M significantly constricted lymphatic vessels as evidenced by dose-dependent increases in lymphatic perfusion pressure. The increase in lymphatic perfusion pressure seen during intralymphatic infusion of endothelin at 1.09 x 10(-8) M during the intra-arterial infusion of phentolamine was not significantly different from that seen prior to phentolamine, indicating that endothelin-mediated lymphatic constriction is not alpha-receptor mediated. Intra-arterial infusion of endothelin at three infusion rates significantly increased forelimb arterial, systemic and lymphatic perfusion pressures. The constriction seen when endothelin (1.09 x 10(-8) M) was infused intralymphatically in the intact lymphatic system was not significantly different from that observed when only the prenodal lymph vessel was perfused. This indicated that the lymph nodes and efferent lymph vessels do not contribute significantly to the lymphatic constriction produced by endothelin. These data are consistent with the hypothesis that endothelin may modulate lymphatic function under either normal or pathophysiological conditions.     

Simulation of a chain of collapsible contracting lymphangions with progressive valve closure

The aim of this investigation was to achieve the first step toward a comprehensive model of the lymphatic system. A numerical model has been constructed of a lymphatic vessel, consisting of a short series chain of contractile segments (lymphangions) and of intersegmental valves. The changing diameter of a segment governs the difference between the flows through inlet and outlet valves and is itself governed by a balance between transmural pressure and passive and active wall properties. The compliance of segments is maximal at intermediate diameters and decreases when the segments are subject to greatly positive or negative transmural pressure. Fluid flow is the result of time-varying active contraction causing diameter to reduce and is limited by segmental viscous and valvular resistance. The valves effect a smooth transition from low forward-flow resistance to high backflow resistance. Contraction occurs sequentially in successive lymphangions in the forward-flow direction. The behavior of chains of one to five lymphangions was investigated by means of pump function curves, with variation of valve opening parameters, maximum contractility, lymphangion size gradation, number of lymphangions, and phase delay between adjacent lymphangion contractions. The model was reasonably robust numerically, with mean flow-rate generally reducing as adverse pressure was increased. Sequential contraction was found to be much more efficient than synchronized contraction. At the highest adverse pressures, pumping failed by one of two mechanisms, depending on parameter settings: either mean leakback flow exceeded forward pumping or contraction failed to open the lymphangion outlet valve. Maximum pressure and maximum flow-rate were both sensitive to the contractile state; maximum pressure was also determined by the number of lymphangions in series. Maximum flow-rate was highly sensitive to the transmural pressure experienced by the most upstream lymphangions, suggesting that many feeding lymphatics would be needed to supply one downstream lymphangion chain pumping at optimal transmural pressure.     

Physiological roles of endogenous nitric oxide in lymphatic pump activity of rat mesentery in vivo

Physiological roles of endogenous nitric oxide (NO) in the lymphatic pump activity of rat mesenteries in vivo were evaluated using an intravital video microscope system. Changes in the pumping frequency (F), the end diastolic diameter (EDD), and the end systolic diameter (ESD) of the mesenteric lymph microvessels were measured with the microscope system and then the pump flow index (PFI) was calculated. A 15-min superfusion of 30 microM N(omega)-nitro-L-arginine methyl ester (L-NAME) in the mesenteries caused significant increases of F and PFI and a significant decrease of the EDD and ESD. Simultaneous superfusion of 1 mM L-arginine with 30 microM L-NAME produced a significant reversal of the L-NAME-mediated increase of F and decrease of ESD. A 15-min superfusion of 100 microM aminoguanidine caused no significant effects on F, EDD, and ESD of the mesenteric lymph vessels in vivo. These findings suggest that endogenous NO has physiologically modulated the lymphatic pump activity in rat mesentery in vivo and that the production and release of NO may be mediated by constitutive NO synthase but not by inducible NO synthase.