Lymphatic Pumping Research Articles

59 Assessment of lymphatic collector pump pressure in healthy and lymphoedematous arms by a new lymphoscintigraphic technique

Modi, S.1 3; Svensson, W.E.3; Peters, A.M.4; Stanton, A.W.B.1 3; Levick, J.R.2; Mortimer, P.S.1 Author Information

Abdominal pump treatment increases leukocyte count and flux in thoracic duct lymph

Previous studies suggest that rhythmic compression of the abdomen (abdominal lymphatic pump, ALP) enhances immunity and resistance to infectious disease, but direct evidence of this has not been documented. Thus, the hypothesis that ALP increases leukocyte numbers in thoracic duct lymph was tested in five anesthetized, open-chest dogs. The lymph duct was catheterized, lymph flow was measured, and lymph was collected under 1) baseline conditions, 2) during ALP. ALP increased lymph flow from 0.5±0.1 to 2.7±0.3 ml/min and increased leukocytes from 1.7±0.2 × 106 to 6.2±0.4 × 106cells/ml. Relative proportions macrophages, neutrophils, total lymphocytes, T cells, and B cells did not change during ALP. Considering increases in flow and leukocyte counts, ALP enhanced leukocyte flux from 0.75±0.04 × 106to 17 ± 2 × 106cells/min. In summary, ALP significantly increased both thoracic duct lymph flow and leukocyte count, so leukocyte lymphatic flux was markedly enhanced. Enhanced mobilization and lymphatic transport of immune cells during abdominal compression is likely an important mechanism responsible for the enhanced immune responses of patients treated with ALP. (Support: NIH grant U19 AT002023)

Intrinsic pump-conduit behavior of lymphangions

Lymphangions, segments of lymphatic vessels bounded by valves, have characteristics of both ventricles and arteries. They can act primarily like pumps when actively transporting lymph against a pressure gradient. They also can act as conduit vessels when passively transporting lymph down a pressure gradient. This duality has implications for clinical treatment of several types of edema, since the strategy to optimize lymph flow may depend on whether it is most beneficial for lymphangions to act as pumps or conduits. To address this duality, we employed a simple computational model of a contracting lymphangion, predicted the flows at both positive and negative axial pressure gradients, and validated the results with in vitro experiments on bovine mesenteric vessels. This model illustrates that contraction increases flow for normal axial pressure gradients. With edema, limb elevation, or external compression, however, the pressure gradient might reverse, and lymph may flow passively down a pressure gradient. In such cases, the valves may be forced open during the entire contraction cycle. The vessel thus acts as a conduit, and contraction has the effect of increasing resistance to passive flow, thus inhibiting flow rather than promoting it. This analysis may explain a possible physiological benefit of the observed flow-mediated inhibition of the lymphatic pump at high flow rates.

Lymph flow modulation: the tricks of a performant machinery

The lymphatic system displays two unique and remarkable features: (a) it collects excess fluid from interstitial compartments whose pressure is mostly subatmospheric and propels the newly formed lymph against a pressure gradient back into the venous vascular bed; and (b) it adjusts its own activity to cope with the specific draining requirements of the tissue in relation to the microvascular filtration rate. These two targets are achieved through the development, within a single functional lymphatic unit, the lymphangion, of cyclic pressure waves that may either drive interstitial fluid into the lymphangion, allowing lymph formation, or squeeze the lymphangion content through the unidirectional intraluminar valves, thus sustaining centripetal lymph propulsion. Hence, two key questions in the unveiling of the secrets of the lymphatic system are how are intraluminar pressure waves generated and how can they be modulated in response to increased tissue fluid volume? Two mechanisms have been proposed (Aukland & Reed, 1993; Schmidt-Schonbein, 1990): (a) the extrinsic, or passive mechanism, which consists in the transmission of mechanical tissue forces elicited by cardiogenic or respiratory displacements or skeletal muscle contraction to the lymphatic vessel wall; and (b) the intrinsic mechanism, which consists instead in the spontaneous contraction of lymphatic smooth muscle cells triggered by phasic action potentials arising in pace-maker smooth muscles cells dispersed in the vessel wall (McHale & Roddie, 1976; McHale & Meharg, 1992). Under conditions of increased tissue fluid volume, both the contractile force and frequency may be modulated according to the tissue drainage requirements, up to attainment of maximal lymph flow at which tissue oedema eventually develops. Modulation of lymphatic tone and pump function has been attributed to various substances released by lymphatic endothelial cells in response to increased lymph flow and shear stress. According to Gashev et al. (2002 and 2004), increased shear stress in mesenteric and thoracic lymphatic ducts reduces end-systolic tone, ejection fraction and pumping frequency. Such a negative inotropic and chronotropic response was attributed to the shear stress-dependent release of nitric oxide (NO), which determines relaxation of the smooth muscle cells and flow-dependent vasodilatation. A completely different scenario was proposed by Koller et al. (1999). In small iliac lymphatic vessels, they found that higher perfusion pressure gradients determined an increased end-diastolic and end-systolic tone, leading to reduced amplitude of the diameter oscillation during the contractile phase, coupled with increased contractile frequency. This response implies a progressive increase in viscous resistances with increasing lymph flow, is abolished by removal of the lymphatic endothelium and depends upon prostaglandins and tromboxane, but not NO, release. The controversy between these two views is not trivial and concerns several aspects of the strategy adopted to modulate lymph function: (a) is the overall tone and flow resistance decreased or increased with high flows? (b) is high flow attained with stronger and energetically more expensive active pumping or with passive, low resistive vasodilation? (c) is the cellular response to increased flow fast (NO-mediated) or slow (prostanoids-mediated)? and, finally, (d) does the extrinsic mechanism play any role? A significant contribution to clarifying these intriguing questions is provided by Gasheva et al. (2006) in this issue of The Journal of Physiology. In this rigorous study, phasically active and phasically non-active segments of rat thoracic duct were challenged with increasing transmural pressures at no imposed flow, in the presence of the NO inhibitor l-NAME, or the cyclooxygenase inhibitor indo-methacine. Since local intraluminar fluid fluxes are generated in the lymphangion of the phasically active segments due to their own spontaneous contractile activity, this study allows the detection of the self-modulatory response of the lymphatic machinery to the shear stress generated by the intrinsic activity itself. Their data clearly demonstrate that, at any distending pressure, phasically active segments display, compared to non-contracting ones, a lower resting tone accompanied by a more efficient pumping amplitude during the contraction phase. This behaviour is entirely dependent upon NO release, while eucosanoids do not seem to be significantly involved. This finding implies that the lymphatic system attains the required efficiency by exploiting a combination of intrinsic and extrinsic mechanisms, thus attaining a high flow with reduced energy expenses. This new paradigm opens new questions concerning the mechanisms by which mechanotransduction of external tissue forces acting on the lymphatic wall and internal shear stress interfere with the self-regulatory intracellular pathways in order to achieve the required pumping efficiency. Hence, this elegant study by Gasheva et al. (2006), while providing some convincing answers, opens new perspectives on the function of the last almost unexplored section of the cardiovascular system.

Mast Cell Degranulation Alters Lymphatic Contractile Activity Through Action of Histamine

Mast cells reside in most tissues and in close association with blood vessels and nerves, areas where lymphatic vessels are also present. Mast cells and lymphatic vessels are two important players in the development of the inflammatory process. This study was designed to examine the effects of mast cell degranulation on the contractile activity of mesenteric lymphatic vessels. Lymphatic vessel contractile activity was assessed in vitro by video microscopy of the mesentery of cow's milk-sensitized guinea pigs upon application of beta-lactoglobulin and compared to the response measured in sham animals. Application of 5-10 microM beta-lactoglobulin increased lymphatic vessel constriction frequency and decreased constriction amplitude (n = 12). This effect was not seen in sham-treated animals (n = 16) and was not due to an increased number of mast cells in the mesentery of the milk-sensitized animals, as revealed by histological examination. Two known mast cell-derived mediators, histamine and thromboxane A2, via stable mimetic U46619 also altered lymphatic pumping in a similar manner, but only pretreatment with the histamine H1 receptor antagonist pyrilamine (1 microM) could reduce the beta-lactoglobulin-induced response. The thromboxane A2 receptor antagonist, SQ 29548, and the 5-lipoxygenase inhibitor, caffeic acid, were without significant effect. In the in vitro mesenteric preparation, mast cell degranulation altered lymphatic contractile activity via the release of a mediator suggested to be histamine and the subsequent activation of H1 receptors. This action could potentially interfere with the expected ability of lymphatic vessels to reduce edema during inflammation.

Differential functional and molecular effects on lymphatic pumping in aged‐rat mesenteric lymphatics and thoracic duct

The mechanisms of age-related changes in lymphatic pumping remain unclear. The general objectives of this study were to characterize the lymphatic intrinsic contractile activity during aging and to investigate the age-dependent changes in the contractile machinery of lymphatic muscle. Active lymphatic pump parameters were evaluated in experiments with isolated, cannulated and pressurized rat mesenteric lymphatics (ML) and thoracic ducts (TD) taken from adult (9 mo) and old (24 mo) rats (Fischer-344 NIA). Both aged ML and TD exhibited a decreased tonic and inotropic (contraction amplitude) responses to the increases of transmural pressure, but chronotropic (contraction frequency) reactions remained rather intact. We also found that age-related changes developed differentially in regional lymphatic beds. Particularly lymphatics that usually behave more as pumping vessels (ML) affected by aging in larger degree than predominantly conductive TD. Molecular analyses found that both cardiac and vascular α-actin expression is decreased significantly in TD of 24 months old rats when compared to 9 months-old samples. Furthermore, smooth muscle MHC gene analyses showed reduced expression of SM1 MHC in both ML and TD in aged samples. In addition protein analyses in TD isolated from aged animals demonstrated that both tropomyosins as well as vascular actin were reduced. (Supported by TAMU CERH 5 P30 ES09106-08)

Effects of C‐reactive protein on rat mesenteric lymphatic contractility

C-reactive protein (CRP) is the classic acute phase reactant with serum levels rapidly increasing up to a thousand-fold during inflammatory responses. Sequential serum CRP levels in the 2–10 μg/ml range have been suggested as prognostic indication of subsequent cardiovascular event. Lymphatics play an important role in the cardiovasculature providing fluid transport of proteins, products of tissue disintegration and infectious agents, and thereby play a major role in edema formation and the immune response. We proposed that a correlation existed between increased serum CRP levels and lymphatic function and hypothesized that increased CRP levels in the lymphatics modulated pumping activity. Rat mesenteric lymphatics were isolated, cannulated and perfused. Contraction frequency and systolic and diastolic diameters were determined from lymphatic diameter tracings. Pressure-induced changes in lymphatic pumping parameters were investigated in the presence and absence of CRP (10 μg/ml). Addition of CRP increased contraction frequency and led to a decrease in tone (increased diastolic diameter). Such CRP-induced changes in lymphatic pump parameters may reflect an important role of CRP in amplification of lymphatic pumping to resolve tissue edema, stabilize of intercellular fluid balance and stimulate immune response during inflammation (NIH HL080526-02, MM; HL051199-09, RMH).

Inhibition of myosin light chain phosphorylation decreases rat mesenteric lymphatic pump function

Molecular mechanisms involved in the regulation of lymphatic muscle contraction are not well understood. We hypothesized that phosphorylation of myosin light chain (MLC) might play an essential role in lymphatic muscle contractility. Molecular analyses showed the presence of MLC20 mRNA and protein in lymphatics from mesenteric, femoral, cervical and thoracic duct. Interestingly differential expression of MLC kinase and phosphatase was found in these lymphatics. To understand the functional roles of MLC phosphorylation, the pumping activities of the isolated and cannulated rat mesenteric lymphatic vessels were measured, determined and detected over a range of transmural pressures (1~5 cm H2O) in the presence or absence of a MLCK specific inhibitor ML-7 (1–100 μM). Results showed that ML-7 produced a significant inhibition of lymph pump activity. The lymphatic systolic diameter was increased. The diastolic diameter was not altered. The contraction frequency, stroke volume, ejection fraction, and pump flow were completely inhibited by ML-7 at 100 μM. Immunohistochemical and Western blot analyses demonstrated a reduced staining of the phospho-MLC in the vessels treated with ML-7. These results are the first evidence that the phosphorylation of MLC involved in the regulation of lymphatic contractile activities. (NIH HL075199 and HL80526).

Stimulation of the lymphatic pump inhibits passive flow

Lymphatic vessels act as pumps to promote lymph flow up a pressure gradient and as conduits for flow down a pressure gradient. We hypothesized that stimulation of the pumping lymphatic will decrease average diameter, increasing vessel resistance to passive flow. Bovine post-nodal mesenteric lymphatic vessels (n = 4) were harvested locally from freshly slaughtered cattle. Vessels were instrumented in a tubular organ bath and bathed with a balanced polyionic 1% albumin solution set to 37° C, gassed with room air, and buffered to a pH of 7.4. Inlet pressure to the vessel was set at 5 cm H2O while outlet pressure was varied from about 6.5 to 3.5 cm H2O. Lymphatic vessels were exposed to solution containing 10−7 M U46619, a thromboxane analogue, and solution without U46619 in random order and instantaneous diameter and flow rate were recorded. Mean diameter of the lymphatic vessel was significantly decreased in the presence of U46619 (1.81±0.032 mm vs. 1.55±0.057 mm, p = 0.001, Student's t-test). Flow rates were significantly inhibited during positive pressure gradients (mean difference 2.63±0.62 g/min, p < 0.001, ANCOVA) while no difference in flow was detected during pumping against a pressure gradient (p = 0.26, ANCOVA). We conclude that stimulation of post-nodal mesenteric lymphatic vessels inhibits passive flow by decreasing average diameter and thereby increasing resistance to flow. TAMU-LSTF, AHA-0365127Y, CDC-623086.

VEGF‐C increases lymph pump flow in rat mesenteric microlymphatics

VEGF-C is a well-described promoter of lymphangiogenesis, however little is known about acute effects of VEGF-C on lymphatic pump activity. We tested the hypothesis that VEGF-C increases microlymphatic pump output. We examined rat mesenteric microlymphatics (approximately 75-100 μm diameter) in vivo using intravital microscopy. Digital videos were recorded during baseline and at 1, 10, 20, 30, 45, and 60 minutes after treatment with 1 nM recombinant VEGF-C (C156S). Contraction frequency, end diastolic diameter (EDD), end systolic diameter, stroke volume, and lymph pump flow (LPF) were determined at these time points. We also assessed arteriolar diameter and microvascular extravasation of FITC-albumin. The results show that VEGF-C significantly increased contraction frequency and EDD within one minute, and increased stroke volume and LPF at 10 minutes. These parameters remained elevated for 30 minutes. VEGF-C did not significantly alter arteriolar diameter or microvascular permeability. Immunofluorescence staining revealed co-localization of the VEGF-C receptor, VEGFR-3, with the lymphatic endothelial cell marker LYVE-1. Taken together, the data 1) demonstrate that VEGF-C increases microlymphatic pump output, and 2) provide evidence of endothelium-associated regulation of microlymphatic phasic contractions and flow. Supported by NIH F32 HL76079, R01 HL61507, R01 HL70752, and R01 HL73324.

Inhibition of active lymph pump by simulated microgravity in rats

During spaceflight the normal head-to-foot hydrostatic pressure gradients are eliminated and body fluids shift toward the head, resulting in a diminished fluid volume in the legs and an increased fluid volume in the head, neck, and upper extremities. Lymphatic function is important in the maintenance of normal tissue fluid volume, but it is not clear how microgravity influences lymphatic pumping. We performed a detailed evaluation of the influence of simulated microgravity on lymphatic diameter, wall thickness, elastance, tone, and other measures of phasic contractility in isolated lymphatics. Head-down tail suspension (HDT) rats were used to simulate the effects of microgravity. Animals were exposed to HDT for 2 wk, after which data were collected and compared with the control non-HDT group. Lymphatics from four regional lymphatic beds (thoracic duct, cervical, mesenteric, and femoral lymphatics) were isolated, cannulated, and pressurized. Input and output pressures were adjusted to apply a range of transmural pressures and flows to the lymphatics. Simulated microgravity caused a potent inhibition of pressure/stretch-stimulated pumping in all four groups of lymphatics. The greatest inhibition was found in cervical lymphatics. These findings presumably are correlated to the cephalic fluid shifts that occur in HDT rats as well as those observed during spaceflight. Flow-dependent pump inhibition was increased after HDT, especially in the thoracic duct. Mesenteric lymphatics were less strongly influenced by HDT, which may support the idea that lymph hydrodynamic conditions in the mesenteric lymphatic during HDT are not dramatically altered.

Arnold Heller and the lymph pump

This article reviews studies on lymph propulsion in the lymph vessels by active contraction of the vessels, first described by Arnold Heller in 1869 in German language, and here translated into English. His observations were first confirmed by Beatrice Carrier (1926) and Howard Flory et al. (1927), and several groups were active up to World war II. Few publications appeared in the period 1940--1960, followed by increasing activity and development of new experimental techniques for use both in various experimental animals and in humans. Recently it has been shown that passive lymph flow may add to active propulsion. Both mechanisms depend on lymph formation, i.e. the uptake of interstitial fluid by the initial lymph vessels which is still not well understood.

Increased lymphatic flow in the thoracic duct during manipulative intervention.

The thoracic pump and the abdominal pump are osteopathic manipulative (OM) lymphatic pump techniques frequently used by osteopathic physicians to treat patients with infections (eg, pneumonia, otitis media). Although there is a widely accepted belief among the osteopathic medical profession that increasing lymphatic flow is beneficial, no measurements of lymph flow during osteopathic manipulative treatment have been reported. The authors surgically instrumented five mongrel dogs to record lymphatic flow in the thoracic duct (TDF) and cardiac variables during three intervention protocols. After recovery from surgery, canine subjects were placed in a standing-support sling, and TDF, cardiac output, mean aortic blood pressure, and heart rate were recorded during two randomized 30-second sessions of manipulative intervention using the osteopathic thoracic pump and abdominal pump techniques on two successive days. Lymph flow in the thoracic duct increased from 1.57+/-0.20 mL x min(-1) to a peak TDF of 4.80+/-1.73 mL x min(-1) during abdominal pump, and from 1.20+/-0.41 mL x min(-1) to 3.45+/-1.61 mL x min(-1) during thoracic pump. Lymph flow in the thoracic duct and cardiac variables were also recorded for canine subjects during physical activity (ie, treadmill exercise at 3 miles per hour at 0% incline). During physical activity, TDF increased from 1.47+/-0.33 mL x min(-1) to 5.81+/-1.30 mL x min(-1). Although cardiac variables did not change significantly during manipulative intervention with lymphatic pump techniques, cardiac output and heart rate did increase during physical activity. The authors conclude that physical activity and manipulative intervention using thoracic pump and abdominal pump techniques produced net increases in TDF (P<.05).

Lymphatic Biology and the Microcirculation: Past, Present and Future

Because of the role that lymphatics have in fluid and macromolecular exchange, lymphatic function has been tightly tied to the study of the microcirculation for decades. Despite this, our understanding of many basic tenets of lymphatic function is far behind that of the blood vascular system. This is in part due to the difficulty inherent in working in small, thin-walled, clear lymphatic vessels and the relative lack of lymphatic specific molecular/cellular markers. The application of cellular and molecular tools to the field of lymphatic biology has recently produced some significant developments in lymphatic endothelial cell biology. These have propelled our understanding of lymphangiogenesis and related fields forward. Whereas the use of some of these techniques in lymphatic muscle biology has somewhat lagged behind those in the endothelium, recent developments in lymphatic muscle contractile and electrical physiology have also led to advances in our understanding of lymphatic transport function, particularly in the regulation of the intrinsic lymph pump. However, much work remains to be done. This paper reviews significant developments in lymphatic biology and discusses areas where further development of lymphatic biology via classical, cellular, and molecular approaches is needed to significantly advance our understanding of lymphatic physiology.

Proteinase-activated receptor 2 activation modulates guinea-pig mesenteric lymphatic vessel pacemaker potential and contractile activity.

Lymphatic vessels rhythmically constrict to avoid fluid and protein accumulation in the interstitial space. This activity is critical during inflammation to prevent excessive oedema. Lymphatic pumping is intrinsic to the smooth muscle in the vessel wall and is due to the spontaneous occurrence of action potentials, the pacemaker of which is proposed to be spontaneous transient depolarizations (STDs). This function is highly susceptible to the fluid load and modulated by chemical agents, amongst which inflammatory mediators are important players. Activation of proteinase-activated receptors (PARs) has been involved in inflammation and affects vascular smooth muscle tone. The present study aims to investigate the role of PAR2, a member of the PAR family, in lymphatic vessel pumping. RT-PCR experiments revealed that PAR2 message is present in lymphatic vessels of the guinea-pig mesentery. Agonists of PAR2 such as trypsin and the activating peptide, SLIGRL-NH2, caused a decrease in the contractile activity of intraluminally perfused lymphatic vessels. Moreover, intracellular microelectrode recordings from isolated vessels revealed that PAR2 activation hyperpolarized the lymphatic smooth muscle membrane potential and altered STD amplitude and frequency. The decreases in constriction frequency and STD activity as well as the hyperpolarization were dependent on a functional endothelium, not affected by NO synthase or guanylyl-cyclase inhibition, but mimicked by PGE2 and iloprost and blocked by indomethacin (10 microM) and glibenclamide (1 microM). These results show that PAR2 activation alters guinea-pig lymphatic vessel contractile and electrical activity via the production of endothelium-derived cyclo-oxygenase metabolites.

ATP inhibits pump activity of lymph vessels via adenosine A1 receptor-mediated involvement of NO- and ATP-sensitive K+ channels.

We examined the effects of ATP on intrinsic pump activity in lymph vessels isolated from the rat. ATP caused significant dilation with a cessation of lymphatic pump activity. Removal of the endothelium or pretreatment with Nomega-nitro-L-arginine methyl ester (L-NAME) significantly reduced ATP-induced inhibitory responses of lymphatic pump activity, whereas reduction was not suppressed completely by 10(-6) M ATP. L-arginine significantly restored ATP-induced inhibitory responses in the presence of L-NAME. ATP-induced inhibitory responses in lymph vessels with endothelium were also significantly, but not completely, suppressed by pretreatment with glibenclamide. 8-Cyclopentyl-1,3-dipropylxanthine (a selective adenosine A1 receptor antagonist), but not suramine (a P2X and P2Y receptor antagonist) or 3,7-dimethyl-1-proparglyxanthine (a selective adenosine A2 receptor antagonist), significantly decreased ATP-induced inhibitory responses. alpha,beta-methylene ATP (a selective P2X and P2Y receptor agonist) had no significant effect on lymphatic pump activity. In some lymph vessels with endothelium (24 of 30 preparations), adenosine also caused dose-dependent dilation with a cessation of lymphatic pump activity. L-NAME significantly reduced the inhibitory responses induced by the lower (3 x 10(-8)-3 x 10(-7) M) concentrations of adenosine. Glibenclamide or 8-cyclopentyl-1,3-dipropylxanthine also significantly suppressed adenosine-induced inhibitory responses. These findings suggest that ATP-induced dilation and inhibition of pump activity of isolated rat lymph vessels are endothelium-dependent and -independent responses. ATP-mediated inhibitory responses may be, in part, related to production of endogenous nitric oxide, involvement of ATP-sensitive K+ channels, or activation of adenosine A1 receptors in lymphatic smooth muscle and endothelium.

Effects of Substance P on Mesenteric Lymphatic Contractility in the Rat

Substance P (SP) is associated with lymphatic tissue and is a putative mediator of inflammation. The lymph pump is one of the major "safety factors" preventing edema and its activity is altered by inflammatory mediators. The impact of SP on lymphatics was studied in the rat mesentery. Rats were prepared for in situ lymphatic observation using intravital videomicroscopic techniques. Sections of the small intestine and mesentery were exteriorized and superfused. Lymphatic diameters were measured and pumping activity was determined from the lymphatic diameter tracing. Lymph pump parameters evaluated included diastolic diameter, systolic diameter, contraction frequency, stroke volume, and lymph pump flow. After a control period, the tissues were exposed to SP (10(-9), 10(-8), 10(-7), 10(-6) M). SP reduced both diameters and increased contraction frequency in a concentration-dependent manner. SP (1.0 microM) produced the following changes (% of control): reductions in diastolic diameter (50%), systolic diameter (55%), and stroke volume (63%); a large increase in contraction frequency (661%); and a modest increase in lymph pump flow (44%). SP also stimulated vessels that were not contracting phasically to develop typical contraction patterns. Although SP produced a tonic constriction of the lymphatics, lymph pump flow was maintained or slightly elevated via an increase in contraction frequency. SP also induced pumping activity in lymphatics that were previously quiescent, thereby further stimulating flow. These effects may serve to minimize the formation of edema in face of the inflammatory edemagenic conditions produced by SP.

Lymphodynamic properties governing sentinel lymph nodes.

Biological properties of lymph microvessels include characteristics of spontaneous contractions of lymph vessels, nitric oxide (NO)-mediated modulation of active lymph pump activity, flow-induced production and release of NO from lymphatic endothelial cells, and localization of NO synthase in cultured lymphatic endothelial cells. B16-BL6 melanoma cells release factors that inhibit active pump activity in isolated lymph microvessels. We have found that basic fibroblast growth factor (bFGF) induces significant proliferation and migration of canine cultured lymphatic endothelial cells. This growth factor facilitates the formation of capillary-like tubes by the cultured endothelial cells.

Lymphatic smooth muscle: the motor unit of lymph drainage

Embedded into the wall of collecting lymphatic vessels and trunks, the lymphatic smooth muscles are cardinal to the functions of the lymphatic system. Their intrinsic contractile property—the intrinsic lymph pump—through rhythmical and phasic contractions of the vessels, represents the principal mechanism by which lymph flow is generated. Through changes in tonic constrictions, lymphatic smooth muscles also modulate lymph flow resistance. Lymphatic smooth muscles are sensitive to physical and chemical stimuli, mediating changes in their activity and modulating lymphatic drainage. Because lymphatic smooth muscles play such an important role in fluid transport, their dysfunction may be a component of many inflammatory disease states. This review presents recent findings on the physiology and cellular biology of lymphatic smooth muscles and discusses the importance of these cells for the function of the lymphatic system in physiological and pathophysiological situations. Cell facts • Home of the intrinsic contractile activity of lymphatic vessels. • Cardinal to the efficient drainage of interstitial fluid through phasic and tonic contractions. • Targeted by a wide variety of stimuli, which modulate their contractile properties/activity. • Critically regulated by luminal flow and pressure and by the lymphatic endothelium. • Dysfunction may lead to pathologies of the lymphatic system like lymphedema, and could be involved in inflammatory pathologies.

Octreotide in Intestinal Lymphangiectasia: Lack of a Clinical Response and Failure to Alter Lymphatic Function in a Guinea Pig Model

Intestinal lymphangiectasia, which can be classified as primary or secondary, is an unusual cause of protein-losing enteropathy. The main clinical features include edema, fat malabsorption, lymphopenia and hypoalbuminemia. Clinical management generally includes a low-fat diet and supplementation with medium chain triglycerides. A small number of recent reports advocate the use of octreotide in intestinal lymphangiectasia. It is unclear why octreotide was used in these studies; although octreotide can alter splanchnic blood flow and intestinal motility, its actions on lymphatic function has never been investigated. A case of a patient with intestinal lymphangiectasia who required a shunt procedure after failing medium chain triglycerides and octreotide therapy is presented. During the management of this case, all existing literature on intestinal lymphangiectasia and all the known actions of octreotide were reviewed. Because some of the case reports suggested that octreotide may improve the clinical course of intestinal lymphangiectasia by altering lymphatic function, a series of experiments were undertaken to assess this. In an established guinea pig model, the role of octreotide in lymphatic function was examined. In this model system, the mesenteric lymphatic vessels responded to 5-hydroxytryptamine with a decrease in constriction frequency, while histamine administration markedly increased lymphatic constriction frequency. Octreotide failed to produce any change in lymphatic function when a wide range of concentrations were applied to the mesenteric lymphatic vessel preparation. In conclusion, in this case, octreotide failed to induce a clinical response and laboratory studies showed that octreotide did not alter lymphatic function. Thus, the mechanisms by which octreotide induced clinical responses in the cases reported elsewhere in the literature remain unclear, but the present study suggests that it does not appear to act via increasing lymphatic pumping.