Antihistamines Increase Body Mass Index Percentiles and Z-Scores in Hispanic Children

The prevalence of childhood obesity has increased over the years in the United States and contributed to a rise in metabolic syndrome and non-alcoholic fatty liver disease (NAFLD). Animal studies suggested the role of histamine blockade on mesenteric lymphatics tone, contributing to weight gain and hepatic steatosis. This study aimed to investigate an association between antihistamines (AH) use in children and obesity. A single-center retrospective cohort study on children with a diagnosis of NAFLD, followed in the gastroenterology clinic, was performed between January 2018 and April 2019. The demographics, medications, and body mass index (BMI) were assessed. Participants were divided into an AH group with documented use and comparison group, antihistamine naïve. Of the 32 participants in the study, 13 used AH, and 19 did not. Antihistamine users had a mean increase in BMI percentile per year of 1.17 compared to a decrease of 0.06 in comparison group (p = 0.0008). AH usage correlated with a mean increase in BMI z-score of 0.23 per year, as opposed to a decrease by 0.012 in comparison group (p = 0.0016). No difference was found in triglycerides (TG), glucose, and liver enzymes. AH use increases BMI percentiles and z-scores over time and is associated with obesity in children.

Emerging Roles of Mast Cells in the Regulation of Lymphatic Immuno-Physiology

Mast cells (MCs) are abundant in almost all vascularized tissues. Furthermore, their anatomical proximity to lymphatic vessels and their ability to synthesize, store and release a large array of inflammatory and vasoactive mediators emphasize their significance in the regulation of the lymphatic vascular functions. As a major secretory cell of the innate immune system, MCs maintain their steady-state granule release under normal physiological conditions; however, the inflammatory response potentiates their ability to synthesize and secrete these mediators. Activation of MCs in response to inflammatory signals can trigger adaptive immune responses by dendritic cell-directed T cell activation. In addition, through the secretion of various mediators, cytokines and growth factors, MCs not only facilitate interaction and migration of immune cells, but also influence lymphatic permeability, contractility, and vascular remodeling as well as immune cell trafficking through the lymphatic vessels. In summary, the consequences of these events directly affect the lymphatic niche, influencing inflammation at multiple levels. In this review, we have summarized the recent advancements in our understanding of the MC biology in the context of the lymphatic vascular system. We have further highlighted the MC-lymphatic interaction axis from the standpoint of the tumor microenvironment.

Histamine-mediated autocrine signaling in mesenteric perilymphatic mast cells

Lymphatic vessels play a critical role in mounting a proper immune response by trafficking peripheral immune cells to draining lymph nodes. Mast cells (MCs) are well known for their roles in type I hypersensitivity reactions, but little is known about their secretory regulation in the lymphatic niche. MCs, as innate sensor and effector cells, reside close to mesenteric lymphatic vessels (MLVs), and their activation and ability to release histamine influences the lymphatic microenvironment in a histamine-NF-κB-dependent manner. Using an established experimental protocol involving surgical isolation of rat mesenteric tissue segments, including MLVs and surrounding perilymphatic tissues, we tested the hypothesis that perilymphatic mesenteric MCs possess histamine receptors (HRs) that bind and respond to the histamine released from these same MCs. Under various experimental conditions, including inflammatory stimulation by LPS, we measured histamine in mesenteric perilymphatic tissues, evaluated expression of histidine decarboxylase in MCs along with the degree of MC degranulation, assessed the functional status of HRs in MCs, and evaluated the ability of histamine itself to induce MC activation. Finally, we evaluated the importance of MCs and HR1 and -2 for MLV-directed trafficking of CD11b/c-positive cells during acute tissue inflammation. Our data indicate the existence of a functionally potent MC-histamine autocrine regulatory loop, the elements of which are crucially important for acute inflammation-induced trafficking of the CD11b/c-positive cells toward MLVs. This MC-histamine loop serves as a first-line cellular servo control system, playing a key role in the innate and adaptive immune response as well as NF-κB-mediated maintenance of body homeostasis.

Modulation of the Tryptophan Hydroxylase 1/Monoamine Oxidase-A/5-Hydroxytryptamine/5-Hydroxytryptamine Receptor 2A/2B/2C Axis Regulates Biliary Proliferation and Liver Fibrosis During Cholestasis

BACKGROUND AND AIMS

Serotonin (5HT) is a neuroendocrine hormone synthetized in the central nervous system (CNS) as well as enterochromaffin cells of the gastrointestinal tract. Tryptophan hydroxylase (TPH1) and monoamine oxidase (MAO-A) are the key enzymes for the synthesis and catabolism of 5HT, respectively. Previous studies demonstrated that 5-hydroxytryptamine receptor (5HTR)1A/1B receptor agonists inhibit biliary hyperplasia in bile-duct ligated (BDL) rats, whereas 5HTR2B receptor antagonists attenuate liver fibrosis (LF) in mice. Our aim was to evaluate the role of 5HTR2A/2B/2C agonists/antagonists in cholestatic models.

APPROACH AND RESULTS

While in vivo studies were performed in BDL rats and the multidrug resistance gene 2 knockout (Mdr2-/- ) mouse model of PSC, in vitro studies were performed in cell lines of cholangiocytes and hepatic stellate cells (HSCs). 5HTR2A/2B/2C and MAO-A/TPH1 are expressed in cholangiocytes and HSCs from BDL rats and Mdr2-/- - mice. Ductular reaction, LF, as well as the mRNA expression of proinflammatory genes increased in normal, BDL rats, and Mdr2-/- - mice following treatment 5HTR2A/2B/2C agonists, but decreased when BDL rats and Mdr2-/- mice were treated with 5HTR2A/2B/2C antagonists compared to BDL rats and Mdr2-/- mice, respectively. 5HT levels increase in Mdr2-/- mice and in PSC human patients compared to their controls and decrease in serum of Mdr2-/- mice treated with 5HTR2A/2B/2C antagonists compared to untreated Mdr2-/- mice. In vitro, cell lines of murine cholangiocytes and human HSCs express 5HTR2A/2B/2C and MAO-A/TPH1; treatment of these cell lines with 5HTR2A/2B/2C antagonists or TPH1 inhibitor decreased 5HT levels as well as expression of fibrosis and inflammation genes compared to controls.

CONCLUSIONS

Modulation of the TPH1/MAO-A/5HT/5HTR2A/2B/2C axis may represent a therapeutic approach for management of cholangiopathies, including PSC.

Lymphatic Cannulation for Lymph Sampling and Molecular Delivery

Unlike the blood, the interstitial fluid and the deriving lymph are directly bathing the cellular layer of each organ. As such, composition analysis of the lymphatic fluid can provide more precise biochemical and cellular information on an organ's health and be a valuable resource for biomarker discovery. In this study, we describe a protocol for cannulation of mouse and rat lymphatic collectors that is suitable for the following: the "omic" sampling of pre- and postnodal lymph, collected from different anatomical districts; the phenotyping of immune cells circulating between parenchymal organs and draining lymph nodes; injection of known amounts of molecules for quantitative immunological studies of nodal trafficking and/or clearance; and monitoring an organ's biochemical omic changes in pathological conditions. Our data indicate that probing the lymphatic fluid can provide an accurate snapshot of an organ's physiology/pathology, making it an ideal target for liquid biopsy.

Prolonged intake of desloratadine: mesenteric lymphatic vessel dysfunction and development of obesity/metabolic syndrome

This study aimed to establish mechanistic links between the prolonged intake of desloratadine, a common H1 receptor blocker (i.e., antihistamine), and development of obesity and metabolic syndrome. Male Sprague-Dawley rats were treated for 16 wk with desloratadine. We analyzed the dynamics of body weight gain, tissue fat accumulation/density, contractility of isolated mesenteric lymphatic vessels, and levels of blood lipids, glucose, and insulin, together with parameters of liver function. Prolonged intake of desloratadine induced development of an obesity-like phenotype and signs of metabolic syndrome. These alterations in the body included excessive weight gain, increased density of abdominal subcutaneous fat and intracapsular brown fat, high blood triglycerides with an indication of their rerouting toward portal blood, high HDL, high fasting blood glucose with normal fasting and nonfasting insulin levels (insulin resistance), high liver/body weight ratio, and liver steatosis (fatty liver). These changes were associated with dysfunction of mesenteric lymphatic vessels, specifically high lymphatic tone and resistance to flow together with diminished tonic and abolished phasic responses to increases in flow, (i.e., greatly diminished adaptive reserves to respond to postprandial increases in lymph flow). The role of nitric oxide in this flow-dependent adaptation was abolished, with remnants of these responses controlled by lymphatic vessel-derived histamine. Our current data, considered together with reports in the literature, support the notion that millions of the United States population are highly likely affected by underevaluated, lymphatic-related side effects of antihistamines and may develop obesity and metabolic syndrome due to the prolonged intake of this medication. NEW & NOTEWORTHY Prolonged intake of desloratadine induced development of obesity and metabolic syndrome associated with dysfunction of mesenteric lymphatic vessels, high lymphatic tone, and resistance to flow together with greatly diminished adaptive reserves to respond to postprandial increases in lymph flow. Data support the notion that millions of the USA population are highly likely affected by underevaluated, lymphatic-related side effects of antihistamines and may develop obesity and metabolic syndrome due to the prolonged intake of this medication.

Temporal Dynamics of the Rat Thoracic Duct Contractility in the Presence of Imposed Flow

BACKGROUND

The initial periods of increased flow inside lymphatic vessels demonstrate specific temporary patterns of self-tuning of lymphatic vessel contractility that are heterogeneous across regional lymphatic networks. The current literature primarily refers to the immediate and fast reactions of the lymphangions to increases in basal flow. Until now, there were no available data on how the lymphatic vessels react to comparatively longer periods of imposed flow.

METHODS AND RESULTS

In this study, we measured and analyzed the contractility of the rat thoracic duct segments, isolated, cannulated, and pressurized at 3 cm H₂O at no imposed flow conditions and during 4 hours of imposed flow (constant transaxial pressure gradient of 2 cm H₂O). We found the development of a progressing lymphatic tonic relaxation and inhibition of the lymphatic contraction frequency over 4 hours of imposed flow. After a short initial decrease, lymphatic phasic contraction amplitude rose significantly during the first hour of imposed flow, and it demonstrated a trend to return toward control levels after 3 hours of imposed flow. As a result, the fractional pump flow (active lymph pumping per minute) of isolated thoracic duct segments reached and maintained a statistically significant decrease (from control no-flow conditions) at the end of the third hour of imposed flow.

CONCLUSIONS

Our new findings provide a better understanding of how lymphatic contractility changes during the development of prolonged periods of steady lymph flow. The latter may occur during the initial phases of development of an inflammatory-related tissue edema.

Quantitative Profiling of the Lymph Node Clearance Capacity

Transport of tissue-derived lymphatic fluid and clearance by draining lymph nodes are pivotal for maintenance of fluid homeostasis in the body and for immune-surveillance of the self- and non-self-proteomes. Yet a quantitative analysis of nodal filtration of the tissue-derived proteome present in lymphatic fluid has not been reported. Here we quantified the efficiency of nodal clearance of the composite proteomic load using label-free and isotope-labeling proteomic analysis of pre-nodal and post-nodal samples collected by direct cannulation. These results were extended by quantitation of the filtration efficiency of fluorophore-labeled proteins, bacteria, and beads infused at physiological flow rates into pre-nodal lymphatic collectors and collected by post-nodal cannulation. We developed a linear model of nodal filtration efficiency dependent on pre-nodal protein concentrations and molecular weight, and uncovered criteria for disposing the proteome incoming from defined anatomical districts under physiological conditions. These findings are pivotal to understanding the maximal antigenic load sustainable by a draining node, and promote understanding of pathogen spreading and nodal filtration of tumor metastasis, potentially helping to improve design of vaccination protocols, immunization strategies and drug delivery.

Aging-associated alterations in lymphatic vessels and mast cells in perilymphatic tissues

Aging alters lymphatic vessel structure (by increasing the size of low muscle cell investiture zones), ultrastructure (through loss of the glycocalyx), and proteome composition causing a concomitant increase in lymphatic permeability. Our recent studies have shown that the contractile function of aged lymphatic vessels is depleted with abolished role of nitric oxide and an increased role of lymphatic-born histamine in flow-dependent regulation of lymphatic phasic contractions and tone. Thus, aging-related reduced vessel functionality and increased oxidative stress of the lymphatic vasculature facilitate the spread of pathogens into perilymphatic tissues. In addition, aging causes the basal activation of the perilymphatic mast cells, which, in turn, restricts the recruitment/activation of MHCII+ -positive cells and eosinophils in perilymphatic tissues. This aging-associated basal activation of perilymphatic mast cells limits proper functioning of the mast cell/histamine/NF-kB axis that is essential for the regulation of lymphatic vessel transport and barrier functions as well as for both the interaction and trafficking of immune cells near and within lymphatic collecting vessels. Cumulatively these alterations play crucial roles in the progression of aging-related pathogenesis in inflammation and immunity.

Optical assessment of changes in mechanical and chemical properties of adipose tissue in diet-induced obese rats

Obesity is becoming a leading cause of health problems world-wide. Obesity and overweight are associated with the structural and chemical changes in tissues; however, few methods exist that allow for concurrent measurement of these changes. Using Brillouin and Raman microspectroscopy, both the mechanical and chemical differences can be assessed simultaneously. We hypothesized that Brillouin spectroscopy can measure the adipose tissues' stiffness, which increases in obesity. Samples of brown and white adipose tissues obtained from control and diet-induced obese adult rats were analyzed. The results show that both adipose tissues of the obese group exhibit a greater high-frequency longitudinal elastic modulus than the control samples, and that the brown fat is generally stiffer than white adipose. The Raman spectra indicate that the lipids' accumulation in adipose tissue outpaces the fibrosis, and that the high-fat diet has a greater effect on the brown adipose than the white fat. Overall, the powerful combination of Brillouin and Raman microspectroscopies successfully assessed both the mechanical properties and chemical composition of adipose tissue simultaneously for the first time. The results indicate that the adipose tissue experiences an obesity-induced increase in stiffness and lipid content, with the brown adipose tissue undergoing a more pronounced change compared to white adipose.

Mast cells and histamine are triggering the NF-κB-mediated reactions of adult and aged perilymphatic mesenteric tissues to acute inflammation

This study aimed to establish mechanistic links between the aging-associated changes in the functional status of mast cells and the altered responses of mesenteric tissue and mesenteric lymphatic vessels (MLVs) to acute inflammation. We used an in vivo model of acute peritoneal inflammation induced by lipopolysaccharide treatment of adult (9-month) and aged (24-month) F-344 rats. We analyzed contractility of isolated MLVs, mast cell activation, activation of nuclear factor-κB (NF-κB) without and with stabilization of mast cells by cromolyn or blockade of all types of histamine receptors and production of 27 major pro-inflammatory cytokines in adult and aged perilymphatic mesenteric tissues and blood. We found that the reactivity of aged contracting lymphatic vessels to LPS-induced acute inflammation was abolished and that activated mast cells trigger NF-κB signaling in the mesentery through release of histamine. The aging-associated basal activation of mesenteric mast cells limits acute inflammatory NF-κB activation in aged mesentery. We conclude that proper functioning of the mast cell/histamine/NF-κB axis is necessary for reactions of the lymphatic vessels to acute inflammatory stimuli as well as for interaction and trafficking of immune cells near and within the collecting lymphatics.

Aged Lymphatic Vessels and Mast Cells in Perilymphatic Tissues

This review provides a comprehensive summary of research on aging-associated alterations in lymphatic vessels and mast cells in perilymphatic tissues. Aging alters structure (by increasing the size of zones with low muscle cell investiture), ultrastructure (through loss of the glycocalyx), and proteome composition with a concomitant increase in permeability of aged lymphatic vessels. The contractile function of aged lymphatic vessels is depleted with the abolished role of nitric oxide and an increased role of lymphatic-born histamine in flow-dependent regulation of lymphatic phasic contractions and tone. In addition, aging induces oxidative stress in lymphatic vessels and facilitates the spread of pathogens from these vessels into perilymphatic tissues. Aging causes the basal activation of perilymphatic mast cells, which, in turn, restricts recruitment/activation of immune cells in perilymphatic tissues. This aging-associated basal activation of mast cells limits proper functioning of the mast cell/histamine/NF-κB axis that is essential for the regulation of lymphatic vessel transport and barrier functions as well as for both the interaction and trafficking of immune cells near and within lymphatic collecting vessels. Cumulatively, these changes play important roles in the pathogenesis of alterations in inflammation and immunity associated with aging.

Histamine as an Endothelium-Derived Relaxing Factor in Aged Mesenteric Lymphatic Vessels

BACKGROUND

Knowledge of the mechanisms by which aging affects contracting lymphatic vessels remains incomplete; therefore, the functional role of histamine in the reaction of aged lymphatic vessels to increases in flow remains unknown.

METHODS AND RESULTS

We measured and analyzed parameters of lymphatic contractility in isolated and pressurized rat mesenteric lymphatic vessels (MLVs) obtained from 9- and 24-month Fischer-344 rats under control conditions and after pharmacological blockade of nitric oxide (NO) by Nω-Nitro-L-arginine methyl ester hydrochloride (L-NAME, 100 μM) or/and blockade of histamine production by α-methyl-DL-histidine dihydrochloride (α-MHD, 10 μM). We also quantitatively compared results of immunohistochemical labeling of the histamine-producing enzyme, histidine decarboxylase (HDC) in adult and aged MLVs. Our data provide the first demonstration of an increased functional role of histamine as an endothelial-derived relaxing factor in aged MLVs, which appears in parallel with the abolished role of NO in the reactions of these lymph vessels to increases in flow. In addition, we found an increased expression of HDC in endothelium of aged MLVs.

CONCLUSIONS

Our findings provide the basis for better understanding of the processes of aging in lymphatic vessels and for setting new important directions for investigations of the aging-associated disturbances in lymph flow and the immune response.

Aging-related anatomical and biochemical changes in lymphatic collectors impair lymph transport, fluid homeostasis, and pathogen clearance

The role of lymphatic vessels is to transport fluid, soluble molecules, and immune cells to the draining lymph nodes. Here, we analyze how the aging process affects the functionality of the lymphatic collectors and the dynamics of lymph flow. Ultrastructural, biochemical, and proteomic analysis indicates a loss of matrix proteins, and smooth muscle cells in aged collectors resulting in a decrease in contraction frequency, systolic lymph flow velocity, and pumping activity, as measured in vivo in lymphatic collectors. Functionally, this impairment also translated into a reduced ability for in vivo bacterial transport as determined by time-lapse microscopy. Ultrastructural and proteomic analysis also indicates a decrease in the thickness of the endothelial cell glycocalyx and loss of gap junction proteins in aged lymph collectors. Redox proteomic analysis mapped an aging-related increase in the glycation and carboxylation of lymphatic’s endothelial cell and matrix proteins. Functionally, these modifications translate into apparent hyperpermeability of the lymphatics with pathogen escaping from the collectors into the surrounding tissue and a decreased ability to control tissue fluid homeostasis. Altogether, our data provide a mechanistic analysis of how the anatomical and biochemical changes, occurring in aged lymphatic vessels, compromise lymph flow, tissue fluid homeostasis, and pathogen transport.

Involvement of histamine in endothelium-dependent relaxation of mesenteric lymphatic vessels

OBJECTIVES

The knowledge of the basic principles of lymphatic function, still remains, to a large degree, rudimentary and will require significant research efforts. Recent studies of the physiology of the MLVs suggested the presence of an EDRF other than NO. In this study, we tested the hypothesis that lymphatic endothelium-derived histamine relaxes MLVs.

METHODS

We measured and analyzed parameters of lymphatic contractility in isolated and pressurized rat MLVs under control conditions and after pharmacological blockade of NO by L-NAME (100 μM) or/and histamine production by α-MHD (10 μM). Effectiveness of α-MHD was confirmed immunohistochemically. We also used immunohistochemical labeling and Western blot analysis of the histamine-producing enzyme, HDC. In addition, we blocked HDC protein expression in MLVs by transient transfection with vivo-morpholino oligos.

RESULTS

We found that only combined pharmacological blockade of NO and histamine production completely eliminates flow-dependent relaxation of lymphatic vessels, thus confirming a role for histamine as an EDRF in MLVs. We also confirmed the presence of HDC and histamine inside lymphatic endothelial cells.

CONCLUSIONS

This study supports a role for histamine as an EDRF in MLVs.

The position- and lymphatic lumen-controlled tissue chambers to study live lymphatic vessels and surrounding tissues ex vivo

BACKGROUND

Until now, there has been no tool available to provide lymphatic researchers the ability to perform experiments in tissue explants containing lymphatic vessels under tissue position- and lymphatic lumen-controlled conditions.

METHODS AND RESULTS

In this article we provide technical details and description of the method of using the newly developed and implemented the position- and lymphatic lumen-controlled tissue chambers to study live lymphatic vessels and surrounding tissues ex vivo. In this study, we, for the first time, performed detailed comparative analysis of the contractile and pumping activity of rat mesenteric lymphatic vessels (MLVs) situated within tissue explants mounted in new tissue chambers and isolated, cannulated, and pressurized rat MLVs maintained in isolated vessel setups. We found no significant differences of the effects of both transmural pressure- and wall shear stress sensitivities of MLVs in tissue chambers and isolated MLVs.

CONCLUSIONS

We conclude that this new experimental tool, a position- and lymphatic lumen-controlled tissue chamber, allows precise investigation of lymphatic function of MLVs interacting with elements of the tissue microenvironment. This method provides an important new set of experimental tools to investigate lymphatic function.

Collecting lymphatic vessel permeability facilitates adipose tissue inflammation and distribution of antigen to lymph node-homing adipose tissue dendritic cells

Collecting lymphatic vessels (CLVs), surrounded by fat and endowed with contractile muscle and valves, transport lymph from tissues after it is absorbed into lymphatic capillaries. CLVs are not known to participate in immune responses. In this study, we observed that the inherent permeability of CLVs allowed broad distribution of lymph components within surrounding fat for uptake by adjacent macrophages and dendritic cells (DCs) that actively interacted with CLVs. Endocytosis of lymph-derived Ags by these cells supported recall T cell responses in the fat and also generated Ag-bearing DCs for emigration into adjacent lymph nodes (LNs). Enhanced recruitment of DCs to inflammation-reactive LNs significantly relied on adipose tissue DCs to maintain sufficient numbers of Ag-bearing DCs as the LN expanded. Thus, CLVs coordinate inflammation and immunity within adipose depots and foster the generation of an unexpected pool of APCs for Ag transport into the adjacent LN.

Mast cell-directed recruitment of MHC class II positive cells and eosinophils towards mesenteric lymphatic vessels in adulthood and elderly

BACKGROUND

Aging impairs mesenteric lymph flow, which is crucial for fluid and macromolecule homeostasis, fat absorption, and immune function. Previously, we demonstrated that mast cells (MCs) line mesenteric lymphatic vessels (MLVs) with a greater degree of basal activation of MCs in aged mesentery. The number of intact MCs available to react acutely to inflammatory stimuli was decreased with age. However, the role of mast cells in recruiting other immune cells towards MLVs and its aging-associated alterations has not been explored before in great detail.

METHODS AND RESULTS

In this study we treated live mesenteric tissue isolated from Sprague Dawley (SD) rats, as well as adult 9-mo and aged 24-mo Fischer-344 (F-344) rats for 2 hours with MC activators (48/80 and Substance P) and performed whole mount IHC and vital dye staining of the mesenteric segments containing MLVs to identify immune cell recruitment towards MLVs after mast cell (MC) activation. Number of major histocompatibility complex (MHC) class II positive APCs and eosinophils near MLVs was counted and compared between treatments and ages.

CONCLUSIONS

With greater density of MCs near MLVs, we for the first time demonstrated that mesenteric MC activation by compound 48/80 and Substance P resulted in recruitment of MHC class II positive cells and eosinophils towards MLVs. This effect was reduced in cromolyn-injected rats, thus confirming that MCs are necessary for such recruitment. The immune cell presence near MLVs after MC activation was reduced in aged tissues. We link these findings to our previous report of lesser number of intact MCs available for initiating an acute immune response in aged mesentery. Cumulatively, these findings serve as the first step in study of the aging-associated mechanisms that link MCs, lymphatic vessels, and disordered immune function in the elderly.

Functional adaptation of bovine mesenteric lymphatic vessels to mesenteric venous hypertension

Lymph flow is the primary mechanism for returning interstitial fluid to the blood circulation. Currently, the adaptive response of lymphatic vessels to mesenteric venous hypertension is not known. This study sought to determine the functional responses of postnodal mesenteric lymphatic vessels. We surgically occluded bovine mesenteric veins to create mesenteric venous hypertension to elevate mesenteric lymph flow. Three days after surgery, postnodal mesenteric lymphatic vessels from mesenteric venous hypertension (MVH; n = 7) and sham surgery (Sham; n = 6) group animals were evaluated and compared. Contraction frequency (MVH: 2.98 ± 0.75 min(-1); Sham: 5.42 ± 0.81 min(-1)) and fractional pump flow (MVH: 1.14 ± 0.30 min(-1); Sham: 2.39 ± 0.32 min(-1)) were significantly lower in the venous occlusion group. These results indicate that postnodal mesenteric lymphatic vessels adapt to mesenteric venous hypertension by reducing intrinsic contractile activity.

Aged lymphatic contractility: recent answers and new questions

Abstract An overview is presented of recent findings related to biology of aging of the lymph transport system. The authors discuss recently obtained data on the aging-associated alterations of lymphatic contractility in thoracic duct and mesenteric lymphatic vessels; on comparisons of function of aged mesenteric lymphatic vessels in situ versus isolated specimens and important conclusions which arose from these studies; on aging-associated changes in functional status of mast cells located close to aged mesenteric lymphatic vessels; on evidence of presence of oxidative stress in aged lymphatic vessels and changes in arrangement of muscle cells in their walls. The authors conclude that future continuation of the research efforts in this area is necessary and will be able to provide not only novel fundamental knowledge on the biology of lymphatic aging, but also will create solid foundation for the subsequent developments of lymphatic-oriented therapeutic interventions in many diseases of the elderly.

Lymphatic muscle cells in rat mesenteric lymphatic vessels of various ages

BACKGROUND

Recent studies on aging-associated changes in mesenteric lymph flow in situ demonstrated predominance of the severe negative chronotropic effect of aging on the contractility of aged mesenteric lymphatic vessels (MLV). At the same time, contraction amplitude of the aged vessels was only slightly diminished by aging and can be rapidly stimulated within 5-15 minutes. However, the detailed quantitative evaluation of potential aging-associated changes in muscle cells investiture in MLV has never been performed.

METHODS AND RESULTS

In this study we, for the first time, performed detailed evaluation of muscle cells investiture in MLV in reference to the position of lymphatic valve in different zones of lymphangion within various age groups (3-mo, 9-mo and 24-mo Fischer-344 rats). Using visual and quantitative analyses of the images of MLV immunohistochemically labeled for actin, we confirmed that the zones located close upstream (pre-valve zones) and above lymphatic valves (valve zones) possess the lowest investiture of lymphatic muscle cells. Most of the high muscle cells investiture zones exist downstream to the lymphatic valve (post-valve zones). The muscle cells investiture of these zones is not affected by aging, while pre-valve and valve zones demonstrate significant aging-associated decrease in muscle cells investiture.

CONCLUSIONS

The low muscle cells investiture zones in lymphatic vessels consist of predominantly longitudinally oriented muscle cells which are positioned in pre-valve and valve zones and connect adjacent lymphangions. These cells may provide important functional impact on the biomechanics of the lymphatic valve gating and electrical coupling between lymphangions, while their aging-associated changes may delimit adaptive reserves of aged lymphatic vessels.

Maximum shortening velocity of lymphatic muscle approaches that of striated muscle

Lymphatic muscle (LM) is widely considered to be a type of vascular smooth muscle, even though LM cells uniquely express contractile proteins from both smooth muscle and cardiac muscle. We tested the hypothesis that LM exhibits an unloaded maximum shortening velocity (Vmax) intermediate between that of smooth muscle and cardiac muscle. Single lymphatic vessels were dissected from the rat mesentery, mounted in a servo-controlled wire myograph, and subjected to isotonic quick release protocols during spontaneous or agonist-evoked contractions. After maximal activation, isotonic quick releases were performed at both the peak and plateau phases of contraction. Vmax was 0.48 ± 0.04 lengths (L)/s at the peak: 2.3 times higher than that of mesenteric arteries and 11.4 times higher than mesenteric veins. In cannulated, pressurized lymphatic vessels, shortening velocity was determined from the maximal rate of constriction [rate of change in internal diameter (-dD/dt)] during spontaneous contractions at optimal preload and minimal afterload; peak -dD/dt exceeded that obtained during any of the isotonic quick release protocols (2.14 ± 0.30 L/s). Peak -dD/dt declined with pressure elevation or activation using substance P. Thus, isotonic methods yielded Vmax values for LM in the mid to high end (0.48 L/s) of those the recorded for phasic smooth muscle (0.05-0.5 L/s), whereas isobaric measurements yielded values (>2.0 L/s) that overlapped the midrange of values for cardiac muscle (0.6-3.3 L/s). Our results challenge the dogma that LM is classical vascular smooth muscle, and its unusually high Vmax is consistent with the expression of cardiac muscle contractile proteins in the lymphatic vessel wall.

Cyclic guanosine monophosphate and the dependent protein kinase regulate lymphatic contractility in rat thoracic duct

  We have previously demonstrated a principal role for nitric oxide (NO) in the endothelium/shear-dependent regulation of contractility in rat thoracic duct (TD). In this study we tested the hypothesis that cyclic guanosine monophosphate (cGMP) and the dependent protein kinase (PKG) are central to the intrinsic and extrinsic flow-dependent modulation of lymphatic contractility. Lymphatic diameters and indices of pumping in isolated, cannulated and pressurized segments of rat TD were measured. The influences of increased transmural pressure (1-5 cmH2O) and imposed flow (1-5 cm H2O transaxial pressure gradients) on lymphatic function were studied before and after: (1) inhibition of guanylate cyclase (GC) with and without a NO donor; (2) application of stable cGMP analogue; and (3) inhibition of the cGMP activation of PKG. Additionally, Western blotting and immunofluorescent tissue staining were used to analyse the PKG isoforms expressed in TD. We found that the GC inhibitor ODQ induced changes in TD contractility similar to NO synthase blockade and prevented the relaxation induced by the NO donor S-nitroso-N-acetylpenicillamine. The cGMP analogue, 8-(4-Chlorophenylthio)-guanosine 3,5-cyclic monophosphate sodium salt (8pCPTcGMP), mimicked the extrinsic flow-induced relaxation in a dose-dependent manner, whereas treatment with the cGMP/PKG inhibitor, guanosine 3,5-cyclic monophosphorothioate, 8-(4-chlorophenylthio)-, Rp-isomer, triethylammonium salt (Rp-8-Br-PETcGMPS), eliminated intrinsic flow-dependent relaxation, and largely inhibited extrinsic flow-dependent relaxation. Western blotting demonstrated that both PKG-Iα and -Iβ isoforms are found in TD, with ∼10 times greater expression of the PKG-Iα protein in TD compared with the aorta and vena cava. The PKG-Iβ isoform expressed equally in TD and vena cava, both being ∼2 times higher than that in the aorta. Immunofluorescent labelling of PKG-Iα protein in the wall of rat thoracic duct confirmed its localization inside TD muscle cells. These findings demonstrate that cGMP is critical to the flow-dependent regulation of TD contractility; they also indicate an important involvement of PKG, especially PKG-Iα in these processes and identifies PKG protein as a potential therapeutic target.

Intrinsic increase in lymphangion muscle contractility in response to elevated afterload

Collecting lymphatic vessels share functional and biochemical characteristics with cardiac muscle; thus, we hypothesized that the lymphatic vessel pump would exhibit behavior analogous to homeometric regulation of the cardiac pump in its adaptation to elevated afterload, i.e., an increase in contractility. Single lymphangions containing two valves were isolated from the rat mesenteric microcirculation, cannulated, and pressurized for in vitro study. Pressures at either end of the lymphangion [input pressure (P(in)), preload; output pressure (P(out)), afterload] were set by a servo controller. Intralymphangion pressure (P(L)) was measured using a servo-null micropipette while internal diameter and valve positions were monitored using video methods. The responses to step- and ramp-wise increases in P(out) (at low, constant P(in)) were determined. P(L )and diameter data recorded during single contraction cycles were used to generate pressure-volume (P-V) relationships for the subsequent analysis of lymphangion pump behavior. Ramp-wise P(out) elevation led to progressive vessel constriction, a rise in end-systolic diameter, and an increase in contraction frequency. Step-wise P(out) elevation produced initial vessel distention followed by time-dependent declines in end-systolic and end-diastolic diameters. Significantly, a 30% leftward shift in the end-systolic P-V relationship accompanied an 84% increase in dP/dt after a step increase in P(out), consistent with an increase in contractility. Calculations of stroke work from the P-V loop area revealed that robust pumps produced net positive work to expel fluid throughout the entire afterload range, whereas weaker pumps exhibited progressively more negative work as gradual afterload elevation led to pump failure. We conclude that lymphatic muscle adapts to output pressure elevation with an intrinsic increase in contractility and that this compensatory mechanism facilitates the maintenance of lymph pump output in the face of edemagenic and/or gravitational loads.

Independent and interactive effects of preload and afterload on the pump function of the isolated lymphangion

We tested the responses of single, isolated lymphangions to selective changes in preload and the effects of changing preload on the response to an imposed afterload. The methods used were similar to those described in our companion paper. Step-wise increases in input pressure (P(in); preload) over a pressure range between 0.5 and 3 cmH(2)O, at constant output pressure (P(out)), led to increases in end-diastolic diameter, decreases in end-systolic diameter, and increases in stroke volume. From a baseline of 1 cmH(2)O, P(in) elevation by 2-7 cmH(2)O consistently produced an immediate fall in stroke volume that subsequently recovered over a time course of 2-3 min. Surprisingly, this adaptation was associated with an increase in the slope of the end-systolic pressure-volume relationship, indicative of an increase in contractility. Lymphangions subjected to P(out) levels exceeding their initial ejection limit would often accommodate by increasing diastolic filling to strengthen contraction sufficiently to match P(out). The lymphangion adaptation to various pressure combinations (P(in) ramps with low or high levels of P(out), P(out) ramps at low or intermediate levels of P(in), and combined P(in) + P(out) ramps) were analyzed using pressure-volume data to calculate stroke work. Under relatively low imposed loads, stroke work was maximal at low preloads (P(in) ∼2 cmH(2)O), whereas at more elevated afterloads, the optimal preload for maximal work displayed a broad plateau over a P(in) range of 5-11 cmH(2)O. These results provide new insights into the normal operation of the lymphatic pump, its comparison with the cardiac pump, and its potential capacity to adapt to increased loads during edemagenic and/or gravitational stress.

Aging-associated shifts in functional status of mast cells located by adult and aged mesenteric lymphatic vessels

We had previously proposed the presence of permanent stimulatory influences in the tissue microenvironment surrounding the aged mesenteric lymphatic vessels (MLV), which influence aged lymphatic function. In this study, we performed immunohistochemical labeling of proteins known to be present in mast cells (mast cell tryptase, c-kit, prostaglandin D(2) synthase, histidine decarboxylase, histamine, transmembrane protein 16A, and TNF-α) with double verification of mast cells in the same segment of rat mesentery containing MLV by labeling with Alexa Fluor 488-conjugated avidin followed by toluidine blue staining. Additionally, we evaluated the aging-associated changes in the number of mast cells located by MLV and in their functional status by inducing mast cell activation by various activators (substance P; anti-rat DNP Immunoglobulin E; peptidoglycan from Staphyloccus aureus and compound 48/80) in the presence of ruthenium red followed by subsequent staining by toluidine blue. We found that there was a 27% aging-associated increase in the total number of mast cells, with an ∼400% increase in the number of activated mast cells in aged mesenteric tissue in resting conditions with diminished ability of mast cells to be newly activated in the presence of inflammatory or chemical stimuli. We conclude that higher degree of preactivation of mast cells in aged mesenteric tissue is important for development of aging-associated impairment of function of mesenteric lymphatic vessels. The limited number of intact aged mast cells located close to the mesenteric lymphatic compartments to react to the presence of acute stimuli may be considered contributory to the aging-associated deteriorations in immune response.

Evidence of increased oxidative stress in aged mesenteric lymphatic vessels

BACKGROUND

We have previously shown that aging is associated with weakened rat mesenteric lymphatic vessel (MLV) contractility. However, the specific mechanisms contributing to this aging-associated contractile degeneration remain unknown. Aging is often associated with elevations in oxidative stress, and reactive oxygen species (ROS) have been shown to reduce the contractility of MLV. Thus in the present study, we sought to assess whether aging is associated with increased levels of oxidative stress and oxidative damage in MLV.

METHODS AND RESULTS

MLV were isolated from 9-mo- and 24-mo-old Fischer-344 rats and subjected to the following experimental techniques: measurement of total superoxide dismutase (SOD) activity; estimation of lipid peroxidation levels via measurement of thiobarbituric acid reactive substances (TBARS); detection of superoxide and mitochondrial ROS in live MLV; Western blot analysis, and immunohistochemical labeling of the SOD isoforms and nitro-tyrosine proteins. We found that aging is associated with increased levels of cellular superoxide and mitochondrial ROS concomitant with a reduction in Cu/Zn-SOD protein expression and total SOD enzymatic activity in MLV. This increase in oxidative stress and decrease in antioxidant activity was associated with evidence of increased lipid (as indicated by TBARS) and protein (as indicated by nitro-tyrosine labeling) oxidative damage.

CONCLUSIONS

Thus for the first time, we demonstrate that aging-associated increases in oxidative stress and oxidative damage is indeed present in the walls of MLV and may contribute to the aging-associated lymphatic pump dysfunction we previously reported.

Regional heterogeneity of length-tension relationships in rat lymph vessels

BACKGROUND

Heterogeneity of the length-tension relationships in lymph vessels has never been evaluated systematically.

METHODS AND RESULTS

In this study we measured the length-tension relationships in lymph vessels from three different regions of the rat: thoracic duct, cervical, and femoral lymph vessels, and compared the results to our previous measurements of rat mesenteric lymph vessels. We performed isometric force measurements on activated and passive lymph vessel segments using a small-vessel wire myograph. We found that all groups of vessels had relatively broad plateaus in their active tension versus length relationships, suggesting that they are adapted to generate near-maximal tensions over a relatively wide range of preloads (at least 0.85-1.05 L(0)). Thoracic duct exhibited the flattest active tension curve, particularly for peak active tension, in which there was less than a 5% change in peak active tension from 0.75 to 1.30 of optimal length. Femoral lymph vessels were able to withstand the highest estimated pressures, followed by mesenteric and cervical vessels and then thoracic duct.

CONCLUSIONS

We conclude that lymph vessels effectively adapt their contractile force to the particular hydrodynamic conditions (transmural pressures and intraluminal flows) that exist in different regions of the lymphatic system.

Aging-associated alterations in contractility of rat mesenteric lymphatic vessels

OBJECTIVE

  To evaluate the age-related changes in pumping of mesenteric lymphatic vessels in 9- and 24-month-old male Fisher-344 rats.

METHODS

  Lymphatic diameters, contraction amplitude, contraction frequency, and fractional pump flow were determined in isolated MLV before and after l-NAME application.

RESULTS

  The data demonstrate a severe weakening of the lymphatic pump in aged MLV including diminished lymphatic contraction amplitude, contraction frequency, and as a result, lymphatic pump activity. The data also suggest that the imposed flow gradient-generated shear-dependent relaxation does not exist in aged rat MLV, and the sensitivity of both adult and aged MLV to such shear cannot be eliminated by nitric oxide (NO) synthases blockade.

CONCLUSIONS

  These data provide new evidence of lymphatic regional heterogeneity for both adult and aged MLV. In MLV, a constant interplay between the tonic and phasic components of the myogenic response and the shear-dependent release of NO predominantly determine the level of contractile activity; the existence of another shear-dependent, but NO-independent regulatory mechanism is probably present. Aging remarkably weakens MLV contractility, which would predispose this lymphatic network to lower total lymph flow in resting conditions and limit the ability to respond to an edemagenic challenge in the elderly.

Long-distance transportation of live isolated lymphatic vessels

BACKGROUND

We performed experiments to test whether isolated lymphatic vessels would remain viable after overnight long-distance shipment.

METHODS AND RESULTS

Freshly isolated segments of rat mesenteric lymphatic vessels were placed into tubes filled by chilled D-MEM/F12 supplemented with antibiotics and shipped via overnight by express mail from College Station, TX to Columbia, MO. Standard physiological and pharmacological tests were performed to investigate the influence of the long-distance transport procedure on the spontaneous contractility of isolated, cannulated, and pressurized rat mesenteric lymphatic vessels. The results demonstrate that normal contractile function of isolated lymphatic vessels can be preserved with long-distance shipping and subsequent overnight recovery if the proper precautions are taken.

CONCLUSIONS

The method of lymphatic vessel transportation described in this report opens up the opportunity to perform in vitro functional tests on lymphatic tissues harvested and initially processed in a remote location. In addition, the described procedures may expand the options for potential sources of fresh human tissue, harvested during surgery or autopsy and allowed to be available to lymphatic researchers in remote locations.

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.

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.

Measuring contraction propagation and localizing pacemaker cells using high speed video microscopy

Previous studies have shown the ability of many lymphatic vessels to contract phasically to pump lymph. Every lymphangion can act like a heart with pacemaker sites that initiate the phasic contractions. The contractile wave propagates along the vessel to synchronize the contraction. However, determining the location of the pacemaker sites within these vessels has proven to be very difficult. A high speed video microscopy system with an automated algorithm to detect pacemaker location and calculate the propagation velocity, speed, duration, and frequency of the contractions is presented in this paper. Previous methods for determining the contractile wave propagation velocity manually were time consuming and subject to errors and potential bias. The presented algorithm is semiautomated giving objective results based on predefined criteria with the option of user intervention. The system was first tested on simulation images and then on images acquired from isolated microlymphatic mesenteric vessels. We recorded contraction propagation velocities around 10 mm/s with a shortening speed of 20.4 to 27.1 μm/s on average and a contraction frequency of 7.4 to 21.6 contractions/min. The simulation results showed that the algorithm has no systematic error when compared to manual tracking. The system was used to determine the pacemaker location with a precision of 28 μm when using a frame rate of 300 frames per second.

Methods for lymphatic vessel culture and gene transfection

OBJECTIVE

To develop the techniques needed for the specific gene/protein targeting transfection experiments in isolated lymphatic vessels, we completed two major tasks: 1) optimize the experimental conditions to maintain the viability of isolated rat lymphatic vessels in culture for sufficiently long periods of time to permit knockdown or overexpression of selected proteins/genes and 2) develop effective transfection protocols for lymphatic muscle and endothelial cells in intact lymphatic vessels without nonspecific impairment of lymphatic contractile function due to the transfection protocol itself.

METHODS

Experimental protocols were developed for the maintenance of isolated lymphatic vessels under nonpressurized and pressurized conditions for 3-12 days in culture and for adenoviral gene transfection of the lymphatic muscle and endothelial cells.

RESULTS

The data demonstrate the effectiveness of the newly developed experimental protocols for the maintenance of isolated rat mesenteric lymphatic vessels and thoracic duct in culture up to 3-12 days without significant impairment of the parameters of their pumping and effective adenoviral/GFP transfection of lymphatic endothelial and muscle cells in isolated rat mesenteric lymphatic vessels.

CONCLUSIONS

These experimental techniques will extend the set of the modern experimental tools available to researchers investigating the physiology of lymphatic function.

Indocyanine green and lymphatic imaging: current problems

BACKGROUND

Near-infrared (NIR) fluorescence imaging using indocyanine green (ICG) has recently been presented as a comparatively easy and informative technique to image lymphatic channels in vivo. However, no data or references have been provided concerning the impact of ICG application on normal lymphatic contractility and lymph transport. Thus, the imaging agent and/or the method of administration may introduce a significant artifact.

METHODS AND RESULTS

Standard pharmacological tests were performed to investigate the influence of ICG on the spontaneous contractility of isolated, cannulated, and pressurized rat mesenteric lymphatic vessels. The data demonstrate that non-irradiated ICG dramatically and dynamically influences the contractility of rat lymphatic vessels in both a dose- and diluent-dependent manner with low ICG concentrations principally altering contractile frequency and higher ICG concentrations completely blocking lymphatic contractility.

CONCLUSIONS

Currently, both researchers and doctors should exercise caution in extrapolating the data obtained with ICG imaging to normal lymphatic function regardless of whether it was obtained in mice, pigs, or humans. Careful and extended pharmacological tests must be performed to evaluate the mechanism of action of ICG on the contractility and physiology of lymphatic vessels with consideration of dose, diluent, and duration of irradiation.

Basic mechanisms controlling lymph transport in the mesenteric lymphatic net

This minireview summarizes an oral presentation given at the National Institute of Diabetes and Digestive and Kidney Diseases National Institutes of Health workshop "Lymphatics in the Digestive System: Physiology, Health, and Disease" in Bethesda, Maryland on November 3-4, 2009. The concepts of extrinsic and intrinsic pumps, as well as intrinsic and extrinsic flows, are discussed in relation to the lymph transport in mesenteric lymphatic vessels. Age-related alterations in the structure and regulatory mechanisms of lymph flow in mesenteric lymphatic vessels may provide the basis for their diminished ability to work during the periods of increased functional loads in them. The recent development of modern experimental tools provides the opportunity to extend the knowledge on lymph transport function of lymphatic vessels that is absolutely necessary to maintain fluid and macromolecular homeostasis and to provide a transportation route for lipids adsorbed in gut and to immune cells.

Hydrodynamic regulation of lymphatic transport and the impact of aging

To accomplish its normal roles in body fluid regulation/macromolecular homeostasis, immune function, and lipid absorption; the lymphatic system must transport lymph from the interstitial spaces, into and through the lymphatics, through the lymphatic compartment of the nodes, back into the nodal efferent lymphatics and eventually empty into the great veins. The usual net pressure gradients along this path do not normally favor the passive movement of lymph. Thus, lymph transport requires the input of energy to the lymph to propel it along this path. To do this, the lymphatic system uses a series of pumps to generate lymph flow. Thus to regulate lymph transport, both lymphatic pumping and resistance must be controlled. This review focuses on the regulation of the intrinsic lymph pump by hydrodynamic factors and how these regulatory processes are altered with age. Intrinsic lymph pumping is generated via the rapid/phasic contractions of lymphatic muscle, which are modulated by local physical factors (pressure/stretch and flow/shear). Increased lymph pressure/stretch will generally activate the intrinsic lymph pump up to a point, beyond which the lymph pump will begin to fail. The effect of increased lymph flow/shear is somewhat more complex, in that it can either activate or inhibit the intrinsic lymph pump, depending on the pattern and magnitude of the flow. The pattern and strength of the hydrodynamic regulation of the lymph transport is different in various parts of the lymphatic tree under normal conditions, depending upon the local hydrodynamic conditions. In addition, various pathophysiological processes can affect lymph transport. We have begun to evaluate the influence of the aging process on lymphatic transport characteristics in the rat thoracic duct. The pressure/stretch-dependent activation of intrinsic pumping is significantly impaired in aged rat thoracic duct (TD) and the flow/shear-dependent regulatory mechanisms are essentially completely lacking. The loss of shear-dependent modulation of lymphatic transport appears to be related to a loss of normal eNOS expression and a large rise in iNOS expression in these vessels. Therefore, aging of the lymph transport system significantly impairs its ability to transport lymph. We believe this will alter normal fluid balance as well as negatively impact immune function in the aged animals. Further studies are needed to detail the mechanisms that control and alter lymphatic transport during normal and aged conditions.

Inhibition of myosin light chain phosphorylation decreases rat mesenteric lymphatic contractile activity

Muscular lymphatics use both phasic and tonic contractions to transport lymph for conducting their vital functions. The molecular mechanisms regulating lymphatic muscle contractions are not well understood. Based on the well-established finding that the phosphorylation of myosin light chain 20 (MLC(20)) plays an essential role in blood vessel smooth muscle contraction, we investigated if phosphorylated MLC(20) (pMLC(20)) would modulate the tonic and/or phasic contractions of lymphatic muscle. The effects of ML-7, a MLC kinase inhibitor (1-10 microM), were tested on the contractile parameters of isolated and cannulated rat mesenteric lymphatics during their responses to the known modulators, pressure (1-5 cm H(2)O) and substance P (SP; 10(-7) M). Immunohistochemical and Western blot analyses of pMLC(20) were also performed on isolated lymphatics. The results showed that 1) increasing pressure decreased both the lymphatic tonic contraction strength and pMLC(20)-to-MLC(20) ratio; 2) SP increased both the tonic contraction strength and phosphorylation of MLC(20); 3) ML-7 decreased both the lymphatic tonic contraction strength and pMLC(20)-to-MLC(20) ratio; and 4) the increase in lymphatic phasic contraction frequency in response to increasing pressure was diminished by ML-7; however, the phasic contraction amplitude was not significantly altered by ML-7 either in the absence or presence of SP. These data provide the first evidence that tonic contraction strength and phasic contraction amplitude of the lymphatics can be differentially regulated, whereby the increase in MLC(20) phosphorylation produces an activation in the tonic contraction without significant changes in the phasic contraction amplitude. Thus, tonic contraction of rat mesenteric lymphatics appears to be MLC kinase dependent.