Inhibition of the active lymph pump by flow in rat mesenteric lymphatics and thoracic duct

Calcitonin gene-related peptide activates different signaling pathways in mesenteric lymphatics of guinea pigs

The effects of calcitonin gene-related peptide (CGRP) on constriction frequency, smooth muscle membrane potential (V(m)), and endothelial V(m) of guinea pig mesenteric lymphatics were examined in vitro. CGRP (1-100 nM) caused an endothelium-dependent decrease in the constriction frequency of perfused lymphatic vessels. The endothelium-dependent CGRP response was abolished by the CGRP-1 receptor antagonist CGRP-(8-37) (1 microM) and pertussis toxin (100 ng/ml). This action of CGRP was also blocked by the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (L-NNA; 10 microM), an action that was reversed by the addition of L-arginine (100 microM). cGMP, adenylate cyclase, cAMP-dependent protein kinase (PKA), and ATP-sensitive K+ (K+(ATP)) channels were all implicated in the endothelium-dependent CGRP response because it was abolished by methylene blue (20 microM), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM), dideoxyadenosine (10 microM), N-[2-(p-bromociannamylamino)-ethyl]-5-isoquinolinesulfonamide-dichloride (H89; 1 microM) and glibenclamide (10 microM). CGRP (100 nM), unlike acetylcholine, did not alter endothelial intracellular Ca2+ concentration or V(m). CGRP (100 nM) hyperpolarized the smooth muscle V(m), an effect inhibited by L-NNA, H89, or glibenclamide. CGRP (500 nM) also caused a decrease in constriction frequency. However, this was no longer blocked by CGRP-(8-37). CGRP (500 nM) also caused smooth muscle hyperpolarization, an action that was now not blocked by L-NNA (100 microM). It was most likely mediated by the activation of the cAMP/PKA pathway and the opening of K+(ATP) channels because it was abolished by H89 or glibenclamide. We conclude that CGRP, at low to moderate concentrations (i.e., 1-100 nM), decreases lymphatic constriction frequency primarily by the stimulation of CGRP-1 receptors coupled to pertussis toxin-sensitive G proteins and the release of NO from the endothelium or enhancement of the actions of endogenous NO. At high concentrations (i.e., 500 nM), CGRP also directly activates the smooth muscle independent of NO. Both mechanisms of activation ultimately cause the PKA-mediated opening of K+(ATP) channels and resultant hyperpolarization.

Molecular Regulation of Microlymphatic Formation and Function: Role of Nitric Oxide

Inhibition of lymphangiogenesis prevents lymphatic cancer metastasis, whereas induction of lymphangiogenesis alleviates lymphedema in experimental animal models. The number of known molecular players that govern the formation and function of the microlymphatic system is growing. Here, we review the role of nitric oxide within the regulation of lymphatic formation and function and point out key unanswered questions for its translation into clinical therapy.

Lymphatic muscle: a review of contractile function

A recent surge in lymphangiogenesis research has led to a greater understanding of lymphatic endothelial cell biology. However, a general understanding of lymphatic muscle cell biology lags far behind its endothelial counterpart. Lymphatics at the level of the collecting vessels and higher contain muscular walls capable of both tonic and phasic contractions, which both generate and regulate lymph flow. Because lymphatic contraction is crucial to lymphatic function, a solid understanding of lymphatic muscle development and function is necessary to understand lymphatic biology. This review summarizes the current body of lymphatic muscle research and addresses important questions that are currently unanswered.

Development and characterization of endothelial cells from rat microlymphatics

BACKGROUND

The lymphatic endothelium is important to the functioning of the lymphatic system, including lymphatic remodeling, control of vessel tone, and lymphatic movement of fluids, macromolecules, and cells. Many of these events occur principally at the level of the microlymphatics. To evaluate the role of the microlymphatic endothelium, a suitable cultured cell line would be useful. We have developed a technique to isolate and culture endothelial cells from microscopic lymphatics, approximately 100 microm in diameter.

METHODS AND RESULTS

To isolate the rat mesenteric lymphatic endothelial cells (RMLEC), the rat was anesthetized and the mesentery carefully exteriorized. A suitable microlymphatic was located and carefully microdissected from the surrounding mesentery. The vessel was carefully cleaned, cannulated, everted, and then incubated on a gelatin-coated plastic culture dish until small patches of cells migrated off of the vessel (3-4 days later.) The explanted vessel was then removed. The remaining cells were cultured and screened for endothelial phenotype. Nonendothelial cells were destroyed. The endothelial nature of the remaining cells was verified by: 1) morphology, 2) uptake of fluorescent acetylated-LDL, 3) staining for von Wille-brand factor, PECAM-1, ecNOS, LYVE-1, VEGFR-3, and 4) essentially negative alpha-vascular smooth muscle actin staining. The defined RMLEC were passed and the profile of adhesion molecules present on the RMLEC was then determined using PCR and immunofluorescence.

CONCLUSIONS

We developed and partially characterized a line of cultured microlymphatic endothelium. RMLEC express known endothelial- and lymphatic-specific markers as well as the following adhesion molecules: N-cadherin, E-cadherin, PECAM-1, alpha-catenin, beta-catenin, gamma-catenin, p120, and a variety of integrins.

Current topics of physiology and pharmacology in the lymphatic system

We have reviewed physiological significance of rhythmical spontaneous contractions of collecting lymph vessels, which play a pivotal role in lymph transport and seem to control lymph formation through changing the pacemaker sites of the rhythmic contractions and contractile patterns of the lymphangions. A characteristic feature that the rhythmic pump activity works in vivo physiologically under the specific environment of lower oxygen tension in lymph (25-40 mm Hg) has been evaluated. With the characteristic feature, generation of endogenous nitric oxide (NO) from lymphatic endothelial cells and/or activation of ATP-sensitive potassium channels (K(ATP)) are reviewed to play crucial roles in the regulation of lymph transport at physiological or pathophysiological conditions. Chemical substances released from malignant tumor cells and tumor-derived parathyroid hormone-related peptide (PTHr-P) are also shown to cause a significant reduction of lymphatic pump activity through generation of endogenous NO and activation of K(ATP) channels. Finally, we have discussed physiological significance and roles of the lower oxygen tension in lymph, generation of endogenous NO, and activation of K(ATP) in lymph formation, lymph transport, and the functions of lymph nodes.

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.

Endothelial nitric oxide synthase regulates microlymphatic flow via collecting lymphatics

Functional interactions between the initial and collecting lymphatics, as well as the molecular players involved, remain elusive. In this study, we assessed the influence of nitric oxide (NO) on lymphatic fluid velocity and flow, using a mouse tail model that permits intravital microscopy and microlymphangiography. We found that NO synthase (NOS) inhibition decreased lymphatic fluid velocity in the initial lymphatics, without any effect on their morphology. Using the same model, we found a similar effect in eNOS-/- mice and in mice treated with a selective endothelial NOS (eNOS) inhibitor. Next, we uncoupled the superficial initial lymphatics from the deeper collecting lymphatics by ligating the latter and found that lymphatic fluid velocity in NOS-inhibited mice became equal to that in control animals. Surprisingly, lymphatic fluid velocity was significantly increased after ligating the collecting lymphatics, and there was a concomitant increase in injection rate and mean lymphatic vessel diameter. Our results provide the first in vivo evidence that eNOS affects function of the whole microlymphatic system and that it is regulated via the collecting lymphatics.

Stretch-induced calcium sensitization of rat lymphatic smooth muscle

The relationships between smooth muscle calcium and isometric tension generation to spontaneous lymphatic pump activity and its modulation by stretch equivalent from 0 to approximately 6 cmH2O were investigated. Excised preparations of the rat thoracic duct were mounted on a wire myograph and loaded with the calcium-sensitive fluorochrome indo-1. Calcium-dependent fluorescence and isometric force were simultaneously recorded. The thoracic duct segments developed spontaneous rhythmic contractile activity. Each contraction was preceded by an increase in intracellular calcium. When the vessels were normalized and stabilized at a preload equal to 3 cmH2O, the peak generation in tension occurred 0.70 +/- 0.11 s after that of calcium. Incremental stretch enhanced the frequency of the phasic activity and amplitude of isometric force generation but not the basal calcium level or the amplitude of the calcium transient. These findings suggest that stretch enhances lymphatic pump activity by increasing the pacemaker activity and the calcium sensitivity of the contractile apparatus.

Rho-Rho kinase pathway is involved in the regulation of myogenic tone and pump activity in isolated lymph vessels

To evaluate whether or not Rho-Rho kinase pathway is involved in the regulation of mechanical activity of lymph vessels, effects of Y-27632 and okadaic acid on lymph pump activity and myogenic, pressure- and agonist-induced tone were examined in isolated rat lymph vessels. Y-27632 caused a significant dilation with a cessation of the lymph pump activity. Y-27632 also produced a dose-related dilation of the lymph vessels precontracted by norepinephrine (NE)-, U-46619- or 80 mM KCl. Okadaic acid significantly constricted the lymph vessels and reduced the frequency of the lymph pump activity. Okadaic acid also produced a dose-related constriction of the lymph vessels precontracted by NE or U-46619. The Y-27632-induced decrease of the frequency of lymph pump activity was significantly reversed by the pretreatment with okadaic acid. In the presence of Y-27632, the pressure-mediated tone of the lymph vessel was significantly decreased. On the other hand, okadaic acid significantly increased the pressure-mediated tone. These findings suggest that Rho kinase and myosin phosphatase activity in lymphatic smooth muscles may contribute to the regulation of lymph pump activity and may be also involved in the control of myogenic pressure- and agonist-induced tone.

Physiologic aspects of lymphatic contractile function: current perspectives

The lymphatic system plays an important role in fluid/macromolecular balance, lipid absorption, and immune functions, and is involved in many different pathologic conditions, like inflammation, spread of cancer cells, and lymphedema. There are several forces that drive lymph centripetally. Extrinsic driving forces, or the passive lymph pump, include lymph formation, arterial pulsations, skeletal muscles contractions, fluctuations of central venous pressure, gastrointestinal peristalsis, and respiration. Intrinsic forces, or the active lymph pump, are the result of coordinated contractions of lymphangions, the morpho-functional units of the lymphatic vessels, which include the valve and portion of the vessel extending to the next valve. The contractions of the lymphangions are initiated by the pacemaker activity of the smooth muscle cells of lymphangion wall. Transmural pressure is an important hydrodynamic factor that modulates pacemaking. Under conditions of low filling, lymphangions might produce negative intraluminal pressures and a suction effect. Because of the complicated hydrodynamic conditions in lymphatic beds, the passive and active lymph pumps sometimes work together to propel lymph centripetally. In other cases (i.e., under conditions of enhanced lymph flow), flow-mediated inhibition of the active lymph pump could serve to decrease lymphatic outflow resistance and save metabolic energy when the driving force of the passive lymph pump is enough to propel lymph. We have recently found that there are profound differences in the pressure and flow sensitivities of lymphatic vessels derived from different tissues, such as the thoracic duct and mesenteric lymphatics. Such results, when considered in light of the controversy surrounding some studies performed in different animals, lead to the idea that the active lymph pumps in humans may have greater regional differences in contractile function than has been seen in animals, because of the upright posture in bipedal humans. This posture creates an additional outflow resistance for lymphatics of the lower part of the body. Thus, despite the ongoing attempts to determine the mechanisms of lymphatic diseases and useful therapies to treat them, there are many disputable or unknown issues regarding the physiology of lymph transport in humans.