A High Salt Diet Alters Pressure-Induced Mechanical Activity of the Rat Lymphatics with Enhancement of Myogenic Characteristics

Sodium, Interstitium, Lymphatics and Hypertension-A Tale of Hydraulics

Blood pressure is regulated by vascular resistance and intravascular volume. However, exchanges of electrolytes and water between intra and extracellular spaces and filtration of fluid and solutes in the capillary beds blur the separation between intravascular, interstitial and intracellular compartments. Contemporary paradigms of microvascular exchange posit filtration of fluids and solutes along the whole capillary bed and a prominent role of lymphatic vessels, rather than its venous end, for their reabsorption. In the last decade, these concepts have stimulated greater interest in and better understanding of the lymphatic system as one of the master regulators of interstitial volume homeostasis. Here, we describe the anatomy and function of the lymphatic system and focus on its plasticity in relation to the accumulation of interstitial sodium in hypertension. The pathophysiological relevance of the lymphatic system is exemplified in the kidneys, which are crucially involved in the control of blood pressure, but also hypertension-mediated cardiac damage. Preclinical modulation of the lymphatic reserve for tissue drainage has demonstrated promise, but has also generated conflicting results. A better understanding of the hydraulic element of hypertension and the role of lymphatics in maintaining fluid balance can open new approaches to prevent and treat hypertension and its consequences, such as heart failure.

Lymphatics in Cardiovascular Physiology

The lymphatic vessels play an essential role in maintaining immune and fluid homeostasis and in the transport of dietary lipids. The discovery of lymphatic endothelial cell-specific markers facilitated the visualization and mechanistic analysis of lymphatic vessels over the past two decades. As a result, lymphatic vessels have emerged as a crucial player in the pathogenesis of several cardiovascular diseases, as demonstrated by worsened disease progression caused by perturbations to lymphatic function. In this review, we discuss the major findings on the role of lymphatic vessels in cardiovascular diseases such as hypertension, obesity, atherosclerosis, myocardial infarction, and heart failure.

Lymphatics - not just a chylomicron conduit

PURPOSE OF REVIEW

Lymphatics are known to have active, regulated pumping by smooth muscle cells that enhance lymph flow, but whether active regulation of lymphatic pumping contributes significantly to the rate of appearance of chylomicrons (CMs) in the blood circulation (i.e., CM production rate) is not currently known. In this review, we highlight some of the potential mechanisms by which lymphatics may regulate CM production.

RECENT FINDINGS

Recent data from our lab and others are beginning to provide clues that suggest a more active role of lymphatics in regulating CM appearance in the circulation through various mechanisms. Potential contributors include apolipoproteins, glucose, glucagon-like peptide-2, and vascular endothelial growth factor-C, but there are likely to be many more.

SUMMARY

The digested products of dietary fats absorbed by the small intestine are re-esterified and packaged by enterocytes into large, triglyceride-rich CM particles or stored temporarily in intracellular cytoplasmic lipid droplets. Secreted CMs traverse the lamina propria and are transported via lymphatics and then the blood circulation to liver and extrahepatic tissues, where they are stored or metabolized as a rich energy source. Although indirect data suggest a relationship between lymphatic pumping and CM production, this concept requires more experimental evidence before we can be sure that lymphatic pumping contributes significantly to the rate of CM appearance in the blood circulation.

Salt-sensitive blood pressure rise in type 1 diabetes patients is accompanied by disturbed skin macrophage influx and lymphatic dilation-a proof-of-concept study

Type 1 diabetes patients are more prone to have hypertension than healthy individuals, possibly mediated by increased blood pressure (BP) sensitivity to high salt intake. The classical concept proposes that the kidney is central in salt-mediated BP rises, by insufficient renal sodium excretion leading to extracellular fluid volume expansion. Recent animal-derived findings, however, propose a causal role for disturbance of macrophage-mediated lymphangiogenesis. Its relevance for humans, specifically type 1 diabetes patients, is unknown. The present study aimed to assess responses of type 1 diabetes patients to a dietary salt load with regard to BP, extracellular fluid volume (using precise iohexol measurements), and CD163+ macrophage and lymphatic capillary density in skin biopsies. Also, macrophage expression of HLA-DR (a proinflammatory marker) and CD206 (an anti-inflammatory marker) was assessed. Type 1 diabetes patients (n = 8) showed a salt-sensitive BP increase without extracellular fluid volume expansion. Whereas healthy controls (n = 12), who had no BP increase, showed increased skin CD163+ and HLA-DR+ macrophages and dilation of lymphatic skin vasculature after the dietary salt load, these changes were absent (and in case of HLA-DR more heterogenic) in type 1 diabetes patients. In conclusion, we show that salt sensitivity in type 1 diabetes patients cannot be explained by the classical concept of extracellular fluid volume expansion. Rather, we open up a potential role for macrophages and the lymphatic system. Future studies on hypertension and diabetes need to scrutinize these phenomena.

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.

Fluid Osmolarity Acutely and Differentially Modulates Lymphatic Vessels Intrinsic Contractions and Lymph Flow

Lymph formation and propulsion rely on an extrinsic mechanism based on the forces that surrounding tissues exert upon the vessel wall and lumen and an intrinsic mechanism based on spontaneous, rhythmic contractions of the lymphatic muscle layer of collecting vessels. The two spontaneous pacemakers described in literature involve chloride-dependent depolarizations (STDs) and I_f-like currents, both giving rise to a variable contraction frequency (f_c) of lymphatic vessels functional units (lymphangions). Several stimuli have been shown to modulate f_c, such as temperature, shear stress, and several tissue chemical modulators (prostaglandins, norepinephrine, acetylcholine, substance P, and others). However, no detailed description is present in literature on the acute modulation of f_c by means of osmolarity change of the surrounding interstitial space. Using a well-developed ex-vivo rat diaphragmatic preparation, in which osmolarity was changed by varying the concentration of D-mannitol in the perfusing solution and in later experiments the concentration of NaCl and then of Na⁺ and Cl⁻ ions separately by ionic substitution, we provide detailed experimental evidences that a stepwise increase in osmolarity from control value (308 mOsm) up to 324 mOsm caused a reduction of f_c down to ~-70% within the first 14 min, and that a stepwise decrease in osmolarity up to 290 mOsm induced an early f_c increase to ~+34% of control, followed by a decline to an f_c of ~-18% of control value. These variations were more dramatic when the same osmolarity changes were obtained by varying NaCl and/or Na⁺ or Cl⁻ ions concentration, which caused an almost complete arrest of spontaneous contractility within 14 min from the application. Diastolic and systolic diameters and stroke volume were not affected by osmolarity changes, so that modulation of lymph flow closely followed that of f_c. Modulation of lymph flow secondary to osmolarity changes is relevant if one considers that interstitial fluid balance is also dependent upon lymph drainage, and thus it is possible that, at least in the acute phase following variations of interstitial fluid osmolarity, its volume control might eventually be impaired due to the reduced or in the worst scenario null lymph drainage.

The interstitium conducts extrarenal storage of sodium and represents a third compartment essential for extracellular volume and blood pressure homeostasis

The role of salt in the pathogenesis of arterial hypertension is not well understood. According to the current understanding, the central mechanism for blood pressure (BP) regulation relies on classical studies linking BP and Na⁺ balance, placing the kidney at the very centre of long-term BP regulation. To maintain BP homeostasis, the effective circulating fluid volume and thereby body Na⁺ content has to be maintained within very narrow limits. From recent work in humans and rats, the notion has emerged that Na⁺ could be stored somewhere in the body without commensurate water retention to buffer free extracellular Na⁺ and that previously unidentified extrarenal, tissue-specific regulatory mechanisms are operative regulating the release and storage of Na⁺ from a kidney-independent reservoir. Moreover, immune cells from the mononuclear phagocyte system not only function as local on-site sensors of interstitial electrolyte concentration, but also, together with lymphatics, act as systemic regulators of body fluid volume and BP. These studies have established new and unexpected targets in studies of BP control and thus the pathophysiology of hypertension: the interstitium/extracellular matrix of the skin, its inherent interstitial fluid and the lymphatic vasculature forming a vessel network in the interstitium. Aspects of the interstitium in relation to Na⁺ balance and hypertension are the focus of this review. Taken together, observations of salt storage in the skin to buffer free extracellular Na⁺ and macrophage modulation of the extracellular matrix and lymphatics suggest that electrolyte homeostasis in the body cannot be achieved by renal excretion alone, but also relies on extrarenal regulatory mechanisms.

Cutaneous control of blood pressure

PURPOSE OF REVIEW

Textbook theory holds that blood pressure (BP) is regulated by the brain, by blood vessels, or by the kidney. Recent evidence suggests that BP could be regulated in the skin.

RECENT FINDINGS

The skin holds a complex capillary counter current system, which controls body temperature, skin perfusion, and apparently systemic BP. Epidemiological data suggest that sunlight exposure plays a role in controlling BP. Ultraviolet A radiation produces vasodilation and a fall in BP. Keratinocytes and immune cells control blood flow in the extensive countercurrent loop system of the skin by producing nitric oxide, a key regulator of vascular tone. The balance between hypoxia-inducible factor-1α and hypoxia-inducible factor-2α activity in keratinocytes controls skin perfusion, systemic thermoregulation, and systemic BP by nitric oxide-dependent mechanisms. Furthermore, the skin accumulates Na which generates a barrier to promote immunological host defense. Immune cells control skin Na metabolism and the clearance of Na via the lymphatic system. Reduced lymphatic clearance increases BP.

SUMMARY

Apart from the well-known role of the brain, blood vessels, and the kidney, the skin is important for systemic BP control in humans and in experimental animals.

A High-Salt Diet Differentially Modulates Mechanical Activity of Afferent and Efferent Collecting Lymphatics in Murine Iliac Lymph Nodes

BACKGROUND

The lymphatic system contributes to fluid homeostasis in various tissues. Recent evidence suggests that lymphangiogenesis induced by a high-salt diet (HSD) is associated with blood pressure regulation. Lymph nodes, located along lymphatic pathways, are not only important secondary lymphoid tissues for cancer metastasis, inflammation, and immune responses, but are also important for fluid homeostasis. Afferent lymphatics collect lymph from the pre-nodal area and efferent lymphatics drain lymph out of the lymph nodes. However, the difference in mechanical activity between afferent and efferent lymphatics and the effect of a HSD on these vessels have not been shown.

METHODS AND RESULTS

Changes in mechanical activity of isolated afferent and efferent lymphatics in normal salt diet (NSD) and 4-week HSD mice in response to increases in intraluminal pressures from 3 to 7 cmH2O were measured using video-microscopy. The higher intramural pressure equivalently decreased pumping activity of afferent and efferent lymphatics in NSD mice. A HSD suppressed the amplitude, ejection fraction, and stroke volume of afferent lymphatics, leading to marked reductions in pumping activity. In contrast, the pumping activities of efferent lymphatics were resistant to a HSD and were preserved by enhancing the contraction frequency.

CONCLUSIONS

A HSD differentially modulated the mechanical activity of afferent and efferent collecting lymphatics in murine iliac lymph nodes.