Immune cells control skin lymphatic electrolyte homeostasis and blood pressure

Sodium balance is not just a renal affair

PURPOSE OF REVIEW

The equilibration theory of extracellular body fluids is tightly linked to sodium (Na) metabolism. It is accepted that with changes in salt intake, renal sodium elimination will prevent any change in interstitial Na content and concentration. This review summarizes recent anomalous findings regarding salt and water homeostasis that are inconsistent with current assumptions.

RECENT FINDINGS

Recent findings from chemical analysis studies of laboratory animals, as well as noninvasive quantitative Na MRI (Na-MRI) studies in patients, have shown that remarkable amounts of Na are stored in muscle and in skin without commensurate water retention. Furthermore, an ultra-long Na balance study in humans suggests the presence of endogenous clocks that generate weekly and monthly infradian rhythmicity of Na storage independent of salt intake. Animal experiments suggest that fluids in the skin interstitium are hypertonic compared with plasma, and that interstitial osmotic stress induces local extrarenal immune cell and lymph-capillary driven mechanisms for electrolyte clearance and maintenance of the internal environment.

SUMMARY

Recent quantitative evidence challenges current ideas on salt and water homeostasis, and suggests that Na homeostasis cannot be maintained without additional previously unappreciated extrarenal regulatory mechanisms.

Effect of intensified diuretic therapy on overnight rostral fluid shift and obstructive sleep apnoea in patients with uncontrolled hypertension

OBJECTIVES

Fluid displacement from the lower extremities to the upper body during sleep is strongly associated with obstructive sleep apnoea in hypertensive patients. The present pathophysiological study tests the hypothesis that intensified diuretic therapy will reduce the apnoea-hypopnoea index and blood pressure of uncontrolled hypertensive patients with obstructive sleep apnoea in proportion to the reduction in overnight change in leg fluid volume.

METHODS

Uncontrolled treated hypertensive patients underwent overnight polysomnography and measurement of overnight changes in leg fluid volume and neck circumference. Those with an apnoea-hypopnoea index at least 20 events per hour (n=16) received metolazone 2.5 mg and spironolactone 25 mg daily for 7 days after which the daily dose was doubled for 7 additional days. Baseline testing was again repeated.

RESULTS

Intensified diuretic therapy reduced the apnoea-hypopnoea index from 57.7 ± 33.0 to 48.5 ± 28.2 events per hour (P=0.005), overnight change in leg fluid volume from -418.1 ± 177.5 to -307.5 ± 161.9 ml (P<0.001) and overnight change in neck circumference from 1.2 ± 0.6 to 0.7 ± 0.4 cm (P<0.001). There was an inverse correlation between the reduction in overnight change in leg fluid volume and decrease in apnoea-hypopnoea index (r=-0.734, P=0.001). The reduction in overnight change in leg fluid volume was also significantly correlated with the change in morning blood pressure (r=0.708, P=0.002 for SBP; r=0.512, P=0.043 for DBP).

CONCLUSION

The findings provide further evidence that fluid redistribution from the legs to the neck during sleep contributes to the severity of obstructive sleep apnoea in hypertension and may be an important link between these two conditions.

Role of "Western diet" in inflammatory autoimmune diseases

Developed societies, although having successfully reduced the burden of infectious disease, constitute an environment where metabolic, cardiovascular, and autoimmune diseases thrive. Living in westernized countries has not fundamentally changed the genetic basis on which these diseases emerge, but has strong impact on lifestyle and pathogen exposure. In particular, nutritional patterns collectively termed the "Western diet", including high-fat and cholesterol, high-protein, high-sugar, and excess salt intake, as well as frequent consumption of processed and 'fast foods', promote obesity, metabolic syndrome, and cardiovascular disease. These factors have also gained high interest as possible promoters of autoimmune diseases. Underlying metabolic and immunologic mechanisms are currently being intensively explored. This review discusses the current knowledge relative to the association of "Western diet" with autoimmunity, and highlights the role of T cells as central players linking dietary influences to autoimmune pathology.

Connexins in lymphatic vessel physiology and disease

Connexins are transmembrane proteins that form gap junction- and hemi-channels. Once inserted into the membrane, hemi-channels (connexons) allow for diffusion of ions and small molecules (<1 kDa) between the extracellular space and the cytosol. Gap junction channels allow diffusion of similar molecules between the cytoplasms of adjacent cells. The expression and function of connexins in blood vessels has been intensely studied in the last few decades. In contrast, only a few studies paid attention to lymphatic vessels; convincing in vivo data with respect to expression patterns of lymphatic connexins and their functional roles have only recently begun to emerge. Interestingly, mutations in connexin genes have been linked to diseases of lymphatic vasculature, most notably primary and secondary lymphedema. This review summarizes the available data regarding lymphatic connexins. More specifically it addresses (i) early studies aimed at presence of gap junction-like structures in lymphatic vessels, (ii) more recent studies focusing on lymphatic connexins using genetically engineered mice, and (iii) results of clinical studies that have reported lymphedema-linked mutations in connexin genes.

The plasticity of human Treg and Th17 cells and its role in autoimmunity

CD4(+) T helper cells are a central element of the adaptive immune system. They protect the organism against a wide range of pathogens and are able to initiate and control many immune reactions in combination with other cells of the adaptive and the innate immune system. Starting from a naive cell, CD4(+) T cells can differentiate into various effector cell populations with specialized function. This subset specific differentiation depends on numerous signals and the strength of stimulation. However, recent data have shown that differentiated CD4(+) T cell subpopulations display a high grade of plasticity and that their initial differentiation is not an endpoint of T cell development. In particular, FoxP3(+) regulatory T cells (Treg) and Th17 effector T cells demonstrate a high grade of plasticity, which allow a functional adaptation to various physiological situations during an immune response. However, the plasticity of Treg and Th17 cells might also be a critical factor for autoimmune disease. Here we discuss the recent developments in CD4(+) T cell plasticity with a focus on Treg and Th17 cells and its role in human autoimmune disease, in particular multiple sclerosis (MS).

Serum Chloride Is an Independent Predictor of Mortality in Hypertensive Patients

Chloride (Cl − ) is the major extracellular anion in the body, accompanying sodium (Na + ), and is primarily derived from dietary sources. Data suggest that increased dietary Cl − intake increases blood pressure, yet paradoxically, higher serum Cl − appears associated with lower mortality and cardiovascular risk. This implies that serum Cl − also reflects risk pathways independent of blood pressure, serum Na + , and bicarbonate (HCO 3 − ). We analyzed 12 968 hypertensive individuals followed up for 35 years, using Cox proportional hazards model to test whether baseline serum Cl − was an independent predictor of mortality. To distinguish the effect of Cl − from Na + and HCO 3 − , we adjusted for these electrolytes and also performed the analysis stratified by Na + /HCO 3 − and Cl − levels. Generalized estimating equation was used to determine the effect of baseline Cl − on follow-up blood pressure. The total time at risk was 197 101 person-years. The lowest quintile of serum Cl − (<100 mEq/L) was associated with a 20% higher mortality (all-cause, cardiovascular and noncardiovascular) compared with the remainder of the subjects. A 1 mEq/L increase in serum Cl − was associated with a 1.5% (hazard ratio, 0.985; 95% confidence interval, 0.98–0.99) reduction in all-cause mortality, after adjustment for baseline confounding variables and Na + , K + , and HCO3 − levels. The group with Na + >135 and Cl − >100 had the best survival, and compared with this group, the Na + >135 and Cl − <100 group had significantly higher mortality (hazard ratio, 1.21; 95% confidence interval, 1.11–1.31). Low, not high Serum Cl − (<100 mEq/L), is associated with greater mortality risk independent of obvious confounders. Further studies are needed to elucidate the relation between Cl − and risk.

Permeability and contractile responses of collecting lymphatic vessels elicited by atrial and brain natriuretic peptides

Atrial and brain natriuretic peptides (ANP and BNP, respectively) are cardiac hormones released into the bloodstream in response to hypervolaemia or fluid shifts to the central circulation. The actions of both peptides include natriuresis and diuresis, a decrease in systemic blood pressure, and inhibition of the renin-angiotensin-aldosterone system. Further, ANP and BNP elicit increases in blood microvessel permeability sufficient to cause protein and fluid extravasation into the interstitium to reduce the vascular volume. Given the importance of the lymphatic vasculature in maintaining fluid balance, we tested the hypothesis that ANP or BNP (100 nM) would likewise elevate lymphatic permeability (Ps) to serum albumin. Using a microfluorometric technique adapted to in vivo lymphatic vessels, we determined that rat mesenteric collecting lymphatic Ps to rat serum albumin increased by 2.0 ± 0.4-fold (P = 0.01, n = 7) and 2.7 ± 0.8-fold (P = 0.07, n = 7) with ANP and BNP, respectively. In addition to measuring Ps responses, we observed changes in spontaneous contraction amplitude and frequency from the albumin flux tracings in vivo. Notably, ANP abolished spontaneous contraction amplitude (P = 0.005) and frequency (P = 0.006), while BNP augmented both parameters by ∼2-fold (P < 0.01 each). These effects of ANP and BNP on contractile function were examined further by using an in vitro assay. In aggregate, these data support the theory that an increase in collecting lymphatic permeability opposes the absorptive function of the lymphatic capillaries, and aids in the retention of protein and fluid in the interstitial space to counteract volume expansion.

Lymphatics in health and disease: a new thematic series in vascular cell

Vascular Cell is launching new series on lymphatics, a vascular system required for physiological fluid balance and immunity, and whose damage leads to edema.