Joshua S. Speed

Hypertension: Physiology and Pathophysiology

Despite major advances in understanding the pathophysiology of hypertension and availability of effective and safe antihypertensive drugs, suboptimal blood pressure (BP) control is still the most important risk factor for cardiovascular mortality and is globally responsible for more than 7 million deaths annually. Short-term and long-term BP regulation involve the integrated actions of multiple cardiovascular, renal, neural, endocrine, and local tissue control systems. Clinical and experimental observations strongly support a central role for the kidneys in the long-term regulation of BP, and abnormal renal-pressure natriuresis is present in all forms of chronic hypertension. Impaired renal-pressure natriuresis and chronic hypertension can be caused by intrarenal or extrarenal factors that reduce glomerular filtration rate or increase renal tubular reabsorption of salt and water; these factors include excessive activation of the renin-angiotensin-aldosterone and sympathetic nervous systems, increased formation of reactive oxygen species, endothelin, and inflammatory cytokines, or decreased synthesis of nitric oxide and various natriuretic factors. In human primary (essential) hypertension, the precise causes of impaired renal function are not completely understood, although excessive weight gain and dietary factors appear to play a major role since hypertension is rare in nonobese hunter-gathers living in nonindustrialized societies. Recent advances in genetics offer opportunities to discover gene-environment interactions that may also contribute to hypertension, although success thus far has been limited mainly to identification of rare monogenic forms of hypertension.

Endothelin type A receptor antagonist attenuates placental ischemia–induced hypertension and uterine vascular resistance

OBJECTIVE We sought to determine the effect of an endothelin type A receptor antagonist (ETA) on uterine artery resistive index (UARI) and mean arterial pressure (MAP) in a placental ischemia rat model of preeclampsia produced by reduction in uterine perfusion pressure (RUPP). STUDY DESIGN UARI was assessed by Doppler velocimetry in RUPP and normal pregnant controls (NP) on gestational days (GD) 12, 15, and 18. UARI was also determined on GD 18 in NP and RUPP pregnant dams after pretreatment with ETA. MAP was recorded on GD 19. RESULTS The RUPP group had a higher MAP and UARI on GD 15 and 18 than the NP group. Pretreatment with ETA attenuated both the MAP and GD-18 UARI in the RUPP group without affecting these parameters in the NP group. CONCLUSION The improvement in UARI could be one potential mechanism for the reduction in MAP in response to ETA in pregnant dams with ischemic placentas.

hypertension in response to IL-6 during pregnancy: role of AT1-receptor activation

BACKGROUND: Increases in interleukin 6 (IL-6) and agonistic autoantibodies to the angiotensin II type 1 receptor (AT1-AA) are proposed to be important links between placental ischemia and hypertension in preeclampsia. METHODS: The purpose of this study was to determine whether IL-6 (5 ng/day), infused into normal pregnant (NP) rats, increased mean arterial pressure (MAP) and AT1-AA. MAP was analyzed in the presence and absence of an angiotensin type 1 receptor (AT1R) antagonist, losartan, L. RESULTS: MAP and AT1-AA increased from 102 ± 2 to 118 ± 4 mmHg and 0.7 ± 0.3 NP to 14.1 ± 1.4 chronotropic units with chronic IL-6 infusion. MAP responses to IL-6 were abolished in losartan pretreated rats (85 ± 4 in NP + L vs 85 ± 3 mmHg in IL-6 + L). CONCLUSION: These data indicate that IL-6 stimulates AT1-AA and that activation of the AT1R mediates IL-6 induced hypertension during pregnancy.

Endothelin, Kidney Disease, and Hypertension

Since its discovery in 1988, endothelin-1 (ET-1) has been widely studied in a diverse number of fields, including neurology, cardiology, development, and to a greater extent, nephrology and hypertension.1,2 Through the activation of its 2 receptors, ETA and ETB, ET-1 influences blood pressure by numerous mechanisms, making it an attractive target for treatment of hypertension and other diseases.3–10 Although antagonists of the ET-1 system are highly effective in experimental models of hypertension and recently have been shown effective in resistant essential hypertension,7,11,12 their translation to the clinic has been disappointing thus far because of various side effects, including fluid retention/edema and liver toxicity.13,14 In fact, only 2 ET-1 receptor antagonists have been approved by the Food and Drug Administration for the use in humans, ambrisentan and bosentan, an ETA antagonist and dual ETA/ETB antagonist, respectively. The sole indication thus far approved is for the treatment of pulmonary hypertension.15 However, several receptor-specific antagonists are available and are going through animal testing and clinical trials. This review will focus on the recent progress of how ET-1 affects blood pressure and the future use of ET-1 receptor antagonists for the treatment of kidney disease and hypertension.14 Either ETA or ETB or both receptors are located on almost every cell type throughout the body. ETA receptors are mostly located on vascular smooth muscle cells, and activation is not only normally prohypertensive through potent vasoconstriction but also have significant effects to increase inflammation, oxidative stress, and increases in proteinuria through direct changes on renal glomerular permeability.12,16,17 ETB receptors, however, function quite the opposite, being mostly antihypertensive. Vascular ETB receptors are mainly located …

ET-1 increases reactive oxygen species following hypoxia and high salt diet in the mouse glomerulus

This study was designed to determine whether ET‐1 derived from endothelial cells contributes to oxidative stress in the glomerulus of mice subjected to a high‐salt diet and/or hypoxia.

Endothelin-1 as a master regulator of whole-body Na+ homeostasis.

The current study was designed to determine whether vascular endothelial‐derived endothelin‐1 (ET‐1) is important for skin Na+ buffering. In control mice (C57BL/6J), plasma Na+ and osmolarity were significantly elevated in animals on high‐ vs. low‐salt (HS and LS, respectively) intake. The increased plasma Na+ and osmolarity were associated with increased ET‐1 mRNA in vascular tissue. There was no detectable difference in skin Na+:H2O in HS fed mice (0.119 ± 0.005 mM vs. 0.127 ± 0.007 mM; LS vs. HS); however, skin Na+:H2O was significantly increased by blockade of the endothelin type A receptor with ABT‐627 (0.116 ± 0.006 mM vs. 0.137 ± 0.007 mM; LS vs. HS; half‐maximal inhibitory concentration, 0.055 nM). ET‐1 peptide content in skin tissue was increased in floxed control animals on HS (85.9 ± 0.9pg/mg vs. 106.4 ± 6.8 pg/mg; P< 0.05), but not in vascular endothelial cell endothelin‐1 knockout (VEET KO) mice (76.4 ± 5.7 pg/mg vs. 65.7 ± 7.9 pg/mg; LS vs. HS). VEET KO mice also had a significantly elevated skin Na+:H2O (0.113 ± 0.007 mM vs. 0.137 ± 0.005 mM; LS vs. HS; P < 0.05). Finally, ET‐1 production was elevated in response to increasing extracellular osmolarity in cultured human endothelial cells. These data support the hypothesis that increased extrarenal vascular ET‐1 production in response to HS intake is mediated by increased extracellular osmolarity and plays a critical role in regulating skin storage of Na+.—Speed, J. S., Heimlich, J. B., Hyndman, K. A., Fox, B. M., Patel, V., Yanagisawa, M., Pollock, J. S., Titze, J. M., Pollock, D. M. Endothelin‐1 as a master regulator of whole‐body Na+ homeostasis. FASEB J. 29, 4937–4944 (2015). www.fasebj.org

Sex differences in ET-1 receptor expression and Ca2+ signaling in the IMCD

The inner medullary collecting duct (IMCD) is the nephron segment with the highest production of endothelin-1 (ET-1) and the greatest expression of ET-1 receptors that function to adjust Na(+) and water balance. We have reported that male rats have reduced natriuresis in response to direct intramedullary infusion of ET-1 compared with female rats. Our aim was to determine whether alterations of ET-1 receptor expression and downstream intracellular Ca(2+) signaling within the IMCD could account for these sex differences. IMCDs from male and female rats were isolated for radioligand binding or microdissected for intracellular Ca(2+) ([Ca(2+)]i) measurement by fluorescence imaging of fura-2 AM. IMCD from male and female rats had similar ETB expression (655 ± 201 vs. 567 ± 39 fmol/mg protein, respectively), whereas male rats had significantly higher ETA expression (436 ± 162 vs. 47 ± 29 fmol/mg protein, respectively; P < 0.05). The [Ca(2+)]i response to ET-1 was significantly greater in IMCDs from male compared with female rats (288 ± 52 vs. 118 ± 32 AUC, nM × 3 min, respectively; P < 0.05). In IMCDs from male rats, the [Ca(2+)]i response to ET-1 was significantly blunted by the ETA antagonist BQ-123 but not by the ETB antagonist BQ-788 (control: 137 ± 27; BQ-123: 53 ± 11; BQ-788: 84 ± 25 AUC, nM × 3 min; P < 0.05), consistent with greater ETA receptor function in male rats. These data demonstrate a sex difference in ETA receptor expression that results in differences in ET-1 Ca(2+) signaling in IMCD. Since activation of ETA receptors is thought to oppose ETB receptor activation, enhanced ETA function in male rats could limit the natriuretic effects of ETB receptor activation.

Role of 20-Hydroxyeicosatetraenoic Acid in Mediating Hypertension in Response to Chronic Renal Medullary Endothelin Type B Receptor Blockade

Background The renal medullary endothelin (ET-1) system plays an important role in the control of sodium excretion and arterial pressure (AP) through the activation of renal medullary ET-B receptors. We have previously shown that blockade of endothelin type B receptors (ET-B) leads to salt-sensitive hypertension through mechanisms that are not fully understood. One possible mechanism is through a reduction in renal medullary production of 20-hydroxyeicosatetraenoic acid (20-HETE). 20-HETE, a metabolite of arachidonic acid, has natriuretic properties similar to ET-B activation. While these findings suggest a possible interaction between ET-B receptor activation and 20-HETE production, it is unknown whether blockade of medullary ET-B receptors in rats maintained on a high sodium intake leads to reductions in 20-HETE production. Methodology/Principal Findings The effect of increasing sodium intake from low (NS = .8%) to high (HS = 8%) on renal medullary production of 20-HETE in the presence and absence of renal medullary ET-B receptor antagonism was examined. Renal medullary blockade of ET-B receptors resulted in salt sensitive hypertension. In control rats, blood pressure rose from 112.8±2.4 mmHg (NS) to 120.7±9.3 mmHg (HS). In contrast, when treated with an ET-B receptor blocker, blood pressure was significantly elevated from 123.7±3.2 (NS) to 164.2±7.1 (HS). Furthermore, increasing sodium intake was associated with elevated medullary 20-HETE (5.6±.8 in NS vs. 14.3±3.7 pg/mg in HS), an effect that was completely abolished by renal medullary ET-B receptor blockade (4.9±.8 for NS and 4.5±.6 pg/mg for HS). Finally, the hypertensive response to intramedullary ET-B receptor blockade was blunted in rats pretreated with a specific 20-HETE synthesis inhibitor. Conclusion These data suggest that increases in renal medullary production of 20-HETE associated with elevating salt intake may be, in part, due to ET-B receptor activation within the renal medulla.

Renal Medullary Endothelin-1 is Decreased in Dahl Salt Sensitive Rats

Although it is well established that the renal endothelin (ET-1) system plays an important role in regulating sodium excretion and blood pressure through activation of renal medullary ET(B) receptors, the role of this system in Dahl salt-sensitive (DS) hypertension is unclear. The purpose of this study was to determine whether the DS rat has abnormalities in the renal medullary endothelin system when maintained on a high sodium intake. The data indicate that Dahl salt-resistant rats (DR) on a high-salt diet had a six-fold higher urinary endothelin excretion than in the DR rats with low Na(+) intake (17.8 ± 4 pg/day vs. 112 ± 44 pg/day). In sharp contrast, urinary endothelin levels increased only twofold in DS rats in response to a high Na(+) intake (13 ± 2 pg/day vs. 29.8 ± 5.5 pg/day). Medullary endothelin concentration in DS rats on a high-Na(+) diet was also significantly lower than DR rats on a high-Na(+) diet (31 ± 2.8 pg/mg vs. 70.9 ± 5 pg/mg). Furthermore, DS rats had a significant reduction in medullary ET(B) receptor expression compared with DR rats while on a high-Na(+) diet. Finally, chronic infusion of ET-1 directly into the renal medulla blunted Dahl salt-sensitive hypertension. These data indicate that a decrease in medullary production of ET-1 in the DS rat could play an important role in the development of salt-sensitive hypertension observed in the DS rat.

Endothelin-1 and the kidney: new perspectives and recent findings.

Purpose of reviewThe role of endothelin-1 (ET-1) in the kidney has been under study for many years; however, the complex mechanisms by which endothelin controls the physiology/pathophysiology of this organ are not fully resolved. This review aims to summarize recent findings in the field, especially regarding glomerular and tubular damage, Na+/water homeostasis and sex differences in ET-1 function. Recent findingsPodocytes have been recently identified as a target of ET-1 in the glomerular filtration barrier via ETA receptor activation. Activation of the ETA receptor by ET-1 leads to renal tubular damage by promoting endoplasmic reticulum stress and apoptosis in these cells. In addition, high flow rates in the nephron in response to high salt intake induce ET-1 production by the collecting ducts and promote nitric oxide-dependent natriuresis through epithelial sodium channel inhibition. Recent evidence also indicates that sex hormones regulate the renal ET-1 system differently in men and women, with estrogen suppressing renal ET-1 production and testosterone upregulating that production. SummaryBased on the reports reviewed in here, targeting of the renal endothelin system is a possible therapeutic approach against the development of glomerular injury. More animal and clinical studies are needed to better understand the dimorphic control of this system by sex hormones.