Normotension, hypertension and body fluid regulation: brain and kidney

The fraction of hypertensive patients with essential hypertension (EH) is decreasing as the knowledge of mechanisms of secondary hypertension increases, but in most new cases of hypertension the pathophysiology remains unknown. Separate neurocentric and renocentric concepts of aetiology have prevailed without much interaction. In this regard, several questions regarding the relationships between body fluid and blood pressure regulation are pertinent. Are all forms of EH associated with sympathetic overdrive or a shift in the pressure-natriuresis curve? Is body fluid homoeostasis normally driven by the influence of arterial blood pressure directly on the kidney? Does plasma renin activity, driven by renal nerve activity and renal arterial pressure, provide a key to stratification of EH? Our review indicates that (i) a narrow definition of EH is useful; (ii) in EH, indices of cardiovascular sympathetic activity are elevated in about 50% of cases; (iii) in EH as in normal conditions, mediators other than arterial blood pressure are the major determinants of renal sodium excretion; (iv) chronic hypertension is always associated with a shift in the pressure-natriuresis curve, but this may be an epiphenomenon; (v) plasma renin levels are useful in the analysis of EH only after metabolic standardization and then determination of the renin function line (plasma renin as a function of sodium intake); and (vi) angiotensin II-mediated hypertension is not a model of EH. Recent studies of baroreceptors and renal nerves as well as sodium intake and renin secretion help bridge the gap between the neurocentric and renocentric concepts.

Renal effects of a novel endogenous natriuretic agent xanthurenic acid 8-o-β-d-glucoside in rats

Xanthurenic acid 8-o-β-d-glucoside is an endogenous derivative of tryptophan metabolism, isolated from urine of patients with chronic renal disease. This compound was suggested previously to act as a natriuretic hormone based on its ability to block short circuit currents in a frog skin assay and to induce a sustained natriuresis when injected into rats (C. D. Cain et al., Proc. Natl. Acad. Sci. USA 2007: 17873-17878). The present communication describes the effects of the compound on renal clearance and hemodynamic parameters in male Sprague-Dawley rats maintained on a normal salt (0.4-0.5%) diet. Intravenous administration of synthetic xanthurenic acid 8-o-β-d-glucoside in two consecutive incremental doses (6.3 and 31.5 nmol) resulted in a significant increase (P < 0.05), in urine flow (43.91 ± 6.31 μL/min vs. 10.54 ± 2.21 μL/min), absolute rate of sodium excretion (3.99 ± 0.95 μEq/min vs. 1.15 ± μEq/min), and percentage sodium excretion (1.63 ± 0.46% vs. 0.37 ± 0.12%, peak response vs. baseline, respectively). The natriuretic/diuretic effect was associated also with a significant increase in potassium excretion. These effects were not related to changes in renal hemodynamics or in arterial blood pressure. Pretreatment with the sodium channel blocker, amiloride, completely abolished the natriuretic and kaluretic actions of the compound. Administration of the xanthurenic acid derivative caused a dose-related increase in urinary nitrite/nitrate excretion. Moreover, under chronic nitric oxide blockade by l-NG-Nitro-Arginine-Methyl-Esther (l-NAME) sodium excretion was similar in rats treated or untreated with the compound. Our data demonstrate that xanthurenic acid 8-o-β-d-glucoside has significant diuretic/natriuretic and kaluretic properties. An intact amiloride-sensitive sodium channel is required for the renal effects of the compound. The data further suggest that the natriuretic effect is mediated in part by a nitric oxide-dependent mechanism.

Does Goal-directed Fluid Therapy Affect Postoperative Orthostatic Intolerance?

Background:

Early mobilization is important for postoperative recovery but is limited by orthostatic intolerance (OI) with a prevalence of 50% 6 h after major surgery. The pathophysiology of postoperative OI is assumed to include hypovolemia besides dysregulation of vasomotor tone. Stroke volume–guided fluid therapy, so-called goal-directed therapy (GDT), corrects functional hypovolemia, and the authors hypothesized that GDT reduces the prevalence of OI after major surgery and assessed this in a prospective, double-blinded trial.

Methods:

Forty-two patients scheduled for open radical prostatectomy were randomized into standard fluid therapy (control group) or GDT groups. Both groups received a fixed-volume crystalloid regimen supplemented with 1:1 replacement of blood loss with colloid, and in addition, the GDT group received colloid to obtain a maximal stroke volume (esophageal Doppler). The primary outcome was the prevalence of OI assessed with a standardized mobilization protocol before and 6 h after surgery. Hemodynamic and hormonal orthostatic responses were evaluated.

Results:

Twelve (57%) versus 15 (71%) patients in the control and GDT groups (P = 0.33), respectively, demonstrated OI after surgery, group difference 14% (CI, −18 to 45%). Patients in the GDT group received more colloid during surgery (1,758 vs. 1,057 ml; P = 0.001) and reached a higher stroke volume (102 vs. 89 ml; P = 0.04). OI patients had an increased length of hospital stay (3 vs. 2 days; P = 0.02) and impaired hemodynamic and norepinephrine responses on mobilization.

Conclusion:

GDT did not reduce the prevalence of OI, and patients with OI demonstrated impaired cardiovascular and hormonal responses to mobilization.

Nitric oxide and the A and B of endothelin of sodium homeostasis

PURPOSE OF REVIEW

In recent years, renal collecting duct-specific endothelin-1 (ET1), endothelin A (ETA) and endothelin B (ETB) receptors as well as nitric oxide synthase 1 (NOS1) knockout mice have been developed with subsequent identification for an integral role in regulation of sodium water homeostasis and ultimately blood pressure. The focus of this review is to integrate these models and to propose a scheme for the control of sodium excretion by the collecting duct and the endothelin/ETB/NOS system.

RECENT FINDINGS

NOS1 splice variants are expressed in the kidney, especially in the collecting duct. Mice express predominantly NOS1β in the medulla, with NOS1α and NOS1β in the cortex, whereas rats express NOS1α and NOS1β in both the cortex and medulla. Novel transcription of collecting duct ET1 mediated by epithelial sodium channels, mitochondrial Na/Ca exchangers and glucocorticoids has been determined. ET1 via the ETB receptor increases nitric oxide production in both rat and mouse collecting ducts, suggesting that NOS1β is linked to ET1-dependent NOS activation in the kidney. As well, genetic deletion of NOS1 splice variants in the collecting duct results in a salt-sensitive hypertensive phenotype in mice, much like the collecting duct ET1 and collecting duct ETB knockout mice.

SUMMARY

In the collecting duct, the ET1/nitric oxide pathways are intimately linked, and deletion of collecting duct ET1, ETB receptor or NOS1β results in a salt-sensitive phenotype, which is at least partially dependent on dysregulation of sodium and water reabsorption.

Renal cortical and medullary blood flow during modest saline loading in humans

AIM

Renal medullary blood flow (RMBF) is considered an important element of sodium homeostasis, but the experimental evidence is incongruent. Studies in anaesthetized animals generally support the concept in contrast to measurements in conscious animals. We hypothesized that saline-induced natriuresis is associated with changes in RMBF in humans.

METHODS

After 4 days of low-sodium diet, healthy men were subjected to slow intravenous saline loading (12 μmol kg(-1) min(-1)) for 4 h. Renal medullary and cortical blood flow was determined by positron emission tomography with H(2)(15)O before and after saline infusion using two independent imaging processing methods. One based on a previously published algorithm (voxel peeling) and a novel method based on contrast-enhanced computed tomography (CT). Blood pressure was measured oscillometrically every 10 min. Cardiac output, heart rate and total peripheral resistance were recorded continuously.

RESULTS

Saline loading increased the urinary sodium excretion by 3.6-fold (21-76 μmol min(-1) , P < 0.01). The RMBF was 2.6 ± 0.2 mL g(-1) tissue min(-1) before and 2.7 ± 0.1 mL g(-1) tissue min(-1) after saline (n.s.). Cortical blood flow was 3.6 ± 0.1 before and 3.4 ± 0.2 after saline (n.s.). Mean arterial blood pressure did not change measurably (90 vs. 90 mmHg). Bland-Altman analysis suggested agreement between results obtained with voxel peeling (2.6 ± 0.2 mL g(-1) tissue min(-1)) and contrast-enhanced CT (2.0 ± 0.1 mL g(-1) tissue min(-1)).

CONCLUSION

In normal humans, changes in RMBF are not necessarily involved in the natriuretic response to modest saline loading. This result is in line with data from conscious rodents.