Blood volume, blood pressure and total body sodium: internal signalling and output control

Emerging therapeutic landscape: Incretin agonists in chronic kidney disease management

The increasing incidence of chronic kidney disease (CKD) poses a significant public health concern, prompting heightened attention to its treatment. Incretins, including glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide, are intestinal peptides released after nutrient intake, known for their hypoglycemic effects in diabetes management. Recent advancements highlight the promising outcomes of GLP-1 receptor agonists in reducing CKD risk factors and improving renal outcomes. The multifaceted functions of GLP-1, such as its anti-obesity, anti-hypertensive, anti-hyperglycemic, anti-lipid, anti-inflammatory, and endothelial function protective properties, contribute to its potential as a therapeutic agent for CKD. Although experiments suggest the potential benefits of incretin in CKD, a comprehensive understanding of its specific mechanisms is still lacking. This review aims to provide a detailed examination of current evidence and potential future directions, emphasizing the promising yet evolving landscape of incretin agonists in the context of CKD.

Salt Sensitivity: Causes, Consequences, and Recent Advances

Salt (sodium chloride) is an essential nutrient required to maintain physiological functions. However, for most people, daily salt intake far exceeds their physiological need and is habitually greater than recommended upper thresholds. Excess salt intake leads to elevation in blood pressure which drives cardiovascular morbidity and mortality. Indeed, excessive salt intake is estimated to be responsible for ≈5 million deaths per year globally. For approximately one-third of otherwise healthy individuals (and >50% of those with hypertension), the effect of salt intake on blood pressure elevation is exaggerated; such people are categorized as salt sensitive and salt sensitivity of blood pressure is considered an independent risk factor for cardiovascular disease and death. The prevalence of salt sensitivity is higher in women than in men and, in both, increases with age. This narrative review considers the foundational concepts of salt sensitivity and the underlying effector systems that cause salt sensitivity. We also consider recent updates in preclinical and clinical research that are revealing new modifying factors that determine the blood pressure response to high salt intake.

Anemia and Hypoxia Impact on Chronic Kidney Disease Onset and Progression: Review and Updates

Chronic kidney disease (CKD) is caused by hypoxia in the renal tissue, leading to inflammation and increased migration of pathogenic cells. Studies showed that leukocytes directly sense hypoxia and respond by initiating gene transcription, encoding the 2-integrin adhesion molecules. Moreover, other mechanisms participate in hypoxia, including anemia. CKD-associated anemia is common, which induces and worsens hypoxia, contributing to CKD progression. Anemia correction can slow CKD progression, but it should be cautiously approached. In this comprehensive review, the underlying pathophysiology mechanisms and the impact of renal tissue hypoxia and anemia in CKD onset and progression will be reviewed and discussed in detail. Searching for the latest updates in PubMed Central, Medline, PubMed database, Google Scholar, and Google search engines were conducted for original studies, including cross-sectional studies, cohort studies, clinical trials, and review articles using different keywords, phrases, and texts such as "CKD progression, anemia in CKD, CKD, anemia effect on CKD progression, anemia effect on CKD progression, and hypoxia and CKD progression". Kidney tissue hypoxia and anemia have an impact on CKD onset and progression. Hypoxia causes nephron cell death, enhancing fibrosis by increasing interstitium protein deposition, inflammatory cell activation, and apoptosis. Severe anemia correction improves life quality and may delay CKD progression. Detection and avoidance of the risk factors of hypoxia prevent recurrent acute kidney injury (AKI) and reduce the CKD rate. A better understanding of kidney hypoxia would prevent AKI and CKD and lead to new therapeutic strategies.

KCC3a, a Strong Candidate Pathway for K+ Loss in Alkalemia

Loss-of-function mutations in the human potassium chloride cotransporter-3 (KCC3) cause a hereditary motor sensory neuropathy associated with agenesis of the corpus callosum. While recapitulating the neuropathy, KCC3-knockout mice also exhibit high blood pressure. This phenotype is believed to have neurogenic and/or vascular origins. The role of KCC3 in the kidney is poorly understood. KCC3 is encoded by two major isoforms originating from alternative promoters: KCC3a and KCC3b, with KCC3b being the predominant transcript in the kidney. Although the transporter has previously been localized to the proximal tubule, we show here the unique expression of the KCC3a isoform in the connecting tubule. Using a KCC3a-specific polyclonal antibody validated for both immunofluorescence and immunoblotting, we showed an intense KCC3a signal restricted to cortical intercalated cells. No overlap is detected between KCC3a and sodium chloride cotransporter (NCC), a distal convoluted tubule (DCT) marker; or between KCC3a and ENaC or calbindin, which are both principal cell markers. KCC3a signal was observed in cells expressing the apical V-ATPase and pendrin, establishing a unique expression pattern characteristic of intercalated cells of type-B or type-nonA/nonB. We further show that treatment of wild-type mice with hydrochlorothiazide, amiloride, or fed a K+-deficient diet up-regulates KCC3a level, suggesting that volume depletion increases KCC3a abundance. This hypothesis was confirmed by showing a higher abundance of KCC3a protein after 23-h water restriction or after placing the mice on a low-salt diet. More importantly, abundance of the Cl−/HCO3− exchanger, pendrin, which is known to secrete bicarbonate in alkalotic conditions, was significantly diminished in KCC3-knockout mice. In addition, KCC3a abundance increased significantly alongside pendrin abundance in bicarbonate-treated alkalotic mice, providing a credible mechanism for K+ loss in metabolic alkalosis.

Weight gain attempts and diet modification efforts among adults in five countries: a cross-sectional study

BACKGROUND

Recent research has emphasized a growing trend of weight gain attempts, particularly among adolescents and boys and young men. Little research has investigated these efforts among adults, as well as the specific diet modifications individuals who are trying to gain weight engage in. Therefore, the aims of this study were to characterize the diet modification efforts used by adults across five countries who reported engaging in weight gain attempts and to determine the associations between weight gain attempts and concerted diet modification efforts.

METHODS

Cross-sectional data from the 2018 and 2019 International Food Policy Study, including participants from Australia, Canada, Mexico, the United Kingdom, and the United States (N = 42,108), were analyzed. In reference to the past 12 months, participants reported on weight gain attempts and diet modification efforts related to increased consumption of calories, protein, fiber, fruits and vegetables, whole grains, dairy products, all meats, red meat only, fats, sugar/added sugar, salt/sodium, and processed foods. Unadjusted (chi-square tests) and adjusted (modified Poisson regressions) analyses were conducted to examine associations between weight gain attempts and diet modification efforts.

RESULTS

Weight gain attempts were significantly associated with higher likelihood of each of the 12 forms of diet modification efforts among male participants, and 10 of the diet modification efforts among female participants. Notably, this included higher likelihood of efforts to consume more calories (males: adjusted prevalence ratio [aPR] 3.25, 95% confidence interval [CI] 2.94-3.59; females: aPR 4.05, 95% CI 3.50-4.70) and fats (males: aPR 2.71, 95% CI 2.42-3.03; females: aPR 3.03, 95% CI 2.58-3.55).

CONCLUSIONS

Overall, the patterns of association between weight gain attempts and diet modification efforts may be indicative of the phenomenon of muscularity-oriented eating behaviors. Findings further highlight the types of foods and nutrients adults from five countries may try to consume in attempts to gain weight.

Natriuresis During an Acute Intravenous Sodium Chloride Infusion in Conscious Sprague Dawley Rats Is Mediated by a Blood Pressure-Independent α1-Adrenoceptor-Mediated Mechanism

The mechanisms that sense alterations in total body sodium content to facilitate sodium homeostasis in response to an acute sodium challenge that does not increase blood pressure have not been fully elucidated. We hypothesized that the renal sympathetic nerves are critical to mediate natriuresis via α₁- or β-adrenoceptors signal transduction pathways to maintain sodium balance in the face of acute increases in total body sodium content that do not activate the pressure-natriuresis mechanism. To address this hypothesis, we used acute bilateral renal denervation (RDNX), an anteroventral third ventricle (AV3V) lesion and α₁- or β-antagonism during an acute 1M NaCl sodium challenge in conscious male Sprague Dawley rats. An acute 1M NaCl infusion did not alter blood pressure and evoked profound natriuresis and sympathoinhibition. Acute bilateral RDNX attenuated the natriuretic and sympathoinhibitory responses evoked by a 1M NaCl infusion [peak natriuresis (μeq/min) sham 14.5 ± 1.3 vs. acute RDNX: 9.2 ± 1.4, p < 0.05; plasma NE (nmol/L) sham control: 44 ± 4 vs. sham 1M NaCl infusion 11 ± 2, p < 0.05; acute RDNX control: 42 ± 6 vs. acute RDNX 1M NaCl infusion 25 ± 3, p < 0.05]. In contrast, an AV3V lesion did not impact the cardiovascular, renal excretory or sympathoinhibitory responses to an acute 1M NaCl infusion. Acute i.v. α₁-adrenoceptor antagonism with terazosin evoked a significant drop in baseline blood pressure and significantly attenuated the natriuretic response to a 1M NaCl load [peak natriuresis (μeq/min) saline 17.2 ± 1.4 vs. i.v. terazosin 7.8 ± 2.5, p < 0.05]. In contrast, acute β-adrenoceptor antagonism with i.v. propranolol infusion did not impact the cardiovascular or renal excretory responses to an acute 1M NaCl infusion. Critically, the natriuretic response to an acute 1M NaCl infusion was significantly blunted in rats receiving a s.c. infusion of the α₁-adrenoceptor antagonist terazosin at a dose that did not lower baseline blood pressure [peak natriuresis (μeq/min) sc saline: 18 ± 1 vs. sc terazosin 7 ± 2, p < 0.05]. Additionally, a s.c. infusion of the α₁-adrenoceptor antagonist terazosin further attenuated the natriuretic response to a 1M NaCl infusion in acutely RDNX animals. Collectively these data indicate a specific role of a blood pressure-independent renal sympathetic nerve-dependent α₁-adrenoceptor-mediated pathway in the natriuretic and sympathoinhibitory responses evoked by acute increases in total body sodium.

Further evidence against the role renal medullary perfusion in short-term control of arterial pressure in normotensive and mildly or overtly hypertensive rats

Earlier evidence from studies of rat hypertension models undermines the widespread view that the rate of renal medullary blood flow (MBF) is critical in control of arterial pressure (MAP). Here, we examined the role of MBF in rats that were normotensive, with modest short-lasting pressure elevation, or with overt established hypertension. The groups studied were anaesthetised Sprague-Dawley rats: (1) normotensive, (2) with acute i.v. norepinephrine-induced MAP elevation, and (3) with hypertension induced by unilateral nephrectomy followed by administration of deoxycorticosterone-acetate (DOCA) and 1% NaCl drinking fluid for 3 weeks. MBF was measured (laser-Doppler probe) and selectively increased using 4-h renal medullary infusion of bradykinin. MAP, renal excretion parameters and post-experiment medullary tissue osmolality and sodium concentration were determined. In the three experimental groups, baseline MAP was 117, 151 and 171 mmHg, respectively. Intramedullary bradykinin increased MBF by 45%, 65% and 70%, respectively, but this was not associated with a change in MAP. In normotensive rats a significant decrease in medullary tissue sodium was seen. The intramedullary bradykinin specifically increased renal excretion of water, sodium and total solutes in norepinephrine-treated rats but not in the two other groups. As previously shown in models of rat hypertension, in the normotensive rats and those with acute mild pressure elevation (resembling labile borderline human hypertension), 4-h renal medullary hyperperfusion failed to decrease MAP. Nor did it decrease in DOCA-salt model mimicking low-renin human hypertension. Evidently, within the 4-h observation, medullary perfusion was not a critical determinant of MAP in normotensive and hypertensive rats.

Altered renal medullary blood flow: A key factor or a parallel event in control of sodium excretion and blood pressure?

In the context of the ongoing debate on the mechanism of blood pressure (BP) regulation and pathophysiology of arterial hypertension ("renocentric" vs "neural" concepts), attention is focused on the putative regulatory role of changes in renal medullary blood flow (MBF). Experimental evidence is analysed with regard to the question whether an elevation of BP and renal perfusion pressure (RPP) is likely to increase MBF due to its impaired autoregulation. It is concluded that such increases have been clearly documented only in rats with extracellular fluid volume expansion. A possible translation of this finding to BP regulation in health and hypertension in humans may only be a matter of speculation. Within the "renocentric" theory, the key event leading to restoration of initial BP level is pressure natriuresis. Its relation to elevation of renal interstitial hydrostatic pressure and to the phenomenon of "wash-out" of renal medullary solutes by increasing MBF is discussed. We also assessed the validity of data supporting the putative mechanism of short-term restoration of elevated BP owing to the release of a vasodilator lipid (medullipin) by the medulla. The structure of the proposed medullary lipid is still undefined, and there is no sound evidence on its mediatory role in lowering elevated BP level. In conclusion, MBF change can hardly be regarded as a crucial event in the regulation of BP: it can be involved in the control of sodium excretion and BP only in some circumstances, although its contributory role cannot be excluded.

The dopamine receptor D5 gene shows signs of independent erosion in toothed and baleen whales

To compare gene loci considering a phylogenetic framework is a promising approach to uncover the genetic basis of human diseases. Imbalance of dopaminergic systems is suspected to underlie some emerging neurological disorders. The physiological functions of dopamine are transduced via G-protein-coupled receptors, including DRD₅ which displays a relatively higher affinity toward dopamine. Importantly, DRD₅ knockout mice are hypertense, a condition emerging from an increase in sympathetic tone. We investigated the evolution of DRD₅, a high affinity receptor for dopamine, in mammals. Surprisingly, among 124 investigated mammalian genomes, we found that Cetacea lineages (Mysticeti and Odontoceti) have independently lost this gene, as well as the burrowing Chrysochloris asiatica (Cape golden mole). We suggest that DRD₅ inactivation parallels hypoxia-induced adaptations, such as peripheral vasoconstriction required for deep-diving in Cetacea, in accordance with the convergent evolution of vasoconstrictor genes in hypoxia-exposed animals. Our findings indicate that Cetacea are natural knockouts for DRD₅ and might offer valuable insights into the mechanisms of some forms of vasoconstriction responses and hypertension in humans.

Does mean arterial blood pressure scale with body mass in mammals? Effects of measurement of blood pressure

For at least the last 30 years, it has been discussed whether mean arterial blood pressure (MAP) is independent of body mass or whether it increases in accordance with the vertical height between the heart and the brain. The debate has centred on the most appropriate mathematical models for analysing allometric scaling and phylogenetic relationships; there has been previously little focus on evaluating the validity of underlying physiological data. Currently, the 2 most comprehensive scaling analyses are based on data from 47 species of mammals, based on 114 references. We reviewed all available references to determine under which physiological conditions MAP had been recorded. In 44 (38.6%) of the cited references, MAP was measured in anaesthetized animals. Data from conscious animals were reported in 59 (51.8%) of references; of these, 3 (2.6%) were radiotelemetric studies. In 5 species, data were reported from both anaesthetized and conscious animals, and the mean difference in the MAP between these settings was 20 ± 29 mm Hg. From a literature search, we identified MAP measurements performed by radiotelemetry in 11 of the 47 species included in the meta-analyses. A Bland-Altman analysis showed a bias of 1 mm Hg with 95% confidence interval (from -35 to 36 mm Hg); that is, the limits of agreement between radiotelemetric studies and studies in restrained animals were double the supposed difference in the MAP between the mouse and elephant. In conclusion, the existing literature does not provide evidence for either a positive or neutral scaling of arterial pressure to body mass across taxa.

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.

Logical Issues With the Pressure Natriuresis Theory of Chronic Hypertension

The term "abnormal pressure natriuresis" refers to a subnormal effect of a given level of blood pressure (BP) on sodium excretion. It is widely believed that abnormal pressure natriuresis causes an initial increase in BP to be sustained. We refer to this view as the "pressure natriuresis theory of chronic hypertension." The proponents of the theory contend that all forms of chronic hypertension are sustained by abnormal pressure natriuresis, irrespective of how hypertension is initiated. This theory would appear to follow from "the three laws of long-term arterial pressure regulation" stated by Guyton and Coleman more than 3 decades ago. These "laws" articulate the concept that for a given level of salt intake, the relationship between arterial pressure and sodium excretion determines the chronic level of BP. Here, we review and examine the recent assertion by Beard that these "laws" of long-term BP control amount to nothing more than a series of tautologies. Our analysis supports Beard's assertion, and also indicates that contemporary investigators often use tautological reasoning in support of the pressure natriuresis theory of chronic hypertension. Although the theory itself is not a tautology, it does not appear to be testable because it holds that abnormal pressure natriuresis causes salt-induced hypertension to be sustained through abnormal increases in cardiac output that are too small to be detected.

How Do Antihypertensive Drugs Work? Insights from Studies of the Renal Regulation of Arterial Blood Pressure

Though antihypertensive drugs have been in use for many decades, the mechanisms by which they act chronically to reduce blood pressure remain unclear. Over long periods, mean arterial blood pressure must match the perfusion pressure necessary for the kidney to achieve its role in eliminating the daily intake of salt and water. It follows that the kidney is the most likely target for the action of most effective antihypertensive agents used chronically in clinical practice today. Here we review the long-term renal actions of antihypertensive agents in human studies and find three different mechanisms of action for the drugs investigated. (i) Selective vasodilatation of the renal afferent arteriole (prazosin, indoramin, clonidine, moxonidine, α-methyldopa, some Ca(++)-channel blockers, angiotensin-receptor blockers, atenolol, metoprolol, bisoprolol, labetolol, hydrochlorothiazide, and furosemide). (ii) Inhibition of tubular solute reabsorption (propranolol, nadolol, oxprenolol, and indapamide). (iii) A combination of these first two mechanisms (amlodipine, nifedipine and ACE-inhibitors). These findings provide insights into the actions of antihypertensive drugs, and challenge misconceptions about the mechanisms underlying the therapeutic efficacy of many of the agents.

High maternal sodium intake alters sex-specific renal renin-angiotensin system components in newborn Wistar offspring

This study aimed to evaluate the systemic and renal renin-angiotensin-aldosterone system (RAAS) at birth in male and female offspring and in mothers fed a high sodium diet (HSD) before and during gestation. Female Wistar rats were fed a HSD (8.0% NaCl) or a normal sodium diet (1.3% NaCl) from 8 weeks of age until delivery of their first litter. Maternal body weight, tail blood pressure, and food and water intake were evaluated. The litter sizes were assessed, and the body and kidney weights of the offspring were measured. Both mothers and offspring were euthanized immediately following the birth of the pups to evaluate plasma renin activity (PRA), renal renin content (RRC), renal angiotensin-converting enzyme (ACE) activity, renal angiotensin (Ang) II content, serum aldosterone (ALDO) levels, and renal cortical and medullary renin messenger RNA expression. In mothers in the HSD group, water intake and kidney mass were higher, whereas renal ACE activity, Ang II, PRA, ALDO and RRC were decreased. In the offspring of HSD-fed dams, the body and kidney mass were lower in both genders, renal ACE activity was lower in females and renal Ang II was lower in males. PRA, RRC, renin gene expression and ALDO levels did not differ between the groups of offspring. The data presented herein showed that a maternal HSD during pregnancy induces low birth weight and a sex-specific response in the RAAS in offspring.

Role of the kidney in the pathogenesis of hypertension: time for a neo-Guytonian paradigm or a paradigm shift?

The "Guytonian paradigm" places the direct effect of arterial pressure, on renal excretion of salt and water, at the center of long-term control of blood pressure, and thus the pathogenesis of hypertension. It originated in the sixties and remains influential within the field of hypertension research. However, the concept of one central long-term feedback loop, through which arterial pressure is maintained by its influence on renal function, has been questioned. Furthermore, some concepts in the paradigm are undermined by experimental observations. For example, volume retention and increased cardiac output induced by high salt intake do not necessarily lead to increased arterial pressure. Indeed, in multiple models of salt-sensitive hypertension the major abnormality appears to be failure of the vasodilator response to increased cardiac output, seen in salt-resistant animals, rather than an increase in cardiac output itself. There is also evidence that renal control of extracellular fluid volume is driven chiefly by volume-dependent neurohumoral control mechanisms rather than through direct or indirect effects of changes in arterial pressure, compatible with the concept that renal sodium excretion is controlled by parallel actions of different feedback systems, including hormones, reflexes, and renal arterial pressure. Moreover, we still do not fully understand the sequence of events underlying the phenomenon of "whole body autoregulation." Thus the events by which volume retention may develop to hypertension characterized by increased peripheral resistance remain enigmatic. Finally, by definition, animal models of hypertension are not "essential hypertension;" progress in our understanding of essential hypertension depends on new results on system functions in patients.

Brain Gαi 2 -subunit proteins and the prevention of salt sensitive hypertension

To counter the development of salt-sensitive hypertension, multiple brain G-protein-coupled receptor (GPCR) systems are activated to facilitate sympathoinhibition, sodium homeostasis, and normotension. Currently there is a paucity of knowledge regarding the role of down-stream GPCR-activated Gα-subunit proteins in these critically important physiological regulatory responses required for long-term blood pressure regulation. We have determined that brain Gαi₂-proteins mediate natriuretic and sympathoinhibitory responses produced by acute pharmacological (exogenous central nociceptin/orphanin FQ receptor (NOP) and α₂-adrenoceptor activation) and physiological challenges to sodium homeostasis (intravenous volume expansion and 1 M sodium load) in conscious Sprague–Dawley rats. We have demonstrated that in salt-resistant rat phenotypes, high dietary salt intake evokes site-specific up-regulation of hypothalamic paraventricular nucleus (PVN) Gαi₂-proteins. Further, we established that PVN Gαi₂ protein up-regulation prevents the development of renal nerve-dependent sympathetically mediated salt-sensitive hypertension in Sprague–Dawley and Dahl salt-resistant rats. Additionally, failure to up-regulate PVN Gαi₂ proteins during high salt-intake contributes to the pathophysiology of Dahl salt-sensitive (DSS) hypertension. Collectively, our data demonstrate that brain, and likely PVN specific, Gαi₂ protein pathways represent a central molecular pathway mediating sympathoinhibitory renal-nerve dependent responses evoked to maintain sodium homeostasis and a salt-resistant phenotype. Further, impairment of this endogenous “anti-hypertensive” mechanism contributes to the pathophysiology of salt-sensitive hypertension.

Renal extraction and acute effects of glucagon-like peptide-1 on central and renal hemodynamics in healthy men

The present experiments were performed to elucidate the acute effects of intravenous infusion of glucagon-like peptide (GLP)-1 on central and renal hemodynamics in healthy men. Seven healthy middle-aged men were examined on two different occasions in random order. During a 3-h infusion of either GLP-1 (1.5 pmol·kg⁻¹·min⁻¹) or saline, cardiac output was estimated noninvasively, and intraarterial blood pressure and heart rate were measured continuously. Renal plasma flow, glomerular filtration rate, and uptake/release of hormones and ions were measured by Fick's Principle after catheterization of a renal vein. Subjects remained supine during the experiments. During GLP-1 infusion, both systolic blood pressure and arterial pulse pressure increased by 5±1 mmHg (P=0.015 and P=0.002, respectively). Heart rate increased by 5±1 beats/min (P=0.005), and cardiac output increased by 18% (P=0.016). Renal plasma flow and glomerular filtration rate as well as the clearance of Na⁺ and Li⁺ were not affected by GLP-1. However, plasma renin activity decreased (P=0.037), whereas plasma levels of atrial natriuretic peptide were unaffected. Renal extraction of intact GLP-1 was 43% (P<0.001), whereas 60% of the primary metabolite GLP-1 9-36amide was extracted (P=0.017). In humans, an acute intravenous administration of GLP-1 leads to increased cardiac output due to a simultaneous increase in stroke volume and heart rate, whereas no effect on renal hemodynamics could be demonstrated despite significant extraction of both the intact hormone and its primary metabolite.

A mathematical model of salt-sensitive hypertension: the neurogenic hypothesis

Salt sensitivity of arterial pressure (salt-sensitive hypertension) is a serious global health issue. The causes of salt-sensitive hypertension are extremely complex and mathematical models can elucidate potential mechanisms that are experimentally inaccessible. Until recently, the only mathematical model for long-term control of arterial pressure was the model of Guyton and Coleman; referred to as the G-C model. The core of this model is the assumption that sodium excretion is driven by renal perfusion pressure, the so-called 'renal function curve'. Thus, the G-C model dictates that all forms of hypertension are due to a primary shift of the renal function curve to a higher operating pressure. However, several recent experimental studies in a model of hypertension produced by the combination of a high salt intake and administration of angiotensin II, the AngII-salt model, are inconsistent with the G-C model. We developed a new mathematical model that does not limit the cause of salt-sensitive hypertension solely to primary renal dysfunction. The model is the first known mathematical counterexample to the assumption that all salt-sensitive forms of hypertension require a primary shift of renal function: we show that in at least one salt-sensitive form of hypertension the requirement is not necessary. We will refer to this computational model as the 'neurogenic model'. In this Symposium Review we discuss how, despite fundamental differences between the G-C model and the neurogenic model regarding mechanisms regulating sodium excretion and vascular resistance, they generate similar haemodynamic profiles of AngII-salt hypertension. In addition, the steady-state relationships between arterial pressure and sodium excretion, a correlation that is often erroneously presented as the 'renal function curve', are also similar in both models. Our findings suggest that salt-sensitive hypertension is not due solely to renal dysfunction, as predicted by the G-C model, but may also result from neurogenic dysfunction.

Dominant factors that govern pressure natriuresis in diuresis and antidiuresis: a mathematical model

We have developed a whole kidney model of the urine concentrating mechanism and renal autoregulation. The model represents the tubuloglomerular feedback (TGF) and myogenic mechanisms, which together affect the resistance of the afferent arteriole and thus glomerular filtration rate. TGF is activated by fluctuations in macula densa [Cl(-)] and the myogefnic mechanism by changes in hydrostatic pressure. The model was used to investigate the relative contributions of medullary blood flow autoregulation and inhibition of transport in the proximal convoluted tubule to pressure natriuresis in both diuresis and antidiuresis. The model predicts that medullary blood flow autoregulation, which only affects the interstitial solute composition in the model, has negligible influence on the rate of NaCl excretion. However, it exerts a significant effect on urine flow, particularly in the antidiuretic kidney. This suggests that interstitial washout has significant implications for the maintenance of hydration status but little direct bearing on salt excretion, and that medullary blood flow may only play a signaling role for stimulating a pressure-natriuresis response. Inhibited reabsorption in the model proximal convoluted tubule is capable of driving pressure natriuresis when the known actions of vasopressin on the collecting duct epithelium are taken into account.

Brain Gαi2-subunit protein-gated pathways are required to mediate the centrally evoked sympathoinhibitory mechanisms activated to maintain sodium homeostasis

OBJECTIVE

We have previously demonstrated a role of GPCR-activated brain Gαi(2)-subunit protein-gated pathways in the natriuretic responses evoked by exogenous central α(2)-adrenoceptor activation and acute intravenous (i.v.) volume expansion in vivo. Our objective was to examine the role of brain Gαi(2) proteins in the integrated neural-humoral responses evoked by i.v. isovolumetric sodium loading, which does not alter mean arterial blood pressure or total blood volume, to maintain sodium homeostasis in conscious Sprague-Dawley rats.

METHODS

Intact or chronic bilateral renal denervated (RDNX) rats were pretreated intracerebroventricularly (i.c.v.) with a scrambled or Gαi(2) oligodeoxynucleotide to selectively downregulate brain Gαi(2) proteins. On the day of study, an i.v. isovolumetric sodium load (1 mol/l NaCl) was administered.

RESULTS

In naive and scrambled oligodeoxynucleotide groups, i.v. sodium loading evoked profound natriuresis, suppression of plasma renin activity (PRA) and global sympathoinhibition. Prior downregulation of brain Gαi(2) proteins significantly attenuated the natriuretic response [peak ΔUNaV (μeq/μl); scrambled 22 ± 2 vs. Gαi(2) 13 ± 2, P < 0.05] and abolished the sympathoinhibitory response [peak Δplasma norepinephrine (% control); SCR -72 ± 8 vs. Gαi(2) -7 ± 5, P < 0.05] without attenuating PRA suppression to sodium loading. In RDNX rats, Gαi(2) oligodeoxynucleotide pretreatment failed to attenuate the natriuretic response [peak ΔUNaV (μeq/μl); RDNX and scrambled 19 ± 3 vs. RDNX and Gαi(2) 20 ± 2] and only partially prevented the sympathoinhibitory response to i.v. sodium loading.

CONCLUSION

These studies reveal a brain Gαi(2)-subunit protein-mediated (renin-angiotensin system-independent) sympathoinhibitory pathway that has a critical role in the central neural mechanisms activated to maintain fluid and electrolyte homeostasis.

Systemic, renal, and colonic effects of intravenous and enteral rehydration in horses

BACKGROUND

Intravenous (IV) and intragastric (IG) administration of fluid therapy are commonly used in equine practice, but there are limited data on the systemic, renal, and enteric effects.

HYPOTHESIS

IV fluid administration will increase intestinal and fecal hydration in a rate-dependent manner after hypertonic dehydration, but will be associated with significant urinary water and electrolyte loss. Equivalent volumes of IG plain water will result in comparatively greater intestinal hydration with less renal loss.

ANIMALS

Six Thoroughbred geldings.

METHODS

Experimental study. 6 by 6 Latin square design investigating constant rate IV administration at 50, 100, and 150 mL/kg/d over 24 hours in horses dehydrated by water deprivation. Equivalent volumes of IG plain water were administered by 4 bolus doses over 24 hours.

RESULTS

Water deprivation resulted in a significant decrease in the percentage of fecal water, and increases in serum and urine osmolality. IV fluids administered at 100 and 150 mL/kg/d restored fecal hydration, but increasing the rate from 100 to 150 mL/kg/d did not confer any additional intestinal benefit, but did result in significantly greater urine production and sodium loss. Equivalent 24-hour volumes of plain water resulted in greater intestinal water and less urine output.

CONCLUSIONS AND CLINICAL IMPORTANCE

IV polyionic isotonic fluids can be used to hydrate intestinal contents in situations where enteral fluids are impractical. IV fluids administered at three times maintenance are no more efficacious and might be associated with adverse physiological findings after withdrawal. Bolus dosing of IG water can be used to restore intestinal water with minimal adverse effects.

Information coding in vasopressin neurons--the role of asynchronous bistable burst firing

The task of the vasopressin system is homeostasis, a type of process which is fundamental to the brain's regulation of the body, exists in many different systems, and is vital to health and survival. Many illnesses are related to the dysfunction of homeostatic systems, including high blood pressure, obesity and diabetes. Beyond the vasopressin system's own importance, in regulating osmotic pressure, it presents an accessible model where we can learn how the features of homeostatic systems generally relate to their function, and potentially develop treatments. The vasopressin system is an important model system in neuroscience because it presents an accessible system in which to investigate the function and importance of, for example, dendritic release and burst firing, both of which are found in many systems of the brain. We have only recently begun to understand the contribution of dendritic release to neuronal function and information processing. Burst firing has most commonly been associated with rhythm generation; in this system it clearly plays a different role, still to be understood fully.

Oxidative stress and neuronal NOS activity: putative determinants of rapid blood pressure increase after renal denervation in anesthetized rats

Long-term effects of renal denervation (DNX) commonly include a decrease in blood pressure (BP), observed in both normotensive animals and various models of hypertension. On the other hand, short term BP responses vary. We examined how post-DNX increase in BP observed in this study depends on baseline metabolic and functional status of animals, with a special interest for the role of oxidative stress. Anesthetized Wistar rats on standard (STD), low-sodium (LS) or high-sodium (HS) diet were used, untreated or pre-treated with tempol, a superoxide scavenger, or N(omega)-propyl-L-arginine (L-NPA), an inhibitor of neuronal NOS (nNOS). Early BP and renal hemodynamic responses were examined to right- and then left-side DNX performed using an own relatively non-invasive technique. Left kidney cortical, outer- and inner-medullary blood flows (CBF, OMBF, IMBF) were continuously recorded as laser-Doppler fluxes. Sequential denervations significantly increased BP to final 19 %, 12 %, and 6 % above control level in HS, LS, and STD groups, respectively. CBF, a measure of total renal perfusion, increased in LS and STD but not in HS rats. Tempol pretreatment prevented the post-denervation BP increase on each diet. Selective inhibition of nNOS prevented BP increase in STD and HS groups, a modest increase persisted in LS rats. We propose that enhanced afferent impulsation from intrarenal chemoreceptors related to oxidative stress in the kidney was the background for acute BP increase after DNX. The response was triggered by a release of brain sympatho-excitatory centers from inhibition by renal afferents, this was followed by widespread sympathetic cardiovascular stimulation.

Gastrin induces sodium-hydrogen exchanger 3 phosphorylation and mTOR activation via a phosphoinositide 3-kinase-/protein kinase C-dependent but AKT-independent pathway in renal proximal tubule cells derived from a normotensive male human

Gastrin is natriuretic, but its renal molecular targets and signal transduction pathways are not fully known. In this study, we confirmed the existence of CCKBR (a gastrin receptor) in male human renal proximal tubule cells and discovered that gastrin induced S6 phosphorylation, a downstream component of the phosphatidylinositol 3 kinase (PI3 kinase)-mammalian target of rapamycin pathway. Gastrin also increased the phosphorylation of sodium-hydrogen exchanger 3 (NHE3) at serine 552, caused its internalization, and decreased its expression at the cell surface and NHE activity. The phosphorylation of NHE3 and S6 was dependent on PI3 kinases because it was blocked by 2 different PI3-kinase inhibitors, wortmannin and LY294,002. The phosphorylation of NHE3 and S6 was not affected by the protein kinase A inhibitor H-89 but was blocked by a pan-PKC (chelerythrine) and a conventional PKC (cPKC) inhibitor (Gö6976) (10 μM) and an intracellular calcium chelator, 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester, suggesting the importance of cPKC and intracellular calcium in the gastrin signaling pathway. The cPKC involved was probably PKCα because it was phosphorylated by gastrin. The gastrin-mediated phosphorylation of NHE3, S6, and PKCα was via phospholipase C because it was blocked by a phospholipase C inhibitor, U73122 (10 μM). The phosphorylation (activation) of AKT, which is usually upstream of mammalian target of rapamycin in the classic PI3 kinase-AKT-p70S6K signaling pathway, was not affected, suggesting that the gastrin-induced phosphorylation of NHE3 and S6 is dependent on both PI3 kinase and PKCα but not AKT.

A new conceptual paradigm for the haemodynamics of salt-sensitive hypertension: a mathematical modelling approach

A conceptually novel mathematical model of neurogenic angiotensin II-salt hypertension is developed and analysed. The model consists of a lumped parameter circulatory model with two parallel vascular beds; two distinct control mechanisms for both natriuresis and arterial resistances can be implemented, resulting in four versions of the model. In contrast with the classical Guyton-Coleman model (GC model) of hypertension, in the standard version of our new model natriuresis is assumed to be independent of arterial pressure and instead driven solely by sodium intake; arterial resistances are driven by increased sympathetic nervous system activity in response to the elevated plasma angiotensin II and increased salt intake (AngII-salt). We compare the standard version of our new model against a simplified Guyton-Coleman model in which natriuresis is a function of arterial pressure via the pressure-natriuresis mechanism, and arterial resistances are controlled via the whole-body autoregulation mechanism. We show that the simplified GC model and the new model correctly predict haemodynamic and renal excretory responses to induced changes in angiotensin II and sodium inputs. Importantly, the new model reproduces the pressure-natriuresis relationship--the correlation between arterial pressure and sodium excretion--despite the assumption of pressure-independent natriuresis. These results show that our model provides a conceptually new alternative to Guyton's theory without contradicting observed haemodynamic changes or pressure-natriuresis relationships. Furthermore, the new model supports the view that hypertension need not necessarily have a renal aetiology and that long-term arterial pressure could be determined by sympathetic nervous system activity without involving the renal sympathetic nerves.

Experimental selective elevation of renal medullary blood flow in hypertensive rats: evidence against short-term hypotensive effect

AIM

Renal medullary blood flow (MBF) can be selectively increased by intrarenal or systemic infusion of bradykinin (Bk) in anaesthetized normotensive rats. We reproduced this effect in a number of rat models of arterial hypertension and examined whether increased perfusion of the renal medulla can cause a short-term decrease in blood pressure (BP) that is not mediated by increased renal excretion and depletion of body fluids.

METHODS

In uninephrectomized Sprague-Dawley rats, BP was elevated to approx. 145 mmHg by acute i.v. infusion of noradrenaline (NA) or angiotensin II (Ang II) (groups 1, 2), 2-week exposure to high-salt diet (3), high-salt diet + chronic low-dose infusion of Ang II using osmotic minipumps (4) or chronic high-dose Ang II infusion on normal diet (5). Uninephrectomized spontaneous hypertensive rats (SHR) were also examined (6,7). To selectively increase medullary perfusion, in anaesthetized rats, bradykinin was infused during 30-75 min into the renal medullary interstitium or intravenously.

RESULTS AND CONCLUSION

Bradykinin increased outer- and inner-medullary blood flow (laser-Doppler fluxes) by 10-20% in groups (1, 2), by 30-50% in groups (3, 4, 5) and approx. 20% in SHR (6, 7). The concurrent increase in total renal blood flow (Transonic probe) was < 3%. A minor (<3%) decrease in BP was seen only in rats acutely rendered hypertensive by NA or Ang II infusions; however, the decreases in BP and increases in medullary perfusion were not correlated. Thus, there was no evidence that in hypertensive rats, substantial selective increases in medullary perfusion can cause a short-term decrease in BP.

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.

Proof of principle: hydration by low-osmolar mannitol-glucose solution alleviates undesirable renal effects of an iso-osmolar contrast medium in rats

OBJECTIVE

Saline infusion is widely used to prevent contrast media (CM)-induced acute kidney injury, because it fosters diuresis. Osmodiuretics have a stronger diuretic effect than saline, yet previous trials indicate that osmodiuretic mannitol tends to promote rather than to prevent CM-induced acute kidney injury. However, these studies used hypertonic mannitol solutions that will result in rebound volume contraction. We hypothesize that combining the osmodiuretic effects of a nonhypertonic mannitol solution with sustained volume expansion alleviates undesirable renal effects of CM.

MATERIALS AND METHODS

Forty-four anesthetized rats were studied by 4 protocols. Urine flow rate, urine viscosity, and glomerular filtration rate (GFR) were measured. Intravenous infusions of hydration solutions were initiated 60 minutes before CM administration and continued throughout the observation period. Hydration by a 3.2% mannitol and 3.2% glucose solution infused at 12 mL/kg per hour (Mannit-Gluc regimen) was compared with a standard regimen of isotonic saline at 4 mL/kg per hour (NaCl regimen); greater infusion rates are required for the Mannit-Gluc regimen because of the profound diuretic effect of mannitol. Two CM were studied: iso-osmolar iodixanol (320 mg I/mL) and low-osmolar iopromide (370 mg I/mL), they were administered as 1.5-mL bolus injection into the thoracic aorta.

RESULTS

The Mannit-Gluc regimen resulted in higher urine flow rates than the standard NaCl regimen, yet maintained a good volume status. By virtue of its stronger diuretic effect, the Mannit-Gluc regimen greatly diminished the increase in urine viscosity and completely prevented the transient decrease in GFR caused by iodixanol with the NaCl regimen. After iopromide, the differences between the hydration regimens were much less, as iopromide increased urine flow rates much more than iodixanol, thus resulting in a much smaller increase in viscosity than iodixanol and no decrease in GFR even with the NaCl regimen.

CONCLUSION

This proof of principle study shows that a hydration regimen that combines the osmodiuretic effect of a low-osmolar mannitol-glucose solution with sustained volume expansion is effective in reducing high urine viscosity and preventing GFR reduction caused by iso-osmolar iodixanol. For low-osmolar CM, the beneficial effects seem negligible, because these compounds per se exert greater osmodiuretic action.

Lowering of blood pressure by chronic suppression of central sympathetic outflow: insight from prolonged baroreflex activation

Device-based therapy for resistant hypertension by electrical activation of the carotid baroreflex is currently undergoing active clinical investigation, and initial findings from clinical trials have been published. The purpose of this mini-review is to summarize the experimental studies that have provided a conceptual understanding of the mechanisms that account for the long-term lowering of arterial pressure with baroreflex activation. The well established mechanisms mediating the role of the baroreflex in short-term regulation of arterial pressure by rapid changes in peripheral resistance and cardiac function are often extended to long-term pressure control, and the more sluggish actions of the baroreflex on renal excretory function are often not taken into consideration. However, because clinical, experimental, and theoretical evidence indicates that the kidneys play a dominant role in long-term control of arterial pressure, this review focuses on the mechanisms that link baroreflex-mediated reductions in central sympathetic outflow with increases in renal excretory function that lead to sustained reductions in arterial pressure.

Basal and postprandial serum levels of gastrin in normotensive and hypertensive adults

Gastrin is a peptide hormone, which acts not only to regulate gastric acid secretion, but also to exert physiological actions such as the regulation of sodium balance. From a case (n = 95)-control (n = 82) study in Fuyang People's Hospital, Anhui Province, China, we found that the fasting serum gastrin levels are similar in normotensive and hypertensive adults but increased to higher levels in the latter group than in the former group after a mixed meal. We suggest that gastrin is involved in the regulation of blood pressure, possibly via the regulation of sodium and water metabolism and/or renin-angiotensin-aldosterone system. However, the mechanism remains to be determined.

Exaggerated natriuresis during clamping of systemic NO supply in healthy young men

NO (nitric oxide) may be involved in fluid homoeostasis. We hypothesized that increases in NO synthesis contribute to acute, saline-induced natriuresis, which, therefore, should be blunted when NO availability is stabilized. Young men were studied during simultaneous infusions of L-NAME [NG-nitro-L-arginine methyl ester; bolus of 750 μg·kg−1 of body weight and 8.3 μg·min−1·kg−1 of body weight] and SNP (sodium nitroprusside), the latter at a rate preventing L-NAME from increasing total peripheral resistance (‘NO-clamping’). Slow volume expansion (saline, 20 μmol of NaCl·min−1·kg−1 of body weight for 3 h) was performed with and without concomitant NO-clamping. NO-clamping itself decreased RPF (renal plasma flow; P~0.02) and tended to decrease arterial blood pressure [MABP (mean arterial blood pressure)]. Volume expansion markedly decreased the plasma levels of renin, AngII (angiotensin II) and aldosterone (all P<0.001), while MABP (oscillometry), heart rate, cardiac output (impedance cardiography), RPF (by p-aminohippurate), GFR [glomerular filtration rate; by using 51Cr-labelled EDTA] and plasma [Na+] and [K+] remained constant. Volume expansion increased sodium excretion (P<0.02) at constant filtered load, but more so during NO-clamping than during control (+184% compared with 52%; P<0.0001). Urinary nitrate/nitrite excretion increased during volume expansion; plasma cGMP and plasma vasopressin were unchanged. The results demonstrate that NO-clamping augments sodium excretion in response to volume expansion at constant MABP and GFR, reduced RPF and decreased renin system activity, a response termed hypernatriuresis. The results indicate that mediator(s) other than MABP, RPF, GFR and renin system activity contribute significantly to the homoeostatic response to saline loading, but the specific mechanisms of hypernatriuresis remain obscure.

Lower vascular tone and larger plasma volume in Parkinson's disease with orthostatic hypotension

The pathophysiology of orthostatic hypotension in Parkinson's disease (PD) is incompletely understood. The primary focus has thus far been on failure of the baroreflex, a central mediated vasoconstrictor mechanism. Here, we test the role of two other possible factors: 1) a reduced peripheral vasoconstriction (which may contribute because PD includes a generalized sympathetic denervation); and 2) an inadequate plasma volume (which may explain why plasma volume expansion can manage orthostatic hypotension in PD). We included 11 PD patients with orthostatic hypotension (PD + OH), 14 PD patients without orthostatic hypotension (PD − OH), and 15 age-matched healthy controls. Leg blood flow was examined using duplex ultrasound during 60° head-up tilt. Leg vascular resistance was calculated as the arterial-venous pressure gradient divided by blood flow. In a subset of 9 PD + OH, 9 PD − OH, and 8 controls, plasma volume was determined by indicator dilution method with radiolabeled albumin (125I-HSA). The basal leg vascular resistance was significantly lower in PD + OH (0.7 ± 0.3 mmHg·ml−1·min) compared with PD − OH (1.3 ± 0.6 mmHg·ml−1·min, P < 0.01) and controls (1.3 ± 0.5 mmHg·ml−1·min, P < 0.01). Leg vascular resistance increased significantly during 60° head-up tilt with no significant difference between the groups. Plasma volume was significantly larger in PD + OH (3,869 ± 265 ml) compared with PD − OH (3,123 ± 377 ml, P < 0.01) and controls (3,204 ± 537 ml, P < 0.01). These results indicate that PD + OH have a lower basal leg vascular resistance in combination with a larger plasma volume compared with PD − OH and controls. Despite the increase in leg vascular resistance during 60° head-up tilt, PD + OH are unable to maintain their blood pressure.

Genetics of salt-sensitive hypertension

The assessment of salt sensitivity of blood pressure is difficult because of the lack of universal consensus on definition. Regardless of the variability in the definition of salt sensitivity, increased salt intake, independent of the actual level of blood pressure, is also a risk factor for cardiovascular morbidity and mortality and kidney disease. A modest reduction in salt intake results in an immediate decrease in blood pressure, with long-term beneficial consequences. However, some have suggested that dietary sodium restriction may not be beneficial to everyone. Thus, there is a need to distinguish salt-sensitive from salt-resistant individuals, but it has been difficult to do so with phenotypic studies. Therefore, there is a need to determine the genes that are involved in salt sensitivity. This review focuses on genes associated with salt sensitivity, with emphasis on the variants associated with salt sensitivity in humans that are not due to monogenic causes. Special emphasis is given to gene variants associated with salt sensitivity whose protein products interfere with cell function and increase blood pressure in transgenic mice.

Effect of low sodium intake and β-blockade on renin synthesis and secretion in mice with unilateral ureteral ligation

We previously reported that sodium depletion increased renin secretion from the normal kidney in mice. We postulated that the combined procedures of sodium depletion and β-adrenoceptor blockade would affect the activity of the renin-angiotensin system. To test this hypothesis, we investigated the interaction of low sodium intake (LSI) and propranolol (PRO) on renin synthesis and secretion. To prevent the influence of tubule flow on renin secretion, mice with a left hydronephrotic kidney were used. LSI increased plasma renin concentration (PRC) 5.6-fold in the right renal vein (P<0.01). There was no net increase of PRC in the left renal vein. Tissue renin concentration (TRC) was elevated 3.6-fold and 1.3-fold in the right and left kidneys (P<0.01), respectively. After administration of PRO, PRC decreased by 34% in the right renal vein and 47% in the aorta (P<0.05); TRC was reduced by 37.5% in the right and 29.3% in the hydronephrotic kidneys (P<0.05). The combination of LSI and PRO increased PRC 3.4-fold and 1.8-fold in the right (P<0.01) and left renal veins (P<0.05), respectively. TRC increased 3.4-fold in the right (P<0.01) but only 61% in the left kidneys (P<0.05). The pattern in change of renin mRNA levels was similar to TRC but the absolute amount was smaller. There were correlations between PRC and renin mRNA, and between TRC and renin mRNA in both kidneys (P<0.001). Thus, LSI increased renin synthesis in both kidneys. However, there was no apparent renin secretion in the hydronephrotic kidney. PRO treatment suppressed renin synthesis and renin secretion, irrespective of hydronephrosis and LSI. The macula densa is critical for renin secretion under all of the circumstances studied.

Effect of short-term high-protein compared with normal-protein diets on renal hemodynamics and associated variables in healthy young men

BACKGROUND

High-protein diets are effective for weight reduction; however, little is known about the potential adverse renal effects of such diets.

OBJECTIVE

The aim of our study was to compare the effect of a high-protein (HP) with a normal-protein (NP) diet on renal hemodynamics and selected clinical-chemical factors.

DESIGN

We prospectively studied the effect of an HP diet (2.4 g x kg(-1) x d(-1)) with that of an NP diet (1.2 g x kg(-1) x d(-1)) on the glomerular filtration rate (assessed on the basis of sinistrin-an inulin analog-clearance) and renal plasma flow (para-aminohippuric acid clearance) by using the constant infusion technique. Filtration fraction and renal vascular resistance were calculated. Twenty-four healthy young men followed the 2 diet protocols for 7 d each in a crossover design. They were individually advised by a dietitian to achieve the planned protein intake by selecting normal foods under isocaloric conditions. Serum and urinary variables and renal hemodynamics were measured on day 7 of both diets.

RESULTS

The glomerular filtration rate (NP: 125 +/- 5 mL/min; HP: 141 +/- 8 mL/min; P < 0.001) and filtration fraction (NP: 23 +/- 5%; HP: 28 +/- 5%; P < 0.05) increased significantly with the HP diet. Renal plasma flow was not significantly different between the HP (496 +/- 25 mL/min) and NP (507 +/- 18 mL/min) phases. Renal vascular resistance was not significantly different between the NP (94 +/- 6 mm Hg x mL(-1) x min(-1)) and HP (99 +/- 8 mm Hg x mL(-1) x min(-1)) phases. Blood urea nitrogen, serum uric acid, glucagon, natriuresis, urinary albumin, and urea excretion increased significantly with the HP diet.

CONCLUSIONS

A short-term HP diet alters renal hemodynamics and renal excretion of uric acid, sodium, and albumin. More attention should be paid to the potential adverse renal effects of HP diets.

Current computational models do not reveal the importance of the nervous system in long-term control of arterial pressure

Arterial pressure is regulated over long periods of time by neural, hormonal and local control mechanisms, which ultimately determine the total blood volume and how it is distributed between the various vascular compartments of the circulation. A full understanding of the complex interplay of these mechanisms can be greatly facilitated by the use of mathematical models. In 1967, Guyton and Coleman published a model for long-term control of arterial pressure that focused on renal control of body sodium and water and thus total blood volume. The central point of their model is that the long-term level of arterial pressure is determined exclusively by the 'renal function curve', which relates arterial pressure to urinary excretion of salt and water. The contribution of the sympathetic nervous system to setting the long-term level of arterial pressure in the model is limited. In light of the overwhelming evidence for a major role of the sympathetic nervous system in long-term control of arterial pressure and the pathogenesis of hypertension, new mathematical models for long-term control of arterial pressure may be necessary. Despite the prominence and general acceptance of the Guyton-Coleman model in the field of hypertension research, we argue here that it overestimates the importance of renal control of body fluids and total blood volume in blood pressure regulation. Furthermore, we suggest that it is possible to construct an alternative model in which sympathetic nervous system activity plays an important role in long-term control of arterial pressure independent of its effects on total blood volume.