Stimulation Of Renin Secretion Research Articles

Excess renin is attributed to the combination of forward and backward failure in HFrEF.

Regulation of the renin-angiotensin system (RAS) in heart failure (HF) with reduced ejection fraction (HFrEF) still raises questions, as a large proportion of patients show normal renin levels despite manifest disease. Experimental venous congestion results in reduced renal perfusion pressure and stimulates renin secretion. We hypothesized that excess renin levels are mainly a result of right ventricular failure as a sequalae of left ventricular dysfunction. The study aimed to link right ventricular function (RVF) with renin levels and to investigate further contributors to excess RAS activation. Three hundred thirty-two chronic HFrEF patients undergoing routine ambulatory care were consecutively enrolled in a prospective, registry-based, observational study. Laboratory parameters, including cardiac-specific markers renin, aldosterone, and N-terminal pro-brain natriuretic peptide (NT-proBNP), echocardiographic examination (n=247), and right heart catheterization (n=85), were documented. The relationship between renin and its respective parameters was analysed. Renin concentration was not associated with the New York Heart Association class or NT-proBNP. Systolic blood pressure, systemic vascular resistance, serum sodium, aldosterone, and lactate dehydrogenase were associated with increased renin levels (P<0.035 for all). Renin levels similarly increased with worsening of RVF parameters such as fractional area change, tricuspid annular plane systolic excursion, tissue Doppler imaging, and inferior vena cava diameter (P<0.011 for all), but not with pulmonary pressure. Excess renin levels were observed when worsening RVF was combined with reduced renal perfusion {625μIU/mL [interquartile range (IQR): 182-1761] vs. 67μIU/mL [IQR: 16-231], P<0.001}, which was associated with worse survival. While unrelated to classical indices of HF severity, circulating renin levels increase with the worsening of RVF, especially in the combined presence of forward and backward failure. This might explain normal renin levels in HFrEF patients but also excess renin levels in poor haemodynamic conditions.

JS-NG-2: LONG-TERM EFFICACY AND SAFETY OF ALISKIREN IN FOUR CASES WITH MALIGNANT HYPERTENSION

Objective: Malignant hypertension (MH) is a form of hypertensive emergency defined by a severe increase in blood pressure (BP) with multiple organ injuries. One possible pathophysiology of MH is an acute activation of the renin-angiotensin system (RAS) driven by the intense stimulation of renin secretion in the juxtaglomerulosa cells. Considering this mechanism, aliskiren, a direct renin inhibitor (DRI), can be a treatment option for MH. However, there is little information on the efficacy and safety of DRI in controlling MH. We herein report four cases with MH that have been successfully controlled by DRI for a long term. Case description: All of the four cases (three male and one female; 34 to 57 years of age) developed advanced retinopathy (III or IV grade on Keith-Wagener classification) and severe hypertension (&gt; 200/120 mmHg) at presentation, meeting the criteria for MH according to the 2020 ISH global hypertension practice guideline. Mean systolic/diastolic BP was 215 ± 5/148 ± 8 mmHg. All developed acute kidney injury, with the mean serum creatinine (sCr) of 4.47 ± 1.13 mg/dl on admission. The ranges of plasma renin activity (PRA) and plasma aldosterone concentration (PAC, as measured by RIA) were 11–33 ng/mL/hr and 371–1370 pg/mL, respectively. Followed by initial treatment with an intra-venous calcium channel blocker, these patients were prescribed aliskiren, starting at 150 mg/day once daily, after confirmation of the elevated PRA levels. In general, aliskiren was well tolerated and BP was successfully controlled in all of the four cases (at discharge, mean systolic/diastolic BP was 123/74 mmHg and mean sCr was 3.84 mg/dL). Patients continued to receive aliskiren (at a dose of 150 or 300 mg/day) after the discharge and mean systolic/diastolic BP and sCr were 131 ± 4/84 ± 3 mmHg and 2.02 ± 0.16 mg/dl at two years of follow-up, respectively. The ranges of PRA and PAC of four cases have fallen to 0.2–0.8 ng/mL/hr and 105–203 pg/mL, respectively. Conclusions: In our cases, severe hypertension and target organ damage were fairly controlled with aliskiren on acute and chronic phases of MH. In all of the four cases, PRA declined with anti-hypertensive treatment and was useful in guiding the doses of medications. Although DRI is not a first-line anti-hypertensive drug class, it may be useful in a certain form of MH with high PRA, which merits further study.

SLC26A6 and NADC‑1: Future direction of nephrolithiasis and calculus‑related hypertension research (Review).

Nephrolithiasis is the most common type of urinary system disease in developed countries, with high morbidity and recurrence rates. Nephrolithiasis is a serious health problem, which eventually leads to the loss of renal function and is closely related to hypertension. Modern medicine has adopted minimally invasive surgery for the management of kidney stones, but this does not resolve the root of the problem. Thus, nephrolithiasis remains a major public health issue, the causes of which remain largely unknown. Researchers have attempted to determine the causes and therapeutic targets of kidney stones and calculus‑related hypertension. Solute carrier family 26 member 6 (SLC26A6), a member of the well‑conserved solute carrier family 26, is highly expressed in the kidney and intestines, and it primarily mediates the transport of various anions, including OXa2‑, HCO3‑, Cl‑ and SO42‑, amongst others. Na+‑dependent dicarboxylate‑1 (NADC‑1) is the Na+‑carboxylate co‑transporter of the SLC13 gene family, which primarily mediates the co‑transport of Na+ and tricarboxylic acid cycle intermediates, such as citrate and succinate, amongst others. Studies have shown that Ca2+ oxalate kidney stones are the most prevalent type of kidney stones. Hyperoxaluria and hypocitraturia notably increase the risk of forming Ca2+ oxalate kidney stones, and the increase in succinate in the juxtaglomerular device can stimulate renin secretion and lead to hypertension. Whilst it is known that it is important to maintain the dynamic equilibrium of oxalate and citrate in the kidney, the synergistic molecular mechanisms underlying the transport of oxalate and citrate across kidney epithelial cells have undergone limited investigations. The present review examines the results from early reports studying oxalate transport and citrate transport in the kidney, describing the synergistic molecular mechanisms of SLC26A6 and NADC‑1 in the process of nephrolithiasis formation. A growing body of research has shown that nephrolithiasis is intricately associated with hypertension. Additionally, the recent investigations into the mediation of succinate via regulation of the synergistic molecular mechanism between the SLC26A6 and NADC‑1 transporters is summarized, revealing their functional role and their close association with the inositol triphosphate receptor‑binding protein to regulate blood pressure.

The D allele of angiotensin-converting enzyme gene is associated with a worse autonomic heart control in community-based older adults.

The relationship between the Renin-Angiotensin-Aldosterone System (RAAS) and modulation of the Autonomic Nervous System (ANS) is reported as complex and bidirectional. Thus, the present study aimed to compare autonomous heart control, through symbolic analysis parameters, in the older adults carrying different ACE I/D gene polymorphisms (rs4646994). Two hundred four older adults comprised the study population and were grouped according to the ACE I/D gene polymorphism: II+ID genotype and DD genotype. Successive RR intervals were recorded along 5min and analyzed with Symbolic Analysis (SA) method. 0V%, 1V% and 2V% patterns were quantified and compared between groups. A linear mixed model analysis was used to determine between-group differences in symbolic analysis parameters (0V%, 1V% and 2V%), taken groups as fixed factor and sex, use of ACE inhibitors and Angiotensin II receptor type I blockers as random factors. The Bayesian analysis was carried out to check the magnitude of the evidence. The results of this study indicate that older adults carrying the DD genotype have statistically greater sympathetic modulation (0V%) and poor sympathovagal balance (1V%), but the Bayesian analysis indicate only an anecdotal effect. Sympathetic predominance is recognized a worrying condition, since may predisposes to cardiovascular risk.

Lack of connexin 40 decreases the calcium sensitivity of renin-secreting juxtaglomerular cells.

The so-called calcium paradoxon of renin describes the phenomenon that exocytosis of renin from juxtaglomerular cells of the kidney is stimulated by lowering of the extracellular calcium concentration. The yet poorly understood effect of extracellular calcium on renin secretion appears to depend on the function of the gap junction protein connexin 40 (Cx40) in renin-producing cells. This study aimed to elucidate the role of Cx40 for the calcium dependency of renin secretion in more detail by investigating if Cx40 function is really essential for the influence of extracellular calcium on renin secretion, if and how Cx40 affects intracellular calcium dynamics in renin-secreting cells and if Cx40-mediated gap junctional coupling of renin-secreting cells with the mesangial cell area is relevant for the influence of extracellular calcium on renin secretion. Renin secretion was studied in isolated perfused mouse kidneys. Calcium measurements were performed in renin-producing cells of microdissected glomeruli. The ultrastructure of renin-secreting cells was examined by electron microscopy. We found that Cx40 was not essential for stimulation of renin secretion by lowering of the extracellular calcium concentration. Instead, Cx40 increased the sensitivity of renin secretion response towards lowering of the extracellular calcium concentration. In line, the sensitivity and dynamics of intracellular calcium in response to lowering of extracellular calcium were dampened when renin-secreting cells lacked Cx40. Disruption of gap junctional coupling of renin-secreting cells by selective deletion of Cx40 from mesangial cells, however, did not change the stimulation of renin secretion by lowering of the extracellular calcium concentration. Deletion of Cx40 from renin cells but not from mesangial cells was associated with a shift of renin expression from perivascular cells of afferent arterioles to extraglomerular mesangial cells. Our findings suggest that Cx40 is not directly involved in the regulation of renin secretion by extracellular calcium. Instead, it appears that in renin-secreting cells of the kidney lacking Cx40, intracellular calcium dynamics and therefore also renin secretion are desensitized towards changes of extracellular calcium. Whether the dampened calcium response of renin-secreting cells lacking Cx40 function results from a direct involvement of Cx40 in intracellular calcium regulation or from the cell type shift of renin expression from perivascular to mesangial cells remains to be clarified. In any case, Cx40-mediated gap junctional coupling between renin and mesangial cells is not relevant for the calcium paradoxon of renin secretion.

Analysis of the calcium paradox of renin secretion

The secretion of the protease renin from renal juxtaglomerular cells is enhanced by subnormal extracellular calcium concentrations. The mechanisms underlying this atypical effect of calcium have not yet been unraveled. We therefore aimed to characterize the effect of extracellular calcium concentration on calcium handling of juxtaglomerular cells and on renin secretion in more detail. For this purpose, we used a combination of experiments with isolated perfused mouse kidneys and direct calcium measurements in renin-secreting cells in situ. We found that lowering of the extracellular calcium concentration led to a sustained elevation of renin secretion. Electron-microscopical analysis of renin-secreting cells exposed to subnormal extracellular calcium concentrations revealed big omega-shaped structures resulting from the intracellular fusion and subsequent emptying of renin storage vesicles. The calcium concentration dependencies as well as the kinetics of changes were rather similar for renin secretion and for renovascular resistance. Since vascular resistance is fundamentally influenced by myosin light chain kinase (MLCK), myosin light chain phosphatase (MLCP), and Rho-associated protein kinase (Rho-K) activities, we examined the effects of MLCK-, MLCP-, and Rho-K inhibitors on renin secretion. Only MLCK inhibition stimulated renin secretion. Conversely, inhibition of MCLP activity lowered perfusate flow and strongly inhibited renin secretion, which could not be reversed by lowering of the extracellular calcium concentration. Renin-secreting cells and smooth muscle cells of afferent arterioles showed immunoreactivity of MLCK. These findings suggest that the inhibitory effect of calcium on renin secretion could be explained by phosphorylation-dependent processes under control of the MLCK.

Free radical scavenging reverses fructose-induced salt-sensitive hypertension.

We have previously reported that a moderate dietary supplementation of 20% fructose but not glucose leads to a salt-sensitive hypertension related to increased proximal sodium–hydrogen exchanger activity and increased renal sodium retention. We also found that while high salt increased renal nitric oxide formation, this was retarded in the presence of fructose intake. We hypothesized that at least part of the pathway leading to fructose-induced salt-sensitive hypertension could be due to fructose-induced formation of reactive oxygen species and inappropriate stimulation of renin secretion, all of which would contribute to an increase in blood pressure. We found that both 20% fructose intake and a high-salt diet stimulated 8-isoprostane excretion. The superoxide dismutase (SOD) mimetic tempol significantly reduced this elevated excretion. Next, we placed rats on a high-salt diet (4%) for 1 week in combination with normal rat chow or 20% fructose with or without chronic tempol administration. A fructose plus high-salt diet induced a rapid increase (15 mmHg) in systolic blood pressure and reversed high salt suppression of plasma renin activity. Tempol treatment reversed the pressor response and restored high salt suppression of renin. We conclude that fructose-induced salt-sensitive hypertension is driven by increased renal reactive oxygen species formation associated with salt retention and an enhanced renin–angiotensin system.

Effects of voluntary exercise on blood pressure, angiotensin II, aldosterone, and renal function in two-kidney, one-clip hypertensive rats.

Spontaneous dynamic exercise promotes sympathoinhibition and decreases arterial pressure in two-kidney, one-clip (2K-1C) hypertensive rats. Renal sympathetic nerves stimulate renin secretion and increase renal tubular sodium reabsorption. We hypothesized that daily voluntary wheel running exercise by 2K-1C rats will decrease mean arterial pressure (MAP), plasma angiotensin II (Ang II), and aldosterone as well as normalize urinary sodium and potassium excretion independent of changes in glomerular filtration rate (GFR). Five-week-old male Sprague Dawley rats underwent sham clipping (Sham) or right renal artery clipping (2K-1C). Rats were randomized to standard caging (SED) or cages with running wheels (EX). After 12 weeks, rats were assigned to either collection of aortic blood for measurement of Ang II and aldosterone or assessment of inulin clearances and excretory function. Running distances were comparable in both EX groups. MAP was lower in 2K-1C EX vs 2K-1C SED rats (P<0.05). Plasma Ang II and aldosterone were significantly higher in 2K-1C SED rats and decreased in 2K-1C EX rats to levels similar to Sham SED or Sham EX rats. Clipped kidney weights were significantly lower in both 2K-1C groups, but GFR and urine flow rates were no different from right and left kidneys among the four groups. Total and fractional sodium excretion rates from the unclipped kidney of 2K-1C SED rats were higher vs either Sham group (P<0.05). Values in 2K-1C EX rats were similar to the Sham groups. Potassium excretion paralleled sodium excretion. These studies show that voluntary dynamic exercise in 2K-1C rats decreases plasma Ang II and aldosterone, which contribute to the lower arterial pressure without deleterious effects on GFR. The effects on sodium excretion underscore the impact of pressure natriuresis despite elevated plasma Ang II and aldosterone in sedentary 2K-1C rats. In contrast, potassium excretion is primarily regulated by circulating aldosterone and distal sodium delivery.

Abstract P144: Association Between Hypertension and Thyroid Abnormalities Among Hospitalized United States Patients: Data From National Inpatient Sample

Introduction: Disorders of thyroid, hyperthyroidism and hypothyroidism, are established risk factors for hypertension. Comparison of these risk factors in terms of association has not been done. Methods: National Inpatient Sample database from 2009 - 2011 was utilized to explore relationship between hypertension and three categories of thyroid hormone levels (hyperthyroidism, hypothyroidism and euthyroidism). Multivariate logistic regressions were used to establish the association among all thyroid hormone level groups. We utilized STATA version 13.0 (College Station, TX) to perform the analyses. Results: Hypertension was more common in patients with hypothyroidism and hyperthyroidism in comparison to euthyroidism (Table 1). Multivariate analysis with logistic regression (controlled for age, sex, race, smoking, obesity, dyslipidemia, diabetes, Charlson comorbidity index) demonstrated that hypertension had a greater association with hyperthyroidism (OR 1.20; CI: 1.17 - 1.23) than hypothyroidism (OR 1.09; CI: 1.08 - 1.10). Discussion: T3 dilates peripheral arterioles and decreases systemic vascular resistance which stimulates renin secretion and increases effective arterial blood volume. Also, it increases heart rate and cardiac contractility. These mechanisms lead to increase in blood pressure. On the other hand, the mechanism with hypothyroidism is not fully recognized. It is thought to be secondary to increase in systemic vascular resistance. Through our study, we can conclude that the risk of hypertension is greater in patients with hyperthyroidism than with hypothyroidism.

TRPV4 participates in pressure-induced inhibition of renin secretion by juxtaglomerular cells.

Increase in blood pressure in the renal afferent arteriole is known to induce an increase in cytosolic calcium concentration ([Ca2+ ]i ) of juxtaglomerular (JG) cells and to result in a decreased secretion of renin. Mechanical stimulation of As4.1 JG cells induces an increase in [Ca2+ ]i that is inhibited by HC067047 and RN1734, two inhibitors of TRPV4, or by siRNA-mediated repression of TRPV4. Inhibition of TRPV4 impairs pressure-induced decrease in renin secretion. Compared to wild-type mice, Trpv4-/- mice present increased resting plasma levels of renin and aldosterone and present a significantly altered pressure-renin relationship. We suggest that TRPV4 channel participates in mechanosensation at the juxtaglomerular apparatus. The renin-angiotensin system is a crucial blood pressure regulation system. It consists of a hormonal cascade where the rate-limiting enzyme is renin, which is secreted into the blood flow by renal juxtaglomerular (JG) cells in response to low pressure in the renal afferent arteriole. In contrast, an increase in blood pressure results in a decreased renin secretion. This is accompanied by a transitory increase in [Ca2+ ]i of JG cells. The inverse relationship between [Ca2+ ]i and renin secretion has been called the 'calcium paradox' of renin release. How increased pressure induces a [Ca2+ ]i transient in JG cells, is however, unknown. We observed that [Ca2+ ]i transients induced by mechanical stimuli in JG As4.1 cells were completely abolished by HC067047 and RN1734, two inhibitors of TRPV4. They were also reduced by half by siRNA-mediated repression of TRPV4 but not after repression or inhibition of TRPV2 or Piezo1 ion channels. Interestingly, the stimulation of renin secretion by the adenylate cyclase activator forskolin was totally inhibited by cyclic stretching of the cells. This effect was mimicked by stimulation with GSK1016790A and 4αPDD, two activators of TRPV4 and inhibited in the presence of HC067047. Moreover, in isolated perfused kidneys from Trpv4-/- mice, the pressure-renin relationship was significantly altered. In vivo, Trpv4-/- mice presented increased plasma levels of renin and aldosterone compared to wild-type mice. Altogether, our results suggest that TRPV4 is involved in the pressure-induced entry of Ca2+ in JG cells, which inhibits renin release and allows the negative feedback regulation on blood pressure.

The role of renal nerves in cardiovascular and renal function in normal and pathophysiological states.

Video slideshow introduction to the Symposium by Symposium Speaker Clive May can be found here. The notion that increased sympathetic nervous system activity is important to trigger and maintain hypertension has been under consideration for decades. It is well known that an increase in renal sympathetic nerve activity (RSNA) stimulates renin secretion, which leads to renal vasoconstriction and increases sodium reabsorption (DiBona & Kopp, 1997), indicating that excessive RSNA will contribute to hypertension. Historically, surgical sympathectomy has been used to improve blood pressure control (Smithwick, 1940; Grimson, 1941). Nevertheless, this procedure was abandoned because of its high operative morbidity and mortality. Recently, however, catheter-based interventional strategies that interrupt the RSNA have shown promising results in providing a better long-term blood pressure control in patients with resistant arterial hypertension (Esler, 2010; Esler et al. 2010). These studies have raised enormous interest in understanding the role of renal nerves in the control of cardiovascular and renal functions. Nevertheless, the mechanisms underlying the initiation and maintenance of sympathoexcitation in pathophysiological conditions and how the renal denervation leads to a long-term reduction of blood pressure are not well understood. The symposium entitled ‘The Role of renal nerves in cardiovascular and renal function in normal and pathophysiological states’, which was held at The First Pan-American Congress of Physiological Sciences, Iguassu Falls, Brazil, 2–6 August 2014, focused on the new advances regarding the importance of efferent and afferent renal fibres on sodium homeostasis and the effects of renal denervation and reinnervation on cardiovascular and kidney functions. Regarding the mechanisms underlying sympathoexcitation in hypertension, the importance of increased oxidative stress in the paraventricular nucleus of hypothalamus and the rostral ventrolateral medulla for the autonomic dysfunction associated with renovascular hypertension was discussed (Campos et al. 2015). The effect of interaction between the sympathetic nervous system and angiotensin II on sodium reabsorption is a well-known classical mechanism for the maintenance of extracellular volume homeostasis; however, the underlying molecular signalling is not clearly understood. Recent data were presented in this symposium to demonstrate that renal nerve stimulation increases intrarenal angiotensin II and activates angiotensin II type 1 receptors, triggering a signalling cascade that leads to elevation of Na+–H+ exchanger isoform 3-mediated tubular transport. The findings enhance our understanding in relation to the interaction and synergism between the sympathetic nervous system and intrarenal angiotensin II actions in normal conditions (Bergamaschi et al. 2015). Additionally, the interaction between the brain and the kidneys to maintain homeostasis is still a matter of debate and was discussed in this symposium. Recent evidences suggest that increased RSNA and renal afferent nerve activity to the brain may contribute to the outcome to the disease, such as hypertension or heart failure (Nishi et al. 2014). During renovascular hypertension, increased afferent signalling from the ischaemic kidney may contribute to the increase in blood pressure. Indeed, in renovascular hypertensive rats submitted to treatment with bone marrow-derived mesenchymal stem cells, an improvement was found in the kidney tissue and function. This was accompanied by a reduction of sympathetic vasomotor activity to the kidneys and a reduction of blood pressure, probably as a consequence of reduced afferents from the diseased kidney (Oliveira-Sales et al. 2015). In another interesting recent study, the time course of the effects of renal denervation was investigated in sheep. In this study, 11 months after catheter-based radiofrequency renal denervation, functional and anatomical evidence of afferent and efferent renal nerve reinnervation were found. Therefore, the mechanism by which catheter-based renal denervation causes a prolonged decrease in blood pressure is not well understood. Further studies are required to address this important issue (Booth et al. 2015). In conclusion, the present symposium discussed new ideas about the mechanisms by which the brain and kidneys communicate and how they interact to maintain cardiovascular and renal homeostasis. Understanding the mechanisms of crosstalk between these two important organs (brain and kidneys) related to the control of blood pressure will probably increase the possibility of finding new strategies for the treatment of cardiovascular and renal diseases. Disclaimer: Supporting information has been peer-reviewed but not copyedited. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

The water channel aquaporin-1 contributes to renin cell recruitment during chronic stimulation of renin production.

Both the processing and release of secretory granules involve water movement across granule membranes. It was hypothesized that the water channel aquaporin (AQP)1 directly contributes to the recruitment of renin-positive cells in the afferent arteriole. AQP1(-/-) and AQP1(+/+) mice were fed a low-salt (LS) diet [0.004% (wt/wt) NaCl] for 7 days and given enalapril [angiotensin-converting enzyme inhibitor (ACEI), 0.1 mg/ml] in drinking water for 3 days. There were no differences in plasma renin concentration at baseline. After LS-ACEI, plasma renin concentrations increased markedly in both genotypes but was significantly lower in AQP1(-/-) mice compared with AQP1(+/+) mice. Tissue renin concentrations were higher in AQP1(-/-) mice, and renin mRNA levels were not different between genotypes. Mean arterial blood pressure was not different at baseline and during LS diet but decreased significantly in both genotypes after the addition of ACEI; the response was faster in AQP1(-/-) mice but then stabilized at a similar level. Renin release after 200 μl blood withdrawal was not different. Isoprenaline-stimulated renin release from isolated perfused kidneys did not differ between genotypes. Cortical tissue norepinephrine concentrations were lower after LS-ACEI compared with baseline with no difference between genotypes. Plasma nitrite/nitrate concentrations were unaffected by genotype and LS-ACEI. In AQP1(-/-) mice, the number of afferent arterioles with recruitment was significantly lower compared with AQP1(+/+) mice after LS-ACEI. We conclude that AQP1 is not necessary for acutely stimulated renin secretion in vivo and from isolated perfused kidneys, whereas recruitment of renin-positive cells in response to chronic stimulation is attenuated or delayed in AQP1(-/-) mice.

Control of renin secretion from kidneys with renin cell hyperplasia

In states of loss-of-function mutations of the renin-angiotensin-aldosterone system, kidneys develop a strong hyperplasia of renin-producing cells. Those additional renin cells are located outside the classic juxtaglomerular areas, mainly in the walls of preglomerular vessels and most prominently in multilayers surrounding afferent arterioles. Since the functional behavior of those ectopic renin cells is yet unknown, we aimed to characterize the control of renin secretion from kidneys with renin cell hyperplasia. As a model, we used kidneys from mice lacking aldosterone synthase (AS⁻/⁻ mice), which displayed 10-fold elevations of renin mRNA and plasma renin concentrations. On the absolute level, renin secretion from isolated AS⁻/⁻ kidneys was more than 10-fold increased over wild-type kidneys. On the relative level, the stimulation of renin secretion by the β-adrenergic activator isoproterenol or by lowering of the concentration of extracellular Ca²⁺ was very similar between the two genotypes. In addition, the inhibitory effects of ANG II and of perfusion pressure were similar between the two genotypes. Deletion of connexin40 blunted the pressure dependency of renin secretion and the stimulatory effect of low extracellular Ca²⁺ on renin secretion in the same manner in kidneys of AS⁻/⁻ mice as in wild-type mice. Our findings suggest a high degree of functional similarity between renin cells originating during development and located at different positions in the adult kidney. They also suggest a high similarity in the expression of membrane proteins relevant for the control of renin secretion, such as β₁-adrenergic receptors, ANG II type 1 receptors, and connexin40.

Convergence of major physiological stimuli for renin release on the Gs-alpha/cyclic adenosine monophosphate signaling pathway

Control of the renin system by physiological mechanisms such as the baroreceptor or the macula densa (MD) is characterized by asymmetry in that the capacity for renin secretion and expression to increase is much larger than the magnitude of the inhibitory response. The large stimulatory reserve of the renin-angiotensin system may be one of the causes for the remarkable salt-conserving power of the mammalian kidney. Physiological stimulation of renin secretion and expression relies on the activation of regulatory pathways that converge on the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) pathway. Mice with selective Gs-alpha (Gsα) deficiency in juxtaglomerular granular cells show a marked reduction of basal renin secretion, and an almost complete unresponsiveness of renin release to furosemide, hydralazine, or isoproterenol. Cyclooxygenase-2 generating prostaglandin E(2) (PGE(2)) and prostacyclin (PGI(2)) in MD and thick ascending limb cells is one of the main effector systems utilizing Gsα-coupled receptors to stimulate the renin-angiotensin system. In addition, β-adrenergic receptors are critical for the expression of high basal levels of renin and for its release response to lowering blood pressure or MD sodium chloride concentration. Nitric oxide generated by nitric oxide synthases in the MD and in endothelial cells enhances cAMP-dependent signaling by stabilizing cAMP through cyclic guanosine monophosphate-dependent inhibition of phosphodiesterase 3. The stimulation of renin secretion by drugs that inhibit angiotensin II formation or action results from the convergent activation of cAMP probably through indirect augmentation of the activity of PGE(2) and PGI(2) receptors, β-adrenergic receptors, and nitric oxide.

Stimulation of Renin Secretion by Catecholamines Is Dependent on Adenylyl Cyclases 5 and 6

The sympathetic nervous system stimulates renin release from juxtaglomerular cells via the β-adrenoreceptor-cAMP pathway. Recent in vitro studies have suggested that the calcium-inhibited adenylyl cyclases (ACs) 5 and 6 possess key roles in the control of renin exocytosis. To investigate the relative contribution of AC5 and AC6 to the regulation of renin release in vivo we performed experiments using AC5 and AC6 knockout mice. Male AC5(-/-) mice exhibited normal plasma renin concentrations, renal renin synthesis (mRNA and renin content), urinary volume, and systolic blood pressure. In male AC6(-/-) mice, plasma renin concentration (AC6(-/-): 732 ± 119; AC6 (+/+): 436 ± 78 ng of angiotensin I per hour*mL(-1); P<0.05), and renin synthesis were stimulated associated with an increased excretion of dilute urine (1.55-fold; P<0.05) and reduced blood pressure (-10.6 mm Hg; P<0.001). Stimulation of plasma renin concentration by a single injection of the β-adrenoreceptor agonist isoproterenol (10 mg/kg IP) was significantly attenuated in AC5(-/-) (male: -20%; female: -33%) compared with wild-type mice in vivo. The mitigation of the plasma renin concentration response to isoproterenol was even more pronounced in AC6(-/-) (male: -63%; female: -50% versus AC6(+/+)). Similarly, the effects of isoproterenol, prostaglandin E2, and pituitary adenylyl cyclase-activating polypeptide on renin release from isolated perfused kidneys were attenuated to a higher extent in AC6(-/-) (-51% to -98% versus AC6(+/+)) than in AC5(-/-) (-31% to 46% versus AC5(+/+)). In conclusion, both AC5 and AC6 are involved in the stimulation of renin secretion in vivo, and AC6 is the dominant isoforms in this process.

Stimulation of Renin Secretion by Angiotensin II Blockade is Gsα-Dependent

Angiotensin II converting enzyme inhibitors (ACEI) or angiotensin II receptor blockers (ARB) presumably stimulate renin secretion by interrupting angiotensin II feedback inhibition. The increase in cytosolic calcium caused by activation of Gq-coupled AT1 receptors may mediate the renin-inhibitory effect of angiotensin II at the cellular level, implying that ACEI and ARB may work by reducing intracellular calcium. Here, we investigated whether angiotensin II blockade acts predominantly through Gs-mediated stimulation of adenylyl cyclase (AC) by testing the effect of ACEI and ARB in mice with juxtaglomerular cell-specific deficiency of the AC-stimulatory Gsalpha. The ACEI captopril and quinaprilate and the ARB candesartan significantly increased plasma renin concentration (PRC) to 20 to 40 times basal PRC in wild-type mice but did not significantly alter PRC in Gsalpha-deficient mice. Captopril also completely abrogated renin stimulation in wild-type mice after co-administration of propranolol, indomethacin, and L-NAME. Treatment with enalapril and a low-NaCl diet for 7 days led to a 35-fold increase in PRC among wild-type mice but no significant change in PRC among Gsalpha-deficient mice. Three different pharmacologic inhibitors of AC reduced the stimulatory effect of captopril by 70% to 80%. In conclusion, blockade of angiotensin II stimulates renin synthesis and release indirectly through the action of ligands that activate the cAMP/PKA pathway in a Gsalpha-dependent fashion, including catecholamines, prostaglandins, and nitric oxide.

Renin Enhancer Is Crucial for Full Response in Renin Expression to an In Vivo Stimulus

We showed recently that deletion of a strong enhancer located 2.7 kb upstream of the renin gene in mice produces a strain with mild hypotension and salt-sensitivity. Here we set out to compare responses in renin expression in kidney and extrarenal tissues in these "REKO" mice. REKO and wild-type mice were placed on a low NaCl/enalapril regimen for 1 week, and then Ren-1(c) mRNA and renin enzyme activities were measured in tissues and plasma. In untreated REKO mice, renin and Ren-1(c) mRNA were reduced significantly in kidney, submandibular gland, adrenal, heart, and brain. In situ hybridization indicated a marked reduction in Ren-1(c) mRNA in juxtaglomerular cells and granular ducts of submandibular gland. After the chronic stimulus response in renal Ren-1(c) mRNA in REKO mice was blunted by 54% compared with wild-type mice, and was accompanied by almost complete exhaustion of renin stores. Response in plasma renin was blunted by 47%, this being mirrored in heart (54% decline), in which renin is derived mostly from the bloodstream. In adrenal a 55% reduction was seen. These data are consistent with inability of REKO mice to adequately replenish renal renin stores during chronic stimulation of renin secretion. In conclusion, the renin enhancer is critical for replenishment of renin stores and response in renin to a chronic in vivo stimulus.

Pituitary Adenylate Cyclase–Activating Polypeptide Stimulates Renin Secretion via Activation of PAC1 Receptors

Besides of its functional role in the nervous system, the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is involved in the regulation of cardiovascular function. Therefore, PACAP is a potent vasodilator in several vascular beds, including the renal vasculature. Because the kidney expresses both PACAP and PACAP-binding sites, it was speculated that PACAP might regulate cardiovascular function by direct vascular effects and indirectly by regulating renin release from the kidneys. PACAP (1-27) stimulated renin secretion from isolated perfused kidneys of rats 4.9-fold with a half-maximum concentration of 1.9 nmol/L. In addition, PACAP stimulated renin release and enhanced membrane capacitance of isolated juxtaglomerular cells, indicating a direct stimulation of exocytotic events. The effect of PACAP on renin release was mediated by the specific PACAP receptors (PAC1), because PACAP (1-27) applied in concentrations in the physiologic range (10 and 100 pmol/L) did not enhance renin release from isolated kidneys of PAC1 receptor knockout mice (PAC1-/-), whereas it stimulated renin release 1.38- and 2.5-fold in kidneys from wild-type mice. Moreover, plasma renin concentration was significantly lower in PAC1-/- compared with their wild-type littermates under control conditions as well as under a low- or high-salt diet and under treatment with the angiotensin-converting enzyme inhibitor ramipril, whereas no differences in plasma renin concentration between the genotypes were detectable after water deprivation. These data show that PACAP acting on PAC1 receptors potently stimulates renin release, serving as a tonic enhancer of the renin system in vivo.

Renin-angiotensin system affects endothelial morphology and permeability of renal afferent arteriole

The afferent arteriole (AA) is an important regulatory site of renal function and blood pressure. We have demonstrated endothelial fenestration and high permeability in the vicinity of renin granulated epithelioid cells in the juxtaglomerular portion of the afferent arteriole in different mammals. The permeability of fenestrated endothelium of afferent arteriole may be important in connection to various physiologic and pathophysiologic processes. We have assumed that the permeable fenestration may serve as a communication channel between the intravascular circulation and a pathway for renin secretion. Utilising the multiphoton image technique we were able to visualise the endothelial fenestration and renin granules of the in vitro microperfused AA and in vivo AA. We demonstrated that ferritin-positive, i.e., permeable portion of the afferent arteriole, under control conditions is on average 45 microm, which is about one-third to half of the total length of the afferent arteriole. The length of this portion is not constant and can change by physiologic and pharmacologic manipulation of renin formation. The permeability of the afferent arteriole is not changing only parallel with the pharmacologically stimulated renin secretion as already demonstrated in adult rats, but also with the change of renin appearance in afferent arteriole within the very first few days of life after birth. Independently from the age there is a significant correlation between the renin-positive and permeable portion of the AA. Further studies are necessary to clarify the physiological significance of afferent arteriolar permeability and its changes in the postnatal development of the kidney, as well as in correlation with activity of renin- angiotensin system.

Lack of Connexin 40 Causes Displacement of Renin-Producing Cells from Afferent Arterioles to the Extraglomerular Mesangium

In the adult kidney, renin-producing cells are typically located in the walls of afferent arterioles at the transition into the glomerular capillary network. The mechanisms that are responsible for restricting renin expression to the juxtaglomerular position are largely unknown. This study showed that in mice that lack connexin 40 (Cx40), the predominant connexin of renin-producing cells, renin-positive cells are absent in the vessel walls and instead are found in cells of the extraglomerular mesangium, glomerular tuft, and periglomerular interstitium. Blocking macula densa transport function by acute administration of loop diuretics strongly enhances renin secretion in vivo and in isolated perfused kidneys of wild-type mice. This effect of loop diuretics is markedly attenuated in vivo and even blunted in vitro in Cx40-deficient mice. Even after prolonged stimulation of renin secretion by severe sodium depletion, renin expression is not seen in juxtaglomerular cells or in cells of more proximal parts of the arterial vessel wall as occurs normally. Instead, renin remains restricted to the extra-/periglomerular interstitium in Cx40-deficient mice. In contrast to the striking displacement of renin-expressing cells in the adult kidney, renin expression in the vessels of the developing kidney was found to be normal. This is the first evidence to indicate that cell-to-cell communication via gap junctions is essential for the correct juxtaglomerular positioning and recruitment of renin-producing cells. Moreover, these findings support the notion that gap junctions are relevant for the macula densa signaling to renin-producing cells.