Inhibition Of Renin Secretion Research Articles

Abstract MP17: Angiotensin II Inhibits Renin Secretion Directly By Eliciting Robust Calcium Oscillations In Juxtaglomerular Cells

Background: Renin secretion from the juxtaglomerular (JG) cells controls circulating Angiotensin II (AngII), a critical blood pressure determinant. AngII is believed to suppress renin release from JG cells to prevent excessive blood pressure elevation. However, a direct negative feedback mechanism has yet to be established in JG cells. In JG cells, Ca 2+ is a crucial second messenger that governs renin secretion by suppressing cAMP, a potent renin secretion stimulator. Thus, in contrast to most secretory cells, renin secretion from JG cells is inversely related to the intracellular Ca 2+ level. However, the role of AngII in modulating JG cell Ca 2+ dynamics remains unclear. Objective: Define intracellular Ca 2+ responses to AngII and identify the critical channels to suppress renin secretion in JG cells. Methods: Kidney slices from mice expressing JG-specific GCaMP6f, a genetically encoded Ca 2+ indicator under the control of the Ren1 c promoter, were imaged ex vivo on a widefield fluorescent microscope for 30 min per experiment (n = 5). In each experiment, slices were continuously perfused with buffer containing variable Ca 2+ and AngII concentrations, ± specific Ca 2+ channel inhibitors. Images were captured at 10 Hz and analyzed using Mesmerize Ca 2+ imaging software to identify and plot fluorescent intensity over time for each JG cell. Additional kidney slices were incubated in 24-well plates with buffer containing vehicle or AngII ± Ca 2+ channel inhibitors for 30 min. ELISA determined renin concentration. Results: 1) AngII dose-dependently evoked robust and periodic Ca 2+ -oscillations (Ca 2+ spikes) in JG cells (mean spikes/min; 50 pM: 5.4, 300 pM: 7.7, 3 nM: 13.2, and 300 nM: 17.6). 2) AngII-elicited Ca 2+ activity was markedly reduced with “Ca 2+ -free” buffer. 3) L-type voltage-dependent Ca 2+ channel (VDCC) blocker nifedipine moderately decreased AngII-elicited Ca 2+ spikes, while T-type VDCC blocker TTA-P2 did not. 4) Blocking endoplasmic reticulum (ER) Ca 2+ -release drastically reduced Ca 2+ activity. 5) AngII dose-dependently decreased renin secretion in kidney slices, congruent with the AngII-elicited Ca 2+ activity in imaging studies. Conclusion: AngII dose-dependently and directly elicited large, periodic Ca 2+ -oscillations in JG cells that suppressed renin release. Both extracellular Ca 2+ influx and intracellular Ca 2+ release from ER stores are required for sustained Ca 2+ activity. L-type, but not T-type, Ca 2+ channels participate in JG cell Ca 2+ -oscillations.

Use of beta blockers in patients with asthma and chronic obstructive pulmonary disease

Introduction. Beta blockers play an essential role in the treatment of cardiovascular diseases, but also various other endocrinological, gastroenterological, ophthalmological and neurological disorders. The most important effects of beta blockers are a reduction in myocardial oxygen consumption and inhibition of renin secretion. Beta blockers are divided into three generations according to their selectivity - non-selective, cardioselective and vasodilating beta blockers. Beta blockers and obstructive pulmonary diseases. Patients with obstructive pulmonary diseases are significantly more likely to develop cardiovascular diseases compared to general population, largely due to common risk factors such as smoking, systemic inflammation, age, and genetic predisposition. The use of nonselective beta blockers carries a great risk for patients with obstructive pulmonary diseases, while cardioselective beta blockers can be used more extensively. Reversible airway obstruction is predominantly present in asthma, so that the adverse effects of beta blockers on the airways are significantly more pronounced in asthma compared to chronic obstructive pulmonary disease. Conclusion. In both asthma and chronic obstructive pulmonary disease, the use of highly cardioselective beta blockers such as bisoprolol and nebivolol is preferred. The use of beta blockers in patients with asthma requires great caution due to the possibility of bronchial obstruction, while in patients with chronic obstructive pulmonary disease they are somewhat safer. Patients must be closely monitored by a physician, with special attention focused on clinical signs of airway obstruction such as wheezing, shortness of breath, and prolonged expiration.

A new channel for the control of renin secretion in juxtaglomerular cells

The role of membrane channels in juxtaglomerular cell physiology is only partially understood. Pannexin 1 is a mechanosensitive, nonjunctional channel known for its role in adenosine triphosphate release. The study by DeLalio etal. documents involvement of pannexin 1 in renin secretion by studying mice deficient in pannexin 1 in renin-secreting cells and a prorenin-secreting cell line. Pannexin 1 is believed to suppress renin secretion and thereby modify blood pressure. The commentary addresses the broader physiological implication of these observations for the regulation of renin and blood pressure.

Pannexin 1 channels in renin-expressing cells influence renin secretion and blood pressure homeostasis

Kidney function and blood pressure homeostasis are regulated by purinergic signaling mechanisms. These autocrine/paracrine signaling pathways are initiated by the release of cellular ATP, which influences kidney hemodynamics and steady-state renin secretion from juxtaglomerular cells. However, the mechanism responsible for ATP release that supports tonic inputs to juxtaglomerular cells and regulates renin secretion remains unclear. Pannexin 1 (Panx1) channels localize to both afferent arterioles and juxtaglomerular cells and provide a transmembrane conduit for ATP release and ion permeability in the kidney and the vasculature. We hypothesized that Panx1 channels in renin-expressing cells regulate renin secretion invivo. Using a renin cell-specific Panx1 knockout model, we found that male Panx1 deficient mice exhibiting a heightened activation of the renin-angiotensin-aldosterone system have markedly increased plasma renin and aldosterone concentrations, and elevated mean arterial pressure with altered peripheral hemodynamics. Following ovariectomy, female mice mirrored the male phenotype. Furthermore, constitutive Panx1 channel activity was observed in As4.1 renin-secreting cells, whereby Panx1 knockdown reduced extracellular ATP accumulation, lowered basal intracellular calcium concentrations and recapitulated a hyper-secretory renin phenotype. Moreover, in response to stress stimuli that lower blood pressure, Panx1-deficient mice exhibited aberrant "renin recruitment" as evidenced by reactivation of renin expression in pre-glomerular arteriolar smooth muscle cells. Thus, renin-cell Panx1 channels suppress renin secretion and influence adaptive renin responses when blood pressure homeostasis is threatened.

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.

CGMP kinase: past, presence and future

Signaling via cGMP-dependent protein kinase is the major pathway of the NO/cGMP cascade in vascular smooth muscle (SM), heart, CNS, and other cells. cGMP-dependent protein kinases (cGK) are serine/threonine kinases that are widely distributed in eukaryotes. Two genes – prkg1 and prkg2 - code for cGKs, namely cGKI and cGKII. In mammals, two isozymes, cGKIα and cGKIβ, are generated from the prkg1 gene. The cGKI isozymes are prominent in all types of smooth muscle, platelets and specific neuronal areas such as cerebellar Purkinje cells, hippocampal neurons, and the lateral amygdala. The cGKII prevails in the secretory epithelium of the small intestine, the juxta-glomerular cells, the adrenal cortex, the chondrocytes, and in the nucleus suprachiasmaticus. Both cGKs are major down- stream effectors of many, but not all signalling events of the NO/cGMP and the ANP/BNP/CNP/cGMP pathways. cGKI relaxes smooth muscle tone and prevents platelet aggregation, whereas cGKII inhibits renin secretion, chloride/water secretion in the small intestine, the resetting of the clock during early night and endochondreal bone growth. cGKs are involved in cardiovascular and non-cardiovascular processes and are modulators of cell growth and many other functions. Recent evidence suggest that vascular smooth muscle relaxation is not affected by cGK-dependent phosphorylation of smooth muscle expressed TRPC channels, but by cGK-dependent inhibition of TRPC-6 in endothelial cells and attenuation of endothelial release of smooth muscle relaxing factors. Angiotensin increases blood pressure and induces cardiac hypertrophy. Hypertrophy is reduced in sildenafil treated wild type mice but not in beta rescue mice. This suggests that the sildenafil effects are mediated by inhibition of a cGMP hydrolysing phosphodiesterase. Restricted evidence supports the notion that sildenafil affects cGMP levels in cardiomyofibroblasts and/or in cardiac endothelial cells. It will be shown that cGK are essential modulators of many regulatory processes.

Regulation of renin secretion by renal juxtaglomerular cells

A major rate-limiting step in the renin-angiotensin-aldosterone system is the release of active renin from endocrine cells (juxtaglomerular (JG) cells) in the media layer of the afferent glomerular arterioles. The number and distribution of JG cells vary with age and the physiological level of stimulation; fetal life and chronic stimulation by extracellular volume contraction is associated with recruitment of renin-producing cells. Upon stimulation of renin release, labeled renin granules "disappear;" the number of granules decrease; cell membrane surface area increases in single cells, and release is quantal. Together, this indicates exocytosis as the predominant mode of release. JG cells release few percent of total renin content by physiological stimulation, and recruitment of renin cells is preferred to recruitment of granules during prolonged stimulation. Several endocrine and paracrine agonists, neurotransmitters, and cell swelling converge on the stimulatory cyclic AMP (cAMP) pathway. Renin secretion is attenuated in mice deficient in beta-adrenoceptors, prostaglandin E(2)-EP4 receptors, Gsα protein, and adenylyl cyclases 5 and 6. Phosphodiesterases (PDE) 3 and 4 degrade cAMP in JG cells, and PDE3 is inhibited by cyclic GMP (cGMP) and couples the cGMP pathway to the cAMP pathway. Cyclic AMP enhances K(+)-current in JG cells and is permissive for secretion by stabilizing membrane potential far from threshold that activates L-type voltage-gated calcium channels. Intracellular calcium paradoxically inhibits renin secretion likely through attenuated formation and enhanced degradation of cAMP; by activation of chloride currents and interaction with calcineurin. Connexin 40 is necessary for localization of JG cells in the vascular wall and for pressure- and macula densa-dependent suppression of renin release.

Antihypertensive Role of Tissue Kallikrein in Hyperaldosteronism in the Mouse

Tissue kallikrein (TK) is synthesized in arteries and distal renal tubule, the main target of aldosterone. Urinary kallikrein excretion increases in hyperaldosteronism. We tested the hypothesis that TK is involved in the cardiovascular and renal effects of high aldosterone. Kallikrein-deficient mice (TK-/-), and wild-type (WT) littermates, studied on two different genetic backgrounds, were treated with aldosterone and high-NaCl diet for 1 month. Control mice received vehicle and standard NaCl diet. Treatment induced 5- to 7-fold increase in plasma aldosterone, suppressed renin secretion, and increased urinary TK activity. In 129SvJ-C57BL/6J mice, blood pressure monitored by radiotelemetry was not different between control TK-/- and WT mice. In TK-/- mice, aldosterone induced larger increases in blood pressure than in WT mice (+47 vs. +27 mm Hg; genotype-treatment interaction, P < 0.05). Night-day difference was also exacerbated in treated TK-/- mice (P < 0.01). Moderate cardiac septal hypertrophy was observed in hypertensive animals without major change in heart function. Aldosterone-salt increased kidney weight similarly in both genotypes but induced a 2-fold increase in renal mRNA abundance of epithelial sodium channel subunits only in TK-/- mice. The hypertensive effect of TK deficiency was also documented in treated C57BL/6J mice. In this strain, aldosterone-induced hypertension was only observed in TK-/- mice (+16 mm Hg, P < 0.01). These findings show that TK deficiency exacerbates aldosterone-salt-induced hypertension. This effect may be due at least in part to enhanced sodium reabsorption in the distal nephron aggravating sodium retention. The study suggests that kallikrein plays an antihypertensive role in hyperaldosteronism.

Proteinase-Activated Receptors 1 and 2 Exert Opposite Effects on Renal Renin Release

Proteinase-activated receptors (PARs) 1 to 4 are highly expressed in the kidney and are involved in the regulation of renal hemodynamics and tubular function. Since intravascular infusion of the proteinase thrombin, which activates PARs, has been shown to decrease plasma renin activity in rats, we investigated the effects of the respective PAR subtypes on renin release using the isolated perfused mouse kidney model. Thrombin dose-dependently reduced perfusate flow and inhibited renin secretion rates (RSRs) that had been prestimulated by the β-adrenoreceptor agonist isoproterenol. The suppression of RSRs was prevented by the selective PAR1 inhibitor SCH79797, and direct activation of PAR1 by TFLLR mimicked the effects of thrombin on RSRs and vascular tone. Moreover, TFLLR suppressed the stimulations of RSRs in response to the loop diuretic bumetanide, to prostaglandin E(2), or to a decrease in renal perfusion pressure but not in response to a reduction in extracellular calcium. The PAR2-activating peptide SLIGRL concentration dependently increased RSR and perfusate flow. The stimulation of RSRs by SLIGRL was markedly attenuated by N(G)-nitro-L-arginine methyl ester, suggesting an NO-dependent mechanism. Activation of PAR4 by AYPGKF did not modulate RSRs or perfusate flow. PAR1 and PAR2 immunoreactivity were detected in the juxtaglomerular region and were colocalized with renin immunoreactivity. Our data provide evidence that PAR1 activation inhibits renal renin secretion and induces renal vasoconstriction, whereas PAR2 activation stimulates renin release and induces vasodilation mainly via the release of NO.

Pressure induces intracellular calcium changes in juxtaglomerular cells in perfused afferent arterioles

Calcium (Ca(2+)) has an important role in nearly all types of cellular secretion, with a particularly novel role in the juxtaglomerular (JG) cells in the kidney. In JG cells, Ca(2+) inhibits renin secretion, which is a major regulator of blood pressure and renal hemodynamics. However, whether alterations in afferent arteriolar (Af-Art) pressure change intracellular Ca(2+) concentration ([Ca(2+)](i)) in JG cells and whether [Ca(2+)](i) comes from extracellular or intracellular sources remains unknown. We hypothesize that increases in perfusion pressure in the Af-Art result in elevations in [Ca(2+)](i) in JG cells. We isolated and perfused Af-Art of C57BL6 mice and measured changes in [Ca(2+)](i) in JG cells in response to perfusion pressure changes. The JG cells' [Ca(2+)](i) was 93.3±2.2 nM at 60 mm Hg perfusion pressure and increased to 111.3±13.4, 119.6±7.3, 130.3±2.9 and 140.8±12.1 nM at 80, 100, 120 and 140 mm Hg, respectively. At 120 mm Hg, increases in [Ca(2+)](i) were reduced in mice receiving the following treatments: (1) the mechanosensitive cation channel blocker, gadolinium (94.6±7.5 nM); (2) L-type calcium channel blocker, nifedipine (105.8±7.5 nM); and (3) calcium-free solution plus ethylene glycol tetraacetic acid (96.0±5.8 nM). Meanwhile, the phospholipase C inhibitor, inositol triphosphate receptor inhibitor, T-type calcium channel blocker, N-type calcium channel blocker and Ca(2+)-ATPase inhibitor did not influence changes in [Ca(2+)](i) in JG cells. In summary, JG cell [Ca(2+)](i) rise as perfusion pressure increases; furthermore, the calcium comes from extracellular sources, specifically mechanosensitive cation channels and L-type calcium channels.

The Connexin40 A96S Mutation Causes Renin-Dependent Hypertension

Deletion of the gap-junction-forming protein connexin40 leads to renin-dependent hypertension in mice, but whether observed human variants in connexin40, such as A96S, promote hypertension is unknown. Here, we generated mice with the A96S variant in the mouse connexin40 gene. Although mice homozygous for the A96S mutations had normal expression patterns of connexin40 in the kidney, they were hypertensive, had sixfold higher plasma renin concentrations, and had 40% higher levels of renin mRNA than controls. Renin-expressing cells were aberrantly located outside the media layer of afferent arterioles, and increased renal perfusion pressure did not inhibit renin secretion from kidneys isolated from homozygous A96S mice. Treatment with a low-salt diet in combination with an ACE inhibitor increased renin mRNA levels, plasma renin concentrations, and the number of aberrantly localized renin-producing cells. Taken together, these findings suggest that the A96S mutation in connexin40 leads to renin-dependent hypertension in mice. Modulation of renin secretion by BP critically depends on functional connexin40; with the A96S mutation, the aberrant extravascular localization of renin-secreting cells in the kidney likely impairs the pressure-mediated inhibition of renin secretion.

Monogenic forms of hypertension

Arterial hypertension in childhood is less frequent as compared to adulthood but is more likely to be secondary to an underlying disorder. After ruling out more obvious causes, some patients still present with strongly suspected secondary hypertension of yet unknown etiology. A number of these children have hypertension due to single gene mutations inherited in an autosomal dominant or recessive fashion. The finding of abnormal potassium levels (low or high) in the presence of suppressed renin secretion, and metabolic alkalosis or acidosis should prompt consideration of these familial diseases. However, mild hypertension and the absence of electrolyte abnormalities do not exclude hereditary conditions. In monogenic hypertensive disorders, three distinct mechanisms leading to the common final pathway of increased sodium reabsorption, volume expansion, and low plasma renin activity are documented. The first mechanism relates to gain-of-function mutations with a subsequent hyperactivity of renal sodium and chloride reabsorption leading to plasma volume expansion (e.g., Liddle's syndrome, Gordon's syndrome). The second mechanism involves deficiencies of enzymes that regulate adrenal steroid hormone synthesis and deactivation (e.g., subtypes of congenital adrenal hyperplasia, apparent mineralocorticoid excess (AME)). The third mechanism is characterized by excessive aldosterone synthesis that escapes normal regulatory mechanisms and leading to volume-dependent hypertension in the presence of suppressed renin release (glucocorticoid remediable aldosteronism). Hormonal studies coupled with genetic testing can help in the early diagnosis of these disorders.

Effect of Angiotensin II Type 1 receptor blocker, Candesartan, and β1adrenoceptor blocker, Atenolol, on brain damage in ischemic stroke

This work aims at studying the possible alteration of renal renin secretion after human ischemic stroke and correlating it to the post stroke neurological and renal function alterations using angiotensin II type 1(AT1) receptor blocker (ARB), candesartan, and beta 1 adrenoreceptor blocker atenolol, which inhibits renin secretion, in Wistar rats subjected to middle cerebral artery occlusion. Methods . This study comprised 21 patients with cerebral ischemic stroke. Seventeen normal persons were used for comparison. Recumbent and standing plasma renin activity (PRA), reflex plasma renin sensitivity, plasminogen activator inhibitor and creatinine clearance (Ccr) were estimated at admission and two weeks later. Moreover, 60 male Wistar rats were divided into two groups SHAM and ischemic. Each of the two groups was further subdivided into three subgroups, non-treated, atenolol treated, and candesartan treated. In all rats, mean arterial blood pressure (MAP), systolic blood pressure (SBP), diastolic blood pressure (DBP), pulse pressure (PP), heart rate (HR), neurobehavioral evaluation, Ccr, PRA, and infarct size were measured. Results . Together with the significant deterioration of the neurological score, focal cerebral ischemia in rats resulted in increased PRA and decreased glomerular filtration rate (GFR). In ischemic stroke patients, GFR was significantly decreased at admission and two weeks later, PRA increased at admission and two weeks later while plasma renin reflex secretion sensitivity had decreased significantly at admission relative to controls, but it increased significantly 2 weeks later. Atenolol caused significant improvement of the neurobehavioral score and renal function and decrease infarct size of rats subjected to focal cerebral ischemia whereas candesartan caused significant improvement of the neurobehavioral score and decreased infarct size with no significant change in GFR. Neither atenolol nor candesartan caused significant change in MAP, SBP, DBP, PP and HR Conclusion . (1) Ischemic stroke seems to be associated with a postischemic increase of the plasma renin secretion, which may increase the infarct size in the brain and may induce acute renal insufficiency. (2) This study confirms that Atenolol and ARBs could benefit ischemic stroke patients without altering blood pressure.

Aldosterone-Producing Adenomas

Primary aldosteronism is the most common form of secondary hypertension and is present in about 8% to 10% of patients with hypertension. In primary aldosteronism, aldosterone secretion is excessive and relatively autonomous of the normal regulatory mechanisms.1,2 It was originally described by Jerome Conn from the University of Michigan3 and is thought to be due to an aldosterone-producing adenoma (APA) causing hypertension and hypokalemic alkalosis. Primary aldosteronism is now recognized to include a spectrum of disorders from unilateral APA to bilateral zona glomerulosa (ZG) hyperplasia, or idiopathic hyperaldosteronism. The diagnosis of all forms of primary aldosteronism is based on the finding of elevated levels of aldosterone and suppressed renin secretion.1 Differentiation between unilateral and bilateral aldosteronism is done by a combination of adrenal imaging (computed tomography or MRI) and bilateral adrenal sampling.1,2 Diagnosis of an APA is attractive because surgical therapy with unilateral adrenalectomy should cure the disease, and mechanistically, APA would suggest a single etiology of a benign neoplastic transformation of cells of the ZG producing aldosterone, regardless of the molecular mechanism. However, the histological characteristics of most adrenal tumors classified as APA are heterogeneous when one considers the surrounding adrenal cortex4 and include varying proportions of 4 different types of cells: clear cells with large vacuolated lipid–laden cytoplasm and central round nuclei similar to zona fasciculata cells; lipid-poor ZG-like cells; compact eosinophilic cells similar to those of the zona reticularis; and cells with cytological features of both ZG and zona fasciculata cells, designated hybrid cells.4 The lipid-laden fasciculata-like cells usually predominate, giving the tumors a characteristic golden yellow color, but some have more glomerulosa-like characteristics.4 However, the histological picture is far more complex, as most patients with APA also have hyperplasia of the rest of the adrenal ZG (40%) or hyperplastic …

Enhanced tubuloglomerular feedback in mice with vascular overexpression of A1adenosine receptors

Adenosine 1 receptors (A1AR) in the kidney are expressed in the vasculature and the tubular system. Pharmacological inhibition or global genetic deletion of A1AR causes marked reductions or abolishment of tubuloglomerular feedback (TGF) responses. To assess the function of vascular A1AR in TGF, we generated transgenic mouse lines in which A1AR expression in smooth muscle was augmented by placing A1AR under the control of a 5.38-kb fragment of the rat smooth muscle alpha-actin promoter and first intron (12). Two founder lines with highest expression in the kidney [353 +/- 42 and 575 +/- 43% compared with the wild type (WT)] were used in the experiments. Enhanced expression of A1AR at the expected site in these lines was confirmed by augmented constrictor responses of isolated afferent arterioles to administration of the A1AR agonist N6-cyclohexyladenosine. Maximum TGF responses (0-30 nl/min flow step) were increased from 8.4 +/- 0.9 mmHg in WT (n = 21) to 14.2 +/- 0.7 mmHg in A1AR-transgene (tg) 4 (n = 22; P < 0.0001), and to 12.6 +/- 1.2 mmHg in A1AR-tg7 (n = 12; P < 0.02). Stepwise changes in perfusion flow caused greater numerical TGF responses in A1AR-tg than WT in all flow ranges with differences reaching levels of significance in the intermediate flow ranges of 7.5-10 and 10-15 nl/min. Proximal-distal single-nephron glomerular filtration rate (SNGFR) differences (free-flow micropuncture) were also increased in A1AR-tg, averaging 6.25 +/- 1.5 nl/min compared with 2.6 +/- 0.51 nl/min in WT (P = 0.034). Basal plasma renin concentrations as well as the suppression of renin secretion after volume expansion were similar in A1AR-tg and WT mice, suggesting lack of transgene expression in juxtaglomerular cells. These data indicate that A1AR expression in vascular smooth muscle cells is a critical component for TGF signaling and that changes in renal vascular A1AR expression may determine the magnitude of TGF responses.

Calcium‐Sensing Receptor Activation In Vivo Decreases Plasma Renin Activity

The calcium‐sensing receptor (CaR) transmits changes in extracellular calcium to changes in intracellular signaling and hormonal secretion. The CaR is expressed in many different cell types that indirectly or directly regulate renin secretion including endothelial and vascular smooth muscle cells, as well as the renin‐secreting juxtaglomerular (JG) cells. We have demonstrated that Sensipar, a CaR‐activating calcimimetic, decreases renin release in vitro in primary cultures of JG cells. However, the response to CaR activation on renin secretion in vivo is unknown. As such, we hypothesized that activation of the CaR with Sensipar in vivo would also decrease renin secretion in anesthetized, male Sprague‐Dawley rats. We infused Sensipar i.v. into anesthetized rats at a dose of 60 mg/kg/hr over 60 min. Blood pressure and renal blood flow remained constant at 101±3 mm Hg and 6.21±0.39 ml/min/g kidney weight, respectively, during the infusion of Sensipar. Basal plasma renin activity (PRA) was 15.42 ± 4.41 ngAngI/ml/hr. Sensipar decreased PRA by 58% to 6.53 ± 1.88 ngAngI/ml/hr (p&lt;0.05, n=7). While our result can not necessarily be attributed to the JG cell‐expressed CaR, we conclude that CaR activation inhibits renin secretion in vivo in the absence of changes in renal hemodynamics or blood pressure.

Adenosine and kidney function: Potential implications in patients with heart failure

Therapy of heart failure is more difficult when renal function is impaired. Here, we outline the effects on kidney function of the autacoid, adenosine, which forms the basis for adenosine A(1) receptor (A(1)R) antagonists as treatment for decompensated heart failure. A(1)R antagonists induce a eukaliuretic natriuresis and diuresis by blocking A(1)R-mediated NaCl reabsorption in the proximal tubule and the collecting duct. Normally, suppressing proximal reabsorption will lower glomerular filtration rate (GFR) through the tubuloglomerular feedback mechanism (TGF). But the TGF response, itself, is mediated by A(1)R in the preglomerular arteriole, so blocking A(1)R allows natriuresis to proceed while GFR remains constant or increases. The influence of A(1)R over vascular resistance in the kidney is augmented by angiotensin II while A(1)R activation directly suppresses renin secretion. These interactions could modulate the overall impact of A(1)R blockade on kidney function in patients taking angiotensin II blockers. A(1)R blockers may increase the energy utilized for transport in the semi-hypoxic medullary thick ascending limb, an effect that could be prevented with loop diuretics. Finally, while the vasodilatory effect of A(1)R blockade could protect against renal ischaemia, A(1)R blockade may act on non-resident cells to exacerbate reperfusion injury, where ischaemia to occur. Despite these uncertainties, the available data on A(1)R antagonist therapy in patients with decompensated heart failure are promising and warrant confirmation in further studies.

Renal Function in Mice with Targeted Disruption of the A Isoform of the Na-K-2Cl Co-Transporter

Three different full-length splice isoforms of the Na-K-2Cl co-transporter (NKCC2/BSC1) are expressed along the thick ascending limb of Henle (TAL), designated NKCC2A, NKCC2B, and NKCC2F. NKCC2F is expressed in the medullary, NKCC2B mainly in the cortical, and NKCC2A in medullary and cortical portions of the TAL. NKCC2B and NKCC2A were shown to be coexpressed in the macula densa (MD) segment of the mouse TAL. The functional consequences of the existence of three different isoforms of NKCC2 are unclear. For studying the specific role of NKCC2A in kidney function, NKCC2A-/- mice were generated by homologous recombination. NKCC2A-/- mice were viable and showed no gross abnormalities. Ambient urine osmolarity was reduced significantly in NKCC2A-/- compared with wild-type mice, but water deprivation elevated urine osmolarity to similar levels in both genotypes. Baseline plasma renin concentration and the effects of a high- and a low-salt diet on plasma renin concentration were similar in NKCC2A+/+ and -/- mice. However, suppression of renin secretion by acute intravenous saline loading (5% of body weight), a measure of MD-dependent inhibition of renin secretion, was reduced markedly in NKCC2A-/- mice compared with wild-type mice. Cl and water absorption along microperfused loops of Henle of NKCC2A-/- mice were unchanged at normal flow rates but significantly reduced at supranormal flow. Tubuloglomerular feedback function curve as determined by stop flow pressure measurements was left-shifted in NKCC2A-/- compared with wild-type mice, with maximum responses being significantly diminished. In summary, NKCC2A activity seems to be required for MD salt sensing in the high Cl concentration range. Coexpression of both high- and low-affinity isoforms of NKCC2 may permit transport and Cl-dependent tubuloglomerular feedback regulation to occur over a wider Cl concentration range.

Sodium reduces renin secretion less in natriuretic peptide C receptor deficient mice

The impact of the natriuretic peptide C receptor (NPR-C) on renin secretion was assessed in mice possessing normal or mutated (nonfunctional) NPR-C’s. Heterozygous mice with a mutation of the NPR-C were bred to obtain either wild type, heterozygote or homozygote (NPR-C deficient) mice. The mice were tested at 2 months of age. Half of the mice of each genotype drank 0.9% saline and the other half drank tap water. After five days of saline or water consumption, the mice were anesthetized with pentobarbital sodium and 150 μl of blood were withdrawn from the inferior vena cava. Wild type control mice exhibited a decrease in renin activity from 2062 ± 364 to 1518 ± 178 ng angiotensin I/ml/hr after saline exposure. The decrease in heterozygotes was from 1663 ± 201 to 1366 ± 128, whereas homozygote (deficient) mice experienced a change from 2070 ± 284 to 1951 ± 314. The difference between renin activity while drinking water and saline averaged 544 ng angiotensin I/ml/hr in wildtype mice, 297 in heterozygote mice and 119 in homozygote mice. Thus, a linear relationship existed between the number of functional NPR-C copies and the ability of saline consumption to suppress renin secretion. Prior studies have identified the natriuretic peptide A receptor as the functional natriuretic peptide receptor suppressing renin release. These data suggest an involvement of the NPR-C as well. (This study was supported by the Minnesota Medical Foundation.)

Combined Thyroidectomy and Renal Denervation Suppress Renin Expression and Secretion in Fetal Sheep

Activity of the fetal renin-angiotensin system (RAS) is developmentally regulated, increasing in late gestation toward term. Thyroid hormone and the renal nerves are both important modulators of renal RAS maturation; however, ablation of either influence alone does not totally block the aforementioned developmental late gestation increase in RAS in fetal sheep. In the current study, we used the technique of thyroidectomy combined with bilateral renal denervation (TX+D), which removes thyroid hormone from the circulation and abolishes effects of renal nerve activity, to determine if simultaneous removal of their effects on the kidney would markedly alter renin expression and secretion in late gestation. TX+D was performed at 120 days of gestation age (dGA). Control fetuses were sham-operated. Immediately before necropsy (approximately 138 dGA), fetuses were infused with isoproterenol to examine plasma active and prorenin changes in response to beta-adrenergic stimulation. TX+D decreased plasma thyroid hormone concentrations, renal renin mRNA, renal active and prorenin levels, and plasma active and prorenin concentrations. Isoproterenol-induced increases in plasma active renin were also reduced in TX+D fetuses. TX+D did not alter renal angiotensin (Ang) II subtype receptor (AT2) expression close to term. These findings suggest that TX+D synergize in the suppression of fetal renin expression.