Juxtaglomerular cell CaSR stimulation decreases renin release via activation of the PLC/IP(3) pathway and the ryanodine receptor

Juxtaglomerular apparatus-mediated homeostatic mechanisms: therapeutic implication for chronic kidney disease

INTRODUCTION

Juxtaglomerular apparatus (JGA)-mediated homeostatic mechanism links to how sodium-glucose cotransporter 2 inhibitors (SGLT2is) slow progression of chronic kidney disease (CKD) and may link to how tolvaptan slows renal function decline in autosomal dominant polycystic kidney disease (ADPKD).

AREA COVERED

JGA-mediated homeostatic mechanism has been hypothesized based on investigations of tubuloglomerular feedback and renin-angiotensin system. We reviewed clinical trials of SGLT2is and tolvaptan to assess the relationship between this mechanism and these drugs.

EXPERT OPINION

When sodium load to macula densa (MD) increases, MD increases adenosine production, constricting afferent arteriole (Af-art) and protecting glomeruli. Concurrently, MD signaling suppresses renin secretion, increases urinary sodium excretion, and counterbalances reduced sodium filtration. However, when there is marked increase in sodium load per-nephron, as in advanced CKD, MD adenosine production increases, relaxing Af-art and maintaining sodium homeostasis at the expense of glomeruli. The beneficial effects of tolvaptan on renal function in ADPKD may also depend on the JGA-mediated homeostatic mechanisms since tolvaptan inhibits sodium reabsorption in the thick ascending limb.The JGA-mediated homeostatic mechanism regulates Af-arts, constricting to relaxing according to homeostatic needs. Understanding this mechanism may contribute to the development of pharmacotherapeutic compounds and better care for patients with CKD.

Role of Calcimimetics in Treating Bone and Mineral Disorders Related to Chronic Kidney Disease

Renal osteodystrophy is common in patients with chronic kidney disease and end-stage renal disease and leads to the risks of fracture and extraosseous vascular calcification. Secondary hyperparathyroidism (SHPT) is characterized by a compensatory increase in parathyroid hormone (PTH) secretion in response to decreased renal phosphate excretion, resulting in potentiating bone resorption and decreased bone quantity and quality. Calcium-sensing receptors (CaSRs) are group C G-proteins and negatively regulate the parathyroid glands through (1) increasing CaSR insertion within the plasma membrane, (2) increasing 1,25-dihydroxy vitamin D3 within the kidney and parathyroid glands, (3) inhibiting fibroblast growth factor 23 (FGF23) in osteocytes, and (4) attenuating intestinal calcium absorption through Transient Receptor Potential Vanilloid subfamily member 6 (TRPV6). Calcimimetics (CaMs) decrease PTH concentrations without elevating the serum calcium levels or extraosseous calcification through direct interaction with cell membrane CaSRs. CaMs reduce osteoclast activity by reducing stress-induced oxidative autophagy and improving Wnt-10b release, which promotes the growth of osteoblasts and subsequent mineralization. CaMs also directly promote osteoblast proliferation and survival. Consequently, bone quality may improve due to decreased bone resorption and improved bone formation. CaMs modulate cardiovascular fibrosis, calcification, and renal fibrosis through different mechanisms. Therefore, CaMs assist in treating SHPT. This narrative review focuses on the role of CaMs in renal osteodystrophy, including their mechanisms and clinical efficacy.

Plasma renin, aldosterone, and urinary prostaglandin E2 levels in children with hypocalcemia due to vitamin D deficiency rickets

We investigated the effect of hypocalcemia on plasma renin, aldosterone, and urine PGE₂ levels in children with vitamin D deficiency rickets (VDDR). In the study group, 25 patients with VDDR-induced hypocalcemia were treated with a single dose of 150,000-300,000 IU cholecalciferol and 50 mg/kg/day elemental Ca for 10 days. On any day between 21th and 30th days after the treatment, the patients' clinical, biochemical and radiologic findings were re-evaluated. The healthy children with the same sex and similar age as the study group comprised the control group. Plasma sodium (Na), potassium (K), calcium (Ca), phosphorus (P), alkaline phosphatase (ALP), parathyroid hormone (PTH), 25- hydroxy vitamin D (25OHD), renin, aldosterone; and urinary Ca, creatinine (Cr) and prostaglandin E₂ (PGE₂) levels were measured in both the study (pre-treatment and post-treatment) and the control group. Plasma Ca, P, 25OHD and renin levels and urinary PGE₂/Cr ratio in the post-treatment group were significantly higher than those in the pre-treatment group while K, ALP, and PTH concentrations were significantly lower. Plasma ALP and PTH levels in pre-treatment group were significantly higher than in the control group while Ca, P, 25OHD, aldosterone and renin concentrations and urinary PGE₂/Cr ratio were significantly lower. Post-treatment plasma Ca level was significantly decreased in normal limits compared to the control group while other biochemical parameters were not different from the control group. Plasma Ca concentration was positively correlated with renin level and urinary PGE₂/Cr ratio. The findings suggest that hypocalcemia may inhibit the production of renin, aldosterone and PGE₂ and a blunt aldosterone secretion may develop even after recovery from hypocalcemia.

Effect of MgCl2 and GdCl3 on ORAI1 Expression and Store-Operated Ca2+ Entry in Megakaryocytes

In chronic kidney disease, hyperphosphatemia upregulates the Ca²⁺ channel ORAI and its activating Ca²⁺ sensor STIM in megakaryocytes and platelets. ORAI1 and STIM1 accomplish store-operated Ca²⁺ entry (SOCE) and play a key role in platelet activation. Signaling linking phosphate to upregulation of ORAI1 and STIM1 includes transcription factor NFAT5 and serum and glucocorticoid-inducible kinase SGK1. In vascular smooth muscle cells, the effect of hyperphosphatemia on ORAI1/STIM1 expression and SOCE is suppressed by Mg²⁺ and the calcium-sensing receptor (CaSR) agonist Gd³⁺. The present study explored whether sustained exposure to Mg²⁺ or Gd³⁺ interferes with the phosphate-induced upregulation of NFAT5, SGK1, ORAI1,2,3, STIM1,2 and SOCE in megakaryocytes. To this end, human megakaryocytic Meg-01 cells were treated with 2 mM ß-glycerophosphate for 24 h in the absence and presence of either 1.5 mM MgCl₂ or 50 µM GdCl₃. Transcript levels were estimated utilizing q-RT-PCR, protein abundance by Western blotting, cytosolic Ca²⁺ concentration ([Ca²⁺]_i) by Fura-2 fluorescence and SOCE from the increase in [Ca²⁺]_i following re-addition of extracellular Ca²⁺ after store depletion with thapsigargin (1 µM). As a result, Mg²⁺ and Gd³⁺ upregulated CaSR and blunted or virtually abolished the phosphate-induced upregulation of NFAT5, SGK1, ORAI1,2,3, STIM1,2 and SOCE in megakaryocytes. In conclusion, Mg²⁺ and the CaSR agonist Gd³⁺ interfere with phosphate-induced dysregulation of [Ca²⁺]_i in megakaryocytes.

Current advances of long non-coding RNAs mediated by wnt signaling in glioma

Glioma is the most common and aggressive brain tumor in the central nervous system (CNS), in which Wnt signaling pathway has been verified to play a pivotal role in regulating the initiation and progression. Currently, numerous studies have indicated that long non-coding RNAs (lncRNAs) have critical functions across biological processes including cell proliferation, colony formation, migration, invasion and apoptosis via Wnt signaling pathway in glioma. This review depicts canonical and non-canonical Wnt/β-catenin signaling pathway properties and relative processing mechanisms in gliomas, and summarizes the function and regulation of lncRNAs mediated by Wnt signaling pathway in the development and progression of glioma. Ultimately, we hope to seek out promising biomarkers and reliable therapeutic targets for glioma.

Calcilytic NPS2143 promotes proliferation and inhibits apoptosis of spontaneously hypertensive rat vascular smooth muscle cells via activation of the renin-angiotensin system

Vascular smooth muscle cell (VSMC) proliferation and apoptosis and the renin-angiotensin system (RAS) play critical roles in the development of essential hypertension. The activation of calcium-sensing receptor (CaSR), functionally expressed in VSMCs, inhibits cyclic adenosine monophosphate (cAMP) formation by elevating intracellular calcium ([Ca²⁺]_i) and then suppressing renin release. The present study aimed to investigate the effects of NPS2143-mediated inhibition of CaSR on VSMC proliferation and apoptosis in spontaneously hypertensive rat (SHR) VSMCs and to assess whether these effects were mediated by alterations to RAS signaling. Primary VSMCs were isolated from the aortas of SHRs and Wistar-Kyoto rats. SHR VSMCs were treated with CaSR antagonist NPS2143 and cell proliferation and CaSR and RAS-related protein expression levels were measured to assess the effect. The results indicated that NPS2143 treatment promoted SHR VSMC proliferation, lower CaSR expression levels and higher RAS-related proteins levels when compared with control treatment. Additional measurement of the expression levels of proteins related to proliferation, remodeling, apoptosis and RAS related proteins, as well as cell viability, cell cycle, cell apoptosis ratio, [Ca²⁺]_i, and the concentration of cAMP was performed after treatment with NPS2143, PLC inhibitor U73122, IP3 receptor antagonist 2-aminoethoxydiphenylborane (APB), adenylyl cyclase-V inhibitor MDL12330A, angiotensin converting enzyme inhibitor captopril, angiotensin I receptor (AT1R) inhibitor losartan, NPS2143 + U73122, NPS2143 + 2-APB, NPS2143 + MDL12330A, NPS2143 + captopril and NPS2143 + losartan. The results suggested that NPS2143 promoted cell proliferation, inhibited cell apoptosis, decreased [Ca²⁺]_i and increased the expression of RAS compared with control treatments. NPS2143 + U73122 and NPS2143 + 2-APB enhanced the effects of NPS2143, while NPS2143 + MDL12330A, NPS2143 + captopril, NPS2143 + losartan attenuated the effected of NPS2143 in SHR VSMCs. Furthermore, the knockdown of AT1R by AT1R-short hairpin RNA also attenuated the effects of NPS2143 compared with NPS2143 alone. Collectively, these data indicated that NPS2143 promoted proliferation and inhibited apoptosis of VSMCs in SHRs, the effect of which was achieved by activation of RAS signaling.

Pathways of hepatic and renal damage through non-classical activation of the renin-angiotensin system in chronic liver disease

In liver cirrhosis, renin-angiotensin system (RAS) activation sustains renal sodium retention and hepatic fibrogenesis. New information has recently enlivened the traditional concept of RAS. For instance, renin and prorenin bind their ubiquitous receptors, resulting in the local production of angiotensin (Ang) II; increased serum calcium and calcimimetic agents, through stimulation of extracellular calcium-sensing receptors (CaSR), blunt renin production and lead to natriuretic effects in human and experimental cirrhosis. Alongside systemic production, there is Ang II tissue production within various organs through RAS enzymes different from angiotensin-converting enzyme (ACE), that is chymase, tissue plasminogen activator and several cathepsins. In experimental cirrhosis, inhibition of chymase leads to natriuretic and hepatic antifibrotic effects, without changes in systemic haemodynamics. In the kidney, local RAS coordinates proximal and distal tubular sodium reabsorption. However, renalase, whose plasma and tissue levels are severely altered in experimental cirrhosis, degrades systemic and renal tubule catecholamines, antagonizing the effects of renal RAS. Angiotensinogen-derived natriuretic and vasodilating peptides (Ang1-9, Ang1-7, Ang3-8) and their receptors have been described. Receptor agonists or antagonists are available to affect portal hypertension and sodium retention in cirrhosis. ACE2-dependent generation of Ang1-7 may inhibit experimental liver fibrosis. inhibition of Ang1-7 clearance by means of neprilysin blockade has portal hypotensive and natriuretic effects. Ang1-12, whose production renin does not regulate, is converted to several different angiotensin peptides via chymase. Finally, Ang II behaves as either an antinatriuretic or a natriuretic agent, based on the tissue content of AT₁ R and AT₂ R receptors, their ratio being prone to pharmacological modulation.

CaSR participates in the regulation of vascular tension in the mesentery of hypertensive rats via the PLC‑IP3/AC‑V/cAMP/RAS pathway

Hypertension is a cardiovascular disease that severely impairs human health; however, its specific etiology and pathogenesis are complex. The present study investigated the effects of the calcium sensing receptor (CaSR) on vascular tone in spontaneously hypertensive rats (SHRs), and clarified the role and mechanism of CaSR in regulating this property with respect to the phospholipase C (PLC)-inositol 1,4,5-triphosphate (IP₃)/adenylate cyclase-V(AC-V)/cyclic adenosine monophosphate (cAMP)/renin-angiotensin system (RAS) pathway in these animals. CaSR protein expression in the mesenteric artery (MA) of rats and CaSR protein expression in SHRs were significantly reduced. Based on wire myography studies, vasoconstriction was significantly augmented and vasodilatation was attenuated in SHRs, and this effect was endothelium-independent. The CaSR calcimimetic NPSR568 and inhibitor NPS2143 reduced vasoconstriction and enhanced vasodilation in SHRs. Furthermore, pretreatment with PLC-IP₃/AC-V/cAMP/RAS pathway blockers significantly reduced the vasoconstriction response and enhanced the vasodilator response in SHRs and Wistar-Kyoto rats (WKY), and these effects were partially dependent on the endothelium. Additionally, pretreatment with CaSR inhibitors were determined to cooperate with the PLC-IP₃/AC-V/cAMP/RAS pathway inhibitors to significantly reduce vasoconstriction and enhance vasodilation in SHRs and WKY. Our results demonstrated that CaSR is functionally expressed in the MA of SHRs, and that CaSR expression is decreased in SHRs. Additionally, vasoconstriction was enhanced while vasodilatation was attenuated in SHRs; these processes were determined to be endothelium-independent. CaSR is involved in the regulation of blood pressure and vascular tension in SHRs and WKYs. In association with mechanistic differences, this effect was proposed to be partially endothelium-dependent and mediated by the PLC-IP₃/AC-V/cAMP/RAS pathway.

Calcimimetic R568 reduced the blood pressure and improved aortic remodeling in spontaneously hypertensive rats by inhibiting local renin-angiotensin system activity

Hypertension is a cardiovascular disease that seriously affects human health. Activation of the calcium-sensing receptor (CaSR) inhibits cyclic adenosine monophosphate (cAMP) formation by increasing [Ca²⁺]_i and subsequently inhibiting renin release. The renin-angiotensin system (RAS) plays an important role in the development of essential hypertension (EH). The purpose of this study was to determine the effects of NPSR568 (R568)-activated CaSR on blood pressure (BP), proliferation, and remodeling of vascular smooth muscle cells, and the activity of the RAS in spontaneously hypertensive rats (SHRs). In this study, we treated SHR and Wistar-Kyoto rats with R568 for 8 weeks. The tail-cuff method was used to assess rat BP weekly. Morphological changes in the thoracic aorta were evaluated with hematoxylin-eosin and Masson staining. Western blotting and immunohistochemistry were used to detect the expression of RAS-related proteins and proliferative remodeling proteins in the thoracic aorta. An enzyme-linked immunosorbent assay was used to detect the content of cAMP, the RAS, and the CaSR in plasma and the thoracic aorta. Finally, we found that treatment with R568 for 8 weeks reduced the BP and inhibited arterial vascular proliferation remodeling in SHRs. R568 administration significantly suppressed the activity of local RAS in the thoracic aortas of SHRs. Moreover, R568 treatment reversed the low expression of CaSR in SHRs. R568 may serve as an effective strategy against EH.

Important roles of the Ca2+-sensing receptor in vascular health and disease

Ca²⁺-sensing receptor (CaSR), a member of G protein-coupled receptor family, is widely expressed in the vascular system, including perivascular neurons, vascular endothelial cells (VECs) and vascular smooth muscle cells (VSMCs). When stimulated, CaSR can further increase the cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) in two ways: intracellular Ca²⁺ release from endo/sarcoplasmic reticulum (ER/SR) and extracellular Ca²⁺ entry through Ca²⁺-permeable cation channels. In endothelium, increased Ca²⁺ subsequently activate nitric oxide synthase (NOS) and intermediate conductance Ca²⁺-activated K⁺ channels (IKCa), resulting in vasodilation through NOS-mediated NO release or membrane hyperpolarization. In VSMCs, CaSR-induced intracellular Ca²⁺ increase causes blood vessel constriction. CaSR activation predominantly induces vasorelaxation of whole vascular tissues through VECs-dependent mechanisms; however, CaSR-induced Ca²⁺ signaling in VSMCs may play a braking role in CaSR-mediated vasorelaxation. Emerging evidence reveals the importance of CaSR in the regulation of vascular tone and blood pressure. Here, we summarized recent advances in CaSR-mediated vascular reaction and the underlying mechanisms in different species, including humans. In addition, several studies have demonstrated that CaSR dysfunction may be associated with some fatal vascular diseases, such as pulmonary arterial hypertension, primary hypertension, diabetes, acute myocardial infarction and vascular calcification. With the advance of studies on CaSR in vascular health and disease, it is expected positive modulators or negative modulators of CaSR used for the treatment of specific diseases may be promising therapeutic options for the prevention and/or treatment of vascular diseases.

Dual-Functional Dendritic Mesoporous Bioactive Glass Nanospheres for Calcium Influx-Mediated Specific Tumor Suppression and Controlled Drug Delivery in Vivo

The development of nanomaterials for stable, controlled delivery of drugs and efficient suppression of tumor growth with desirable biosafety remains challenging in the nano-biomedical field. In this study, we prepared and optimized mesoporous bioactive glass (MBG) nanospheres to establish a functional drug delivery system and analyzed the effect of the dendritic mesoporous structure on drug loading and release. We then utilized an in vitro model to examine the biological effects of dendritic MBG nanospheres on normal and tumor cells and studied the molecular mechanism underlying specific tumor suppression by MBG nanospheres. Finally, we investigated the combinational effect of MBG nanospheres and a cancer therapeutic drug with an in vivo tumor xenograft model. Our results show that the dendritic MBG nanospheres have been successfully synthesized by optimizing calcium: silicon ratio. MBG nanospheres exhibit a dendritic mesoporous structure with a large specific surface area, demonstrate high drug loading efficiency, and release drugs in a controlled fashion to effectively prolong drug half-life. Ca²⁺ in nanospheres activates transient receptor potential channels and calcium-sensing receptor on tumor cells, mediates calcium influx, and directly regulates the calpain-1-Bcl-2-caspase-3 signaling pathway to specifically suppress tumor growth without affecting normal cells. In addition, dendritic MBG nanospheres synergize with cancer drugs to improve antitumor efficacy and reduce systemic toxicity. Dendritic MBG nanospheres with antitumor activity and controlled drug release have been successfully achieved and the underlying molecular mechanism was elucidated, paving the way for translational application.

Treatment Based on Cinacalcet Reduces Oxidative Stress in Hemodialysis Patients with Secondary Hyperparathyroidism

BACKGROUND/AIMS

Oxidative stress is one of the leading factors contributing to increased mortality in patients with chronic kidney disease (CKD) and secondary hyperparathyroidism (sHPT). Cinacalcet is now commonly used in the treatment of sHPT in patients with CKD. The aim of this study was to assess the influence of treatment with cinacalcet on the oxidative stress markers in patients on hemodialysis with sHPT.

METHODS

In 58 hemodialysed patients with sHPT (parathyroid hormone [PTH] > 300 pg/mL) plasma Advanced Oxidation Protein Products (AOPP), serum total antioxidant capacity - ImAnOx (TAS/TAC), serum PTH, calcium and phosphate concentrations were assessed before the first dose of cinacalcet and after 6 months of treatment.

RESULTS

Serum PTH concentration decreased significantly from 895 (748-1,070) to 384 (289-510) pg/mL after 6 months of treatment; p < 0.0001. Mean serum concentrations of -calcium and phosphate remained stable. Plasma AOPP concentration decreased significantly from 152 (126-185) to 49 -(43-57) µmol/L after 6 months of treatment; p < 0.0001. ImAnOx significantly increased from 260 (251-270) to 272 (264-280) µmol/L; p = 0.04. After 6 months of treatment, a significant, positive correlation was found between ImAnOx and the daily dose of cinacalcet (r = 0.30; p = 0.02). Also, the change of serum ImAnOx during treatment with cinacalcet significantly correlated with the daily dose of cinacalcet r = 0.35; p = 0.01. No significant correlations were found between plasma AOPP concentration or ImAnOx and PTH, or their changes in time.

CONCLUSIONS

(1) Six-month treatment based on cinacalcet seems to reduce oxidative stress markers in maintenance hemodialysis patients with sHPT. (2) This benefit may be related rather to the direct action of cinacalcet than to the serum PTH concentration decrease.

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.

Bimodal dynamics of granular organelles in primary renin-expressing cells revealed using TIRF microscopy

Renin is the initiator and rate-limiting factor in the renin-angiotensin blood pressure regulation system. Although renin is not exclusively produced in the kidney, in nonmurine species the synthesis and secretion of the active circulatory enzyme is confined almost exclusively to the dense core granules of juxtaglomerular (JG) cells, where prorenin is processed and stored for release via a regulated pathway. Despite its importance, the structural organization and regulation of granules within these cells is not well understood, in part due to the difficulty in culturing primary JG cells in vitro and the lack of appropriate cell lines. We have streamlined the isolation and culture of primary renin-expressing cells suitable for high-speed, high-resolution live imaging using a Percoll gradient-based procedure to purify cells from RenGFP⁺ transgenic mice. Fibronectin-coated glass coverslips proved optimal for the adhesion of renin-expressing cells and facilitated live cell imaging at the plasma membrane of primary renin cells using total internal reflection fluorescence microscopy (TIRFM). To obtain quantitative data on intracellular function, we stained mixed granule and lysosome populations with Lysotracker Red and stimulated cells using 100 nM isoproterenol. Analysis of membrane-proximal acidic granular organelle dynamics and behavior within renin-expressing cells revealed the existence of two populations of granular organelles with distinct functional responses following isoproterenol stimulation. The application of high-resolution techniques for imaging JG and other specialized kidney cells provides new opportunities for investigating renal cell biology.

Reduced Expression of the Extracellular Calcium-Sensing Receptor (CaSR) Is Associated with Activation of the Renin-Angiotensin System (RAS) to Promote Vascular Remodeling in the Pathogenesis of Essential Hypertension

The proliferation of vascular smooth muscle cells (VSMCs), remodeling of the vasculature, and the renin-angiotensin system (RAS) play important roles in the development of essential hypertension (EH), which is defined as high blood pressure (BP) in which secondary causes, such as renovascular disease, are absent. The calcium-sensing receptor (CaSR) is involved in the regulation of BP. However, the underlying mechanisms by which the CaSR regulates BP are poorly understood. In the present study, the role of the CaSR in EH was investigated using male spontaneously hypertensive rats (SHRs) and rat and human plasma samples. The percentages of medial wall thickness to external diameter (WT%), total vessel wall cross-sectional area to the total area (WA%) of thoracic arteries, as well as the percentage of wall area occupied by collagen to total vessel wall area (CA%) were determined. Tissue protein expression and plasma concentrations of the CaSR, cyclic adenosine monophosphate (cAMP), renin, and angiotensin II (Ang II) were additionally assessed. WT%, WA%, and CA% were found to increase with increasing BP, whereas the plasma concentration of CaSR was found to decrease. With increasing BP, the levels of smooth muscle actin and calponin decreased, whereas those of osteopontin and proliferating cell nuclear antigen increased. The CaSR level negatively correlated with the levels of cAMP and Ang II, but positively correlated with those of renin. Our data suggest that reduced expression of the CaSR is correlated with activation of the RAS, which induces increased vascular remodeling and VSMC proliferation, and thereby associated with EH in the SHR model and in the Han Chinese population. Our findings provide new insights into the pathogenesis of EH.

Comparative expression of the extracellular calcium-sensing receptor in the mouse, rat, and human kidney

The calcium-sensing receptor (CaSR) was cloned over 20 years ago and functionally demonstrated to regulate circulating levels of parathyroid hormone by maintaining physiological serum ionized calcium concentration ([Ca(2+)]). The receptor is highly expressed in the kidney; however, intrarenal and intraspecies distribution remains controversial. Recently, additional functions of the CaSR receptor in the kidney have emerged, including parathyroid hormone-independent effects. It is therefore critical to establish unequivocally the localization of the CaSR in the kidney to relate this to its proposed physiological roles. In this study, we determined CaSR expression in mouse, rat, and human kidneys using in situ hybridization, immunohistochemistry (using 8 different commercially available and custom-made antibodies), and proximity ligation assays. Negative results in mice with kidney-specific CaSR ablation confirmed the specificity of the immunohistochemistry signal. Both in situ hybridization and immunohistochemistry showed CaSR expression in the thick ascending limb, distal tubule, and collecting duct of all species, with the thick ascending limb showing the highest levels. Within the collecting ducts, there was significant heterogeneity of expression between cell types. In the proximal tubule, lower levels of immunoreactivity were detected by immunohistochemistry and proximity ligation assays. Proximity ligation assays were the only technique to demonstrate expression within glomeruli. This study demonstrated CaSR expression throughout the kidney with minimal discrepancy between species but with significant variation in the levels of expression between cell and tubule types. These findings clarify the intrarenal distribution of the CaSR and enable elucidation of the full physiological roles of the receptor within this organ.

Calcium-sensing receptor activation in chronic kidney disease: effects beyond parathyroid hormone control

Secondary hyperparathyroidism (SHPT) is an important complication of advanced chronic kidney disease (CKD). Cinacalcet, an allosteric modulator of the calcium-sensing receptor (CaSR) expressed in parathyroid glands, is the only calcimimetic approved to treat SHPT in patients on dialysis. By enhancing CaSR sensitivity for plasma extracellular calcium (Ca(2+)0), cinacalcet reduces serum parathyroid hormone, Ca(2+)0, and serum inorganic phosphorous concentrations, allowing better control of SHPT and CKD-mineral and bone disorders. Of interest, the CaSR also is expressed in a variety of tissues where its activation regulates diverse cellular processes, including secretion, apoptosis, and proliferation. Thus, the existence of potential off-target effects of cinacalcet cannot be neglected. This review summarizes our current knowledge concerning the potential role(s) of the CaSR expressed in various tissues in CKD-related disorders, independently of parathyroid hormone control.

Antialbuminuric actions of calcilytics in the remnant kidney

Hyperphosphatemia accelerates the progression of chronic kidney diseases. In the present study, the effects of ronacaleret, a calcilytic agent, on renal injury were assessed in the following four groups of rats: 5/6-nephrectomized Wistar rats as a control (C group), rats treated with ronacaleret (3 mg·kg(-1)·day(-1); R group), rats treated with calcitriol (30 ng·kg(-1)·day(-1); V group), and rats treated with both ronacaleret and calcitriol (R + V group). Three months later, rats were euthanized under anesthesia, and the remnant kidneys were harvested for analysis. Albuminuria was lower in the R and V groups than in the C group (P < 0.05). Creatinine clearance was elevated in the R and V groups compared with the C group (P < 0.05). Serum Ca(2+) and renal ANG II were higher in the R + V group than in the C group (P < 0.05 for each), and serum phosphate was reduced in the R group compared with the C group (P < 0.05). Fibroblast growth factor-23 was lower in the R group and higher in the V and R + V groups than in the C group. However, parathyroid hormone did not differ significantly among the four groups. Renal klotho expression was elevated in the R and V groups compared with the C group (P < 0.05). The present data indicate that ronacaleret preserves klotho expression and renal function with reductions in serum phosphate and albuminuria in 5/6-nephrectomized rats. Our findings demonstrate that vitamin D prevents declines in klotho expression and renal function, suppressing albuminuria.

The renin-angiotensin-aldosterone system and calcium-regulatory hormones

There is increasing evidence of a clinically relevant interplay between the renin-angiotensin-aldosterone system and calcium-regulatory systems. Classically, the former is considered a key regulator of sodium and volume homeostasis, while the latter is most often associated with skeletal health. However, emerging evidence suggests an overlap in regulatory control. Hyperaldosteronism and hyperparathyroidism represent pathophysiologic conditions that may contribute to or perpetuate each other; aldosterone regulates parathyroid hormone and associates with adverse skeletal complications, and parathyroid hormone regulates aldosterone and associates with adverse cardiovascular complications. As dysregulation in both systems is linked to poor cardiovascular and skeletal health, it is increasingly important to fully characterize how they interact to more precisely understand their impact on human health and potential therapies to modulate these interactions. This review describes the known clinical interactions between these two systems including observational and interventional studies. Specifically, we review studies describing the inhibition of renin activity by calcium and vitamin D, and a potentially bidirectional and stimulatory relationship between aldosterone and parathyroid hormone. Deciphering these relationships might clarify variability in outcomes research, inform the design of future intervention studies and provide insight into the results of prior and ongoing intervention studies. However, before these opportunities can be addressed, more effort must be placed on shifting observational data to the proof of concept phase. This will require reallocation of resources to conduct interventional studies and secure the necessary talent.

Endothelin inhibits renin release from juxtaglomerular cells via endothelin receptors A and B via a transient receptor potential canonical-mediated pathway

Renin is the rate-limiting step in the production of angiotensin II: a critical element in the regulation of blood pressure and in the pathogenesis of hypertension. Renin release from the juxtaglomerular (JG) cell is stimulated by the second messenger cAMP and inhibited by increases in calcium (Ca). Endothelins (ETs) inhibit renin release in a Ca-dependent manner. JG cells contain multiple isoforms of canonical transient receptor potential (TRPC) Ca-permeable channels. The proposed hypothesis is that endothelin inhibits renin release by activating TRPC store-operated Ca channels. RT-PCR and immunofluorescence revealed expression of both ETA and ETB receptors in mouse JG cells. Incubation of primary cultures of JG cells with ET-1 (10 nmol/L) decreased renin release by 28%. Addition of either an ETA or an ETB receptor blocker completely prevented the ET inhibition of renin release. Incubation with the TRPC blocker (SKF 96365, 50 μmol/L) completely reversed the Ca-mediated inhibition of renin release by ETs. These results suggest that endothelin inhibits renin release from JG cells via both ETA and ETB receptors, which leads to the activation of TRPC store-operated Ca channels.

Activation of gill Ca2+-sensing receptor as a protective pathway to reduce Ca2+-induced cytotoxicity

The expression of the Ca(2) (+)-sensing receptor (Casr) in the endocrine gland known as the corpuscle of Stannius (CS) regulates the secretion of the hypocalcemic hormone stanniocalcin-1 (STC1) to inhibit gill Ca(2) (+) uptake. Although numerous studies have reported the branchial expression of Casr and Stc1, the functions of these proteins in gills have not been elucidated yet. On the basis of recent findings regarding the autocrine/paracrine functions of STC1 in mammalian models, we proposed the hypothesis that branchial CaSR has an in situ 'sensing' function to regulate STC1 that maintains local Ca(2) (+) homeostasis. In this study, we investigated Casr-mediated signaling and its regulation of Stc1 and cyclooxygenase-2 (Cox2) expression/function using a primary gill-cell culture model. The biochemical responses of gill cells isolated from Japanese eels to an increasing concentration of extracellular Ca(2) (+) (0.1-1 mM) were tested. This stimulation led to a transient increase in phosphatidylcholine-phospholipase C (PC-PLC) activity, followed by activation of ERK and inositol 1,4,5-trisphosphate-Ca(2) (+)/calmodulin-dependent protein kinase 2 (CaMK2) signaling pathways. Cotreatment with the calcimimetic R467 caused synergistic effects on Ca(2) (+)-stimulated PC-PLC activity, ERK signaling, and CaMK2 signaling. The activation of the CaSR-PLC-ERK pathway was associated with increased expression levels of Stc1 and Cox2 as confirmed by the inhibition of Erk using a chemical inhibitor, PD98059. Functionally, Ca(2) (+)/R-467 pretreatment was found to protect cells from thapsigargin-induced cell death. Inhibition of COX2 activity using NS398 abolished this protection, while transduction of STC1 lentiviral particles in the gill cells increased the protective effects. Collectively, our data revealed the expression of functional CaSR in gill tissues. The identification of the CaSR-STC1/COX2-mediated protective pathway in gill cells sheds light on a possible cellular protective mechanism against an increase in intracellular Ca(2) (+) levels associated with transepithelial Ca(2) (+) transport.

Novel long-acting prostacyclin agonist (ONO-1301) with an angiogenic effect: promoting synthesis of hepatocyte growth factor and increasing cyclic AMP concentration via IP-receptor signaling

The purpose of this study was to evaluate the angiogenic potency of ONO-1301, a novel prostacyclin agonist, using a murine sponge model. Solutions of ONO-1301 or hepatocyte growth factor (HGF), as a positive control, were injected into sponges in the backs of mice, daily for 14 days. Hemoglobin and HGF levels in the sponge were increased for up to 14 days on daily treatment with ONO-1301 while on HGF treatment, they peaked on day 7 and had decreased again by day 14. ONO-1301 also upregulated c-Met expression for 14 days in a dose-dependent manner. When the mice were pretreated with an antibody to HGF or the prostaglandin I (IP)-receptor antagonist CAY10441, the angiogenic effect of ONO-1301 was dramatically reduced. Plasma concentrations of cyclic adenosine monophosphate (cAMP) were increased in a dose-dependent manner by once daily treatment with ONO-1301 for 14 days. This effect was reduced by pretreatment with the IP-receptor antagonist. In conclusion, hemoglobin level was increased by repeated treatment with ONO-1301 for 14 days. It is suggested that ONO-1301 induced angiogenesis by promoting the synthesis of HGF and upregulated c-Met expression, followed by an increase in cAMP concentrations mediated by IP-receptor signaling.

Human interventions to characterize novel relationships between the renin-angiotensin-aldosterone system and parathyroid hormone

Observational studies in primary hyperaldosteronism suggest a positive relationship between aldosterone and parathyroid hormone (PTH); however, interventions to better characterize the physiological relationship between the renin-angiotensin-aldosterone system (RAAS) and PTH are needed. We evaluated the effect of individual RAAS components on PTH using 4 interventions in humans without primary hyperaldosteronism. PTH was measured before and after study (1) low-dose angiotensin II (Ang II) infusion (1 ng/kg per minute) and captopril administration (25 mg×1); study (2) high-dose Ang II infusion (3 ng/kg per minute); study (3) blinded crossover randomization to aldosterone infusion (0.7 µg/kg per hour) and vehicle; and study (4) blinded randomization to spironolactone (50 mg/daily) or placebo for 6 weeks. Infusion of Ang II at 1 ng/kg per minute acutely increased aldosterone (+148%) and PTH (+10.3%), whereas Ang II at 3 ng/kg per minute induced larger incremental changes in aldosterone (+241%) and PTH (+36%; P<0.01). Captopril acutely decreased aldosterone (-12%) and PTH (-9.7%; P<0.01). In contrast, aldosterone infusion robustly raised serum aldosterone (+892%) without modifying PTH. However, spironolactone therapy during 6 weeks modestly lowered PTH when compared with placebo (P<0.05). In vitro studies revealed the presence of Ang II type I and mineralocorticoid receptor mRNA and protein expression in normal and adenomatous human parathyroid tissues. We observed novel pleiotropic relationships between RAAS components and the regulation of PTH in individuals without primary hyperaldosteronism: the acute modulation of PTH by the RAAS seems to be mediated by Ang II, whereas the long-term influence of the RAAS on PTH may involve aldosterone. Future studies to evaluate the impact of RAAS inhibitors in treating PTH-mediated disorders are warranted.

Adenosine inhibits renin release from juxtaglomerular cells via an A1 receptor-TRPC-mediated pathway

Renin is synthesized and released from juxtaglomerular (JG) cells. Adenosine inhibits renin release via an adenosine A1 receptor (A1R) calcium-mediated pathway. How this occurs is unknown. In cardiomyocytes, adenosine increases intracellular calcium via transient receptor potential canonical (TRPC) channels. We hypothesized that adenosine inhibits renin release via A1R activation, opening TRPC channels. However, higher concentrations of adenosine may stimulate renin release through A2R activation. Using primary cultures of isolated mouse JG cells, immunolabeling demonstrated renin and A1R in JG cells, but not A2R subtypes, although RT-PCR indicated the presence of mRNA of both A2AR and A2BR. Incubating JG cells with increasing concentrations of adenosine decreased renin release. Different concentrations of the adenosine receptor agonist N-ethylcarboxamide adenosine (NECA) did not change renin. Activating A1R with 0.5 μM N6-cyclohexyladenosine (CHA) decreased basal renin release from 0.22 ± 0.05 to 0.14 ± 0.03 μg of angiotensin I generated per milliliter of sample per hour of incubation (AngI/ml/mg prot) (P < 0.03), and higher concentrations also inhibited renin. Reducing extracellular calcium with EGTA increased renin release (0.35 ± 0.08 μg AngI/ml/mg prot; P < 0.01), and blocked renin inhibition by CHA (0.28 ± 0.06 μg AngI/ml/mg prot; P < 0. 005 vs. CHA alone). The intracellular calcium chelator BAPTA-AM increased renin release by 55%, and blocked the inhibitory effect of CHA. Repeating these experiments in JG cells from A1R knockout mice using CHA or NECA demonstrated no effect on renin release. However, RT-PCR showed mRNA from TRPC isoforms 3 and 6 in isolated JG cells. Adding the TRPC blocker SKF-96365 reversed CHA-mediated inhibition of renin release. Thus A1R activation results in a calcium-dependent inhibition of renin release via TRPC-mediated calcium entry, but A2 receptors do not regulate renin release.

Control of renal calcium, phosphate, electrolyte, and water excretion by the calcium-sensing receptor

Through regulation of excretion, the kidney shares responsibility for the metabolic balance of calcium (Ca(2+)) with several other tissues including the GI tract and bone. The balances of Ca(2+) and phosphate (PO4), magnesium (Mg(2+)), sodium (Na(+)), potassium (K(+)), chloride (Cl(-)), and water (H2O) are linked via regulatory systems with overlapping effects and are also controlled by systems specific to each of them. Cloning of the calcium-sensing receptor (CaSR) along with the recognition that mutations in the CaSR gene are responsible for two familial syndromes characterized by abnormalities in the regulation of PTH secretion and Ca(2+) metabolism (Familial Hypocalciuric Hypercalcemia, FHH, and Autosomal Dominant Hypocalcemia, ADH) made it clear that extracellular Ca(2+) (Ca(2+)o) participates in its own regulation via a specific, receptor-mediated mechanism. Demonstration that the CaSR is expressed in the kidney as well as the parathyroid glands combined with more complete characterizations of FHH and ADH established that the effects of elevated Ca(2+) on the kidney (wasting of Na(+), K(+), Cl(-), Ca(2+), Mg(2+) and H2O) are attributable to activation of the CaSR. The advent of positive and negative allosteric modulators of the CaSR along with mouse models with global or tissue-selective deletion of the CaSR in the kidney have allowed a better understanding of the functions of the CaSR in various nephron segments. The biology of the CaSR is more complicated than originally thought and difficult to define precisely owing to the limitations of reagents such as anti-CaSR antibodies and the difficulties inherent in separating direct effects of Ca(2+) on the kidney mediated by the CaSR from associated CaSR-induced changes in PTH. Nevertheless, renal CaSRs have nephron-specific effects that contribute to regulating Ca(2+) in the circulation and urine in a manner that assures a narrow range of Ca(2+)o in the blood and avoids excessively high concentrations of Ca(2+) in the urine.