Caveolae, caveolin and cav-p60 in smooth muscle and renin-producing cells in the rat kidney

AIMS: In vascular smooth muscle cells caveolae are important for signalling mechanisms regulating vascular contraction. In smooth muscle layer of the renal afferent arteriole juxtaglomerular cells (JG cells) are non-contractile renin producing cells that have the capacity to change their phenotype into smooth muscle cells and back again by metaplastic transformation. Signalling mechanisms in JG cells are not fully understood and we therefore investigated if caveolae were present, and thereby could be involved as integrators of cellular signalling in both of these phenotypes of smooth muscle cells. METHODS: Using electron microscopy we compared the number of caveolae in JG cells and smooth muscle cells in the afferent arteriole of the rat kidney. The expression of caveolin and cav-p60 was examined using a combination of immunogold electron microscopy and immunofluorescence microscopy. RESULTS: We found that JG cells have sixfold less caveolae per cell surface sectional length than smooth muscle cells. The expression of cavolin-1 and cav-p60 correlated with the number of caveolae. An examination of the general distribution of caveolae, cav-p60 and caveolins in the rat kidney showed that cav-p60, like caveolin-1, is a specific maker of caveolae. CONCLUSION: The number of caveolae in JG cells is very low, and this makes it unlikely that caveolae are of major importance for the renin secretion specific for JG cells.     

Caveolae,caveolin and cav‐p60 in smooth muscle and renin‐producing cells in the rat kidney

Aims: In vascular smooth muscle cells caveolae are important for signalling mechanisms regulating vascular contraction. In smooth muscle layer of the renal afferent arteriole juxtaglomerular cells (JG cells) are non‐contractile renin producing cells that have the capacity to change their phenotype into smooth muscle cells and back again by metaplastic transformation. Signalling mechanisms in JG cells are not fully understood and we therefore investigated if caveolae were present, and thereby could be involved as integrators of cellular signalling in both of these phenotypes of smooth muscle cells. Methods: Using electron microscopy we compared the number of caveolae in JG cells and smooth muscle cells in the afferent arteriole of the rat kidney. The expression of caveolin and cav‐p60 was examined using a combination of immunogold electron microscopy and immunofluorescence microscopy. Results: We found that JG cells have sixfold less caveolae per cell surface sectional length than smooth muscle cells. The expression of cavolin‐1 and cav‐p60 correlated with the number of caveolae. An examination of the general distribution of caveolae, cav‐p60 and caveolins in the rat kidney showed that cav‐p60, like caveolin‐1, is a specific maker of caveolae. Conclusion: The number of caveolae in JG cells is very low, and this makes it unlikely that caveolae are of major importance for the renin secretion specific for JG cells.     

Immunohistochemical localization of renin in renal tumors.

Immunoperoxidase staining for renin was performed with renal tumors, including juxtaglomerular (JG) tumor, Wilms' tumors, renal adenocarcinomas, renal oncocytomas, and cortical adenomas. Compared with the JG apparatus adjacent to the glomerulus, JG tumor cells were less darkly but diffusely stained for renin. One of five Wilms' tumors revealed more numerous renin-containing tumor cells than the adjacent renal cortex, whereas three of ten renal adenocarcinomas and two of three renal oncocytomas revealed only focally renin-positive tumor cell cytoplasms. None of six cortical adenomas were positive for renin. With available fresh tumor tissue, renin activity was studied by measuring newly formed angiotensin I by radioimmunoassay. JG tumor contained markedly elevated renin activity, whereas one Wilms' tumor and two renal adenocarcinomas contained no more than 2% of renin activity of the renal cortex, more than 50% of which was inactive renin. These findings suggest that the JG tumor elaborates enormous amounts of active renin, whereas other renal tumors produce lesser amounts of renin, more than half of which is inactive renin.     

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 E₂–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.     

Interleukin-1 inhibits renin gene expression in As4.1 cells but not in native juxtaglomerular cells

 Cardiovascular effects of inflammatory interleukins (IL) have been suggested to be mediated by the renin-angiotensin system in vivo. To address the direct cellular effect of IL, we examined the influence of IL-1β on renin secretion and renin mRNA in cultures of mouse juxtaglomerular granular (JG) cells and in the mouse tumor cell line As4.1, which expresses renin mRNA. Renin mRNA levels and secretion of active renin were not significantly changed by IL-1β in native JG cells. Activation of adenylyl cyclase by forskolin increased renin secretion and renin mRNA levels three- and fivefold, respectively. These stimulatory responses to forskolin were not altered by IL-1β. In contrast to native JG cells, renin mRNA abundance was markedly suppressed by IL-1β in As4.1 cells, whereas secretion of active renin and the stability of renin mRNA were not changed. In As4.1 cells forskolin did not change renin secretion or renin mRNA abundance in the absence or in the presence of IL-1β. These findings suggest that IL-1β has no direct influence on renin secretion and renin mRNA abundance at the level of native JG cells. Received: 15 December 1997 / Received after revision: 25 April 1998 / Accepted: 27 May 1998     

The influence of extracellular and intracellular calcium on the secretion of renin

Changes in plasma, extracellular, and intracellular calcium can affect renin secretion from the renal juxtaglomerular (JG) cells. Elevated intracellular calcium directly inhibits renin release from JG cells by decreasing the dominant second messenger intracellular cyclic adenosine monophosphate (cAMP) via actions on calcium-inhibitable adenylyl cyclases and calcium-activated phosphodiesterases. Increased extracellular calcium also directly inhibits renin release by stimulating the calcium-sensing receptor (CaSR) on JG cells, resulting in parallel changes in the intracellular environment and decreasing intracellular cAMP. In vivo, acutely elevated plasma calcium inhibits plasma renin activity (PRA) via parathyroid hormone-mediated elevations in renal cortical interstitial calcium that stimulate the JG cell CaSR. However, chronically elevated plasma calcium or CaSR activation may actually stimulate PRA. This elevation in PRA may be a compensatory mechanism resulting from calcium-mediated polyuria. Thus, changing the extracellular calcium in vitro or in vivo results in inversely related acute changes in cAMP, and therefore renin release, but chronic changes in calcium may result in more complex interactions dependent upon the duration of changes and the integration of the body’s response to these changes.     

Parallel regulation of renin and lysosomal integral membrane protein 2 in renin-producing cells: further evidence for a lysosomal nature of renin secretory vesicles

The protease renin is the key enzyme in the renin–angiotensin system (RAS) that regulates extracellular volume and blood pressure. Renin is synthesized in renal juxtaglomerular cells (JG cells) as the inactive precursor prorenin. Activation of prorenin by cleavage of the prosegment occurs in renin storage vesicles that have lysosomal properties. To characterize the renin storage vesicles more precisely, the expression and functional relevance of the major lysosomal membrane proteins lysosomal-associated membrane protein 1 (LAMP-1), LAMP-2, and lysosomal integral membrane protein 2 (LIMP-2) were determined in JG cells. Immunostaining experiments revealed strong coexpression of renin with the LIMP-2 (SCARB2), while faint staining of LAMP-1 and LAMP-2 was detected in some JG cells only. Stimulation of the renin system (ACE inhibitor, renal hypoperfusion) resulted in the recruitment of renin-producing cells in the afferent arterioles and parallel upregulation of LIMP-2, but not LAMP-1 or LAMP-2. Despite the coregulation of renin and LIMP-2, LIMP-2-deficient mice had normal renal renin mRNA levels, renal renin and prorenin contents, and plasma renin and prorenin concentrations under control conditions and in response to stimulation with a low salt diet (with or without angiotensin-converting enzyme (ACE) inhibition). No differences in the size or number of renin vesicles were detected using electron microscopy. Acute stimulation of renin release by isoproterenol exerted similar responses in both genotypes in vivo and in isolated perfused kidneys. Renin and the major lysosomal protein LIMP-2 are colocalized and coregulated in renal JG cells, further corroborating the lysosomal nature of renin storage vesicles. LIMP-2 does not appear to play an obvious role in the regulation of renin synthesis or release.     

Membrane potential and cation channels in rat juxtaglomerular cells

The relationship between membrane potential and cation channels in juxtaglomerular (JG) cells is not well understood. Here we review electrophysiological and molecular studies of JG cells demonstrating the presence of large voltage-sensitive, calcium-activated potassium channels (BK(Ca)) of the ZERO splice variant, which is also activated by cAMP. These channels explain the hyperpolarization, which has been observed after stimulation of renin release with cAMP. In addition, there is now evidence that JG cells express functional L-type voltage-dependent calcium channels (Ca(v) 1.2), which in situations with strong depolarization lead to calcium influx and inhibition of renin release. In most in vivo situations the membrane potential is probably protected against depolarization by the BK(Ca) channels.     

The hydronephrotic kidney of the mouse as a tool for intravital microscopy and in vitro electrophysiological studies of renin-containing cells

Experimental hydronephrosis in mice has been studied with histological, ultrastructural, immunohistochemical, biochemical, and electrophysiological techniques to establish its value as a preparation for the investigation of glomerular microcirculation as well as the electrophysiological and biochemical properties of the renin-containing juxtaglomerular (JG) and vascular smooth muscle (VSM) cells of the afferent glomerular arteriole. During developing hydronephrosis the kidney parenchyma becomes progressively thinner as a result of tubular atrophy, being, after 12 weeks, a tissue sheet of about 200 micron in thickness. In this preparation, the renal arterial tree, in particular the glomerular arterioles, and also the glomeruli can be easily visualized. This permits intravital microscopic studies or direct visual identification of JG and VSM cells for microelectrode impalement. In spite of complete tubular atrophy, the vascular system is well preserved. Ultrastructurally, JG and VSM cells as well as the axon terminals innervating the vessels are intact. The same holds for the glomeruli except for a certain confluence of the podocyte foot processes and a thickening of the basal lamina. Renin immunostaining and kidney renin content in the hydronephrotic organ correspond to those in control kidneys. In addition, renin release from this preparation can be stimulated in a typical manner by isoproterenol and inhibited by angiotensin II, indicating that the receptors controling renin release and the secretory mechanism itself are still intact. Electrophysiological recordings from JG and VSM cells show a high membrane potential (-75 mv), and spontaneous depolarizing junction potentials, owing to transmitter release from the nerve terminals. Inhibitors of renin secretion, e.g. angiotensin II, depolarize both cell types, whereas stimulators such as isoproterenol do not change the membrane potential. We conclude that the hydronephrotic mouse kidney is a suitable model for in vitro studies of the electrophysiology and biochemistry of the media cells of the afferent arteriole, as well as for in vivo studies of glomerular microcirculation.     

Ultrastructural juxtaglomerular cell changes in normotensive rats treated with quinapril, an inhibitor of angiotensin-converting enzyme

Sequential histologic and ultrastructural changes in juxtaglomerular apparatus (JGA) were defined in male rats treated with quinapril, an inhibitor of angiotensin-converting enzyme (ACE). Doses of 0, 10, 100, and 400 mg/kg were administered daily by gavage for up to 4 weeks. Granular juxtaglomerular (JG) cells were normal or hypogranular on Day 1 at all doses and were hypergranular by Day 7 in rats given 100 and 400 mg/kg relative to age-matched controls. Histologically, JGA hypertrophy was apparent by Day 7 at all doses and was most pronounced by Day 14 in intermediate and deep cortical zones of rats given 100 and 400 mg/kg. Ultrastructurally, hypertrophic JG cells had abundant rough endoplasmic reticulum and free ribosomes, and prominent Golgi complexes associated with numerous cytoplasmic coated vesicles. Dose-dependent increases in numbers of protogranules, altered granules, and cytoplasmic vacuoles occurred in association with decreased size and increased pleomorphism of mature secretory granules. Granule alterations included vesicular to lamellar membranous matrical inclusions, irregular patterns of osmiophilia, matrical vacuolation, and flocculent to coarsely granular matrix of greater density than mature granules. We concluded that JG cell hypertrophy and hyperplasia occurred rapidly in response to subchronic ACE inhibition. Further, ultrastructural changes in JG cells were indicative of stimulated renin synthesis by a regulated pathway, renin secretion by exocytosis and cytoplasmic solubilization of granules, and autophagy of granules as a mechanism whereby JG cells regulate levels of stored renin under conditions of excessive stimulation.     

Exocytosis of secretory granules in the juxtaglomerular granular cells of kidneys

Background: There is little agreement as to the secretory process of renin granules in juxtaglomerular granular cells (JG cells) of kidneys, although a large number of studies of the regulation of renin secretion have been reported. Methods: The structural correlation between the stimuli and the secretory process was examined in mouse JG cells on renal cortical slice incubated with the beta-adrenergic agonist, isoproterenol; the loop diuretic, furocemide; the Ca²⁺ chelator, EGTA; and the actin filament-disrupting agent, cytochalasin B. Results and Conclusions: Treatment with isoproterenol (10⁻⁵−10⁻³ M) or furocemide (10⁻³M) in Ca²⁺-containing medium did not significantly affect the ultrastructure of JG cells. In slices incubated with isoproterenol or furocemide in the Ca²⁺-free medium, JG cells occasionally contained a few electron-lucent granules at the cell periphery in addition to the electrondense mature granules observed in the control slices. On rare occasions, the JG cells displayed omega-shaped cavities with electron-lucent matrices, a feature similar to the contents of electron-lucent granules. Cytochalasin B markedly promoted the effects of these stimulants in Ca²⁺-free medium. These findings suggest the participation of actin filament disassembly in the exocytotic process of the mature granules in JG cells. © 1995 Wiley-Liss, Inc.     

巨细胞病毒感染对细胞肾素基因表达的影响及其生物学意义

目的 探讨巨细胞病毒(cytomegalovirus,CMV)感染肾球旁细胞肾素基因表达的变化及其意义.方法 用病毒感染复数(MOI)为10、0.1和0的鼠CMV分别与鼠肾球旁细胞模型As4.1细胞共育5 d作为实验组;用紫外线灭活病毒的假感染(mock感染)对照组.RT-PCR检测感染细胞中CMV即刻早期基因1(IE1)的表达;免疫荧光观察肾素阳性细胞和肾素阳性颗粒在细胞的分布;双色免疫荧光染色观察肾素阳性颗粒是否出现在CMV阳性细胞;RT-PCR和Western blot检测肾素基因在感染细胞内的表达.结果 CMV感染As4.1细胞后出现典型的病毒空斑;病毒感染细胞CMV IE1 RT-PCR产物阳性;肾素阳性细胞集中在病毒空斑周围CMV新感染细胞区,肾素阳性荧光颗粒主要以块状和环状存在于病毒感染细胞质中;双色免疫荧光染色显示肾素阳性颗粒和CMV阳性颗粒出现在同一细胞;CMV感染细胞肾素基因的表达随病毒感染量增加而增加.结论 CMV感染并导致其宿主细胞肾素基因表达,可能涉及CMV加速心血管疾病发生发展的新机制.     

巨细胞病毒感染对细胞肾素基因表达的影响及其生物学意义

目的 探讨巨细胞病毒(cytomegalovirus,CMV)感染肾球旁细胞肾素基因表达的变化及其意义.方法 用病毒感染复数(MOI)为10、0.1和0的鼠CMV分别与鼠肾球旁细胞模型As4.1细胞共育5 d作为实验组;用紫外线灭活病毒的假感染(mock感染)对照组.RT-PCR检测感染细胞中CMV即刻早期基因1(IE1)的表达;免疫荧光观察肾素阳性细胞和肾素阳性颗粒在细胞的分布;双色免疫荧光染色观察肾素阳性颗粒是否出现在CMV阳性细胞;RT-PCR和Western blot检测肾素基因在感染细胞内的表达.结果 CMV感染As4.1细胞后出现典型的病毒空斑;病毒感染细胞CMV IE1 RT-PCR产物阳性;肾素阳性细胞集中在病毒空斑周围CMV新感染细胞区,肾素阳性荧光颗粒主要以块状和环状存在于病毒感染细胞质中;双色免疫荧光染色显示肾素阳性颗粒和CMV阳性颗粒出现在同一细胞;CMV感染细胞肾素基因的表达随病毒感染量增加而增加.结论 CMV感染并导致其宿主细胞肾素基因表达,可能涉及CMV加速心血管疾病发生发展的新机制.     

KATP channels are not essential for pressure-dependent control of renin secretion

 This study aimed to investigate the functional role of ATP-sensitive K⁺ (K_ATP) channels in the control of renin secretion by renal perfusion pressure. We studied the effect of openers and blockers of K_ATP-channels on basal- and low-pressure-induced renin secretion from isolated perfused rat kidneys (IPRK). Cromakalim (0.1–10 μM) stimulated basal renin secretion up to threefold and caused vasorelaxation in the IPRK. Both effects of cromakalim were attenuated by glibenclamide. Cromakalim stimulated renin secretion from isolated juxtaglomerular (JG) cells and from microdissected afferent arterioles, all of which suggests that K _ATP channel openers stimulate renin secretion at the level of JG cells. A decrease in the perfusion pressure from 13.3 to 9.33 kPa (100 mmHg to 70 mmHg) increased renin secretion twofold, and cromakalim further increased renin secretion. At 5.33 kPa (40 mmHg) renin secretion was increased sevenfold and was not further enhanced by cromakalim. The low-pressure-induced stimulation of renin secretion was not changed by glibenclamide. Finally, the dependence on calcium of the cromakalim-induced stimulation of renin was examined. Addition of the calcium antagonist amlodipine to the perfusate stimulated renin secretion, and in this situation cromakalim had no further effect. The stimulation of renin secretion by cromakalim in the IPRK was markedly attenuated by angiotensin II (1 nM). These results suggest that cromakalim could stimulate renin secretion through a pathway that includes a decrease in JG cell calcium. K_ATP channels are not essentially involved in pressure-sensitive renin secretion. Received: 26 March 1997 / Received after revision: 15 November 1997 / Accepted: 1 December 1997     

The juxtaglomerular apparatus of the spontaneously hypertensive rat

Spontaneously hypertensive rats (SHR) are used to study the pathogenesis of essential hypertension. This study investigates the role of the renin-angiotensin system (RAS) in the pathogenesis of hypertension in SHRs and the morphometry of the JGA by a three-dimensional computer reconstruction program "GLOM" and electron microscopy. Systolic blood pressure (SBP) (tail cuff method) was higher in SHRs compared to controls (P less than 0.001). Plasma renin concentration (PRC) was lower in SHRs than in controls (P less than 0.001). Reconstruction of the JGA revealed granulated JG cells in the afferent and efferent arterioles and in the vascular tree away from the JGA area. Electron microscopy showed granulated JG cells in the afferent and efferent arterioles. The percentage volume of the granulated JG cells in SHR was significantly higher than in controls (P less than 0.01). A relationship was found between the percentage volume of granulated JG cells and the SBP in SHRs (r = 0.933, P less than 0.05). The wall/lumen perimeter ratio was also significantly higher in the SHRs compared to the controls (P less than 0.05). Low PRC in SHRs has been reported by several workers. The apparent hyperactivity of the JGA may indicate failure of renin release or an abnormal synthesis/secretion rate.     

Hypothesis:presynaptic receptors controlling renin release.

Presynaptic receptors have recently been identified on nerves supplying vascular smooth muscle. Renin is stored in juxtaglomerular cells in the wall of the afferent glomerular arteriole and the JG cell is probably derived from vascular smooth muscle. Nervous stimuli exert an important influence on release of renin, but the nature of the receptors involved is not agreed. We propose that there are presynaptic receptors associated with the JG apparatus as with vascular muscle and that a mechanism of this sort gives a better account of data on renin release.     

Renin secretion from permeabilized juxtaglomerular cells requires a permeant cation

 The cytosolic concentration of chloride correlates directly with renin secretion from renal juxtaglomerular granular (JG) cells. In the present study, the mechanism by which chloride stimulates renin release was investigated in a preparation of permeabilized rat glomeruli with attached JG cells. An isosmotic increase in the concentration of chloride by 129 mM stimulated renin release 16- to 20-fold. Substitution of K⁺ by the impermeant cation N-methyl-d-glucamine (NMDG) abolished this response, while substitution with Na⁺ caused marginal inhibition. Substitution with Cs⁺ had no effect. Addition of sucrose, which permeates the secretory granules poorly, also abolished the stimulation of renin secretion by KCl. The response to KCl was not affected by K⁺-channel antagonists or by agonists of K⁺ channels. Chloride channel blockers were also without effect on the secretory response to KCl. When the ATP concentration was lowered from 1 to 0.1 mM renin release was stimulated, while an increase in the ATP concentration from 1 to 5 mM had no effect. Blockers of ATP-sensitive (K_ATP) channels did not modify the response to chloride. The present data suggest that chloride stimulates renin release after entry of KCl into the renin secretory granules which results in swelling and release of renin. Received: 3 July 1998 / Received after revision: 9 September 1998 / Accepted: 8 October 1998     

Ultrastructure,renin status,contractile and electrophysiological properties of the afferent glomerular arteriole in the rat hydronephrotic kidney

Summary Histological, ultrastructural, immunohistochemical, intravital microscopic and electrophysiological techniques have been applied to study experimental hydronephrosis in rats in order to assess its value as a preparation for the investigation of renal microcirculation and of the electrophysiological properties of the renin-containing juxtaglomerular (JG) cells of the afferent glomerular arteriole.As hydronephrosis develops, the kidney parenchyma becomes progressively thinner owing to tubular atrophy. Twelve weeks after ureteral ligature, this process results in a transparent tissue sheet of about 150–200 m in thickness. In this preparation, the renal arterial tree as well as the glomeruli can be easily visualized for intravital microscopic studies, e.g. the determination of kidney vessel diameters, or the identification of JG cells for penetration with an intracellular microelectrode. In contrast to the tubular atrophy, the vascular system is well preserved, and the JG cells and the sympathetic axon terminals are ultrastructurally intact. This is also true for the glomeruli, except for a certain confluence of the podocyte foot processes and a thickening of the basal laminae. Renin immunostaining and kidney renin content in the hydronephrotic organ correspond to those in control kidneys. In addition, there are no differences in the plasma renin levels of hydronephrotic and control rats.Intravital microscopic observations reveal that the renal vascular tree reacts in a typical, concentration dependent manner to the vasoconstrictor agent angiotensin II, mainly at the level of the resistance vessels. Electrophysiological recordings from juxtaglomerular granulated cells show a high membrane potential (–60 mV), and spontaneous depolarizing junction potentials, owing to random transmitter release from the nerve terminals. Angiotensin II, an inhibitor of renin release, depolarizes JG cells reversibly.Hence, we may infer that the hydronephrotic rat kidney is a suitable model for in vivo studies of the renal microcirculation as well as for in vitro investigations of the electrophysiological properties of the media cells of the afferent glomerular arteriole.This work was supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich 90 Cardiovaskuläres System and within the Forschergruppe Niere/ Heidelberg