Myosin content and vasoconstrictive ability of the proximal and distal (renin-positive) segments of the preglomerular arteriole

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.

Tubular control of renin synthesis and secretion

The intratubular composition of fluid at the tubulovascular contact site of the juxtaglomerular apparatus serves as regulatory input for secretion and synthesis of renin. Experimental evidence, mostly from in vitro perfused preparations, indicates an inverse relation between luminal NaCl concentration and renin secretion. The cellular transduction mechanism is initiated by concentration-dependent NaCl uptake through the Na-K-2Cl cotransporter (NKCC2) with activation of NKCC2 causing inhibition and deactivation of NKCC2 causing stimulation of renin release. Changes in NKCC2 activity are coupled to alterations in the generation of paracrine factors that interact with granular cells. Among these factors, generation of PGE2 in a COX-2-dependent fashion appears to play a dominant role in the stimulatory arm of tubular control of renin release. [NaCl] is a determinant of local PG release over an appropriate concentration range, and blockade of COX-2 activity interferes with the NaCl dependency of renin secretion. The complex array of local paracrine controls also includes nNOS-mediated synthesis of nitric oxide, with NO playing the role of a modifier of the intracellular signaling pathway. A role of adenosine may be particularly important when [NaCl] is increased, and at least some of the available evidence is consistent with an important suppressive effect of adenosine at higher salt concentrations.

The role of calcium in the regulation of renin secretion

Renin is the enzyme which is the rate-limiting step in the formation of the hormone angiotensin II. Therefore, the regulation of renin secretion is critical in understanding the control of the renin-angiotensin-aldosterone system and its many biological and pathological actions. Renin is synthesized, stored in, and released from the juxtaglomerular (JG) cells of the kidney. While renin secretion is positively regulated by the "second messenger" cAMP, unlike most secretory cells, renin secretion from the JG cell is inversely related to the extracellular and intracellular calcium concentrations. This novel relationship is referred to as the "calcium paradox." This review will address observations made over the past 30 years regarding calcium and the regulation of renin secretion, and focus on recent observations which address this scientific conundrum. These include 1) receptor-mediated pathways for changing intracellular calcium; 2) the discovery of a calcium-inhibitable isoform of adenylyl cyclase associated with renin in the JG cells; 3) calcium-sensing receptors in the JG cells; 4) calcium-calmodulin-mediated signals; 5) the role of phosphodiesterases; and 6) connexins, gap junctions, calcium waves, and the cortical extracellular calcium environment. While cAMP is the dominant second messenger for renin secretion, calcium appears to modulate the integrated activities of the enzymes, which balance cAMP synthesis and degradation. Thus this review concludes that calcium modifies the amplitude of cAMP-mediated renin-signaling pathways. While calcium does not directly control renin secretion, increased calcium inhibits and decreased calcium amplifies cAMP-stimulated renin secretion.

Fluid flow in the juxtaglomerular interstitium visualized in vivo

Earlier electron microscopy studies demonstrated morphological signs of fluid flow in the juxtaglomerular apparatus (JGA), including fenestrations of the afferent arteriole (AA) endothelium facing renin granular cells. We aimed to directly visualize fluid flow in the JGA, the putative function of the fenestrated endothelium, using intravital multiphoton microscopy of Munich-Wistar rats and C57BL6 mice. Renin content of the AA correlated strongly with the length of the fenestrated, filtering AA segment. Fluorescence of the extracellular fluid marker lucifer yellow (LY) injected into the cannulated femoral vein in bolus was followed in the renal cortex by real-time imaging. LY was detected in the interstitium around the JG AA before the plasma LY filtered into Bowman's capsule and early proximal tubule. The fluorescence intensity of LY in the JGA interstitium was 17.9 ± 3.5% of that in the AA plasma ( n = 6). The JGA fluid flow was oscillatory, consisting of two components: a fast (one every 5–10 s) and a slow (one every 45–50 s) oscillation, most likely due to the rapid transmission of both the myogenic and tubuloglomerular feedback (TGF)-mediated hemodynamic changes. LY was also detected in the distal tubular lumen about 2–5 s later than in the AA, indicating the flow of JGA interstitial fluid through the macula densa. In the isolated microperfused JGA, blocking the early proximal tubule with a micropipette caused significant increases in MD cell volume by 62 ± 4% ( n = 4) and induced dilation of the intercellular lateral spaces. In summary, significant and dynamic fluid flow exists in the JGA which may help filter the released renin into the renal interstitium (endocrine function). It may also modulate TGF and renin signals in the JGA (hemodynamic function).

Chronic angiotensin converting enzyme inhibition enhances renal vascular responsiveness to acetylcholine in anaesthetized rabbits

OBJECTIVE

To determine whether 6 weeks continuous treatment with an angiotensin converting enzyme (ACE) inhibitor reduced renal vascular responsiveness in vivo, since this treatment results in extensive phenotypic conversion of afferent arteriolar cells from contractile to endocrine-like, renin secretory cells.

METHODS

Enalapril (10 microg/kg per h s.c.) was delivered continuously for 6 weeks. In anaesthetized rabbits (treated or sham), arterial blood pressure and renal blood flow were measured and renal responsiveness tested by constructing dose-response curves to bolus doses of phenylephrine, angiotensin II and acetylcholine delivered directly into the renal artery.

RESULTS

ACE inhibition resulted in a significant shift to the left in the renal vascular conductance responses to acetylcholine (P < 0.005) and angiotensin II (P < 0.05), indicating enhanced, not reduced, responsiveness to these agents. There were no significant effects of chronic ACE inhibition on the conductance responses to phenylephrine.

CONCLUSIONS

Contrary to our hypothesis, 6 weeks ACE inhibition did not reduce renal vascular responsiveness to three vasoactive agents, suggesting that the phenotypic changes observed in the afferent arterioles and to a lesser extent the interlobular arteries, were either insignificant or compensated for by other changes in renal circulatory control.

Podocytes respond to mechanical stress in vitro

Glomerular capillary pressure is thought to affect the structure and function of glomerular cells. However, it is unknown whether podocytes are intrinsically sensitive to mechanical forces. In the present study, differentiated mouse podocytes were cultured on flexible silicone membranes. Biaxial cyclic stress (0.5 Hz and 5% linear strain) was applied to the membranes for up to 3 d. Mechanical stress reduced the size of podocyte cell bodies, and processes became thin and elongated. Podocytes did not align in the inhomogeneous force field. Whereas the network of microtubules and that of the intermediate filament vimentin exhibited no major changes, mechanical stress induced a reversible reorganization of the actin cytoskeleton: transversal stress fibers (SF) disappeared and radial SF that were connected to an actin-rich center (ARC) formed. Epithelial and fibroblast cell lines did not exhibit a comparable stress-induced reorganization of the F-actin. Confocal and electron microscopy revealed an ellipsoidal and dense filamentous structure of the ARC. Myosin II, alpha-actinin, and the podocyte-specific protein synaptopodin were present in radial SF, but, opposite to F-actin, they were not enriched in the ARC. The formation of the ARC and of radial SF in response to mechanical stress was inhibited by nonspecific blockade of Ca(2+) influx with Ni(2+) (1 mM), by Rho kinase inhibition with Y-27632 (10 microM), but not by inhibition of stretch-activated cation channels with Gd(3+) (50 microM). In summary, mechanical stress induces a unique reorganization of the actin cytoskeleton in podocytes, featuring radial SF and an ARC, which differ in protein composition. The F-actin reorganization in response to mechanical stress depends on Ca(2+) influx and Rho kinase. The present study provides the first direct evidence that podocytes are mechanosensitive.

Cell-specific protein and gene expression in the juxtaglomerular apparatus

1. The juxtaglomerular apparatus (JGA) consists of a tubular component, the macula densa (MD), attached to a vascular component consisting of the afferent and efferent arterioles and the extraglomerular mesangium. The JGA is richly innervated by sympathetic fibres. 2. The MD is morphologically, histochemically and functionally different from the ascending thick portion of the loop of Henle where it is located. 3. The vascular component includes the vascular smooth muscle cells of the arteriole, the renin-producing cells or juxtaglomerular cells, extraglomerular mesangial cells (Goormaghtigh cells) and endothelial cells. They are coupled by gap junctions. 4. Physiological evidence indicates that the composition of tubular fluid at the MD regulates renin secretion and glomerular haemodynamics and that the JGA is important in the maintenance of body salt-water homeostasis. Evidence suggests that the MD exerts its action on the vascular component through a paracrine mechanism.

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 slices 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(-3) M) in Ca(2+)-containing medium did not significantly affect the ultrastructure of JG cells. In slices incubated with isoproterenol or furocemide in the Ca(2+)-free medium, JG cells occasionally contained a few electron-lucent granules at the cell periphery in addition to the electron-dense 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 Ca(2+)-free medium. These findings suggest that participation of actin filament disassembly in the exocytotic process of the mature granules in JG cells.

Diversity and variability of smooth muscle phenotypes of renal arterioles as revealed by myosin isoform expression

The contractility and distensibility of renal arterioles are important in the regulation of glomerular filtration. However, little is known regarding the characteristics of contractile proteins in these arterioles. Recently it was demonstrated that vascular smooth muscles contain two types of myosin heavy chain (MHC) isoforms, SM1 and SM2, which are unique molecular markers of smooth muscle cell phenotypes. SM1 is constitutively expressed in all types of smooth muscles, whereas SM2 exists only in mature smooth muscles. We characterized the expression of MHC isoforms as well as the ultrastructural myofilament assembly of renal arteriolar smooth muscles in human, rat and rabbit by immunohistochemical techniques. SM1 and alpha-smooth muscle actin were localized in both the preglomerular vessels (including the afferent arterioles) and efferent arterioles, whereas SM2 was present only in the preglomerular vessels. Renin-producing cells in the afferent arterioles (juxtaglomerular granular cells, JG cells) were positive for alpha-smooth muscle actin but negative for SM2. When renin synthesis was stimulated, the more proximal afferent arteriolar smooth muscles turned renin-positive and SM2 disappeared. Glomerular mesangial cells did not show immunoreactivities for SM1, SM2 or alpha-smooth muscle actin. The difference in MHC isoform expression in these arterioles was also reflected by ultrastructures; the afferent arteriolar smooth muscles contained abundant myofilaments including thick filaments, whereas the efferent arteriolar smooth muscles had a few myofilaments composed only of thin microfilaments. The JG cells displayed a myofilament assembly similar to that in the efferent arteriolar smooth muscles. We conclude from these observations that smooth muscles in pre-and postglomerular arterioles, the glomerular mesangial cells and JG cells differ in phenotypes, suggesting that they may have different contractile properties which may be critically involved in the regulation of glomerular filtration.

Cell calcium concentration in glomerular afferent and efferent arterioles under the action of noradrenaline and angiotensin II

The glomerular arterioles in the juxtaglomerular apparatus seem to function as effectors of the tubuloglomerular feedback mechanism. In this mechanism increased delivery of fluid to the distal nephron activates the macula densa cells through transport via an Na-2Cl-K cotransporter. This activation may lead to vasoconstriction of the afferent arteriole. Furthermore, vasoactive substances seem to affect both afferent and efferent arterioles. There are morphological differences along the afferent arteriole, some parts containing epithelioid cells with renin granules and others regular smooth muscle cells. The aim of the present experiments was to determine whether noradrenaline (10(-6) M) and angiotensin II (10(-6) M) had differential effects on the cell calcium concentration [Ca2+]i and on contraction in isolated perfused afferent and efferent arterioles and in the mesangial region. [Ca2+]i was measured with fura-2, an intensified videocamera and a digital imaging system. From the proximal to the distal part of the arteriole [Ca2+]i increased from about 100 to 250 nM. A [Ca2+]i increase and a contraction were caused by noradrenaline alone in the proximal part of the afferent arteriole and by angiotensin II alone in the distal part of this arteriole. In the mesangial region there was a high basal [Ca2+]i but no response to the vasoactive substances. In the efferent arteriole, application of both noradrenaline and angiotensin II led to an increase in [Ca2+]i and a contraction. The present experiments indicate that the two vasoactive substances tested act in a similar fashion along the whole length of the efferent arteriole, while in the afferent arteriole their actions are not equally distributed.

Hypothetical interpretation of the calcium paradox in renin secretion

Most renin-positive cells of the preglomerular arteriole are intermediate in morphological appearence between smooth muscle cells and epithelioid cells. Intermediate cells contain, in addition to secretory granules, contractile proteins arranged as a sublemmal network. The paradoxical (inhibitory) role of calcium in renin secretion is explained, on the basis of these findings, by an increased tone of the sublemmal network; this might impair the preexocytotic access of renin granules to the cell membrane.

Cytoplasmic sequestration phenomena in smooth muscle cells of kidney resistance vessels and epithelioid cells of the juxtaglomerular apparatus

The occurrence of vacuoles in cells of contractile tissues and especially in media cells of resistance vessels has been known for quite some time. Recently, it has been widely accepted that these vacuoles, characteristically lined by a double membrane, result from herniation of one vascular smooth muscle cell into the other as a result of vasoconstriction. In our electronmicroscopic investigations we found double membrane-bounded vacuoles not only in kidney resistance vessels of rats and mice under conditions of vasoconstriction, but also in control animals and animals with maximal renal vasodilation. Part of our observations are compatible with the assumption that such vacuoles arise from a damage of club-shaped, musculo-muscular contacts due to shape changes of media cells during maximal vasoconstriction or vasodilation. However, serial thin sectioning revealed that some of the cytoplasmic vacuoles have no connections with neighbouring cells. This finding and various parallels to the generation of autophagic vacuoles indicate that the so-called herniations may also represent demarcations of large cytoplasmic areas within an individual cell. Irrespective of the origin of these vacuoles, their contents show different stages of deterioration. At later stages, the vacuoles appear to be adjacent, with only one membrane, to the extracellular space, into which they are believed to discharge finally. Cytoplasmic vacuolization has not only been observed in smooth muscle cells, but also in juxtaglomerular epithelioid cells of the afferent arteriole. Here the vacuoles-besides other organelles--also contain secretory granules; it is therefore proposed that autophagic phenomena with final extrusion of cytoplasmic material may be involved in the programmed down-regulation of the granular renin store following inhibition of renin synthesis and secretion.