Coexistence of renin and angiotensin II in epitheloid cell secretory granules of rat kidney

Evolving concepts on regulation and function of renin in distal nephron

Sustained stimulation of the intrarenal/intratubular renin-angiotensin system in a setting of elevated arterial pressure elicits renal vasoconstriction, increased sodium reabsorption, proliferation, fibrosis, and eventual renal injury. Activation of luminal AT(1) receptors in proximal and distal nephron segments by local Ang II formation stimulates various transport systems. Augmented angiotensinogen (AGT) production by proximal tubule cells increases AGT secretion contributing to increased proximal Ang II levels and leading to spillover of AGT into the distal nephron segments, as reflected by increased urinary AGT excretion. The increased distal delivery of AGT provides substrate for renin, which is expressed in principal cells of the collecting tubule and collecting ducts, and is also stimulated by AT(1) receptor activation. Renin and prorenin are secreted into the tubular lumen and act on the AGT delivered from the proximal tubule to form more Ang I. The catalytic actions of renin and or prorenin may be enhanced by binding to prorenin receptors on the intercalated cells or soluble prorenin receptor secreted into the tubular fluid. There is also increased luminal angiotensin converting enzyme in collecting ducts facilitating Ang II formation leading to stimulation of sodium reabsorption via sodium channel and sodium/chloride co-transporter. Thus, increased collecting duct renin contributes to Ang II-dependent hypertension by augmenting distal nephron intratubular Ang II formation leading to sustained stimulation of sodium reabsorption and progression of hypertension.

AT1 receptor-mediated enhancement of collecting duct renin in angiotensin II-dependent hypertensive rats

Angiotensin II (ANG II)-infused rats exhibit increases in distal nephron renin expressed in principal cells of connecting tubules and collecting ducts. This study was performed to determine whether the augmentation of distal nephron renin involves ANG II type 1 (AT1) receptor activation. Male Sprague-Dawley rats (200-220 g) were divided into three groups: 1) sham operated (n = 8); 2) ANG II infused (80 ng/min, 13 days, n = 8); and 3) ANG II infused plus AT1 receptor blocker (ARB), olmesartan (5 mg/days, n = 8). ANG II infusion increased systolic blood pressure (BP; 178 +/- 4 vs. 122 +/- 1 mmHg; P < 0.001) and suppressed plasma renin activity (PRA; 0.08 +/- 0.1 vs. 5.3 +/- 0.8 ng ANG I x ml(-1) x h(-1)). ARB treatment prevented the increase in BP (113 +/- 6 mmHg) and led to increases in PRA (15.8 +/- 1.5 ng ANG I x ml(-1) x h(-1)). Renin protein levels measured in the kidney medulla, to avoid contribution from juxtaglomerular apparatus cells, were higher in ANG II-infused rats [1.64 +/- 0.3 vs. 1.00 +/- 0.1 densitometric units (DU) compared with sham-operated rats; P < 0.05], and ARB treatment prevented this increase (1.01 +/- 0.1). Similarly, renin immunoreactivity increased in medullary collecting ducts of ANG II-infused compared with sham-operated rats (2.5 +/- 0.3 vs. 1.0 +/- 0.2 DU; P < 0.001), which was also prevented by ARB (1.01 +/- 0.06). Renin qRTPCR in ANG II-infused rats showed higher mRNA levels in the kidney medulla compared with sham-operated rats (5.5 +/- 2.3 vs. 0.04 +/- 0.02 ratio to GAPDH mRNA levels; P < 0.001); however, renin transcript levels were normalized in the ARB-treated rats. These data demonstrate that the augmentation of distal nephron renin in ANG II-infused hypertensive rats is AT1 receptor mediated. The augmented distal tubular renin may contribute to increased intratubular ANG II levels and distal nephron sodium reabsorption in ANG II-dependent hypertension.

Renal interstitial fluid concentrations of angiotensins I and II in anesthetized rats

Previous studies have indicated that angiotensin II (Ang II) concentrations in renal interstitial fluid are much higher than plasma levels. In the present study, we performed experiments to explore renal interstitial fluid concentrations of Ang I and Ang II further and to determine whether these levels are altered by acute arterial infusion of an ACE inhibitor (enalaprilat) or by volume expansion. Microdialysis probes (molecular weight cutoff point: 30 000 Da) were implanted in the renal cortex of anesthetized rats and were perfused at a rate of 2 microL/min. Using relative equilibrium rates, the basal renal interstitial fluid Ang II concentration averaged 3.07+/-0.43 nmol/L, a value much higher than the plasma Ang II concentration of 107+/-8 pmol/L (n=7). Interstitial fluid Ang I concentrations (0.84+/-0.04 nmol/L) were consistently lower than the Ang II concentrations but higher than the plasma Ang I concentrations (112+/-14 pmol/L). Intra-arterial infusion of enalaprilat (7.5 micromol/kg/min, n=5) for 120 minutes resulted in a significant decrease in mean arterial pressure (from 114+/-4 to 68+/-4 mm Hg) along with reductions in plasma and renal ACE activity (by -99% and -52%, respectively). Enalaprilat resulted in a significant increase in plasma Ang I from 133+/-21 to 1167+/-328 pmol/L and a decrease in plasma Ang II from 110+/-12 to 67+/-9 pmol/L. During enalaprilat infusion, interstitial fluid concentration of Ang I was significantly increased from 0.78+/-0.06 to 0.97+/-0.08 nmol/L; however, Ang II concentrations were not altered significantly (3.67+/-0.28 versus 3.67+/-0.25 nmol/L). Acute volume loading with Ringer's solution containing 1% bovine serum albumin at a rate of 150 microL/min for 2 hours (6% to 7% of body weight) lowered plasma concentrations of Ang I from 110+/-23 to 16+/-2 pmol/L and Ang II from 100+/-23 to 36+/-6 pmol/L; however, renal interstitial fluid concentrations of Ang I and Ang II were not altered significantly during volume expansion (Ang I, from 0.77+/-0.05 to 0.69+/-0.03 nmol/L; Ang II, from 3.76+/-0.43 to 3.59+/-0.39 nmol/L, n=5). These data indicate that renal interstitial fluid concentrations of Ang I and Ang II are substantially higher than the corresponding plasma concentrations. Furthermore, the fact that the high interstitial fluid concentrations of Ang II are not responsive to acute ACE inhibition or volume expansion suggests the compartmentalization and independent regulation of renal interstitial fluid Ang II.

The cardiac renin-angiotensin system: novel signaling mechanisms related to cardiac growth and function

Angiotensin II, the effector peptide of the renin-angiotensin system, has been demonstrated to be involved in the regulation of cellular growth of several tissues in response to developmental, physiological, and pathological processes. The recent identification of renin-angiotensin system components and localization of angiotensin II receptors in cardiac tissue suggests that locally synthesized Ang II can modulate functional and growth responses in cardiac tissue. In this review, regulation of the cardiac RAS is discussed, with an emphasis on growth-related Ang II signal transduction systems.

Elements of a paracrine tubular renin-angiotensin system along the entire nephron

The renin-angiotensin system is a major regulator of body sodium, predominantly through the actions of intrarenal angiotensin II of unclear origin. We show that polarized epithelium of the proximal tubule synthesizes and secretes angiotensinogen at its apical side and that the protein can be detected in urine as a function of dietary sodium. Furthermore, we demonstrate that renin is expressed and secreted in a restricted nephron segment, the connecting tubule, also in a sodium-dependent fashion. A paracrine renin-angiotensin system operating along the entire nephron may contribute to long-term arterial pressure regulation by integrating distant tubular sodium-reabsorbing functions.

The cardiac renin-angiotensin system: conceptual, or a regulator of cardiac function?

Angiotensin II, the effector peptide of the renin-angiotensin system, regulates cellular growth in response to developmental, physiological, and pathological processes. The identification of renin-angiotensin system components and angiotensin II receptors in cardiac tissue suggests the existence of an autocrine/paracrine system that has effects independent of angiotensin II derived from the circulatory system. To be functional, a local renin-angiotensin system should produce sufficient amounts of the autocrine and/or paracrine factor to elicit biological responses, contain the final effector (angiotensin II receptor), and respond to humoral, neural, and/or mechanical stimuli. In this review, we discuss evidence for a functional cardiac renin-angiotensin system.

ACE Inhibitors and Renal Vascular Responses in the Spontaneously Hypertensive Rat

BACKGROUND: Substantial evidence has accumulated for the intrarenal generation of functionally important quantities of angiotensin II (Ang II). To assess the possibility that Ang II generation occurs beyond a barrier to diffusion from the vascular compartment, six angiotensin-converting enzyme (ACE) inhibitors varying widely in their lipid solubility were employed in the spontaneously hypertensive rat (SHR) and their normotensive controls (WKY). The biological end points were renal blood flow and its response to Ang II. RESULTS: Two ACE inhibitors, ramipril and captopril, induced a larger increase in renal blood flow and enhanced the renal vascular response to Ang II substantially more than did enalapril and lisinopril. The two prodrugs, enalapril and ramipril, which are substantially more lipophilic than the respective active drugs, enalaprilat and ramiprilat, showed equivalent responses. The partial agonist saralasin virtually abolished the renal vasodilator response to ramipril. The pattern of response was similar in WKY, but the responses were substantially smaller. CONCLUSIONS: The results support the concept that a functionally important compartment for intrarenal Ang II formation exists in the healthy rat and that this process is amplified in the SHR.

Evidence for intracellular generation of angiotensin II in rat juxtaglomerular cells

The formation of the vasoactive peptide angiotensin II (AII) is dependent on the sequential action of two enzymes, renin and angiotensin converting enzyme (ACE), on the substrate angiotensinogen. Although the renin-producing cells of the kidney do not express angiotensinogen, they contain large amounts of AII in the same storage granules that contain renin. When renin expression is suppressed in these cells, AII also disappears. In the current study, we have tested whether the renin-associated disappearance of AII in renal juxtaglomerular (JG) cells is due to a renin-dependent down-regulation of granule biosynthesis and whether receptor-mediated internalization of AII could account for its concentration in these cells. Our results support a model whereby AII peptides are generated within JG cells, presumably by a mechanism which involves the action of endogenous renin on internalized, exogenous angiotensinogen.

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.

Chronic hypertension and altered baroreflex responses in transgenic mice containing the human renin and human angiotensinogen genes

We have generated a transgenic model consisting of both the human renin and human angiotensinogen genes to study further the role played by the renin-angiotensin system in regulating arterial pressure. Transgenic mice containing either gene alone were normotensive, whereas mice containing both genes were chronically hypertensive. Plasma renin activity and plasma angiotensin II levels were both markedly elevated in the double transgenic mice compared with either single transgenic or nontransgenic controls. The elevation in blood pressure caused by the human transgenes was independent of the genotype at the endogenous renin locus and was equal in mice homozygous for the Ren-1c allele or in mice containing one copy each of Ren-1c, Ren-1d, or Ren-2. Chronic overproduction of angiotensin II in the double transgenic mice resulted in a resetting of the baroreflex control of heart rate to a higher pressure without significantly changing the gain or sensitivity of the reflex. Moreover, this change was not due to the effects of elevated pressure itself since angiotensin-converting enzyme inhibition had minimal effects on the baroreflex in spontaneously hypertensive BPH-2 control mice, which exhibit non-renin-dependent hypertension. This double transgenic model should provide an excellent tool for further studies on the mechanisms of hypertension initiated by the renin-angiotensin system.

Pathophysiology of altered renal function in renal vascular hypertension

It is now clear that specific angiotensin-dependent mechanisms contribute importantly to the pathophysiology of hypertension (HT) and altered renal function in models of two-kidney, one-clip (2-K, 1-C) HT in rats. The discovery of specific antagonists for angiotensin-converting enzyme and the newer angiotensin receptor and kinin receptor antagonists have allowed delineation of the contributions of these hormones to altered renal function in these models. The focus of interest in most of these studies has been the nonclipped kidney, which would be expected to ameliorate elevated blood pressure by exhibiting a pressure diuresis and natriuresis in the environment of systemic HT. Antagonism of the renin-angiotensin system in rat models of renal vascular HT indicates that the effects of angiotensin attenuate renal hemodynamic and excretory behavior, particularly in the nonclipped kidney. Furthermore, angiotensin attenuates the efficiency of autoregulation of renal hemodynamics in the nonclipped kidney. Function of the clipped kidney appears to be both angiotensin and perfusion pressure dependent. Evidence that inhibition of angiotensin reverses or improves these altered hemodynamic and excretory functions indicates that angiotensin may contribute importantly to the pathophysiology of HT in these models by altering or impairing the ability of the nonclipped or "normal" kidney to excrete sodium and volume. The precise roles of altered activity of vasodilator hormones to contribute to these alterations of renal function remains to be defined.

Rat angiotensinogen is secreted only constitutively when transfected into AtT-20 cells

To test whether angiotensinogen might be targeted to dense core secretory granules in cells containing a regulated secretory pathway, we expressed rat angiotensinogen in AtT-20 cells, a mouse pituitary cell line that has the demonstrated ability to correctly sort proteins to the constitutive or regulated pathway. We compared the pattern of secretion of angiotensinogen with that of endogenous adrenocorticotropin hormone, which is secreted by AtT-20 cells through the regulated pathway. When cells were incubated for 5 hours with dibutyryladenosine cyclic monophosphate or KCl, adrenocorticotropin hormone secretion was significantly higher than control, whereas monensin had no effect. In contrast, angiotensinogen secretion was markedly reduced by monensin, but no stimulation was observed with dibutyryladenosine cyclic monophosphate or KCl. These results make it unlikely that angiotensinogen could be cotargeted with active renin in the dense core granules of the regulated pathway. Alternative mechanisms must explain how angiotensin II is synthesized locally by tissue renin-angiotensin systems.

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.

The juxtaglomerular apparatus in Bartter's syndrome and related tubulopathies. An immunocytochemical and electron microscopic study

A comparative immunocytochemical and electron microscopic study was performed on renal biopsies from two children with classical Bartter's syndrome (BS) and three children with a recently described variant, the so-called hyperprostaglandin E-syndrome (HES). Compared to age-matched controls, kidney specimens from patients with BS and HES disclosed a marked hypertrophy and hyperplasia of the juxtaglomerular apparatus (JGA). In addition, in HES focal tubular and interstitial calcifications accompanied by interstitial fibrosis and tubular atrophy were noted. On immunocytochemistry, chronic stimulation of the JGA in BS and HES was characterized by an increase in the number of renin-positive cells, particularly in the media of afferent arterioles, but also in efferent arterioles and in the glomerular stalk. The length of the renin-positive portion of the preglomerular arterioles was significantly increased when compared to controls (100 +/- 32 vs. 49 +/- 17 microns; p less than 0.001). In addition, the immunoreactivity of individual renin-positive cells was markedly enhanced. On electron microscopy, "hypertrophy" of the RER and of Golgi complexes with paracrystalline deposits in dilated RER cisterns and protogranules indicated an increased renin synthesis. Renin could be identified in mature secretory granules as well as protogranules by immune electron microscopy. Angiotensinogen was present in hypertrophied epithelial cells of Bowman's capsule. Converting-enzyme reactivity was observed in controls as well as in BS and HES in the brush border of the proximal tubule. In contrast to previous reports, Angiotensin II was completely negative in control as well as in diseased kidneys. We conclude from our results that both BS and HES are characterized by a marked activation of the JGA and severe stimulation of the renin-angiotensin system. Since activation of this system, however, leads--independently of the primary stimulus--to qualitatively very similar morphological reactions, these results do not implicate a common pathogenetic mechanism to both conditions.

Direct release of angiotensins I and II from isolated rat kidney perfused with angiotensinogen-free medium

A direct measurement of both angiotensins I and II immunoreactive substances was made in the perfusate from isolated rat kidney perfused with Krebs-Ringer solution which was free of any component of the renin-angiotensin system. The identity of the immunoreactive peptides was confirmed as angiotensin I and angiotensin II by high-pressure liquid chromatography in reference to standard compounds. The rate of release of angiotensins was as high as 1313.5 +/- 184.5 and 772.4 +/- 82.5 pg for angiotensins I and II, respectively, during the first perfusion period of 20 min, and it remained stable at least for 2 hours. There was a good relationship between the angiotensin I secretion rate and renin secretion rate simultaneously determined in the perfusate, and also between the angiotensin I secretion rate and angiotensin II secretion rate. These results taken together with the previous observations of the coexistence of renin and angiotensins I and II in juxtaglomerular cells of the kidney provide evidence for intrarenal formation and release of angiotensin II. It does not agree with the notion that these peptides are internalized from circulation. Angiotensin II secreted from the kidney may play diverse functions in intrarenal regulation.

Endothelial renin-angiotensin pathway: evidence for intracellular synthesis and secretion of angiotensins

Cultured bovine aortic endothelial cells (BAEC) contain renin and angiotensinogen. To examine whether angiotensins are synthesized intracellularly and secreted by these cells, we assayed cell extracts as well as serum-free media of intact confluent BAEC. Angiotensins were identified by their retention time on reverse phase high performance liquid chromatography and direct radioimmunoassay. BAEC and their media contain angiotensin II and angiotensin III. The rate of angiotensin accumulation in the media was a nonlinear function of time; the highest rate occurred in the first 15 minutes. The amount of angiotensin II accumulated in 30 minutes exceeded 200% of the intracellular concentration and that of angiotensin III exceeded 500% of the initial intracellular content. Neither renin nor angiotensinogen could be detected in the media. The viability of these cells was supported by low lactic dehydrogenase activity in the media (less than 0.5% of cellular level). These data suggest that BAEC is capable of synthesizing and secreting angiotensins. We postulate that this endothelial-derived angiotensin system may play an important paracrine or autocrine role in influencing local vascular tone.

Earliest renin containing cell differentiation during ontogenesis in the rat. An immunocytochemical study

The differentiation of renin containing cells was studied by immunocytochemistry in normal rat fetuses by the use of highly specific renin, angiotensin I and II antisera. Renin synthesizing cells were detectable as early as the 15th day of gestation outside the nephrogen territories within the walls of mesonephrotic-gonadic and renal arteries. Intrarenal differentiation began at the 17th day and progressed along the intrarenal arterial tree. AII immunostaining appeared concomitantly in the renin containing cells and developed considerably during ontogenesis, suggesting intracellular biosynthesis. It can be suggested that in the fetus newly synthesized AII may contribute to the early systemic and renal blood pressure regulation.

Comparison of angiotensin II staining in rat brain using affinity purified and crude antisera

The use of affinity purified ANG II antiserum as opposed to crude antiserum greatly enhanced the staining resolution in rat brain. This improved resolution was due to a complete loss of background staining and an apparent increase in specific staining that was totally blockable by preabsorption. With the purified antibody it was easily possible to visualise the finest fibres in rats not treated with colchicine. Furthermore, the improved technique permitted a clearer visualisation of an ANG II-like immunoreactive product in cell bodies. This use of affinity purified antibody should greatly facilitate the mapping of central angiotensinergic pathways.

Development and fate of the secretory granules of juxtaglomerular epithelioid cells

The development and fate of the secretory granules in murine, rat and human juxtaglomerular epithelioid cells were examined using ultrastructural and immunocytochemical methods. The formation of mature renin granules occurs by fusion of rhomboid protogranules followed by coalescence of their paracrystalline contents, and by the fusion of roundish juvenile granules having an amorphous internum. Protogranules with paracrystalline contents are prominent in animals with stimulated renin synthesis, indicating an overcharge in processing and/or packaging of the secretory product, renin, under these conditions. Various similarities between lysosomes/multivesicular bodies (MVBs) and juvenile renin granules have been observed. With the exception of small MVBs, no renin-negative organelles that could be regarded as lysosomes were found in epithelioid cells of mice and rats. Therefore, we suggest that renin granules are modified lysosomes. Immunocytochemical findings indicate that juvenile secretory granules of epithelioid cells represent the converting and activating compartment for prorenin. Endocytosed foreign tracers such as HRP or cationized ferritin are preferentially internalized by juvenile renin granules, which hence appear to be outstanding by their fusogeneity. Consequently, juvenile granules are probably responsible for the secretion of prorenin, and mature granules for that of active renin.

Atrial natriuretic peptide inhibits renin release from juxtaglomerular cells by a cGMP-mediated process

We have examined the effect of a synthetic analogue of human alpha-atrial natriuretic peptide (ANP), APII, on renin release in cultured renal juxtaglomerular cells (JGA cells). Using cell cultures containing 80-90% renal juxtaglomerular cells, we found that ANP (10(-13)-10(-9) M) strongly inhibited renin release from the cells in a dose-dependent fashion (ki, 10 pM) to about 10% of control. Inhibition of renin release by ANP was paralleled by an increase in cellular cGMP levels; while in the presence of the cGMP-phosphodiesterase inhibitor M&B 22948 (1 mM), concentrations of ANP lower by a factor of 100 were required to obtain the same effects on renin release and cGMP levels. The guanylate cyclase inhibitor methylene blue (10 microM), on the other hand, shifted the dose-response curves for renin release and cGMP levels to 100-fold higher concentrations of ANP. Neither the influx of 45Ca into the cells nor the intracellular quin-2 signal, which is a measure for changes of intracellular Ca concentration, was in any way altered by ANP. Our results suggest that ANP inhibits renin release from juxtaglomerular cells by a cGMP-dependent process that does not involve changes in intracellular calcium.

Anatomy of the renin-angiotensin system in the normal and pathological kidney

In this review we describe the contributions made by immunocytochemistry to our knowledge of the renin-angiotensin system in the normal and the pathological kidney. Most of the renin-secreting cells appear to be on the outer aspect of the vessel wall, supporting the view that renin is secreted mainly into the interstitium of the kidney rather than into the lumen of the vessel. Angiotensin II immunoreactivity is present within renin-secreting cells. The angiotensin II appears to be present in high concentration in the renin storage granules and is therefore presumably secreted from the cell with renin. The pathways by which renin is secreted from the cell have also been clarified. In pathological kidneys, the reactions of renin-secreting cells to variation in functional demand have been confirmed. Renin-containing cells have also been found in most types of renal tumours and occasional cases probably secrete renin or prorenin into the blood. In renal tumours and in the developing kidney (in all species studied) the renin-containing cells are also intimately associated with blood vessels.

The morphological basis of fluid balance in the interstitium of the juxtaglomerular apparatus

The morphological basis of fluid balance in the interstitium of the juxtaglomerular apparatus (JGA) was reevaluated in rats, mice and Tupaia. Three ultrastructural features in the region of the vascular pole of the renal corpuscle are described that may be important for the fluid balance in this region: (1) podocyte foot processes in the parietal layer of Bowman's capsule, (2) endothelial fenestrations in the wall of the incoming afferent arteriole, both facing Goormaghtigh and epithelioid cells, and (3) the mesangial-type lining of the glomerular stalk. With respect to the relevant pressure gradients, this morphology may provide the basis of bulk-fluid flow directed to the interstitium of the JGA including the Goormaghtigh cell field. Thus, the fluid balance in the lacis area and, consequently, the tubulo-glomerular feedback mechanism, probably does not solely depend upon the reabsorptive transport of the macula densa. Similar considerations may be valid for the humoral control of renin secretion from juxtaglomerular epithelioid cells.

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

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 microns 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.

Long-term culture of renin containing tissue

Thin cortical tissue explants from kidneys of hydronephrotic mice were excised and incubated in different culture media containing growth and proliferation factors. Over a period of several months the content of renin in the explants and in the culture medium was repeatedly measured, to define the conditions necessary for the maintenance of renin production in a long-term culture. The best results were obtained when culturing the renal tissue in Dulbecco's medium (DMEM) with 10% fetal calf serum, 6 units/100 ml platelet-derived growth factor and 200 ng/ml glycylhistidyllysine. Renin was still present within the cells and in the culture medium after more than six months. Prevention of dedifferentiation, as evidenced in this case by the maintenance of renin production, seemed to be dependent on specific extracellular matrix proteins of renal origin. If the explants were dissociated from their matrix components by collagenase, a gradual loss of renin production was observed within 5 days. Complementation of the collagenase-digested cell suspension with different nonrenal extracellular matrix materials did not afford the stabilizing effect of the original pericellular matrix.

Evidence for existence of angiotensins I and II in mature renin granules from rat kidney cortex

Renin granules were isolated by the combination of discontinuous and continuous Percoll density gradient centrifugation. The peak fraction containing the highest concentration of renin granules was found to contain the highest concentration of both angiotensin I and II immunoreactive substances. The identity of the immunoreactive peptides was further confirmed as angiotensin I and angiotensin II by high pressure liquid chromatography in reference to standard compounds. The coexistence of angiotensins I and II with renin indicates the formation of angiotensin II in renin granules. These findings clarify the mechanism of intracellular formation of angiotensin II as opposed to its formation in plasma and provide evidence against the internalization of angiotensin II, a hypothesis supported by the failure to detect angiotensin I in renin granules. Angiotensin II was increased by a low sodium diet while a high sodium diet did not affect its content.

Are the renin-containing granules of juxtaglomerular epithelioid cells modified lysosomes?

Mature secretory granules of epithelioid cells--the so-called renin granules--exhibit certain properties, which in this particular combination are expressed only by lysosomes: Renin granules have autophagic capabilities; they react to the application of lipidosis-inducing, lysosomotropic substances by the gradual accumulation of polar lipids; all secretory granules of epithelioid cells contain acid phosphatase until maturity; and exogenous tracers reach renin granules without labeling the Golgi complex. Several functional implications can therefore be considered. Hydrolytic enzymes, constitutive elements of the granule matrix, might either cleave inactive prorenin to yield active renin within the granules or, by unspecific hydrolysis of renin, participate in the regulation of the overall quantity of secretory product. Autophagic phenomena, the involvement of renin granules in the traffic of exogenous tracers, and the build-up of polar lipids following experimental interference with lipid catabolism indicate a large turnover of membrane material in renin granules. They also suggest that cytoplasmic and extracellular fluid gains access to the granule content and may thus be involved there in the regulation of biochemical reactions by changing the intragranular milieu or via signal molecules. In addition to the lysosome-like properties of epithelioid cell secretory granules, the secretory product, renin, as a carboxyl protease, is structurally related to other acidic proteases. In the case of cathepsin D, even functional similarities exist.