Cyclooxygenase-2 is associated with the macula densa of rat kidney and increases with salt restriction

The kidney is a rich source of prostaglandins. These eicosanoids, formed by cyclooxygenase-dependent metabolism of arachidonic acid, are important physiologic mediators of renal glomerular hemodynamics and tubular sodium and water reabsorption. Two separate isoforms of cyclooxygenase (COX) have now been identified: constitutive COX-1, encoded by a 2.8-kb mRNA, and mitogen-activated COX-2, encoded by a 4.0-4.5-kb mRNA. COX-2 expression increases during development and inflammation, but, except for brain, constitutive expression is low. It has been generally accepted that physiologic renal production of prostaglandins is mediated by COX-1. However, in the absence of inflammation, low levels of COX-2 mRNA are also detectable in the kidney. To examine the role of COX-2 in the kidney and determine its intrarenal localization, we used a 1.3-kb cDNA probe specific for the 3' untranslated region of rat COX-2 and COX-2-specific antiserum. The COX-2-specific cDNA probe hybridized with a 4.4-kb transcript in total RNA from adult rat kidney. Immunoblots of microsomes isolated from kidney cortex and papilla indicated immunoreactive COX-2 in both locations. In situ hybridization and immunohistochemistry indicated that renal cortical COX-2 expression was localized to the macula densa of the juxtaglomerular apparatus and to adjacent epithelial cells of the cortical thick ascending limb of Henle. In addition, COX-2 immunoreactivity was detected in interstitial cells in the papilla. No COX-2 message or immunoreactive protein was detected in arterioles, glomeruli, or cortical or medullary collecting ducts. When animals were chronically sodium restricted, the level of COX-2 in the region of the macula densa increased threefold (from 0.86 +/- 0.08 to 2.52 +/- 0.43/mm2) and the total area of the COX-2 immunoreactive cells in cortex increased from 34 microns2/mm2 of cortex to 226 microns2/mm2 of cortex. The intrarenal distribution of COX-2 and its increased expression in response to sodium restriction suggest that in addition to its proposed role in inflammatory and growth responses, this enzyme may play an important role in the regulation of salt, volume, and blood pressure homeostasis.

Morphology of the kidney of the platypus (Ornithorhynchus anatinus: Monotremata)

The platypus kidney shows morphological similarities to those of other mammals. Macroscopically, the cortex is easily distinguishable from the fairly wide medulla. Within the medulla, no clear border is observed between the inner and outer zones. Light and transmission electron microscopically, the glomeruli show quite similar architecture to those of other mammals; however, the glomerular lobulation is very clear. The glomerular tufts are rather simple, but capillary lumen varies widely in size, which is one of the unique features of the platypus kidney. The urinary tubule is generally similar to that of human and other mammals in shape and segmentation; however, the staining specificities of histochemical reactions and the shape of epithelial cells of the Henle's loop differ from those of other mammals. The most conspicuous features are: 1) although no protein casts are found in the tubular lumina, epithelial cells of the proximal convoluted tubule (PCT) have numerous electron-dense vesicles as in human nephrotic kidneys; and 2) the platypus Henle's loop consists of the thick epithelial cells similar to the mammalian type nephron of birds. As compared to those of other mammals such as humans and rats, our observations suggest that the platypus kidney is less developed, in terms of evolution.

An approach to the structure and function of the glomerular mesangium

The mesangium of the glomerulus is a connective tissue tree arising at the vascular pole of the glomerulus and supporting the glomerular capillaries. It is partly covered by a basement membrane that follows the epithelial cells from the peripheral glomerular capillary wall over the supporting tissue. The capillary endothelium does not normally have a separate basement membrane. The endothelium has fenestrations that open directly into the mesangium and allow blood plasma and tracers to flow into the mesangium. The fenestrations partially restrict (or sieve) particles over 405 A in mean length from entry. Tracers move in intercellular channels and are filtered and concentrated by the basement membrane at the sides of the mesangium or by mesangial matrix filaments in the channels between cells. The irregular distributions of flow, matrix, and concentrations of tracers may account for irregular lobular reactions in glomerular disease. Two main pathways of flow seem to be (1) through the basement membrane and between the epithelial foot processes to form part of the glomerular filtrate and (2) into the efferent capillaries through their mesangial fenestrations. Intrinsic mesangial cells can now be regarded as myofibroblasts associated with the production of the connective tissue matrix. These cells hold the basement membrane to maintain the shape of the glomerular capillaries, they swell readily, and they can constrict like smooth muscle cells with appropriate stimulation. These reactions may enable them to control the flow of blood through the capillary network in glomerular disease. Mesangial cells can take up large amounts of foreign material within 24 h. Intrinsic mesangial cells and monocytes can increase in numbers in disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular mechanisms of renin release

Renin secretion from renal juxtaglomerular cells is controlled by the intrarenal blood pressure, sodium chloride load of the organism, sympathetic nerve activity, and a number of hormones. The mechanisms by which these parameters exert their effects are not well understood. However, it is reasonable to assume that they act via signal transduction systems, implying the role of second messenger molecules in the control of renin secretion. This article summarizes present knowledge about the cellular mechanisms of renin secretion. Moreover, current concepts about the mechanisms by which the blood pressure and the sodium chloride load could influence renin secretion are discussed.

Anatomical relationship between kallikrein-containing tubules and the juxtaglomerular apparatus in the human kidney

Current evidence suggests a functional and biochemical link between the renin and the kallikrein systems. The purpose of this work was to study the localization of kallikrein along the human nephron to elucidate whether there exists an anatomical base for such interrelation. Serial sections of human kidney tissue were stained by immunocytochemical methods with antisera against kallikrein. Kallikrein immunostaining was observed exclusively in segments of the distal nephron lying in the cortical labyrinths and forming arcades in its distal portion. Consistently the tubules containing kallikrein established a close anatomical relationship with the afferent arteriole of the juxtaglomerular apparatus providing an anatomical base for an interaction between the renin and kallikrein systems in the human kidney.

Mammalian renal modifications in dry environments

The literature on the role of the kidney and renal morphological modifications in places of limited water supply is reviewed. The anatomical structures for urine concentration found in animals living in desert or arid environments, although not all occurring in one particular animal, are wide medullae, long loops of Henle, long proximal tubules, long collecting tubules, small renal corpuscles, extension of the renal pelvis, well developed elongated papillae, occurrence of giant vascular bundles, specialized ultrastructure of Henle's loops, epithelial changes in the collecting tubule, zonation of the vasa recta and peculiarity of the arterial supply to the kidney. The renal renin content is moderately high in these species. The renin-angiotensin-aldosterone system is very active, retaining Na+ with water. The urine is concentrated at the expense of other electrolytes. Both the renal blood and urinary flow rates are lower than in species with access to unlimited water supply. The juxtaglomerular apparatus components are topographically intimate for effective tubuloglomerular autoregulation of renal blood flow.

The renin-angiotensin system in nonmammalian vertebrates

The most primitive components of the RAS appeared early in the phylogenetic history of vertebrate animals. It is probable that renin granules were present in the kidneys of ancestral chordates before divergence in the evolution of actinopterygian fish and tetrapods occurred. Granulated juxtaglomerular cells similar to the renin-containing cells of the mammalian nephron are found in most extant vertebrate species although not in agnathan and elasmobranch fish. A macula densa occurs in amphibians, birds and mammals; and an extraglomerular mesangium, only in birds and mammals. Renin-like activity and angiotensin-like pressor material have been demonstrated in all classes of vertebrates. The amino acid sequences of native ANG I have been determined for representative species of teleost fish, amphibian, reptile and bird. These peptides differ from mammalian angiotensins at positions 1, 5 and 9. The RAS appears to be involved in osmoregulation, ionoregulation and the control of blood circulation. Prolonged hypovolemic hypotension or sodium depletion increases renin levels. Angiotensins elicit drinking and stimulate transepithelial ion transport. However, direct steroidogenic and antidiuretic hormone-releasing activities, which would promote mineral and fluid conservation, have not been demonstrated unambiguously in nonmammalian vertebrates. ANG II raises blood pressure by direct vasoconstrictor action on arteriolar muscles in some animals, but perhaps more generally by acting on the nervous system and adrenal paraneurons. In birds the hormone also has a hypotensive effect. ANG II stimulates the SNS in agnathans, elasmobranchs, teleosts, amphibians, reptiles, birds and mammals. Thus, modulation of sympathetic activity may be one of the most primitive and conservative functions of the RAS. For this reason, nonmammalian vertebrates are valuable models for studying the neurogenic actions of angiotensin II relevant to hypertensive disease.

Morphological aspects of the rat kidney preserved by cold storage. II. The juxtaglomerular apparatus

The present paper reports on the changes occurring in the juxtaglomerular apparatus during preservation of the rat kidney by cold storage, using two media: Sacks and Plasmagel, to which a membrane stabilizing "cocktail" was added (hydrocortisone, chlorpromazine, epsilonaminocaproic acid - EACA, propranolol). Evident alterations appeared at 48 hours more accentuated at 72 and 96 hours, and more intense when preserved in Plasmagel. The most affected structure in the juxtaglomerular apparatus was the macula densa, the epithelial cells having a more stable structure.

[Morphometry on the juxtaglomerular apparatus and the glomerulus of the rat (author's transl)]

The area of contact between the macula densa and the terminal vascular components, the vas afferens, the vas efferens and the mesangium (Goormaghtigh cells) of five glomeruli of the rat has been determined morphometrically using serial sections. The surface, the filtration area, the volume and the total length of the capillaries of each glomeruli has been estimated, and these data have been correlated. There is evidence, that a contact between the macula densa and the mesangium always exists, a contact to the vas afferens is not obligatory.

[Morphometric analysis of the glomeruli and juxtaglomerular apparatus of rat kidneys during the course of experimental hypokinesia]

In the hypokinetic study a certain correlation between variations in the means of the volume of cortical and juxtamedullary glomerules and the function of the juxtaglomerular apparatus has been established. It is hypothesized that at early hypokinesia renal vascular changes are nonspecific. The phenomenon of renal excretion of fluids and electrolytes is explained in terms of a simultaneous increase in the reninangiotensin activity and 11-hydroxycorticosteroid content in plasma.

Structural organization of heterogeneous basal laminas in human glomeruli

Differential response to 5 M guanidine, pH 7.0, shows at least two classes of glomerular basal laminas: epithelial and endothelial-mesangial. The former is the predominant element, folding into capillary tufts and loops but never completely encircle the entire circumference of glomerular capillaries. It remains as a single, continuous sheet within each glomerulus. The mesangial basal lamina partitions the vascular space into individual capillary lumen. The endothelial basal lamina, which extends from mesangium to encircle capillary lumens, is poorly developed, and undetectable in most capillaries. The organization of these two classes of basal laminas provides a structural framework for understanding glomerular permeability, especially the spatial relationship between capillary lumens and the mesangium. Interruption of the juxtaglomerular epithelial basal lamina, with sprouting of a new endothelial-mesangial bud, and formation of a new glomerular capillary tuft are noted in a group of patients with diabetic glomerulosclerosis and chronic renal failure. Wrinkling and resorption of the juxtamesangial epithelial basal lamina with formation of a new epithelial basal lamina are also observed. These findings indicate that the mesangium and the juxtamesangial epithelial basal lamina may be the active site of glomerular basal lamina resorption, turnover, and neoangiogenesis.

Anatomy of the juxtaglomerular apparatus

The juxtaglomerular apparatus, located in the glomerular hilum, consists of a vascular component (afferent and efferent arterioles and extraglomerular mesangium) and a tubular component (macula densa). Two types of contact between vascular and tubular components are observed: a) a complex type, involving distal tubule, extraglomerular mesangium, and proximal efferent arteriole, and b) a simple type, consisting of apposition of the basement membranes of the vascular and tubular components. Juxtaglomerular granular cells, the source of renin, are present throughout the vascular component but are more numerous in the afferent arteriole. They can be considered as "myoendocrine" cells, since they contain myofibrils and attachment bodies, together with secretory granules and crystalline protogranules. Macula densa cells differ from those elsewhere in the distal tubule in that their nuclei are closer to each other, the Golgi apparatus is basally located, and their basal membrane infoldings are less prominent. Adrenergic nerves are demonstrable by fluorescence histochemistry in the juxtaglomerular region. Electron microscopy reveals unmyelinated nerve fibers containing small dense-cored vesicles and capable, as shown by ultrastructural autoradiography, of incorporating exogenous tritiated norepinephrine. Neuroeffector junctions occur between nerves and cells of the vascular and, less frequently, the tubular component. In addition, adrenergic axons are observed in a juxtaglomerular cell tumor. Nerve terminals are seen in direct contact with the tumor cells.

Distribution and density of the canine renal cortical lymphatic system

The pattern, distribution, and extent of the lymphatic circulation in the canine renal cortex was studied with light and electron microscopy, in two groups of animals, one with and one without ipsilateral ureteric obstruction for 3 days. Recognition of lymphatics in tissue sections was facilitated by mild dilatation, induced in both groups by ligation of the renal collecting vessels for 4 to 6 hours, and by retrograde injection of tracer in a third group. Of 77 lymphatics present in 180 blocks from six kidneys, approximately one third were intralobular, the remainder being primarily associated with interlobular blood vessels. The cross-sectional area of interlobular lymphatics was almost twice that of intralobular lymphatics. The relationships of these lymphatics were analyzed quantitatively. Intralobular lymphatics had primary relationships with terminal arteries, arterioles, renal corpuscles, and tubular elements. Both inter- and intralobular lymphatics had secondary relationships with a small proportion of all components of the cortical parenchyma including juxtaglomerular complexes. The most common association was between lymphatics and elements of the vascular tree. Morphometric analysis was used to obtain volume density data on the composition of the renal cortex. The volume density of lymphatics was 0.0026 in ureter-obstructed kidneys and 0.0017 in nonobstructed kidneys. The cross-sectional surface area of lymphatics in ureter-obstructed kidneys was significantly larger than those in nonobstructed kidneys. The volume density of other cortical components was found to be in good agreement with published data. From the volume density data, it was concluded that the volume of lymph in the renal cortex, under conditions of mild lymphatic dilatation, was about 1% that of the volume of blood in the cortical peritubular capillaries.

The normal juxtaglomerular apparatus in the human kidney. A morphological study

Out of 49 serially studied juxtaglomerular apparatuses, 6 typical variants from two normal human kidneys were reconstructed graphically. The agranular Goormaghtigh cells filled the entire space between the macula densa, the afferent and the efferent arterioles and the glomerular mesangium. The Goormaghtigh cells were always in direct contact with all the other structures. They also invariably continued into the glomerular mesangium. The distal tubule regularly showed widening in the macula densa segment and, at this level, there was considerable variation in the shape of the distal tubule. Direct contact between the macula densa and the hilar arterioles was not always present, the area of contact was usually greater with the afferent than with the efferent arteriole.

Histochemistry of the juxtaglomerular apparatus in the toad Bufo bufo. The glucose-6-phosphate dehydrogenase activity of the macula densa

An investigation regarding the question of whether there exists a macula densa as part of the juxtaglomerular apparatus in the kidney of amphibians has been carried out. With the aid of a histochemical reaction for glucose-6-phosphate dehydrogenase activity, the presence of a macula densa zone as a specialized part of the distal tubule in the toad Bufo bufo was demonstrated. The functional significance of the high glucose-6-phosphate dehydrogenase activity in the macula densa cells is discussed.

Tubulo-vascular relationships in the developing kidney

The postnatal development of the tubulo-vascular relationships in the kidney was studied in 60 rats. The kidney is relatively immature at birth and only the juxtamedullary glomeruli are present. As their loops of Henle develop they grow down into the medulla around the previously developed collecting ducts, forming tubulovascular 'units' around whose periphery the shorter loops of the more peripheral glomeruli grow down. The bundles of vasa recta develop in loose interstitial tissue which lies between the tubulo-vascular units so that the peripheral vessels of the bundles, which are ascending vasa recta, lie in close relationship with the descending short loops of Henle. Thus, the vascular bundles do not form the central axis around which the medullary components develop but rather take up the available space left after the tubulo-vascular units have developed. Development is virtually complete by the fourth week of postnatal life.

[Modification of Bowie's method of demonstrating specific granules in cells of the human renal juxtaglomerular apparatus fixed in neutral formalin]

A modification of Bowie's method for detection of specific granules of the juxtaglomerular apparatus of the human kidneys, fixed in 10% neutral formalin, is suggested. In order to achieve better staining, sections of material fixed in formalin are additionally treated with Helly's liquid and, following the removal of sublimate deposit, with a 2.5% solution of potassium bichromate. After this the sections are stained by Bowie's method in accordance with Pitcock and Hartroft's prescription.

Renin secretion: an anatomical basis for tubular control

By means of electron microscopy of serial sections and three-dimensional reconstruction of the juxtaglomerular apparatus a quantitative study has been made of the distribution of the areas of contact between the tubular and vascular components. Of the two arterioles the efferent is the only one consistently in contact with the distal tubule. The extraglomerular mesangium is also in contact with the distal tubule in all the apparatuses examined. Two morphologically distinct types of contact are described; one is thought to be permanent and the other reversible. An analysis of the cells of the juxtaglomerular apparatus for their position, granularity, and contact has revealed that the majority of granular cells are not in contact with the distal tubule. The anatomical findings are integrated in a model of the control of renin secretion based on variations in contact between the elements of the juxtaglomerular apparatus.

Some effects of dihydroergocrisine and of phentolamine mesylate on renal function in rats

Single injections of dihydroergocristine (dhe) (25, 50 and 100 μg, i.p.) did not depress the systemic arterial pressure but antagonized the pressor effects of (-)-noradrenaline (100 μg, s.c.) in unanaesthetized rats. The effects of these doses of dhe on water diuresis were minimal: slight K retention, a rise in urinary Na:K and a slight but significant fall in the clearance of p-aminohippuric acid were noted. Single injections of phentolamine mesylate (10, 20 and 40 μg, i.p.) greatly depressed systemic arterial pressure and reduced the glomerular filtration rate and p-aminohippuric acid clearance and the excretion of water and Na proportionately, but markedly. The Na:K in the urine rose. Chemical denervation of the kidneys either with dhe, 32 μg, or phentolamine, 2 mg, thrice daily for 5–7 days decreased, but for 14 days increased, the juxtaglomerular index of the kidneys. Parallel changes were found in the extractable renin. Continued treatment with dhe (32 μg thrice daily for 2 weeks) raised, and hyperduric adrenaline (250 μg twice daily for 1 week) lowered the stores of growth hormone in the adenohypophysis. Continued treatment with dhe, 32 μg thrice daily produced antidiuresis and retention by the third day lasting to the fifth. This condition had reversed by the eighth day.

Effects of high temperature and acute diuresis on glucose-6-phosphate dehydrogenase activity of mascula densa and plasma renin activity in rats

Experiments performed in rats exposed to heat and to acute diuresis revealed: (a) a positive influence of the environmental temperature on plasma renin activity; (b) a dissociation between plasma renin activity and the glucose-6-phosphate dehydrogenase activity of the macula densa following furosemide administration; and (c) the absence of any significant changes in juxtaglomerular cell granularity despite a very significant increase in plasma renin activity.