Characterization of monoclonal antibodies specific for rabbit renal brush-border hydrolases: application to immunohistological localization

By use of immunodepletion studies, we characterized four monoclonal antibodies reactive with rabbit brush-border (BB) as specific for aminopeptidase N (AP), dipeptidylpeptidase IV (DPPIV), neutral endopeptidase (EP), and angiotensin-converting enzyme (ACE), and we used these antibodies for immunohistochemical detection of these four hydrolases. Expression within the kidney was studied by light and electron microscopy. All four hydrolases are expressed on the various segments of the proximal tubule. In addition, EP and DPPIV are detectable on visceral epithelial cells of the glomerulus and AP on the cells of Bowman's capsule. Outside the kidney, the four hydrolases are expressed within the digestive and genital tracts, where AP, EP, and DPPIV predominate on epithelial structures, whereas ACE is essentially located in vascular structures. The latter localization is also characteristic of ACE in the other organs studied, where clear-cut systematic distribution of the other hydrolases was often difficult to demonstrate. In addition, AP, DPPIV, and EP were detected on lymphoid cells. As compared to reports of data obtained essentially by enzymatic or immunoradiometric assays, these observations suggest considerable interspecies variations of extrarenal expression of the major BB hydrolases. This should be taken into account in attempting to define a general physiological role for a given enzyme.

Na+-H+ exchanger in proximal cells isolated from rabbit kidney. I. Functional characteristics

The purpose of this study was to investigate the characteristics of the Na+-H+ exchange in isolated proximal cells from rabbit kidney cortex. The cells were prepared by mechanical dissociation and sequential passages through nylon meshes. The intracellular pH (pHi) was measured in a bicarbonate-free medium [extracellular pH (pHe) = 7.30], using the fluorescent dye 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). Resting pHi was 7.13 +/- 0.04 (n = 11) at 20-22 degrees C. Cells were acid loaded with nigericin in choline solution and H+ efflux, induced by extracellular Na+ (Nae), was calculated using a buffering power of 23.6 +/- 0.6 mmol.1-1.pH unit-1 (n = 19) estimated by NH4Cl exposure. In isolated proximal cells, the Na+-H+ antiporter had an apparent Km for Nae of 86.7 +/- 1.5 mM (n = 4) and was competitively inhibited by amiloride with a Ki of 33.3 +/- 6.4 X 10(-6) M (n = 3). Lowering pHe, inhibited the Na+-H+ exchanger. This inhibition was not purely competitive and the Ki was 40.4 +/- 12.7 nM (n = 3). The Na+-H+ exchange was greatly activated when the cytoplasm was acidified. The intracellular H+ concentration dependence did not follow simple Michaelis-Menten kinetics. Of the different cations tested on pHi recovery, such as Li+, choline+, K+, and tetramethylammonium, only Li+ induced an alkalinization of acidified cells similar to that of Na+. 22Na influx measurements indicated that cellular depletion of Na+ stimulated Na+-H+ exchange. The results permit the conclusion that the isolation procedures did not impair the main features of the Na+-H+ antiporter, at least as compared with those previously described in renal brush-border membrane vesicles or in other cellular systems. The integrity of the transporter in isolated proximal cells would permit the direct study of its hormonal and metabolic control.

Immunological segmentation of the rabbit distal, connecting, and collecting tubules

To obtain monoclonal antibodies (MAB) specific for the different cell types of distal and collecting tubules, BALB/c mice were immunized with cell suspensions highly enriched in cells from the distal segments of the rabbit nephron. Nine MAB were selected and cloned. Four groups could be identified on the basis of double-labeling immunofluorescence (IF) on frozen kidney sections and on microdissected tubules. In addition, binding specificity at the cellular level was studied by immunoelectronmicroscopy (IEM) for selected MAB. A single MAB (group 1) was specific for distal bright cells and a subpopulation of cortical ascending limb cells. Six MAB (group 2) reacted with connecting and collecting tubules. Five of these (group 2A) had similar binding patterns and reacted identically with the two tubular segments. The MAB studied by IEM was specific for connecting and principal cells. One antibody (group 2B) reacted with only a fraction of the cells associated with the connecting tubule (CNT), but with all cells of the cortical collecting tubule (CCT). By IEM, this antibody was found to be specific for intercalated cells in CNT and bound both principal and intercalated cells of the CCT. Two MAB (group 3) reacted with antigen(s) expressed by the various terminal segments of renal tubule. MAB of groups 1 and 2A, which define distal bright cells and connecting-principal cells from the CNT-CCT, respectively, were used for cell fractionation experiments. Heterogeneous rabbit cortical cells were first incubated with the selected MAB. MAB-bearing renal cells were separated on plastic dishes previously coated with an affinity-purified goat anti-mouse immunoglobulin. Using these procedures it was possible to obtain highly purified subpopulations of distal, bright, or connecting-principal cells.

Indirect immunoselection of late distal cell populations from rabbit kidney cortex

This study describes a method for the separation of distal cell populations based on the sequestration of proximal cells on immunoadsorbent columns (CNBr-activated Sepharose 6MB) bound with three brush-border monoclonal antibodies (S6-Mab). A high yield of isolated cell suspension from rabbit kidney cortex was prepared by mechanical dissociation after perfusion and incubation of the kidneys with 10(-3) M EDTA. The sequestration of the proximal cells was achieved in two sequential chromatographic steps. About 92% of the applied cells were first retained on an S6-Mab column after a 60-min stationary stage and the unbound cells were submitted by direct flow to a second S6-Mab column. In such conditions, 8 X 10(6) cells were recovered when starting with 331 X 10(6) cortical cells. The efficiency of the proximal cell depletion process was confirmed by an 80% decrease in brush-border enzymes, a very low phosphoenolpyruvate carboxykinase activity, and absence of cells bearing long microvilli, as ascertained by electron microscopy. This immunodepleted cell population presented the enzymatical characteristics of cells from the more distal segments. As compared with the initial cell suspension, these cells exhibited higher hexokinase (2.3 times), succinate dehydrogenase (1.5 times), and Na+-K+-ATPase (2.6 times) activities. In addition, adenylate cyclase activities remained sensitive to parathormone, arginine vasopressin, and isoproterenol. The functional capacity of these immunodepleted cells was assessed by an almost complete exclusion of eosin dye, a low Na+ and high K+ intracellular content, and a high respiratory rate of oxygen consumption. In conclusion, this immunoselective process makes it possible to obtain subpopulations of renal cortical cells possessing the main characteristics of the distal, connecting, and collecting cells for physiological and metabolic studies.

Characterization of monoclonal antibodies to rabbit renal cortical cells

The immunological heterogeneity of the rabbit nephron was investigated using monoclonal antibodies. Seventeen antibodies have been produced by fusion of NS1 myeloma cells with spleen cells from BALB/c mice immunized with unfractionated rabbit renal cortical cell preparations. Sixteen antibodies reacted with proximal tubular cells: 11 with the brush border and 5 with basolateral membrane or intracytoplasmic components. Only one of the latter was specific for constituents of the proximal tubule. One antibody reacted with the cortical collecting tubule. Eight of the anti-brush-border antibodies were further characterized by immunoprecipitation of detergent-solubilized radiolabeled brush-border membrane vesicles. Seven proteins with subunits ranging in molecular weight from 90,000 to greater than 340,000 were identified. Systematic survey showed that one of these proteins with a subunit molecular weight of 115,000 exhibited leucine aminopeptidase activity. Selected monoclonal antibodies bound to Sepharose 4B immunoadsorbents were used to deplete solubilized brush-border membrane vesicles of a given antigen and to identify leucine aminopeptidase. Furthermore, the obtention of specific antibodies directed against the proximal tubule allowed us to set up a simple method for renal cell separation: isolated renal cortical cells could be depleted by 80% in proximal cells by passage over columns of Sepharose 6MB covalently linked with three different monoclonal anti-brush-border antibodies, thus leading to cell suspensions considerably enriched in tubule cells originating from the more distal segments of the nephron.

Phosphate uptake by proximal cells isolated from rabbit kidney: role of dexamethasone

A single cell suspension was prepared from rabbit kidney cortex by gentle mechanical dissociation. The isolated cells were functionally intact and metabolically active, as indicated by exclusion of eosin dye, respiratory measurement, and ATP content. The isolated cells were shown to possess long microvilli, and their proximal origin was confirmed by enzymatic and glucose production studies. The uptakes of 0.1 mM phosphate (Pi) and 0.05 mM D-glucose at 37 degrees C were strongly dependent on the extracellular Na+. Incubation of the cells with parathyroid hormone (10 U/ml) for 20 min reduced 20-s Pi uptake by 20%. The addition of dibutyryl cAMP decreased Pi uptake, with a maximal effect at 10(-6) M. Incubation of cells with gluconeogenic substrates, under conditions in which glucose production was increased, did not promote any change in Pi accumulation. When incubated for 60 min at 37 degrees C in the presence of dexamethasone (Dex), the Na+-dependent Pi uptake was decreased by 29% at 10(-9) M and by 36% at 10(-7) M without modification of the glucose uptake. Replacing Dex by aldosterone (10(-7) M) remained without effect on Pi uptake. It is concluded that: renal cells isolated by our preparative method are mainly of proximal origin; isolated cells are a good model for studying the regulation of Pi uptake in the proximal tubule; and glucocorticoids have an acute specific effect on Pi transport detectable in vitro and this effect does not seem to be related to modifications of renal gluconeogenesis.

Cellular heterogeneity of uridine incorporation in collecting tubules: effect of DOCA

In previous studies we showed that in vitro uridine incorporation along the renal tubule is heterogeneous and that DOCA induces a stimulation of RNA synthesis in distal cortical and medullary structures. The present work examines by autoradiography of isolated tubules and renal tissue sections the cellular heterogeneity of the connecting (CNT) and cortical collecting (CCT) tubules after in vivo injection of [3H]uridine in normal and DOCA-treated rabbits. Data confirmed the profile of uridine incorporation along the tubule, which was found in in vitro experiments, and the DOCA-induced stimulation of RNA synthesis. In microdissected CNT and CCT of control kidneys, statistical analysis of the distribution of labeling revealed the presence of two distinct cell populations: one with low labeling (2-3 silver grains per nucleus) and one with high labeling (10-13), which represent 64 and 36%, respectively (CNT), and 74 and 26%, respectively (CCT), of the whole population. Histological data showed that the respective proportions of intercalated cells (29% in CNT; 21% in CCT) and connecting tubule cells (65%) or principal cells (79%) are close to those of the populations with high or low labeling. In addition, autoradiographs on renal sections directly demonstrated that the labeling of intercalated cells (19.3 silver grains/100 micron2 nuclear surface in CNT; 14.7 in CCT) was three times higher than that of connecting (6.6) or principal (5.8) cells. In isolated CNT and CCT, DOCA induced similar absolute increases in the labeling of the two populations. However, the relative increase was more than two times higher in the population with low labeling (+131% in CNT, +210% in CCT) than in the one with high labeling (+71% and +98%). We conclude that cell population of the collecting cortical tubule (CNT and CCT) is heterogeneous with regard to uridine incorporation, reflecting RNA synthesis.

Heterogeneity of uridine incorporation along the rabbit nephron. I. Autoradiographic study

We present here an autoradiographic study of uridine labeling in tubular segments micro-dissected from the rabbit kidney. Kidney pyramids were incubated for 60 min with low (66 nM) and high (66 microM) [3H]-uridine concentration. At the two concentrations studied the labeling was almost exclusively nuclear in all segments studied. At the low concentration, labeling predominated in the macula densa (MD = 63.88 +/- 6.15 silver grains/100 micron2, n = 11), cortical ascending limb (CAL = 19.65 +/- 1.65, n = 15), and initial distal tubule (DCTa = 24.31 +/- 2.70, n = 6). It was minimal in the proximal tubule (PCT2 = 9.14 +/- 1.61, n = 16) and in the cortical (CCT = 5.23 +/- 0.75, n = 18) and medullary (MCT = 5.52 +/- 1.10, n = 12) collecting ducts. At a high concentration, the profile of labeling was roughly similar except for a relative increase in labeling much more pronounced in collecting ducts (CCT = +373, MCT = +323%) than in the other structures (MD = -14, CAL = +66, DCTa = +49, PCT = +9%). Pulse-chase experiments do not show evidence for differences in turnover or degradation rates of RNA between segments, at least in the PCT and the connecting part of the CCT. Analysis of the results at low and high concentration suggests that the observed heterogeneity in uridine labeling depends on both variable endogenous nucleoside pools and different rates of uridine incorporation into RNA from one segment to another.

Heterogeneity of uridine incorporation along the rabbit nephron. II. Effect of DOCA

Uridine uptake was investigated in microdissected segments from the entire length of tubules from normal rabbit kidneys and the effect DOCA treatment was studied. Uridine uptake was measured after in vitro incubation of kidney pyramids with [14C]uridine (0.25 mM). Determinations were done on intact segments, and also using TCA precipitation on both the acid-soluble fraction considered as representative of the pyrimidine nucleoside precursor pool and the acid-insoluble one containing the precipitated nucleic acids. In normal rabbits the fraction of uridine incorporated in the insoluble fraction represented about 5% of that in the soluble one for all segments. Maximal values were found in the distal convoluted tubule (DCT) and the connecting tubule (CNT), where uridine uptake was two- to fourfold higher than in other segments. After DOCA treatment, a significant increase in uridine uptake in the soluble fraction was present in the cortical collecting (CCT) (+90%, P less than 0.001) and medullary collecting tubules (MCT) (+63%, P less than 0.05) when values were corrected for the cellular swelling induced by DOCA in all segments. An even more marked rise in uridine incorporation was observed in the insoluble fraction of the DCT (+88%, P less than 0.05), CCT (+291%, P less than 0.001), and MCT (+422%, P less than 0.05). These results indicate that 1) the level of uridine uptake in soluble and insoluble fractions widely differs along the tubular length of the normal rabbit nephron; and 2) DOCA treatment induces an increase of uridine incorporation in both the pyrimidine precursor pool and nucleic acids of the collecting tubule, known to be the target site for corticosteroid hormones.

Bioluminescence procedures for the measurement of NAD(P) dependent enzyme catalytic activities in submicrogram quantities of rabbit and human nephron structures

Reduced flavin mononucleotide dependent luciferase (EC 1.14.14.3) from Photobacterium fischeri has been used to measure NAD(P) dependent enzymes in submicrogram quantities of tissue homogenates and isolated structures of rabbit and human kidney. The procedure for measuring NAD(P)H was optimized, with internal standardization, to give a linear constant signal between 1 and 100 pmol. This method was applied to the measurement of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and 3-hydroxybutyrate dehydrogenase (EC 1.1.1.30) along the various structures of the rabbit nephron, microdissected from fresh tissue slices. Blank and recovery measurements were performed on each structure, and enzyme catalytic activities were calculated on the basis of tubular length and protein. Glucose-6-phosphate dehydrogenase was found to be present in all nephron structures with highest catalytic activities in glomeruli, thin limbs of Henle's loop and medullary collecting tubules. Lowest catalytic activities were detected in the pars recta of proximal tubules, the distal convoluted and the connecting tubule. The mitochondrial 3-hydroxybutyrate dehydrogenase exhibited a different distribution pattern: Highest catalytic activities were found in the cortical ascending limbs of Henle's loop, the proximal and distal convoluted tubule. An unusual internephron heterogeneity for this enzyme was found in the distal convoluted tubule. Catalytic activities in thin limbs of Henle's loop, the medullary ascending limb, and the cortical and medullary collecting tubule were not significantly different from blank activities. The results obtained in isolated nephron segments agreed with those calculated from cortex and medullary homogenates. In a preliminary experiment on human kidney it could be demonstrated that the procedures can be applied to fresh human biopsy samples. The bioluminescence method offers several advantages (simplicity, rapidity) in comparison with the classical techniques used for ultramicro analysis of tissue enzymes (enzymatic cycling, radiochemical tests). The results confirm the biochemical heterogeneity of nephron structures and allow conclusions about species dependent functional differences of the pentose-phosphate cycle and renal ketone body metabolism along the nephron.

Autoradiographic determination of dexamethasone binding sites along the rabbit nephron

Specific binding sites of tritiated dexamethasone ([3H]dex) along the tubule of rabbit kidney were investigated using an autoradiographic method (dry film) on isolated tubular segments. After in vitro incubation of kidney pyramids with [3H]dex (0.15-53 nM) in the presence or absence of an excess (X200) of unlabeled dexamethasone, tubular segments were microdissected and processed for autoradiography. A quantitative analysis of specific labeling over cytoplasm and nuclei was performed. Specific nuclear binding was observed in all tubular segments beyond the pars recta. This binding was dose dependent and reached much higher values than those reported for aldosterone. In the proximal tubule, the specific labeling was also high but remained mostly cytoplasmic. The meaning of these drastically different intracellular localizations is still open to interpretation. Autoradiography was performed after in vivo injection of [3H]dex and [3H]aldosterone. The results were not different from those described here for dexamethasone and from those previously reported for aldosterone after in vitro incubation. We conclude that specific nuclear binding sites for dexamethasone range over the nephron except for proximal tubule, with no great difference among segments, in contrast to specific sites for aldosterone, which are restricted to distal and cortical collecting tubules. The exact significance of the proximal cytoplasmic specific binding of [3H]dex remains to be determined.

Autoradiographic study of aldosterone and dexamethasone binding in isolated glomeruli of rabbit kidney

Specific binding of tritiated aldosterone ([3H]A) and dexamethasone ([3H]Dex) has been investigated in glomeruli isolated from rabbit kidney by microdissection, using an autoradiographic method on dry film. At low physiological concentrations of [3H]A (0.2-2 nM), no specific binding was detected. At high concentration (24 nM), a low specific binding was found in cytoplasm (2.5 specific silver grains/100 micrometers2) and nuclei (3.7), which was about 10 times lower than in distal tubule. No difference was found between superficial and juxtamedullary glomeruli. At 2.4 nM [3H]A, no aldosterone binding was present in glomerular arterioles. Nuclear binding of [3H]Dex was low at 3 nM (6.4 specific silver grains/100 micrometers2) and reached 23.6 at 100 nM without significant cytoplasmic binding. Here again, this binding was much lower than in distal structures of the nephron. In the few arterioles obtained, a clear specific nuclear binding was observed. We conclude that the glomerulus is probably not a target structure for aldosterone. The existence of a specific binding of dexamethasone, although lower than in the distal tubule, raises the problem of its physiologic significance.

Choline kinase activity along the rabbit nephron

Choline kinase catalyzes the phosphorylation of choline to phosphorylcholine which is thus made available for phosphatidylcholine biosynthesis. Choline kinase activity was determined in defined microdissected structures of rabbit nephron with a radiochemical microprocedure. Enzyme activity was present in all segments tested. When referred to tubular length, the highest activities were found in proximal convoluted tubules. Due to the lower protein content of distal structures these segments exhibited higher enzyme activities when referred to micrograms of protein. From this distribution pattern it was concluded that all nephron segments are able to use extracellular choline for phosphatidylcholine biosynthesis.

Cortical cell populations from rabbit kidney isolated by free-flow electrophoresis: characterization by measurement of hormone-sensitive adenylate cyclase

Free-flow electrophoresis allows the separation of different cell populations from a cell suspension isolated from rabbit kidney cortex after perfusion of the kidneys with a calcium-binder, followed by gentle mechanical treatment. After electrophoretic separation, analysis of the adenylate cyclase activities after stimulation by various hormones allows the precise determination of the origin of the cell populations with different electrophoretic mobilities. Adenylate cyclase from the slow-moving main cell population was only sensitive to parathyroid hormone. These cells had also high alkaline phosphatase content, further demonstrating their proximal origin. The various fast-moving cell populations had adenylate cyclase sensitive to isoproterenol and arginine vasopressin but were less sensitive to parathyroid hormone than the slow-moving cells. Their alkaline phosphatase content was also much lower. This indicates that these fast-moving cell populations originate from both the granulous segment of the distal tubule and from the collecting ducts. The adenylate cyclase activity and the cyclic AMP contents of isolated proximal cells maintained in culture medium were also investigated.

Aldosterone binding in isolated tubules II. An autoradiographic study of concentration dependency in the rabbit nephron

Specific binding of aldosterone along the rabbit nephron was investigated using an autoradiographic method on microdissected tubules within a large range of [3H]aldosterone ([3H]A) concentrations (3 X 10(-10) to 2 X 10(-8) M). Paired incubations with [3H]A in the presence or absence of unlabeled A (100-fold excess) were done to determine the specific binding. In addition, competition studies (10 times excess) with A, dexamethasone (Dex), and estradiol (E) were performed. No specific nuclear binding was detected in the proximal convoluted tubule and pars recta. In the cortical collecting tubule (CCT), a high specific labeling occurred at concentrations of A as low as 3 X 10(-10) M that plateaued at 1.5 X 10(-9) M. Aldosterone was the best competitor. In the bright and granular distal tubule, the binding resembled that of CCT, but Dex was nearly as potent a competitor as A. All along the loop of Henle and in the medullary collecting duct, the binding of A was weak at low concentrations and gradually rose with concentration without reaching a plateau. An almost equivalent displacement was obtained by A and Dex in 10-fold excess. We conclude that aldosterone binds mostly to mineralocorticoid sites in the collecting tubule, whereas the binding appears to be to both mineralo- and glucocorticoid sites in the distal tubule, in the different parts of the loop of Henle, and in the medullary collecting tubule.

Aldosterone binding in isolated tubules I. Biochemical determination in proximal and distal parts of the rabbit nephron

Microbiochemical methods were applied to proximal tubules (PCT) and a mixture of distal and cortical collecting tubules (D + C) of rabbit kidney in order to define aldosterone binding sites. For each experiment, after incubation of kidney pyramids with [3H]aldosterone ([3H]A), either alone or in the presence of an excess unlabeled A, 100-150 mm of both categories of tubules were microdissected using collagenase. Specific binding was determined on the nuclear fraction of each sample. Aldosterone concentrations ranged from 2 X 10(-9) to 4.5 X 10(-8) M. No specific binding was detectable in PCT. Specific binding in D + C increased rapidly as a function of [3H]A concentration up to 5 X 10(-9) M and then more slowly. No plateau was reached. Both the absence of saturation of the binding curve and the curvilinear aspect of the Scatchard plot suggested the presence of two binding sites, one of high affinity, presumably a mineralocorticoid site, and the other of lower affinity, possibly a glucocorticoid site. These experiments suggest that the distal structures of the nephron, located in the cortex, are the main sites of binding of aldosterone and contain a high number of specific binding sites for this hormone.

Renal glycerol metabolism and the distribution of glycerol kinase in rabbit nephron

Glycerol and dihydroxyacetone are metabolized by rabbit kidney-cortex tubules, isolated by collagenase treatment. Half-maximal concentrations of both substrates were determined with regard to uptake rates and product formations. Maximal uptake rates were 643 and 329 mumol/h per g of protein for dihydroxyacetone and glycerol respectively. Glucose and lactate were found as major metabolic products. Glycerol kinase, the enzyme catalysing the first step in renal glycerol and dihydroxyacetone metabolism, was measured radiochemically as described by Newsholme, Robinson & Taylor [(1967) Biochim, Biophys. Acta 132, 338-346] and adapted for studies of the localization of this enzyme along the different structures of rabbit nephron. The results show that glycerol kinase is located exclusively in the proximal segments, i.e. the proximal convoluted tubules and the pars recta, but is negligible in the other structures studied. The activities were close to the maximal dihydroxyacetone uptake rates measured in tubule suspensions.

Distribution of hexokinase and phosphoenolpyruvate carboxykinase along the rabbit nephron

Radiochemical microprocedures were developed for the determination of hexokinase and phosphoenolpyruvate carboxykinase (PEPCK) activity in single microdissected segments of the mature rabbit nephron dissected from fresh tissue after collagenase treatment. All results were related to tubular length and tubular protein content. Hexokinase activity was found to be lowest in the proximal convoluted tubule and to increase along the following nephron segments, with highest activity in the connecting tubule. The gluconeogenic enzyme PEPCK, on the other hand, was exclusively found in the proximal tubule. Early and late portions of the convoluted segment exhibited the same specific activity, but only 50% was found in the pars recta. All other renal structures exhibited only insignificant activity of PEPCK. The results show that renal glucose metabolism and gluconeogenesis are clearly separated. As previously shown for the cytosolic rat enzyme, rabbit mitochondrial PEPCK is also exclusively a proximal tubular enzyme, thus confirming the dominant role of this segment in mammalian renal gluconeogenesis. The high activity of hexokinase in the segments of the distal tubule points to the role of glucose as metabolic fuel, glycogen precursor, and other glucose-6-phosphate-using pathways in these structures.

Gentamicin incorporation along the nephron: autoradiographic study on isolated tubules

Gentamicin is a nephrotoxic antibiotic of the aminoglycoside group, which accumulates within the renal cortex. The present study is an attempt to localize precisely the sites of gentamicin accumulation along isolated tubular segments. We performed autoradiography of 3 H-gentamicin (3H-G) uptake on isolated tubules from kidneys of 6 rabbits previously treated by a single dose of this drug (125 muCi/kg of body wt; 140 microgram/kg of body wt). Isolated tubules were obtained by microdissection following collagenase incubation, 4 hours after 3H-G administration. Autoradiography of single isolated tubular segments was performed according to a dry-film technique. Results were as follows. Almost no gentamicin incorporation (less than 2 silver grains per 150 micrometer2) takes place along the distal parts of the nephron, from the beginning of the loop of Henle to the end of the medullary collecting duct. No differences were visible along these parts of the nephron, whatever their localization, cortical or medullary, In the proximal tubule (PT), we observed a gradual regular increase in 3H-G accumulation, from the glomerulus to the end of the pars recta. The silver grain density progressively increased along this structure from the very early PT (5 per 150 micrometer2) to the last millimeter of the pars recta (40 per 150 micrometers). No clear difference between superficial and juxtamedullary nephrons was detected. The possible mechanisms that could account for this observed variation in 3H-G cellular uptake along the PT are discussed.

Aldosterone binding along the rabbit nephron: an autoradiographic study on isolated tubules

The sites of action of aldosterone (A) along the tubule of rabbit kidney were studied by autoradiographic localization of mineralocorticoid-binding sites on microdissected tubular segments. Kidney pyramids were incubated at 30 degrees C for 1 h in a collagenase solution with [3H]aldosterone at a concentration of 1.5 X 10(-9) M with and without an excess unlabeled A. Tubular segments were then microdissected and transferred onto dry film; fixation and staining were done only after exposure of the film 4 mo later in order to avoid diffusion. Specific nuclear labeling was 19.0 +/- 1.3 silver grains/100 micrometers2 in distal convoluted tubules (n = 28) and 21.0 +/- 1.8 in cortical collecting ducts (n = 18). No difference between these two structures was observed (P greater than 0.1, paired t test, n = 15). No specific binding was found in the proximal tubule (0.5 +/- 0.4, n = 17). In the thick ascending limb of Henle's loop, the labeling was low (3.9 +/- 0.9, n = 16). We conclude that, in the rabbit kidney, nuclear mineralocorticoid-binding sites, presumably receptors, are present in the distal and cortical collecting tubule.

Binding of aldosterone to cytoplasmic and nuclear receptors of the rabbit kidney

The binding characteristics of [3H]aldosterone ([3H]A) in the rabbit kidney were compared with those described in other species, and experimental conditions suitable for both the isolation of renal tubules and the retention of aldosterone receptors were examined. Specific binding of [3H]A to cytoplasmic (C) and nuclear (N) receptors was determined on kidney slices incubated at +30 degrees C in rabbits fed a low K+-high Na+ diet. Results indicate the presence of at least two binding sites in N fractions, one of high affinity (Kd (I)) 1.8 X 10(-8) M) and low capacity (Nmax (I) 4.6 X 10(-14) mol/mg DNA), called type I sites, and the other of lower affinity (Kd (II) 8.9 X 10(-8) M) and more numerous sites (Nmax (II) 13.1 X 10(-14) mol/mg DNA). Competition studies showed that type I sites have higher affinity for [3H]A than for other steroids. Incubation at +37 degrees C resulted in a loss of type I sites. In adrenalectomized rabbits the C binding was improved but not the N binding (Kd (I) 1.2 X 10(-8) M; Kd (II) 5.6 X 10(-8) M). In rabbits treated with aminoglutethimide, an inhibitor of steroid synthesis, the N binding was lower than in other series. It was concluded that 1) [3H]A binding in the rabbit kidney does not differ from that described in other species; 2) a low K+-high Na+ diet is a satisfactory condition for study of [3H]A binding; and 3) incubation at +30 degrees C, a temperature necessary for microdissection procedures, is suitable for aldosterone binding studies.

The effect of puromycin on subepithelial deposits induced by antibodies directed against tubular antigens: a quantitative study

The mechanism of the GN induced by the intravenous injection of antibodies directed against renal tubular epithelial antigens (RTE) was studied by the use of antibody specifically purified by affinity chromatography. With this reagent RTE antigens could be demonstrated on frozen sections of glomeruli as well as on glomerular cells in culture. The antibodies were radiolabelled, and using paired label techniques glomerular binding could be measured in vivo. In normal rats, approximately 4% of the antibody injected was specifically bound in the glomeruli. Binding was not detectable on proximal tubular brush borders. In rats treated with 100 mg/kg of aminonucleoside of puromycin, a substance known to induce alterations of glomerular epithelial cells, it could be shown that glomerular binding was selectively decreased. These observations indicate that immune deposits in passive Heyman nephitis are formed mainly in situ and that local alteration of the glomerulus can markedly influence deposit formation.