Molecular and functional characterization of a vasotocin-sensitive aquaporin water channel in quail kidney

Both mammals and birds can concentrate urine hyperosmotic to plasma via a countercurrent multiplier mechanism, although evolutionary lines leading to mammals and birds diverged at an early stage of tetrapod evolution. We reported earlier (Nishimura H, Koseki C, and Patel TB. Am J Physiol Regul Integr Comp Physiol 271: R1535-R1543, 1996) that arginine vasotocin (AVT; avian antidiuretic hormone) increases diffusional water permeability in the isolated, perfused medullary collecting duct (CD) of the quail kidney. In the present study, we have identified an aquaporin (AQP) 2 homolog water channel in the medullary cones of Japanese quail, Coturnix coturnix (qAQP2), by RT-PCR-based cloning techniques. A full-length cDNA contains an 822-bp open reading frame that encodes a 274-amino acid sequence with 75.5% identity to rat AQP2. The qAQP2 has six transmembrane domains, two asparagine-proline-alanine (NPA) sequences, and putative N-glycosylation (asparagine-124) and phosphorylation sites (serine-257) for cAMP-dependent protein kinase. qAQP2 is expressed in the membrane of Xenopus laevis oocytes and significantly increased its osmotic water permeability (P(f)), inhibitable (P < 0.01) by mercury chloride. qAQP2 mRNA (RT-PCR) was detected in the kidney; medullary mRNA levels were higher than cortical levels. qAQP2 protein that binds to rabbit anti-rat AQP2 antibody is present in the apical/subapical regions of both cortical and medullary CDs from normally hydrated quail, and the intensity of staining increased only in the medullary CDs after water deprivation or AVT treatment. The relative density of the approximately 29-kDa protein band detected by immunoblot from the medullary cones was modestly higher in water-deprived/AVT-treated quail. The results suggest that 1) medullary CDs of quail kidneys express a mercury-sensitive functioning qAQP2 water channel, and 2) qAQP2 is at least partly regulated by an AVT-dependent mechanism. This is the first clear identification of AQP2 homolog in nonmammalian vertebrates.     

Two distinct aquaporin-4 cDNAs isolated from medullary cone of quail kidney

Water deprivation or arginine vasotocin upregulates aquaporin-2 (AQP2) expression in apical and subapical regions of medullary collecting duct (CD) cells of Coturnix coturnix quail (q) kidneys. We therefore aimed to determine whether the CD has AQPs mediating water exit from the intracellular to the extracellular (interstitial) space. Using a homologue cloning technique, we isolated two distinct qAQP4 cDNAs from quail medullary cones; long (L, open reading frames) and short (S) cDNA encoded 335 (qAQP4-L) and 301 (qAQP4-S) amino acids with, respectively, 80% and 87% identity to human long- and short-form AQP4. qAQP4-S is identical to qAQP4-L from the second initiation site. Both isoforms have two NPA motifs, but lack cysteine at the known mercury-sensitive site. qAQP4-L and qAQP4-S are expressed in membranes of Xenopus laevis oocytes, but both failed to increase the water permeability (P(f)) of oocytes exposed to a hypotonic solution. Glutamate (Q242) replacement with histidine did not increase P(f). With conventional RT-PCR and real-time PCR, qAQP4-L/S mRNA signals were detected in the brain, lung, heart, intestine, adrenal gland, skeletal muscle, liver, and kidney (higher in medulla than in cortical region). qAQP4-L mRNA was detected only in the brain and adrenal gland. Orthogonal arrays of intramembranous particles were not detected in quail CDs. The results suggest that although qAQP4-L and qAQP4-S have high homology to mammalian AQP4, their physiological function may be different.     

Bilateral ureteral obstruction induces early downregulation and redistribution of AQP2 and phosphorylated AQP2

Bilateral ureteral obstruction (BUO) is characterized by impairment of urine flow from the kidneys and altered expression of specific membrane proteins in the kidney involved in regulation of renal water and salt transport. Importantly, 24-h BUO reduces the abundance of the collecting duct water channel aquaporin-2 (AQP2) and AQP2 phosphorylated at serine 256 (AQP2pS256). To investigate the mechanism behind downregulation of AQP2 in BUO, rats were subjected to BUO and examined after 2, 6, 12, and 24 h. Q-PCR and immunoblotting showed significantly decreased AQP2 mRNA expression after 2-h BUO and decreased abundance of total AQP2 after 12 and 24 h. In parallel, immunohistochemistry showed weaker labeling of AQP2 at the apical surface of inner medullary collecting ducts (IMCD) compared with controls. The abundance of AQP2pS256 was significantly reduced from 6-h BUO and was confirmed by immunohistochemistry. Importantly, immunoblotting showed reduced abundance of AQP2pS261 after 12- and 24-h BUO mimicking total AQP2. Immunohistochemistry demonstrated early changed intracellular localization of AQP2pS261 in BUO, and colocalization studies showed redistribution from the apical membrane to early endosomes and lysosomes. In conclusion, BUO induces a very early regulation of AQP2 both at the level of abundance and on cellular localization. AQP2 and AQP2 phosphorylated at ser261 redistribute to more intracellular localizations and colocalize with the early endosomal marker EEA1 and the lysosomal marker cathepsin D, suggesting that early downregulation of AQP2 could in part be caused by degradation of AQP2 through a lysosomal degradation pathway.     

Expression and Localization of Angiogenic Growth Factors in Developing Porcine Mesonephric Glomeruli

The development and growth of renal glomeruli is regulated by specific angiogenic growth factors, including vascular endothelial growth factor (VEGF) and the angiopoietins (ANGPT1 and ANGPT2). The expression of these factors has already been studied during metanephric glomerulogenesis, but it remains to be elucidated during the development of the embryonic mesonephros, which can function as an interesting model for glomerular development and senescence. In this study, the presence of the angiogenic growth factors was studied in developing porcine mesonephroi, using IHC and real-time RT-qPCR on laser capture microdissected glomeruli. In addition, mesonephric glomerular growth was measured by using stereological methods. ANGPT2 remained upregulated during maturation of glomeruli, which may be explained by the continuous growth of the glomeruli, as observed by stereological examination. The mRNA for VEGFA was expressed in early developing and in maturing glomeruli. The VEGF receptor VEGFR1 was stably expressed during the whole lifespan of mesonephric glomeruli, whereas VEGFR2 mRNA was only upregulated in early glomerulogenesis, suggesting that VEGFR2 is important for the vascular growth but that VEGFR1 is important for the maintenance of endothelial fenestrations. (J Histochem Cytochem 58:1045–1056, 2010)     

Podocytes in metanephric organ culture express characteristic in vivo phenotypes

 Podocytes outgrown from isolated glomeruli in vitro have failed to express fully differentiated in vivo phenotypes. In an attempt to determine whether podocytes in metanephric culture accomplish terminal differentiation, as observed in vivo, we investigated expression of their characteristic phenotypic features in rat metanephric organ cultures using immunohistochemistry and electron microscopy. Rat metanephroi were harvested on embryonic day 12.5 and cultured on transmembrane filters for 9 days. Morphological examination revealed two maturation stages when the podocytes resembled those of the S-shaped body stage and maturational stages of glomeruli in vivo. Electron microscopy revealed that, firstly podocytes lost their intercellular contacts and, simultaneously, the tight junctions shifted into close proximity to cell bases, followed by foot process development. Immunohistochemistry demonstrated that the tight junction protein, ZO-1, and specific podocytic markers, pp44, 5-1–6, podocalyxin and vimentin were expressed in a cell maturity-dependent manner, as observed in newborn rat kidneys. Furthermore, glomerular basement membrane components, collagen type IV and laminin, were expressed in the glomerular center. Our findings that cell maturity-dependent expression of structural and functional phenotypes in podocytes in metanephric culture was the same as that observed in developing kidneys in vivo indicate that podocyte differentiation during glomerulogenesis may be operated by an intrinsic property, such as programmed cell fate. Furthermore, these highly differentiated podocytes in vitro may provide clues that will help to establish a podocyte culture system. Accepted: 26 February 1997     

Development of embryonic stem cells in recombinant kidneys

Embryonic stem cells (ESC) are self-renewing and can generate all cell types during normal development. Previous studies have begun to explore fates of ESCs and their mesodermal derivatives after injection into explanted intact metanephric kidneys and neonatal kidneys maturing in vivo. Here, we exploited a recently described recombinant organ culture model, mixing fluorescent quantum dot labeled mouse exogenous cells with host metanephric cells. We compared abilities of undifferentiated ESCs with ESC-derived mesodermal or non-mesodermal cells to contribute to tissue compartments within recombinant, chimeric metanephroi. ESC-derived mesodermal cells downregulated Oct4, a marker of undifferentiated cells, and, as assessed by locations of quantum dots, contributed to Wilms’ tumor 1-expressing forming nephrons, synaptopodin-expressing glomeruli, and organic ion-transporting tubular epithelia. Similar results were observed when labeled native metanephric cells were recombined with host cells. In striking contrast, non-mesodermal ESC-derived cells strongly inhibited growth of embryonic kidneys, while undifferentiated ESCs predominantly formed Oct4 expressing colonies between forming nephrons and glomeruli. These findings clarify the conclusion that ESC-derived mesodermal cells have functional nephrogenic potential, supporting the idea that they could potentially replace damaged epithelia in diseased kidneys. On the other hand, undifferentiated ESCs and non-mesodermal precursors derived from ESCs would appear to be less suitable materials for use in kidney cell therapies.     

Transplantation of rat metanephroi into mice

To determine whether transplanted metanephroi grow and differentiate after implantation into the omentum in hosts of a different species, we implanted metanephroi from embryonic day 15 (E15) rat embryos into uninephrectomized mice (hosts). Some host mice received human CTLA4Ig (hCTLA4Ig), anti-CD45RB, and anti-CD154 (tolerance-inducing agents). E15 metanephroi contained only metanephric blastema, segments of ureteric bud, and primitive nephrons with no glomeruli. Rat metanephroi did not grow or differentiate in mice that received no tolerance-inducing agents. However, by 2 wk posttransplantation in mice that received hCTLA4Ig, anti-CD45RB, and anti-CD154, metanephroi from E15 rats had enlarged, become vascularized, and formed mature tubules and glomeruli. Rat metanephroi contained cells that stained specifically for mouse CD31, a marker for sprouting endothelial cells. Some rat glomerular capillary loops stained positively for mouse CD31. Here, we show that chimeric kidneys develop from metanephroi transplanted rat-->mouse and that glomeruli are vascularized, at least in part, by host vessels.     

COX-2 disruption leads to increased central vasopressin stores and impaired urine concentrating ability in mice

It was hypothesized that cyclooxygenase-2 (COX-2) activity promotes urine concentrating ability through stimulation of vasopressin (AVP) release after water deprivation (WD). COX-2-deficient (COX-2(-/-), C57BL/6) and wild-type (WT) mice were water deprived for 24 h, and water balance, central AVP mRNA and peptide level, AVP plasma concentration, and AVP-regulated renal transport protein abundances were measured. In male COX-2(-/-), basal urine output and water intake were elevated while urine osmolality was decreased compared with WT. Water deprivation resulted in lower urine osmolality, higher plasma osmolality in COX-2(-/-) mice irrespective of gender. Hypothalamic AVP mRNA level increased and was unchanged between COX-2(-/-) and WT after WD. AVP peptide content was higher in COX-2(-/-) compared with WT. At baseline, plasma AVP concentration was elevated in conscious chronically catheterized COX-2(-/-) mice, but after WD plasma AVP was unchanged between COX-2(-/-) and WT mice (43 ± 11 vs. 70 ± 16 pg/ml). Renal V2 receptor abundance was downregulated in COX-2(-/-) mice. Medullary interstitial osmolality increased and did not differ between COX-2(-/-) and WT after WD. Aquaporin-2 (AQP2; cortex-outer medulla), AQP3 (all regions), and UT-A1 (inner medulla) protein abundances were elevated in COX-2(-/-) at baseline and further increased after WD. COX-2(-/-) mice had elevated plasma urea and creatinine and accumulation of small subcapsular glomeruli. In conclusion, hypothalamic COX-2 activity is not necessary for enhanced AVP expression and secretion in response to water deprivation. Renal medullary COX-2 activity negatively regulates AQP2 and -3. The urine concentrating defect in COX-2(-/-) is likely caused by developmental glomerular injury and not dysregulation of AVP or collecting duct aquaporins.     

mPGES-1 deletion potentiates urine concentrating capability after water deprivation

PGE(2) plays an important role in the regulation of fluid metabolism chiefly via antagonizing vasopressin-induced osmotic permeability in the distal nephron, but its enzymatic sources remain uncertain. The present study was undertaken to investigate the potential role of microsomal PGE synthase (mPGES)-1 in the regulation of urine concentrating ability after water deprivation (WD). Following 24-h WD, wild-type (WT) mice exhibited a significant reduction in urine volume, accompanied by a significant elevation in urine osmolality compared with control groups. In contrast, in response to WD, mPGES-1 knockout (KO) mice had much less urine volume and higher urine osmolality. Analysis of plasma volume by measurement of hematocrit and by using a nanoparticle-based method consistently demonstrated that dehydrated WT mice were volume depleted, which was significantly improved in the KO mice. WD induced a twofold increase in urinary PGE(2) output in WT mice, which was completely blocked by mPGES-1 deletion. At baseline, the KO mice had a 20% increase in V(2) receptor mRNA expression in the renal medulla but not the cortex compared with WT controls; the expression was unaffected by WD irrespective of the genotype. In response to WD, renal medullary aquaporin-2 (AQP2) mRNA exhibited a 60% increase in WT mice, and this increase was greater in the KO mice. Immunoblotting demonstrated increased renal medullary AQP2 protein abundance in both genotypes following WD, with a greater increase in the KO mice. Similar results were obtained by using immunohistochemistry. Paradoxically, plasma AVP response to WD seen in WT mice was absent in the KO mice. Taken together, these results suggest that mPGES-1-derived PGE(2) reduces urine concentrating ability through suppression of renal medullary expression of V(2) receptors and AQP2 but may enhance it by mediating the central AVP response.     

Immunohistochemical identification of aquaporin 2 in the kidneys of young beef cattle

Aquaporin 2 (AQP2) is a small, integral tetrameric plasma membrane protein that is expressed in mammalian kidneys. The specific constitution of this protein and its selective permeability to water means that AQP2 plays an important role in hypertonic urine production. Immunolocalization of AQP2 has been studied in humans, monkeys, sheep, dogs, rabbits, rats, mice and adult cattle. We analyzed the expression of AQP2 in kidneys of 7-month-old Polish-Friesian var. black and white male calves. AQP2 was localized in the principal cells of collecting ducts in medullary rays penetrating the renal cortex and in the collecting ducts of renal medulla. AQP2 was expressed most strongly in the apical plasma membrane, but expression was observed also in the intracellular vesicles and basolateral plasma membrane. Our study provides new information concerning the immunolocalization of AQP2 in calf kidneys.     

Betaglycan is required for the establishment of nephron endowment in the mouse

Betaglycan is an accessory receptor for the transforming growth factor-β (TGFβ) superfamily, many members of which play key roles in kidney development. The purpose of this study was to define the role of this co-receptor on fetal murine kidney development. Stereological examination of embryonic and adult betaglycan heterozygous kidneys revealed augmented nephron number relative to littermate controls. Fetal heterozygous kidneys exhibited accelerated ureteric branching, which correlated with augmented nephron development at embryonic day (e) 15.5. In contrast, betaglycan null kidneys exhibited renal hypoplasia from e13.5 and reduced nephron number at e15.5. Quantitative real-time PCR analysis of e11.5-e14.5 kidneys demonstrated that heterozygous kidneys exhibited a transient decrease in Bmp4 expression at e11.5 and a subsequent cascade of changes in the gene regulatory network that governs metanephric development, including significant increases in Pax2, Eya1, Gdnf, Ret, Wnt4, and Wt1 expression. Conversely, gene expression in null kidneys was normal until e13.5, when significant reductions were detected in the expression of Bmp4 as well as other key metanephric regulatory genes. Tgfb1 and Tgfb2 mRNA expression was down-regulated in both nulls and heterozygotes at e13.5 and e14.5. The opposing morphological and molecular phenotypes in betaglycan heterozygote and null mutants demonstrate that the levels of betaglycan must be tightly regulated for optimal kidney development.     

Glycoconjugates are stage- and position-specific cell surface molecules in the developing olfactory system, 2: Unique carbohydrate antigens are topographic markers for selective projection patterns of olfactory axons.

Three monoclonal antibodies specific for different carbohydrate antigens were used to analyze the development of the olfactory system in rats. CC2 antibodies react with a subset of main olfactory neurons, their axons, and terminals in the olfactory bulb. CC2 antigens are expressed on dorsomedial neurons in the olfactory epithelium (OE) from embryonic (E) day 15 to adults. In the olfactory bulb (OB), only dorsomedially located glomeruli express CC2 glycoconjugates from postnatal day (P) 2 to adults. Thus CC2 defines a dorsomedially organized projection that is established early in embryonic development and continues in adults. P-Path antibodies react with antigens that are expressed on the olfactory nerve in embryos, and are also detected on cell bodies in the neuroepithelium and in glomeruli of the OB at P2. At P14, P-Path staining is weaker, but remains present on many cells in the epithelium and in many glomeruli in the bulb. Postnatally, P-Path immunostaining continues to decrease in most regions of the OE and OB. At P35 and afterwards, only a few P-Path-positive neuronal cells can be detected in the OE. Furthermore, after P35 only two groups of glomeruli in the OB are P-Path immunoreactive. One is situated adjacent to the accessory olfactory bulb (AOB) at the dorsocaudal surface of the OB. The other is adjacent to the AOB at the ventrocaudal surface of the OB. Thus, in adults, P-Path glycoconjugates are expressed in neurons and axons that project only to a specific subset of caudal glomeruli of the OB. Monoclonal antibody 1B2, reacts with beta-galactose-terminating glycolipids and glycoproteins. At P2, 1B2 immunoreactivity is seen on a subset of cell bodies that are distributed throughout the OE and is expressed in most glomeruli in the OB at this age. By P35 and in adults, 1B2 continues to be expressed on a subset of neurons in the OE that project to only a small subset of glomeruli in the OB. Unlike CC2 and P-Path antigens that define specific groups of glomeruli, 1B2-immunoreactive glomeruli do not have a detectable spatial pattern. It is more likely that 1B2 antigens define a specific stage in the maturation of connections between the OE and OB.     

Glycoconjugates are stage- and position-specific cell surface molecules in the developing olfactory system, 2: Unique carbohydrate antigens are topographic markers for selective projection patterns of olfactory axons

Three monoclonal antibodies specific for different carbohydrate antigens were used to analyze the development of the olfactory system in rats. CC2 antibodies react with a subset of main olfactory neurons, their axons, and terminals in the olfactory bulb. CC2 antigens are expressed on dorsomedial neurons in the olfactory epithelium (OE) from embryonic (E) day 15 to adults. In the olfactory bulb (OB), only dorsomedially located glomeruli express CC2 glycoconjugates from postnatal day (P) 2 to adults. Thus CC2 defines a dorsomedially organized projection that is established early in embryonic development and continues in adults. P-Path antibodies react with antigens that are expressed on the olfactory nerve in embryos, and are also detected on cell bodies in the neuroepithelium and in glomeruli of the OB at P2. At P14, P-Path staining is weaker, but remains present on many cells in the epithelium and in many glomeruli in the bulb. Postnatally, P-Path immunostaining continues to decrease in most regions of the OE and OB. At P35 and afterwards, only a few P-Path-positive neuronal cells can be detected in the OE. Furthermore, after P35 only two groups of glomeruli in the OB are P-Path immunoreactive. One is situated adjacent to the accessory olfactory bulb (AOB) at the dorsocaudal surface of the OB. The other is adjacent to the AOB at the ventrocaudal surface of the OB. Thus, in adults, P-Path glycoconjugates are expressed in neurons and axons that project only to a specific subset of caudal glomeruli of the OB. Monoclonal antibody 1B2, reacts with β-galactose-terminating glycolipids and glycoproteins. At P2, 1B2 immunoreactivity is seen on a subset of cell bodies that are distributed throughout the OE and is expressed in most glomeruli in the OB at this age. By P35 and in adults, 1B2 continues to be expressed on a subset of neurons in the OE that project to only a small subset of glomeruli in the OB. Unlike CC2 and P-Path antigens that define specific groups of glomeruli, 1B2-immunoreactive glomeruli do not have a detectable spatial pattern. It is more likely that 1B2 antigens define a specific stage in the maturation of connections between the OE and OB.     

Expression of estrogen receptor-alpha and -beta mRNA in the brain of Japanese quail embryos

The present study was conducted to investigate the mRNA expression of the two estrogen receptor (ER) subtypes ERalpha and ERbeta in the brain of Japanese quail embryos. We found expression of both ERalpha and ERbeta mRNA in homogenate of whole head from 6-day-old embryos, and in brain homogenate from 9- and 12-day-old embryos using real-time PCR. In 9- and 12-day-old embryos the ERalpha expression was higher in females than in males. We used in situ hybridization to examine the localization of the ERs in sections from male and female brains on day 9 and day 17 of incubation. On day 9, ERbeta mRNA was detected in the developing medial preoptic nucleus (POM), in the medial part of the bed nucleus of the striae terminalis (BSTm), and in the tuberal region of the hypothalamus. ERalpha signal could not be detected in the POM, the BSTm or the tuberal region in 9-day-old embryos. In 17-day-old embryos, ERbeta was highly expressed in the preoptic area, the nucleus Taeniae of the Amygdala (TnA) and the BSTm. Expression of ERalpha mRNA was detected in parts of the preoptic area and in the telencephalic TnA. No ERalpha expression was found in the BSTm, an area known to be sexually dimorphic in adults. The high embryonic expression of ERbeta in brain areas linked to sexual behavior indicates that ERbeta plays a role in sexual differentiation of the Japanese quail brain.     

Platelet-derived growth factor receptor beta regulates migration and DNA synthesis in metanephric mesenchymal cells

Platelet-derived growth factor (PDGF) B-chain and PDGF receptor beta (PDGFR beta) are essential for glomerulogenesis. Mice deficient in PDGF B-chain or PDGFR beta exhibit an abnormal glomerular phenotype characterized by total lack of mesangial cells. In this study, we localized PDGFR beta in the developing rat kidney and explored the biological effects of PDGF in metanephric mesenchymal cells in an attempt to determine the mechanism by which PDGF regulates mesangial cell development. Immunohistochemical and in situ hybridization studies of rat embryonic kidneys reveal that PDGFR beta localizes to undifferentiated metanephric mesenchyme and is later expressed in the cleft of the comma-shaped and S-shaped bodies and in more mature glomeruli in a mesangial distribution. We also isolated and characterized cells from rat metanephric mesenchyme. Metanephric mesenchymal cells express vimentin and alpha-smooth muscle actin but not cytokeratin. These cells also express functional PDGFR beta, as demonstrated by autophosphorylation of the receptor as well as activation of phosphatidylinositol 3 kinase in response to PDGF B-chain homodimer. PDGF B-chain also induces migration and proliferation of metanephric mesenchymal cells. Taken together with the fact that PDGF B-chain is expressed in the glomerular epithelium and mesangial area, as demonstrated in the human embryonic kidney, we suggest that PDGF B-chain acts in a paracrine fashion to stimulate the migration and proliferation of mesangial cell precursors from undifferentiated metanephric mesenchyme to the mesangial area. PDGF B-chain also likely stimulates proliferation of mesangial cell precursors in an autocrine fashion once these cells migrate to the glomerular tuft.     

The FgfrL1 receptor is required for development of slow muscle fibers

FgfrL1, which interacts with Fgf ligands and heparin, is a member of the fibroblast growth factor receptor (Fgfr) family. FgfrL1-deficient mice show two significant alterations when compared to wildtype mice: They die at birth due to a malformed diaphragm and they lack metanephric kidneys. Utilizing gene arrays, qPCR and in situ hybridization we show here that the diaphragm of FgfrL1 knockout animals lacks any slow muscle fibers at E18.5 as indicated by the absence of slow fiber markers Myh7, Myl2 and Myl3. Similar lesions are also found in other skeletal muscles that contain a high proportion of slow fibers at birth, such as the extraocular muscles. In contrast to the slow fibers, fast fibers do not appear to be affected as shown by expression of fast fiber markers Myh3, Myh8, Myl1 and MylPF. At early developmental stages (E10.5, E15.5), FgfrL1-deficient animals express slow fiber genes at normal levels. The loss of slow fibers cannot be attributed to the lack of kidneys, since Wnt4 knockout mice, which also lack metanephric kidneys, show normal expression of Myh7, Myl2 and Myl3. Thus, FgfrL1 is specifically required for embryonic development of slow muscle fibers.     

Distribution of histamine in the rat kidney during pregnancy and development

Summary An antiserum against conjugated histamine and two oligonucleotide probes that detect the mRNA encoding L-histidine decarboxylase (HDC) involved in histamine synthesis were used to study the appearance of histamine and its location in the kidneys of fetal, newborn and young postnatal rats and in the kidneys of pregnant rats. On embryonic days 16 and 18 (E16 and E18), some HA-immunoreactive (HA-ir) cells were found within the largest S-shaped bodies. Histamine was found to appear rapidly between the 18th and 20th embryonic days in the convoluted tubules of the kidneys. On postnatal day 0 (P0), the distal convoluted tubules and collecting ducts exhibited bright fluorescence, the intensity of which decreased quickly so that it was faint on day P4 and absent at later stages. In kidneys of pregnant rats HA-ir was found in the epithelium of both the Bowman's capsule, collecting ducts and in a few cells within the tubules. Nonuniform HA-ir was also detected within glomeruli. No evidence for the presence of L-histidine decarboxylase mRNA in kidneys of fetuses or pregnant rats was seen. It is concluded that distinct structures in the developing rat kidney contain histamine during a period around birth from day E20 to day P4. In the pregnant rat, the epithelium that is in direct contact with the urine flow is immunoreactive for histamine from day 16 to 20 of pregnancy. The results suggest that histamine is not synthesized locally in the kidneys but rather originates from other tissues.